Recent Progress in the Core-Shell Nanostructures of the NiMoO₄-Based Composite Materials for Supercapacitor Applications: A Comprehensive Review

Abstract

Supercapacitors have emerged as one of the promising energy storage systems owing to their rapid charge/discharge capability, long-term cycling stability, and high power density. The application of core-shell nanostructures for supercapacitors is one of the effective strategies to achieve a high specific surface area for abundant reaction sites and good electrical conductivity for fast charge transfer, hence improving the performance of supercapacitors. Particularly, the use of NiMoO₄ for the core-shell structure has drawn great attention due to its outstanding advantages, such as its natural abundance, low material cost, superior electrochemical performance, and wide electrochemical potential window in cyclic voltammetry. In this context, this review comprehensively covers the recent progress of the core-shell nanostructures based on the NiMoO₄-composite materials, which find applications in supercapacitors. The composite materials that incorporate metal oxides such as NiMoO₄, metal hydroxides, metal chalcogenides, carbon materials, and conductive polymers are discussed in detail for such core-shell nanostructures with the aim of understanding how the adopted materials and the relevant morphology govern the electrochemical features for supercapacitors. Finally, the existing challenges in current technologies for supercapacitors are discussed, while possible future directions in developing the NiMoO₄-composite-based core-shell nanostructures are proposed for high-performance supercapacitors.

1. Introduction

The fast growth of the global population with industrial development demands the capability of harvesting energy more effectively with minimizing the concomitantly induced ecological hazards. Fossil fuel consumption induces environmental pollution and has increased critical concerns about global climate change as well as the ecological crisis. The International Energy Agency estimated that the energy required to be harvested from conventional fossil fuels shall rise to 18.30 billion tons of oil equivalent (btoe) in 2035 [1]. This prediction that would accelerate environmental concerns has triggered tremendous research interests in exploiting alternative and renewable energy resources, such as solar, wind, geothermal, and tidal energies, from which electricity generation has recently become somewhat effectively possible. However, most renewable energy acquisition would suffer from harvest intermittence, and it is thus crucial to develop large-scale energy storage systems to store electrical energy and offset the gap between electricity generation and energy demand.

There are many energy storage systems available, among which lithium-ion batteries and electrochemical capacitors (ECs) are at the forefront of cutting-edge technologies, as shown in the Ragone plot in Figure 1a [2]. ECs, also called supercapacitors (SCs) or ultracapacitors, occupy a unique position of having modest energy-density (0.1–10 Whkg⁻¹) and moderate power-density (10–10⁶ Wkg⁻¹), consequently bridging the gap between batteries (high energy-density~10–100 Whkg⁻¹; low power-density~2–200 Wkg⁻¹) and conventional capacitors (low energy-density~0.02–0.1 Whkg⁻¹; high power-density~2000–10⁷ Wkg⁻¹) [2]. Over the past decade, many efforts have been devoted to SCs research because of their fascinating advantages of ultrafast charge/discharge rate, high power-density, long cyclic lifespan, high Coulombic efficiency, good safety, and low maintenance cost, with the combination of a variety of applications in consumer electronics, industrial power systems, communication systems, transportation, etc. [3]. Despite such advantages of SCs, the bottleneck that hinders their extensive use is the availability of high energy-density in SC-based devices. Therefore, boosting the energy-density without compromising the other intrinsic benefits of SCs is required. Typically, the SC is composed of a current collector, an electrode, an electrolyte, and a separator, in which the electrode material is the main factor affecting the performance of the supercapacitor.

Based on the energy storage mechanism, supercapacitors are basically classified into electrical double-layer capacitors (EDLCs), pseudocapacitors (PCs), and hybrid supercapacitors, as illustrated in Figure 1b [4]. The EDLCs primarily store charges through the physical adsorption and desorption of ions (non-Faradaic charge storage) taking place in the electrical double layer formed at the electrode/electrolyte interface, which leads to high power-density. Nevertheless, the low specific capacitance and energy-density hinder their widespread applications. Carbon materials, such as porous carbon, activated carbon (AC), carbon fiber, graphene, and carbon nanotube (CNT), are typically employed as electrode materials for EDLCs, owing to their good electrical conductivity, porous structure, and high surface area [5]. On the other hand, PCs store their charges by rapid and reversible electrochemical redox reactions (Faradaic charge storage) at the surface or near the surface of electroactive materials, which can provide remarkably higher specific capacitance compared to EDLCs. Therefore, in the past few years, research attempts have been made to exploit various pseudocapacitor materials, including transition metal oxides (RuO₂ and MnO₂), metal hydroxides, metal sulfides, and conductive polymers (polyaniline, polypyrrole, and polythiophene) for supercapacitor electrodes [6,7]. Meanwhile, the hybrid supercapacitor results from the combination of a high-power EDLC electrode and a high energy-density battery-type electrode (non-capacitive Faradaic charge storage).

As the materials adopted for supercapacitors, mixed transition metal oxides (MTMOs) have gained great attention because of their higher electrochemical activity, higher electrical conductivity, and multiple oxidation states than single metal oxides [8]. Recently, binary transition metal oxides (AB_xO_y) have become promising electrode materials to attain excellent supercapacitive performance. Among them, nickel molybdate (NiMoO₄) with spinel structure can be a potential candidate for supercapacitor application because of its intrinsic properties of high redox potential, high specific capacitance, and natural abundance-based low cost [9]. The rich redox reactions of NiMoO₄ are due to the multiple valence states of nickel ion (Ni²⁺/Ni³⁺) mediated by OH⁻ ions in the alkaline electrolyte (KOH or NaOH). During the electrochemical cycle test, the charge storage on the NiMoO₄ electrode surface arises according to the following redox reaction [10,11,12,13,14],

Ni(OH)₂ + OH⁻ ⇔ NiOOH + H₂O + e⁻

(1)

It is noted that the bulk material of NiMoO₄ still suffers from some issues, for example, low-rate capability and poor cycle performance caused by the dissolution of active materials during cycling tests. This problem, however, can be solved by downsizing the material from bulk to nanoscale, i.e., by synthesizing NiMoO₄ nanostructures [15,16,17]. Compared to the bulk material, the nanostructure can provide more active sites and facilitate electron transport by shortening the diffusion pathway, consequently improving rate capability and cycling stability during the charge/discharge process [18]. In addition to the downsizing benefits, the use of a combination of NiMoO₄ and other transition metal oxides for synthesis can result in greater electrochemical performance. Several reports are available on the core-shell structure, in which an inner “core” material is surrounded by an outer “shell” material having similar or different properties from the core [19]. The core-shell structure is designed such that the core is the active material with functional properties while the shell protects and strengthens the core to augment the supercapacitive performance. The core-shell nanostructure creates hierarchical porous channels for active charge transport and holds high electrical conductivity while maintaining better mechanical stability.

There are many articles solely reporting the development and challenges of transition metal oxides, such as MnO₂, WO₃, CuO/Cu₂O, CuCo₂O₄, and NiCo₂O₄ for supercapacitor applications [7,8,19]. The synthesis strategies, energy storage mechanisms, and electrochemical performance of these core-shell structured composites have been reviewed in detail. But the topic of core-shell structures based on NiMoO₄-composite materials for supercapacitors has not been reviewed so far. Moreover, compared to single metal oxides, the use of mixed transition metal oxides can result in improved electrochemical performance. Considering NiMoO₄, it is interesting to note that Ni and Mo ions individually contribute to redox activity and conductivity. Therefore, this article mainly focuses on the research progress made in the past decade on the core-shell nanostructures of NiMoO₄-based composite materials for supercapacitor applications. Such nanostructures using various NiMoO₄ composites are elaborated in detail with highlights on how the selected material and the corresponding morphology affect the electrochemical features of supercapacitors. Finally, the pre-existing challenges to this approach are addressed with a discussion on how to improve the overall performance of supercapacitors based on NiMoO₄-based core-shell nanostructured composites.

2. Properties of NiMoO₄

2.1. Crystal Structure

At atmospheric pressure, NiMoO₄ exists in 2 phases, namely, a low-temperature α-phase and a high-temperature β-phase. Both phases belong to the monoclinic crystal structure, as shown in Figure 2. The crystal symmetry consists of space group C2/m. The lattice parameters are a = 9.582 Å, b = 8.763 Å, and c = 7.619 Å [20]. A major difference between the 2 phases is in the anisotropic coordination of the Mo⁶⁺ ions. The α- and β- phases of NiMoO₄ are obtained when the Mo⁶⁺ ions occupy octahedral sites (MoO₆) and tetrahedral sites (MoO₄), respectively. But Ni²⁺ ions occupy only octahedral sites in both phases [21,22,23]. Interestingly, the edge-sharing NiO₆ octahedral sites are interconnected by MoO₄ tetrahedral sites to establish a 3D network structure with open channels, which is advantageous for ion diffusion [20]. Further, the α-phase presents doubly ionized vacancies, while the β-phase presents singly ionized vacancies. The α→β phase transition commonly takes place at a temperature of around 600 °C. Hence, the temperature is the main factor in determining the phase and the degree of crystallinity of NiMoO₄. Indeed, when compared to good crystallinity, poor crystallinity is recommended for excellent supercapacitive behavior because it leads to more pathways for electrolyte infiltration and ion transport [24].

2.2. Electrical Conductivity

Generally, in NiMoO₄, the Mo⁶⁺ atom contributes to the intrinsic electrical conductivity, whereas not involved in the redox reaction [25]. The conductivity is of the order of 10⁻⁶ Scm^−1, and notably, β-NiMoO₄ has higher conductivity than α-NiMoO₄ [26,27]. Indeed, the conductivity of NiMoO₄ depends on the band gap energy (E_g~2.81 eV) and compared to carbon materials, is usually unsatisfactory. Fortunately, the process of compositing, doping, and introducing oxygen vacancies could improve the conductivity of NiMoO₄. In addition, the nature of the substrate has a significant influence on the conductivity. If the synthesized material is in a powder form, a binder and conductive reagent are used in the electrode fabrication. The additives can increase the internal resistance and dead volume with an inefficient use of active material, which is unfavorable for electron/ion transfer during the redox reaction, leading to low specific capacitance and poor rate capability. To solve this problem, binder-free NiMoO₄ electrodes can be prepared by directly growing the material on conductive substrates like Ni foam (NF), Ti foil, carbon cloth (CC), and graphite paper [20,28]. These substrates act as the current collector, and their intrinsic advantages include a high specific surface area (SSA), a macroporous structure, and superior electrical conductivity.

2.3. Morphology

The supercapacitive performance is closely associated with the material structure. Since NiMoO₄ is a pseudocapacitive material, its SSA matters for redox reactions that occur at the electrode surface. A larger SSA indicates the presence of a larger number of electroactive sites for the redox process. The SSA of metal oxides mostly depends on the nanostructure. The construction of a nanostructure with unique pore size and large SSA would enhance the capacitance of electrode material. With the aim of developing supercapacitor electrodes of high performance, NiMoO₄ nanostructures with various morphologies have been attempted, such as nanowires [29], nanospheres [30], nanorods [31], nanosheets [20], nanoflowers [32], and nanoflakes [33]. For example, Cai et al. [34] described NiMoO₄ nanorods and nanospheres as α- and β- phases, respectively. NiMoO₄ nanospheres assembled from thin mesoporous nanosheets showed high specific capacitance, good rate capability, and cycling stability, which were attributed to a high surface area (58.2 m²g⁻¹, pore volume is 0.218 cm³g⁻¹) and high electrical conductivity when compared to NiMoO₄ nanorods (13.5 m²g⁻¹, pore volume is 0.030 cm³g⁻¹). At a current density of 1 Ag⁻¹, the specific capacitance was 974.4 Fg⁻¹ with a remarkable energy-density of 20.1 Whkg⁻¹ and a power-density of 2100 Wkg⁻¹.

NiMoO₄ nanosheets are superior to NiMoO₄ nanorods in terms of both specific capacitance and cycling stability. The high porosity of nanosheets ensures a high SSA (surface area~79 m²g⁻¹ and pore size < 10 nm) that is much larger than that of nanorods (surface area~41 m²g⁻¹). This relatively enlarges the electrode/electrolyte contact area and offers abundant active sites for a rapid redox reaction, resulting in improved specific capacitance. Moreover, nanosheets are much thinner than nanorods, supporting fast electron/ion transport, whereas they are likely to resist volume change during the charge/discharge cycles [20].

3. Synthesis

Synthesis methods have a significant impact on the morphology, crystallite size, conductivity, and electrochemical performance of electrode materials. A variety of methods are employed for synthesizing NiMoO₄, such as hydrothermal, microwave-assisted synthesis, electrodeposition, etc. Some common methods are briefly discussed in this section.

3.1. Hydrothermal Method

The hydrothermal method commonly used for efficiently preparing metal oxide nanostructures requires no complex, sophisticated equipment. In this method, the precursors are dissolved in deionized water, and the pH value is regulated by adding acid or base into the precursor solution. The solution is then transferred into a Teflon-lined stainless steel autoclave to complete the reaction at a temperature over 100 °C for a specific duration of time. The end-product is centrifuged, collected, and finally dried in a vacuum. Compared to other chemical techniques, this method can produce nanostructures of high purity with various types of morphology [35,36,37].

Cai et al. [38] produced ultrathin mesoporous NiMoO₄ nanosheets using a one-step hydrothermal method. Even at a high current density of 20 Ag⁻¹, the material still maintained a specific capacitance of up to 1200.5 Fg⁻¹ and capacitance retention of about 75% as the charge/discharge rate changes from 2 to 20 Ag⁻¹. The superior electrochemical performance was attributed to ultrathin mesoporous structure and high electrical conductivity. The hydrothermal method also offered the robust advantage of preparing binder-free nanostructures directly on NF-like conductive substrates. Adhesion between the nanostructured network and NF allowed for fast electron transport, efficiently enhancing the cycling stability and rate capability even at high current density. The 3D NiMoO₄ nanoplate arrays synthesized on NF produced a capacitance of 3.4 Fcm⁻² at 2 mAcm⁻² with notable cycling stability (87% capacity retention after 3000 cycles) [39]. Hong et al. [40] also reported, by a hydrothermal method, the solubility-dependent NiMoO₄ nanostructures, with a capacitance of 1335 Fg⁻¹ at 1 Ag⁻¹ and cycling stability of 81.3% over 3000 charge/discharge cycles. Feng et al. [41] synthesized NiMoO₄ nanoflakes on an N-doped graphene surface, which displayed a specific capacitance of 1913 Fg⁻¹ at 1 Ag⁻¹.

3.2. Microwave-Assisted Method

The microwave-assisted synthesis offers certain advantages over hydrothermal methods, such as cleanliness, high efficiency, low energy consumption, nucleation, and crystallization in a short time duration [42]. The sheet-like NiO/NiMoO₄ hybrid nanostructures synthesized by this method exhibited a higher specific capacitance (1147.5 Fg⁻¹) at 1 Ag⁻¹ compared to the material synthesized using the hydrothermal approach (677.8 Fg⁻¹). In another work, the 2D amorphous NiMoO₄ nanoflakes prepared by this technique showed a high specific capacitance of 1650 Fg⁻¹ [43].

3.3. Electrodeposition Method

The electrodeposition method is also used to develop NiMoO₄ nanostructures in a three-electrode system. The end products are deposited on the conductive substrate in a homogeneous solution comprising the precursor salts. Typically, the method is employed by a subsequent thermal annealing process to prepare a nanoporous structure. For example, Kumbhar et al. [44] synthesized a honeycomb-like NiMoO₄ structure with a range of electrochemical cycles (40, 60, 80, and 100). The electrode obtained by 80 cycles exhibited a high specific capacitance of 1475 Fg⁻¹ at 1 Ag⁻¹ and a good rate capability of 72.8% at 20 Ag⁻¹, while the electrode exhibited 87.9% capacitive retention after 5000 cycles.

4. Composites of NiMoO₄ Core-Shell Nanostructures for Use as Electrode Materials

A core-shell structure usually consists of a sphere-shaped central medium, i.e., a core, with a concentric film around it, i.e., a shell. This structure can be configured by joining two materials with different inherent properties, for instance, a core with high electrochemical activity and a shell with high electrical conductivity. The synergistic effect of the use of heterogeneous materials for a core and a shell can lead the supercapacitor of the core-shell format to possess better energy storage ability compared to a single material-based structure.

The core material efficiently transfers the charges while contributing to electro-capacitance. As the core materials, carbon materials, metals, metal chalcogenides, and metal oxides have been used. Among those, carbon materials and metals are known to benefit from high conductivity that ensures efficient charge transfer during the charge/discharge process. Meanwhile, the shell, i.e., a thin layer that is grown on the core and would thus exhibit similar morphological properties to the core, has the role of affording a high surface area for accumulating charges and providing a huge number of active sites to conduct redox reactions. Carbon materials, conductive polymers, semiconductors, metal sulfides, and metal oxides could be used as shell materials [45,46].

NiMoO₄, a naturally abundant pseudocapacitive metal oxide, has been considered a good candidate for either a core or shell material due to its natural abundance, the material’s low cost, exceptional redox reaction, and wide electrochemical potential window in cyclic voltammetry. Nevertheless, the primary concern regarding the use of the NiMoO₄ nanoparticles would arise from the material-intrinsic low electrical conductivity and the high probability of aggregation of the particles. This could limit electron/ion transport, thus reducing overall electrochemical performance [47,48]. To tackle the challenges, the rational design of a hybrid core-shell nanostructured electrode of NiMoO₄-based composite materials can be an alternative to boost overall electrochemical characteristics, including rapid electron/ion transfer kinetics, high specific capacitance, and fine structural tunability. The NiMoO₄-based composite materials consist of NiMoO₄ with metal oxides (ZnCo₂O₄, SnO₂)/metal hydroxides (NiCo-LDH)/metal chalcogenides (NiCo₂S₄, Ni₃S₂)/carbon materials (CNTs, graphene, activated carbons, etc.)/conductive polymers (PANI, PPy).

4.1. NiMoO₄-Metal Oxide Composite

The NiMoO₄-metal oxide composite has been used for core-shell nanostructured electrodes due to good electrochemical features [49,50,51]. Table 1 summarizes the electrochemical properties of the core-shell nanostructures of this kind of composite reported in the literature. MnO₂, a metal oxide with pseudocapacitive properties, has been incorporated into NiMoO4 nanowires, forming the hybrid core-shell structure, i.e., NiMoO₄-MnO₂ on CC [52]. During the hydrothermal synthesis, the NiMoO₄ nanowire acted as a ‘backbone’ to guide the MnO₂ self-assembling growth in an aqueous solution without surfactant and stabilizer, as shown in Figure 3a. Figure 3b illustrates that NiMoO₄ nanowires were tightly attached to MnO₂ nanoflakes. The presence of spacing between NiMoO₄ nanowires facilitated electrolyte ion diffusion into the inner region of electrodes, and the CC could offer abundant electron transfer channels to enhance the electrochemical performance (Figure 3c). The hybrid nanostructure of NiMoO₄-MnO₂ delivered a higher areal capacitance (3.90 Fcm⁻² at 8 mAcm⁻²) than NiMoO₄ nanowires. NiMoO₄-MnO₂ nanosheet arrays on Ti mesh showed a specific capacitance of 976 Fg⁻¹ at 1 Ag⁻¹ [53]. The 3D α-NiMoO₄-δ-MnO₂/NF nanorod/nanosheet structures were fabricated through hydrothermal and chelation-mediated aqueous processes [54]. The specific capacitance initially decreased at cycles up to 1000 due to the increase of equivalent series resistance. After 1000 cycles, the petal-like δ-MnO₂ nanosheets served as a protective layer to keep the grass-like α-NiMoO₄ structural integrity during the remaining redox reaction. As a result, superior cycling life with a 101.9% capacitance retention rate was attained (5000 cycles), being much better than that of α-NiMoO₄ nanorods (78.5%). The 1D MnO₂-NiMoO₄ nanostructured electrode delivered a maximum specific capacitance of 1123.7 Fg⁻¹ at a scan rate of 5 mVs⁻¹ and a super-long cycling life with a 115.5% retention rate after 5000 cycles [55]. It also possessed tremendous flexibility, without obvious change of specific capacitance after bending (30°–150°).

Co₃O₄ has received special attention because of its superior capacitance (3560 Fg⁻¹), easy synthesis, good electrochemical stability, and great reversibility [57]. Therefore, the NiMoO₄-Co₃O₄ composite was anchored on rGO/NF for different reaction times (1, 3, 5, and 7 h) [58]. With increasing the scan rate (5–50 mVs⁻¹), the CV plots of the NiMoO₄-Co₃O₄-5H electrode maintained a similar form. Besides, the shift of cathodic and anodic peaks toward negative and positive potentials was ascribed to the resistance and polarization effect of the electrode, respectively. At a low scan rate, the rate of ion diffusion was greater than that of the electron release. At a high scan rate, the redox peak shift was due to the limited intercalation of electrolyte ions into the dense center of the composite, while ion diffusion was inadequate to satisfy electron neutralization. The composite combined the advantages of the large specific capacitance of NiMoO₄ and the great rate capability of Co₃O₄. The prepared NiMoO₄-Co₃O₄-5H composite exhibited a specific capacitance of 1722.3 Fg⁻¹ at 1 Ag⁻¹, a good rate capability of 80.8% at 10 Ag⁻¹ and cycling stability of 91.1% (6000 cycles). High Coulombic efficiency of about 99.4% showed good electrochemical reversibility.

Many research groups employed hydrothermal methods to grow the NiMoO₄ nanosheets on the backbone of Co₃O₄ material and prepare the hierarchical 3D Co₃O₄-NiMoO₄ heterostructure on NF and CC [59,60,61,62,63,64]. Hierarchical Co₃O₄-NiMoO₄ core-shell nanostructure may hold the advantages of improved rate capability resulting from Co₃O₄ nanowires and high SSA of NiMoO₄ nanosheets. Hu and co-workers [65] utilized MOF-derived Co₃O₄ nanosheets and NiMoO₄ nanosheets as core and shell materials, respectively. ZIF-67 precursor was converted into leaf-like Co₃O₄ through calcination in air. The areal-specific capacitance achieved a maximum of 2.3 Fcm⁻² at 1 mAcm⁻² due to the high mass loading and the synergistic effect of materials. Dong et al. [66], for the first time, synthesized tube-like Co₃O₄-NiMoO₄ yolk-shell composite via a two-step hydrothermal route. Here, ultrathin NiMoO₄ nanosheets (thickness~200 nm) are vertically arranged and interconnected together to develop a porous shell, covering the Co₃O₄ fiber with interspaces between the core and shell. The unique mesoporous structure enhances the contact area of the electrode and electrolyte, as well as increases the electrochemically active sites. All CV curves tested under different scan rates (5–100 mVs⁻¹) show a pair of strong redox peaks, representing the pseudocapacitive behavior. The specific capacitance reached 998.05 Fg⁻¹ at 0.5 Ag⁻¹. Ultrathin and porous NiMoO₄-CoMoO₄ nanoflakes were constructed on hollow Co₃O₄ nanowires through a hydrothermal route, followed by an activation process in the presence of KOH and calcination treatment at 700 °C [67]. The role of KOH (excessive and without) on the porous structure of Co₃O₄-NiMoO₄-CoMoO₄ heterostructure was analyzed. After the KOH activation, both the pore volume and SSA (253.5 m²g⁻¹) increase. This is due to the Co-precursor decomposition and reaction with KOH at 700 °C, creating K₂CO₃, H₂O, and CO₂, which makes tiny holes in the end- product. The long-range ordered 3D porous structure produces nanopores of~2–5 nm in size. But the excess KOH induces aggregation and strong gas formation, resulting in the bulk composite. The high SSA, improved mass transfer, open and porous nanostructure, and more active sites contributed to electrochemical performance. The enhanced use of NiMoO₄-CoMoO₄ shell layer on the Co₃O₄ core helps to gain extra electrons, which increases the OH⁻ adsorption at the shell surface, leading to high specific capacity (272 mAhg⁻¹ at 1 Ag⁻¹). This work paved the way to synthesize materials with the structure of M₃O₄-MMoO₄-MMoO₄ (M = Fe, Ni, Sn, etc.).

The binary metal oxides usually display better electrochemical performance than the single metal oxides. Particularly, the transition metal cobaltites, such as NiCo₂O₄, ZnCo₂O₄, CuCo₂O₄, and MnCo₂O_4, have multiple oxidation states as well as high electrical conductivity. NiCo₂O₄ is popular because of its fast reversible redox reactions, therefore integrated with NiMoO₄ to increase the specific capacitance and rate capability [68,69,70,71]. Hierarchical NiCo₂O₄-NiMoO₄/NF nanowire/nanosheet arrays (NWSAs) were synthesized by changing hydrothermal reaction time (4, 8, 12, and 16 h) [72]. In Figure 4a, the TEM image reveals that the NiCo₂O₄ nanowire is covered by a NiMoO₄ nanosheet. Even for a long reaction time (12 h), the shell region of the interconnected network consisting of highly porous NiMoO₄ nanosheets is retained well, which can offer more electroactive sites for redox reactions. HRTEM measurements of the shell material exhibit a set of clear lattice fringes with an interplanar spacing of ca. 0.214 nm, corresponding to the (121) planes of NiMoO₄ nanosheets (Figure 4b,c). The EDS mapping images confirm that the NiMoO₄ shell material is effectively coated on the backbone of the NiCo₂O₄ nanowire, as displayed in Figure 4d. The development of a 3D interconnected network is advantageous for facile electrolyte infiltration and rapid electron transport. All the CV curves show a similar shape even at a high scan rate of 50 mVs⁻¹, indicating outstanding electrochemical reversibility and high-rate performance (Figure 4e). Figure 4f represents the discharge curves of the NiCo₂O₄-NiMoO₄ electrode (12 h) with a potential range of 0–0.5 V at different current densities (10–80 mAcm⁻²). The optimized NiCo₂O₄-NiMoO₄ electrode (12 h) delivered a good areal capacitance of 5.80 Fcm⁻² at 10 mAcm⁻² (Figure 4g). The asymmetric supercapacitor (ASC) of NiCo₂O₄-NiMoO₄//AC attained a high energy-density of 21.7 Whkg⁻¹ (Figure 4h,i). Both reaction time and growth temperature strongly influence the morphology of NiCo₂O₄-NiMoO₄/NF nanowires [73]. For an 8 h reaction time, the NiCo₂O₄ core surface was almost covered by small NiMoO₄ nanoflakes. At a low temperature of 80 °C, the rod-like morphology was obtained. When the temperature increased to 120 °C, nanorods were broken and perfectly formed as nanowire arrays at 160 °C. However, at 200 °C, nanowires fully disappeared and mostly grew into nanosheets. At 240 °C, these nanosheets were completely interconnected with each another, creating an intricate transportation network. The areal and mass-specific capacitances were 7.55 Fcm⁻² and 1242 Fg⁻¹, respectively, at 10 mAcm⁻². By changing the reaction time, nanosheets and nanoplates of NiMoO₄ were grown onto the backbone of NiCo₂O₄ (nanowires and nanosheets), and the electrode performance was considerably good [74,75,76]. For NiMoO₄, it is clear that the sheet-like morphology can be observed at a prolonged reaction time.

A flexible and binder-free electrode of NiCo₂O₄-NiMoO₄ nanowires was grown on CC [77]. Because of the high electrical conductivity of NiCo₂O₄, electrons can easily transport between CC and NiMoO₄ nanosheets via the backbone of NiCo₂O₄ nanowires. The areal capacitance was 2.917 Fcm⁻² at 2 mAcm⁻², and cycling stability was 90.6% (after 2000 cycles). Hong and co-workers [78] constructed NiCo₂O₄ and NiCo₂O₄-NiMoO₄ nanostructures on two different substrates (NF and CC). They investigated the effect of substrate on morphology, surface area, and porous structure. Interestingly, the morphology was nearly independent of the substrate. A large surface area and pore volume were attained for the electrode corresponding to CC. However, NiCo₂O₄-NiMoO₄/NF showed better performance due to the high electrical conductivity and the formation of the nickel oxide/hydroxide layer on NF. NiMoO₄ nanosheet arrays were grown on sea urchin-like NiCo₂O₄ to develop a mesoporous 3D NiCo₂O₄-NiMoO₄ structure [79]. The 3D honeycomb-like structure of NiCo₂O₄-NiMoO₄ nanofilm/nanoflake arrays was synthesized on NF [80]. Hierarchical NiCo₂O₄-NiMoO₄ nanostructures were designed with two different morphologies of NiCo₂O₄ scaffolds (uninterrupted nanosheet arrays (UNSAs) and nanoneedle arrays (NNAs)), and the effect of morphology on the electrochemical performance was investigated [81]. At 2 mAcm⁻², the NiCo₂O₄-UNSA-NiMoO₄ electrode delivered a high areal-specific capacitance and mass-specific capacitance of 7.29 Fcm⁻² and 1941 Fg⁻¹, respectively. But the corresponding values for NiCo₂O₄-NNA-NiMoO₄ were 5.96 Fcm⁻² and 1560 Fg⁻¹. Further, the NiCo₂O₄-UNSA-NiMoO₄ electrode showed outstanding rate capability (84.1%) at 60 mAcm⁻², compared to NNA-NiMoO₄ (73.5%). This electrochemical superiority is largely assigned to ultrathin NiMoO₄ nanosheets and hierarchical mesoporous structure, due to which the electroactive surface area increased; therefore, ion transport became easier within the electrode. Moreover, the UNSAs electrode can produce enhanced reversibility and lower charge transfer resistance (0.758 Ω) compared to NNAs (1.438 Ω), particularly at long charge/discharge cycles. Due to the larger lateral size of~2–4 µm and interconnection, NiCo₂O₄ nanosheets provided more electron transport channels, resulting in low resistance. But, in the case of NiCo₂O₄ nanoneedles, the electrical conductivity was very much limited by its cusp feature.

The multi-dimensional NiCo₂O₄-NiMoO₄ nanowire/nanosheet arrays were hydrothermally synthesized on CC [82]. The shell thickness of NiMoO₄ was controlled by urea. The thick NiMoO₄ shell serves as a protective layer to forbid the structural collapse of NiCo₂O₄ nanowires during the redox reaction. Due to the high SSA, large pore volume, low charge transfer resistance, and improved strain accommodation, the NiCo₂O₄-NiMoO₄ electrode with a thick NiMoO₄ shell exhibited favorable electrochemical performance, delivering a high areal capacitance of 2522 mFcm⁻² at 1 mAcm⁻². Even after 5000 cycles, the core-shell structure was sustained, and NiCo₂O₄ nanowires remained largely unaffected without structural collapse. Hong et al. [83] reported that substrate has a great influence on the morphology of NiCo₂O₄-NiMoO₄ rather than a structure-directing agent (SDA). They used different structure-directing agents (urea, hexamethylenetetramine (HMT), and ammonium fluoride (NH₄F)) and substrates (NF and CC). When NF was used, the electrochemical performance was better. The highest specific capacitance (C_F) of 4.05 Fcm⁻² was obtained for the NiCo₂O₄-NiMoO₄/NF electrode synthesized by NH₄F, which is assigned to the smallest R_s (1.24 Ω) and R_ct (0.99 Ω) values arising from the influence of NF substrate, even if the surface area of active material is too small (14.52 m²g⁻¹). The assembled device delivered excellent Coulombic efficiency (more than 95%), as well as flexibility without any capacitance decay under 150^o bending. The highly ordered and vertically aligned NiCo₂O₄ nanoflakes covered by NiMoO₄ nanoparticles were synthesized by electrochemical deposition [84]. The NiCo₂O₄/NiMoO₄ electrode exhibited enhanced performance with a specific capacitance of 3705 Fg⁻¹ at 1.5 Ag⁻¹ and a rate capability of 3525 Fg⁻¹ at 30 Ag⁻¹ (95.1%). The cycling stability was assessed for the first 5000 charge/discharge cycles at 30 Ag⁻¹. With the increasing number of cycles, the specific capacitance slightly improved and attained its maximum at 3530 Fg⁻¹. This was explained by the electromotive force created due to the applied current density so that electrolyte ions enter into the pores of the active material. As a result, the heterogeneous surface transit from the Cassie-Baxter state to the Wenzel state. After 5000 cycles, 94.6% of initial capacitance was retained. This may be due to the electrochemically and/or mechanically detached active material arising from the strain produced during the Faradaic redox reactions. The 3D flower-like NiCo₂O₄-NiMoO₄/rGO hybrid composite was grown on NF [85]. Urchin-like NiCo₂O₄ nanoneedles were covered by crosslinked NiMoO₄ nanoflakes, over which rGO nanosheets were deposited. The NiMoO₄ nanoflakes spatially filled the NiCo₂O₄ nanoneedle surface and gaps between them, producing a porous structure together with high SSA (~79.7 m²g⁻¹, pore volume~0.24 cm³g⁻¹); meanwhile, rGO contributed to improved electrical conductivity. Therefore, a high areal specific capacitance of 9.41 Fcm⁻² (1837.89 Fg⁻¹) at 10mAcm⁻² was achieved, along with 75% capacitance retention after 2000 cycles. After the cycle test, the overall 3D porous structure and morphology were still preserved. Nevertheless, the GO coating realized some damage due to multiple cycles. The honeycomb structure of the NiMoO₄-NiCo₂O₄/NF electrode with folded and silk-like morphology was fabricated using chemical bath deposition [86]. It exhibited a high specific capacitance of 2695 Fg⁻¹ at 20 mAg⁻², which was better compared to NiCo₂O₄ nanoplates (1018 Fg⁻¹) and NiMoO₄ honeycombs (1194 Fg⁻¹).

Recently, attention has been paid to ZnCo₂O₄ owing to its high theoretical capacitance, rich redox reaction, and diverse morphology [87]. A smart strategy was used to construct a reduced-ZnCo₂O₄-NiMoO₄·H₂O core-shell heterostructure [88]. ZnCo₂O₄ and NiMoO₄·H₂O were hydrothermally synthesized, and oxygen vacancies were introduced into ZnCo₂O₄ through chemical reduction (rZnCo₂O₄), which led to the wrinkled surface of ZnCo₂O₄ nanowires, along with poor crystallinity. The excess oxygen vacancies destroyed the integral structure, which resulted in poor electrochemical performance. The optimized electrode of rZnCo₂O₄-NiMoO₄·H₂O (for 3 h) showed the highest areal capacitance of 3.53 Fcm⁻², which was higher than that of rZnCo₂O₄ (3 h) (127.7%) and the pristine ZnCo₂O₄ (320.2%). The improved cycling stability is due to the large electrical conductivity and abundant active sites introduced by oxygen vacancies. In the case of 3D ZnCo₂O₄-NiMoO₄ heterostructure developed on NF, the plate-like ZnCo₂O₄ was confirmed to be covered by interconnected NiMoO₄ nanosheets [89]. The surface of ZnCo₂O₄ plates consists of highly-entangled grains and pore channels (pore size~10–20 nm). HRTEM images and the fast Fourier transform diffraction (FFT) patterns revealed two distinct regions in the ZnCo₂O₄/NiMoO₄ heterostructures: the primary region consists of NiMoO₄ shell and rigid ZnCo₂O₄ core, and the secondary region linked with the layered NiMoO₄ nanosheets. The stable, porous, and conductive features benefitted the high areal and mass-specific capacitances of 6.07 Fcm⁻² and 1480.48 Fg⁻¹, respectively, at 2 mAcm⁻². NiMoO₄ nanosheet arrays were hydrothermally grown on ZnCo₂O₄ nanowires anchored on NF. At 1 Ag⁻¹, the specific capacitance reached a maximum of 1912 Fg⁻¹ [90]. In another work, different nanostructures of ZnCo₂O₄ (nanowires and nanosheets) were used as skeletons to grow NiMoO₄ [91]. The ZnCo₂O₄-NiMoO₄ nanosheets delivered high specific capacity (1158 Cg⁻¹ at 10 mAcm⁻²) and excellent cycling stability (103.4% after 5000 cycles), which was much better than that of ZnCo₂O₄-NiMoO₄ nanowires (913.5 Cg⁻¹). The reason might be the unique nanostructure, where the ZnCo₂O₄ nanosheets are strongly interconnected to maximize the surface exposure and also shorten the ion transport distance. Additionally, the NiMoO₄ shell layer provided structural integrity and low IR drop during the charge/discharge process.

In CuCo₂O₄-NiMoO₄/NF nanowire-nanosheet arrays (diameter~300 nm), NiMoO₄ nanosheets were closely covered on CuCo₂O₄ nanowires, and shell thickness was~100 nm [92]. A pair of strong redox peaks is due to the reversible processes of Ni³⁺/Ni²⁺, Cu²⁺/Cu⁺, and Co⁴⁺/Co³⁺. The specific capacitance reached 2207 Fg⁻¹ at 1.25 Ag⁻¹, and the capacitance loss was only about 4.4% from its initial value after 5000 cycles. Urchin-like CuCo₂O₄-NiMoO₄ architecture was constructed on NF [93]. The CuCo₂O₄ microspheres consist of 1D CuCo₂O₄ nanoneedles oriented and assembled in a radial form from the center to offer open space, enhancing the charge transport. For a 4 h reaction time, a maximum specific capacity of 276 mAhg⁻¹ was achieved at 1 Ag⁻¹. This is ascribed to the 2D NiMoO₄ nanosheets, which can increase the active sites, ease ion transport, and diminish the volume change during long-term cycling. The MnCo₂O₄-NiMoO₄/NF electrode exhibited an improved specific capacitance of 1244 Fg⁻¹ at 1 Ag⁻¹ and a rate capability of 91% at 10 Ag⁻¹ [94]. The MnCo₂O₄ nanowire acted as a template for the homogeneous nucleation of the outer NiMoO₄ nanosheet shell. The average pore size was 8.5 nm, which benefitted the ion diffusion inside the electrode material. The MnCo₂O₄-NiMoO₄ nanoneedles/nanoflakes achieved a specific capacitance of up to 1718 Fg⁻¹ at 1 Ag⁻¹ [95]. Double urchin-like MgCo₂O₄-NiMoO₄/NF displayed good physical and chemical structures, as well as excellent electrochemical properties [96]. At 1 Ag⁻¹, the optimized electrode (12 h reaction time) produced a specific capacitance of 1775 Fg⁻¹. In another work, CoMoO₄ and NiMoO₄ shell materials were separately loaded on MgCo₂O₄ nanosheet arrays supported on NF [97]. The specific capacity of MgCo₂O₄-NiMoO₄ (1111.57 Cg⁻¹ at 1 mAcm⁻²) was higher than that of MgCo₂O₄-CoMoO₄ (1089.94 Cg⁻¹).

Venus flytrap-like NiCoMn-O-NiMoO₄-C nanosheet arrays supported on CC substrate were developed for different reaction temperatures (80, 100, and 120 °C) and reaction times (2, 4, and 6 h) [98]. The NiCoMn-O-NiMoO₄ core nanosheet arrays (thickness~256 nm) with self-decorated nanoneedles act as the conductive backbone because of their trimetallic nature. The NiCoMn-O created extra electrochemically active sites for electrolyte permeation, and the NiMoO₄ layer contributed to the high electrochemical activity. The carbon shell layer (thickness~4 nm) improved the rate capability by shortening the electron transport path and increasing the cycling stability by reducing the volume swelling during the redox process. Therefore, a high specific capacitance value of 2189.5 Fg⁻¹ (at 0.25 Ag⁻¹) was attained. The 3D porous ZnNiCo-O-NiMoO₄ nanowire/nanosheet arrays (NWNSAs) were prepared on NF by an additive-free hydrothermal process [99]. Because of the uniform distribution of mesopores (average pore diameter~3.8 nm) all over the surface of the polycrystalline NiMoO₄ nanosheets, ZnNiCo-O-NiMoO₄ NWNSAs offered excellent conductivity, durability, and appropriate channels for the fast transfer of electrons/ions. At 3 mAcm⁻², a higher specific capacity (338.5 mAhg⁻¹) was achieved compared to the ZnNiCo-O electrode (266.1 mAhg⁻¹). After 10,000 cycles, the capacity retention was 86%. The ASC assembled with Fe₂O₃/graphene hydrogel anode showed an excellent specific capacity of 87.5 mAhg⁻¹ at 4 mAcm⁻².

CoMoO₄-NiMoO₄·xH₂O heterostructure was hydrothermally prepared on carbon fabric (CF) [100]. With increasing reaction time (1–10 h), NiMoO₄ nanosheets were largely covered over CoMoO₄ nanowires. Scanning electron microscopy (SEM) images revealed that NiMoO₄·xH₂O nanosheets are uniformly covered on the CoMoO₄ nanowire surface, creating an interconnected and highly porous structure that will provide a network structure for fast electron transportation, abundant active sites for ion diffusion, and good strain accommodation. Under different scan rates (5–100 mVs⁻¹), each CV curve shows a pair of redox peaks, representing the pseudocapacitive behavior. The specific capacitance is mostly linked with the quasi-reversible electron-transfer kinetics originating from Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ redox reactions and possibly mediated by OH⁻ ions in the electrolyte. The Mo ion is not included in redox reactions but contributes to electrical conductivity. Consequently, the CoMoO₄-NiMoO₄·xH₂O electrode yielded a maximum specific capacitance of 1582 Fg⁻¹ at 1 Ag⁻¹. Also, nanobundles of CoMoO₄-NiMoO₄.xH₂O were synthesized by chemical coprecipitation using CoMoO₄ nanorods as backbone material [101]. The effect of the Ni-Co mass ratio and reaction time on the electrochemical properties was investigated. The specific capacitance improved with increasing Ni-Co mass ratio and the optimum ratio was 1.4:0.6. The specific capacitance increased up to 4 h reaction time because of the large amount of NiMoO₄.xH₂O. Due to the benefits of the excellent rate capability of CoMoO₄ and the high specific capacitance of NiMoO₄.xH₂O, the specific capacitance reached 1039 Fg⁻¹ at 2.5 mAcm⁻². CoMoO₄-NiMoO₄ nanosheet arrays were grown at various reaction times (2, 4, and 6 h) [102]. The high density of NiMoO₄ nanosheets is unfavorable for the diffusion of electrolyte ions, and it may degrade the electrochemical performance. The optimized electrode (4 h) displayed a maximum specific capacitance of 1639.8 Fg⁻¹ (3.30 Fcm⁻²) ever reported for CoMoO₄-NiMoO₄ structure and excellent cycling stability (95% after 3000 cycles). A facile one-step hydrothermal route was used to develop NiMoO₄/CoMoO₄ nanorods [103]. The low internal resistance (R_b = 1.20 Ω), Warburg impedance (1.07 Ω/√s), and pseudo charge transfer resistance (R_ct = 7.027 Ω) confirmed the enhancement of electrical conductivity, resulting from the synergistic effect of NiMoO₄ and CoMoO₄ nanorods. At 1 Ag⁻¹, a specific capacitance of 1445 Fg⁻¹ was achieved. The rod-like NiMoO₄/CoMoO₄ nanostructure delivered a specific capacitance of 1164 Fg⁻¹ (2 Ag⁻¹) [104]. In NiMoO₄-CoMoO₄ core/sheath nanowire arrays anchored on NF, the surface of NiMoO₄ nanowires was fully wrapped with ultrathin and intersected CoMoO₄ nanosheets, resulting in a highly porous structure [105]. The areal capacitance was 5.4 Fcm⁻² at 2 mAcm⁻², and it was preserved up to 3.1 Fcm⁻² (57.4%) at 40 mAcm⁻².

Hierarchical NiMoO₄-Co₃V₂O₈ nanorod/nanosphere clusters were prepared by Hu et al. [106]. The redox peaks in CV curves are owing to Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺/Co⁴⁺, signifying the reversible non-capacitive Faradaic reactions. Among the prepared samples with different concentrations of Co₃V₂O₈ precursor, the NiMoO₄-Co₃V₂O₈-8 composite possessed a large SSA (54.1 m²g⁻¹) with mesoporous structure (pore size~2–10 nm), which could enhance the electrons/ions transport, leading to a maximum specific capacity of 357 Cg⁻¹ at 1 Ag⁻¹. The Co₃V₂O₈ nanospheres could accommodate the volume swelling of NiMoO₄ nanorods during the long-term cycling process, confirming notable capacity retention of 89.7% over 5000 cycles. Also, the Coulombic efficiency was nearly 100% for each charge/discharge cycle. Honeycomb-like NiMoO₄-NiWO₄ nanocomposite was hydrothermally grown on NF, and the corresponding SEM images (Figure 3d,e) exhibited plenty of NiWO₄ nanoparticles with different particle sizes wrapped on the surface of NiMoO₄ nanoflakes [56]. At 2 Ag⁻¹, the composite electrode attained a maximum specific capacitance of 1290 Fg⁻¹. As shown in Figure 3f, cycling stability was 93.1% after 3000 cycles. Sharma et al. [107] experimentally and theoretically investigated Zn-doped NiMoO₄-AWO₄ (A = Co or Mg) core-shell structure using the density-functional theory method. Even though the MgWO₄ (−110) surface is more stable than MgWO₄ (−202) due to the lower surface energy, the result of lattice dynamics obviously shows that Zn-NiMoO₄-MgWO₄ (−110) is dynamically unstable, and MgWO₄ is thus incompatible for being a shell for the Zn-NiMoO₄ core structure. For Zn-NiMoO₄-MgWO₄ (−110), the interface interaction was too strong, so that the MgWO₄ shell pulled out atoms from the Zn-NiMoO₄ surface, resulting in the surface degradation of the Zn-NiMoO₄ core. The dynamically stable Zn-NiMoO₄-CoWO₄ structure improves the ion diffusion, and the interfacial effects are controlled by the Zn dopant and the distinct surface. Therefore, Zn-NiMoO₄-CoWO₄ showed better performance than Zn-NiMoO₄-MgWO₄, and the areal capacitance was 7.12 Fcm⁻² at 2 mAcm⁻². The 3D ZnFe₂O₄-NiMoO₄ nanosheet arrays fabricated on rGO/NF substrate exhibited a maximum specific capacitance of 1854 Fg⁻¹ at 1 Ag⁻¹ for 4 h reaction time [108]. Apart from the above-discussed heterogeneous structures, homogeneous structures (NiMoO₄ itself acting as core and shell materials) were also attempted. NiMoO₄-NiMoO₄ sheet-on-wire nanoarrays displayed good electrochemical performance over the pure NiMoO₄ nanowires by eliminating the potential barrier at the nanowire/nanosheet interface [109]. Also, homogeneous NiMoO₄-NiMoO₄.xH₂O nanorods/nanosheets performed well, compared to the pure NiMoO₄ nanoarrays, in terms of areal-specific capacitance, rate performance, and cycle life [110].

Table 1. Electrochemical performances of the core-shell structures based on the NiMoO₄-metal oxide composite for supercapacitors.

Core-Shell Structure	Role of NiMoO4	Morphology of NiMoO4	Surface Area (m2g−1)	Specific Capacity/Specific Capacitance	Cycling Stability	Rate Capability	Energy-density (Whkg−1)	Power-density (Wkg−1)	Ref.
MoO3-NiMoO4	shell	nanobelt	26	1307 Fg−1 (1 mVs−1) and 748 Fg−1 (0.5 Ag−1)	171% (10,000 cycles)	186 Fg−1 (50 Ag−1)	37.5	425	[49]
CuO-NiMoO4	shell	nanowire	-	2600 Fg−1 (3.9 Fcm−2) at 3 mAcm−2	87.3% (5000 cycles)	1829.3 Fg−1 (40 mAcm−2)	42.28	631.57	[50]
SnO2-NiMoO4	shell	nanosheet	90.67	0.65 mAhcm−2 (5 mAcm−2)	84.2% (5000 cycles)	57.9% (50 mAcm−2)	78.4	895	[51]
NiMoO4-MnO2	core	nanowire	-	3.9 Fcm−2 (8 mAcm−2)	90.5% (4000 cycles)	3.07 Fcm−2 (32 mAcm−2)	-	-	[52]
NiMoO4-MnO2	core	nanosheet	-	976 Fg−1 (1 Ag−1)	90.9% (3000 cycles)	732 Fg−1 (15 Ag−1)	-	-	[53]
α-NiMoO4-δ-MnO2	core	nanorod	-	1136 Fg−1 (2 Ag−1)	101.9% (5000 cycles)	580 Fg−1 (20 Ag−1)	-	-	[54]
MnO2-NiMoO4	shell	nanoflake	-	582.2 Fg−1 (1 Ag−1)	115.5% (5000 cycles)	322.2 Fg−1 (10 Ag−1)	32.5	750	[55]
NiMoO4-Co3O4	core	nanosheet	-	1722.3 Fg−1 (1 Ag−1)	91% (6000 cycles)	80.8% (10 Ag−1)	37.1	798.0	[58]
Co3O4-MMoO4 (M = Ni)	shell	-	-	2041 Fg−1 (0.5 Ag−1)	72% (3000 cycles)	1540 Fg−1 (8 Ag−1)	41.9	298	[59]
Co3O4-NiMoO4	shell	nanosheet	10.28	1526 Fg−1 (3 mAcm−2)	70% (1000 cycles)	72% (30 mAcm−2)	37.8	482	[60]
Co3O4-NiMoO4	shell	nanosheet	251.2	3.61 Fcm−2 (3 mAcm−2)	77.4% (3000 cycles)	2.96 Fcm−2 (15 mAcm−2)	-	-	[61]
Co3O4-NiMoO4	shell	nanosheet	-	636.8 Cg−1 (5 mAcm−2)	84.1% (2000 cycles)	280.2 Cg−1 (40 mAcm−2)	58.5	389	[62]
Co3O4-NiMoO4	shell	nanosheet	-	3.61 Fcm−2 (2 mAcm−2)	101.3% (9000 cycles)	44% (30 mAcm−2)	-	-	[63]
Co3O4-NiMoO4	shell	nanosheet	-	1476 Fg−1 (1 Ag−1)	96% (2000 cycles)	1200 Fg−1 (20 Ag−1)	-	-	[64]
Co3O4-NiMoO4	shell	nanosheet	-	2.3 Fcm−2 (1 mAcm−2)	80% (4000 cycles)	73% (20 Acm−2)	0.249 mWh cm−2	1.6 mW cm−2	[65]
Co3O4-NiMoO4	shell	nanosheet	243.4	998.05 Fg−1 (0.5 Ag−1)	89.9% (3000 cycles)	880 Fg−1 (20 Ag−1)	-	-	[66]
Co3O4-NiMoO4/CoMoO4	NiMoO4/ CoMoO4 as shell	nanoflake	253.5	272.2 mAhg−1 (1 Ag−1)	84.5% (1000 cycles)	114.9 mAhg−1 (25 Ag−1)	53.9	1000	[67]
NiCo2O4-NiMoO4	shell	nanofilm	-	685.7 Cg−1 (1 Ag−1)	100% (10000 cycles)	621 Cg−1 (10 Ag−1)	96.3	4050	[71]
NiCo2O4-NiMoO4	shell	nanosheet	-	5.80 Fcm−2 (10 mAcm−2)	81.8% (5000 cycles)	4.85 Fcm−2 (80 mAcm−2)	21.7	157	[72]
NiCo2O4-NiMoO4	shell	nanoflake	121.9	1242 Fg−1 (10 mAcm−2)	84% (5000 cycles)	987 Fg−1 (80 mAcm−2)	-	-	[73]
NiCo2O4-NiMoO4	shell	nanosheet	-	1770.95 Cg−1 (3 mAcm−2)	102.78% (5000 cycles)	1334.18 Cg−1 (40 mAcm−2)	30.57	676.06	[74]
NiCo2O4-NiMoO4	shell	nanoplate	-	1974 Fg−1 (5 mAcm−2)	76% (5000 cycles)	1117 Fg−1 (100 mAcm−2)	47	400	[75]
NiCo2O4-NiMoO4	shell	nanosheet	-	2806 Fg−1 (5 Ag−1)	87.7% (5000 cycles)	1408 Fg−1 (30 Ag−1)	64.2	750	[76]
NiCo2O4-NiMoO4	shell	nanosheet	91.97	2.917 Fcm−2 (2 mAcm−2)	90.6% (2000 cycles)	1.608 Fcm−2 (40 mAcm−2)	-	-	[77]
NiCo2O4-NiMoO4	shell	nanosheet	NiCo2O4-NiMoO4/NF = 70.06 NiCo2O4-NiMoO4/CC = 74.34	NF = 1.294 Fcm−2 CC = 0.443 Fcm−2 (50 mVs−1)	80% (3000 cycles)	-	NF = 11.90 CC = 5.06	800	[78]
NiCo2O4-NiMoO4	shell	nanosheet	100.3	2474 Fg−1 (1 Ag−1)	95% (1000 cycles)	2080 Fg−1 (20 Ag−1)	42.1	175	[79]
NiCo2O4-NiMoO4	shell	nanoflake	-	6.29 Fcm−2 (5 mAcm−2)	87% (5000 cycles)	3.58 Fcm−2 (100 mAcm−2)	-	-	[80]
NiCo2O4-NiMoO4	shell	nanosheet	-	7.29 Fcm−2 (2 mAcm−2)	82.2% (5000 cycles)	84.1% (60 mAcm−2)	52.6	332.4	[81]
NiCo2O4-NiMoO4	shell	nanosheet	30.56	2522 mFcm−2 (1 mAcm−2)	89.8% (5000 cycles)	-	53.3	750	[82]
NiCo2O4-NiMoO4	shell	nanosheet	NiCo2O4-NiMoO4/NF = 14.52 (NH4F) NiCo2O4-NiMoO4/CC = 74.34 (urea)	NiCo2O4-NiMoO4/NF = 4.05 Fcm−2 (NH4F) NiCo2O4-NiMoO4/CC = 1.62 Fcm−2 (urea)	NiCo2O4-NiMoO4/NF = 80% (5000 cycles) (NH4F)	-	70.78	3250	[83]
NiCo2O4-NiMoO4	shell	nanoparticle	-	3705 Fg−1 (1.5 Ag−1)	94.6% (5000 cycles)	3525 Fg−1 (30 Ag−1)	76.45	370	[84]
NiCo2O4-NiMoO4/rGO	shell	nanoflake	79.7	9.41 Fcm−2 (10 mAcm−2)	75% (2000 cycles)	6.02 Fcm−2 (50 mAcm−2)	-	-	[85]
NiMoO4-NiCo2O4	core	honeycomb nanostructure	-	2695 Fg−1 (20 mAg−2)	98.9% (3000 cycles)	1527 Fg−1 (28 mAg−2)	61.2	371.5	[86]
rZnCo2O4-NiMoO4·H2O	shell	nanosheet	-	3.53 Fcm−2 (1 mAcm−2)	95.4% (5000 cycles)	-	2.55 mWhcm−3	0.033 Wcm−3	[88]
ZnCo2O4-NiMoO4	shell	nanosheet	156.52	1480.48 Fg−1 (2 mAcm−2)	90.6% (15000 cycles)	959.04 Fg−1 (50 mAcm−2)	48.6	2820	[89]
ZnCo2O4-NiMoO4	shell	nanosheet	-	1912 Fg−1 (1 Ag−1)	84.1% (10000 cycles)	1040 Fg−1 (20 Ag−1)	57.5	900	[90]
ZnCo2O4-NiMoO4	shell	nanosheet	-	1238.1 Cg−1 (3 mAcm−2)	103.4% (5000 cycles)	932.8 Cg−1 (40 mAcm−2)	25.3	787.9	[91]
CuCo2O4-NiMoO4	shell	nanosheet	-	2207 Fg−1 (1.25 Ag−1)	95.6% (5000 cycles)	1560.35 Fg−1 (25 Ag−1)	40	-	[92]
CuCo2O4-NiMoO4	shell	nanosheet	-	276 mAhg−1 (1 Ag−1)	98.3% (8000 cycles)	133 mAhg−1 (10 Ag−1)	44.8	374.2	[93]
MnCo2O4-NiMoO4	shell	nanosheet	119.2	1244 Fg−1 (1 Ag−1)	81% (2500 cycles)	1132 Fg−1 (10 Ag−1)	42	852.3	[94]
MnCo2O4-NiMoO4	shell	nanoflake	-	1718 Fg−1 (1 Ag−1)	84% (6000 cycles)	1200 Fg−1 (8 Ag−1)	42.3	797	[95]
MgCo2O4-NiMoO4	shell	nanosheet	-	1775 Fg−1 (1 Ag−1)	74.7% (5000 cycles)	1191 Fg−1 (20 Ag−1)	37.5	480	[96]
MgCo2O4-MMoO4 (M = Ni)	shell	nanosheet	-	1111.57 Cg−1 (1 mAcm−2)	90.04% (5000 cycles)	788.09 Cg−1 (20 mAcm−2)	23.46	102.6	[97]
NiCoMn-O -NiMoO4-C	shell	nanolayer	-	2189.5 Fg−1 (0.25 Ag−1)	81.6% (1500 cycles)	1361.1 Fg−1 (20 Ag−1)	59.9	214.1	[98]
ZnNiCo-O-NiMoO4	shell	nanowire/ nanosheet	185	338.5 mAhg−1 (3 mAcm−2)	86% (10,000 cycles)	71% (25 mAcm−2)	35.3	5115.1	[99]
CoMoO4-NiMoO4·xH2O	shell	nanosheet	100.79	1582 Fg−1 (1 Ag−1)	97.1% (3000 cycles)	1050 Fg−1 (15 Ag−1)	41.8	700	[100]
CoMoO4-NiMoO4.xH2O	shell	nanorod	17.0	1039 Fg−1 (2.5 mAcm−2)	75.1% (1000 cycles)	750 Fg−1 (100 mAcm−2)	-	-	[101]
CoMoO4-NiMoO4	shell	nanosheet	31.77	1639.8 Fg−1 (10 mAcm−2)	95% (3000 cycles)	1106.9 Fg−1 (60 mAcm−2)	28.7	267	[102]
NiMoO4-CoMoO4	core	nanorod	-	1445 Fg−1 (1 Ag−1)	78.8% (3000 cycles)	815 Fg−1 (10 Ag−1)	-	-	[103]
NiMoO4-CoMoO4	core	nanorod	-	1164 Fg−1 (2 Ag−1)	75% (3000 cycles)	974 Fg−1 (20 Ag−1)	23.1	375	[104]
NiMoO4-CoMoO4	core	nanowire	-	5.4 Fcm−2 (2 mAcm−2)	82.6% (8000 cycles)	3.1 Fcm−2 (40 mAcm−2)	49.3	630	[105]
NiMoO4-Co3V2O8	core	nanorod	54.1	357 Cg−1 (1 Ag−1)	89.7% (5000 cycles)	77.8% (5 Ag−1)	48.5	839.1	[106]
NiMoO4-NiWO4	core	nanoflake	-	1290 Fg−1 (2 Ag−1)	93.1% (3000 cycles)	101.3 Fg−1 (18 Ag−1)	-	-	[56]
Zn-doped NiMoO4-AWO4 (A = Co or Mg)	Zn-doped NiMoO4 core	nanoneedle	-	6.41 Fcm−2 (Zn-NiMoO4-MgWO4) 7.12 Fcm−2 (Zn-NiMoO4-CoWO4) at 2 mAcm−2	96% (1000 cycles)	-	-	-	[107]
ZnFe2O4-NiMoO4	shell	nanosheet	-	1854 Fg−1 (1 Ag−1)	91.6% (7000 cycles)	1220 Fg−1 (20 Ag−1)	58.6	799	[108]
NiMoO4-NiMoO4	core/ shell	nanowire/nanosheet	33.2	413 mAhg−1 (1 Ag−1)	361.2 mAhg−1 (3000 cycles)	220 mAhg−1 (20 Ag−1)	47.2	1380	[109]
NiMoO4-NiMoO4.xH2O	NiMoO4 as core NiMoO4.xH2O as shell	NiMoO4 as nanorod and NiMoO4.xH2O as nanosheet	-	6.34 Fcm−2 (4 mAcm−2)	89% (5000 cycles)	3.13 Fcm−2 (70 mAcm−2)	141 mWhcm−2	0.38 mWcm−2	[110]

Table 1. Electrochemical performances of the core-shell structures based on the NiMoO₄-metal oxide composite for supercapacitors.

Core-Shell Structure	Role of NiMoO4	Morphology of NiMoO4	Surface Area (m2g−1)	Specific Capacity/Specific Capacitance	Cycling Stability	Rate Capability	Energy-density (Whkg−1)	Power-density (Wkg−1)	Ref.
MoO3-NiMoO4	shell	nanobelt	26	1307 Fg−1 (1 mVs−1) and 748 Fg−1 (0.5 Ag−1)	171% (10,000 cycles)	186 Fg−1 (50 Ag−1)	37.5	425	[49]
CuO-NiMoO4	shell	nanowire	-	2600 Fg−1 (3.9 Fcm−2) at 3 mAcm−2	87.3% (5000 cycles)	1829.3 Fg−1 (40 mAcm−2)	42.28	631.57	[50]
SnO2-NiMoO4	shell	nanosheet	90.67	0.65 mAhcm−2 (5 mAcm−2)	84.2% (5000 cycles)	57.9% (50 mAcm−2)	78.4	895	[51]
NiMoO4-MnO2	core	nanowire	-	3.9 Fcm−2 (8 mAcm−2)	90.5% (4000 cycles)	3.07 Fcm−2 (32 mAcm−2)	-	-	[52]
NiMoO4-MnO2	core	nanosheet	-	976 Fg−1 (1 Ag−1)	90.9% (3000 cycles)	732 Fg−1 (15 Ag−1)	-	-	[53]
α-NiMoO4-δ-MnO2	core	nanorod	-	1136 Fg−1 (2 Ag−1)	101.9% (5000 cycles)	580 Fg−1 (20 Ag−1)	-	-	[54]
MnO2-NiMoO4	shell	nanoflake	-	582.2 Fg−1 (1 Ag−1)	115.5% (5000 cycles)	322.2 Fg−1 (10 Ag−1)	32.5	750	[55]
NiMoO4-Co3O4	core	nanosheet	-	1722.3 Fg−1 (1 Ag−1)	91% (6000 cycles)	80.8% (10 Ag−1)	37.1	798.0	[58]
Co3O4-MMoO4 (M = Ni)	shell	-	-	2041 Fg−1 (0.5 Ag−1)	72% (3000 cycles)	1540 Fg−1 (8 Ag−1)	41.9	298	[59]
Co3O4-NiMoO4	shell	nanosheet	10.28	1526 Fg−1 (3 mAcm−2)	70% (1000 cycles)	72% (30 mAcm−2)	37.8	482	[60]
Co3O4-NiMoO4	shell	nanosheet	251.2	3.61 Fcm−2 (3 mAcm−2)	77.4% (3000 cycles)	2.96 Fcm−2 (15 mAcm−2)	-	-	[61]
Co3O4-NiMoO4	shell	nanosheet	-	636.8 Cg−1 (5 mAcm−2)	84.1% (2000 cycles)	280.2 Cg−1 (40 mAcm−2)	58.5	389	[62]
Co3O4-NiMoO4	shell	nanosheet	-	3.61 Fcm−2 (2 mAcm−2)	101.3% (9000 cycles)	44% (30 mAcm−2)	-	-	[63]
Co3O4-NiMoO4	shell	nanosheet	-	1476 Fg−1 (1 Ag−1)	96% (2000 cycles)	1200 Fg−1 (20 Ag−1)	-	-	[64]
Co3O4-NiMoO4	shell	nanosheet	-	2.3 Fcm−2 (1 mAcm−2)	80% (4000 cycles)	73% (20 Acm−2)	0.249 mWh cm−2	1.6 mW cm−2	[65]
Co3O4-NiMoO4	shell	nanosheet	243.4	998.05 Fg−1 (0.5 Ag−1)	89.9% (3000 cycles)	880 Fg−1 (20 Ag−1)	-	-	[66]
Co3O4-NiMoO4/CoMoO4	NiMoO4/ CoMoO4 as shell	nanoflake	253.5	272.2 mAhg−1 (1 Ag−1)	84.5% (1000 cycles)	114.9 mAhg−1 (25 Ag−1)	53.9	1000	[67]
NiCo2O4-NiMoO4	shell	nanofilm	-	685.7 Cg−1 (1 Ag−1)	100% (10000 cycles)	621 Cg−1 (10 Ag−1)	96.3	4050	[71]
NiCo2O4-NiMoO4	shell	nanosheet	-	5.80 Fcm−2 (10 mAcm−2)	81.8% (5000 cycles)	4.85 Fcm−2 (80 mAcm−2)	21.7	157	[72]
NiCo2O4-NiMoO4	shell	nanoflake	121.9	1242 Fg−1 (10 mAcm−2)	84% (5000 cycles)	987 Fg−1 (80 mAcm−2)	-	-	[73]
NiCo2O4-NiMoO4	shell	nanosheet	-	1770.95 Cg−1 (3 mAcm−2)	102.78% (5000 cycles)	1334.18 Cg−1 (40 mAcm−2)	30.57	676.06	[74]
NiCo2O4-NiMoO4	shell	nanoplate	-	1974 Fg−1 (5 mAcm−2)	76% (5000 cycles)	1117 Fg−1 (100 mAcm−2)	47	400	[75]
NiCo2O4-NiMoO4	shell	nanosheet	-	2806 Fg−1 (5 Ag−1)	87.7% (5000 cycles)	1408 Fg−1 (30 Ag−1)	64.2	750	[76]
NiCo2O4-NiMoO4	shell	nanosheet	91.97	2.917 Fcm−2 (2 mAcm−2)	90.6% (2000 cycles)	1.608 Fcm−2 (40 mAcm−2)	-	-	[77]
NiCo2O4-NiMoO4	shell	nanosheet	NiCo2O4-NiMoO4/NF = 70.06 NiCo2O4-NiMoO4/CC = 74.34	NF = 1.294 Fcm−2 CC = 0.443 Fcm−2 (50 mVs−1)	80% (3000 cycles)	-	NF = 11.90 CC = 5.06	800	[78]
NiCo2O4-NiMoO4	shell	nanosheet	100.3	2474 Fg−1 (1 Ag−1)	95% (1000 cycles)	2080 Fg−1 (20 Ag−1)	42.1	175	[79]
NiCo2O4-NiMoO4	shell	nanoflake	-	6.29 Fcm−2 (5 mAcm−2)	87% (5000 cycles)	3.58 Fcm−2 (100 mAcm−2)	-	-	[80]
NiCo2O4-NiMoO4	shell	nanosheet	-	7.29 Fcm−2 (2 mAcm−2)	82.2% (5000 cycles)	84.1% (60 mAcm−2)	52.6	332.4	[81]
NiCo2O4-NiMoO4	shell	nanosheet	30.56	2522 mFcm−2 (1 mAcm−2)	89.8% (5000 cycles)	-	53.3	750	[82]
NiCo2O4-NiMoO4	shell	nanosheet	NiCo2O4-NiMoO4/NF = 14.52 (NH4F) NiCo2O4-NiMoO4/CC = 74.34 (urea)	NiCo2O4-NiMoO4/NF = 4.05 Fcm−2 (NH4F) NiCo2O4-NiMoO4/CC = 1.62 Fcm−2 (urea)	NiCo2O4-NiMoO4/NF = 80% (5000 cycles) (NH4F)	-	70.78	3250	[83]
NiCo2O4-NiMoO4	shell	nanoparticle	-	3705 Fg−1 (1.5 Ag−1)	94.6% (5000 cycles)	3525 Fg−1 (30 Ag−1)	76.45	370	[84]
NiCo2O4-NiMoO4/rGO	shell	nanoflake	79.7	9.41 Fcm−2 (10 mAcm−2)	75% (2000 cycles)	6.02 Fcm−2 (50 mAcm−2)	-	-	[85]
NiMoO4-NiCo2O4	core	honeycomb nanostructure	-	2695 Fg−1 (20 mAg−2)	98.9% (3000 cycles)	1527 Fg−1 (28 mAg−2)	61.2	371.5	[86]
rZnCo2O4-NiMoO4·H2O	shell	nanosheet	-	3.53 Fcm−2 (1 mAcm−2)	95.4% (5000 cycles)	-	2.55 mWhcm−3	0.033 Wcm−3	[88]
ZnCo2O4-NiMoO4	shell	nanosheet	156.52	1480.48 Fg−1 (2 mAcm−2)	90.6% (15000 cycles)	959.04 Fg−1 (50 mAcm−2)	48.6	2820	[89]
ZnCo2O4-NiMoO4	shell	nanosheet	-	1912 Fg−1 (1 Ag−1)	84.1% (10000 cycles)	1040 Fg−1 (20 Ag−1)	57.5	900	[90]
ZnCo2O4-NiMoO4	shell	nanosheet	-	1238.1 Cg−1 (3 mAcm−2)	103.4% (5000 cycles)	932.8 Cg−1 (40 mAcm−2)	25.3	787.9	[91]
CuCo2O4-NiMoO4	shell	nanosheet	-	2207 Fg−1 (1.25 Ag−1)	95.6% (5000 cycles)	1560.35 Fg−1 (25 Ag−1)	40	-	[92]
CuCo2O4-NiMoO4	shell	nanosheet	-	276 mAhg−1 (1 Ag−1)	98.3% (8000 cycles)	133 mAhg−1 (10 Ag−1)	44.8	374.2	[93]
MnCo2O4-NiMoO4	shell	nanosheet	119.2	1244 Fg−1 (1 Ag−1)	81% (2500 cycles)	1132 Fg−1 (10 Ag−1)	42	852.3	[94]
MnCo2O4-NiMoO4	shell	nanoflake	-	1718 Fg−1 (1 Ag−1)	84% (6000 cycles)	1200 Fg−1 (8 Ag−1)	42.3	797	[95]
MgCo2O4-NiMoO4	shell	nanosheet	-	1775 Fg−1 (1 Ag−1)	74.7% (5000 cycles)	1191 Fg−1 (20 Ag−1)	37.5	480	[96]
MgCo2O4-MMoO4 (M = Ni)	shell	nanosheet	-	1111.57 Cg−1 (1 mAcm−2)	90.04% (5000 cycles)	788.09 Cg−1 (20 mAcm−2)	23.46	102.6	[97]
NiCoMn-O -NiMoO4-C	shell	nanolayer	-	2189.5 Fg−1 (0.25 Ag−1)	81.6% (1500 cycles)	1361.1 Fg−1 (20 Ag−1)	59.9	214.1	[98]
ZnNiCo-O-NiMoO4	shell	nanowire/ nanosheet	185	338.5 mAhg−1 (3 mAcm−2)	86% (10,000 cycles)	71% (25 mAcm−2)	35.3	5115.1	[99]
CoMoO4-NiMoO4·xH2O	shell	nanosheet	100.79	1582 Fg−1 (1 Ag−1)	97.1% (3000 cycles)	1050 Fg−1 (15 Ag−1)	41.8	700	[100]
CoMoO4-NiMoO4.xH2O	shell	nanorod	17.0	1039 Fg−1 (2.5 mAcm−2)	75.1% (1000 cycles)	750 Fg−1 (100 mAcm−2)	-	-	[101]
CoMoO4-NiMoO4	shell	nanosheet	31.77	1639.8 Fg−1 (10 mAcm−2)	95% (3000 cycles)	1106.9 Fg−1 (60 mAcm−2)	28.7	267	[102]
NiMoO4-CoMoO4	core	nanorod	-	1445 Fg−1 (1 Ag−1)	78.8% (3000 cycles)	815 Fg−1 (10 Ag−1)	-	-	[103]
NiMoO4-CoMoO4	core	nanorod	-	1164 Fg−1 (2 Ag−1)	75% (3000 cycles)	974 Fg−1 (20 Ag−1)	23.1	375	[104]
NiMoO4-CoMoO4	core	nanowire	-	5.4 Fcm−2 (2 mAcm−2)	82.6% (8000 cycles)	3.1 Fcm−2 (40 mAcm−2)	49.3	630	[105]
NiMoO4-Co3V2O8	core	nanorod	54.1	357 Cg−1 (1 Ag−1)	89.7% (5000 cycles)	77.8% (5 Ag−1)	48.5	839.1	[106]
NiMoO4-NiWO4	core	nanoflake	-	1290 Fg−1 (2 Ag−1)	93.1% (3000 cycles)	101.3 Fg−1 (18 Ag−1)	-	-	[56]
Zn-doped NiMoO4-AWO4 (A = Co or Mg)	Zn-doped NiMoO4 core	nanoneedle	-	6.41 Fcm−2 (Zn-NiMoO4-MgWO4) 7.12 Fcm−2 (Zn-NiMoO4-CoWO4) at 2 mAcm−2	96% (1000 cycles)	-	-	-	[107]
ZnFe2O4-NiMoO4	shell	nanosheet	-	1854 Fg−1 (1 Ag−1)	91.6% (7000 cycles)	1220 Fg−1 (20 Ag−1)	58.6	799	[108]
NiMoO4-NiMoO4	core/ shell	nanowire/nanosheet	33.2	413 mAhg−1 (1 Ag−1)	361.2 mAhg−1 (3000 cycles)	220 mAhg−1 (20 Ag−1)	47.2	1380	[109]
NiMoO4-NiMoO4.xH2O	NiMoO4 as core NiMoO4.xH2O as shell	NiMoO4 as nanorod and NiMoO4.xH2O as nanosheet	-	6.34 Fcm−2 (4 mAcm−2)	89% (5000 cycles)	3.13 Fcm−2 (70 mAcm−2)	141 mWhcm−2	0.38 mWcm−2	[110]

4.2. NiMoO₄-Metal Hydroxide Composite

Transition metal hydroxides are deemed promising candidates for supercapacitor electrodes. The core material with rod-like morphology is very appropriate for designing core-shell nanostructures. Besides, poor crystallinity is more auspicious for ion transport and beneficial for good electrochemical performance [24,111,112]. NiMoO₄-Ni(OH)₂ nanorods were successfully constructed on NF via hydrothermal and electrodeposition processes [113]. The Ni(OH)₂ nanosheets were uniformly covered on the surface of NiMoO₄ nanorods in such a way that they were aligned with their planes more or less perpendicular to NiMoO₄ nanorods. The nonlinearity of the galvanostatic charge/discharge curves in the voltage range of 0–0.4 V indicates the Faradaic process, and a high value of areal capacitance of 7.43 Fcm⁻² at 4 mAcm⁻² is due to the quasi-reversible electron transfer process. NiMoO₄-Co(OH)₂ nanowire arrays were grown on NF via hydrothermal and electrochemical deposition routes [114]. As displayed in Figure 5a–c, ultrathin Co(OH)₂ nanoflakes (thickness~10–20 nm) were uniformly wrapped on NiMoO₄ nanowires, due to which the areal capacitance reached 2.335 Fcm⁻² at 5 mAcm^−2, and capacitance retention rate was 83% after 5000 cycles. Such a good performance is due to the large surface area of Co(OH)₂ and open space that enables the rapid intercalation/deintercalation of ions by affording abundant active sites. The NiMoO₄-Co(OH)₂ produces more ion migration channels and facilitates the electrolyte ion transport in 3D space, as displayed in Figure 5d. The large electroactive surface area of the electrode is confirmed by low R_ct (Faradaic interfacial charge transfer resistance) from EIS measurement, and the value increases after 5000 cycles, which is probably due to the loss of some active material and the corrosion of NF caused by the dissolved oxygen in KOH electrolyte.

Layered double hydroxides (LDHs) are a group of materials in which the layers contain divalent (M²⁺) and trivalent (M³⁺) metal ions, and the interlayer region is occupied by charge-balancing anions. Their advantages include large interlayer spacing, high surface area, and easy synthesis [115]. Ni-Co LDHs are commonly applied with NiMoO₄ nanosheets owing to their high theoretical capacitance (>3000 Fg⁻¹) and low cost [116]. To construct NiMoO₄-Ni-Co LDH-NiCo₂O₄ nanorod/nanosheet structure, NiCo₂O₄ nanorods were hydrothermally prepared on NF, and then Ni-Co LDH nanosheets were grown on NiCo₂O₄ by electrodeposition [117]. Afterward, Ni-Co LDH was utilized as a template to provide a larger area for the NiMoO₄ nanosheet growth. The size and thickness of nanosheets were controlled by varying the electrodeposition time (50, 100, 150, and 200 s). The NiMoO₄-Ni-Co LDH shell protects the NiCo₂O₄ core from structural damage and further improves material stability as well as capacity. A maximum specific capacity of 1035 Cg⁻¹ was attained by NiMoO₄-Ni-Co LDH150-NiCo₂O₄. Hydrothermal synthesis of Ni-Co LDH-NiMoO₄/GO on carbon fiber is illustrated in Figure 5e [116]. The graphene oxide (GO) layer was attached with Ni-Co LDH-NiMoO₄ by physical adsorption. The surface of GO contains more oxygen-containing functional groups, and the carbon fiber is adsorbed with GO to create a stable 3D framework. As evident from the XRD pattern in Figure 5f, the Ni-Co LDH is a mixture of rhombohedral Ni(OH)₂ and Co(OH)₂. The poor crystallinity of NiMoO₄ is due to the presence of many pores. Figure 5g shows the resistance properties of Ni-Co LDH-Ni-MoO₄ and Ni-Co LDH-NiMoO₄/GO, investigated by EIS measurements. It is clear that the R_s value of the Ni-Co LDH-NiMoO₄/GO (6.79 Ω) electrode is less compared to Ni-Co LDH-NiMoO₄ (9.21 Ω). Therefore the former electrode exhibited good performance in all aspects. Double-shell hierarchical structure of P-doped cobalt carbonate hydroxide-NiMoO₄ (P-CoCH-NiMoO₄) was constructed through hydrothermal growth of densely-packed NiMoO₄ nanosheets on NF, followed by the growth of CoCH nanowires [118]. Afterward, in-situ vapor phase P-doping was applied to change the crystal structure and surface heteroatoms distribution of the composite. The large mass loading of active material supports the energy storage capacity and electrochemically active sites. The unique structure with porous gradient channels and hydrophilic nature can increase the electrolyte permeation and ion diffusion efficiency. The conductive NiMoO₄ arrays can facilitate the electron transfer between NF and P-CoCH. Besides, due to the phosphorization process, lattice distortion, surface defects, and oxygen vacancies were introduced on the surface of CoCH nanowires, which can efficiently increase the electron transfer and create additional catalytic active sites. Owing to these facts, the as-prepared electrode reached a high areal capacitance (5.08 Fcm⁻² at 2 mAcm⁻²) and good cycling stability (82.7% after 2000 cycles).

4.3. NiMoO₄-Metal Chalcogenide Composite

A common issue with metal oxides is their limited electron transport. Metal sulfides are recently found to have improved performance owing to their diversified crystal structure, high electrical conductivity, and good electrochemical activity of sulfur than oxygen [119,120]. The electrochemical performance of NiMoO₄-metal hydroxide, NiMoO₄-metal chalcogenide, NiMoO₄-carbon material, and NiMoO₄-conductive polymer core-shell structures is shown in Table 2. Funnel-shaped NiMoO₄-Co₃S₄/NF nanostructure was synthesized using different hydrothermal reaction times (8, 12, and 16 h) and electrochemical deposition [121]. The SEM images revealed that 2D NiMoO₄ nanosheets have sufficient open space and large lateral size, which are beneficial to the deposition of Co₃S₄. The Co₃S₄ nanosheets were uniformly deposited over the NiMoO₄ porous structure by tight interconnection and arranged in the same orientation with NiMoO₄, forming a double-layer open-up network (Figure 6a,b). Consequently, this improves electrolyte penetration and charge transport. As shown in Figure 6c, the closed region of the cyclic voltammetry (CV) curve of NiMoO₄-Co₃S₄ (12 h) is larger than that of the pure NiMoO₄ and Co₃S₄. Therefore, the ultrathin architecture offered a large specific capacitance of 359.31 mAhg⁻¹ (2589.6 Fg⁻¹) at 0.5 Ag⁻¹. This great improvement was due to the parallel orientation of NiMoO₄ and Co₃S₄ nanosheets. In NiMoO₄-MoS₂ nanorods, the pseudocapacitive property arises due to the redox reactions of Ni²⁺/Ni³⁺ from NiMoO₄ and Mo⁴⁺ ions from MoS₂ [122]. The heterostructure electrode offered an outstanding specific capacitance of 2246.7 Fg⁻¹ at 1 Ag⁻¹ and improved cycling stability (88.4% after 5000 cycles). In the high-frequency region of Nyquist plots, the charge transfer resistance (R_ct) value of NiMoO₄-MoS₂ (1.24 Ω) is relatively smaller compared to NiMoO₄ (2.77 Ω) and MoS₂ (4.81 Ω), which is due to the rapid electron transport between NiMoO₄-MoS₂ nanorods and KOH electrolyte. In the low-frequency region, the NiMoO₄-MoS₂ electrode shows a larger slope compared to NiMoO₄ and MoS₂, implying a lower diffusion resistance. This is ascribed to a highly porous structure with mesopores and macropores, enabling more active sites. The ASC device composed of NiMoO₄-MoS₂ as cathode and N, S-codoped porous carbon as anode delivered a high energy-density of 47.5 Whkg⁻¹ at a power-density of 440 Wkg⁻¹.

The 3D NiMoO₄-Ni₃S₂ heterostructure was made up of numerous Ni₃S₂ nanosheets wrapped on NiMoO₄ nanorods [124]. This unique morphology helps to effectively inhibit the volume change of the electrode material during the long-term cycle process. The abundant pore channels can reduce the transmission and diffusion path of ions. Additionally, the intrinsic resistance (IR drop) of the electrode is small. Because of these reasons, the areal capacitance reached 2.3 Fcm⁻² at 1 mAcm⁻² (1.8 times larger than that of NiMoO₄). The 1D Ni₃S₂-NiMoO₄ structure exhibited a high areal capacity of 1327.3 μAhcm⁻² at 2 mAcm⁻² and a rate capability of 67.8% (900.9 μAhcm⁻² at 40 mAcm⁻²) [125]. The hair-like Ni₃S₂ nanowires provided high electrical conductivity and inhibited the aggregation of NiMoO₄ nanosheets. Hence, Ni₃S₂-NiMoO₄ (diameter~150 nm) nanowires exhibited good cycling stability (96.7% after 6000 cycles). NiMoO₄-NiS₂/MoS₂ (S-NiMoO₄-x) nanocomposite was developed via in-situ sulfurization [sulfur with the mass ratio of 1:x (x = 1, 2, and 3)] of the hydrothermally-prepared NiMoO₄ nanowires [126]. The S-NiMoO₄-2 exhibited good performance (970 Fg⁻¹ at 5 Ag⁻¹) due to the porous structure and the long-range continuous interfaces between outer NiS₂/MoS₂ nanosheets and inner NiMoO₄ nanowires, which offers more accessible sites and fast charge transfer channels. NiMoO₄-Ni₉S₈/MoS₂ nanorods were constructed using hydrothermal-assisted direct sulfurization [127]. The hierarchical porous structure is a combination of both micropores and mesopores contributed by outer Ni₉S₈/MoS₂ nanoflakes and inner NiMoO₄ nanorods, respectively, due to the introduction of sulfur during the calcination process. Because of the high porosity and synergistic effect of bimetal sulfides, the specific capacity attained a maximum of 488.9 Fg⁻¹ at 1 Ag⁻¹. The 1D core-double shell arrays of NiMoO₄-C-Ni₃S₂ were prepared on NF by successively using hydrothermal, carbonization and electrodeposition processes [128]. The order of capacitive responses was NiMoO₄-C-Ni₃S₂ > NiMoO₄-Ni₃S₂ > NiMoO₄ > NiMoO₄-C for 50 mVs⁻¹ scan rate in CV measurement. The reason for less capacitance after carbon coating is that the carbon layer covers the active sites of NiMoO₄. NiMoO₄-C-Ni₃S₂ shows a larger capacitance compared to NiMoO₄-Ni₃S₂ because the inner shell of carbon helps to strengthen the linkage between the NiMoO₄ core nanowire and Ni₃S₂ outer shell, which can eliminate the interfacial resistance and load a large amount of Ni₃S₂ nanosheets. The specific capacity reached 7.9 Fcm⁻² at 5 mAcm⁻².

Binary metal sulfides usually have richer redox reactions and better conductivity compared to single metal sulfides, showing improved electrochemical performance. Among them, NiCo₂S₄ nanostructure is the most extensively studied material with NiMoO₄ due to its superior ion transfer rate and rapid electrochemical response. NiCo₂S₄-NiMoO₄.xH₂O nanoneedle arrays were developed on NF using hydrothermal and sulfur anion exchange processes [129]. NiCo₂S₄ nanoneedles served as the backbone for the NiMoO₄.xH₂O nanosheet growth. This could increase the conductivity between NiCo₂S₄ and NiMoO₄ and also efficiently reduce the agglomeration of NiMoO₄.xH₂O nanosheets, enabling abundant electroactive sites for redox reactions. Consequently, a specific capacity of 830.2 Fg⁻¹ was attained at 2 Ag⁻¹. NiCo₂S₄-NiMoO₄ nanosheet arrays were constructed by rationally growing NiCo₂S₄ nanosheets on NF by anion exchange reaction with Na₂S, followed by decorating with porous NiMoO₄ nanosheets [130]. With the increasing reaction time (2, 4, and 6 h), plenty of NiMoO₄ nanosheets coated over NiCo₂S₄, forming thick NiCo₂S₄-NiMoO₄ nanosheets with enlarged surface area for fast Faradaic redox reactions. The specific capacitance reached 1487.6 Fg⁻¹ at 1 Ag⁻¹ for 4 h reaction time. NiCo₂S₄-NiMoO₄ nanoarrays were fabricated by developing grass-like NiCo₂S₄ nanotubes covered by abundant interlinked NiMoO₄ nanosheets [131]. At 5 mAcm⁻², the specific capacitance reached a higher value of 2006 Fg⁻¹ compared to bare NiCo₂S₄ (1264 Fg⁻¹). To construct NiCo₂S₄-NiMoO₄ nanospheres, NiCo₂S₄ nanoballs were synthesized first [132]. Diethanolamine was used as a weak alkali precipitant that caused the slow reaction rate to favor nucleation and growth. At 1 Ag⁻¹, a maximum specific capacitance of 1714 Fg⁻¹ was achieved. NiCo₂S₄-NiMoO₄ nanostructures were prepared by developing ultrathin, mesoporous, and interconnected NiMoO₄ nanosheets on the surface of NiCo₂S₄ nanotubes [133]. The areal capacity was 673.3 μAhcm⁻² at 5 mAcm⁻². NiCo₂O_xS_y-NiMoO₄ nanotube/nanosheet structure was hydrothermally grown by Chiu et al. [134]. Ni-Co LDH nanowires were calcined to form NiCo₂O₄ nanowires, which were further transformed into NiCo₂S₄ core nanotube arrays. NiCo₂S₄ is dissolved to form NiCo₂O_xS_y nanotubes during the NiMoO₄ shell growth. The nanoparticle-assembled wall of the NiCo₂O_xS_y nanotubes has a large active surface area for enabling more redox reactions with the electrolyte. Hence, a high specific capacity of 168.18 mAhg⁻¹ was gained at 10 mAcm⁻².

NiMoO₄-Ni-Co-S nanorods were constructed by adopting hydrothermal for NiMoO₄ growth [135]. Then, Ni-Co-S was electrodeposited under a scan rate of 5 mVs⁻¹ for 6, 8, and 10 cycles. The electrode showed good performance (1892 Fg⁻¹ at 5 mAcm⁻²) for 8 cycles. Single-crystalline NiMoO₄/CoMoO₄ nanorods enclosed by polycrystalline Ni-Co-S nanosheets (NCMOS) were effectively deposited on CC using the hydrothermal method and electrochemical deposition [123]. After the hydrothermal method, the 3D textile structure of NiMoO₄/CoMoO₄ (NCMO) was maintained, as shown in Figure 6d. The NCMO nanorods radially grow on CC and form a flower-like structure. As shown in Figure 6e, Ni-Co-S nanosheets create the NCMO surface more wrinkled, which increases the contact area with electrolyte and helps to improve the electrochemical performance. The average diameter of NCMO nanorods is~210 nm, and the TEM image of NCMOS confirmed that nanorods are covered by nanosheets (Figure 6f,g). The bulk solution resistance (R_s) and charge transfer resistance (R_ct) values of NCMOS/CC are higher (0.92, 0.19 Ω) when compared to NCMO/CC (0.63, 0.08 Ω) due to the more loading of active material, as evident from Figure 6h. The NCMOS material exhibited a good specific capacitance of 778.1 Fg⁻¹ at 0.5 Ag⁻¹ and an excellent rate capability of 648.4 Fg⁻¹ at 4 Ag⁻¹ (Figure 6i). A flexible ASC was further fabricated with NCMOS and AC, delivering an energy-density of 33.1 WhKg⁻¹ at a power-density of 199.6 Wkg⁻¹. Acharya and co-workers [136] scrupulously designed a hollow-tubular rGO-NiMoO₄-Ni-Co-S hybrid nanostructure in a fashionable way. First, NiMoO₄ nanorods were prepared on rGO-coated NF using a facile hydrothermal method. Next, the as-coated NF was simply dipped into the Co-precursor solution containing 2-methylimidazole to obtain rGO-NiMoO₄-Co-MOF (metal-organic framework). This Co-MOF (ZIF-67) was then modified into rGO-NiMoO₄-Ni-Co-LDH via an etching process in a Ni-precursor solution. At last, the synthesized material was used for the sulfidation process in thioacetamide to obtain rGO-NiMoO₄-Ni-Co-S architecture. The specific capacity reached 318 mAhg⁻¹ at 1 Ag⁻¹, and cycling stability was 88.87% (after 10,000 cycles). Moreover, the fabricated rGO-NiMoO₄-Ni-Co-S//rGO-MDC (MOF-derived carbon) ASC device delivered an energy-density of 57.24 Whkg⁻¹ at a power-density of 801.8 Wkg⁻¹ with a notable life span of 90.89% after 10,000 cycles.

Metal selenides have a lower band gap than the corresponding sulfide compounds. Therefore, very recently, the Ni-Co-Se-NiMoO₄ hybrid structure was developed on rGO-coated NF [137]. As shown in Figure 7a, first, porous Ni-Co-Se nanorods were vertically deposited on an rGO-NF substrate via oxalic acid template and selenization processes. Then, the surface of Ni-Co-Se nanorods was decorated with hydrothermally-prepared NiMoO₄ nanosheets to attain a Ni-Co-Se-NiMoO₄ structure with a high SSA (114 m²g⁻¹) and pore volume (0.42 cm³g⁻¹). As shown in Figure 7b,c, hollow Ni-Co-Se nanorods interconnect with NiMoO₄ nanosheets, which leads to the Ni-Co-Se-NiMoO₄-rGO-NF structure. The as-prepared electrode shows a magnificent electrochemical performance in terms of a maximum specific capacity of 396.1 mAhg⁻¹ at 1 Ag⁻¹ and excellent capacity retention of 87.6% after 8000 cycles (Figure 7d,f). As illustrated in Figure 7e, the diffusion-controlled current contribution is leading at a low scan rate. With increasing scan rate, the capacitive contribution increases, enhancing the interaction of electrolyte ions towards the electrode surface. An HSC was assembled by sandwiching the battery-type electrode with an EDLC-type OA-MOF-PCCNT-NF electrode (oxalic acid and MOF-derived porous carbon/CNT coated NF). The device showed a high energy-density of 63.1 Whkg⁻¹ at a power-density of 799.8 Wkg^−1, along with 89.4% capacitance retention after 8000 cycles. The 1D-NiMoO₄-2D-NiMoS₄ (NMS) porous nanostructure was designed by growing NiMoO₄ nanorods first and then treating it with Na₂S solution at different concentrations (5, 10, and 20 mM) [138]. When the S₂⁻ concentration increased to 20 mM, NiMoO₄ nanorods dissolved into nanoparticles and agglomerated into clumps. Because of this structural collapse, the surface area decreased, and hence the electrochemical performance became poor. The porous structure of NMS-10 mainly consists of mesopores, macropores, and a few micropores. The mesopores and micropores existed on NiMoO₄ nanorods and NiMoS₄ nanosheets, while the macropores were due to the staggering growth of NiMoS₄ nanosheets. The NMS-10 produced a maximum specific capacitance of 832.3 Fg⁻¹ at 5 Ag⁻¹.

Table 2. Electrochemical performances of core-shell structures using composites of NiMoO₄-metal hydroxide, NiMoO₄-metal chalcogenide, NiMoO₄-carbon material, NiMoO₄-conductive polymer for supercapacitors.

Core-Shell Structure	Role of NiMoO4	Morphology of NiMoO4	Surface Area (m2g−1)	Specific Capacity/Specific Capacitance	Cycling Stability	Rate Capability	Energy Density (Whkg−1)	Power Density (Wkg−1)	Ref.
NiMoO4-metal hydroxide composite
NiMoO4-Ni(OH)2	core	nanorod	-	7.43 Fcm−2 (4 mAcm−2)	72% (1000 cycles)	3.06 Fcm−2 (112 mAcm−2)	-	-	[113]
NiMoO4-Co(OH)2	core	nanowire	-	2.335 Fcm−2 (5 mAcm−2)	83% (5000 cycles)	0.909 Fcm−2 (50 mAcm−2)	-	-	[114]
Ni-Co LDH-NiMoO4	shell	nanosheet	100.6	2100 Fg−1 (1 Ag−1)	91% (5000 cycles)	780 Fg−1 (10 Ag−1)	538.3	2522	[116]
NiMoO4-Ni-Co LDH-NiCo2O4	shell	nanosheet	-	1035 Cg−1 (2587.5 Fg−1) at 1 Ag−1	80.6% (5000 cycles)	688 Cg−1 (20 Ag−1)	66.8	900	[117]
P-CoCH-NiMoO4	core	nanosheet	-	5.08 Fcm−2 (2 mAcm−2)	82.7% (2000 cycles)	3.27 Fcm−2 (20 mAcm−2)	-	-	[118]
NiMoO4-metal chalcogenide composite
NiMoO4-Co3S4	core	nanosheet	62.04	359.31 mAhg−1 (0.5 Ag−1)	82.9% (10000 cycles)	56.94 mAhg−1 (10 Ag−1)	33.4	387.50	[121]
NiMoO4-MoS2	core	nanorod	58.8	2246.7 Fg−1 (1 Ag−1)	88.4% (5000 cycles)	1200.4 Fg−1 (20 Ag−1)	47.5	440	[122]
NiMoO4-Ni3S2	core	nanorod	-	2.3 Fcm−2 (1 mAcm−2)	84.4% (6000 cycles)	69.6% (40 mAcm−2)	158.4 mWhcm−2	2.199 Wcm−2	[124]
Ni3S2-NiMoO4	shell	nanosheet	-	1327.3 μAhcm−2 (2 mAcm−2)	96.7% (6000 cycles)	900.9 μAhcm−2 (40 mAcm−2)	121.5	2.285 kW kg−1	[125]
NiMoO4-NiS2/MoS2	core	nanowire	27.5	970 Fg−1 (5 Ag−1)	-	711 Fg−1 (20 Ag−1)	26.8	700	[126]
NiMoO4-Ni9S8/MoS2	core	nanorod	27.96	488.9 Fg−1 (1 Ag−1)	81% (10000 cycles)	52.9 Fg−1 (20 Ag−1)	-	-	[127]
NiMoO4-C-Ni3S2	core	nanowire	-	7.9 Fcm−2 (5 mAcm−2)	78.9% (3000 cycles)	1.57 Fcm−2 (50 mAcm−2)	1.29 mWhcm−3	13.99 Wcm−3	[128]
NiCo2S4-NiMoO4·xH2O	shell	nanosheet	-	830.2 Fg−1 (2 Ag−1)	89.9% (5000 cycles)	380.9 Fg−1 (20 Ag−1)	19.3	795.7	[129]
NiCo2S4-NiMoO4	shell	nanosheet	-	1487.6 Fg−1 (1 Ag−1)	89.7% (8000 cycles)	1154.3 Fg−1 (20 Ag−1)	53.2	560	[130]
NiCo2S4-NiMoO4	shell	nanosheet	-	2006 Fg−1 (5 mAcm−2)	75% (2000 cycles)	1305 Fg−1 (50 mAcm−2)	21.4	58	[131]
NiCo2S4-NiMoO4	shell	nanosheet	95	1714 Fg−1 (1 Ag−1)	96% (5000 cycles)	1314 Fg−1 (20 Ag−1)	29.1	172	[132]
NiCo2S4-NiMoO4	shell	nanosheet	10.08	673.3 μAhcm−2 (5 mAcm−2)	84.2% (2000 cycles)	636.7 μAhcm−2 (100 mAcm−2)	33.1	219	[133]
NiCo2OxSy-NiMoO4	shell	nanosheet	-	17.75 Fcm−2 (1345 Fg−1) at 10mAcm−2	69.3% (2000 cycles)	43% (50 mAcm−2)	5.28	329	[134]
NiMoO4-Ni-Co-S	core	nanorod	-	1892 Fg−1 (5 mAcm−2)	91.7% (6000 cycles)	842 Fg−1 (40 mAcm−2)	2.45 mWh cm−3	0.131 Wcm−3	[135]
NiMoO4/CoMoO4-Ni-Co-S	NiMoO4/CoMoO4 core	nanorod	-	778.1 Fg−1 (0.5 Ag−1)	98% (5000 cycles)	648.4 Fg−1 (4 Ag−1)	33.1	199.6	[123]
rGO-NiMoO4-Ni-Co-S	core	NiMoO4 hollow nanotube	-	318 mAhg−1 (1 Ag−1)	88.87% (10000 cycles)	212 mAhg−1 (20 Ag−1)	57.24	801.8	[136]
Ni-Co-Se-NiMoO4	shell	nanosheet	114	396.1 mAhg−1 (1 Ag−1)	87.6% (8000 cycles)	283.3 mAhg−1 (20 Ag−1)	63.1	799.8	[137]
NiMoO4-NiMoS4	core	nanorod	18.6	832.3 Fg−1 (5 A g−1)	81.4% (1000 cycles)	555 Fg−1 (15 Ag−1)	22.84	3750	[138]
NiMoO4-carbon material composite
C-NiMoO4	shell	nanograin	-	268.8 Fg−1 (0.5 Ag−1)	88.4% (2000 cycles)	168.4 Fg−1 (10 Ag−1)	-	-	[139]
Carbon nanofiber-NiMoO4	shell	nanosheet	280	1840 Fg−1 (1 Ag−1)	78.3% (10000 cycles)	78% (20 Ag−1)	23.9	750	[140]
N-C-NiMoO4	shell	nanosheet	258.2	1242 Fg−1 (1 Ag−1)	82.9% (2000 cycles)	581 Fg−1 (20 Ag−1)	44.6	250.4	[141]
XMoO4-carbon submicrofiber (X = Ni, Co)	shell	nanosheet	75.722	1600 Fg−1 (1 Ag−1)	90.7% (3000 cycles)	1166 Fg−1 (10 Ag−1)	55.33	999.89	[142]
NiMoO4-CNTs-CuO	shell	nanosheet	7.08	23.40 Fcm−2 (2 mAcm−2)	82.53% (10000 cycles)	12.9 Fcm−2 (22 mAcm−2)	96.40 mWhcm−3	0.4 Wcm−3	[143]
NiMoO4/V2CTx-rGO	NiMoO4/ V2CTx as yolk	nanoparticle	100.74	1022 Fg−1 (1 Ag−1)	88.9% (3000 cycles)	827 Fg−1 (10 Ag−1)	56.1	800	[144]
NiMoO4-conductive polymer structures
NiMoO4-PANI	core	nanorod	56.14	1214 Fg−1 (1 Ag−1)	80.7% (2000 cycles)	813 Fg−1 (20 Ag−1)	33.07	240	[145]
PPy-NiMoO4	core	nanowire	-	3.4 Fcm−2 (5 mAcm−2)	94% (5000 cycles)	2.30 Fcm−2 (50 mAcm−2)	0.5 mWcm−2	3.7 mWhcm−2	[146]
NiCo2O4-NiMoO4/PANI	shell	nanoplate	-	2.38 Fcm−2 (1 mAcm−2)	92.36% (5000 cycles)	1.508 Fcm−2 (10 mAcm−2)	90	443.2	[147]
NiO-NiMoO4-PPy	NiMoO4 and PPy as shell	porous spherical nanostructure	170.10	1645.1 Fg−1 (1 Ag−1)	77.1% (30000 cycles)	843.2 Fg−1 (20 Ag−1)	-	-	[148]

Table 2. Electrochemical performances of core-shell structures using composites of NiMoO₄-metal hydroxide, NiMoO₄-metal chalcogenide, NiMoO₄-carbon material, NiMoO₄-conductive polymer for supercapacitors.

Core-Shell Structure	Role of NiMoO4	Morphology of NiMoO4	Surface Area (m2g−1)	Specific Capacity/Specific Capacitance	Cycling Stability	Rate Capability	Energy Density (Whkg−1)	Power Density (Wkg−1)	Ref.
NiMoO4-metal hydroxide composite
NiMoO4-Ni(OH)2	core	nanorod	-	7.43 Fcm−2 (4 mAcm−2)	72% (1000 cycles)	3.06 Fcm−2 (112 mAcm−2)	-	-	[113]
NiMoO4-Co(OH)2	core	nanowire	-	2.335 Fcm−2 (5 mAcm−2)	83% (5000 cycles)	0.909 Fcm−2 (50 mAcm−2)	-	-	[114]
Ni-Co LDH-NiMoO4	shell	nanosheet	100.6	2100 Fg−1 (1 Ag−1)	91% (5000 cycles)	780 Fg−1 (10 Ag−1)	538.3	2522	[116]
NiMoO4-Ni-Co LDH-NiCo2O4	shell	nanosheet	-	1035 Cg−1 (2587.5 Fg−1) at 1 Ag−1	80.6% (5000 cycles)	688 Cg−1 (20 Ag−1)	66.8	900	[117]
P-CoCH-NiMoO4	core	nanosheet	-	5.08 Fcm−2 (2 mAcm−2)	82.7% (2000 cycles)	3.27 Fcm−2 (20 mAcm−2)	-	-	[118]
NiMoO4-metal chalcogenide composite
NiMoO4-Co3S4	core	nanosheet	62.04	359.31 mAhg−1 (0.5 Ag−1)	82.9% (10000 cycles)	56.94 mAhg−1 (10 Ag−1)	33.4	387.50	[121]
NiMoO4-MoS2	core	nanorod	58.8	2246.7 Fg−1 (1 Ag−1)	88.4% (5000 cycles)	1200.4 Fg−1 (20 Ag−1)	47.5	440	[122]
NiMoO4-Ni3S2	core	nanorod	-	2.3 Fcm−2 (1 mAcm−2)	84.4% (6000 cycles)	69.6% (40 mAcm−2)	158.4 mWhcm−2	2.199 Wcm−2	[124]
Ni3S2-NiMoO4	shell	nanosheet	-	1327.3 μAhcm−2 (2 mAcm−2)	96.7% (6000 cycles)	900.9 μAhcm−2 (40 mAcm−2)	121.5	2.285 kW kg−1	[125]
NiMoO4-NiS2/MoS2	core	nanowire	27.5	970 Fg−1 (5 Ag−1)	-	711 Fg−1 (20 Ag−1)	26.8	700	[126]
NiMoO4-Ni9S8/MoS2	core	nanorod	27.96	488.9 Fg−1 (1 Ag−1)	81% (10000 cycles)	52.9 Fg−1 (20 Ag−1)	-	-	[127]
NiMoO4-C-Ni3S2	core	nanowire	-	7.9 Fcm−2 (5 mAcm−2)	78.9% (3000 cycles)	1.57 Fcm−2 (50 mAcm−2)	1.29 mWhcm−3	13.99 Wcm−3	[128]
NiCo2S4-NiMoO4·xH2O	shell	nanosheet	-	830.2 Fg−1 (2 Ag−1)	89.9% (5000 cycles)	380.9 Fg−1 (20 Ag−1)	19.3	795.7	[129]
NiCo2S4-NiMoO4	shell	nanosheet	-	1487.6 Fg−1 (1 Ag−1)	89.7% (8000 cycles)	1154.3 Fg−1 (20 Ag−1)	53.2	560	[130]
NiCo2S4-NiMoO4	shell	nanosheet	-	2006 Fg−1 (5 mAcm−2)	75% (2000 cycles)	1305 Fg−1 (50 mAcm−2)	21.4	58	[131]
NiCo2S4-NiMoO4	shell	nanosheet	95	1714 Fg−1 (1 Ag−1)	96% (5000 cycles)	1314 Fg−1 (20 Ag−1)	29.1	172	[132]
NiCo2S4-NiMoO4	shell	nanosheet	10.08	673.3 μAhcm−2 (5 mAcm−2)	84.2% (2000 cycles)	636.7 μAhcm−2 (100 mAcm−2)	33.1	219	[133]
NiCo2OxSy-NiMoO4	shell	nanosheet	-	17.75 Fcm−2 (1345 Fg−1) at 10mAcm−2	69.3% (2000 cycles)	43% (50 mAcm−2)	5.28	329	[134]
NiMoO4-Ni-Co-S	core	nanorod	-	1892 Fg−1 (5 mAcm−2)	91.7% (6000 cycles)	842 Fg−1 (40 mAcm−2)	2.45 mWh cm−3	0.131 Wcm−3	[135]
NiMoO4/CoMoO4-Ni-Co-S	NiMoO4/CoMoO4 core	nanorod	-	778.1 Fg−1 (0.5 Ag−1)	98% (5000 cycles)	648.4 Fg−1 (4 Ag−1)	33.1	199.6	[123]
rGO-NiMoO4-Ni-Co-S	core	NiMoO4 hollow nanotube	-	318 mAhg−1 (1 Ag−1)	88.87% (10000 cycles)	212 mAhg−1 (20 Ag−1)	57.24	801.8	[136]
Ni-Co-Se-NiMoO4	shell	nanosheet	114	396.1 mAhg−1 (1 Ag−1)	87.6% (8000 cycles)	283.3 mAhg−1 (20 Ag−1)	63.1	799.8	[137]
NiMoO4-NiMoS4	core	nanorod	18.6	832.3 Fg−1 (5 A g−1)	81.4% (1000 cycles)	555 Fg−1 (15 Ag−1)	22.84	3750	[138]
NiMoO4-carbon material composite
C-NiMoO4	shell	nanograin	-	268.8 Fg−1 (0.5 Ag−1)	88.4% (2000 cycles)	168.4 Fg−1 (10 Ag−1)	-	-	[139]
Carbon nanofiber-NiMoO4	shell	nanosheet	280	1840 Fg−1 (1 Ag−1)	78.3% (10000 cycles)	78% (20 Ag−1)	23.9	750	[140]
N-C-NiMoO4	shell	nanosheet	258.2	1242 Fg−1 (1 Ag−1)	82.9% (2000 cycles)	581 Fg−1 (20 Ag−1)	44.6	250.4	[141]
XMoO4-carbon submicrofiber (X = Ni, Co)	shell	nanosheet	75.722	1600 Fg−1 (1 Ag−1)	90.7% (3000 cycles)	1166 Fg−1 (10 Ag−1)	55.33	999.89	[142]
NiMoO4-CNTs-CuO	shell	nanosheet	7.08	23.40 Fcm−2 (2 mAcm−2)	82.53% (10000 cycles)	12.9 Fcm−2 (22 mAcm−2)	96.40 mWhcm−3	0.4 Wcm−3	[143]
NiMoO4/V2CTx-rGO	NiMoO4/ V2CTx as yolk	nanoparticle	100.74	1022 Fg−1 (1 Ag−1)	88.9% (3000 cycles)	827 Fg−1 (10 Ag−1)	56.1	800	[144]
NiMoO4-conductive polymer structures
NiMoO4-PANI	core	nanorod	56.14	1214 Fg−1 (1 Ag−1)	80.7% (2000 cycles)	813 Fg−1 (20 Ag−1)	33.07	240	[145]
PPy-NiMoO4	core	nanowire	-	3.4 Fcm−2 (5 mAcm−2)	94% (5000 cycles)	2.30 Fcm−2 (50 mAcm−2)	0.5 mWcm−2	3.7 mWhcm−2	[146]
NiCo2O4-NiMoO4/PANI	shell	nanoplate	-	2.38 Fcm−2 (1 mAcm−2)	92.36% (5000 cycles)	1.508 Fcm−2 (10 mAcm−2)	90	443.2	[147]
NiO-NiMoO4-PPy	NiMoO4 and PPy as shell	porous spherical nanostructure	170.10	1645.1 Fg−1 (1 Ag−1)	77.1% (30000 cycles)	843.2 Fg−1 (20 Ag−1)	-	-	[148]

4.4. NiMoO₄-Carbon Material Composite

Carbon materials such as porous carbon, activated carbon, carbon fiber, carbon nanotube, and graphene are often exploited as supercapacitor materials. They have a high active surface area, excellent electrical conductivity, good chemical stability, and robust mechanical strength. Nevertheless, their practical applications are limited because the specific capacitance is less compared to transition metal oxides. Combining carbon materials with NiMoO₄ can significantly improve cycling stability and rate capability. The grouping can take complete benefits of superior electrical conductivity, great SSA from carbon materials, and high specific capacitance from NiMoO₄.

The hierarchical C-NiMoO₄ composite was synthesized via a two-step hydrothermal method [139]. The NiMoO₄ nanograins were strongly attached to the surface of carbon spheres (CSs), creating a rough silkworm-cocoon-like composite material. The unique structured electrode exhibited good performance due to the following features: (i) highly conductive CSs considerably decrease the charge transfer resistance of the electrode, (ii) the strong bonding between NiMoO₄ and CSs leads to improved electron/ion diffusion, (iii) the exclusive structure has excellent mechanical adhesion, therefore, improving the electrode stability. The specific capacitance (C_sp) was maximum up to 268.8 Fg⁻¹ at 0.5 Ag⁻¹ and the value reduced to 168.4 Fg⁻¹ at 10 Ag⁻¹ because of the inadequate interaction between the electrode and electrolyte ions. The coaxial carbon-NiMoO₄ composite nanofibers were reported for the supercapacitor electrode [140]. The uniform NiMoO₄ nanosheets were deposited on electrospun carbon nanofibers (CNFs) via a microwave-assisted hydrothermal method followed by a thermal process. The rough surface of conductive CNFs (diameter~400 nm) offers abundant nucleation sites for the growth of the NiMoO₄ shell layer, which is made up of numerous nanosheets, as evident from TEM analysis. The thickness of the mesoporous NiMoO₄ layer can be tuned by varying the concentration of the precursor solution. The nanosheets create intimate contact with the CNFs, which favors the electron transfer and increases the redox active surface (SSA~280 m²g⁻¹). Also, the high porosity of CNFs enables the infiltration of electrolytes into the composite, which is essential for high-rate capability. Due to these benefits, the specific capacitance reached 1840 Fg⁻¹ at 1 Ag⁻¹ with 78.3% capacity retention (10,000 cycles). N-C-NiMoO₄ double-shelled hollow microtubes were developed from nitrogen-doped carbon (N-C) nanotubes (inner shell) and NiMoO₄ nanosheets (outer shell) [141]. Initially, PPy nanotubes were converted into N-C nanotubes by calcining in an argon atmosphere. Next, a SiO₂ layer was applied to the N-C nanotube surface. Finally, NiMoO₄ nanosheets were in-situ grown on the N-C nanotube surface through a hydrothermal process, after which the N-C-NiMoO₄ composite was attained by eliminating residual SiO₂ via calcination and etching processes. The N-C nanotubes prevent NiMoO₄ nanosheets from stacking and aggregation, which ultimately leads to a hollow tube structure with high SSA (258.2 m²g⁻¹, average pore size~9.164 nm) and creates NiMoO₄ nanosheets with increased active sites. At 1 Ag⁻¹, the specific capacitance attained a maximum of 1242 Fg⁻¹.

NiMoO₄-CMFs (carbon submicrofibers), CoMoO₄-CMFs, and NiMoO₄-HPCMFs (hollow porous carbon submicrofibers) were constructed via electrospinning followed by the hydrothermal method [142]. The SEM images of NiMoO₄-CMFs and NiMoO₄-HPCMFs are shown in Figure 8a,c. At 1 Ag⁻¹, NiMoO₄-CMFs delivered a higher specific capacitance and cycling stability (1485.53 Fg⁻¹, 87.6% after 3000 cycles) compared to CoMoO₄-CMFs (1407.1 Fg⁻¹, 80.6% after 3000 cycles). The growth of thin NiMoO₄ nanosheets (ca. 250 nm) over CMFs and HPCMFs was confirmed by TEM images (Figure 8b,d). Benefiting from the hollow and mesoporous structure with a pore size of ca. 3 nm (Figure 8e), HPCMFs can offer large SSA for electrolyte infiltration and provide additional channels for electron/ion transport. At 1 Ag⁻¹, the specific capacitance of NiMoO₄-HPCMFs was 1600 Fg⁻¹, and cycling stability was 90.7% after 3000 cycles. Novel NiMoO₄ NSs-CNTs-CuO nanowire arrays (NWAs) were rationally constructed on Cu foam using electrodeposition and hydrothermal methods by Yao et al. [143]. Carbon nanotubes (CNTs) strongly wrapped over Cu(OH)₂ nanowires supported on Cu skeleton to form conductive network channels, contributing to the rapid electron/ion diffusion. CNTs-Cu(OH)₂ NWAs were enclosed by NiMoO₄ nanosheets (NSs) to create core-shell nanowires. After calcination, the resulting highly porous NiMoO₄ NSs-CNTs-CuO NWAs demonstrate a large surface area with abundant active sites for redox reactions, which can improve energy storage. At 2 mAcm⁻², the supercapacitor electrode exhibited an ultrahigh areal specific capacitance of 23.4 Fcm⁻². Chen et al. [144] constructed a hollow NiMoO₄/V₂CT_x-rGO yolk-shell composite employing a novel room temperature ionic liquid (RTIL)-assisted hydrothermal method, as illustrated in Figure 8f. First, NiMoO₄ was incorporated into the V₂CT_x MXene layer in the presence of RTIL, named 1-butyl-3-methylimidazole tetrafluoroborate ([Bmim]BF₄). Then, rGO was applied to the NiMoO₄/V₂CT_x yolk under electrostatic forces. The [Bmim]BF₄ avoids over-oxidation of V₂CT_x MXene during the preparation of NiMoO₄ and decreases its surface energy, which makes the NiMoO₄/V₂CT_x-rGO composite with high stability. In Figure 8g,h, the TEM images of NiMoO₄/V₂CT_x-rGO confirmed the formation of a yolk-shell structure with a definite void space between the inner core and the outer shell. HRTEM images revealed the uniform distribution of MXene sheets, which further confirmed that NiMoO₄ nanoparticles did not leave the MXene sheet to create large clusters due to the magnetic properties (Figure 8i,j). As shown in Figure 8k, the CV curves are similar in shape, and a pair of redox peaks represent pseudocapacitive behavior. After 3000 cycles, the electrode retained 88.9% of its initial capacity, specifying good cycling stability (Figure 8l). At 1 Ag⁻¹, the specific capacitance was 1022 Fg⁻¹, which can be assigned to the following reasons. (1) The yolk-shell structure with large SSA and porosity significantly improves ion diffusion, (2) The petal-folded NiMoO₄ nanoparticles are dispersed over the surface or layered structure of V₂CT_x, resulting in abundant active sites; meanwhile, aggregation of rGO is impeded, (3) The rGO shell develops conductive channels for electrons. Also, a bamboo-shaped MoO₂-Fe₂O₃/NC (N-doped carbon) anode was designed on CC using electrochemical deposition. The NiMoO₄/V₂CT_x-rGO//MoO₂-Fe₂O₃/NC device attained a high energy-density of 56.1 Whkg⁻¹ at a power density of 800 Wkg⁻¹. Wang et al. [149] hydrothermally developed the 2D/2D NiMoO₄/MXene nanosheets with an interconnected porous network. The hydrophilic properties of Ti₃C₂T_x and its superior electrical conductivity, as well as the synergistic interactions between NiMoO₄ and Ti₃C₂T_x, all contributed to the high specific capacity of 545.5 Cg⁻¹ at 0.5 Ag⁻¹.

4.5. NiMoO₄-Conductive Polymer Composite

Conductive polymers of polyaniline (PANI) and polypyrrole (PPy) exhibit interesting features of high electrical conductivity, fast reversible Faradaic reactions, high charge density, and high flexibility [145]. Meanwhile, low SSA, limited charge/discharge cycles, and difficult processing hinder their practical applications. Therefore, these materials can be grouped with NiMoO₄ to improve electrochemical performance. Highly-conductive NiMoO₄-PANI nanocomposite was synthesized via solvothermal followed by chemical polymerization [145]. The surface of mesoporous NiMoO₄ nanorods is uniformly covered by PANI, which could effectively reduce the pore size (~7.8 nm) and prevent the aggregation of nanorods as well as afford active sites for electron/ion diffusion. At 1 Ag⁻¹, the specific capacitance reached 1214 Fg⁻¹. After 2000 cycles, the cycling stability was 80.7% at 5 Ag⁻¹. The 3D PPy/NiMoO₄/CC heterostructure was developed by adopting hydrothermal and chemical polymerization methods [146]. NiMoO₄ and PPy/NiMoO₄ were evenly distributed on CC (Figure 9a,c). The smooth surface of NiMoO₄ nanowires became rough due to the introduction of PPy, and the diameter of the nanowire reasonably increased, as depicted in Figure 9b,d. As shown in Figure 9e, PPy/NiMoO₄/CC displayed a better areal-specific capacitance of 3.4 Fcm⁻² at 5 mAcm⁻² compared to NiMoO₄/CC due to the decrease of ion diffusion length and charge transfer impedance. The exterior bulge-like PPy nanostructure could overcome the volume change and improve material utilization during the charge/discharge process, contributing to excellent cycling stability of 94% over 5000 cycles (Figure 9f). The PPy/NiMoO₄//AC device provided an energy density of 0.5 mWcm⁻² at a power density of 3.7 mWhcm⁻² and exhibited good flexibility without any obvious change in the CV curves under different deformation conditions.

Shen and co-workers [147] demonstrated holothurian-like NiCo₂O₄-NiMoO₄/PANI nanocomposites. NiCo₂O₄ nanowires and NiMoO₄ nanoplates were hydrothermally grown on CC, and then highly conductive 3D PANI nanorods were deposited on the surface of NiCo₂O₄-NiMoO₄ nanoflowers by in-situ polymerization. At 1 mAcm⁻², the areal specific capacitance reached a maximum of 2.38 Fcm⁻², and cycling stability was 92.36% after 5000 cycles. This is because the holothurian-like morphology significantly improves the surface area and reduces the charge transfer distance. Additionally, the active channels in NiCo₂O₄-NiMoO₄/PANI structure and polar groups on PANI can facilitate ion diffusion, resulting in high ionic conductivity. A spherical NiO-NiMoO₄-PPy nanostructure was constructed along with NiO, NiMoO₄, and NiO-NiMoO₄ [148]. Only NiO shows flower-like morphology, and all other nanostructures of NiMoO₄, NiO-NiMoO₄, and NiO-NiMoO₄-PPy exhibit porous structures composed of interconnected nanosheets. The SSA of NiO-NiMoO₄-PPy (170.10 m²g⁻¹) is lower than that of NiO-NiMoO₄ (258.78 m²g⁻¹) because PPy occupies the interspace. The introduction of NiMoO₄ and PPy shell structures can efficiently reduce the charge transfer resistance and increase the redox reaction by providing high electrical conductivity; meanwhile, the PPy also offers additional mechanical strength to the nanostructure. All CV curves are similar in shape with the increasing scan rate, indicating the pseudocapacitive behavior of NiO-NiMoO₄-PPy. Figure 9g illustrates that GCD curves exhibit nearly symmetric, nonlinear triangular shapes and steady discharge plateaus at different current densities (1–20 Ag⁻¹). Specifically, NiO-NiMoO₄-PPy displays the longest discharge times at each current density, signifying the largest specific capacitance (Figure 9h). At 1 Ag⁻¹, the specific capacitance attained a maximum of 1645.1 Fg⁻¹, and it was reduced to 843.2 Fg⁻¹ at 20 Ag⁻¹, showing its exceptional rate capability. The capacity retention was 77.1% after 30,000 cycles (Figure 9i). From density functional theory (DFT) calculations, it was suggested that much stronger Mo-O bonding is crucial to stabilize NiO-NiMoO₄ nanostructure, which would result in good cycling stability.

5. Conclusions

SCs (or ECs) have great potential in the field of energy storage devices. Fabricating the electrode material in a core-shell structure can be one of the effective ways to improve the SCs overall performance. The use of the carefully chosen composite material for the core-shell structure can also benefit from synergistic material effects for better SCs performance. The excellent electrode characteristics would stem from their inherent properties, such as unique morphology and/or porous structure, good electrical conductivity, high redox activity, and good stability.

NiMoO₄, a promising pseudocapacitive material, has been used to form a composite material with additional materials, including metal oxides, metal hydroxides, metal chalcogenides, carbon, and conductive polymers. These composite materials could be fabricated into the core-shell nanostructures to boost the electrochemical performance for SCs application. This paper reviewed those core-shell structures in detail and summarized them in terms of methodological aspects and their relevant properties for SCs applications.

6. Future Perspectives

We provide several points which can lend themselves to the future research focus to improve the performance of the hybrid core-shell structure using the NiMoO₄-based composite materials for SCs application as follows:

• The porous structure of the core-shell-based devices possesses the advantages of the enhancement of the electrode-electrolyte contact area and the shortened distance of electron/ion diffusion from the active medium to the current collector. The morphological feature of the porous core-shell structure is nearly independent of the choice of substrate used to grow the NiMoO₄ composite through the hydrothermal method. Those advantages can lead to superior specific capacitance rate capability and long cycling stability. However, its poor crystallinity that possibly occurs may pose the challenge of inadequate electrical conductivity. This possible shortcoming can be compensated for by the use of the proper substrate. Additional focus can be placed on the substrate for optimizing the SCs’ performances;
• When compared with other synthesis methods, the hydrothermal approach offers a direct and efficient way to synthesize distinctive core-shell composites based on NiMoO₄ and to get improved electrochemical performances by modifying the temperature, reaction time, type, and concentration of surfactants. This method plays a crucial role in the morphology of the composite material. For example, when the reaction time exceeds that required, the shell thickness becomes increased and disordered, with the consequence of distorting/collapsing the original structure and eventually producing cracks. This would, in turn, result in poor electrolyte infiltration. Moreover, mass loading should be optimized as it affects the specific capacitance and energy storage capacity. Future research needs to focus on such parameter optimization for better SCs performance;
• The choice of electrolyte based on conductivity and ionic mobility is crucial for good supercapacitive performance. Generally, aqueous electrolytes have higher conductivity than non-aqueous and solid electrolytes due to their low dynamic viscosity. Aqueous electrolytes, such as alkali metal-based hydroxy (KOH, NaOH) and sulfate electrolytes (Na₂SO₄, H₂SO₄), are commonly used for supercapacitors. The KOH electrolyte is the most popular among all due to its smaller hydrated ions (ionic radius~3.31 Å), therefore often used in all types of the core-shell structures of the NiMoO₄-based composites;
• Various morphologies for the core-shell structures of the NiMoO₄-based composite materials have been investigated, such as the nanosheets, the nanowires, the nanorods, and the nanoflakes. Among those reported, the nanosheets exhibited a higher specific capacitance than the others due to a large SSA. They could also facilitate fast electron/ion transport with increasing conductive channels and hold up their volume during the long charge/discharge cycles. These merits can lead nanosheets to gather more research interests for finding SCs applications;
• The combination of Ni-Co-based oxides with NiMoO₄ has often been reported in the literature. When compared to all other core-shell composites, Ni-Co-based sulfides coupled with NiMoO₄ for the core-shell structures showed superior electrochemical performance due to the participation of Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ in redox reactions. Moreover, cobalt sulfides, as opposed to their oxide counterparts, have a more varied crystal structure, high electrical conductivity, and good electrochemical activity;
• To date, only a few reports are available on NiMoO₄-based core-shell composites consisting of metal sulfides, carbon materials, and conductive polymers. Therefore, other types of mixed transition metal oxides (metal tungstates, metal vanadates)/sulfides can be exploited with NiMoO₄ for core-shell composites to achieve a higher reversible redox activity and specific capacitance than the single metal oxides/sulfides;
• Compact and flexible energy storage devices can be developed for advanced thin and wearable electronics. The complex, hollow, and branched core-shell nanostructured composites can be designed to fit the future supercapacitor.