Section 5 Hybrid Capacitors

## **Chapter 7**

## Supercapacitor Supported by Nickel, Cobalt and Conducting Polymer Based Materials: Design Techniques and Current Advancement

*Satish P. Mardikar, Sagar D. Balgude and Santosh J. Uke*

## **Abstract**

The recent advanced electronic appliances demand special high power devices with lightweight, flexible, inexpensive, and environment friendly in nature. In addition, for many industrial and automotive applications, we need energy storage systems that can store energy in a short time and deliver an intense pulse of energy for long duration. Till date the Li-ion battery is the only choice for fulfilling all our energy storage demands. However, the high cost, limited availability and nonenvironmental nature of electrodes and electrolyte material of Li-ion battery limits its applicability. Hence, the world demands an alternative replacement for the Li-ion battery. In this regard, the supercapacitor is one of the most emerging and potential energy storage devices. The electrode plays an important role in supercapacitors. The nickel and cobalt based oxide, hydroxides, and their composites with conducting polymer are promising and highly appreciated electrode materials for supercapacitors. This chapter covers the recent advances in supercapacitors supported by nickel, cobalt and conducting polymer based materials and their applications predominantly described in the recent literature. Recent advances are reviewed including new methods of synthesis, nanostructuring, and self-assembly using surfactant and modifiers. This chapter also covered the applications of supercapacitors in powering the light weight, flexible and wearable electronics.

**Keywords:** supercapacitors, mixed ternary metal oxides, nanostructured, NiCo2O4, energy density, etc.

#### **1. Introduction**

Supercapacitor (SCp) is also known as ultracapacitor. SCP is the advanced electrochemical energy storage device. At present, the lithium ion battery (LiBs), lead acid battery and SCP are the major available energy storage systems. Over the other energy storage systems, the SCP is stands out to be a promising energy storage device with very attractive properties such as high specific capacitance, high power density, moderate energy density, good cyclic stability, low cost, environmental friendly nature, etc. SCP has been utilized in various electrical applications viz.

hybrid vehicles, power backup, military services, and portable electronic devices like laptops, mobile phones, roll-up displays, electronic papers, etc. [1–3].

The performance of SCP is strongly depends on types electrode materials i.e. active material used in supercapacitor. Based on the type of active material used and process energy storage, the SCP can be divided into three main categories, including pseudocapacitors (PCs), electric double-layer capacitors (EDLCs) and hybrid capacitors [4–6]. In PCs metal oxides, metal hydroxides and conducting polymers are employed as active material. On the other hand, carbon base materials such carbon nanotubes, graphene and carbon black, etc. are used as active material in EDLCs employed. Likewise, the used of combination of metal oxide, conducting polymer and carbon based material as active material results hybrid capacitor. The PCs delivered high specific capacitance, high energy density than EDLCs, but demonstrates poor power density and cycle stability. Nevertheless, owing to the high active surface area, the EDLCs delivered high specific capacitance but suffer poor energy density than PCs [4]. The hybrid capacitors retain the advantage of both PCs and EDLCs and hence they delivered high specific capacitance, high energy density, and large cycle life [7, 8]. Moreover, the performance of supercapacitors is equally rely on different aspect of active material viz. quality, electric conductivity, material, size, porosity, synthesis method, etc. More specifically, the synthesis method can bring many attractive advantages in active material for extraordinary electrochemical performance of supercapacitor. Therefore, synthesis of active materials with high porosity, stable performance and good electrical conductivity has a very wide research potential.

Recently, to enhance the energy density, cycle life and electrochemical performance of the supercapacitors, the use of electrode material with desired structure with uniform porosity is one of the appealing strategies. From many decades, the nanostructured single transition metal oxides such as RuO2 [9], MnO2 [10], CeO2 [11], Fe2O3 [12], Fe3O4 [13], Co3O4,, Mn3O4,, etc., and the nanostructured mixed ternary metal oxide (TMOs) such as ZnFe2O4, NiFe2O4 CuFe2O4, CoFe2O4, MnCo2O4 ZnCo2O4, NiCo2O4, and etc., and conducting polymers such as, polyaniline (PANi), polypyrrole (Ppy), polythiophene (Pth), etc. has been extensively used as active material for all types of supercapacitors. Out of the different materials used as the electrodes for SCp applications. The TMOs are highly studied and excessively used as active material in all types of supercapacitors. The mixed TMOs are also called the spinel metal oxide. The spinel TMOs have the general formula AB2O4. In AB2O4, the cubic crystal structure of TMOs consists of closely packed O2− anions and an octahedral and tetrahedral space of the lattice occupied by the transition metal cations A and B, respectively. Due to this closed packed structure, the mixed TMOs show the extraordinary characteristics over single metal oxides, such as two order higher electrical conductivity, superior electrochemical performance and excellent stability over single metal oxides. Moreover, the recent research reports shows the TMOs have better structural advantages and higher surface area and porosity [14]. More specifically, the TMOs show low cost, natural abundance, low toxicity and environmental friendly nature. Hence, TMOs have drowned more research attention in recent years. In addition, the extraordinary electrochemical performance of TMOs in solid as well liquid electrolyte makes it a promising and potential candidate as electrode material for PCs. The various mixed TMOs viz. ZnFe2O4, NiFe2O4 CuFe2O4, CoFe2O4, MnCo2O4 ZnCo2O4, NiCo2O4, etc. has been utilized as electrode material for PCs. Out of the different TMOs the nickel and cobalt based TMOs have gained more research attention as electrode material in supercapacitors due to their attracting properties such as low cost, natural abundance, low toxicity and environmental friendly nature. More specifically, these materials show variable structures, diverse morphologies, high specific surface and uniform porosity and outstanding

#### *Supercapacitor Supported by Nickel, Cobalt and Conducting Polymer Based Materials: Design… DOI: http://dx.doi.org/10.5772/intechopen.98355*

electrical conductivity. The NiCo2O4 demonstrated high electrical conductivity due to the presence of Ni in it. Whereas, Co enhances the electrochemical activity of oxides, further, the synergistic effect among Ni in Co offers high electrical conductivity with an excellent electrochemical behavior in supercapacitors [12]. The NiCo2O4 demonstrated a high theoretical capacity [15]. The nickel and cobalt based TMOs show diverse morphologies, this includes various nanostructures ranging from 0 to 3 D architectures viz. quantum dots, nanowires, nanosheets, platelets like nanoparticles, porous network like framework, coral- like porous crystals, ordered mesoporous particles, urchin-like microstructures and urchin-like nanostructures. Till date many recent attractive reviews have presented recent development in mixed TMOs as electrode material for SCs [7, 16–19]. We recommend few of them for readers who are new to this field of energy research.

In the present chapter we provide the recent advancements in synthesis of nanostructured nickel and cobalt base mixed TMOs and their composites with conducting polymer based materials as electrode material for supercapacitors predominantly described in the recent literature. Moreover, here our special emphasis will be on new methods of synthesis, nanostructuring, and self-assembly using surfactant and modifiers. In addition, we provide a summary of structural and morphological advancements regarding the electrochemical properties of supercapacitors. Finally, we link our discussion to the recent applications in powering the light weight, flexible and wearable electronics real world applications.

#### **2. Synthesis of nickel and cobalt base mixed TMOs**

Compared to the micro sized the nanostructured cobalt and nickel based TMOs show higher specific capacitance and long cycle life. Therefore many recent research strategies have drowned to synthesized nanosize mixed cobalt and nickel based TMOs. The various synthesis methods for synthesis of nanostructured cobalt and nickel based mixed TMOs viz., hydrothermal method sol–gel, thermal evaporation method, chemical bath deposition, electrodeposition, oil/water interfacial selfassembly strategy, etc. have been extensively reported in literature. Hydrothermal method is one of the excessively adopted synthesis methods for hierarchical nanostructure synthesis. This method is cost effective, simple, and easy to scale-up at room temperature. This method is mostly used for fine tuning the morphology and controlling the size of nanostructures. Agglomeration of NiCo2O4 results in low electrical conductivity and decreases the specific capacitance and cycle life of SCp [20]. Therefore, to enhance the electrical conductivity the use of high surface area with high porosity conductive substrates are highly recommended. These substrates enhance the contact between electrode and electrolyte and allowed more electrolyte ions penetration in active material. The various conductive substrates such as textiles, sponges, carbon clothes, carbon fibers, conventional paper, cables, etc. are used as substrates to fabricate SCs. Such conductive substrates are advantageous for enhancing the electrochemical performance via providing short diffusion path, high electrical conductivity, ample electroactive sites [21]. In this regard, Yang *et al.* synthesized the nanoneedle arrays of on filter carbon paper substrate via facial hydrothermal synthesis method. For fabrication the filter carbon paper submerged into the precursor of NiCo2O4 followed by calcination in the Argon atmosphere in the range of 250-400°C for 2 hours. Using this approach they have reported urchin like and nanoneedle arrays and further adopted this nanostructure for SCp applications [20]. The synthesis parameters like reaction temperature reaction temperature and reaction time controls the structures. Further, the calcination temperature after synthesis plays crucial role in improving the surface morphology, specific surface

area, porosity, etc. [22]. Siwatch *et al.* [22] have reported the formation of NiCo2O4 quantum dots via hydrothermal synthesis and studied the effect of synthesis parameters like reaction temperature and time, and calcination temperature on the morphology of NiCo2O4 quantum dots. Further, the highly porous flower-like structure of NiCo2O4 quantum dots obtained at the calcination temperature 300o C is highly useful for SCp applications. Lu *et al.* reported the synthesis of mesoporous NiCo2O4 via reagents assisted hydrothermal method and studied the effect of reagent cetyltrimethylammonium bromide (CTAB) on morphology and electrochemical behavior of NiCo2O4 for SCp applications. Moreover, the reagent during synthesis enhanced the specific surface area and charge transport of NiCo2O4. As a result, the cyclic stability, rate performance and specific capacitance of NiCo2O4 quantum dots based in asymmetric SCp found to be increased [23]. Binder used during the fabrication of electrode increase the electrode resistance which further decreases the electrochemical performance and cycle life of SCp. Therefore recently binder free fabrication approaches such as direct growth on conductive substrate is more popular. Furthermore, over the conventional substrates the direct growth on three dimensional (3D) conductive substrate offers many advantages such as shorten the diffusion path, healthy synergy between the active material and electrolyte, provides ample electroactive site, etc. which further help to enhance the electrochemical performance of SCp. For example, Yang *et al.* [24] directly grown NiCo2O4 on gelatin-based carbonenickel foam (3D) by facial hydrothermal method followed by calcination at 350°C for 2 hrs under an Argon atmosphere.

The morphology of NiCo2O4 is reported to be nanoflower-like. The fabricated 3D electrode provides fast ions and electrons transfer rate and enhances the electroactive surface area of the NiCo2O4 via forming a complex 3D network. In addition, the nickel foam as substrate adds the electric conductivity whereas the gelatin based carbon on the nickel foam provides high surface area for uniform growth of NiCo2O4 during synthesis. In our previous study, we have reported the synthesis of nanostructured NiCo2O4 via surfactant assisted hydrothermal method and studied the effect surfactant and reaction parameters on the morphology of nanostructured NiCo2O4. From this synthesis, we got two distinct morphologies viz. platelet-like and nanorod-like using surfactants TEA ethoxylate and polyethylene glycol (PEG), respectively. We further used this nanostructured NiCo2O4 for SCp applications [18].

In addition to the hydrothermal method, the combustion method is one of the simple and easy to scalable synthesis methods. Over the hydrothermal method the combustion method does not requires Teflon-lined stainless steel autoclaves and centrifuge for product washing, is less time consuming and provides high phase purities. This regard, Kumar *et al.* reported the growth of NiCo2O4 on conductive substrate nickel foam using combustion method. For the synthesis, honeycomb-like NiCo2O4 the nitrate and glycine used as oxidizer and fuel, respectively [6].

For enhancing the surface area and porosity and electrochemical activities of NiCo2O4, the formation of composites of NiCo2O4 with carbon based material is one of the appealing strategies, for example, NiCo2O4/CNT, NiCo2O4/MWCNT, NiCo2O4/ graphene, NiCo2O4/reduced graphene oxides (r-GO), etc. demonstrated to be a potential candidates for SCp applications. Carbon base material viz. CNT, MWCNT, graphene, r-GO, etc. provide excellent flexibility, high specific surface areas, remarkable electrical conductivity, good thermal and chemical stability [5, 25–27]. For example, Li *et al.* [25]*.* reported the synthesis of NiCo2O4/CNT composites, and studied the structure formation of NiCo2O4 with and without surfactant for supercapacitive applications. The nanoflakes and nanocorn like morphology for NiCo2O4 is obtained by using surfactant sodium dodecyl sulfate. Pathak *et al.* synthesized NiCo2O4 and NiCo2O4@ MWCNT composite using facile hydrothermal method.

*Supercapacitor Supported by Nickel, Cobalt and Conducting Polymer Based Materials: Design… DOI: http://dx.doi.org/10.5772/intechopen.98355*

NiCo2O4@ MWCNT demonstrated superior electrochemical performance and demonstrated a good electrode for SCp applications. In addition, using density functional they reveal the enhanced density of states near the Fermi level and increased quantum capacitance of the NiCo2O4 @SWCNT is one of the important reasons for high specific capacitance, high power density and energy density [28]. The PCs use reversible fast faradaic reactions to store electrical charges, which allow them to achieve higher capacitance by at least one order of magnitude than those obtained by EDLCs. Materials sustaining such redox reactions on their surfaces include, for example, conducting polymers and transition metal oxides.

#### **3. Applications of Ni and Co based metal oxides and their composites**

#### **3.1 Pseudocapacitor (PCs)**

Recently, PCs received considerable attention due to the one order higher capacitance, higher volumetric capacitance, higher energy density and use of low cost and easily synthesized active material than EDLCs. [28–30]. For example, Eskandari *et al..* fabricated NiCo2O4 and its composite with PANi and MWCNTs and reduced graphene oxide r-GO and studied their SCp performance in 3 M KOH. Out of the different composites the NiCo2O4/PANi demonstrated superior performance and exhibited specific capacitance of 1760 Fg−1 (900 F/g and 734 F/g for NiCo2O4/ MWCNTs and NiCo2O4/r-GO, respectively) at current density of 1 Ag−1, respectively. The highest specific capacitance in NiCo2O4/PANi is due to supplementary conductive pathways provided by PANi and synergistic effect of the rooted pseudo-reaction. Moreover the composite NiCo2O4/MWCNTs shows stable cycle is life and demonstrated to be best retention over all other composites as 89% over 2000 charge discharge cycles. Composite NiCo2O4 /r-GO also exhibited good cycle performance and shows retention in specific capacitance of 87% over 2000 charge discharge cycles. In addition, the pristine NiCo2O4 shows higher retention than NiCo2O4/PANi, i.e. 70% and 73%, respectively. The highest cyclic stability in NiCo2O4/MWCNTs is due to the high electrical conductivity and high mechanical strength of MWCNTs. In fact, the good cyclic performance in NiCo2O4/r-GO is the results of higher electrical conductivity, high surface morphology and good mechanical strength than PANi and pristine NiCo2O4 [31] Moreover, the representative Ni and Co based material and their performance in PCs are summarized in **Table 1**.

#### **3.2 Hybrid capacitors**

Even if the Ni and Co based TMOs are advantageous for SCp applications, however, in the long cycling process the rapid degradation of NiCo2O4 electrode materials is the major obstacle among the commercialization of NiCo2O4 based SCp. By increasing the electrical conductivity of NiCo2O4 this hurdle can be minimized and the higher rate capabilities can be attained. Therefore, from the last two decades, researchers devoted more efforts to enhance the electrical conductivity of NiCo2O4, this includes fabrication of hybrid composite with other conducting electrode materials, viz. carbon based material (CNts, SWCNts, MWCNts, activated carbon, doped and undoped reduced graphene oxides, etc.), conducting polymers, etc. In addition, recent formation of composite of NiCo2O4 with other mixed TMOs has gain enormous attention. For example, Mary *et al.* reported the fabrication of NiCo2O4 and ZnCo2O4 composites and studied their morphology dependent electrochemical behavior for hybrid SCp applications. In addition, the hybrid SCp


*Supercapacitors for the Next Generation*

**Table 1.** *Overview of representative Ni and Co based material and their performance in PCs.*

**128**


*Supercapacitor Supported by Nickel, Cobalt and Conducting Polymer Based Materials: Design… DOI: http://dx.doi.org/10.5772/intechopen.98355*

> **Table 2.**

*Overview of representative Ni and Co based material and their performance hybrid supercapacitor.*


*Supercapacitors for the Next Generation*

**Table 3.** *Supercapacitor Supported by Nickel, Cobalt and Conducting Polymer Based Materials: Design… DOI: http://dx.doi.org/10.5772/intechopen.98355*

fabricated using the NiCo2O4 and ZnCo2O4 composite and nitrogen doped activated carbon. The high surface area and uniform porosity of activated carbon in hybrid SCp enhances the capacitance via enabling the more electrolyte ions into active material. Interestingly, NiCo2O4 @ ZnCo2O4 composite shows high specific capacitance of 236 C g−1 at a current density of 1 A g−1. Moreover, the aforementioned hybrid SCp results in high energy density of 101.6 Whkg−1 and high retention in capacitance at 78.5% over 12000 charge–discharge cycles. Moreover, the representative Ni and Co based material and their performance in hybrid SCp are summarized in **Table 2**.

#### **3.3 Asymmetric capacitors**

The symmetrical SCp limits their specific capacitance due to narrow potential windows. Moreover, the use of aqueous base liquid electrolyte in symmetrical SCp decreases the specific capacitance energy density and cycle life. To overcome such drawback the fabrication of SCp with two different kinds of active material based electrode is demonstrated to be an effective strategy. The SCp fabricated using two different electrodes is termed an asymmetric supercapacitor. In asymmetric SCp positive electrode is fabricated using metal oxide base material, while negative electrode is fabricated by carbon based material. The combination of different active materials in a single device with higher operating potential result in higher the specific capacitance and energy density [40]. However, aqueous-based symmetric supercapacitors suffer from narrow potential windows, due to the limitation of the water decomposition. Therefore, an effective way is to construct asymmetric supercapacitor, which consists of two kinds of electrode materials, for instance positive electrode having pseudocapacitive nature and negative electrode having electric double layer capacitance with higher operating potential, for obtaining higher energy density [19, 20]. In the case of positive electrode materials, transition metal oxide based nanoparticles, conducting polymers based materials have been widely utilized, which exhibits pseudocapacitance as well as reversible redox Faradaic reaction. As negative electrode materials, carbon based materials like carbon nanotubes (CNT), graphene oxide (GO), activated carbon, and mesoporous carbon materials displaying electric double layer capacitance have been used. Among the carbon allotropes, mesoporous carbons have been extensively used as negative electrode material due to its high surface area and good electrical conductivity. For more understanding the recent advancements in NiCo2O4 and their composites and their performance in asymmetric SCp are summarized in **Table 3**.

#### **4. Conclusions and outlooks**

With ever increasing energy demands, day by day the SCp gaining much interest as an energy storage device. From a many years, the nickel and cobalt based TMOs and their composites have been studied and successively employed as an active material in all types of SCp. The nanostructured NiCo2O4 is low cost, in abundance, environmentally friendly in nature and has high electrical conductivity. In addition, due to the enhanced mobility of charge carriers the nanostructured NiCo2O4 demonstrated to be higher electrochemical performance than the single metal oxides. In this regard, the recent advances in synthesis of pristine NiCo2O4 and their composites with diverse morphologies and their applications for electrochemical performance in all types of SCp have been summarized in this chapter. Out of the different synthesized methods used for synthesis nanostructured NiCo2O4, the hydrothermal method is found to be excessively used. Moreover, the hydrothermal

method is demonstrated to be more advantageous for the synthesis of diverse morphologies ranging from 0 D to 3 D, and resulted in high specific surface area and uniform porosity. The pristine nanostructured NiCo2O4 has many limitations for its commercial supercapacitor applications. Therefore, advanced strategies like synthesis of hierarchical nanostructures of NiCo2O4 and the fabrication of composite with other mixed TMOs, carbon based material and conducting polymers can enhance the specific capacitance, energy density and rate capability of Ni and Co based supercapacitors.

#### **Conflict of interest**

The authors declare no conflict of interest.

#### **Author details**

Satish P. Mardikar1 , Sagar D. Balgude2 and Santosh J. Uke3 \*

1 Smt. R.S. College, SGB Amravati University, Amravati, India

2 Department of Chemistry, D.Y. Patil University, Pune, India

3 JDPS College, SGB Amravati University, Amravati, India

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

© 2021 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.

*Supercapacitor Supported by Nickel, Cobalt and Conducting Polymer Based Materials: Design… DOI: http://dx.doi.org/10.5772/intechopen.98355*

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[36] F. Ren, Z. Tong, S. Tan, J. Yao, L. Pei, A. Abulizi, Ultrathin and porous NiCo2O4 nanosheets based 3D hierarchical electrode materials for high-performance asymmetric supercapacitor, Journal of Electrochemical Energy Conversion and Storage. (2021) 1-11.

[37] Y. Li, Z. Zhang, Y. Chen, H. Chen, Y. Fan, Y. Li, D. Cui, C. Xue, Facile synthesis of a Ni-based NiCo2O4-PANI composite for ultrahigh specific capacitance, Applied Surface Science. 506 (2020) 144646. doi:10.1016/j. apsusc.2019.144646.

[38] Z. Cao, C. Liu, Y. Huang, Y. Gao, Y. Wang, Z. Li, Y. Yan, M. Zhang, Oxygenvacancy-rich NiCo2O4 nanoneedles electrode with poor crystallinity for high energy density all-solid-state symmetric supercapacitors, Journal of Power Sources. 449 (2020) 227571. doi:10.1016/j.jpowsour.2019.227571.

[39] R. Bai, X. Luo, D. Zhen, C. Ci, J. Zhang, D. Wu, M. Cao, Y. Liu, Facile fabrication of comb-like porous NiCo2O4 nanoneedles on Ni foam as an advanced electrode for highperformance supercapacitor, International Journal of Hydrogen

Energy. 45 (2020) 32343-32354. doi:10.1016/j.ijhydene.2020.08.156.

[40] S.S. Jayaseelan, S. Radhakrishnan, B. Saravanakumar, M.-K. Seo, M.-S. Khil, H.-Y. Kim, B.-S. Kim, Mesoporous 3D NiCo2O4/MWCNT nanocomposite aerogels prepared by a supercritical CO2 drying method for high performance hybrid supercapacitor electrodes, Colloids and Surfaces A: Physicochemical and Engineering Aspects. 538 (2018) 451-459. doi:10.1016/j.colsurfa.2017.11.037.

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[42] C. Young, R. Salunkhe, S. Alshehri, T. Ahamad, Z. Huang, J. Henzie, Y. Yamauchi, High Energy Density Supercapacitors Composed of Nickel Cobalt Oxide Nanosheets on Nanoporous Carbon Nanoarchitectures, Journal of Materials Chemistry A. 5 (2017). doi:10.1039/C7TA01362K.

[43] N.V. Nguyen, T.V. Tran, S.T. Luong, T.M. Pham, K.V. Nguyen, T.D. Vu, H.S. Nguyen, N.V. To, Facile Synthesis of a NiCo2O4 Nanoparticles Mesoporous Carbon Composite as Electrode Materials for Supercapacitor, ChemistrySelect. 5 (2020) 7060-7068.

[44] J. Fang, C. Kang, L. Fu, S. Li, Q. Liu, Fabrication of hollow bamboo-shaped NiCo2O4 with controllable shell morphologies for high performance hybrid supercapacitors, Journal of Alloys and Compounds. 849 (2020) 156317. doi:10.1016/j. jallcom.2020.156317.

[45] P. Siwatch, K. Sharma, N. Singh, N. Manyani, S.K. Tripathi, Enhanced supercapacitive performance of reduced graphene oxide by incorporating NiCo2O4 quantum dots using aqueous

electrolyte, Electrochimica Acta. 381 (2021) 138235. doi:10.1016/j. electacta.2021.138235.

[46] L. Hou, W. Yang, X. Xu, B. Deng, J. Tian, S. Wang, F. Yang, Y. Li, In-situ formation of oxygen-vacancy-rich NiCo2O4/nitrogen-deficient graphitic carbon nitride hybrids for highperformance supercapacitors, Electrochimica Acta. 340 (2020) 135996. doi:10.1016/j.electacta. 2020.135996.

[47] J. Chen, T. Ma, M. Chen, Z. Peng, Z. Feng, C. Pan, H. Zou, W. Yang, S. Chen, Porous NiCo2O4@Ppy core-shell nanowire arrays covered on carbon cloth for flexible all-solid-state hybrid supercapacitors, Journal of Energy Storage. 32 (2020) 101895. doi:10.1016/j. est.2020.101895.

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[49] P. Salarizadeh, M.B. Askari, M. Seifi, S.M. Rozati, S.S. Eisazadeh, Pristine NiCo2O4 nanorods loaded rGO electrode as a remarkable electrode material for asymmetric supercapacitors, Materials Science in Semiconductor Processing. 114 (2020) 105078. doi:10.1016/j.mssp.2020.105078.

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#### **Chapter 8**

## Effect of Different Metals Doped in Nickel Oxide Nanomaterials on Electrochemical Capacitive Performance

*Amar Laxman Jadhav, Sharad Laxman Jadhav and Anamika Vitthal Kadam*

#### **Abstract**

Recently, the various porous nano metal oxides used for the electrochemical energy storage supercapacitor applications. Some researchers focus on the binary as well as ternary metal oxides and more metal oxide complex composite materials used for the supercapacitors. In the review article focused on the effect of different metals doped in a nickel oxide nano material on the electrochemical capacitive performance, discussion on methodologies, charge storage mechanism, latest research articles and prepared nanostructures. Nowadays nickel oxide is developing electrode material for storage of charge due to its higher thermal stability, excellent chemical stability, cost effective materials, higher theoretical values of specific capacitance, naturally rich and environment friendliness material. The various metals doped in NiO and their composite oxides have shown good structural stability, reversible capacity, long cycling stability and have been also studied nano structured electrode materials for electrochemical supercapacitor applications.

**Keywords:** Metal oxide, Nickel Oxide, electrochemical supercapacitor, Nanomaterial

#### **1. Introduction**

Nowadays, in this research on the rapid growth of electronic portable energy storage devices and hybrids electrical vehicles, the call for high power density and energy density resources has been increased manifold. Supercapacitor also called ultracapacitor or electrochemical capacitors, exhibits higher power density than the normal capacitor and higher energy density than the batteries. The electrochemical capacitor shows faster charge–discharge mechanism behavior and also exhibits long cycle stability. Therefore, supercapacitor or electrochemical capacitor indicates bridge between the normal capacitor and fuel cell, batteries. The electrochemical supercapacitor has two main types based on the charge storage mechanism (i) Electrochemical double layer capacitor (EDLc) is based on electrostatically charge storage mechanism and (ii) pseudocapacitor is electrochemical charge storage mechanism. The carbonbased materials (Activated carbon materials, graphene oxide) used for the preparation of EDLc supercapacitor, transition metal oxide also used for the preparation of active electrode materials for pseudocapacitor and hybrid supercapacitor exhibits

**Figure 1.** *Classifications of the electrochemical supercapacitor.*

intermediate properties between the EDLc and pseudocapacitor behaviors schematic diagram shown **Figure 1**.

The effect of different morphologies on the charge storage at the active electrode materials for the supercapacitor application. The 1D, 2D, and 3D morphology increase the active materials surface area due to an increase in the power density and specific capacitance. It is an important role in electrochemical capacitors. The active electrode materials should be higher specific capacitance, structural stability, and good mechanical to provides long cycling lifetimes. The nano porous active electrode materials prepared by using carbon materials, metal oxides, and conducting polymers such as graphene oxide [1, 2], activated carbon [3] and derivatives of carbide materials [4–10], CuO, NiO, RuO2, Cu2O, Fe2O4, CoO, MnO [11–17] and polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh) [18–22], The transition metal oxide electrode shows excellent properties of electrochemical performance (Specific capacitance, power density, and cycling stability) than the other types of the electrodes.

Generally, research has been carried out on the various transition metal oxide materials like cobalt oxide [23], iridium oxide [24], nickel oxide (NiO) [25], manganese oxide [26], iron Oxide [15], ruthenium oxide [13], and zinc oxide [27]. Currently, research on nickel oxide with other composite electrode materials just like a NiO//graphene oxide, Carbon nanotubes (CNT)//NiO, Ru doped nickel oxide, Cu doped nickel oxide, Cerium doped nickel oxide, and so on [28]. In the fabrications of high-performance supercapacitor at the laboratory, some interruptions occur due to nanostructured morphological structures with the large surface area of the electrode materials. Therefore, the nanostructures of the SEM images are an important parameter for supercapacitor applications. The electrochemical specific capacitance, power density, and cycling stability depend on the morphological structures due to so many researchers work on the synthesis of various types of nanostructured morphologies. But the Nickel oxide electrochemical specific capacitance does not get at maximum (In case of practical its value is 1000F/g) and the theoretical value is 2584F/g. Thus, gets maximum capacitance value, the growth of the nanostructured morphological materials with increased conductivity, lower interfacial resistance, and large surface area of the promising electrode materials is a promising solution.

There are several reports available on the synthesis of nanomaterials and electrochemical characterizing with more conductivity. NiO nanomaterials show excellent physical as well as chemical properties such as mesoporous and hierarchical porous

#### *Effect of Different Metals Doped in Nickel Oxide Nanomaterials on Electrochemical Capacitive… DOI: http://dx.doi.org/10.5772/intechopen.99326*

nature, large surface area, and more electronic conductivity. The nanoporous electrode provides a large surface area that can enhance the electrochemical performance because increases the interactions between electrolytes with active electrode materials occurs faradic reaction at the interfacing sites. Further, an available large quantity of porosity nature can be more diffusion of the electrolytic electrons or ions in the electrodes and improve the volume alteration during the charge– discharge cyclic process due to enhances the cycling life of the active electrode materials. The oxide materials can be prepared by the various method can occur different type morphologies such as nanorod, nanoparticles, nanowire, nanoflower, nanotube, nanosheet, nanoneedles by Hydrothermal, Successive ionic adsorption and reaction (SILLAR), chemical precipitation, chemical bath deposition (CBD), sol–gel, solvothermal and electrodeposition methods.

Nickel oxide (NiO) exhibits multiple oxidations states these properties more suitable for the redox reaction or faradic reactions which gets maximum specific capacitance. One of the disadvances is the less electronic conductivity of the electrode. Therefore, large efforts have been dedicated to the manufacture of nanomaterials electrodes the exceptional advantages of some metal oxide doped NiO nanomaterials enhance the higher conductivity. Recently, the literature survey found that the Cu doped NiO nanomaterials, Co-doped nickel oxide, Mn-doped nickel oxide, Cerium doped nickel oxide were found to be very promising for the supercapacitor applications. In this review article, metal oxide doped nickel oxides materials for electrochemical supercapacitor applications have also been discussed briefly. However, to our best knowledge, there is no review found on the development of metal oxides doped NiO nanomaterials electrodes for supercapacitors applications. The energy storage mechanism in metal oxide doped nickel oxide electrode materials also discussed in this review article.

#### **2. Principle of energy storage mechanism in electrochemical supercapacitor**

In recent year, the energy storage and energy conversion are a big challenge and concern to the researchers. The excellent electrochemical performance depends on the properties of electrode materials. The electrochemical supercapacitor consists of a three importance parts one is electrode materials and another is an electrolyte, separator. The electrode materials are a most important part in an electrochemical capacitor. In the literature survey, the electrochemical electrical double layer capacitor made by carbon materials as a graphene oxide, activated carbon, carbon nanotubes and derivatives of a carbon materials, pseudocapacitor made by using a metal oxide or conducting polymers and hybrid supercapacitor is a made by using combination of the carbon-based materials and metal oxide. The charge storage mechanism of Electrical double layer capacitor (EDLc) is based on the electrostatically. The electrolytes and electrode materials interfaces on the surface layer in EDLc. The charge storage mechanism of the pseudocapacitor based on the electrochemically i.e. faradic reactions occur in the electrode and electrolytes interface and in the advance hybrid's supercapacitor consist of a both electrostatics and electrochemical charge storage mechanism.

The ideal electrical double layer capacitor electrode materials show rectangular in shape of cyclic charging -discharging cures but if the electrode shows pseudocapacitor behaviors then the curves show the nonlinear rectangular shapes, these nonlinear curves consist of oxidation- reduction peak. This peaks clearly indicates that the electrode materials and electrolytes interfaces occur faradic reactions during the cyclic voltammetry process **Figure 2**.

**Figure 2.**

*The charging -discharging cyclic voltammetry curve of electrochemical supercapacitor (a) EDLC curve and (b) pseudocapacitor curve.*

#### **2.1 Charge storage mechanism within NiO nano materials**

Generally, the metal oxide electrodes show higher power density than the carbon materials and higher electrochemical stability than the conducting polymer material electrodes. The charge store on the surface of the carbon based EDLCs supercapacitors and in puedocapacitor the charge store in porous nano materials, it occurs faradic reactions between the electrode materials and electrolytes. The NiO materials more suitable for supercapacitors applications because they exhibit several required properties.


NiO have been promising materials for supercapacitor applications due to the exhibits higher theoretical specific capacitance, but in practical case these NiO materials does not get or shows highest specific capacitance values. Sometimes, achieve higher specific capacitance of NiO materials because the NiO shows higher charge storage at the highly porous nanostructure materials, low resistance between electrode and electrolytes, highly conductive substrate. In the practical case NiO based supercapacitors observed in the literature survey, the specific capacitance values 50 to ~1000F/g. The NiO nano materials shows pseudocapacitive nature, during the cyclic voltammetry process these nanomaterials exhibits redox reaction mechanism and it's converted to NiOOH and reversible state. Sometimes, during the redox cycles, the cathodic current peaks and anodic current peaks shifted more towards the positive and negative axes as the scan rate was increased. The shifting of peaks currents which is maybe due to the highly accessible surface area of the porous NiO nanostructures and the fast ionic/electronic diffusion rate during redox reaction.

*Effect of Different Metals Doped in Nickel Oxide Nanomaterials on Electrochemical Capacitive… DOI: http://dx.doi.org/10.5772/intechopen.99326*

NiO and its binary as well as ternary composite materials prepared as various techniques by using some additives and binder free method. Therefore, the prepared NiO based binary as well as ternary composite or metal doped NiO materials shows lower conductivity and higher interfacing resistance. Due to metal oxides directly synthesized on conductive electrode substrate have the advantage as it can not only result in higher capacitance but also minimize the contact resistance. Various conductive substrates like stainless steel (SS), ITO glass, FTO glass, carbon cloth, carbon mesh, Ni foam, and copper strip electrodes have been used to preparations of the nanostructured metal oxides. It should be mentioned that the electrode metal oxide/hydroxides material deposited directly on to the current collector electrodes is the best choice to minimize the resistance and enhances the electrochemical performance of the electrode materials.

#### **3. Synthesis techniques**

Different techniques are used for the synthesis of the nanomaterials. The synthesized nano materials have been formed various nanostructures and proper required porous materials. It is well known that the nano structure morphology of the synthesized electrodes plays an important role in electrochemical redox reactions. The nanostructured electrodes increase the interactions between the electrode and electrolytes due to enhances the performance of the electrochemical performance.

In the chapter, we place advancing a comprehensive summary of the synthesis techniques, pure NiO and metal doped NiO based nanostructures with electrochemical analysis. Following methods briefly discussed one by one.

#### **3.1 Hydrothermal method**

Hydrothermal synthesis method is one of the most common used for one-pot synthesis techniques to prepare a wide range of metal oxides [29, 30]. Hydrothermal synthesis process is solution based. We know that these hydrothermal methods provide good crystallinity structures and highly nano porous morphological shape selectivity to oxides-based materials [31]. Generally, the precursors of metal oxides are formed by mixture of reaction ingredients being heated at sealed Teflon-lined stainless-steel autoclave. Solvents under the room temperature to high temperature range and various pressure ranges are used to formations of the nanostructure materials. Generally, the temperature is used higher than 1000C and a pressure will be established mechanically in a closed autoclave system. In hydrothermal techniques to the reaction temperature other parameter like volume of solvent, reaction time also have importance impact on the final synthesis morphology of the materials. By controlling the various parameters such as reaction time, pH value of percussor solutions, reaction temperature and concentration of the precursor solution, it can produce various dimensional (0D, 1D, 2D and 3D) morphologies with large surface area based porous nanostructures. A lot of researcher groups have made optimize the reaction conditions and prepares superior morphologies to enhance the electrochemical performance of the nano materials, which will be discussed in details later section.

#### **3.2 SILAR method**

These SILAR methods is a solution-based techniques, it is widely used for the synthesis of the various type of metal oxide/hydroxide thin films, these techniques more commonly used to preparations of the different type nanostructured materials. SILLAR is a simple, cost effective, binder free method and it is appropriate method for synthesis of large-scale of nanostructure materials [32].

#### **3.3 Chemical bath deposition method**

The chemical bath deposition (CBD) techniques were first discovered by Nagayama in 1988 [33]. This technique mostly used in prepare of the metal oxide thin films for various applications. The principle of CBD method deals with the immersion of a substrate in a precursor solution [34, 35]. Then the grown-on metal oxide/hydroxide precipitates on the substrate surface to produces a thin well adherent and binder less film. These techniques beneficial due to its low cost, low temperature, binder less, and various adjustment parameter for preparations of different nanostructured materials and also more suitable for large-scale deposition particularly for the preparation of uniform oxide thin films on samples. This method usually needs a strong chemical oxidant or reducing agent to drive reactions to take place. Recently, this technique is very popular in today's for preparations of different types metal oxides as well as hydroxides like NiO thin film nanostructures.

#### **3.4 Electrodeposition method**

The electrodeposition is a one of the most widely used method to formations of different metal oxides as well as hydroxides nano materials [36]. This electrodeposition method is based on electrochemical oxidation- reduction (redox) reaction and the metal oxides/hydroxides are deposited on to the conducting substrate electrode. In these techniques, the optimal of the anion and adjustment of appropriate pH of the solution during the deposition is very crucial parameter. Although, this is a simple method to preparations of metal oxides/hydroxides with uniform grown morphology on electrode.

#### **3.5 Spray pyrolysis method**

Among the chemical techniques mentioned above, spray pyrolysis is most popular today for the large area thin film formation. Spray solution results directly into oxide formation. It has number of advantages. (1) Doping is easy as required amount of dopant can be added by mixing proper amount of solution of the dopant. (2) Like vapor deposition technique, spray pyrolysis does not require high quality target or vacuum at any stage hence this is one of the great advantages of this technique in the industrial applications. (3) Deposition rate and the thickness of the film can be easily controlled by controlling spray parameters. (4) Deposition in the moderate temperature range 150°C - 500°C is possible. (5) There is no restriction on the size and surface morphology of the substrate. (6) It is possible to prepare multilayer or multi compositional films. Due to these advantages, numbers of conducting and semiconducting materials were prepared by spray pyrolysis technique [37, 38]. In the present work, spray pyrolysis set up (Labotronics make) was used to prepare thin films of cobalt oxide, manganese oxide, manganese doped cobalt oxide, ruthenium oxide, ruthenium doped cobalt oxide and ruthenium doped manganese: cobalt oxide (ternary oxide) thin films by both aqueous as well as non-aqueous routes.

#### **3.6 Microwave-assisted method**

Although, the hydrothermal method and solvothermal synthesis method are the most useful methods to preparations of the different types nanostructures materials *Effect of Different Metals Doped in Nickel Oxide Nanomaterials on Electrochemical Capacitive… DOI: http://dx.doi.org/10.5772/intechopen.99326*

with controllable structure, size and morphology [39, 40], In hydrothermal method synthesis nanomaterials required more time for the reactions. Therefore, the microwave-assisted method is being used widely for the synthesis of different types nanostructure materials in few minutes. Microwave synthesis has become a popular method. Which substantially reduces the reaction time. The microwave-assisted method can suppress side reactions and provide rapid kinetics of crystallization growth. Using the microwave- solvothermal coupled method; one can not only effectively reduce the reaction time but can also control the morphology. It can produce narrow particle size distribution with high purity and large surface area of the active electrode materials. Therefore, microwave-solvothermal technique is an effective technique to fabricate the different types metal oxide and hydroxides with desired morphology.

#### **4. Pure NiO electrode for supercapacitor applications**

The different method used to preparation of the different type's nanostructure nickel oxides/hydroxides. There have been many reports on NiO nanostructures including porous nano/microspheres [41, 42], nanosheets [43], nanoflowers [44] and nanofibers [45]. In general, the proper porous nanostructure plays an important role in electrochemical charge storage mechanism due excellent electrical conductivity and the large surface area [46–48]. The overview of pure NiO prepared by using different synthesis methods and their electrochemical performance is tabulated in **Table 1** [49–63]. The following section briefly discusses on various synthetic routes for the fabrication of pure NiO nanostructures and their supercapacitor properties.

#### **4.1 Hydrothermal method**

In the hydrothermal synthesis route provides by the 3D nanostructures of materials with large surface area, these properties more suitable for the supercapacitor applications. Generally, 3D nanostructured electrodes are prepared on the conducting substrate foams like nickel foam. Ni foams exhibits highly porous structures and conductive substrate for synthesis of NiO. In the hydrothermal method's various adjustable parameters like reaction temperature, reaction time, concentration of percussor solutions, pH and so on are used to preparation of the different type's nanostructures. This is due to the increases in the conductivity of the electrodes of the pure NiO. The nanosheets and flower-like morphology of NiO composed of flabbergasted lotus-root- like nanosheets were also fabricated by hydrothermal method [44]. The concentrations of reaction reagent in the reaction medium also have the key role to control the morphology of metal hydroxides and metal oxides.

#### **4.2 SILLAR and chemical bath deposition**

This method compared to hydrothermal method, relatively a smaller number of researchers work on preparations of the pure NiO. The specific capacitance SC of NiO were observed in aqueous NaOH and KOH electrolytes observed 129.5 F/g and 69.8 F/g respectively [34]. Xia et al. [57] successfully prepared the NiO monolayer hollow-sphere composed of porous net-like NiO nanoflakes film (SSA 325 m2/g) by chemical bath deposition using polystyrene sphere template. The SC value for this porous NiO films was found to be 311 F/g with an excellent capacitance retention which is due to the porous structure that could alleviate the structure distortion caused by volume expansion during the cycling process [56].


*Supercapacitors for the Next Generation*

#### *Effect of Different Metals Doped in Nickel Oxide Nanomaterials on Electrochemical Capacitive… DOI: http://dx.doi.org/10.5772/intechopen.99326*


## **Table 1.**

*Pure NiO prepared using various methods and their electrochemical supercapacitor performances.*

#### **4.3 Electrodeposition method**

In 1997 year, the porous NiO nanostructure materials were reported by Srinivasan et al. [65], where shows a very little capacitance value is 59 F/g. However, in 2004 year, the capacitance value observed 138 F/g has been reported for 3D NiO on stainless steel conducting substrate [66]. 1D mesoporous core shell structure shows the hexagonal lyotropic Ni(OH)2 synthesized by electrodeposition [67]. The main drawback of NiO is a lower electrical conductivity, and the achieving higher conductivity has a great, the NiO/ITO showed increased conductivity and thus improved the SC (1025 F/g) compared to NiO/Ti (416 F/g) [68].

#### **4.4 Spray pyrolysis method**

Among numerous chemical techniques mentioned in schematic (**Figure 3**) SPM is the most popular today because of its applicability to produce variety of doped and undoped metal oxide films [69]. The basic principle involved in SPM is the pyrolytic decomposition of salts of a desired compound onto the preheated substrates. The atomization of the spray solution into a spray of fine droplets also depends on the geometry of the spraying nozzle and pressure of a carrier gas. Every sprayed droplet reaching the surface of the hot substrate undergoes pyrolytic (endothermic) decomposition and forms a single crystallite or clusters of crystallites as a product. The remaining volatile byproducts and solvents escape out in the form of vapor phase. The substrates provide thermal energy for the decomposition and subsequent recombination of the constituent species, followed by sintering and crystallization of the clusters of crystallites and thus coherent films are formed. The required thermal energy is different for the different materials and the solvents used.

Nickel oxide (NiO) films can be prepared using various chemical methods. Among these, spray method is a mechanically simple, cost-effective, and large

#### **Figure 3.**

*Schematic spray pyrolysis deposition method. (Adapted with permission from Ref. [69]. Copyright 2021, Elsevier.)*

*Effect of Different Metals Doped in Nickel Oxide Nanomaterials on Electrochemical Capacitive… DOI: http://dx.doi.org/10.5772/intechopen.99326*

surface deposition method. The various precursors are used for preparation of NiO thin films electrodes using different in gradient sources like nickel nitrate, nickel acetate and nickel chloride [70]. Yadav et.al reported specific capacitance of the pure NiO is 564F/g at 1A/g in 2 M KOH electrolytes with 1000 cyclic stability [70]. Kate et.al reported that the NiO thin films were successfully deposited using spray method, the observed specific capacitance values is 1000F/g at 5 mV/s scan rate [71].

#### **5. Metals doped in NiO nanomaterials for supercapacitor applications**

A great development has been achieved in developing low cost, higher conductivity, porous materials and more simple methods for the synthesis of various metal doped oxide electrodes for electrochemical supercapacitor applications. In the previous point discussed, pure NiO has drawn rigorous research interests due to its promising properties and some drawback of the electrochemical analysis. However, pure NiO exhibits lower specific capacitance values (SC) and it is providing lower electrochemical stability. There are several reasons for the low stability and low capacitance of pure NiO. Such as conductivity of NiO materials is very poor, does not proper electrolytes interactions of the nanomaterials and so on. But if we want to increase the electrochemical stability with capacitance then we have to dope the proper metals, so that the conductivity of the nanomaterials will increases and also the capacitance will be increased. In the following **Table 2** [28, 72–80] shows metal oxide doped in NiO and its effect on electrochemical performance. Various method is used to formations of the metal doped NiO nanostructures like hydrothermal, spray, sol–gel, chemical bath, facile chemical synthesis and so on.

In the hydrothermal method various adjustable parameters used for the for the synthesis of different types nanostructured morphologies. Such as temperature controlled, concentration of the percussor, reaction time and different types


#### **Table 2.**

*Different Metal doped in NiO prepared using various methods and their structural, electrochemical supercapacitor performances.*

substrate foams are used for the preparations of the large surface area of the nanostructured morphologies. In schematic **Figure 4** shows effect of all above parameters on the nanomaterials and formations of the different nano structures [81]. The 1D or 2D structures like NiO nanorod, Ni(OH)2 nano-wall and Co3O4 nanowire/ nanosheet arrays can be attained by using simple hydrothermal of directly putting the substrates into precursor solutions (contain metal salts and alkali), maintaining for a certain time at appropriate temperature and following annealing treatment (**Figure 4**). The morphological structures and their size strongly depend only on the reaction conditions, such as reaction temperature, reaction time, and concentration of the precursor solutions and its ratios proportionality of the reactants. The observed structures of the NiO are the small nano array and Nickel hydroxides small nano wire like morphological structures.

In sol gel synthesis is a one of the widely used method for the formation of the large number nano materials. Saraynya et.al shows that the cerium doped nickel oxide more active and suitable materials for the supercapacitor applications [28]. They are observed highest specific capacitance value is 2444 F/g at 5 mV/s at a 1%

#### **Figure 4.**

*Schematic hydrothermal deposition with using various parameter. (Adapted with permission from Ref. [81]. copyright 2021, Elsevier).*

#### **Figure 5.**

*The effect on nanostructure by using various doping percentage in NiO. (Adapted with permission from Ref. [28]. Copyright 2021, Elsevier).*

*Effect of Different Metals Doped in Nickel Oxide Nanomaterials on Electrochemical Capacitive… DOI: http://dx.doi.org/10.5772/intechopen.99326*

#### **Figure 6.**

*(a and b) The SEM images of pure NiO and (c and d) The SEM images of 1% Ce doped NiO. (Adapted with permission from Ref. [28]. Copyright 2021, Elsevier).*

cerium doped nickel oxide. In a **Figure 5** shows the pure NiO is lower value of specific capacitance because these NiO clearly indicates agglomerated nanostructured morphologies, but in the cerium doped NiO exhibits highest specific capacitance due to the morphologies flake like and large active electrode surface area for the access of the electrons. Here in **Figure 6** shows percentage of the metal dopant increases the structure of the nanomaterials change, it clearly indicates mostly important parameter for the proper formations of the nanostructured morphology for charge storage supercapacitor applications. The 1% Cerium doped NiO formatted 3D nanoflower like structure with accessible large surface [28].

#### **6. Materials selection and challenges of the supercapacitor**

The effect of transition metal oxide doped NiO by spray method shows more electrochemical performance than the pure NiO. Kate et.al show that the cobalt doped nickel oxide highest specific capacitance value is 835F/g at 5 mV/s in a 2MKOH electrolytes. Sharanya et.al shows that the the cerium doped nickel oxide is exhibits pseudocapacitive nature and good candidate materials for the supercapacitor applications, his shows that the highest specific capacitance of the Ce doped NiO is 2444F/g [28].

#### **6.1 Selection of electrode materials for supercapacitor applications**

Electrochemical energy storage devices play an important role in developing green energy for future to the society. After evaluating the published literature survey, we noticed that a great research has been carried out on electrochemical supercapacitor applications. A great challenge on the to investigate the electrochemical

highly performed electrode materials. The various researches focused on the preparation of the large surface of the material morphologies and enhancing conductivity, and electrolytes to obtain high energy and power densities with long cycle life. Therefore, it is necessary to the selection of electrode materials for the electrochemical supercapacitor. Therefore, herein we proposed some designing high-performance electrode materials for supercapacitors, such as specific surface area, proper selecting electrolytes, conductivity of the electrode's materials and design more porous different types nanostructures.

#### **7. Challenges in the supercapacitor applications**

Various challenges in the electrochemical supercapacitor such as enhances specific surface area of the electrode materials, enhances the conductivity of the electrode materials, maintain proper thickness of the electrodes, proper electrolytes used in charge storage mechanism, fabrications of the device, suitable separator used between the electrodes, leakage problem in devices and maintain the equivalent series resistance of the electrode materials. All of the above challenges most important in the supercapacitor application.

#### **8. Conclusions**

In the chapter, we have scientifically drawn the recent progress on a transition metal doped nickel oxide (NiO) as the energy storage materials for supercapacitor applications. The effect of the metal doped nickel oxide on the supercapacitor and developing nanostructures of pure NiO and metal oxide doped NiO based pseudocapacitor electrodes have been discussed. If you want to get maximum capacitance, you need to have a specific surface area, it is crucial parameter to obtain suitable morphologies of electrode materials. Clearly indicates that the various nanostructures of the electrodes such as flower, flake, nanobelt, nanowire, nanorod, hollow, core-shell, granular particles thin films are needed to improve the electrochemical performances further. The specific surface area and conductivity of the electrodes are two most important critical parameters that determine the supercapacitor performance which has to be optimized.

Nickel oxide is a semiconductor material, it shows lower electric conductivity so it has the same effect on the electron motion and hence the effect on the specific capacitance of the supercapacitor. To improve electric conductivity, NiO is often combined with nanostructured conductive transition metal oxides such as cerium, copper, aluminum, and magnesium to produces Metal doped NiO based electrodes. In this way, good electrical conductivity and rich electroactive sites for the electrolyte ions are obtained. The metal dope nickel oxide is shows pseudocapacitive nature. The doping of other metal oxides can also introduce impurity band effects and can enhance the electrochemical performance.

It is critical for researchers to improve both synthesis conditions and material qualities in order to fully leverage the potential of NiO-based electrode materials. High specific capacitance and long-term cycle stability are also concerning that must be addressed. This is the focus of the authors' on-going work. Other hand the synthesis methodologies, there are many issues pertaining to the measurement techniques and electrode preparation process that require attention. Furthermore, engineering factors like fabrication of electrodes, choice of electrolytes, membrane separators and packaging are not well established and thus need extensive investigation.

*Effect of Different Metals Doped in Nickel Oxide Nanomaterials on Electrochemical Capacitive… DOI: http://dx.doi.org/10.5772/intechopen.99326*

#### **Conflict of interest**

The authors declare no conflict of interest.

#### **Author details**

Amar Laxman Jadhav\*, Sharad Laxman Jadhav and Anamika Vitthal Kadam Lab of Electrochemical Energy Studies, Department of Physics, The Institute of Science, Mumbai (Dr. Homi Bhabha State University), Mumbai, India

\*Address all correspondence to: jadhavamar50@yahoo.com

© 2021 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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Section 6
