Recent Advances in Metal Chalcogenides (MX; X = S, Se) Nanostructures for Electrochemical Supercapacitor Applications: A Brief Review

Supercapacitors (SCs) have received a great deal of attention and play an important role for future self-powered devices, mainly owing to their higher power density. Among all types of electrical energy storage devices, electrochemical supercapacitors are considered to be the most promising because of their superior performance characteristics, including short charging time, high power density, safety, easy fabrication procedures, and long operational life. An SC consists of two foremost components, namely electrode materials, and electrolyte. The selection of appropriate electrode materials with rational nanostructured designs has resulted in improved electrochemical properties for high performance and has reduced the cost of SCs. In this review, we mainly spotlight the non-metallic oxide, especially metal chalcogenides (MX; X = S, Se) based nanostructured electrode materials for electrochemical SCs. Different non-metallic oxide materials are highlighted in various categories, such as transition metal sulfides and selenides materials. Finally, the designing strategy and future improvements on metal chalcogenide materials for the application of electrochemical SCs are also discussed.


Introduction
A substantial global upsurge in the depletion of fossil fuels from the rapid growth of global economy has generated two vital concerns: the first is the exhaustion of existing fossil fuel reserves, and the second is associated with an increase in greenhouse gas emissions, in particular, and environmental pollution, in general. Hence, it is necessary to develop and commercialize sustainable environment friendly energy sources and their related technologies are being developed globally as a matter of urgency [1][2][3][4][5][6]. Also, the development of associated energy conversion devices to gather these intermittent energy sources efficiently is in demand. In this specific backdrop, electrochemical supercapacitors (SCs) have overriding importance because of their exceptional power density and storage properties compared to other contemporary energy storage devices. SCs have a number of great advantages including long life cycle, high power density, high efficiency, high specific capacitance, flexible operating temperature, and environmental friendliness. Moreover, they are where m is mass of the used electrode material (g), I is the voltammetric current, ∆V is the potential window (V), and v is the scan rate (mV·s −1 ), respectively. The electrode materials' specific capacitances are evaluated from a (CD) analysis: where I is the discharging current (A), t is the time (s), and V is the potential difference (V), respectively. Furthermore, the energy and power densities are calculated by and P = I∆V m (4) respectively. In the aforementioned Equations (3) and (4), the I, ∆V, ∆t, and m are the current potential difference, discharging time, and mass of an electroactive material, respectively. Criteria for electrode materials selection are (i) Multiple oxidation states (ii) Superior electrical conductivity (iii) High surface area & chemical stability (iv) Electrochemical activity (electrolyte ions can freely interact into the electrode surface) To improve the capacitance of supercapacitors, four key factors are required: (i) Doping of the metals to increase the conductivity and redox activity (ii) A wide potential window (iii) High surface area for the redox reaction (iv) High charge/discharge rate In the present review, the recent advances in the fabrication of metal sulfides and metal selenide-based nanostructured electrode materials for electrochemical SCs are discussed. Finally, the benefits of both metal sulfide-and selenide-based nanostructured electrode materials in the designing strategy for electrochemical SC applications are also systematically presented.

Metal Chalcogenides for Electrochemical SCs
The industrially vital and scientifically significant metal chalcogenides (MCs) (S, Se, and Te) have received a great deal of attention in the past two decades due to their anisotropic property. In general, transition elements of groups IV to VII B combine with VI A group elements, such as S, Se, and Te to form binary stable layered crystalline structures [21]. These layered transition MCs possess the general formula of MX 2 , where M is a transition element in groups IV B (Ti, Zr, Hf), V B (V, Nb, Ta), VI B (Mo, W), or VII B (Tc, Re) and X is a chalcogen atom in the VI A group (S, Se, Te). The structure and properties of most of the transition MCs almost resemble semimetal pristine graphene, except for the band gap [22], which is nearly zero in pristine graphene whereas in transition MCs, it depends on the elemental combination, the number of layers, and the presence or lack of adopting atoms. Hence, their band gap values lie between 0 and 2 eV. Due to the variation in band gap, different transition MC structures are tunable, and so have become industrially important materials [23].
In this part of the review article, we particularly describe the application of nanostructured transition MCs in electrochemical SCs. They have gained considerable attention due to their high specific power, and long stability and life cycle, and they offer better safety tolerance relative to batteries in a wide range of applications in consumer electronics, electric tools, buffer powers, hybrid electronic vehicles, and so forth [22]. On the other hand, MCs have been applied in the fields of fuel cells, solar cells, light-emitting diodes, sensors, lithium-ion batteries, electrocatalysts, thermoelectric devices, and memory devices, as well as being widely utilized in SCs, due to their excellent properties. These include (i) improved life cycle; (ii) flexibility; (iii) providing additional reactive sites and catalytic activity; (iv) improving conductivity as well as reduction of inner resistance and ohmic loss; (v) short path lengths for electron transport; and (vi) displaying quantum-sized effects. Furthermore, we describe the future promising areas of transition metal group sulfides and selenide nanostructures covering both their properties and their applications in SCs. Specifically, metal sulfides exhibit greatly improved electrochemical performance, which largely originates from their higher electronic conductivity, higher electrochemical activity, and mechanical and thermal stability. On the other hand, it has been well reported that the performance of electrochemical energy storage devices depends greatly on the crystalline phase, size of the electroactive materials, structural and morphological features, and composition and the design of electrodes [22]. Metal selenides, as a new class of battery-like electrode materials, have gained increasing interests as promising supercapacitor electrode materials, not only possessing rich redox chemistry, but also better electronic conductivity, and mechanical and thermal stability. Compared to metal sulfides, metal selenides are for less reported than that of metal sulfides. The details are presented herein.

Nickel Sulfides
In recent years, nanometer-sized metal sulfides have played a significant role in the field of electronics, especially optical and optoelectronic devices, due to their distinct excellent physical and chemical properties. Certainly, nickel sulfide is of particular interest because of its different phases, such as NiS, Ni 3 S 2 , NiS 2 , Ni 3 S 4 , Ni 7 S 10 , and Ni 9 S 8 , and its different morphologies [24]. However, the different phases and morphologies of nickel sulfides sometimes coexist as a combination of more than two different phases [25]. Hence, obtaining an even morphology with pure nickel sulfides is still a challenge that has attracted a great deal of attention. Some of the important phases of nickel sulfides and their application in SCs are briefly discussed under the following subsections.
(a) Ni 3 S 2 In the midst of the different types of nickel sulfides, Ni 3 S 2 has exhibited a better performance as an electrode material for energy storage devices, due to its different types of morphology and advantages, including its low capital cost, high specific capacitance, and simple synthesis route. These are anticipated to help it meet the increasing necessities of energy storage systems, especially for SCs [25]. In addition, it occurs abundantly in nature as minerals in the form of heazlewoodite. Hence, in recent years, it has been investigated widely for SC applications. However, Ni 3 S 2 , unfortunately, has low conductivity, which restricts the fast electron transport required for high rate capability, and can even act as an insulator. This sort of issue has been overcome by way of incorporating highly conductive electrode materials in the pseudocapacitive Ni 3 S 2 material.
Chou et al. [26] first synthesized the flaky Ni 3 S 2 nanostructure on Ni-foam by a simple potentiodynamic deposition method and employed it for SCs. This material showed a maximum specific capacitance of 717 F·g −1 at 2 A·g −1 rate in 1 M KOH solution with remarkable capacitance retention of 91%. On the other hand, Karthikeyan et al. [27] used a one-pot hydrothermal synthesis method of Ni 3 S 2 to increase the electrochemical properties and specific capacitance of Ni 3 S 2 further. They grew hierarchical Ni 3 S 2 nanostructures in a Ni foam cell and evaluated its capacitance behavior. The cell offered a maximum specific capacitance of 1293 F·g −1 at a current density of 5 mA·cm −2 . Moreover, a different kind of preparation method has been extensively studied and reported for other similar type of electrode materials [28,29]. Zhou et al. [30] further used a hydrothermal method to synthesize Ni(OH) 2 nanosheets coated onto single-crystal Ni 3 S 2 nanorods grown on the surface of three-dimensional (3-D) graphene nanosheets (Ni 3 S 2 @Ni(OH) 2 /3-D-GN), which were able to achieve a relatively high capacitance of 1277 F·g −1 at 2 mV·s −1 and 1037.5 F·g −1 at 5.1 A·g −1 . They also investigated the structural evaluation of Ni 3 S 2 @Ni(OH) 2 /3-D-GN with respect to hydrothermal reaction time, and concluded that as the reaction time increases from 6 h to 12 h, the evolution of the structure from Ni 3 S 2 nanorods to Ni 3 S 2 @Ni(OH) 2 occurred, followed by conversion to pure Ni(OH) 2 nanosheets. After a hydrothermal reaction time of 6 h, Ni 3 S 2 nanorods were obtained, as exhibited in Figure 1. and (e) a proposed mechanism for the growth of the Ni 3 S 2 @Ni(OH) 2 /3-D-GN structure. Reproduced with permission from [31]. Royal Society of Chemistry, 2016.
Later on, Zhu et al. [32] reported the preparation of Ni 3 S 2 nanosheets on a CNT backbone with a specific capacitance of 514 F·g −1 at a current density of 4 A·g −1 and excellent cycling stability. Likewise, Pan et al. [33] designed and compared the capacitance behavior between Ni 3 S 2 and Ni 3 S 2 /graphene on Ni-foam. Obviously, compared to pristine Ni 3 S 2 , the Ni 3 S 2 /graphene nanocomposites showed better electrochemical behavior and achieved a specific capacitance value of around 278.3 F·g −1 for the first 20 cycles. Afterwards, the capacitance started to decrease to 230.6 F·g −1 over 35 cycles, and finally reached 223 F·g −1 until 50 cycles, which might have been due to the detachment of electrode material from the Ni-foam.
To improve the specific capacitance of Ni 3 S 2 /graphene composites, a simple process controlled by adjusting the extent of sulfidation was proposed by Ou et al. [34] who achieved the highest specific capacitance of 1022 F·g −1 . The same group also studied the one-step hydrogen reduction synthesis of Ni 3 S 2 /graphene composites reported elsewhere [35]. Moreover, the biomolecule-assisted hydrothermal synthesis of Ni 3 S 2 nanospheres/reduced graphene oxide (Ni 3 S 2 /rGO) nanocomposites was investigated using L-cysteine as the reducing agent, and their application to SCs characterized [36]. They displayed very high specific capacitances of 1169 F·g −1 and 761 F·g −1 at 5 A·g −1 and 50 A·g −1 current rates, respectively, with good cycling stability, while bare Ni 3 S 2 /rGO on Ni-foam offered a specific capacitance of 2188.8 F·g −1 at 2.9 A·g −1 [37].
In recent times, a series of Ni 3 S 2 nanowires, such as Ni 3 S 2 -Ni, Ni 3 S 2 -NiS, and Ni 3 S 2 -NiS-Ni, have been grown on nickel nanowire templates, and their capacitance behavior compared elaborately [38]. Among these, Ni 3 S 2 -NiS nanowires presented superior redox reactivity with a high specific capacitance of 1077.3 F·g −1 at 5 A·g −1 , due to their excellent aspect ratio and electrical conductivity. On the contrary, the other two nanowire electrodes (Ni 3 S 2 -Ni and Ni 3 S 2 -NiS-Ni) possessed 100% capacitance retention compared to the Ni 3 S 2 -NiS electrode (76.3%). A rationally designed two-step method to fabricate self-supported Ni 3 S 2 nanosheet arrays on a metal-organic framework has been investigated by Chen et al. [39] who achieved a maximum specific capacitance of 200 F·g −1 at a current density of 10 A·g −1 .

(b) NiS
As discussed earlier, uniform morphology with a pure phase of nickel sulfide is still a challenge, and currently, plenty of research is focused on resolving this problem [40]. Nevertheless, few studies have dealt with morphological control during the synthesis of the NiS and NiS 2 phases with a pyrite structure [41,42]. In addition, those consisting of nickel sulfide phases are less toxic and highly abundant in nature, and possess high redox activity [43][44][45][46]. For instance, flower-like β-NiS was successfully synthesized and reported by Yang et al. [47], in which the electrodes displayed a specific capacitance of 966 F·g −1 at a current rate of 0.5 A·g −1 . Similarly, Wang et al. [48] prepared one-dimensional (1-D) (110)-oriented NiS nanorods with a high specific capacitance of 1403.8 F·g −1 at a current density of 1 A·g −1 . This high specific capacitance of the electrode material might have been due to the designed 1-D electron-transport pathway and large specific surface area of NiS. Likewise, successful SC performances of α-NiS and β-NiS were reported by Wei et al. [49].
However, the pure phases of these electrodes suffer from poor cycling stability owing to the agglomeration and pulverization of NiS during consecutive cycling of the CD process. The cycling stability of NiS electrodes has been improved by changing the experimental conditions, and including conducting nanomaterials along with a NiS matrix, as reported earlier, some of which are listed later. The phase-controlled synthesis of α-NiS embedded in carbon nanorods was synthesized by Sun et al. [50]; the electrodes delivered a high electrochemical stability with 100% capacitance retention with a specific capacitance of 1092 F·g −1 at 1 A·g −1 . Similarly, NiS nanoparticles on Ni-foam, [51][52][53] activated carbon, [53] N-doped carbon fiber aerogels [46], and rGO [54], have been reported recently.

(c) Ni 3 S 4
One of the rarely reported nickel sulfide phases, Ni 3 S 4 , exists in nature as polydymite. Still, the scientific community is facing the challenge to obtain the purest phase of Ni 3 S 4 by conventional solid-state reactions for SC applications. Hence, to date, the electrochemical properties of Ni 3 S 4 remain hidden. Only a few studies in the literature discussed earlier are on Ni 3 S 4 for SC applications. In recent times, Zhang et al. [55] prepared the 3-D rigid Ni 3 S 4 nanosheet frames by controlled solvothermal synthesis, and evaluated their electrochemical performances for SC applications. Interestingly, the 3-D rigid Ni 3 S 4 nanosheet frames possessed better capacitance performances than that of flat Ni 3 S 4 . The 3-D rigid Ni 3 S 4 nanosheet frames achieved a maximum capacitance value of 1213 F·g −1 , which was due to high free volume and high compressive length. The proposed mechanism for both flat Ni 3 S 4 and 3-D Ni 3 S 4 nanosheet frames is schematically represented in Figure 2. Furthermore, the synergistic effects of the layered Ni 3 S 4 , MoS 2 , and conductive carbon fibers were analyzed by Huang et al. [56] who reported a capacitance value of 1296 with 96.2% capacitance retention. Similarly, the Ni 3 S 4 @amorphous MoS 2 nanosphere electrodes have exhibited a high specific capacitance of 1440.9 F·g −1 at 2 A·g −1 [57].
Easily self-assembled Ni 3 S 4 -MoS 2 hetero-junction electrode materials assisted by an ionic liquid 1-butyl-3-methylimidazolium thiocyanate have been prepared for the first time with the electrode attaining high specific capacitance of 985.21 F·g −1 at a current density of 1 A·g −1 [58]. The role of ionic liquid in this hetero-junction electrode synthesis was that it provides a sulfur source for the sulfidation reaction, and also influences the formation of Ni 3 S 4 -MoS 2 with different precursor reactions. Other phases of nickel sulfides, such as Ni 9 S 8 [59] and NiS 2 [24], were also produced but rarely reported for SC applications, due to their unstable phase nature.
In this section, we briefly discuss the salient features and potential applications of CuS in the field of electrochemical SCs. Studies on the electrochemical behavior of CuS are very limited, and so an investigation into its use as an electrode material is highly significant. Recently, it has been reported as a suitable SC electrode material, due to its high theoretical capacitance [64][65][66][67]. For example, Peng et al. [68] synthesized CuS with different morphologies using a low-temperature solvothermal method, and employed it for SC applications. The high surface area flower-like CuS provided a good specific capacitance of 597 F·g −1 with an excellent discharging rate and cycling stability. The sonochemical-assisted synthesis of CuS has been studied elaborately, and yielded a specific capacitance of 62.77 F·g −1 at 5 mV·s −1 [69].
The important metal chalcogenide CuS provides an electronic conductivity of 10 −3 S·cm −1 and theoretical specific capacity of 561 mA·h·g −1 . However, this is not favorable for SC applications because pure CuS is a semiconductor with relatively low conductivity when compared to carbon nanomaterials and conducting polymers, and its volume change during cycling causes poor cycling stability [70]. Hence, it is desirable to geometrically control the preparation of CuS composites and combine them with electronically conductive substance to enhance SC performance greatly.
Ultrafine CuS nanoneedle arrays grown on a CNT backbone have also been investigated as electrodes for SC applications in the past. Interestingly, these reported 1-D hierarchical electrodes offered better capacitance values with excellent cyclability, owing to the abundant surface area between the electrode and electrolyte. A schematic illustration of the formation of CuS nanoneedles on a CNT backbone is depicted in Figure 3. Later, Huang et al. [71] have applied a different hydrothermal approach to synthesis CuS/MWCNT (multi-walled CNT) electrodes and analyzed its electrochemical performance (2831 F·g −1 ). The CNT-incorporated porous 3-dimensional CuS microsphere composite electrodes had peony-like microspheres with a diameter of 1 µm, and each microsphere was composed of a few tens of bundled nanosheets of 15-30 nm thickness [72]. They showed excellent cyclability and rate capability, with an average reversible capacitance of 1960 F·g −1 at 10 mA·cm −2 . The electrochemical SC performances of different important metal sulfides are tabulated in Table 1. A high-performance SC based on CuS@PPy composite has been developed by in situ oxidation polymerization recently [73]. The composite had uniform spheres with an average thickness of 1 µm, which in turn were composed of plenty of intertwined sheet-like subunits. The electrodes exhibited a high specific capacitance of 427 F·g −1 at 1 A·g −1 . Currently, CuS nanowires on a copper mesh have also served as working electrode in SCs. These CuS-nanowire-based electrodes were free from the binder and conductive material, and had well-arrayed structures with nanosized grains and a high aspect ratio and density. In addition, the other electronically conducting substances like rGO, acetylene black, polyaniline (PANI), and CNTs have also been combined with CuS with the resultant electrodes showing very good capacitance performance and great retention [70,[74][75][76][77].For instance, the schematic illustration of synthesis of CuS@rGO composites was displayed in Figure 4 [70].

Cobalt Sulfides
In the past decade, cobalt sulfide has received a great deal of interest, due to its applications in versatile fields such as SCs, lithium ion batteries, alkaline rechargeable batteries, magnetic materials, and catalysts [88][89][90][91]. To date, various nanostructures of cobalt sulfide have been examined and reported as electrode materials for SCs. However, the controlled synthesis of cobalt sulfides with high purity and well-defined complex morphology is highly complicated. This may be due to the following factors. (i) Since it exists in nature as different chemical compositions (Co 1−x S, CoS, CoS 2 , Co 9 S 8 , and Co 3 S 4 ), it can easily transform from one phase to another phase; (ii) During preparation, it is very difficult to remove impurities such as cobalt oxide and cobalt hydroxide, because cobalt ions have a very strong affinity to oxygen; (iii) Controlling the reaction temperature is challenging for the reason that cobalt sulfides possess a complicated phase diagram. In order to deal with these factors as well to prepare high purity cobalt sulfide nanostructures, various types of synthetic routes have been employed in the past. There are several reports on the synthesis and electrochemical evaluation of nanostructured cobalt sulfides pertinent to SCs and will be discussed in this section. Chen et al. [92] fabricated a high-performance electrochemical SC using Co 3 S 4 nanosheet arrays on Ni-foam as electrodes, which were prepared by an anion exchange reaction of the Co 3 O 4 nanosheet arrays. Furthermore, they compared the electrochemical performances of Co 3 S 4 nanosheet arrays with its corresponding metal oxide analog Co 3 O 4 nanosheet arrays. Interestingly, the specific capacitance and cycling stability of Co 3 S 4 nanosheet arrays electrodes were 4.1 times higher than that of Co 3 O 4 nanosheet arrays, as shown in Figure 5, and achieved a maximum areal capacitance of 1.81 F·cm −2 at a current density of 24 mA·cm −2 . Recently, rGO nanosheets wrapped around Co 3 S 4 nanoflake electrodes were developed, and their electrochemical performance thoroughly investigated by Patil et al. [93]; the electrode offered a highest specific capacitance of 2314 F·g −1 at 2 mV·s −1 . 3-D flower-like hierarchical CoS nanostructure electrodes have been prepared using 6 M KOH solution and employed in SCs, which yielded 586 F·g −1 at 1 A·g −1 after 1000 cycles [94]. Nevertheless, one-step hydrothermally synthesized two-dimensional (2-D) CoS nanosheet electrodes exhibited superior performance with a higher specific capacitance of around 1314 F·g −1 at 3 A·g −1 [95]. Later, Wan et al. synthesized and reported the performance of CoS nanotubes for high performance SCs [78], while Justin et al. studied the synthesis of CoS nanospheres using a hydrothermal method and evaluated their applications in SCs [96], and recently, a flower-like CoS hollow sphere electrodes for energy storage devices have been reported [97]. Accordingly, other CoS nanostructures and composites with rGO, titania, and CNT have also been synthesized and studied for SC applications [79,[98][99][100].
(c) CoS 1.097 As with CoS, Wang et al. [101] developed a simple solvothermal method to prepare flower-like 3-D hierarchical CoS 1.097 and employed it as an SC electrode, which exhibited high specific capacitances of 555 F·g −1 and 464 F·g −1 at 5 mA·cm −2 and 100 mA·cm −2 , respectively, while 1-D hierarchical CoS 1.097 on CNT nanostructured electrodes delivered a remarkable specific capacitance of 640 F·g −1 at 8 A·g −1 after 3000 consecutive CD cycles [102]. Another nanostructure consisting of an ultralong CoS 1.097 nanotube network provided high specific capacitance, good capacitance retention, and excellent coulombic efficiency, due to its hollow structure and large surface area [103].

(d) CoS 2
Pyrite-phase cobalt disulfide (CoS 2 ) is intrinsically a conductive metal that has been considered as one of the promising materials for wide potential application in SCs [104]. Moreover, it is earth abundant and low cost, and has long-term stability under acidic operating conditions. Furthermore, the thermal stability and Gibbs free energy (−146 kJ·mol −1 ) of CoS 2 is much higher than that of other metal sulfides, indicating that it has superior capacitive behavior compared to activated carbon positive electrodes for hybrid SCs [105]. As we know, the electrochemical properties of SC electrode materials strongly depend on particle size, shape, and porosity, as well as pore size distribution. Superior electrochemical and pseudocapacitive properties were observed for single phased CoS 2 ellipsoids, nanoflake thin films, nanowires, octahedrons, and hollow spheres [106][107][108]. The hierarchical mesoporous CoS 2 electrodes offered a high specific capacitance of 718.7 F·g −1 at 1 A·g −1 [80], whereas 3-D hollow CoS 2 nanoframe electrodes fabricated by anion replacement had a maximum capacitance of 568 F·g −1 at 0.5 A·g −1 [109]. Nevertheless, single component CoS 2 was intrinsically unstable, which caused several problems, such as relatively low capacitance, poor cycling stability, and rate capability. These could be overcome by an effective synthetic strategy for direct growth of a CoS 2 active material on a conductive support, which dramatically enhanced the capacitance performance [110]. For instance, CoS 2 -rGO composites which possessed better electrochemical properties than pure individual components have been prepared and investigated recently [111]. Furthermore, a CoS 2 /MoS 2 on carbon fiber cloth hierarchical electrode exhibited excellent long life cycle stability and achieved a maximum capacitance value of 406 F·g −1 [112].
(e) Co 9 S 8 Various nanostructures of Co 9 S 8 including nanosheets, nanoneedles, nanospheres, a yolk-shell structure, as well as various heterostructures with CNT and rGO were reported as potential anodes for lithium-ion batteries and dye-sensitized solar cells [113][114][115][116][117][118][119][120]. However, the reports on Co 9 S 8 nanostructured electrodes leading to SC applications are very scarce. For instance, high purity Co 9 S 8 thin films on Ni foam have been developed by atomic layer deposition and employed as high-performance SC electrodes which possessed a specific capacitance of 1645 F·g −1 at 3 A·g −1 [121]. Later Ramachandran et al. [122] suggested a low cost synthetic route for Co 9 S 8 nanoflake/graphene composite electrodes that offered a maximum specific capacitance of 808 F·g −1 at 5 mV·s −1 in 6 M KOH electrolyte solution. Mashikwa et al. [123] developed a new type of SC electrode consisting of Co 9 S 8 nanoparticle clusters embedded in an activated graphene foam structure using a microwave-assisted hydrothermal method; the electrode was capable of delivering a specific capacitance of 1150 F·g −1 at 5 mV·s −1 . Furthermore, 3-D petal-like two-mixed metal sulfide-graphene composite electrodes (Co 9 S 8 /rGO/Ni 3 S 2 /Ni foam) fabricated for high-performance SCs exhibited superior capacitive performance with the high capability (2611.9 F·g −1 at 3.9 A·g −1 ), excellent rate capability, and enhanced electrochemical stability with remarkable capacitance retention [124].

Binary Metal Sulfides
Although many transition metal sulfides have been investigated as electrodes for SCs, binary metal sulfides are quite interesting, due to their higher active redox sites, as well as mechanical and thermal stability compared to that of their corresponding single component counterparts. Most binary metal sulfide nanostructures have been synthesized by applying the Kirkendall effect [125], and recently, various binary metal sulfides have been prepared based on it [82]. In brief, the Kirkendall effect is based on the mutual diffusion process of two metals through an interface so that vacancy diffusion occurs to compensate for the inequality of the material flow and that the initial interface moves. Nevertheless, reports on binary metal sulfides as SC electrode materials are still limited.

(a) NiCo 2 S 4
The urchin-like porous NiCo 2 S 4 nanotubes have been synthesized and employed as pseudocapacitor electrodes with excellent electrochemical performance in the past [107,108]. Later Pu et al. [82] successfully synthesized hollow hexagonal NiCo 2 S 4 nanoplates, which exhibited a high specific capacitance of 437 F·g −1 at 1 A·g −1 using 3 M KOH electrolyte solution. CoNi 2 S 4 electrode materials were successfully fabricated by Du et al. [126]. Self-templating synthesized NiCo 2 S 4 hollow spheres have shown excellent electrochemical properties, such as an intrinsic electronic conductivity hundreds of times higher than that of its corresponding binary metal oxides [127]; an electrode cell made with it achieved a maximum capacitance of 1263 F·g −1 at 2 A·g −1 with remarkable rate capability. In the meantime, NiCo 2 S 4 nanostructures prepared by, for instance, hydrothermal, solvothermal, and polyol methods also exhibited high specific capacitance with fabulous capacitance retention, and were reported as potential pseudocapacitor electrodes for SC applications [81,128,129]. Recently, the NiCo 2 S 4 on carbon fiber cloth and carbon fiber paper have been investigated, and their electrochemical performances compared to SC applications. NiCo 2 S 4 carbon fiber paper demonstrated favorable charge-transfer kinetics and fast electron transport compared to NiCo 2 S 4 carbon fiber cloth, and thus showed superior electrochemical performance compared to its counterpart [130].

(b) Manganese Cobalt Sulfides (MCS)
Great attention has been paid to MCS-based electrodes in the past three years, due to its eco-friendly nature and high redox properties. As with NiCo 2 S 4 , reports on MCS are very few. Previously, Chen et al. [131] synthesized hollow tubular MCS for pseudocapacitor applications. Currently, the ultrathin mesoporous MCS nanosheets have been grown on Ni foam using an electrodeposition technique and characterized for its applications in SCs [132]. Very recently, a high specific capacitance of 1938 F·g −1 at 5 A·g −1 with long-term cycling stability and capacitance retention have been reported for nano honeycomb-like MCS/3 D-graphene on Ni-foam electrodes [133].
Apart from the above binary metal sulfides, there has only been one report on a 3-D yolk-shell NiGa 2 S 4 structure confined with nanosheets for high-performance SC applications [134].

Molybdenum Disulfide
In the past decade, MoS 2 has received a great deal of attention, due to its unique physical and chemical properties and find applications in various fields including electrochromic devices hydrogen storage, catalysis, capacitors, lubricants, and batteries [135][136][137]. In brief, MoS 2 is a graphene-like 2-D material in which the middle layer of molybdenum is sandwiched between two sulfur layers. All three layers are stacked over each other and held together by weak van der Waals forces [138,139]. In recent times, researchers have focused on the utilization of MoS 2 to develop high-performance SCs, due to its higher theoretical capacitance (1000 F·g −1 ) than graphite and fast intrinsic ionic conductivity [140,141].
Ajayan and co-workers [142] prepared 2-DMoS 2 film-based micro-SCs by a low-cost spray painting process and subsequent laser printing. The prepared SCs exhibited a better electrochemical performance than graphene-based micro-SCs and delivered a high voltammetric capacitance of 178 F·cm −3 with better cycling performance. Later on, several groups have also reported the same range of capacitance values for hydrothermally synthesized MoS 2 at current density rate of 1 A·g −1 [143][144][145][146]. In another typical case, Soon et al. [147] investigated MoS 2 nanosheets as potential electrodes for SCs, and reported that the SC performance of the electrodes was comparable to that of CNT array electrodes. Recently, chemically deposited MoS 2 thin films have been synthesized by Pujari et al. [148] using 1 M Na 2 SO 4 as an electrolyte solution. They showed a specific capacitance value of 180 F·g −1 with 82% capacitance retention. Karthikeyan et al. [149] reported MoS 2 -based electrodes prepared using a ball-milling process, and employed them in wire type solid-state SCs. However, in practice, the electronic conductivity of MoS 2 (10 −5 Ω −1 ·cm −1 ) is still lower compared to graphene and the specific capacitance of MoS 2 is very limited when used on its own in SCs [150,151], a deficiency which has been overcome by combining it with other conducting materials (as discussed earlier for metal sulfides).
Huang et al. [152] fabricated a new class of PANI/MoS 2 composites in which the short rod PANI was anchored onto the surface of MoS 2 . The resultant electrode offered a specific capacitance of 575 F·g −1 at 20 mV·s −1 . The same group extended their research on MoS 2 -graphene nanocomposites and concluded that the capacitance behavior of MoS 2 -graphene composite (243 F·g −1 ) was quite higher than that of bare MoS 2 (120 F·g −1 ) and bare graphene (35 F·g −1 ) at 1 A·g −1 , and was comparable with other reported results on MoS 2 -graphene electrodes [83,[153][154][155][156][157]. Recently, MoS 2 decorated laser-induced graphene on polyimide foil-based flexible electrodes [158] have been reported, and showed excellent electrochemical performance. Furthermore, Mandal et al. [159] reported a high specific capacitance value of 253 F·g −1 for MoS 2 /rGO composites at 1 A·g −1 current density rate, which implies the superiority of MoS 2 nanocomposites for SCs as high-performance electrodes. Meanwhile, multi-walled CNT/MoS 2 composites have shown a better specific capacitance and achieved a maximum of (452.7 F·g −1 ) compared to bare MoS 2 (149.6 F·g −1 ) and bare MWCNT (69.2 F·g −1 ) at a current density rate of 1 A·g −1 [160].
Moreover, the utilization of a conducting template along with molybdenum sulfide also improved the surface area and electrochemical performance, and a few classical references are discussed herein. Porous tubular C/MoS 2 composites using porous aluminum oxide as a template were prepared for the first time by Hu et al. [161], and the prepared electrodes delivered a high capacitance of 210 F·g −1 at 1 A·g −1 with a very good cycling rate. In another typical case, hydrothermally synthesized C/MoS 2 having flower-like morphology exhibited a capacitance value of 201.4 F·g −1 at 1 A·g −1 [162]. Kumuthini et al. [163] prepared MoS 2 @C nanofiber electrodes using an electrospinning process, and achieved high capacitance with 100% life cycle, due to their prominent electrochemical properties with improved stability. In addition, conducting templates like Mo foil, PANI, and PPy have been used along with MoS 2 as binder-free electrodes for SCs in recent years [164][165][166], but the cycling stability and performances of the MoS 2 -based SCs are not satisfactory and are still a challenge.

Other Transition Metal Sulfides
(a) Bi 2 S 3 Bi 2 S 3 is a direct band gap (1.4 eV) layered semiconductor material, and exists mostly in orthorhombic form. In recent years, more attention has been paid to it due to its specific electrical and optical properties, and it has found potentially applicable in the fields of SCs, photocatalysis, sensors, and batteries [167,168]. The important properties of Bi 2 S 3 leading to SC applications are discussed herein. Rod-like Bi 2 S 3 micro flowers have been synthesized and characterized for their application in SCs; they provided a maximum specific capacitance of 185.7 F·g −1 at 1 A·g −1 [169]. Similarly, a recent report on hetero-structured Bi 2 S 3 nanorod/MoS 2 nanosheet electrodes showed a specific capacitance of 1258 F·g −1 at 10 A·g −1 with 92.6% of capacitance retention [85]. Later on, Raut et al. synthesized Bi 2 S 3 thin films on stainless steel using a successive ionic layer adsorption and reaction (SILAR) method, which improved capacitance performance with long-term cyclability [84].
(b) La 2 S 3 Due to its stable transition state, the rare earth element lanthanum-based chalcogenides are considered as promising for use in SC electrodes in the current era. Depending on the experimental conditions, lanthanum sulfide exists in different forms, including LaS, La 2 S 3 , and La 3 S 4 , which possess excellent pseudocapacitive behavior and high electronic conductivity similar to other metal sulfides [170]. However, reports on these electrode materials are highly limited, due to their synthetic routes [171]. Most of the reported results on La 2 S 3 leading to asymmetric SCs have been synthesized using the SILAR method. For instance, Patil et al. [172] prepared La 2 S 3 thin films on a stainless steel substrate using the SILAR method and studied its electrochemical performance. The resultant electrode delivered a specific capacitance of 256 F·g −1 using LiClO 4 /PC electrolyte, while the same La 2 S 3 electrodes in aqueous electrolytes, such as KOH and Na 2 SO 4 , offered a maximum capacitance of 358 F·g −1 at 5 mV·s −1 [86]. Later on, they extended their studies to the effect of annealing on these La 2 S 3 electrodes prepared using chemical bath method, which improved the specific capacitance of the electrodes drastically [173]. Their air-annealed La 2 S 3 electrodes achieved a maximum of 294 F·g −1 at 0.5 mV·s −1 , which was much higher than bare La 2 S 3 electrodes.
(c) WS 2 WS 2 -based electrode materials are receiving increased attention for applications in SCs, owing to their high specific surface area and adaptable electronic structures. In brief, naturally occurring WS 2 possesses a hexagonal crystal structure with space group P63/mmc. Each WS 2 monolayer consists of an individual layer of W atoms with six-fold coordination symmetry, which is then hexagonally packed between two trigonal atomic layers of S atoms [87]. Though it possesses a number of advantages, it did not have electronic conductivity as high as zero band gap graphene, which hampers the direct stand-alone application of WS 2 in SCs. Quite a few reported results are displayed herein. Ratha and coworkers [87] reported the fabrication of WS 2 /rGO electrodes using a one-step hydrothermal method; these electrodes were capable of delivering 350 F·g −1 at 2 mV·S −1 . Furthermore, the mechanism of WS 2 /rGO nanosheet electrodes was explained by Tu et al. [174]. In the short-term, the charges were stored in the pseudocapacitor via the redox reactions of W 6+ and W 4+ on WS 2 , as well as by the O-containing surface functionality on the surface of rGO. It showed excellent specific capacitance with remarkable capacitance retention. Later on, a series of 2-D transition metal carbides (TMCs) , including WS 2 , were investigated, and their strong influence on capacitance studied by Martinez et al. [175]. Bissett et al. [176] analyzed liquid-phase exfoliated WS 2 electrodes for SCs that offered a maximum specific capacitance of 3.5 F·g −1 at 10 mV·S −1 .

Transition Metal Selenides
The charge storage mechanism and electrochemical properties of transition metal sulfides for SC applications were discussed in the previous section. This section is purely devoted to a discussion on selenium-based metal chalcogenides for SC applications. Selenium, the nearest neighbor of sulfur in the VI A group, possesses the same valence electrons and oxidation number as sulfur [177]. Hence, the chemical and electrochemical activities of metal selenides almost resemble a metal sulfide, which indicates that the metal selenides may also have promising applications in SCs [178]. Some of the important metal selenides are discussed herein.

Nickel Selenide
Among the transition metal chalcogenides studied, nickel selenides are of particular interest due to their tunable electronic configuration and multiple oxidation states. In addition, they possess resistivity below 10 −3 Ohm·cm −1 , due to their paramagnetic nature, which makes them suitable candidates for energy storage devices, especially for SCs [179]. To date, reports on NiSe 2 -based SCs are very limited, due to their highly complicated synthetic routes. The synthesis of NiSe 2 involves multiple steps, which has led to more expensive capital cost in bulk scale preparations.
Recently, Wang et al. [180] synthesized truncated cube-like NiSe 2 single crystals using a simple hydrothermal approach, and deeply studied its electrochemical performance. These electrodes offered a maximum specific capacitance of 1044 F·g −1 at 3 A·g −1 with an excellent rate capability. Similarly, hexapod-like two-dimensional NiSe 2 single crystals have been investigated; their electrode delivered a maximum capacitance value of 75 F·g −1 at a current density of 1 mA·cm −2 with a capacitance retention rate of 94% [181].

Copper Selenide
Inexpensive, semiconducting CuSe has been applied in the fields of optoelectronics, thermoelectrics, and solar cells [182,183]. Due to its variable oxidation states and high electrical conductivity, it is capable of delivering good electrochemical properties. Nevertheless, no reports have hitherto become available on the electrochemical properties of CuSe and only a few studies have been published on CuSe-based SCs. The binder-free pseudo capacitive CuSe 2 /Cu electrodes have been synthesized using a simple hydrothermal method, and the reported electrodes delivered a high specific capacitance of 1037.5 F·g −1 at 0.25 mA·cm −2 [184]. Moreover, vertically oriented CuSe nanosheet films have recently been developed, and their use in solid-state flexible SCs explored; they exhibited a specific capacitance value of 209 F·g −1 [185]. Shinde et al. [186] reported Cu 2 Se nanodentrites as electrodes for high-performance SCs. However, the electrochemical properties of CuSe have not yet been fully identified, and future research is likely to be in the direction of developing high-performance SCs using CuSe electrodes.

Molybdenum Diselenide
In MoSe 2 , the molybdenum atom is squashed between two selenium atoms by means of strong covalent bonds that characterize the Se-Mo-Se interaction. It has high theoretical capacitance and comprises low-cost and abundant elements. The stacked layers are held together by weak van der Waals forces responsible for ion migration during the CD process. To the best of our knowledge, only very few studies have become available on MoSe 2 for SC applications.
Balasingam et al. [187] reported layered MoSe 2 in a two-electrode configuration using H 2 SO 4 electrolyte, in which the electrodes possessed very good specific capacitance of 199 F·g −1 at 2 mV·s −1 . The electrode cell's specific capacitance was increased further by combining MoSe 2 with rGO to attain a maximum of 211 F·g −1 with excellent cyclability. Later on, Haung et al. [188] studied and reported the electrochemical performances of MoSe 2 /graphene composites for SC applications, and the same group recently grew MoSe 2 -based electrodes on Mo-foil and reported their capacitance behavior [189]. Furthermore, low-cost MoSe 2 /MWCNT electrodes have been prepared recently using dip and dry, followed by a chemical bath deposition method [190]. The remarkable performance of the electrodes implies that they would be a potential candidate for high-performance SCs.

Cobalt Selenides
In recent years, cobalt selenide-based materials have been a new research hot spot in the field of electrochemical SCs, due to their cost-effectiveness and highly reversible nature. There are a variety of compounds including CoSe 2 , CoSe, Co 0.85 Se, Co 3 Se 4 , and Co 2 Se 3 [191], which have been synthesized using various synthetic routes. To date, very few pleasing results for cobalt selenides and their composites in electrochemical energy storage systems have been published in this regard, which is discussed in this section.
(a) Co 0.85 Se Polycrystalline Co 0.85 Se nanotubes having a hollow nanostructure were successfully prepared and investigated by Wang et al. [191]. They also compared the electrochemical and cycling properties of Co 0.85 Se nanotubes with Co 0.85 Se nanoparticles [192], the obtained results indicating that the specific capacitance, cycling stability, and rate capability of Co 0.85 Se nanotubes were superior to those of Co 0.85 Se nanoparticles. Additionally, Co 0.85 Se hollow nanowires have been previously efficaciously synthesized and used as efficient pseudocapacitive electrodes for SCs [193]. Interestingly, Peng et al. [194] employed Co 0.85 Se nanosheets as the positive electrode, and nitrogen-doped porous carbon network as the negative electrode, to fabricate asymmetric SCs, which yielded an energy density of 21.1 W·h·kg −1 at a power density of 400 W·kg −1 with excellent capacitance retention of 93.8%. Later on, Zhao et al. [195,196] used activated carbon as the negative electrode instead of nitrogen-doped porous carbon network, and reported an energy density of 17.8 W·h·kg −1 at a power density of 3.57 kW·kg −1 for Co 0.85 Se/AC asymmetric SCs. Meanwhile, Gong et al. [197] replaced Co 0.85 Se nanosheet positive electrodes with Co 0.85 Se nanosheet/Ni-foam, which provided a significant increase in energy density with outstanding cycling stability (39.7 W·h·kg −1 at a power density of 789.6 W·kg −1 ).

(b) CoSe 2
There has hardly been any investigation into using CoSe 2 as an electrode material for SCs, and up until now, very few studies have reported using it. However, systematic investigations on the electrochemical performances of metal chalcogenides, such as CoSe 2 and CoTe 2 , have successfully analyzed and studied their applicability as high performance SCs [198,199]. The CoSe 2 electrodes delivered a maximum specific capacitance of 951 F·g −1 at 5 mV·s −1 , which is three times higher than that of CoTe 2 . Later on Zhang et al. [200] assembled solid state asymmetric SCs using N-doped CoSe 2 /C as pseudocapacitive electrode whose electrochemical properties have not yet been fully studied. Hence, much effort will be focused on developing these electrodes in the near future.

Binary Metal Selenides
The binary metal selenides are currently highly fascinating, and only a scarce number of reports on their use in SC applications are available, and their electrochemical performance has not yet been fully studied. Nevertheless, some classical examples are presented here. Xia et al. [204] showed that (Ni, Co) 0.85 Se was able to deliver a highest areal capacitance of 2.33 F·cm −2 at 4 mA·cm −2 current rate. This super-hydrophilic electrode had metal-like electronic conductivity and offered a maximum conductivity of 1.67 × 10 6 S·cm −1 . Similarly, Ni-Co-Se nanowires have shown a high specific capacitance of 86 F·g −1 at a current density of 1 A·g −1 and excellent cycling stability, with virtually no decrease in capacitance after 2000 continuous CD cycles [201]. More recently, Peng et al. [205,206] prepared two different selenide nanosheet-array electrodes comprising NiSe@MoSe 2 and Ni 0.85 Se@MoSe 2 , using a hydrothermal method, and studied their use in asymmetric SC applications.

Summary and Outlook
Currently, the development of SCs as electrochemical energy storage devices is of major importance, with the spotlight on their high power density. A typical SC is composed of electrode material and electrolyte. In an assessment of electrochemical SC devices other than those with a long lifecycle, both their energy and power densities are the two most essential properties. In view of this, current major research focused on SCs is on increasing these characteristics and their life cycle to decrease the cost of the electrode materials. The choice of suitable electrode materials with rational nanostructured designs has resulted in improved electrochemical properties for high performance and cost reduction of SCs. In this review, we conferred recent progress in the advancement of non-metallic oxides, transition metal sulfides and selenides were especially highlighted for SC applications.
The major advantages of electrochemical supercapacitors are high power density (1-10 Kw/kg), lifetime (estimated to be up to 30 years), cycle efficiency (98-100%), operating temperatures (−40 to 70 • C), environmental friendliness, and safety. However, the challenges to be focused on supercapacitor are (i) Energy density: For practical application, high energy density electrochemical system is required.
In view of this, the energy density of electrochemical supercapacitors is less than less than of batteries. (ii) Cost efficiency: The commonly employed electrode materials such as high porous surface area carbon materials and RuO 2 are more expensive. Also, the cost of organic electrolytes is far from negligible. (iii) Self-discharge rate: Electrochemical supercapacitors have high in self discharge rate 10-40%/day. Nanostructured transition metal chalcogenides have gained huge consideration due to their distinctive chemical stability, electronic properties, and remarkable structure. Among these, transition metal sulfides have been proven to exhibit superior electrochemical performance compared to their bulk counterparts, because of their novel properties associated with decreased size, unique shape, and defective nature.
Nanoscale structures can effectively improve electrochemical reaction efficiency and utilization of active materials with improved energy and power densities. Extraordinary investigation ought to be done to construct novel electrode materials for SCs, and new ideas and/or design strategies are required in this field. While designing and constructing electrode materials, the researcher ought to take into consideration that they should be abundant, cheap, and eco-friendly for clean technology and potentially be of use in a broad selection of applications.

Conflicts of Interest:
The authors declare no conflict of interest.