**4.1 Carbon-ruthenium oxide (RuO2)-based composite electrode materials**

Among the metal oxides, ruthenium oxide (RuO2) has been considered as very common electrode materials for SCs in acidic medium due to their excellent pseudocapacity which is arising from high conductivity, good thermal stability, highly reversible redox reactions, three different oxidation states within 1.2 V, and high specific capacitance natures. The pseudocapacitance mechanism of RuO2 for SC electrodes can be described as equation [24]:

$$\text{RuO}\_2 + \text{xH}^+ + \text{xe}^- = \text{RuO}\_{2-\text{x}} \left( \text{OH} \right)\_\text{x} \left( 0 \le \text{x} \le 2 \right) \tag{1}$$

Or

$$\text{RuO}\_2 + \text{H}^+ + \text{e}^- = \text{RuOOH} \tag{2}$$

**121**

equation [28].

**Figure 3**

*density [26].*

*Carbon-Based Nanocomposite Materials for High-Performance Supercapacitors*

solid-state supercapacitors. RuO2/NGA composite with finely tuned mass loading of 16.3 μg cm−2 and transmittance of 34.1% (λ = 550 nm) demonstrated maximum areal energy of 0.074 μW h cm−2 and power of 64 μW cm−2 with cyclic stability of 100% over 2000 cycles [27]. This RuO2/NGA based high transparent SC can be

*(a) Schematic presentation of RuOx deposited on the vertically aligned porous carbon nanotubes porous through ALD by sequential pulsing of Ru (EtCp)2 and oxygen. (b) and (c) SEM and TEM images of vertically aligned CNTs coated with ALD RuOx [25]. (d) Microfabrication process of 3D porous RuO2/LSG interdigitated micro-supercapacitors through direct laser writing on a DVD disc using a LightScribe DVD burner. (e) A high-magnification TEM image of 3D porous RuO2/LSG composite showing complete wrapping of the RuO2 nanoparticles (NP) by multiple layers of the graphene sheets. (f) The gravimetric capacitance retention of laser scribed graphene (LSG) and RuO2/LSG electrodes as a function of the applied current* 

**4.2 Carbon-manganese oxides (MnO2)-based composite electrode materials**

MnO2 has been considered as a promising pseudocapacitive electrode materials for energy storage applications due to low price, abundant reserve, high specific capacitance, and environmental environment benign nature and low toxicity in comparison to other transition-metal oxides. In general, the charge storage mechanism of MnO2 involves change in manganese oxidation state from +3 to +4 and the contribution of protons or alkali cations, which can be shown in the following

MnO C e MnOOC <sup>2</sup>

+ − + +↔ (3)

practically used in many advanced transparent electrical devices.

*DOI: http://dx.doi.org/10.5772/intechopen.95460*

However, its scarcity and high cost limits the fabrication of RuO2 based electrodes for potential applications. But, smartly use of composite materials by synergic integration of pseudocapacitive RuO2 materials with conductive carbonaceous substrates not only improves the capacitance but also reduces the cost of the electrode. Recent studies are more focus about the selecting the best carbonaceous substrate and the synthesis procedures to fabricate ruthenium oxide (RuO2)-coated on the porous carbonaceous substrates.

RuO2-CNT composite has been prepared by uniformly coating of RuO2 on the vertically aligned porous carbon nanotubes porous through atomic layer deposition (ALD) technique and further activation by voltammetry potential coulometry (**Figure 3(a-c)**) [25]. This ALD technique has many advantages such as deposition on large surface area, accurate thickness and exceptional uniformity for electrode designing in energy storage devises. The as-prepared RuO2-CNT composite shows excellent electrochemical performance as an electrode material for SC in respect of capacitance, power density and stability. Several publications have been reported the specific capacitance and power density of RuO2-CNT composite, which are around 650 F g−1 and 17 kW kg−1, respectively. Kaner *et.al* recently demonstrated the synthesis and processing of 3D porous RuO2/laser-scribed graphene (LSG) composite electrode for miniaturized and interdigitated SC that exhibit ultrahigh energy and power density (**Figure 3(d)**) [26]. The high-resolution TEM (HRTEM) image of 3D porous RuO2/LSG composite in **Figure 3(e)** shows that multiple layers of the graphene sheets wrap around each RuO2 nanoparticle. 3D porous RuO2/LSG composite electrode showed an ultrahigh specific capacitance of 1139 Fg−1 with outstanding rate capability and the asymmetric supercapacitor (ASC) made of 3D porous RuO2/LSG composite electrode as positive electrode exhibited an extremely high energy density of 55 W h kg−1 at a power density of 12 kW kg−1 (**Figure 3(f )**). Other interesting composite of RuO2 made of RuO2 decorated nitrogen-doped reduced graphene oxide aerogel (NGA) are used as high-performance transparent

*Carbon-Based Nanocomposite Materials for High-Performance Supercapacitors DOI: http://dx.doi.org/10.5772/intechopen.95460*

#### **Figure 3**

*Novel Nanomaterials*

Or

layer capacitance mechanisms [21–23].

electrodes can be described as equation [24]:

on the porous carbonaceous substrates.

these regards but suffer from comparatively limited specific capacitance. Hence, the synergic integration of metal oxides with conducting carbon supports may form high potential carbon-metal oxide composite electrodes materials for SCs and hybrid devices because of their enhanced electrochemical performance through the combined effect of pseudocapacitive/faradaic charge storage and electrical double

**4.1 Carbon-ruthenium oxide (RuO2)-based composite electrode materials**

Among the metal oxides, ruthenium oxide (RuO2) has been considered as very common electrode materials for SCs in acidic medium due to their excellent pseudocapacity which is arising from high conductivity, good thermal stability, highly reversible redox reactions, three different oxidation states within 1.2 V, and high specific capacitance natures. The pseudocapacitance mechanism of RuO2 for SC

RuO H e RuOOH <sup>2</sup>

RuO2-CNT composite has been prepared by uniformly coating of RuO2 on the vertically aligned porous carbon nanotubes porous through atomic layer deposition (ALD) technique and further activation by voltammetry potential coulometry (**Figure 3(a-c)**) [25]. This ALD technique has many advantages such as deposition on large surface area, accurate thickness and exceptional uniformity for electrode designing in energy storage devises. The as-prepared RuO2-CNT composite shows excellent electrochemical performance as an electrode material for SC in respect of capacitance, power density and stability. Several publications have been reported the specific capacitance and power density of RuO2-CNT composite, which are around 650 F g−1 and 17 kW kg−1, respectively. Kaner *et.al* recently demonstrated the synthesis and processing of 3D porous RuO2/laser-scribed graphene (LSG) composite electrode for miniaturized and interdigitated SC that exhibit ultrahigh energy and power density (**Figure 3(d)**) [26]. The high-resolution TEM (HRTEM) image of 3D porous RuO2/LSG composite in **Figure 3(e)** shows that multiple layers of the graphene sheets wrap around each RuO2 nanoparticle. 3D porous RuO2/LSG composite electrode showed an ultrahigh specific capacitance of 1139 Fg−1 with outstanding rate capability and the asymmetric supercapacitor (ASC) made of 3D porous RuO2/LSG composite electrode as positive electrode exhibited an extremely high energy density of 55 W h kg−1 at a power density of 12 kW kg−1 (**Figure 3(f )**). Other interesting composite of RuO2 made of RuO2 decorated nitrogen-doped reduced graphene oxide aerogel (NGA) are used as high-performance transparent

However, its scarcity and high cost limits the fabrication of RuO2 based electrodes for potential applications. But, smartly use of composite materials by synergic integration of pseudocapacitive RuO2 materials with conductive carbonaceous substrates not only improves the capacitance but also reduces the cost of the electrode. Recent studies are more focus about the selecting the best carbonaceous substrate and the synthesis procedures to fabricate ruthenium oxide (RuO2)-coated

<sup>2</sup> 2 x ( ) ( ) <sup>x</sup> RuO xH xe RuO OH 0 x 2 + − + += <sup>−</sup> ≤ ≤ (1)

+ − + += (2)

**120**

*(a) Schematic presentation of RuOx deposited on the vertically aligned porous carbon nanotubes porous through ALD by sequential pulsing of Ru (EtCp)2 and oxygen. (b) and (c) SEM and TEM images of vertically aligned CNTs coated with ALD RuOx [25]. (d) Microfabrication process of 3D porous RuO2/LSG interdigitated micro-supercapacitors through direct laser writing on a DVD disc using a LightScribe DVD burner. (e) A high-magnification TEM image of 3D porous RuO2/LSG composite showing complete wrapping of the RuO2 nanoparticles (NP) by multiple layers of the graphene sheets. (f) The gravimetric capacitance retention of laser scribed graphene (LSG) and RuO2/LSG electrodes as a function of the applied current density [26].*

solid-state supercapacitors. RuO2/NGA composite with finely tuned mass loading of 16.3 μg cm−2 and transmittance of 34.1% (λ = 550 nm) demonstrated maximum areal energy of 0.074 μW h cm−2 and power of 64 μW cm−2 with cyclic stability of 100% over 2000 cycles [27]. This RuO2/NGA based high transparent SC can be practically used in many advanced transparent electrical devices.

#### **4.2 Carbon-manganese oxides (MnO2)-based composite electrode materials**

MnO2 has been considered as a promising pseudocapacitive electrode materials for energy storage applications due to low price, abundant reserve, high specific capacitance, and environmental environment benign nature and low toxicity in comparison to other transition-metal oxides. In general, the charge storage mechanism of MnO2 involves change in manganese oxidation state from +3 to +4 and the contribution of protons or alkali cations, which can be shown in the following equation [28].

$$\text{MnO}\_2 + \text{C}^\* + \text{e}^- \leftrightarrow \text{MnOOC} \tag{3}$$

Where C+ represents protons or alkali cations (Li+ , Na+ , K+ ).

However, MnO2 based electrodes limits the capacity and power density due to their low surface area and poor electronic/ionic conductivity. Therefore, the composite of MnO2 with high-surface area and conducting carbonaceous materials may improve the electrochemical performance in terms of specific capacity, energy and power densities by providing the larger interfacial area between the MnO2 particles and the electrolyte solution [29].

Gao *et al.* fabricated a MnO2/activated carbon (AC) based hybrid SC, where AC not only acted as a conducting support but also increase the capacitance as well as energy and power densities [30]. In addition, engineering the morphology of MnO2 into different nanostructures is considered to be a practical approach to increase its electrochemical performance. It is reported that the pore sizes of the mesoporous-MnO2/AC are greatly affected the specific capacitance and the rate capability of the SCs. Huang *et al.* demonstrated the influence of CNT on the electrochemical properties of MnO2-CNT composite electrode by controlling the growth of MnO2 nanostructures on CNTs through a facile redox approach (**Figure 4(a-c)**) [31]. The as-prepared MnO2-CNT composite electrode showed a maximum specific capacitance of 247.9 F g−1 with outstanding cyclic stability of 92.8% after 5000 cycles. In addition, it has been noticed that the aligned CNTs are more favoured as SC electrodes over nonaligned CNTs due to their large specific surface area, low contact resistance, and fast electron-transfer kinetics. Graphene is being used as a supporting material for MnO2 nanostructures due to its large surface area, high conductivity, and high stability nature. For example, microwave

#### **Figure 4.**

*(a) Controlled growth of MnO2 nanostructured on CNT surface through facile redox method. (b) TEM images displaying coverage of MnO2 on the surface of CNT. (c) The cyclic curve of a MnO2-CNT nanowire composite at current density of 2 A g−1 [31]. (d) Schematic illustration of fabrication of Co3O4 nanoflake/graphene@ Ni hybrid electrode materials by in situ synthesis method. (e) Top-view SEM images of the Co3O4 nanoflake/ graphene/Ni hybrid electrode. (f) GCD curves of Co3O4 nanoflake/graphene/Ni hybrid electrode at current density of 1 mA cm−2 [39]. (g) A schematic of the synthesis of the porous Co3O4 nanoball/CA hybrid. (h) FE-SEM images of the porous Co3O4 nanoball/CA hybrid. (i) The specific capacitance test of the porous Co3O4 nanoball/CA hybrid electrode at a current density of 1 A g−1 as a function of cycle number (inset: 11 cycles continuous GCD curves obtained for porous Co3O4 nanoball/CA hybrid electrode for the different cycle numbers) [41].*

**123**

CoO:

Co3O4:

*Carbon-Based Nanocomposite Materials for High-Performance Supercapacitors*

retention of 61% at a current density of 40 mA cm−2 [34].

originates from the following redox reaction: [36]

irradiation synthesised MnO2-graphene composites exhibited the maximum capacitance of 310 F g−1, which is much higher than the bare graphene and MnO2 (110 F g−1) [32]. Beside their high capacitance, MnO2-graphene composites have better cyclic stability of 95% over 15000 cycles. The excellent electrochemical performance of MnO2-graphene composites is due to large surface area and high conductivity of graphene network. Recently, Zhang *et al.* reported highly flexible ASCs based on graphene hydrogel (GH)/copper wire (CW) as the negative electrode and hierarchical MnO2/graphene/carbon fiber (CF) as the positive electrode, which exhibited excellent areal energy density of 18.1 μW h cm−2 and operated reversibly at potential window of 0-1.6 V [33]. 3D porous carbon nanostructures can also be used as MnO2 support for supercapacitor (SC) electrodes as they provided large surface area, well-defined pathways to electrolyte access, and better mechanical stability. Fang *et al.* demonstrated a novel solid-state symmetric supercapacitor (SSC) based on 3D rGO@MnO2 foam electrode and Polyacrylic Acid (PAA)-Portland cement-KOH electrolyte, which showed a very high areal capacity of 1.84 F cm−2 at current density of 0.5 mA cm−2 and excellent capacitance

**4.3 Carbon-cobalt oxides (CoO/Co3O4)-based composite electrode materials**

Cobalt oxides has been received considerable attention as highly promising SC electrode materials due to their non-toxic, low cost, easy synthesis, environmentally friendly, and more importantly high theoretical capacitance (CoO: 4292 F g−1, Co3O4: 3560 F g−1) [35]. In addition, cobalt oxides exhibits outstanding electrochemical behaviour in alkaline as well as organic electrolyte, which is possible due to their ability to interact with the ions at the electrolyte surface as well as through the bulk of the material. The pseudocapacitance of cobalt oxides (CoO/Co3O4) are

> Co OH 4 CoOOH e CoOOH OH 4 CoO H O e *O* − −

> Co O OH H O 3CoOOH e CoOOH OH CoO H O e

However, the low electrical/ionic conductivity of cobalt oxides hinders their practical performance as SC electrodes. Most efficient way to improve their electrochemical performance is to form composites of cobalt oxides by incorporation into a carbon-based conducting supports. A Co3O4/AC composite SC electrode was reported by Iqbal *et al*. [37]. The electrode exhibited maximum achievable specific capacitance 567 F g−1 and maximum energy density of 63 W h kg−1 at 0.7 A g−1. In addition to the high specific capacitance, Co3O4/AC composite of capacitive retentivity is 82% after 6000 charge/discharge cycles and safe to handle due to no leakage. The specific capacitance of the cobalt oxide strongly depends on the microstructure and morphology of the materials, which facilitate the electrolyte ion transport through the material more effectively. Sun *et al.* demonstrated a

++↔ + + ↔ ++

3 4 2

+↔ + + ↔ ++

2 2

2 2

− − − −

(4)

(5)

− −

*DOI: http://dx.doi.org/10.5772/intechopen.95460*

*Carbon-Based Nanocomposite Materials for High-Performance Supercapacitors DOI: http://dx.doi.org/10.5772/intechopen.95460*

irradiation synthesised MnO2-graphene composites exhibited the maximum capacitance of 310 F g−1, which is much higher than the bare graphene and MnO2 (110 F g−1) [32]. Beside their high capacitance, MnO2-graphene composites have better cyclic stability of 95% over 15000 cycles. The excellent electrochemical performance of MnO2-graphene composites is due to large surface area and high conductivity of graphene network. Recently, Zhang *et al.* reported highly flexible ASCs based on graphene hydrogel (GH)/copper wire (CW) as the negative electrode and hierarchical MnO2/graphene/carbon fiber (CF) as the positive electrode, which exhibited excellent areal energy density of 18.1 μW h cm−2 and operated reversibly at potential window of 0-1.6 V [33]. 3D porous carbon nanostructures can also be used as MnO2 support for supercapacitor (SC) electrodes as they provided large surface area, well-defined pathways to electrolyte access, and better mechanical stability. Fang *et al.* demonstrated a novel solid-state symmetric supercapacitor (SSC) based on 3D rGO@MnO2 foam electrode and Polyacrylic Acid (PAA)-Portland cement-KOH electrolyte, which showed a very high areal capacity of 1.84 F cm−2 at current density of 0.5 mA cm−2 and excellent capacitance retention of 61% at a current density of 40 mA cm−2 [34].

## **4.3 Carbon-cobalt oxides (CoO/Co3O4)-based composite electrode materials**

Cobalt oxides has been received considerable attention as highly promising SC electrode materials due to their non-toxic, low cost, easy synthesis, environmentally friendly, and more importantly high theoretical capacitance (CoO: 4292 F g−1, Co3O4: 3560 F g−1) [35]. In addition, cobalt oxides exhibits outstanding electrochemical behaviour in alkaline as well as organic electrolyte, which is possible due to their ability to interact with the ions at the electrolyte surface as well as through the bulk of the material. The pseudocapacitance of cobalt oxides (CoO/Co3O4) are originates from the following redox reaction: [36]

CoO:

*Novel Nanomaterials*

Where C+

and the electrolyte solution [29].

represents protons or alkali cations (Li+

However, MnO2 based electrodes limits the capacity and power density due to their low surface area and poor electronic/ionic conductivity. Therefore, the composite of MnO2 with high-surface area and conducting carbonaceous materials may improve the electrochemical performance in terms of specific capacity, energy and power densities by providing the larger interfacial area between the MnO2 particles

Gao *et al.* fabricated a MnO2/activated carbon (AC) based hybrid SC, where AC not only acted as a conducting support but also increase the capacitance as well as energy and power densities [30]. In addition, engineering the morphology of MnO2 into different nanostructures is considered to be a practical approach to increase its electrochemical performance. It is reported that the pore sizes of the mesoporous-MnO2/AC are greatly affected the specific capacitance and the rate capability of the SCs. Huang *et al.* demonstrated the influence of CNT on the electrochemical properties of MnO2-CNT composite electrode by controlling the growth of MnO2 nanostructures on CNTs through a facile redox approach (**Figure 4(a-c)**) [31]. The as-prepared MnO2-CNT composite electrode showed a maximum specific capacitance of 247.9 F g−1 with outstanding cyclic stability of 92.8% after 5000 cycles. In addition, it has been noticed that the aligned CNTs are more favoured as SC electrodes over nonaligned CNTs due to their large specific surface area, low contact resistance, and fast electron-transfer kinetics. Graphene is being used as a supporting material for MnO2 nanostructures due to its large surface area, high conductivity, and high stability nature. For example, microwave

*(a) Controlled growth of MnO2 nanostructured on CNT surface through facile redox method. (b) TEM images displaying coverage of MnO2 on the surface of CNT. (c) The cyclic curve of a MnO2-CNT nanowire composite at current density of 2 A g−1 [31]. (d) Schematic illustration of fabrication of Co3O4 nanoflake/graphene@ Ni hybrid electrode materials by in situ synthesis method. (e) Top-view SEM images of the Co3O4 nanoflake/ graphene/Ni hybrid electrode. (f) GCD curves of Co3O4 nanoflake/graphene/Ni hybrid electrode at current density of 1 mA cm−2 [39]. (g) A schematic of the synthesis of the porous Co3O4 nanoball/CA hybrid. (h) FE-SEM images of the porous Co3O4 nanoball/CA hybrid. (i) The specific capacitance test of the porous Co3O4 nanoball/CA hybrid electrode at a current density of 1 A g−1 as a function of cycle number (inset: 11 cycles continuous GCD curves obtained for porous Co3O4 nanoball/CA hybrid electrode for the different cycle* 

, Na+ , K+ ).

**122**

*numbers) [41].*

**Figure 4.**

$$\begin{aligned} \text{CoO} + \text{OH}^- &\leftrightarrow \text{4} \cdot \text{CoOOH} + \text{e}^-\\ \text{CaOOH} + \text{OH}^- &\leftrightarrow \text{4} \cdot \text{CoO}\_2 + \text{H}\_2\text{O} + \text{e}^- \end{aligned} \tag{4}$$

Co3O4:

$$\begin{aligned} \text{Ca}\_3\text{O}\_4 + \text{OH}^- + \text{H}\_2\text{O} &\leftrightarrow \text{3CaOOH} + \text{e}^-\\ \text{CaOOH} + \text{OH}^- &\leftrightarrow \text{CaO}\_2 + \text{H}\_2\text{O} + \text{e}^- \end{aligned} \tag{5}$$

However, the low electrical/ionic conductivity of cobalt oxides hinders their practical performance as SC electrodes. Most efficient way to improve their electrochemical performance is to form composites of cobalt oxides by incorporation into a carbon-based conducting supports. A Co3O4/AC composite SC electrode was reported by Iqbal *et al*. [37]. The electrode exhibited maximum achievable specific capacitance 567 F g−1 and maximum energy density of 63 W h kg−1 at 0.7 A g−1. In addition to the high specific capacitance, Co3O4/AC composite of capacitive retentivity is 82% after 6000 charge/discharge cycles and safe to handle due to no leakage. The specific capacitance of the cobalt oxide strongly depends on the microstructure and morphology of the materials, which facilitate the electrolyte ion transport through the material more effectively. Sun *et al.* demonstrated a

simple and effective approach to grow well-aligned 3D cobalt oxide nanowire arrays (Co3O4 NWAs) directly on carbon nanotube fibers (CNTFs) through CVD process [38]. The Co3O4 NWAs/CNFs showed a specific capacitance of 734.25 F cm−3 (2210 mF cm−2) at 1.0 A cm−3 and a high energy density of 13.2 mW h cm−3 at a current density of 1.0 A cm−3. Graphene along with cobalt oxides can be used as a composite material for SCs because of its high conductivity, high surface area, high carrier mobility, and excellent mechanical strength. For example, an in situ synthesised Co3O4/graphene@NF hybrid composite electrode with a thickness of 13 nm exhibited a high specific capacitance of 1.75 F cm−2 at 1 mA cm−2 and a capacitance increase of 12.2% after 5000 cycles at 10 mA cm−2 (**Figure 4(d-f ))** [39]. Tseng *et al.* demonstrate a binder-free and flexible SC based on CoO/graphene hollow nanoballs (GHBs) composite electrode [40]. The as fabricated CoO/GHBs composite electrode exhibits high specific capacitance of 2238 F g−1 at a current density of 1 A g−1 and good rate capability of 1170 F g−1 at a current density of 15 A g−1. The excellent capacitive performance and high rate capability were accomplished by the synergistic combination of conductive GHBs with large surface areas and highly pseudocapacitive CoO. In addition, as fabricated SSC demonstrated a very high power density (6000 W kg−1 at 8.2 W h kg−1), high energy density (16 W h kg−1 at 800 W kg−1), good cycling stability (∼100% capacitance retention after 5000 cycles), and excellent mechanical flexibility at various bending positions. Recently, 3D-carbon aerogels (3D-CA) with appropriate electrical conductivity, high specific surface area and rich dielectric electrochemical stability when combined with the porous cobalt oxides can enabled the fabrication of an composite electrode with outstanding electrochemical performance.Co3O4/CA composite electrode which was synthesized through in situ growth method showed a specific capacitance of 350 F g−1 at 1 A g−1 and Energy density of 23.82 kW kg−1 at a power density of 95.96 W kg−1 (**Figure 4(g-i))** [41]. The as-prepared ASC device could be cycled reversibly in a potential range of 0.0 to 1 V at 1 A g−1 and showed a capacity retention of 210% over 6000 cycles. Zhu *et al.* adopted a facile hydrothermal method to synthesize self-assembled cobalt oxide (CoO) nanorod cluster on 3D-graphene foam (CoO-3DGF) which exhibits a very high performance compared with CoO nanorod clusters grown on Ni foam (680 F g−1) in terms of specific capacitance 980 F g−1 at 1 A g−1 and cycling stability of 103% over 10,000 cycles [42].

#### **4.4 Carbon-binary metal oxide based composite electrode materials**

Recently, binary metal oxides such as NiCo2O4, NiFe2O4, CoFe2O4, ZnMnO4, and ZnCo2O4 have attracted much attention due to higher electrical conductivity than individual metal oxide and provide higher capacitance due to more affluent redox reaction than individual components [43]. Even though binary meal oxides possess better electrochemical performance than individual metal oxide extremely, they still suffer from inferior rate performance, low utilization rate and poor cycle stability. However, by incorporating carbon based materials improve their conductivity as well as power density due to high surface area, high conductivity and stable chemical properties of carbon based materials [44]. Kumar *et al.* fabricated Carbon black (CB) decorated Ni/Co oxide composite electrode through by using the successive ionic layer adsorption and reaction (SILAR) method [45]. Carbon black (CB) decorated Ni/Co oxide composite electrode with 7% weight percentage of CB exhibited a high specific capacitance of 1811 F g**<sup>−</sup>**<sup>1</sup> at 0.5 mA cm**<sup>−</sup>**<sup>2</sup> with excellent cyclic retention of 92% over 8000 cycles and delivered an impressive high energy density of 91 W h Kg**<sup>−</sup>**<sup>1</sup> at a power density of 151 W Kg**<sup>−</sup>**<sup>1</sup> , which is significantly higher than pure Ni/Co oxide composite electrode as well as other carbon embedded composites. Veerasubramani *et al.* have adopted a novel approach to fabricate

**125**

*Carbon-Based Nanocomposite Materials for High-Performance Supercapacitors*

CNT-deposited CoMoO4/Ni foam through a hydrothermal method followed by dry reforming reaction (DRR) of propane and CO2 [46, 47]. The as fabricated CNTdeposited CoMoO4/Ni foam electrode achieved a maximum areal capacity of 160

showed 22-fold higher performance than the heat-treated CoMoO4/Ni foam. The high electrochemical performance is due to the presence of CNTs on the surface of CoMoO4/Ni foam electrode, which increases the conductivity of the electrode and enhances the ion transport kinetics. Further as fabricated ASC device, consists of CNT-deposited CoMoO4/Ni foam as the positive electrode and reduced graphene oxide (rGO)-coated carbon cloth (CC) as the negative electrode stored a maximum

(29.04 Wh kg**<sup>−</sup>**<sup>1</sup>

at a scan rate of 5 mV/sec, which is almost 4 times larger than pure

) and showed the capacitance retention of 95% over 1000 cycles.

(1835 W kg**<sup>−</sup>**<sup>1</sup>

capacitance retention of more than 95% of its initial capacitance over 1500 cycles. Soam *et al.* synthesized porous type of NiFe2O4/graphene nanocomposite electrode by a solution based process for supercapacitor application [48]. The as-prepared NiFe2O4/graphene nanocomposite electrode exhibited a maximum specific capaci-

The significantly enhanced specific capacitance of the NiFe2O4/graphene nanocomposite electrode material is due to the synergic effect of high porous graphene sheets and NiFe2O4 particles, which are strongly interconnected together leading to a good electric/ionic conduction on the electrode and better contact of ions with the electrode materials. Zhou *et al.* reported a novel and green Cu2O template-assisted route based on "coordinating etching and precipitating" process for the synthesis of 3D porous reduced graphene (rGN)/NiCo2O4 film [49]. The as-synthesized 3D rGN/NiCo2O4 film exhibited high specific capacitance of 708.36 F g−1 at a current density of 1 A g−1 with a rate retention of 82.2% as current density ranges from 1 to 16 Ag−1, and remarkable capacitance retention of 94.3% after 6000 cycles at a high

**5. Carbon-metal hydroxide composites electrode materials**

interlayer space to occupy more ions hence larger capacitance.

**5.1 Carbon-nickel hydroxide (Ni(OH)2) composite electrode materials**

Ni(OH)2 is being considered as an attractive candidate as electrode in SCs because of its high theoretical capacitance (2358 F g−1). It can be prepared by a simple and low cost process. It has demonstrated good stability in alkaline electrolytes. Its low electrical conductivity is a barrier to achieve higher capacitance. Therefore, a thin region near the surface of nickel hydroxide contributes to the charge storage process due to diffusion-limited redox reactions. To obtain larger capacitance, it has to be utilized completely in the charge storage process. In this regard, researchers have generally adopted conductive additives to effectively improve utilization of active materials and result in larger capacitance. Kang *et al.* have used the same concept

Among the active materials, metal hydroxides have also been considered promising electrode materials for electrochemical SCs because of extremely high specific capacitance. Metal hydroxide in several forms such as Ni(OH)2, Co(OH)2, NiCo(OH)2, Cu(OH)2, FeOOH have been investigated as electrodes for SC [50–52]. These materials have large internal spaces for fast insertion and desertion of electrolyte ions. Moreover, these metal hydroxides can be synthesized using simple synthetic approaches. Metal hydroxide consists of stacked layers intercalated having

with excellent cyclic stability of ~105% over 3000 cycles and

) at 2 mA cm**<sup>−</sup>**<sup>2</sup>

) 10 mA cm**<sup>−</sup>**<sup>2</sup>

and delivered

with excellent

*DOI: http://dx.doi.org/10.5772/intechopen.95460*

at 1 mA cm**<sup>−</sup>**<sup>2</sup>

areal energy density of 122 μWh cm**<sup>−</sup>**<sup>2</sup>

a high power density of 7,727 μWcm**<sup>−</sup>**<sup>2</sup>

μAh cm**<sup>−</sup>**<sup>2</sup>

tance of 207 Fg**<sup>−</sup>**<sup>1</sup>

NiFe2O4 (60 Fg**<sup>−</sup>**<sup>1</sup>

current density of 10 A g−1.

*Carbon-Based Nanocomposite Materials for High-Performance Supercapacitors DOI: http://dx.doi.org/10.5772/intechopen.95460*

CNT-deposited CoMoO4/Ni foam through a hydrothermal method followed by dry reforming reaction (DRR) of propane and CO2 [46, 47]. The as fabricated CNTdeposited CoMoO4/Ni foam electrode achieved a maximum areal capacity of 160 μAh cm**<sup>−</sup>**<sup>2</sup> at 1 mA cm**<sup>−</sup>**<sup>2</sup> with excellent cyclic stability of ~105% over 3000 cycles and showed 22-fold higher performance than the heat-treated CoMoO4/Ni foam. The high electrochemical performance is due to the presence of CNTs on the surface of CoMoO4/Ni foam electrode, which increases the conductivity of the electrode and enhances the ion transport kinetics. Further as fabricated ASC device, consists of CNT-deposited CoMoO4/Ni foam as the positive electrode and reduced graphene oxide (rGO)-coated carbon cloth (CC) as the negative electrode stored a maximum areal energy density of 122 μWh cm**<sup>−</sup>**<sup>2</sup> (29.04 Wh kg**<sup>−</sup>**<sup>1</sup> ) at 2 mA cm**<sup>−</sup>**<sup>2</sup> and delivered a high power density of 7,727 μWcm**<sup>−</sup>**<sup>2</sup> (1835 W kg**<sup>−</sup>**<sup>1</sup> ) 10 mA cm**<sup>−</sup>**<sup>2</sup> with excellent capacitance retention of more than 95% of its initial capacitance over 1500 cycles. Soam *et al.* synthesized porous type of NiFe2O4/graphene nanocomposite electrode by a solution based process for supercapacitor application [48]. The as-prepared NiFe2O4/graphene nanocomposite electrode exhibited a maximum specific capacitance of 207 Fg**<sup>−</sup>**<sup>1</sup> at a scan rate of 5 mV/sec, which is almost 4 times larger than pure NiFe2O4 (60 Fg**<sup>−</sup>**<sup>1</sup> ) and showed the capacitance retention of 95% over 1000 cycles. The significantly enhanced specific capacitance of the NiFe2O4/graphene nanocomposite electrode material is due to the synergic effect of high porous graphene sheets and NiFe2O4 particles, which are strongly interconnected together leading to a good electric/ionic conduction on the electrode and better contact of ions with the electrode materials. Zhou *et al.* reported a novel and green Cu2O template-assisted route based on "coordinating etching and precipitating" process for the synthesis of 3D porous reduced graphene (rGN)/NiCo2O4 film [49]. The as-synthesized 3D rGN/NiCo2O4 film exhibited high specific capacitance of 708.36 F g−1 at a current density of 1 A g−1 with a rate retention of 82.2% as current density ranges from 1 to 16 Ag−1, and remarkable capacitance retention of 94.3% after 6000 cycles at a high current density of 10 A g−1.
