Advances in Energy Storage Systems

Transactions on Energy Conversion. 2007;22(2):439-449. DOI: 10.1109/

[16] Harrabi N, Soussi M, Aitouche A.

Maximum power control for photovoltaic power based on fuzzy Takagi-Sugeno model. Journal of Electrical Engineering. 2015;25:1-11

[17] Pouresmaeil E, Miracle DM, Bellmunt OG. Control scheme of threelevel NPC inverter for integration of renewable energy resources into AC grid. IEEE Systems Journal. 2012;6(2):

[18] Subsingha W. Design and analysis three phase three level diode-clamped grid connected inverter. Energy Procedia. 2016;89:130-136

[19] Powerex Company Catalogue: Application Notes to Three-Level Inverter Technology, First Release;

[20] Tremblay O, Dessaint L-A. Experimental validation of a battery dynamic model for EV applications. World Electric Vehicle Journal. 2009;3

Engineering. 2015;15

[21] El Shahat A, Haddad R, Kalaani Y. Lead acid battery modeling for photovoltaic applications. Journal of Electrical

TEC.2006.874230

Energy Storage Devices

242-253

2009:1-12

68

**71**

**Chapter 5**

**Abstract**

600 cycles at 1 A g<sup>−</sup><sup>1</sup>

**1. Introduction**

of 3578 mAh g<sup>−</sup><sup>1</sup>

SiO*x* as a Potential Anode Material

Despite the high energy density of SiO*x*, its practical use as an anode material for Li-ion batteries is hindered by its low electronic conductivity and sluggish electron transport kinetics. These disadvantageous properties result from the insulating nature of SiO2, which leads to electrical contact loss and poor cyclability. Herein, we synthesized a C-SiO*x* composite based on amorphous carbon and a SiO*x* matrix via the alcoholysis reaction between SiCl4 and ethylene glycol. We then used nonpolar benzene to simultaneously achieve homogenous dispersion of the Si source and the formation of a carbon coating layer, resulting in the formation of a (C-SiO*x*)@C composite with exceptional electrochemical properties. Next, we performed structural modifications using Ti doping and a multiple-carbon matrix to successfully fabricate a (C-Ti*x*Si1−*x*O*y*)@C composite. The combination of Ti doping and carbon coating greatly enhanced the conductivity of SiO*x*; moreover, the incorporated carbon acted as an effective oxide buffer, preventing structural degradation. The (C-Ti*x*Si1−*x*O*y*)@C composite exhibited excellent capacity retention of 88.9% over

.

for carbon-based electrodes) [4–6].

for Li-Ion Batteries: Role of

Structural Modifications

*Hyeon-Woo Yang and Sun-Jae Kim*

Carbon Coating, Doping, and

with a capacity of 828 mAh g<sup>−</sup><sup>1</sup>

(compared with 372 mAh g<sup>−</sup><sup>1</sup>

**Keywords:** lithium ion battery, SiO*x* anode, multiple carbon matrix, doping

Silicon (Si) is a key anode material for fabricating next-generation Li-ion batteries (LIBs) with longer cycle life and higher energy density to help meet the growing market demand for electric vehicles (EVs) and hybrid cars [1–3]. As a host material for lithium, Si is earth-abundant and delivers a high theoretical capacity

Nevertheless, the large volumetric expansion (∼400%) of Si anodes results in degradation of Si particles and destruction of the solid-electrolyte interphase (SEI) [7–9]. These issues can induce drastic capacity fade and even overall damage to the electrodes, thereby hindering the commercial application of Si anodes in LIBs.

Silicon suboxide (SiO*x*, 0 < x < 2) has attracted considerable interest as a potential alternative to Si because of its enhanced cycling stability. SiO*x* not only exhibits a relatively small volume expansion but also forms Li2O and Li silicates that serve as buffer media for Si during the first lithiation process [10–12]. As a result, SiO*x*

#### **Chapter 5**

## SiO*x* as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping, and Structural Modifications

*Hyeon-Woo Yang and Sun-Jae Kim*

### **Abstract**

Despite the high energy density of SiO*x*, its practical use as an anode material for Li-ion batteries is hindered by its low electronic conductivity and sluggish electron transport kinetics. These disadvantageous properties result from the insulating nature of SiO2, which leads to electrical contact loss and poor cyclability. Herein, we synthesized a C-SiO*x* composite based on amorphous carbon and a SiO*x* matrix via the alcoholysis reaction between SiCl4 and ethylene glycol. We then used nonpolar benzene to simultaneously achieve homogenous dispersion of the Si source and the formation of a carbon coating layer, resulting in the formation of a (C-SiO*x*)@C composite with exceptional electrochemical properties. Next, we performed structural modifications using Ti doping and a multiple-carbon matrix to successfully fabricate a (C-Ti*x*Si1−*x*O*y*)@C composite. The combination of Ti doping and carbon coating greatly enhanced the conductivity of SiO*x*; moreover, the incorporated carbon acted as an effective oxide buffer, preventing structural degradation. The (C-Ti*x*Si1−*x*O*y*)@C composite exhibited excellent capacity retention of 88.9% over 600 cycles at 1 A g<sup>−</sup><sup>1</sup> with a capacity of 828 mAh g<sup>−</sup><sup>1</sup> .

**Keywords:** lithium ion battery, SiO*x* anode, multiple carbon matrix, doping

#### **1. Introduction**

Silicon (Si) is a key anode material for fabricating next-generation Li-ion batteries (LIBs) with longer cycle life and higher energy density to help meet the growing market demand for electric vehicles (EVs) and hybrid cars [1–3]. As a host material for lithium, Si is earth-abundant and delivers a high theoretical capacity of 3578 mAh g<sup>−</sup><sup>1</sup> (compared with 372 mAh g<sup>−</sup><sup>1</sup> for carbon-based electrodes) [4–6]. Nevertheless, the large volumetric expansion (∼400%) of Si anodes results in degradation of Si particles and destruction of the solid-electrolyte interphase (SEI) [7–9]. These issues can induce drastic capacity fade and even overall damage to the electrodes, thereby hindering the commercial application of Si anodes in LIBs.

Silicon suboxide (SiO*x*, 0 < x < 2) has attracted considerable interest as a potential alternative to Si because of its enhanced cycling stability. SiO*x* not only exhibits a relatively small volume expansion but also forms Li2O and Li silicates that serve as buffer media for Si during the first lithiation process [10–12]. As a result, SiO*x*

exhibits better cycling performance than Si. Nevertheless, the low electronic conductivity and sluggish electron transport kinetics of SiO*x* resulting from the insulating property of SiO2 lead to poor electrochemical performance and have hindered the application of SiO*x* as anode materials for commercialized LIBs [13–16]. Many researchers have proposed strategies to address these issues, resulting in progress such as the development of carbon-coated SiO*x* composites. Although the improved electrical conductivity achieved by carbon coating can improve the electrochemical performance of SiO*x*, complicated, multi-step, and high-temperature processes are required [17–20]. For instance, Liu et al. developed a Si-Void@SiO*x* nanowire composite using thermal evaporation/chemical etching of a mixed powder of SiO and ZnS at high temperatures (1250 and 1650°C) [18]. In addition, Han et al. prepared a SiO@C composite using a two-step process with SiO powder as the raw material; after ball milling for 3 h at 3000 rpm, the ball-milled SiO particles were calcined at 700°C using sodium dodecylbenzene sulfonate [19].

To avoid the complicated and costly processes adopted in previous studies, in this study, a simple and cost-effective one-pot synthesis method was developed to fabricate a carbon-incorporated/carbon-coated SiO*x* ((C-SiO*x*)@C) composite. We attempted to simultaneously form interconnected carbon paths in the composite and encapsulate the surface with carbon using ethylene glycol and benzene. We further attempted to fabricate a SiO*x* composite with superior electrochemical performance by maximizing the electrical conductivity through Ti doping. Ti doping can result in the formation of TiSi alloys, which are beneficial for improving the cyclic stability of LIB electrode materials [21, 22]. In addition, black TiO2−*x* has been reported to exhibit higher conductivity than pristine white TiO2 because of the existence of Ti3+ (corresponding to an oxygen deficiency) in the structure. Thus, we suspected that Ti3+ doping might lead to outstanding electrochemical performance [23–25].

In the current study, we prepared a Ti3+-doped and carbon-incorporated/ carbon-coated SiO*x* ((C-Ti*x*Si1−*x*O*y*)@C) composite and investigated the effects of these structural modifications. The electrochemical performance of the (C-Ti*x*Si1−*x*O*y*)@C composite was greatly improved compared with that of carbonincorporated SiO*x* (C-SiO*x*) and a (C-SiO*x*)@C composite. The electrochemical performance of the (C-Ti*x*Si1−*x*O*y*)@C composite was greatly improved compared with that of the C-SiO*x* and (C-SiO*x*)@C composite. The initial discharge capacity of the (C-Ti*x*Si1−*x*O*y*)@C composite at 0.1 A g<sup>−</sup><sup>1</sup> was ~1304 mAh g<sup>−</sup><sup>1</sup> , which was ~4 times higher than that of C-SiO*x* under the same conditions. Furthermore, the (C-Ti*x*Si1−*x*O*y*)@C composite delivered a capacity retention of ~88.9% over 600 cycles at a higher current density of 1 A g<sup>−</sup><sup>1</sup> with a high coulombic efficiency of ~99%.

### **2. Experimental**

*Preparation of C-SiOx*: First, 13 mL of ethylene glycol (EG, 99.9%, Samchun Co.) was added to 20 mL of SiCl4 (99%, Wako Co.) under vigorous stirring. The mixture was rapidly transformed into a mineral-like solid, which was converted into C-SiO*<sup>x</sup>* powder by heat treatment at 725°C for 1 h under vacuum.

*Preparation of (C-SiOx)@C*: First, 13 mL of EG was poured into a mixture of 20 mL of SiCl4 and 50 mL of benzene (99.5%, Daejung Co.) under vigorous stirring. Benzene was used to control the reaction between EG and SiCl4 necessary for the synthesis of the powder. The mineral-like solid formed through the alcoholysis reaction was transformed into a (C-SiO*x*)@C composite by heat treatment at 725°C for 1 h under vacuum.

**73**

follows:

Li<sup>+</sup>

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping…*

*Preparation of (C-TixSi*<sup>1</sup>−*xOy)@C*: First, 50 μL of TiCl4 (99.9% SHOWA) was dissolved in EG (13 mL) by stirring for 1 day. Then, nonpolar SiCl4 (20 mL) was uniformly distributed in 50 mL of benzene for 30 min. After the solution was added to the mixture of EG and TiCl4 under vigorous stirring, the solution-state mixture was converted into a yellow mineral-like solid containing Si, Ti, O, and C. Finally, the (C-Ti*x*Si1−*x*O*y*)@C composite was obtained by heat treatment in a tube furnace

*Materials characterization:* The morphologies of all the samples were characterized using field-emission scanning electron microscopy (FESEM; SU-8010 and S-4700, Hitachi Co.), high-resolution transmission electron microscopy (HRTEM; JEM 2100F, JEOL); and Cs-corrected TEM with cold FEG (Cs-TEM, JEM-ARM200F, JEOL). Energy-dispersive X-ray spectroscopy (EDS) coupled with TEM was used for local elemental analyses. The crystal structures were characterized using X-ray diffraction (XRD; Rigaku, D/MAX-2500). The state of carbon was analyzed using Raman spectroscopy (FEX, Nost Co., Ltd.; 532-nm wavelength), and X-ray photoelectron spectroscopy (XPS; K-alpha, Thermo Scientific Inc.) was employed to obtain further information about Si2p, C1s, O1s, Li1s, and F1s. The carbon content of the composite was measured using a carbon/sulfur analyzer

*Electrochemical characterization:* For the electrochemical characterization, all the powders were crushed using a 3D mixer (Turbula mixer, DM-T2, Daemyoung Co.) with 5-mm zirconia balls at 50 rpm for 24 h to achieve a uniform particle distribution. All the samples were first mixed with Super P (SP, TIMCAL, Super P Li) and sodium-carboxymethyl cellulose (Na-CMC, Sigma Aldrich Co.) in an active material/Super P/CMC weight ratio of 70/20/10; deionized (DI) water was added to form a homogeneous slurry. Electrochemical characterization of the electrode was performed using CR2032 coin-type cells, with a lithium metal foil used as the counter electrode. The electrodes were dried in a vacuum oven at 80°C for 24 h before being transferred to an Ar-filled glove box for cell assembly. The electrolyte used was a solution of 1.2 M LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (3:7 v/v, Panax Etec) containing 3% vinylene carbonate additive. The coin cells were charged and discharged between 0.01 and 1.5 V (vs. Li/

chemical characterization. Electrochemical impedance spectroscopy (EIS) analysis (Bio-Logic Co., VMP3) was performed in the frequency range of 1 MHz to 1 mHz

**3. Synthesis of SiO***x* **active materials for highly enhanced electrochemical** 

Images of the SiO*x* composites formed via the alcoholysis reaction before and after heat treatment are presented in **Figure 1a**. The alcoholysis mechanism between the silicon precursor, silicon tetrachloride (SiCl4), and ethanol was as

SiCl4 + 4C2H5OH → Si (OC2H5)4 + 4HCl↑ (1)

at 25°C for the electro-

C) → SiO2 + 2O(C2H5)2 (2)

) by applying various currents ranging from 0.1 to 5 A g<sup>−</sup><sup>1</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.82379*

at 725°C for 1 h under vacuum.

(CS-2000, ELTRA GmbH).

with an AC amplitude of 10 mV.

**3.1 Amorphous SiO***x* **and carbon matrix**

Si (OC2H5)4 + Heat treatment (> 600°

**performance**

#### *SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping… DOI: http://dx.doi.org/10.5772/intechopen.82379*

*Preparation of (C-TixSi*<sup>1</sup>−*xOy)@C*: First, 50 μL of TiCl4 (99.9% SHOWA) was dissolved in EG (13 mL) by stirring for 1 day. Then, nonpolar SiCl4 (20 mL) was uniformly distributed in 50 mL of benzene for 30 min. After the solution was added to the mixture of EG and TiCl4 under vigorous stirring, the solution-state mixture was converted into a yellow mineral-like solid containing Si, Ti, O, and C. Finally, the (C-Ti*x*Si1−*x*O*y*)@C composite was obtained by heat treatment in a tube furnace at 725°C for 1 h under vacuum.

*Materials characterization:* The morphologies of all the samples were characterized using field-emission scanning electron microscopy (FESEM; SU-8010 and S-4700, Hitachi Co.), high-resolution transmission electron microscopy (HRTEM; JEM 2100F, JEOL); and Cs-corrected TEM with cold FEG (Cs-TEM, JEM-ARM200F, JEOL). Energy-dispersive X-ray spectroscopy (EDS) coupled with TEM was used for local elemental analyses. The crystal structures were characterized using X-ray diffraction (XRD; Rigaku, D/MAX-2500). The state of carbon was analyzed using Raman spectroscopy (FEX, Nost Co., Ltd.; 532-nm wavelength), and X-ray photoelectron spectroscopy (XPS; K-alpha, Thermo Scientific Inc.) was employed to obtain further information about Si2p, C1s, O1s, Li1s, and F1s. The carbon content of the composite was measured using a carbon/sulfur analyzer (CS-2000, ELTRA GmbH).

*Electrochemical characterization:* For the electrochemical characterization, all the powders were crushed using a 3D mixer (Turbula mixer, DM-T2, Daemyoung Co.) with 5-mm zirconia balls at 50 rpm for 24 h to achieve a uniform particle distribution. All the samples were first mixed with Super P (SP, TIMCAL, Super P Li) and sodium-carboxymethyl cellulose (Na-CMC, Sigma Aldrich Co.) in an active material/Super P/CMC weight ratio of 70/20/10; deionized (DI) water was added to form a homogeneous slurry. Electrochemical characterization of the electrode was performed using CR2032 coin-type cells, with a lithium metal foil used as the counter electrode. The electrodes were dried in a vacuum oven at 80°C for 24 h before being transferred to an Ar-filled glove box for cell assembly. The electrolyte used was a solution of 1.2 M LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (3:7 v/v, Panax Etec) containing 3% vinylene carbonate additive. The coin cells were charged and discharged between 0.01 and 1.5 V (vs. Li/ Li<sup>+</sup> ) by applying various currents ranging from 0.1 to 5 A g<sup>−</sup><sup>1</sup> at 25°C for the electrochemical characterization. Electrochemical impedance spectroscopy (EIS) analysis (Bio-Logic Co., VMP3) was performed in the frequency range of 1 MHz to 1 mHz with an AC amplitude of 10 mV.

#### **3. Synthesis of SiO***x* **active materials for highly enhanced electrochemical performance**

#### **3.1 Amorphous SiO***x* **and carbon matrix**

Images of the SiO*x* composites formed via the alcoholysis reaction before and after heat treatment are presented in **Figure 1a**. The alcoholysis mechanism between the silicon precursor, silicon tetrachloride (SiCl4), and ethanol was as follows:

$$\text{SiCl}\_4 + 4\text{C}\_2\text{H}\_5\text{OH} \rightarrow \text{Si} \left( \text{OC}\_2\text{H}\_5 \right)\_4 + 4\text{HCl} \\ \uparrow \tag{1}$$

$$\text{Si} \left( \text{OC}\_2\text{H}\_5 \right)\_4 \text{ + Heat treatment} \left( > 60 \, \text{0} \, ^\circ \text{C} \right) \rightarrow \text{SiO}\_2 + 2 \text{O} \left( \text{C}\_2\text{H}\_5 \right)\_2 \tag{2}$$

*Energy Storage Devices*

performance [23–25].

**2. Experimental**

for 1 h under vacuum.

exhibits better cycling performance than Si. Nevertheless, the low electronic conductivity and sluggish electron transport kinetics of SiO*x* resulting from the insulating property of SiO2 lead to poor electrochemical performance and have hindered the application of SiO*x* as anode materials for commercialized LIBs [13–16]. Many researchers have proposed strategies to address these issues, resulting in progress such as the development of carbon-coated SiO*x* composites. Although the improved electrical conductivity achieved by carbon coating can improve the electrochemical performance of SiO*x*, complicated, multi-step, and high-temperature processes are required [17–20]. For instance, Liu et al. developed a Si-Void@SiO*x* nanowire composite using thermal evaporation/chemical etching of a mixed powder of SiO and ZnS at high temperatures (1250 and 1650°C) [18]. In addition, Han et al. prepared a SiO@C composite using a two-step process with SiO powder as the raw material; after ball milling for 3 h at 3000 rpm, the ball-milled SiO particles were calcined at

To avoid the complicated and costly processes adopted in previous studies, in this study, a simple and cost-effective one-pot synthesis method was developed to fabricate a carbon-incorporated/carbon-coated SiO*x* ((C-SiO*x*)@C) composite. We attempted to simultaneously form interconnected carbon paths in the composite and encapsulate the surface with carbon using ethylene glycol and benzene. We further attempted to fabricate a SiO*x* composite with superior electrochemical performance by maximizing the electrical conductivity through Ti doping. Ti doping can result in the formation of TiSi alloys, which are beneficial for improving the cyclic stability of LIB electrode materials [21, 22]. In addition, black TiO2−*x* has been reported to exhibit higher conductivity than pristine white TiO2 because of the existence of Ti3+ (corresponding to an oxygen deficiency) in the structure. Thus, we suspected that Ti3+ doping might lead to outstanding electrochemical

In the current study, we prepared a Ti3+-doped and carbon-incorporated/ carbon-coated SiO*x* ((C-Ti*x*Si1−*x*O*y*)@C) composite and investigated the effects of these structural modifications. The electrochemical performance of the

(C-Ti*x*Si1−*x*O*y*)@C composite was greatly improved compared with that of carbonincorporated SiO*x* (C-SiO*x*) and a (C-SiO*x*)@C composite. The electrochemical performance of the (C-Ti*x*Si1−*x*O*y*)@C composite was greatly improved compared with that of the C-SiO*x* and (C-SiO*x*)@C composite. The initial discharge capacity

~4 times higher than that of C-SiO*x* under the same conditions. Furthermore, the (C-Ti*x*Si1−*x*O*y*)@C composite delivered a capacity retention of ~88.9% over 600 cycles

*Preparation of C-SiOx*: First, 13 mL of ethylene glycol (EG, 99.9%, Samchun Co.) was added to 20 mL of SiCl4 (99%, Wako Co.) under vigorous stirring. The mixture was rapidly transformed into a mineral-like solid, which was converted into C-SiO*<sup>x</sup>*

*Preparation of (C-SiOx)@C*: First, 13 mL of EG was poured into a mixture of 20 mL of SiCl4 and 50 mL of benzene (99.5%, Daejung Co.) under vigorous stirring. Benzene was used to control the reaction between EG and SiCl4 necessary for the synthesis of the powder. The mineral-like solid formed through the alcoholysis reaction was transformed into a (C-SiO*x*)@C composite by heat treatment at 725°C

was ~1304 mAh g<sup>−</sup><sup>1</sup>

with a high coulombic efficiency of ~99%.

, which was

700°C using sodium dodecylbenzene sulfonate [19].

of the (C-Ti*x*Si1−*x*O*y*)@C composite at 0.1 A g<sup>−</sup><sup>1</sup>

powder by heat treatment at 725°C for 1 h under vacuum.

at a higher current density of 1 A g<sup>−</sup><sup>1</sup>

**72**

#### **Figure 1.**

*Image of synthesized SiOx composite prepared using SiCl4 and water (a and b), SiCl4 and ethanol (c and d), and SiCl4 and ethylene glycol (e and f) respectively.*

$$\text{C}\_2\text{H}\_5\text{OH} \rightarrow \text{C} + \text{3H}\_2\uparrow \text{ + CO or CO}\_2\uparrow \tag{3}$$

During the reaction between SiCl4 and ethanol, tetraethoxysilane (TEOS) was formed (1). The TEOS was converted into SiO2 phase by the heat treatment (>600°C) (2). Simultaneously, the residual ethanol generated carbon and CO or CO2 gas (3). As observed in **Figure 1a**, the mineral-like solid transformed into a gray SiO*<sup>x</sup>* powder after heat treatment. In addition, SiO2 and HCl were formed by the reaction between SiCl4 and water, and a white SiO2 powder was obtained after the heat treatment.

In contrast to these reactions, the alcoholysis mechanism between SiCl4 and EG was as follows:

$$\text{xSiCl}\_4 + \text{yC}\_2\text{H}\_4\text{(OH)}\_2 \rightarrow \text{Si}\_x\text{(O}\_2\text{C}\_2\text{H}\_2\text{)}\_y + \text{aHCl}\uparrow\tag{4}$$

**75**

**Figure 2.**

*composite.*

sized at a current density of 0.1 A g<sup>−</sup><sup>1</sup>

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping…*

As observed in **Figure 2b**, the characteristic peaks of amorphous SiO2 were detected at 103.8 eV in the Si 2p spectra, whereas those for Si–Si bonding were not observed, indicating that the Si particles were completely surrounded by SiO2. **Figure 2c** shows that various carbon-related bonds, such as C▬C (285 eV), C▬O (286.7 eV), and C═O (298.4 eV), were detected in the C 1s spectra, confirming the existence of the carbon-based complex in the C-SiO*x* composite. The XPS analysis confirmed that the C-SiO*x* composite was composed of Si4+ and C▬C, C▬O, and C═O bonds. The carbon matrix in the C-SiO*x* composite was further characterized using Raman spectroscopy, and the results are presented in **Figure 2d**. After carbonization was achieved by the heat treatment, strong peaks centered at 1360 and 1580 cm<sup>−</sup><sup>1</sup> appeared in the Raman spectra of the C-SiO*x* composite, corresponding to the disordered carbon band (D band) and graphitic carbon band (G band), respectively. **Figure 3** presents the cycle performance profiles of the SiO*x* composites synthe-

*(a) XRD patterns, XPS spectra showing (b) Si 2p peak and (c) C 1s peak, and (d) Raman spectra of C-SiOx*

The C-SiO*x* composite exhibited a first discharge capacity of 330 mAh g<sup>−</sup><sup>1</sup>

delivered first discharge capacities of 31 and 60 mAh g<sup>−</sup><sup>1</sup>

great reversibility, whereas the SiO*x* composites prepared using water and ethanol

SiO*x* structure resulted in a low reversible capacity, the low electrical conductivity of SiO2 was overcome by synthesizing the C-SiO*x* composite with a carbon matrix. In addition, the SiO*x* structure was advantageous for achieving good cyclability because the presence of SiO2 buffers effectively reduced the large volume change of

using water, ethanol, and EG, respectively.

with

, respectively. Although the

*DOI: http://dx.doi.org/10.5772/intechopen.82379*

$$\text{Si}\_x\text{(O}\_2\text{C}\_2\text{H}\_2\text{)}\_\text{y} + \text{Heat treatment} \left(\text{> } 60\text{ }\text{\textdegree C}\right) \rightarrow \text{Si}\_x\text{(OC)}\_\text{y} + \text{yCO}^\dagger\text{ + yH}\_2\text{[}^\circ\text{ (5)]}$$

Si*x*(O2C2H2)*y* was formed by the alcoholysis reaction between SiCl4 and EG (4). The obtained Si*x*(O2C2H2)*y* was transformed into the completely black Si*x*(OC)*<sup>y</sup>* after heat treatment (5). The Si*x*(OC)*y* powder composed of a carbon and SiO*<sup>x</sup>* matrix was labeled as the carbon-incorporated SiO*x* (C-SiO*x*) composite.

We used several characterization techniques to confirm the carbon-based complex of the C-SiO*x* composite and the mechanism proposed above. The XRD pattern in **Figure 2a** reveals broad peaks over the range of 10–30°, which can be indexed to the amorphous phase of the C-SiO*x* composite. In addition, the elemental bonding properties of the C-SiO*x* composite were investigated using XPS analyses.

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping… DOI: http://dx.doi.org/10.5772/intechopen.82379*

**Figure 2.**

*Energy Storage Devices*

C2H5OH → C + 3H2↑ + CO or CO2↑ (3)

*Image of synthesized SiOx composite prepared using SiCl4 and water (a and b), SiCl4 and ethanol (c and d),* 

In contrast to these reactions, the alcoholysis mechanism between SiCl4 and EG

xSiCl4 + yC2H4 (OH)2 → Si*<sup>x</sup>* (O2C2H2)*<sup>y</sup>* + αHCl↑ (4)

Si*x*(O2C2H2)*y* was formed by the alcoholysis reaction between SiCl4 and EG (4). The obtained Si*x*(O2C2H2)*y* was transformed into the completely black Si*x*(OC)*<sup>y</sup>* after heat treatment (5). The Si*x*(OC)*y* powder composed of a carbon and SiO*<sup>x</sup>* matrix was labeled as the carbon-incorporated SiO*x* (C-SiO*x*) composite.

We used several characterization techniques to confirm the carbon-based complex of the C-SiO*x* composite and the mechanism proposed above. The XRD pattern in **Figure 2a** reveals broad peaks over the range of 10–30°, which can be indexed to the amorphous phase of the C-SiO*x* composite. In addition, the elemental bonding properties of the C-SiO*x* composite were investigated using XPS analyses.

C) → Si*<sup>x</sup>* (OC)*<sup>y</sup>* + yCO↑ + yH2↑ (5)

During the reaction between SiCl4 and ethanol, tetraethoxysilane (TEOS) was formed (1). The TEOS was converted into SiO2 phase by the heat treatment (>600°C) (2). Simultaneously, the residual ethanol generated carbon and CO or CO2 gas (3). As observed in **Figure 1a**, the mineral-like solid transformed into a gray SiO*<sup>x</sup>* powder after heat treatment. In addition, SiO2 and HCl were formed by the reaction between SiCl4 and water, and a white SiO2 powder was obtained after the heat

**74**

treatment.

**Figure 1.**

was as follows:

Si*<sup>x</sup>* (O2C2H2)*<sup>y</sup>* + Heat treatment (> 600°

*and SiCl4 and ethylene glycol (e and f) respectively.*

*(a) XRD patterns, XPS spectra showing (b) Si 2p peak and (c) C 1s peak, and (d) Raman spectra of C-SiOx composite.*

As observed in **Figure 2b**, the characteristic peaks of amorphous SiO2 were detected at 103.8 eV in the Si 2p spectra, whereas those for Si–Si bonding were not observed, indicating that the Si particles were completely surrounded by SiO2. **Figure 2c** shows that various carbon-related bonds, such as C▬C (285 eV), C▬O (286.7 eV), and C═O (298.4 eV), were detected in the C 1s spectra, confirming the existence of the carbon-based complex in the C-SiO*x* composite. The XPS analysis confirmed that the C-SiO*x* composite was composed of Si4+ and C▬C, C▬O, and C═O bonds. The carbon matrix in the C-SiO*x* composite was further characterized using Raman spectroscopy, and the results are presented in **Figure 2d**. After carbonization was achieved by the heat treatment, strong peaks centered at 1360 and 1580 cm<sup>−</sup><sup>1</sup> appeared in the Raman spectra of the C-SiO*x* composite, corresponding to the disordered carbon band (D band) and graphitic carbon band (G band), respectively.

**Figure 3** presents the cycle performance profiles of the SiO*x* composites synthesized at a current density of 0.1 A g<sup>−</sup><sup>1</sup> using water, ethanol, and EG, respectively. The C-SiO*x* composite exhibited a first discharge capacity of 330 mAh g<sup>−</sup><sup>1</sup> with great reversibility, whereas the SiO*x* composites prepared using water and ethanol delivered first discharge capacities of 31 and 60 mAh g<sup>−</sup><sup>1</sup> , respectively. Although the SiO*x* structure resulted in a low reversible capacity, the low electrical conductivity of SiO2 was overcome by synthesizing the C-SiO*x* composite with a carbon matrix. In addition, the SiO*x* structure was advantageous for achieving good cyclability because the presence of SiO2 buffers effectively reduced the large volume change of

**Figure 3.** *Cyclic performance of synthesized SiOx composite prepared using water, ethanol, and ethylene glycol.*

Si during charge/discharge cycles. Therefore, the electrochemical tests revealed the excellent electrochemical performance of the C-SiO*x* composite.

#### **3.2 Effect of carbon coating on the surface of SiO***x* **particles**

To improve the poor reversible capacity resulting from the low electrical conductivity, we next designed a novel carbon-incorporated/carbon-coated SiO*<sup>x</sup>* ((C-SiO*x*)@C) composite using EG with benzene to achieve the homogenous distribution of the Si source and simultaneous formation of a multiple-carbon matrix in the composite. In particular, the use of nonpolar benzene enabled the formation of uniformly disperse nonpolar SiCl4 via dispersion forces, which potentially minimized the aggregation of Si nanoparticles and contributed to the formation of a carbon framework in the SiO*x* composite. Moreover, the conductive carbon was completely coated on the surface of each SiO*x* particle, which not only provided a fast electron transport path but also effectively prevented structural failure resulting from the large volume expansion during charge/discharge, which led to great enhancement of the electrochemical properties of the SiO*x* composite.

To verify the successful preparation of the (C-SiO*x*)@C composite and our above hypotheses, we performed several experiments. Structural variation using benzene was identified using XPS analyses, which confirmed the chemical states of each element in the composite. As observed in **Figure 4a** and **b**, the (C-SiO*x*)@C composite was based on SiO2, similar to the C-SiO*x* composite. However, the higher intensity of the C▬C bond compared with that of the C▬O bond in the C 1s spectra of C for the (C-SiO*x*)@ composite indicates the presence of a multiple-carbon matrix derived from EG and benzene. A pitch-coated C-SiO*x* composite was also prepared using ~11 wt% pitch carbon to confirm the effects of the benzene-based carbon coating, as the C content in the (C-SiO*x*)@C composite measured using a carbon/sulfur determinator was estimated to be 11 wt%.

In the XRD patterns of the obtained powders of the C-SiO*x*, pitch-coated C-SiO*<sup>x</sup>* and (C-SiO*x*)@C composite, only a broad peak at approximately 25° was observed without the appearance of a crystalline Si peak (**Figure 5a**). The Raman spectra of the samples revealed two strong peaks at 1360 and 1580 cm<sup>−</sup><sup>1</sup> , which were assigned to the D band and G band from the carbon, respectively, as observed in **Figure 5b**. The intensity ratio between the D and G bands indicated the crystallinity of graphitic carbon. In contrast, much weaker peaks were observed for the C-SiO*x* and pitch-coated C-SiO*x*, suggesting the presence of a multiple-carbon matrix composed of highly graphitic carbon in the (C-SiO*x*)@C composite.

**77**

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping…*

*XPS spectra showing (a) Si 2p peak and (b) C 1s peak of (C-SiOx)@C composite.*

To further evaluate the carbon-based complex of the (C-SiO*x*)@C composite in detail, we examined the morphology of (C-SiO*x*)@C using TEM and compared it with those of the C-SiO*x* and pitch-coated C-SiO*x*. As observed in **Figure 6a**–**c**, TEM analysis confirmed that an amorphous matrix surrounded the uniformly dispersed Si nanoparticles (3–4 nm size); the amorphous layer was also homogeneously coated on the surface of all the particles of the (C-SiO*x*)@C composite. TEM mapping analysis indicated that the outer coating layer was mainly composed of C, whereas the core was composed of C, Si, and O from the interconnected structures consisting of amorphous phases such as SiO2 and C (**Figure 6d**). These analyses indicated that the interconnected carbon paths and homogenous carbon coating were successfully prepared, resulting in considerable improvement of the electrochemical performance of the novel (C-SiO*x*)@C composite. However, the crystalline Si nanoparticles and carbon coating layer were not observed in the TEM images of the C-SiO*x*, whereas for the pitch-coated C-SiO*x*, a pitch carbon layer coated on amorphous SiO*x* and a C

*(a) XRD patterns and (b) Raman spectra of C-SiOx, pitch coated C-SiOx, and (C-SiOx)@C composite.*

The electrochemical behavior of the (C-SiO*x*)@C composite was investigated using galvanostatic measurements in Li cells and was compared with that of C-SiO*<sup>x</sup>* and pitch-coated C-SiO*x*. **Figure 7a** presents the charge/discharge capacities of

(C-SiO*x*)@C electrode delivered a high initial discharge capacity (925 mAh g<sup>−</sup><sup>1</sup>

high stability during repeated charge/discharge cycles. Notably, up to ~92% of the initial discharge capacity was maintained, whereas the capacity of the pitch-coated

in the voltage range of 0.01–1.5 V. The

) and

matrix are observed in **Figure 6e** and **f**, respectively.

the samples at a current density of 0.1 A g<sup>−</sup><sup>1</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.82379*

**Figure 4.**

**Figure 5.**

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping… DOI: http://dx.doi.org/10.5772/intechopen.82379*

**Figure 4.** *XPS spectra showing (a) Si 2p peak and (b) C 1s peak of (C-SiOx)@C composite.*

**Figure 5.** *(a) XRD patterns and (b) Raman spectra of C-SiOx, pitch coated C-SiOx, and (C-SiOx)@C composite.*

To further evaluate the carbon-based complex of the (C-SiO*x*)@C composite in detail, we examined the morphology of (C-SiO*x*)@C using TEM and compared it with those of the C-SiO*x* and pitch-coated C-SiO*x*. As observed in **Figure 6a**–**c**, TEM analysis confirmed that an amorphous matrix surrounded the uniformly dispersed Si nanoparticles (3–4 nm size); the amorphous layer was also homogeneously coated on the surface of all the particles of the (C-SiO*x*)@C composite. TEM mapping analysis indicated that the outer coating layer was mainly composed of C, whereas the core was composed of C, Si, and O from the interconnected structures consisting of amorphous phases such as SiO2 and C (**Figure 6d**). These analyses indicated that the interconnected carbon paths and homogenous carbon coating were successfully prepared, resulting in considerable improvement of the electrochemical performance of the novel (C-SiO*x*)@C composite. However, the crystalline Si nanoparticles and carbon coating layer were not observed in the TEM images of the C-SiO*x*, whereas for the pitch-coated C-SiO*x*, a pitch carbon layer coated on amorphous SiO*x* and a C matrix are observed in **Figure 6e** and **f**, respectively.

The electrochemical behavior of the (C-SiO*x*)@C composite was investigated using galvanostatic measurements in Li cells and was compared with that of C-SiO*<sup>x</sup>* and pitch-coated C-SiO*x*. **Figure 7a** presents the charge/discharge capacities of the samples at a current density of 0.1 A g<sup>−</sup><sup>1</sup> in the voltage range of 0.01–1.5 V. The (C-SiO*x*)@C electrode delivered a high initial discharge capacity (925 mAh g<sup>−</sup><sup>1</sup> ) and high stability during repeated charge/discharge cycles. Notably, up to ~92% of the initial discharge capacity was maintained, whereas the capacity of the pitch-coated

*Energy Storage Devices*

**Figure 3.**

Si during charge/discharge cycles. Therefore, the electrochemical tests revealed the

*Cyclic performance of synthesized SiOx composite prepared using water, ethanol, and ethylene glycol.*

To improve the poor reversible capacity resulting from the low electrical conductivity, we next designed a novel carbon-incorporated/carbon-coated SiO*<sup>x</sup>* ((C-SiO*x*)@C) composite using EG with benzene to achieve the homogenous distribution of the Si source and simultaneous formation of a multiple-carbon matrix in the composite. In particular, the use of nonpolar benzene enabled the formation of uniformly disperse nonpolar SiCl4 via dispersion forces, which potentially minimized the aggregation of Si nanoparticles and contributed to the formation of a carbon framework in the SiO*x* composite. Moreover, the conductive carbon was completely coated on the surface of each SiO*x* particle, which not only provided a fast electron transport path but also effectively prevented structural failure resulting from the large volume expansion during charge/discharge, which led to great

enhancement of the electrochemical properties of the SiO*x* composite.

carbon/sulfur determinator was estimated to be 11 wt%.

the samples revealed two strong peaks at 1360 and 1580 cm<sup>−</sup><sup>1</sup>

posed of highly graphitic carbon in the (C-SiO*x*)@C composite.

To verify the successful preparation of the (C-SiO*x*)@C composite and our above hypotheses, we performed several experiments. Structural variation using benzene was identified using XPS analyses, which confirmed the chemical states of each element in the composite. As observed in **Figure 4a** and **b**, the (C-SiO*x*)@C composite was based on SiO2, similar to the C-SiO*x* composite. However, the higher intensity of the C▬C bond compared with that of the C▬O bond in the C 1s spectra of C for the (C-SiO*x*)@ composite indicates the presence of a multiple-carbon matrix derived from EG and benzene. A pitch-coated C-SiO*x* composite was also prepared using ~11 wt% pitch carbon to confirm the effects of the benzene-based carbon coating, as the C content in the (C-SiO*x*)@C composite measured using a

In the XRD patterns of the obtained powders of the C-SiO*x*, pitch-coated C-SiO*<sup>x</sup>* and (C-SiO*x*)@C composite, only a broad peak at approximately 25° was observed without the appearance of a crystalline Si peak (**Figure 5a**). The Raman spectra of

to the D band and G band from the carbon, respectively, as observed in **Figure 5b**. The intensity ratio between the D and G bands indicated the crystallinity of graphitic carbon. In contrast, much weaker peaks were observed for the C-SiO*x* and pitch-coated C-SiO*x*, suggesting the presence of a multiple-carbon matrix com-

, which were assigned

excellent electrochemical performance of the C-SiO*x* composite.

**3.2 Effect of carbon coating on the surface of SiO***x* **particles**

**76**

#### **Figure 6.**

*(a)–(c) TEM images of (C-SiOx)@C composite and (d) TEM elemental mapping images of C, Si, and O in the (C-SiOx)@C composite. TEM images of (e) C-SiOx and (f) pitch-coated C-SiOx.*

SiO*x* gradually decreased under the same conditions. The pitch-coated SiO*x* electrode exhibited a high initial charge/discharge capacity of 1891/786 mAh g<sup>−</sup><sup>1</sup> but only 88.8% retention of its initial capacity over 100 cycles. In addition, the first discharge capacity of the pristine SiO*x* was only ~400 mAh g<sup>−</sup><sup>1</sup> , which is less than half of that of the (C-SiO*x*)@C composite. EIS characterization was performed for the C-SiO*x*, pitch-coated C-SiO*x*, and (C-SiO*x*)@C electrodes before cycling, and the electrical conductivity was substantially improved by carbon coating. As observed in **Figure 7b**, the (C-SiO*x*)@C composite exhibited a smaller charge-transfer resistance than the C-SiO*x* and pitch-coated C-SiO*x*, indicating that the carbon framework derived from EG and benzene enhanced the electrical conductivity of

**79**

g<sup>−</sup><sup>1</sup>

**Figure 7.**

*density of 0.5 A g<sup>−</sup><sup>1</sup>*

*performance at a current density of 0.1 A g<sup>−</sup><sup>1</sup>*

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping…*

the SiO*x* particles. Additionally, the (C-SiO*x*)@C composite exhibited satisfactory performance at a higher current density. As observed in **Figure 7c** and **d**, 591 mAh

*with an AC amplitude of 10 mV. (c) Cycling performance of C-SiOx and (C-SiOx)@C composite at a current* 

*. (d) Charge/discharge curves of (C-SiOx)@C composites over 600 cycles.*

 *and (b) Nyquist plots in the frequency range of 1 MHz to 1 mHz* 

*Electrochemical characteristics of C-SiOx, pitch-coated SiOx, and (C-SiOx)@C composite. (a) Cycling* 

tained over 600 cycles with a coulombic efficiency of ~99%, whereas the capacity of C-SiO*x* drastically declined under the same conditions. These results indicate that the multiple-carbon matrix in the (C-SiO*x*)@C composite not only provided high electrical conductivity but also prevented the severe structural degradation that generally accompanies the large volume change during charge/discharge.

The structural differences between the C-SiO*x* and (C-SiO*x*)@C composite electrodes were clearly determined using XPS analyses. The chemical state of each element in the compound was identified, as shown in **Figure 8**. For the Si 2p spectra, the electrodes exhibited peaks at 101.6 and 103.5 eV, corresponding to lithium silicates (Li*x*SiO*y*) and Si▬O bonding, respectively. These lithium silicate phases are known irreversible products formed during the first cycle, and their detection in the SEI layer of Si-based electrodes has been previously reported [26, 27]. The presence of the higher lithium silicates peak indicates that the rate of irreversible consumption was higher than that in the C-SiO*x* electrode. For the C 1s spectrum of the C-SiO*x* and (C-SiO*x*)@C electrodes, the peak at 285.0 eV was assigned to the C▬C bonds in the carbon-based complex of the SiO*x* anode. The peak at 287.0 eV corresponding to C▬O originates from the SiO*x* composite and CMC binder. For the C-SiO*x* electrode, the absence of the peak at 289.1 eV indicates the disappearance of C═O bonding. Instead, a new peak appeared at 290.3 eV, which is attributed to the formation of SEI layer components such as lithium carbonate (Li2CO3) and lithium alkyl carbonates [28]. For the Li 1s spectra, the peaks at 54.5, 56.0, and 56.8 eV are assigned to the formation of SEI layer components such as Li2O, lithium

was main-

of the discharge capacity of the (C-SiO*x*)@C composite at 0.5 A g<sup>−</sup><sup>1</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.82379*

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping… DOI: http://dx.doi.org/10.5772/intechopen.82379*

**Figure 7.**

*Energy Storage Devices*

**78**

**Figure 6.**

SiO*x* gradually decreased under the same conditions. The pitch-coated SiO*x* electrode exhibited a high initial charge/discharge capacity of 1891/786 mAh g<sup>−</sup><sup>1</sup>

*(a)–(c) TEM images of (C-SiOx)@C composite and (d) TEM elemental mapping images of C, Si, and O in* 

charge capacity of the pristine SiO*x* was only ~400 mAh g<sup>−</sup><sup>1</sup>

*the (C-SiOx)@C composite. TEM images of (e) C-SiOx and (f) pitch-coated C-SiOx.*

only 88.8% retention of its initial capacity over 100 cycles. In addition, the first dis-

of that of the (C-SiO*x*)@C composite. EIS characterization was performed for the C-SiO*x*, pitch-coated C-SiO*x*, and (C-SiO*x*)@C electrodes before cycling, and the electrical conductivity was substantially improved by carbon coating. As observed in **Figure 7b**, the (C-SiO*x*)@C composite exhibited a smaller charge-transfer resistance than the C-SiO*x* and pitch-coated C-SiO*x*, indicating that the carbon framework derived from EG and benzene enhanced the electrical conductivity of

but

, which is less than half

*Electrochemical characteristics of C-SiOx, pitch-coated SiOx, and (C-SiOx)@C composite. (a) Cycling performance at a current density of 0.1 A g<sup>−</sup><sup>1</sup> and (b) Nyquist plots in the frequency range of 1 MHz to 1 mHz with an AC amplitude of 10 mV. (c) Cycling performance of C-SiOx and (C-SiOx)@C composite at a current density of 0.5 A g<sup>−</sup><sup>1</sup> . (d) Charge/discharge curves of (C-SiOx)@C composites over 600 cycles.*

the SiO*x* particles. Additionally, the (C-SiO*x*)@C composite exhibited satisfactory performance at a higher current density. As observed in **Figure 7c** and **d**, 591 mAh g<sup>−</sup><sup>1</sup> of the discharge capacity of the (C-SiO*x*)@C composite at 0.5 A g<sup>−</sup><sup>1</sup> was maintained over 600 cycles with a coulombic efficiency of ~99%, whereas the capacity of C-SiO*x* drastically declined under the same conditions. These results indicate that the multiple-carbon matrix in the (C-SiO*x*)@C composite not only provided high electrical conductivity but also prevented the severe structural degradation that generally accompanies the large volume change during charge/discharge.

The structural differences between the C-SiO*x* and (C-SiO*x*)@C composite electrodes were clearly determined using XPS analyses. The chemical state of each element in the compound was identified, as shown in **Figure 8**. For the Si 2p spectra, the electrodes exhibited peaks at 101.6 and 103.5 eV, corresponding to lithium silicates (Li*x*SiO*y*) and Si▬O bonding, respectively. These lithium silicate phases are known irreversible products formed during the first cycle, and their detection in the SEI layer of Si-based electrodes has been previously reported [26, 27]. The presence of the higher lithium silicates peak indicates that the rate of irreversible consumption was higher than that in the C-SiO*x* electrode. For the C 1s spectrum of the C-SiO*x* and (C-SiO*x*)@C electrodes, the peak at 285.0 eV was assigned to the C▬C bonds in the carbon-based complex of the SiO*x* anode. The peak at 287.0 eV corresponding to C▬O originates from the SiO*x* composite and CMC binder. For the C-SiO*x* electrode, the absence of the peak at 289.1 eV indicates the disappearance of C═O bonding. Instead, a new peak appeared at 290.3 eV, which is attributed to the formation of SEI layer components such as lithium carbonate (Li2CO3) and lithium alkyl carbonates [28]. For the Li 1s spectra, the peaks at 54.5, 56.0, and 56.8 eV are assigned to the formation of SEI layer components such as Li2O, lithium

#### **Figure 8.**

*XPS spectra of surface of C-SiOx and (C-SiOx)@C electrodes in the first cycle: (a) Si 2p, (b) C 1s, (c) Li 1s, and (d) F 1s branches.*

fluoride (LiF), and Li*x*PF*y*, respectively. Compared with the C-SiO*x* electrode, weak Li2O and Li*x*PF*y* peaks and a strong LiF peak were observed for the (C-SiO*x*)@C electrode, indicating that LiF was the main component of the SEI layer. The F 1s spectra for the (C-SiO*x*)@C anode contained a very strong peak at 685.2 eV attributable to LiF in addition to very weak peaks assigned to Li*x*PF*y*O*z* (686.5 eV) and Li*x*PF*y* (687.5 eV).

Additionally, the morphological differences between the surfaces of the C-SiO*<sup>x</sup>* and (C-SiO*x*)@C electrodes during charge/discharge cycles were investigated using SEM and TEM analysis, as observed in **Figure 9**. After 200 cycles, extensive cracking and partial fracture of the C-SiO*x* electrode was observed, whereas the surface of the (C-SiO*x*)@C electrode was stably retained. These results indicate that the benzene-derived multiple-carbon matrix could play an important role in improving the cyclic stability and electrical conductivity of SiO*x* to enable its use as a promising anode for LIBs.

#### **3.3 Boosting the performance by Ti doping on SiO***x* **sites**

We previously demonstrated the exceptional improvement of the electrochemical performance of the (C-SiO*x*)@C composite achieved through the formation of interconnected carbon paths and a homogenous carbon coating. These results confirmed that improvement of the electrical conductivity of the SiO*x* composite affected the electrochemical properties. Herein, we prepared a Ti3+-doped and

**81**

follows:

**Figure 9.**

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping…*

carbon-incorporated/carbon-coated SiO*x* ((C-Ti*x*Si1−*x*O*y*)@C) composite via the alcoholysis-based reaction using SiCl4, TiCl4, and EG with benzene. The detailed reaction mechanism of the formation of the (C-Ti*x*Si1−*x*O*y*)@C composite was as

*SEM images and TEM image of (a)–(c) C-SiOx electrode and (d)–(f) (C-SiOx)@C electrode before cycling* 

*.*

Ti*<sup>x</sup>* Si1−*<sup>x</sup>* (O2C2H2)*<sup>y</sup>* + zC6H6 + αHCl↑ (6)

+ 6zC + yCO↑ + (y + 3z)H2↑ (7)

Ti*<sup>x</sup>* Si1−*<sup>x</sup>* (OC)*<sup>y</sup>* + 6zC → (C − Ti*<sup>x</sup>* Si1−*x*O*y*)@C composite. (8)

After the precursors were mixed, the solution-state mixture was rapidly converted into mineral-like solids containing Si, Ti, O, and C. In these reactions, TiCl4 should preferentially react with EG to achieve a homogenous dispersion of Ti ions in the (C-Ti*x*Si1−*x*O*y*)@C composite. Additionally, the use of nonpolar benzene, an additional carbon source, enabled the formation of a homogeneous distribution of

the nonpolar SiCl4 and TiCl4 through dispersion forces during the reaction. As shown in **Figure 10a**, XRD patterns of the (C-Ti*x*Si1−*x*O*y*)@C composite revealed the presence of typical amorphous phases of SiO2, which was comparable to the results for SiO*x* reported above. The existence of carbon in the sample was verified using Raman spectroscopy. In **Figure 10b**, peaks at 1350 and 1690 cm<sup>−</sup><sup>1</sup>

corresponding to the D band and G band, respectively, were clearly revealed for the (C-Ti*x*Si1−*x*O*y*)@C composite. To further characterize the composition of the (C-Ti*x*Si1−*x*O*y*)@C composite, XPS analysis was performed. As shown in **Figure 10c**, two characteristic peaks for Si4+ corresponding to the previously synthesized C-SiO*<sup>x</sup>* and (C-SiO*x*)@C composite in the Si 2p spectra were observed. However, for the (C-Ti*x*Si1−*x*O*y*)@C composite, peaks corresponding to Si3+ were observed, which resulted from electrons trapped in the Ti3+/Ti4+ state or oxygen vacancies. As shown

C) → Ti*<sup>x</sup>* Si1−*<sup>x</sup>* (OC)*<sup>y</sup>*

,

xTiCl4 + (1 − x)SiCl4 + yC2H4 (OH)2 + zC6H6 →

Ti*<sup>x</sup>* Si1−*<sup>x</sup>* (O2C2H2)*<sup>y</sup>* + zC6H6 (> 600°

*DOI: http://dx.doi.org/10.5772/intechopen.82379*

*and after 200 cycles at a current density of 0.5 A g<sup>−</sup><sup>1</sup>*

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping… DOI: http://dx.doi.org/10.5772/intechopen.82379*

#### **Figure 9.**

*Energy Storage Devices*

fluoride (LiF), and Li*x*PF*y*, respectively. Compared with the C-SiO*x* electrode, weak Li2O and Li*x*PF*y* peaks and a strong LiF peak were observed for the (C-SiO*x*)@C electrode, indicating that LiF was the main component of the SEI layer. The F 1s spectra for the (C-SiO*x*)@C anode contained a very strong peak at 685.2 eV attributable to LiF in addition to very weak peaks assigned to Li*x*PF*y*O*z* (686.5 eV) and

*XPS spectra of surface of C-SiOx and (C-SiOx)@C electrodes in the first cycle: (a) Si 2p, (b) C 1s, (c) Li 1s,* 

Additionally, the morphological differences between the surfaces of the C-SiO*<sup>x</sup>* and (C-SiO*x*)@C electrodes during charge/discharge cycles were investigated using SEM and TEM analysis, as observed in **Figure 9**. After 200 cycles, extensive cracking and partial fracture of the C-SiO*x* electrode was observed, whereas the surface of the (C-SiO*x*)@C electrode was stably retained. These results indicate that the benzene-derived multiple-carbon matrix could play an important role in improving the cyclic stability and electrical conductivity of SiO*x* to enable its use as a promis-

We previously demonstrated the exceptional improvement of the electrochemical performance of the (C-SiO*x*)@C composite achieved through the formation of interconnected carbon paths and a homogenous carbon coating. These results confirmed that improvement of the electrical conductivity of the SiO*x* composite affected the electrochemical properties. Herein, we prepared a Ti3+-doped and

**3.3 Boosting the performance by Ti doping on SiO***x* **sites**

**80**

Li*x*PF*y* (687.5 eV).

*and (d) F 1s branches.*

**Figure 8.**

ing anode for LIBs.

*SEM images and TEM image of (a)–(c) C-SiOx electrode and (d)–(f) (C-SiOx)@C electrode before cycling and after 200 cycles at a current density of 0.5 A g<sup>−</sup><sup>1</sup> .*

carbon-incorporated/carbon-coated SiO*x* ((C-Ti*x*Si1−*x*O*y*)@C) composite via the alcoholysis-based reaction using SiCl4, TiCl4, and EG with benzene. The detailed reaction mechanism of the formation of the (C-Ti*x*Si1−*x*O*y*)@C composite was as follows:

$$\text{xTiCl}\_4 + \text{(1 - x)SiCl}\_4 + \text{yC}\_2\text{H}\_4\text{ (OH)}\_2 + \text{zC}\_6\text{H}\_6 \rightarrow$$

$$\text{Ti}\_x\text{Si}\_{1-x}\text{(O}\_2\text{C}\_2\text{H}\_2\text{)}\_2 + \text{zC}\_6\text{H}\_6 + \text{aHCl}\uparrow\tag{6}$$

$$\text{Ti}\_{\text{x}}\text{Si}\_{\text{1-x}}\text{(O}\_{2}\text{C}\_{2}\text{H}\_{2}\text{)}\_{\text{y}} + \text{zC}\_{6}\text{H}\_{6} \left(\text{> 600}\text{ }^{\circ}\text{C}\right) \rightarrow \text{Ti}\_{\text{x}}\text{Si}\_{\text{1-x}}\text{(O}\text{C}\text{)}\_{\text{y}}$$

$$\text{\* } 6\text{zC} + \text{yCO}^{\bullet}\text{ + (y + 3z)H}\_{2}\text{ }^{\circ}\text{ }\tag{7}$$

$$\text{Ti}\_{\text{x}}\text{Si}\_{\text{1-x}}\text{(OC)}\_{\text{y}} + \text{6zC} \rightarrow \text{(C - Ti}\_{\text{x}}\text{Si}\_{\text{1-x}}\text{O}\_{\text{y}}\text{)} \oplus \text{C composite.} \tag{8}$$

After the precursors were mixed, the solution-state mixture was rapidly converted into mineral-like solids containing Si, Ti, O, and C. In these reactions, TiCl4 should preferentially react with EG to achieve a homogenous dispersion of Ti ions in the (C-Ti*x*Si1−*x*O*y*)@C composite. Additionally, the use of nonpolar benzene, an additional carbon source, enabled the formation of a homogeneous distribution of the nonpolar SiCl4 and TiCl4 through dispersion forces during the reaction.

As shown in **Figure 10a**, XRD patterns of the (C-Ti*x*Si1−*x*O*y*)@C composite revealed the presence of typical amorphous phases of SiO2, which was comparable to the results for SiO*x* reported above. The existence of carbon in the sample was verified using Raman spectroscopy. In **Figure 10b**, peaks at 1350 and 1690 cm<sup>−</sup><sup>1</sup> , corresponding to the D band and G band, respectively, were clearly revealed for the (C-Ti*x*Si1−*x*O*y*)@C composite. To further characterize the composition of the (C-Ti*x*Si1−*x*O*y*)@C composite, XPS analysis was performed. As shown in **Figure 10c**, two characteristic peaks for Si4+ corresponding to the previously synthesized C-SiO*<sup>x</sup>* and (C-SiO*x*)@C composite in the Si 2p spectra were observed. However, for the (C-Ti*x*Si1−*x*O*y*)@C composite, peaks corresponding to Si3+ were observed, which resulted from electrons trapped in the Ti3+/Ti4+ state or oxygen vacancies. As shown

**Figure 10.**

*(a) XRD patterns, (b) Raman spectra, and XPS spectra showing (c) Si 2p, (d) O 1s, (e) C 1s, and (f) Ti 2p of (C-TixSi1−xOy)@C composite.*

in **Figure 10d**, Si▬O and C▬O bonding were detected for the (C-Ti*x*Si1−*x*O*y*)@C composite. As observed in **Figure 10e**, characteristic peaks were detected in the C 1s spectra of the (C-Ti*x*Si1−*x*O*y*)@C composite, indicating the existence of the carbon framework in the composites. The existence of Ti in the (C-Ti*x*Si1−*x*O*y*)@C composite was also clearly confirmed through XPS analysis. In **Figure 10f**, four characteristic peaks appeared in the Ti 2p spectra of the (C-Ti*x*Si1−*x*O*y*)@C composite. The peaks at ~464.4 and ~458.5 eV correspond to the Ti3+ ion of Ti2O3, and the peaks at ~460.2 and ~465.8 eV were attributed to the Ti4+ ion of TiO2 [29–31], which indicates the formation of Ti▬O bonds in the (C-Ti*x*Si1−*x*O*y*)@C composite

**83**

SiO*x* composite.

at 1 A g<sup>−</sup><sup>1</sup>

5 cycles at 0.1 A g<sup>−</sup><sup>1</sup>

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping…*

compared with that of the C-SiO*x* and (C-SiO*x*)@C composite.

during the heat-treatment process. However, because far fewer Ti ions than Si ions were present in the composite, we suspected that the Ti▬O bond peak was covered by the Si▬O bond peak in the XPS spectrum. The existence of Ti3+ ions implies that the (C-Ti*x*Si1−*x*O*y*)@C composite could possess improved electrical conductivity

TEM–EDS analysis was employed to examine the elemental dispersion of the (C-Ti*x*Si1−*x*O*y*)@C. As displayed in **Figure 11a**, the elements of Si, O, C, and Ti almost overlapped, indicating the homogeneous distribution of Ti in the SiO*<sup>x</sup>* composite. The carbon component also indicated that each particle was encircled by a carbon-rich region with a thickness of ~20 nm. However, it was verified that despite existence of Ti, any peaks were not observed in XRD and Raman of the (C-Ti*x*Si1−*x*O*y*)@C composite, which indicates that the intensity resulting from Ti is very low and overall phase of this composite is amorphous. Thus, we supposed that Ti was not detected by XRD and Raman, although TEM mapping images and XPS spectrum showed existence of Ti in the composite. The TEM images in **Figure 11b** and **c** confirm the presence of crystalline Si with sizes of ~15 nm and reveal (111) planes with an interplanar spacing of 3.1 Å. Si nanoparticles were distributed in the SiO*x* and carbon matrix, contributing to the improved capacity of the (C-Ti*x*Si1−*x*O*y*)@C composite. Additionally, **Figure 12** presents SEM images of the C-SiO*x*, (C-SiO*x*)@C, and (C-Ti*x*Si1−*x*O*y*)@C composite; the morphology and size of the particles were not affected by the addition of benzene and the Ti-based source. The elemental compositions and calculated atomic ratios of oxygen to silicon of the samples are listed in **Table 1**. The O/Si ratio decreased with the addition of benzene and the Ti source, suggesting an increase in the carbon content and electron trapping resulting from the presence of Ti3+/Ti4+ ions. These experimental results indicate that the (C-Ti*x*Si1−*x*O*y*)@C composite was successfully prepared, that the electrical conductivity was enhanced by the presence of Ti3+ ions, and that the carbon coating might result in outstanding electrochemical performance of the

**Figure 13a** and **b** present voltage profiles and show the cycling performance,

pared with those of the C-SiO*x* and (C-SiO*x*)@C composite. The initial discharge

retention was 95.2% after 100 cycles, corresponding to a capacity loss of 0.048% per cycle. To determine the power capability of each sample, the electrochemical properties were measured at various current densities. As observed in **Figure 13c**,

was ~5 times higher than that of C-SiO*x* under the same conditions. In addition, as shown in **Figure 13d**, the capacity retentions of the C-SiO*x* and (C-SiO*x*)@C composites were only 58.9 and 86.8%, respectively, over 600 cycles at 1 A g<sup>−</sup><sup>1</sup>

(C-Ti*x*Si1−*x*O*y*)@C composite was retained after 600 cycles under the same condi-

To further understand the differences in the electrochemical performance of the three electrodes, EIS measurements were performed after 50 cycles at 0.1 A g<sup>−</sup><sup>1</sup> over the frequency range of 1 MHz to 1 mHz with an AC amplitude of 10 mV. As observed in **Figure 14a**, the Nyquist plots of all of the samples consisted of a semicircle at high frequency and a straight line at low frequency. The first intersection of the semicircle at high frequency with the real axis is related to the electrolyte solution resistance (Rel), and the diameter of the semicircle is related to the chargetransfer resistance (Rct) resulting from the reaction at the electrode-electrolyte interface. The straight line at low frequency is related to the Warburg impedance (Zre) corresponding to the Li-ion diffusion; the Warburg impedance coefficient

of this discharge capacity was retained, which

. In contrast, up to ~88.9% of the initial discharge capacity of the

for 100 cycles com-

, and the capacity

after

respectively, of the (C-Ti*x*Si1−*x*O*y*)@C composite at 0.1 A g<sup>−</sup><sup>1</sup>

, up to ~985 mAh g<sup>−</sup><sup>1</sup>

tions with a high coulombic efficiency of ~99%.

capacity of the (C-Ti*x*Si1−*x*O*y*)@C composite was ~1304 mAh g<sup>−</sup><sup>1</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.82379*

#### *SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping… DOI: http://dx.doi.org/10.5772/intechopen.82379*

during the heat-treatment process. However, because far fewer Ti ions than Si ions were present in the composite, we suspected that the Ti▬O bond peak was covered by the Si▬O bond peak in the XPS spectrum. The existence of Ti3+ ions implies that the (C-Ti*x*Si1−*x*O*y*)@C composite could possess improved electrical conductivity compared with that of the C-SiO*x* and (C-SiO*x*)@C composite.

TEM–EDS analysis was employed to examine the elemental dispersion of the (C-Ti*x*Si1−*x*O*y*)@C. As displayed in **Figure 11a**, the elements of Si, O, C, and Ti almost overlapped, indicating the homogeneous distribution of Ti in the SiO*<sup>x</sup>* composite. The carbon component also indicated that each particle was encircled by a carbon-rich region with a thickness of ~20 nm. However, it was verified that despite existence of Ti, any peaks were not observed in XRD and Raman of the (C-Ti*x*Si1−*x*O*y*)@C composite, which indicates that the intensity resulting from Ti is very low and overall phase of this composite is amorphous. Thus, we supposed that Ti was not detected by XRD and Raman, although TEM mapping images and XPS spectrum showed existence of Ti in the composite. The TEM images in **Figure 11b** and **c** confirm the presence of crystalline Si with sizes of ~15 nm and reveal (111) planes with an interplanar spacing of 3.1 Å. Si nanoparticles were distributed in the SiO*x* and carbon matrix, contributing to the improved capacity of the (C-Ti*x*Si1−*x*O*y*)@C composite. Additionally, **Figure 12** presents SEM images of the C-SiO*x*, (C-SiO*x*)@C, and (C-Ti*x*Si1−*x*O*y*)@C composite; the morphology and size of the particles were not affected by the addition of benzene and the Ti-based source. The elemental compositions and calculated atomic ratios of oxygen to silicon of the samples are listed in **Table 1**. The O/Si ratio decreased with the addition of benzene and the Ti source, suggesting an increase in the carbon content and electron trapping resulting from the presence of Ti3+/Ti4+ ions. These experimental results indicate that the (C-Ti*x*Si1−*x*O*y*)@C composite was successfully prepared, that the electrical conductivity was enhanced by the presence of Ti3+ ions, and that the carbon coating might result in outstanding electrochemical performance of the SiO*x* composite.

**Figure 13a** and **b** present voltage profiles and show the cycling performance, respectively, of the (C-Ti*x*Si1−*x*O*y*)@C composite at 0.1 A g<sup>−</sup><sup>1</sup> for 100 cycles compared with those of the C-SiO*x* and (C-SiO*x*)@C composite. The initial discharge capacity of the (C-Ti*x*Si1−*x*O*y*)@C composite was ~1304 mAh g<sup>−</sup><sup>1</sup> , and the capacity retention was 95.2% after 100 cycles, corresponding to a capacity loss of 0.048% per cycle. To determine the power capability of each sample, the electrochemical properties were measured at various current densities. As observed in **Figure 13c**, at 1 A g<sup>−</sup><sup>1</sup> , up to ~985 mAh g<sup>−</sup><sup>1</sup> of this discharge capacity was retained, which was ~5 times higher than that of C-SiO*x* under the same conditions. In addition, as shown in **Figure 13d**, the capacity retentions of the C-SiO*x* and (C-SiO*x*)@C composites were only 58.9 and 86.8%, respectively, over 600 cycles at 1 A g<sup>−</sup><sup>1</sup> after 5 cycles at 0.1 A g<sup>−</sup><sup>1</sup> . In contrast, up to ~88.9% of the initial discharge capacity of the (C-Ti*x*Si1−*x*O*y*)@C composite was retained after 600 cycles under the same conditions with a high coulombic efficiency of ~99%.

To further understand the differences in the electrochemical performance of the three electrodes, EIS measurements were performed after 50 cycles at 0.1 A g<sup>−</sup><sup>1</sup> over the frequency range of 1 MHz to 1 mHz with an AC amplitude of 10 mV. As observed in **Figure 14a**, the Nyquist plots of all of the samples consisted of a semicircle at high frequency and a straight line at low frequency. The first intersection of the semicircle at high frequency with the real axis is related to the electrolyte solution resistance (Rel), and the diameter of the semicircle is related to the chargetransfer resistance (Rct) resulting from the reaction at the electrode-electrolyte interface. The straight line at low frequency is related to the Warburg impedance (Zre) corresponding to the Li-ion diffusion; the Warburg impedance coefficient

*Energy Storage Devices*

**82**

**Figure 10.**

*(C-TixSi1−xOy)@C composite.*

in **Figure 10d**, Si▬O and C▬O bonding were detected for the (C-Ti*x*Si1−*x*O*y*)@C composite. As observed in **Figure 10e**, characteristic peaks were detected in the C 1s spectra of the (C-Ti*x*Si1−*x*O*y*)@C composite, indicating the existence of the carbon framework in the composites. The existence of Ti in the (C-Ti*x*Si1−*x*O*y*)@C composite was also clearly confirmed through XPS analysis. In **Figure 10f**, four characteristic peaks appeared in the Ti 2p spectra of the (C-Ti*x*Si1−*x*O*y*)@C composite. The peaks at ~464.4 and ~458.5 eV correspond to the Ti3+ ion of Ti2O3, and the peaks at ~460.2 and ~465.8 eV were attributed to the Ti4+ ion of TiO2 [29–31], which indicates the formation of Ti▬O bonds in the (C-Ti*x*Si1−*x*O*y*)@C composite

*(a) XRD patterns, (b) Raman spectra, and XPS spectra showing (c) Si 2p, (d) O 1s, (e) C 1s, and (f) Ti 2p of* 

#### **Figure 11.**

*TEM characterization of (C-TixSi1−xOy)@C composite: (a) TEM elemental mapping images of C, Ti, Si, and O. (b) TEM image of ~20-nm-thick carbon layer on amorphous SiO2 matrix. (c) TEM image showing lattice fringes of Si nanoparticles. The white dashed circles identify crystalline nano-Si with a planar distance of 3.1 Å at (111).*

**85**

**Figure 13.**

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping…*

**Sample Element content (wt%) Atomic ratio**

C-SiO*<sup>x</sup>* 45.8 7.3 46.9 1.80 (C-SiO*x*)@C 46.3 10.4 43.3 1.64 (C-Ti*x*Si1−*x*O*y*)@C 47.2 10.7 41.8 0.25 1.55

*Elemental composition and atomic ratio of O/Si of pristine C-SiOx, (C-SiOx)@C and (C-TixSi1−xOy)@C* 

**Si C O Ti O/Si**

(σw) can be estimated from the relations between Zre and ω−1/2 at low frequency

*Electrochemical characteristics of C-SiOx, (C-SiOx)@C composite and (C-TixSi1−xOy)@C composite.* 

*(a) Charge/discharge profiles and (b) cycling performance at current density of 0.1 A g<sup>−</sup><sup>1</sup>*

*at various current densities. (d) Cycling performance at a current density of 1 A g<sup>−</sup><sup>1</sup>*

Zre = Rct + Rel + σ<sup>w</sup> ω−1/2 (9)

The Li-ion diffusion coefficient (DLi) can be calculated using Eq. (10), where R, T, A, n, F, and C refer to the gas constant, temperature, surface area, number of electrons per molecule participating in the redox reaction, Faraday constant, and

T2

mol cm<sup>−</sup><sup>3</sup>

), respectively.

/2A2n4F4C2σ<sup>2</sup> (10)

*.*

*. (c) Power capability* 

using Eq. (9), where ω is the angular frequency [32–34].

maximum ion concentration (7.69 × 10<sup>−</sup><sup>3</sup>

DLi = R<sup>2</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.82379*

**Table 1.**

*composite.*

#### **Figure 12.** *SEM images of (a) C-SiOx, (b) (C-SiOx)@C composite and (c) (C-TixSi1−xOy)@C composite.*

**Sample Element content (wt%) Atomic ratio Si C O Ti O/Si** C-SiO*<sup>x</sup>* 45.8 7.3 46.9 1.80 (C-SiO*x*)@C 46.3 10.4 43.3 1.64 (C-Ti*x*Si1−*x*O*y*)@C 47.2 10.7 41.8 0.25 1.55

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping… DOI: http://dx.doi.org/10.5772/intechopen.82379*

**Table 1.**

*Energy Storage Devices*

**84**

**Figure 12.**

**Figure 11.**

*at (111).*

*TEM characterization of (C-TixSi1−xOy)@C composite: (a) TEM elemental mapping images of C, Ti, Si, and O. (b) TEM image of ~20-nm-thick carbon layer on amorphous SiO2 matrix. (c) TEM image showing lattice fringes of Si nanoparticles. The white dashed circles identify crystalline nano-Si with a planar distance of 3.1 Å* 

*SEM images of (a) C-SiOx, (b) (C-SiOx)@C composite and (c) (C-TixSi1−xOy)@C composite.*

*Elemental composition and atomic ratio of O/Si of pristine C-SiOx, (C-SiOx)@C and (C-TixSi1−xOy)@C composite.*

**Figure 13.**

*Electrochemical characteristics of C-SiOx, (C-SiOx)@C composite and (C-TixSi1−xOy)@C composite. (a) Charge/discharge profiles and (b) cycling performance at current density of 0.1 A g<sup>−</sup><sup>1</sup> . (c) Power capability at various current densities. (d) Cycling performance at a current density of 1 A g<sup>−</sup><sup>1</sup> .*

(σw) can be estimated from the relations between Zre and ω−1/2 at low frequency using Eq. (9), where ω is the angular frequency [32–34].

$$\mathbf{Z\_{rc}} = \mathbf{R\_{ct}} + \mathbf{R\_{cl}} + \sigma\_{\mathbf{w}} \,\mathrm{o}\,\mathrm{o}^{-1/2} \tag{9}$$

The Li-ion diffusion coefficient (DLi) can be calculated using Eq. (10), where R, T, A, n, F, and C refer to the gas constant, temperature, surface area, number of electrons per molecule participating in the redox reaction, Faraday constant, and maximum ion concentration (7.69 × 10<sup>−</sup><sup>3</sup> mol cm<sup>−</sup><sup>3</sup> ), respectively.

$$\mathbf{D}\_{\rm Li} = \mathbf{R}^2 \mathbf{T}^2 / 2\mathbf{A}^2 \mathbf{n}^4 \mathbf{F}^4 \mathbf{C}^2 \sigma^2 \tag{10}$$

#### **Figure 14.**

*(a) Nyquist plots of EIS results and (b) Zre–ω−1/2 plots in the low-frequency range for C-SiOx, (C-SiOx)@C composite, and (C-TixSi1−xOy)@C composite after 50 cycles at a current density of 0.1 A g<sup>−</sup><sup>1</sup> over the frequency range of 1 MHz to 1 mHz with an AC amplitude of 10 mV.*


#### **Table 2.**

*Rel, Rct, σw, and Li-ion diffusion coefficients of C-SiOx, (C-SiOx)@C composite, and (C-TixSi1−xOy)@C composite.*

#### **Figure 15.**

*TEM images of (a) C-SiOx, (b) (C-SiOx)@C composite, and (c) (C-TixSi1−xOy)@C composite electrodes after 300 cycles at a current density of 1 A g<sup>−</sup><sup>1</sup> .*

As shown in **Table 2**, the calculated DLi of the (C-Ti*x*Si1−*x*O*y*)@C composite was 2.04 × 10<sup>−</sup>13 cm2 s<sup>−</sup><sup>1</sup> , which indicates that the Li-ion diffusion was ~4 times faster than that for C-SiO*x* (5.06 × 10–14 cm2 s<sup>−</sup><sup>1</sup> ). Furthermore, Rct of the C-SiO*x*, (C-SiO*x*)@C, and (C-Ti*x*Si1−*x*O*y*)@C electrodes were determined to be 230.7, 204.9, and 134.9 Ω, respectively. Thus, it can be concluded that the multiple structural modifications resulting from the Ti doping and carbon coating led to increased DLi and reduced Rct of the SiO*x* composite, indicating that the (C-Ti*x*Si1−*x*O*y*)@C composite could exhibit highly enhanced electrochemical performance as a promising anode for LIBs.

The stable cycle life of the (C-Ti*x*Si1−*x*O*y*)@C composite resulted from its high structural stability because of the multiple-carbon matrix derived from EG and benzene. As shown in **Figure 15a**–**c**, whereas severe particle degradation was

**87**

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping…*

detected for C-SiO*x* after 300 cycles at high current density, the morphologies of the (C-SiO*x*)@C and (C-Ti*x*Si1−*x*O*y*)@C composites were retained under the same conditions. Thus, the (C-SiO*x*)@C and (C-Ti*x*Si1−*x*O*y*)@C composites exhibited improved structural stability compared with C-SiO*x*. The fracture of particles during cycling leads to the loss of electrical contact and increase of the charge-transfer resistance, resulting in gradual capacity fade during cycling. Therefore, these results indicate that the (C-SiO*x*)@C and (C-Ti*x*Si1−*x*O*y*)@C composites would exhibit

In summary, we successfully fabricated SiO*x* active materials using a simple and cost-effective one-pot synthesis method via an alcoholysis-based reaction. We also demonstrated the exceptional improvement of the electrochemical performance of (C-Ti*x*Si1−*x*O*y*)@C compared with that of SiO*x* achieved by structural modifications using Ti doping and a multiple-carbon matrix. The (C-Ti*x*Si1−*x*O*y*)@C composite consisted of uniformly dispersed Ti ions in an amorphous carbon and SiO*x* matrix, which was homogenously encapsulated by ∼20-nm-thick carbon, enabling the achievement of high power capability and outstanding cyclability. At 1 A g<sup>−</sup><sup>1</sup>

(C-Ti*x*Si1−*x*O*y*)@C electrode retained a discharge capacity of up to ~995 mAh g<sup>−</sup><sup>1</sup>

which was ~3 times higher than that retained by C-SiO*x* under the same conditions. Furthermore, the structural modifications also provided an effective buffer that prevented the severe structural degradation caused by the large volumetric expansion during the charge/discharge cycles. As a result, the (C-Ti*x*Si1−*x*O*y*)@C composite exhibited superior cycle life stability. The C-SiO*x* and (C-SiO*x*)@C composite retained up to ~58.9 and ~86.8% of their initial capacities after 600 cycles at 1 A g<sup>−</sup><sup>1</sup>

respectively, whereas the (C-Ti*x*Si1−*x*O*y*)@C composite delivered a capacity reten-

This work is supported by the National Research Foundation of Korea (NRF)

We obtained permission for the figures and tables used in this paper from

Journal of The Electrochemical Society and Journal of Power Sources.

grant funded by the Ministry of Science, ICT and Future Planning (NRF-

2016R1A2B4014521) and (NRF-2015M3D1A1069713).

The authors declare no competing financial interest.

, the

,

,

*DOI: http://dx.doi.org/10.5772/intechopen.82379*

improved cyclability compared with C-SiO*x*.

**4. Conclusion**

tion of ~88.9%.

**Acknowledgements**

**Conflict of interest**

**Notes/thanks/other declarations**

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping… DOI: http://dx.doi.org/10.5772/intechopen.82379*

detected for C-SiO*x* after 300 cycles at high current density, the morphologies of the (C-SiO*x*)@C and (C-Ti*x*Si1−*x*O*y*)@C composites were retained under the same conditions. Thus, the (C-SiO*x*)@C and (C-Ti*x*Si1−*x*O*y*)@C composites exhibited improved structural stability compared with C-SiO*x*. The fracture of particles during cycling leads to the loss of electrical contact and increase of the charge-transfer resistance, resulting in gradual capacity fade during cycling. Therefore, these results indicate that the (C-SiO*x*)@C and (C-Ti*x*Si1−*x*O*y*)@C composites would exhibit improved cyclability compared with C-SiO*x*.

#### **4. Conclusion**

*Energy Storage Devices*

**86**

2.04 × 10<sup>−</sup>13 cm2

**Figure 15.**

DLi/cm2

**Table 2.**

**Figure 14.**

*composite.*

s<sup>−</sup><sup>1</sup>

*after 300 cycles at a current density of 1 A g<sup>−</sup><sup>1</sup>*

that for C-SiO*x* (5.06 × 10–14 cm2

As shown in **Table 2**, the calculated DLi of the (C-Ti*x*Si1−*x*O*y*)@C composite was

and (C-Ti*x*Si1−*x*O*y*)@C electrodes were determined to be 230.7, 204.9, and 134.9 Ω, respectively. Thus, it can be concluded that the multiple structural modifications resulting from the Ti doping and carbon coating led to increased DLi and reduced Rct of the SiO*x* composite, indicating that the (C-Ti*x*Si1−*x*O*y*)@C composite could exhibit

*TEM images of (a) C-SiOx, (b) (C-SiOx)@C composite, and (c) (C-TixSi1−xOy)@C composite electrodes* 

The stable cycle life of the (C-Ti*x*Si1−*x*O*y*)@C composite resulted from its high structural stability because of the multiple-carbon matrix derived from EG and benzene. As shown in **Figure 15a**–**c**, whereas severe particle degradation was

highly enhanced electrochemical performance as a promising anode for LIBs.

s<sup>−</sup><sup>1</sup>

*.*

, which indicates that the Li-ion diffusion was ~4 times faster than

**C-SiO***<sup>x</sup>* **(C-SiO***x***)@C (C-Ti***x***Si1***−x***O***y***)@C**

 *over the frequency* 

Rel/Ω 1.023 1.045 1.124 Rct/Ω 230.7 204.9 134.9 σw/Ω s<sup>−</sup>1/2 70.7 49.7 35.2

*(a) Nyquist plots of EIS results and (b) Zre–ω−1/2 plots in the low-frequency range for C-SiOx, (C-SiOx)@C* 

*composite, and (C-TixSi1−xOy)@C composite after 50 cycles at a current density of 0.1 A g<sup>−</sup><sup>1</sup>*

*range of 1 MHz to 1 mHz with an AC amplitude of 10 mV.*

*Rel, Rct, σw, and Li-ion diffusion coefficients of C-SiOx, (C-SiOx)@C composite, and (C-TixSi1−xOy)@C* 

s<sup>−</sup><sup>1</sup> 5.06 × 10<sup>−</sup><sup>14</sup> 1.02 × 10<sup>−</sup><sup>13</sup> 2.04 × 10<sup>−</sup><sup>13</sup>

). Furthermore, Rct of the C-SiO*x*, (C-SiO*x*)@C,

In summary, we successfully fabricated SiO*x* active materials using a simple and cost-effective one-pot synthesis method via an alcoholysis-based reaction. We also demonstrated the exceptional improvement of the electrochemical performance of (C-Ti*x*Si1−*x*O*y*)@C compared with that of SiO*x* achieved by structural modifications using Ti doping and a multiple-carbon matrix. The (C-Ti*x*Si1−*x*O*y*)@C composite consisted of uniformly dispersed Ti ions in an amorphous carbon and SiO*x* matrix, which was homogenously encapsulated by ∼20-nm-thick carbon, enabling the achievement of high power capability and outstanding cyclability. At 1 A g<sup>−</sup><sup>1</sup> , the (C-Ti*x*Si1−*x*O*y*)@C electrode retained a discharge capacity of up to ~995 mAh g<sup>−</sup><sup>1</sup> , which was ~3 times higher than that retained by C-SiO*x* under the same conditions. Furthermore, the structural modifications also provided an effective buffer that prevented the severe structural degradation caused by the large volumetric expansion during the charge/discharge cycles. As a result, the (C-Ti*x*Si1−*x*O*y*)@C composite exhibited superior cycle life stability. The C-SiO*x* and (C-SiO*x*)@C composite retained up to ~58.9 and ~86.8% of their initial capacities after 600 cycles at 1 A g<sup>−</sup><sup>1</sup> , respectively, whereas the (C-Ti*x*Si1−*x*O*y*)@C composite delivered a capacity retention of ~88.9%.

#### **Acknowledgements**

This work is supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning (NRF-2016R1A2B4014521) and (NRF-2015M3D1A1069713).

#### **Conflict of interest**

The authors declare no competing financial interest.

#### **Notes/thanks/other declarations**

We obtained permission for the figures and tables used in this paper from Journal of The Electrochemical Society and Journal of Power Sources.

*Energy Storage Devices*

#### **Author details**

Hyeon-Woo Yang and Sun-Jae Kim\* Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul, Republic of Korea

\*Address all correspondence to: sjkim1@sejong.ac.kr

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**89**

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping…*

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[12] Sun L, Su T, Xu L, Liu M, Du H-B. Two-dimensional ultra-thin SiOx (0 < x < 2) nanosheets with long-term cycling stability as lithium ion battery anodes. Chemical Communications. 2016;**52**:4341-4344. DOI: 10.1039/

[13] Guo C, Wang D, Liu T, Zhu J, Lang X. A three dimensional SiOx/C@rGO nanocomposite as a high energy anode material for lithium-ion batteries. Journal of Materials Chemistry A. 2014;**2**:3521-3527. DOI: 10.1039/

[14] Zhang J, Zhang C, Liu Z, Zheng J, Zuo Y, Xue C, et al. High-performance ball-milled SiOx anodes for lithium ion batteries. Journal of Power Sources. 2017;**339**:86-92. DOI: 10.1016/j.

electacta.2016.12.071

celc.201700316

c6cc00723f

c3ta13746e

jpowsour.2016.11.044

*DOI: http://dx.doi.org/10.5772/intechopen.82379*

[1] Liu N, Lu Z, Zhao J, McDowell MT, Lee H-W, Zhao W, et al. A pomegranateinspired nanoscale design for largevolume-change lithium battery anodes. Nature Nanotechnology. 2014;**9**:187-192.

DOI: 10.1038/NNANO.2014.6

Chemical Reviews. 2014;**114**:

[2] Obrovac M, Chevrier V. Alloy negative electrodes for Li-ion batteries.

11444-11502. DOI: 10.1021/cr500207g

[3] Choi S, Kwon T-W, Coskun A, Choi JW. Highly elastic binders integrating polyrotaxanes for silicon microparticle anodes in lithium ion batteries. Science. 2017;**357**:279-283. DOI: 10.1126/science.

[4] Casimir A, Zhang H, Ogoke O, Amine JC, Lu J, Wu G. Silicon-based anodes for lithium-ion batteries: Effectiveness of materials synthesis and electrode preparation. Nano Energy. 2016;**27**:359-376. DOI: 10.1016/j.

[5] Xing Y, Shen T, Guo T, Wang X, Xia X, Gu C, et al. A novel durable doubleconductive core-shell structure applying to the synthesis of silicon anode for lithium ion batteries. Journal of Power Sources. 2018;**384**:207-213. DOI: 10.1016/j.jpowsour.2018.02.051

[6] Shang H, Zuo Z, Yu L, Wang F, He F, Li Y. Low-temperature growth of allcarbon graphdiyne on a silicon anode for high-performance lithium-ion batteries. Advanced Materials. 2018;**30**:1801459.

[7] Li Z, He Q, He L, Hu P, Li W, Yan H, et al. Self-sacrificed synthesis of carboncoated SiOx nanowires for high capacity lithium ion battery anodes. Journal of Materials Chemistry A. 2017;**5**: 4183-4189. DOI: 10.1039/x0xx00000x

[8] Parimalam BS, Mac Intosh AD, Kadam R, Lucht BL. Decomposition

DOI: 10.1002/adma.201801459

**References**

aal4373

nanoen.2016.07.023

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping… DOI: http://dx.doi.org/10.5772/intechopen.82379*

#### **References**

*Energy Storage Devices*

**88**

**Author details**

provided the original work is properly cited.

Hyeon-Woo Yang and Sun-Jae Kim\*

University, Seoul, Republic of Korea

\*Address all correspondence to: sjkim1@sejong.ac.kr

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Department of Nanotechnology and Advanced Materials Engineering, Sejong

[1] Liu N, Lu Z, Zhao J, McDowell MT, Lee H-W, Zhao W, et al. A pomegranateinspired nanoscale design for largevolume-change lithium battery anodes. Nature Nanotechnology. 2014;**9**:187-192. DOI: 10.1038/NNANO.2014.6

[2] Obrovac M, Chevrier V. Alloy negative electrodes for Li-ion batteries. Chemical Reviews. 2014;**114**: 11444-11502. DOI: 10.1021/cr500207g

[3] Choi S, Kwon T-W, Coskun A, Choi JW. Highly elastic binders integrating polyrotaxanes for silicon microparticle anodes in lithium ion batteries. Science. 2017;**357**:279-283. DOI: 10.1126/science. aal4373

[4] Casimir A, Zhang H, Ogoke O, Amine JC, Lu J, Wu G. Silicon-based anodes for lithium-ion batteries: Effectiveness of materials synthesis and electrode preparation. Nano Energy. 2016;**27**:359-376. DOI: 10.1016/j. nanoen.2016.07.023

[5] Xing Y, Shen T, Guo T, Wang X, Xia X, Gu C, et al. A novel durable doubleconductive core-shell structure applying to the synthesis of silicon anode for lithium ion batteries. Journal of Power Sources. 2018;**384**:207-213. DOI: 10.1016/j.jpowsour.2018.02.051

[6] Shang H, Zuo Z, Yu L, Wang F, He F, Li Y. Low-temperature growth of allcarbon graphdiyne on a silicon anode for high-performance lithium-ion batteries. Advanced Materials. 2018;**30**:1801459. DOI: 10.1002/adma.201801459

[7] Li Z, He Q, He L, Hu P, Li W, Yan H, et al. Self-sacrificed synthesis of carboncoated SiOx nanowires for high capacity lithium ion battery anodes. Journal of Materials Chemistry A. 2017;**5**: 4183-4189. DOI: 10.1039/x0xx00000x

[8] Parimalam BS, Mac Intosh AD, Kadam R, Lucht BL. Decomposition reactions of anode solid electrolyte interphase (SEI) components with LiPF6. The Journal of Physical Chemistry C. 2017;**121**:22733-22738. DOI: 10.1021/acs.jpcc.7b08433

[9] Haruta M, Okubo T, Masuo Y, Yoshida S, Tomita A, Takenaka T, et al. Temperature effects on SEI formation and cyclability of Si nanoflake powder anode in the presence of SEI-forming additives. Electrochimica Acta. 2017;**224**:186-193. DOI: 10.1016/j. electacta.2016.12.071

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[11] Park E, Park MS, Lee J, Kim KJ, Jeong G, Kim JH, et al. A highly resilient mesoporous SiOx lithium storage material engineered by oil–water templating. ChemSusChem. 2015;**8**: 688-694. DOI: 10.1002/cssc.201402907

[12] Sun L, Su T, Xu L, Liu M, Du H-B. Two-dimensional ultra-thin SiOx (0 < x < 2) nanosheets with long-term cycling stability as lithium ion battery anodes. Chemical Communications. 2016;**52**:4341-4344. DOI: 10.1039/ c6cc00723f

[13] Guo C, Wang D, Liu T, Zhu J, Lang X. A three dimensional SiOx/C@rGO nanocomposite as a high energy anode material for lithium-ion batteries. Journal of Materials Chemistry A. 2014;**2**:3521-3527. DOI: 10.1039/ c3ta13746e

[14] Zhang J, Zhang C, Liu Z, Zheng J, Zuo Y, Xue C, et al. High-performance ball-milled SiOx anodes for lithium ion batteries. Journal of Power Sources. 2017;**339**:86-92. DOI: 10.1016/j. jpowsour.2016.11.044

[15] Shi L, Wang W, Wang A, Yuan K, Jin Z, Yang Y. Scalable synthesis of coreshell structured SiOx/nitrogen-doped carbon composite as a high-performance anode material for lithium-ion batteries. Journal of Power Sources. 2016;**318**: 184-191. DOI: 10.1016/j.jpowsour. 2016.03.111

[16] Xu Q, Sun JK, Yin YX, Guo YG. Facile synthesis of blocky SiOx/C with graphite-like structure for highperformance lithium-ion battery anodes. Advanced Functional Materials. 2018;**28**:1705235. DOI: 10.1002/ adfm.201705235

[17] Park E, Yoo H, Lee J, Park M-S, Kim Y-J, Kim H. Dual-size silicon nanocrystal-embedded SiOx nanocomposite as a high-capacity lithium storage material. ACS Nano. 2015;**9**:7690-7696. DOI: 10.1021/ acsnano.5b03166

[18] Liu Q, Cui Z, Zou R, Zhang J, Xu K, Hu J. Surface coating constraint induced anisotropic swelling of silicon in Si– Void@SiOx nanowire anode for lithiumion batteries. Small. 2017;**13**:1603754. DOI: 10.1002/smll.201603754

[19] Han J, Chen G, Yan T, Liu H, Shi L, An Z, et al. Creating graphene-like carbon layers on SiO anodes via a layer-by-layer strategy for lithium-ion battery. Chemical Engineering Journal. 2018;**347**:273-279. DOI: 10.1016/j. cej.2018.04.100

[20] Dou F, Shi L, Song P, Chen G, An J, Liu H, et al. Design of orderly carbon coatings for SiO anodes promoted by TiO2 toward high performance lithiumion battery. Chemical Engineering Journal. 2018;**338**:488-495. DOI: 10.1016/j.cej.2018.01.048

[21] Lee K-M, Lee Y-S, Kim Y-W, Sun Y-K, Lee S-M. Electrochemical characterization of Ti–Si and Ti–Si–Al alloy anodes for Li-ion batteries produced by mechanical ball milling.

Journal of Alloys and Compounds. 2009;**472**:461-465. DOI: 10.1016/j. jallcom.2008.04.102

[22] Wang Y, He Y, Xiao R, Li H, Aifantis K, Huang X. Investigation of crack patterns and cyclic performance of Ti–Si nanocomposite thin film anodes for lithium ion batteries. Journal of Power Sources. 2012;**202**:236-245. DOI: 10.1016/j.jpowsour.2011.11.027

[23] Ren Y, Li J, Yu J. Enhanced electrochemical performance of TiO2 by Ti3+ doping using a facile solvothermal method as anode materials for lithium-ion batteries. Electrochimica Acta. 2014;**138**:41-47. DOI: 10.1016/j. electacta.2014.06.068

[24] Seok D-I, Wu M, Shim KB, Kang Y, Jung H-K. High-rate performance of Ti3+ self-doped TiO2 prepared by imidazole reduction for Li-ion batteries. Nanotechnology. 2016;**27**:435401. DOI: 10.1088/0957-4484/27/43/435401

[25] Chen J, Song W, Hou H, Zhang Y, Jing M, Jia X, et al. Ti3+ self-doped dark rutile TiO2 ultrafine nanorods with durable high-rate capability for lithiumion batteries. Advanced Functional Materials. 2015;**25**:6793-6801. DOI: 10.1002/adfm.201502978

[26] Miyachi M, Yamamoto H, Kawai H, Ohta T, Shirakata M. Analysis of SiO anodes for lithium-ion batteries. Journal of the Electrochemical Society. 2005;**152**:A2089-A2091. DOI: 10.1149/1.2013210

[27] Philippe B, Dedryvère RM, Allouche J, Lindgren F, Gorgoi M, Rensmo HK, et al. Nanosilicon electrodes for lithiumion batteries: Interfacial mechanisms studied by hard and soft x-ray photoelectron spectroscopy. Chemistry of Materials. 2012;**24**:1107, 1115. DOI: 10.1021/cm2034195

[28] Verma P, Maire P, Novák P. A review of the features and analyses

**91**

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping…*

*DOI: http://dx.doi.org/10.5772/intechopen.82379*

of the solid electrolyte interphase in Li-ion batteries. Electrochimica Acta. 2010;**55**:6332-6341. DOI: 10.1016/j.

[29] Hashimoto S, Tanaka A. Alteration of Ti 2p XPS spectrum for titanium

bombardment. Surface and Interface Analysis. 2002;**34**:262-265. DOI:

[30] Chrcanovic BR, Pedrosa AR, Martins MD. Chemical and topographic analysis of treated surfaces of five different commercial dental titanium

implants. Materials Research. 2012;**15**:372-382. DOI: 10.1590/ S1516-14392012005000035

Materials & Interfaces. 2015;**7**: 18483-18490. DOI: 10.1021/acsami.

[32] Jiang Z, Pei B, Manthiram A. Randomly stacked holey graphene anodes for lithium ion batteries with enhanced electrochemical performance. Journal of Materials Chemistry A. 2013;**1**:7775-7781. DOI: 10.1039/

[33] Li S, Cao X, Schmidt CN, Xu Q, Uchaker E, Pei Y, et al. TiNb2O7/ graphene composites as high-rate anode materials for lithium/sodium ion batteries. Journal of Materials Chemistry A. 2016;**4**:4242-4251. DOI:

[34] Tan Y, Wong KW, Ng KM. Novel silicon doped tin oxide–carbon microspheres as anode material for lithium ion batteries: The multiple effects exerted by doped Si. Small. 2017;**13**:1702614. DOI: 10.1002/

5b04652

c3ta10457e

10.1039/c5ta10510b

smll.201702614

[31] Park AR, Son D-Y, Kim JS, Lee JY, Park N-G, Park J, et al. Si/Ti2O3/reduced graphene oxide nanocomposite anodes for lithium-ion batteries with highly enhanced cyclic stability. ACS Applied

electacta.2010.05.072

10.1002/sia.1296

oxide by low-energy Ar ion

*SiO*x *as a Potential Anode Material for Li-Ion Batteries: Role of Carbon Coating, Doping… DOI: http://dx.doi.org/10.5772/intechopen.82379*

of the solid electrolyte interphase in Li-ion batteries. Electrochimica Acta. 2010;**55**:6332-6341. DOI: 10.1016/j. electacta.2010.05.072

*Energy Storage Devices*

2016.03.111

adfm.201705235

acsnano.5b03166

cej.2018.04.100

10.1016/j.cej.2018.01.048

[21] Lee K-M, Lee Y-S, Kim Y-W, Sun Y-K, Lee S-M. Electrochemical characterization of Ti–Si and Ti–Si–Al alloy anodes for Li-ion batteries produced by mechanical ball milling.

[15] Shi L, Wang W, Wang A, Yuan K, Jin Z, Yang Y. Scalable synthesis of coreshell structured SiOx/nitrogen-doped carbon composite as a high-performance anode material for lithium-ion batteries. Journal of Power Sources. 2016;**318**: 184-191. DOI: 10.1016/j.jpowsour.

Journal of Alloys and Compounds. 2009;**472**:461-465. DOI: 10.1016/j.

10.1016/j.jpowsour.2011.11.027

[23] Ren Y, Li J, Yu J. Enhanced

method as anode materials for lithium-ion batteries. Electrochimica Acta. 2014;**138**:41-47. DOI: 10.1016/j.

electacta.2014.06.068

[22] Wang Y, He Y, Xiao R, Li H, Aifantis K, Huang X. Investigation of crack patterns and cyclic performance of Ti–Si nanocomposite thin film anodes for lithium ion batteries. Journal of Power Sources. 2012;**202**:236-245. DOI:

electrochemical performance of TiO2 by Ti3+ doping using a facile solvothermal

[24] Seok D-I, Wu M, Shim KB, Kang Y, Jung H-K. High-rate performance of Ti3+ self-doped TiO2 prepared by imidazole reduction for Li-ion batteries. Nanotechnology. 2016;**27**:435401. DOI: 10.1088/0957-4484/27/43/435401

[25] Chen J, Song W, Hou H, Zhang Y, Jing M, Jia X, et al. Ti3+ self-doped dark rutile TiO2 ultrafine nanorods with durable high-rate capability for lithiumion batteries. Advanced Functional Materials. 2015;**25**:6793-6801. DOI:

[26] Miyachi M, Yamamoto H, Kawai H, Ohta T, Shirakata M. Analysis of SiO anodes for lithium-ion batteries. Journal of the Electrochemical Society. 2005;**152**:A2089-A2091. DOI:

[27] Philippe B, Dedryvère RM, Allouche J, Lindgren F, Gorgoi M, Rensmo HK, et al. Nanosilicon electrodes for lithiumion batteries: Interfacial mechanisms studied by hard and soft x-ray

photoelectron spectroscopy. Chemistry of Materials. 2012;**24**:1107, 1115. DOI:

[28] Verma P, Maire P, Novák P. A review of the features and analyses

10.1002/adfm.201502978

10.1149/1.2013210

10.1021/cm2034195

jallcom.2008.04.102

[16] Xu Q, Sun JK, Yin YX, Guo YG. Facile synthesis of blocky SiOx/C with graphite-like structure for highperformance lithium-ion battery anodes. Advanced Functional Materials.

2018;**28**:1705235. DOI: 10.1002/

[17] Park E, Yoo H, Lee J, Park M-S, Kim Y-J, Kim H. Dual-size silicon nanocrystal-embedded SiOx nanocomposite as a high-capacity lithium storage material. ACS Nano. 2015;**9**:7690-7696. DOI: 10.1021/

DOI: 10.1002/smll.201603754

[19] Han J, Chen G, Yan T, Liu H, Shi L, An Z, et al. Creating graphene-like carbon layers on SiO anodes via a layer-by-layer strategy for lithium-ion battery. Chemical Engineering Journal. 2018;**347**:273-279. DOI: 10.1016/j.

[20] Dou F, Shi L, Song P, Chen G, An J, Liu H, et al. Design of orderly carbon coatings for SiO anodes promoted by TiO2 toward high performance lithiumion battery. Chemical Engineering Journal. 2018;**338**:488-495. DOI:

[18] Liu Q, Cui Z, Zou R, Zhang J, Xu K, Hu J. Surface coating constraint induced anisotropic swelling of silicon in Si– Void@SiOx nanowire anode for lithiumion batteries. Small. 2017;**13**:1603754.

**90**

[29] Hashimoto S, Tanaka A. Alteration of Ti 2p XPS spectrum for titanium oxide by low-energy Ar ion bombardment. Surface and Interface Analysis. 2002;**34**:262-265. DOI: 10.1002/sia.1296

[30] Chrcanovic BR, Pedrosa AR, Martins MD. Chemical and topographic analysis of treated surfaces of five different commercial dental titanium implants. Materials Research. 2012;**15**:372-382. DOI: 10.1590/ S1516-14392012005000035

[31] Park AR, Son D-Y, Kim JS, Lee JY, Park N-G, Park J, et al. Si/Ti2O3/reduced graphene oxide nanocomposite anodes for lithium-ion batteries with highly enhanced cyclic stability. ACS Applied Materials & Interfaces. 2015;**7**: 18483-18490. DOI: 10.1021/acsami. 5b04652

[32] Jiang Z, Pei B, Manthiram A. Randomly stacked holey graphene anodes for lithium ion batteries with enhanced electrochemical performance. Journal of Materials Chemistry A. 2013;**1**:7775-7781. DOI: 10.1039/ c3ta10457e

[33] Li S, Cao X, Schmidt CN, Xu Q, Uchaker E, Pei Y, et al. TiNb2O7/ graphene composites as high-rate anode materials for lithium/sodium ion batteries. Journal of Materials Chemistry A. 2016;**4**:4242-4251. DOI: 10.1039/c5ta10510b

[34] Tan Y, Wong KW, Ng KM. Novel silicon doped tin oxide–carbon microspheres as anode material for lithium ion batteries: The multiple effects exerted by doped Si. Small. 2017;**13**:1702614. DOI: 10.1002/ smll.201702614

**93**

**Chapter 6**

**Abstract**

recycling purposes.

**1. Introduction**

Na-ion battery, Li-S battery, Li-air battery

Progress on Free-Standing

*Karthick Ramalingam and Fuming Chen*

Future Scenario

Graphene Hybrid: Advantages and

Free-standing graphene (FSG) paper like electrodes has paid attention to the energy storage device application in the past decade. It befits to fabricate flexible devices due to its remarkable mechanical strength and offers high electrical conductivity. In this chapter, we explore the advantages and future prospects of FSG fresh candidate in rechargeable batteries. Herein, we summarized the synthetic strategies used for FSG fabrication and its properties, followed by its application in rechargeable batteries. Extensively, this chapter deals with fabrication of FSG hybrid composite papers for battery applications to understand the overall device performance. Specifically, we discuss the benefits of FSG electrodes over conventional electrode material and its fabrication in battery system. Ultimately, we conclude with the significance of FSG paper in battery application and forthcoming advantage for

**Keywords:** free-standing graphene paper, electroactive, Li-ion battery,

Pertaining to the day-to-day energy usage increases, various technologies were addressed to satisfy the current energy demand. Based on this circumstance, the electronic devices for energy conversion (solar cells and fuel cells) and energy storage (batteries and supercapacitors) were extensively studied throughout the world [1]. Basically, the performance of these devices depends on the materials' design with different nanostructures and material interfaces. In particular, advanced materials including carbon nanomaterials, viz., carbon black, carbon nanotubes, carbon nanofibers, graphene, and so on, play a vital role in an attempt to lead the breakthrough and challenges from laboratory scale to technology ideas [2]. Among them, graphene, since its discovery, has been stirring enthusiasm among the scientific community owing to its attractive properties. Properties such as high electrocatalytic activity, good conductivity with immense surface area, and low costs make it an ideal candidate to implement in electrochemical application. Subsequently, graphene has been utilized as a promising candidate in energy storage applications such as battery and supercapacitors (SCs) [3, 4]. Due to its high electrical conductivity, charge carrier mobility, and transparency, it has been potentially used as an electrode for electrochemical energy device application [5, 6]. Processing of graphene electrodes differs according to their application

#### **Chapter 6**

## Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario

*Karthick Ramalingam and Fuming Chen*

#### **Abstract**

Free-standing graphene (FSG) paper like electrodes has paid attention to the energy storage device application in the past decade. It befits to fabricate flexible devices due to its remarkable mechanical strength and offers high electrical conductivity. In this chapter, we explore the advantages and future prospects of FSG fresh candidate in rechargeable batteries. Herein, we summarized the synthetic strategies used for FSG fabrication and its properties, followed by its application in rechargeable batteries. Extensively, this chapter deals with fabrication of FSG hybrid composite papers for battery applications to understand the overall device performance. Specifically, we discuss the benefits of FSG electrodes over conventional electrode material and its fabrication in battery system. Ultimately, we conclude with the significance of FSG paper in battery application and forthcoming advantage for recycling purposes.

**Keywords:** free-standing graphene paper, electroactive, Li-ion battery, Na-ion battery, Li-S battery, Li-air battery

#### **1. Introduction**

Pertaining to the day-to-day energy usage increases, various technologies were addressed to satisfy the current energy demand. Based on this circumstance, the electronic devices for energy conversion (solar cells and fuel cells) and energy storage (batteries and supercapacitors) were extensively studied throughout the world [1]. Basically, the performance of these devices depends on the materials' design with different nanostructures and material interfaces. In particular, advanced materials including carbon nanomaterials, viz., carbon black, carbon nanotubes, carbon nanofibers, graphene, and so on, play a vital role in an attempt to lead the breakthrough and challenges from laboratory scale to technology ideas [2].

Among them, graphene, since its discovery, has been stirring enthusiasm among the scientific community owing to its attractive properties. Properties such as high electrocatalytic activity, good conductivity with immense surface area, and low costs make it an ideal candidate to implement in electrochemical application. Subsequently, graphene has been utilized as a promising candidate in energy storage applications such as battery and supercapacitors (SCs) [3, 4]. Due to its high electrical conductivity, charge carrier mobility, and transparency, it has been potentially used as an electrode for electrochemical energy device application [5, 6]. Processing of graphene electrodes differs according to their application by fabrication techniques and synthetic strategies. As graphene is an electrode focusing on rechargeable battery application, the device performance is based on the presence of electroactive sites in graphene sheets [7, 8]. Therefore, graphene sheets composited with suitable electroactive materials like metal chalcogenides, metal oxides/hydroxides, metal nanostructures, and even the heteroatom-doped graphene provide better activity for rechargeable batteries [9–11]. Conventionally, the electrode materials were deposited on metal foils by doctor-blade technique, drop-casting, spray-coating, or spin coating to construct the batteries. This electrode material was mixed with foreign materials (binders and conducting agent) to make into ink, paste, colloidal dispersion, etc., for deposition purposes. In the case of self-supported graphene foams or FSGs, the foreign materials are avoided, and on the whole, they act as electrodes directly [12]. This chapter outlines few reported literature on FSG performance for rechargeable battery applications. Moreover, we summarized the synthetic strategies and fabrication of free-standing graphene/ hybrid functional materials for particular device application.

### **2. Graphene: properties and nomenclature**

Graphene is a 2D one atom thin sheet that consists of hexagonal sp2 carbon, which is densely packed into honey-comb lattice and large benzene-like aromatic hydrocarbon. It is considered as fundamental basis for all carbon allotropes, and their conceptual depiction are shown in **Figure 1**. It represents that 2D graphene sheet can be enclosed into 0D like fullerene structure and rolled up into 1D-like carbon nanotube structure, and 10 layers of graphene can be stacked up into 3D graphitic-like structure. Hence, it is considered as "mother of carbon allotropes" [13]. The fabrication of graphene film by different synthetic routes was adapted accordingly to its required

#### **Figure 1.**

*Carbon allotropes in different forms: 0D Bucky ball, 1D nanotubes, 2D sheets, and 3D graphite form (without permission from Ref. [13]).*

**95**

**Figure 2.**

*Properties of graphene and its appropriate application.*

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario*

methods), and wet chemical process (oxidation of graphite) [14].

–106

properties for many applications. Current technologies addressed to synthesize graphene via several routes are as follows: mechanical exfoliation (liquid exfoliation and scotch tape method), epitaxial growth (chemical vapor deposition (CVD) and from organic molecules method), unzipping CNT (chemical and electrochemical

Graphene possesses exclusive chemical, physical, mechanical, and thermal properties, which focuses on the field of electrochemical applications as an electrode material to enhance the stability and durability of the devices. Graphene application in any devices is adopted according to its properties as shown in **Figure 2**. Prominently, the conductivity of anode and cathode electrodes plays a vital role in batteries, which collect or disperse the electrons that tune up the performance

the number of layers [15, 16]. Additionally, the electrode surface area is an essential part for batteries, which has high theoretical surface area of graphene, and

thickness, the spring constants were observed between 1 and 5 N/m, and pristine graphene exhibits Young's modulus of 1.05 TPa and intrinsic strength of 110 GPa, which has high mechanical property [18, 19]. The electrochemical property is a perspective for energy storage and generation technologies. The rate of heterogeneous electron transfer occurs on graphene materials; in the meantime, the rate of

carbon networks of 2D graphene sheet exhibit high

S/cm than any other carbon materials depending on

/g [17]. For suspended graphene sheets below 10 nm

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

**2.1 Graphene: properties**

to device. The conjugated sp2

conductivity around 104

is reported to be ∼2600 m<sup>2</sup>

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

properties for many applications. Current technologies addressed to synthesize graphene via several routes are as follows: mechanical exfoliation (liquid exfoliation and scotch tape method), epitaxial growth (chemical vapor deposition (CVD) and from organic molecules method), unzipping CNT (chemical and electrochemical methods), and wet chemical process (oxidation of graphite) [14].

#### **2.1 Graphene: properties**

*Energy Storage Devices*

by fabrication techniques and synthetic strategies. As graphene is an electrode focusing on rechargeable battery application, the device performance is based on the presence of electroactive sites in graphene sheets [7, 8]. Therefore, graphene sheets composited with suitable electroactive materials like metal chalcogenides, metal oxides/hydroxides, metal nanostructures, and even the heteroatom-doped graphene provide better activity for rechargeable batteries [9–11]. Conventionally, the electrode materials were deposited on metal foils by doctor-blade technique, drop-casting, spray-coating, or spin coating to construct the batteries. This electrode material was mixed with foreign materials (binders and conducting agent) to make into ink, paste, colloidal dispersion, etc., for deposition purposes. In the case of self-supported graphene foams or FSGs, the foreign materials are avoided, and on the whole, they act as electrodes directly [12]. This chapter outlines few reported literature on FSG performance for rechargeable battery applications. Moreover, we summarized the synthetic strategies and fabrication of free-standing graphene/

hybrid functional materials for particular device application.

Graphene is a 2D one atom thin sheet that consists of hexagonal sp2

is densely packed into honey-comb lattice and large benzene-like aromatic hydrocarbon. It is considered as fundamental basis for all carbon allotropes, and their conceptual depiction are shown in **Figure 1**. It represents that 2D graphene sheet can be enclosed into 0D like fullerene structure and rolled up into 1D-like carbon nanotube structure, and 10 layers of graphene can be stacked up into 3D graphitic-like structure. Hence, it is considered as "mother of carbon allotropes" [13]. The fabrication of graphene film by different synthetic routes was adapted accordingly to its required

*Carbon allotropes in different forms: 0D Bucky ball, 1D nanotubes, 2D sheets, and 3D graphite form (without* 

carbon, which

**2. Graphene: properties and nomenclature**

**94**

**Figure 1.**

*permission from Ref. [13]).*

Graphene possesses exclusive chemical, physical, mechanical, and thermal properties, which focuses on the field of electrochemical applications as an electrode material to enhance the stability and durability of the devices. Graphene application in any devices is adopted according to its properties as shown in **Figure 2**. Prominently, the conductivity of anode and cathode electrodes plays a vital role in batteries, which collect or disperse the electrons that tune up the performance to device. The conjugated sp2 carbon networks of 2D graphene sheet exhibit high conductivity around 104 –106 S/cm than any other carbon materials depending on the number of layers [15, 16]. Additionally, the electrode surface area is an essential part for batteries, which has high theoretical surface area of graphene, and is reported to be ∼2600 m<sup>2</sup> /g [17]. For suspended graphene sheets below 10 nm thickness, the spring constants were observed between 1 and 5 N/m, and pristine graphene exhibits Young's modulus of 1.05 TPa and intrinsic strength of 110 GPa, which has high mechanical property [18, 19]. The electrochemical property is a perspective for energy storage and generation technologies. The rate of heterogeneous electron transfer occurs on graphene materials; in the meantime, the rate of

**Figure 2.** *Properties of graphene and its appropriate application.*

reaction varies selectively at edges and basal plane according to their electroactive sites by adding impurities or doping. Graphene-based materials were potentially applied in electrochemical devices due to their inherent electrochemical activity nature [20]. These amazing properties of graphene such as electrical, mechanical, and electrochemical were attracted for rechargeable batteries.

#### **2.2 Graphene: nomenclature**

It is well known that graphene can be synthesized by several routes and named according to the recovered final product. Graphene research has elevated gradually in the past 5 years for its tremendous properties, but the scientific community ends up with the confusion in naming the material. Even though researchers have synthesized up to 100 layers of carbon sheets, they were naming them as graphene. This provides different changes in properties compared with the single-layer graphene sheet for their practical applications [21]. Hence, carbon journal community raised a nomenclature for graphene family, which is shown in (**Table 1**).

The descriptive term is an essential thing for researchers in the area of graphene material because the properties will change accordingly with recovered product with different synthetic strategies. For example, the graphene-based transparent conducting film adopted by the CVD method obtained 600 ohms/sq. at 96.5% transmittance at 550 nm, whereas solution processed graphene increases above 10 K ohms at the same transmittance [22–24]. Even the electrochemical behavior fluctuates according to the synthetic strategies; for instance, the presence of oxygen functional groups in graphene oxide (GO) shows an excellent electrochemical behavior rather than the pristine graphene [25]. Hence, the electrochemical device


**97**

104

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario*

**3. Free-standing graphene: synthesis and its properties**

of applications due to the tuning of their properties.

applications based on graphene electrodes depend on the architecture and hybrid composites to improve the active sites. Recently, 3D architecture like graphene materials such as foams, hydrogel, aerogel, and free-standing was utilized in

For designing and fabricating large scale macroscopic or microscopic architecture like materials, the choice of precursor signifies the synthetic strategies. Graphene sheets synthesized by wet chemical process commenced for several applications due to the presence of functional groups. As discussed in the previous section, the methods utilized for the preparation of graphene sheets conclude their suitable application based on their properties. Noteworthy, there is a challenge for high dispersion of graphene either in aqueous or in organic solvents. It has been achieved by dispersing agent introduced into hydrophobic graphene sheets for good dispersion, whereas it submerges the graphene properties [26]. In the view of fact, large scale solution processable GO has several advantages such as cost effective, eco-friendly solvent and facile to introduce any foreign material due to the presence of functional groups [27, 28]. The copious amount of functional groups attached to the graphene surface contains hydroxyl and epoxy groups at basal planes and carboxyl groups at edges. This leads to affinity with water molecules, which provides a higher dispersion and further it assists with other inorganic or organic molecules for facile composite preparation. In the choice of precursor for free-standing material preparation, GO dominates as a building block due to its features of large scale solution processable with high colloidal dispersion. The resultant macroscopic FSG holds as an excellent mechanical, electrical, and light-weight material. Further, the 3D architecture of FSG enhances the surface area, porous nature, and structural active sites by merging with other functional host materials such as semiconducting material, metal nanoparticles, and polymers. The synergy of graphene sheets and functional host materials in the 3D macroscopic architecture attracted wide variety

In 1998, Smalley prepared CNT buckypaper by vacuum filtration, in prior it is well dispersed in Triton X-100 surfactant to break up the pi-pi interaction between the bundled ropes of CNT [29]. Further, CNT buckypapers were prepared by domino pushing technique, and they are strong, robust, and flexible. The obtained paper exhibits 26 micron thickness; the electrical conductivity was found to be 2.0 ×

 S/m and thermal conductivity shows 153 W/mK [30]. These papers were directly applied for supercapacitor application. Thus, the carbon paper–like materials were potentially applied in a variety of applications due to their light-weight, highly flexible, robust, and eco-friendly nature. On the basis of cost, the CNT papers lag behind for the practical applications, and they have been replaced by graphene sheets. Similar to CNT buckypaper, GO paper was fabricated by flow-assisted vacuum filtration or evaporation techniques. **Figure 3a** and **b** shows the photograph of flexible GO paper and mechanical properties comparison chart of GO paper, buckypapers, vermiculite paper-like material, and graphite foil, respectively. Young's modulus is as high as in GO papers with 42 GPa for vacuum-assisted technique, and similar tensile strength but lowest Young's modulus (12.7 GPa) was obtained for evaporationinduced self-assembly technique [31, 34]. Thus, the high mechanical properties of GO paper can be used in several applications such as supercapacitors and other flexible substrates [35]. Moreover, the mechanical properties of GO papers depend on the alignment of GO sheets by any chemical modification between the layers and at the edges. The modifications are made either by crosslinking or grafting between

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

electrochemistry-oriented topics.

#### **Table 1.**

*Nomenclature of graphene based on the structure.*

applications based on graphene electrodes depend on the architecture and hybrid composites to improve the active sites. Recently, 3D architecture like graphene materials such as foams, hydrogel, aerogel, and free-standing was utilized in electrochemistry-oriented topics.

#### **3. Free-standing graphene: synthesis and its properties**

For designing and fabricating large scale macroscopic or microscopic architecture like materials, the choice of precursor signifies the synthetic strategies. Graphene sheets synthesized by wet chemical process commenced for several applications due to the presence of functional groups. As discussed in the previous section, the methods utilized for the preparation of graphene sheets conclude their suitable application based on their properties. Noteworthy, there is a challenge for high dispersion of graphene either in aqueous or in organic solvents. It has been achieved by dispersing agent introduced into hydrophobic graphene sheets for good dispersion, whereas it submerges the graphene properties [26]. In the view of fact, large scale solution processable GO has several advantages such as cost effective, eco-friendly solvent and facile to introduce any foreign material due to the presence of functional groups [27, 28]. The copious amount of functional groups attached to the graphene surface contains hydroxyl and epoxy groups at basal planes and carboxyl groups at edges. This leads to affinity with water molecules, which provides a higher dispersion and further it assists with other inorganic or organic molecules for facile composite preparation. In the choice of precursor for free-standing material preparation, GO dominates as a building block due to its features of large scale solution processable with high colloidal dispersion. The resultant macroscopic FSG holds as an excellent mechanical, electrical, and light-weight material. Further, the 3D architecture of FSG enhances the surface area, porous nature, and structural active sites by merging with other functional host materials such as semiconducting material, metal nanoparticles, and polymers. The synergy of graphene sheets and functional host materials in the 3D macroscopic architecture attracted wide variety of applications due to the tuning of their properties.

In 1998, Smalley prepared CNT buckypaper by vacuum filtration, in prior it is well dispersed in Triton X-100 surfactant to break up the pi-pi interaction between the bundled ropes of CNT [29]. Further, CNT buckypapers were prepared by domino pushing technique, and they are strong, robust, and flexible. The obtained paper exhibits 26 micron thickness; the electrical conductivity was found to be 2.0 × 104 S/m and thermal conductivity shows 153 W/mK [30]. These papers were directly applied for supercapacitor application. Thus, the carbon paper–like materials were potentially applied in a variety of applications due to their light-weight, highly flexible, robust, and eco-friendly nature. On the basis of cost, the CNT papers lag behind for the practical applications, and they have been replaced by graphene sheets. Similar to CNT buckypaper, GO paper was fabricated by flow-assisted vacuum filtration or evaporation techniques. **Figure 3a** and **b** shows the photograph of flexible GO paper and mechanical properties comparison chart of GO paper, buckypapers, vermiculite paper-like material, and graphite foil, respectively. Young's modulus is as high as in GO papers with 42 GPa for vacuum-assisted technique, and similar tensile strength but lowest Young's modulus (12.7 GPa) was obtained for evaporationinduced self-assembly technique [31, 34]. Thus, the high mechanical properties of GO paper can be used in several applications such as supercapacitors and other flexible substrates [35]. Moreover, the mechanical properties of GO papers depend on the alignment of GO sheets by any chemical modification between the layers and at the edges. The modifications are made either by crosslinking or grafting between

*Energy Storage Devices*

**2.2 Graphene: nomenclature**

reaction varies selectively at edges and basal plane according to their electroactive sites by adding impurities or doping. Graphene-based materials were potentially applied in electrochemical devices due to their inherent electrochemical activity nature [20]. These amazing properties of graphene such as electrical, mechanical,

It is well known that graphene can be synthesized by several routes and named according to the recovered final product. Graphene research has elevated gradually in the past 5 years for its tremendous properties, but the scientific community ends up with the confusion in naming the material. Even though researchers have synthesized up to 100 layers of carbon sheets, they were naming them as graphene. This provides different changes in properties compared with the single-layer graphene sheet for their practical applications [21]. Hence, carbon journal community

The descriptive term is an essential thing for researchers in the area of graphene material because the properties will change accordingly with recovered product with different synthetic strategies. For example, the graphene-based transparent conducting film adopted by the CVD method obtained 600 ohms/sq. at 96.5% transmittance at 550 nm, whereas solution processed graphene increases above 10 K ohms at the same transmittance [22–24]. Even the electrochemical behavior fluctuates according to the synthetic strategies; for instance, the presence of oxygen functional groups in graphene oxide (GO) shows an excellent electrochemical behavior rather than the pristine graphene [25]. Hence, the electrochemical device

raised a nomenclature for graphene family, which is shown in (**Table 1**).

Graphene Two-dimensional sheet with one atom thickness

Few layer graphene Subset of multilayer graphene

Exfoliated graphite Exfoliation of bulk graphite

Reduced graphene oxide Reduction or restoration of sp2

Turbostratic graphene Arrangement of graphene sheets in rotational fault structure Bi-,tri-, or multilayer graphene Stacking of graphene sheets (2 - bi, 3 - tri, & 4 - 10 – multi) in

Graphene nanoribbon Length dimension in micron and width in the range of nanometer Graphene quantum dots Lateral dimension less than 10 nm with photoluminescence property Graphene oxide Graphene sheets that contain functional groups (epoxy,

Graphite oxide Exfoliation of bulk graphite by strong oxidation process

Graphenization Growth of graphene by small molecules (bottom-up approach)

AB, ABA, or rotational order

hydroxyl, and carboxyl)

Graphene sheets arranged in 3D forms

Lateral/thickness of graphene sheets <100 nm.

carbon of graphene oxide

**Materials Description**

Graphite nanosheets, nanoflakes, and

Free-standing graphene, graphene foam,

*Nomenclature of graphene based on the structure.*

hydrogel, and aerogel

nanoplates

and electrochemical were attracted for rechargeable batteries.

**96**

**Table 1.**

#### **Figure 3.**

*(a) Photograph of flexible graphene oxide paper, (b) comparison chart of mechanical properties of GO paper with other flexible paper materials, (c) effect of FSG electrical conductivity changes w.r.t its properties upon HI treatment in different scale of time, and (d) electrical conductivity versus the Raman and XPS data of GO paper reduced by different metal halides (without permission from Refs. [31–33]).*

the two sheets as GO has several functional groups that covalently attached to other molecules [36, 37]. The intercalation, functionalization, and interaction between the GO sheets provide high mechanical stiffness for paper-like material. Moreover, the atmospheric humidity affects the mechanical property of the GO paper, increase in the relative humidity to 100%, the GO colloidal solution absorbs water from moisture and it bulges to 70% which decreases the tensile strength [34]. The functionalization on graphene surface also affects the mechanical properties depending on the functional moieties as well as the bonding nature [38–40]. The electrical properties of GO papers depend on the synthetic methods as several changes were observed in structures and reduction ratios of C/O. Upon exposing to the hydrazine vapor, the conductivity of GO papers increased by four order of magnitude from 8.5 × 10<sup>−</sup><sup>4</sup> to 170 S/cm. Further enhancement in conductivities of GO paper was developed by treating the paper with mixture of argon/hydrogen/hydrazine vapors [41]. The removal of the oxygen group is the main factor to restore the sp2 carbon network by chemical or thermal treatment. The chemical reductive treatment efficiently removes the oxygen moieties from the GO paper, whereas the thermal treatment shows high restoration of sp2 carbon network but less removal of oxygen functional groups.

**99**

**4.1 Li-ion battery**

during the discharging process.

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario*

place of Al, Cu, Ni foam, etc., for energy storage applications.

battery (SIB), sodium-sulfur, Li-air, Zn-air, and flow batteries.

Conventionally, LIBs are made up of graphite anode and LiCoO2 layered material as cathode sandwiched between LiPF6 (1.0 mol/L) as an organic electrolyte dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) in 1:1 volume ratio [45]. While LIB is charging, deintercalation happens at cathode, where the Li ions are removed from the layered LiCoO2 by releasing electrons to cathode. The released Li ions are transported to anode with the help of the electrolyte system and finally intercalated into graphite by gaining electrons. The same process is reversed

Designing of anode materials for LIBs has focused much attention on retaining large reversible specific capacity. Beyond the graphite anode, few metal oxides and

**4. Free-standing graphene electrodes for batteries**

Recently, a rapid reduction treatment was proposed by immersing the GO papers in hydrohalic acids, viz., HI and HBr, which shows a remarkable electrical conductivity around 298 and 3220 S/cm, respectively [32, 42]. Based on the facile chemical treatment, the electrical conductivity of FSG improvement was shown by treating the GO papers in metal halides like MgI2, AlI3, ZnI2, and FeI2 that exhibit 550 S/cm [33].

Owing to these attractive mechanical and electrical properties of FSG material, it played vital role in flexible device technologies based on electrochemical energy storage and generation, actuators, sensors, and catalysts. Based on the attractive graphene properties and its nomenclature, the graphene oxide has fascinating properties which has layered structure similar to graphene that containing oxygen functional groups such as carboxyl, hydroxyl and epoxy. These functional groups were highly dispersed in DI water; hence, it is well aligned over vacuum filtration process. The GO paper is peeled off after vacuum drying and subjected to reducing treatment, as synthesized FSG material is directly utilized as current collector in

Battery is an electrochemical energy storage device that is cost-effective and ecofriendly and with cyclic durability, excellent overall performance, and long-term stability. In this decade, lithium ion battery (LIB) is successfully commercialized worldwide for portable electronic devices, and it has approximately 200 kWh scale for transportation and stationary storage [43]. On comparison with other secondary-based batteries such as sodium sulfur, redox flow, Ni-Cd, etc., Li ion cells have gathered the most commercial interest because they provide high energy and power densities, respectively. In contrast, other secondary batteries are under development stage for consideration in commercial package over LIB due to its major drawback as follows: large scale storage, cost of materials, toxicity, cyclic performance, or stability issues. However, the better system in secondary batteries credited for LIB because the redox potential of −3.04 V vs. SHE (standard hydrogen electrode) for Li/Li+ which has high electropositive in periodic table and light weight material with small ionic radius. Henceforth, the charge-discharge rates enhance and power densities vary in the ranges of 500–2000 W/kg [44]. In commercialized LIBs, the existing negative electrode is a graphite-layered structure material coupled with the host material and LiCoO2 has positive electrodes. Similar to LIBs, the other systems were also focused since it lags behind to reach the theoretical specific capacity (400 Wh/kg) that requires for electric vehicles for long term usage. Hence, other kinds of secondary batteries have been discovered such as Li-sulfur, sodium-ion

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

#### *Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

Recently, a rapid reduction treatment was proposed by immersing the GO papers in hydrohalic acids, viz., HI and HBr, which shows a remarkable electrical conductivity around 298 and 3220 S/cm, respectively [32, 42]. Based on the facile chemical treatment, the electrical conductivity of FSG improvement was shown by treating the GO papers in metal halides like MgI2, AlI3, ZnI2, and FeI2 that exhibit 550 S/cm [33].

Owing to these attractive mechanical and electrical properties of FSG material, it played vital role in flexible device technologies based on electrochemical energy storage and generation, actuators, sensors, and catalysts. Based on the attractive graphene properties and its nomenclature, the graphene oxide has fascinating properties which has layered structure similar to graphene that containing oxygen functional groups such as carboxyl, hydroxyl and epoxy. These functional groups were highly dispersed in DI water; hence, it is well aligned over vacuum filtration process. The GO paper is peeled off after vacuum drying and subjected to reducing treatment, as synthesized FSG material is directly utilized as current collector in place of Al, Cu, Ni foam, etc., for energy storage applications.

#### **4. Free-standing graphene electrodes for batteries**

Battery is an electrochemical energy storage device that is cost-effective and ecofriendly and with cyclic durability, excellent overall performance, and long-term stability. In this decade, lithium ion battery (LIB) is successfully commercialized worldwide for portable electronic devices, and it has approximately 200 kWh scale for transportation and stationary storage [43]. On comparison with other secondary-based batteries such as sodium sulfur, redox flow, Ni-Cd, etc., Li ion cells have gathered the most commercial interest because they provide high energy and power densities, respectively. In contrast, other secondary batteries are under development stage for consideration in commercial package over LIB due to its major drawback as follows: large scale storage, cost of materials, toxicity, cyclic performance, or stability issues. However, the better system in secondary batteries credited for LIB because the redox potential of −3.04 V vs. SHE (standard hydrogen electrode) for Li/Li+ which has high electropositive in periodic table and light weight material with small ionic radius. Henceforth, the charge-discharge rates enhance and power densities vary in the ranges of 500–2000 W/kg [44]. In commercialized LIBs, the existing negative electrode is a graphite-layered structure material coupled with the host material and LiCoO2 has positive electrodes. Similar to LIBs, the other systems were also focused since it lags behind to reach the theoretical specific capacity (400 Wh/kg) that requires for electric vehicles for long term usage. Hence, other kinds of secondary batteries have been discovered such as Li-sulfur, sodium-ion battery (SIB), sodium-sulfur, Li-air, Zn-air, and flow batteries.

#### **4.1 Li-ion battery**

Conventionally, LIBs are made up of graphite anode and LiCoO2 layered material as cathode sandwiched between LiPF6 (1.0 mol/L) as an organic electrolyte dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) in 1:1 volume ratio [45]. While LIB is charging, deintercalation happens at cathode, where the Li ions are removed from the layered LiCoO2 by releasing electrons to cathode. The released Li ions are transported to anode with the help of the electrolyte system and finally intercalated into graphite by gaining electrons. The same process is reversed during the discharging process.

Designing of anode materials for LIBs has focused much attention on retaining large reversible specific capacity. Beyond the graphite anode, few metal oxides and

*Energy Storage Devices*

**98**

**Figure 3.**

restoration of sp2

the two sheets as GO has several functional groups that covalently attached to other molecules [36, 37]. The intercalation, functionalization, and interaction between the GO sheets provide high mechanical stiffness for paper-like material. Moreover, the atmospheric humidity affects the mechanical property of the GO paper, increase in the relative humidity to 100%, the GO colloidal solution absorbs water from moisture and it bulges to 70% which decreases the tensile strength [34]. The functionalization on graphene surface also affects the mechanical properties depending on the functional moieties as well as the bonding nature [38–40]. The electrical properties of GO papers depend on the synthetic methods as several changes were observed in structures and reduction ratios of C/O. Upon exposing to the hydrazine vapor, the conductivity of GO papers increased by four order of magnitude from 8.5 × 10<sup>−</sup><sup>4</sup> to 170 S/cm. Further enhancement in conductivities of GO paper was developed by treating the paper with mixture of argon/hydrogen/hydrazine vapors [41]. The

*(a) Photograph of flexible graphene oxide paper, (b) comparison chart of mechanical properties of GO paper with other flexible paper materials, (c) effect of FSG electrical conductivity changes w.r.t its properties upon HI treatment in different scale of time, and (d) electrical conductivity versus the Raman and XPS data of GO* 

chemical or thermal treatment. The chemical reductive treatment efficiently removes the oxygen moieties from the GO paper, whereas the thermal treatment shows high

carbon network but less removal of oxygen functional groups.

carbon network by

removal of the oxygen group is the main factor to restore the sp2

*paper reduced by different metal halides (without permission from Refs. [31–33]).*

metal alloys were developed as anode material, and the lithiation and delithiation processes were investigated. Specifically, FSG paper outpaces the other candidates such as carbon nanotube (CNT) paper or graphite foil due to their tremendous properties as discussed earlier. Importantly, the electrical and mechanical properties of FSG are potentially applied for flexible device application. However, the FSG electrode itself does not provide higher capacity (approximately 100 mAh/g), which is not applicable as anode in LIB; instead, it has good cycling stability. Therefore, the host material that has high electrochemical active sites is incorporated into FSG for improvement of capacity in the device. This extends the large volume expansion in FSG electrodes for an efficient Li ions intercalation. One of the advantages of this FSG hybrid electrode is that it excludes the nonconducting polymer binders as additives. Conventional electrode-based materials were obtained as powders and coated on the metal foils in the form of ink using additives like polymer binders and conducting additive, whereas the FSG hybrid electrode plays dual role as a current collector and conductive additive.

In 2005, LIBs were fabricated with free-standing electrode based on CNTs prepared by vacuum filtration method [46]. Significantly, the free-standing electrode fabrication is a facile route in comparison with the conventional electrode since the mixture of active material, polymer binder, and conductive additive in solvent coated on metal foils. The CNT free-standing electrode provides reversible discharge capacity of 200 mAh/g at 0.08 mA/cm<sup>2</sup> . Further, the specific capacity was enhanced by the CVD grown free-standing CNT that delivers 572 mAh/g at 0.2 mA/cm<sup>2</sup> [47]. This is a quite interesting result obtained for free-standing electrodes rather than the conventional electrodes. Meanwhile, the usage of high-cost material CNTs as free-standing electrodes lags behind manufacturing process. From this point of view, inexpensive material graphene prepared by chemical methods provides large scale production as dispersion in many solvents. This dispersion is readily subjected to vacuum filtration to prepare FSG paper with desired thickness. Usually, the discharge capacity of 298 mAh/g decreased to 240 mAh/g after 50 cycles for graphite electrodes with 81% retention capacity. But the FSG paper itself as anode provides huge irreversible discharge capacity, i.e., 680 mAh/g at initial cycle dropped to 84 mAh/g second cycle. The retention capacity is very poor compared to graphite electrode and therefore it is concluded to be not a suitable candidate for anode material [48]. This helps infer that solid electrolyte interface (SEI) formation is a significant parameter to reduce the storage capacity in FSG electrodes.

To potentially apply FSG as anode material in LIBs, the second phase material with highly electrochemical active sites should be composited to enhance the capacity. In this regard, Lee et al. composited Si NPs on GO sheets, vacuum filtered, and followed by thermal treatment to produce FSG/Si nanoparticle (NP) paper. This work delivers high Li ion storage when compared to pristine FSG electrodes. Si NPs intercalated between the graphene sheets of FSG paper that facilitates good 3D graphite-like framework and provides high Li ion storage even at high current density [49]. Another work has been reported with similar hybrid FSG/Si NPs, whereas a facile route has been introduced to fabricate. The specific capacity of 708 mAh/g was observed without any loss even after 100 cycles and this is mainly due to the larger volume change in graphene-Si composite. It also denotes the performance of device with an efficient electron and charge transfer contributed by graphene sheets that minimize the internal resistance of the electrodes [50]. Zhang et al. prepared Si hollow nanosheets using Mg as template and connected with graphene sheets to obtain free-standing electrodes by layer-by-layer method followed by HI reduction treatment. The specific capacity was examined during flat and bent state, which delivers similar results without any loss. Remarkably, Si/FSG paper anodes

**101**

efficient Li ion storage.

lower capacity of 43 mAh/cm3

shows higher volumetric capacity of 260 mAh/cm3

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario*

retain high reversible capacities even at long cycles, which reveals their retention capacity. They exhibit specific capacity of 660 mAh/g at 0.2 A/g current density after 150 cycles with 99% coulombic efficiency [51]. As mentioned earlier, all the Si NPs are highly expensive in terms of manufacturing process and hence a low cost method plays a significant factor. To tackle this issue, Cai et al. prepared Si NPs on CNT surface using low-cost Al-Si alloy as starting material and further inserted with graphene sheets to form a self-standing hybrid anodes for LIBs. Comparing with bare Si/CNT or Si/Graphene anodes, Si-CNT/FSG hybrid electrode, it delivers 1100 mAh/g at 0.2 A/g current density after 100 cycles. Addition of CNT was involved to disperse the Si NPs on the surface and provide network between the graphene sheets for conductivity enhancement as well as improved Li ion intercala-

Metal oxides (MOs) play an important role in LIBs as anode material and their poor conductivity restricts their application. Hence, introducing the conductive phase into MOs provides high retention capacity with long-life cycling stability. The theoretical reversible capacity of SnO2 is 782 mAh/g and its poor performance is due to low cycling with serious volume expansion. With this regard, SnO2 NPs dispersed on GO surface, followed by vacuum filtration to obtain free-standing electrodes and used as two different LIB anodes by thermally reduced and chemically reduced respectively [53, 54]. The specific capacity of 438.5 mAh/g at 0.1 A/g and 700 mAh/g at 0.2 A/g has been delivered for the two different reduction methods for SnO2 NPs/FSG electrodes. In both the cases, capacity fading is not observed even after the 50 cycles owing to the good anchoring of SnO2 and graphene sheets. Further, other metal oxides TiO2, Mn3O4, Fe3O4, and CuO nanostructured materials are incorporated into the FSG and are investigated for their performance in anode application for LIBs that delivers 269 mAh/g at 0.2 A/g, 692 mAh/g at 0.05 A/g, 544 mAh/g at 10 A/g, and 698.7 mAh/g at 0.67 A/g capacities, respectively [55–59]. Commonly, all these metal oxides' specific capacity shows a reasonable capacity with the long-life cycling after incorporating the MOs into FSG electrodes due to the following aspects: (1) Interaction of GO and MO precursors increases, which enhances the well dispersive growth of MO NPs on graphene sheets. (2) Anchoring of MOs and graphene enhances the volume expansion/contraction for lithiation/delithiation process. (3) The cycling stability increases compared to pristine MO anodes even after several cycles owing to its structural phase remain stable after alloying/de-alloying process of lithium ions. (4) MOs avoid the aggregation of graphene stacking that leads to larger void space to penetrate the electrolyte and make a strong interface with the electrochemical active MOs for an

Further, with the controlled synthesis of oxygen, functionalized CNT/FSG electrodes were fabricated for anode application in LIBs. The battery performance is based on the oxygen functional groups in the electrodes that have been investigated. An optimization in weight ratios of CNT/FSG and heat treatment improves the volumetric and gravimetric capacitances. The CNT/GO hybrid at a ratio of 1:1

current densities, the role of oxygen in capacity role suppress for 200°C larger than the 900°C [60]. This implies the importance of CNT intercalation between the graphene sheets of FSG electrodes. Zhang et al. demonstrated the defect-rich MoS2 NSs/graphene/CNT hybrid paper as anode material for LIBs. In this design, MoS2 facilitates the lithium ion storage due to the high active sites at the edges and the electrical conductivity improved by the network of CNTs attached to the graphene sheets. In addition to the conductivity enhancement, the porosity of the FSG electrodes increased by the network of CNT sandwiched graphene sheets. On the

that reduced at 200°C, while

for 900°C treated CNT/GO. Whereas, at high

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

tion for efficient charge transfer [52].

#### *Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

retain high reversible capacities even at long cycles, which reveals their retention capacity. They exhibit specific capacity of 660 mAh/g at 0.2 A/g current density after 150 cycles with 99% coulombic efficiency [51]. As mentioned earlier, all the Si NPs are highly expensive in terms of manufacturing process and hence a low cost method plays a significant factor. To tackle this issue, Cai et al. prepared Si NPs on CNT surface using low-cost Al-Si alloy as starting material and further inserted with graphene sheets to form a self-standing hybrid anodes for LIBs. Comparing with bare Si/CNT or Si/Graphene anodes, Si-CNT/FSG hybrid electrode, it delivers 1100 mAh/g at 0.2 A/g current density after 100 cycles. Addition of CNT was involved to disperse the Si NPs on the surface and provide network between the graphene sheets for conductivity enhancement as well as improved Li ion intercalation for efficient charge transfer [52].

Metal oxides (MOs) play an important role in LIBs as anode material and their poor conductivity restricts their application. Hence, introducing the conductive phase into MOs provides high retention capacity with long-life cycling stability. The theoretical reversible capacity of SnO2 is 782 mAh/g and its poor performance is due to low cycling with serious volume expansion. With this regard, SnO2 NPs dispersed on GO surface, followed by vacuum filtration to obtain free-standing electrodes and used as two different LIB anodes by thermally reduced and chemically reduced respectively [53, 54]. The specific capacity of 438.5 mAh/g at 0.1 A/g and 700 mAh/g at 0.2 A/g has been delivered for the two different reduction methods for SnO2 NPs/FSG electrodes. In both the cases, capacity fading is not observed even after the 50 cycles owing to the good anchoring of SnO2 and graphene sheets. Further, other metal oxides TiO2, Mn3O4, Fe3O4, and CuO nanostructured materials are incorporated into the FSG and are investigated for their performance in anode application for LIBs that delivers 269 mAh/g at 0.2 A/g, 692 mAh/g at 0.05 A/g, 544 mAh/g at 10 A/g, and 698.7 mAh/g at 0.67 A/g capacities, respectively [55–59]. Commonly, all these metal oxides' specific capacity shows a reasonable capacity with the long-life cycling after incorporating the MOs into FSG electrodes due to the following aspects: (1) Interaction of GO and MO precursors increases, which enhances the well dispersive growth of MO NPs on graphene sheets. (2) Anchoring of MOs and graphene enhances the volume expansion/contraction for lithiation/delithiation process. (3) The cycling stability increases compared to pristine MO anodes even after several cycles owing to its structural phase remain stable after alloying/de-alloying process of lithium ions. (4) MOs avoid the aggregation of graphene stacking that leads to larger void space to penetrate the electrolyte and make a strong interface with the electrochemical active MOs for an efficient Li ion storage.

Further, with the controlled synthesis of oxygen, functionalized CNT/FSG electrodes were fabricated for anode application in LIBs. The battery performance is based on the oxygen functional groups in the electrodes that have been investigated. An optimization in weight ratios of CNT/FSG and heat treatment improves the volumetric and gravimetric capacitances. The CNT/GO hybrid at a ratio of 1:1 shows higher volumetric capacity of 260 mAh/cm3 that reduced at 200°C, while lower capacity of 43 mAh/cm3 for 900°C treated CNT/GO. Whereas, at high current densities, the role of oxygen in capacity role suppress for 200°C larger than the 900°C [60]. This implies the importance of CNT intercalation between the graphene sheets of FSG electrodes. Zhang et al. demonstrated the defect-rich MoS2 NSs/graphene/CNT hybrid paper as anode material for LIBs. In this design, MoS2 facilitates the lithium ion storage due to the high active sites at the edges and the electrical conductivity improved by the network of CNTs attached to the graphene sheets. In addition to the conductivity enhancement, the porosity of the FSG electrodes increased by the network of CNT sandwiched graphene sheets. On the

*Energy Storage Devices*

collector and conductive additive.

at 0.2 mA/cm<sup>2</sup>

FSG electrodes.

discharge capacity of 200 mAh/g at 0.08 mA/cm<sup>2</sup>

metal alloys were developed as anode material, and the lithiation and delithiation processes were investigated. Specifically, FSG paper outpaces the other candidates such as carbon nanotube (CNT) paper or graphite foil due to their tremendous properties as discussed earlier. Importantly, the electrical and mechanical properties of FSG are potentially applied for flexible device application. However, the FSG electrode itself does not provide higher capacity (approximately 100 mAh/g), which is not applicable as anode in LIB; instead, it has good cycling stability. Therefore, the host material that has high electrochemical active sites is incorporated into FSG for improvement of capacity in the device. This extends the large volume expansion in FSG electrodes for an efficient Li ions intercalation. One of the advantages of this FSG hybrid electrode is that it excludes the nonconducting polymer binders as additives. Conventional electrode-based materials were obtained as powders and coated on the metal foils in the form of ink using additives like polymer binders and conducting additive, whereas the FSG hybrid electrode plays dual role as a current

In 2005, LIBs were fabricated with free-standing electrode based on CNTs prepared by vacuum filtration method [46]. Significantly, the free-standing electrode fabrication is a facile route in comparison with the conventional electrode since the mixture of active material, polymer binder, and conductive additive in solvent coated on metal foils. The CNT free-standing electrode provides reversible

was enhanced by the CVD grown free-standing CNT that delivers 572 mAh/g

electrodes rather than the conventional electrodes. Meanwhile, the usage of high-cost material CNTs as free-standing electrodes lags behind manufacturing process. From this point of view, inexpensive material graphene prepared by chemical methods provides large scale production as dispersion in many solvents. This dispersion is readily subjected to vacuum filtration to prepare FSG paper with desired thickness. Usually, the discharge capacity of 298 mAh/g decreased to 240 mAh/g after 50 cycles for graphite electrodes with 81% retention capacity. But the FSG paper itself as anode provides huge irreversible discharge capacity, i.e., 680 mAh/g at initial cycle dropped to 84 mAh/g second cycle. The retention capacity is very poor compared to graphite electrode and therefore it is concluded to be not a suitable candidate for anode material [48]. This helps infer that solid electrolyte interface (SEI) formation is a significant parameter to reduce the storage capacity in

To potentially apply FSG as anode material in LIBs, the second phase material with highly electrochemical active sites should be composited to enhance the capacity. In this regard, Lee et al. composited Si NPs on GO sheets, vacuum filtered, and followed by thermal treatment to produce FSG/Si nanoparticle (NP) paper. This work delivers high Li ion storage when compared to pristine FSG electrodes. Si NPs intercalated between the graphene sheets of FSG paper that facilitates good 3D graphite-like framework and provides high Li ion storage even at high current density [49]. Another work has been reported with similar hybrid FSG/Si NPs, whereas a facile route has been introduced to fabricate. The specific capacity of 708 mAh/g was observed without any loss even after 100 cycles and this is mainly due to the larger volume change in graphene-Si composite. It also denotes the performance of device with an efficient electron and charge transfer contributed by graphene sheets that minimize the internal resistance of the electrodes [50]. Zhang et al. prepared Si hollow nanosheets using Mg as template and connected with graphene sheets to obtain free-standing electrodes by layer-by-layer method followed by HI reduction treatment. The specific capacity was examined during flat and bent state, which delivers similar results without any loss. Remarkably, Si/FSG paper anodes

[47]. This is a quite interesting result obtained for free-standing

. Further, the specific capacity

**100**

whole, the binder-free and substrate-free hybrid anode papers deliver high reversible capacity of 1137.2 mAh/g at 0.1 A/g current density with good cycling stability [61]. This framework induces a novel pathway to incorporate other host materials to understand the CNT/FSG electrodes. Recently, several transition metal oxides provide high reversible theoretical capacities compared with the commercialized graphite anode. To the CNT/FSG electrode network, transition metal oxides such as Fe2O3 [62], CuO [63], MnO [64], and CoSnO3 [65] were incorporated as electrochemical active phase into the framework and investigated as anode material performance for LIBs. All these hybrid papers exhibit high reversible capacity of 716 and 600 mAh/g at 0.5 A/g current density more than 50 cycles for Fe2O3 and CuO nanobox, respectively. Apart from this, an enhanced capacity was observed for CoSnO3 and MnO NPs at high current density of 2 A/g, which delivers 676 and 530 mAh/g, respectively. Individually, the CNT/FSG and transition metal oxide anodes were found to have a drastic decrease of specific capacity upon increasing the current density, whereas a slight decrease of specific capacity was observed after hosting the metal oxides into CNT/FSG framework. Reasons for high reversible capacity and good cyclic stability of metal oxide-CNT/FSG electrodes are very similar due to the following merits: (1) incorporation of metal oxides improves the Li ion kinetics and enhances the charge transfer due to highly conductive CNT network between the graphene sheets; (2) 3D framework of CNT/FSG has highly porous nature, large specific surface area, and large volume change, which has well dispersion of metal oxide NPs onto the carbon surfaces; and (3) long cycling due to good attachment of metal oxide with CNT/FSG, whereas greater the volume expansion, higher the Li ion intercalation.

Interestingly, Cao et al. designed a unique layered nanostructure of porous ternary ZnCo2O4 on graphene sheets and fabricated as flexible anode and investigated its electrochemical performance. And also they constructed full cell with LiFePO4 as cathode material that deposited on FSG paper as slurry by homogenous mixing of conductive additive and polymer binder [66]. **Figure 4a** shows the photograph of flexible Li-ion battery fabricated by FSG hybrid electrodes. The half-cell of ZnCo2O4/FSG anode delivers higher specific capacity of 791 mAh/g at 1 A/g after 1000 cycles with 97.3% of capacity retention and concludes that it has an excellent cycling stability. **Figure 4b** shows the rate capability of the flexible battery with different current densities ranging from 0.5 to 10 C. This full cell delivers 40 mAh/g even at 10 C rate and the specific capacitance remains the same after the current density decreased to 2 C, which shows a good reversibility. The full cell has FSG paper as current collector for both the anode and cathode that are composited with ZnCo2O4 and LiFePO4 as host materials, respectively. It operates at 2 V with initial charge of 143 mAh/g and coulombic efficiency of 97.2%, which is comparable to existing LIB. The specific capacity is maintained at 90 mAh/g with high capacity retention under flat and bent states over 100 cycling process, which implies the flexibility of the device as shown in **Figure 4d**. It represents that graphene conductivity is unchanged while bending the device.

#### **4.2 Sodium-ion battery**

Ahead of LIBs, SIBs have attracted the research community as the resources of Na are inexhaustible across the globe. In comparison with LIBs, the redox potential is −2.71 V vs. SHE and only the radius is 55% larger than the Li ions. Larger radius influences to focus on suitable material for insertion/extraction of Na ions effectively. The researchers focused on developing an efficient anode material for SIBs that involves carbon-based families and Na intermetallic compounds. The first cycle-specific capacity of sodium-antimony and sodium-phosphorous shows 600 and 2596 mAh/g,

**103**

**Figure 4.**

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario*

respectively [67–69]. Specific capacities drop after first cycles due to the internal cracking in the electrodes upon Na ion insertion. It leads to hinder the electrical properties and dissolution of electrode materials to electrolyte. The hard carbon with large interlayer distance that functions as anode material for SIBs and delivers more

*(a) Photograph of flexible full cell Li-ion battery with FSG/ZnCo2O4 as anode and FSG/LiFePO4 as cathode, (b) charge-discharge curve of full cell at 0.5 C rate, (c) charge-discharge rate capability at different rates, and (d) capacity variation on flat and bent state during cycling at 2 C rate (without permission from Ref. [66]).*

The porous nature and structure of the FSG could facilitate the accommodation of host materials such as transition metal chalcogenides (TMCs), which are electrochemically active for the Na ions for alloying process. David et al. reported that the MoS2/FSG composite papers exhibit an excellent cyclic stability with high reversible capacity of 338 mAh/g at 0.025 A/g. It is the first report and opens the pathway to apply free-standing electrodes for SIB anode [70]. The cyclic stability was enhanced in flower-like MoS2 incorporated on graphene foam prepared by onestep microwave-assisted synthesis. It offers stable capacity of 290 mAh/g at 0.1 A/g after 50 cycles compared to previous MoS2/FSG electrode. The cycling performance is enhanced due to highly conductive 3D graphene foam and well-dispersed MoS2, which shields as well as avoids the strain during the sodiation/desodiation process at anode [71]. With the significance of MoS2 TMC for SIB anodes, further investigation was followed by incorporating other TMCs such as WS2 and Co0.85Se into FSG [72, 73]. As mentioned in LIBs, the electrochemical behavior can be increased by introducing the heteroatoms into the graphene sheets. Heteroatom-doped FSG electrode performance was investigated for SIB anode, where the nitrogen improves the electronic conductivity and fluorine expands the interlayer for an efficient accommodation of Na ions. This delivers a reversible capacity of 56.3 mAh/g at 1 A/g for 5000 cycles. It indicates that the doping of heteroatoms enhances the cycling stability of SIB anodes. **Figure 5a** shows the discharge/charge profile before and after the bent state, which remains with the same capacity at current density

than 200 mAh/g of capacity even after 100 cycles was reported elsewhere.

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

**Figure 4.**

*Energy Storage Devices*

ion intercalation.

is unchanged while bending the device.

**4.2 Sodium-ion battery**

whole, the binder-free and substrate-free hybrid anode papers deliver high reversible capacity of 1137.2 mAh/g at 0.1 A/g current density with good cycling stability [61]. This framework induces a novel pathway to incorporate other host materials to understand the CNT/FSG electrodes. Recently, several transition metal oxides provide high reversible theoretical capacities compared with the commercialized graphite anode. To the CNT/FSG electrode network, transition metal oxides such as Fe2O3 [62], CuO [63], MnO [64], and CoSnO3 [65] were incorporated as electrochemical active phase into the framework and investigated as anode material performance for LIBs. All these hybrid papers exhibit high reversible capacity of 716 and 600 mAh/g at 0.5 A/g current density more than 50 cycles for Fe2O3 and CuO nanobox, respectively. Apart from this, an enhanced capacity was observed for CoSnO3 and MnO NPs at high current density of 2 A/g, which delivers 676 and 530 mAh/g, respectively. Individually, the CNT/FSG and transition metal oxide anodes were found to have a drastic decrease of specific capacity upon increasing the current density, whereas a slight decrease of specific capacity was observed after hosting the metal oxides into CNT/FSG framework. Reasons for high reversible capacity and good cyclic stability of metal oxide-CNT/FSG electrodes are very similar due to the following merits: (1) incorporation of metal oxides improves the Li ion kinetics and enhances the charge transfer due to highly conductive CNT network between the graphene sheets; (2) 3D framework of CNT/FSG has highly porous nature, large specific surface area, and large volume change, which has well dispersion of metal oxide NPs onto the carbon surfaces; and (3) long cycling due to good attachment of metal oxide with CNT/FSG, whereas greater the volume expansion, higher the Li

Interestingly, Cao et al. designed a unique layered nanostructure of porous ternary ZnCo2O4 on graphene sheets and fabricated as flexible anode and investigated its electrochemical performance. And also they constructed full cell with LiFePO4 as cathode material that deposited on FSG paper as slurry by homogenous mixing of conductive additive and polymer binder [66]. **Figure 4a** shows the photograph of flexible Li-ion battery fabricated by FSG hybrid electrodes. The half-cell of ZnCo2O4/FSG anode delivers higher specific capacity of 791 mAh/g at 1 A/g after 1000 cycles with 97.3% of capacity retention and concludes that it has an excellent cycling stability. **Figure 4b** shows the rate capability of the flexible battery with different current densities ranging from 0.5 to 10 C. This full cell delivers 40 mAh/g even at 10 C rate and the specific capacitance remains the same after the current density decreased to 2 C, which shows a good reversibility. The full cell has FSG paper as current collector for both the anode and cathode that are composited with ZnCo2O4 and LiFePO4 as host materials, respectively. It operates at 2 V with initial charge of 143 mAh/g and coulombic efficiency of 97.2%, which is comparable to existing LIB. The specific capacity is maintained at 90 mAh/g with high capacity retention under flat and bent states over 100 cycling process, which implies the flexibility of the device as shown in **Figure 4d**. It represents that graphene conductivity

Ahead of LIBs, SIBs have attracted the research community as the resources of Na are inexhaustible across the globe. In comparison with LIBs, the redox potential is −2.71 V vs. SHE and only the radius is 55% larger than the Li ions. Larger radius influences to focus on suitable material for insertion/extraction of Na ions effectively. The researchers focused on developing an efficient anode material for SIBs that involves carbon-based families and Na intermetallic compounds. The first cycle-specific capacity of sodium-antimony and sodium-phosphorous shows 600 and 2596 mAh/g,

**102**

*(a) Photograph of flexible full cell Li-ion battery with FSG/ZnCo2O4 as anode and FSG/LiFePO4 as cathode, (b) charge-discharge curve of full cell at 0.5 C rate, (c) charge-discharge rate capability at different rates, and (d) capacity variation on flat and bent state during cycling at 2 C rate (without permission from Ref. [66]).*

respectively [67–69]. Specific capacities drop after first cycles due to the internal cracking in the electrodes upon Na ion insertion. It leads to hinder the electrical properties and dissolution of electrode materials to electrolyte. The hard carbon with large interlayer distance that functions as anode material for SIBs and delivers more than 200 mAh/g of capacity even after 100 cycles was reported elsewhere.

The porous nature and structure of the FSG could facilitate the accommodation of host materials such as transition metal chalcogenides (TMCs), which are electrochemically active for the Na ions for alloying process. David et al. reported that the MoS2/FSG composite papers exhibit an excellent cyclic stability with high reversible capacity of 338 mAh/g at 0.025 A/g. It is the first report and opens the pathway to apply free-standing electrodes for SIB anode [70]. The cyclic stability was enhanced in flower-like MoS2 incorporated on graphene foam prepared by onestep microwave-assisted synthesis. It offers stable capacity of 290 mAh/g at 0.1 A/g after 50 cycles compared to previous MoS2/FSG electrode. The cycling performance is enhanced due to highly conductive 3D graphene foam and well-dispersed MoS2, which shields as well as avoids the strain during the sodiation/desodiation process at anode [71]. With the significance of MoS2 TMC for SIB anodes, further investigation was followed by incorporating other TMCs such as WS2 and Co0.85Se into FSG [72, 73]. As mentioned in LIBs, the electrochemical behavior can be increased by introducing the heteroatoms into the graphene sheets. Heteroatom-doped FSG electrode performance was investigated for SIB anode, where the nitrogen improves the electronic conductivity and fluorine expands the interlayer for an efficient accommodation of Na ions. This delivers a reversible capacity of 56.3 mAh/g at 1 A/g for 5000 cycles. It indicates that the doping of heteroatoms enhances the cycling stability of SIB anodes. **Figure 5a** shows the discharge/charge profile before and after the bent state, which remains with the same capacity at current density

#### **Figure 5.**

*(a) Discharge/charge profile of heteroatoms (N and F)-doped FSG electrode at bent and normal state for SIBs. (inset: The photograph of FSG pouch cell illuminated with LED), (b) comparison of specific capacity and coulombic efficiency of bare FSG and N-doped FSG for Li-S battery. Cross-sectional SEM images of (c) discharged and (d) re-charged macroporous FSG electrodes (without permission from Refs. [74–76]).*

of 0.05 A/g. It reveals the mechanical strength of the FSG electrodes that is suitable to fabricate flexible pouch cell [74]. Even though the above said materials show an excellent cyclic stability, still it is necessary to improve the specific capacity of SIBs. It is well known that Na3P has theoretical capacity of 2600 mAh/g, where its demerits are very similar to those of Si electrode in LIBs. Because of high pulverization, fast capacity fading and also it hinders the electrical contact which lags behind in the electrochemical stability. Lots of effort have been made by assembling red P into carbon matrix to overcome these problems. Red P was composited on carbon nanofibers (CNFs) and dipped in GO solution followed by HI treatment providing P-CNF/FSG electrodes. In this architecture, CNF network enhances the pathway of electron transport rapidly and the role of graphene sheets to improve the conductivity as well as to avoid the breakup of bonds P–P from electrodes. This work demonstrates a significant capacity of 406.6 mAh/g at 1 A/g after 180 cycles [77]. Moreover, the graphene sheets have been utilized as a multifunctional conductive binder, and hard carbon/FSG as anodes for SIBs was constructed. It delivers high reversible capacity of 372.4 mAh/g and shows capacity retention of 90% over 200 cycling. A superior performance is observed in the absence of PVDF binder with higher rate capabilities and converting the rigid nature of hard carbon into flexible graphene sheets [78].

#### **4.3 Li-S battery**

Akin to SIBs, FSG electrodes play a major role in other rechargeable secondary batteries such as Li-S, Li-air, and Zn-air. The higher specific energy is a significant parameter for transportation and stationary applications, and in that case, Li-S batteries offer advantages but it is limited with few challenges discussed later. The

**105**

**4.4 Metal-air battery**

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario*

change and thus improves the cycling stability of Li-S battery [82].

Recently, metal-air batteries have inspired much attention apart from the above said battery systems due to their high theoretical capacity than the metal-ion and Li-S batteries. The metal-air batteries can be operated in aqueous or nonaqueous medium based on the selection of metals. The nonaqueous medium is well suited for the Li-air batteries that deliver high capacity than in aqueous medium but still there are some issues when it comes to the practical application. The development of cathode in Li-air is significant as it is the main compartment to breathe oxygen

highest theoretical capacity of Li-S system is 2600 Wh/kg, which is highest than the LIB due to highest capacity of Li-S cathode sulfur has 1675 mAh/g. The most challenging part is to improve the electronic conductivity of cathodes of Li-S as the sulfur exhibits poor conductivity of 10–17 S/cm as well as the formation of polysulfides at cathodes. These polysulfides oxidize the Li anode and get back to cathodes and re-oxidize, thus lowering the performance of Li-S system. An extensive effort has been made to improve the cathodes by incorporating the carbon additives to sulfur to minimize the unnecessary reactions. Initially, mesoporous FSG was prepared and the sulfur was deposited by vapor treatment and was utilized as cathodes for Li-S system. It delivers charging capacity of 1288 mAh/g with high coulombic efficiency that reveals the restriction of sulfur to dissolute polysulfides in mesoporous FSG framework [79]. Similar to LIB and SIBs, the electrochemical behavior of cathode in Li-S system enhanced for heteroatom-doped FSG electrodes. **Figure 5b** shows the comparison of FSG and N-doped FSG capacity and coulombic efficiency with different cycle number. The heteroatom-doped FSG shows superior performance than the bare FSG due to the high interaction of polysulfides with heteroatoms that increase specific capacity. The nitrogen doping effect in FSG minimizes the concentration of polysulfides and forms a uniform layer of Li2S at cathode. This system delivers 1000 mAh/g at 0.335 A/g after 100 cycles [75]. In another work, Zhu et al. developed free-standing cathodes by CNTs that were interconnected with the sulfur-graphene walls and investigated the electrochemical behavior that delivers 1346 mAh/g at 0.17 A/g current density. It is due to sulfur at graphene walls that deals to provide dual response as follows: (i) hinder the dissolution of polysulfides minimizing the shuttle phenomenon and (ii) offer volume expansion even at high quantity of sulfur. Moreover, its capacity retention shows 40% when current density is increased to 16.7 A/g owing to the good electron pathway by CNTs connected with graphene nanosheets [80]. Further, nanosized Li2S (25–50 nm) particles incorporated into FSG papers by vacuum filtration process demonstrated an excellent cycling and rate capability with reversible capacity of 816.1 mAh/g at 0.1675 A/g (150 cycles) and 597 mAh/g at 11.7 A/g (200 cycles). This shows excellent performance in electrochemical behavior due to the uniform distribution of Li2S particles on graphene sheets that minimize the barrier for Li ion transport and particularly it has superior wetting nature to interconnect the polysulfides with graphene network into the paper electrodes [81]. Similarly, Chen et al. designed an efficient hierarchical nanostructure like nanobundled forest with Li2S/few-walled CNTs at FSG obtained solution processing followed by self-assembly method as cathodes. In this design, CNTs assembled in shaft-like structure and Li2S as active material, whereas the graphene sheets act as barrier for Li2S. It achieves high capacity of 868 and 433 mAh/g at current density of 335 and 16.7 A/g, respectively. This originates from the good framework between CNTs and graphene sheets as well as the uniform distribution of Li2S, and moreover, the barrier of graphene sheets for Li2S reduces the dissolution of polysulfides. Overall, the influence of void space enhances the volume

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

#### *Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

*Energy Storage Devices*

of 0.05 A/g. It reveals the mechanical strength of the FSG electrodes that is suitable to fabricate flexible pouch cell [74]. Even though the above said materials show an excellent cyclic stability, still it is necessary to improve the specific capacity of SIBs. It is well known that Na3P has theoretical capacity of 2600 mAh/g, where its demerits are very similar to those of Si electrode in LIBs. Because of high pulverization, fast capacity fading and also it hinders the electrical contact which lags behind in the electrochemical stability. Lots of effort have been made by assembling red P into carbon matrix to overcome these problems. Red P was composited on carbon nanofibers (CNFs) and dipped in GO solution followed by HI treatment providing P-CNF/FSG electrodes. In this architecture, CNF network enhances the pathway of electron transport rapidly and the role of graphene sheets to improve the conductivity as well as to avoid the breakup of bonds P–P from electrodes. This work demonstrates a significant capacity of 406.6 mAh/g at 1 A/g after 180 cycles [77]. Moreover, the graphene sheets have been utilized as a multifunctional conductive binder, and hard carbon/FSG as anodes for SIBs was constructed. It delivers high reversible capacity of 372.4 mAh/g and shows capacity retention of 90% over 200 cycling. A superior performance is observed in the absence of PVDF binder with higher rate capabilities and converting the rigid nature of hard carbon into flexible

*(a) Discharge/charge profile of heteroatoms (N and F)-doped FSG electrode at bent and normal state for SIBs. (inset: The photograph of FSG pouch cell illuminated with LED), (b) comparison of specific capacity and coulombic efficiency of bare FSG and N-doped FSG for Li-S battery. Cross-sectional SEM images of (c) discharged and (d) re-charged macroporous FSG electrodes (without permission from Refs. [74–76]).*

Akin to SIBs, FSG electrodes play a major role in other rechargeable secondary batteries such as Li-S, Li-air, and Zn-air. The higher specific energy is a significant parameter for transportation and stationary applications, and in that case, Li-S batteries offer advantages but it is limited with few challenges discussed later. The

**104**

graphene sheets [78].

**4.3 Li-S battery**

**Figure 5.**

highest theoretical capacity of Li-S system is 2600 Wh/kg, which is highest than the LIB due to highest capacity of Li-S cathode sulfur has 1675 mAh/g. The most challenging part is to improve the electronic conductivity of cathodes of Li-S as the sulfur exhibits poor conductivity of 10–17 S/cm as well as the formation of polysulfides at cathodes. These polysulfides oxidize the Li anode and get back to cathodes and re-oxidize, thus lowering the performance of Li-S system. An extensive effort has been made to improve the cathodes by incorporating the carbon additives to sulfur to minimize the unnecessary reactions. Initially, mesoporous FSG was prepared and the sulfur was deposited by vapor treatment and was utilized as cathodes for Li-S system. It delivers charging capacity of 1288 mAh/g with high coulombic efficiency that reveals the restriction of sulfur to dissolute polysulfides in mesoporous FSG framework [79]. Similar to LIB and SIBs, the electrochemical behavior of cathode in Li-S system enhanced for heteroatom-doped FSG electrodes. **Figure 5b** shows the comparison of FSG and N-doped FSG capacity and coulombic efficiency with different cycle number. The heteroatom-doped FSG shows superior performance than the bare FSG due to the high interaction of polysulfides with heteroatoms that increase specific capacity. The nitrogen doping effect in FSG minimizes the concentration of polysulfides and forms a uniform layer of Li2S at cathode. This system delivers 1000 mAh/g at 0.335 A/g after 100 cycles [75]. In another work, Zhu et al. developed free-standing cathodes by CNTs that were interconnected with the sulfur-graphene walls and investigated the electrochemical behavior that delivers 1346 mAh/g at 0.17 A/g current density. It is due to sulfur at graphene walls that deals to provide dual response as follows: (i) hinder the dissolution of polysulfides minimizing the shuttle phenomenon and (ii) offer volume expansion even at high quantity of sulfur. Moreover, its capacity retention shows 40% when current density is increased to 16.7 A/g owing to the good electron pathway by CNTs connected with graphene nanosheets [80]. Further, nanosized Li2S (25–50 nm) particles incorporated into FSG papers by vacuum filtration process demonstrated an excellent cycling and rate capability with reversible capacity of 816.1 mAh/g at 0.1675 A/g (150 cycles) and 597 mAh/g at 11.7 A/g (200 cycles). This shows excellent performance in electrochemical behavior due to the uniform distribution of Li2S particles on graphene sheets that minimize the barrier for Li ion transport and particularly it has superior wetting nature to interconnect the polysulfides with graphene network into the paper electrodes [81]. Similarly, Chen et al. designed an efficient hierarchical nanostructure like nanobundled forest with Li2S/few-walled CNTs at FSG obtained solution processing followed by self-assembly method as cathodes. In this design, CNTs assembled in shaft-like structure and Li2S as active material, whereas the graphene sheets act as barrier for Li2S. It achieves high capacity of 868 and 433 mAh/g at current density of 335 and 16.7 A/g, respectively. This originates from the good framework between CNTs and graphene sheets as well as the uniform distribution of Li2S, and moreover, the barrier of graphene sheets for Li2S reduces the dissolution of polysulfides. Overall, the influence of void space enhances the volume change and thus improves the cycling stability of Li-S battery [82].

#### **4.4 Metal-air battery**

Recently, metal-air batteries have inspired much attention apart from the above said battery systems due to their high theoretical capacity than the metal-ion and Li-S batteries. The metal-air batteries can be operated in aqueous or nonaqueous medium based on the selection of metals. The nonaqueous medium is well suited for the Li-air batteries that deliver high capacity than in aqueous medium but still there are some issues when it comes to the practical application. The development of cathode in Li-air is significant as it is the main compartment to breathe oxygen

for delivering high capacity of the system. There are a lot of reports for cathode development based on metal oxides grown on Ni foam as binder-free electrodes. The role of FSG electrodes was also investigated as cathodes for Li-air batteries. First, Kim et al. developed graphene nanoplates (GNP)/GO composite paper-like electrodes as cathodes for Li-air battery system. The wrinkled nature of the paper electrodes induces the high surface area and also delivers higher discharge capacity of 9760 mAh/g at 0.1 A/g current density. This superior performance is due to the reduced overpotential, and the difference in consumption/evolution of O2 is minimized. On the whole, the system exhibits higher efficiency in OER (oxygen evolution reaction)/ORR (oxygen reduction reaction) of 87% [83]. The same group developed macroporous FSG paper with surface area of 373 m2 /g and pore volume of 10.9 cm3 /g with 91.6% of porosity that exhibits a high specific capacity of 12,200 mAh/g at 0.2 A/g. The rate capability is enhanced where it shows high cycling performance even at higher current density of 0.5 and 2 A/g that delivers approximately 1000 mAh/g. This is attributed to the minimized volume expansion that limits the decomposition and formation of Li2O2 at the macroporous nature of FSG. While discharging/charging the macroporous FSG, the nature of FSG electrode decomposes the discharge products completely that reveals its highly porous structure as shown in the **Figure 5c** and **d** [76]. Researchers investigated the effect of FSG cathodes in Li-air upon introduction of metal oxides, namely, α-MnO2 and NiCo2O4. Upon insertion of α-MnO2 into FSG electrodes, the overpotential decrease was caused during charge/discharge process. It delivers 2900 mAh/g for the higher content of α-MnO2 that was reported and shows the catalytic improvement in this study [84]. And Jiang et al. reported an excellent reversible capacity of 5000 mAh/g at 0.4 A/g by incorporating mesoporous NiCo2O4 into macropores of FSG. It also lowers about 0.18 and 0.54 V of overpotential for discharge and charge, respectively [85].

### **5. Conclusions**

In this chapter, FSG electrodes in battery applications signify their potential advantages to the fabrication technology. The fabrication of FSG electrode is facile as well as it excludes some additives applied in conventional electrodes. At present, the electrode of spent batteries contains active materials, binder, and metal foil, which set hurdles for recycling process. Herein, the FSG hybrid electrodes provide good capacity and cycling for battery application without binder and metal current collector. This exclusion provides light weight and flexible batteries and also there is a pathway to discover a facile route to recover the materials from FSG hybrid–based spent batteries in future.

#### **Acknowledgements**

This work was supported by South China Normal University. F.C. thanks the support from Outstanding Young Scholar Project (8S0256), the Project of Blue Fire Plan (CXZJHZ201709), and the Scientific and Technological Plan of Guangdong Province (2018A050506078).

**107**

**Author details**

P.R. China

Karthick Ramalingam and Fuming Chen\*

provided the original work is properly cited.

\*Address all correspondence to: fmchen@m.scnu.edu.cn

Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, Guangdong Engineering Technology Research Center of Efficient Green Energy and Environment Protection Materials, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario*

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

#### **Conflict of interest**

The authors declare that there is no conflict of interest.

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

### **Author details**

*Energy Storage Devices*

volume of 10.9 cm3

respectively [85].

**5. Conclusions**

spent batteries in future.

**Acknowledgements**

**Conflict of interest**

Province (2018A050506078).

for delivering high capacity of the system. There are a lot of reports for cathode development based on metal oxides grown on Ni foam as binder-free electrodes. The role of FSG electrodes was also investigated as cathodes for Li-air batteries. First, Kim et al. developed graphene nanoplates (GNP)/GO composite paper-like electrodes as cathodes for Li-air battery system. The wrinkled nature of the paper electrodes induces the high surface area and also delivers higher discharge capacity of 9760 mAh/g at 0.1 A/g current density. This superior performance is due to the reduced overpotential, and the difference in consumption/evolution of O2 is minimized. On the whole, the system exhibits higher efficiency in OER (oxygen evolution reaction)/ORR (oxygen reduction reaction) of 87% [83]. The same

group developed macroporous FSG paper with surface area of 373 m2

of 12,200 mAh/g at 0.2 A/g. The rate capability is enhanced where it shows high cycling performance even at higher current density of 0.5 and 2 A/g that delivers approximately 1000 mAh/g. This is attributed to the minimized volume expansion that limits the decomposition and formation of Li2O2 at the macroporous nature of FSG. While discharging/charging the macroporous FSG, the nature of FSG electrode decomposes the discharge products completely that reveals its highly porous structure as shown in the **Figure 5c** and **d** [76]. Researchers investigated the effect of FSG cathodes in Li-air upon introduction of metal oxides, namely, α-MnO2 and NiCo2O4. Upon insertion of α-MnO2 into FSG electrodes, the overpotential decrease was caused during charge/discharge process. It delivers 2900 mAh/g for the higher content of α-MnO2 that was reported and shows the catalytic improvement in this study [84]. And Jiang et al. reported an excellent reversible capacity of 5000 mAh/g at 0.4 A/g by incorporating mesoporous NiCo2O4 into macropores of FSG. It also lowers about 0.18 and 0.54 V of overpotential for discharge and charge,

In this chapter, FSG electrodes in battery applications signify their potential advantages to the fabrication technology. The fabrication of FSG electrode is facile as well as it excludes some additives applied in conventional electrodes. At present, the electrode of spent batteries contains active materials, binder, and metal foil, which set hurdles for recycling process. Herein, the FSG hybrid electrodes provide good capacity and cycling for battery application without binder and metal current collector. This exclusion provides light weight and flexible batteries and also there is a pathway to discover a facile route to recover the materials from FSG hybrid–based

This work was supported by South China Normal University. F.C. thanks the support from Outstanding Young Scholar Project (8S0256), the Project of Blue Fire Plan (CXZJHZ201709), and the Scientific and Technological Plan of Guangdong

The authors declare that there is no conflict of interest.

/g with 91.6% of porosity that exhibits a high specific capacity

/g and pore

**106**

Karthick Ramalingam and Fuming Chen\* Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, Guangdong Engineering Technology Research Center of Efficient Green Energy and Environment Protection Materials, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou, P.R. China

\*Address all correspondence to: fmchen@m.scnu.edu.cn

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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2017;**43**:7588-7593. DOI: 10.1016/j. ceramint.2017.03.051

[65] Zhao X, Wang G, Zhou Y, Wang H. Flexible free-standing ternary CoSnO3/graphene/carbon nanotubes composite papers as anodes for enhanced performance of lithiumion batteries. Energy. 2017;**118**: 172-180. DOI: 10.1016/j.energy. 2016.12.018

[66] Cao H, Zhou X, Deng W, Ma Z, Liu Y, Liu Z. Layer structured graphene/ porous ZnCo2O4 composite film for high performance flexible lithiumion batteries. Chemical Engineering Journal. 2018;**343**:654-661. DOI: 10.1016/j.cej.2018.03.001

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[68] Darwiche A,Marino C, Sougrati MT, Fraisse B, Stievano L, Monconduit L. Better cycling performances of bulk Sb in Na-ion batteries compared to Li-ion systems: An unexpected electrochemical mechanism. Journal of the American Chemical Society. 2012;**134**:20805- 20811. DOI: 10.1021/ja310347x

[69] Qian J, Wu X, Cao Y, Ai X, Yang H. High capacity and rate capability of amorphous phosphorus for sodium ion batteries. Angewandte Chemie, International Edition. 2013;**52**:4633- 4636. DOI: 10.1002/anie.201209689

[70] David L, Bhandavat R, Singh G. MoS2/Graphene composite paper for sodium-ion battery electrodes. ACS Nano. 2014;**8**:1759-1770. DOI: 10.1021/nn406156b

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ion batteries. Journal of Materials Chemistry A. 2018;**6**:1574-1581. DOI:

[78] Sun N, Guan Y, Liu YT, Zhu Q, Shen J, Liu S, et al. Facile synthesis of free-standing, flexible hard carbon anode for high performance sodium ion batteries using graphene as a multifunctional binder. Carbon. 2018;**137**:475-483. DOI: 10.1016/j.

[79] Huang X, Sun B, Li K, Chen S, Wang G. Mesoporous graphene paper immobilised sulfur as a flexible electrode for lithium-sulfur batteries. Journal of Materials Chemistry A. 2013;**1**:13484-13489. DOI: 10.1039/

[80] Zhu L, Peng HJ, Liang J, Huang JQ, Chen CM, Guo X, et al. Interconnected carbon nanotube/graphene nanosphere

scaffolds as free-standing paper electrode for high-rate and ultra-stable lithium-sulfur batteries. Nano Energy. 2015;**11**:746-755. DOI: 10.1016/j.

[81] Wang C, Wang X, Yang Y, Kushima A, Chen J, Huang Y, et al. Slurryless Li2S/reduced graphene oxide cathode paper for high-performance lithium sulfur battery. Nano Letters. 2015;**15**:1796-1802. DOI: 10.1021/acs.

[82] Chen Y, Lu S, Zhou J, Qin W, Wu X. Synergistically assembled Li2S/FWNTs@ reduced graphene oxide nanobundle

performance Li2S cathodes. Advanced Functional Materials. 2017;**27**:1700987.

[83] Kim DY, Kim M, Kim DW, Suk J, Park OO, Kang Y. Flexible binder-free graphene paper cathodes for highperformance Li-O2 batteries. Carbon. 2015;**93**:625-635. DOI: 10.1016/j.

forest for free-standing high-

DOI: 10.1002/adfm.201700987

carbon.2015.05.097

nanoen.2014.11.062

nanolett.5b00112

10.1039/C7TA07762A

carbon.2018.05.056

C3TA12826A

*DOI: http://dx.doi.org/10.5772/intechopen.84275*

synthesized self-standing electrode of MoS2 nanosheets assembled on graphene foam for high-performance Li-ion and Na-ion batteries. Journal of Alloys and Compounds. 2016;**660**:11-16. DOI: 10.1016/j.jallcom.2015.11.040

[72] Wang Y, Kong D, Shi W, Liu B, Sim G, Ge Q, et al. Ice Templated freestanding hierarchically WS2/CNTrGO aerogel for high-performance rechargeable lithium and sodium ion batteries. Advanced Energy Materials.

2016;**6**:1601057. DOI: 10.1002/

10.1016/j.jallcom.2017.10.094

carbon.2017.01.101

10.1002/cssc.201402329

[77] Ma X, Chen L, Ren X,

[74] An H, Li Y, Gao Y, Cao C, Han J, Feng Y, et al. Free-standing fluorine and nitrogen co-doped graphene paper as a high-performance electrode for flexible sodium-ion batteries. Carbon. 2017;**116**:338-346. DOI: 10.1016/j.

[75] Han K, Shen J, Hao S, Hao S, Ye H, Wolverton C, et al. Free-standing nitrogen-doped graphene paper as electrodes for high-performance lithium/dissolved polysulfide batteries. ChemSusChem. 2014;**7**:2545-2553. DOI:

[76] Kim DY, Kim M, Kim DW, Suk J, Park JJ, Park OO, et al. Graphene paper with controlled pore structure for high-performance cathodes in Li2O batteries. Carbon. 2016;**100**:265-272. DOI: 10.1016/j.carbon.2016.01.013

Hou G, Chen L, Zhang L, et al. Highperformance red phosphorus/ carbon nanofibers/graphene freestanding paper anode for sodium

[73] Zhang G, Liu K, Liu S, Song H, Zhou J. Flexible Co0.85Se nanosheets/ graphene composite film as binder-free anode with high Li- and Na-ion storage performance. Journal of Alloys and Compounds. 2018;**731**:714-722. DOI:

aenm.201601057

*Progress on Free-Standing Graphene Hybrid: Advantages and Future Scenario DOI: http://dx.doi.org/10.5772/intechopen.84275*

synthesized self-standing electrode of MoS2 nanosheets assembled on graphene foam for high-performance Li-ion and Na-ion batteries. Journal of Alloys and Compounds. 2016;**660**:11-16. DOI: 10.1016/j.jallcom.2015.11.040

*Energy Storage Devices*

10.1016/j.cej.2016.10.041

am403136e

adfm.201200697

10.1039/c5ra05038c

electacta.2015.09.080

graphene paper@Fe3O4 nanorod array@ carbon as integrated anode for robust lithium storage. Chemical Engineering Journal. 2017;**309**:272-277. DOI:

2017;**43**:7588-7593. DOI: 10.1016/j.

[66] Cao H, Zhou X, Deng W, Ma Z, Liu Y, Liu Z. Layer structured graphene/ porous ZnCo2O4 composite film for high performance flexible lithiumion batteries. Chemical Engineering Journal. 2018;**343**:654-661. DOI:

[67] Qian J, Chen Y, Wu L, Cao Y, Ai X, Yang H. High capacity Na-storage and superior cyclability of nanocomposite Sb/C anode for Na-ion batteries. Chemical Communications. 2012;**48**:

10.1016/j.cej.2018.03.001

[68] Darwiche A,Marino C,

[70] David L, Bhandavat R,

10.1021/nn406156b

Singh G. MoS2/Graphene composite paper for sodium-ion battery electrodes. ACS Nano. 2014;**8**:1759-1770. DOI:

[71] Xiang J, Dong D, Wen F, Zhao J, Zhong X, Wang L, et al. Microwave

Sougrati MT, Fraisse B, Stievano L, Monconduit L. Better cycling performances of bulk Sb in Na-ion batteries compared to Li-ion systems: An unexpected electrochemical mechanism. Journal of the American Chemical Society. 2012;**134**:20805- 20811. DOI: 10.1021/ja310347x

[69] Qian J, Wu X, Cao Y, Ai X, Yang H. High capacity and rate capability of amorphous phosphorus for sodium ion batteries. Angewandte Chemie, International Edition. 2013;**52**:4633- 4636. DOI: 10.1002/anie.201209689

[65] Zhao X, Wang G, Zhou Y, Wang H. Flexible free-standing ternary CoSnO3/graphene/carbon nanotubes composite papers as anodes for enhanced performance of lithiumion batteries. Energy. 2017;**118**: 172-180. DOI: 10.1016/j.energy.

ceramint.2017.03.051

2016.12.018

7070-7072

[59] Liu Y, Wang W, Gu L, Wang Y, Ying Y, Mao Y, et al. Flexible CuO nanosheets/reduced-graphene oxide composite paper: Binder-free anode for high performance lithium-ion batteries. ACS Applied Materials and Interfaces. 2013;**5**:9850-9855. DOI: 10.1021/

[60] Byon HR, Gallant BM, Lee SW, Yang SH. Role of oxygen functional groups in carbon nanotube/graphene freestanding electrodes for high performance lithium batteries. Advanced Functional Materials. 2013;**23**:1037-1045. DOI: 10.1002/

[61] Zhang L, Fan W, Liu T. A flexible free-standing defect-rich MoS2/ graphene/carbon nanotube hybrid paper as a binder-free anode for highperformance lithium ion batteries. RSC Advances. 2015;**5**:43130-43140. DOI:

[62] Wang J, Wang G, Wang H. Flexible

[63] Liu Y, Cai X, Shi W. Free-standing graphene/carbon nanotubes/CuO aerogel paper anode for lithium ion batteries. Materials Letters. 2016;**172**: 72-75. DOI: 10.1016/j.matlet.2016.02.068

[64] Li Y, Wang P, Bao Y, Huang K. A flexible nanostructured paper of MnO NPs@MWCNTs/r-GO multilayer sandwich composite for high-performance lithium-ion batteries. Ceramics International.

free-standing Fe2O3/graphene/ carbon nanotubes hybrid films as anode materials for high performance lithium-ion batteries. Electrochimica Acta. 2015;**182**:192-201. DOI: 10.1016/j.

**112**

[72] Wang Y, Kong D, Shi W, Liu B, Sim G, Ge Q, et al. Ice Templated freestanding hierarchically WS2/CNTrGO aerogel for high-performance rechargeable lithium and sodium ion batteries. Advanced Energy Materials. 2016;**6**:1601057. DOI: 10.1002/ aenm.201601057

[73] Zhang G, Liu K, Liu S, Song H, Zhou J. Flexible Co0.85Se nanosheets/ graphene composite film as binder-free anode with high Li- and Na-ion storage performance. Journal of Alloys and Compounds. 2018;**731**:714-722. DOI: 10.1016/j.jallcom.2017.10.094

[74] An H, Li Y, Gao Y, Cao C, Han J, Feng Y, et al. Free-standing fluorine and nitrogen co-doped graphene paper as a high-performance electrode for flexible sodium-ion batteries. Carbon. 2017;**116**:338-346. DOI: 10.1016/j. carbon.2017.01.101

[75] Han K, Shen J, Hao S, Hao S, Ye H, Wolverton C, et al. Free-standing nitrogen-doped graphene paper as electrodes for high-performance lithium/dissolved polysulfide batteries. ChemSusChem. 2014;**7**:2545-2553. DOI: 10.1002/cssc.201402329

[76] Kim DY, Kim M, Kim DW, Suk J, Park JJ, Park OO, et al. Graphene paper with controlled pore structure for high-performance cathodes in Li2O batteries. Carbon. 2016;**100**:265-272. DOI: 10.1016/j.carbon.2016.01.013

[77] Ma X, Chen L, Ren X, Hou G, Chen L, Zhang L, et al. Highperformance red phosphorus/ carbon nanofibers/graphene freestanding paper anode for sodium

ion batteries. Journal of Materials Chemistry A. 2018;**6**:1574-1581. DOI: 10.1039/C7TA07762A

[78] Sun N, Guan Y, Liu YT, Zhu Q, Shen J, Liu S, et al. Facile synthesis of free-standing, flexible hard carbon anode for high performance sodium ion batteries using graphene as a multifunctional binder. Carbon. 2018;**137**:475-483. DOI: 10.1016/j. carbon.2018.05.056

[79] Huang X, Sun B, Li K, Chen S, Wang G. Mesoporous graphene paper immobilised sulfur as a flexible electrode for lithium-sulfur batteries. Journal of Materials Chemistry A. 2013;**1**:13484-13489. DOI: 10.1039/ C3TA12826A

[80] Zhu L, Peng HJ, Liang J, Huang JQ, Chen CM, Guo X, et al. Interconnected carbon nanotube/graphene nanosphere scaffolds as free-standing paper electrode for high-rate and ultra-stable lithium-sulfur batteries. Nano Energy. 2015;**11**:746-755. DOI: 10.1016/j. nanoen.2014.11.062

[81] Wang C, Wang X, Yang Y, Kushima A, Chen J, Huang Y, et al. Slurryless Li2S/reduced graphene oxide cathode paper for high-performance lithium sulfur battery. Nano Letters. 2015;**15**:1796-1802. DOI: 10.1021/acs. nanolett.5b00112

[82] Chen Y, Lu S, Zhou J, Qin W, Wu X. Synergistically assembled Li2S/FWNTs@ reduced graphene oxide nanobundle forest for free-standing highperformance Li2S cathodes. Advanced Functional Materials. 2017;**27**:1700987. DOI: 10.1002/adfm.201700987

[83] Kim DY, Kim M, Kim DW, Suk J, Park OO, Kang Y. Flexible binder-free graphene paper cathodes for highperformance Li-O2 batteries. Carbon. 2015;**93**:625-635. DOI: 10.1016/j. carbon.2015.05.097

[84] Ozcan S, Tokur M, Cetinkaya T, Guler A, Uysal M, Guler MO, et al. Free standing flexible graphene oxide +α-MnO2 composite cathodes for Li–air batteries. Solid State Ionics. 2016;**286**:34-39. DOI: 10.1016/j. ssi.2015.12.016

Chapter 7

Sangwon Kim

peak power density

1. Introduction

115

energy, a shift in the energy mix.

technology should be secured.

Abstract

Vanadium Redox Flow Batteries:

The importance of reliable energy storage system in large scale is increasing to replace fossil fuel power and nuclear power with renewable energy completely because of the fluctuation nature of renewable energy generation. The vanadium redox flow battery (VRFB) is one promising candidate in large-scale stationary energy storage system, which stores electric energy by changing the oxidation numbers of anolyte and catholyte through redox reaction. This chapter covers the basic principles of vanadium redox flow batteries, component technologies, flow configurations, operation strategies, and cost analysis. The thermodynamic analysis of the electrochemical reactions and the electrode reaction mechanisms in VRFB systems have been explained, and the analysis of VRFB performance according to the flow field and flow rate has been described. It is shown that the limiting current density of "flow-by" design is more than two times greater than that of "flowthrough" design. In the cost analysis of 10 kW/120 kWh VRFB system, stack and

Electrochemical Engineering

electrolyte account for 40 and 32% of total cost, respectively.

Keywords: vanadium electrolyte, carbon electrode, overpotential, polarization, state of charge, flow-through, flow-by, flow rate, limiting current density,

The global environmental is changing rapidly. The established world's first energy demand and biggest carbon emitter countries are being replaced by emerging countries. The use of renewable energy is expanding due to technological development and environmental problems. The global energy market is moving toward the reduction of fossil fuels and the expansion of environment friendly

For stable supply of renewable energy with high volatility such as sunlight or wind power, securing stability of power system is the most important. To do this, an intelligent power network should be built up, and grid-based energy storage

The vanadium redox flow battery is one of the most promising secondary batteries as a large-capacity energy storage device for storing renewable energy [1, 2, 4]. Recently, a safety issue has been arisen by frequent fire accident of a largecapacity energy storage system (ESS) using a lithium ion battery. The vanadium electrolyte is a nonflammable aqueous solution and has a high heat capacity to limit the temperature rise. Therefore, VRFB has no risk of ignition and explosion.

[85] Jiang Y, Zou L, Cheng J, Huang Y, Jia L, Chi B, et al. Needle-like NiCo2O4 coated on graphene foam as a flexible cathode for lithium-oxygen batteries. ChemElectroChem. 2017;**4**:3140-3147. DOI: 10.1002/celc.201700864

#### Chapter 7

*Energy Storage Devices*

ssi.2015.12.016

[84] Ozcan S, Tokur M, Cetinkaya T, Guler A, Uysal M, Guler MO, et al. Free standing flexible graphene oxide +α-MnO2 composite cathodes for Li–air batteries. Solid State Ionics. 2016;**286**:34-39. DOI: 10.1016/j.

[85] Jiang Y, Zou L, Cheng J, Huang Y, Jia L, Chi B, et al. Needle-like NiCo2O4 coated on graphene foam as a flexible cathode for lithium-oxygen batteries. ChemElectroChem. 2017;**4**:3140-3147.

DOI: 10.1002/celc.201700864

**114**

## Vanadium Redox Flow Batteries: Electrochemical Engineering

Sangwon Kim

#### Abstract

The importance of reliable energy storage system in large scale is increasing to replace fossil fuel power and nuclear power with renewable energy completely because of the fluctuation nature of renewable energy generation. The vanadium redox flow battery (VRFB) is one promising candidate in large-scale stationary energy storage system, which stores electric energy by changing the oxidation numbers of anolyte and catholyte through redox reaction. This chapter covers the basic principles of vanadium redox flow batteries, component technologies, flow configurations, operation strategies, and cost analysis. The thermodynamic analysis of the electrochemical reactions and the electrode reaction mechanisms in VRFB systems have been explained, and the analysis of VRFB performance according to the flow field and flow rate has been described. It is shown that the limiting current density of "flow-by" design is more than two times greater than that of "flowthrough" design. In the cost analysis of 10 kW/120 kWh VRFB system, stack and electrolyte account for 40 and 32% of total cost, respectively.

Keywords: vanadium electrolyte, carbon electrode, overpotential, polarization, state of charge, flow-through, flow-by, flow rate, limiting current density, peak power density

#### 1. Introduction

The global environmental is changing rapidly. The established world's first energy demand and biggest carbon emitter countries are being replaced by emerging countries. The use of renewable energy is expanding due to technological development and environmental problems. The global energy market is moving toward the reduction of fossil fuels and the expansion of environment friendly energy, a shift in the energy mix.

For stable supply of renewable energy with high volatility such as sunlight or wind power, securing stability of power system is the most important. To do this, an intelligent power network should be built up, and grid-based energy storage technology should be secured.

The vanadium redox flow battery is one of the most promising secondary batteries as a large-capacity energy storage device for storing renewable energy [1, 2, 4]. Recently, a safety issue has been arisen by frequent fire accident of a largecapacity energy storage system (ESS) using a lithium ion battery. The vanadium electrolyte is a nonflammable aqueous solution and has a high heat capacity to limit the temperature rise. Therefore, VRFB has no risk of ignition and explosion.

The power of VRFB depends on the performance of the stack, and the energy storage capacity depends on the electrolyte concentration and the electrolyte reservoir size, which greatly increases the degree of freedom in system design [7, 24]. A schematic diagram of the vanadium redox flow battery is shown in Figure 1.

Flow batteries suffer from the capacity imbalance due to the mixing of the both

side active materials caused by the electrolyte diffusion across the membrane, resulting in an irreversible loss of capacity as well as an efficiency loss [10–14]. Since the vanadium redox flow battery uses vanadium as the active material of both electrolytes, the use of appropriate rebalancing techniques can mitigate capacity

The vanadium ion may have various oxidation numbers from bivalent to pentavalent. Using this property, vanadium is used as the electrolyte redox

+

Negative electrode: <sup>V</sup><sup>2</sup><sup>þ</sup> \$ V3<sup>þ</sup> <sup>þ</sup> <sup>e</sup>� <sup>E</sup><sup>0</sup> ¼ �0:255 V (1)

The permeation of the vanadium ions through the membrane occurs since any membrane cannot block the crossover of the redox species completely. The vanadium ions diffused to the counter electrolyte cause a cross-contamination reaction as below:

The self-discharging reactions caused by the vanadium ions permeated into the

<sup>2</sup> <sup>þ</sup> <sup>e</sup>� <sup>þ</sup> 2H<sup>þ</sup> \$ VO<sup>2</sup><sup>þ</sup> <sup>þ</sup> H2O <sup>E</sup><sup>0</sup> ¼ þ1:004 V (2)

<sup>2</sup> <sup>þ</sup> V2<sup>þ</sup> <sup>þ</sup> 2H<sup>þ</sup> \$ VO<sup>2</sup><sup>þ</sup> <sup>þ</sup> V3<sup>þ</sup> <sup>þ</sup> H2O <sup>E</sup><sup>0</sup> ¼ þ1:259 V (3)

VO<sup>2</sup><sup>þ</sup> <sup>þ</sup> 2H<sup>þ</sup> <sup>þ</sup> <sup>e</sup>� \$ <sup>V</sup><sup>3</sup><sup>þ</sup> <sup>þ</sup> H2O <sup>E</sup><sup>0</sup> ¼ þ0:34 V (4)

VO2<sup>þ</sup> <sup>þ</sup> <sup>V</sup><sup>2</sup><sup>þ</sup> <sup>þ</sup> 2H<sup>þ</sup> ! 2V<sup>3</sup><sup>þ</sup> <sup>þ</sup> H2O (5)

V2<sup>þ</sup> <sup>þ</sup> VO<sup>2</sup><sup>þ</sup> <sup>þ</sup> 2H<sup>þ</sup> ! 2 V<sup>3</sup><sup>þ</sup> <sup>þ</sup> H2O (10)

<sup>2</sup> <sup>þ</sup> 2V<sup>2</sup><sup>þ</sup> <sup>þ</sup> 4H<sup>þ</sup> ! 3V2<sup>þ</sup> <sup>þ</sup> 2H2O (6)

<sup>2</sup> <sup>þ</sup> V3<sup>þ</sup> ! 2 VO2<sup>þ</sup> (7)

<sup>2</sup> <sup>þ</sup> 2H<sup>þ</sup> ! 3VO<sup>2</sup><sup>þ</sup> <sup>þ</sup> H2O (8)

<sup>2</sup> ! 2 VO2<sup>þ</sup> (9)

by V(V), V(IV), V(III), and V(II) for explanation. Solution of V(III) is added to the negative electrolyte tank, and solution of V(IV) is added to the positive electrolyte tank as shown in Figure 1. When the electricity is applied to the electrodes, the V(III) ion of the negative electrolyte is reduced to V(II), and the V(IV) ion of the positive electrolyte is oxidized to V(V). This means that when the VRFB is charged, the difference in the oxidation number between the positive electrolyte and negative electrolyte increases from +1 to +3, and it can be understood conceptually that the electric energy is stored in the increased bivalent oxidation number. When the VRFB is discharged, V(II) in negative electrolyte is oxidized to V(III), and V(V) in positive electrolyte is reduced to V(IV). The chemical

, VO2 +, V3 +, and V2 + are represented

loss though vanadium crossovers can lead to loss of efficiency.

2. Electrochemical reactions and kinetics

Vanadium Redox Flow Batteries: Electrochemical Engineering

DOI: http://dx.doi.org/10.5772/intechopen.85166

reactions for charge-discharge are expressed as follows:

counter electrolytes can be described as below:

VO<sup>þ</sup>

VO<sup>þ</sup>

<sup>V</sup><sup>3</sup><sup>þ</sup> <sup>þ</sup> VO<sup>þ</sup>

<sup>V</sup><sup>2</sup><sup>þ</sup> <sup>þ</sup> 2VO<sup>þ</sup>

couple material of the flow battery. VO2

Positive electrode: VO<sup>þ</sup>

Overall reaction: VO<sup>þ</sup>

Negative electrode:

Positive electrode:

117

#### Figure 1.

Schematic of vanadium redox flow batteries: (a) charging and (b) discharging. Reproduced with permission from [3]. Copyright 2017 by Elsevier.

The power of VRFB depends on the performance of the stack, and the energy

reservoir size, which greatly increases the degree of freedom in system design [7, 24]. A schematic diagram of the vanadium redox flow battery is shown in Figure 1.

Schematic of vanadium redox flow batteries: (a) charging and (b) discharging. Reproduced with permission

storage capacity depends on the electrolyte concentration and the electrolyte

Energy Storage Devices

Figure 1.

116

from [3]. Copyright 2017 by Elsevier.

Flow batteries suffer from the capacity imbalance due to the mixing of the both side active materials caused by the electrolyte diffusion across the membrane, resulting in an irreversible loss of capacity as well as an efficiency loss [10–14]. Since the vanadium redox flow battery uses vanadium as the active material of both electrolytes, the use of appropriate rebalancing techniques can mitigate capacity loss though vanadium crossovers can lead to loss of efficiency.

#### 2. Electrochemical reactions and kinetics

The vanadium ion may have various oxidation numbers from bivalent to pentavalent. Using this property, vanadium is used as the electrolyte redox couple material of the flow battery. VO2 + , VO2 +, V3 +, and V2 + are represented by V(V), V(IV), V(III), and V(II) for explanation. Solution of V(III) is added to the negative electrolyte tank, and solution of V(IV) is added to the positive electrolyte tank as shown in Figure 1. When the electricity is applied to the electrodes, the V(III) ion of the negative electrolyte is reduced to V(II), and the V(IV) ion of the positive electrolyte is oxidized to V(V). This means that when the VRFB is charged, the difference in the oxidation number between the positive electrolyte and negative electrolyte increases from +1 to +3, and it can be understood conceptually that the electric energy is stored in the increased bivalent oxidation number. When the VRFB is discharged, V(II) in negative electrolyte is oxidized to V(III), and V(V) in positive electrolyte is reduced to V(IV). The chemical reactions for charge-discharge are expressed as follows:

$$\text{Negative electrode: } \mathbf{V}^{2+} \leftrightarrow \mathbf{V}^{3+} + \mathbf{e}^- \ E^0 = -0.255 \text{ V} \tag{1}$$

$$\text{Positive electrode: } \text{VO}\_2^+ + \text{e}^- + 2\text{H}^+ \leftrightarrow \text{VO}^{2+} + \text{H}\_2\text{O} \, E^0 = +1.004 \text{ V} \tag{2}$$

$$\text{Overall reaction:}\ \mathrm{VO}\_{2}^{+} + \mathrm{V}^{2+} + 2\mathrm{H}^{+} \leftrightarrow \mathrm{VO}^{2+} + \mathrm{V}^{3+} + \mathrm{H}\_{2}\mathrm{O}\ E^{0} = +1.259\,\mathrm{V}\tag{3}$$

The permeation of the vanadium ions through the membrane occurs since any membrane cannot block the crossover of the redox species completely. The vanadium ions diffused to the counter electrolyte cause a cross-contamination reaction as below:

$$\text{VO}^{2+} + 2\text{H}^+ + \text{e}^- \leftrightarrow \text{V}^{3+} + \text{H}\_2\text{O} \, E^0 = +0.34 \text{ V} \tag{4}$$

The self-discharging reactions caused by the vanadium ions permeated into the counter electrolytes can be described as below:

Negative electrode:

$$\text{V}\text{O}^{2+} + \text{V}^{2+} + 2\text{H}^{+} \rightarrow 2\text{V}^{3+} + \text{H}\_{2}\text{O} \tag{5}$$

$$\text{VO}\_2^+ + 2\text{V}^{2+} + 4\text{H}^+ \rightarrow 3\text{V}^{2+} + 2\text{H}\_2\text{O} \tag{6}$$

$$\text{VO}\_2^+ + \text{V}^{3+} \rightarrow 2\text{VO}^{2+} \tag{7}$$

Positive electrode:

$$\text{V}^{2+} + 2\text{VO}\_2^{+} + 2\text{H}^{+} \rightarrow 3\text{VO}^{2+} + \text{H}\_2\text{O} \tag{8}$$

$$\text{V}^{\text{3}^+} + \text{VO}\_2^+ \rightarrow 2\text{VO}^{\text{2}+} \tag{9}$$

$$\text{V}^{2+} + \text{VO}^{2+} + 2\text{H}^{+} \rightarrow 2\text{ V}^{3+} + \text{H}\_{2}\text{O} \tag{10}$$

When the VRFB is overcharged, hydrogen and oxygen gas can be generated at the negative and positive electrodes, respectively. Additionally, the carbon dioxide gas can be generated by corrosion of graphite plate with the produced oxygen gas.

Negative electrode:

$$\text{2H}^+ + \text{2e}^- \rightarrow \text{H}\_{2(gas)}\tag{11}$$

where φ<sup>s</sup> is the electric potential of the solid electrode and φ<sup>l</sup> is the electrolyte

The standard open-circuit voltage of VRFB, E0 = 1.26 V, can be derived from

However, the actual operating voltage of VRFB differs from this thermodynamic

At discharge, the operating voltage becomes smaller than theoretical value. As the current density increases, the overpotential and iR drop increase, so the charging voltage increases and the discharging voltage decreases as shown in Figure 3c. Energy density and power density can be calculated in Eqs. (25) and (26), respectively.

<sup>L</sup> � <sup>26</sup>:<sup>8</sup> Ah

where n is the number of electrons transferred during reactions, C is a vanadium

electrolyte concentration, 1.6 mol/L, Vdischarge is averaged discharge voltage, and

Charge-discharge voltage of vanadium redox flow battery: Current vs. voltage and overpotential and open-

mol � 1:3 V

Power denisty ¼ current density � Vdischarge (26)

overpotential is required in addition to the thermodynamic voltage. Figure 2 shows the relationship of the voltage and current during charging and discharging at the two electrodes of VRFB, assuming that the overall kinetics are determined by the

value. Charging voltage should be larger than 1.26 V since the amount of

Echarge <sup>¼</sup> <sup>E</sup><sup>0</sup>

Edischarge <sup>¼</sup> <sup>E</sup><sup>0</sup>

where η<sup>a</sup> is anodic overpotential and η<sup>c</sup> is cathodic overpotential.

<sup>¼</sup> <sup>1</sup> � <sup>1</sup>:6mol

<sup>Δ</sup>G<sup>0</sup> <sup>¼</sup> <sup>Δ</sup>H<sup>0</sup> � <sup>T</sup>ΔS<sup>0</sup> ¼ �nFE<sup>0</sup> ¼ �119:<sup>3</sup> kJ=mol (22)

cell þ η<sup>a</sup> þ η<sup>c</sup> þ iRtotal (23)

cell � η<sup>a</sup> � η<sup>c</sup> � iRtotal (24)

<sup>2</sup> <sup>¼</sup> <sup>27</sup>:<sup>872</sup> Wh=<sup>L</sup> (25)

potential.

Gibbs free energy relation as below:

DOI: http://dx.doi.org/10.5772/intechopen.85166

Energy density <sup>¼</sup> nCFVdis

Figure 2.

119

charge transfer in the electrochemical reaction.

Vanadium Redox Flow Batteries: Electrochemical Engineering

Ntank

circuit voltage at positive electrode and negative electrode.

Positive electrode:

$$\text{C} + 2\text{H}\_2\text{O} \rightarrow \text{CO}\_{2(gas)} + 4\text{H}^+ + 4\text{e}^- \tag{12}$$

$$\text{2H}\_2\text{O} \rightarrow \text{O}\_{2(gas)} + 4\text{H}^+ + 4\text{e}^- \tag{13}$$

$$\text{C} + \text{O}\_{2(gas)} \rightarrow \text{CO}\_{2(gas)}\tag{14}$$

The equilibrium cell potentials, Eeq for each reaction, are calculated using Nernst equation according to

$$E\_{eq, \text{neg}} = E\_{\text{neg}}^0 + \frac{RT}{F} \ln\left(\frac{C\_{V^{\flat+}}}{C\_{V^{2+}}}\right) \tag{15}$$

$$E\_{eq,pos} = E\_{pos}^0 + \frac{RT}{F} \ln\left(\frac{C\_{VO\_2^+} \left(c\_{H^+}\right)^2}{C\_{VO^{2+}}}\right) \tag{16}$$

$$E\_{eq,overall} = E\_{overall}^0 + \frac{RT}{F} \ln\left(\frac{C\_{VO\_2^+} \left(C\_{H^+}\right)^2}{C\_{VO^{2+}}} \frac{C\_{V^{2+}}}{C\_{V^{3+}}}\right) \tag{17}$$

where Ci\* is the concentration of the i species; E<sup>0</sup> is the standard cell potential for the electrode reaction; R is the ideal gas constant, 8.314 J/mol K; T is the cell temperature; and F is Faraday's constant, 96,485 As/mol.

The exchange current density is the magnitude of the current when the electrode reactions reach the equilibrium and can be described as

$$i\_{0, \text{neg}} = Fk\_{\text{neg}}^0 \mathbf{C}\_{V^{\beta+}}^{\*\left(1 - a\_{\text{neg}}\right)} \mathbf{C}\_{V^{2+}}^{\*a\_{\text{neg}}} \tag{18}$$

$$\dot{a}\_{0,pos} = Fk\_{pos}^0 \mathcal{C}\_{VO\_2^+}^{\*\left(1-a\_{pu}\right)} \mathcal{C}\_{VO^{2+}}^{\*a\_{pu}} \tag{19}$$

where k<sup>0</sup> is the standard rate constant.

Following the Butler-Volmer equation [5, 24], the currents at negative electrode and positive electrode are described as

$$\dot{\mathbf{u}}\_{\text{neg}} = \dot{\mathbf{u}}\_{0,\text{neg}} \left[ \left( \frac{\mathbf{C}\_{V^{\beta+}}(\mathbf{0}, t)}{\mathbf{C}\_{V^{\beta+}}^{\*}} \right) \exp\left( -\frac{a\_{\text{neg}} F}{RT} \eta\_{\text{neg}} \right) - \left( \frac{\mathbf{C}\_{V^{2+}}(\mathbf{0}, t)}{\mathbf{C}\_{V^{2+}}^{\*}} \right) \exp\left( \frac{(\mathbf{1} - a\_{\text{neg}})F}{RT} \eta\_{\text{neg}} \right) \right] \tag{20}$$

$$i\_{pas} = i\_{0, pas} \left[ \left( \frac{\mathbf{C}\_{VO\_2^+} (0, t)}{\mathbf{C}\_{VO\_2^+}^\*} \right) \exp\left( -\frac{a\_{pas} F}{RT} \eta\_{pas} \right) - \left( \frac{\mathbf{C}\_{VO^{2+}} (0, t)}{\mathbf{C}\_{VO^{2+}}^\*} \right) \exp\left( \frac{(1 - a\_{pas}) F}{RT} \eta\_{pas} \right) \right] \tag{21}$$

where α is the transfer coefficient or symmetry factor and η is the overpotential, defined as η ¼ ϕ<sup>s</sup> � ϕ<sup>l</sup> � Eeq:

Vanadium Redox Flow Batteries: Electrochemical Engineering DOI: http://dx.doi.org/10.5772/intechopen.85166

When the VRFB is overcharged, hydrogen and oxygen gas can be generated at the negative and positive electrodes, respectively. Additionally, the carbon dioxide gas can be generated by corrosion of graphite plate with the produced

The equilibrium cell potentials, Eeq for each reaction, are calculated using Nernst

RT

<sup>F</sup> ln CV<sup>3</sup><sup>þ</sup> CV<sup>2</sup><sup>þ</sup> � �

CVO<sup>þ</sup>

CVO<sup>þ</sup>

<sup>2</sup> cH ð Þ <sup>þ</sup> <sup>2</sup> CVO<sup>2</sup><sup>þ</sup> !

> <sup>2</sup> CH ð Þ <sup>þ</sup> <sup>2</sup> CVO<sup>2</sup><sup>þ</sup>

!

CV<sup>2</sup><sup>þ</sup> CV<sup>3</sup><sup>þ</sup>

<sup>V</sup>2<sup>þ</sup> (18)

VO2<sup>þ</sup> (19)

1 � αneg � �F

1 � αpos � �F RT <sup>η</sup>pos

RT <sup>η</sup>neg

(20)

(21)

exp

exp

neg þ

RT <sup>F</sup> ln

RT <sup>F</sup> ln

where Ci\* is the concentration of the i species; E<sup>0</sup> is the standard cell potential for the electrode reaction; R is the ideal gas constant, 8.314 J/mol K; T is the cell

The exchange current density is the magnitude of the current when the electrode

negC<sup>∗</sup> ð Þ <sup>1</sup>�αneg

posC<sup>∗</sup> ð Þ <sup>1</sup>�αpos VOþ 2

Following the Butler-Volmer equation [5, 24], the currents at negative electrode

<sup>V</sup>3<sup>þ</sup> <sup>C</sup><sup>∗</sup> <sup>α</sup>neg

C<sup>∗</sup> <sup>α</sup>pos

� CV<sup>2</sup><sup>þ</sup> ð Þ <sup>0</sup>; <sup>t</sup> C∗ V2<sup>þ</sup> � �

� CVO<sup>2</sup><sup>þ</sup> ð Þ <sup>0</sup>; <sup>t</sup> C∗ VO2<sup>þ</sup>

!

� � � �

" # � �

where α is the transfer coefficient or symmetry factor and η is the overpotential,

pos þ

overall þ

<sup>i</sup>0,neg <sup>¼</sup> Fk<sup>0</sup>

<sup>i</sup>0,pos <sup>¼</sup> Fk<sup>0</sup>

exp � <sup>α</sup>negF

exp � <sup>α</sup>posF

RT <sup>η</sup>neg � �

RT <sup>η</sup>pos � �

Eeq,neg <sup>¼</sup> <sup>E</sup><sup>0</sup>

Eeq,pos <sup>¼</sup> <sup>E</sup><sup>0</sup>

Eeq, overall <sup>¼</sup> <sup>E</sup><sup>0</sup>

temperature; and F is Faraday's constant, 96,485 As/mol.

reactions reach the equilibrium and can be described as

where k<sup>0</sup> is the standard rate constant.

and positive electrode are described as

CV<sup>3</sup><sup>þ</sup> ð Þ 0; t C∗ V3<sup>þ</sup> � �

!

CVO<sup>þ</sup> <sup>2</sup> ð Þ 0; t C∗ VOþ 2

defined as η ¼ ϕ<sup>s</sup> � ϕ<sup>l</sup> � Eeq:

ineg ¼ i0,neg

ipos ¼ i0,pos

118

2H<sup>þ</sup> þ 2e� ! H2 gas ð Þ (11)

(15)

(16)

(17)

C þ 2H2O ! CO2 gas ð Þ þ 4H<sup>þ</sup> þ 4e� (12) 2H2O ! O2 gas ð Þ þ 4H<sup>þ</sup> þ 4e� (13) C þ O2 gas ð Þ ! CO2 gas ð Þ (14)

oxygen gas.

Negative electrode:

Energy Storage Devices

Positive electrode:

equation according to

where φ<sup>s</sup> is the electric potential of the solid electrode and φ<sup>l</sup> is the electrolyte potential.

The standard open-circuit voltage of VRFB, E0 = 1.26 V, can be derived from Gibbs free energy relation as below:

$$
\Delta G^{0} = \Delta H^{0} - T\Delta S^{0} = -nFE^{0} = -\mathbf{1} \mathbf{1} \mathbf{9}.\mathbf{3}\text{ kJ/mol} \tag{22}
$$

However, the actual operating voltage of VRFB differs from this thermodynamic value. Charging voltage should be larger than 1.26 V since the amount of overpotential is required in addition to the thermodynamic voltage. Figure 2 shows the relationship of the voltage and current during charging and discharging at the two electrodes of VRFB, assuming that the overall kinetics are determined by the charge transfer in the electrochemical reaction.

$$E\_{charge} = E\_{cell}^{0} + \eta\_a + \eta\_c + iR\_{total} \tag{23}$$

$$E\_{discharge} = E\_{cell}^0 - \eta\_a - \eta\_c - iR\_{total} \tag{24}$$

where η<sup>a</sup> is anodic overpotential and η<sup>c</sup> is cathodic overpotential.

At discharge, the operating voltage becomes smaller than theoretical value. As the current density increases, the overpotential and iR drop increase, so the charging voltage increases and the discharging voltage decreases as shown in Figure 3c. Energy density and power density can be calculated in Eqs. (25) and (26), respectively.

$$\text{Energy density} = \frac{nCFV\_{di}}{N\_{tank}} = \frac{1 \times \frac{1.6 \text{mol}}{L} \times 26.8 \frac{Ah}{mol} \times 1.3 \text{ V}}{2} = 27.872 \text{ Wh/L} \tag{25}$$

$$\text{Power density} = \text{current density} \times \overline{V}\_{dickary} \tag{26}$$

where n is the number of electrons transferred during reactions, C is a vanadium electrolyte concentration, 1.6 mol/L, Vdischarge is averaged discharge voltage, and

Figure 2.

Charge-discharge voltage of vanadium redox flow battery: Current vs. voltage and overpotential and opencircuit voltage at positive electrode and negative electrode.

EE <sup>¼</sup> discharge energy Wh ð Þ

DOI: http://dx.doi.org/10.5772/intechopen.85166

Ð

disV tð Þdt=tdis <sup>Ð</sup> chV tð Þdt=tch

Vanadium Redox Flow Batteries: Electrochemical Engineering

<sup>¼</sup> Idis <sup>∙</sup>tdis Ich ∙tch

vanadium ions [8, 9]:

3. Electrode

charge energy Wh ð Þ � 100% <sup>¼</sup>

SOC <sup>¼</sup> CV<sup>2</sup><sup>þ</sup>

stack and overall battery system efficiency [15, 16].

3.1 Reaction mechanism at carbon felt electrode

[17, 18]. Oxidation and reduction mechanisms of the VO2

operating potential window.

121

CV<sup>2</sup><sup>þ</sup> þ CV<sup>3</sup><sup>þ</sup>

in the reaction. The electrode material influences the performance of VRFB diversely. The electrode should be electrochemically stable in the operating potential window of VRFB. The electrochemical activity of electrode affects the chargedischarge voltages and consequently the voltage efficiency during battery cycle operation. The electrode must have high electrical conductivity to increase the charge transfer speed. The charge transfer speed is related the ohmic losses, cell voltage, and energy efficiency. The vanadium can be dissolved in strong acidic aqueous solution; therefore the electrode should be chemically stable in strong acidic condition. The chemical stability of the electrode in acid electrolyte is related to the corrosion resistance when oxygen is generated at the positive electrode during overcharged and determines the lifetime of VRFB. The porosity of the electrode affects the pumping energy loss, which affects pressure drop across the

The electrode provides the active sites for the redox reaction of redox couples dissolved in the electrolyte notwithstanding the electrode itself does not participate

Various carbon materials including carbon felt, graphite felt, and carbon paper have been extensively studied as electrodes for VRFB. Especially, carbon felts are considered to be suitable for use as electrodes of VRFB because of their wide specific surface area, high electrical conductivity, high chemical stability, and wide

Sun and Skyllas-Kazacos reported that the C-OH functional group acts as an active site for oxidation of VO2+ and reduction of V3+ on the surface of the electrode

+

/VO2+ and V2+/V3+

Ð

Ð

<sup>¼</sup> CVO<sup>þ</sup>

CVO<sup>þ</sup>

2

<sup>2</sup> þ CVO<sup>2</sup><sup>þ</sup>

� 100% ¼ CE � VE

Voltage efficiency (VE) is the average discharge voltage to the average charge voltage. Figure 3a shows the charging and discharging curves of VRFB in constant current mode, in which the current is maintained as constant value during chargedischarge cycle. While the current is constant during charge-discharge, the voltage is not constant but gradually changing in the whole cycle. Voltage efficiency represents a measure of electrical resistance loss and the polarization properties of battery. The polarization plot in Figure 3c coincides with the voltage efficiency trend in Figure 3b. Energy efficiency is the ratio of available energy to stored energy, which can be calculated as the product of voltage efficiency and current efficiency. It is important to monitor the charging status of VRFB since especially overcharging the battery results in gas evolution side reactions, cell resistance increase, and capacity loss. Normally, VRFB is operated in charge range of 20–80%. The status of charge (SOC) is defined as the following using the concentrations of

disI tð ÞV tð Þdt

chI tð ÞV tð Þdt � 100%

(29)

(30)

Figure 3.

Vanadium redox flow battery performance: (a) cell voltage and open-circuit voltage profiles at current density of 60 mA/cm<sup>2</sup> , (b) efficiencies depending on current densities, (c) polarization plot of the unit cell, and (d) energy density and power density.

Ntank is a number of tank. There are only three variables that contribute to increasing energy density and power density: the vanadium ion concentration, discharging voltage, and current density. However, the concentration of the vanadium ions is limited by low solubilities of vanadium ions in aqueous solution. The discharging voltage and current density are restricted by the electrochemical activities of vanadium electrolyte. Figure 3d shows that as a current density increases, energy density decreases, and power density increases. Normal operating current density range is 50–80 mA/cm<sup>2</sup> , and stored energy density is in the range of 25–35 Wh/L or 20–32 Wh/kg. The corresponding power density is less than 0.1 W/cm<sup>2</sup> .

The performance of VRFB can be measured with three efficiencies: current efficiency, voltage efficiency, and energy efficiency, which are defined in Eqs. (27), (28), and (29), respectively. The current efficiency (CE, Coulombic efficiency) is defined as the ratio of the amount of usable charge to the stored charge amount, that is, the discharge capacity divided by the charge capacity. CE is a measure of the storage capacity loss during charge-discharge process. The capacity loss is mainly caused by the crossover of the electrolyte ions through the membrane. The mixed active materials result in a capacity imbalance between the anode and cathode electrolytes and an irreversible capacity loss.

$$\begin{split} \text{CE} &= \frac{\text{discharge capacity}}{\text{charge capacity}} \times 100\text{\%} = \frac{\int\_{di} I(t)dt}{\int\_{ch} I(t)dt} \times 100\text{\%} = \frac{I\_{dis} \bullet t\_{dis}}{I\_{ch} \bullet t\_{ch}} \times 100\text{\%} \\ &= \frac{t\_{dis}}{t\_{ch}} \times 100\text{\% (If } I\_{dis} = I\_{ch} \text{ )} \end{split} \tag{27}$$
 
$$\text{VE} = \frac{\text{average discharge voltage}}{\text{average charge voltage}} \times 100\text{\%} = \frac{\int\_{di} V(t)dt \Big/\_{t\_{dis}}}{\int\_{di} V(t)dt \Big/\_{t\_{dis}}} \times 100\text{\%} \tag{28}$$

Vanadium Redox Flow Batteries: Electrochemical Engineering DOI: http://dx.doi.org/10.5772/intechopen.85166

$$\begin{split} EE &= \frac{\text{discharge energy } (Wh)}{\text{charge energy } (Wh)} \times 100\% = \frac{\int\_{di} I(t)V(t)dt}{\int\_{ch} I(t)V(t)dt} \times 100\% \\ &= \frac{I\_{di} \bullet t\_{di} \int\_{di} \int\_{di}^{V(t)dt} /\_{t\_{di}}}{I\_{ch} \bullet t\_{ch} \int\_{d}^{V(t)dt} /\_{t\_{di}}} \times 100\% = CE \times VE \end{split} \tag{29}$$

Voltage efficiency (VE) is the average discharge voltage to the average charge voltage. Figure 3a shows the charging and discharging curves of VRFB in constant current mode, in which the current is maintained as constant value during chargedischarge cycle. While the current is constant during charge-discharge, the voltage is not constant but gradually changing in the whole cycle. Voltage efficiency represents a measure of electrical resistance loss and the polarization properties of battery. The polarization plot in Figure 3c coincides with the voltage efficiency trend in Figure 3b. Energy efficiency is the ratio of available energy to stored energy, which can be calculated as the product of voltage efficiency and current efficiency.

It is important to monitor the charging status of VRFB since especially overcharging the battery results in gas evolution side reactions, cell resistance increase, and capacity loss. Normally, VRFB is operated in charge range of 20–80%. The status of charge (SOC) is defined as the following using the concentrations of vanadium ions [8, 9]:

$$\text{SOC} = \frac{\text{C}\_{V^{2+}}}{\text{C}\_{V^{2+}} + \text{C}\_{V^{3+}}} = \frac{\text{C}\_{VO\_2^{+}}}{\text{C}\_{VO\_2^{+}} + \text{C}\_{VO^{2+}}} \tag{30}$$

#### 3. Electrode

Ntank is a number of tank. There are only three variables that contribute to increasing energy density and power density: the vanadium ion concentration, discharging voltage, and current density. However, the concentration of the vanadium ions is limited by low solubilities of vanadium ions in aqueous solution. The discharging voltage and current density are restricted by the electrochemical activities of vanadium electrolyte. Figure 3d shows that as a current density increases, energy density decreases, and power density increases. Normal operating current density

Vanadium redox flow battery performance: (a) cell voltage and open-circuit voltage profiles at current density

, (b) efficiencies depending on current densities, (c) polarization plot of the unit cell, and (d)

The performance of VRFB can be measured with three efficiencies: current efficiency, voltage efficiency, and energy efficiency, which are defined in Eqs. (27), (28), and (29), respectively. The current efficiency (CE, Coulombic efficiency) is defined as the ratio of the amount of usable charge to the stored charge amount, that is, the discharge capacity divided by the charge capacity. CE is a measure of the storage capacity loss during charge-discharge process. The capacity loss is mainly caused by the crossover of the electrolyte ions through the membrane. The mixed active materials result in a capacity imbalance between the anode and cathode

> Ð disI tð Þdt Ð

20–32 Wh/kg. The corresponding power density is less than 0.1 W/cm<sup>2</sup>

, and stored energy density is in the range of 25–35 Wh/L or

chI tð Þdt � 100% <sup>¼</sup> Idis <sup>∙</sup>tdis

Ð

disV tð Þdt=tdis <sup>Ð</sup> chV tð Þdt=tch

Ich ∙tch

.

� 100%

� 100% (28)

(27)

range is 50–80 mA/cm<sup>2</sup>

energy density and power density.

Energy Storage Devices

Figure 3.

of 60 mA/cm<sup>2</sup>

electrolytes and an irreversible capacity loss.

charge capacity � 100% <sup>¼</sup>

� 100% If ð Þ Idis ¼ Ich

VE <sup>¼</sup> average discharge voltage

average charge voltage � 100% <sup>¼</sup>

CE <sup>¼</sup> discharge capacity

<sup>¼</sup> tdis tch

120

The electrode provides the active sites for the redox reaction of redox couples dissolved in the electrolyte notwithstanding the electrode itself does not participate in the reaction. The electrode material influences the performance of VRFB diversely. The electrode should be electrochemically stable in the operating potential window of VRFB. The electrochemical activity of electrode affects the chargedischarge voltages and consequently the voltage efficiency during battery cycle operation. The electrode must have high electrical conductivity to increase the charge transfer speed. The charge transfer speed is related the ohmic losses, cell voltage, and energy efficiency. The vanadium can be dissolved in strong acidic aqueous solution; therefore the electrode should be chemically stable in strong acidic condition. The chemical stability of the electrode in acid electrolyte is related to the corrosion resistance when oxygen is generated at the positive electrode during overcharged and determines the lifetime of VRFB. The porosity of the electrode affects the pumping energy loss, which affects pressure drop across the stack and overall battery system efficiency [15, 16].

#### 3.1 Reaction mechanism at carbon felt electrode

Various carbon materials including carbon felt, graphite felt, and carbon paper have been extensively studied as electrodes for VRFB. Especially, carbon felts are considered to be suitable for use as electrodes of VRFB because of their wide specific surface area, high electrical conductivity, high chemical stability, and wide operating potential window.

Sun and Skyllas-Kazacos reported that the C-OH functional group acts as an active site for oxidation of VO2+ and reduction of V3+ on the surface of the electrode [17, 18]. Oxidation and reduction mechanisms of the VO2 + /VO2+ and V2+/V3+

redox couples at the electrode surface can be explained in three steps as shown in Figure 4. At first step of charge process, the vanadium ions are diffused from the bulk electrolytes to the vicinity of the electrodes and absorbed on the surface of the electrodes. The absorbed vanadium ions are connected to the electrode through the exchange with functional group hydrogen ions. In the second step, the electron and oxygen transfer reactions occur in the VO2 + /VO2+ redox couple, and only the electron transfer reaction occurs in the V2+/V3+ redox couple. At the positive electrode, an oxygen atom of C-O functional group moves to the VO2+, and an electron of the VO2+ is transferred to the electrode following the C-O-V bond, and the oxidation number of vanadium ion increases from +4 to +5. At the negative electrode, an electron is transferred from the electrode to the V3+ along the C-O-V

bond, and the oxidation number of vanadium ion is reduced from +3 to +2. In the third step, the ion exchange process between the V ion attached to the electrode surface and the H+ ion in the electrolyte occurs, and the produced reactants (VO2

To improve the electrochemical performance of VRFB, it is necessary to promote the reaction kinetics of vanadium ion redox couples. For this purpose, the electrode should have high electrical conductivity and the sufficient amount of

Cyclic voltammetry (CV) is used to monitor the reaction rates of redox couples

and to evaluate the electrode performance of flow batteries. The CV curves in Figure 5 show the electrode characteristics of the VRFB cell. The negative potential region of CV indicates the redox reaction of V2+/V3+ ions, and the positive potential

+

Figure 5a compares the electrode characteristics of the standard sulfuric acid electrolyte and the mixed acid electrolyte containing 6 M Cl. The peak current of the vanadium redox reaction is higher in the mixed electrolyte than in the standard sulfuric acid solution. This indicates that the reaction kinetics is improved due to the excellent fluidity of the electrolyte by adding sulfate chloride. The reaction voltage of the redox couples in the mixed solution increases slightly comparing to the sulfate solution, but there is no significant difference in the electrochemical

Figure 5b shows the reaction characteristics of carbon paper and catalytic behavior of biomass-derived activated carbon (AC) in the vanadium electrolyte. The V3+/ VO2+ redox couple peaks appear clearly in AC-coated carbon paper CV curve, and

Park et al. [21] investigated the change of VRFB performance according to the compression ratio of the carbon felt electrode and suggested the optimal compression ratio of the electrode. Oh et al. [22] conducted a numerical study of the VRFB model to investigate the effect of electrode compression on the charging and discharging behavior of VRFB. Yoon et al. [23] studied the flow distribution depending on local porosity of the electrode both numerically and experimentally.

(a) Cyclic voltammograms on a graphite felt electrode of a standard sulfate VRFB electrolyte (1.5 M V4+ and

0.5 mV/s. Reproduced with permission with [19]. Copyright 2011 Wiley. (b) Cyclic voltammograms on Toray carbon sheets with and without mesoporous AC loading in the presence of 1.7 M V3.5+ in 4 M H2SO4 solutions

<sup>2</sup>, and 6 M Cl) at a scan rate of

<sup>2</sup>) and a mixed electrolyte solution (2.5 M V4+, 2.5 M SO4

at a scan rate of 5 mV/s. Reproduced with permission from [20]. Copyright 2015 Elsevier.

/VO2+ ions in electrolyte.

and V2+) diffuse back into the originated electrolytes, respectively.

reversibility between the sulfuric acid and the mixed electrolyte.

these multivalent peaks reveal the superior catalytic activity of AC coating.

oxygen and nitrogen functional groups at the surface.

Vanadium Redox Flow Batteries: Electrochemical Engineering

DOI: http://dx.doi.org/10.5772/intechopen.85166

3.2 Electrochemical characters

Figure 5.

5.0 M SO4

123

region implies the redox reaction of VO2

+

#### Figure 4.

Schematic illustration of the redox reaction mechanism for (a) VO2 + /VO2+ redox couples in the catholyte and (b) V2+/V3+ redox couple in the anolyte on the surface of the carbon felt electrode in VRFB. Reproduced with permission from [16]. Copyright 2015 by the Royal Society of Chemistry.

#### Vanadium Redox Flow Batteries: Electrochemical Engineering DOI: http://dx.doi.org/10.5772/intechopen.85166

bond, and the oxidation number of vanadium ion is reduced from +3 to +2. In the third step, the ion exchange process between the V ion attached to the electrode surface and the H+ ion in the electrolyte occurs, and the produced reactants (VO2 + and V2+) diffuse back into the originated electrolytes, respectively.

To improve the electrochemical performance of VRFB, it is necessary to promote the reaction kinetics of vanadium ion redox couples. For this purpose, the electrode should have high electrical conductivity and the sufficient amount of oxygen and nitrogen functional groups at the surface.

#### 3.2 Electrochemical characters

redox couples at the electrode surface can be explained in three steps as shown in Figure 4. At first step of charge process, the vanadium ions are diffused from the bulk electrolytes to the vicinity of the electrodes and absorbed on the surface of the electrodes. The absorbed vanadium ions are connected to the electrode through the exchange with functional group hydrogen ions. In the second step, the electron

electron transfer reaction occurs in the V2+/V3+ redox couple. At the positive electrode, an oxygen atom of C-O functional group moves to the VO2+, and an electron of the VO2+ is transferred to the electrode following the C-O-V bond, and the oxidation number of vanadium ion increases from +4 to +5. At the negative electrode, an electron is transferred from the electrode to the V3+ along the C-O-V

+

/VO2+ redox couple, and only the

and oxygen transfer reactions occur in the VO2

Energy Storage Devices

Figure 4.

122

Schematic illustration of the redox reaction mechanism for (a) VO2

permission from [16]. Copyright 2015 by the Royal Society of Chemistry.

+

(b) V2+/V3+ redox couple in the anolyte on the surface of the carbon felt electrode in VRFB. Reproduced with

/VO2+ redox couples in the catholyte and

Cyclic voltammetry (CV) is used to monitor the reaction rates of redox couples and to evaluate the electrode performance of flow batteries. The CV curves in Figure 5 show the electrode characteristics of the VRFB cell. The negative potential region of CV indicates the redox reaction of V2+/V3+ ions, and the positive potential region implies the redox reaction of VO2 + /VO2+ ions in electrolyte.

Figure 5a compares the electrode characteristics of the standard sulfuric acid electrolyte and the mixed acid electrolyte containing 6 M Cl. The peak current of the vanadium redox reaction is higher in the mixed electrolyte than in the standard sulfuric acid solution. This indicates that the reaction kinetics is improved due to the excellent fluidity of the electrolyte by adding sulfate chloride. The reaction voltage of the redox couples in the mixed solution increases slightly comparing to the sulfate solution, but there is no significant difference in the electrochemical reversibility between the sulfuric acid and the mixed electrolyte.

Figure 5b shows the reaction characteristics of carbon paper and catalytic behavior of biomass-derived activated carbon (AC) in the vanadium electrolyte. The V3+/ VO2+ redox couple peaks appear clearly in AC-coated carbon paper CV curve, and these multivalent peaks reveal the superior catalytic activity of AC coating.

Park et al. [21] investigated the change of VRFB performance according to the compression ratio of the carbon felt electrode and suggested the optimal compression ratio of the electrode. Oh et al. [22] conducted a numerical study of the VRFB model to investigate the effect of electrode compression on the charging and discharging behavior of VRFB. Yoon et al. [23] studied the flow distribution depending on local porosity of the electrode both numerically and experimentally.

#### Figure 5.

(a) Cyclic voltammograms on a graphite felt electrode of a standard sulfate VRFB electrolyte (1.5 M V4+ and 5.0 M SO4 <sup>2</sup>) and a mixed electrolyte solution (2.5 M V4+, 2.5 M SO4 <sup>2</sup>, and 6 M Cl) at a scan rate of 0.5 mV/s. Reproduced with permission with [19]. Copyright 2011 Wiley. (b) Cyclic voltammograms on Toray carbon sheets with and without mesoporous AC loading in the presence of 1.7 M V3.5+ in 4 M H2SO4 solutions at a scan rate of 5 mV/s. Reproduced with permission from [20]. Copyright 2015 Elsevier.

#### Figure 6.

(a) Specific resistance and porosity vs. percentage of compression for FA-30A carbon felt electrodes and (b) polarization curves of VRFB cells with electrodes of various levels of compression. Reproduced with permission from [21]. Copyright 2014 Elsevier.

As the percentage of electrode compression increases, the specific resistance and porosity of the electrode decrease as shown in Figure 6a. Compressed electrodes with reduced resistivity promote electron transfer, which increases the discharge time and maximum power of the VRFB cell and significantly increases VRFB performance efficiencies and discharge capacities, especially under high current density (Figure 6b). However, decreased porosity reduces the electrolyte flow passages through the electrode and increases pumping losses. The energy efficiency of the battery increases with increasing electrode compression ratio of up to 20%. When the carbon felt electrode is compressed more than 20%, the energy efficiency can be reduced due to the combined effect of deteriorated electrolyte transport and enhanced electron transfer. Overall, it can be concluded that the compression of the carbon felt electrode has a positive effect on cell performance, and the compression ratio optimization can generate significant improvement of VRFB performance without additional cost.

explained as the increase in capacity of the VRFB, which means that the battery can store more energy. Figure 7b shows SOC increasing corresponding to the flow rate increase. On this basis, it is clear that a large mass flow rate can enhance the utilization of vanadium ions. This result explains the increase in the VFB capacity as the stoichiometric number increases. The variation of the efficiencies according to the flow rate is shown in Figure 7c and similar to the efficiency behavior according

Current density of 75 mA/cm<sup>2</sup> at various flow rates; (a) charge–discharge curve, (b) SOC, and (c) efficiencies as a function of stoichiometric number (λ ¼ Qreal =Qtheo ). Reproduced with permission from [8]. Copyright

Vanadium Redox Flow Batteries: Electrochemical Engineering

DOI: http://dx.doi.org/10.5772/intechopen.85166

Flow patterns of RFB can be categorized into two types: "flow-through" type without flow field and "flow-by" type which has a flow field design on the bipolar plate. Leung et al. [25] explained that the structure in which the flow direction is parallel to the current direction is "flow-through" type and the structure in which the flow direction is perpendicular to the current direction is "flow-by" type. However, this definition does not match the concept we are dealing with here. In the scheme described here, the directions of electrolyte flow and electric current are perpendicular to each other in both "flow-through" and "flow-by" configurations. Figure 8 shows the flow battery stack configuration and conceptual schematics of both flow designs. The classical "flow-through" type is the configuration in which the electrolyte flows through the porous carbon felt electrode. A "flow-by" type is the structure in which the electrolyte flows by the surface of an electrode following the flow field at the bipolar plate like a fuel cell. A "flow-by" type can choose relatively thinner carbon felt or carbon paper as an electrode material. Zawodzinski's group first reported better electrochemical performance and improved limiting current density and peak power density of VRFB with a "zerogap" serpentine flow field design comparing to "flow-through" configuration [29]. This results from reduced ohmic loss and enhanced localized mass transfer due to thinner thickness and larger surface area-to-volume ratio of carbon paper used as electrode than those of carbon felt. Elgammal et al. [30] achieved normalized limiting current density of 2961 mA/cm<sup>2</sup> mol and peak power density of 2588 mW/ cm2 of VRFB with serpentine flow field. However, "flow-through" configuration distributes the electrolyte flow more uniformly and results in less pressure drops

The electrolyte flow behavior is indicated schematically in Figure 9. The electrolyte is flowing mainly following channel over the electrode and partly penetrating into the porous electrode forced by pressure gradient. The flow velocity through the porous carbon media is lower than mean velocity of fully developed channel flow. The amount of the electrolyte penetrated into the porous electrode is associated with the stoichiometric availability of electrolyte reactants and the battery

to the current density.

Figure 7.

2018 Elsevier.

performance.

125

and pumping losses than "flow-by" configuration.

#### 4. Electrolyte flow

The flow characteristics have a significant effect on the performance of redox flow battery. The flow distribution is related to the supply of reactant and participation of active species in redox reaction. The uniform flow distribution represents the uniform current density distribution. If the electrolyte flows nonuniformly, the reactants are not fully employed to the electrochemical reaction, which will lead to the degradation of the VRFB performance and durability.

Electrolyte flow rate is the speed of supplying reactants to the active site of electrode. If the flow rate is not enough, the capacity of the electrolytes is not fully utilized. If the flow rate is too high, the pumping loss increases, and the overall system efficiency is reduced accordingly. Therefore, optimizing flow rate is necessary in VRFB operation, and the importance increases significantly as storing capacity increases. The theoretical flow rate can be calculated as below [8]:

$$Q\_{hoo} = \frac{I}{n \times F \times \text{C} \times \text{SOC}\_{min}} \tag{31}$$

where I is the current; n is the number of electrons transferred during the reaction, which is 1 for VFB; C is the total vanadium concentration for each reservoir (1.6 M); and SOCmin is the minimum state of charge, which is 20% normally.

The stoichiometric number, λ, is defined as the ratio of the actual flow rate to theoretical flow rate. Figure 7 shows that as the stoichiometric number increases, the charge-discharge cycle time increases. The extension of the cycle time can be

Vanadium Redox Flow Batteries: Electrochemical Engineering DOI: http://dx.doi.org/10.5772/intechopen.85166

Figure 7.

As the percentage of electrode compression increases, the specific resistance and porosity of the electrode decrease as shown in Figure 6a. Compressed electrodes with reduced resistivity promote electron transfer, which increases the discharge time and maximum power of the VRFB cell and significantly increases VRFB performance efficiencies and discharge capacities, especially under high current density (Figure 6b). However, decreased porosity reduces the electrolyte flow passages through the electrode and increases pumping losses. The energy efficiency of the battery increases with increasing electrode compression ratio of up to 20%. When the carbon felt electrode is compressed more than 20%, the energy efficiency can be reduced due to the combined effect of deteriorated electrolyte transport and enhanced electron transfer. Overall, it can be concluded that the compression of the carbon felt electrode has a positive effect on cell performance, and the compression ratio optimization can generate significant improvement of VRFB

(a) Specific resistance and porosity vs. percentage of compression for FA-30A carbon felt electrodes and (b) polarization curves of VRFB cells with electrodes of various levels of compression. Reproduced with permission

The flow characteristics have a significant effect on the performance of redox flow battery. The flow distribution is related to the supply of reactant and participation of active species in redox reaction. The uniform flow distribution represents the uniform current density distribution. If the electrolyte flows nonuniformly, the reactants are not fully employed to the electrochemical reaction, which will lead

Electrolyte flow rate is the speed of supplying reactants to the active site of electrode. If the flow rate is not enough, the capacity of the electrolytes is not fully utilized. If the flow rate is too high, the pumping loss increases, and the overall system efficiency is reduced accordingly. Therefore, optimizing flow rate is necessary in VRFB operation, and the importance increases significantly as storing capacity increases. The theoretical flow rate can be calculated as below [8]:

n � F � C � SOCmin

(31)

Qtheo <sup>¼</sup> <sup>I</sup>

where I is the current; n is the number of electrons transferred during the reaction, which is 1 for VFB; C is the total vanadium concentration for each reservoir (1.6 M); and SOCmin is the minimum state of charge, which is 20% normally. The stoichiometric number, λ, is defined as the ratio of the actual flow rate to theoretical flow rate. Figure 7 shows that as the stoichiometric number increases, the charge-discharge cycle time increases. The extension of the cycle time can be

to the degradation of the VRFB performance and durability.

performance without additional cost.

4. Electrolyte flow

124

Figure 6.

from [21]. Copyright 2014 Elsevier.

Energy Storage Devices

Current density of 75 mA/cm<sup>2</sup> at various flow rates; (a) charge–discharge curve, (b) SOC, and (c) efficiencies as a function of stoichiometric number (λ ¼ Qreal =Qtheo ). Reproduced with permission from [8]. Copyright 2018 Elsevier.

explained as the increase in capacity of the VRFB, which means that the battery can store more energy. Figure 7b shows SOC increasing corresponding to the flow rate increase. On this basis, it is clear that a large mass flow rate can enhance the utilization of vanadium ions. This result explains the increase in the VFB capacity as the stoichiometric number increases. The variation of the efficiencies according to the flow rate is shown in Figure 7c and similar to the efficiency behavior according to the current density.

Flow patterns of RFB can be categorized into two types: "flow-through" type without flow field and "flow-by" type which has a flow field design on the bipolar plate. Leung et al. [25] explained that the structure in which the flow direction is parallel to the current direction is "flow-through" type and the structure in which the flow direction is perpendicular to the current direction is "flow-by" type. However, this definition does not match the concept we are dealing with here. In the scheme described here, the directions of electrolyte flow and electric current are perpendicular to each other in both "flow-through" and "flow-by" configurations. Figure 8 shows the flow battery stack configuration and conceptual schematics of both flow designs. The classical "flow-through" type is the configuration in which the electrolyte flows through the porous carbon felt electrode. A "flow-by" type is the structure in which the electrolyte flows by the surface of an electrode following the flow field at the bipolar plate like a fuel cell. A "flow-by" type can choose relatively thinner carbon felt or carbon paper as an electrode material. Zawodzinski's group first reported better electrochemical performance and improved limiting current density and peak power density of VRFB with a "zerogap" serpentine flow field design comparing to "flow-through" configuration [29]. This results from reduced ohmic loss and enhanced localized mass transfer due to thinner thickness and larger surface area-to-volume ratio of carbon paper used as electrode than those of carbon felt. Elgammal et al. [30] achieved normalized limiting current density of 2961 mA/cm<sup>2</sup> mol and peak power density of 2588 mW/ cm2 of VRFB with serpentine flow field. However, "flow-through" configuration distributes the electrolyte flow more uniformly and results in less pressure drops and pumping losses than "flow-by" configuration.

The electrolyte flow behavior is indicated schematically in Figure 9. The electrolyte is flowing mainly following channel over the electrode and partly penetrating into the porous electrode forced by pressure gradient. The flow velocity through the porous carbon media is lower than mean velocity of fully developed channel flow. The amount of the electrolyte penetrated into the porous electrode is associated with the stoichiometric availability of electrolyte reactants and the battery performance.

#### Figure 8.

(a) Schematic of flow battery stack configuration. Reproduced with permission from [31]. Copyright 2015 by Elsevier. (b) Bipolar plate and two-dimensional configuration of "flow-through" design and (c) "flow-by" design. Reproduced with permission from [26]. Copyright 2018 by the Royal Society of Chemistry.

Limiting current density is a key factor evaluating flow battery performance. High current density allows fast electrochemical reactions and reduces charging time. Newman et al. developed the limiting current density model as below [6]:

$$i\_{lim} = 0.9783 \frac{nF D c}{L} \int\_0^L \left(\frac{u\_f}{hDX}\right)^{\frac{1}{3}} dX \tag{32}$$

imax <sup>¼</sup> nFcQp

(a) Diagram of electrolyte flow through a single flow channel and over the porous electrode in RFBs, (b) twodimensional flow distributions in the flow channel-porous electrode layered system, and (c) the case of current density limited by the diffusion boundary layer formed between one flat plate and one electrode, which does not allow electrolyte reactant penetration. (d) the case of current density limited by the stoichiometric availability of the electrolyte reactants penetrate through the porous electrode from the flow channel. Reproduced with

where Qp is the volumetric flow of electrolyte reactant penetration through the interface between the flow channel and porous electrode and A is the cross-section

The entrance flow rate of "flow-by" type is higher than "flow-through" type. If entrance flow rate is increased, the penetrating electrolyte flow into the porous electrode is increased because the diffusion boundary layer is decreased, and the

Zawodzinski et al. have shown how the discharge polarization curves of VRFB behave with the flow field and flow rate variations [28]. The flow-through type shows a limiting current density of 165 mA/cm <sup>2</sup> at an electrolyte circulation rate of 30 ml/min

(a) Discharging polarization curve of the flow-through type VRFB (0.5 M V/2.0 M H2SO4 electrolyte with 30 ml/min) and (b) iR free discharge polarization curves illustrating the effect of the electrolyte flow rate on flow-by type VRFB (1.0 M V/5.0 M H2SO4 electrolyte). Reproduced with permission from [28]. Copyright

area of porous electrode that is perpendicular to the current direction.

maximum current density is increased according to Eq. (33).

permission from [26]. Copyright 2018 by the Royal Society of Chemistry.

Vanadium Redox Flow Batteries: Electrochemical Engineering

DOI: http://dx.doi.org/10.5772/intechopen.85166

Figure 9.

Figure 10.

127

2011 by springer.

<sup>A</sup> (33)

where n is the number of electrons transferred during reactions, D is the diffusion coefficient, c is the bulk electrolyte concentration, L is the length of the flow channel, uf is the averaged electrolyte flow velocity along the flow channel, and h is the distance between one electrode and one flat plate. Newman's model predicts the limiting current density of an electrolyte flowing between one flat plate and one electrode as shown in Figure 9c assuming no electrolyte penetration into the electrode surface.

The limiting current density dominated by the stoichiometric availability of reactant in the porous electrode as shown in Figure 9d is called "maximum current density" and can be expressed in Eq. (33) [26, 27]:

Vanadium Redox Flow Batteries: Electrochemical Engineering DOI: http://dx.doi.org/10.5772/intechopen.85166

#### Figure 9.

(a) Diagram of electrolyte flow through a single flow channel and over the porous electrode in RFBs, (b) twodimensional flow distributions in the flow channel-porous electrode layered system, and (c) the case of current density limited by the diffusion boundary layer formed between one flat plate and one electrode, which does not allow electrolyte reactant penetration. (d) the case of current density limited by the stoichiometric availability of the electrolyte reactants penetrate through the porous electrode from the flow channel. Reproduced with permission from [26]. Copyright 2018 by the Royal Society of Chemistry.

$$i\_{\text{max}} = \frac{nFcQ\_p}{A} \tag{33}$$

where Qp is the volumetric flow of electrolyte reactant penetration through the interface between the flow channel and porous electrode and A is the cross-section area of porous electrode that is perpendicular to the current direction.

The entrance flow rate of "flow-by" type is higher than "flow-through" type. If entrance flow rate is increased, the penetrating electrolyte flow into the porous electrode is increased because the diffusion boundary layer is decreased, and the maximum current density is increased according to Eq. (33).

Zawodzinski et al. have shown how the discharge polarization curves of VRFB behave with the flow field and flow rate variations [28]. The flow-through type shows a limiting current density of 165 mA/cm <sup>2</sup> at an electrolyte circulation rate of 30 ml/min

#### Figure 10.

Limiting current density is a key factor evaluating flow battery performance. High current density allows fast electrochemical reactions and reduces charging time. Newman et al. developed the limiting current density model as below [6]:

(a) Schematic of flow battery stack configuration. Reproduced with permission from [31]. Copyright 2015 by Elsevier. (b) Bipolar plate and two-dimensional configuration of "flow-through" design and (c) "flow-by" design. Reproduced with permission from [26]. Copyright 2018 by the Royal Society of Chemistry.

L

where n is the number of electrons transferred during reactions, D is the diffusion coefficient, c is the bulk electrolyte concentration, L is the length of the flow channel, uf is the averaged electrolyte flow velocity along the flow channel, and h is the distance between one electrode and one flat plate. Newman's model predicts the limiting current density of an electrolyte flowing between one flat plate and one electrode as shown in Figure 9c assuming no electrolyte penetration into the

The limiting current density dominated by the stoichiometric availability of reactant in the porous electrode as shown in Figure 9d is called "maximum current

ð L

uf hDX � �<sup>1</sup> 3

dX (32)

0

ilim <sup>¼</sup> <sup>0</sup>:<sup>9783</sup> nFDc

density" and can be expressed in Eq. (33) [26, 27]:

electrode surface.

126

Figure 8.

Energy Storage Devices

(a) Discharging polarization curve of the flow-through type VRFB (0.5 M V/2.0 M H2SO4 electrolyte with 30 ml/min) and (b) iR free discharge polarization curves illustrating the effect of the electrolyte flow rate on flow-by type VRFB (1.0 M V/5.0 M H2SO4 electrolyte). Reproduced with permission from [28]. Copyright 2011 by springer.


#### Table 1.

Comparison of theoretical limiting current density and observed current density in flow-by configuration of VRFB at various electrolyte flow rates. Reproduced with permission from [28]. Copyright 2011 by springer.

(Figure 10a). Figure 10b shows that the limiting current density of the flow-by type increases from 40 to 321 mA/cm <sup>2</sup> as the flow rate increases from 0.5 to 25 ml/min. The values of the theoretical and observed limit current density according to the flow rate are summarized in Table 1. The theoretical limiting current density was calculated by converting the transfer rate of the electrolyte to the bipolar plate into the number of available electrons, assuming that all vanadium was converted in a single pass.

the total cost. Electrolyte accounts for 32% of the total cost, which is the largest portion as a single component. In order to increase the energy content of the flow battery, the additional active material and the tank are required, so that the cost proportion of the electrolyte may increase depending on the storage capacity increase and the fluctuation of vanadium market price. In this analysis, the energy storage cost for VRFB system is presented at € 1078/kWh, which is expected to

10 kW/120 kWh VRFB system cost analysis. Reproduced with permission from [32]. Copyright 2016 by

Vanadium redox flow battery is one of the most promising devices for a large energy storage system to substitute the fossil fuel and nuclear energy with renewable energy. The VRFB is a complicated device that combines all the technologies of electrochemistry, mechanical engineering, polymer science, and materials science similar to the fuel cell. To optimize the flow battery design, it is necessary to understand the flow distribution, local current distribution, limits, and maximum current density. Understanding the shunt current and pressure distribution allows to design the flow battery stack with high power, large capacity, and high system efficiencies. Both experimental and modeling approaches are required to develop advanced vana-

Since Skyllas-Kazacos group at the University of New South Wales invented the VRFB in 1986, many researchers have conducted VRFB research. It is true that the VRFB are closer to commercialization than any other flow batteries. However still many of the reaction mechanisms and material characteristics must be further studied, and it is sure that the vanadium redox flow batteries are still very attractive research topics.

This research was supported by the basic research project of Korea Institute of Science and Technology (KIST) Europe, "Electrochemical energy transformation

dium redox flow battery stacks with high electrochemical performance.

decrease with increasing production quantities.

Vanadium Redox Flow Batteries: Electrochemical Engineering

DOI: http://dx.doi.org/10.5772/intechopen.85166

6. Conclusions

Figure 11.

Noack J. et al.

Acknowledgements

and energy storage".

129

#### 5. Cost analysis

Various batteries compete to become renewable energy storage devices in the power grid. One of the most important factors in practical implementation is the battery installation cost (capital cost). Noack et al. [32] conducted a technoeconomic modeling analysis based on a 10 kW/120 kWh VRFB system. The costs and ratios of each component are summarized in Table 2 and Figure 11, respectively. The largest portion of the VRFB cost is the stack, which accounts for 40% of


#### Table 2.

Cost analysis of 10 kW/120 kWh VRFB system. Reproduced with permission from [32]. Copyright 2016 by Noack J. et al.

#### Vanadium Redox Flow Batteries: Electrochemical Engineering DOI: http://dx.doi.org/10.5772/intechopen.85166

#### Figure 11.

(Figure 10a). Figure 10b shows that the limiting current density of the flow-by type increases from 40 to 321 mA/cm <sup>2</sup> as the flow rate increases from 0.5 to 25 ml/min. The values of the theoretical and observed limit current density according to the flow rate are summarized in Table 1. The theoretical limiting current density was calculated by converting the transfer rate of the electrolyte to the bipolar plate into the number of available electrons, assuming that all vanadium was converted in a single pass.

Comparison of theoretical limiting current density and observed current density in flow-by configuration of VRFB at various electrolyte flow rates. Reproduced with permission from [28]. Copyright 2011 by springer.

Various batteries compete to become renewable energy storage devices in the power grid. One of the most important factors in practical implementation is the battery installation cost (capital cost). Noack et al. [32] conducted a technoeconomic modeling analysis based on a 10 kW/120 kWh VRFB system. The costs and ratios of each component are summarized in Table 2 and Figure 11, respectively. The largest portion of the VRFB cost is the stack, which accounts for 40% of

VRFB system parameter Cost VRFB stack component Cost Electrolyte € 41,000 Bipolar plate € 11,211 Tank € 9082 Felt electrode € 11,047 System assembling € 9000 Frame € 3066 Power electronics € 5000 Membrane € 6656 Fluid components € 3420 Gasket € 16,974 Control engineering € 9160 Assembling € 2782 VRFB stack € 52,646 End plate € 435 VRFB stack specific cost € 5265 /kW Isolation plate € 217 Total system cost € 129,310 Current collector € 141 Total system specific cost € 1078 / kWh Connection € 119

Cost analysis of 10 kW/120 kWh VRFB system. Reproduced with permission from [32]. Copyright

5. Cost analysis

Flow rate (ml/min)

Energy Storage Devices

Table 1.

Theoretical limiting current density (mA/cm<sup>2</sup>

)

0.5 161 40 25.2 643 105 16.3 1287 159 12.4 2573 209 8.12 3860 250 6.48 5147 261 5.07 6433 306 4.76 8042 321 3.99

Observed limiting current density (mA/cm2

)

Percent of max current

Table 2.

128

2016 by Noack J. et al.

10 kW/120 kWh VRFB system cost analysis. Reproduced with permission from [32]. Copyright 2016 by Noack J. et al.

the total cost. Electrolyte accounts for 32% of the total cost, which is the largest portion as a single component. In order to increase the energy content of the flow battery, the additional active material and the tank are required, so that the cost proportion of the electrolyte may increase depending on the storage capacity increase and the fluctuation of vanadium market price. In this analysis, the energy storage cost for VRFB system is presented at € 1078/kWh, which is expected to decrease with increasing production quantities.

#### 6. Conclusions

Vanadium redox flow battery is one of the most promising devices for a large energy storage system to substitute the fossil fuel and nuclear energy with renewable energy. The VRFB is a complicated device that combines all the technologies of electrochemistry, mechanical engineering, polymer science, and materials science similar to the fuel cell. To optimize the flow battery design, it is necessary to understand the flow distribution, local current distribution, limits, and maximum current density. Understanding the shunt current and pressure distribution allows to design the flow battery stack with high power, large capacity, and high system efficiencies. Both experimental and modeling approaches are required to develop advanced vanadium redox flow battery stacks with high electrochemical performance.

Since Skyllas-Kazacos group at the University of New South Wales invented the VRFB in 1986, many researchers have conducted VRFB research. It is true that the VRFB are closer to commercialization than any other flow batteries. However still many of the reaction mechanisms and material characteristics must be further studied, and it is sure that the vanadium redox flow batteries are still very attractive research topics.

#### Acknowledgements

This research was supported by the basic research project of Korea Institute of Science and Technology (KIST) Europe, "Electrochemical energy transformation and energy storage".

Energy Storage Devices

#### Author details

Sangwon Kim1,2

1 Korea Institute of Science and Technology (KIST) Europe, Saarbrücken, Germany

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DOI: http://dx.doi.org/10.5772/intechopen.85166

Vanadium Redox Flow Batteries: Electrochemical Engineering

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JH, Kim HJ, Han J, et al. Nafion membranes with a sulfonated organic additive for the use in vanadium redox flow batteries. Journal of Applied Polymer Science. 2019;136:47547. DOI:

acsaem.8b01116

[12] Chen C, Henkensmeier D, Kim S, Yoon SJ, Zinkevich T, Indris S. Improved all-vanadium redox flow batteries using catholyte additive and a cross-linked methylated polybenzimidazole membrane. ACS Applied Energy Materials. 2018;1:

batteries: Fundamentals and applications. In: Khalid M, editor. Redox: Principles and Advance Applications. London: InTech; 2017.

pp. 103-118. DOI: 10.5772/

10.1016/j.rser.2016.11.188

[5] Bard AJ, Faulkner LR. Electrochemical Methods:

Hoboken: Wiley; 2000. 832 p

Batteries: Fundamentals and

[8] Kim D, Yoon S, Lee J, Kim S. Parametric study and flow rate optimization of all-vanadium redox flow batteries. Applied Energy. 2018;

228:891-901. DOI: 10.1016/j. apenergy.2018.06.094

2018. 432 p

131

[6] Newman J, Thomas-Alyea KE. Electrochemical Systems. 3rd ed. Hoboken: Wiley; 2004. 647 p

[4] Alotto P, Guarnieri M, Moro F. Redox flow batteries for the storage of renewable energy: A review. Renewable and Sustainable Energy Reviews. 2014; 29:325-335. DOI: 10.1016/j.rser.2013.

Fundamentals and Applications. 2nd ed.

[7] Zhang H, Li X, Zhang J. Redox Flow

Applications. Boca Raton: CRC Press;

[2] Ye R, Henkensmeier D, Yoon SJ, Huang Z, Kim DK, Chang Z, et al. Redox flow batteries for energy storage: A technology review. ASME Journal of Electrochemical Energy Conversion and Storage. 2018;15:010801. DOI: 10.1115/

[3] Choi C, Kim S, Kim R, Choi Y, Kim S, Jung H, et al. A review of vanadium electrolytes for vanadium redox flow batteries. Renewable and Sustainable Energy Reviews. 2017;69:263-274. DOI:

intechopen.68752

1.4037248

08.001

2 Transfercenter Sustainable Electrochemistry, Saarland University, Saarbrücken, Germany

\*Address all correspondence to: sangwon.kim@kist-europe.de

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Vanadium Redox Flow Batteries: Electrochemical Engineering DOI: http://dx.doi.org/10.5772/intechopen.85166

#### References

[1] Chen R, Kim S, Chang Z. Redox flow batteries: Fundamentals and applications. In: Khalid M, editor. Redox: Principles and Advance Applications. London: InTech; 2017. pp. 103-118. DOI: 10.5772/ intechopen.68752

[2] Ye R, Henkensmeier D, Yoon SJ, Huang Z, Kim DK, Chang Z, et al. Redox flow batteries for energy storage: A technology review. ASME Journal of Electrochemical Energy Conversion and Storage. 2018;15:010801. DOI: 10.1115/ 1.4037248

[3] Choi C, Kim S, Kim R, Choi Y, Kim S, Jung H, et al. A review of vanadium electrolytes for vanadium redox flow batteries. Renewable and Sustainable Energy Reviews. 2017;69:263-274. DOI: 10.1016/j.rser.2016.11.188

[4] Alotto P, Guarnieri M, Moro F. Redox flow batteries for the storage of renewable energy: A review. Renewable and Sustainable Energy Reviews. 2014; 29:325-335. DOI: 10.1016/j.rser.2013. 08.001

[5] Bard AJ, Faulkner LR. Electrochemical Methods: Fundamentals and Applications. 2nd ed. Hoboken: Wiley; 2000. 832 p

[6] Newman J, Thomas-Alyea KE. Electrochemical Systems. 3rd ed. Hoboken: Wiley; 2004. 647 p

[7] Zhang H, Li X, Zhang J. Redox Flow Batteries: Fundamentals and Applications. Boca Raton: CRC Press; 2018. 432 p

[8] Kim D, Yoon S, Lee J, Kim S. Parametric study and flow rate optimization of all-vanadium redox flow batteries. Applied Energy. 2018; 228:891-901. DOI: 10.1016/j. apenergy.2018.06.094

[9] Knehr KW, Kumbur EC. Open circuit voltage of vanadium redox flow batteries: Discrepancy between models and experiments. Electrochemistry Communications. 2011;13:342-345. DOI: 10.1016/j.elecom.2011.01.020

[10] Hwang G, Kim S, In D, Lee D, Ryu C. Application of the commercial ion exchange membranes in the allvanadium redox flow battery. Journal of Industrial and Engineering Chemistry. 2018;60:360-365. DOI: 10.1016/j. jiec.2017.11.023

[11] Jung M, Lee W, Krishan NN, Kim S, Gupta G, Komsyska L, et al. Porous-Nafion/PBI composite membranes and Nafion/PBI blend membranes for vanadium redox flow batteries. Applied Surface Science. 2018;450:301-311. DOI: 10.1016/j.apsusc.2018.04.198

[12] Chen C, Henkensmeier D, Kim S, Yoon SJ, Zinkevich T, Indris S. Improved all-vanadium redox flow batteries using catholyte additive and a cross-linked methylated polybenzimidazole membrane. ACS Applied Energy Materials. 2018;1: 6047-6055. DOI: 10.1021/ acsaem.8b01116

[13] Strużyńska-Pirona I, Jung M, Maljuschc A, Conradic O, Kim S, Jang J, et al. Imidazole based ionenes, their blends with PBI-OO and applicability as membrane in a vanadium redox flow battery. European Polymer Journal. 2017;96:383-392. DOI: 10.1016/j. eurpolymj.2017.09.031

[14] Lee Y, Kim S, Hempelmann R, Jang JH, Kim HJ, Han J, et al. Nafion membranes with a sulfonated organic additive for the use in vanadium redox flow batteries. Journal of Applied Polymer Science. 2019;136:47547. DOI: 10.1002/app.47547

Author details

Energy Storage Devices

Sangwon Kim1,2

Germany

130

1 Korea Institute of Science and Technology (KIST) Europe, Saarbrücken, Germany

2 Transfercenter Sustainable Electrochemistry, Saarland University, Saarbrücken,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: sangwon.kim@kist-europe.de

provided the original work is properly cited.

[15] Ulaganathana M, Aravindan V, Yan Q, Madhavi S, Skyllas-Kazacos M, Lim TM. Recent advancements in allvanadium redox flow batteries. Advanced Materials Interfaces. 2015;3: 1500309. DOI: 10.1002/admi.201500309

[16] Kim KJ, Park MS, Kim YJ, Kim JH, Dou SX, Skyllas-Kazacos M. A technology review of electrodes and reaction mechanisms in vanadium redox flow batteries. Journal of Materials Chemistry A. 2015;3:16913-16933. DOI: 10.1039/C5TA02613J

[17] Sum E, Skyllas-Kazacos M. A study of the V(II)/V(III) redox couple for redox flow cell applications. Journal of Power Sources. 1985;15:179-190. DOI: 10.1016/0378-7753(85)80071-9

[18] Sum E, Rychcik M, Skyllas-Kazacos M. Investigation of the V(V)/V(IV) system for use in the positive half-cell of a redox battery. Jounral of Power Sources. 1985;16:85-95. DOI: 10.1016/ 0378-7753(85)80082-3

[19] Li L, Kim S, Wang W, Vijayakumar M, Nie Z, Chen B, et al. A stable vanadium redox-flow battery with high energy density for large-scale energy storage. Applied Energy Materials. 2011; 1:394-400. DOI: 10.1002/aenm. 201100008

[20] Ulaganathana M, Jain A, Aravindan V, Jayaraman S, Ling WC, Lim TM, et al. Bio-mass derived mesoporous carbon as superior electrode in all vanadium redox flow battery with multicouple reactions. Jounral of Power Sources. 2015;15:846-850. DOI: 10.1016/ j.jpowsour.2014.10.176

[21] Park SK, Shim J, Yang JH, Jin CS, Lee BS, Lee YS, et al. The influence of compressed carbon felt electrodes on the performance of a vanadium redox flow battery. Electrochimica Acta. 2014; 116:447-452. DOI: 10.1016/j.electacta. 2013.11.073

[22] Oh K,Won S, Ju H. Numerical study of the effects of carbon felt electrode compression in all-vanadium redox flow batteries. Electrochimica Acta. 2015;181: 13-23. DOI: 10.1016/j.electacta.2015.02.212

vanadium redox flow batteries through modified cell architecture. Journal of Power Sources. 2012;206:450-453. DOI:

DOI: http://dx.doi.org/10.5772/intechopen.85166

Vanadium Redox Flow Batteries: Electrochemical Engineering

10.1016/j.jpowsour.2011.12.026

03.131

[30] Elgammal RA, Tang Z, Sun CN, Lawton J, Zawodzinski TA. Species uptake and mass transport in

membranes for vanadium redox flow batteries. Electrochimica Acta. 2017;237: 1-11. DOI: 10.1016/j.electacta.2017.

[31] Ha S, Gallagher KG. Estimating the system price of redox flow batteries for grid storage. Journal of Power Sources. 2015;296:122-132. DOI: 10.1016/j.

Roznyatovskaya N, Pinkwart K, Tübke J. Techno-economic modeling and analysis of redox flow battery systems. Energies. 2016;9:627. DOI: 10.3390/

jpowsour.2015.07.004

en9080627

133

[32] Noack J, Wietschel L,

[23] Yoon SJ, Kim S, Kim DK. Optimization of local porosity in the electrode as an advanced channel for allvanadium redox flow battery. Energy. 2019;172:26-35. DOI: 10.1016/j. energy.2019.01.101

[24] Weber AZ, Mench MM, Meyers JP, Ross PN, Gostick JT, Liu Q. Redox flow batteries: A review. Journal of Applied Electrochemistry. 2011;41:1137-1164. DOI: 10.1007/s10800-011-0352-6

[25] Leung P, Li X, Leon CP, Berlouis L, Low CTJ, Walsh FC. Progress in redox flow batteries, remaining challenges and their applications in energy storage. RSC Advances. 2012;2:10125-10156. DOI: 10.1039/C2RA21342G

[26] Ke X, Prahl JM, Alexander JID, Wainright JS, Zawodzinski TA, Savinell R. Rechargeable redox flow batteries: Flow fields, stacks and design considerations. Chemical Society Reviews. 2018;47:8721-8743. DOI: 10.1039/C8CS00072G

[27] Ke X, Alexander JID, Prahl JM, Savinell RF. Flow distribution and maximum current density studies in redox flow batteries with a single passage of the serpentine flow channel. Journal of Power Sources. 2014;270: 646-657. DOI: 10.1016/j.jpowsour. 2014.07.155

[28] Aaron A, Tang Z, Papandrew AB, Zawodzinski TA. Polarization curve analysis of all-vanadium redox flow batteries. Journal of Applied Electrochemistry. 2011;41:1175-1182. DOI: 10.1007/s10800-011-0335-7

[29] Aaron DS, Liu Q, Tang Z, Grim GM, Papandrew AB, Turhan A, et al. Dramatic performance gains in

Vanadium Redox Flow Batteries: Electrochemical Engineering DOI: http://dx.doi.org/10.5772/intechopen.85166

vanadium redox flow batteries through modified cell architecture. Journal of Power Sources. 2012;206:450-453. DOI: 10.1016/j.jpowsour.2011.12.026

[15] Ulaganathana M, Aravindan V, Yan Q, Madhavi S, Skyllas-Kazacos M, Lim TM. Recent advancements in allvanadium redox flow batteries. Advanced Materials Interfaces. 2015;3: 1500309. DOI: 10.1002/admi.201500309 [22] Oh K,Won S, Ju H. Numerical study of

[24] Weber AZ, Mench MM, Meyers JP, Ross PN, Gostick JT, Liu Q. Redox flow batteries: A review. Journal of Applied Electrochemistry. 2011;41:1137-1164. DOI: 10.1007/s10800-011-0352-6

[25] Leung P, Li X, Leon CP, Berlouis L, Low CTJ, Walsh FC. Progress in redox flow batteries, remaining challenges and their applications in energy storage. RSC Advances. 2012;2:10125-10156. DOI:

[26] Ke X, Prahl JM, Alexander JID, Wainright JS, Zawodzinski TA, Savinell R. Rechargeable redox flow batteries:

[27] Ke X, Alexander JID, Prahl JM, Savinell RF. Flow distribution and maximum current density studies in redox flow batteries with a single passage of the serpentine flow channel. Journal of Power Sources. 2014;270: 646-657. DOI: 10.1016/j.jpowsour.

[28] Aaron A, Tang Z, Papandrew AB, Zawodzinski TA. Polarization curve analysis of all-vanadium redox flow

Electrochemistry. 2011;41:1175-1182. DOI: 10.1007/s10800-011-0335-7

Papandrew AB, Turhan A, et al. Dramatic performance gains in

[29] Aaron DS, Liu Q, Tang Z, Grim GM,

batteries. Journal of Applied

Flow fields, stacks and design considerations. Chemical Society Reviews. 2018;47:8721-8743. DOI:

the effects of carbon felt electrode compression in all-vanadium redox flow batteries. Electrochimica Acta. 2015;181: 13-23. DOI: 10.1016/j.electacta.2015.02.212

[23] Yoon SJ, Kim S, Kim DK. Optimization of local porosity in the electrode as an advanced channel for allvanadium redox flow battery. Energy. 2019;172:26-35. DOI: 10.1016/j.

energy.2019.01.101

10.1039/C2RA21342G

10.1039/C8CS00072G

2014.07.155

[16] Kim KJ, Park MS, Kim YJ, Kim JH,

[17] Sum E, Skyllas-Kazacos M. A study of the V(II)/V(III) redox couple for redox flow cell applications. Journal of Power Sources. 1985;15:179-190. DOI: 10.1016/0378-7753(85)80071-9

[18] Sum E, Rychcik M, Skyllas-Kazacos M. Investigation of the V(V)/V(IV) system for use in the positive half-cell of a redox battery. Jounral of Power Sources. 1985;16:85-95. DOI: 10.1016/

[19] Li L, Kim S, Wang W, Vijayakumar

[20] Ulaganathana M, Jain A, Aravindan V, Jayaraman S, Ling WC, Lim TM, et al. Bio-mass derived mesoporous carbon as superior electrode in all vanadium redox flow battery with multicouple reactions. Jounral of Power Sources. 2015;15:846-850. DOI: 10.1016/

[21] Park SK, Shim J, Yang JH, Jin CS, Lee BS, Lee YS, et al. The influence of compressed carbon felt electrodes on the performance of a vanadium redox flow battery. Electrochimica Acta. 2014; 116:447-452. DOI: 10.1016/j.electacta.

M, Nie Z, Chen B, et al. A stable vanadium redox-flow battery with high energy density for large-scale energy storage. Applied Energy Materials. 2011;

1:394-400. DOI: 10.1002/aenm.

Dou SX, Skyllas-Kazacos M. A technology review of electrodes and reaction mechanisms in vanadium redox flow batteries. Journal of Materials Chemistry A. 2015;3:16913-16933. DOI:

10.1039/C5TA02613J

Energy Storage Devices

0378-7753(85)80082-3

j.jpowsour.2014.10.176

201100008

2013.11.073

132

[30] Elgammal RA, Tang Z, Sun CN, Lawton J, Zawodzinski TA. Species uptake and mass transport in membranes for vanadium redox flow batteries. Electrochimica Acta. 2017;237: 1-11. DOI: 10.1016/j.electacta.2017. 03.131

[31] Ha S, Gallagher KG. Estimating the system price of redox flow batteries for grid storage. Journal of Power Sources. 2015;296:122-132. DOI: 10.1016/j. jpowsour.2015.07.004

[32] Noack J, Wietschel L, Roznyatovskaya N, Pinkwart K, Tübke J. Techno-economic modeling and analysis of redox flow battery systems. Energies. 2016;9:627. DOI: 10.3390/ en9080627

Chapter 8

Abstract

chapter.

135

quantity of hydrogen produced

Hydrogen Energy Storage

The dominating trend of variable renewable energy sources (RES) continues to underpin the early retirement of baseload power generating sources such as coal, nuclear, and natural gas steam generators; however, the need to maintain system reliability remains the challenge. Implementing energy storage with conventional power plants provides a method for load leveling, peak shaving, and time shifting allowing power quality improvement and reduction in grid energy management issues, implementing energy storage with RES smooth their intermittency, by storing the surplus in their generation for later use during their shortfall, thus enabling their high penetration into the electricity grid. Energy storage technologies (EST) can be classified according to many criteria like their application (permanent or portable), capacity, storage duration (short or long), and size (weight and volume). EST suited for short duration storage and low-to-medium power outputs are seen performing better in improving power quality, while those providing medium-to-high power outputs with long durations are seen better suited for energy management of electrical networks. With the growing deployment of renewable energy systems, EST must be utilized to allow the grid to absorb the increased integration of RES generation. The recent advances in hydrogen energy storage technologies (HEST) have unlocked their potential for use with constrained renewable generation. HEST combines hydrogen production, storage, and end use technologies with the renewable generation either in a directly connected configuration or in an indirectly connected configuration via the existing power network. This chapter introduces the hydrogen energy storage technology and its implementation in conjunction with renewable energy sources. The efficiency of renewable hydrogen energy storage systems (RHESS) will be explored with a techno-economic assessment. A levelized cost (LC) model that identifies the financial competitiveness of HEST in different application scenarios is given, where five scenarios are investigated to demonstrate the most financially competitive configuration. To address the absence of a commercial software tool that can quickly size an energy system incorporating HEST while using limited data, a deterministic modeling approach that enables a quick initial sizing of hybrid renewable hydrogen energy systems (HRHES) is given in this chapter. This modeling approach can achieve the initial sizing of a HRHES using only two input data, namely the available renewable energy resource and the load profile. A modeling of the effect of the electrolyzer thermal transients at start-up, when operated in conjunction with an intermittent renewable generation, on the quantity of hydrogen produced is also given in this

Keywords: hydrogen energy storage technology, renewable hydrogen energy storage systems, levelized cost modeling, sizing hybrid renewable hydrogen energy system for a specified demand and renewable resource, modeling the effect of the electrolyzer thermal transients at start-up when powered by renewables on the

Dallia Mahmoud Morsi Ali

## Chapter 8 Hydrogen Energy Storage

Dallia Mahmoud Morsi Ali

#### Abstract

The dominating trend of variable renewable energy sources (RES) continues to underpin the early retirement of baseload power generating sources such as coal, nuclear, and natural gas steam generators; however, the need to maintain system reliability remains the challenge. Implementing energy storage with conventional power plants provides a method for load leveling, peak shaving, and time shifting allowing power quality improvement and reduction in grid energy management issues, implementing energy storage with RES smooth their intermittency, by storing the surplus in their generation for later use during their shortfall, thus enabling their high penetration into the electricity grid. Energy storage technologies (EST) can be classified according to many criteria like their application (permanent or portable), capacity, storage duration (short or long), and size (weight and volume). EST suited for short duration storage and low-to-medium power outputs are seen performing better in improving power quality, while those providing medium-to-high power outputs with long durations are seen better suited for energy management of electrical networks. With the growing deployment of renewable energy systems, EST must be utilized to allow the grid to absorb the increased integration of RES generation. The recent advances in hydrogen energy storage technologies (HEST) have unlocked their potential for use with constrained renewable generation. HEST combines hydrogen production, storage, and end use technologies with the renewable generation either in a directly connected configuration or in an indirectly connected configuration via the existing power network. This chapter introduces the hydrogen energy storage technology and its implementation in conjunction with renewable energy sources. The efficiency of renewable hydrogen energy storage systems (RHESS) will be explored with a techno-economic assessment. A levelized cost (LC) model that identifies the financial competitiveness of HEST in different application scenarios is given, where five scenarios are investigated to demonstrate the most financially competitive configuration. To address the absence of a commercial software tool that can quickly size an energy system incorporating HEST while using limited data, a deterministic modeling approach that enables a quick initial sizing of hybrid renewable hydrogen energy systems (HRHES) is given in this chapter. This modeling approach can achieve the initial sizing of a HRHES using only two input data, namely the available renewable energy resource and the load profile. A modeling of the effect of the electrolyzer thermal transients at start-up, when operated in conjunction with an intermittent renewable generation, on the quantity of hydrogen produced is also given in this chapter.

Keywords: hydrogen energy storage technology, renewable hydrogen energy storage systems, levelized cost modeling, sizing hybrid renewable hydrogen energy system for a specified demand and renewable resource, modeling the effect of the electrolyzer thermal transients at start-up when powered by renewables on the quantity of hydrogen produced

#### 1. Introduction

The green-house gas emissions associated with conventional electricity generation will lead to an increase in the average global temperature over the upcoming years, which in turn will lead to raised sea levels and more frequent extreme weather conditions and droughts. To mitigate such global climate changes, the world needs an energy transition that allows a cleaner and more sustainable energy supply.

2. Preface: role of energy storage in the world energy transition

ingly address the climate change with its associated political, economic, and

diverted to the EST operational costs [4].

DOI: http://dx.doi.org/10.5772/intechopen.88902

Hydrogen Energy Storage

increased amounts of RES [8].

Energy storage for Load Leveling and Peak Shaving.

Figure 1.

137

as a step toward achieving the energy transition [6].

2.1 The need for energy storage in modern power systems

Twenty-four percent of the global green-house gas emissions are produced from the electrical power generation sector [2]. Implementing RES into the power generation sector can play a vital role in reducing this emission percentage and accord-

environmental pressures [3]. To enable the high penetration of fluctuating RES into the electrical power network, the network should be able to absorb and store the excess in the power fed so that they remain stable and should utilize the RES generation in the most possible effective manner. Delivering the RES generation to the load when needed and to the storage when the generation exceeds consumption allows the absorption of the excess in RES generation and thus reduces the need for the grid weak interconnections upgrading and remove a considerable amount of constraint issues. Additionally, implementing energy storage reduces the spinning reserve requirements and thus allows spinning reserve operational costs to be

The flexibility that the storage brings to the grid reduces the electrical supply and demand imbalance associated with increased RES integration, and thus facilitates energy transition. While the inclusion of energy storage brings some additional capital and operational costs together with energy conversion loss efficiencies [5] and although there is limited experience with implementing large-scale energy storage technologies (except for pumped hydro), the implementation of EST is still considered vital for enabling the projected increase of RES into the power network

Energy storage increases the power grid capacity in accommodating the increasing fluctuations in supply and demand, and thus it plays a crucial role in supporting the wider integration of distributed RES in modern electrical networks [7]. Integrating EST into electrical networks allows more flexibility in accommodating the

The decarbonization of the world's energy system has started in 2015 after the signature of the legally binding agreement by 195 countries to keep the global warming well below 2°C [1]. Since then, significant amounts of RES have been installed and integrated into the grid while securing the supply and the system resilience. Expanding the utilization of RES into the electricity grid however requires large-scale electricity storage to cover for their energy intermittency. Even a small-scale grid that handles only 10–30 gigawatts could not rely entirely on intermittent RES without having a gigawatt-scale storage that can work for many hours; so, for example, securing 3 GW for 2 days requires a 144 gigawatt-hours storage. There are many storage options; some of these are the flow batteries, which store the energy directly in the electrolyte, but are still in an early stage of deployment; sodium-sulfur batteries, with higher energy density than Li-ion ones; however, their hot liquid metal electrolyte is inconvenient; supercapacitors, which cannot provide electricity over a long enough time; and compressed air and flywheels have made it only to small and midsize installations due to location restrictions. Hydrogen energy storage however offers a clean, sustainable, and flexible storage option that can be scaled up to enable large-scale energy storage over long periods of time with no restrictions on location, and therefore it has the potential to enable the energy transition. While the transition toward using more variable RES into the power grid will unbalance the supply and demand, using the excess in the RES power supply in electrolysis to produce hydrogen and store it for future use during RES supply deficit can help balancing the grid. The stored hydrogen can also be used in other sectors like transport, industry, residential heat, etc. Implementing hydrogen energy storage with renewables therefore have the potential to improve the economic efficiency of renewable investments, enhance the security of power supply, and serve as a carbon-free seasonal storage supplying energy when the RES production is low or the energy demand is high.

This chapter explores the context of hydrogen as an energy vector and the role of hydrogen energy storage in the world energy transition. An exploration into the hydrogen technology techno-economic potential, its applications, achievements, and challenges to its deployment as well as recommendations for accelerating its deployment are covered in this chapter. This chapter also includes a model that has been developed to enable the quick sizing of a hybrid renewable hydrogen energy system (HRHES) that integrates solar and wind renewable resources combined with hydrogen energy storage to meet a specific electrical load. The effect of the electrolyzer thermal transients at start-up, when operated in conjunction with the intermittent renewable generation, on the quantity of hydrogen produced is included in the developed model. Implementing the developed model as a tool for identifying the performance issues within installed hydrogen systems during their operation is also given in this chapter. Two case studies are provided to verify the developed model, and to validate that thermally compensated electrolyzer models are essential for designing new hydrogen installations as well as for monitoring the performance issues within running installations.

1. Introduction

Energy Storage Devices

supply.

is high.

installations.

136

The green-house gas emissions associated with conventional electricity generation will lead to an increase in the average global temperature over the upcoming years, which in turn will lead to raised sea levels and more frequent extreme weather conditions and droughts. To mitigate such global climate changes, the world needs an energy transition that allows a cleaner and more sustainable energy

The decarbonization of the world's energy system has started in 2015 after the signature of the legally binding agreement by 195 countries to keep the global warming well below 2°C [1]. Since then, significant amounts of RES have been installed and integrated into the grid while securing the supply and the system resilience. Expanding the utilization of RES into the electricity grid however requires large-scale electricity storage to cover for their energy intermittency. Even a small-scale grid that handles only 10–30 gigawatts could not rely entirely on intermittent RES without having a gigawatt-scale storage that can work for many hours; so, for example, securing 3 GW for 2 days requires a 144 gigawatt-hours storage. There are many storage options; some of these are the flow batteries, which store the energy directly in the electrolyte, but are still in an early stage of deployment; sodium-sulfur batteries, with higher energy density than Li-ion ones; however, their hot liquid metal electrolyte is inconvenient; supercapacitors, which cannot provide electricity over a long enough time; and compressed air and flywheels have made it only to small and midsize installations due to location restrictions. Hydrogen energy storage however offers a clean, sustainable, and flexible storage option that can be scaled up to enable large-scale energy storage over long periods of time with no restrictions on location, and therefore it has the potential to enable the energy transition. While the transition toward using more variable RES into the power grid will unbalance the supply and demand, using the excess in the RES power supply in electrolysis to produce hydrogen and store it for future use during RES supply deficit can help balancing the grid. The stored hydrogen can also be used in other sectors like transport, industry, residential heat, etc. Implementing hydrogen energy storage with renewables therefore have the potential to improve the economic efficiency of renewable investments, enhance the security of power supply, and serve as a carbon-free seasonal storage supplying energy when the RES production is low or the energy demand

This chapter explores the context of hydrogen as an energy vector and the role of hydrogen energy storage in the world energy transition. An exploration into the hydrogen technology techno-economic potential, its applications, achievements, and challenges to its deployment as well as recommendations for accelerating its deployment are covered in this chapter. This chapter also includes a model that has been developed to enable the quick sizing of a hybrid renewable hydrogen energy system (HRHES) that integrates solar and wind renewable resources combined with hydrogen energy storage to meet a specific electrical load. The effect of the electrolyzer thermal transients at start-up, when operated in conjunction with the intermittent renewable generation, on the quantity of hydrogen produced is included in the developed model. Implementing the developed model as a tool for identifying the performance issues within installed hydrogen systems during their operation is also given in this chapter. Two case studies are provided to verify the developed model, and to validate that thermally compensated electrolyzer models are essential for designing new hydrogen installations as well as for monitoring the performance issues within running

### 2. Preface: role of energy storage in the world energy transition

Twenty-four percent of the global green-house gas emissions are produced from the electrical power generation sector [2]. Implementing RES into the power generation sector can play a vital role in reducing this emission percentage and accordingly address the climate change with its associated political, economic, and environmental pressures [3]. To enable the high penetration of fluctuating RES into the electrical power network, the network should be able to absorb and store the excess in the power fed so that they remain stable and should utilize the RES generation in the most possible effective manner. Delivering the RES generation to the load when needed and to the storage when the generation exceeds consumption allows the absorption of the excess in RES generation and thus reduces the need for the grid weak interconnections upgrading and remove a considerable amount of constraint issues. Additionally, implementing energy storage reduces the spinning reserve requirements and thus allows spinning reserve operational costs to be diverted to the EST operational costs [4].

The flexibility that the storage brings to the grid reduces the electrical supply and demand imbalance associated with increased RES integration, and thus facilitates energy transition. While the inclusion of energy storage brings some additional capital and operational costs together with energy conversion loss efficiencies [5] and although there is limited experience with implementing large-scale energy storage technologies (except for pumped hydro), the implementation of EST is still considered vital for enabling the projected increase of RES into the power network as a step toward achieving the energy transition [6].

#### 2.1 The need for energy storage in modern power systems

Energy storage increases the power grid capacity in accommodating the increasing fluctuations in supply and demand, and thus it plays a crucial role in supporting the wider integration of distributed RES in modern electrical networks [7]. Integrating EST into electrical networks allows more flexibility in accommodating the increased amounts of RES [8].

Figure 1. Energy storage for Load Leveling and Peak Shaving.

In power systems, energy storage provides a method for "Load Leveling" by storing the power during periods of light loading and delivering it during periods of high demand, thus avoiding the high costs during peak demand and postponing investments in grid upgrades or in building new generating capacity. It also provides a method for "peak shaving," which works like load leveling but aims to reduce the peak demand. Energy storage can also be used for "time shifting" by storing the energy during low price times, and discharging it during high price times. All these actions allow reduction in the grid's energy management issues and improve the power quality. Figure 1 depicts how energy storage allows load leveling and peak shaving with conventional power plants, and Figure 2 depicts how implementing bulk energy storage with intermittent RES facilitates their high

penetration into the electricity grid through storing the surplus in their energy to be

Energy storage is crucial for many applications and while implementing them in large size at the supply side can assist in the network bulk energy management, implementing them distributed near the consumer can assist in reducing power quality issues. EST energy storage duration ranges from few seconds of operation to several hours [9]. Figure 3 shows a summary of the different storage technologies with the applications in which they are best suited as conducted by the AEA

It can be seen from Figure 3 that energy storage technologies that are best suited for short duration storage and low to medium power outputs are typically seen as performing better in improving the power quality, while EST that provide medium to high power outputs with long durations are better suited for the energy manage-

Traditionally, the electrical network infrastructure has been designed to deliver electricity from several large-scale centralized fossil-fuelled electrical power stations to several domestic, industrial, and commercial consumers through a transmission network that is suited for power flow in one direction from generation to load. The hydrocarbon-fuelled and nuclear power stations are conventionally load following and can adjust their electrical generation output to follow demand, and thus the electrical grid maintains its equilibrium. When the network experiences any imbalance condition, the network operators implement control mechanisms to return the network to its balanced state. So, when generation cannot meet demand,

the spinning reserve is connected to the electrical network and is loaded up according to demand. However, spinning reserve is a highly costly, carbon intensive, and inefficient method for maintaining the network stability as it requires the generating stations to remain running and consuming fuel to be brought online very

The admission of the RES into the grid provides genuinely green energy at the points of entry; however, it creates a huge operational problem in managing the central generators to cover any transient variations in the renewable power input and consumer demand, thus hindering the renewable potential. The power network operators will have to manage the imbalance by either increasing the operation of the spinning reserve leading to increased costs [11], or by implementing other ways different from what they use with traditional power stations [12]. So, owners of RES connected to an increasingly congested electrical transmission and distribution network will have to incur financial penalties when they produce power at times of low demand and the network operators will have to pay compensations to RES owners when they introduce constraints [13]. An example of this is what happened at Scotland in April 2014, when strong winds made the Scottish grid not able to absorb all the wind power generated and had to constrain it off the grid while paying compensations to the owners of wind generators. Approximately £890,000

Given that the global renewable energy contribution (excluding bio-fuels) is predicted to increase by over 50% between 2010 and 2035 [14], the electrical power networks therefore need to have the capacity to store the excess power fed into the grid from fluctuating and intermittent power sources [15]. The electrical power networks also need to continually achieve equilibrium under the increasing demand conditions while reducing their dependence on the expensive and inefficient

quickly when needed to maintain the supply and demand balance.

was paid over few hours to six wind farms.

139

used later to match the fluctuating load demand curve.

DOI: http://dx.doi.org/10.5772/intechopen.88902

technologies for the Scottish Government [10].

3. Context for hydrogen as an energy vector

ment of electrical networks.

Hydrogen Energy Storage

Figure 2.

Energy storage with RES to enable their high grid penetration.

#### Figure 3.

Energy storage technology overview, Scottish Government Report [10]. \* SMES = superconducting magnetic energy storage.

#### Hydrogen Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.88902

In power systems, energy storage provides a method for "Load Leveling" by storing the power during periods of light loading and delivering it during periods of high demand, thus avoiding the high costs during peak demand and postponing investments in grid upgrades or in building new generating capacity. It also provides a method for "peak shaving," which works like load leveling but aims to reduce the peak demand. Energy storage can also be used for "time shifting" by storing the energy during low price times, and discharging it during high price times. All these actions allow reduction in the grid's energy management issues and improve the power quality. Figure 1 depicts how energy storage allows load leveling and peak shaving with conventional power plants, and Figure 2 depicts how implementing bulk energy storage with intermittent RES facilitates their high

Figure 2.

Energy Storage Devices

Figure 3.

138

energy storage.

Energy storage with RES to enable their high grid penetration.

Energy storage technology overview, Scottish Government Report [10]. \*

SMES = superconducting magnetic

penetration into the electricity grid through storing the surplus in their energy to be used later to match the fluctuating load demand curve.

Energy storage is crucial for many applications and while implementing them in large size at the supply side can assist in the network bulk energy management, implementing them distributed near the consumer can assist in reducing power quality issues. EST energy storage duration ranges from few seconds of operation to several hours [9]. Figure 3 shows a summary of the different storage technologies with the applications in which they are best suited as conducted by the AEA technologies for the Scottish Government [10].

It can be seen from Figure 3 that energy storage technologies that are best suited for short duration storage and low to medium power outputs are typically seen as performing better in improving the power quality, while EST that provide medium to high power outputs with long durations are better suited for the energy management of electrical networks.

#### 3. Context for hydrogen as an energy vector

Traditionally, the electrical network infrastructure has been designed to deliver electricity from several large-scale centralized fossil-fuelled electrical power stations to several domestic, industrial, and commercial consumers through a transmission network that is suited for power flow in one direction from generation to load. The hydrocarbon-fuelled and nuclear power stations are conventionally load following and can adjust their electrical generation output to follow demand, and thus the electrical grid maintains its equilibrium. When the network experiences any imbalance condition, the network operators implement control mechanisms to return the network to its balanced state. So, when generation cannot meet demand, the spinning reserve is connected to the electrical network and is loaded up according to demand. However, spinning reserve is a highly costly, carbon intensive, and inefficient method for maintaining the network stability as it requires the generating stations to remain running and consuming fuel to be brought online very quickly when needed to maintain the supply and demand balance.

The admission of the RES into the grid provides genuinely green energy at the points of entry; however, it creates a huge operational problem in managing the central generators to cover any transient variations in the renewable power input and consumer demand, thus hindering the renewable potential. The power network operators will have to manage the imbalance by either increasing the operation of the spinning reserve leading to increased costs [11], or by implementing other ways different from what they use with traditional power stations [12]. So, owners of RES connected to an increasingly congested electrical transmission and distribution network will have to incur financial penalties when they produce power at times of low demand and the network operators will have to pay compensations to RES owners when they introduce constraints [13]. An example of this is what happened at Scotland in April 2014, when strong winds made the Scottish grid not able to absorb all the wind power generated and had to constrain it off the grid while paying compensations to the owners of wind generators. Approximately £890,000 was paid over few hours to six wind farms.

Given that the global renewable energy contribution (excluding bio-fuels) is predicted to increase by over 50% between 2010 and 2035 [14], the electrical power networks therefore need to have the capacity to store the excess power fed into the grid from fluctuating and intermittent power sources [15]. The electrical power networks also need to continually achieve equilibrium under the increasing demand conditions while reducing their dependence on the expensive and inefficient

spinning reserve, and thus it is crucial to increase their capacity through the implementation of energy storage technologies [16]. Therefore, it can be concluded that implementing EST is essential in modern power grids.

50% compared with a maximum of 37% for a small combustion engine [17]. Alternatively, the stored hydrogen can be used for other end uses and thus hydrogen and

A key barrier to realize the potential of hydrogen energy storage systems is the limitation in the available modeling software and tools [18]. Another challenge is the ability to quantify the energy capacity and economic viability of the hydrogen energy storage technology (HEST) when integrated into the electrical power grid to enable the projected increase of renewables. Addressing these challenges is the main

4.1 Challenges to the hydrogen energy storage deployment

4.2 Potential of the hydrogen energy storage technology

energy generation in constrained power networks

key for accelerating the wide deployment of the hydrogen technology.

4.2.1 Role of hydrogen energy storage in allowing increased integration of renewable

Hydrogen, as a form of energy storage, can deliver a fuel for making power or heat or for fueling a car while absorbing the intermittent power inputs from RES. Hydrogen production systems (electrolyzers) can be operated as deferrable and controllable loads within a smart grid infrastructure to allow the absorption of increased renewable energy generation in constrained power networks. The stored hydrogen can be used later in generating electricity when needed, or it can be used in other energy intensive sectors such as the gas grid, transport as a fuel, and industrial processes. Hydrogen storage is not geographically restricted and offers the potential to shift constrained renewable generation into other energy intensive

Large industrial and commercial consumers can play a vital role in balancing the grid through the intelligent use of their electrical loads while implementing hydrogen production and storage technologies. One example which demonstrates that hydrogen technology can be used for balancing the grid is what happens in

"Tessenderlo Group," a company which utilizes both oxygen and hydrogen gases in its chemical processing activities [19]. Tessenderlo utilizes one of the world's largest scale hydrogen production systems, in the order of multiple Mega Watts (MW) scale [20, 21], and is charged at a lower cost/kWh in return to allow the local distribution network operator (DNO) to adjust its electrolytic hydrogen and oxygen production in maintaining the electrical network in the balanced state [22]. The DNO makes these adjustments in accordance with the demands on the electrical network using demand side management (DSM) techniques. The hydrogen production is reduced when the electrical network is experiencing a period of high demand and low energy production, and increased when the generation in electrical network exceeds consumption. Such a trading arrangement with a preferential tariff minimizes the need for the local network operators to waste money on highly inefficient spinning reserves. So, while utilizing the electrolyzer in maintaining the grid balance both hydrogen and oxygen gases are produced to be used in the

Hydrogen is in a strong position to be applied widely as an energy storage vector for balancing the grid while increasing the RES integration. Over the last decade, several renewable hydrogen concepts have been investigated [23] and several

oxygen gases are sold as commodities.

DOI: http://dx.doi.org/10.5772/intechopen.88902

Hydrogen Energy Storage

sectors.

chemical processes.

141

4.2.2 Position of the hydrogen energy storage technology

Hydrogen energy storage technologies are slowly but surely unlocking the potential of RES. Integrating HEST into the power networks not only allow the absorption of excess energy from fluctuating RES, but also allow the supply of supplementary energy needed when the RES production is insufficient to meet demand. Thus, HEST enable balancing the supply and demand while allowing the increased implementation of variable output RES.

#### 4. The hydrogen energy storage technology

Chemical energy storage in the form of hydrogen (gas or liquid) has the potential to store energy over long periods of time and can be scaled up with no restrictions on its location. Hydrogen can be used as an energy carrier, stored and delivered to where it is needed. The storage mechanism does not have high rate of self-discharge or degradation in performance. The basic elements of a hydrogen energy storage system (HESS) can be recognized in Figure 4. The electrolyzer (hydrogen generator) is used to convert the electrical energy from an energy source (typically renewable) into hydrogen for storage. The hydrogen storage system can store the hydrogen in several forms (pressurized gas, metal hydride, or liquid Dewar tank). A hydrogen energy conversion system then converts the stored chemical energy in the hydrogen back to electrical energy while giving off water and heat as by-products with no carbon emissions. The hydrogen energy conversion system that is commonly used is the fuel cell, given that its typical average electrical conversion efficiency, as recorded for installed projects, ranges between 40 and

Figure 4.

Basic elements of a hydrogen energy storage system (HESS).

spinning reserve, and thus it is crucial to increase their capacity through the implementation of energy storage technologies [16]. Therefore, it can be concluded that

Hydrogen energy storage technologies are slowly but surely unlocking the potential of RES. Integrating HEST into the power networks not only allow the absorption of excess energy from fluctuating RES, but also allow the supply of supplementary energy needed when the RES production is insufficient to meet demand. Thus, HEST enable balancing the supply and demand while allowing the

Chemical energy storage in the form of hydrogen (gas or liquid) has the potential to store energy over long periods of time and can be scaled up with no restrictions on its location. Hydrogen can be used as an energy carrier, stored and delivered to where it is needed. The storage mechanism does not have high rate of self-discharge or degradation in performance. The basic elements of a hydrogen energy storage system (HESS) can be recognized in Figure 4. The electrolyzer (hydrogen generator) is used to convert the electrical energy from an energy source (typically renewable) into hydrogen for storage. The hydrogen storage system can store the hydrogen in several forms (pressurized gas, metal hydride, or liquid Dewar tank). A hydrogen energy conversion system then converts the stored chemical energy in the hydrogen back to electrical energy while giving off water and heat as by-products with no carbon emissions. The hydrogen energy conversion system that is commonly used is the fuel cell, given that its typical average electrical conversion efficiency, as recorded for installed projects, ranges between 40 and

implementing EST is essential in modern power grids.

Energy Storage Devices

increased implementation of variable output RES.

4. The hydrogen energy storage technology

Figure 4.

140

Basic elements of a hydrogen energy storage system (HESS).

50% compared with a maximum of 37% for a small combustion engine [17]. Alternatively, the stored hydrogen can be used for other end uses and thus hydrogen and oxygen gases are sold as commodities.

#### 4.1 Challenges to the hydrogen energy storage deployment

A key barrier to realize the potential of hydrogen energy storage systems is the limitation in the available modeling software and tools [18]. Another challenge is the ability to quantify the energy capacity and economic viability of the hydrogen energy storage technology (HEST) when integrated into the electrical power grid to enable the projected increase of renewables. Addressing these challenges is the main key for accelerating the wide deployment of the hydrogen technology.

#### 4.2 Potential of the hydrogen energy storage technology

#### 4.2.1 Role of hydrogen energy storage in allowing increased integration of renewable energy generation in constrained power networks

Hydrogen, as a form of energy storage, can deliver a fuel for making power or heat or for fueling a car while absorbing the intermittent power inputs from RES. Hydrogen production systems (electrolyzers) can be operated as deferrable and controllable loads within a smart grid infrastructure to allow the absorption of increased renewable energy generation in constrained power networks. The stored hydrogen can be used later in generating electricity when needed, or it can be used in other energy intensive sectors such as the gas grid, transport as a fuel, and industrial processes. Hydrogen storage is not geographically restricted and offers the potential to shift constrained renewable generation into other energy intensive sectors.

Large industrial and commercial consumers can play a vital role in balancing the grid through the intelligent use of their electrical loads while implementing hydrogen production and storage technologies. One example which demonstrates that hydrogen technology can be used for balancing the grid is what happens in "Tessenderlo Group," a company which utilizes both oxygen and hydrogen gases in its chemical processing activities [19]. Tessenderlo utilizes one of the world's largest scale hydrogen production systems, in the order of multiple Mega Watts (MW) scale [20, 21], and is charged at a lower cost/kWh in return to allow the local distribution network operator (DNO) to adjust its electrolytic hydrogen and oxygen production in maintaining the electrical network in the balanced state [22]. The DNO makes these adjustments in accordance with the demands on the electrical network using demand side management (DSM) techniques. The hydrogen production is reduced when the electrical network is experiencing a period of high demand and low energy production, and increased when the generation in electrical network exceeds consumption. Such a trading arrangement with a preferential tariff minimizes the need for the local network operators to waste money on highly inefficient spinning reserves. So, while utilizing the electrolyzer in maintaining the grid balance both hydrogen and oxygen gases are produced to be used in the chemical processes.

#### 4.2.2 Position of the hydrogen energy storage technology

Hydrogen is in a strong position to be applied widely as an energy storage vector for balancing the grid while increasing the RES integration. Over the last decade, several renewable hydrogen concepts have been investigated [23] and several

installations have been implemented to demonstrate the role of energy storage in the form of hydrogen in balancing the supply and demand in constrained grids. Many of these installations were based around small-scale RES of only a few tens of kilowatts, with exceptions to the hydrogen mini grid system (HMGS) in Rotherham, the Yorkshire [24, 25], the Utsira (Norway) energy system [26], and the Hydrogen Office [27], where large-scale RES have been utilized. All these systems have utilized commercially available alkaline electrolyzers with rated hydrogen production capacity in the range of 0.2–10 Nm3 /h and operating pressures in the range of 7–20 bar, except the Hydrogen Office electrolyzer of 3.5 Nm3 /h at 55 bar.

Hydrogen storage technologies can be divided into physical storage, where hydrogen molecules are stored (this includes pure hydrogen storage via compression and liquefaction), and chemical storage, where hydrides are stored. While chemical storage could offer high storage performance due to the strong binding of hydrogen and the high storage densities, the regeneration of storage material remains an issue with a large number of chemical storage systems still under investigation.

Demonstration projects have showed that hydrogen has a flexibility with RES, which is not available in other energy storage technologies. It has been found out that energy storage employing hydrogen technologies is best suited with renewable energy sources through the absorption of their surplus generation via electrolysis and storing it in the form of compressed hydrogen gas for later re-use in many applications such as the following:

a role to play in durations between several minutes to hours and is best suited for applications larger than 100 kW, and thus can be identified as appropriate for the

• injected into the gas grids (since it is mixable with other gases);

• used to power a fuel cell (FC) or a combustion engine' vehicle; or

4.2.5 Limitations toward the adoption of the hydrogen energy storage technology

5. Techno-economic assessment of hydrogen energy storage

efficiency, energy density, power density, and technological-maturity [37].

• used in many industrial processes (like fertilizer production)

• used to generate electricity and heat via a fuel cell;

HES, when compared to the other ESTs, is seen to be suitable for use with RES [34]. In summary, the stored hydrogen produced during the RES excess generation

H2 for other end use applications-

Figure 5 overviews the implementation of hydrogen energy storage with RES.

Despite of the benefits and potential that HEST presents, the high capital cost and the low turn-around efficiency (i.e., electricity to hydrogen stored then back to electricity) are two noteworthy limitations [35]. Significant efforts are being made by industry to address cost and efficiency concerns. Additionally, many countries have started the process of publishing draft guidelines for the use of hydrogen

To contribute to the effective and wider implementation of the hydrogen technology, especially where HESS are operated in combination with variable RES, appropriate financial mechanisms and effective modeling techniques are developed in this chapter.

Energy storage technologies are generally compared in terms of their lifetime,

Energy Management of Electricity Networks.

Implementing hydrogen energy storage with RES.

energy storage technologies [36].

143

periods can be:

Figure 5.

Hydrogen Energy Storage

DOI: http://dx.doi.org/10.5772/intechopen.88902


As the governments around the world are strategically moving toward a low carbon economy, hydrogen storage will undoubtedly play an important role in making use of the grid constrained "green" energy within a rapidly growing market [28].

#### 4.2.3 Opportunities of the hydrogen energy storage technology

Hydrogen can be stored for long periods of time without degradation. Hydrogen can be stored in a gaseous or liquid form, or in some instances adsorbed onto a solid form in the case of metal hydride storage technology. Hydrogen is mixable with other gases making it suitable for mixing into the existing natural gas grid in a process known as sector shifting [29]. Additionally, hydrogen can be used as reactive agent in the chemical transformations of synthetic natural gas and fuels.

Hydrogen is seen by many of the energy industry experts as a mean of storing the surplus renewable energy from sources such as wind, solar, wave, and tide [30, 31] for later use. It is also seen to have a market potential for vehicle fueling in both urban and remote rural areas [32, 33].

#### 4.2.4 Additional benefits of the hydrogen energy storage technology

It can be noticed from Figure 3, which has overviewed the different storage technologies together with the applications in which they are best suited as conducted by the AEA technologies for the Scottish government, that hydrogen has installations have been implemented to demonstrate the role of energy storage in the form of hydrogen in balancing the supply and demand in constrained grids. Many of these installations were based around small-scale RES of only a few tens of kilowatts, with exceptions to the hydrogen mini grid system (HMGS) in Rotherham, the Yorkshire [24, 25], the Utsira (Norway) energy system [26], and the Hydrogen Office [27], where large-scale RES have been utilized. All these systems have utilized commercially available alkaline electrolyzers with rated hydrogen

range of 7–20 bar, except the Hydrogen Office electrolyzer of 3.5 Nm3

with a large number of chemical storage systems still under investigation.

Hydrogen storage technologies can be divided into physical storage, where hydrogen molecules are stored (this includes pure hydrogen storage via compression and liquefaction), and chemical storage, where hydrides are stored. While chemical storage could offer high storage performance due to the strong binding of hydrogen and the high storage densities, the regeneration of storage material remains an issue

Demonstration projects have showed that hydrogen has a flexibility with RES, which is not available in other energy storage technologies. It has been found out that energy storage employing hydrogen technologies is best suited with renewable energy sources through the absorption of their surplus generation via electrolysis and storing it in the form of compressed hydrogen gas for later re-use in many

• Controllable generation reserve via fuel cells and/or gas turbines and/or

• A means to transfer the renewable energy into the gas grid

4.2.3 Opportunities of the hydrogen energy storage technology

4.2.4 Additional benefits of the hydrogen energy storage technology

both urban and remote rural areas [32, 33].

142

• A chemical process gas for other end uses in food, fertilizers, etc.

As the governments around the world are strategically moving toward a low carbon economy, hydrogen storage will undoubtedly play an important role in making use of the grid constrained "green" energy within a rapidly growing market [28].

Hydrogen can be stored for long periods of time without degradation. Hydrogen can be stored in a gaseous or liquid form, or in some instances adsorbed onto a solid form in the case of metal hydride storage technology. Hydrogen is mixable with other gases making it suitable for mixing into the existing natural gas grid in a process known as sector shifting [29]. Additionally, hydrogen can be used as reactive agent in the chemical transformations of synthetic natural gas and fuels.

Hydrogen is seen by many of the energy industry experts as a mean of storing the surplus renewable energy from sources such as wind, solar, wave, and tide [30, 31] for later use. It is also seen to have a market potential for vehicle fueling in

It can be noticed from Figure 3, which has overviewed the different storage

conducted by the AEA technologies for the Scottish government, that hydrogen has

technologies together with the applications in which they are best suited as

/h and operating pressures in the

/h at 55 bar.

production capacity in the range of 0.2–10 Nm3

Energy Storage Devices

applications such as the following:

internal combustion engines (ICE)

• Fuel for transport applications

Figure 5. Implementing hydrogen energy storage with RES.

a role to play in durations between several minutes to hours and is best suited for applications larger than 100 kW, and thus can be identified as appropriate for the Energy Management of Electricity Networks.

HES, when compared to the other ESTs, is seen to be suitable for use with RES [34]. In summary, the stored hydrogen produced during the RES excess generation periods can be:


Figure 5 overviews the implementation of hydrogen energy storage with RES.

#### 4.2.5 Limitations toward the adoption of the hydrogen energy storage technology

Despite of the benefits and potential that HEST presents, the high capital cost and the low turn-around efficiency (i.e., electricity to hydrogen stored then back to electricity) are two noteworthy limitations [35]. Significant efforts are being made by industry to address cost and efficiency concerns. Additionally, many countries have started the process of publishing draft guidelines for the use of hydrogen energy storage technologies [36].

To contribute to the effective and wider implementation of the hydrogen technology, especially where HESS are operated in combination with variable RES, appropriate financial mechanisms and effective modeling techniques are developed in this chapter.

#### 5. Techno-economic assessment of hydrogen energy storage

Energy storage technologies are generally compared in terms of their lifetime, efficiency, energy density, power density, and technological-maturity [37].

They are also often compared based on application-specific benefits and specific characteristics of interest [38, 39]; however, such comparisons did not take into consideration their financial competitiveness.

5.1 The proposed levelized cost model (LCM) for HEST

storage technology can be expressed as shown in Eq. (1) [41]:

LSC ¼

LSC ¼

P<sup>n</sup> t¼1

> P<sup>n</sup> t¼1

where H2t is the value of sold hydrogen gas year (t) and O2t is the value of sold

To evaluate the economic competitiveness of hydrogen energy storage systems utilizing the "surplus" or "grid constrained" renewable energy generation, several configurations are considered here to conclude the most economic scenario. Scenario 1: Selling 100% of the hydrogen and oxygen gases produced by electrolyzer, and no electricity to sell (i.e., no fuel cell electricity generation).

Scenario 2: Selling 100% of the H2 gas stored as electricity injected back to the power grid through the FC electricity generation (i.e., no H2 gas to sell), and selling

Scenario 3: Selling 50% of the produced hydrogen as gas while the other 50% is sold as electricity to the grid via the fuel cell, and selling 50% of the oxygen as gas while the remaining 50% is vented to the atmosphere (i.e., not making use of half

Scenario 4: Selling 100% of the hydrogen as gas (i.e., no fuel cell generation and no electricity to sell) and no selling of oxygen gas (i.e., not making use of all the

Scenario 5: Selling 100% of the stored hydrogen as electricity back to the power grid (i.e., no H2 gas selling), and no O2 gas selling (i.e., not making use of all the

Note that, Scenarios 4 and 5 do not utilize the by-product oxygen gas.

energy [44].

Hydrogen Energy Storage

DOI: http://dx.doi.org/10.5772/intechopen.88902

as shown in Eq. (2):

oxygen gas in year (t).

100% of the O2 gas.

the produced oxygen value).

produced oxygen value).

produced oxygen value).

145

The costs of energy storage are considered using a levelized cost of ownership approach. Typically, the capital costs of an energy storage facility are expressed as £/kW installed, where it includes all expenses involved in the purchase and installation of facility. The £/kW capital expenditure (CapEx) multiplied by the size of the facility produces the total cost of the project. In the proposed model, all the costs related to an energy storage facility are expressed as (i) total £/kW of usable discharge capacity (in kW) and (ii) total £/kWh of usable energy

storage capacity. EST with deeper Depth of Discharge (i.e., the ability of an ESS to release its stored energy) and higher turn-around efficiency (i.e., ratio between input energy and output energy) will have a lower unit cost of usable power and

Using the levelized cost approach, the levelized storage cost (LSC) of energy

P<sup>n</sup> t¼1 EOt ð Þ <sup>1</sup>þ<sup>r</sup> <sup>t</sup>

where ISCt is the invested storage capital in year (t); SOMt is the storage operation and maintenance costs in year (t); ECt is the input energy cost (t); r is the annual discount rate (typically 10%); and EOt is the value of released energy in year (t). The LSC for the hydrogen storage technology will have additional revenue potential realized in the sale of both hydrogen and oxygen gases as a commodity. Equation (1) is therefore expanded to include H2t and O2t, and the LSC is expressed

ISCtþSOMtþECt ð Þ <sup>1</sup>þ<sup>r</sup> <sup>t</sup>

ISCtþSOMtþECt ð Þ <sup>1</sup>þ<sup>r</sup> <sup>t</sup>

EOtþH2tþO2<sup>t</sup> ð Þ <sup>1</sup>þ<sup>r</sup> <sup>t</sup>

(1)

(2)

P<sup>n</sup> t¼1

Financial competitiveness of EST is to define the price of stored energy per kWh over the lifetime of the energy storage system. The Electric Power Research Institute (EPRI) has developed and documented a method that analyses the costs associated with grid connected energy storage applications [40]. The EPRI utilizes a levelized cost model (LCM) approach to perform the cost benefit analysis for energy storage technologies. The LC, in its basic form, is calculated by dividing the annual expenditures by the annual income and correcting for inflationary effects. The LC reflects the capital and operational expenditures including the upfront capital costs, the fuel expenses, the operating and maintenance (O&M) charges, the financing costs, etc. levelized cost analysis is often used in regulatory review and longer-term resource planning [41]. Levelized costs can be done using limited input data, thus useful for evaluating technologies with limited operating experience or available data.

The HEST has not been included in the EPRI analysis [42] or in other cost analysis modeling techniques available in literature. HEST appears to be commonly excluded from the cost comparative studies due to its high capital cost and low turnaround efficiency compared to other bulk ESTs. Even the studies that examined HEST have not included its additional revenue streams. So, a study that has been completed by the National Renewable Energy Laboratories (NREL) has introduced the value of grid connected stored hydrogen for transport applications [43], but did not consider the potential value of the oxygen gas as a by-product from electrolysis. Since the by-product oxygen would either have a positive or a negative effect on the cost competitiveness of HEST, therefore, a new LCM is developed in this section to explore this.

Figure 6 illustrates the economic revenue streams of both the conventional and hydrogen energy storage technologies. While conventional energy storage systems allow for energy to be stored and released in the form of electrical energy, hydrogen as an energy storage mechanism allows surplus RES electrical energy to be stored and released as electricity in addition to hydrogen and oxygen gases that could be sold as commodities offering greater financial competitiveness. Since this could offset the HEST low energy efficiency and high capital costs, it is considered in the developed HEST financial competitiveness model.

Figure 6. HEST possible economic revenue streams compared to conventional EST.

They are also often compared based on application-specific benefits and specific characteristics of interest [38, 39]; however, such comparisons did not take into

The HEST has not been included in the EPRI analysis [42] or in other cost analysis modeling techniques available in literature. HEST appears to be commonly excluded from the cost comparative studies due to its high capital cost and low turnaround efficiency compared to other bulk ESTs. Even the studies that examined HEST have not included its additional revenue streams. So, a study that has been completed by the National Renewable Energy Laboratories (NREL) has introduced the value of grid connected stored hydrogen for transport applications [43], but did not consider the potential value of the oxygen gas as a by-product from electrolysis. Since the by-product oxygen would either have a positive or a negative effect on the cost competitiveness of HEST, therefore, a new LCM is developed in this section to

Figure 6 illustrates the economic revenue streams of both the conventional and hydrogen energy storage technologies. While conventional energy storage systems allow for energy to be stored and released in the form of electrical energy, hydrogen as an energy storage mechanism allows surplus RES electrical energy to be stored and released as electricity in addition to hydrogen and oxygen gases that could be sold as commodities offering greater financial competitiveness. Since this could offset the HEST low energy efficiency and high capital costs, it is considered in the

Financial competitiveness of EST is to define the price of stored energy per kWh over the lifetime of the energy storage system. The Electric Power Research Institute (EPRI) has developed and documented a method that analyses the costs associated with grid connected energy storage applications [40]. The EPRI utilizes a levelized cost model (LCM) approach to perform the cost benefit analysis for energy storage technologies. The LC, in its basic form, is calculated by dividing the annual expenditures by the annual income and correcting for inflationary effects. The LC reflects the capital and operational expenditures including the upfront capital costs, the fuel expenses, the operating and maintenance (O&M) charges, the financing costs, etc. levelized cost analysis is often used in regulatory review and longer-term resource planning [41]. Levelized costs can be done using limited input data, thus useful for evaluating technologies with limited operating experience or

consideration their financial competitiveness.

developed HEST financial competitiveness model.

HEST possible economic revenue streams compared to conventional EST.

available data.

Energy Storage Devices

explore this.

Figure 6.

144

#### 5.1 The proposed levelized cost model (LCM) for HEST

The costs of energy storage are considered using a levelized cost of ownership approach. Typically, the capital costs of an energy storage facility are expressed as £/kW installed, where it includes all expenses involved in the purchase and installation of facility. The £/kW capital expenditure (CapEx) multiplied by the size of the facility produces the total cost of the project. In the proposed model, all the costs related to an energy storage facility are expressed as (i) total £/kW of usable discharge capacity (in kW) and (ii) total £/kWh of usable energy storage capacity. EST with deeper Depth of Discharge (i.e., the ability of an ESS to release its stored energy) and higher turn-around efficiency (i.e., ratio between input energy and output energy) will have a lower unit cost of usable power and energy [44].

Using the levelized cost approach, the levelized storage cost (LSC) of energy storage technology can be expressed as shown in Eq. (1) [41]:

$$LSC = \frac{\sum\_{t=1}^{n} \frac{ISC\_t + SOM\_t + EC\_t}{(1+r)^t}}{\sum\_{t=1}^{n} \frac{EO\_t}{(1+r)^t}} \tag{1}$$

where ISCt is the invested storage capital in year (t); SOMt is the storage operation and maintenance costs in year (t); ECt is the input energy cost (t); r is the annual discount rate (typically 10%); and EOt is the value of released energy in year (t).

The LSC for the hydrogen storage technology will have additional revenue potential realized in the sale of both hydrogen and oxygen gases as a commodity. Equation (1) is therefore expanded to include H2t and O2t, and the LSC is expressed as shown in Eq. (2):

$$L\text{SCC} = \frac{\sum\_{t=1}^{n} \frac{I\text{SC}\_t + \text{SOM}\_t + EC\_t}{(1+r)^t}}{\sum\_{t=1}^{n} \frac{EO\_t + H\text{2}\_t + O\text{2}\_t}{(1+r)^t}} \tag{2}$$

where H2t is the value of sold hydrogen gas year (t) and O2t is the value of sold oxygen gas in year (t).

To evaluate the economic competitiveness of hydrogen energy storage systems utilizing the "surplus" or "grid constrained" renewable energy generation, several configurations are considered here to conclude the most economic scenario.

Scenario 1: Selling 100% of the hydrogen and oxygen gases produced by electrolyzer, and no electricity to sell (i.e., no fuel cell electricity generation).

Scenario 2: Selling 100% of the H2 gas stored as electricity injected back to the power grid through the FC electricity generation (i.e., no H2 gas to sell), and selling 100% of the O2 gas.

Scenario 3: Selling 50% of the produced hydrogen as gas while the other 50% is sold as electricity to the grid via the fuel cell, and selling 50% of the oxygen as gas while the remaining 50% is vented to the atmosphere (i.e., not making use of half the produced oxygen value).

Scenario 4: Selling 100% of the hydrogen as gas (i.e., no fuel cell generation and no electricity to sell) and no selling of oxygen gas (i.e., not making use of all the produced oxygen value).

Scenario 5: Selling 100% of the stored hydrogen as electricity back to the power grid (i.e., no H2 gas selling), and no O2 gas selling (i.e., not making use of all the produced oxygen value).

Note that, Scenarios 4 and 5 do not utilize the by-product oxygen gas.

These five scenarios are tested on the "hydrogen office" energy storage system, as a case study, and the levelized cost per unit output is calculated for each scenario using Eq. (2). The Hydrogen Office, in Methil Docks Business Park in Scotland, employs a wind/hydrogen energy storage system that has been installed to demonstrate the potential of HES in storing surplus renewable energy. The Hydrogen Office's main components are shown in Figure 7, the Capital expenditure (CapEx) and Operational Expenditure (Opex) data are given in Table 1, and the market value for the by-product H2 and O2 gases is given in Table 2 [34].

Figure 8 shows the LSC results for the five scenarios. It can be seen from figure that the most financially competitive configuration for the hydrogen energy storage technology is realized in Scenario 1. A favorable result is also seen in Scenarios 2 and 3. The least competitive configuration is seen in Scenario 5 when none of the gases is sold as commodity. Although hydrogen has a high financial value when sold as a gas, Scenario 4 demonstrates that it is not competitive when sold on its own.

It can be concluded from Figure 8 that the hydrogen energy storage technology has a great potential and financial benefit in enabling the projected increase of renewable generation into the electrical network as it allows alternative economic pathways for

the surplus renewable generation. The stored hydrogen is not only limited for electricity production, but can also be sold for another end uses. Moreover, HEST has the

O2 Sale (£/Ton) £ 3000.00 H2 Sale (£/Ton) £ 5000.00

The HEST levelized cost is then compared to the levelized costs of other conventional energy storage technologies as obtained from a research conducted by NREL and summarized in Table 3 [45]. Figure 9 illustrates this comparison.

It can be seen from Figure 9 that CAES and PHS are more cost competitive than the five HEST scenarios proposed. This shows that there is still a need for reducing the HEST CapEx or increasing its turn-around efficiency to increase its financial competitiveness. However, HEST can compete with NaS and RFB technologies when it is used in conjunction with the oxygen gas by-product selling. Additionally, HEST competes with NiCd battery technology when only 50% of the oxygen gas is sold and half the hydrogen is sold as gas and the other half as electricity. HEST is not competitive when used for only selling electricity (Scenario 5) or only selling H2 gas

5.2 Efficiency of renewable hydrogen energy storage systems (RHESS)

All energy storage systems have varying degrees of inefficiency (turn-around efficiency), with typical efficiency ranging from 45 to 80%. Hydrogen energy storage systems' efficiency can be considered higher especially when implemented

• They commonly utilize a fuel cell that has a conversion efficiency a lot higher

most economic potential when its by-product oxygen is sold as well.

(Scenario 4) without selling the O2 gas.

The LCM simulated output costs for the HEST scenarios.

Market data

Hydrogen Energy Storage

Market value for hydrogen and oxygen gases.

DOI: http://dx.doi.org/10.5772/intechopen.88902

Table 2.

Figure 8.

147

with RES because of the following:

• The efficiency of electrolysis is high.

than that of the combustion engine technology.

Figure 7. The Hydrogen Office simplified system overview.


#### Table 1.

HEST capital and operational expenditure data.


#### Table 2.

These five scenarios are tested on the "hydrogen office" energy storage system, as a case study, and the levelized cost per unit output is calculated for each scenario using Eq. (2). The Hydrogen Office, in Methil Docks Business Park in Scotland, employs a wind/hydrogen energy storage system that has been installed to demonstrate the potential of HES in storing surplus renewable energy. The Hydrogen Office's main components are shown in Figure 7, the Capital expenditure (CapEx) and Operational Expenditure (Opex) data are given in Table 1, and the market

Figure 8 shows the LSC results for the five scenarios. It can be seen from figure that the most financially competitive configuration for the hydrogen energy storage technology is realized in Scenario 1. A favorable result is also seen in Scenarios 2 and 3. The least competitive configuration is seen in Scenario 5 when none of the gases is sold as commodity. Although hydrogen has a high financial value when sold as a gas, Scenario 4 demonstrates that it is not competitive when sold on its own.

It can be concluded from Figure 8 that the hydrogen energy storage technology has a great potential and financial benefit in enabling the projected increase of renewable generation into the electrical network as it allows alternative economic pathways for

Electrolyzer (£/kW) £ 2500.00 Storage (£/kWh) £ 27.00 Fuel cell (£/kWh) £ 4000.00

Electrolyzer (£/kW) £ 50.00 Storage (£/kWh) £ 2.00 Fuel cell (£/kW) £ 100.00

value for the by-product H2 and O2 gases is given in Table 2 [34].

Figure 7.

Table 1.

146

The Hydrogen Office simplified system overview.

HEST capital and operational expenditure data.

CapEx & OpEx data Hydrogen CapEx

Energy Storage Devices

Hydrogen OpEX

Market value for hydrogen and oxygen gases.

#### Figure 8.

the surplus renewable generation. The stored hydrogen is not only limited for electricity production, but can also be sold for another end uses. Moreover, HEST has the most economic potential when its by-product oxygen is sold as well.

The HEST levelized cost is then compared to the levelized costs of other conventional energy storage technologies as obtained from a research conducted by NREL and summarized in Table 3 [45]. Figure 9 illustrates this comparison.

It can be seen from Figure 9 that CAES and PHS are more cost competitive than the five HEST scenarios proposed. This shows that there is still a need for reducing the HEST CapEx or increasing its turn-around efficiency to increase its financial competitiveness. However, HEST can compete with NaS and RFB technologies when it is used in conjunction with the oxygen gas by-product selling. Additionally, HEST competes with NiCd battery technology when only 50% of the oxygen gas is sold and half the hydrogen is sold as gas and the other half as electricity. HEST is not competitive when used for only selling electricity (Scenario 5) or only selling H2 gas (Scenario 4) without selling the O2 gas.

#### 5.2 Efficiency of renewable hydrogen energy storage systems (RHESS)

All energy storage systems have varying degrees of inefficiency (turn-around efficiency), with typical efficiency ranging from 45 to 80%. Hydrogen energy storage systems' efficiency can be considered higher especially when implemented with RES because of the following:



hybrid renewable energy system, and those which can [like HOMER, BALMORAL,

SimREN, and UniSyD3.0] require a significant quantity of input data and substantial computing resources to perform well and still some of them are not capable of sizing the hydrogen energy system. The more advanced standalone modeling techniques, like genetic algorithms (GA), particle swarm optimization (PSO), and sim-

To address the large input data requirements of the commercially existing software and the significant computing resources needed by advanced GA, PSO, and SA techniques, a new deterministic sizing methodology that offers a rapid initial system sizing of a hybrid renewable hydrogen energy system (HRHES) with the minimal amount of input data and computer resources is given here. This simple technical sizing technique, referred to as deterministic [48, 49], can provide a rapid and reasonably accurate system sizing [50] while using limited number of input data. This approach can therefore play an important role at the initial design phase of HRHES.

To demonstrate the proposed new deterministic sizing methodology that offers a rapid initial system sizing of a hybrid renewable hydrogen energy system (HRHES) with the minimal amount of input data and computer resources and thus supports its initial design, a HRHES sizing model is developed here based on the presence of solar and wind as the renewable resources combined with HEST to meet the demands of an electrical load. The outline of the developed deterministic sizing algorithm is shown in Figure 10 and is detailed in the following subsections.

6.1.1 Sizing the integrated renewable energy sources (wind turbine and solar

The first criterion in sizing a HRHES is sizing the renewable energy sources (RES). The objective of including the RES generation into the sizing routine is to minimize the difference between the average load demand (Pdem) and the average renewable energy generation (Pgen). Typically, the duration of the analysis is 1 year to allow incorporating the seasonal variation of the demand and renewable generation. In the developed model, the wind turbine is considered as the primary renewable energy source and the photovoltaic (PV) as the secondary. Hence, the capacity

The capacity factor (CF) is defined as the average power output (P) from the renewable device as a percentage of the maximum power output (P). The capacity factor of a wind turbine (CFwind) is given by Eq. (3), and that for a PV (CFPV) is

> CFw <sup>¼</sup> Pw Pw

CFPV <sup>¼</sup> PPV

PV, respectively, while Pw and PPV are their rated power outputs.

where Pw and PPV are the average power outputs from the wind turbine and the

Given that sizing the wind turbine is usually restricted to the unit sizes available in the market, which in general are 3, 5, 6, 10, 15, 20, 50, 250, 330, 500, 850, 900,

PPV

(3)

(4)

H2RES, ENPEP-BALANCE, HYDROGEMS (incorporated into Transys16),

ulated annealing (SA) need significant computing resources.

6.1 The developed deterministic sizing algorithm

factor for each technology is first determined.

photovoltaic)

Hydrogen Energy Storage

DOI: http://dx.doi.org/10.5772/intechopen.88902

given by Eq. (4).

149

#### Table 3.

Levelized costs of other energy storage technology [45].

#### Figure 9.

The levelized cost of HEST versus the levelized costs of other conventional energy storage technologies.


#### 6. Sizing a hybrid renewable hydrogen energy system (HRHES)

The available sizing models are either Commercial software (like EMCAS, EnergyPLAN, energyPRO, GTMax, IKARUS, Invert, MiniCAM, NEMS, ORCED, PERSEUS, ORCED, PERSEUS, PRIMES, ProdRisk, RAMSES, RETScreen, SIVAEL, STREAM, WASP, and WILMAR [46, 47]) or standalone modeling techniques. The commercially available software does not offer simulating the HEST as part of a

#### Hydrogen Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.88902

hybrid renewable energy system, and those which can [like HOMER, BALMORAL, H2RES, ENPEP-BALANCE, HYDROGEMS (incorporated into Transys16), SimREN, and UniSyD3.0] require a significant quantity of input data and substantial computing resources to perform well and still some of them are not capable of sizing the hydrogen energy system. The more advanced standalone modeling techniques, like genetic algorithms (GA), particle swarm optimization (PSO), and simulated annealing (SA) need significant computing resources.

To address the large input data requirements of the commercially existing software and the significant computing resources needed by advanced GA, PSO, and SA techniques, a new deterministic sizing methodology that offers a rapid initial system sizing of a hybrid renewable hydrogen energy system (HRHES) with the minimal amount of input data and computer resources is given here. This simple technical sizing technique, referred to as deterministic [48, 49], can provide a rapid and reasonably accurate system sizing [50] while using limited number of input data. This approach can therefore play an important role at the initial design phase of HRHES.

#### 6.1 The developed deterministic sizing algorithm

To demonstrate the proposed new deterministic sizing methodology that offers a rapid initial system sizing of a hybrid renewable hydrogen energy system (HRHES) with the minimal amount of input data and computer resources and thus supports its initial design, a HRHES sizing model is developed here based on the presence of solar and wind as the renewable resources combined with HEST to meet the demands of an electrical load. The outline of the developed deterministic sizing algorithm is shown in Figure 10 and is detailed in the following subsections.

#### 6.1.1 Sizing the integrated renewable energy sources (wind turbine and solar photovoltaic)

The first criterion in sizing a HRHES is sizing the renewable energy sources (RES). The objective of including the RES generation into the sizing routine is to minimize the difference between the average load demand (Pdem) and the average renewable energy generation (Pgen). Typically, the duration of the analysis is 1 year to allow incorporating the seasonal variation of the demand and renewable generation. In the developed model, the wind turbine is considered as the primary renewable energy source and the photovoltaic (PV) as the secondary. Hence, the capacity factor for each technology is first determined.

The capacity factor (CF) is defined as the average power output (P) from the renewable device as a percentage of the maximum power output (P). The capacity factor of a wind turbine (CFwind) is given by Eq. (3), and that for a PV (CFPV) is given by Eq. (4).

$$CF\_w = \frac{\overline{P}\_w}{P\_w} \tag{3}$$

$$\text{CF}\_{PV} = \frac{\overline{P}\_{PV}}{P\_{PV}} \tag{4}$$

where Pw and PPV are the average power outputs from the wind turbine and the PV, respectively, while Pw and PPV are their rated power outputs.

Given that sizing the wind turbine is usually restricted to the unit sizes available in the market, which in general are 3, 5, 6, 10, 15, 20, 50, 250, 330, 500, 850, 900,

• Their efficiency can be increased by utilizing the output heat from

The levelized cost of HEST versus the levelized costs of other conventional energy storage technologies.

• When utilized with RES, they capture and store the excess in renewable energy that would have otherwise been dumped and this in turn adds to their

Technology Levelized storage cost (LSC)

NiCd battery £ 0.52 NaS battery £ 0.16 Radox flow battery (RFB) £ 0.17 Pumped hydrostorage (PHS) £ 0.08 Compressed air energy storage (CAES) £ 0.06

• They do not only store the electrical energy for future re-use like all other conventional ESS, but also allow both hydrogen and oxygen gases to be sold as commodities thus increasing the system economic efficiency. The 3:1 increase in revenue options, shown in Figure 6, opens the potential for downstream applications like car fuelling, fertilizer production, and high and low-grade

6. Sizing a hybrid renewable hydrogen energy system (HRHES)

The available sizing models are either Commercial software (like EMCAS, EnergyPLAN, energyPRO, GTMax, IKARUS, Invert, MiniCAM, NEMS, ORCED, PERSEUS, ORCED, PERSEUS, PRIMES, ProdRisk, RAMSES, RETScreen, SIVAEL, STREAM, WASP, and WILMAR [46, 47]) or standalone modeling techniques. The commercially available software does not offer simulating the HEST as part of a

electrolyzers and fuel cells in process heating.

heat applications in addition to electricity.

efficiency.

Figure 9.

148

Table 3.

Energy Storage Devices

Levelized costs of other energy storage technology [45].

where λ is the tip speed ratio, ratio of the wind speed, and the speed at which the wind turbines rotor tips are traveling, and it is found using Eq. (8); and B is the

> πnD 60 v

where n is the turbine RPM; D is the turbine rotor diameter; and v is the wind

The second step is to define the rated power of the supporting PV array (PPV)

PPV <sup>¼</sup> Pdem � ð Þ CFwPw CFPV

When the load demand exceeds the renewable generation, this deficit is met by the fuel cell generation. The fuel cell converts the chemical energy of the stored hydrogen into electrical energy to supply the demand. The fuel cell size is selected to meet or exceed the load maximum power demand PdemMAX ð Þ. Typically, a margin of 20% is added to the size of the fuel cell to accommodate any modest increase in peak demands [50]. The fuel cell power output (PFC) is therefore calculated as

The RES are first sized to supply a specified load on an annual average basis using Eqs. (3)–(9). Sizes of the appropriate electrolyzer and fuel cell are then identified using Eqs. (10) and (11). Because there could be times when there is no or insufficient renewable generation to supply the demand, assessing the correlation of the load demand and renewable generation is therefore needed. Simulating the energy system without storage, as shown in Figure 11, using the load and calculated sizes for PV and wind turbine allows to identify the correlation between the load

The difference between the load demand and the renewable generation at different timings can be found by subtracting the load demand from the renewable

PEL ¼ ð Þ PPV þ Pwind � Pdem\_ min =2 (10)

PFC ffi 1:2 PdemMAX ð Þ (11)

(8)

(9)

λ ¼

using Eq. (9). In Eq. (9), PPV is calculated by using the values obtained from

After sizing the RES, the size of the electrolyzer (PEL) is calculated by subtracting the minimum load demand (Pdem\_ min ) from the total rated power that can be delivered by the renewable energy sources. However, it has been found out that such a size can be very large and underutilized and because electrolyzers can be very expensive, it is desirable to operate them with a high level of utilization. Reducing this calculated value by around 50% [51], as shown in Eq. (10), increases the electrolyzer level of utilization but there will be times when the total renewable generation exceeds the total power that can be absorbed by the load and the

6.1.2 Sizing the electrolyzer (hydrogen generator)

blade's pitch angle.

Hydrogen Energy Storage

DOI: http://dx.doi.org/10.5772/intechopen.88902

speed.

Eqs. (3)–(5).

electrolyzer of the RHESS.

6.1.3 Sizing the fuel cell

shown in Eq. (11).

151

6.1.4 Sizing the hydrogen storage tank

demand and renewable generation.

Figure 10.

The proposed deterministic sizing algorithm.

1200, 2200, 3200 kW, and thus the wind turbine is sized first. The average power of the wind turbine (Pw) is first selected to be close to the average demand of the defined load (Pdem); i.e., Pw ffi Pdem:

The rated power output for the wind turbine is then calculated using the proposed wind turbine model given by Eq. (5). Note that, the annual average site wind speed is calculated using Eq. (6) and the wind turbine rotor coefficient of performance (Cp), which is a measure of wind turbines blade rotor effectiveness at converting the power in the wind to mechanical power, is calculated using Eq. (7).

$$P\_w = \left(\frac{1}{2}C\_P\rho A v^3\right) \tag{5}$$

$$v = \mathbf{1} + \mathbf{S}\_{nr} \cos \left( t\_{hr} \left( \frac{\mathbf{360}}{8760} \right) \left( \frac{\pi}{180} \right) \right) \exp \left\{ \left( - \left( \frac{\overline{\mathbf{v}}}{\mathbf{C}} \right)^{k} \right) \right\} \tag{6}$$

$$\mathbf{C}\_{p}(\lambda) = \mathbf{0}.5(\lambda - 0.02\beta^2 - 2.9)e^{\lambda} - \mathbf{0}.0303\lambda\tag{7}$$

Hydrogen Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.88902

where λ is the tip speed ratio, ratio of the wind speed, and the speed at which the wind turbines rotor tips are traveling, and it is found using Eq. (8); and B is the blade's pitch angle.

$$
\lambda = \frac{\frac{\pi nD}{60}}{v} \tag{8}
$$

where n is the turbine RPM; D is the turbine rotor diameter; and v is the wind speed.

The second step is to define the rated power of the supporting PV array (PPV) using Eq. (9). In Eq. (9), PPV is calculated by using the values obtained from Eqs. (3)–(5).

$$P\_{PV} = \frac{\overline{P}\_{dem} - (\mathcal{C}F\_w P\_w)}{\mathcal{C}F\_{PV}} \tag{9}$$

6.1.2 Sizing the electrolyzer (hydrogen generator)

After sizing the RES, the size of the electrolyzer (PEL) is calculated by subtracting the minimum load demand (Pdem\_ min ) from the total rated power that can be delivered by the renewable energy sources. However, it has been found out that such a size can be very large and underutilized and because electrolyzers can be very expensive, it is desirable to operate them with a high level of utilization. Reducing this calculated value by around 50% [51], as shown in Eq. (10), increases the electrolyzer level of utilization but there will be times when the total renewable generation exceeds the total power that can be absorbed by the load and the electrolyzer of the RHESS.

$$P\_{\rm EL} = (P\_{PV} + P\_{wind} - P\_{dem\_{\rm -}min})/2 \tag{10}$$

#### 6.1.3 Sizing the fuel cell

When the load demand exceeds the renewable generation, this deficit is met by the fuel cell generation. The fuel cell converts the chemical energy of the stored hydrogen into electrical energy to supply the demand. The fuel cell size is selected to meet or exceed the load maximum power demand PdemMAX ð Þ. Typically, a margin of 20% is added to the size of the fuel cell to accommodate any modest increase in peak demands [50]. The fuel cell power output (PFC) is therefore calculated as shown in Eq. (11).

$$P\_{\rm FC} \cong \mathbf{1.2}(P\_{dem\_{MAX}}) \tag{11}$$

#### 6.1.4 Sizing the hydrogen storage tank

The RES are first sized to supply a specified load on an annual average basis using Eqs. (3)–(9). Sizes of the appropriate electrolyzer and fuel cell are then identified using Eqs. (10) and (11). Because there could be times when there is no or insufficient renewable generation to supply the demand, assessing the correlation of the load demand and renewable generation is therefore needed. Simulating the energy system without storage, as shown in Figure 11, using the load and calculated sizes for PV and wind turbine allows to identify the correlation between the load demand and renewable generation.

The difference between the load demand and the renewable generation at different timings can be found by subtracting the load demand from the renewable

1200, 2200, 3200 kW, and thus the wind turbine is sized first. The average power of the wind turbine (Pw) is first selected to be close to the average demand of the

The rated power output for the wind turbine is then calculated using the proposed wind turbine model given by Eq. (5). Note that, the annual average site wind speed is calculated using Eq. (6) and the wind turbine rotor coefficient of performance (Cp), which is a measure of wind turbines blade rotor effectiveness at converting the power in the wind to mechanical power, is calculated using

> Pw <sup>¼</sup> <sup>1</sup> 2

360 8760 � � π

Cpð Þ¼ <sup>λ</sup> <sup>0</sup>:<sup>5</sup> <sup>λ</sup> � <sup>0</sup>:02β<sup>2</sup> � <sup>2</sup>:<sup>9</sup> � �<sup>e</sup>

� � � �

CPρAv<sup>3</sup> � �

180

exp � <sup>v</sup>

C � �<sup>k</sup> ( ) !

<sup>λ</sup> � <sup>0</sup>:0303<sup>λ</sup> (7)

(5)

(6)

defined load (Pdem); i.e., Pw ffi Pdem:

The proposed deterministic sizing algorithm.

v ¼ 1 þ Svar cos thr

Eq. (7).

150

Figure 10.

Energy Storage Devices

Figure 11. Simulating the system without energy storage.

resource value for each recorded sample. Summing the differences for all the sample intervals yields a negative value, indicating a supply deficit, which defines the energy storage size. Equation (12) defines the size of energy storage (ES) needed to cover this deficit.

$$
\overline{E}\_{\mathbb{S}} = \sum E\_{bal} < 0 \tag{12}
$$

not the case, any leakage will impact the overall system efficiency and the operation

To address these challenges, a model that includes the thermal compensation effect to accurately simulate the hydrogen production from an electrolyzer fed by RES is developed in this section. The developed thermally compensated electrolyzer model can also be used as a tool to detect any H2 leakage and identify performance issues within an operational electrolyzer. The developed model, when implemented on working HES systems, allows the identification of hydrogen leakages without the

Ignoring the effect of temperature on the electrolyzer hydrogen production, as the case with HOMER, may lead to unrealistic simulation results because the electrolyzer production efficiency at lower temperatures is lower than that at full production temperature. This result has been illustrated when the developed model was implemented on a 30 kW electrolyzer, as a case study, to simulate the effect of temperature across its full range of hydrogen production. Figure 12 demonstrates the impact of heat compensation on the electrolyzer hydrogen production efficiency. The developed model is based on the combination of heat transfer theory, fundamental thermo-dynamics, and empirical electrochemical relationships, as measured from operating systems. The developed model is detailed in the following

7.1 The algorithm proposed for identifying the effect of thermal transients on

To identify the effects of thermal transients on the overall hydrogen production from an electrolyzer, a three-step algorithm is developed as shown in Figure 13. The first step involves simulating the hydrogen production from a renewable-powered electrolyzer with the electrolyzer model compensated using the effects of temperature on its hydrogen production. The second step involves repeating the simulation, but with the electrolyzer temperature fixed at its full working temperature (i.e., effects of thermal transients ignored). In the final step, the overall effect of thermal transients on hydrogen production is calculated by subtracting the step 1 hydrogen

cost and could result into a potential safety hazard as well.

the electrolyzer hydrogen production

Impact of heat compensation on the efficiency of a 30-kW electrolyzer.

subsections.

Hydrogen Energy Storage

DOI: http://dx.doi.org/10.5772/intechopen.88902

Figure 12.

153

need for maintenance inspection thus reducing the operating costs.

The hydrogen storage tank must be sized to hold enough hydrogen for the fuel cell to deliver the energy requirements (ES), thus the average fuel cell conversion efficiency is considered. Therefore, the energy that is required to be stored within the hydrogen storage tank (Etank) can be defined as shown in Eq. (13).

$$
\overline{E}\_{tank} = \overline{E}\_t \cdot \frac{1}{\overline{\eta}\_{\rm FC}} \tag{13}
$$

where ηFC is the fuel cell conversion efficiency considering the lower heating value (LHV) of the hydrogen gas.

The average volume of hydrogen (VLtank) that needs to be stored in the hydrogen tank is then calculated, based on the absolute energy content of the hydrogen gas, using Eq. (14). The absolute energy content of the hydrogen gas is known as the higher heating value (HHV) of the hydrogen gas which is known to be 3.55 kWh/ Nm3 [52].

$$
\overline{VL}\_{tank} = \frac{\overline{E}\_{tank}}{3.55} \tag{14}
$$

#### 7. Modeling the effect of thermal transients on the hydrogen production of renewably powered electrolyzers and utilizing the developed model as a tool to identify performance issues within operational hydrogen systems

A great challenge that faces the application of renewable-powered hydrogen energy storage systems is the ability to accurately determine the hydrogen production of an electrolyzer running on RES. Other challenges include their high costs and the need to guarantee their reliable and safe operation. A way forward toward achieving cost reduction is to lower their operation and maintenance costs by improving the hydrogen production efficiency and the system performance [53]. To achieve this, while ensuring safe system operation, the HES system must be able to handle the hydrogen gas securely and any leak must be quickly identified. If this is

#### Hydrogen Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.88902

not the case, any leakage will impact the overall system efficiency and the operation cost and could result into a potential safety hazard as well.

To address these challenges, a model that includes the thermal compensation effect to accurately simulate the hydrogen production from an electrolyzer fed by RES is developed in this section. The developed thermally compensated electrolyzer model can also be used as a tool to detect any H2 leakage and identify performance issues within an operational electrolyzer. The developed model, when implemented on working HES systems, allows the identification of hydrogen leakages without the need for maintenance inspection thus reducing the operating costs.

Ignoring the effect of temperature on the electrolyzer hydrogen production, as the case with HOMER, may lead to unrealistic simulation results because the electrolyzer production efficiency at lower temperatures is lower than that at full production temperature. This result has been illustrated when the developed model was implemented on a 30 kW electrolyzer, as a case study, to simulate the effect of temperature across its full range of hydrogen production. Figure 12 demonstrates the impact of heat compensation on the electrolyzer hydrogen production efficiency.

The developed model is based on the combination of heat transfer theory, fundamental thermo-dynamics, and empirical electrochemical relationships, as measured from operating systems. The developed model is detailed in the following subsections.

#### 7.1 The algorithm proposed for identifying the effect of thermal transients on the electrolyzer hydrogen production

To identify the effects of thermal transients on the overall hydrogen production from an electrolyzer, a three-step algorithm is developed as shown in Figure 13. The first step involves simulating the hydrogen production from a renewable-powered electrolyzer with the electrolyzer model compensated using the effects of temperature on its hydrogen production. The second step involves repeating the simulation, but with the electrolyzer temperature fixed at its full working temperature (i.e., effects of thermal transients ignored). In the final step, the overall effect of thermal transients on hydrogen production is calculated by subtracting the step 1 hydrogen

Figure 12. Impact of heat compensation on the efficiency of a 30-kW electrolyzer.

resource value for each recorded sample. Summing the differences for all the sample intervals yields a negative value, indicating a supply deficit, which defines the energy storage size. Equation (12) defines the size of energy storage (ES) needed to

The hydrogen storage tank must be sized to hold enough hydrogen for the fuel cell to deliver the energy requirements (ES), thus the average fuel cell conversion efficiency is considered. Therefore, the energy that is required to be stored within

Etank <sup>¼</sup> Es: <sup>1</sup>

where ηFC is the fuel cell conversion efficiency considering the lower heating

The average volume of hydrogen (VLtank) that needs to be stored in the hydrogen tank is then calculated, based on the absolute energy content of the hydrogen gas, using Eq. (14). The absolute energy content of the hydrogen gas is known as the higher heating value (HHV) of the hydrogen gas which is known to be 3.55 kWh/

VLtank <sup>¼</sup> Etank

7. Modeling the effect of thermal transients on the hydrogen production of renewably powered electrolyzers and utilizing the developed model as a tool to identify performance issues within operational

A great challenge that faces the application of renewable-powered hydrogen energy storage systems is the ability to accurately determine the hydrogen production of an electrolyzer running on RES. Other challenges include their high costs and the need to guarantee their reliable and safe operation. A way forward toward achieving cost reduction is to lower their operation and maintenance costs by improving the hydrogen production efficiency and the system performance [53]. To achieve this, while ensuring safe system operation, the HES system must be able to handle the hydrogen gas securely and any leak must be quickly identified. If this is

ηFC

the hydrogen storage tank (Etank) can be defined as shown in Eq. (13).

ES <sup>¼</sup> <sup>X</sup>Ebal <sup>&</sup>lt; <sup>0</sup> (12)

<sup>3</sup>:<sup>55</sup> (14)

(13)

cover this deficit.

Energy Storage Devices

Figure 11.

Nm3 [52].

152

value (LHV) of the hydrogen gas.

Simulating the system without energy storage.

hydrogen systems

Figure 13.

Proposed algorithm to identify the impact of thermal transients on the electrolyzer hydrogen production.

output of thermally compensated model from the hydrogen output of the fixed temperature model of step 2.

The voltage (U) required to breakdown the water to produce hydrogen can be

where U is the water breakdown (or hydrogen production) voltage (V); Urev is the overvoltage beyond reversible electrochemical cell voltage; r1,2 is the empirical

The reversible cell voltage (Urev) is calculated using the empirical Nernst equa-

<sup>T</sup> <sup>þ</sup> <sup>9</sup>:<sup>523</sup> � <sup>10</sup>�<sup>5</sup>

The Faraday efficiency is the ratio between the actual and maximum theoretical hydrogen mass that can be produced by an electrolyzer. Faraday efficiency losses are caused by parasitic current losses within the electrolysis cell stack. The parasitic current loss increases as a percentage of the overall current with the decreasing current densities and increasing temperatures. Therefore, the percentage of parasitic current loss to the total current flow increases with decreasing current densities. An empirical equation for the Faraday efficiency is shown in Eq. (17).

<sup>t</sup><sup>1</sup> <sup>þ</sup> <sup>t</sup><sup>2</sup> <sup>T</sup> <sup>þ</sup> <sup>t</sup><sup>3</sup> T2 <sup>A</sup> <sup>I</sup> !

!

þ 1

); T is the temperature (K); t1,2,3 is

); s is the overvoltage

); and I is the current (A).

TlnT <sup>þ</sup> <sup>9</sup>:<sup>84</sup> � <sup>10</sup>�<sup>8</sup>T<sup>2</sup> (16)

(15)

expressed in terms of (Urev). The voltage required to facilitate the electrolytic dissociation of water molecules is temperature dependent and can be expressed as

<sup>A</sup> <sup>I</sup> <sup>þ</sup> slog

<sup>U</sup> <sup>¼</sup> Urev <sup>þ</sup> ð Þ <sup>r</sup><sup>1</sup> <sup>þ</sup> <sup>r</sup>2<sup>T</sup>

Incorporating the effect of thermal transients in electrolyzer model.

the empirical overvoltage parameter of electrode (mA�<sup>1</sup> m<sup>2</sup>

parameter of electrode (V); A is the electrode area (m<sup>2</sup>

ohmic resistance parameter of electrolyte (Ωm<sup>2</sup>

tion for electrolysis given by Eq. (16) [20].

Urev,T Kð Þ <sup>¼</sup> <sup>1</sup>:<sup>5184</sup> � <sup>1</sup>:<sup>5421</sup> � <sup>10</sup>�<sup>3</sup>

7.2.2 The Faraday efficiency

155

shown in Eq. (15).

Hydrogen Energy Storage

DOI: http://dx.doi.org/10.5772/intechopen.88902

Figure 14.

#### 7.2 Developing the electrolyzer model

A robust electrolyzer model, shown in Figure 14, is developed in this section to be used in the proposed algorithm to identify the effects of thermal transients on the overall hydrogen production. To formulate an accurate and robust electrolyzer model that can accurately predict the electrochemical and thermal dynamic behavior of an advanced alkaline electrolyzer, the model is developed based on Øystein Ulleberg model [19] while integrating the voltage/current U-I relationship, the faraday efficiency, as well as the thermal and the pressurized hydrogen storage modeling components.

#### 7.2.1 The voltage/current U-I curve

An electrolyzer operating characteristic is determined by its voltage and current profile. The quantity of hydrogen produced by an electrolyzer varies with the amount of current passing through the electrolytic cell stack. The electrolytic cell voltage develops as more current is absorbed by the electrolyzer to increase the gas output flow. This U-I relationship would be a straight line for an ideal electrolyzer; however, it is a nonlinear relationship due to losses occurring in the electrochemistry and cell structure. The relationship is affected by the ohmic resistance of the electrolyte and electrodes as well as the parasitic loss of "stray" electrolysis. The parasitic loss of stray electrolysis is a phenomenon where the electrons flow down the electrolyte fluid channels instead of flowing directly between the electrodes themselves.

Figure 14. Incorporating the effect of thermal transients in electrolyzer model.

The voltage (U) required to breakdown the water to produce hydrogen can be expressed in terms of (Urev). The voltage required to facilitate the electrolytic dissociation of water molecules is temperature dependent and can be expressed as shown in Eq. (15).

$$U = U\_{rev} + \frac{(r\_1 + r\_2T)}{A}I + sl \log\left(\left(\frac{t\_1 + \frac{t\_2}{T} + \frac{t\_3}{T^2}}{A}I\right) + 1\right) \tag{15}$$

where U is the water breakdown (or hydrogen production) voltage (V); Urev is the overvoltage beyond reversible electrochemical cell voltage; r1,2 is the empirical ohmic resistance parameter of electrolyte (Ωm<sup>2</sup> ); T is the temperature (K); t1,2,3 is the empirical overvoltage parameter of electrode (mA�<sup>1</sup> m<sup>2</sup> ); s is the overvoltage parameter of electrode (V); A is the electrode area (m<sup>2</sup> ); and I is the current (A).

The reversible cell voltage (Urev) is calculated using the empirical Nernst equation for electrolysis given by Eq. (16) [20].

$$U\_{nv,T(K)} = 1.5184 - 1.5421 \times 10^{-3}T + 9.523 \times 10^{-5}Tl\'{n}T + 9.84 \times 10^{-8}T^2 \tag{16}$$

#### 7.2.2 The Faraday efficiency

The Faraday efficiency is the ratio between the actual and maximum theoretical hydrogen mass that can be produced by an electrolyzer. Faraday efficiency losses are caused by parasitic current losses within the electrolysis cell stack. The parasitic current loss increases as a percentage of the overall current with the decreasing current densities and increasing temperatures. Therefore, the percentage of parasitic current loss to the total current flow increases with decreasing current densities. An empirical equation for the Faraday efficiency is shown in Eq. (17).

output of thermally compensated model from the hydrogen output of the fixed

Proposed algorithm to identify the impact of thermal transients on the electrolyzer hydrogen production.

A robust electrolyzer model, shown in Figure 14, is developed in this section to be used in the proposed algorithm to identify the effects of thermal transients on the overall hydrogen production. To formulate an accurate and robust electrolyzer model that can accurately predict the electrochemical and thermal dynamic behavior of an advanced alkaline electrolyzer, the model is developed based on Øystein Ulleberg model [19] while integrating the voltage/current U-I relationship, the faraday efficiency, as well as the thermal and the pressurized hydrogen storage

An electrolyzer operating characteristic is determined by its voltage and current

profile. The quantity of hydrogen produced by an electrolyzer varies with the amount of current passing through the electrolytic cell stack. The electrolytic cell voltage develops as more current is absorbed by the electrolyzer to increase the gas output flow. This U-I relationship would be a straight line for an ideal electrolyzer; however, it is a nonlinear relationship due to losses occurring in the electrochemistry and cell structure. The relationship is affected by the ohmic resistance of the electrolyte and electrodes as well as the parasitic loss of "stray" electrolysis. The parasitic loss of stray electrolysis is a phenomenon where the electrons flow down the electrolyte fluid channels instead of flowing directly between the electrodes

temperature model of step 2.

Figure 13.

Energy Storage Devices

modeling components.

themselves.

154

7.2.1 The voltage/current U-I curve

7.2 Developing the electrolyzer model

$$\eta\_F = \frac{\left(\frac{I}{A}\right)^2}{f\_1 + \left(\frac{I}{A}\right)^2} f\_2 \tag{17}$$

where Q\_ gen is the thermal energy created by electrolysis process; Q\_ loss is the thermal energy lost to the environment; Q\_ cool is the thermal energy dissipated by cooling system; Ct is the thermal capacity (or inertia) of electrolyzer (JK�<sup>1</sup>

electrolysis stack; η<sup>e</sup> is the energy efficiency (%); Utn is the thermo-neutral voltage (V); U is the cell voltage (V); T is the electrolyzer temperature (K); Ta is the

To calculate the electrolyzer temperature as time passes (T), it is assumed that the electrolyzer exhibits a constant heat generation and heat transfer profile for a small-time interval of not more than a few seconds. An intra-time-step steady-state thermal model can be expressed, as shown in Eq. (24), where Tini is the initial

When hydrogen is produced by the electrolyzer, there is a need to store it and therefore there is a need to include hydrogen storage modeling to the proposed model. The two main components needed to model pressurized hydrogen storage is the formula for the pressure considering the gas behavior and the compressibility factor Z. The ideal gas relationship can be used to describe the behavior of real hydrogen gas accurately only at relatively low pressures up to approximately 1450 psig and at normal ambient temperatures, results then become increasingly inaccurate at higher pressures. One of the easiest ways to account for this additional compression is through the addition of a compressibility factor, designated by the symbol Z. The Z factor is derived from the data obtained through experimentation and it depends on temperature, pressure, and on the nature of the gas. The Z factor is used as a multiplier to adjust the ideal gas law to fit into the actual gas behavior, as shown in Eq. (25).

where P is the absolute pressure in Pascal; ρ is the density; T is the absolute temperature in Kelvin; and R is the universal gas constant, 8.31434 Nm/mol K.

<sup>ρ</sup>RT <sup>¼</sup> <sup>1</sup> <sup>þ</sup><sup>X</sup>

7.3 Developing the electrolyzer model using MATLAB/Simulink

way to assess the integrity of operational real-life hydrogen installations.

Calculating Z: The National Institute for Standards and Technology has developed a mathematical method for calculating compressibility factors accurately using a virial equation based on pressure (MPa) and temperature (K) [22]. The compressibility factor for hydrogen at different pressures and temperatures can be calculated to a high degree of accuracy by using Eq. (26) and the constants listed in Table 4 [23, 24].

9

100 T � �bi P

i¼1 ai

where the equation and its constants are defined for pressures in Mega-Pascal

The previously developed modeling equations are used in this section to develop a MATLAB-Simulink model. The developed MATLAB model is then used in a novel

the thermal resistance of electrolyzer (W�<sup>1</sup>

DOI: http://dx.doi.org/10.5772/intechopen.88902

Hydrogen Energy Storage

temperature and Δ<sup>t</sup> is the change in time.

7.2.4 Pressurized hydrogen storage modeling

ambient temperature (K); and t is the time (seconds).

T ¼ Tini þ

Z Pð Þ¼ , <sup>T</sup> <sup>P</sup>

(MPa) and temperatures in Kelvin (K).

157

Δt Ct

); Rt is

K); nc is the number of cells in the

<sup>Q</sup>\_ gen � <sup>Q</sup>\_ loss � <sup>Q</sup>\_ cool � � (24)

P ¼ ZρRT (25)

1 � �ci

(26)

where η<sup>F</sup> is the Faraday efficiency; A is the electrode area (m<sup>2</sup> ); I is the current (A); f1 is the Faraday efficiency parameter mA2 cm�<sup>4</sup> ; f2 is the Faraday efficiency parameter (number between 0 and 1); and f1 and f2 are selected empirically.

Faraday's law also models the production rate of hydrogen in an electrolytic cell. The production rate of hydrogen is directly proportional to the transfer rate of electrons at the electrodes. This is equivalent to the electrical current provided by the power supply. Therefore, the total hydrogen production rate in an electrolysis stack consisting of several cells connected in series can be expressed, as shown in Eq. (18).

$$
\dot{m}\_{H\_2} = \eta\_F \frac{n\_c I}{zF} \tag{18}
$$

where n\_ H2 is the molar flow rate (mol s�<sup>1</sup> ); η<sup>F</sup> is the Faraday efficiency; z is 2 (number of electrons transferred per reaction); I is the current (A); F is the Faraday constant 96,485 C mol�<sup>1</sup> ; and nc is the number of series cells in electrolyzer cell stack.

#### 7.2.3 The thermal model

The production of heat in an electrolyzer is primarily caused by electrical inefficiencies. The energy efficiency can be calculated from the thermo-neutral voltage (Utn) and the cell voltage (U) using Eq. (19).

$$
\eta\_e = \frac{U\_{tn}}{U} \tag{19}
$$

where η<sup>e</sup> is the energy efficiency; Utn is the thermo-neutral voltage ffi 1.477 V; and U is the cell voltage.

The value for Utn remains almost constant within the pressure and temperature range considered here (0–1200 kPa pressure, 0–80°C temperature), this value is 1.477 V [21].

The operating temperature of an electrolyzer can be found from the overall thermal energy balance of the electrolysis system. The thermal energy balance of the electrolyzer can be expressed, as shown in Eq. (20), where Eq. (21) calculates the thermal energy created by the electrolysis process, and Eq. (22) is used to calculate the thermal losses of the electrolyzer system. Equation (23) is applied to maintain the electrolyzer temperature at or below the maximum temperature specified by manufacturer; it is assumed that electrolyzer cooling system is sufficient to remove the excess heat generated by the electrolysis process.

$$\mathbf{C}\_{t}\frac{dT}{dt} = \dot{\mathbf{Q}}\_{\text{gen}} - \dot{\mathbf{Q}}\_{\text{loss}} - \dot{\mathbf{Q}}\_{\text{cool}} \tag{20}$$

$$
\dot{Q}\_{gen} = n\_c (U - U\_{tn}) I = n\_c U I (1 - \eta\_e) \tag{21}
$$

$$
\dot{Q}\_{loss} = \frac{1}{R\_t}(T - T\_a) \tag{22}
$$

$$
\dot{Q}\_{col} > \dot{Q}\_{gen} - \dot{Q}\_{loss} \tag{23}
$$

η<sup>F</sup> ¼

parameter (number between 0 and 1); and f1 and f2 are selected empirically.

The production rate of hydrogen is directly proportional to the transfer rate of electrons at the electrodes. This is equivalent to the electrical current provided by the power supply. Therefore, the total hydrogen production rate in an electrolysis stack consisting of several cells connected in series can be expressed, as shown in

n\_<sup>H</sup><sup>2</sup> ¼ η<sup>F</sup>

(number of electrons transferred per reaction); I is the current (A); F is the Faraday

The production of heat in an electrolyzer is primarily caused by electrical inefficiencies. The energy efficiency can be calculated from the thermo-neutral voltage

<sup>η</sup><sup>e</sup> <sup>¼</sup> Utn

where η<sup>e</sup> is the energy efficiency; Utn is the thermo-neutral voltage ffi 1.477 V;

The value for Utn remains almost constant within the pressure and temperature range considered here (0–1200 kPa pressure, 0–80°C temperature), this value is

The operating temperature of an electrolyzer can be found from the overall thermal energy balance of the electrolysis system. The thermal energy balance of the electrolyzer can be expressed, as shown in Eq. (20), where Eq. (21) calculates the thermal energy created by the electrolysis process, and Eq. (22) is used to calculate the thermal losses of the electrolyzer system. Equation (23) is applied to maintain the electrolyzer temperature at or below the maximum temperature specified by manufacturer; it is assumed that electrolyzer cooling system is sufficient to

remove the excess heat generated by the electrolysis process.

<sup>Q</sup>\_ los<sup>s</sup> <sup>¼</sup> <sup>1</sup> Rt

Ct dT ncI

; and nc is the number of series cells in electrolyzer cell

where η<sup>F</sup> is the Faraday efficiency; A is the electrode area (m<sup>2</sup>

(A); f1 is the Faraday efficiency parameter mA2 cm�<sup>4</sup>

where n\_ H2 is the molar flow rate (mol s�<sup>1</sup>

(Utn) and the cell voltage (U) using Eq. (19).

Eq. (18).

Energy Storage Devices

stack.

constant 96,485 C mol�<sup>1</sup>

7.2.3 The thermal model

and U is the cell voltage.

1.477 V [21].

156

I A <sup>2</sup>

<sup>2</sup> <sup>f</sup> <sup>2</sup> (17)

zF (18)

); η<sup>F</sup> is the Faraday efficiency; z is 2

<sup>U</sup> (19)

dt <sup>¼</sup> <sup>Q</sup>\_ gen � <sup>Q</sup>\_ loss � <sup>Q</sup>\_ cool (20)

<sup>Q</sup>\_ cool <sup>&</sup>gt; <sup>Q</sup>\_ gen � <sup>Q</sup>\_ loss (23)

ð Þ T � Ta (22)

<sup>Q</sup>\_ gen <sup>¼</sup> ncð Þ <sup>U</sup> � Utn <sup>I</sup> <sup>¼</sup> ncUI <sup>1</sup> � <sup>η</sup><sup>e</sup> ð Þ (21)

); I is the current

; f2 is the Faraday efficiency

<sup>f</sup> <sup>1</sup> <sup>þ</sup> <sup>I</sup> A

Faraday's law also models the production rate of hydrogen in an electrolytic cell.

where Q\_ gen is the thermal energy created by electrolysis process; Q\_ loss is the thermal energy lost to the environment; Q\_ cool is the thermal energy dissipated by cooling system; Ct is the thermal capacity (or inertia) of electrolyzer (JK�<sup>1</sup> ); Rt is the thermal resistance of electrolyzer (W�<sup>1</sup> K); nc is the number of cells in the electrolysis stack; η<sup>e</sup> is the energy efficiency (%); Utn is the thermo-neutral voltage (V); U is the cell voltage (V); T is the electrolyzer temperature (K); Ta is the ambient temperature (K); and t is the time (seconds).

To calculate the electrolyzer temperature as time passes (T), it is assumed that the electrolyzer exhibits a constant heat generation and heat transfer profile for a small-time interval of not more than a few seconds. An intra-time-step steady-state thermal model can be expressed, as shown in Eq. (24), where Tini is the initial temperature and Δ<sup>t</sup> is the change in time.

$$T = T\_{ini} + \frac{\Delta t}{\mathcal{C}\_t} \left( \dot{Q}\_{gen} - \dot{Q}\_{loss} - \dot{Q}\_{cool} \right) \tag{24}$$

#### 7.2.4 Pressurized hydrogen storage modeling

When hydrogen is produced by the electrolyzer, there is a need to store it and therefore there is a need to include hydrogen storage modeling to the proposed model. The two main components needed to model pressurized hydrogen storage is the formula for the pressure considering the gas behavior and the compressibility factor Z.

The ideal gas relationship can be used to describe the behavior of real hydrogen gas accurately only at relatively low pressures up to approximately 1450 psig and at normal ambient temperatures, results then become increasingly inaccurate at higher pressures. One of the easiest ways to account for this additional compression is through the addition of a compressibility factor, designated by the symbol Z. The Z factor is derived from the data obtained through experimentation and it depends on temperature, pressure, and on the nature of the gas. The Z factor is used as a multiplier to adjust the ideal gas law to fit into the actual gas behavior, as shown in Eq. (25).

$$P = Z\rho RT\tag{25}$$

where P is the absolute pressure in Pascal; ρ is the density; T is the absolute temperature in Kelvin; and R is the universal gas constant, 8.31434 Nm/mol K.

Calculating Z: The National Institute for Standards and Technology has developed a mathematical method for calculating compressibility factors accurately using a virial equation based on pressure (MPa) and temperature (K) [22]. The compressibility factor for hydrogen at different pressures and temperatures can be calculated to a high degree of accuracy by using Eq. (26) and the constants listed in Table 4 [23, 24].

$$Z(P,T) = \frac{P}{\rho RT} = 1 + \sum\_{i=1}^{9} a\_i \left(\frac{100}{T}\right)^{b\_i} \left(\frac{P}{1}\right)^{c\_i} \tag{26}$$

where the equation and its constants are defined for pressures in Mega-Pascal (MPa) and temperatures in Kelvin (K).

#### 7.3 Developing the electrolyzer model using MATLAB/Simulink

The previously developed modeling equations are used in this section to develop a MATLAB-Simulink model. The developed MATLAB model is then used in a novel way to assess the integrity of operational real-life hydrogen installations.


7.4 Developing the thermally compensated electrolyzer model: compensating

Simulating an electrolyzer without considering the thermal compensation leads to a nonnegligible error in the simulation which can lead to a miscalculation of the return on investment (ROI). This is especially true during a cold start of an electrolyzer. An electrolyzer is said to be in a cold start when it is switched on in any of the following situations: (i) the electrolyzer is cold (not heated up and not at its standard operating temperature), (ii) not under pressure. Note that a standard operating temperature for an alkaline electrolyzer is about 80°C; for a proton exchange membrane (PEM) electrolyzer, it is about 70°C; and for a solid oxide electrolyzer (SOE), this varies with the material being used to construct the cells. In a cold start situation, the electrolyzer is cold and not under pressure and thus its efficiency is low as it requires pressurizing and heating itself up. This takes a short time if the electrolyzer is small; however, this time dramatically increases as the size of the electrolyzer increases. For alkaline electrolyzers, this time may be 1 s for small ones but can take up to several minutes for large ones; however, time is less for the PEM technology. The developed thermally compensated model will therefore be focused on alkaline electrolyzers as they are the

To simulate the practical operation of an electrolysis system, it is important to include compensation of the effects of temperature on the electrolytic process. The exothermic thermal reaction that takes place during electrolysis in an alkaline electrolyzer impacts the energy efficiency of the gas generation process and especially the UI relationships. In other words, as hydrogen is being produced by the electrolyzer, an electrochemical reaction takes place at the electrodes. This reaction heats up the electrolyte and the associated electrode materials (this is known as the exothermic reaction) resulting into an increase in temperature which leads to reduction in the cell voltage and cell current needed to generate the hydrogen gas. In other words, the increase in the electrolyzer temperature reduces the power requirements of the electrolytic cells for the same hydrogen production, thereby

Integrating the thermal model within the previously developed electrolyzer model enables the thermal energy efficiency to be incorporated, and thus allows the model to generate output data that is extremely close to the real-world electrolyzer performance. Figure 16 depicts the thermally compensated architecture included in the developed Simulink model. The new model considers the thermal energy generated by electrolysis (Q gen), the thermal capacity of the electrolyzer itself to absorb and dissipate thermal energy (Q loss), and the cooling system to maintain the thermal equilibrium required for an efficient hydrogen generation (Q cool) and utilizes

Two case studies are provided in this section to verify the developed model and validate that thermally compensated electrolyzer models are critical not only for designing new hydrogen installations, but also for monitoring the performance of operational ones. These case studies are carried out on two real-world field installed systems. The first one is used to verify that the developed model can accurately simulate hydrogen energy systems. The second is used to verify that the developed model can be used as a tool for investigating the operational integrity of operating electrolyzer systems and checking their performance while identifying any failures

Eq. (23) to calculate the thermal balance of the electrolytic process.

the temperature effect in an electrolysis system

Hydrogen Energy Storage

DOI: http://dx.doi.org/10.5772/intechopen.88902

ones that suffer the most from heat compensation effect.

increasing the system efficiency.

7.5 Verification of the developed model

159

to support the reduction in maintenance requirements.

Molar mass: M = 2.01588 g/mol. Universal Gas Constant: R = 8.314472 J/(mol K).

#### Table 4.

Constants to calculate Z.

Figure 15. Interaction between the U-I, the Faraday efficiency and the thermal subsystem models in MATLAB/Simulink.

The hydrogen generation and storage mathematical model, described by Eqs. (15)–(26), is implemented under the Simulink framework to develop the MATLAB Simulink model. Figure 15 illustrates the interaction between the developed MATLAB Simulink subsystems for the U-I model, the Faraday efficiency model, and the thermal model but without any thermal compensation. The pressurized modeling is not shown in Figure 15, but it has been accounted for through the H2 Molar Mass.

However, the simulation results for this model do not reflect the real hydrogen installations results, and thus the developed model is modified to include a newly added thermal compensation factor which is detailed in the next subsection.

#### 7.4 Developing the thermally compensated electrolyzer model: compensating the temperature effect in an electrolysis system

Simulating an electrolyzer without considering the thermal compensation leads to a nonnegligible error in the simulation which can lead to a miscalculation of the return on investment (ROI). This is especially true during a cold start of an electrolyzer. An electrolyzer is said to be in a cold start when it is switched on in any of the following situations: (i) the electrolyzer is cold (not heated up and not at its standard operating temperature), (ii) not under pressure. Note that a standard operating temperature for an alkaline electrolyzer is about 80°C; for a proton exchange membrane (PEM) electrolyzer, it is about 70°C; and for a solid oxide electrolyzer (SOE), this varies with the material being used to construct the cells. In a cold start situation, the electrolyzer is cold and not under pressure and thus its efficiency is low as it requires pressurizing and heating itself up. This takes a short time if the electrolyzer is small; however, this time dramatically increases as the size of the electrolyzer increases. For alkaline electrolyzers, this time may be 1 s for small ones but can take up to several minutes for large ones; however, time is less for the PEM technology. The developed thermally compensated model will therefore be focused on alkaline electrolyzers as they are the ones that suffer the most from heat compensation effect.

To simulate the practical operation of an electrolysis system, it is important to include compensation of the effects of temperature on the electrolytic process. The exothermic thermal reaction that takes place during electrolysis in an alkaline electrolyzer impacts the energy efficiency of the gas generation process and especially the UI relationships. In other words, as hydrogen is being produced by the electrolyzer, an electrochemical reaction takes place at the electrodes. This reaction heats up the electrolyte and the associated electrode materials (this is known as the exothermic reaction) resulting into an increase in temperature which leads to reduction in the cell voltage and cell current needed to generate the hydrogen gas. In other words, the increase in the electrolyzer temperature reduces the power requirements of the electrolytic cells for the same hydrogen production, thereby increasing the system efficiency.

Integrating the thermal model within the previously developed electrolyzer model enables the thermal energy efficiency to be incorporated, and thus allows the model to generate output data that is extremely close to the real-world electrolyzer performance. Figure 16 depicts the thermally compensated architecture included in the developed Simulink model. The new model considers the thermal energy generated by electrolysis (Q gen), the thermal capacity of the electrolyzer itself to absorb and dissipate thermal energy (Q loss), and the cooling system to maintain the thermal equilibrium required for an efficient hydrogen generation (Q cool) and utilizes Eq. (23) to calculate the thermal balance of the electrolytic process.

#### 7.5 Verification of the developed model

Two case studies are provided in this section to verify the developed model and validate that thermally compensated electrolyzer models are critical not only for designing new hydrogen installations, but also for monitoring the performance of operational ones. These case studies are carried out on two real-world field installed systems. The first one is used to verify that the developed model can accurately simulate hydrogen energy systems. The second is used to verify that the developed model can be used as a tool for investigating the operational integrity of operating electrolyzer systems and checking their performance while identifying any failures to support the reduction in maintenance requirements.

The hydrogen generation and storage mathematical model, described by Eqs. (15)–(26), is implemented under the Simulink framework to develop the MATLAB Simulink model. Figure 15 illustrates the interaction between the developed MATLAB Simulink subsystems for the U-I model, the Faraday efficiency model, and the thermal model but without any thermal compensation. The pressurized modeling is not shown in Figure 15, but it has been accounted for through

Interaction between the U-I, the Faraday efficiency and the thermal subsystem models in MATLAB/Simulink.

I ai bi ci 0.05888460 1.325 1.0 0.06136111 1.87 1.0 0.002650473 2.5 2.0 0.002731125 2.8 2.0 0.001802374 2.938 2.42 0.001150707 3.14 2.63 0.9588528 <sup>10</sup><sup>4</sup> 3.37 3.0 0.1109040 <sup>10</sup><sup>6</sup> 3.75 4.0 0.1264403 <sup>10</sup><sup>9</sup> 4.0 5.0

However, the simulation results for this model do not reflect the real hydrogen installations results, and thus the developed model is modified to include a newly added thermal compensation factor which is detailed in the next

the H2 Molar Mass.

Molar mass: M = 2.01588 g/mol.

Constants to calculate Z.

Energy Storage Devices

Table 4.

Universal Gas Constant: R = 8.314472 J/(mol K).

subsection.

158

Figure 15.

Variable Description Unit Value r1 Electrolyte ohmic resistive parameter Ωm<sup>2</sup> 0.0000805 r2 Electrolyte ohmic resistive parameter <sup>Ω</sup>m<sup>2</sup> 0.0000025 A Electrode area m<sup>2</sup> 0.37 S Overvoltage parameter of electrode V 0.19 t1 Empirical overvoltage parameter of electrode A<sup>1</sup> m<sup>2</sup> 1.002 t2 Empirical overvoltage parameter of electrode A<sup>1</sup> m<sup>2</sup> 8.424 t3 Empirical overvoltage parameter of electrode A<sup>1</sup> m<sup>2</sup> 247.3 f1 Faraday efficiency parameter mA <sup>2</sup> cm<sup>4</sup> 200 f2 Faraday efficiency parameter 0….1 0.94 Vstd Volume of ideal gas at STP m<sup>3</sup> mol<sup>1</sup> 0.0224136

Rt Thermal resistance of electrolyzer W<sup>1</sup>

Electrolyzer variables—variables for the 30 kW alkaline electrolyzer.

Variables for the hydrogen storage—4800 L void capacity.

Data-log of the current consumed by 30-kW electrolyzer while in operation.

Table 5.

Hydrogen Energy Storage

DOI: http://dx.doi.org/10.5772/intechopen.88902

Table 6.

Figure 18.

161

Ct Thermal capacity of electrolyzer JK<sup>1</sup> 300,000 nc Number of cells in electrolysis stack N 180 Ta Ambient temperature °C 20 Tmax Maximum operating temperature of electrolyzer °C 60 Thyst Cooling hysteresis thermal band °C 3

Variable Description Unit Value V Tank volume m<sup>3</sup> 4.8 Ta Ambient temperature °C 20

K 0.018

Figure 16. The thermally compensated model developed under MATLAB/Simulink framework.

#### 7.5.1 Verifying the developed model can accurately simulate hydrogen energy generators

A 30 kW real-world alkaline electrolyzer, which is operational within an existing hydrogen installation, is chosen to verify that the developed electrolyzer Simulink model can accurately simulate it. This 30 kW alkaline electrolyzer can develop 5.3 Nm3 /h of hydrogen at a pressure up to 1200 kPa. It consists of two electrolytic cell stacks; each stack has 90 cells configured in a series connected array. The electrolyzer is designed to operate at a temperature of 60°C. Figure 17 shows the electrolyzer (i.e., the hydrogen generator) connected to a 4800 L gas bottle array for the storage of the generated hydrogen at a pressure up to 1200 kPa.

Table 5 gives the values for the electrolyzer variables, while values for the variables of the modular hydrogen storage system which is connected directly to the electrolysis system are given in Table 6.

A data-log of the current consumed by the real-world electrolyzer while in operation is given in Figure 18. The electrolyzer temperature and pressure responses to the current are also recorded and compared to the results from the developed Simulink model, as illustrated in Figures 19 and 20, respectively. Both figures demonstrate that the developed model results are very close to the data collected from the operating electrolyzer, thus confirming that the developed model can be used for accurately simulating real-world installations.

On further analyzing Figure 19, it can be noticed that the electrolyzer is switched on at time 133 s (cold start) and it reaches its operating temperature (and pressure) at time 309 s; thus, the time taken from the cold start to the operating temperature is 176 s. This means that the electrolyzer almost took 3 min to reach its operating conditions and substantial amounts of hydrogen will not be produced

Figure 17. Hybrid renewable H2 generation and storage.

#### Hydrogen Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.88902


#### Table 5.

7.5.1 Verifying the developed model can accurately simulate hydrogen energy generators

cell stacks; each stack has 90 cells configured in a series connected array. The electrolyzer is designed to operate at a temperature of 60°C. Figure 17 shows the electrolyzer (i.e., the hydrogen generator) connected to a 4800 L gas bottle array

for the storage of the generated hydrogen at a pressure up to 1200 kPa.

The thermally compensated model developed under MATLAB/Simulink framework.

can be used for accurately simulating real-world installations.

electrolysis system are given in Table 6.

5.3 Nm3

Figure 17.

160

Hybrid renewable H2 generation and storage.

Figure 16.

Energy Storage Devices

A 30 kW real-world alkaline electrolyzer, which is operational within an existing hydrogen installation, is chosen to verify that the developed electrolyzer Simulink model can accurately simulate it. This 30 kW alkaline electrolyzer can develop

/h of hydrogen at a pressure up to 1200 kPa. It consists of two electrolytic

Table 5 gives the values for the electrolyzer variables, while values for the variables of the modular hydrogen storage system which is connected directly to the

A data-log of the current consumed by the real-world electrolyzer while in operation is given in Figure 18. The electrolyzer temperature and pressure responses to the current are also recorded and compared to the results from the developed Simulink model, as illustrated in Figures 19 and 20, respectively. Both figures demonstrate that the developed model results are very close to the data collected from the operating electrolyzer, thus confirming that the developed model

On further analyzing Figure 19, it can be noticed that the electrolyzer is switched on at time 133 s (cold start) and it reaches its operating temperature (and pressure) at time 309 s; thus, the time taken from the cold start to the operating temperature is 176 s. This means that the electrolyzer almost took 3 min to reach its operating conditions and substantial amounts of hydrogen will not be produced

Electrolyzer variables—variables for the 30 kW alkaline electrolyzer.


#### Table 6.

Variables for the hydrogen storage—4800 L void capacity.

#### Figure 18. Data-log of the current consumed by 30-kW electrolyzer while in operation.

7.5.2 Verifying that the developed model can be used as a tool for identifying performance

The thermally compensated electrolyzer model is further tested in one of the most unexploited applications for any model—its use in a postinstallation scenario. When an electrolyzer model is developed, it is usually used in the preinstallation stage to investigate if the planned system will operate as anticipated when installed in the field. In this section, the developed model is further used in a postinstallation scenario to demonstrate that it is also capable to detect issues within operating

The developed Simulink model was used to simulate an operating hydrogen generator when its performance was detected to be not as anticipated for a couple of weeks. It was suspected that the electrolyzer has developed an internal issue, so the aim was to examine if the developed model can be used in a post-installation situation to determine this performance issue. The operating hydrogen system, on which the developed Simulink model was tested in a postinstallation scenario, is been operating in Africa for over 8 years and it employs a 30 kW alkaline electrolyzer identical to that given in Table 5 connected to a storage system of 2499 L void volume capacity. On comparing the model output results to the on-site collected data, two performance issues were doubted. The first was an early degradation of the stack; however, this was disregarded as none of the other similar installed stacks illustrated such a drastic performance issue. The second was the presence of a hydrogen leak; and this was clear from the divergence between the modeled and recorded data shown in Figure 21. The figure shows that there was more H2 production in the model results than what was achieved in the practical installation, and this in turn suggests a leak within the installed system. This suggestion was confirmed by an on-site inspection of the hydrogen system which revealed that a fitting in the pipe that carries the H2 gas from the electrolyzer to the storage system had developed a premature failure. The fast rate of detected leak spray bubbling, shown in Figure 21, indicates the presence of a leak on two sides of the faulty pipe fitting shown in Figure 22. This finding clearly demonstrates the apparent benefit of the developed model in identifying leakages during operation,

systems removing the need for maintenance crew on-site inspection.

Divergence between the modeled and recorded pressures indicating a suspected hydrogen gas leak.

issues within operational hydrogen systems

DOI: http://dx.doi.org/10.5772/intechopen.88902

Hydrogen Energy Storage

Figure 21.

163

Figure 19. Recorded electrolyzer temperature versus model output.

Figure 20. Recorded electrolyzer pressure versus model output.

during this time. If such a small electrolyzer took 3 min to reach its steady-state operation, then it can be tangibly assumed that this time will be much higher for a larger scale electrolyzer and substantial loss in hydrogen production could be realized. Considering the financials, it will be consequently affected by the loss in the hydrogen production during the cold start period. It can therefore be concluded that the inaccurate hydrogen production numbers generated from nonthermally compensated hydrogen generators simulation models will generate misleading higher ROI values. Thus, it can be also concluded that the developed thermally compensated simulation model is essential for accurately calculating the potential for financial return of a hydrogen system since it allows the accurate computation of hydrogen production.

#### 7.5.2 Verifying that the developed model can be used as a tool for identifying performance issues within operational hydrogen systems

The thermally compensated electrolyzer model is further tested in one of the most unexploited applications for any model—its use in a postinstallation scenario. When an electrolyzer model is developed, it is usually used in the preinstallation stage to investigate if the planned system will operate as anticipated when installed in the field. In this section, the developed model is further used in a postinstallation scenario to demonstrate that it is also capable to detect issues within operating systems removing the need for maintenance crew on-site inspection.

The developed Simulink model was used to simulate an operating hydrogen generator when its performance was detected to be not as anticipated for a couple of weeks. It was suspected that the electrolyzer has developed an internal issue, so the aim was to examine if the developed model can be used in a post-installation situation to determine this performance issue. The operating hydrogen system, on which the developed Simulink model was tested in a postinstallation scenario, is been operating in Africa for over 8 years and it employs a 30 kW alkaline electrolyzer identical to that given in Table 5 connected to a storage system of 2499 L void volume capacity. On comparing the model output results to the on-site collected data, two performance issues were doubted. The first was an early degradation of the stack; however, this was disregarded as none of the other similar installed stacks illustrated such a drastic performance issue. The second was the presence of a hydrogen leak; and this was clear from the divergence between the modeled and recorded data shown in Figure 21. The figure shows that there was more H2 production in the model results than what was achieved in the practical installation, and this in turn suggests a leak within the installed system. This suggestion was confirmed by an on-site inspection of the hydrogen system which revealed that a fitting in the pipe that carries the H2 gas from the electrolyzer to the storage system had developed a premature failure. The fast rate of detected leak spray bubbling, shown in Figure 21, indicates the presence of a leak on two sides of the faulty pipe fitting shown in Figure 22. This finding clearly demonstrates the apparent benefit of the developed model in identifying leakages during operation,

during this time. If such a small electrolyzer took 3 min to reach its steady-state operation, then it can be tangibly assumed that this time will be much higher for a larger scale electrolyzer and substantial loss in hydrogen production could be realized. Considering the financials, it will be consequently affected by the loss in the hydrogen production during the cold start period. It can therefore be concluded that the inaccurate hydrogen production numbers generated from nonthermally compensated hydrogen generators simulation models will generate misleading higher ROI values. Thus, it can be also concluded that the developed thermally compensated simulation model is essential for accurately calculating the potential for financial return of a hydrogen system since it allows the accurate computation of

hydrogen production.

162

Figure 19.

Energy Storage Devices

Figure 20.

Recorded electrolyzer temperature versus model output.

Recorded electrolyzer pressure versus model output.

• An algorithm for modeling the impact of thermal transients, especially in alkaline electrolyzers, on the overall hydrogen production has been developed.

The prolonged thermal transients, associated with electrolyzers fed by renewable energy sources, result into extended periods of time where the electrolyzer does not produce hydrogen at its highest efficiency, and thus resulting into an overall reduction in its hydrogen production. The typical effect of thermal transients on the electrolyzer hydrogen production can be found by using the proposed algorithm, and a reduction in the cumulative hydrogen production was found to be in the range between 1 and 3%.

• The thermally compensated electrolyzer model has been developed in Simulink and has proven, through a case study, to be able to accurately simulate

hydrogen generation and storage systems. The developed model presents a key finding for the hydrogen industry as it does not only allow the investigation of hydrogen systems performance in a preinstallation scenario prior to embarking

into the expensive capital investment, but also proven to be useful in postinstallation scenarios. The developed model was found to be able to simulate operational installed hydrogen systems and assist in identifying their

performance issues accurately.

Hydrogen Energy Storage

DOI: http://dx.doi.org/10.5772/intechopen.88902

Author details

165

Dallia Mahmoud Morsi Ali

provided the original work is properly cited.

Robert Gordon University, Aberdeen, Scotland, United Kingdom

\*Address all correspondence to: dolly.ali@hotmail.co.uk; d.ali@rgu.ac.uk

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Figure 22. Source of leakage identified after on-site inspection.

and thus it can save the time and cost of maintenance inspection as well as preventing the safety hazards associated with the hydrogen vented in atmosphere.

The developed thermally compensated model has been able to reveal this hydrogen gas leak, which was about 10.89 g, equating to a 2.3% reduction in the overall system efficiency. This leak was so small for the leak detection system to detect; therefore, if many small leaks like this occur at different locations of a largescale system without being detected by the safety alarm system this could lead to more financial losses and potentially a hazardous situation. Therefore, the developed model can be used as a tool to provide an early warning of leakages or other issues, and thus provided an extra layer of safety and a potential for increasing the financial return through the development of a predictive maintenance system.

#### 8. Conclusion

In conclusion, the contributions to knowledge that has been presented within this chapter can be summarized as follows:


#### Hydrogen Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.88902


#### Author details

and thus it can save the time and cost of maintenance inspection as well as

8. Conclusion

164

Figure 22.

Energy Storage Devices

this chapter can be summarized as follows:

Source of leakage identified after on-site inspection.

preventing the safety hazards associated with the hydrogen vented in atmosphere. The developed thermally compensated model has been able to reveal this hydrogen gas leak, which was about 10.89 g, equating to a 2.3% reduction in the overall system efficiency. This leak was so small for the leak detection system to detect; therefore, if many small leaks like this occur at different locations of a largescale system without being detected by the safety alarm system this could lead to more financial losses and potentially a hazardous situation. Therefore, the developed model can be used as a tool to provide an early warning of leakages or other issues, and thus provided an extra layer of safety and a potential for increasing the financial return through the development of a predictive maintenance system.

In conclusion, the contributions to knowledge that has been presented within

• A novel levelized cost model has been developed for investigating the financial competitiveness of the hydrogen energy storage technology. It has been identified that hydrogen use as an energy storage mechanism achieves the most

financial competitiveness when the by-product oxygen is utilized.

• A new deterministic sizing methodology that offers a rapid initial sizing of renewable hydrogen energy storage systems has been given. The proposed method requires a very limited number of input data to offer an initial system size for a hybrid renewable hydrogen energy storage system (HRHES) very quickly, and thus it is useful at the very early initial design phase to assist in the early decision-making for system implementation. To develop this sizing model, a model has been developed for every single item in the proposed HRHES (the implemented renewable energy sources, the electrolyzer, H2 storage, and fuel cell). These models were then integrated together.

Dallia Mahmoud Morsi Ali Robert Gordon University, Aberdeen, Scotland, United Kingdom

\*Address all correspondence to: dolly.ali@hotmail.co.uk; d.ali@rgu.ac.uk

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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### *Edited by M. Taha Demirkan and Adel Attia*

Energy storage will be a very important part of the near future, and its effectiveness will be crucial for most future technologies. Energy can be stored in several different ways and these differ in terms of the type and the conversion method of the energy. Among those methods; chemical, mechanical, and thermal energy storage are some of the most favorable methods for containing energy. Current energy storage devices are still far from meeting the demands of new technological developments. Therefore, much effort has been put to improving the performance of different types of energy storage technologies in the last few decades.

Published in London, UK © 2019 IntechOpen © spainter\_vfx / iStock

Energy Storage Devices

Energy Storage Devices

*Edited by M. Taha Demirkan and Adel Attia*