Research progress in the development of transition metal chalcogenides and their composite-based electrode materials for supercapacitors

Abstract

Supercapacitors revealing excellent power density have arisen as the most promising candidates for supporting the major developments in energy storage devices. Supercapacitor attracts many emerging mobile devices for addressing energy storage and harvesting issues. The supercapacitor is similar to a conventional capacitor. Moreover, many researchers studied the improvement of energy and power density so that they can be applied extensively. The electrochemical performance of supercapacitor depends on various factors like electrode materials, electrolyte, and the range of voltage used. Most researchers mainly focused on the development of new electrode materials which yield better performance for the application of supercapacitors. This review work summarizes the introduction of supercapacitors and the recent advanced development of a variety of electrode materials in supercapacitors and production methods. In particular, transition metal chalcogenide–based electrode materials are focused here. Also, this review précises the improvement of the electrochemical performance of supercapacitor by incorporating or doping highly active materials like MWCNT, graphene, CNT, reduced graphene oxide, metal-based compounds, and polymers. The enhancement of specific capacity by altering the morphology and developing electrode with new morphological structures is deeply discussed in this review. Recently, trimetallic chalcogenides and its composites are emerged as new promising electrode materials which deliver large specific capacitance with excellent cycling stability and rate performance have also been reported here.

1 Introduction

The increase in population and economic growth across the world leads to increase in the usage of fossil fuels for various purposes. This leads to two main environmental issues: exhaustion of existing fossil fuels and environmental pollution, i.e., global warming due to the emission of greenhouse gas. These issues compel us to create and market environmentally responsible, economically viable, and sustainable energy sources [1]. Sustainable renewable energy from nature can be utilized as the energy source using various technologies from the sun, wind, ocean, hydropower, etc. [2]. Energy storage systems play the prominent role in intermittent nature of the renewable energy sources and increase the power transmission into the grid [3, 4]. Batteries, supercapacitors, and fuel cells are the major energy storage systems that store and deliver the energy at the time of requirement by the principle of electrochemical energy conversion. Batteries are well established and have wide range of application from simple electronic equipment such as clock and toys to automobile vehicle such as EMV. But there are some drawbacks in battery technologies such as less power densities, low cycle life, higher recharging time period, and rise in temperature during operation and hazardous to the environment [5]. These downside impulses to develop new alternative and efficient energy storage systems lead to the supercapacitors. Supercapacitors (SCs) or ultracapacitor is an electrochemical device with high power density than batteries and larger capacitance than the conventional capacitors. SCs have greater advantages such as higher power densities, high efficiency, high specific capacitance, and wide operating temperature. Moreover, they are environmental friendliness and charge quickly and deliver high power at a short period and build a gap between batteries and conventional capacitors. Due to these advantages, they are used for specific applications that require quick charge discharge and long life cycle than the long period of energy storage.

Supercapacitors can be classified as either electrochemical double-layer capacitor (EDLC) or pseudocapacitors based on their energy storage potential. While considering the charge storage mechanism, there are two main charge storage principles in capacitors: double-layer capacitance, electrostatic storage of energy by separation of charge in a Helmholtz double layer, and pseudocapacitance, electrochemical storage of energy by faradic redox reaction. Variation in mechanism and choice of electrodes lead to three different classifications: electric double-layer capacitors (EDLCs), pseudocapacitors (PCs), and hybrid capacitors (HCs).

EDLCs store charges electrostatically in the formed double layer at the interface of two electrodes. It uses carbon-based materials such as activated carbon (AC), carbon nanosheets, single-walled and multi-walled carbon nanotubes (CNT), carbon aerogel, graphene, graphene oxides, and mesoporous carbon to store energy [5]. PCs are a type of supercapacitors that store charges by a Faradaic mechanism based on fast and highly reversible surface or near-surface redox reactions. When a potential is applied, fast and reversible Faradaic reactions (redox reactions) take place on the electrode material similar to batteries results in Faradic current. PCs uses metal oxides (RuO₂, MnO₂, NiO, MoO₂, V₂O₅, etc.), metal chalcogenides (MnSe, CoSe, MoS, etc.), metal nitrides (VN, TiN, RuN, etc.), and conducting polymers (polypyrrole, polyaniline, polythiophene, etc.) as electrode materials for SCs. In EDLCs, non-Faradic reaction takes places where the electrode material is undisturbed leads to long life cycle of the SCs but outcomes with low energy density. On the other hand, pseudocapacitors undergo Faradic mechanism for the charging and discharging process on the surface of electrodes offer high energy density than the EDLC at the price of low cycle life and rate capability. Since both types have different disadvantages, the combination of both types can improve the efficiency of the supercapacitors. Hybrid supercapacitors are another type with a combination of both electric double capacitors and pseudocapacitors working principle for the charging and discharging process in supercapacitors. The different parameters of EDLCs, PCs, and HSCs are given in Table 1. The carbon-based electrode's non-Faradaic charge-discharge reaction and the other electrodes surface confined Faradaic reaction combine to provide large energy densities and good cyclic stability. In both mechanisms, large surface area, appropriate pore-size distribution, and high conductivity are essential properties of the electrode materials to attain large capacitance [6].

Table 1 Parameter comparisons of electric double-layer capacitors, pseudocapacitors, and hybrid supercapacitors

Supercapacitors are devices that are accomplished for managing and providing high power densities than the batteries at the time of requirement. It also has high cycle life (greater than 100 times) than the conventional batteries [8, 9]. Even though it delivers hundred to thousand times higher power at the same volume, its energy density is much lower (5–50 times) than the batteries. This limits the application of SCs to the necessity of high power bursts and is not essential for high energy storage capacity (high energy density). The application of SCs in the industrial field is mostly for military purposes, and in automotive industries, they are used in electric vehicles coupled with batteries for maximum efficiency [10]. SCs serve as a bridge between traditional capacitors and batteries since the batteries have higher energy density but lower power density and cycle lives when compared to traditional capacitors. The characteristics of the capacitors, supercapacitors, and batteries are given in Table 2. This energy and power densities characteristic between energy storage devices were clearly explained using Ragone plot that plots the value of specific energy (Wh kg⁻¹) versus specific power (W kg⁻¹). This plot can only relate the energy and power densities of the energy storage system and does not deal with other parameters such as eco-friendliness, cost, safety, and life cycle [6]. These need to be analyzed separately for the complete understanding of the energy storage device limitation and its advantages.

Table 2 Shortened characteristics of capacitors, supercapacitors, and batteries

Supercapacitors consists of two electrode separated by ion permeable membrane called separator, to avoid conduct between two electrodes that leads to short circuit. Separator is soaked in electrolyte and provides the ionic charge transfer between the two electrodes. The separator membranes should also have high electrical resistance, high ionic conductivity, and low thickness for the better performance. The major issues in supercapacitor are the energy density which is much lower than the batteries. In order to provide for the better application, the energy density needs to be increased by various measures. The energy and power densities of the SCs are calculated by [11]

$$\begin{array}{cc}E=\frac{1}{2}{CV}^{2}& (Wh\; {kg}^{-1})\end{array}$$

$$\begin{array}{cc}P=\frac{{V}^{2}}{4R}& (W {kg}^{-1})\end{array}$$

where V is the cell voltage (V), C is the specific capacitance (F g⁻¹), and R is the internal resistance of the supercapacitor. To increase the power densities of SCs, the internal resistance of the cell which is the sum of electrode resistance and electrolyte resistance has to be reduced, although the internal resistance of the SCs is much lower than batteries due to the fast and rapid recombination of positive and negative charges even in Faradic process. To increase the energy density of the SCs, both specific capacitance and cell voltage have to be increased since they are linearly proportional. Cell voltage of the SCs depends on the electrode material and electrolyte. But the operating voltage of the SCs is determined by electrolyte stability window. For aqueous electrolyte, the operating voltage will be around 1.2 V since water molecules decompose into oxygen and hydrogen at voltage above 1.23 V. But organic electrolyte has the operating voltage around 3.5 V, and ionic electrolyte has the wide stability window ranging from 3 to 6 V. Hence, choosing suitable electrolyte for the SCs can improve its operating potential window and thus increases the energy density of the device. Then, energy density is also directly proportional to its capacitance, so increasing the capacitance increases the energy density. In order to increase the overall cell capacitance, specific capacitance of the individual electrode has to be improved. Thus, the development of electrode materials for the SCs becomes the hot area of research work in energy storage system. The specific capacitance of the material C_s is [11]

$$\begin{array}{cc}{C}_{s}=\upvarepsilon \frac{A}{d}& (F {g}^{-1})\end{array}$$

where ε is the permittivity of the free space and A and d are the surface area of the electrode and distance between two electrodes. Specific capacitance is directly proportional to the surface area of the electrode to hold the charge physically by electric double layer; thus, the nanomaterials which have increased surface area to volume ratio can be used as electrode materials for SCs. Furthermore, PCs also require high active sites for Faradic reaction (redox reaction) on the electrode surface, and thus, nanomaterials increase high active sites in the surface of the electrode. The supercapacitors are made of different nanomaterials as the electrodes such as carbon-based materials (activated carbon, carbon nanotubes, graphene, mesoporous carbon, etc.), metal oxides (ruthenium oxide, manganese oxide, nickel oxide, etc.), conducting polymers (polyaniline, polypyrrole, etc.), and their composite materials.

2 Electrode materials

The capacitance of the SCs depends on the specific surface area of the material, but they are not fully accessible when it comes in contact with the electrolyte used and the capacitance of the electrode materials also not linearly increased with increases in specific surface area. Along with surface area, pore size and pore-size distribution also play a prominent role in the capacitance of the SCs electrode. Largeot et al., in their experiments, show that when pore size of the electrode material is almost close to the size of the ions in the electrolyte yields the maximum double-layer capacitance [12]. Higher or smaller pore size decreases the capacitance of the materials. Thus, the capacitance is strongly depending on the electrochemically accessible surface area of the electrode.

2.1 Carbon materials

The carbon-based materials are mainly known for their specific surface area. Different carbon materials are used as electrodes for SCs. From the research work suggested by Conway et al., the carbon-based materials are used for the double-layer-type capacitor with three main unique properties such as (1) high specific surface area, (2) good inter- and intraparticle conductivity in pore matrices, and (3) good accessibility by electrolyte in the pores of the carbon materials [13]. These carbon materials mainly include activated carbons, carbon aerogels, carbon nanotubes, mesoporous carbon, and graphene.

Activated carbons are the first choice of the electrode materials for the EDLC-type capacitor. These are porous carbon materials with porous structure consists of micropores (less than 2 nm), mesopores (2–50 nm), and macropores (greater than 50 nm) to attain high specific surface area but has low electrical conductivity (1200–2500 S m⁻¹) [14]. An activated carbon electrode having a specific surface area of approximately 1000 m² g⁻¹ has the capacitance of 100 F g⁻¹ (100 µF cm⁻²) [15]. Carbon aerogels are known to be one of the world’s lightest material with high specific surface area (SSA) and density. It has low internal resistance, thus provides high power density [16]. Chien et al. developed carbon aerogels with a capacitance of 104 F cm⁻³ that yields an energy density of 90 Wh kg⁻¹ and a power density of 20 Wh kg⁻¹ [17]. Carbon nanotubes (CNTs) are produced by the physical process of decomposition of hydrocarbons that forms two types of CNTs based on precursors single-walled CNT and multi-walled CNTs. The SWCNTs have the high theoretical specific surface area (1315 m² g⁻¹), but MWCNTs have lower SSA [18]. Chen et al. developed 50-nm diameter SWCNT on graphitic foil, with good electrochemical stability and yields specific capacitance of 115.7 F g⁻¹. Emmenegger et al. produced well-aligned MWCNTs that grow on aluminum films with diameters ranging from 5 to 100 nm, producing a high volumetric capacitance of 120 F cm⁻³ [19]. Graphene is the one carbon atom thick sheet made up of sp² lattice in polyaromatic honeycomb crystal lattice. They are suitable for good performance energy storage devices due to their excellent physicochemical properties. The highlighted properties are large surface area, good chemical and thermal stability, wide potential window, and abundant surface functional group [20]. Stoller et al. reported that specific capacitance of the prepared graphene is 205 F g⁻¹, and its energy density is 28.5 Wh kg⁻¹ [21]. The carbon-based material has many advantages such as high surface area, good chemical, and thermal stability, but the major limitation is electrochemically accessible surface area by the electrolyte in the electrode and low energy density due to the formation of electric double layer.

2.2 Metal oxides

The metal oxide–based electrode materials provide higher energy density than the carbon-based materials due to its charge storage mechanism. These materials store charge by electrochemical Faradic reaction between the electrode and electrolyte in the appropriate potential windows. Some unique properties are required for the metal oxides to be used as the electrode materials for the supercapacitors are as follows: (1) the metal can exist in two or more oxidation states, (2) the oxides should be electronically conductive, and (3) free movement of proton intercalation between oxide lattices [13]. Transition metal oxides are explored widely due to its high conductivity, chemical stability, and variable valence.

Apart from the various transition metal oxides, ruthenium oxide (RuO₂) is one of the most explored TMOs due to its high reversible faradic reaction (redox reaction), three distinct oxidation states by Ru, wide potential window (1.2 V), high specific capacitance, superior proton conductivity, better thermal stability, and long cycle life [22]. In acidic medium, fast reversible electron transfer and electron adsorption take place resulting in high specific capacitance. Zheng et al. reported that the amorphous RuO₂ in the sulfuric acid electrolyte (acidic medium) exhibits a maximum capacitance of 720 F g⁻¹ [23]. The internal resistance of the RuO₂ is much lower than the other electrodes that yield high power and energy densities, but the availability of the materials is fewer which causes higher cost and shows poor performance in higher current densities. Long et al. reported the hydrous ruthenium oxide (RuO₂ 0.5 H₂O) that has a specific capacitance of approximately 900 F g⁻¹ and high electrical conductivity [24]. Due to the environmental hazards and high cost of RuO₂ leads to the better replacement by manganese oxide (MnO₂). In comparison with RuO₂, MnO₂ shows relatively low cost, low toxicity, environment safety, and high theoretical capacitance almost equal to 1300 F g⁻¹ (for RuO₂, 1358 F g⁻¹) [25]. Hu et al. reported several MnO₂ thin film–based systems that reach the specific capacitances of about 600 F g⁻¹ in some aqueous electrolytes such as KCl, KOH, K₂SO₄, and Na₂SO₄ at the operating potential window of between 0.9 and 1.2 V [26]. Cobalt oxide (Co₃O₄) is another transition metal oxide and is also investigated due to its high theoretical capacitance of about 3560 F g⁻¹, better reversibility, and better electrochemical performance. Wang et al. investigated the 3D hollow cobalt oxide which yields the capacitance of around 820 F g⁻¹ at 5 mV s⁻¹, and nanoporous Co₃O₄ prepared by solvothermal method yields an energy density of about 42.3 Wh kg⁻¹ [27, 28]. Nickel oxide is also considered one of the prominent electrodes for the supercapacitors mainly for the alkaline electrolyte due to its high theoretical capacitance, low cost, and environmental friendliness. Yang et al. synthesized NiOO in cubic structure with various calcination temperatures; the maximum capacitance of the material yields around 700 F g⁻¹ at 250 °C [29]. Vanadium oxide (V₂O₅) has also been investigated due to its wide potential window and its variation in oxidation state that yields fast redox reaction in bulk and surface of the vanadium material. Lee and Goodenough prepared the amorphous V₂O₅ by quenching the bulk vanadium pentoxide powders at 950 °C, yielding the maximum capacitance of 350 F g⁻¹ in aqueous KCl electrolyte [30].

2.3 Conducting polymers

Conducting polymer–based electrodes are used for the supercapacitors due to its various properties such as low cost, flexibility, and low internal resistance and also have high potential densities, high porosity, and adjustable redox activity by the surface modification through various chemical processes. In conducting polymers, the charge storage takes place by faradic redox process, i.e., during the charging and discharging process, the movement of the ions occurs in the backbone of the polymer chain without any structural alternations. The conducting polymer SCs are classified into three types based on the p-doped polymer and n-doped polymer such as (1) type Ι (symmetric) p-p doped same polymer, (2) type II (asymmetric) p-p′ doped different polymer (e.g., polypyrrole/polythiophene), and (3) type III (symmetric) n-p doped polymers. Among these types, type III is considered the advanced conducting polymer supercapacitors based on their design and their energy storage mechanism [31, 32]. The conducting polymers in supercapacitors are polyaniline, polypyrrole, polythiophene, and PEDOT. Polyaniline (PANI) is the lightweight polymer and has high conductivity, low cost, and mechanical stability and possess high theoretical capacitance used as the electrode materials for the SCs. Lie et al. investigated PANI in pure form as the electrode materials and yields a capacitance of 815 F g⁻¹ [33]. Polypyrrole (PPY) is also a conducting polymer mainly known for its simple preparation method, p-doped valence, high conductivity, and stability. It also has greater density and higher flexibility than the other conducting polymer electrodes. Kim et al. studied the polymer-based electrodes with carbon materials by developing polypyrrole with carbon fiber yields the specific capacitance of about 600 F g⁻¹ at the scan rates of 30 mV s⁻¹ [34]. Polythiophene and its derivatives are both p- and n-doped polymers that can be prepared by chemical oxidative polymerization. Zhou et al. reported that the polythiophene prepared from Triton X-100 shows a maximum capacitance of 117 F g⁻¹ [35]. The main limitation of using polymers based supercapacitors as the electrode is the swelling and shrinkage of the polymer electrode while continuing for long life cycle.

2.4 Transition metal chalcogenides

The main criteria for the electrode material selections are multiple oxidation states, superior conductivity, and electrochemically active. Even though conducting polymers, metal oxides, and carbon-based materials exhibit these key characteristics, their applications are limited. For example, conducting polymers are inexpensive and environmentally friendly but have limited operation across the potential window, and swelling and shrinkage of the electrode leads to the need for alternative measures in supercapacitor. Metal oxides have a high energy density, but electrochemical instability and surface deformation lead to further applications [10]. Transition metals have various advantages such as multiple oxidation state and good pseudocapacitive behavior, and they received great attention due to their anisotropic properties. These transition metals with chalcogenides (S, Se, Te) have gained attention in past decades due to their high specific power, stability, and life cycle and offer better tolerance in environmental safety measures than the other energy storage devices in electronic devices and in EMV [36]. TMCs have application of the various fields such as in energy harvesting (solar cells, fuel cells), energy storages (batteries, supercapacitors), electronics (LED, sensors), and memory-based devices due to their excellent properties such as flexibility, additional reactive sites for catalysis and redox reaction, improved conductivity by reduction of internal resistance, low mean path, and quantum effect [37, 38]. Generally, Chalcogenide-based materials have improved electrochemical performance with high electrical conductivity and stability; selenide-based materials are a new class of electrode material with rich redox chemistry, superior electrical conductivity and stability. Chalcogenide-based research is highly concentrated in SC-based devices because of these improved applications. Compared with sulfide and selenides, telluride with transition metals are rarely reported, since in periodic table, telluride is placed in the same group of chalcogenides between metal and non-metals, then it possess some unique properties such as good conductivity and better stability like the other chalcogenides that are reported [39].

Transition metal chalcogenides are appealing for a variety of applications, including energy conversion and storage, due to their changing band gaps, distinctive stoichiometry, tunable structure, and materials. Due to their benefits of high theoretical capacitance, transition metal chalcogenides are anticipated to meet the high energy storage requirements of supercapacitors. The most remarkable feature of metal chalcogenides is their changeable active sites, which are made possible by their rich component and electronic structure and make them suitable materials for the creation of extremely effective electrodes for supercapacitors. The low symmetry anisotropic (1D and 2D) transition metal chalcogenides (TMCs) have attracted a lot of attention due to their novel electrical and catalytic properties, which have applications in electrochemical energy storage, chemical sensing, and next-generation optoelectronics. Metal tellurides, metal selenides, and metal sulfides have demonstrated exceptional cycle stability and high power density and have been used in supercapacitor applications. In an electrochemical energy storage system, the electrochemical performance of electrodes is influenced by their morphology, chemical content, synthesis method, and crystalline structure. The most often reported crystal structures in electrochemical energy storage applications are layer and spinel structures. At the same time, the disadvantage of TMCs as supercapacitors regarding low energy density has been identified as a significant challenge in the furtherance of supercapacitor technologies. It also has limited synthesis strategies and low stability, which lowers the performance of supercapacitors. The development of high-performance electrode materials is one of the most involved strategies for overcoming the problem of low energy density. Based on the researchers report on the transition metal chalcogenides, herein, we have detailed the use of transition metal chalcogenides as the electrode material for the supercapacitors.

3 Synthesis methods

3.1 Co-precipitation method

Co-precipitation is a technique for making multi-component oxides by precipitating intermediate chemicals like oxalates or hydroxides. If the reaction is well controlled, dopants can also be added to the mixed oxides. This technology has been shown to be useful for the synthesis of advanced energy materials such as cathodes in rechargeable M-ion batteries and solid fossil fuels. The washing and drying of the hydroxide or oxalate intermediate chemical cause little difficulty in generating nanoparticles by co-precipitation. It is possible to create high-quality nanomaterials via co-precipitation techniques.

3.2 Sol–gel method

The wet chemical approach of sol–gel synthesis of metal oxides is widely utilized to make materials for coatings, optical, energy, catalysis, separation (chromatography), electronics, and sensors. A typical sol–gel approach starts with the addition of a metal salt or metal alkoxide to water (elevated temperature or catalyst may also be used), which then undergoes a hydrolyzed reaction to produce a metal hydroxide colloid or nanoparticles. After the precursor has been hydrolyzed to the required quantity, a suspension is created by adding a stabilizing agent, such as nitric acid. A sol or stable suspension of hydroxide nanoparticles is obtained at this moment. One of the most significant advantages of sol–gel procedure is that the sol can be employed in a variety of processes to create a film of material, including dip coating, spin coating, drop position, aerosol processing, coating, and many more. After the solution has been deposited and the water has been removed during drying, a stiff gel will form. The pace of gelation determines the particle size and porosity of the finished material; therefore, it is a crucial processing step. The gel is an amorphous substance that can be burned to convert it into the final oxide layer once it has dried (Fig. 1). Sol–gel process can be used to make a wide range of oxide nanoparticles, including Al₂O₃, Fe₂O₃, NiO, SnO₂, TiO₂, WO₃, ZnO, ZrO₂, and BaTiO₃. Many of these pure oxides can be employed as the active element in gas sensors, and various investigations have been conducted in this sector using the sol–gel process. Sol–gel processes can be used to introduce dopants to materials with fine stoichiometry control. Sol–gel produced antimony-doped SnO₂ and niobium-doped TiO₂ which are two specific instances in gas sensing research. These are only a few examples of the current interest in sol–gel method as a means of producing high-performance oxide nanoparticles. Sol–gel technique for producing oxide nanoparticles has various advantages, including a vast number of precursors accessible, which allows reactions to be customized to create an almost infinite variety of specially doped materials to meet the specific application. Particle size control is achieved by carefully controlling the reaction condition. The initial step in the sol–gel method is to establish a stable suspension, which offers a lot of benefits for depositing materials on microsensors.

Fig. 1

Schematic representation of synthesis of nanoparticles. a, b Electrodeposition method image is reproduced with permission from ref. [40] under Creative Commons Attribution-Non Commercial 3.0 Unported License by Royal Society of Chemistry. c Hydrothermal method image is reproduced with permission from ref. [41] under Copyright © 2020 Elsevier B.V. d Emulsion method image is reproduced with permission from ref. [42] under Creative Commons Attribution License CC BY, Copyright © 2020 by MDPI. e Sol–gel method image is reproduced with permission from ref. [43] under Creative Commons Attribution License CC BY, Copyright © 2021 Bokov et al.

3.3 Hydrothermal synthesis

Many of the mechanisms were first clarified for mineral systems by Morey, who coined the term hydrothermal. When it comes to ceramic nanomaterial processing, hydrothermal synthesis is described as an aqueous chemical reaction in a sealed container at a temperature that generates an elevated pressure on its own. The synthesis of completely crystalline nanoparticles can be achieved at low temperatures, between 100 and 374 °C, which are the boiling and critical points for water, respectively. Hydrothermal processing has several advantages, including high purity (> 99.5%) and chemical homogeneity, small particle size (up to 5 nm), narrow particle size distribution, single-step processing, low energy consumption, quick reaction time, low-cost equipment, and the ability to generate metastable compounds, and, most importantly, no calcination is required for many materials because they are fully crystallized by the reaction. One of the reasons oxide nanostructures created using hydrothermal synthesis has increased in popularity over the last decade is the fact that nanomaterials including one-, two-, and three-dimensional structures can be synthesized to a fully crystalline state (Fig. 1).

3.4 Solvothermal method

Solvothermal synthesis is analogue to hydrothermal synthesis, but instead of water, it uses organic solvents. It provides various advantages over other approaches. First, solvothermal conditions allow for fast solution convection. The mild environment allows for the careful control of nanoparticle size, shape distribution, and crystallinity, as well as the formation of crystals with few lattice defects. Second, the low boiling point of the organic solvent involved can result in a larger reaction pressure when carried out at high temperatures, which aids in the crystallization process. Third, the mild temperature allows specific structural properties of precursors to be transmitted into products, allowing for product morphology control. Solvents can also give functional groups, which can be used to synthesis new materials by reacting with the precursors or products. Finally, solvothermal synthesis can limit the release of hazardous vapor during some reaction systems, such as those involving toxic starting ingredients. The sealed system not only helps with “green chemistry,” but it also effectively lowers the risk of oxidation and contamination from the atmosphere or oxygen, which is critical for high-purity products. As a result, the solvothermal approach was investigated as a convenient route for synthesizing CulnS₂, AglnS₂, and CulnSe2 nanoparticles from single-source precursors in order to control the size and shape of nanoparticles. One-step solvothermal synthesis can be used to generate nanoparticles directly through singe molecules of relative precursors acting as building blocks.

3.5 Emulsion synthesis

Microemulsion processing is a crucial synthetic technology because it may produce oxide nanoparticles, whereas large-scale emulsion processing produces particles in the order of micrometers. Microemulsions are optically transparent dispersions of two immiscible liquids, such as water and oil, that are thermodynamically stable. Surfactant mixtures are used to lower the interfacial tension to near zero (less than 0.001 mN m⁻¹ in some situations), allowing the two phases to disperse spontaneously by thermal motion. Domain sizes in equilibrium water (assuming a water-in-oil microemulsion) range from 10 to 100 nm, depending on the surfactant type and concentration. The nucleation, nanoparticle formation, intermediate growth (for colloids), and eventual coagulation and flocculation of colloidal and nanoparticles are a complicated process that includes relationships between nucleation, nanoparticle formation, intermediate growth (for colloids), and eventual coagulation and flocculation. All these variables are influenced by the interactions of molecular species in the microemulsion. The presence of surfactant film acting as stabilizer for the oxide particles allows for successful microemulsion synthesis of oxide materials because particle nucleation occurs simultaneously in a large number of micelles (spherical water droplets stabilized by surfactants in the oil medium) with well-isolated nucleation sites. The sizes of the colloids or nanoparticles generated are directly determined by the sizes of these emulsified droplets. Surfactant-stabilized water micelles act as nano-sized reactors for executing synthetic processes. Only when the nucleation and growth stages are tightly segregated do monodispersed particles emerge (one of the advantages of microemulsion synthesis). Controlling the molar ratio of water to surfactant allows the size of emulsified water droplets to be adjusted, making it possible to regulate the size of oxide nanoparticles generated using this process. The microemulsion synthesis process provides excellent dispersion, small particle size distribution, and shape control, making it a very appealing method for oxide nanoparticle creation (Fig. 1). According to Shi and Verweij, one component of microemulsion preparation that must be considered is particle purification following synthesis. Their findings revealed that non-agglomerated nanoparticles with diameters smaller than 5 nm could be created and that homogenous coatings could be made after thorough cleaning method. Microemulsion processing has been used to make SnO₂ nanoparticles in the size range of 3–5 nm, but no sensing data with materials made this way is available.

3.6 Electrodeposition method

Electrodeposition is a process of transfer of electrons to the ions in a solution; a thin layer of one metal is deposited on top of a thin layer of another metal to adjust its surface properties. This bottom-up fabrication method is adaptable and can be used for a broad range of applications. Due to its capacity to fabricate one-dimensional nanostructures such as nanoribbons, nanorods, nanowires, and nanotubes, electrodeposition has gained prominence in recent years.

4 Single metal chalcogenides and its composites

4.1 Cadmium chalcogenides

Cadmium sulfide (CdS) is broadly studied ultracapacitive TMSs because it possesses variable sulfide states which help fast and successive redox reactions. It is an n-type semiconductor with a narrow band gap of approximately 2.4 eV. It has advantages like high discharge rates, excellent morphology with high surface area, high energy density, and virtuous environmental stability with long cycle life. Also, cadmium has a very little toxicity in nature. Various groups reported the use of CdS as electrodes in the application of energy storage devices. For the first time, Xu and his co-workers reported CdS on nickel foam as electrodes with a gravimetric capacitance of 909 F g⁻¹. Oloore et al. developed CdS QDs and organohalide perovskite-based bilayer electrodes through facile and inexpensive solution process coating for the use of symmetric electrochemical capacitors [44]. Finally, he showed that the supercapacitors developed with cadmium sulfide with methyl ammonium bismuth iodide (MAPI3) electrodes displayed the highest areal capacitance of 141 µF cm⁻² and power density of 12.7 mW cm⁻² and energy density of 23.8 mWh cm⁻² with stability retention of 87% after 4000 cycles.

Chen et al. prepared CdS NPs anchored 3D graphite cage for supercapacitor study [45]. As a result, the 3D CdS/graphite cage displayed a better specific capacitance of 511 F g⁻¹ at 5 A g⁻¹. Also, he prepared a 3D CdS/graphite/rGO asymmetric supercapacitor with an energy density of 30.4 Wh kg⁻¹ at a power density of 800 W kg⁻¹ with cycling stability of 90.1% after 5000 cycles at 10 A g⁻¹. Patil et al. synthesized Ag NWs@CdS core–shell nanostructured electrodes and showed the areal capacitance of ~ 2662 mF. cm⁻² at 10 mV s⁻¹ and 810 mF. cm⁻² at 45 mA [46]. Later, Patil et al. synthesized core–shell nanostructures of Co₃O₄@CdS on a nickel foam using a one-pot hydrothermal method and SILAR method [47]. The core–shell Co₃O₄@CdS electrode-based symmetric supercapacitor showed a better specific capacitance of 360 F g⁻¹ and 99 F g⁻¹ at 10 mV s⁻¹ with stability retention of 92% after 2000 cycles. He et al. suggested that G-CdS nanocomposite through a one-pot solvothermal process and displayed high electrochemical behavior with excellent stability [48]. Wang et al. synthesized hierarchical Ni₃S₂@CdS core–shell nanostructures on nickel foam using the hydrothermal method for the first time [49]. Ni₃S₂@CdS core–shell nanostructure electrode-based asymmetric supercapacitors exhibit an excellent energy density of 127.5 Wh kg⁻¹ at 2 mA. cm⁻² with cycling stability of 130% after 4000 charge–discharge cycles at 6 mA. cm⁻². Based on this, Safdar et al. enhanced the capacitive performance of electrochemical capacitors by developing Ni₃S₂/CdS through hydrothermal method followed by successive ionic layer adsorption and reaction deposition (SILAR) techniques [50]. They showed a specific capacity of 545.6 C g⁻¹ at 1 A g⁻¹ with excellent stability of 103% after 5000 charge–discharge cycles at 5 A g⁻¹. Also, recently, mixed metal oxide–based materials like Mn/Fr oxides, Ni/Mn oxides, and Mn/Ni/Co oxide have explored high enhancement in the performance of electrochemical devices. Based on this concern, De Adhikari et al. synthesized a mixed system of CdS-CoFe₂O₄@rGO nanohybrid through a simple hydrothermal method for high-performance supercapacitors, which exhibits a high specific capacitance of 1487 F g⁻¹ at 5 A g⁻¹ current density with good capacitance retention of 78% even after 5000 cycles.

Related to sulfides, selenides could be potentially better candidates, as selenium displays more metallicity with higher electronic conductivity (1 × 10⁻³ S m⁻¹). During the pseudocapacitive reaction, selenides retain better cycling stability without the formation of polyselenide intermediates. Though selenides have good electrochemical properties, preparing selenides with better rate performance and higher specific capacity is still a challenge. To overcome these obstacles, Zhai et al. proposed a system of mixed metal selenide (Co₉Se₈/CdSe) on nickel foam via selenization method and developed a cell (Fig. 2). He stated that the developed cell shows excellent power density and energy (57.6 Wh kg⁻¹ at 10.9 kW kg⁻¹ or 68.0 Wh kg⁻¹ at 1.20 kW kg⁻¹) with better cycling stability of 80.9% after 1000 cycles at 2 A g⁻¹.

Fig. 2

a, b High resolution transmission electron micrographs of RuS₂ nanoparticles c–e 3-cell electrochemical performance of RuS₂ in 0.5 M H₂SO₄ electrolyte. f–h Electrochemical performance of RuS₂ symmetric cell supercapacitors, image is reproduced with permission from ref [51] under Copyright © 2016 Elsevier Ltd

Cadmium selenide belongs to the II–VI group compound semiconductor, and it is extensively used in the application of optoelectronics, solar cells, light-emitting diodes, laser diodes, etc. For the first time, Bae et al. reported on semiconducting CdSe QDs as electrodes on electrochemical capacitors. In this study, they used a complex hot injection method to prepare CdSe QDs [52]. Following this, Pawar et al. prepared CdSe electrodes through the chemical bath deposition method with various reaction times for the application of electrochemical supercapacitors [53]. He reported that CdSe deposited at a reaction time of 8 h exhibited superior supercapacitive performance with an excellent areal capacitance and energy density of 1.285 mF. cm⁻² and 4.015 Wh kg⁻¹.

Cadmium telluride (CdTe) belongs to the II–VI semiconducting periodic table with a direct band gap of 1.47 eV under room temperature. It is an appropriate material for the application of photovoltaics because of its high absorption coefficient property. However, it is a single crystal with a zinc blende structure, but it also exists in the wurtzite and hexagonal structure. Even though the preparations of CdTe are well studied, most of the study compacts with the optical and structural characterization. Concerned by the excellent potential and properties of CdTe NRs, Manikandan et al. made an effort to know its electrochemical behavior and its role in energy storage devices. In this study, he proposed that the prepared CdTe NRs offers an outstanding specific capacitance performance of 438 F g⁻¹ at 2 mA. cm⁻² over the other CdTe-based supercapacitors [54].

4.2 Mercury chalcogenides

Mercury sulfide (HgS) is a binary compound belonging to II and VI group elements with an optical band gap between 1.9 and 2.6 eV. Due to its band gap property, mercury sulfides show its auspicious applications in catalysts, IR detectors, photoconductors, photo-electrochemical cells, solid-state solar cells, etc. Various methods such as hydrothermal microwave-assisted and wet chemical route are used to synthesis HgS nanostructures with various structural morphologies like nanoparticles, dendrite-form, star-shape, rod-like structures. HgS has explored several applications like electrostatic imaging, ultrasonic transducing, image sensors, and photoelectric conversion devices. Though HgS has excellent characteristics, more consideration has not been paid yet towards energy storage uses. For the first time, Pande et al. synthesized HgS with cauliflower-like surface architecture by using the SILAR method under room temperature for the application of supercapacitors [55]. HgS electrode-based capacitor exhibits a maximum specific capacitance and energy density of 446 F g⁻¹ at 2 mV s⁻¹ and 15.45 Wh kg⁻¹. Also, Pande et al. synthesized mercury sulfide/MWCNT nanocomposite through a “dip and dry” process followed by successive ionic layer adsorption and reaction method [56]. This nanocomposite exhibits a higher specific capacitance of 946.43 F g⁻¹ at the scan rate of 2 mV s⁻¹ with excellent cycling stability of 93% even after 4000 charge–discharge cycles. Also, HgS/MWCNT nanocomposite-based electrodes possess high specific energy and power densities of 42.97 Wh kg⁻¹ and 1.60 kW kg⁻¹. This shows that HgS-based materials have a decent application in the domain of energy storage devices for the future scope.

4.3 Ruthenium chalcogenides

A ruthenium semiconducting compound is one of the most promising TMCs with excellent technological uses such as fuel cells and water splitting. It has a narrow band gap of 1.85 eV with superior stability towards photoelectrolysis of water under visible light. Because of its multiple valence states and rich redox chemistry, ruthenium-based material is recognized as the remarkable electrochemical properties. Ruthenium-based materials like metallic ruthenium, Ru complexes, amorphous and crystalline RuO₂, and hydrous RuO₂ are investigated for several electrochemical applications such as electrocatalysis, sensors, batteries, and supercapacitors. Although studies on energy storage of TMCs are rapidly increasing and ongoing, the effectiveness of RuS₂ as an electrode for energy storage devices is not examined yet. On the other hand, ruthenium disulfide (RuS₂) is proven to be an excellent catalyst material compared to MoS₂. Thus, for the first time, Krishnamoorthy et al. demonstrated the preparation of cubic RuS₂ NPs through the sonochemical method and used as an electrode for the application of electrochemical supercapacitors [51]. The prepared cubic RuS₂ NP-based electrode delivered an excellent specific capacitance of 85 F g⁻¹ at 0.5 mA. cm⁻¹² with retention stability of 96.15% capacitance over 5000 cycles (Fig. 2).

Bolagam et al. synthesized RuS₂/TRGO composite through the hydrothermal method which delivers a specific capacitance of 193 F g⁻¹ at 5 mV s⁻¹ with excellent cyclic stability and rate performance. But so far, there is still no report on the use of ruthenium disulfide (RuSe₂) as the electrode material for the use of supercapacitors. Thus, Yun et al. prepared crystalline RuSe₂ nanoparticles through the hydrothermal method followed by thermal treatment at 650 °C under an argon atmosphere (shown in Fig. 2) [57]. For the first time, the prepared crystalline RuSe₂ NPs were utilized as a pseudocapacitive supercapacitor electrode for which it showed a specific capacitance of 100.8 F g⁻¹ at 0.2 A g⁻¹ with superior cycle stability and excellent rate performance.

4.4 Silver chalcogenides

Mo et al. synthesized graphene sheets/Ag₂S composite through a facile solvothermal method [58]. The electrochemical performance of graphene sheets/Ag₂S was carried out on a modified glassy carbon electrode in a three-electrode cell. Gs-Ag₂S showed an enhanced specific capacitance of 1063 F g⁻¹ and could be employed as better supercapacitor materials for future electronic devices. Carbon allotropes such as activated carbons, single- and multi-walled carbon nanotubes, graphene, and fullerenes have been investigated thoroughly due to their moderate specific capacitance and long cycle stability. Hybrid materials showed enhanced physical and chemical properties with excellent mechanical flexibility and conductivity. But they have some limitations such as expensive and time-consuming synthesis process, high contact resistance at the interface of electrode/current collector, and low power density. Therefore, an effort has been developed in adding new materials which can be carbon-free and synthesized easily. Based on this, Nair et al. synthesized Cds/Ag₂S through the cation exchange process for the application of supercapacitors [59]. They used toxic cadmium ions in the initial synthesis but obtained Ag₂S nanowires and showed the highest specific capacitance of 268.4 F g⁻¹ at 1.5 mA. cm⁻² in a 1.5 M NaOH electrolyte. Pawar et al. used a facile, low-cost, successive ionic layer adsorption and reaction (SILAR) method for the preparation of Cu₂S-Ag₂S composite electrode on nickel foam [60]. The prepared electrodes showed a high specific capacity of 772 C/g at a scan rate of 10 mV s⁻¹ than the individual electrodes with a capacity retention of 89% after 2000 cycles (Fig. 3).

Fig. 3

a Schematic diagram of the different stages in the SILAR process for Cu₂S-AgS composite electrode. b Schematic illustration of the fabrication of symmetric supercapacitor device. c–f Electrochemical studies of symmetric Cu₂S-Ag₂S supercapacitor electrode, image is reproduced with permission from ref [60] under Copyright © 2017 Elsevier Ltd

4.5 Tantalum chalcogenides

Till now, various TMDs have been studied extensively. Especially, tantalum diselenide (TaSe₂), as a metallic nature with a layered low dimension electric conductor, has been studied for its charge density wave property, field emission, phonon, thermal property, and superconductivity. Depending on the coordination of tantalum atoms, there are various polymorphic forms for tantalum diselenide crystal. They are 1 T (T, trigonal), 2H (H, hexagonal), and 3R (R, rhombohedral) polytypes. In the past few decades, polymorphism in tantalum diselenide leads to various physical properties like the metallic property of 2H and semiconducting behavior for 1 T and 3R polytypes. Up to date, there were no reports about the direct preparation of TaSe₂ nanobelts. For the first time, Wang et al. synthesized a 3D conductive quasi-array based on 2H-TaSe₂ nanobelts directly on a tantalum foil through one-step surface-assisted chemical vapor transport technique [61]. Also, they used conductive quasi-array-based 2H-TaSe₂ as a substrate for the in situ electrodeposition of polypyrrole to form cylinder-like composite nanostructures and developed a symmetric supercapacitor from the composite. It showed a high areal capacitance of 835 mF.cm⁻² at 2 mV s⁻¹ with excellent initial capacitance maintained at 98.7% even after 10,000 cycles.

Other than sulfides and selenides, telluride-based materials have not prepared widely due to its deficiency of phase diagrams and high melting points. This type of crystals exists due to the interaction of ionic, metallic, and covalent bonds. Especially, quite low electronegative nature of tellurium in transition metal tellurides commonly leads to complex opposition between metals and non-metals for the electrons and, as significant, to valence electron localization. It is quite interesting to note that tantalum telluride (TaTe₂) has been studied to have a distorted 1 T structural property with a space group of C2/m. Generally, Te-based materials have remained unexplored practically. But in the past decades, the thermoelectric and electronic properties of this TaTe₂ were studied. For the first time, Chakravarty et al. make an effort to prepare TaTe₂ nanosheets through a simple microwave-assisted method and employed for the use of supercapacitors [62]. They stated that the supercapacitor developed with TaTe₂ nanosheets as anode and platinum as cathode exhibits a coulombic efficiency of 95%, while the cycle-to-cycle decrease in capacity was less than 5% and also, the maximum discharging or charging capacity was less than 2.4 mV s⁻¹ which is desirable for the characteristic behavior of supercapacitor.

4.6 Titanium chalcogenides

Titanium chalcogenides are one of the most important TMDCs, which have sizable band gaps, so these materials are used for the application of electronic and optoelectronic components. Titanium-based chalcogenides belong to IV or V group elements of the periodic table with a narrow band gap. Especially, titanium disulfide (TiS₂) is the layered structures like molybdenum and tungsten with the form of S-Ti-S unit structure, whereas the sulfur atoms in two hexagonal planes are separated by a plane of Ti atoms. The bond between Ti and S is the covalent bond of interactions, whereas the adjacent S-Ti-S layers are bonded through weak Van der Waals force of interactions. Various methods such as hydrothermal method, chemical vapor deposition (CVD) technique, solid assisted reaction, and physical ablation were employed for the preparation of TiS₂. But the electrochemical performance of TiS₂ was not reported widely. Parvaz et al. prepared TiS₂ nanodiscs through solid-state reaction (SSR) method and studied their electrochemical behavior for supercapacitor applications [63]. The observed results indicate the possibility of TiS₂ as a better electrode material for the application of supercapacitor.

4.7 Zinc chalcogenides

Like other metal oxides, nanostructured metal sulfides have received more attention towards researchers and exposed approachable pseudocapacitive performance. The two or more valence states present in the sulfide and better theoretical capacity of sulfur deliver good characteristic behavior of the capacitor. Among various metal sulfides, zinc sulfide (ZnS) is a significant II–VI semiconductor material with a wide band gap of 3.5–3.8 eV and two possible crystal structures, namely, sphalerite and wurtzite. Much application has been proposed with ZnS nanomaterials like electronics, optoelectronics, photovoltaics, and energy storage devices. Jayalakshmi et al. studied the performance of the capacitor of ZnS nanoparticles in various electrolytes. Recent studies demonstrated that the usage of carbon-based materials like CNTs and graphene into ZnS NP has decreased the band gap and could be applied for several applications including energy storage devices. Based on this, Ramachandran et al. prepared ZnS-decorated graphene nanocomposites through the facile solvothermal method and used as an electrode to examine their electrochemical behavior in 6 M KOH electrolyte towards supercapacitor applications [64]. This nanocomposite electrode achieved a specific capacitance of 197.1 F g⁻¹ at 5 mV s⁻¹ with initial capacitance maintained at 94.1% after 1000 cycles. Similarly, Hou et al. prepared CNT-decorated hierarchical ultrathin ZnS nanosheets through the facile method as an electrode for supercapacitor applications [65]. The results revealed that the ZnS@CNT electrode-based flexible all-solid-state supercapacitor obtained the specific capacitance of 159.6 F g⁻¹ with long cycling stability. Iqbal et al. deposited ZnS nanoweb onto the Ni foam having pre-deposited graphene oxide thin layer through the hydrothermal method [66]. This electrode holds an excellent specific capacitance of 3052 F g⁻¹ at a scan rate of 2 mV s⁻¹. According to the galvanostatic charge–discharge profile, the specific capacitance is calculated to be 2400.3 F g⁻¹ at 3 mA. cm⁻². The power and energy densities obtained for GO-supported ZnS nanoweb is 4407.73 W kg⁻¹ and 120 Wh kg⁻¹, respectively. Javed et al. grow ZnS nanospheres on a flexible carbon textile (CT) using the hydrothermal method [67]. The ZnS-CT-based electrode possesses a specific capacitance of 747 F g⁻¹ at 5 mV s⁻¹ and directly applied as a binder-free electrode for the fabrication of symmetric flexible full solid-state supercapacitor. The ZnS-CT-based supercapacitor delivers a better capacitive behavior with a maximum areal capacitance of 56.25 F cm⁻², C_sp of 540 F g⁻¹ at 5 mV s⁻¹ with initial capacitance maintained at 94.6% even after 5000 cycles.

However, ZnS-based electrode material still experiences a low operating voltage, energy density, and specific capacitance for practical commercial uses. To rectify these drawbacks, an effective methodology is to fabricate composites of zinc sulfides with an excellent metal-based pseudocapacitive material. Based on this concern, Sabari Arul et al. have attempted to fabricate ZnS/MnS nanocomposite-based symmetric supercapacitor device with high specific energy and power densities and long cycle stability [68]. The results revealed that ZnS/MnS nanocomposite-based electrode achieved a specific capacitance of 884 F g⁻¹ at 2 mV s⁻¹ (Fig. 4). Finally, the utmost energy and power densities of ZnS/MnS NC-based symmetric supercapacitor are 91 Wh kg⁻¹ and 7.78 kW kg⁻¹ with long cycling stability after 5000 cycles. Li et al. synthesized a novel cactus-like ZnS/Ni₃S₂ nanohybrid for the first time through a facile two-step hydrothermal method as an electrode material for high-performance asymmetric supercapacitor [69]. It delivered a high specific capacitance of 2093 F g⁻¹ at 1 A g⁻¹ with excellent initial capacitance maintained even at higher current density (72% at 10 A g⁻¹). The asymmetric supercapacitor assembled with ZnS/Ni₃S₂ electrode exhibits high energy and power density of 51.2 Wh kg⁻¹ and 849.4 W kg⁻¹, respectively.

Fig. 4

a Schematic illustration of the growth mechanism of ZnS/MnS NCs. b Assembled ZnS/MnS NCs//ZnS/MnS NC SSC device. c–f Electrochemical performance of SSC device (c) at different operating voltages, d CV curves at diverse scan rates, e GCD curves at various current densities, f cycling stability for 5000 cycles. Inset shows the GCD curves at initial/final cycles, and symmetric devices lit a green LED image are reproduced with permission from ref. [68] under Copyright © 2017, Springer-Verlag GmbH, Germany

Recent reports demonstrate that the composite of two kinds of transition metal sulfides with carbon-based materials has enhanced the electrochemical properties with excellent photocatalytic efficiency and a wider range of solar energy absorption than the single component sulfide. On this concern, for the first time, Li et al. prepared ZnS NWs/Cu₇S₄ NPs/rGO nanocomposite through a one-pot hydrothermal method and utilized as an electrode material for supercapacitor [70]. The ZnS/Cu₇S₄/rGO nanocomposite-based electrode achieved an ultimate specific capacitance of 1114 F g⁻¹ at 1 A g⁻¹ with initial capacitance holding at 88% after 5000 cycles. The ZnS/Cu₇S₄/rGO NC-based asymmetric supercapacitor was fabricated, which achieved the energy and power densities of 22 Wh kg⁻¹ and 595 W kg⁻¹ with 77% of initial capacitance sustained after 5000 cycles.

Wei et al. synthesized graphitic C₃N₄/ZnS-based electrode using a one-step calcination process with various mass ratios under the N₂ atmosphere [71]. The specific capacitance of g-C₃N₄/ZnS exhibits a maximum specific capacitance of 497.7 F g⁻¹ at 1 A g⁻¹ with capacitance retaining at 80.4% at 5 A g⁻¹ after 1000 cycles. The symmetric supercapacitor developed with a g-C₃N₄/ZnS-based electrode achieved an energy and power density of 10.4 Wh kg⁻¹ and 187.3 W kg⁻¹ with long cycling stability. Cao et al. prepared a double-layer hollow structure of Cu₇S₄/NiS through a self-generated sacrificial template method and used as an electrode material for high-performance supercapacitor [72]. The Cu₇S₄/NiS composites with three different particle sizes are prepared, and they exhibit a high specific capacitance of 1204 F g⁻¹, 1028 F g⁻¹, and 857 F g⁻¹ at a current density of 1 A g⁻¹, and 85.8%, 84.32%, and 80.13% of its initial capacitance were sustained after 1000 cycles. Binder-free cupric ion-containing zinc sulfide nanoplate-like structures were developed by Hussain et al. utilizing a practical solvothermal method. The developed electrode material displays exceptional coulombic efficiency (99%) and a specific capacitance of 545 F g⁻¹ at a current density of 1 A g⁻¹ after 5000 cycles [73].

4.8 Vanadium chalcogenides

Vanadium disulfide (VS₂) is a family of transition metal sulfides, comprised of two-dimensional layered structures with the vanadium metal sandwiched by two sulfur atoms. This slack structure is bonded by weak Van der Waals force of interactions, forming an interlayer spacing of 5.76 Å, which enables the electrons and ions to transport from one valence shell to another. At the same time, because of its low cost and wide range of sources, vanadium disulfide is studied extensively towards energy storage devices. To date, very few reports have emerged regarding the preparation of vanadium disulfide through the chemical route for the application of supercapacitors. Feng et al. used VS₂ ultrathin nanosheets as an electrode material for supercapacitors and obtained a maximum specific capacitance of 4760 µF.cm⁻². According to literature, for the first time, Pandit et al. developed nanostructured VS₂ through the SILAR method and functioned as an electrode material for a flexible symmetric all-solid-state supercapacitor [74]. It yields a specific power of 1.5 kW kg⁻¹ (specific capacitance of 25.9 Wh kg⁻¹) at a voltage window of 1.6 V. Guo et al. reported the synthesis of ultrathin VS₂ TMD nanoplates with in-plane and out-of-plane defects through simple colloidal chemical method [75]. These rich-defect nanoplates are used as anode material for supercapacitor application, which provides an ultrahigh specific capacitance of 2200 F g⁻¹ at 1 A g⁻¹. The fabricated asymmetric supercapacitor obtained a better energy density of 66.54 Wh kg^-1 at a power density of 0.75 kW kg⁻¹ with long cycling constancy over 5000 cycles.

The addition of carbonaceous materials to vanadium disulfide helps to enhance its energy storage properties. In TMD/carbonaceous composites, carbonaceous-based materials are known to be a conductive channel that enhances the contact between the electrolyte and electrode interface. Masikhwa et al. developed an asymmetric supercapacitor with VS₂ nanosheets and activated carbon and achieved a maximum specific capacitance of 155 F g⁻¹ at 1 A g⁻¹ current density as an asymmetric supercapacitor and exhibits 42 Wh kg⁻¹ of energy density, 700 W kg⁻¹ of power density with ~ 99% of initial capacitance maintained over 5000 cycles at 2 A g⁻¹ [76]. Later, Pandit et al. prepared hexagonal structured VS₂ NPs onto the MWCNTs matrix through a facile chemical route [77]. It exhibits a maximum capacitance of 830 F g⁻¹ with long cycle steadiness at 95.9% over 10,000 cycles. Fabricated VS₂/MWCNT-based flexible solid-state symmetric supercapacitor obtained a high specific capacitance of 182 F g⁻¹ at 2 mV s⁻¹ with 42 Wh kg⁻¹ of specific energy and 93.2% of long cycling stability over 5000 cycles. Meyer et al. successfully synthesized carbon-supported vanadium disulfide nanocomposites through hydrothermal synthesis which exhibits specific capacitance of 33 F g⁻¹ at 1 mA. cm⁻² [78]. Also, Fang et al. reported the preparation of cauliflower-like nanocomposite made up of ultrathin VS₂ nanosheets and ZnO nanospheres through in situ growth of ZnO nanospheres on the ultrathin VS₂ nanosheets by simple solution method and examined as electrode materials for supercapacitors [79]. The cauliflower-like nanocomposite exhibits a high specific capacitance of 2695.7 F g⁻¹ at 1 A g⁻¹ with initial capacitance holding at 92.7% over 5000 cycles.

As another form of vanadium sulfide, vanadium tetrasulfide (VS₄) stands out in TMSmaterials. It exhibits a 1D chain structure with an interchain distance of 5.83 Å. The sulfur atom present in vanadium tetrasulfide is in the form of S₂²⁻ and bonded to the adjacent V atoms. This sparse chain structure offers enough space for the insertion and extraction of electrolyte ions and accelerates the progress of the reaction. The capacity of VS₄ is higher than that of VS₂ since the sulfur content in VS₄ is high and it plays a very predominant role as an active reactant in the electrochemical reaction. The use of graphene or other carbon-based material with vanadium will surely improve both the specific capacitance and cycle performance compared to the pure form. Ou et al. synthesized V₅S₈/graphite nanosheets which exhibits a specific capacitance of 1112 mA h/g at 0.1 A g⁻¹ in Li battery and remains 846 mAh g⁻¹ after 700 cycles. Kalam et al. prepared hierarchal porous vanadium sulfide/rGO (V₃S₄/rGO) composite using the hydrothermal method and used as electrode material for supercapacitor [80]. The V₃S₄/rGO composite electrode offers a maximum specific capacitance of 520 F g⁻¹ at 1 A g⁻¹ current density with excellent cyclic stability of 99.6% over 2000 cycles (Fig. 5). Zhang et al. synthesized flower-like VS₄/rGO composite and used in aluminum-ion batteries which provides a promising capacity and coulombic efficiency of 90% after 1000 cycles. Sun et al. developed a promising anode with vanadium sulfide on reduced graphene oxide (VS₄/rGO) for sodium-ion battery and exhibits a capacity of 362 mAh/g [81]. Wang et al. successfully prepared patronite VS₄ anchored on carbon nanocubes with a petal-shaped structure consisting of nanolayers using a one-step hydrothermal method [82]. The VS₄/CNT composite-based electrode offers a specific capacitance of 330 F g⁻¹ at 1 A g⁻¹ which is exceeding that of pure VS₂. The VS₄/CNT electrode-based symmetric supercapacitor (SSC) exhibits a spectacular areal capacitance of 676 mF. cm⁻² with an energy and power density of 51.2 Wh kg⁻¹ and 30.95 W kg⁻¹ at 2.2 V. Ratha et al. prepared both VS₄/rGO- and VS₄/CNT-based electrodes and examined their electrochemical behavior in 0.5 M of K₂SO₄ solution. The VS₄/CNT nanosheets exhibit a capacitance of 231 F g⁻¹, while VS₄/rGO brought a specific capacitance of 492 F g⁻¹ at 1 mV s⁻¹ under the two-electrode system. Based on this concern, Wang et al. planned to combine CNTs and rGO with VS₄, which show better electrochemical performance than the pure phase [83]. They prepared 3D VS₄/CNTs/rGO through the hydrothermal method and used as an electrode material for the application of supercapacitors. It explored a remarkable capacitance of 497 F g⁻¹ (Fig. 5). The symmetric supercapacitor developed with VS₄/CNTs/rGO electrode exhibits an areal capacitance of 1003.5 mF. cm⁻² with energy and power density of 72.07 Wh kg^-1 and 14.69 W kg⁻¹ at 0.5 mA. cm⁻².

Fig. 5

Structural formation of a VS₂ and b Ni_xV_3-xSe₄ and V₃Se₄, images are reproduced with permission from ref. [84] under Copyright © 2019, American Chemical Society. c–f Different schematics for the synthesis of various vanadium chalcogenides: c synthesis of VS₄/CNTs/rGO composites by simple one-step hydrothermal method, images are reproduced with permission from ref. [83] under Copyright © 2020 Elsevier Inc.; d synthesis of V₃S₄/rGO composites by hydrothermal method, images are reproduced with permission from ref. [80] under Copyright © 2018 Taiwan Institute of Chemical Engineers, published by Elsevier B.V.; e synthesis of VS₂ hexagons on MWCNTs by using simple and cost-effective successive ionic layer adsorption and reaction (SILAR) method, images are reproduced with permission from ref. [77] under Copyright © 2017 American Chemical Society; f synthesis of all-in-solution VS₂ ultrathin nanosheets, images are reproduced with permission form ref. under Copyright © 2011 American Chemical Society

Vanadium diselenide (VSe₂) is consists of two Se atoms sandwiching with one V atom in between them forming layered structures stacked through weak Van der Waals force of interaction. VSe₂ has hexagonal crystal structures and exists in both 2H and 1 T phase with metallic nature. Due to the strong electron correlation between the adjacent V atoms and high charge density wave induced structural instability, VSe₂ obtains an excellent potential to be exploited in the application of energy storage and conversion devices. According to the report, to date, both VS₂ and VSe₂ are shown metallic nature, while other TMCs are insulators or semiconductors with few of them possess superconducting behavior as well. Recently, it has been observed that in contrast to TMCs with semiconducting nature, those having metallic nature show ultrahigh conductivity and used as an appropriate material for supercapacitor applications. Also, vanadium diselenide (VSe₂) has been reported to have versatile electronics behavior which is beneficial for the realization of futuristic nano-devices. Additionally, its unique activities towards intercalation reactions, VSe₂, have been stated to display promising energy storage properties as active components in the cathode material of Li-ion batteries. Hybridization of these highly conductive TMCs with carbon-based material would surely enhance their overall supercapacitor performances. Since the graphene hybrid of VS₂ is proved to be a better alternative for energy storage devices, enhanced performance is expected from VSe₂/rGO hybrid. On this concern, Marri et al. reported the preparation of VSe₂/rGO with various concentrations of GO through a one-step hydrothermal method for the first time [85]. The supercapacitor performance of VSe₂/rGO_0.3 showed a high specific capacitance of ~ 680 F g⁻¹ at a current density of 1 A g⁻¹. Moreover, it acquires a high energy and power density of ~ 212 Wh kg⁻¹ and ~ 3.3 kW kg⁻¹ with capacitance retention of ~ 81% over 10,000 charge–discharge cycles. Chemical synthesis was employed by Xu et al. in 2021 to create VSe₂ nanoparticles anchored on an N-doped hollow carbon sphere. An improved specific capacitance of 1030 F g⁻¹ at 1 A g⁻¹ is observed in the produced electrode material. With a power density of 701.91 W kg⁻¹, the asymmetric supercapacitor made with an activated carbon anode and VSe2@NC cathode has a high energy density of 85.41 Wh kg⁻¹. It also has a high-stable cycling performance with 90% retention after 2000 cycles [86]. Additionally, Ramu et al. created binder-free patronite (VS4) flower-like nanostructures that are facilely fabricated on carbon cloth (CC) using a simple hydrothermal process [87]. With exceptional energy and power densities of 74.4 Wh cm⁻² (28.6 Wh kg⁻¹) and 10,154 W cm⁻² (9340 W kg⁻¹), respectively, the IL-based symmetric supercapacitor is built and exhibits a high areal capacitance of 536 mF. cm⁻² (206 F g⁻¹) and remarkable cycling endurance (93%) as well. Molybdenum sulfide (MoS₂) @ vanadium sulfide (VS₂) and tungsten sulfide (WS₂) @ VS₂ hybrid nanoarchitectures were created by Hussain et al. using a simple one-step hydrothermal process. The electrodes produced have high specific capacitances of 513 and 615 F g⁻¹, respectively, at 2.5 A g⁻¹. The asymmetric device, which was created using WS₂@VS₂ electrodes, has a high specific capacitance of 222 F g⁻¹ at an applied current of 2.5 A g⁻¹ and a specific energy of 52 Wh kg⁻¹ at power density of 1 kW kg⁻¹ [88].

4.9 Tungsten chalcogenides

Many researchers have stated that among various transition metal dichalcogenides (TMDCs), tungsten disulfide (WS₂) is a low-cost, graphene-like alternative material that offers many applications in the field of energy-related devices such as solar cells, photocatalysts, supercapacitors, and Li-ion batteries. Tungsten disulfide (WS₂) has a hexagonal crystal structure (space group of P63/mnc) made up of three stacked atomic layers (S-W-S) bonded together by weak Van der Waals force. The interlayer spacing of the adjacent atoms is ~ 0.61 nm, which is higher than that of graphite (0.33 nm). These stacked layers will provide space for the insertion and extraction of Li⁺ ions. Although WS₂ is a potentially promising material for energy storage devices, its poor electronic conductivity, the catalytic property of the bulk form, low specific capacitance, poor volume expansion, and agglomeration properties have restricted its practical applications. To overcome these demerits, several approaches have been explored. Use of carbon-based materials, conducting polymers, metal oxides could suppress the abovesaid defects. Chen et al. reported a novel 3D WS₂ nanotubes/graphene hybrid with unique sandwich-type geometry through one-pot hydrothermal for Li-ion batteries. Ratha et al. synthesized WS₂ nanosheet with reduced graphene oxide through the hydrothermal method [89]. The WS₂/rGO hybrids exhibit a specific capacitance of 350 F g⁻¹ at 2 mV s⁻¹, which is about 5 times higher than the pure WS₂ and and 2.5 times higher than the pure rGO nanosheets. Tu et al. synthesized well-constructed WS₂/rGO nanosheets by a simple molten salt process as the electrode material for SCs [90]. It yields a massive specific capacitance of 2508.07 F g⁻¹ at a scan rate of 1 mV s⁻¹ with capacitance retention of 98.6% over 5000 cycles. Similarly, several methods have been taken to address the limitations of WS₂ and to improve its electrical contact between the catalyst. Generally, 2D mesoporous nanosheets have received much attention because of their superior properties compared to their conventional bulk materials. Shang et al. prepared interwoven WS₂ nanoplates supported on carbon fiber cloth (WS₂/CFC) through a facile solvothermal method [91]. The electrochemical behavior of WS₂/CFC delivers a specific capacitance of 399 F g⁻¹ at 1 A g⁻¹. Moreover, WS₂/CFC exhibits long cycling stability 99% of initial capacitance retaining for over 500 cycles. Similarly, Qiu et al. reported the preparation of tungsten disulfide/active carbon fiber nanocomposite through electrospinning, one-pot carbonization, and activation, and followed by a hydrothermal process [92]. The nanocomposite displays a high specific capacitance of 600 F g⁻¹ at 1 A g⁻¹. The fabricated quasi-solid-state asymmetric supercapacitor obtains an approachable specific capacitance of 237.7 F g⁻¹ at 1 A g⁻¹.

The performance of the nanostructured electrode is deeply dependent upon the surface area, electric and ionic conductivity, compact dimensions, reactivity, etc. Normally, mesoporous structured based materials offer a high specific surface area for easy ionic transportation. In the previous report, mesoporous structure materials display several attractive features and have recognized to be potentially favorable anode materials for Li-ion batteries. Based on this literature report, herein, Ansari et al. have prepared porous WS₂ consists of few-layered nanosheets through the hydrothermal method and used as electrode material for supercapacitor [93]. It displays a specific capacitance of 241.5 F g⁻¹ at 0.75 A g⁻¹ with long cycling stability over 2000 cycles.

Like sulfide, tungsten selenide (WSe₂) is also considered a promising electrode material for the use of energy storage devices due to its wide and direct band gap. But, due to its low electronic conductivity and easy aggregation because of a high specific area, it limits its future application. To solve these hindrances, a profitable way is to associate metal selenide with nanostructured carbon-based materials like mesoporous carbon, CNT, graphene, and rGO. Inspired by previous reports, for the first time, Gopi et al. successfully prepared nanosheet-like tungsten diselenide with rGO hybrid through facile one-step hydrothermal route for the application of supercapacitor [94]. The WSe₂/rGO nanocomposite electrode-based supercapacitor displays a high specific capacitance of 389 F g⁻¹ at a current density of 1 A g⁻¹ with long capacitance retention of 98.7% over 3000 cycles, and it also delivered an energy and power density of 34.5 Wh kg⁻¹ and 400 W kg⁻¹, respectively.

Compared to two-dimensional metal sulfides and selenides, telluride (Te)-based TMDs are metallic, which results in a fast transport rate of ions at the electrodes/electrolyte interface to raise the specific capacitance. Thus, the electrochemical studies of 2D Te-based TMDs are vital. Recent reports have stated that 1Td WTe₂ is a type-II Weyl semimetal, making it show abnormal physical performance like a positive quantum spin hall gap for monolayers and extreme magnetoresistance. Also, the transport rate properties of atomically thick 1Td WTe₂ exposed a superconducting nature below 2.5 K. The outstanding physical characteristics of 1Td WTe₂, especially high electrical conductivity, encourage us that atomically thick 1 Td WTe₂ deserves specific consideration as an auspicious electrode material for energy storage devices, particularly, supercapacitors. However, to date, the reports on the application of Te-based TMDs in supercapacitors are quite infrequent. Here, for the first time, Yu et al. reported the single-crystal 1Td WTe₂ exfoliated nanosheets by liquid phase exfoliation method, which are assembled into air-stable films and further all-solid-state flexible supercapacitors [95]. The 1Td WTe₂ nanosheet-based supercapacitor delivers a mass capacitance of 221 F g⁻¹ and stack capacitance of 74 F. cm⁻³. Moreover, they also display excellent volumetric energy and power density of 0.01 Wh cm⁻³ and 83.6 W cm⁻³ with capacitance retention of ~ 91% over 5500 cycles.

4.9.1 Iron chalcogenides

Iron sulfides (commonly known as fool’s gold) are one of the most promising candidates for energy storage devices due to their cost-effectiveness, environmental benignity, and low abundance nature. The solubility of sulfur in iron results in a multitude of stoichiometry, which leads to the formation of iron sulfides in multi-valent states like troilite (FeS), pyrite (FeS₂), and greigite (Fe₃S₄). Furthermore, for the past decades, many researchers have attempted to prepare FeS thin film using various synthesis approaches. Karade et al. first reported the preparation of FeS thin-film electrodes was investigated by the SILAR method at room temperature. They also reported the use of FeS thin films as efficient electrodes in liquid configuration and developed the symmetric flexible solid-state supercapacitor device using PVA-LiClO₄ as a gel electrolyte at a potential window of 2 V [96]. The fabricated supercapacitor gained a specific capacitance of 4.62 F g⁻¹ at 0.75 mA with an energy density of 2.56 Wh kg⁻¹. It yields capacitance retention of 91% over 1000 cycles along with bending of device up to 175°. Moreover, few uses of FeS in supercapacitors may attribute to the large change in volume during the charging and discharging process. The expansions of volume can extent up to 200% to cause the pulverization of FeS which results in poor cycle stability. To solve this, an effective technique is coating a carbon layer on the FeS surface or reducing the size of FeS. Also, combining with active materials may alleviate the volume change, thereby increasing the cyclability, which enhances their conductivity. Based on the concern, Zhao et al., a porous FeS/rGO composite was prepared by in situ grown on Fe foil surface and directly used as an electrode material for supercapacitor [97]. It displays an excellent specific capacitance of 300 F g⁻¹ (900 mF. cm⁻²) with 97.5% of capacitance retention over 2000 cycles.

As one kind of iron sulfide, FeS₂ has been investigated as an electrode material for supercapacitors. Chen et al. prepared pyrite (FeS₂) nanobelts through the facile hydrothermal method for enhancing the performance of aqueous pseudocapacitor. But the practical application of iron-based electrodes was hindered by some problems like large volumetric expansion, low rate capability, low capacity retention, and poor inherent conductivity. To rectify these defects, composite of iron sulfides with carbon-based materials, conducting polymers or metal oxides, is approachable. Thus, Sridhar et al. reported the one-pot two-step method for the preparation of carbon nanofiber (CNF) cross-linked FeS₂ networks through the microwave method [98]. The prepared 3D mesoporous FeS₂/CNF electrodes yield a maximum capacitance of 612 and 342 F g⁻¹ at 5 and 100 mV s⁻¹ with initial capacitance retained at 97% even after 2000 cycles (Fig. 6). Javed et al. synthesized FeS₂ nanospheres supported on carbon paper which exhibits a better electrochemical performance towards energy storage device and yields a high energy and power density of 44 Wh kg⁻¹ and 175 W kg⁻¹, respectively. Zhong et al. developed a supercapacitor with a hierarchical FeS₂@Fe₂O₃ heterostructure which displays an excellent capacitance performance to the bare Fe₂O₃ [99]. Pei et al. developed the FeS₂/GNS electrode for supercapacitor which yields a theoretical capacitance of 313.6 F g⁻¹. Sun et al. synthesized FeS₂ nanoellipsoids through a rapid microwave-assisted method to use as an anode material for supercapacitor [100]. It displays a specific capacitance of 515 C/g at 1 A g⁻¹ with energy and power density of 64 Wh kg⁻¹ and 271.2 W kg⁻¹ with initial capacitance maintained at 91% of initial capacitance after 5000 cycles. Zhang et al. prepared a novel Fe₇S₈@Fe₅Ni₄S₈ flower center/petal hierarchical nanostructure via a one-step solvothermal method, which achieved a specific capacitance of 670.4 C/g at 1 A g⁻¹. The constructed supercapacitor based on Fe₇S₈@FeNi₄S₈ composite electrode exhibits high energy and power density of 49.9 Wh kg⁻¹ and 770.0 W kg⁻¹ with 88.9% of capacitance retention after 10,000 cycles. Selenium-enriched hybrid NiSe₂@Fe₃Se₄ (NFS) nanocomposites were prepared by Manikandan et al. and which are easily deposited on Ni-foam utilizing the chemical bath deposition (CBD) method. The maximal areal capacity of the deposited NiSe₂@Fe₃Se₄ for 36 h (NFS@36 h) is 6.05 C/cm² at 6 mA. cm⁻². The NFS@36 is used as the positive electrode in a hybrid supercapacitor (HSC), and biomass-derived O, N enriched activated carbon is used as the negative electrode and exhibits superior specific energy of 52 Wh kg⁻¹ at 398 W kg⁻¹ specific power. Additionally, after 10,000 charge/discharge cycles at 5 A g⁻¹, the device shows a remarkable cycling endurance, with a specific capacitance retention rate of 92% [101].

Fig. 6

Schematic representation of the formation of iron chalcogenides. a FeS/RGO nanocomposite by in situ growth method, image is reproduced with permission from ref. [97] under Copyright © 2017 Elsevier Ltd. b Ni_XFe_3-XSe₄ and c structural configuration of Ni_XFe_3-XSe₄ nanoarray, images are reproduced with permission from ref. [84] under Copyright © 2019, American Chemical Society

4.9.2 Manganese chalcogenides

Manganese sulfide (MnS) is typically a p-type semiconductor with a wide band gap of 3.1 eV, since Mn has multiple oxidation states, due to outstanding properties such as economical and eco-friendly nature, and high electronic conductivity of ~ 3.2 × 10³ S cm⁻¹ than their corresponding hydroxides or oxides [102]. It exists in three different phases, namely, rock-salt structure with α-phase, zinc blende structure with β-phase, and wurtzite structure with γ-phase, respectively (Fig. 7). MnS could be used for charge storage purposes through redox reactions along with non-Faradaic processes. Furthermore, the layered crystal structure of MnS could facilitate the easy intercalation and de-intercalation of electrolyte ions thereby boosting the electrochemical stability of a supercapacitor. Although among various polymorphs of MnS, alpha-phase of MnS is the most stable one, and few reports are available for the use of electrode material in energy storage devices. Tang et al. prepared MnS nanocrystals of hollow spindle-like nanospheres and tetrapod nanorods via the hydrothermal method and yields a specific capacitance of 704 F g⁻¹ and 400.6 F g⁻¹, respectively [103]. Li et al. synthesized 2D MnS nanosheets through the hydrothermal method and investigated their electrochemical performance for supercapacitors [104]. The alpha-MnS nanosheet electrode demonstrated a high specific capacitance of 667.40 F g⁻¹ at 1 mV s⁻¹ and 344.51 F g⁻¹ at 0.5 A g⁻¹ with initial capacitance retained at 93% over 5000 cycles. Pujari et al. prepared cubic microfibers MnS thin films through the chemical bath deposition (CBD) method which revealed a high specific capacitance of 747 F g⁻¹ at 1 mA. cm⁻² with initial capacitance sustained at 85% over 2000 cycles (Fig. 7) [105]. Quan et al. reported the preparation of α-MnS NPs with nitrogen-doped rGO through a simple one-step solvothermal method and fabricated MnS/N-rGO//N-rGO electrode-based asymmetric supercapacitor with a specific capacitance of 77.9 F g⁻¹. Mohamed et al. prepared α-MnS nanoflakes/rGO nanosheets through the facile one-step hydrothermal method and used for the application of supercapacitors [106]. The hybrid supercapacitor device was fabricated using α-MnS/rGO and activated carbon as a positive and negative electrode. It exhibits a high energy and power density of 38.13 Wh kg⁻¹ and 850 W kg⁻¹, respectively. Tang et al. used the hydrothermal approach to create porous manganese sulfide (MnS/GO-NH3) nanocrystals based on the Kirkendall effect [107]. It exhibited a high specific capacitance of 390.8 F g⁻¹, and the developed MnS/GO-NH₃ electrode-based ASC devices demonstrate specific capacitance of 73.63 F g⁻¹ with energy and power density of 14.9 Wh kg⁻¹ and 4.6 kW kg⁻¹, respectively. Naveenkumar et al. successfully electrodeposited MnS on graphene-wrapped Ni foam substrate as an electrode for supercapacitor application [108]. It delivered a specific capacitance of 2220 F g⁻¹ at 0.5 A g⁻¹ with initial capacitance retained at 94.6% over 1000 cycles. Ragupathi et al. employed sol–gel method to prepare graphitic carbon nitride–doped MnS nanocomposites for supercapacitor application. It yields a maximum specific capacitance of 463.32 F g⁻¹ at 10 mV s⁻¹ with initial capacitance maintained at 98.6% over 2000 cycles [109].

Fig. 7

Structural formation of different manganese chalcogenides: a MnS₂, b MnSe, c MnSe₂. d–f Schematic representation of various synthesis processes of manganese sulfides. d Controlled sulfurization of MnCO₃ thin film into different morphological MnS₂, image is reproduced with permission from ref. [110] under Copyright © 2019 Elsevier Ltd. e Hydrothermal synthesis of γ-MnS2 with ASCs device, image is reproduced with permission from ref. [111] under Copyright © 2016 Springer Nature. f Facial in situ hydrothermal approach combined with etching and pre-oxidization process for the synthesis of Ni₃S₂@MnS composite, image is reproduced with permission from ref [112]. under Copyright © 2017 Elsevier Ltd. g Structures of manganese sulfide: α-MnS (rock salt type), β-MnS (zinc blende type), and γ-MnS (wurtzite type), image is reproduced with permission from ref. [111] under Copyright © 2016 Springer Nature

Chen et al. synthesized MnS nanocrystals through the hydrothermal method and developed MnS/activated carbon electrode-based asymmetric supercapacitor which displayed a maximum specific capacitance of 73.63 F g⁻¹ at 1 mV s⁻¹. Javed et al. prepared MnS nanoparticles onto the carbon textile through the hydrothermal method and developed a solid-state symmetric supercapacitor with high energy and power density of 52.03 Wh kg⁻¹ and 1.2 kW kg⁻¹, respectively. Kumbhar et al. successfully prepared novel MnS nanoclusters on nickel foam by the SILAR method and used them as an electrode to examine their capacitance performance [113]. It presents a high specific capacitance of 828 F g⁻¹ at 5 mV s⁻¹ with capacitance retention of 85.2% over 5000 cycles. Also, the ASC was fabricated using MnS@NF and reduced graphene oxide as a positive and negative electrode, which displays high energy and power density of 34.1 Wh kg⁻¹ and 12.8 kW kg⁻¹ with initial capacitance holding at 86.5% after 2000 cycles.

Additionally, the laminar nanostructure of manganese sulfide (wurtzite structure) accelerates the penetration of electrolyte and the easy ionic intercalation, which promotes its intrinsic electrochemical reactivity for the capacitive property. Chen et al. fabricated asymmetric supercapacitor using rod-like γ-MnS nanocrystal and porous eggplant derives activated carbon as a positive and negative electrode. They showed a specific capacitance of 110.4 F g⁻¹ at 0.5 A g⁻¹ and possess an energy and power density of 37.6 Wh kg⁻¹ and 181.2 W kg⁻¹ with the initial capacitance were upholding at 89.87% over 5000 cycles. But, due to poor cycle life and low electrical conductivity, only a little attention has been paid for γ-MnS to use as an electrode material for supercapacitor. As an electrode material, the coating of graphene is one of the efficient ways to raise conductivity. If nanostructured graphene is used as the matrix material for MnS-based composite, it not only well accommodates the MnS particles, but additionally offers a significant electrode and electrolyte contact for the charge transfer process. Based on the abovesaid considerations, Li et al. synthesized gamma-phase manganese sulfide (γ-MnS)/rGO composite through a one-pot solvothermal method. The electrochemical performance of fabricated γ-MnS/rGO electrode-based supercapacitor possesses an enhanced specific capacitance of 802.5 F g⁻¹ at 5 A g⁻¹, and there is no decrease of its initial capacitance values even after 2000 cycles. Arul et al. prepared γ-MnS NPs through a simple chemical process [114]. For the first time, they attempted to deposit the synthesized MnS NPS on homemade graphite/scotch tape, as a binder-free flexible conducting electrode with a maximum specific capacitance of 112 F g⁻¹ at 5 mV s⁻¹ and cycling permanency of 93% of its initial capacitance even after 2000 cycles. Zhang et al. successfully prepared γ-MnS/rGO composite through a facile one-pot hydrothermal method and used as electrode materials for ASC [115]. It exhibits a specific capacitance of 547.6 F g⁻¹ at 1 A g⁻¹ with initial capacitance sustaining at 65% over 5000 cycles (Fig. 8).

Fig. 8

a–c Digital snapshots of prepared two MnS thin films with PVA-KOH gel electrolyte as a device. d Schematic of MnS thin-film symmetric supercapacitor. f A photograph presenting red LED illumination by connecting two symmetric devices in series, image is reproduced with permission from ref [105] under Copyright © 2016 Elsevier Ltd. e, g A red LED powered by two assembled MnS//EDAC ASC devices in series. h Electrochemical studies of MnS and EDAC electrodes, image is reproduced with permission from ref. [111] under Copyright © 2016 Springer Nature. i Electrochemical performance of α-MnS/N-rGO hybrid//N-rGO asymmetric devices, image is reproduced with permission from ref. [116] under Copyright © 2016 Elsevier Ltd

As comparable to manganese sulfide (MnS), manganese selenide (MnSe) is a typical p-type semiconductor with a band gap of 2.0 eV. It possesses a low electron resistance with high ionic conductivity. MnSe has high electrical conductivity than MnO (~ 4.2 eV) and MnS (~ 3.2 eV). There are three different phases of MnSe, namely, the cubic NaCl phase or rock-salt phase (α-MnSe), the cubic zinc blende phase (β-phase), and hexagonal wurtzite phase (γ-phase) (Fig. 7). Among other phases, the rock-salt phase is thermodynamically stable, whereas the zinc blende phase is highly stable and observed as a minor impurity phase and the wurtzite phase is said to be metastable. Due to harsh growth form particularly in liquid, it is very difficult to synthesis MnSe nanostructures. Very few reports are available so far for the synthesis of MnSe nanostructures including α-MnSe nanoparticles and nanocubes, β-MnSe nanowires, and γ-MnSe nanorods. Kim et al. synthesized α-phase MnSe nanoparticles with a maximum specific capacitance of 96.76 F g⁻¹ at 0.1 mA. cm⁻², although preparation of thermodynamically stable rock-salt (α-phase) phase MnSe nanostructures is still a challenge. For the first time, Javed et al. successfully synthesized single-phase hierarchical MnSe microflowers assembled by nanosheets via a facile-solvothermal method used as efficient electrode material for symmetric supercapacitor [117]. Remarkably, it showed outstanding electrochemical specific capacitance of 200 F g⁻¹ at 1 A g⁻¹ with an energy density of 55.42 Wh kg⁻¹ and 97.15% of initial capacitance maintained over 5000 cycles.

Tang et al. reported a simple one-step solvothermal method to synthesis the nanocellular rock-salt phase of MnSe and investigated its electrochemical behavior towards energy storage devices [118]. The MnSe faradic electrode displays a high capacity of 84.7 mAh/g at 10 mV s⁻¹ with long cycling stability and good rate capability. The MnSe electrode-based supercapacitor was developed which exhibits large energy and power density of 39.6 µWh cm⁻² and 0.96 mW cm⁻², respectively. Sahoo et al. used hydrothermally prepared α-MnSe as an electrode material for symmetric supercapacitor [119]. It delivered a maximum specific capacitance of 96.76 F g⁻¹ at 0.1 mA. cm⁻² with a corresponding energy density of 8.60 Wh kg⁻¹ over 2000 cycles. The developed α-MnSe electrode-based symmetric supercapacitor exhibits a specific capacitance of 23.44 F g⁻¹ at 0.1 mA. cm⁻² with a potential window of 0.8 V. Ranganatha et al. prepared γ-MnS/rGO composite through a one-pot solvothermal method and studied their electrochemical performance as supercapacitor electrode materials (Fig. 9) [120]. The maximum specific capacitance of γ-MnS/rGO is 1009 C/g at 1 A g⁻¹ with an initial capacitance retained at 82% after 2000 cycles, whereas pristine γ-MnS offers only 480 C/g of specific capacitance 1 A g⁻¹ with a capacity maintained at 64%.

Fig. 9

a Fabrication of electrode on flexible carbon textile using α-MnSe nanospheres by one-step solvothermal method. b Schematically representation of flexible MnSe@CT symmetric SC. b Schematically representation of flexible MnSe@CT symmetric SC e Electrochemical measurements of MnSe@CT electrode in aqueous electrolyte. f Electrochemical performance of symmetrical MnSe@CT based symmetrical SC in LiCl hydrogel electrolyte, image is reproduced with permission from ref [117] under Copyright © 2019 Elsevier B.V. c The schematic representation of the synthesis of the cubic G-MnSe₂ hybrid material. f Electrochemical performance of cubic MnSe₂ and G-MnSe₂ symmetric cells, image is reproduced with permission from ref [121]. under Copyright © 2017 Wiley‐VCH Verlag GmbH & Co

On the other hand, MnSe₂ also possess a similar crystal structure and other intrinsic physical and chemical properties as like other metal TMDs. MnSe₂ has many advantages including low toxicity, low cost, and earth-abundant material; basically, pristine metal dichalcogenides have some intrinsic disadvantages like low chemical stability, low specific capacitance, rate capability, and poor cycle life which hinders their electrochemical energy storage performance on a device scale. The abovementioned drawbacks are overcome by combining them with high-conductive materials like carbon and graphene to form a hybrid structure. On this concept, Balamuralitharan et al. synthesized 2D cubic MnSe₂ and reduced graphene oxide–decorated MnSe₂ (MnSe₂/rGO) through a facile hydrothermal method [121]. The electrochemical energy storage performance of MnSe₂ and MnSe₂/rGO hybrid-based electrodes was examined and used for supercapacitor application for the first time. The specific capacitance of MnSe₂ electrode is ~ 57.8 mF. cm⁻², whereas MnSe₂/rGO hybrid electrode has a high specific capacitance of 93.3 mF. cm⁻². The fabricated MnSe₂ symmetric supercapacitor exhibits excellent capacitance retention of 80% over 4500 cycles, whereas MnSe₂/rGO-based SC displays 106% of its initial capacitance sustained over 4500 cycles under similar conditions (Fig. 9). Wet chemical synthesis of manganese sulfoselenide nanoparticles anchored graphene oxide nanocomposite was carried out by Yasoda et al. As a supercapacitor electrode, GO-MnSSe produced a specific capacitance of 603 F g⁻¹ at 0.1 A g⁻¹ in 1 M KCl. The constructed two-electrode device displayed a decent retention of 67% after 9000 cycles, with a specific capacitance of 98.5 mF. cm⁻² at 80 µA cm^-2 [122].

4.9.3 Copper chalcogenides

Copper sulfide (CuS) is typically a p-type semiconductor with a band gap of 1.2⁻² eV and becomes a potentially promising candidate for SCs owing to low cost, abundant availability, large theoretical capacity, and environmental benignity. It has diverse stoichiometric forms based on their crystal structure extending from orthogonal to hexagonal, which includes covellite (CuS), spionkopite (Cu_1.39S), geerite (Cu_1.6S), anilite (Cu_1.75), digenite (Cu_1.8S), djurleite (Cu_1.95S), chalcocite (Cu₂S), and villamaninite (CuS₂) (Fig. 10). Till now, there are only a few reports on CuS on supercapacitor applications. Huang et al. reported CuS nanosheets using a solvothermal method and used them as electrode for supercapacitor. It exhibits an outstanding electrochemical capacitance of 833.3 F g⁻¹. Excellent electrochemical behavior with a better specific capacitance of 101.34 F g⁻¹ was reported by Krishnamoorthy et al. by developing SC based on CuS nanoparticles by the hydrothermal method. Huang et al. reported a one-step solvothermal method with different morphologies of CuS nanosheets and used it as a supercapacitor electrode material [123]. The prepared CuS nanosheets displayed a specific capacitance of 833.3 F g⁻¹ at 1 A g⁻¹ as compared to CuS-CTAB (378.9 F g⁻¹) and CuS-SDBS (232.4 F g⁻¹). Heydari et al. synthesized CuS nano-hollow spheres with nanoporous structure through a facile method (Fig. 11) [124]. It displayed a marvelous specific capacitance of 948 F g⁻¹ at 1 A g⁻¹ with a rate capability of 46% of the initial capacitance retention at 50 A g⁻¹. Yu-Kuei et al. reported CuS nanowire preparation through liquid–solid reaction, which exhibits a specific capacitance of 305 F g⁻¹ and 87% of original capacitance obtained over 5000 cycles (Figs. 11 and 12) [125].

Fig. 10

a–c Schematic illustration of the different CuS: a CuS, b Cu₂S, c Cu₇S₄. d, e Phase transformation: d Cu(Tu)₃Cl into CuS, e [Bmim]⁺ ions on the surface of the CuS layer, images are reproduced with permission from ref [126] under Copyright © 2015 American Chemical Society

Fig. 11

a–h Different synthesis methods for the preparation of copper sulfides: a CuS nanowire array on Cu foil by simple wet chemical process, image is reproduced with permission from ref [125]. under Copyright © 2014 Elsevier Ltd. c Preparation of CuS at carbon cloth by electrodeposition technique, image is reproduced with permission from ref [127]. under Copyright © 2018 Elsevier Ltd. b, d Preparation of CuS thin films on flexible stainless steel by SILAR and hydrothermal method, image is reproduced with permission from ref. [128] under Copyright © 2019 Elsevier Ltd. e CuS@CD–GO 3D porous hydrogels prepared using one-pot hydrothermal method, image is reproduced with permission from ref [129]. under Copyright © 2017 Elsevier Ltd. f Preparation of CuS nanosheets using microwave irradiation method, image is reproduced with permission from ref. [130] under Copyright © 2019 Elsevier Ltd. g CuS nano-hollow sphere prepared by hydrothermal method using PVP, image is reproduced with permission from ref [124]. under Copyright © 2016 Elsevier B.V. h Solvothermal method for 3D CuS structure using ionic liquid precursors, images are reproduced with permission from ref [126] under Copyright © 2015 American Chemical Society

Fig. 12

a Schematic illustration of CuS nano-hollow spheres, image is reproduced with permission from ref [124]. under Copyright © 2016 Elsevier B.V. b Two-electrode device components and photograph of CuS//AC device lit red LED, image is reproduced with permission from ref. [130] under Copyright © 2019 Elsevier Ltd. c, f Schematic illustration of g-CuS/CC symmetric devices and its bending photograph, image is reproduced with permission from ref [127]. under Copyright © 2018 Elsevier Ltd. d Schematic illustration of the EDTA-Cu_1.8S composite flexible SSC device (optical photographs of the SSC device under normal and bent), image is reproduced with permission from ref [131] under Copyright © 2018 Elsevier Ltd. e, g Schematic illustrations of CuS asymmetric flexible solid-state supercapacitor devices on flexible stainless steel. g AFSScs devices lit red and green LED method, image is reproduced with permission from ref. [128] under Copyright © 2019 Elsevier Ltd

To date, many CuS architectures have been developed for SCs like microspheres, nanowires, nanoflowers, nanosheets, nanotubes, and nanoplatelets. Unfortunately, their energy density, rate performance, and cycle stability are less well than expected. So, to overwhelm these issues, structural and compositional engineering may pay an effective way to improve the electrochemical performance of CuS. Durga et al. studied coriander leaf-like CuS nanostructures on nickel foam for SC applications [132]. It showed a high specific capacitance of 5029.28 F g⁻¹ at 4 A g⁻¹ with an energy density of 169.73 Wh kg^-1 and 107% of capacitance retention over 2000 cycles. Bulakhe et al. synthesized three-dimensional copper sulfide with various morphologies through the SILAR method and used as an electrode material for high-performance SCs [133]. The maximum specific capacitance of flowers like and integrated nanotubes were observed to be 761 and 470 F g⁻¹ at 5 mV s⁻¹.

Various nanostructure CuS-based electrode materials were investigated and fabrication for the application of supercapacitor. Still, owing to the lack of rate capability and the dead surface of CuS, it is far beyond the commercial application. Recently, a hybrid supercapacitor has received much greater attention towards energy storage devices. The combination of several materials with CuS would improve their capacitor efficiency. Based on this concern, Zeraati et al. prepared CuS nanowires through a chemical route on Cu substrates, and their electrochemical performance was compared with the case that these nanowires are coated with SiC (Fig. 11) [134]. The SiC-coated CuS exhibits a specific capacitance of 3370 F g⁻¹ with 98.4% capacity sustained over 1000 cycles. Cui et al. reported rGO-wrapped 3D CuS nano-erythrocytes through the solvothermal method, which exhibits an energy and power density of 16.7 Wh kg⁻¹ and 681 W kg⁻¹ and 90.8% of the initial capacitance maintained over 10,000 cycles at 2 A g⁻¹ [135]. The fiber-shaped SC based on PANI/CuS/PET was developed by Ba et al. and reported a specific capacitance of 29 mF. cm⁻² with 93.1% of capacitance retention over 1000 cycles [136]. Hou et al. prepared hierarchical structured CuS grown on MWCNT with a specific capacitance of 566.4 F g⁻¹ and retention of 94.5% of its initial capacitance after 5000 cycles [137].

Copper sulfide (Cu₂S or chalcocite)–based supercapacitor displayed less specific capacitance owing to its inadequate charge transfer barriers at the electrode and electrolyte interface and low specific capacitance. Hence, researchers exploited the composites of Cu₂S electrodes for the study of the supercapacitor. Alshammari et al. reported the preparation of a new hybrid core silver nanowires (Ag NWs) with shell copper sulfide (Cu₂S) nanostructure by the SILAR method, which unveils a specific capacitance of 603 F g⁻¹ (stainless steel substrate) and 707 F g⁻¹ (Ni foam substrate) with an energy and power density of 10.01 Wh kg⁻¹ and 25.33 Wh kg⁻¹ at 0.2 mA (Fig. 11) [138]. Hong et al. prepared 1D single-crystalline Cu₂S nanostructures by solution-based direct synthesis process and used as electrode materials for SC [139]. It displayed a specific capacitance of 750 mF. cm⁻² at 2 mA. cm⁻² with 90.5% of the initial capacitance sustained over 20,000 cycles. Liu et al. prepared hierarchical Cu₂S nanorods with various crystal phases by a simple hydrothermal method, which exhibits a specific capacitance of 346 mF. cm⁻² (hexagonal phase) at 5 mA. cm⁻² with 90% of original capacitance retention over 2000 cycles [140]. Zhao et al. prepared the Cu₂S microsphere by reducing copper sulfate with ascorbic acid in sodium thiosulfate solution and used as electrode materials for SC [141]. It showed a specific capacitance of 444.2 F g⁻¹ at 1 A g⁻¹ with 87% of initial primary capacitance retained over 6000 cycles.

As one of the non-stoichiometric CuS, copper sulfide with Cu:S ratio of 1.75 is usually known as anilite (Cu_1.75S or Cu₇S₄). It is the most stable Cu-rich crystal structure in the system of Cu_xS, where the S atoms in Cu₇S₄ form a rigid cubic lattice, thereby offers a crystalline pathway for the embedding of electrolyte ions (Fig. 10). While the Cu ions around the S sublattice are superionic with “liquid-like mobility,” such definite transportation of the ionic behavior of Cu is vital to enhance the pseudocapacitive performance. For the first time, Javed et al. fabricated a flexible solid-state supercapacitor based on faradic redox active material of Cu₇Se₄ nanowires through a modified hydroxide-mediated approach [142]. It showed a high specific capacitance of 400 F g⁻¹ at 10 mV s⁻¹ and energy and power density of 35 Wh kg⁻¹ and 200 W kg⁻¹ with 95% capacitance retention over 5000 cycles. Also, Liu et al. adopted the calcination-vulcanization method to prepare Cu₉S₈@C for the first time [143]. Then, they deposited it onto a carbon fiber cloth. Furthermore, they prepared polypyrrole/Cu₉S₈@C-CC nanocomposite-based electrodes through the electrochemical deposition method and used for SC application. The specific capacitance of PPy/Cu₉S₈@C-CC electrodes was found to be 270.72 F g⁻¹ at 10 mV s⁻¹ with 80.36% of capacitance retention after 3000 cycles. Zhou et al. synthesized porous Cu_7.2S₄ sub-microspheres through ion-exchange reaction, which displayed a specific capacitance of 491.5 F g⁻¹ at 1 A g⁻¹ and 82% of capacitance retention even after 1000 cycles [144]. Xu et al. prepared Cu_1.92S nanorod accompanying CuS nanoribbons grown on copper foam and as electrode materials for asymmetric supercapacitor application [145]. It showed a high energy and power density of 35 Wh kg⁻¹ and 266 W kg⁻¹ with 88% of capacitance retention after 5000 charge–discharge cycles. Wang et al. prepared several kinds of copper sulfides, namely CuS, Cu₇S₄, and Cu₉S₅, through the liquid phase synthesis process [146]. They reported that the snowflake-like morphology of Cu₇S₄ offers a special path for the diffusion of ions. It exhibits a high specific capacitance of 1303.01 F g⁻¹ at 5 A g⁻¹ with 98.84% of capacitance retention after 1000 cycles.

Copper selenides (Cu₂Se) is a p-type semiconducting material. Copper selenides are binary composites and can be produced in many stoichiometric arrangements as reported in various forms like CuSe, CuSe₂, Cu₂Se, Cu₃Se₂, Cu₅Se₄, and Cu₇Se₄ as well as non-stoichiometric compositions like Cu_2-xSe. For Cu_2-xSe, Se atoms are placed in face-centered cubic positions, while Cu ions are placed in superionic states. Copper selenide is prepared in several phases such as monoclinic, cubic, tetragonal, and hexagonal. Several researchers have reported the various syntheses of zero-dimensional, one-dimensional, and two-dimensional Cu₂Se nanomaterials through several methods such as hydrothermal, solvothermal, hot injection technique, liquid phase deposition, and sonochemical method. It has advantages including multiple oxidation states and high electrical conductivity than metal oxides, which could offer better electrochemical properties. Thus, copper selenide is widely used in the application of gas sensors, catalysts, thermoelectric devices, rechargeable lithium, and sodium batteries, but only a few reports are available for copper selenides towards energy storage applications. Pazhamalai et al., using a straightforward hydrothermal technique, created hierarchical CuSe₂ nanoneedles synthesized on Cu foil and tested their electrochemical properties to serve as a binder-free electrode for supercapacitor applications [147]. The galvanostatic charge–discharge technique displayed that CuSe₂ nanoneedles/Cu electrode displayed a high specific capacitance of 1037.5 F g⁻¹ at 0.25 mA. cm⁻². Also, since morphology is a key factor in designing nanomaterials with controlled functional properties, Shinde et al. demonstrated the supercapacitor application of Cu₂Se electrodes with various morphologies through electrodeposition method [148]. The well-designed morphologies of Cu₂Se nanostructures have been used for fabricating supercapacitor devices. Among various morphologies, the nanodendrite-like morphology of Cu₂Se exhibits a maximum specific capacitance of 688 F g⁻¹ at 5 mV s⁻¹. To enhance the intrinsic conductivity of metal chalcogenides and to reduce the dissolution of selenide species during cycling, the only suitable approach is to prepare nanometer-sized materials with conductive additives. On this concern, Jin et al. prepared 1D CNTs@C with Cu_2-xSe nanospheres through a facile solvothermal method and investigated as an electrode material for supercapacitor (Fig. 13) [149]. It exhibits better specific capacitance of 302.7 F g⁻¹ at a constant current density of 1 A g⁻¹ with opening capacitance retained at 86.9% over 2000 cycles. The summary on copper chalcogenide–based electrode materials with various synthesis methods and their supercapacitor performances are given in Table 3.

Fig. 13

Electrochemical performance of various copper chalcogenides and its composites, images are reproduced with permission from ref. [124] and [150] under Copyright © 2016 Elsevier B.V

Table 3 Summary of copper chalcogenide–based electrode materials with various synthesis methods and their supercapacitor performances

4.9.4 Cobalt chalcogenides

Cobalt sulfide is one of the semiconducting TMCs with multiple oxidation states for high electronic conduction and easy charge transfer. It acts as an excellent electrode material for energy storage devices owing to inexpensive, good electrochemical performance, and environmentally friendly nature. It exists in various phases like CoS, CoS₂, Co₃S₄, Co₉S₈, and CoS_1.097, respectively. Each phase of cobalt sulfide has its own merits. Various studies on CoS₂ denote that CoS₂ micro/nanostructures with several structural morphologies like octahedrons, hollow sphere, hierarchical mesoporous microsphere, ellipsoids, worm-like, nanocomposites, and nanocubes, which improve the electrochemical behavior of CoS₂ for energy storage devices. Many synthesis methods like hydrothermal, microwave assisted, solid-phase reaction, solvothermal, and ion-exchange reaction were studied so far. Kumar et al. prepared CoS electrodes through a facile chemical bath deposition method with various solvent on nickel foam [167]. When utilized as electrode material for high-performance supercapacitor, CoS prepared with ethanol solvent exhibits a high specific capacitance of 41.36 F g⁻¹ at 1.5 A g⁻¹ with excellent cycling permanency and rate performance. Zhang et al. synthesized cobalt disulfide (CoS₂) nanodendrites through a one-step solvothermal method, which delivered a maximum specific capacitance of 311.06 F g⁻¹ at 1 A g⁻¹ with 80.22% of initial capacitance preservation after 3000 charge–discharge cycles [168]. Amaresh and co-workers fabricated cubic CoS₂ nanoparticles based supercapacitor through single-step microwave-mediated method [169]. The specific capacitance of phase pure cubic CoS₂ nanoparticles was found to be 52 F g⁻¹ at 0.7 A g⁻¹ with 80% initial capacitance retaining even after 10,000 cycles. Liu et al. successfully prepared hierarchical cobalt sulfide/cobalt basic salt nanocomposite using a vapor-phase hydrothermal method for the application of supercapacitor [170]. It yields a high specific capacitance of 1984 F g⁻¹ at 1 A g⁻¹, excellent rate capability of 78.6% capacitance retention, and cycle stability of 90.2% of its initial capacitance maintained even after 5000 cycles.

Chen et al. prepared Co₃S₄ nanosheet arrays through in situ shape and phase transformation synthesis and used as efficient electrode material for high-performance supercapacitors [171]. It showed a specific capacitance of 1081 F g⁻¹ at 1.61 A g⁻¹, areal capacitance of 2.69 F. cm⁻² and 2.37 F. cm⁻² at 4.12 mA. cm⁻² with the primary capacitance holding at 96.2% after 3000 cycles. Aloqayli et al. prepared cobalt sulfide (Co₉S₈) for flexible, durable, and high-performance supercapacitors via a facile technique [172]. It showed a high specific capacitance of 7358 mF. cm⁻² with excellent cycle stability after 5000 cycles. Ghosh et al. prepared hierarchical Co₃S₄ on reduced graphene oxide hydrogel@Ni foam through the hydrothermal method and utilized it as electrode material for the fabrication of the supercapacitor. The aqueous asymmetric supercapacitor fabricated with the Ni@rGO-Co₃S₄ electrode displayed a specific capacitance of 1369 F g⁻¹ at 1.5 A g⁻¹ and holds 96.6% of the initial capacitance after 3000 cycles at even higher current density of 12 A g⁻¹ [173].

Liu and co-workers for the first time used a one-step solvothermal process to create new networked ultralong CoS_1.097 nanotubes, which they used as an active material for supercapacitor applications [174]. It showed a maximum specific capacitance of 764 F g⁻¹ at 2 A g⁻¹ and 85% of original capacitance retaining after 500 cycles. Cao et al. synthesized 2D CoS_1.097/N-doped carbon nanocomposite through a facile method for the first time [175]. The supercapacitor developed with CoS_1.097/N-doped carbon yields a high specific capacitance of 360.1 F g⁻¹ at 1.5 A g⁻¹ with high-rate capability and recollects 56.8% of its initial capacitance at a current density of 1.5 A g⁻¹. Chang et al. utilized a facile synthesis for the preparation of Co₉S₈/Ni₃S₂ double-size nanoparticles decorated on rGO [176]. The supercapacitor was assembled with Co₉S₈/Ni₃S₂/rGO-based electrode material, which exhibits a maximum specific capacitance of 1929.1 F g⁻¹ at 1 A g⁻¹ with 92.8% of retained capacitance after 1000 cycles at a higher current density of 10 A g⁻¹. Zhu et al. prepared hexagonal prism-like hierarchical Co₉S₈@Ni(OH)₂ core–shell nanotubes on carbon fibers through electrodeposition process [177]. The high-performance supercapacitor fabricated with Co₉S₈@Ni(OH)₂ electrode possesses a high specific capacitance of 149.44 mAh/g at 1 A g⁻¹ with initial capacitance engaged at 97.3% after 5000 cycles. Han et al. designed a supercapacitor based on a porous Co₉S₈ nanosheet array on nickel foam through a facile synthesis [178]. It exhibits a good electrochemical performance with a specific capacitance of 1098.8 F g⁻¹ at 0.5 A g⁻¹ and holds 87.4% of its initial capacitance of the first cycle even after 1000 cycles and has a good rate performance of 54.6% at 10 A g⁻¹.

Cobalt selenide (CoSe) has semiconducting nature with a lower optical band gap. Among Co-based compounds, CoSe has potentially outstanding electrical conductivity owing to its metallic property of Se. Also, compared with the sulfide and oxides, selenium possesses lower electronegativity with a larger ionic radius. The electron in the outermost orbital of cobalt has a weak attraction with Se; hence, the weakly bound electrons offer electroactive reaction sites for redox reaction and thereby enhance the overall kinetics of the electrochemical reaction. Additionally, CoSe₂ has been proved to possess excellent electrochemical activity and studied to be a negative electrode for supercapacitor. Consequently, it is expected that CoSe₂ could be used as positive material for supercapacitor application. However, the study on CoSe₂ for SC positive electrode is worth exploring and rarely reported. Chen et al. synthesized bifunctional bamboo-like CoSe₂ arrays through thermal annealing technique for high-performance supercapacitor [179]. It obtained a specific capacitance of 544.6 F g⁻¹ at 1 mA. cm⁻². The fabricated ACS based on the CoSe₂ electrode possesses an energy and power density of 20.2 Wh kg⁻¹ and 144.1 W kg⁻¹ at 10 mA. cm⁻². Bose et al. prepared Co₃Se₄ nanosheets embedded on N-CNT as an electrode active material for supercapacitor through pyrolysis and solvothermal method, which exhibits a specific capacitance of 114 F g⁻¹ at 2 mV s⁻¹ with 96% of its initial capacitance obtained even after 5000 cycles [180]. Peng and co-workers, using a straightforward low-temperature solvothermal technique, created an asymmetric supercapacitor constructed on nanosheets of cobalt selenide that resemble petal-like morphology [181]. It yields an energy and power density of 21.1 Wh kg⁻¹ and 400 W kg⁻¹, and after 5000 cycles, the initial capacitance retained at 93.8%.

Kirubasankar et al. hydrothermally prepared in situ grown CoSe onto graphene nanosheets as electrode material for ASC [182]. The CoSe-G nanohybrid electrode presented a high specific capacitance of 1037 F g⁻¹ at 5 mV s⁻¹. The asymmetric supercapacitor fabricated with the CoSe-G electrode possesses an energy and power density of 45.5 Wh kg⁻¹ and 1.1 kW kg⁻¹ and recollects 81% of its initial capacitance after 5000 cycles. Zhang et al. successfully prepared ASC based on porous cobalt selenide thin films (CoSe) through electrodeposition method, which yields a specific capacitance of 510 F g⁻¹ at 1 A g⁻¹ and retains 91% of its original capacitance over 5000 cycles [183]. Zhao et al. synthesized tremelliform Co_0.85Se nanosheets through a simple solvothermal technique and utilized them as an electrode active material for supercapacitor [184]. It yields an energy and power density of 17.8 Wh kg⁻¹ and 3.57 kW kg⁻¹, and after 2000 charge–discharge cycles, the primary capacitance of the material remains 93%. Zhang et al. employed self-templated technique to fabricate a high-performance supercapacitor based on N-doped CoSe₂/C double-shelled dodecahedral [185]. It exhibits a specific capacitance of 658 F g⁻¹ at 2 A g⁻¹ and retains 62.6% of its initial capacitance after 2000 charge–discharge cycles.

Due to good thermal, electrical, and magnetic properties, cobalt telluride (CoTe) has received much attention in the field of electrocatalysis, solar cells, photocatalysis, batteries, biosensors, and water splitting. The nanostructures CoTe have revealed several applications, especially in energy generation and storage. For the preparation of two-dimensional CoTe-based material, high temperature and reducing atmosphere are required, which makes it difficult and expensive for the large-scale production and fabrication of supercapacitors. Hence, for the first time, Manikandan et al. synthesized cobalt telluride through a hydrothermal route and used it as electrode material for the fabrication of SC [186]. It showed a high specific capacitance of 170 C/g at 0.5 A g⁻¹. The fabricated CoTe-based supercapacitor displayed an energy and power density of 40.7 Wh kg⁻¹ and 800 W kg⁻¹ at 1 A g⁻¹ with capacitance holding at 85% over 10,000 cycles. Xiao et al. successfully synthesized highly dispersed CoTe electro through a one-step solvothermal route [187]. The synthesized regular CoTe nanowire electrode material exhibits a specific capacitance of 643.6 F g⁻¹ at 1 A g⁻¹ with 76.9% retention of its initial capacitance after 5000 cycles. The asymmetric supercapacitor-based CoTe nanowire displayed an energy and power density of 32.9 Wh kg⁻¹ and 800.27 W kg⁻¹ at 1 A g⁻¹, and after 5000 charge–discharge, it exhibits 90.5% of capacitance retention, showing good cycle performance. Mao et al. prepared CoTe₂ nanoflowers through a facile solvothermal method for supercapacitor applications [188]. The electrochemical performance of CoTe₂-based supercapacitor exhibits a high specific capacitance of 460 F g⁻¹ at 1.5 A g⁻¹ with 91% of its initial capacitance holding over 5000 cycles. Bhat and co-workers prepared CoTe₂ nanomaterial via the anion-exchange reaction method and utilized as electrode material for the application of supercapacitor, stating an electrochemical capacitance of 360 F g⁻¹. Ye et al. improved the supercapacitor performance with CoTe//AC electrode material through a facile hydrothermal route [189]. It displayed a maximum specific capacitance of 622.8 F g⁻¹ at 1 A g⁻¹. The summary of various methods for the synthesis of cobalt chalcogenides based electrodes and their supercapacitor performances are given in Table 4.

Table 4 Summary of cobalt chalcogenide–based electrode materials with various synthesis methods and their supercapacitor performances

4.9.5 Nickel chalcogenides

Among the family of transition metal sulfides, NiS-based material earned much attention owing to its low cost, simple fabrication, high electrical conductivity, and low toxicity nature. It exists in various stoichiometric forms and thermodynamically stable crystal structures like NiS, NiS₂, Ni₃S₂, Ni₃S₄, Ni₆S₅, Ni₇S₆, and Ni₉Se₈. Till now, many studies reported the use of nickel sulfide–based supercapacitor. But, the practical applications of nickel sulfides are hesitated because of their poor cycling stability and rate performance. To enhance the performance, designing, fabricating hierarchical nanostructures, compositing pseudocapacitive materials, and preparing materials directly on the current collectors are the main strategies. Du et al. prepared nanosheet-assembled hollow micro- and nanostructure NiS with different shapes like ellipsoid shaped, cube shaped, and capsule shaped through various morphological α-Fe₂O₃ templates [235]. Among these, the capsule-shaped NiS-based electrode yields a maximum specific capacitance of 1159 F g⁻¹ at 2 A g⁻¹ current density. The assembled ASC device with capsule-shaped NiS//rGO@Fe₃O₄ exhibits high energy and power density of 43.7 Wh kg⁻¹ and 664 W kg⁻¹ with 83.3% of initial capacitance retention even after 5000 charge–discharge cycles. Gaikar et al. worked on the growth of interconnected nanorods and nanoplates of NiS on the Ti substrate via a simple chemical bath deposition method and used it as electrode material for SCs [236]. It yields a high specific capacitance of 788 F g⁻¹ at 1 mA. cm⁻² current density with a better rate capability of 640 F g⁻¹ at 50 mA. cm⁻², and 98% of the initial capacitance was retained after 1000 cycles. The assembled NiS electrode-based supercapacitor exhibits an energy and power density of 27.4 Wh kg⁻¹ and 3.05 kW kg⁻¹, respectively. Fu et al. synthesized a novel honeycomb-like Ni₃S₂ nanosheet array electrode through a facile synthesis for supercapacitor applications [237]. It displayed a high specific capacity of 151.2 mAh/g at 3 A g⁻¹ with excellent cycling stability and rate performance. Akbarzadeh et al. reported the electrochemical performance of NiS₂ nanocubes prepared by using a facile solvothermal method for high-performance supercapacitor [238]. It yields a high specific capacitance of 2077 F g⁻¹ at a current density of 0.65 A g⁻¹ with excellent cycling stability and rate performance. Gou et al. synthesized a hollow sphere of NiS₂ by employing a two-step hydrothermal method and castoff as electrode material for the supercapacitor [239]. It achieves a high specific capacitance of 1382.0 F g⁻¹ at 1 A g⁻¹ and 506.1 F g⁻¹ at 20 A g⁻¹ of current density. It retained the specific capacitance of 451.1 F g⁻¹ after 5000 charge–discharge cycles at 10 A g⁻¹. For the first time, Li et al. successfully prepared Ni₇S₆ hollow spheres with mesoporous shells by a facile hydrothermal method and employed as an electrode material for supercapacitor [240]. It exhibits a high specific capacitance of 2283.2 F g⁻¹ at a current density of 1 A g⁻¹ with 97% of capacitance retention after 1000 cycles.

Recently, a combination of various synthesis methods appears to be a significant methodology to prepare nanophase materials with different shapes, sizes, etc. Using this perception, Nandhini et al. used the combination of microwave and hydrothermal methods for the preparation of NiS nanostructures and investigated their electrochemical performance for the fabrication of supercapacitors [241]. The result displayed that the NiS-based electrode exhibits a specific capacitance of 964 F g⁻¹ at 1 A g⁻¹ current density with undiminished capacitance retention after 2000 cycles. Cheng et al. studied a facile electrode fabricated using ion implantation and hydrothermal sulfurization method for the synthesis of spicules-like Ni₃S₂ shell grown on Mo nanoparticle-doped Ni foam, which displayed a special hierarchical structure [242]. It employed as electrode material for the application of supercapacitor showing a specific capacitance of 361 C/g at 1 mA. cm⁻², and 168% of original capacitance was obtained after 2000 cycles at a current density of 20 mA. cm⁻².

Compared with a single component, sulfide, the composite of binary metal sulfides, can provide rich active sites for the redox reaction and thereby increase the performance of the electrochemical reaction. Chang et al. prepared the composite of Ni₃S₂ and Co₉S₈ NPs decorated on rGO through a facile synthesis and utilized as electrode material for a high-performance supercapacitor. It showed a high specific capacitance of 1929 F g⁻¹ at 1 A g⁻¹ of current density which is much higher than that of Co-Ni₃Se₂ (1075.5 F g⁻¹)-based electrode. It exhibits a high-rate performance of 1669.2 F g⁻¹ at 20 A g⁻¹ with initial capacitance retention of 92.8% after 1000 cycles. Also, Gao et al. prepared novel amorphous Ni_xS_y@CoS double-shelled polyhedral nanocages through a simple facile process for the application of supercapacitor [243]. At a current density of 2 A g⁻¹, it exhibits a remarkably high specific capacitance of 2091 F g⁻¹ with long-term cycle stability and excellent rate performance. Ghosh et al. prepared hierarchical Ni₃S₄ on reduced graphene oxide hydrogel@Ni foam through the hydrothermal method and utilized as electrode material for the fabrication of supercapacitor [173]. The aqueous asymmetric supercapacitor fabricated with the Ni@rGO-Ni₃S₄ electrode displayed a specific capacitance of 987.9 F g⁻¹ at 1.5 A g⁻¹ and retains 97.9% of primary capacitance after 3000 cycles at a current density of 12 A g⁻¹. Chen et al. fabricated a high-performance supercapacitor with interconnected 2D/3D NiS/Ni₃S₄ composite using a one-pot hydrothermal method [244]. The 2D/3D/NiS/Ni₃S₄ composite at a current density of 1 A g⁻¹ exhibits a maximum specific capacitance 1796 F g⁻¹ with an original capacitance holding at 80.5% after 1000 charge–discharge cycles. Li et al. prepared 3D Ni₉S₈ nanorods embedded in oxygen incorporated MoS₂ nanosheets on carbon cloth as an efficient electrode material for supercapacitor [245]. It exhibits a high specific capacitance of 907 F g⁻¹ at 2 A g⁻¹ with excellent cycling stability after 1200 cycles owing to its unique mutual embedding 3D nanostructure [245].

As we know, nickel and selenium have the same electronic configuration and nearby electronegative value (Ni = 1.9, Se = 2.4). Nickel selenide can occur in various compositions like NiSe, Ni_0.85Se, NiSe₂, and Ni₃Se₂ at room temperature. The intrinsic metallization of nickel selenide provides high electrical conductivity, and hence, it acts as an ideal electrode material. Gu et al. prepared a novel NiSe₂ nanoarray supported on nickel foam by using in situ hydrothermal methods and used as an efficient electrode material for flexible hybrid supercapacitor [246]. The electrochemical behavior of NiSe₂ nanoarray-based electrode exhibits a high specific capacitance (262 mAh/g) with an energy density of 33 Wh kg⁻¹ and cycle retention of 90.3% of its initial capacitance over 5000 charge–discharge cycles. Li et al. synthesized nickel selenide thin films with different phases through a facile electrodeposition technique and utilized as electrode material for the fabrication of asymmetric supercapacitor [247]. The result obtained shows that the specific capacitance of Ni₃Se₂ and NiSe was found to be 581.1 F g⁻¹ at 1 A g⁻¹ and 1644.7 F g⁻¹ at 2 A g⁻¹ current density, respectively. These values are much comparable to those of transition metal oxides and higher than the carbonaceous materials. The cycle stability performance of Ni₃Se₂ and NiSe thin films is 90.1% and 75.0% of initial capacitance have been achieved even after 10,000 cycles. The ASC device based on the NiSe//AC device attained an energy and power density of 0.36 mWh cm^-3 and 33.35 mW cm^-3, respectively. Yu et al. prepared a network of porous ultrathin NiSe nanosheets supported on Ni foam for the fabrication of a high-performance hybrid supercapacitor [248]. The inherent nature of NiSe attained the highest specific capacity of 443 mAh/g at a current density 3 A g⁻¹. The fabricated NiSe nanosheet network/porous carbon-based hybrid supercapacitor achieved a high energy and power density of 66.6 Wh kg⁻¹ and 425 W kg⁻¹ with good rate performance and cycling stability. Du et al. prepared honeycomb-like metallic Ni_0.85Se nanosheet through a one-step hydrothermal approach [249]. The specific capacitance of Ni_0.85Se was found to be 3105 F g⁻¹ at 1 A g⁻¹ with capacitance retention of 90.1% after 5000 cycles.

Very recently, Eu et al. studied the electrochemical performance of Ni₃Se₂ grown on Ni foam through a simple one-step hydrothermal method with different morphologies like irregular film, nanowire array, and microspheres [250]. The result displayed that the electrode of Ni₃Se₂@Ni with irregular film, nanowire array, and microsphere exhibits high specific capacitance of 504 F g⁻¹, 592 F g⁻¹, and 816 F g⁻¹ at a current density of 10 mA. cm⁻². Among different morphologies, the Ni₃Se₂@Ni electrode with microsphere morphology exhibits excellent electrochemical performance for the supercapacitor application and retains 85.5% of its initial capacitance after 1000 cycles.

To further enhance the electrochemical performance of a selenide-based electrode material, only vital strategy is to dope using pseudocapacitive metal ion, which may lead to a greater abundance in the redox reaction and thereby raise the specific capacitance and improve the electrochemical performances owing to the advantage of doped metal ions and the synergistic effect of host ion and the doped metal ions. Also, it provides a free diffusion path for the fast transportation of ions and facile ion accessibility to storage sites. Gu et al. hydrothermally prepared Co-doped NiSe₂ nanowire for the fabrication of high performance of asymmetric supercapacitor [251]. The Co@NiSe₂-based electrode material delivers a high specific capacitance of 3167.6 F g⁻¹ at 1 A g⁻¹ current density. The asymmetric supercapacitor based on the Co@NiSe₂ electrode attained an energy and power density of 50.0 Wh kg⁻¹ and 779.0 W kg⁻¹ with excellent rate performance and cycle stability. Arul et al. fabricated supercapacitor device based on NiSe₂/Ni(OH)₂ nanocomposite electrode through a facile hydrothermal method followed by ultrasonication process [252]. It attained a high specific capacitance of 2212 F g⁻¹ at 2 mA. cm⁻², which is higher than the pure NiSe₂ (326 F g⁻¹) at the same current density with a capacitance retention of 95% over 5000 cycles. Gu et al. successfully prepared nitrogen-doped rGO with NiSe₂ nanoparticles for a high-performance supercapacitor via a two-step process with a combination of the hydrothermal and solvothermal methods [253]. At a current density of 1 A g⁻¹, the as-prepared N-rGO/NiSe₂ electrode displayed a high specific capacitance of 2451.4 F g⁻¹. The assembled N-rGO/NiSe₂ electrode-based ASC device exhibits an energy density of 40.5 Wh kg⁻¹ at a power density of 841.5 W kg⁻¹ with good cycling stability and rate performance. Jiang et al. used hydrothermally synthesized Ni₃Se₂ nanosheets grown on Ni foam as electrode material for supercapacitor [254]. The as-prepared Ni₃Se₂/Ni electrode delivered a high specific capacitance of 854 F g⁻¹ at a current density of 1 A g⁻¹. The device based on Ni₃Se₂/Ni//AC ASC exhibits an energy density of 23.3 Wh kg⁻¹ at a power density of 398.1 W kg⁻¹ with initial capacitance sustained by 91.11% after 5000 cycles. Subramania et al. prepared in situ grown NiSe-G nanohybrid through a hydrothermal process and utilized as an electrode material for the application of asymmetric supercapacitor [255]. The result has shown that the NiSe-G nanohybrid electrode possesses a high specific capacitance of 1280 F g⁻¹ at 1 A g⁻¹ with 98% capacitance retention after 2500 cycles. The fabricated NiSe-G electrode-based ASC delivers an energy and power density of 50.1 Wh kg⁻¹ and 816 W kg⁻¹, respectively. Peng et al. prepared NiSe@MoSe₂ nanosheet array through a facile one-step hydrothermal method and used as electrode material for ASC [256]. The prepared NiSe@MoSe2-based electrode delivered a specific capacitance of 223 F g⁻¹ at 1 A g⁻¹. The fabricated asymmetric supercapacitor maintained the initial capacitance by 91.4% even after 5000 cycles and also yields an energy and power density of 32.6 Wh kg⁻¹ and 415 W kg⁻¹.

Owing to the high surface area, smaller ionization energy, high mechanical stability, increasing redox-active structures, and high electrical conductivity (Te, 2 × 10² S/m), nickel telluride (NiTe) has influenced the applications of energy storage devices. Manikandan et al. have successfully prepared NiTe nanorods through the hydrothermal method by using CTAB as a surfactant and ascorbic acid as a reducing agent [257]. NiTe-based electrode material exhibits a specific capacitance of 618 F g⁻¹ at 1 A g⁻¹ of current density and retains 75% of its initial capacitance after 5000 charge–discharge cycles. During this process, it exhibits a coulombic efficiency of 99% which indicates that NiTe-based electrode material has good reversibility for the supercapacitor applications. Pei Zhou et al. used a facile hydrothermal method for the preparation of NiTe rods grown on Ni foam and used as positive electrode material for the fabrication of asymmetric supercapacitors [258]. It delivered a high specific capacitance of 804 F g⁻¹ at 1 A g⁻¹ with remarkable cycling stability 81% of capacitance retention after 3000 cycles.

Additionally, doping is the vital approach to enhance the electrochemical performance of materials, since the crystal structure, electrical behavior, and conductivity of the materials could be altered after doping of different metal and non-metal ions. Based on this concern, Ye et al. synthesized Co ion-doped NiTe supported on Ni foam through a one-step hydrothermal method and utilized as electrode material for supercapacitor. The result reveals that the specific capacitance of the NiTe:Co electrode is significantly improved, and the highest specific capacitance is 1645.6 F g⁻¹ at 1 A g⁻¹ which is higher than the pristine NiTe-based electrode (872.7 F g⁻¹). Furthermore, the supercapacitor was fabricated with NiTe:Co//AC, which delivers an energy density of 36.8 Wh kg⁻¹ than NiTe//AC-based supercapacitor (24.4 Wh kg⁻¹) at same power density. Deshagani et al. changed the crystal structure of NiTe by doping selenide by using a facile hydrothermal method [259]. The prepared material exhibits a specific capacitance of 943 F g⁻¹ and compared with the NiTe-based electrode shows 1.5 times. Ye et al. used a one-step hydrothermal method for the preparation of Se-doped NiTe electrode materials for SC [260]. The result shows that the Se-doped NiTe electrode exhibits a high specific capacitance of 998.2 F g⁻¹ at 1 A g⁻¹ which is much better than the specific capacitance of NiTe electrode (603.6 F g⁻¹). Furthermore, the ASC based on Se-doped NiTe electrode delivered a superior energy and power density of 42.7 Wh kg⁻¹ and 800.6 W kg⁻¹ at 1 A g⁻¹ with remarkable cycling stability of 76.4% retention after 10,000 cycles.

Also, Ye et al. fabricated a high-performance asymmetric supercapacitor based on NiTe/NiSe composites in situ grown on Ni foam [261]. The NiTe/NiSe-based electrode delivered a high specific capacitance of 1868 F g⁻¹ at 1 A g⁻¹. The fabricated ASC based on NiTe/NiSe//AC device showed a high energy and power density of 33.7 Wh kg⁻¹ and 800 W kg⁻¹ with good cycling stability of 86.2% of its initial capacitance after 5000 cycles at 2 A g⁻¹. The summary of various synthesis approaches of nickel chalcogenide–based electrode materials and their supercapacitor performances are shown in Table 5.

Table 5 Summary of nickel chalcogenide–based electrode materials with various synthesis methods and their supercapacitor performances

4.9.6 Molybdenum chalcogenides

Among several pseudocapacitive dichalcogenide materials, nanostructured MoS₂ could be employed as the most auspicious two-dimensional material for the application of supercapacitors. Molybdenum disulfide (MoS₂) is the first TMDCs used as electrode material in lithium-ion batteries since it has high energy and current density, and also, it possesses high intrinsic ionic conductivity and high redox properties. MoS₂ has a two-dimensional layered structure like graphene which offers an extended surface area for the storage of charges and possesses high theoretical capacity than the graphite material. The charge storage in nanostructured MoS₂ happens through the diffusion of electrolyte ions into the inter-layer or double-layer charging on the external surface. Manuraj et al. fabricated high-performance supercapacitors with MoS₂ nanostructures grown Ni foam substrates through a simple hydrothermal process at various times. The nanostructured MoS₂ electrode exhibits a specific capacitance of 244 F g⁻¹ at 1 mV. s⁻¹ with corresponding capacitance retention of 92% after 9000 charge–discharge cycles. Choudhary et al. used a direct magnetron sputtering method to develop MoS₂ thin-film supercapacitor electrodes. The 3D MoS₂ film delivered a specific capacitance of ~ 330 F. cm⁻³ with a retention of 97% of its primary capacitance after 5000 cycles [334]. Huang et al. worked on the hydrothermal synthesis of MoS₂ nanosheets as an electrode for a supercapacitor, which displayed a specific capacitance of 129.2 F g⁻¹ at 1 A g⁻¹ withholding 85.1% of its initial capacitance after 500 cycles [335]. Karade et al. prepared ultrathin MoS₂ nanoflake electrodes through a chemical bath deposition method for the fabrication of high-performance supercapacitors [336]. At 5 mV. s⁻¹, the homogeneous ultrathin MoS₂ nanoflake electrodes achieved a specific capacitance of 576 F g⁻¹ at a current density, and after 3000 cycles, it sustained with the initial capacitance by 82%. Li et al. reported the electrochemical performance of transparent 1 T-MoS₂ nanofilm through layer-by-layer self-assembly technique for high-performance supercapacitors. It exhibits a volumetric capacitance of 220 F cm⁻³ at 0.04 mA. cm⁻² and obtained 130.6% of the original capacitance after 5000 cycles [337].

Although MoS₂ has many merits, its lower capacitance and low cycle life result in lower electrical conductivity. To overcome these demerits, doping of TMDCS with carbonaceous or any other pseudo-active materials is the only known strategy. Gupta et al. prepared two-dimensional MoS₂ and activated functionalized CNT hybrid through a one-step hydrothermal method and employed as electrode material for supercapacitor [338]. At a current density of 0.5 A g⁻¹, it exhibits a high specific capacitance of 516 F g⁻¹ with a respective energy density of 71.76 Wh kg⁻¹. Li et al. synthesized vertical MoS₂ on rGO nanosheets through the hydrothermal method, which delivered a specific capacitance of 331 F g⁻¹ at 0.75 A g⁻¹ with good cycling retention of 110.7% of its initial capacitance even after 15,000 cycles [339]. The assembled ASC with the MoS₂/rGO electrode achieved an energy density of 29.2 Wh kg⁻¹ and a power density of 4517.7 W kg⁻¹. Ali et al. worked on a mechanically exfoliated MoS₂ sheet coupled with conductive PANI as an electrode material for the supercapacitor which exhibits a specific capacitance of 510.12 F g⁻¹ at a current density of 1 A g⁻¹ with corresponding capacitance retention of ~ 80% after 2500 cycles [340]. Bai et al. synthesized MoS₂/rGO/PANI through a facile two-step approach involving hydrothermal and in situ polymerization method [341]. The MoS₂/rGO/PANI composites achieved a high specific capacitance of 570 F g at a current density of 1 A g⁻¹ with corresponding capacitance retention of 78.6% after 500 cycles. Chanda et al. prepared hierarchical heterostructure of MoS₂ flake anchored on the TiO₂ sphere as an electrode for SCs, which achieved a specific capacitance of 152.22 F g⁻¹ at 0.1 A g⁻¹ current density [342].

Chang et al. synthesized MoS₂/PPy nanocomposite via hydrothermal approach and employed as an electrode material for the application of supercapacitor [343]. The MoS₂/PPy electrode material delivered a specific capacitance of 307.5 F g⁻¹ which is much higher than the pristine MoS₂ (138.5 F g⁻¹) and polypyrrole (106.3 F g⁻¹) at a current density of 1 A g⁻¹ with excellent cycling stability of 96.47% after 1000 charge–discharge cycles. For the first time, Chao et al. synthesized oxygen-incorporated MoS₂/PANI/rGO hierarchical nanosheet composite through oxygen incorporation and polyaniline intercalation method and used it as an efficient electrode material for the fabrication of SCs [344]. At a current density of 1 A g⁻¹, MoS₂/PANI/rGO hierarchical nanosheet composite exhibits a specific capacitance of 752.0 F g⁻¹ using a three-electrode system. Fan et al. reported the electrochemical performance of mesoporous MoS₂/C composite through a facile hydrothermal method and employed it as an electrode for SC. The specific capacitance of MoS₂/C composite was found to be 201.4 F g⁻¹ at 0.2 A g⁻¹ with excellent cycling stability and rate performance, which is much higher than that of pristine MoS₂ and carbon [345]. For the first time, Niu et al. synthesized Mo₂S₃@Ni₃S₂ nanowires on a nickel foam through a simple CTAB-assisted hydrothermal method [346]. The Mo₂S₃@Ni₃S₂ nanowire electrode unveils a high specific capacitance of 998.9 F g⁻¹ at 1 A g⁻¹ with outstanding retention of 90.55% of its initial capacitance after 650 cycles.

The monolayer MoS₂ is known to have two phases, namely the trigonal prismatic phase and the octahedral phase. The trigonal prismatic phase is labeled as 2H with a space group of D_3h, while the octahedral phase is labeled as 1 T with a space group of O_h. The 2H phase is relatively stable but semiconducting and of poor conductivity, while the 1 T phase is metastable under room temperature but metallic and of better conductivity. Recently, the metallic 1 T phase was reported to be of great advantage for MoS₂ NS-based supercapacitors. But if the higher conductivity of 1 T phase and the higher stability nature of the 2H phase can be hybridized in monolayer MoS₂, both high charge transportation and large specific surface area will be gained which are the most beneficial factors for supercapacitors. Based on this perspective, Jiang et al. synthesized 1 T-2H phase hybridization of monolayer MoS₂ through a chemical exfoliated method. The electrochemical performance of 1 T-2H monolayer MoS₂ was observed to be 366.9 F g⁻¹ at 0.5 A g⁻¹ withholding the stability at 92.2% of its initial capacitance after 1000 cycles [347].

As like graphene, molybdenum chalcogenides, MX (where M = Mo, and X = S, Se, and Te), has a layered structure which arose as one of the most promising candidates for the sensor, phototransistors, catalysis, and energy storage devices owing to their distinctive crystal structures and diverse material properties. These types of chalcogenide materials interact through strong chemical bonds in the molecular layers, while the individual layers interact via weak Van der Waals force of attraction, materializing a graphene-like layered structure. This layered structure is favorable for the insertion and extraction of a variety of ions in the electrolyte. Among various Mo-based chalcogenides, MoSe₂ has received much attention in the field of energy storage devices. MoSe₂ has a layered structure of Se-Mo-Se, with a narrow band gap and interlayer spacing of 0.646 nm. The interlayer spacing of MoSe₂ is much higher than that of MoS₂ (0.615 nm) and graphite (0.335 nm). Gao et al. prepared a sphere-feature MoSe₂ with excellent electrochemical activity through a facile hydrothermal method [348]. The electrochemical performance of MoSe₂ spheres yields a high specific capacitance of 243 F g⁻¹ at 0.5 A g⁻¹ and recollects 90.3% of its initial capacitance over 1000 cycles at a current density of 1 A g⁻¹. For the first time, Aziz and co-workers synthesized a hierarchical nanostructure of orthorhombic Mo₉Se₁₁ through colloidal processing and utilized as an electrode for the application of asymmetric supercapacitor [349]. At a current density of 5 mV. s⁻¹, it yields a specific capacitance of ~ 510 F g⁻¹. The cycle life of the Mo₉Se₁₁ electrode retains about 60% of its initial capacitance after 4000 cycles. Jia et al. used a simple and facile solvothermal method for the preparation of a MoSe₂ microsphere composed of 2D nanosheets, and at a current density of 1 A g⁻¹, it exhibits a specific capacitance of 272 F g⁻¹ [350].

Although MoSe₂ has several advantages, the poor electrical conductivity of MoSe₂ obstructs its electrochemical performance and practical application. Hence, a frequently employed strategy to further raise the electrochemical performance of MoSe₂ includes a novel design of hybrid nanostructures with carbon-based or any other electrically active materials. Balasingam et al. developed a new MoSe₂/rGO composite nanosheet using a simple and straightforward hydrothermal process and used as an electrode material for supercapacitor applications [351]. The MoSe₂/rGO-based electrode delivered a high specific capacitance of 211 F g⁻¹ at a scan rate of 5 mV. s⁻¹ and gained 180% of its primary capacitance over 10,000 cycles. Lately, isoelectronic doping in Mo-based chalcogenides has received considerable attention attributing to the impeding generation of defects and dislocations and ease of alloy formation. In this perspective, Bhat et al. worked on the preparation of tungsten-doped MoSe₂/graphene through a facile hydrothermal method for the supercapacitor. The W-MoSe₂/G electrode possesses a specific capacitance of 248 F g⁻¹ with capacitance retention of 102% after 20,000 cycles. Karade et al. prepared a 2D cryptomelane-like MoSe₂ on MWCNT hybrid film through a “dip and dry” method followed by the CVD technique. The MoSe₂/MWCNT hybrid electrode exhibits a specific capacitance of 232 F g⁻¹ at a current density of 1.4 A g⁻¹ with an outstanding cyclic stability of 93% after 1000 cycles [352]. Huang et al. prepared a porous layered MoSe₂-graphene composite on nickel foam for the application of the high performance of supercapacitor [353]. It showed a high specific capacitance of 1422 F g⁻¹ at 1 A g⁻¹ with retention of 100.7% even after 1500 cycles.

Li et al. synthesized the 3D MoSe₂-acetylene black electrode through a facile hydrothermal method [354]. At a current density of 1 A g⁻¹, the 3D MoSe₂/AB composite shows a high specific capacitance of 2020 F g⁻¹ with good cycling stability of 107.5% after 1500 cycles. He et al. synthesized MoSe₂ nanosheets wrapped on carbon aerogel nanospheres as an efficient electrode material for supercapacitor which showed a specific capacitance of 775.3 C/g with capacitance retention of 98% after 1500 cycles at a current density of 1 A g⁻¹ [355]. Kirubasankar et al. synthesized a 2D-2D MoSe₂/graphene nanohybrid through a sonochemical method and employed as an electrode for supercapacitor [356]. The MoSe₂/graphene nanohybrid shows a maximum specific capacitance of 945 F g⁻¹ at a current density of 1 A g⁻¹. The fabricated asymmetric supercapacitor with MoSe₂/graphene hybrid electrode yields an energy and power density of 26.6 Wh kg⁻¹ and 0.8 kW kg⁻¹ with better cycling stability of 88% of its initial capacitance even after 3000 cycles. Also, Kirubasankar et al. prepared 2D MoSe₂/Ni(OH)₂ nanohybrid through a one-step hydrothermal method [357]. The 2D MoSe₂/Ni(OH)₂ electrode delivered a high specific capacitance of 1175 F g⁻¹ at 1 A g⁻¹ which is much higher than the Ni(OH)₂ nanosheets (933 F g⁻¹) under same current density. The fabricated asymmetric supercapacitor based on 2D MoSe₂/Ni(OH)₂ nanohybrid achieved an energy and power density of 43 Wh kg⁻¹ and 8181 W kg⁻¹ with a retention of 91.6% of its initial capacitance after 5000 charge–discharge cycles. The summary on various synthesis methods of molybdenum-based electrode materials and their supercapacitor performances are shown in Table 6.

Table 6 Summary of molybdenum chalcogenide–based electrode materials with various synthesis methods and their supercapacitor performances

5 Bimetal chalcogenides and its composites

The combination of two different metals will improve the redox chemistry of the electroactive materials and their performance when compared to single-metal sulfides. Binary metal chalcogenides and their composites have been studied vastly owing to their extensive properties like good electrical conductivity, high catalytic activity, and redox potential and low electronegativity. But very recently, bimetal chalcogenides and its composites showed superior electrochemical performance compared to their corresponding counterparts, owing to their multiple oxidation states and high redox properties. Cai et al. developed a composite of honeycomb-like nickel manganese sulfide nanosheet on carbon cloth via a facile two-step approach, which delivers a specific capacitance of 205 mAh/g at 2 mA. cm⁻² [397]. The fabricated supercapacitor exhibits an energy and power density of 27.3 Wh kg⁻¹ and 505.2 W kg⁻¹ with retaining at 75.3% of its initial capacitance after 6000 cycles. Cao et al. used a one-pot approach for the preparation of porous nickel-manganese sulfides with tunable compositions [398]. The optimized nickel-manganese sulfides employed as an electrode exhibits a specific capacitance of 1068 F g⁻¹ at 1 A g⁻¹ current density. Chen et al. studied the electrochemical performance of Co_xMn_3-x sulfides by changing Co/Mn ratios through solvothermal techniques [399]. Among various compositions, Co_2.5Mn_0.5 sulfide showed a high specific capacitance of 289 C/g at 1 A g⁻¹ with outstanding cycling stability of 95.1% after 2000 cycles. Peng et al. reported the electrochemical performance of supercapacitor fabricated with heterostructure cobalt manganese sulfide (CMS) nanoneedle arrays by a low-temperature hydrothermal method [400]. The as-obtained CMS nanoneedles exhibit a specific capacity of 0.53 mAh. cm⁻² at 2 mA. cm⁻² with a cycle life of 93.7% of capacitance retaining after 1500 cycles. Bolagam et al. successfully fabricated the pseudocapacitor based on cobalt ruthenium sulfides through a simple hydrothermal method, which exhibits a specific capacitance of 75 F g⁻¹ at 1 A g⁻¹ with retention of 81% of its initial capacitance after 1000 cycles [401]. Pazhamalai et al. developed an ASC based on copper tungsten sulfide grown on Ni foam binder-free electrodes, which showed an outstanding specific capacitance of 2666.6 F g⁻¹ at a current density of 10 mA. cm⁻² [402]. The electrochemical behavior of copper tungsten sulfides/Ni/graphene possesses a high energy and power density of 48.57 Wh kg⁻¹ and 102 µWh cm⁻² with better cycle life over 10,000 cycles. Du et al. prepared a high-performance hybrid supercapacitor based on nanoporous nickel-copper sulfide/carbon cloth through an anion-exchange reaction, which delivers a high specific capacitance of 936 F g⁻¹ at 1 A g⁻¹ with better rate performance of 76% [403].

Nguyen et al. demonstrated the synthesized procedure of the bimetal selenide system of nickel-vanadium selenide (Ni_xV_3-xSe₄) and nickel–iron selenide (Ni_xFe_3-xSe₄) through a facile and simple hydrothermal method followed by selenization for flexible asymmetric supercapacitors (Fig. 14) [84]. The prepared NiV₂Se₄ and NiFe₂Se₄ electrodes showed a specific capacitance of ~ 329 and 261 mAh/g at a current density of 1 mA. cm⁻² with rate performance of 79.33% and 77.78% and excellent cycling stability of 98.6% and 97.9% after 10,000 cycles, respectively. The fabricated flexible ASC based on NiV₂Se₄//NiFe₂Se₄ electrodes exhibits a high energy and power density of 73.5 Wh kg⁻¹ and 0.733 kW kg⁻¹ with capacitance retention of 96.6% after 10,000 cycles (Fig. 15). Deka et al. fabricated a supercapacitor based on copper-cobalt selenide nanowire-anchored woven carbon fiber, which showed an energy density of 191.64 mWh kg⁻¹ and power density of 36.65 W kg⁻¹ with retention of 77.3% of its initial capacitance [404]. Du et al. used a two-step method involving hydrothermal and cation-exchange process for the preparation of (Ni_0.5Co_0.5)_0.85Se nanosheet arrays which possesses high electrochemical properties. The supercapacitors fabricated with (Ni_0.5Co_0.5)_0.85Se possesses an energy and power density of 70.58 Wh kg⁻¹ and 320.02 W kg⁻¹ with 91.88% capacitance retention after 8000 charge–discharge cycles.

Fig. 14

Schematic representation for the design and fabrication of hierarchical Ni_xV_3-xSe₄ and Ni_xFe_3-xSe₄ nanostructures for solid-state ASCs, images are reproduced with permission from ref. [84] under Copyright © 2019, American Chemical Society

Fig. 15

Electrochemical performance of the NiV₂Se₄//NiFe₂Se₄ ASC device, a schematic illustration of the assembled ASC with NiV₂Se₄ and NiFe₂Se₄ electrodes, b CV curves of the flexible ASC at scan rates from 10 to 100 mV s⁻¹, c GCD curves of the flexible ASC at current densities from 1 to 50 A g⁻¹, d specific capacity values as a function of applied current densities for the flexible ASC, e cycling performance of the flexible ASC exemplified at 20 mA. cm⁻² with 10,000 charge–discharge cycles (the inset shows the first and last ten GCD cycles), and f Ragone plot of the flexible ASC as compared with the reported literature, images are reproduced with permission from ref. [84] under Copyright © 2019, American Chemical Society

Guo et al. used different ratios of Ni and Co for preparing a series of ternary materials through the co-exchange method and fabricated a supercapacitor based on Ni@Ni_0.8Co_0.2Se electrode which exhibits high energy and power density of 17 Wh kg⁻¹ and 1526.8 W kg⁻¹, respectively [405]. Hu et al. designed a series of hierarchical nickel cobalt selenide NPs/NSs through a low-temperature selenization method for fabricating a high-performance supercapacitor [406]. The electrochemical performance of Ni_0.67Co_0.33Se NPs/NSs at a current density of 1 A g⁻¹ exhibited a large specific capacitance of 447 C/g and retains 97% of initial capacitance after 2000 cycles. Quan et al. designed a hierarchical nanostructure of nickel cobalt selenide (Ni_0.33Co_0.67)Se₂ complex with improved electrochemical performance via a facile ion-exchange reaction [407]. The specific capacitance of the optimized complex is observed to be 827.9 F g⁻¹ at a current density of 1 A g⁻¹. Wang et al. explored the improvement of the electrochemical performance of supercapacitors by preparing Ni_0.6Co_0.4Se₂ electrode through a hydrothermal method [408]. At 1 A g⁻¹, it exhibits a superior specific capacitance of 606.6 C/g retaining 91.0% of initial capacitance after 5000 cycles. Xie et al. investigated the electrochemical performance of the Ni_xCo_1-xSe₂ series by fabricating a high-performance asymmetric supercapacitor [409]. At 1 A g⁻¹ current density, it exhibits a high specific capacitance of 1580 F g. The assembled ASC with Ni_0.6Co_0.4Se₂-based electrode exhibits high energy and power density of 44.1 Wh kg⁻¹ and 691.3 W kg⁻¹ with long cycling stability. Cheng et al. designed a novel core–shell structure ZnCo₂S₄ electrode through a solvothermal method, which delivered a specific capacitance of 1045.3 F g⁻¹ at 2 A g⁻¹ and retains 95.5% of its initial capacitance after 5000 charge–discharge cycles [410]. Elshahawy et al. prepared sulphospinel MnCo₂S₄ material through a controlled sulfurization method, which offers an excellent specific capacitance of 938 F g⁻¹ at 20 A g⁻¹ and retains about 95% of its capacitance after 5000 cycles [411]. The assembled hybrid supercapacitor delivered an energy and power densities of 43 Wh kg⁻¹ at 0.801 kW kg⁻¹, respectively. Guo et al. used a straightforward hydrothermal process to create crystalline and amorphous copper-cobalt sulfide, which is then examined as an electrode material for supercapacitor applications [412]. Among various samples, the sample CuCo₂S₄ prepared at 150 °C attained a highest specific capacitance of 515 F g⁻¹ at 1 A g⁻¹ with ~ 93.3% of capacitance retention over 10,000 cycles. Huang et al. works on tip-welded ferric-cobalt sulfide hollow nanoneedles on conductive carbon fibers through a two-step sulfidation technique and researched as electrode material for supercapacitor [413]. It showed a high specific capacitance of 2282 F g⁻¹ at 1 A g⁻¹ with 82.3% of capacitance retention after 5000 cycles. Ai et al. worked on nanostructured CoNi₂S₄ with various morphologies grown on carbon cloth through a facile precursor transformation method by adjusting the anions in nickel and cobalt salts [414]. The as-prepared CoNi₂S₄ electrodes attained a specific capacitance of 2714 F g⁻¹ at a current density of 1 A g⁻¹ and retain long-term cycling stability and excellent rate capability. Beka et al. used a simple two-step hydrothermal method for the preparation of coral-like CoNi₂S₂ grown on NF and researched as supercapacitor electrode material [415]. The as-obtained sample exhibits a high specific capacitance of 2864 F g⁻¹ at 1 A g⁻¹ with extraordinary cycling life of ~ 117% over 10,000 continuous cycles. Liang et al. designed a novel hierarchical core–shell and hollow structure of CoNi₂S₄ using TEOA-assisted hydrothermal method and investigated their electrochemical performance for supercapacitor application [416]. The prepared CoNi₂S₄ nanospheres achieved an ultrahigh specific capacitance of 2035 F g⁻¹ at 1 A g⁻¹ with superior cycling stability of about 91.3% after 3000 cycles. Anthuvan et al. synthesized a rambutan-like cobalt–nickel sulfide (CoNiS₄) through a one-step hydrothermal method for improving the electrochemical performance of supercapacitor [417]. The as-synthesized CoNi₂S₄ sample at 1 A g⁻¹ achieved a specific capacitance of 1102.22 F g⁻¹ with 75% capacitance retention over 3000 cycles.

For the first time, Zhang et al. improved the electrochemical performance of supercapacitors by synthesizing nickel cobalt telluride grown on Ni foam through a simple solvothermal method followed by an ion-exchange reaction [418]. The constructed Ni_0.33Co_0.67Te electrode supercapacitor possesses a specific capacity of 131.2 mAh/g at 1 A g⁻¹ current density with high energy and power density of 54.0 Wh kg⁻¹ and 918 W kg⁻¹ and retains about 90% of initial capacitance over 5000 cycles. Chandrasekaran et al. employed a microwave-assisted approach to synthesize nanostructured tin nickel sulfide (SnNi₂S₄) composite and utilized it as an active material for supercapacitors [419]. The as-synthesized SnNi₂S₄ sample reached a high specific capacitance of 1483.42 F g⁻¹ at 2 A g⁻¹ with excellent retention of 97.34% of initial capacitance after 5000 charge–discharge cycles. Balamurugan et al. prepared a hierarchical copper-nickel sulfide (Cu_1-xNi_xS) nanosheets for improving the performance of asymmetric solid-state supercapacitors using an anion-exchange method, which exhibits a high specific capacitance of 2672 F g⁻¹ at 2 mA. cm⁻² current density [420]. The constructed ASC based on Cu_1-xNi_xS showed high energy of ~ 94.05 Wh kg⁻¹ at a power density of 1.09 kW kg⁻¹ with outstanding cycling stability of 95.86% after 10,000 charge–discharge cycles. Ke et al. designed a high-performance supercapacitor based on a porous, hierarchical structured ammonium nickel molybdate/nickel sulfide/rGO composite electrode prepared by a two-step hydrothermal method [421]. At 1 A g⁻¹, the active material achieved a high specific capacitance of 150 mAh/g with better rate performance and cycling stability. Elkholy et al. reported the electrochemical activity of the ZnMoS₄ electrode-based supercapacitor prepared by a simple solvothermal method [422]. It exhibits a specific capacitance of 280 F g⁻¹ at 0.7 A g⁻¹ with 86.79% of capacitance retention after 1000 cycles. Sahoo et al. first prepared a binder-free electrode material based on copper-molybdenum sulfide on Ni foam, which delivered a specific capacitance of 663 mAh/g with a superior energy density of 23.61 Wh kg⁻¹ and long-term cycling stability. Zhang et al. synthesized a novel potassium copper selenide nanowire (KCu4Se8) through a modified composite-hydroxide mediated (M-CHM) method. The synthesized KCu₄Se₈ NWs with 30 µm of length is studied for the application of solid-state supercapacitor, and their electrochemical performance was tested. It showed a specific capacitance of 25.3 F g⁻¹ at 5 mV s⁻¹ with excellent long-term cycling stability over 5000 cycles [423].

Even though the abovesaid binary TMCs have lots of advantages, meager cycling stability, low specific capacitance, low rate capability, etc. have hindered its practical applications. Generally, composite materials help to raise the performance of electroactive materials. Al Haj et al. synthesized N-doped graphene-encapsulated cobalt iron sulfide indicated as Co₈FeS₈@NG through the in situ hydrothermal method and utilized as an efficient electrode material for SCs [424]. It exhibits a specific capacitance of ~ 1374 F g⁻¹ at 2 A g⁻¹ with ~ 96.1% of capacitance retention after 10,000 cycles. The fabricated Co₈FeS₈@NG//FeS@NG ASC showed an outstanding energy and power densities of ~ 70.4 Wh kg⁻¹ and 0.598 kW kg⁻¹, respectively. Xu et al. demonstrated a facile solid-state synthesis of ultrathin Mo_0.91W_0.09S₂ nanosheets/amorphous carbon composites for supercapacitor applications [425]. The optimized Mo_0.91W_0.09S₂/amorphous carbon nanosheets possess a high specific capacitance of 432.7 F g⁻¹ at 1 A g⁻¹ with retention of 93.8% of its initial capacitance after 500 cycles.

Diggikar et al. studied the performance of silver vanadium sulfide/PANI composite prepared by the in situ polymerization method for supercapacitor applications [426]. It revealed a high specific capacitance of 440 F g⁻¹ which is exceeding the pristine PANI (128 F g⁻¹) under the same current density. Guo et al. employed a novel strategy for preparing a high-quality cobalt copper sulfide NPs anchored on N-graphene NSs (Co₂CuS₄/NG) through a simple solvothermal method [427]. This composite at a current density of 1 A g⁻¹ displayed a high specific capacitance of ~ 1005 F g⁻¹ with 96.3% of capacitance after 5000 cycles. Annamalai et al. synthesized a highly exposed NiCo₂S₄-rGO nanoporous through a simple facile technique [428]. The prepared NiCo₂S₄-rGO nanoporous electrode offered a specific capacitance of 1527 F g at a scan rate of 10 mV s⁻¹. The fabricated supercapacitor shows a high energy density of 60.9 Wh kg⁻¹ at a power density of 1.4 kW kg⁻¹ with excellent cycling stability and rate performance. Bahaa et al. proposed to design and prepare a hierarchal copper-cobalt sulfide (CuCo₂S₄) nanosheet arrays from a metal–organic framework, which offers an improved electroactive site for the diffusion of electrolyte ions [429]. The as-prepared CuCo₂S₄ exhibits a high specific capacity of ~ 409.2 mAh/g at 3 mA. cm⁻² with ~ 94.2% of cycling stability after 10,000 cycles. The assembled supercapacitor with CuCo₂S₄ NS//Fe₂O₃/NG electrode exhibits an energy and power densities of ~ 89.6 Wh kg⁻¹ and ~ 663 W kg⁻¹, respectively. Han et al. investigated the electrochemical activity of ternary metal sulfides of manganese cobalt sulfide (MCS) with RGO has grown on nickel foam for the application of supercapacitors [215]. The prepared electrode exhibits an outstanding specific capacity of 1356 C/g at 1 A g⁻¹ with long cycle stability of 92.9% after 3000 cycles. Li et al. successfully synthesized NiCo₂S₄ nanosheets on porous graphitic carbon nitride (g-C₃N₄) NSs and employed as electrode material for supercapacitor [430]. The NiCo₂S₄/P-g-C₃N₄ electrode showed a high specific capacitance of 506 C/g at 1 A g⁻¹ with 99% of cycling stability after 5000 continuous cycles. Du et al. studied the superior electrochemical performance of the CoNi₂S₄/graphene nanocomposite electrode prepared by a facile physical approach [431]. At a current density of 1 A g⁻¹, the constructed CoNi₂S₄/graphene composite achieved a specific capacitance of 2009.1 F g⁻¹ and maintained 755.4 F g⁻¹ at 4 A g⁻¹ even after 2000 continuous charge–discharge cycles. Lv et al. constructed a high-performance asymmetric solid-state supercapacitor based on hierarchical zinc cobalt sulfide@nickel sulfide (Zn_0.76Co_0.24S@Ni₃S₂) nanosheet cores through a simple hydrothermal method followed by sulfurization technique. The designed ASC achieved a high specific capacitance of 1209 C/g at 2 A g⁻¹ with 94.9% of capacitance retention after 5000 cycles. Also, it exhibits an energy density of 53.8 Wh kg⁻¹ at a power density of 853 W kg⁻¹ with excellent rate performance. Li et al. created 3D manganese molybdenum sulfide with rGO/NF using a two-step hydrothermal process and employed it as an electrode material for supercapacitor applications [432]. The result obtained showed that MMS/rGO/NF composite achieved a specific capacitance of 1637.1 C/g at 1 A g⁻¹ with long-term cycling stability of 96.5% after 8000 cycles. Sahoo et al. synthesized a novel Cu₂MoS₄ NPs embedded rGO sheets through a simple one-pot hydrothermal method [433]. It exhibits a specific capacitance of 231.51 F g⁻¹ at 5 mV s⁻¹ than the pristine Cu₂MoS₄ (135.78 F g⁻¹) electrode. Sazonov et al. demonstrated the electrochemical performance of CoMoS₄/Co₃V₂O₈ nanocomposite prepared by a chemical precipitation method. It exhibits a specific capacitance of 584 F g⁻¹ at 0.5 A g⁻¹ with superior cycling stability after 3000 cycles [434].

Also, morphology and structures are very significant to enhance the electrochemical properties of electroactive materials and thereby facilitate excellent contact with electrolyte. On this concern, Zhu et al. reported a novel architecture of sea urchin–like cobalt manganese sulfide NWs arrays through a two-step hydrothermal method followed by anion-exchange sulfuration process [435]. It displayed a specific capacitance of 502 C/g at current density of 1 A g⁻¹ with excellent capacitance maintenance at 107% after 2000 cycles. An et al. synthesized a coral-like Ni_0.9Co_1.92Se₄ nanostructural electrode through a two-step solvothermal method that exhibits a specific capacitance of 1021.1 F g⁻¹ at 2 mA. cm⁻² with excellent rate capability of 77% over 2–5 mA. cm⁻² and cycling stability of 88.39% after 5000 cycles [436]. Yang et al. synthesized a highly exposed active surface of (Ni_xCo_1-x)₉Se₈ series through a one-step growth solid solution reaction for high-performance solid-state supercapacitor [437]. The prepared (Ni_0.1Co_0.9)₉Se₈ nanodendrites delivered a specific capacitance of 3762 F g⁻¹ at 5 A g⁻¹ and retain 94.8% of its initial capacitance after 5000 cycles. For the first time, Zhang et al. investigated the formation and electrochemical performance of double-shelled zinc cobalt sulfide dodecahedral cages through a sequential chemical etching and sulfurization method [438]. The optimized zinc cobalt sulfide showed enhanced electrochemical performance with an outstanding specific capacitance of 1266 F g⁻¹ at 1 A g⁻¹ and retains 91% of the capacitance over 10,000 cycles. Yu and Lin studied the morphological variation and electrochemical performance of nickel ions delivered by Ni foam and nickel salt in the hydrothermal route for preparing nickel cobalt sulfide [439]. The as-prepared NCS electrode at a current density of 4 A g⁻¹ exhibits a specific capacitance of 2206 F g⁻¹ with a good rate capacity of 1655.8 F g⁻¹ and retains about 94.6% of capacitance after 2000 cycles. Nan et al. studied the intrinsic energy storage mechanism of low crystallinity NiCo₂S₄ for supercapacitor application, which delivered a high specific capacitance of 666.27 F g⁻¹ at 5 A g⁻¹ and retains about 65.29% of its initial capacitance after 10,000 cycles [440]. Talha et al. used a one-step facile method for preparing copper-cobalt sulfide for the application of supercapacitors [441]. The copper-cobalt sulfide electrode achieved a high specific capacitance of ~ 516 F g⁻¹ even at higher current density of 10 A g⁻¹ with a rate performance of ~ 72% and retains ~ 66% of capacitance after 10,000 cycles. The assembled supercapacitor exhibits high energy and power densities of ~ 35.2 Wh kg⁻¹ and ~ 6.6 kW kg⁻¹, respectively. Chen et al. synthesized hierarchical core–shell NiMoO₄@NiCoS nanorods grown on Ni foam by two-step method [442]. The well-designed nanorods exhibit a specific capacitance of 1892 F g⁻¹ at 5 mA.cm⁻² with a retention of 91.7% capacitance after 6000 cycles. He et al. studied the improved supercapacitors performance by introducing the hierarchical Ni-Co-S@Ni-W–O core–shell NSAs on nickel foam by a facile three-step hydrothermal method [443]. These hybrid nanosheet arrays provide a high specific capacitance of 1988 F g⁻¹ at 2 A g⁻¹ current density, and the constructed supercapacitor using this hybrid NSAs offers a high energy and power densities of 55.1 Wh kg⁻¹ and 799.8 W kg⁻¹, respectively. Tang et al. prepared a highly electronic conductive cobalt–nickel sulfide dendrite/quasi-spherical nanocomposite through a facile hydrothermal method. The constructed Co_1.5Ni_1.5SS₄ electrode-based supercapacitor at a power density of 103.4 W kg⁻¹ achieved an energy density of 32.4 Wh kg⁻¹ and served with better cyclic stability.

Hussain et al. prepared rod-like zinc cobalt sulfide through a single-step hydrothermal method and used as an electrode material for supercapacitor [444]. At a current density of 1 A g⁻¹, the ZCS-based electrode exhibits an outstanding specific capacitance of 2418 F g⁻¹ and 83% of long cycling stability over 10,000 cycles. Ai et al. studied the application of a supercapacitor by using a novel 3D flower-like CoNi₂S₄/carbon nanotube composite through a simple and facile precursor transformation approach [445]. The electrochemical activity of CoNi₂S₄/carbon nanotubes showed a specific capacitance of 2094 F g⁻¹ at 1 A g⁻¹ with a 72% rate capacity even at 10 A g⁻¹. Wang et al. first prepared a novel rhombic dodecahedron ZIF-67-derived amorphous CoNi₂S₄ nanocage structure through a sulfurization technique and employed as electrode material for supercapacitor [446]. The electrochemical performance of CoNi₂S₄ nanocage reached an ultrahigh specific capacitance of 1890 F g at 4 A g⁻¹ with superior capacitance retention of 89.9% over 1000 charge–discharge cycles. Moreover, the constructed supercapacitor attained outstanding energy and power density of 35 Wh kg⁻¹ and 640 W kg⁻¹, respectively. Wang et al. designed a kelp-like structured NiCo₂S₄-C-MoS₂ composite electrode-based supercapacitor, which achieved a specific capacitance of 1601 F g⁻¹ at a current density of 0.5 A g⁻¹ [447]. The ASC based on this composite exhibits superior energy and power densities of 27.7 Wh kg⁻¹ and 400 W kg⁻¹, respectively, with cycling stability of 60% after 1000 cycles.

Due to their relatively high cycle stability and specific capacitance, transition metal oxysulfides (TMOS) have been considered promising electrode materials for energy storage devices. Liu prepared cobalt–nickel oxysulfide through a hydrothermal method, which delivered a specific capacitance of 592 F g⁻¹ at 5 A g⁻¹ current density with retention of 81.5% of its initial capacitance after 2000 cycles [448]. In another work, Liu synthesized manganese cobalt oxysulfide grown on Ni foam via a two-step hydrothermal route that parades a specific capacitance of 490 C/g at 2 A g⁻¹ with excellent cycling stability of 86.5% after 3000 cycles. Also, Liu studied the electrochemical performance of zinc cobalt oxysulfide for energy storage applications and revealed a specific capacity of 645.5 C/g at 1 A g⁻¹ with capacitance retention of 76% after 1000 cycles. Yao et al. used the electrochemical deposition technique for the synthesis of manganese oxysulfide, which exhibits an enhanced specific capacitance of 214 F g⁻¹ at 1 mA. cm⁻² with 75.4% of cycling stability after 1000 cycles. Based on this works, Asen et al. employed an electrochemical deposition method for the preparation of iron vanadium oxysulfide nanostructures with different ratios [449]. The iron vanadium oxysulfide at 2:1 ratio showed an improved specific capacitance of 217 F g⁻¹ at 3 A g⁻¹ with 92% of capacitance retention after 4000 cycles. The summary of bimetal chalcogenide-based electrode materials and their supercapacitor performances are shown in Table 7.

Table 7 Summary of bimetal chalcogenide–based electrode materials with various synthesis methods and their supercapacitor performances

6 Trimetallic chalcogenides and its composites

Compared to monometallic (NiS, CoS, FeS, etc.) and bimetallic sulfides (nickel cobalt, nickel manganese, etc.), the electrochemical influences of cobalt, nickel, manganese, and ions in the trimetallic sulfides delivered rich redox reactions resulting in outstanding specific capacitance. Generally, Co, Ni, and Mn-based supercapacitors displayed a high specific capacitance, energy density, and power density at low current density. Also, it is reported that electrodes of mixed transition metals offer superior electrochemical performance than single TMSs. Thus, the coupling of three metal species could render the mixed TMO/TMSs and will surely increase the redox reaction with high electrical conductivity, which is favorable for the application of energy storage devices. Wei et al. prepared a yolk-shell hollow sphere of nickel cobalt manganese sulfide via a self-templating strategy, which possesses a high specific capacitance of 1360 F g⁻¹ at a current density of 1 A g⁻¹ [524]. The fabricated nickel cobalt manganese sulfide supercapacitor device showed an energy and power densities of 49.8 Wh kg⁻¹ and 1700 W kg⁻¹, respectively, with only a 1.8% loss of its initial capacitance even after 6000 cycles. Sahoo et al. used a cathodic electrodeposition method for preparing nickel cobalt manganese sulfide (NCMS) nanosheets on Ni foam for the fabrication of high-performance supercapacitors [525]. The optimized NCMS, at 1 A g⁻¹, offers a larger specific capacitance of 2717 F g⁻¹ with great cycle life and energy density (94.7 Wh kg⁻¹). Gao et al. synthesized a single-phase CuCo_2-xNi_xS₄ electrode through a facile two-step hydrothermal method to improve the electrochemical activity of supercapacitor [526]. The electrochemical performance of the CuCo_2-xNi_xS₄ electrode reached up to 647 F g⁻¹ of specific capacitance at 1 A g⁻¹ of current density with ~ 98% of capacitance retention after 10,000 cycles. Verma et al. investigated the pseudocapacitive behavior of cobalt manganese nickel sulfide (CoMnNiS) nanosheet grown on Ni-foam through a simple electrodeposition method [527]. The designed CoMnNiS electrode achieved a specific capacitance of 257.4 mAh/g at 2.5 A g⁻¹. They fabricated an asymmetric supercapacitor based on both CoMnNiS/NiCuO and CoMnNiS/CNT electrodes with a superior energy density of 8.4 and 6.3 Wh kg⁻¹ at a power density of 985 and 211 W kg⁻¹, respectively. Isacfranklin et al. developed a newer form of chalcogenides using copper, iron, and tin to form Cu2FeSnS4/PVP/rGO-decorated nanocomposite that are prepared by simple hydrothermal method and followed by nucleation process in later studies in quaternary chalcogenides for use as electrode material [528]. The developed electrode material exhibits an excellent specific capacitance value of 328 F g⁻¹ (45.55 mA h/g) at 0.5 A g⁻¹, and the symmetric cell made using this electrode exhibits energy and power densities of 73 Wh kg⁻¹ and 749 W kg⁻¹ at 1 A g⁻¹. With 20,000 cycles, it has a coulombic efficiency of 99.99% and a 63% capacity retention. They also developed marigold flower-like structured Cu₂NiSnS₄ by employing simple solvothermal process, and the electrode revealed high 1029 F g⁻¹ specific capacitance at 0.5 A g⁻¹ current density. To prove its application towards practical technology development, a full-cell asymmetric solid-state device is fabricated which delivers 41.25 Wh kg⁻¹ and 750 W kg energy and power density at 0.5 A g⁻¹ [529]. Song et al. developed Ni_xCo_yMn_zS/Ni(SeO₃) (NCMS/NSeO) heterostructure that is prepared on Ni-plated carbon cloth and employed them as electrode for supercapacitor; the optimized electrodes exhibit a high capacity of 536 mAh g⁻¹ at 1 A g⁻¹, the assembled asymmetric supercapacitor achieves an ultrahigh energy density of 141 Wh kg⁻¹, and the main highlight of this work is an impressive high-rate capability and cyclability combination with 124% capacitance retention after 10,000 cycles at a large current density of 50 A g⁻¹ [530].

7 Outlook and future prospects

For high energy storage, long cycle, and reliability, supercapacitors are the most predominant technology that can complement the strength of batteries. Firstly, we pointed out the advantages and disadvantages of carbonaceous materials like graphene, MWCNT, metal oxides, and polymers that fail to provide high specific capacitance and cycling stability in supercapacitors. In this study, we collected many transition metal chalcogenides and their composites with different morphologies-based electrode materials that offer high ratings in supercapacitors. From the above discussion, the TMCs and their composite-based electrode material possess better specific capacitance with high-rate capability and long-term stability than carbon-based materials. Despite being promising candidates, it has some demerits of having low energy and power density compared to batteries. Hence, many morphological changes and doping of metals have been done to rectify these limitations. Recently, researchers have developed supercapacitors with a novel ternary meta for energy storage systems due to their large surface area, high conductivity, and feasibility towards electrochemical performance. In addition, the morphology-controlled ternary metals are attractive at high-temperature operations. Also, a few recent reports show that the trimetallic-based electrode is a promising candidate in supercapacitor with extraordinary specific capacitance and power density. Hence, we conclude that among various electrodes, bi- and trimetallic-based electrodes offer high efficiency in comparison with lithium-based batteries. Lastly, we conclude that the efficient electrode materials used in testing supercapacitors must be practically developed and used in the commercial market.