6. TiO2-V2O5 nanocomposites as supercapacitor applications

Among various transition metal oxides, vanadium oxides (V2O5) also known as vanadium pentoxides have already been studies as a promising supercapacitor electrode material for energy storage application due to its excellent physical properties,

pseudocapacitance. Underpotential deposition arises when reversible adsorptions as well as removal of atoms occur at metal surface in two dimensional Faradic reactions. Redox pseudocapacitance exists when reversible redox reactions taken place at the electrode surface. In case of intercalation pseudocapacitance, ions are elec-

Although these three mechanisms are physically different from each other but they can be electrochemically governed by the Nernst equation. According to this equation, if the reaction potential E can be approximated by a linear function of

> 1 þ Q <sup>r</sup> Q <sup>r</sup>

where, n is the number of electron, F is the Faraday constant, m is the molecular weight of active electrode and Qr is the reaction quotient. Transition metal oxides are chosen as the active materials for supercapacitor electrode and they store charge via Faradic or redox mechanism. They exhibit large theoretical specific capacitance with multiple valence states which enables them one of the most studied materials

Hybrid supercapacitors are third type of supercapacitors which combine the

Besides the electrodes, another most important factor which can expressively influence the electrochemical performance of supercapacitor device is electrolyte. Generally, electrolyte exists in inside the separator as well as inside the active material layers. The important factors for an electrolyte are one wide potential window which is key factors to achieve higher energy density and the other is the high ionic concentration, low resistivity, low viscosity etc. which can also influence the power density of the supercapacitor device. There are three types of electrolyte usually used in supercapacitors: aqueous electrolyte, organic electrolytes and ionic electrolytes. Aqueous electrolytes (such as H2SO4, KOH, Na2SO4, HCl, NaCl and NH4Cl aqueous solution and so on) limit the cell voltage window of supercapacitor to typically 0–1 V due to their low electrochemical stability, which effectively reduces the energy density of the cell. It can also provide a higher ionic concentra-

trolyte may exhibit higher charge storage capacity but the main drawback is in terms of improving both energy and power densities due to their narrow working

. Supercapacitors containing aqueous elec-

supercapacitors are made with composite materials that include EDLC materials (carbonaceous materials such as activated carbon, graphene, CNT etc.) and pseudocapacitive materials (transition metal oxides and conducting polymers). There can also be asymmetric supercapacitor with one pseudocapacitive electrode and another EDLC electrode or hybrid electrode or vice-versa. Several binary and ternary composite based on polymer and CNTs have been prepared for electrochemical capacitive energy storage application. They offer large specific capacitance compare to individual one, which is due to the strong interaction between polymer and CNTs. Gupta and Miural were the first to propose that SWNT/PANI composite can be effectively used as the electrodes for supercapacitors. The highest specific

features of both EDLCs and pseudocapacitors. The electrodes of hybrid

capacitance value of 463 F g�<sup>1</sup> was obtained for 27 wt% CNT.

(2)

trochemically intercalated into the structure of redox materials.

Science,Technology and Advanced Application of Supercapacitors

(1 + Qr)/Qr, the specific capacitance can be obtained from Eq. (2)

group in the field of supercapacitor [1, 19].

3.3 Hybrid supercapacitor

4. Electrolytes

92

tion with conductivity up to 1.0 S cm�<sup>1</sup>

Cm <sup>¼</sup> nF mE layered structure, non-toxic in nature, easy synthesis process, presence of several oxidation states (+2 to +5) and high specific capacity. But its low electrical conductivity limits its practical device application. The charge storage mechanism of a supercapacitor strongly depends on the surface properties of the electrode materials so the nanomaterial is suitable for electrode fabrication. Nanomaterial possesses high surface area along with high surface energy but the aggregation of nanoparticles is the most challenging problem. This effectively increases the strain for electrolyte ions diffusion within the nanoparticles at the surface of the electrode. Therefore, people are working to design and fabricate three-dimensional (3D), ordered and mesoporous nanomaterial to overcome such type of problem. There are four fundamental steps to control the charge storage behaviors of nanomaterials: (i) electron hopping between two nanoparticles; (ii) electron hopping within single nanoparticle; (iii) electron hopping between active electrode materials and current collectors; finally (iv) diffusion of proton within nanoparticles. It is well known that the proton diffusion and electron hoping within the nanoparticles are intrinsic properties of nanoparticles but the resistance due to intra-particle electron hopping can be diminished by loading it on a stable metal oxide [2]. It has also been observed practically that the loading on a stable metal oxide affects the increase of diffusion barrier of proton within the active materials and which follows the loss of effective sites. Because of the combination of TiO2 with V2O5 Ti-O-V bonds are formed instead of metal-metal bonds which can result in an overall improvement of electrochemical activity and chemical stability due to the drop in the intra-particle electron-hopping resistance. Thus, TiO2-V2O5 nanocomposite can be a promising candidate for supercapacitor electrode in the near future. In this work, interconnected mesoporous TiO2-V2O5 nanocomposite has been synthesized which demonstrate tube like structure. The as synthesized electrodes of TiO2-V2O5 composite demonstrate kinetically fast charge-discharge properties along with longcycle stability, which are commanding properties for supercapacitors [12, 23].

area of this material has also been further characterized by nitrogen adsorption and

surface area provides large contact area between the electrode/electrolyte interface enabling fast ion transfer which can improve the electrochemical performances of the electrode material. The corresponding pore size distribution of this electrode material measured from the isotherm BJH model is shown in Figure 1b (inset). The average pore size for this TiO2-V2O5 nanostructure is around 8.90 nm, which implies that the material possesses a mesoporous like wide pore size distribution.

The CV curves of TiO2-V2O5 composite (Figure 2(a)) with different scan rates

. The synergistic effect of these two metal oxides enhances the conductiv-

The plot of variation of specific capacitance with scan rate (Figure 2(b)) shows that at lower scan rate the TiO2-V2O5 nanocomposite electrode demonstrate higher capacitance value and it falls with increase of scan rate. This variation can be

explained on the basis of movement of ions from electrolyte to electrode material. At lower scan rate, the electrolyte ions get enough time to contact the outer and interior active sites of the material which lead a large number of charge accumulation correspond to high specific capacitance value. Consequently, as the scan rate increase the mobility of charges per unit time increase as well as capacitance decrease due to less number of charge accretions on the outer surface of the electrode material. The charge storage mechanism of the electrode material can be studied according to the Power law, which explains that the total CV current is the sum of non-Faradic capacitive current and adsorptions/desorption currents. According to the Power law,

scan rate dependent CV current of the electrode can be written as Eq. (6)

(a) CV curves at different scan rates; (b) specific capacitance vs. scan rate plot and (c) log(ip) vs. log(ν) plot

no O2 or H2 gas evolution observed at the ends of the potential windows which infers that this electrode can work in the wide potential window (�0.5 to +1.3 V) without suffering any degradation. The total charge stored in the electrode due to the redox reactions and pseudocapacitive behavior can be obtained from the total area enclosed by the CV curve for a particular scan rate. Same nature of CV curve for all scan rates reveals the good electrochemical performance of the electrode. This composite offers maximum specific capacitance of 310 F g�<sup>1</sup> at a scan rate

) signify a good electrochemical redox process. There is

<sup>i</sup> <sup>¼</sup> av<sup>n</sup> (6)

. This large

desorption BET surface area measurement. The adsorption-desorption plot (Figure 1(b)) of the composite reveals the typical type-IV isotherm and also demonstrates the mesoporous nature of TiO2-V2O5 nanostructure. The specific surface area calculated from the BET measurement is around 44m2 g�<sup>1</sup>

Transition Metal Oxide-Based Nano-materials for Energy Storage Application

6.2 Electrochemical analysis

DOI: http://dx.doi.org/10.5772/intechopen.80298

(2, 10, 50 and 100 mV s�<sup>1</sup>

ity of the composition.

2 mV s�<sup>1</sup>

Figure 2.

95

for the TiO2-V2O5 composite.

## 6.1 Morphological analysis

The field emission scanning electron microscopy (FESEM) images (Figure 1(a)) of TiO2-V2O5 nanocomposite shows ordered array of tube-like mesoporous structure. Because of large surface area, these mesoporous tube-like nanostructure can provide large number of active sites for competent ion diffusion. Due to increase in the contact area of the material with electrolyte ions, such type of surface improves the electrochemical activities of the electrode material. The active specific surface

#### Figure 1.

(a) FESEM image of TiO2-V2O5 composite; (b) N2 adsorption-desorption isotherms and pore size distribution curve (inset) of as prepared TiO2-V2O5.

Transition Metal Oxide-Based Nano-materials for Energy Storage Application DOI: http://dx.doi.org/10.5772/intechopen.80298

area of this material has also been further characterized by nitrogen adsorption and desorption BET surface area measurement. The adsorption-desorption plot (Figure 1(b)) of the composite reveals the typical type-IV isotherm and also demonstrates the mesoporous nature of TiO2-V2O5 nanostructure. The specific surface area calculated from the BET measurement is around 44m2 g�<sup>1</sup> . This large surface area provides large contact area between the electrode/electrolyte interface enabling fast ion transfer which can improve the electrochemical performances of the electrode material. The corresponding pore size distribution of this electrode material measured from the isotherm BJH model is shown in Figure 1b (inset). The average pore size for this TiO2-V2O5 nanostructure is around 8.90 nm, which implies that the material possesses a mesoporous like wide pore size distribution.

#### 6.2 Electrochemical analysis

layered structure, non-toxic in nature, easy synthesis process, presence of several oxidation states (+2 to +5) and high specific capacity. But its low electrical conductivity limits its practical device application. The charge storage mechanism of a supercapacitor strongly depends on the surface properties of the electrode materials so the nanomaterial is suitable for electrode fabrication. Nanomaterial possesses

nanoparticles is the most challenging problem. This effectively increases the strain for electrolyte ions diffusion within the nanoparticles at the surface of the electrode. Therefore, people are working to design and fabricate three-dimensional (3D), ordered and mesoporous nanomaterial to overcome such type of problem. There are four fundamental steps to control the charge storage behaviors of nanomaterials: (i) electron hopping between two nanoparticles; (ii) electron hopping within single nanoparticle; (iii) electron hopping between active electrode materials and current collectors; finally (iv) diffusion of proton within nanoparticles. It is well known that the proton diffusion and electron hoping within the nanoparticles are intrinsic properties of nanoparticles but the resistance due to intra-particle electron hopping can be diminished by loading it on a stable metal oxide [2]. It has also been observed practically that the loading on a stable metal oxide affects the increase of diffusion barrier of proton within the active materials and which follows the loss of effective sites. Because of the combination of TiO2 with V2O5 Ti-O-V bonds are formed instead of metal-metal bonds which can result in an overall improvement of electrochemical activity and chemical stability due to the drop in the intra-particle electron-hopping resistance. Thus, TiO2-V2O5 nanocomposite can be a promising

high surface area along with high surface energy but the aggregation of

Science,Technology and Advanced Application of Supercapacitors

candidate for supercapacitor electrode in the near future. In this work,

6.1 Morphological analysis

Figure 1.

94

curve (inset) of as prepared TiO2-V2O5.

interconnected mesoporous TiO2-V2O5 nanocomposite has been synthesized which demonstrate tube like structure. The as synthesized electrodes of TiO2-V2O5 composite demonstrate kinetically fast charge-discharge properties along with longcycle stability, which are commanding properties for supercapacitors [12, 23].

The field emission scanning electron microscopy (FESEM) images (Figure 1(a)) of TiO2-V2O5 nanocomposite shows ordered array of tube-like mesoporous structure. Because of large surface area, these mesoporous tube-like nanostructure can provide large number of active sites for competent ion diffusion. Due to increase in the contact area of the material with electrolyte ions, such type of surface improves the electrochemical activities of the electrode material. The active specific surface

(a) FESEM image of TiO2-V2O5 composite; (b) N2 adsorption-desorption isotherms and pore size distribution

The CV curves of TiO2-V2O5 composite (Figure 2(a)) with different scan rates (2, 10, 50 and 100 mV s�<sup>1</sup> ) signify a good electrochemical redox process. There is no O2 or H2 gas evolution observed at the ends of the potential windows which infers that this electrode can work in the wide potential window (�0.5 to +1.3 V) without suffering any degradation. The total charge stored in the electrode due to the redox reactions and pseudocapacitive behavior can be obtained from the total area enclosed by the CV curve for a particular scan rate. Same nature of CV curve for all scan rates reveals the good electrochemical performance of the electrode. This composite offers maximum specific capacitance of 310 F g�<sup>1</sup> at a scan rate 2 mV s�<sup>1</sup> . The synergistic effect of these two metal oxides enhances the conductivity of the composition.

The plot of variation of specific capacitance with scan rate (Figure 2(b)) shows that at lower scan rate the TiO2-V2O5 nanocomposite electrode demonstrate higher capacitance value and it falls with increase of scan rate. This variation can be explained on the basis of movement of ions from electrolyte to electrode material. At lower scan rate, the electrolyte ions get enough time to contact the outer and interior active sites of the material which lead a large number of charge accumulation correspond to high specific capacitance value. Consequently, as the scan rate increase the mobility of charges per unit time increase as well as capacitance decrease due to less number of charge accretions on the outer surface of the electrode material. The charge storage mechanism of the electrode material can be studied according to the Power law, which explains that the total CV current is the sum of non-Faradic capacitive current and adsorptions/desorption currents. According to the Power law, scan rate dependent CV current of the electrode can be written as Eq. (6)

$$
\dot{a} = av^n \tag{6}
$$

#### Figure 2.

(a) CV curves at different scan rates; (b) specific capacitance vs. scan rate plot and (c) log(ip) vs. log(ν) plot for the TiO2-V2O5 composite.

where, 'a' and 'n' are adjustable parameters and ν is scan rate [1, 22]. The value of 'n' can be obtained from slope of the linear fit log(i) vs. log(ν) at a fixed potential. The 'n' value varies from zero to one. For pure resistor n = 0, for ideal diffusion control process n = 0.5 and for ideal capacitive process n = 1.0 i.e. non Faradic process. Figure 2(c) shows the plot of log(i) vs. log(ν) of TiO2-V2O5 nanocomposite. For this case, the 'n' value 0.69 reveals that the adsorption/desorption process dominates over capacitive mechanism i.e. the total current can be written as the combination of capacitive and adsorption/desorption current as

$$i = k\_1(v) + k\_2 \left(v^{1/2}\right) \tag{7}$$

charge transfer resistance within the electrode material. The diameter of the semicircular portion gives the value of charge transfer resistance of the material. This semi-circular loop at high frequency can be modeled by a combination of parallel "RC" (Rct-Cdl) circuit along with a series resistance (Rs). But, to describe the impedance behavior on the whole frequency range a more detailed circuit have to be considered. Moreover, as seen in FESEM, the electrode materials are particle in nature, the electrolyte can easily access the active material and thus a thin electrolyte film can locally separate the nanoparticles from each other resisting the electronic contact between the materials as well as with the current collector. Because of these two effects the interface resistance Rct is increased. The high frequency loop is formed due to the current collector/ active material interface capacitance Cdl in association with the interface resistance Rct. The electrolyte can easily penetrate within the porous electrode materials in the mid frequency range. A straight line corresponding to the frequency dependent Warburg impedance (W) is observed which arises due to the linear diffusion process of ions at the outer surface of the electrode material from electrolyte solution. At very low frequency almost a straight vertical line is observed which mainly originated due to the ions diffusion behavior,

Transition Metal Oxide-Based Nano-materials for Energy Storage Application

In order to further study the charge storage ability of synthesized TiO2-V2O5 nanocomposites electrode, GCD at various current densities (0.5, 0.8, 1.0, 2.0 and

pseudocapacitive behavior and superior capacitive retention. Very small IR drop is also observed at the starting point of discharge time even at high current densities signifies that the electrode material offers very low internal series resistance (Rs) due to the electrolyte solution. However, the sample, due to its mesoporous morphology, accelerated the movement of K<sup>+</sup> and Cl ions through its channels inside the pores and reveals superior redox nature at higher current densities. The GCD cycling curves have a nearly symmetric shape at high current densities, indicating that the composite has a good electrochemical capacitive characteristic and superior capacitive retention. The maximum specific capacitance of 307 F g<sup>1</sup> has been obtained for TiO2-V2O5 nanocomposites at 0.5 mA cm<sup>2</sup> current density, which is almost equal to the value calculated from CV plots. It is also observed that the specific capacitance value decreases with increase of current densities which is mainly due to the decrease of accessibilities of electrolyte ions into the inner surface of the electrode material. The long-term cycle stabilities of prepared TiO2-V2O5 nanocomposite has been studied up to 10,000 cycles at a current density of

5.0 mA cm<sup>2</sup> as shown in Figure 3(c). The electrode exhibits very good cycle life of 94% over 10,000 cycles. It has been well explained that both the transition metal oxide play significant role for improvement of excellent electrochemical behavior providing additional possibility of fast redox process even at higher current densi-

The asymmetric supercapacitor (ASC) was fabricated with activated porous carbon (AC) as negative electrode, TiO2-V2O5 nanocomposite as positive electrode and 1 M Na2SO4 as the electrolyte with Whatman filter paper (pore diameter 25 μm) as separator. The mass ratio of the active materials for negative and positive electrodes was around 2. All the supercapacitive studies of ASCs were

6.3 Asymmetric supercapacitor device performance

6.3.1 Fabrication of asymmetric supercapacitor

CV measurement. At lower current densities the composite exhibits good

) has been performed. Figure 3(b) represents the GCD plots with potential windows from 0.5 to +1.3 V, which is reliable with the potential range of

indicating very low diffusion resistances.

DOI: http://dx.doi.org/10.5772/intechopen.80298

5 mA cm<sup>2</sup>

ties [23, 25].

97

where k1(ν) and k2(ν1/2) signify the non-Faradic capacitive current and adsorption/desorption current, respectively. To calculate the value of k1 and k2 one can plot i(V)/ν 1/2 along y-axis and ν1/2 along x-axis. The slope and intercept of the linear fit gives the values of k1 and k2, which can explain the contribution of the Faradic adsorption/desorption current and capacitive current to the total current. It can be concluded that the maximum amount of charge in the working electrode is accumulated based on the adsorption/desorption mechanism instead of capacitive mechanism. On the other hand, Trassati et al. first time reported that the total specific capacitance of an electrode material is the sum of two specific capacitance values provided by the outer and inner surface of the electrode i.e.

$$\mathbf{C}\_{total} = \mathbf{C}\_{in} + \mathbf{C}\_{out} \left(\mathbf{F} \mathbf{g}^{-1}\right) \tag{8}$$

The two specific capacitances due to the influence of inner and outer surface of the electrode strongly depend on the scan rate (ν). The intercept of the linear fit of specific capacitance vs. ν1/2 plot at ν = 0 gives the value of total specific capacitance (320 F g�<sup>1</sup> ) due to the diffusion of ions into the electrode which is nearly equal to the value obtained from CV curve (310 F g�<sup>1</sup> ). Similarly, the value of specific capacitance due to the outer surface of the electrode can be calculated from the plot of total specific capacitance vs. ν�1/2 plot and taking the intercept at ν = ∞ and the value in this case is given by 81 F g�<sup>1</sup> . Thus it can be concluded that the maximum capacitance value is mainly due to the contribution of inner active sites of the electrode which suggests that the charge storage mechanism is strongly based on adsorption/desorption process rather than capacitive process [1, 24].

To investigate the high performance electrochemical properties and kinetic information of TiO2-V2O5 nanocomposite electrode materials EIS measurements have been done in the frequency range of 0.1 Hz to 100 kHz with AC perturbation amplitude of 10 mV. The EIS plot (Figure 3(a)) show frequency dependent three regions which can provide information about the kinetic nature of the electrode material. The small distorted semicircular curve at high frequency is due to the

Figure 3.

(a) EIS plot; (b) GCD plot at different current densities and (c) 10000-cycle stability analysis for the TiO2-V2O5 composites.

#### Transition Metal Oxide-Based Nano-materials for Energy Storage Application DOI: http://dx.doi.org/10.5772/intechopen.80298

charge transfer resistance within the electrode material. The diameter of the semicircular portion gives the value of charge transfer resistance of the material. This semi-circular loop at high frequency can be modeled by a combination of parallel "RC" (Rct-Cdl) circuit along with a series resistance (Rs). But, to describe the impedance behavior on the whole frequency range a more detailed circuit have to be considered. Moreover, as seen in FESEM, the electrode materials are particle in nature, the electrolyte can easily access the active material and thus a thin electrolyte film can locally separate the nanoparticles from each other resisting the electronic contact between the materials as well as with the current collector. Because of these two effects the interface resistance Rct is increased. The high frequency loop is formed due to the current collector/ active material interface capacitance Cdl in association with the interface resistance Rct. The electrolyte can easily penetrate within the porous electrode materials in the mid frequency range. A straight line corresponding to the frequency dependent Warburg impedance (W) is observed which arises due to the linear diffusion process of ions at the outer surface of the electrode material from electrolyte solution. At very low frequency almost a straight vertical line is observed which mainly originated due to the ions diffusion behavior, indicating very low diffusion resistances.

In order to further study the charge storage ability of synthesized TiO2-V2O5 nanocomposites electrode, GCD at various current densities (0.5, 0.8, 1.0, 2.0 and 5 mA cm<sup>2</sup> ) has been performed. Figure 3(b) represents the GCD plots with potential windows from 0.5 to +1.3 V, which is reliable with the potential range of CV measurement. At lower current densities the composite exhibits good pseudocapacitive behavior and superior capacitive retention. Very small IR drop is also observed at the starting point of discharge time even at high current densities signifies that the electrode material offers very low internal series resistance (Rs) due to the electrolyte solution. However, the sample, due to its mesoporous morphology, accelerated the movement of K<sup>+</sup> and Cl ions through its channels inside the pores and reveals superior redox nature at higher current densities. The GCD cycling curves have a nearly symmetric shape at high current densities, indicating that the composite has a good electrochemical capacitive characteristic and superior capacitive retention. The maximum specific capacitance of 307 F g<sup>1</sup> has been obtained for TiO2-V2O5 nanocomposites at 0.5 mA cm<sup>2</sup> current density, which is almost equal to the value calculated from CV plots. It is also observed that the specific capacitance value decreases with increase of current densities which is mainly due to the decrease of accessibilities of electrolyte ions into the inner surface of the electrode material. The long-term cycle stabilities of prepared TiO2-V2O5 nanocomposite has been studied up to 10,000 cycles at a current density of 5.0 mA cm<sup>2</sup> as shown in Figure 3(c). The electrode exhibits very good cycle life of 94% over 10,000 cycles. It has been well explained that both the transition metal oxide play significant role for improvement of excellent electrochemical behavior providing additional possibility of fast redox process even at higher current densities [23, 25].

### 6.3 Asymmetric supercapacitor device performance

#### 6.3.1 Fabrication of asymmetric supercapacitor

The asymmetric supercapacitor (ASC) was fabricated with activated porous carbon (AC) as negative electrode, TiO2-V2O5 nanocomposite as positive electrode and 1 M Na2SO4 as the electrolyte with Whatman filter paper (pore diameter 25 μm) as separator. The mass ratio of the active materials for negative and positive electrodes was around 2. All the supercapacitive studies of ASCs were

where, 'a' and 'n' are adjustable parameters and ν is scan rate [1, 22]. The value of 'n' can be obtained from slope of the linear fit log(i) vs. log(ν) at a fixed potential. The 'n' value varies from zero to one. For pure resistor n = 0, for ideal diffusion control process n = 0.5 and for ideal capacitive process n = 1.0 i.e. non Faradic

nanocomposite. For this case, the 'n' value 0.69 reveals that the adsorption/desorption process dominates over capacitive mechanism i.e. the total current can be written as the combination of capacitive and adsorption/desorption current as

<sup>i</sup> <sup>¼</sup> <sup>k</sup>1ð Þþ <sup>v</sup> <sup>k</sup><sup>2</sup> <sup>v</sup>1=<sup>2</sup>

fit gives the values of k1 and k2, which can explain the contribution of the Faradic adsorption/desorption current and capacitive current to the total current. It can be concluded that the maximum amount of charge in the working electrode is accumulated based on the adsorption/desorption mechanism instead of capacitive mechanism. On the other hand, Trassati et al. first time reported that the total specific capacitance of an electrode material is the sum of two specific capacitance

where k1(ν) and k2(ν1/2) signify the non-Faradic capacitive current and adsorption/desorption current, respectively. To calculate the value of k1 and k2 one can

The two specific capacitances due to the influence of inner and outer surface of the electrode strongly depend on the scan rate (ν). The intercept of the linear fit of specific capacitance vs. ν1/2 plot at ν = 0 gives the value of total specific capacitance

capacitance due to the outer surface of the electrode can be calculated from the plot of total specific capacitance vs. ν�1/2 plot and taking the intercept at ν = ∞ and the

capacitance value is mainly due to the contribution of inner active sites of the electrode which suggests that the charge storage mechanism is strongly based on

(a) EIS plot; (b) GCD plot at different current densities and (c) 10000-cycle stability analysis for the

To investigate the high performance electrochemical properties and kinetic information of TiO2-V2O5 nanocomposite electrode materials EIS measurements have been done in the frequency range of 0.1 Hz to 100 kHz with AC perturbation amplitude of 10 mV. The EIS plot (Figure 3(a)) show frequency dependent three regions which can provide information about the kinetic nature of the electrode material. The small distorted semicircular curve at high frequency is due to the

adsorption/desorption process rather than capacitive process [1, 24].

) due to the diffusion of ions into the electrode which is nearly equal to

1/2 along y-axis and ν1/2 along x-axis. The slope and intercept of the linear

Ctotal <sup>¼</sup> Cin <sup>þ</sup> Cout Fg�<sup>1</sup> (8)

). Similarly, the value of specific

. Thus it can be concluded that the maximum

(7)

process. Figure 2(c) shows the plot of log(i) vs. log(ν) of TiO2-V2O5

Science,Technology and Advanced Application of Supercapacitors

values provided by the outer and inner surface of the electrode i.e.

the value obtained from CV curve (310 F g�<sup>1</sup>

value in this case is given by 81 F g�<sup>1</sup>

plot i(V)/ν

(320 F g�<sup>1</sup>

Figure 3.

96

TiO2-V2O5 composites.

performed at 300 K. The specific energy (Ecell) (W h kg�<sup>1</sup> ) and specific power (Pcell) (W kg�<sup>1</sup> ) of the device have been calculated by using the Eqs. (9–11)

$$C\_s = \frac{I \times \Delta t}{m \left(V\_f - V\_i\right)}\tag{9}$$

rate. The GCD data at different current densities are shown in Figure 4(c) and the curves represent that the discharge time decrease with increasing current densities. The specific capacitances were calculated by using Eq. (10) and the value of maxi-

Transition Metal Oxide-Based Nano-materials for Energy Storage Application

the specific capacitance decreases with increase of current densities. The specific energy and specific power of this device have also been calculated using Eqs. (10) and (11) and are shown in the form of Ragone plot in Figure 4(d). The highest specific energy (Ecell) and specific power (Pcell) is of 20.18 W h kg<sup>1</sup> and

, respectively. Overall electrochemical study of TiO2-V2O5 nanocomposite shows that it is an excellent positive electrode material for future

Recently, single phase spinel-structured NiMn2O4, a low cost, non-toxic ternary metal oxide, has received a great interests over many other metal oxides due to its excellent electrochemical performance. As has been stated earlier, to be used as electrodes in supercapacitors, materials with good electrical conductivity and excellent electrochemical performance are needed for achieving high energy density and high power density. In this context, nanostructured materials are more suitable than traditional bulk materials as electrode materials for supercapacitor, as they offer higher surface to volume ratio and shorter electron-ions transport channels. In fact, metal oxide nanostructures become the target of modern research for their utilization in high performance energy storage devices. Moreover, the morphology of those nanostructures is seen to affect their electrochemical performance. Therefore, designing metal oxide nanostructures of controlled morphology and size with good electrical conductivity is a challenge for their utilization in energy storage

FESEM image shows (Figure 5(a)) densely packed agglomerated spherical NiMn2O4 nanoparticles of sizes 6–10 nm (8 nm average size) are formed. Formation of such densely packed small NiMn2O4 nanoparticles effectively generates

(a) FESEM image of the NiMn2O4 nanoparticles; (b) N2 adsorption-desorption isotherms and pore size

. From the GCD curves it is clear that

mum is 86 F g<sup>1</sup> at current density of 4.1 A g<sup>1</sup>

DOI: http://dx.doi.org/10.5772/intechopen.80298

7. NiMn2O4 as supercapacitor applications

devices such as electrochemical supercapacitors [12, 26].

7.1 Morphological characterization

distribution curve (inset) of as prepared NiMn2O4.

5.94 kW kg<sup>1</sup>

applications.

Figure 5.

99

$$E\_{cell} = \frac{1}{2} \left[ \frac{C\_s \left(V\_f - V\_i\right)^2}{3.6} \right] \tag{10}$$

$$P\_{cell} = \frac{\text{3600} \times E\_{cell}}{\Delta t} \tag{11}$$

where Cs is the specific capacitance of the ASC devices (F g�<sup>1</sup> ), I is the discharge current (A), Δt is the discharge time (s), m is the combined mass of the both active electrode materials (g) and (Vf - Vi) is the potential window within which the supercapacitor operate (V) [2]. The cyclic voltammograms (CV) of this fabricated ASCs device have been studied at different potential windows ranging between 0.5 and 1.3 V at a fixed scan rate of 100 mV s�<sup>1</sup> (Figure 4(a)) and it shows no deviation in the shape of the voltammogram at 1.3 V which ensures that the ASC can operate up to 1.3 V. The CV curves of the ASC at different scan rates are shown in Figure 4(b). These curves show the increase of CV current with increasing scan

#### Figure 4.

(a) CV curves at 100 mV s-1 for different window potentials; (b) CV curves at different scan rates for a fixed window; (c) GCD curves at different current densities and (d) energy density vs. power density plot for the TiO2-V2O5 composite device.

Transition Metal Oxide-Based Nano-materials for Energy Storage Application DOI: http://dx.doi.org/10.5772/intechopen.80298

rate. The GCD data at different current densities are shown in Figure 4(c) and the curves represent that the discharge time decrease with increasing current densities. The specific capacitances were calculated by using Eq. (10) and the value of maximum is 86 F g<sup>1</sup> at current density of 4.1 A g<sup>1</sup> . From the GCD curves it is clear that the specific capacitance decreases with increase of current densities. The specific energy and specific power of this device have also been calculated using Eqs. (10) and (11) and are shown in the form of Ragone plot in Figure 4(d). The highest specific energy (Ecell) and specific power (Pcell) is of 20.18 W h kg<sup>1</sup> and 5.94 kW kg<sup>1</sup> , respectively. Overall electrochemical study of TiO2-V2O5 nanocomposite shows that it is an excellent positive electrode material for future applications.
