A Review of Cobalt-Based Metal Hydroxide Electrode for Applications in Supercapacitors

Abstract

Supercapacitors are the cutting-edge, high performing, and emerging energy storage devices in the future of energy storage technology. It delivers high energy and produces higher specific capacitances. This research study provides insights into supercapacitor materials and their potential applications by examining different battery technologies compared with supercapacitors’ advantages and disadvantages. Transition metal hydroxides (cobalt hydroxides) have been studied to develop electrodes for supercapacitors and their use in various fields of energy and conversion devices. Cobalt-based metal oxides and hydroxides provide high-capacitance electrodes for supercapacitors. Metal hydroxides combine high electrical conductivity and excellent stability over time. The metal oxides used to prepare the electrodes for supercapacitors are cobalt-based metal oxides and hydroxides. It is stronger than most of the other oxides and has tremendous electrical conductivity. Cobalt hydroxides are also used in supercapacitors instead of other metal hydroxides, such as aluminum hydroxide, copper hydroxide, and nickel hydroxide. This study gives a complete overview of the preparation, synthesis, analysis, and characterization of cobalt hydroxide thin film electrodes by using the electrochemical deposition technique, parameters measurements, important characteristics, material properties, various applications, and future enhancement in supercapacitors.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Supercapacitors, as sustainable energy storage devices, can contribute to the natural ecological environment and society. Cobalt hydroxide is a kind of metal oxide material with cobalt and oxygen as the main elements. Cobalt hydroxide is a material that has been explored as a potential electrode material for use in supercapacitors. It has several properties that make it attractive for this application, including a high specific capacitance, good electrical conductivity, and good stability over a wide range of pH conditions. Cobalt hydroxide has been shown to have a high energy density, making it an interesting choice for use in supercapacitors. In addition, it has good cycling stability, meaning it can maintain its performance over a large number of charge/discharge cycles. One of the main challenges in using cobalt hydroxide as an electrode material for supercapacitors is its relatively high cost compared to other materials. Further, there is ongoing research into improving the performance and stability of cobalt hydroxide-based supercapacitors. Cobalt hydroxide nanoflakes can be prepared, and the cathodic potentials can also be observed by various research tools. Nanoflakes have been studied structurally and morphologically by using scientific research tools such as X-ray diffraction (XRD) for determining the crystal structure of the cobalt hydroxide thin films, Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), and scanning electron microscopy (SEM) for measuring the nanoparticle size, galvanostatic charge-discharge (GCD), energy dispersive spectroscopy (ESD), and electrochemical impedance spectroscopy (EIP). Its properties are recognized by cyclic voltammetry (CV) for observing the behavior of electrodes concerning voltage-current characteristics and electrochemical impedance spectroscopy (EIP) [1]. Thin films of cobalt hydroxide Co(OH)₂ were developed on the substrate of stainless steel (SS) under galvanostatic conditions [2]. The growth of the cobalt hydroxide coating was controlled on stainless steel (SS), and alkaline conditions were used as the desired catalyst for oxygen evolution [3]. The silver deposition method developed, characterized, and synthesized thin films of α-Co(OH)₂ with natural porosity and uniform structure. High-impedance measurements have been performed to evaluate the pseudocapacitance behavior and validate its uniform and porous natural properties [4].

The cobalt (Co) thin film prepared on the metal surface was patterned to obtain new Co pattern images. The vertical, horizontal, and circular patterns were obtained by SEM [5]. A thin film was fabricated that uses cobalt-doped nickel phosphate as the active material for the high-performance electrochemical advanced energy storage device [6, 7]. The nanomaterials were deposited directly onto porous nickel foam based on transition metal oxide. The resulting electrode exhibited supercapacitor-type performance and mechanical durability against repeated charging cycles, with a specific capacitance (230 F/g) of the electrode. The current density is measured as 0.2 A/g [8]. Metal-organic frameworks (MOFs) and Co(OH)₂ crystals have been synthesized and characterized for the fabrication of electrodes as charge storage applications in electrochemical ultracapacitor technology [9, 10]. The study showed that the power density of the fabricated MOF/Co(OH)₂-based supercapacitors is more than that of capacitors fabricated using conventional carbon-based electrodes [11]. The three materials useful in this work are cobalt nitrate, aluminum nitrate, and graphite. Cobalt nitrate or Co(NO₃) is the best choice because of its high purity of 99.99%. Aluminum nitrate or Al(NO₃) is a second choice to increase capacitance because it is close to pure 99%. Graphite electrodes are looked at here as catalysts because they have a high discharge potential than other materials [12]. An EDX analysis of the Co(OH)₂ alpha nanosheets confirmed that the Co³⁺ ions were in the tetrahedral coordination for pure Co(OH)₂ [13]. The local structural order and extensibility are increased about 2.4 times to 7.1% for cobalt hydroxide [Co(OH)₂] alpha-LTS related to pure Co(OH)₂. Both room-temperature capacitances of Co(OH)₂ electrode (27 F/g at 0.5 A/g) over 5 cycles with a charge/discharge cutoff of 3 V improved 4 times than those of commercialized activated carbon anode (7 F/g at 0.5 A/g) [14].

Zinc-cobalt-sulfide (ZCS) disc microspheres were synthesized at three levels of sulfidation using ethylene glycol (EG) as an additive by a co-reduction reaction of zinc and cobalt picolinate [15]. The presence of this ordered structure provides evidence supporting the use of ordered-mesoporous film prepared by electrochemical deposition as solid acid catalysts [16]. There are increasing demands for energy storage devices and electrode materials due to industrialization, population growth, and urbanization. Cu electrodes with Co-Co(OH)₂-modified surface were fabricated by a spraying technique [17]. Its specific capacitance values were amplified to 127 F/g and 544 F/g, respectively. It could be applied to energy storage devices [18]. In the study, scientists made films from a material called Co(OH)₂, which can dissolve large amounts of H₂ gas, and in the near future, it may be possible to make fuel tanks from this material [19]. Cobalt hydroxide is a kind of metal oxide material with cobalt and oxygen as the main elements [20, 21]. The transition metal oxide 2D materials are gaining so much interest in various fields of energy and storage like supercapacitors, particularly for energy storage, and may potentially meet the upcoming demand for better performance [22]. A cobalt metal hydroxide-modified electrode was deposited on the surface area of the substrate made by a glassy carbon electrode (GCE) [23]. The electrochemical capacitor is one of the developing next-generation energy sources and provides a greener, more efficient, low-cost, and safe energy storage.

Renewable energies such as solar and wind or hybrid vehicle batteries are eco-friendly [24]. The cobalt hydroxide (Co(OH)₂) electrode is a cathode in lithium-ion batteries for high-power backup [25]. Transition metal-based cobalt oxides and hydroxides are used for advanced energy storage devices which are still under the research and development phase [26, 27]. A research work reported the electrochemical performance of carbon microsphere/MnO₂ nanosheets which were fabricated by using the in situ self-limiting deposition method [28]. Novel hierarchical porous carbon membranes were prepared which consist of source of carbon polyacrylonitrile (PAN) by using phase inversion technique [29]. Vanadium nitride nanoparticles have been prepared on graphene surface by sing electrochemical deposition technique [30]. Molybdenum nitride (Mo₂N) and molybdenum nitride/polyaniline composites (Mo₂N@PANI) were fabricated by using in situ growth methods [31]. Cobalt oxides (CoO, Co₃O₄, and Co₂O₃) are conductive substances due to the development of a 1D and 2D network structure [32–34]. There have been significant advances in energy storage, particularly since the 1980s when electronic devices evolved dramatically [35, 36]. Supercapacitors have primarily high theoretical capacitance and chemical stability [37]. The theoretical and simulation values of the specific capacitance have been measured more than one million times [38–40]. Capacitance values are within 60% of the theoretical value. Supercapacitor cells can be assembled into packs to achieve large capacitances, high power, and long shelf life [41–43]. Supercapacitors have almost unlimited cycle life—more than 100000 cycles. Energy density is on par with batteries that are currently in use [44–47]. Co₃O₄-based materials have recently drawn interest for lithium-ion battery (LIB) positive electrodes owing to their superior specific capacity, excellent safety, and environmental compatibility [48–50].

1.1. Motivation

Supercapacitors can be charged and discharged very rapidly, making them well-suited for use in applications that require high power density. In addition, they have a long lifespan and can withstand many charge/discharge cycles, making them a potentially attractive option for use in sustainable energy storage systems. A lot of investigation is under research on metal-based supercapacitor electrodes like cobalt oxide and hydroxide. A cobalt hydroxide electrode is a key element of a supercapacitor. Therefore, the cobalt hydroxide electrodes will play important role in supercapacitors and energy storage sustainable technologies in the future. Overall, cobalt hydroxide has the potential to be a useful electrode material for supercapacitors, but further research and development are needed to fully realize its’ potential.

1.2. Purpose

The purpose of this study is to review cobalt metal-based hydroxide electrodes and their applications in supercapacitors, to review the published research papers that are relevant to cobalt hydroxide and its application in supercapacitors in details, and to review specific details of cobalt compound to prepare the cobalt hydroxide electrodes by using the electrochemical deposition technique. The research approach is to identify the classification, characterization, and performance of cobalt hydroxides for supercapacitors. Additionally, comparative assessment of supercapacitors with conventional capacitors and batteries is also outlined.

2. Materials and Methods

2.1. Supercapacitor Technology

Compared to conventional batteries, supercapacitor has high energy with a high power rate for energy storage applications. Supercapacitors are considered a sustainable source of energy and energy backup devices [51, 52]. They have distinct features such as high power and energy densities, cost-effectiveness, quick charging and slow discharging, high specific capacitance, and high performance [53–55]. Electrochemical capacitors offer tremendous potential for energy storage, for flashlights and other small appliances to help power them on, for use as a battery backup in communication systems and other equipment where power output is not continuous, and for use as gas sensor in chemical sensors [55]. Capacitance can range from 1 farad to several megafarads per unit cell [56]. This has made the electrochemical capacitor an attractive medium to store electricity since they are fabricated by inserting different electrode materials into their electrolyte solution [57–60]. We have studied several research papers and determined the conclusion of where to develop supercapacitors by using metal hydroxides for suitable applications. Cobalt-based metal oxide and hydroxide thin films can be prepared for supercapacitor electrodes by using electrochemical deposition methods. They are low-cost and eco-friendly, have high performance, and have a high energy storage capability. They are used in several applications in renewable energy production, portable devices, remote sensing devices, electric vehicles, and industries. Cobalt-based metal oxide and hydroxide thin films can be prepared for supercapacitor electrodes by using electrochemical deposition methods.

2.1.1. Classification of Supercapacitors

Supercapacitors are categorized into three types: EDLC, hybrid, and pseudocapacitors, as shown in Figure 1. Supercapacitors are differentiated according to materials, manufacturing, electrodes, and their applications. EDLC has three types: activated carbon, carbon nanotubes, and graphene-based supercapacitors [61–65]. Pseudocapacitors are classified as polymers and metal oxides/hydroxides [65–70].

[figure(s) omitted; refer to PDF]

The fundamentals of electrochemical capacitors are shown in Figure 2 such as electrostatic capacitor, electrolyte capacitor, and electrical double-layer capacitors. Double-layered capacitors can deliver high capacitance, high stability, and vibration resistance [71]. Double-layer capacitors are divided into two different classes depending on construction and materials. In the first one, the electrodes are divided into two layers by a thin dielectric or air, whereas in the second one, they are placed together in contact with each other to form one monolithic structure and function [72]. Pseudocapacitors are special types of capacitors based on the chemical reactions at the electrode. The electrode has a thin layer or coating of one or more metals, such as nickel, silver, manganese, or platinum [73]. Hybrid capacitors are produced by stacking different electrolytes or gel in an asymmetric capacitor [74, 75]. The electrostatic capacitor is a passive component that stores charge [76]. Electrolytic capacitors come in two varieties: dry type and liquid type. Dry-type capacitors have a solid dielectric separating the two electrodes; they operate by storing electrical energy as they slowly release it into a circuit [77–79]. Electrical double-layer capacitor types are based on the same principle as a metal plate capacitor, which absorbs electrical energy at the metal interface [80].

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2.1.2. Electrolyte for Supercapacitors

Electrolyte materials are of different types for different electrodes or chemical compounds, such as aqueous and nonaqueous materials, in supercapacitors. The behavior of electrolytes varies with their temperature and polarization [81]. The performance of supercapacitors significantly depends on electrolytes. Electrolytes for aqueous supercapacitors are obtained from mixed solutions of ions with different types of conductivity [82, 83]. In contrast, most of the organic electrolytes are formed by a self-assembly process of ions based on solvating agents, such as phenol and methanol [84]. Therefore, organic electrolyte materials are used for both anode and cathode of supercapacitors [85]. Recently studied electrolyte materials for hybrid supercapacitors are aqueous, organic, and ionic liquids. Aqueous electrolyte materials such as KOH, H₂SO₄, and KCl are the most generally used due to their uniqueness and cost-effectiveness [86–88]. The capacitances of the supercapacitor can change with the different types of electrolytes. Aqueous electrolytes enhance the specific capacitance, but they have some restrictions regarding voltage windows for batteries [89, 90]. The electrolyte material and voltage/potential window ($\mathrm{v}$) are given in Table 1.

Table 1

Details of voltage window of different electrolytes [90].

Sr. No.	Electrolyte	Potential window ($\mathrm{v}$)
1	H2SO4	1.0
2	KOH	1.0
3	Na2SO4	1.8
4	Li2SO4	2.2
5	H3PO4	0.8

The electrochemical method uses two electrodes: one working and one reference, in a three-electrode system. All three electrodes are connected to a potentiostat instrument to monitor and control the changes in current concerning a given potential [91, 92]. The electrodes connect to a potentiostat, which can be used to measure the change in current concerning a given potential or an analyte [93]. This three-electrode method is used to measure galvanic corrosion at a constant potential. A working electrode (W) is placed in water of known resistivity while the reference electrode (R) is held on the surface being tested. The counter electrode (C) is inserted into the sample being tested and allows it to be set up in its correct position for testing [94, 95].

Electrolysis is shown in Figure 3 which involves a chemical reaction in which an electric current is used to divide water molecules into hydrogen and oxygen. The electrolysis of water to make hydrogen and oxygen is a process that takes place in a device called an electrolyzer. A voltage is applied between these electrodes. The focus of this project will be on the use of electrocatalysts in such applications as batteries, fuel cells, and metal oxide supercapacitors. It will also explore the advantages that the electrocatalytic process has over other clean energy methods [95–100].

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2.1.3. Cobalt Hydroxide Electrodes for Supercapacitors

The potential electrode material is cobalt hydroxide. The high capacitance, stability, and cyclability of cobalt hydroxide make it promising for supercapacitors, but the energy storage and conversion mechanism at the atomic level have not been fully investigated [101, 102]. To date, cobalt hydroxide has only been subjected to a few ionic conductivity studies and computational modeling [103]. The resulting electrodes exhibit large specific capacities at low temperatures and high rate capability (high power density) [104]. These electrode materials are fully flexible for large applications including automotive batteries and hybrid vehicles (e.g., fuel cells). Cobalt (II) nitrate hexahydrate [Co(NO₃)₂·6H₂O] and potassium hydroxide (KOH) chemical materials are required for the reaction [105]. The synthesis procedure involves preparing Co(NO₃)₂ by treating CoCl₂ with HNO₃ and NaNO₃ at room temperature [106]. The reaction mixture is then added to hot water containing urea (CH₄N₂O), and polyvinyl pyrrolidone (C₆H₉NO), in double distilled water (DDW), for one hour [4]. CuSO₄ (CoSO₄) was deposited on the Cu-platinum (0.05 mol%) substrate at room temperature in double distilled water [107]. Some electrodes are made of Co(OH)₂ and Ni foam, and they are used in medicine, cleaning, and air conditioning. They have better conductivity than silver and gold electrodes and high performance [108–110]. Cobalt nitrate, aluminum nitrate, and graphite oxide are promising candidates as layers for supercapacitors [111–115]. Further research shows that ordered mesoporous Co(OH)₂ thin films consisted of a surfactant, aqueous solution of Co(NO₃)₂, and NaNO₃ [116]. The electrolyte used for the electrodeposition of Co(OH)₂ films consists of a surfactant, NaNO_3, and a source of Co(NO₃) [116]. The supercapacitor electrode of a single crystalline cobalt-based metal hydroxide has been studied [117, 118].

Research has investigated electrodes containing cobalt metal hydroxides. It has a high specific capacitance with a large interlayer spacing structure [120, 121]. The supercapacitor electrode of a single crystalline cobalt-based metal hydroxide has been studied [122–125]. Cobalt hydroxide thin film deposited on a plastic substrate by screen printing is shown in Figure 4 [119].

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2.2. Important Characterizations of Cobalt Hydroxide Thin Films

Several important characteristics should be considered when evaluating cobalt hydroxide thin films, including the following.

2.2.1. Thickness

The thickness of the cobalt hydroxide thin film can affect its overall performance and stability.

2.2.2. Surface Morphology

The surface morphology of the thin film, including its roughness and grain size, can impact its electrochemical properties.

2.2.3. Composition

The chemical composition of the thin film, including the ratio of cobalt to hydroxide ions, can affect its properties.

2.2.4. Electrical Conductivity

The electrical conductivity of the thin film is important for its use as an electrode material in electronic devices.

2.2.5. Capacitance

The capacitance of the thin film is a measure of its ability to store electrical charge and is an important factor in its performance as an electrode material for supercapacitors.

2.2.6. Stability

The stability of the thin film over a range of pH and temperature conditions is important for its long-term performance and reliability.

2.2.7. Corrosion Resistance

The corrosion resistance of the thin film is important for its durability and reliability in various environments.

3. Structural, Morphological, and Capacitance Analysis

3.1. XRD Characterization

X-ray diffraction is the process by which X-rays are diffracted by a crystal. The atoms in a crystal arranged to form an orderly interface between two different materials form waves of energy. When X-rays leave these planes, they interfere with one another and create patterns of diffraction that can be measured and recorded by a detector. XRD analysis results of cobalt hydroxide thin film are obtained by spray pyrolysis. The XRD pattern by JCPDS data shows all diffraction peaks in the brucite phase of β-Co(OH)₂ material. Peak (001) was sharply strengthened and showed preferential growth. XRD Pattern shows the peaks (001), (100), (101), (102), (110), (003), and (111) of the cobalt hydroxide material deposition onto the stainless steel substrate. High purity of cobalt hydroxide was observed on the substrate shown in Figure 5 which produces the XRD pattern of cobalt hydroxide [126].

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3.2. FTIR Characterization

The FTIR spectrum showed the typical bi-functionalized structure of the cobalt hydroxide thin films. The OH group was described via the brucite structure where the interlayer water molecule and H bonds are stretched. These modes are characteristic of metal-oxygen bonding as well as cobalt hydroxide-water interactions in Figure 6 which shows the IR spectrum of cobalt hydroxide [2].

[figure(s) omitted; refer to PDF]

3.3. SEM Characterization

Scanning electron microscopes (SEMs) offer a wealth of important and complex information about materials. Understanding the heterogeneities and structural features that makeup SEM images is notoriously difficult; image processing also brings many unique challenges. This produces an image of the sample on a fluorescent screen or storage device. Using a scanning beam to image samples is what gives SEMs their name—their beams are shown on the screen as they move across the surface being imaged. These technologies include something called X-ray fluorescence (XRF), which uses X-ray energy in the same range as our atomic processes that can detect elements in materials to determine whether they contain certain minerals; diffraction, which demonstrates how objects are formed into their final shape by well-honed sharp edges; and dark scattering techniques that use optical techniques for imaging most objects in solutions and acids without damaging them. Characterization methods such as XRD and FTIR were studied. SEM images were viewed in the different structures. It described the discrete plate-like structure under SEM in large quantities. When it was observed under high magnification of SEM as shown in Figures 7(a)–7(d), the images showed plate-like structures not in hexagonal shape perfectly. It was irregular in shape and evident in the edge length [127].

[figure(s) omitted; refer to PDF]

3.4. CV Characterization

It describes the measured CV curves of as prepared β-Co(OH)₂. nanostructures on conductive textiles at a scan rate of 30 mV/s within a potential window of about −0.1 to 0.5 V for both nanoplates and cabbage like β-Co(OH)2 nanostructures. Scan rate influences both capacitive drift and capacitive drift, where I (V) depends on electrode surface area, volume, and mass (probably also time constants). Figure 8 shows the CV graph of the given sample [128].

[figure(s) omitted; refer to PDF]

3.5. Charging and Discharging Characterization

The charging characteristics of energy storage devices or electrodes are shown as current and voltage versus time. The battery should be charged up to the end of charge voltages and then discharged through an LED load. From the charge-discharge curve and chronopotentiogram, the specific capacitance can be measured. The difference of two peaks is 10 s between nanoplate and cabbage. The nature of the charging and discharging cycles is apparent from the chronopotentiogram in Figure 9 which shows the cobalt hydroxide performance.

[figure(s) omitted; refer to PDF]

3.6. EIS Spectrum Characterization

The EIS spectrum in a three-electrode system was used to study the electrochemical behavior of the β-Co(OH)₂ electrode. The applied potential is 0.3 V at different frequency ranges in a 1 M KOH electrolyte. In this paper, the DC doubling process during oxygen evolution and its relationship with electrocatalysis were discussed in detail. The Nyquist diagram and equivalent circuit of the cobalt hydroxide electrode are shown in Figure 10.

[figure(s) omitted; refer to PDF]

4. Comparison of Electrical Energy Storage Devices

4.1. Comparison of Supercapacitor with Capacitor and Battery

Supercapacitors, also known as ultracapacitors, are energy storage devices that have a high power density and can deliver large amounts of electrical energy quickly [130]. They are similar to capacitors, which are also used for storing electrical energy but have much higher capacitance values. This means that supercapacitors can store much more energy than traditional capacitors. Batteries are also used for storing electrical energy, but they work differently than supercapacitors and capacitors [126, 127]. Batteries store energy through a chemical reaction, and they release this energy slowly over time. This makes them well suited for applications that require a long-term, stable power source. Overall, supercapacitors and batteries are both useful for storing electrical energy, but they have different strengths and are best suited for different applications [128, 129]. Supercapacitors have a high power density and can deliver large amounts of electrical energy quickly, making them well suited for applications that require a quick burst of power. Batteries, on the other hand, have a high energy density and can store and release electrical energy over a longer period, making them well suited for applications that require a stable, long-term power source [131]. The characteristic comparison of supercapacitor with general batteries is given in Table 2.

Table 2

Comparison of conventional capacitors, supercapacitors, and batteries [131].

Sr. No.	Factors	Supercapacitor	Capacitor	Battery
1	Specific power (W/kg)	1000–2000	10,000	50–200
2	Specific energy (Wh/kg)	1–10	0.1	20–100
3	Cycle life	>1,00,000	>5,00,000	500–2000
4	Charge time	1–30 s	10−6–10−3	0.3–3 h
5	Discharge time	1–30 s	10−6–10−3	1–5 h
6	Cycle efficiency (100%)	90–95	100	70–85

4.2. Comparison of Supercapacitor with Lithium-Ion Battery

Supercapacitors and lithium-ion batteries are both used for storing electrical energy, but they have some key differences that make them better suited for different types of applications [119, 130]. One of the main differences between supercapacitors and lithium-ion batteries is their energy density. Supercapacitors have a relatively low energy density compared to lithium-ion batteries, meaning they cannot store as much energy in a given volume [126]. However, supercapacitors have a much higher power density, meaning they can deliver large amounts of electrical energy very quickly [127]. This makes them well suited for applications that require a quick burst of power, such as hybrid and electric vehicles. Lithium-ion batteries, on the other hand, have a much higher energy density, meaning they can store more energy in a given volume. This makes them well suited for applications that require a stable, long-term power source, such as portable electronic devices [128]. Lithium-ion batteries also have a relatively low power density, meaning they cannot deliver large amounts of electrical energy as quickly as supercapacitors [129, 131]. Overall, supercapacitors and lithium-ion batteries are both useful for storing electrical energy, but they have different strengths and are best suited for different types of applications [132]. The characteristic comparison of supercapacitor with lithium-ion battery is given in Table 3.

Table 3

General comparison of supercapacitor and lithium-ion battery [132].

Sr. No	Factors	Supercapacitor	Lithium-ionbattery
1	Specific power (W/kg)	>10,000	1000–3000
2	Specific energy (Wh/kg)	5–10	120–240
3	Cost per kWh	10,000$	250–1000$
4	Charge time	1–10 seconds	10–60 minutes
5	Cell potential	2.3–2.75 v	3.6 v
6	Charge temperature	−40 to 65°C	0 to 45°C
7	Discharge temperature	−40 to 65°C	−20 to 60°C

5. Applications of Supercapacitors in Different Sectors

Supercapacitors, also known as ultracapacitors, are energy storage devices that have a high power density and can deliver large amounts of electrical energy quickly. They have several potential applications, including the following.

5.1. Powering Electronic Devices

Supercapacitors can be used to provide a quick burst of power to electronic devices, such as cell phones, laptops, and tablets.

5.2. Energy Storage

Supercapacitors can be used to store excess energy generated by renewable energy sources, such as solar and wind, and release it when needed.

5.3. Transportation

Supercapacitors can be used in electric and hybrid vehicles to provide a quick burst of power when needed, such as during acceleration.

5.4. Power Conditioning

Supercapacitors can be used to smooth out power fluctuations in electrical grids, improving the stability and reliability of the grid.

5.5. Uninterruptible Power Supply (UPS)

Supercapacitors can be used in UPS systems to provide a quick burst of power in the event of a power outage.

5.6. Consumer Electronics

Supercapacitors can be used in consumer electronics, such as digital cameras and portable music players, to provide a quick burst of power and extend battery life. Overall, supercapacitors have the potential to play a valuable role in a wide range of applications due to their ability to quickly store and release electrical energy.

6. Future Scope of Supercapacitors

There is significant interest in the development and commercialization of supercapacitors, also known as ultracapacitors, due to their potential to revolutionize the way we store and use electrical energy. In the future, it is likely that supercapacitors will become an increasingly important component of various energy storage systems and will be used in a wide range of applications. Supercapacitors are expected to have a range of other applications, including in the power grid, consumer electronics, and portable power systems. As research and development in this area continue, supercapacitors will likely play an increasingly important role in the way we generate, store, and use electrical energy.

Supercapacitors can be used to generate high power density, high specific capacitance, fast recharge capability, and long cycle life. In the past few decades, many research efforts have been conducted on pseudocapacitive or supercapacitor materials such as metal hydroxide. It targets high specific capacitances and high energy densities for energy storage devices. Supercapacitors have been studied for decades and are now being used in a wide variety of commercially available devices. SCs are attractive for their high performance, cost-effectiveness, eco-friendliness, and maintenance. The energy storage device of pseudocapacitance plays a crucial role in electric vehicles, personal electronics, and other applications. This article reviews the basic theory of pseudocapacitance and its physical and chemical properties. While the most common examples of pseudocapacitive materials are zeolites and metal oxides, recent research has been focused on cobalt hydroxide materials.

7. Conclusions

This review article is based on the current scenario of cobalt hydroxide supercapacitors. The electrode of cobalt hydroxide has several properties that make it attractive for use in a variety of applications, including as an electrode material in electronic devices such as supercapacitors, also known as ultracapacitors. It has a high specific capacitance, meaning it can store a large amount of electrical charge in a small amount of material, and it has good electrical conductivity and stability over a wide range of pH conditions. There are also challenges to using cobalt hydroxide, including its relatively high cost compared to other materials and the need for further research and development to improve its performance and stability.

It provides a new approach for various materials used for supercapacitors as per the demands of the present world. It includes all aspects related to this emerging field, including recent work and prospects in catalysis, for example, the electrocatalytic deposition process. The authors have also included an additional section that offers a brief introduction to electrocapacitive materials, their properties, and their applications. This research work focuses on the analysis of the principles, mechanisms, and preparation of energy storage devices along with their characteristics. The need for cobalt-based metal hydroxide supercapacitors can be justified due to the reported specific capacitance. Supercapacitors provide a means for storing power for a short time and then delivering it again rapidly. Supercapacitors can prove to be sustainable energy-storing devices with high feasibility and eco-friendliness.

Additional Points

Highlights. (i) The electrochemical performance of Co(OH)₂ electrode materials for supercapacitors has been reviewed. (ii) Cobalt-based metal hydroxide thin film electrodes have been studied by using the electrochemical deposition technique. (iii) Important aspects and characterization of Co(OH)₂ thin films have been studied. (iv) The outlooks and performance improvement of Co(OH)₂-based materials for supercapacitors have been given.

Authors’ Contributions

All authors contributed equally to the preparation of this manuscript.

Acknowledgments

The authors would like to express their gratitude to MGV’s LVH Research Centre, ASCC Panchavati, Nashik (Maharashtra), affiliated with Savitribai Phule Pune University (Pune), India.

References

[1] N. C. Maile, R. T. Patil, S. K. Shinde, D. Y. Kim, A. V. Fulari, D. S. Lee, V. J. Fulari, "Facial growth of Co(OH)2 nanoflakes on stainless steel for supercapacitors: effect of deposition potential," Journal of Materials Science: Materials in Electronics,DOI: 10.1007/s10854-019-00849-5, 2019.

[2] I. Barauskienė, E. Valatka, "Synthesis, structure and capacitive properties of cobalt hydroxide films on stainless steel substrates," Open Chemistry, vol. 12 no. 11, pp. 1206-1211, DOI: 10.2478/s11532-014-0570-y, 2014.

[3] M. Dinamani, P. V. Kamath, "Electrocatalysis of oxygen evolution at stainless steel anodes by electro-synthesized cobalt hydroxide coatings," Journal of Applied Electrochemistry, vol. 30 no. 10, pp. 1157-1161, DOI: 10.1023/a:1004020825529, 2000.

[4] F. Shaikh, T. D. Dongale, R. K. Kamat, "Facile method for synthesis of α -Co(OH) 2 and their supercapacitor properties," MicroElectronics International, vol. 35 no. 4, pp. 220-230, DOI: 10.1108/MI-10-2017-0057, 2018.

[5] M. S. M. Gadwal, S. D. Sartale, V. L. Mathe, H. M. Pathan, "Substrate assisted electrochemical deposition of patterned cobalt thin films," Electrochemistry Communications, vol. 11 no. 8, pp. 1711-1713, DOI: 10.1016/j.elecom.2009.07.002, 2009.

[6] H. Li, Y. Gao, C. Wang, G. Yang, "Advanced energy materials," A Simple Electrochemical Route to Access Amorphous Mixed-Metal Hydroxides for Supercapacitor Electrode Materials,DOI: 10.1002/aenm.201401767, 2014.

[7] B. Li, Y. Shi, K. Huang, M. Zhao, J. Qiu, H. Xue, H. Pang, "Cobalt-doped nickel phosphite for high performance of electrochemical energy storage," Small, vol. 14 no. 13,DOI: 10.1002/smll.201703811, 2018.

[8] R. R. Salunkhe, B. P. Bastakoti, C.-T. Hsu, N. Suzuki, J. H. Kim, S. X. Dou, C.-C. Hu, Y. Yamauchi, "Direct growth of cobalt hydroxide rods on nickel foam and its application for energy storage," Chemistry - A European Journal, vol. 20 no. 11, pp. 3084-3088, DOI: 10.1002/chem.201303652, 2014.

[9] B. K. Kim, S. Y. Serubbable, A. Yu, J. Zhang, "Electrochemical Supercapacitors for Energy Storage and Conversion, Handbook of Clean Energy Systems," John Wiley & Sons,DOI: 10.1002/9781118991978.hces11, 2015.

[10] P. Vialat, C. Mousty, C. Taviot-Gueho, G. Renaudin, H. Martinez, J.-C. Dupin, E. Elkaim, F. Leroux, "High-performing monometallic cobalt layered double hydroxide supercapacitor with defined local structure," Advanced Functional Materials, vol. 24 no. 30, pp. 4831-4842, DOI: 10.1002/adfm.201400310, 2014.

[11] T. Deng, Y. Lu, W. Zhang, M. Sui, X. Shi, D. Wang, W. Zheng, "Inverted design for high-performance supercapacitor via Co(OH)2-Derived highly oriented MOF electrodes," Advanced Energy Materials, vol. 8 no. 7,DOI: 10.1002/aenm.201702294, 2017.

[12] Y. H. Chen, Z. Hu, Y. Chang, H. Wang, G. Fu, X. Jin, L. Xie, "Layered Al-substituted cobalt hydroxides/GO composites for electrode materials of supercapacitors," Chinese Journal of Chemistry, vol. 29 no. 11, pp. 2257-2262, DOI: 10.1002/cjoc.201180389, 2011.

[13] S. Zhang, N. Pan, "Supercapacitors performance evaluation," Advanced Energy Materials, vol. 5 no. 6,DOI: 10.1002/aenm.201401401, 2015.

[14] Z. Gao, W. Yang, Y. Yan, J. Wang, J. Ma, X. Zhang, B. Xing, L. Liu, "Synthesis and exfoliation of layered α -Co(OH)2 nanosheets and their electrochemical performance for supercapacitors," European Journal of Inorganic Chemistry, vol. 2013 no. 27, pp. 4832-4838, DOI: 10.1002/ejic.201300525, 2013.

[15] X. Gao, Q. Chang, J. Hong, D. Long, G. Jin, X. Xiao, "Zinc cobalt sulfide microspheres as a high-performance electrode material for supercapacitors," ChemistrySelect, vol. 3 no. 48, pp. 13751-13758, DOI: 10.1002/slct.201803095, 2018.

[16] W.-J. Zhou, J. Zhang, X. Tong, D.-D. Zhao, H.-L. Li, "Electrodeposition of ordered mesoporous cobalt hydroxide film from lyotropic liquid crystal media for electrochemical capacitors," Journal of Materials Chemistry,DOI: 10.1039/b715070a, 2008.

[17] A. Yavuz, M. Yakup Hacıibrahimoğlu, M. Bedir, "Synthesis and characterisation of Co–Co(OH) 2 composite anode material on Cu current collector for energy storage devices," Materials Research Express, vol. 4,DOI: 10.1088/2053-1591/aa6acc, 2017.

[18] I. Hussain, C. Lamiel, S. G. Mohamed, S. Vijayakumar, A. Ali, J.-J. Shim, "Controlled synthesis and growth mechanism of zinc cobalt sulfide rods on Ni-foam for high-performance supercapacitors," Journal of Industrial and Engineering Chemistry, vol. 71, pp. 250-259, DOI: 10.1016/j.jiec.2018.11.033, 2019.

[19] W.-J. Zhou, M.-W. Xu, D.-D. Zhao, C.-L. Xu, H. L. Li, "Electrodeposition and characterization of ordered mesoporous cobalt hydroxide films on different substrates for supercapacitors," Microporous and Mesoporous Materials, vol. 117 no. 1-2, pp. 55-60, DOI: 10.1016/j.micromeso.2008.06.004, 2009.

[20] T. Deng, W. Zhang, O. Arcelus, J.-G. Kim, J. Carrasco, S. J. Yoo, W. Zheng, J. Wang, H. Tian, H. Zhang, X. Cui, T. F. Rojo, "Atomic-level energy storage mechanism of cobalt hydroxide electrode for pseudocapacitors," Nature Communications, vol. 8 no. 1,DOI: 10.1038/ncomms15194, 2017.

[21] A. A. S. Rovetta, M. P. Browne, A. Harvey, I. J. Godwin, J. N. Coleman, M. E. G. Lyons, "Cobalt hydroxide nanoflakes and their application as supercapacitors and oxygen evolution catalysts," Nanotechnology, vol. 28 no. 37,DOI: 10.1088/1361-6528/aa7f1b, 2017.

[22] M. Aghazadeh, I. Karimzadeh, A. Ahmadi, M. R. Ganjali, "Electrochemical grown cobalt hydroxide three-dimensional nanostructures on Ni foam as high performance supercapacitor electrode material," Journal of Materials Science: Materials in Electronics, vol. 29 no. 17, pp. 14567-14573, DOI: 10.1007/s10854-018-9591-1, 2018.

[23] Y. Fan, L. Fan, S. Meng, Y. Guo, Y. Liu, "Preparation of cobalt hydroxide film modified electrode and its analytical application," Journal of Analytical Chemistry, vol. 67 no. 4, pp. 370-377, DOI: 10.1134/s1061934812040144, 2012.

[24] Y. Yang, T. Yuan, P. Zhu, L. Zhang, T. Li, H. Zheng, C. R. Sun, C. Wong, "Cobalt hydroxide-based compressible electrode material for asymmetrical all-solid supercapacitor," Railway System Controls,DOI: 10.1039/x0xx00000x, 2013.

[25] D. T. Dam, X. Wang, J. M. Lee, "Fabrication of a mesoporous Co(OH)2/ITO nanowire composite electrode and its application in supercapacitors," RSC Advances, vol. 2 no. 28, pp. 10512-10518, DOI: 10.1039/c2ra21747c, 2012.

[26] A. Gulino, P. Dapporto, P. Rossi, I. Fragala, "A novel self-generating liquid MOCVD precursor for Co3O4 thin films," Chemistry of Materials, vol. 15 no. 20, pp. 3748-3752, DOI: 10.1021/cm034305z, 2003.

[27] J. S. Choi, C. H. Yo, "Nonstoichiometric compositions of cobaltous oxide," Inorganic Chemistry, vol. 13 no. 7, pp. 1720-1724, DOI: 10.1021/ic50137a037, 1974.

[28] X. Zhang, F. Ran, H. Fan, Y. Tan, L. Zhao, X. Li, L. Kong, L. Kang, "A dandelion-like carbon microsphere/MnO 2 nanosheets composite for supercapacitors," Journal of Energy Chemistry, vol. 23 no. 1, pp. 82-90, DOI: 10.1016/s2095-4956(14)60121-2, 2014.

[29] H. Fan, "A hierarchical porous carbon membrane from polyacrylonitrile/polyvinylpyrrolidone blending membranes: preparation, characterization and electrochemical capacitive performance," Journal of Energy Chemistry, vol. 23 no. 6, pp. 684-693, DOI: 10.1016/s2095-4956(14)60200-x, 2014.

[30] T. He, Z. Wang, X. Li, Y. Tan, Y. Liu, L. Kong, L. Kang, C. Chen, F. Ran, "Intercalation structure of vanadium nitride nanoparticles growing on graphene surface toward high negative active material for supercapacitor utilization," Journal of Alloys and Compounds, vol. 781, pp. 1054-1058, DOI: 10.1016/j.jallcom.2018.12.149, 2019.

[31] T. He, W. Zhang, P. Manasa, F. Ran, "Quantum dots of molybdenum nitride embedded in continuously distributed polyaniline as novel electrode material for supercapacitor," Journal of Alloys and Compounds, vol. 812,DOI: 10.1016/j.jallcom.2019.152138, 2020.

[32] X. Wang, W. Tian, T. Zhai, C. Zhi, Y. Bando, D. Golberg, "Cobalt(II, III) oxide hollow structures: fabrication, properties, and applications," Journal of Materials Chemistry, vol. 22 no. 44, pp. 23310-23326, DOI: 10.1039/c2jm33940d, 2012.

[33] W. L. Smith, A. D. Hobson, "The structure of cobalt oxide, Co3O4," Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry, vol. 29 no. 2, pp. 362-363, DOI: 10.1107/s0567740873002505, 1973.

[34] Q. He, E. J. Cairns, "Review—recent progress in electrocatalysts for oxygen reduction suitable for alkaline anion exchange membrane fuel cells," Journal of the Electrochemical Society, vol. 162 no. 14, pp. F1504-F1539, DOI: 10.1149/2.0551514jes, 2015.

[35] J. Lee, B. Jeong, J. D. Ocon, "Oxygen electrocatalysis in chemical energy conversion and storage technologies," Current Applied Physics, vol. 13 no. 2, pp. 309-321, DOI: 10.1016/j.cap.2012.08.008, 2013.

[36] C. R. Raj, A. Samanta, S. H. Noh, S. Mondal, T. Okajima, T. Ohsaka, "Emerging new generation electrocatalysts for the oxygen reduction reaction," Journal of Materials Chemistry, vol. 4 no. 29, pp. 11156-11178, DOI: 10.1039/c6ta03300h, 2016.

[37] M. Tahir, L. Pan, F. Idrees, X. Zhang, L. Wang, J. J. Zou, "Oxygen evolution reaction for energy conversion and storage: a comprehensive review," Nano Energy, vol. 37, pp. 136-157, 2017.

[38] Y. Jiao, Y. Zheng, M. Jaroniec, S. Z. Qiao, "Design of electrocatalysts for oxygen and hydrogen-involving energy conversion reactions," Chemical Society Reviews, vol. 44 no. 8, pp. 2060-2086, DOI: 10.1039/c4cs00470a, 2015.

[39] N.-T. Suen, S.-F. Hung, Q. Quan, N. Zhang, Y.-J. Xu, H. M. Chen, "Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives," Chemical Society Reviews, vol. 46 no. 2, pp. 337-365, DOI: 10.1039/c6cs00328a, 2017.

[40] T. Reier, H. N. Nong, D. Teschner, R. Schlögl, P. Strasser, "Electrocatalytic oxygen evolution reaction in acidic environments – reaction mechanisms and catalysts," Advanced Energy Materials, vol. 7 no. 1,DOI: 10.1002/aenm.201601275, 2017.

[41] L. Lin, J. Liu, T. Liu, J. Hao, K. Ji, R. Sun, W. Zeng, Z. Wang, "Growth-controlled NiCo 2 S 4 nanosheet arrays with self-decorated nanoneedles for high-performance pseudocapacitors," Journal of Materials Chemistry, vol. 3 no. 34, pp. 17652-17658, DOI: 10.1039/C5TA04054J, 2015.

[42] Y. P. Zhu, C. Guo, Y. Zheng, S.-Z. Qiao, "Surface and interface engineering of noble-metal-free electrocatalysts for efficient energy conversion processes," Accounts of Chemical Research, vol. 50 no. 4, pp. 915-923, DOI: 10.1021/acs.accounts.6b00635, 2017.

[43] L. Zhang, J. Xiao, H. Wang, M. Shao, "Carbon-based electrocatalysts for hydrogen and oxygen evolution reactions," ACS Catalysis, vol. 7 no. 11, pp. 7855-7865, DOI: 10.1021/acscatal.7b02718, 2017.

[44] M. Shao, Q. Chang, J. P. Dodelet, R. Chenitz, "Recent advances in electrocatalysts for oxygen reduction reaction," Chemistry Review, vol. 116 no. 6, pp. 3594-3657, DOI: 10.1021/acs.chemrev.5b00462, 2016.

[45] G. Wu, P. Zelenay, "Nanostructured nonprecious metal catalysts for oxygen reduction reaction," Accounts of Chemical Research, vol. 46 no. 8, pp. 1878-1889, DOI: 10.1021/ar400011z, 2013.

[46] C. Zhu, S. Dong, "Recent progress in graphene-based nanomaterials as advanced electrocatalysts towards oxygen reduction reaction," Nanoscale, vol. 5, pp. 1753-1767, DOI: 10.1039/c2nr33839d, 2013.

[47] P. Poizot, S. Laruelle, S. Grugeon, L. DuPont, J.-M. Tarascon, "Nanosized transition-metal oxides as negative-electrode materials for lithium-ion batteries," Nature, vol. 407 no. 6803, pp. 496-499, DOI: 10.1038/35035045, 2000.

[48] D. Larcher, G. Sudant, J. Leriche, Y. Chabre, J. Tarascon, "The electrochemical reduction of Co3O4 in a lithium cell," J Electrochem Soc, vol. 149, pp. A234-A241, 2002.

[49] R. B. Rakhi, W. Chen, D. Cha, H. N. Alshareef, "Substrate-dependent self-organization of mesoporous cobalt oxide nanowires with remarkable pseudocapacitance," Nano Letters, vol. 12 no. 5, pp. 2559-2567, DOI: 10.1021/nl300779a, 2012.

[50] Z.-L. Wang, D. Xu, J.-J. Xu, X.-B. Zhang, "Oxygen electrocatalysts in metal-air batteries: from aqueous to non-aqueous electrolytes," Chemical Society Reviews, vol. 43 no. 22, pp. 7746-7786, DOI: 10.1039/c3cs60248f, 2014.

[51] H. J. Qiu, L. Liu, Y. P. Mu, H. J. Zhang, Y. Wang, "Designed synthesis of cobalt-oxide-based nanomaterials for superior electrochemical energy storage devices," Nano Research, vol. 8 no. 2, pp. 321-339, DOI: 10.1007/s12274-014-0589-6, 2015.

[52] J. Pettersson, B. Ramsey, D. Harrison, "A review of the latest developments in electrodes for unitised regenerative polymer electrolyte fuel cells," Journal of Power Sources, vol. 157 no. 1, pp. 28-34, DOI: 10.1016/j.jpowsour.2006.01.059, 2006.

[53] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, "Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction," Nature Materials, vol. 10, pp. 780-786, DOI: 10.1038/nmat3087, 2011.

[54] H. Sun, H. M. Ang, M. O. Tadé, S. Wang, "Co3O4 nanocrystals with predominantly exposed facets: synthesis, environmental and energy applications," Journal of Materials Chemistry, vol. 1 no. 46, pp. 14427-14442, DOI: 10.1039/c3ta12960h, 2013.

[55] W. Zhang, Y. Zeng, N. Xiao, H. H. Hng, Q. Yan, "One-step electrochemical preparation of graphene-based heterostructures for Li storage," Journal of Materials Chemistry, vol. 22 no. 17, pp. 8455-8461, DOI: 10.1039/c2jm16315b, 2012.

[56] X. Xie, W. Shen, "Morphology control of cobalt oxide nanocrystals for promoting their catalytic performance," Nanoscale, vol. 1, pp. 50-60, DOI: 10.1039/b9nr00155g, 2009.

[57] V. H. Grassian, "When size really matters: size-dependent properties and surface chemistry of metal and metal oxide nanoparticles in gas and liquid phase environments," Journal of Physical Chemistry C, vol. 112 no. 47, pp. 18303-18313, DOI: 10.1021/jp806073t, 2008.

[58] J. Mei, Y. Zhang, T. Liao, Z. Sun, S. X. Dou, "Storage performance of 2D nanomaterials," National Science Review, vol. 5, pp. 389-416, 2018.

[59] J. Wang, X. Li, B. Wei, R. Sun, W. Yu, H. Y. Hoh, H. Xu, J. Li, X. Ge, Z. Chen, C. Su, Z. Wang, "Activating basal planes of NiPS3 for hydrogen evolution by nonmetal heteroatom doping," Advanced Functional Materials, vol. 30 no. 12,DOI: 10.1002/adfm.201908708, 2020.

[60] M. Armand, J.-M. Tarascon, "Building better batteries," Nature, vol. 451 no. 7179, pp. 652-657, DOI: 10.1038/451652a, 2008.

[61] M. Winter, R. J. Brodd, "What are batteries, fuel cells, and supercapacitors?," Chemical Reviews, vol. 104 no. 10, pp. 4245-4270, DOI: 10.1021/cr020730k, 2004.

[62] J. B. Goodenough, K. S. Park, "The Li-ion rechargeable battery: a perspective," Journal of the American Chemical Society, vol. 135 no. 4, pp. 1167-1176, DOI: 10.1021/ja3091438, 2013.

[63] G. Zhou, F. Li, H.-M. Cheng, "Progress in flexible lithium batteries and future prospects," Energy & Environmental Science, vol. 7 no. 4, pp. 1307-1338, DOI: 10.1039/c3ee43182g, 2014.

[64] J. Cabana, L. Monconduit, D. Larcher, M. R. Palacín, "Beyond intercalation-based Li-ion batteries: the state of the art and challenges of electrode materials reacting through conversion reactions," Advanced Materials, vol. 22 no. 35, pp. E170-E192, DOI: 10.1002/adma.201000717, 2010.

[65] M. V. Reddy, G. V. Subba Rao, B. V. R. Chowdari, "Metal oxides and oxysalts as anode materials for Li-ion batteries," Chemistry Review, vol. 113 no. 7, pp. 5364-5457, DOI: 10.1021/cr3001884, 2013.

[66] J. R. Miller, P. Simon, "Electrochemical capacitors for energy management," Science, vol. 321 no. 5889, pp. 651-652, DOI: 10.1126/science.1158736, 2008.

[67] G. A. Snook, P. Kao, A. S. Best, "Conducting-polymer-based supercapacitor devices and electrodes," Journal of Power Sources, vol. 196,DOI: 10.1016/j.jpowsour.2010.06.084, 2011.

[68] M. Zhi, C. Xiang, J. Li, M. Li, N. Wu, "Nanostructured carbon-metal oxide composite electrodes for supercapacitors: a review," Nanoscale, vol. 5 no. 1, pp. 72-88, DOI: 10.1039/c2nr32040a, 2013.

[69] A. González, E. Goikolea, J. A. Barrena, R. Mysyk, "Review on supercapacitors: technologies and materials," Renewable and Sustainable Energy Reviews, vol. 58, pp. 1189-1206, DOI: 10.1016/j.rser.2015.12.249, 2016.

[70] V. Chabot, D. Higgins, A. Yu, X. Xiao, Z. Chen, J. Zhang, "A review of graphene and graphene oxide sponge: material synthesis and applications to energy and the environment," Energy & Environmental Science, vol. 7 no. 5, pp. 1564-1596, DOI: 10.1039/c3ee43385d, 2014.

[71] Y. B. Tan, J.-M. Lee, "Graphene for supercapacitor applications," Journal of Materials Chemistry, vol. 1 no. 47, pp. 14814-14843, DOI: 10.1039/c3ta12193c, 2013.

[72] G. Wang, L. Zhang, J. Zhang, "A review of electrode materials for electrochemical supercapacitors," Chemical Society Reviews, vol. 41 no. 2, pp. 797-828, DOI: 10.1039/c1cs15060j, 2012.

[73] Z. Yu, L. Tetard, L. Zhai, J. Thomas, "Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions," Energy & Environmental Science, vol. 8, pp. 702-730, DOI: 10.1039/c4ee03229b, 2015.

[74] C. Liu, Z. Yu, D. Neff, A. Zhamu, B. Z. Jang, "Graphene-based supercapacitor with an ultrahigh energy density," Nano Letters, vol. 10 no. 12, pp. 4863-4868, DOI: 10.1021/nl102661q, 2010.

[75] J. Mei, L. Zhang, "Facile and economic synthesis of nitrogen-doped graphene/manganese dioxide composites in aqueous solution for energy storage devices," Materials Letters, vol. 143, pp. 163-166, DOI: 10.1016/j.matlet.2014.12.095, 2015.

[76] J. Mei, L. Zhang, "Anchoring high-dispersed MnO2 nanowires on nitrogen-doped graphene as electrode materials for supercapacitors," Electrochimica Acta, vol. 173, pp. 338-344, DOI: 10.1016/j.electacta.2015.05.064, 2015.

[77] J. Mei, L. Zhang, "Novel MnOOH-graphene nanocomposites: preparation, characterization and electrochemical properties for supercapacitors," Journal of Solid State Chemistry, vol. 221, pp. 178-183, DOI: 10.1016/j.jssc.2014.08.038, 2015.

[78] Y. F. Yuan, X. H. Xia, J. B. Wu, X. H. Huang, Y. B. Pei, J. L. Yang, S. Guo, "Hierarchically porous Co3O4 film with mesoporous walls prepared via liquid crystalline template for supercapacitor application," Electrochemistry Communications, vol. 13 no. 10, pp. 1123-1126, DOI: 10.1016/j.elecom.2011.07.012, 2011.

[79] Y. Xiao, S. Liu, F. Li, A. Zhang, J. Zhao, S. Fang, D. Jia, "3D hierarchical Co3O4 twin-spheres with an urchin-like structure: large-scale synthesis, multistep-splitting growth, and electrochemical pseudocapacitors," Advanced Functional Materials, vol. 22 no. 19, pp. 4052-4059, DOI: 10.1002/adfm.201200519, 2012.

[80] X. Wang, S. Yao, X. Wu, Z. Shi, H. Sun, R. Que, "High gas-sensor and supercapacitor performance of porous Co3O4 ultrathin nanosheets," RSC Advances, vol. 5 no. 23, pp. 17938-17944, DOI: 10.1039/c4ra14450c, 2015.

[81] S. Chen, Y. Zhao, B. Sun, Z. Ao, X. Xie, Y. Wei, G. Wang, "Microwave-assisted synthesis of mesoporous Co 3 O 4 nanoflakes for applications in lithium ion batteries and oxygen evolution reactions," ACS Applied Materials & Interfaces, vol. 7 no. 5, pp. 3306-3313, DOI: 10.1021/am508136k, 2015.

[82] C. W. Kung, H. W. Chen, C. Y. Lin, R. Vittal, K. C. Ho, "Synthesis of Co3O4 nanosheets via electrodeposition followed by ozone treatment and their application to high-performance supercapacitors," Journal of Power Sources, vol. 214, pp. 91-99, DOI: 10.1016/j.jpowsour.2012.04.076, 2012.

[83] M. Vezvaie, P. Kalisvaart, H. Fritzsche, Z. Tun, D. Mitlin, "The penetration depth of chemical reactions in a thin-film Co3O4 supercapacitor electrode," Journal of the Electrochemical Society, vol. 161 no. 5, pp. A798-A802, DOI: 10.1149/2.081405jes, 2014.

[84] G. Godillot, L. Guerlou-Demourgues, P.-L. Taberna, P. Simon, C. Delmas, "Original conductive nano-Co3O4 investigated as electrode material for hybrid supercapacitors," Electrochemical and Solid-State Letters, vol. 14 no. 10, pp. A139-A142, DOI: 10.1149/1.3609259, 2011.

[85] W. Du, R. Liu, Y. Jiang, Q. Lu, Y. Fan, F. Gao, "Facile synthesis of hollow Co3O4 boxes for high capacity supercapacitor," Journal of Power Sources, vol. 227, pp. 101-105, DOI: 10.1016/j.jpowsour.2012.11.009, 2013.

[86] F. Meng, Z. Fang, Z. Li, W. Xu, M. Wang, Y. Liu, J. Zhang, W. Wang, D. Zhao, X. Guo, "Porous Co3O4 materials prepared by solid-state thermolysis of a novel Co-MOF crystal and their superior energy storage performances for supercapacitors," Journal of Materials Chemistry, vol. 1 no. 24, pp. 7235-7241, DOI: 10.1039/c3ta11054k, 2013.

[87] Y. Fan, G. Shao, Z. Ma, G. Wang, H. Shao, S. Yan, "Ultrathin nanoflakes assembled 3D hierarchical mesoporous Co3O4 nanoparticles for high-rate pseudocapacitors," Particle & Particle Systems Characterization, vol. 31 no. 10, pp. 1079-1083, DOI: 10.1002/ppsc.201400066, 2014.

[88] X. Wang, A. Sumboja, E. Khoo, C. Yan, P. S. Lee, "Cryogel synthesis of hierarchical interconnected macro/mesoporous Co3O4 with superb electrochemical energy storage," Journal of Physical Chemistry C, vol. 116 no. 7, pp. 4930-4935, DOI: 10.1021/jp211339t, 2012.

[89] C. Feng, J. Zhang, Y. He, C. Zhong, W. Hu, L. Liu, Y. Deng, "Sub 3nm Co3O4 nanofilms with enhanced supercapacitor properties," ACS Nano, vol. 9 no. 2, pp. 1730-1739, DOI: 10.1021/nn506548d, 2015.

[90] Y. Wang, Y. Lei, J. Li, L. Gu, H. Yuan, D. Xiao, "Synthesis of 3D-nanonet hollow structured Co3O4 for high capacity supercapacitor," ACS Applied Materials & Interfaces, vol. 6 no. 9, pp. 6739-6747, DOI: 10.1021/am500464n, 2014.

[91] L. Lin, T. Liu, J. Liu, R. Sun, J. Hao, K. Ji, Z. Wang, "Facile synthesis of groove-like NiMoO4 hollow nanorods for high-performance supercapacitors," Applied Surface Science, vol. 360, pp. 234-239, DOI: 10.1016/j.apsusc.2015.11.018, 2016.

[92] M. Aghazadeh, "Electrochemical preparation and properties of nanostructured Co3O4 as supercapacitor material," Journal of Appled Electrochemistry, vol. 42 no. 2, pp. 89-94, DOI: 10.1007/s10800-011-0375-z, 2012.

[93] H. Wang, L. Zhang, X. Tan, C. M. B. Holt, B. Zahiri, B. C. Olsen, D. Mitlin, "Supercapacitive properties of hydrothermally synthesized Co3O4 nanostructures," Journal of Physical Chemistry C, vol. 115 no. 35, pp. 17599-17605, DOI: 10.1021/jp2049684, 2011.

[94] C. Yuan, L. Yang, L. Hou, L. Shen, F. Zhang, D. Li, X. Zhang, "Large-scale Co3O4 nanoparticles growing on nickel sheets via a one-step strategy and their ultra highly reversible redox reaction toward supercapacitors," Journal of Materials Chemistry, vol. 21 no. 45,DOI: 10.1039/c1jm14173b, 2011.

[95] R. Tummala, R. K. Guduru, P. S. Mohanty, "Nanostructured Co3O4 electrodes for supercapacitor applications from plasma spray technique," Journal of Power Sources, vol. 209, pp. 44-51, DOI: 10.1016/j.jpowsour.2012.02.071, 2012.

[96] T. Guo, L. Li, Z. Wang, "Recent development and future perspectives of amorphous transition metal-based electrocatalysts for oxygen evolution reaction," Advanced Energy Materials, vol. 12 no. 24,DOI: 10.1002/aenm.202200827, 2022.

[97] D. Kong, J. Luo, Y. Wang, W. Ren, T. Yu, Y. Luo, Y. Yang, C. Cheng, "Three-dimensional Co3O4@MnO2 hierarchical nanoneedle arrays: morphology control and electrochemical energy storage," Advanced Functional Materials, vol. 24, pp. 3815-3826, DOI: 10.1002/adfm.201304206, 2014.

[98] G. Zhang, T. Wang, X. Yu, H. Zhang, H. Duan, B. Lu, "Nanoforest of hierarchical Co3O4@NiCo2O4 nanowire arrays for high-performance supercapacitors," Nano Energy, vol. 2 no. 5, pp. 586-594, DOI: 10.1016/j.nanoen.2013.07.008, 2013.

[99] J. Lang, X. Yan, Q. Xue, "Facile preparation and electrochemical characterization of cobalt oxide/multiwalled carbon nanotube composites for supercapacitors," Journal of Power Sources, vol. 196 no. 18, pp. 7841-7846, DOI: 10.1016/j.jpowsour.2011.04.010, 2011.

[100] R. Madhu, V. Veeramani, S. M. Chen, A. Manikandan, A. Y. Lo, Y. L. Chueh, "Honeycomb-like porous carbon–cobalt oxide nanocomposite for high-performance enzymeless glucose sensor and supercapacitor applications," ACS Applied Materials & Interfaces, vol. 7 no. 29, pp. 15812-15820, DOI: 10.1021/acsami.5b04132, 2015.

[101] X. Wang, M. Li, Z. Chang, Y. Yang, Y. Wu, X. Liu, "Co3O4@MWCNT nano cable as cathode with superior electrochemical performance for supercapacitors," ACS Applied Materials & Interfaces, vol. 7 no. 4, pp. 2280-2285, DOI: 10.1021/am5062272, 2015.

[102] Q. Liao, N. Li, S. Jin, G. Yang, C. Wang, "All-solid-state symmetric supercapacitor based on Co3O4 nanoparticles on vertically aligned graphene," ACS Nano, vol. 9 no. 5, pp. 5310-5317, DOI: 10.1021/acsnano.5b00821, 2015.

[103] N. Wang, Q. Liu, D. Kang, J. Gu, W. Zhang, D. Zhang, "Facile self-cross-linking synthesis of 3D nanoporous Co3O4/carbon hybrid electrode materials for supercapacitors," ACS Applied Materials & Interfaces, vol. 8 no. 25, pp. 16035-16044, DOI: 10.1021/acsami.6b03527, 2016.

[104] J. Zhang, J. Fu, J. Zhang, H. Ma, Y. He, F. Li, E. Xie, D. Xue, H. Zhang, Y. Peng, "Co@Co3O4 core-shell three-dimensional nanonetwork for high-performance electrochemical energy storage," Small, vol. 10 no. 13, pp. 2618-2624, DOI: 10.1002/smll.201303926, 2014.

[105] W. Zhou, J. Liu, T. Chen, K. S. Tan, X. Jia, Z. Luo, C. Cong, H. Yang, C. M. Li, T. Yu, "Fabrication of Co3O4-reduced graphene oxide scrolls for high-performance supercapacitor electrodes," PCCP: Physical Chemistry Chemical Physics, vol. 13 no. 32, pp. 14462-14465, DOI: 10.1039/c1cp21917k, 2011.

[106] L. Ma, H. Zhou, X. Shen, Q. Chen, G. Zhu, Z. Ji, "Facile synthesis of Co3O4 porous nanosheets/reduced graphene oxide composites and their excellent supercapacitor performance," RSC Advances, vol. 4 no. 95, pp. 53180-53187, DOI: 10.1039/c4ra07136k, 2014.

[107] C. M. A. Parlett, K. Wilson, A. F. Lee, "Hierarchical porous materials: catalytic applications," Chemical Society Reviews, vol. 42 no. 9, pp. 3876-3893, DOI: 10.1039/c2cs35378d, 2013.

[108] X. Cai, L. Lai, J. Lin, Z. Shen, "Recent advances in air electrodes for Zn-air batteries: electrocatalysis and structural design," Materials Horizons, vol. 4 no. 6, pp. 945-976, DOI: 10.1039/c7mh00358g, 2017.

[109] G. Wang, Y. Yang, D. Han, Y. Li, "Oxygen defective metal oxides for energy conversion and storage," Nano Today, vol. 13, pp. 23-39, DOI: 10.1016/j.nantod.2017.02.009, 2017.

[110] G. Wang, H. Wang, Y. Ling, Y. Tang, X. Yang, R. C. Fitzmorris, C. Wang, J. Z. Zhang, Y. Li, "Hydrogen-treated TiO2 nanowire arrays for photoelectrochemical water splitting," Nano Letters, vol. 11 no. 7, pp. 3026-3033, DOI: 10.1021/nl201766h, 2011.

[111] X. Chen, L. Liu, P. Y. Yu, S. S. Mao, "Black hydrogenated titanium dioxide nanocrystals," Science, vol. 331, pp. 746-750, 2011.

[112] G. Wang, Y. Ling, H. Wang, X. Yang, C. Wang, J. Z. Zhang, Y. Li, "Hydrogen-treated WO3 nanoflakes show enhanced photostability," Energy & Environmental Science, vol. 5 no. 3, pp. 6180-6187, DOI: 10.1039/c2ee03158b, 2012.

[113] H.-S. Kim, J. B. Cook, H. Lin, J. S. Ko, S. H. Tolbert, V. Ozolins, B. Dunn, "Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO3− x," Nature Materials, vol. 16 no. 4, pp. 454-460, DOI: 10.1038/nmat4810, 2017.

[114] M. Vasilopoulou, A. M. Douvas, D. G. Georgiadou, L. C. Palilis, S. Kennou, L. Sygellou, A. Soultati, I. Kostis, G. Papadimitropoulos, D. Davazoglou, P. Argitis, "The influence of hydrogenation and oxygen vacancies on molybdenum oxides work function and gap states for application in organic optoelectronics," Journal of the American Chemical Society, vol. 134 no. 39, pp. 16178-16187, DOI: 10.1021/ja3026906, 2012.

[115] E. D. Hanson, L. Lajaunie, S. Hao, B. D. Myers, F. Shi, A. A. Murthy, C. Wolverton, R. Arenal, V. P. Dravid, "Systematic study of oxygen vacancy tunable transport properties of few-layer MoO3- x enabled by vapor-based synthesis," Advanced Functional Materials, vol. 27 no. 17,DOI: 10.1002/adfm.201605380, 2017.

[116] G. Zhu, J. Zhu, W. Jiang, Z. Zhang, J. Wang, Y. Zhu, Q. Zhang, "Surface oxygen vacancy induced α -MnO2 nanofiber for highly efficient ozone elimination," Applied Catalysis B: Environmental, vol. 209, pp. 729-737, DOI: 10.1016/j.apcatb.2017.02.068, 2017.

[117] D. A. Tompsett, S. C. Parker, M. S. Islam, "Rutile (2014) ( β -) MnO 2 surfaces and vacancy formation for high electrochemical and catalytic performance," Journal of the American Chemical Society, vol. 136 no. 4, pp. 1418-1426, DOI: 10.1021/ja4092962, 2014.

[118] Y. Wang, T. Zhou, K. Jiang, P. Da, Z. Peng, J. Tang, B. Kong, W. B. Cai, Z. Yang, G. Zheng, "Reduced mesoporous Co3O4 nanowires as efficient water oxidation electrocatalysts and supercapacitor electrodes," Advanced Energy Materials, vol. 4 no. 16,DOI: 10.1002/aenm.201400696, 2014.

[119] J. P. Cheng, J. Zhang, F. Liu, "The recent development of metal hydroxides as the electrode material of electrochemical capacitors," RSC Advances, vol. 4, 2014.

[120] T. Zhai, L. Wan, S. Sun, Q. Chen, J. Sun, Q. Xia, "Phosphate ion functionalized Co3O4 ultrathin nanosheets with greatly improved surface reactivity for high-performance pseudocapacitors," Advanced Materials, vol. 29, 2017.

[121] Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K. Nørskov, T. F. Jaramillo, "Combining theory and experiment in electrocatalysis: insights into materials design," Science, vol. 355, .

[122] V. R. Stamenkovic, D. Strmcnik, P. P. Lopes, N. M. Markovic, "Energy and fuels from electrochemical interfaces," Nature Materials, vol. 16, pp. 57-69, 2017.

[123] F. Cao, P. S. Tang, "Hydrothermal-synthesized Co(OH)2 nanocone arrays for supercapacitor applications, Chen," Journal of Power Sources, vol. 216, 2012.

[124] J. Libich, J. Máca, "Supercapacitors: properties and applications," Journal of Energy Storage, vol. 17, 2018.

[125] T. M. Gür, "Review of electrical energy storage technologies, materials, and systems: challenges and prospects for large-scale grid storage," Energy & Environmental Science, vol. 11, 2018.

[126] Z. Wenzhong, N. Wang, Q. Zhou, L. Wang, T. Yang, G. Zhang, "A room temperature solution-phase process to synthesize pure phase single-crystalline hexagonal cobalt hydroxide nanoplates," Journal of Crystal Growth, vol. 312, pp. 3485-3489, 2010.

[127] A. Roy, H. S. Jadhav, "Electrochemical growth of Co(OH)2 nanoflakes on Ni foam for methanol electro-oxidation," New Journal of Chemistry, vol. 41, 2017.

[128] N. Goli, J. Su, "Self-assembled hierarchical β -cobalt hydroxide nanostructures on conductive textiles by one-step electrochemical deposition," CrystEngComm, vol. 16, pp. 11027-11034, DOI: 10.1039/C4CE01696C, 2014.

[129] I. Jour, "Carboxymethyl cellulose aided fabrication of flaky structured mesoporous β -Co(OH)2/C nanocomposite for supercapacitor," Journal of Materials Science: Materials in Electronics,DOI: 10.1007/s10854-018-0482-2, 2019.

[130] K. K. Lee, W. S. Chin, C. H. Sow, "“Cobalt-based compounds and composites as electrode materials for high-performance electrochemical capacitors," Royal Society of Chemistry, vol. 2, 2014.

[131] P. Kumari, M. Shandilya, M. Lal, R. Rai, High Dielectric Materials for Supercapacitors, Smart Materials for Smart Living, 2017.

[132] Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, L. Zhang, "Progress of electrochemical capacitor electrode materials: A review," International Journal of Hydrogen Energy, vol. 34 no. 11, pp. 4889-4899, 2009.