7. NiMn2O4 as supercapacitor applications

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 devices such as electrochemical supercapacitors [12, 26].

#### 7.1 Morphological characterization

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

#### Figure 5.

(a) FESEM image of the NiMn2O4 nanoparticles; (b) N2 adsorption-desorption isotherms and pore size distribution curve (inset) of as prepared NiMn2O4.

performed at 300 K. The specific energy (Ecell) (W h kg�<sup>1</sup>

Science,Technology and Advanced Application of Supercapacitors

Ecell <sup>¼</sup> <sup>1</sup> 2

where Cs is the specific capacitance of the ASC devices (F g�<sup>1</sup>

) of the device have been calculated by using the Eqs. (9–11)

Cs Vf � Vi � �<sup>2</sup> 3:6 " #

Pcell <sup>¼</sup> <sup>3600</sup> � Ecell

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

(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

Cs <sup>¼</sup> <sup>I</sup> � <sup>Δ</sup><sup>t</sup> m Vf � Vi

(Pcell) (W kg�<sup>1</sup>

Figure 4.

98

TiO2-V2O5 composite device.

) and specific power

(10)

), I is the discharge

� � (9)

<sup>Δ</sup><sup>t</sup> (11)

porous surface of high specific surface area, which enhances the electrochemical performance of the electrodes due to high contact area of the material with electrolyte. The porous nature of the NiMn2O4 nanostructures is confirmed form their Brunauer-Emmett-Teller (BET) surface area measurement. The adsorptiondesorption plots of the sample presented in Figure 5(b) clearly revealed a typical type IV isotherm which corresponds to the mesoporous nature of NiMn2O4 nanocrystals. The BET estimated specific surface area of the material is 43.6 m<sup>2</sup> g�<sup>1</sup> with average pore size 13.3 nm, which offers large number of active sites in the electrochemical process. Such a high specific surface area of the nanostructures can also provide a large contact area between the electrolyte solution and the electrode, ensuing fast ion transfer at the interface.

#### 7.2 Electrochemical characterization

The cyclic voltammogram (CV) curves (Figure 6(a)) of the NiMn2O4 electrode at various scan rates in the potential range �1.0 to 1.3 V indicate the typical Faradic charge transfer behavior due to the presence of functional groups or pore size distribution. The non-rectangular shape of the CV curves specifies the redox nature of the electrode material and provides the information on the pseudocapacitive behavior of the electrode in a suitable electrolyte solution. There are several oxidation and reduction peaks (Mn3+\$Mn4+ and Ni2+\$Ni3+) in the CV curves, which can be clearly identified due to the faradic redox processes related to Eq. (12) [27].

$$\text{NiMn}\_2\text{O}\_4 + \text{Na}^+ + e^- \leftrightarrow \text{NaNiMn}\_2\text{O}\_4 \tag{12}$$

be explained considering ion diffusion mechanism. At lower current densities, a large surface area of the electrode is occupied by Na<sup>+</sup> ions from electrolyte solution as they get enough time to access the maximum active sites of the electrode material, offering higher specific capacitance value. Conversely, due to limited accessibility of the Na+ ions inside the electrode material, the specific capacitance of the electrode is lower at higher current densities. The specific capacitance value can also be calculated from the GCD profile at a given current density, using Eq. (8) and the

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

using NiMn2O4 nanoparticles exhibit very high cycle life of about 91% over 10,000 cycles, indicating the oxides of both the elements (Ni and Mn) play significant roles for the improvement of electrochemical performance of the electrode.

windows (ΔV). The ratio of two electrode mass essential to follow:

m<sup>þ</sup> m�

It is known that for asymmetric supercapacitors (ASCs) the charge stored at two opposite electrodes (positive and negative) should be equal and opposite i.e. q+ = q� [28, 29]. The amount of charge stored by the each electrode generally depends on the specific capacitance (Cm), mass of the electrode (m) and potential

> <sup>¼</sup> <sup>C</sup>� � ð Þ <sup>Δ</sup><sup>E</sup> � <sup>C</sup><sup>þ</sup> � ð Þ <sup>Δ</sup><sup>E</sup> <sup>þ</sup>

Figure 7(a) represents the cyclic voltammograms (CV) of this fabricated ASCs device at different potential windows (0–0.9, 0–1.1, 0–1.3, 0–1.5 and 0–1.8 V). All CV curves show same nature which indicates that this device can performs within

(a) CV curves at 100 mV s�<sup>1</sup> 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

. The electrodes fabricated

(13)

maximum capacitance obtained 820 F g�<sup>1</sup> at 4.0 A g�<sup>1</sup>

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

7.3 Device characterization

Figure 7.

101

NiMn2O4 composite.

The specific capacitance (Cm) of the electrode for each scan rate has been calculated from the CV curves by using Eqs. (7) and (8) and the maximum specific capacitance of 875 F g�<sup>1</sup> is obtained for a scan rate 2 mV s�<sup>1</sup> . The GCD curves at different current densities (Figure 6(b)) indicate the pseudocapacitor type behavior with very low current densities at the potential corresponding to the Faradic reactions. A very small potential drop (IR-drop) has also been observed at the beginning of the discharge curve, even at high current densities, which indicates the NiMn2O4 electrode has a very low internal series resistance (Rs) within Na2SO4 electrolyte solution, as well as low contact resistance at the interface of current collector and electrolyte solution. A decrease of charging/discharging time with increasing current density can be clearly perceived from the GCD curves, which can

Figure 6. (a) CV curves at different scan rates and (b) GCD curves at different current densities for the NiMn2O4 composite.

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

be explained considering ion diffusion mechanism. At lower current densities, a large surface area of the electrode is occupied by Na<sup>+</sup> ions from electrolyte solution as they get enough time to access the maximum active sites of the electrode material, offering higher specific capacitance value. Conversely, due to limited accessibility of the Na+ ions inside the electrode material, the specific capacitance of the electrode is lower at higher current densities. The specific capacitance value can also be calculated from the GCD profile at a given current density, using Eq. (8) and the maximum capacitance obtained 820 F g�<sup>1</sup> at 4.0 A g�<sup>1</sup> . The electrodes fabricated using NiMn2O4 nanoparticles exhibit very high cycle life of about 91% over 10,000 cycles, indicating the oxides of both the elements (Ni and Mn) play significant roles for the improvement of electrochemical performance of the electrode.

#### 7.3 Device characterization

porous surface of high specific surface area, which enhances the electrochemical performance of the electrodes due to high contact area of the material with electrolyte. The porous nature of the NiMn2O4 nanostructures is confirmed form their Brunauer-Emmett-Teller (BET) surface area measurement. The adsorptiondesorption plots of the sample presented in Figure 5(b) clearly revealed a typical type IV isotherm which corresponds to the mesoporous nature of NiMn2O4 nanocrystals. The BET estimated specific surface area of the material is 43.6 m<sup>2</sup> g�<sup>1</sup> with average pore size 13.3 nm, which offers large number of active sites in the electrochemical process. Such a high specific surface area of the nanostructures can also provide a large contact area between the electrolyte solution and the electrode,

Science,Technology and Advanced Application of Supercapacitors

The cyclic voltammogram (CV) curves (Figure 6(a)) of the NiMn2O4 electrode at various scan rates in the potential range �1.0 to 1.3 V indicate the typical Faradic charge transfer behavior due to the presence of functional groups or pore size distribution. The non-rectangular shape of the CV curves specifies the redox nature of the electrode material and provides the information on the pseudocapacitive behavior of the electrode in a suitable electrolyte solution. There are several oxidation and reduction peaks (Mn3+\$Mn4+ and Ni2+\$Ni3+) in the CV curves, which can be clearly identified due to the faradic redox processes related to Eq. (12) [27].

� \$ NaNiMn2O4 (12)

. The GCD curves at

NiMn2O4 þ Na<sup>þ</sup> þ e

capacitance of 875 F g�<sup>1</sup> is obtained for a scan rate 2 mV s�<sup>1</sup>

The specific capacitance (Cm) of the electrode for each scan rate has been calculated from the CV curves by using Eqs. (7) and (8) and the maximum specific

different current densities (Figure 6(b)) indicate the pseudocapacitor type behavior with very low current densities at the potential corresponding to the Faradic reactions. A very small potential drop (IR-drop) has also been observed at the beginning of the discharge curve, even at high current densities, which indicates the NiMn2O4 electrode has a very low internal series resistance (Rs) within Na2SO4 electrolyte solution, as well as low contact resistance at the interface of current collector and electrolyte solution. A decrease of charging/discharging time with increasing current density can be clearly perceived from the GCD curves, which can

(a) CV curves at different scan rates and (b) GCD curves at different current densities for the NiMn2O4

ensuing fast ion transfer at the interface.

7.2 Electrochemical characterization

Figure 6.

composite.

100

It is known that for asymmetric supercapacitors (ASCs) the charge stored at two opposite electrodes (positive and negative) should be equal and opposite i.e. q+ = q� [28, 29]. The amount of charge stored by the each electrode generally depends on the specific capacitance (Cm), mass of the electrode (m) and potential windows (ΔV). The ratio of two electrode mass essential to follow:

$$\frac{m\_+}{m\_-} = \frac{\mathbf{C}\_- \times (\Delta E)\_-}{\mathbf{C}\_+ \times (\Delta E)\_+} \tag{13}$$

Figure 7(a) represents the cyclic voltammograms (CV) of this fabricated ASCs device at different potential windows (0–0.9, 0–1.1, 0–1.3, 0–1.5 and 0–1.8 V). All CV curves show same nature which indicates that this device can performs within

#### Figure 7.

(a) CV curves at 100 mV s�<sup>1</sup> 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 NiMn2O4 composite.

maximum potential window 0–1.8 V without any degradation. Thus, all electrochemical studies have been done within maximum potential window from 0 to 1.8 V and the CV curves (Figure 7(b)) at different scan rates show relatively rectangular nature without presence of any redox peaks. It clearly indicates that the charge storage mechanism is mainly due to the electric double layer of the device. The CV profiles also remain almost same without any distortion with increasing scan rates, indicating suitable fast charge–discharge property. To study the rate capability of this ASC device, the GCD test at different current densities (1.0, 1.25, 1.50, 1.75, 2.00, 2.25 and 2.50 A g<sup>1</sup> ) has been performed. The GCD plots (Figure 7(c)) show that the discharge time decreases with increasing current density. For practical purpose it is expected that a good supercapacitor device provides high specific capacitance and high energy density. The relationship between specific capacitance vs. current density of the fabricated ASC device shows that the device delivers maximum specific capacitance of 166.7 F g<sup>1</sup> at current density 1Ag<sup>1</sup> . The specific capacitance also decreases when current density increases, since diffusion of electrolyte ions and electrons most likely are restricted due to the time constrain. The specific energy and power densities of the ASC have been calculated from the discharge curves at different current densities in the voltage window of 0–1.8 V and the Ragone plot is shown in Figure 7(d). This device delivers offers specific energy density and power density of 75.01 W h kg<sup>1</sup> and 2.25 kW kg<sup>1</sup> , respectively. The ASC device configuration (Figure 7d) presents a columbic efficiency 97.6% indicating that the device is suitable for highperformance supercapacitor applications in future.

Acknowledgements

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

Conflict of interest

Author details

103

A. Ray (File No.–09/096(0927)/2018-EMR-I) and S. Saha (File No.–09/096 (0898)/2017–EMR-I) wish to thank CSIR, Government of India for financial support. S. Das is thankful to the Department of Science and Technology (DST), Government of India, for providing research support through the 'INSPIRE Faculty Award' (IFA13-PH-71). A. Roy (IF140920) is also thankful to the Department of Science and Technology (DST), INSPIRE, Government of India, for providing

research support through the 'INSPIRE Fellowship'.

All the authors declare that there is no conflict of interest.

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

Apurba Ray, Atanu Roy, Samik Saha and Sachindranath Das\*

\*Address all correspondence to: sachindas15@gmail.com

provided the original work is properly cited.

Department of Instrumentation Science, Jadavpur University, Kolkata, India

© 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,
