Recent Design and Synthesis Strategies for High-Performance Supercapacitors Utilizing ZnCo₂O₄-Based Electrode Materials

Abstract

ZnCo₂O₄ has emerged as a promising electrode material for supercapacitor applications due to its unique properties and potential for high-performance energy storage. As a transition metal oxide, ZnCo₂O₄ offers eco-friendly characteristics and favorable diffusion properties, making it an attractive candidate for sustainable energy storage systems. However, the poor conductivity and low surface area of ZnCo₂O₄ have posed challenges for its optimal utilization in supercapacitors. Various innovative approaches have been explored to overcome these limitations, including the development of ZnCo₂O₄ with different morphologies such as core-shell and porous structures. This review work aims to provide a comprehensive analysis of diverse synthesis methods employed in recent studies, including hydrothermal growth, solvothermal synthesis, wet chemical methods, and miscellaneous synthesis techniques, each offering unique advantages and influencing the properties of the synthesized materials. The synthesis conditions, such as precursor concentrations, temperature, annealing time, and the incorporation of dopants or additional materials, were found to play a crucial role in determining the electrochemical performance of ZnCo₂O₄-based supercapacitor electrodes. Core-shell heterostructures based on ZnCo₂O₄ exhibited versatility and tunability, with the choice of shell material significantly impacting the electrochemical performance. The incorporation of different materials in composite electrodes, as well as doping strategies, proved effective in enhancing specific capacitance, stability, surface area, and charge transfer characteristics. Controlled synthesis of ZnCo₂O₄ with diverse morphologies and porosity was crucial in improving mechanical strength, surface area, and ion diffusion capabilities. The findings provide valuable insights for the design and engineering of high-performance supercapacitor electrodes based on ZnCo₂O₄, and suggest exciting avenues for further exploration, including advanced characterization techniques, novel doping strategies, scale-up of synthesis methods, and integration into practical supercapacitor devices. Continued research and development in this field will contribute to the advancement of energy storage technologies and the realization of efficient and sustainable energy storage systems.

1. Introduction

The environmental challenges posed by fossil fuels, including climate change and carbon-emissions-induced global warming, have prompted a shift towards clean renewable alternatives in the energy sector [1,2,3]. This transition has led to an increased focus on the development of functional materials for fabricating diverse energy storage devices, including aqueous Zn-ion [4], Na-ion [5], lithium–selenium [6], Li-ion [7], and Zn-air batteries [8]. Among various energy storage technologies, supercapacitors are considered a promising energy storage technology [9,10]. Supercapacitors offer several advantages, such as safety, lightweight construction, high efficiency, and stability [11,12]. They possess comprehensive characteristics, including fast charging and discharging, high power density, cyclic stability, and reversibility [13,14,15]. Supercapacitors have potential applications in numerous fields, making them an area of intense research and development [16,17].

The performance of a supercapacitor primarily depends on the characteristics of the electrode materials used [18,19,20,21]. Supercapacitors can be broadly categorized into two types based on the electrodes employed. The first type uses electrodes made of activated carbon, carbon nanotubes, and graphene to achieve electrical double-layer capacitors (EDLCs) [22,23,24]. However, EDLCs based on carbon electrodes suffer from low specific capacitance, limiting their energy storage capacity. To address this limitation, researchers have explored the use of pseudocapacitors, which are supercapacitors made with transition metal oxides, transition metal dichalcogenides, conducting polymers, and metal sulfides/selenides [25,26,27,28]. Pseudocapacitors have been found to exhibit superior performance compared to EDLCs [29,30].

Among the various transition metal oxides, ZnCo₂O₄ has emerged as a highly competent cathode material for pseudocapacitor-based supercapacitors. This compound, consisting of Zn²⁺ and Co³⁺ ions in a spinel structure, provides a continuous pathway for ion diffusion [31]. ZnCo₂O₄ possesses several desirable characteristics, including a relatively eco-friendly nature, low cost, high theoretical capacitance, and a large morphological diversity that includes nanosized sheets, wires, rods, and porous structures [32]. These nanostructures can be grown on conducting substrates such as nickel (Ni) foam, carbon cloth, and stainless-steel substrates [33].

However, ZnCo₂O₄ faces certain challenges, including poor conductivity, a low surface area, and limited rate capability, resulting in the premature fading of specific capacitance over multiple charge-discharge cycles [34]. To overcome these issues, considerable efforts have been made to improve the performance of ZnCo₂O₄ as an electrode material. Several strategies have been explored, such as synthesizing ZnCo₂O₄ with varying morphologies, creating composites with highly conducting fillers, and doping with metal ions to facilitate easy ion and electron diffusion.

To further improve the conductivity and rate capability of ZnCo₂O₄ electrodes, composite materials have been developed by incorporating highly conducting fillers. For instance, carbon-based materials such as carbon nanotubes and graphene have been successfully integrated with ZnCo₂O₄ to form hybrid structures [35,36,37]. These composites exhibit synergistic effects, combining the high specific capacitance of ZnCo₂O₄ with the excellent electrical conductivity of carbon-based materials [38]. Additionally, metal ion doping has been explored to enhance the ion diffusion kinetics within ZnCo₂O₄, thereby improving its electrochemical performance [39]. The performance of ZnCo₂O₄-based supercapacitors is typically evaluated in terms of specific capacitance and cyclic stability. Specific capacitance refers to the amount of charge stored per unit mass or the surface area of the electrode material and is a key parameter to assess the energy storage capacity. Cyclic stability measures the ability of the supercapacitor to maintain its specific capacitance over repeated charge-discharge cycles without significant degradation [40]. Recent studies have reported impressive improvements in both specific capacitance and cyclic stability through the optimization of ZnCo₂O₄ synthesis parameters and electrode configurations.

In recent years, significant progress has been made in the synthesis and performance optimization of ZnCo₂O₄ as an electrode material for supercapacitors. Various synthesis methods, including hydrothermal, solvothermal, and wet chemical approaches, have been employed for this purpose. Hydrothermal synthesis enables the production of ZnCo₂O₄ with improved crystallinity and structural properties. Furthermore, post-annealing processes, typically conducted above 250 °C for a few hours to 24 h, are employed to further enhance or tune the crystallinity and structural properties of ZnCo₂O₄-based electrodes. Binary to tertiary composites have also been developed using the hydrothermal method, expanding the possibilities for tailoring the properties of ZnCo₂O₄-based electrodes. Hydrothermal methods have proven particularly effective in producing ZnCo₂O₄ with desired characteristics.

Another area of research focuses on surface engineering and the modification of ZnCo₂O₄ electrodes. Surface modification techniques, including the deposition of conductive polymers or metal oxide coatings, have been employed to improve the overall performance of ZnCo₂O₄-based supercapacitors. These modifications enhance the electrical conductivity, stability, and ion accessibility of the electrode material, leading to improved energy storage capabilities and prolonged cycle life.

This review presents the significant progress achieved in designing and synthesizing ZnCo₂O₄-based electrode materials for high-performance supercapacitors. Scheme 1 summarizes the key points discussed in this review regarding the preparation of supercapacitor electrode materials. It emphasizes the importance of tailored morphologies, synthesis conditions, and the incorporation of different materials and doping strategies in enhancing the electrochemical performance of these materials. The discussed studies showcase the effectiveness of various synthesis methods, including hydrothermal growth, solvothermal synthesis, wet chemical methods, and miscellaneous synthesis techniques, in influencing the properties of the synthesized materials. The controlled synthesis of ZnCo₂O₄ with diverse morphologies and porosity has been crucial in improving their mechanical strength, surface area, and ion diffusion capabilities. These findings provide valuable insights for the design and engineering of high-performance supercapacitor electrodes based on ZnCo₂O₄. The future prospects in this field are also discussed in detail, including optimizing synthesis parameters, exploring sustainable synthesis routes, and investigating the compatibility and integration of these electrode materials into practical supercapacitor devices.

2. Hydrothermal Method

Hydrothermal synthesis is a widely employed method for the fabrication of various advanced materials, including ZnCo₂O₄-based electrode materials [41,42]. This technique involves the use of high-temperature and high-pressure aqueous solutions, providing a controlled environment for the nucleation and growth of nanoscale materials. With its distinct advantages in achieving desirable morphologies and enhanced electrochemical properties, hydrothermal synthesis has gained significant attention in the field of energy storage applications. Through hydrothermal synthesis, researchers have been able to precisely control the synthesis parameters to obtain ZnCo₂O₄ materials with uniform particle size distribution, improved crystallinity, and tailored morphologies [43,44,45]. The synthesis process typically involves the reaction between zinc and cobalt precursors in a hydrothermal reactor, where specific temperature and pressure conditions are carefully controlled [46]. The choice of precursor materials, reaction parameters, and subsequent post-treatment processes significantly influence the structural and electrochemical properties of the synthesized ZnCo₂O₄ materials. By manipulating reaction parameters such as temperature, pressure, precursor concentration, and reaction time, researchers have achieved a wide range of ZnCo₂O₄ morphologies, including nanoparticles, nanowires, nanosheets, and hierarchical structures. These diverse morphologies offer increased surface area and enhanced electrochemical performance. Various nanostructured ZnCo₂O₄-based materials have been prepared using the hydrothermal method and have been demonstrated as promising electrode materials for supercapacitors, which are discussed in this study.

2.1. Core-Shell Nanostructures

In order to enhance the electrical conductivity and structural rigidity of materials, the incorporation of ZnCo₂O₄ has been explored. One effective approach is the formation of core-shell electrodes by integrating ZnCo₂O₄ as the core and other binary metal oxides as the shell. This core-shell structure provides several advantages, including facilitating the rapid transport of ions and electrons, as well as efficient charge-discharge processes. Additionally, the core-shell heterostructure offers an increased number of electrochemically active sites. The core-shell architecture offers synergistic effects derived from both the core and shell components. This unique structure is particularly advantageous in energy storage applications. By directly growing core/shell hybrid nanostructures on conductive substrates such as Ni foam or Carbon cloth, the need for binders, which are often non-conductive polymeric materials that can hinder ion and electron transport, is eliminated. Typically, the synthesis process involves first hydrothermally growing ZnCo₂O₄ core materials either on a conductive substrate or as a powder material. Subsequently, a 1D nanostructure, such as nanowires, nanorods, or nanotubes, is grown on the ZnCo₂O₄ core. This is followed by the growth of a 2D nanosheet or nanoflake as the shell material. The incorporation of a 2D shell component significantly increases the surface area, thereby enhancing the overall electrochemical performance of the core-shell electrode. By adopting this core-shell approach, materials with improved electrical conductivity and structural integrity can be achieved. The utilization of conductive substrates and the careful design of core-shell structures offer promising prospects for advancing energy storage technologies.

Table 1. Hydrothermal synthesis conditions and electrochemical performance of core-shell ZnCo₂O₄-based electrode materials.

S. No.	Material	Hydrothermal Conditions	Specific Capacitance (F g−1)	Cycle Stability (%)	Energy Density (Wh kg−1)	Power Density (W kg−1)	Rate Capability (%)	Ref. (Publication Year)
1	3D array of ZnCo2O4 core and NiMoO4 shell	120 °C for 5 h	1912	84.1% after 10,000 cycles	57.5	18,000	55 at 20 A g−1	[47] (2020)
2	ZnCo2O4@NiCo2O4	120 °C for 6 h	1728	97.8% after 10,000 cycles	-	-	92.9 at 20 A g−1	[48] (2022)
3	ZnCo2O4@Ni-Co-S	120 °C for 6 h	1762.6	81.4% after 5000 cycles	-	-	81.3 at 50 A g−1	[49] (2020)
4	ZnCo2O4@Ni2.5Mo6S6.7	120 °C for 6 h	848	90.4% after 10,000 cycles	177.9	2700	-	[50] (2023)
5	ZnCo2O4@CoMoO4	120 °C for 7 h	903	95.1% after 8000 cycles	135.6	2704.1	-	[51] (2020)
6	ZnCo2O4@NiCoMn-S	120 °C for 6 h	1671	75% after 5000 cycles	64.36	950	-	[52] (2021)
7	Co3O4/ZnCo2O4	120 °C for 3 h	1804	93% after 3000 cycles	-	-	-	[53] (2021)

shows the hydrothermal synthesis conditions and electrochemical performance of core-shell ZnCo₂O₄-based electrode materials.

In the field of supercapacitor research, the development of ZnCo₂O₄-based core-shell heterostructures has garnered significant attention due to their promising electrochemical properties. Wang et al., (2020) conducted a study where they prepared a three-dimensional (3D) heterogeneous array consisting of a ZnCo₂O₄ nanowire core and a NiMoO₄ nanosheet shell (ZnCo₂O₄@NiMoO₄). The synthesis involved a two-step sequential combination of hydrothermal and annealing processes. Firstly, Zn(NO₃)₂·6H₂O (zinc nitrate hexahydrate) and Co(NO₃)₂·6H₂O (cobalt nitrate hexahydrate) were used as precursors to grow ZnCo₂O₄ nanowires on Ni foam through hydrothermal conditions at 120 °C for 5 h. Subsequently, a hydrothermal process at 120 °C for 5 h followed by annealing at 450 °C for 2 h led to the growth of NiMoO₄ nanosheets as the shell material. This ZnCo₂O₄@NiMoO₄ heterostructure exhibited a specific capacitance of 1912 F g⁻¹ and excellent cyclic stability of 84.1% after 10,000 cycles. The reported synergistic effect is a result of the ZnCo₂O₄/Ni foam, which enables the efficient transfer of ions and electrons, in conjunction with the NiMoO₄ nanosheet that facilitates the intercalation and deintercalation of OH⁻ ions. This synergistic combination leads to improved charge storage and transfer kinetics, resulting in enhanced specific capacitance and overall performance [47].

In a similar vein, Wu et al., (2022) reported the synthesis of a ZnCo₂O₄@NiCo₂O₄ hierarchical core-shell heterostructure. The fabrication process involved the hydrothermal growth of ZnCo₂O₄ nanowires on Ni foam at 120 °C for 6 h, followed by electrodeposition of NiCo₂O₄ nanosheets using a conventional three-electrode system at 25 °C. Subsequent annealing at 350 °C for 2 h further improved the structural integrity of the heterostructure. A schematic diagram depicting the synthesis process of ZnCo₂O₄@NiCo₂O₄ is presented in Figure 1. The resulting ZnCo₂O₄@NiCo₂O₄ core-shell heterostructure exhibited a specific capacitance of 1728 F g⁻¹ and exceptional cyclic stability of 97.8% after 10,000 cycles. These impressive results were attributed to the strong interfacial interaction between the electrode materials, the presence of numerous active sites, and the facilitated ion transport within the heterostructure [48].

Expanding the scope of ZnCo₂O₄-based core-shell heterostructures, Xuan et al., (2020) investigated the hierarchical ZnCo₂O₄@Ni-Co-S core-shell composite. Following a similar synthesis strategy to Wu et al., (2020) they first hydrothermally grew ZnCo₂O₄ nanowires on Ni foam and subsequently electrodeposited Ni-Co-S nanosheets. The resulting core-shell composite exhibited a specific capacitance of 1762.6 F g⁻¹ and demonstrated good cyclic stability, retaining 81.4% of its initial capacitance after 5000 cycles. The superior performance of the ZnCo₂O₄@Ni-Co-S core-shell composite was attributed to the synergistic effects resulting from the combination of the two materials, which promoted efficient charge storage and transport within the supercapacitor [49].

Incorporating different shell materials, Yang et al., (2023) presented a study on ZnCo₂O₄@Ni_2.5Mo₆S_6.7 core-shell hybrids. The synthesis process involved a two-step sequential hydrothermal and annealing process. Firstly, ZnCo₂O₄ nanowires were hydrothermally grown on Ni foam at 120 °C for 6 h. After the growth of the ZnCo₂O₄ nanowire core, Ni_2.5Mo₄ nanosheets were formed as the shell material. To further enhance the electrochemical performance, the Ni_2.5Mo₄ shell was subjected to sulfurization through hydrothermal treatment at 120 °C for 6 h using sodium sulfide nonahydrate, resulting in the formation of Ni_2.5Mo₆S_6.7. The resulting ZnCo₂O₄@Ni_2.5Mo₆S_6.7 core-shell hybrid exhibited excellent mechanical flexibility and demonstrated an energy density of 177.9 W kg⁻¹ and a remarkable capacitance retention of 90.4% after 10,000 cycles, highlighting its potential for high-performance supercapacitor applications [50].

Exploring the utilization of ternary molybdenum oxide, Liu et al., (2020) synthesized a ZnCo₂O₄@CoMoO₄ core-shell electrode. The synthesis process involved the growth of CoMoO₄ nanosheets, known for their outstanding conductivity, on a ZnCo₂O₄ core. The ZnCo₂O₄ core was first hydrothermally grown at 120 °C for 7 h, followed by the growth of the CoMoO₄ shell at 160 °C for 6 h. Subsequent annealing at 320 °C for 2 h enhanced the structural integrity of the core-shell heterostructure. Figure 2 shows the synthesis schematic and SEM images of the prepared samples. The resulting ZnCo₂O₄@CoMoO₄ electrode exhibited a specific capacitance of 903 C g⁻¹. Furthermore, when assembled into an asymmetric supercapacitor, the device achieved an impressive energy density of 135.6 Wh kg⁻¹ and exhibited a capacitance retention of 95.1% after 8000 cycles, demonstrating its potential for long-term stable energy storage [51].

Investigating the effect of the composition of the shell material on the electrochemical performance, Wang et al., (2021) optimized the changes in the composition of the NiCoMn-S shell in a three-dimensional ZnCo₂O₄@NiCoMn-S core-shell electrode. By tuning the concentration of precursor salts (MnCl₂·4H₂O, Co(NO₃)₂·6H₂O and Ni(NO₃)₂·6H₂O) during electrochemical deposition, the Mn, Co, and Ni content of the shell was varied. Prior to forming the NiCoMn-S shell, ZnCo₂O₄ nanorods were hydrothermally grown and annealed. The optimized ZnCo₂O₄@NiCoMn-S core-shell heterostructure, with an elemental ratio of Ni:Co:Mn = 1:2:1, exhibited the highest specific capacitance of 1671 F g⁻¹ and a cycle retention rate of 76.48% after 3000 cycles. When incorporated into an asymmetric capacitor, this material demonstrated an energy density of 64.36 Wh kg⁻¹ and cyclic stability of 75% after 5000 cycles, showcasing its potential for practical energy storage applications [52].

Furthermore, in an attempt to capitalize on the advantages of Co₃O₄, such as high conductivity, large surface area, and resistance to corrosion, Lin et al., (2021) developed Co₃O₄/ZnCo₂O₄ core-shell electrodes. The synthesis process involved the hydrothermal growth of Co₃O₄ nanorods on Ni foam at 120 °C for 3 h, followed by annealing at 300 °C for 3 h. Subsequently, flaky ZnCo₂O₄ nanosheets, with a thickness of approximately 40–100 nm, were formed on the Co₃O₄ nanorods, which had a diameter of around 150 nm. These core-shell electrodes were further annealed at 400 °C for 3 h to enhance the structural stability. The Co₃O₄/ZnCo₂O₄ core-shell electrode demonstrated a specific capacitance of 1804 F g⁻¹, highlighting its excellent electrochemical performance. Moreover, the electrode exhibited remarkable cyclic stability, retaining 93% of its initial capacitance after 3000 cycles. This enhanced performance was attributed to the superior structural support provided by the Co₃O₄ scaffold, which promoted efficient charge transfer and ion diffusion within the electrode material [53].

Comparing the synthesis conditions, it is evident that hydrothermal growth followed by annealing is a common approach in most studies. The specific temperatures and durations slightly vary depending on the desired materials and structures. In terms of performance, the specific capacitance values achieved in these studies range from 848 F g⁻¹ to 1912 F g⁻¹, indicating the high capacitance capabilities of the synthesized electrodes. Cyclic stability values after thousands of cycles range from 75% to 97.8%, demonstrating the robustness and durability of the core-shell heterostructures. Comparing the different shell materials, it can be observed that each composition exhibits unique electrochemical properties. NiMoO₄, NiCo₂O₄, Ni-Co-S, and NiCoMn-S shells demonstrate good specific capacitance values and cyclic stability. The presence of these materials as the shell layer enhances the overall electrochemical performance of the supercapacitors. CoMoO₄ and Co₃O₄ provide excellent conductivity and structural support, resulting in high specific capacitance and cyclic stability. The synthesis of ZnCo₂O₄@NiCo₂O₄ stands out with improved specific capacitance and capacitance retention. Furthermore, the optimization of the NiCoMn-S shell composition by Wang et al., (2020) highlights the importance of controlling the elemental ratios to achieve the highest specific capacitance and reasonable cyclic stability.

Overall, these studies demonstrate the versatility and tunability of core-shell heterostructures based on ZnCo₂O₄ for supercapacitor applications. The choice of the shell material and its composition significantly influences the electrochemical performance of the electrodes, including specific capacitance, cyclic stability, and energy density. The results provide valuable insights for designing and engineering high-performance supercapacitor electrodes with enhanced energy storage capabilities.

2.2. ZnCo₂O₄-Based Nanocomposites

Composite materials are multifunctional materials created by blending various components with distinct capabilities, including high electrical conductivity, mechanical strength, and energy storage properties. The hydrothermal method enables the growth of crystalline materials from precursor states, allowing for the synthesis of composite crystals with a synergistic effect through the combination of different precursors. This approach addresses the inherent limitations of ZnCo₂O₄, such as low electrical conductivity, cyclic stability, capacitance, and charge-transfer properties. By incorporating components with excellent conductivity and high ion/electron diffusion, composites with enhanced supercapacitor performance can be achieved.

Table 2. Hydrothermal synthesis conditions and electrochemical performance of ZnCo₂O₄-based nanocomposite electrode materials.

S. No.	Material	Hydrothermal Conditions	Specific Capacitance (F g−1)	Cycle Stability (%)	Energy Density (Wh kg−1)	Power Density (W kg−1)	Ref. (Publication Year)
1	ZnWO4/ZnCo2O4 nanocomposite	120 °C for 5 h	180.121 mA h g−1	97.83% after 4000 cycles	-	-	[54] (2021)
2	N-doped C and ZnCo2O4 composite	120 °C for 24 h	146	81.9% after 10,000 cycles	81.32	2668.3	[55] (2021)
3	PANI/g-C3N4 and ZnCo2O4composite	150 °C for 3 h	738 at 2 A g−1	100% after 2000 cycles	6.35	375	[56] (2021)
4	ZnCo2O4 RGO composite	140 °C for 12 h	365	94% after 2000 cycles	-	-	[57] (2022)
5	ZnCo2O4—GNP composite	190 °C for 12 h	593.6 F g−1 at 0.25 A g−1		-	-	[58] (2020)

shows the hydrothermal synthesis conditions and electrochemical performance of ZnCo₂O₄-based nanocomposite electrode materials.

CdS is the transition metal sulfide, known for its excellent conductivity, environmental stability, high theoretical capacity (1675 F g⁻¹), and low cost. Patil prepared CdS- ZnCo₂O₄/composite electrode by hydrothermal method. First, ZnCo₂O₄ was hydrothermally coated on Ni foam (140 °C, 16 h), and then CdS was grown on ZnCo₂O₄-coated Ni foam by successive ionic layer adsorption and reaction (SILAR) method. This composite exhibited higher areal capacity (2658 mC cm⁻²), energy density (517 μWh cm⁻²), and power density (17.5 mW cm⁻² at 25 mA) than ZnCo₂O₄ alone [59]. The synthesis schematic, SEM images, and performance comparison graphs of the prepared samples are shown in Figure 3. In a separate study, Raghavendra et al., (2021) prepared a jasmine petal-like nanoflower WS₂/ZnCo₂O₄ composite electrode. The precursors of constituting elements were mixed and WS₂/ZnCo₂ was hydrothermally formed (180 °C for 9 h) on Ni foam. The WS₂/ZnCo₂O₄ electrode exhibited a specific capacity of 154.74 mA h g⁻¹, and a cyclic stability of 96.34% after 4000 cycles [60].

ZnCo₂O₄ possessed with Zn²⁺ and Co³⁺ sites have been considered useful for constructing efficient supercapacitors. Raghavendra et al., (2021) prepared a ZnWO₄/ZnCo₂O₄ nanocomposite electrode using a single-step hydrothermal method, where the precursors of both components were mixed. The reaction was performed at 120 °C for 5 h, then the sample was dried at 60 °C for 12 h. There was no annealing step. This electrode showed a specific capacitance of 180.121 mA h g⁻¹, and 97.83% cyclic stability was observed after 4000 cycles. The observed performance was attributed to the rapid diffusion of electrons/ions in the ZnWO₄/ZnCo₂O₄ composite [54].

To address the issue of poor electrical conductivity associated with ZnCo₂O₄, Gunasekaran et al., (2021) prepared N-doped carbon nanocomposite. ZnCo₂O₄ was hydrothermally prepared at 120 °C for 24 h, and annealed at 250 °C for 2 h. For doping nitrogen, pulverized carbon derived from bamboo wood was impregnated with urea and activated at 900 °C for 2 h under inert conditions. Both ZnCo₂O₄ and N-doped activated carbon were mixed by solution blending and the composite was obtained after the stirring and drying process. This composite electrode resulted in a specific capacitance of 146 F g⁻¹ and cyclic stability of 81.9% after 10,000 cycles. N-functionality in the carbon backbone decreased oxygen content and favored rapid electron transfer, and also improved stability, conductivity, and performance [55].

In another approach, Kumar et al., (2021) used graphitic carbon nitride (g-C₃N₄) and polyaniline (PANI) as the nitrogen source for improving the supercapacitor performance of ZnCo₂O₄ electrodes. The g-C₃N₄ can enhance active sites, electron donor/acceptor properties, and high capacitance. PANI can enhance the conductivity as well as overall electrochemical properties. ZnCo₂O₄ was hydrothermally grown on Ni foam, which allowed the g-C₃N₄ to coat the immersed g-C₃N₄ dispersion. Meanwhile, the PANI monomer was added to the g-C₃N₄ dispersion. In the presence of g-C₃N₄, PANI formed via in situ chemical oxidation polymerization on the surface of ZnCo₂O₄. This hybrid ternary composite had higher surface-active species and rendered a synergistic effect. The specific capacitance of the hybrid ternary composite was 738 F g⁻¹ at 2 A g⁻¹ in 6 M KOH [56].

To further enhance the electrochemical performance by improving the surface area and electrical conductivity, Samiei et al., (2022) prepared ZnCo₂O₄/rGO composite by hydrothermal method and subjecting it to gamma ray exposure. As a result of exposure to gamma ray, the size of the particle size was decreased, and this led to an increase in surface area. Gamma irradiation with an average energy of 1.25 MeV was found to be optimum for obtaining ZnCo₂O₄/rGO electrodes with the highest performance with a specific capacitance of 365 F g⁻¹ and a stability of 94% after 5000 cycles [57].

In order to obtain high-energy density carbon nanoparticles (CNPs), the material with excellent electrical double-layer capacitance was prepared with ZnCo₂O₄ by Shaikh et al., (2020) CNPs were synthesized by carbonizing D-(+)-sucrose under hydrothermal conditions (190 °C for 12 h) and sintered (400 °C for 4 h). To prepare the composite, GNP dispersion was blended with the precursors of ZnCo₂O₄ and subjected to hydrothermal conditions (180 °C for 12 h) and annealed at 350 °C for 4 h. The specific capacitance of 593.6 F g⁻¹ at 0.25 A g⁻¹ was noticed [58].

These studies demonstrate the potential of ZnCo₂O₄-based composite electrodes for supercapacitor applications. The incorporation of different materials, such as CdS, WS₂, ZnWO₄, N-doped carbon, g-C₃N₄, PANI, rGO, and CNPs, offers enhanced electrochemical performance in terms of specific capacitance, cyclic stability, and energy density. The synthesis methods involve hydrothermal growth, SILAR, in-situ chemical oxidation polymerization, and gamma ray exposure, showcasing the versatility in fabricating composite electrodes. In terms of performance, the specific capacitance values achieved in these studies range from 146 F g⁻¹ to 738 F g⁻¹, indicating the high capacitance capabilities of the synthesized composite electrodes.

The cyclic stability values after thousands of cycles range from 81.9% to 97.83%, demonstrating the robustness and durability of the composite electrodes. Comparing the different shell materials used in the composite electrodes, it is evident that each composition exhibits unique electrochemical properties. For example, Cd_S, WS₂, ZnWO₄, and N-doped carbon provide improved specific capacitance and cyclic stability. The incorporation of these materials enhances the overall electrochemical performance of the supercapacitors.

The synthesis of ZnCo₂O₄ with graphitic carbon nitride (g-C₃N₄) and polyaniline (PANI) as nitrogen sources stands out, as it shows a high specific capacitance of 738 F g⁻¹ at 2 A g⁻¹ in 6M KOH. This indicates the effectiveness of incorporating g-C₃N₄ and PANI in improving the energy storage capabilities of ZnCo₂O₄ electrodes. Furthermore, the optimization of the composite electrodes by gamma ray exposure and the addition of carbon nanoparticles (CNPs) show enhanced performance in terms of specific capacitance and stability. These modifications result in increased surface area and improved electrical double-layer capacitance, contributing to higher energy storage. Finally, these studies demonstrate the versatility and tunability of ZnCo₂O₄-based composite electrodes for supercapacitor applications.

2.3. Modification of ZnCo₂O₄ through Elemental Doping and Decoration

The performance of electrode materials has been demonstrated to improve through the doping of inherently conducting transition metal ions. The introduction of heteroatoms through doping induces structural changes and distortions in ZnCo₂O₄, thereby increasing the number of active sites available. This increase in active sites is attributed to the impact of the doped elements, which in turn enhances the rate capability of the material. Interestingly, rather than forming distinct phases such as cobalt sulfide, zinc sulfide, or zinc cobalt sulfide, the structural flexibility and distortion induced by elemental doping play a significant role in achieving these improvements.

For instance, Yang et al., (2020) utilized the hydrothermal method to prepare S-doped ZnCo₂O₄. The incorporation of S into the ZnCo₂O₄ crystal structure is known to enhance its performance, similar to the effects observed with doping heteroatoms such as phosphorus (P), boron (B), and nitrogen (N). Initially, ZnCo₂O₄ micro-spindles were synthesized via hydrothermal synthesis at 180 °C for 12 h and subsequently annealed at 400 °C for 2 h. These micro-spindles were then subjected to hydrothermal treatment with thioacetamide at 120 °C for 2 h. The resulting samples were characterized using transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and elemental mapping (Figure 4). The S-doped ZnCo₂O₄ exhibited a high specific capacity of 522 F g⁻¹ at 0.5 A g⁻¹ and retained 78% of its capacitance after 5000 cycles. The introduction of S increased the disorder and flexibility within the ZnCo₂O₄ structure without disrupting its crystalline nature. Consequently, the charge transfer characteristics of ZnCo₂O₄ were enhanced through S-doping [61].

Due to their high electronic conductivity and capability for redox reactions, mixed metal sulfides demonstrate superior electrochemical performance compared to single metal sulfides. Vijayakumar et al., (2021) fabricated Zn-Co-S nanostrip cluster arrays by subjecting ZnCo₂O₄, which was hydrothermally grown on Ni foam, to an anion exchange reaction. The Zn-Co-S nanostrips had a length of 8 µm. The Zn-Co-S electrode exhibited a threefold increase in specific capacity (830 F g⁻¹) compared to the ZnCo₂O₄ electrode. This improvement can be attributed to the larger size of S^2– ions that substituted oxygen, resulting in higher polarizability and lower electronegativity. These factors played a crucial role in enhancing the performance of the asymmetric supercapacitor [62].

Table 3. Hydrothermal synthesis conditions and electrochemical performance of modified ZnCo₂O₄-based electrode materials.

S. No.	Material	Hydrothermal Conditions	Specific Capacitance (F g−1)	Cycle Stability (%)	Energy Density (Wh kg−1)	Power Density (W kg−1)	Ref. (Publication Year)
8	S-doped ZnCo2O4	180 °C for 12 h	522 F g−1	78% after 5000 cycles	-	-	[61] (2020)
9	Zn-Co-S from ZnCo2O4	120 °C for 12 h	830 F g−1	76% after 2000 cycles	19.0	514	[62] (2021)
10	Cu-doped ZnCo2O4	120 °C for 6 h	1425	96% after 2000 cycles	55	2621	[63] (2019)
6	Au-coated rGO-ZnCo2O4	150 °C for 12 h	113.8 at 10 mA cm−2	97% after 2000 cycles	31	2121	[64] (2020)
7	CNS-decorated Co-rich ZnCo2O4	180 °C for 6 h	1116.24 at 0.35 A g−1	93% after 2000 cycles	-	-	[65] (2020)

presents the hydrothermal synthesis conditions and electrochemical performance of electrode materials based on modified ZnCo₂O₄.

To address issues associated with ZnCo₂O₄ electrodes such as poor electrical conductivity, low rate capability, and low surface area, Sharma et al., (2019) doped ZnCo₂O₄ with Cu, which is a cost-effective and highly conductive material. To obtain Cu-doped ZnCo₂O₄, an appropriate mixture of precursors was blended and subjected to hydrothermal treatment (120 °C, 6 h), followed by calcination (450 °C for 2 h). Cu doping significantly increased the surface area of ZnCo₂O₄ by a factor of two and reduced the charge transfer resistance. As a result of Cu doping, the specific capacitance of the electrode increased by 1.55 times compared to ZnCo₂O₄, reaching 1425 F g⁻¹, while ZnCo₂O₄ exhibited a specific capacitance of 917 F g⁻¹. Moreover, the Cu-doped ZnCo₂O₄ electrode demonstrated stability of 96% after 2000 cycles. The improved performance can be attributed to the increased surface area and enhanced electronic conductivity achieved through Cu doping. Figure 5 provides a schematic depiction of the device preparation process and includes performance and cyclic stability graphs [63].

In addition, Patil et al., (2020) employed a gold-reduced graphene oxide composite (Au@rGO) to enhance the electrochemical performance of ZnCo₂O₄ electrodes. The presence of Au nanoparticles prevented the restacking of rGO sheets and increased the surface area of the electrode. To obtain the Au-coated rGO−ZnCo₂O₄ hybrid, HAuCl₄ was added to the hydrothermally prepared rGO-ZnCo₂O₄ (synthesized at 150 °C for 12 h and annealed at 400 °C for 2 h), followed by treatment at 90 °C for 30 min. This process resulted in the formation of cauliflower-like morphology in the Au@rGO-ZnCo₂O₄ hybrid, as illustrated in Figure 6. The electrode exhibited a specific capacitance of 288.5 mAh g⁻¹ at 2 mV s⁻¹ and demonstrated 97% stability after 2000 cycles. The distribution of Au nanoparticles on rGO was instrumental in improving the specific capacity of Au@rGO−ZnCo₂O₄ [64].

Furthermore, Reddy et al., (2020) adjusted the non-stoichiometric ratio of Zn/Co in ZnCo₂O₄ to create Co-rich ZnCo₂O₄, which was reported to exhibit higher electronic conductivity and superior supercapacitor performance. Additionally, carbon nanospheres (CNS) were decorated onto the ZCO nanosheets, and both materials were synthesized using the hydrothermal method. The interconnected CNS network was prepared from D-(+)-glucose at 180 °C for 9 h. To synthesize Co-rich ZnCo₂O₄/CNS, the CNS dispersion was mixed with ZnCo₂O₄ precursors and subjected to hydrothermal treatment at 180 °C for 6 h. The resulting electrode exhibited a specific capacitance of 1116.2 F g⁻¹ at 0.35 A g⁻¹, with 93% of the specific capacitance retained after 2000 cycles. The enhanced supercapacitor performance of this material was attributed to its high conductivity, short ion diffusion distance, reduced aggregation, and improved percolation of the electron-conducting pathway [65].

Various synthesis methods and doping strategies have been employed to enhance the electrochemical performance of ZnCo₂O₄ electrodes. S-doping, mixed metal sulfides, Cu-doping, Au@rGO composite, and Co-rich ZnCo₂O₄/CNS decoration have demonstrated improvements in specific capacitance, stability, surface area, and charge transfer characteristics. The choice of synthesis method and doping strategy depends on the specific requirements and desired performance characteristics for the application of ZnCo₂O₄-based supercapacitors. Sharma et al., (2019) achieved a remarkable performance (1425 F g⁻¹) enhancement in ZnCo₂O₄ electrodes by doping them with Cu, leading to a retention rate of 96% after 2000 cycles. This improvement can be attributed to the increased surface area and enhanced electronic conductivity resulting from Cu doping [63].

2.4. Tailoring ZnCo₂O₄ Morphology

The performance of supercapacitor electrode materials can vary based on their morphology, which determines factors such as surface area, structural connectivity, and porosity. ZnCo₂O₄ with different morphologies has been synthesized using the hydrothermal method, leading to the formation of micro flowers, microspheres, nanosheets, nanorods, nanoflakes, nanowires, nanotubes, and hierarchical sheet-like structures. In some cases, morphology-tuning agents have been employed to achieve specific shapes. Extensive research has been conducted to investigate the morphology-dependent properties of ZnCo₂O₄ electrodes. Table 4 provides a summary of the hydrothermal synthesis conditions for preparing ZnCo₂O₄ with various morphologies and compares their electrochemical performance.

Moreover, 2D hierarchical sheet-like ZnCo₂O₄ microstructures for supercapacitor applications were prepared using a simple hydrothermal method. The 2D thin sheet morphologies with hierarchical structures have been considered useful for obtaining functional materials with sheet-like morphologies and a large active surface area. Prasad et al., (2020) prepared a 2D hierarchical sheet-like ZnCo₂O₄ microstructure electrode using a hydrothermal method at 160 °C for 6 h, followed by annealing at 500 °C for 5 h. These 2D sheets, with unequal sizes, possessed numerous irregular pores and demonstrated an areal capacitance of 16.13 mF cm⁻² at a current density of 10 µA cm⁻¹. The authors claimed that the capacitance increased by 170% after 1000 cycles due to full activation. The increase in the active surface area resulting from the diffusion of the electrolyte into the bulk structure and circulation was reported as the reason for the unusually high capacitance increase of 170% after 1000 cycles [66].

Well-organized 3D hierarchical micro nano-structures have emerged as potential supercapacitors due to their mechanical stability and easy diffusion of ions and electrons. Reddy et al., (2020) synthesized 3D hierarchical ZnCo₂O₄ using a hydrothermal method at 180 °C for 6 h. They utilized polyvinylpyrrolidone (PVP) as a stabilizer and annealed the structure at 400 °C for 4 h. The use of PVP resulted in the formation of peony-like ZnCo₂O₄ structures due to the coordination of PVP ligands with metal oxides. Since PVP possesses both hydrophilic and hydrophobic functional groups, it effectively prevents the agglomeration of ZnCo₂O₄ and facilitates the dynamic growth of specific crystal structures. This material exhibited a specific capacitance of 421.05 F g⁻¹ (31.52 C g⁻¹) at 1 A g⁻¹ and maintained 88% stability after 2000 cycles. The peony-like architectures improved the mechanical strength and surface area as they generated crevices, allowing for the quick diffusion of ions and electrolytes [67].

ZnCo₂O₄ is known to suffer from low energy density and capacitance fading during charge/discharge cycles. To address this, Javed et al., (2020) deposited ZnCo₂O₄ nanobelts onto an activated carbon-cloth composite (ZCO@CC) using a hydrothermal method at 120 °C for 8 h. The ZCO@CC composite was then thermally sintered at 500 °C for 2 h. Figure 7 illustrates the synthesis process and SEM images of the ZCO nanobelts. ZCO@CC exhibited a specific capacitance of 1197.14 F g⁻¹ (838 C g⁻¹) at 2 A g⁻¹ and a rate capability of 75.18% at 10 A g⁻¹. A cyclic stability of 95.01% was observed after 5000 cycles. The remarkable electrochemical performance observed was attributed to the 3D interconnected nanobelt network with a mesoporous structure. This porous structure provided a high surface area, allowed for easy ionic diffusion, and ensured robust adhesion with the CC substrate. The direct deposition of ZCO on flexible CC was reported to be useful for avoiding binders and conductive fillers, while also ensuring the full utilization of the ZCO electrode material and providing robust mechanical stability [68].

Furthermore, Kumar et al., (2020) synthesized ZnCo₂O₄ nanomaterials by varying the growth agents. ZnCo-ammonium fluoride (ZC-AF), ZnCo-urea (ZC-UA), ZnCo-hexamethylenetetramine (ZC-HT), and ZnCo-urea/ammonium fluoride/hexamethylenetetramine (ZC-UAH) were used as growth agents. The hydrothermal synthesis was performed at 110 °C for 15 h, and the samples were annealed at 250 °C for 2 h. ZC-UAH resulted in the formation of ZnCo₂O₄ nanomaterials with a large surface area, growth modification, high electrical conductivity, and abundant active sites. The ZC-UAH electrode exhibited a specific capacity of 462.5 C g⁻¹ (at 1 A g⁻¹) with 97.4% capacitance retention after 5000 cycles [69].

Moreover, in pursuit of synthesis strategies enabling control over the morphology of ZnCo₂O₄, the ability to obtain electrodes with high surface-to-volume ratios and electronic conductivity becomes crucial. In this regard, Rajesh et al., (2021) successfully synthesized mesoporous necklace-type ZnCo₂O₄ nanowires using a hydrothermal method at 140 °C for 12 h, followed by annealing at 400 °C for 2 h. The mesoporous nanowires were formed through the assembly and growth of several tiny building blocks of ZnCo₂O₄. The supercapacitor fabricated with these nanowires demonstrated a specific capacity of 1099 F g⁻¹ (at 1 A g⁻¹), with a stability of 84.82% after 5000 cycles. The exceptional electrochemical performance was attributed to the high surface area of the mesoporous nanowires and the synergistic effect resulting from the composition of Zn and Co [70].

In addition to that, Naskar et al., (2021) employed a hydrothermal method at 120 °C for 5 h to grow ZnCo₂O₄ micro-stars on Ni foam, which were subsequently annealed at 350 °C for 2 h. To enhance its performance, ZnO was electrodeposited onto the ZnCo₂O₄ structure. Moreover, the researchers coupled this composite with a porous flaky activated carbon derived from green tea leaves. The resulting asymmetric supercapacitor exhibited a specific capacitance (SC) of 557 F g⁻¹, an energy density of 173 Wh kg⁻¹, and a power density of approximately 3 kW kg⁻¹. After 5000 cycles, the stability of the supercapacitor remained over 95%. Notably, the ZnO overlayer contributed to good electrical conductivity (14.6 mS cm⁻¹) and significantly enhanced the overall electron transport capability within the composite structure [71].

Stainless steel (SS) demonstrates stronger conductivity as a backbone for zinc cobaltite compared to Ni foam and carbon cloth. Additionally, SS serves as an inert substrate that does not interfere with the electrochemical characteristics of the deposited electrode material. Inherently, SS possesses high thermal stability, enabling better capacitance retention over prolonged charge-discharge cycles. Without the use of a binder, Tiwari et al., (2021) directly deposited nanostructured ZnCo₂O₄ on SS through hydrothermal (at 120 °C for 12 h) and electrodeposition methods, followed by annealing at 350 °C for 2 h. The hydrothermal route resulted in the formation of ZnCo₂O₄ microspheres (exhibiting a specific capacitance of 430.75 F g⁻¹ at 5 mV s⁻¹), while electrodeposition yielded ZnCo₂O₄ nanosheets (with a specific capacitance of 171.03 F g⁻¹ at 5 mV s⁻¹). The hydrothermally grown ZnCo₂O₄ microspheres, characterized by a high aspect ratio and purity, demonstrated better electrochemical performance compared to ZnCo₂O₄ nanosheets. Consequently, the improved electrochemical performance of hydrothermally deposited ZnCo₂O₄ microspheres positions them as a superior choice for supercapacitor applications compared to electrodeposited nanosheets [72].

Table 4. Hydrothermal synthesis conditions for the preparation of various morphological ZnCo₂O₄ and their electrochemical performance comparison.

S. No.	Material	Hydrothermal Conditions	Specific Capacitance (F g−1)	Cycle Stability (%)	Energy Density (Wh kg−1)	Power Density (W kg−1)	Ref. (Publication Year)
1	3D hierarchical ZnCo2O4	180 °C for 6 h	421.05	88% after 5000 cycles	-	-	[67] (2020)
2	ZnCo2O4 Nanowires	140 °C for 12 h	1099 F g−1	84.82% after 5000 cycles	41.87	4000	[70] (2021)
3	Star-shaped ZnCo2O4	120 °C for 5 h	557 F g−1	95% after 5000 cycles	173	3000	[71] (2021)
4	ZnCo2O4 microspheres by hydrothermal	120 °C for 12 h	430.75	85% after 500 cycles	70	3750	[72] (2021)

Table 4. Hydrothermal synthesis conditions for the preparation of various morphological ZnCo₂O₄ and their electrochemical performance comparison.

S. No.	Material	Hydrothermal Conditions	Specific Capacitance (F g−1)	Cycle Stability (%)	Energy Density (Wh kg−1)	Power Density (W kg−1)	Ref. (Publication Year)
1	3D hierarchical ZnCo2O4	180 °C for 6 h	421.05	88% after 5000 cycles	-	-	[67] (2020)
2	ZnCo2O4 Nanowires	140 °C for 12 h	1099 F g−1	84.82% after 5000 cycles	41.87	4000	[70] (2021)
3	Star-shaped ZnCo2O4	120 °C for 5 h	557 F g−1	95% after 5000 cycles	173	3000	[71] (2021)
4	ZnCo2O4 microspheres by hydrothermal	120 °C for 12 h	430.75	85% after 500 cycles	70	3750	[72] (2021)

In another study, Tiwari et al., (2021) utilized a hydrothermal method at 160 °C for 12 h to directly deposit ZnCo₂O₄ microspheres composed of crumbled nanosheets onto stainless steel, followed by annealing at 350 °C for 2 h. The resulting ZnCo₂O₄ microspheres electrode exhibited a specific capacitance of 593 F g⁻¹ at 10 mV s⁻¹. Furthermore, a columbic efficiency of 96.5% was achieved [73].

The hydrothermal method has been reported to be successful in synthesizing ZnCo₂O₄ with diverse morphologies. The use of hierarchical micro nano-structures, such as 3D-hierarchical and mesoporous architectures, has shown promise in enhancing the mechanical strength, surface area, and ion diffusion capabilities of the materials. The choice of growth agents, such as PVP, ammonium fluoride, urea, and hexamethylenetetramine, has also influenced the properties of the synthesized materials. In terms of electrochemical performance, specific capacitance values ranging from 421.05 F g⁻¹ to 1197.14 F g⁻¹ have been achieved at different current densities. The stability of the materials after numerous charge-discharge cycles varied between 84.82% and 97.4%. The incorporation of additional components, such as ZnO overlayers and activated carbon composites, has further enhanced the performance by improving electron transport and increasing energy density.

The choice of substrate material has also been explored, with stainless steel (SS) emerging as a promising option due to its high thermal stability, conductivity, and capacitance retention over prolonged cycling. The direct deposition of ZnCo₂O₄ on SS has resulted in microsphere and nanosheet structures, each exhibiting distinct electrochemical properties. Overall, the synthesis strategies and performance values discussed in the reviewed studies offer valuable insights for the development of ZnCo₂O₄-based materials with high surface-to-volume ratios, electronic conductivity, and superior electrochemical performance for supercapacitor applications.

Advantages of the Hydrothermal Method: The hydrothermal method allows for the synthesis of diverse morphologies and compositions of ZnCo₂O₄, including core-shell nanostructures, nanocomposites, and doped/decorated materials. This versatility enables the tailoring of the material properties to meet specific application requirements. The studies indicate that ZnCo₂O₄-based materials synthesized via the hydrothermal method exhibit high specific capacitance values up to 1912 F g⁻¹. This demonstrates the excellent energy storage capabilities of these materials in supercapacitors. The synthesized ZnCo₂O₄ materials demonstrate good cyclic stability, with a maximum of up to 97.8% retention after thousands of charge-discharge cycles. This highlights their robustness and durability for long-term usage. The hydrothermal method allows for the incorporation of different materials, such as CdS, WS₂, ZnWO₄, N-doped carbon, g-C₃N₄, PANI, rGO, and CNPs, resulting in composite electrodes with enhanced electrochemical performance. The addition of these materials improves specific capacitance, cyclic stability, and energy density. The optimization of shell composition in core-shell heterostructures and elemental doping strategies offer the ability to control the elemental ratios in ZnCo₂O₄-based materials. This control allows for the achievement of higher specific capacitance and reasonable cyclic stability.

Limitations of the Hydrothermal Method: The specific temperatures and durations required for hydrothermal growth may slightly vary depending on the desired materials and structures. The optimization of synthesis conditions for different compositions can be time-consuming and may require experimental trial and error. Scaling up the hydrothermal synthesis process to produce ZnCo₂O₄-based materials on a larger scale may pose challenges. Maintaining consistent product quality, uniformity, and reproducibility can be more difficult when working with larger volumes. The hydrothermal method allows for the incorporation of various additional materials, doping strategies, and morphological modifications. While this provides flexibility, it also adds more complexity to material synthesis and design. Understanding the interactions between different components and optimizing their combination can be challenging. Different morphologies and compositions of ZnCo₂O₄ synthesized using the hydrothermal method exhibit distinct electrochemical properties. However, it may be necessary to compromise on certain material qualities in order to concurrently achieve high specific capacitance, cyclic stability, and energy density.

3. Solvothermal Synthesis of ZnCo₂O₄

Similar to the hydrothermal method, solvothermal synthesis is a commonly used wet-chemical technique for synthesizing nanomaterials under high pressure and temperature conditions. In the solvothermal method, organic solvents, either alone or in combination with water, are used as the synthesis medium. The choice of solvent mixture plays a crucial role in solvothermal synthesis as it allows for the control of physicochemical characteristics, such as desired composition, structural orientation, shape, and redox properties. The surface tension of solvents also affects the formation of metal oxide nano/microstructures. Solvent systems with lower surface tension can modify the surface tension of the precursors, influencing the nucleation kinetics, crystal growth, and morphology.

In solvothermal synthesis, water-based solvent mixtures have been found to offer advantages over organic solvents such as acetone, chloroform, NMP, DMF, DMSO, and isopropanol. This section discusses the solvothermal synthesis of ZnCo₂O₄, and

Table 5. Solvothermal synthesis conditions and electrochemical performance of ZnCo₂O₄-based electrode materials.

S. No.	Material	Solvothermal Conditions	Specific Capacitance (F g−1)	Cycle Stability (%)	Energy Density (Wh kg−1)	Power Density (W kg−1)	Ref. (Publication Year)
1	ZnCo2O4 with Zn/Co vacancies	180 °C for 24 h	1608.95	89% after 3500 cycles	-	-	[74] (2020)
2	Porous ZnCo2O4 quasi-cubes	180 °C for 10 h	804	74.5% after 5000 cycles	34.4	860.1	[75] (2020)
3	ZnCo2O4/NiCo2S4 nanosheet arrays	150 °C for 12 h	2385	91.5% after 5000 cycles	57.28	8000	[76] (2022)
4	ZnCo2O4-ZnO/ZnCo2O4 core–shell microarchitecture composite	110 °C for 5 h	2487	95% after 6000 cycles	55.46	1500	[77] (2022)
5	ZnCo2O4 nanocubes with 3D porous structure	180 °C for 6 h	542.6	87% after 8000 cycles	-	-	[78] (2022)

provides an overview of the solvothermal synthesis conditions and electrochemical performance of ZnCo₂O₄-based electrode materials.

Inducing non-stoichiometric metal and oxygen vacancies in the structure of ZnCo₂O₄ has been found to be beneficial for improving its electrochemical performance. These vacancies serve as sites for absorption, electron donation, and charge transport. Typically, these vacancies are created through annealing under vacuum, Ar, N₂, O₂, or by treating with solvents. As an alternative approach, Reddy et al., (2020) developed self-assemblies of ZnCo₂O₄ with Zn/Co vacancies by adjusting the concentration of precursors without the need for external energy in the form of annealing or surface treatment. In their study, Reddy et al., (2020) introduced Zn/Co vacancies in the ZnCo₂O₄ self-assemblies by varying the concentration of precursors, namely Zn(CH₃COO)₂·2H₂O, Co(CH₃COO)₂·4H₂O, and CO(NH₂)₂. The solvothermal process was performed at 180 °C for 24 h using ethylene glycol as the medium. The concentrations of these precursors were adjusted at a ratio of 1:2:3, and this ratio was increased by twofold and threefold. The resulting morphologies were rhombus-, spindle-, and peanut-like structures, as depicted in Figure 8. The specific capacitance at 0.35 A g⁻¹ and the Zn/Co ratio of these samples were as follows: 1608.95 F g⁻¹ and 0.549; 629.05 F g⁻¹ and 0.533; 1007.48 F g⁻¹ and 0.503. Therefore, by tuning the volume ratio of the precursors, both the Zn/Co vacancies and the morphology of the ZnCo₂O₄ self-assemblies were effectively controlled [74].

The porosity of an electrode significantly influences the performance of a supercapacitor, as the pores serve as electroactive sites and provide ample contact for the electrolytes. Chen et al., (2020) aimed to develop an affordable solvothermal method for producing porous ZnCo₂O₄. Instead of using N,N’-dimethylformamide and isopropyl alcohol, they employed a mixture of glycerol and water as the solvent. Precursors such as Zn(CH₃COO)₂·2H₂O, Co(CH₃COO)₂·4H₂O, and urea were utilized, and the synthesis was conducted at 180 °C for 10 h. The resulting materials were then annealed in air at 350 °C for 4 h. This process yielded quasi-cubes (QCs) of ZnCo₂O₄ assembled with numerous pores. The material exhibited a surface area of 83.9 m² g⁻¹ and a specific capacitance of 804 F g⁻¹. Furthermore, the cyclic stability of the electrode was 79.2% after 3000 cycles [75].

To prepare electrodes with high power density, Jiao et al., (2022) utilized metal-organic framework (MOF)-derived ZnCo₂O₄ to deposit a ZnCo₂O₄/NiCo₂S₄ composite nanosheet array onto a hydrophilic carbon cloth. The ZCO/NCS composite electrodes were obtained through electrochemical deposition using a mixed solution of MOF-derived ZnCo₂O₄ and precursors of Ni, Co, and S. A medium consisting of DMF and methanol was employed for the solvothermal synthesis of MOF-derived ZnCo₂O₄ at 150 °C for 12 h. Subsequently, the synthesized material was treated with tannic acid and annealed at 600 °C for 2 h. The samples prepared under the electrodeposition condition (0.9 V, 10 min) exhibited a specific capacitance of 2385 F g⁻¹ (at 1 A g⁻¹) and a power density of 8.0 kW kg⁻¹ at 10 A g⁻¹. Furthermore, a capacitance retention of 91.5% was observed after 5000 cycles [76].

In line with previous studies, Li et al., (2022) explored the solvothermal method to prepare a composite of ZnCo₂O₄-ZnO/ZnCo₂O₄ core-shell microarchitecture. Initially, ZnCo₂O₄-ZnO microspheres and pristine ZnCo₂O₄ were separately prepared. The uncalcined ZnCo₂O₄-ZnO was then mixed with ZnCo₂O₄ precursors (Zn(NO₃)₂⋅6H₂O, Co(NO₃)₂⋅6H₂O, and hexamethylenetetramine), treated under identical solvothermal synthesis conditions (110 °C for 5 h), and calcined at 350 °C for 2 h. The synthetic illustration and corresponding SEM images of the prepared microspheres are provided in Figure 9. The morphology of the ZnCo₂O₄-ZnO microspheres consisted of interconnected rough nanowires. In the composite, vertically aligned, cross-linked, and dense nanosheets of ZnCo₂O₄ covered the ZnCo₂O₄-ZnO microsphere cores as a shell. The specific capacity, cyclic stability, energy density, and power density exhibited by this composite were 2487 F g⁻¹ (at 1 A g⁻¹), 93.3% over 6000 cycles, 55.46 Wh kg⁻¹, and 1500 W kg⁻¹, respectively. The abundance of active sites at the surface and interfaces of these composites holds promise for the fabrication of high-performance supercapacitors [77].

ZnCo₂O₄ with 3D porous structures is expected to provide a large surface area, easy access to electrolytes, and mechanical robustness. In line with this, Kuchi et al., (2022) synthesized 3D mesoporous ZnCo₂O₄ nanocubes self-assembled with nanoparticles. The precursors of ZnCo₂O₄ were mixed with an ethanol and water mixture, stirred, and treated at 180 °C for 6 h with slow heating. The resulting precipitate was collected and annealed at 400 °C for 4 h. These nanocubes exhibited a specific capacitance of 542.6 F g⁻¹ (at 1 A g⁻¹), and the stability was 87% after 8000 cycles. The high surface area, accelerated ion/electron diffusion, and ability to accommodate volume changes were attributed to the promising performance of ZnCo₂O₄ nanocubes [78].

In continuation of exploring different approaches to enhance the performance of ZnCo₂O₄, Gedi et al., (2022) synthesized ZnCo₂O₄ with the same morphology but varying in porosity by tuning the mixed solvent. Two solvent mixtures, ethylene glycol + deionized water (EGD) and ethanol + deionized water (ED), were used. The precursors were treated at 200 °C for 5 h and then annealed at 400 °C for 7 h. The resulting ZnCo₂O₄ microstructures exhibited polygonal morphology with a porous surface when using EGD, while ED resulted in polygons with a smooth surface. The surface areas obtained for EGD and ED were 50.17 m² g⁻¹ and 38.14 m² g⁻¹, respectively. The areal capacitance exhibited by the porous and smooth polygons was 24.4 and 20.1 mF cm⁻², respectively. EGD demonstrated 85% cyclic stability when tested up to 4000 cycles [79].

The studies demonstrate that the morphology and porosity of ZnCo₂O₄ can be controlled by adjusting the synthesis conditions and precursors used. The introduction of metal and oxygen vacancies in the structure of ZnCo₂O₄ proved to be useful for improving its electrochemical performance. The obtained specific capacitance values were ranging from 542.60 F g⁻¹ to 2487 F g⁻¹. Porosity also played a crucial role in the supercapacitor performance, with surface area values ranging from 38.14 m² g⁻¹ to 83.9 m² g⁻¹. The cyclic stability of the ZnCo₂O₄ samples was also an important factor, with all studies reporting retention values above 74.5% and up to 95% after several thousand cycles.

It is worth noting that the synthesis conditions greatly varied between the studies, with differences in solvent, precursor concentration, temperature, and annealing time. In terms of performance, the study by Li et al., (2022) reported the highest specific capacitance value of 2487 F g⁻¹ at 1 A g⁻¹, while the use of MOF-derived ZnCo₂O₄ in combination with the precursors of Ni, Co, S resulted in high energy density and power density of 57.2 Wh kg⁻¹ and 8000 W kg⁻¹, respectively. Additionally, the study by Chen et al., (2020) demonstrated an inexpensive solvothermal route for obtaining porous ZnCo₂O₄, which could be advantageous for large-scale production.

Advantages of the Solvothermal Method: The solvothermal method offers control over the morphology and porosity of ZnCo₂O₄ by adjusting the synthesis conditions and precursors used. This control allows for the optimization of material properties and performance for specific applications. The introduction of metal and oxygen vacancies in the ZnCo₂O₄ structure through the solvothermal method has been shown to improve its electrochemical performance, indicating the high energy storage capabilities of the synthesized materials. The porosity of ZnCo₂O₄ obtained through the solvothermal method plays a crucial role in supercapacitor performance, which provides an increased electrode-electrolyte interface and enhanced charge storage capacity. The cyclic stability of the ZnCo₂O₄ samples synthesized via solvothermal method is reported to up to 95% after several thousand charge-discharge cycles. This indicates the durability and long-term stability of the materials for supercapacitor applications. The solvothermal method offers flexibility in terms of solvent, precursor concentration, temperature, and annealing time. This versatility allows for the optimization of synthesis conditions to achieve desired material properties and performance characteristics. One study demonstrated an inexpensive solvothermal route for obtaining porous ZnCo₂O₄. This suggests the potential for large-scale production of ZnCo₂O₄-based materials using the solvothermal method, which could be advantageous for practical applications.

Limitations of the Solvothermal Method: The synthesis conditions, such as the solvent, precursor concentration, temperature, and annealing time, can greatly vary between studies. This variability makes it challenging to establish standardized protocols and may require further optimization for specific applications. The solvothermal method may involve complex synthesis procedures and the use of specific precursors and solvents. The complexity of the solvothermal method can increase the overall cost and make large-scale production more challenging. Additionally, due to the different synthesis conditions and precursors used, there may be compromises between specific capacitance, cyclic stability, surface area, and other material properties.

4. Wet Chemical Synthesis of ZnCo₂O₄

The wet chemical route has been proven to be an effective method for the large-scale synthesis of functional materials. This approach involves the mixing of precursor solutions, which initiates the chemical reactions leading to the formation of the desired material. The resulting pre-product can take various forms, such as a precipitate, sol-gel, solid residue, or even as a coating on a substrate. To obtain the final desired material, the pre-product is subjected to a high-temperature annealing step. Unlike hydrothermal and solvothermal methods, the wet chemical route eliminates the need for time-consuming high-pressure conditions. Consequently, the wet chemical method is considered an economical and rapid approach for synthesizing ZnCo₂O₄ and other similar materials.

Table 6. Wet chemical method-derived ZnCo₂O₄-based electrode materials and electrochemical performance.

S. No.	Material	Specific Capacitance (F g−1)	Cycle Stability (%)	Energy Density (Wh kg−1)	Power Density (W kg−1)	Ref. (Publication Year)
1	ZnCo2O4/NiCo2S4@ppy core-shell	2507	83.2% after 5000 cycles	44.15	4250	[80] (2021)
2	MOF-derived ZnCo2O4	94.4	87.2% after 3500 cycles	28.6	100	[81] (2020)
3	Hollow Nanospheres of ZnCo2O4	199	114% after 15,000 cycles	54.9	6105	[82] (2020)
4	Ni2+ and Cr3+ in to ZnCo2O4	575	90.24% after 2000 cycles	16.3	900	[39] (2021)
5	Ni2+-doped ZnCo2O4	176	95% after 10,000 cycles	40	3875	[83] (2021)
6	ZnCo2O4 Nanosheets Coated on Nanowires	1890	97.14% after 10,000 cycles	67.78	800	[84] (2022)
7	ZnCo2O4/CNT composite	888	94.72% after 5000 cycles	-	-	[85] (2022)
8	ZnCo2O4/ZnO nanobelts	481	67% after 6000 cycles	23.77	399.98	[86] (2023)

provides an overview of ZnCo₂O₄-based electrode materials synthesized using the wet-chemical method, along with their corresponding electrochemical performance.

In their study, Zhu et al., (2021) developed a core-shell nanosheets material consisting of spinel zinc cobalt oxide and nickel cobalt oxide, @PPy. The researchers achieved this by directly growing leaf-like nanosheets of ZnCo₂O₄, derived from a metal-organic framework (MOF), on Ni foam. The synthesis process involved the initial formation of Zn/Co-MOF using a wet chemical method. A solution containing precursors of Zn, Co, and Ni, along with the coordination regulator 2-methylimidazole, was mixed and applied to the Ni foam substrate. Subsequently, the Ni foam coated with MOF-derived ZnCo₂O₄ was annealed at 300 °C for 2 h. To complete the core-shell structure, a nanosheets shell layer of NiCo₂S₄@polypyrrole (PPy) was formed on the Ni foam coated with leaf-like ZnCo₂O₄ through chemical bath deposition using a mixture of NiCo₂S₄ and polypyrene [80].

MOFs, which are crystalline porous coordination polymers composed of organic ligands and metal ion clusters, have been utilized as templates for the synthesis of ZnCo₂O₄ due to their large specific surface area. Bimetallic MOFs, in particular, are well suited for facilitating fast charge transfer and redox reactions. In their study, He et al., (2020) synthesized MOF-based ZnCo₂O₄ using a wet chemical method. The schematic depiction of the synthesis process is illustrated in Figure 10. Initially, the MOF and Zn/Co-MOF were synthesized by stirring the respective precursors with methylimidazole. To obtain ZnCo₂O₄, the Zn/Co-MOF was annealed at 450 °C for 1 h under a N₂ atmosphere, followed by exposure to ambient air. The resulting carbon-rich ZnCo₂O₄ exhibited a specific capacitance of 94.4 F g⁻¹ and an energy density of 28.6 Wh kg⁻¹. The cyclic stability of the material was measured at 87.2% over 5000 cycles. The improved electrochemical performance can be attributed to the hierarchical porous structures of the MOF-derived ZnCo₂O₄ [81].

With the objective of fabricating an ultra-long-life supercapacitor, Hussain et al., (2020) employed a wet chemical method to prepare hierarchical hollow nanospheres of ZnCo₂O₄, with or without Ni-ion doping. In the synthesis of ZnCo₂O₄, zinc acetate dihydrate and cobalt acetate tetrahydrate were added to an ethylene glycol (EG) solution. The mixture was then gradually combined with NH₄F and hexamethylenetetramine in the EG solution containing the precursors. After stirring at 90 °C for 6 h, a ZnCo-glycolate residue was obtained. Calcination of the ZnCo-glycolate residue at 400 °C for 3 h yielded porous and crystalline nanospheres. In the case of Ni-doped materials, nickel acetate was added to the precursor mixture during the aforementioned procedure. The Ni-doped ZnCo₂O₄ exhibited a specific capacitance of approximately 199 F g⁻¹ (at 1 A g⁻¹), along with energy and power densities of approximately 54.9 Wh kg⁻¹ and 6105 W kg⁻¹, respectively. The cyclic stability was measured at 114% after 15,000 cycles. The hierarchical nanostructure of ZnCo₂O₄ provided a higher surface area, resulting in a supercapacitor with ultralong cyclic stability and high energy density [82].

Incorporating Ni²⁺ and Cr³⁺ into ZnCo₂O₄ was chosen as a strategy to increase conductivity. Soundarya et al., (2021) prepared Ni²⁺- and Cr³⁺-doped ZnCo₂O₄ through a sol-gel synthesis method. Zinc chloride and cobalt chloride hexahydrate were used as precursors, while citric acid was employed to solubilize the precursors. Chromium chloride hexahydrate and nickel chloride hexahydrate were utilized as the sources of Cr³⁺ and Ni²⁺, respectively. The mixture was heated at 80 °C for 6 h, and the powder of Ni²⁺ and Cr³⁺ incorporated into ZnCo₂O₄ was obtained by calcining this mixture at 550 °C for 5 h. The specific capacitance delivered by this material was 575 F g⁻¹ (at 1 A g⁻¹). The power density and energy density were measured at 900 W kg⁻¹ and 16 Wh kg⁻¹, respectively. The findings indicated that Ni²⁺ and Cr³⁺ served as conductivity boosters for ZnCo₂O₄, while reducing the particle size and enhancing the number of active sites [39].

Sandhiya et al., (2021) also prepared Ni-doped ZnCo₂O₄ using the sol-gel method. Nickel nitrate hexahydrate, zinc nitrate tetrahydrate, and cobalt nitrate hexahydrate were employed as precursors. The mixture was stirred at 80 °C for 30 min at room temperature, and the resultant mixture was left to stand for 12 h. Subsequently, it was calcined at 375 °C for 4 h in an air atmosphere. The Ni-doped ZnCo₂O₄ exhibited a higher specific capacitance of 176 C g⁻¹ compared to ZnCo₂O₄ (65 C g⁻¹). The capacitance retention after 10,000 cycles was 90%. The Ni²⁺ doping of ZnCo₂O₄ significantly improved both the energy and stability of the supercapacitor [83].

In the pursuit of multifunctional ZnCo₂O₄ materials, Wang et al., (2022) developed a single-step water bath heating method to fabricate 3D-structured ZnCo₂O₄ nanosheet-coated nanowires. In this method, an aqueous mixture of precursors was subjected to treatment with Ni foam at 75 °C for 12 h. The resulting Ni foam was then annealed at 550 °C for 5 h in an air atmosphere, resulting in the formation of nanowires with lengths ranging from 2 to 4 µm. Figure 11 provides a schematic representation of the synthesis process, as well as the morphology and EDS mapping of the ZnCo₂O₄ nanosheets coated nanowires. The specific capacitance of this material reached 1890 F g⁻¹ (at 3 A g⁻¹), and the cyclic stability was observed to be 96.1% after 10,000 cycles, indicating the robustness of the morphology against cycling-induced damage. Additionally, this material exhibited photocatalytic properties under sunlight, making it suitable for environmental pollutant treatment. This work presents a scalable strategy for synthesizing multifunctional ZnCo₂O₄ materials [84].

Using the co-precipitation method, Isacfranklin et al., (2022) synthesized a composite of ZnCo₂O₄ nanoparticles and carbon nanotubes (CNT). Chemical vapor deposition was employed to prepare the CNT, which served to enhance the electrochemical properties of the composite by improving conductivity and electrode stability. The precursors, ZnCl₂ and CoCl₂, were thoroughly mixed and precipitated using NaOH and urea. The resulting precipitate was collected and subjected to calcination at 450 °C for 3 h. The composite exhibited a specific capacitance of 888 F g⁻¹ and a remarkable capacitance retention of 94.72% over 5000 cycles. These findings conclusively demonstrate the positive impact of CNT on the electrochemical properties of the composite, primarily by enhancing material conductivity [85].

Electrodes comprising heterostructures formed by coupling components with different band gaps have been recognized as beneficial for enhancing energy storage and conversion properties. The interface of such heterostructures can induce charge redistribution, thereby generating a built-in electric field that enhances the ion and electron transfer capabilities of the electrode. The coupling of the p-type semiconductor ZnCo₂O₄ with the n-type semiconductor ZnO, for instance, can induce a built-in electric field when their Fermi levels reach equilibrium, resulting in the spontaneous migration of carriers around the interface. In this context, Ma et al., (2023) synthesized a ZnCo₂O₄/ZnO hybrid electrode using a wet chemical synthesis method. Initially, a mixture of a previously prepared methacrylic acid complex of Zn and Co with ethanol was prepared, leading to the formation of Co/Zn-nCPs nanobelts. Subsequent annealing of these nanobelts in an air atmosphere at temperatures ranging from 250 to 400 °C resulted in the formation of the ZnCo₂O₄/ZnO hybrid. Notably, the electrode annealed at 350 °C exhibited the highest specific capacitance of 481.0 F g⁻¹ at 1 A g⁻¹. Furthermore, an energy density of 23.77 Wh kg⁻¹ was observed at a power density of 399.98 W kg⁻¹. However, the cyclic stability of the electrode was found to be 67% after 6000 cycles. The enhanced electrochemical performance of the ZnCo₂O₄/ZnO hybrid can be attributed to the built-in electric field created within the heterostructure, which improves conductivity and charge transport. A detailed illustration of the mechanism underlying the improved performance of the ZnCo₂O₄/ZnO hybrid can be found in Figure 12 [86].

Cyclic stability, which measures the retention of capacitance over a certain number of charge-discharge cycles, is an important parameter for the long-term durability of supercapacitors. The studies reported retention values ranging from 67% to 114% after 5000 to 15,000 cycles, indicating good cyclic stability for most of the synthesized ZnCo₂O₄ materials. The synthesis methods and conditions significantly varied among the studies. Different techniques of wet chemical methods, such as sol-gel synthesis and water bath heating, were employed. Variations in precursor materials, annealing temperatures, reaction times, and the use of additional substances such as MOFs, polymers, or carbon nanotubes resulted in diverse morphologies and compositions of the ZnCo₂O₄ materials.

The studies highlighted the importance of tailoring the synthesis conditions and precursor materials to control the morphology, porosity, and composition of ZnCo₂O₄-based materials. The incorporation of dopants, hierarchical structures, heterostructure interfaces, and the use of templates such as MOFs or carbon nanotubes have shown significant improvements in specific capacitance, energy density, and cyclic stability.

These studies demonstrate that the synthesis conditions, including precursor concentrations, temperature, annealing time, and the incorporation of dopants or additional materials, play a crucial role in determining the electrochemical performance of ZnCo₂O₄-based supercapacitor electrodes. In terms of specific capacitance, the reported values varied across the studies, ranging from 94.4 F g⁻¹ to 2507.0 F g⁻¹. This wide range can be attributed to the different synthesis methods and strategies employed, as well as the variations in the morphology, porosity, and composition of the materials. Similarly, the energy density values varied from 16 Wh kg⁻¹ to 44.15 Wh kg⁻¹, indicating the diverse performance capabilities of the synthesized materials.

Advantages of the Wet Chemical Method: The wet chemical method allows for flexibility in adjusting synthesis conditions, such as precursor concentrations, temperature, annealing time, and the incorporation of dopants or additional materials. This versatility enables the optimization of electrochemical performance and material properties. The synthesis conditions and precursor materials can be tailored to control the morphology, porosity, and composition of ZnCo₂O₄-based materials. This control offers the ability to design materials with desired characteristics for specific supercapacitor applications. The studies demonstrate a wide range of specific capacitance values up to 2507.0 F g⁻¹, and energy density values, with a maximum of 44.15 Wh kg⁻¹, indicating the high-performance capabilities of the synthesized materials. This allows for the selection of materials with suitable properties for different energy storage requirements. The wet chemical synthesis method yields ZnCo₂O₄ materials with good cyclic stability, demonstrating the materials’ ability to maintain capacitance over a prolonged period and ensuring the long-term durability of supercapacitors. The wet chemical method allows for the incorporation of dopants, hierarchical structures, heterostructure interfaces, and the use of templates such as MOFs or carbon nanotubes. These additions have shown significant improvements in specific capacitance, energy density, and cyclic stability, expanding the performance capabilities of ZnCo₂O₄-based materials.

Limitations of the Wet-Chemical Method: The wet chemical method encompasses various synthesis techniques, such as sol-gel synthesis and water bath heating, leading to variations in precursor materials, annealing temperatures, reaction times, and the use of additional substances. These differences can make it challenging to establish standardized protocols and compare results across studies. Achieving the desired morphology, porosity, and composition of ZnCo₂O₄ materials through the wet-chemical method requires careful optimization of synthesis conditions and precursor materials. This complexity adds to the challenge of obtaining consistent and reproducible results. The synthesis conditions and the inclusion of dopants or additional materials in the wet chemical method can impact specific capacitance, energy density, and cyclic stability. Balancing these properties to achieve an optimal combination can pose significant challenges.

5. Miscellaneous Methods for the Preparation of ZnCo₂O₄-Based Electrodes

In addition to hydrothermal, solvothermal, and wet chemical methods, alternative approaches have been investigated to minimize the synthesis time and equipment requirements for fabricating ZnCo₂O₄ electrodes. Several methods, including combustion, ultrasonic synthesis, ionothermal, and vacuum filtration, have been explored with the aim of achieving this objective. These methods offer potential advantages in terms of efficiency and simplicity in the synthesis process of ZnCo₂O₄ electrodes. By exploring these alternative methods, researchers aim to streamline the synthesis procedures and improve the overall feasibility of producing ZnCo₂O₄ electrodes for various applications.

Table 7. Diverse methods for the synthesis of ZnCo₂O₄-based electrodes and their electrochemical performance comparison.

S. No.	Material	Method	Specific Capacitance (F g−1)	Cycle Stability (%)	Energy Density (Wh kg−1)	Power Density (W kg−1)	Ref. (Publication Year)
1	ZnCo2O4 NPs by combustion method	Combustion	843	97% after 5000 cycles	26.28	716	[87] (2020)
2	Porous nano ZnCo2O4 Nanoflakes	Ionothermal	90.6	100% after 10,000 cycles	21.2	800	[88] (2021)
3	ZnCo2O4/rGO	Vacuum Filtration	1075.4	89.3% after 10,000 cycles	-	-	[89] (2021)
4	ZnCo2O4@NiO composites	Refluxing	882	90.2% after 4000 cycles	46.66	800	[90] (2022)
5	ZnCo2O4-Nanocelluose Composite	Ultrasonic	346	97% after 5000 cycles	15.8	138.4	[91] (2022)
6	ZnCo2O4-	Coprecipitation	195.8 C g−1	89.5% after 4000 cycles	5.6	551.1	[92] (2022)

shows diverse methods explored for the synthesis of ZnCo₂O₄-based electrodes and their electrochemical performance comparison.

With the aim of reducing the synthesis time and the requirement for sophisticated instruments in the production of ZnCo₂O₄ nanoparticles, Bhagwan et al., (2020) developed a combustion method. They mixed the precursors with a small amount of water (5 mL) and a precipitating agent. The mixture was stirred for 10–15 min and then subjected to combustion at 300 °C for 15 min. Subsequently, the resulting mixture was calcined at temperatures ranging from 400 to 800 °C for 4 h. By employing simple mixing followed by thermal treatments, ZnCo₂O₄ nanoparticles with sizes below 50 nm were obtained within 5 h. Among the various samples, the samples calcined at 600 °C exhibited a high capacitance of 843 F g⁻¹ at 1 A g⁻¹. Additionally, an energy density of 26.28 Wh kg⁻¹ was achieved at a power density of 716 W kg⁻¹. The samples also demonstrated 97% stability after 5000 cycles. The superior performance of the sample calcined at 600 °C can be attributed to its high crystallinity and interconnected morphology, which contribute to enhanced electrochemical properties [87].

Deep eutectic solvents (DES), analogs of ionic liquids, have also been utilized for the synthesis of ZnCo₂O₄. DES is soluble in water, non-volatile, and non-toxic, making it an environmentally friendly solvent that can be produced at a low cost using quaternary ammonium salts. The use of DES in synthesis is expected to yield nanomaterials with a porous framework. Eum et al., (2021) successfully prepared 2D porous nanoflakes of ZnCo₂O₄ through DES-mediated synthesis. In their method, DES was produced by combining choline chloride and urea in a 1:2 ratio, which was then heated to 80 °C in an airtight flask. After sufficient stirring, a homogeneous and colorless DES was obtained. To synthesize ZnCo₂O₄, the desired number of precursors was dissolved in DES and treated in a closed autoclave at 140 °C for 12 h. The resulting 2D ZnCo₂O₄ nanoflakes exhibited a highly porous structure and cubic crystalline morphology. The surface area and pore volume were measured to be 115.18 m² g⁻¹ and 0.411 cm³ g⁻¹, respectively. The specific capacitance of the material was calculated to be 90.6 F g⁻¹. Furthermore, the energy density of the material was found to be 21.2 Wh kg⁻¹ at a power density of 800 W kg⁻¹. Remarkably, the cyclic stability test demonstrated 100% capacity retention even after 10,000 cycles. The exceptional electrochemical performance of the material can be attributed to its porous framework and large surface area, which contribute to enhanced energy storage and conversion capabilities [88].

Electrode materials with a hollow structure are considered more beneficial for improving capacitance due to their large surface area, good permeability, and low density. Yan et al., (2021) successfully synthesized a ZnCo₂O₄/rGO electrode with a hollow sphere morphology. The process involved first preparing the metal oxide through hydrothermal treatment at 180 °C for 6 h, followed by annealing at 350 to 400 °C for 2 h. Then, a dispersion of metal oxide/GO (graphene oxide) was prepared. A film was formed by vacuum filtration using the metal oxide/GO dispersion. The prepared metal oxide/GO film was further subjected to thermal annealing at 180 °C to convert GO into rGO (reduced graphene oxide). Figure 13 illustrates the production of asymmetric supercapacitors and NiCo₂O₄/rGO hybrid lamellar films. The specific capacitance exhibited by the ZnCo₂O₄/rGO electrode (at a ratio of 1:2) was measured to be 1075.4 F g⁻¹, and the cyclic stability was observed to be 89.3% after 10,000 cycles. The outstanding electrochemical properties were attributed to the synergy between the high conductivity of rGO and the rich redox performance of the metal oxide. This combination contributed to the enhanced electrochemical performance of the electrode material [89].

The binder-free and direct deposition of core/shell electrodes on conducting substrates, such as flexible stainless-steel mesh (FSSM), is an effective approach for fabricating high-performance supercapacitors. Kamble et al., (2022) achieved this by directly depositing ZnCo₂O₄@NiO composites onto FSSM by refluxing an aqueous precursor solution at 95 °C for 8 h, with the FSSM vertically mounted. Following the reflux reaction, the FSSM was annealed at 350 °C for 2 h, resulting in the hierarchical deposition of NiO nanoflakes on the ZnCo₂O₄ nanorods. This core-shell structure with a high electroactive surface area exhibited a specific capacitance of 882 F g⁻¹ (at 4 mA cm⁻²). The maximum energy density observed was 46.66 Wh kg⁻¹. Furthermore, the cyclic stability of the electrode was 90.2% after 4000 cycles. The presence of a large number of electroactive sites is the key factor contributing to the high electrochemical performance exhibited by the ZnCo₂O₄@NiO composites [90].

The use of nature-driven functional materials as carbon sources for the fabrication of metal oxide/carbon composite electrodes has gained attention. Examples of such materials include natural biopolymers such as chitin, phytagel, cellulose, and chitosan. Palem et al., (2022) employed nanocrystalline cellulose (NC) to create a hierarchical ZnCo₂O₄-NC electrode using an ultrasonic synthesis method. The NC, obtained through acid hydrolysis, was mixed with ZnCo₂O₄ precursors and subjected to probe sonication (60% amplitude, 10 s pulse, 750 W, and 20 kHz) for 60 min. The resulting precipitate was separated, dried, and annealed at 450 °C for 6 h. The synthesis process and EDX mapping results of the nanocomposites are illustrated in Figure 14. The ZnCo₂O₄-NC composite exhibited a cyclic stability of 97% after 5000 cycles, with an initial specific capacitance of 346 F g⁻¹ compared to 236 F g⁻¹ for pristine ZnCo₂O₄ (at 0.5 A g⁻¹). The energy density was measured at 5.8 Wh kg⁻¹. The enhanced specific capacitance of this composite can be attributed to improved morphological and surface properties. The combination of NC and the ultrasonic synthesis method proved to be effective in the efficient synthesis of ZnCo₂O₄-NC within a relatively short preparation time [91].

Similar to reduced graphene oxide (rGO), MXenes, which are conductive 2D sheets of transition metal nitrides, carbides, and carbonitrides, have also been investigated for their potential in enhancing the electrochemical performance of electrodes. Wang et al., (2022) synthesized ZnCo₂O₄-MXenes composites using a coprecipitation method. Initially, Ti₃C₂ was delaminated into nanosheets through HCL/LiF etching. Subsequently, ZnCo₂O₄ precursors, trisodium citrate, and delaminated-Ti₃C₂ (d-TC) were mixed and refluxed at 90 °C for 6 h under a N₂ atmosphere. The composite was then collected and annealed under N₂ flow at 350 °C for 2 h. The synthesis process and charge storage mechanism of the composites are depicted in Figure 15. The ZnCo₂O₄/d-TC heterostructure composites exhibited an energy density of 15.6 Wh kg⁻¹, a specific capacity of 195.8 C g⁻¹, and capacitance retention of 89.5% after 4000 cycles. The presence of homogeneously vertically grown aligned ZnCo₂O₄ porous nanoplates anchored on ultrathin delaminated-Ti₃C₂ nanosheets contributed to improved stability and enhanced charge transfer [92].

All of the synthesis methods explored in this section have their advantages and disadvantages. The combustion method is a simple and cost-effective way to synthesize ZnCo₂O₄ nanoparticles, but high temperatures are required for calcination. The ionothermal synthesis method using deep eutectic solvents (DES) is an eco-friendly solvent and can produce highly porous ZnCo₂O₄ electrodes, but the process can be time-consuming. The vacuum filtration method can produce hollow structured ZnCo₂O₄ electrodes with high specific capacitance, but the process can be complex. The refluxing method can deposit ZnCo₂O₄ electrodes on flexible substrates, but the process requires a high-temperature annealing step.

The ultrasonic synthesis method can produce ZnCo₂O₄ electrodes with a short preparation time, but the specific capacitance is relatively low. Finally, the coprecipitation method can produce ZnCo₂O₄ composites with MXenes which can increase the electrochemical performance of the electrodes. The choice of synthesis method for ZnCo₂O₄ electrodes should depend on the specific application requirements, such as the desired specific capacitance, energy density, and cycling stability. ZnCo₂O₄/rGO, prepared using the vacuum filtration method as reported by Yan et al., (2021) demonstrated high performance with a specific capacitance of 1075.4 F g⁻¹ and a promising cyclic stability of 89.3% after 10,000 cycles. The authors attributed this exceptional performance to the material’s large surface-active area, good permeability, and low density.

6. Conclusions and Future Scope

This review article provided a comprehensive analysis of the design and synthesis strategies employed for high-performance supercapacitors utilizing ZnCo₂O₄-based electrode materials. ZnCo₂O₄ has emerged as a promising transition metal oxide with its eco-friendly nature and favorable diffusion characteristics. However, the inherent challenges of poor conductivity and low surface area have impeded its optimal utilization in supercapacitor applications. To overcome these limitations, various innovative approaches have been explored. The development of ZnCo₂O₄ with different morphologies, such as core-shell and porous structures, has demonstrated significant improvements in specific capacitance and surface area, respectively. The studies reviewed encompassed various synthesis methods, including hydrothermal growth, solvothermal synthesis, wet chemical methods, and various miscellaneous synthesis, each offering unique advantages and influencing the properties of the synthesized materials. Hydrothermal synthesis has been a particularly effective method for achieving crystalline ZnCo₂O₄ with diverse structures, including core-shell, doped, and composite architectures. This synthesis route offers precise control over the growth process, resulting in tailored electrode materials with enhanced electrochemical properties. Moreover, solvothermal and wet chemical synthesis methods have also contributed to the synthesis of ZnCo₂O₄-based electrodes, providing alternative routes for material design and optimization. The synthesis conditions, such as precursor concentrations, temperature, annealing time, and the incorporation of dopants or additional materials, were found to play a crucial role in determining the electrochemical performance of ZnCo₂O₄-based supercapacitor electrodes. The choice of synthesis method should depend on specific application requirements, considering factors such as desired specific capacitance, energy density, and cycling stability.

The studies demonstrated the versatility and tunability of core-shell heterostructures based on ZnCo₂O₄ for supercapacitor applications. In the core-shell preparation, the choice of the shell material and its composition significantly influenced the electrochemical performance, including specific capacitance, cyclic stability, and energy density. Incorporation of different materials, such as CdS, WS₂, ZnWO₄, N-doped carbon, g-C₃N₄, PANI, rGO, and CNPs, in composite electrodes offered enhanced electrochemical performance. Doping strategies, such as S-doping, mixed metal sulfides, Cu-doping, Au@rGO composite, and Co-rich ZnCo₂O₄/CNS decoration, proved effective in improving specific capacitance, stability, surface area, and charge transfer characteristics of ZnCo₂O₄ electrodes. Notably, Cu-doped ZnCo₂O₄ demonstrated remarkable performance enhancement with a retention rate of 96% after 2000 cycles, attributed to increased surface area and enhanced electronic conductivity. The controlled synthesis of ZnCo₂O₄ with diverse morphologies, hierarchical micro nano-structures, and porosity proved crucial in enhancing the mechanical strength, surface area, and ion diffusion capabilities of the materials. The choice of growth agents and substrate materials, such as stainless steel (SS), also influenced the electrochemical properties of the synthesized materials.

Overall, the findings from these studies provide valuable insights for the design and engineering of high-performance supercapacitor electrodes based on ZnCo₂O₄. Looking ahead, there are several exciting avenues for further exploration in the field of high-performance supercapacitors utilizing ZnCo₂O₄-based electrode materials. Firstly, the integration of advanced characterization techniques can provide deeper insights into the structure-property relationships, facilitating the rational design of electrode materials with improved performance. Secondly, the exploration of novel doping strategies and the synthesis of hybrid composites can offer synergistic effects, enabling even higher energy storage capabilities and long-term stability. Additionally, efforts should be directed towards the scale-up of these synthesis methods for large-scale production, as well as investigating the compatibility and integration of these electrode materials into practical supercapacitor devices. In conclusion, the synthesis strategies and performance values discussed in this review article demonstrate the immense potential of ZnCo₂O₄-based electrode materials for high-performance supercapacitors. Continuous efforts should be made to optimize the synthesis parameters, enhance the reproducibility of results, and explore sustainable synthesis routes to ensure the commercial viability of these electrode materials. Continued research and development in this field will undoubtedly contribute to the advancement of energy storage technologies and the realization of efficient and sustainable energy storage systems.