In recent time, supercapacitors (SCs) are one of the emerging technologies used for clean energy prospect. The higher power density, low specific energy, longer cycle life, and environmental affability made the SCs superior compared to conventional batteries. However, the scientific community is working towards increasing the specific energy of SCs by finding a suitable electrode material. Carbon materials, conducting polymers, and metal oxide or hydroxides are reported to be suitable candidates as electrodes for SC [1-3]. Carbon materials such as activated carbon, carbon nanotube and graphene provide excellent electrical conductivity and chemical stability [4], however, they come with narrow charge storage capacity and relatively low energy density [1]. On the other hand, the conducting polymers are a good choice as a pseudocapacitor [3]. Nevertheless, the electrochemical stability of conducting polymer is poor. Towards this end, transition metal oxides (TMOs) are alternative candidates due to their multiple oxidation states and rapid redox kinetics [2,5-7]. Amongst other TMOs [8-10], vanadium oxides have received recent attention owing to their lowcost, variety of valence states, and abundant sources [11-13]. V is a transition metal ([Ar]3d³4s²) with valences in the range of +2 to +5 with major oxides as VO, V₂O₃, VO₂, and V₂O₅ [14]. However, the V-O phase diagram comprises mixed-valence oxides comprehending two oxidation states, e.g. V₄O₉, V₆O₁₃, V₈O₁₅, V₇O₁₃, V₆O₁₁, among others which permit conversion between oxides of different stoichiometry easily and make it unstable during chargingdischarging cycle. As the composition, oxidization state, and structural phase of a material have a significant role on electrochemical properties; the exchange of valency along with structural instability for these materials result in poor electrochemical and cycle performance. Issues related to the presence of multiple valence states of V [13], as well as its stability affecting retention of capacitance and its efficiency, are found to hinder further utility in SCs. The poor charge storage properties and electrical conductivity of vanadium oxides are reported to be succeeded by fabricating directly on the current collector, doping element, or by nanostructure engineering. There are several reports on SC properties of VO_x composites as well as individual vanadium oxides such as VO₂, V₂O₃, and V₂O₅ [3,15-26]. In this context, this mini review presents a summary of recent developments in vanadium oxide based supercapacitor along with future developments, prospects, and challenges.

Vanadium dioxide (VO₂) is known to be stabilized in different polymorphs, including VO₂(A), VO₂(B), VO₂(C), VO₂(D), among others. [27]. Among the VO₂ polymorphs, VO₂(B) attracts much attention for its well known MIT at a technologically important temperature of 340K, which is very close to room temperature [28]. VO₂(B) crystallizes in rutile tetragonal (R; space group P42/mnm) and monoclinic (M1; space group P21/c) structure above and below the transition temperature, respectively [29,30]. In the hightemperature R phase, V atoms are equally spaced, forming linear chains along the cR axis with each V atom surrounded by an oxygen octahedron [31]. The lattice parameters are cR = 2.85 Å, and aR =bR = 4.55 Å. Whereas in lowtemperature monoclinic phase, the volume of the unit cell becomes double than that of R phase with lattice parameters aM₁ = 5.70 Å, bM₁ = 4.55 Å, cM₁ = 5.38 Å, and ßM₁ = 123° [32]. The approximate crystallographic relationship between M₁ and R phase is  b_R- c_R [33]. In the M₁ phase, there are significant differences in the arrangement of V along c_R axis. The V atoms form pair, and the pairs tilt along the c_R axis making the surrounding octahedron deformed. Besides M1, two more metastable phases of monoclinic M₂ having space group C2/m and triclinic T (alternatively monoclinic M3) with space group C? 1 are also reported in the process of the phase transition from M₁ to R [34].

There are several reports on the supercapacitive performance of VO₂(B) in the M1 phase. However, its low rate capability and cycling instability become obstacles to serve as a commercial supercapacitor. The modification in structure designing has been adopted to overcome the barrier. Zhang et al. [35] prepared template-free 3D hollow spherical cages (shown in figures 1a-b) by hydrothermal method, which showed a specific capacitance of 1175 mF·cm⁻² (336 F·g⁻¹) with adequate stability, and 68% of the capacitance was retained after 10,000 cycles.However, 2D nanosheet of VO₂ is reported to be a more eligible candidate for electrochemical performance than that of its 3D counterpart because of large specific surface area shortening the diffusion path of ion and thereby enhancing the redox reaction. Ndiaye et al. [36] obtained specific capacitance of 663 F·g⁻¹ at the scan rate of 5 mV·s⁻¹ and excellent cycling stability after 5000 cycles at the current density of 10 A·g⁻¹ for VO₂ nanosheets. The 2D nanosheets (Figures 1c and 1d), while assembled with the structure of carbonized iron-polyaniline (C-FP), exhibited a specific capacity of 47 mAh·g⁻¹ at a current density of 1 A·g⁻¹ [37]. In 2D nanosheets, a large specific surface area diminishes the path length of the ion diffusion enabling the execution of the redox reaction effectively. Rakhi et al. [38] reported a specific capacitance of ~ 405 F·g⁻¹ at the current density of 1 A·g⁻¹ for VO₂ nanosheets in an organic gel electrolyte (1 M LiClO₄ in propylene carbonate) with nearly 82% capacitance retention.

The thin layer of 1D VO₂ nanorods on indium tin oxide - coated glass substrates are also reported [39] to produce a specific capacitance of ~486 mF·cm⁻² at the scan rate of 10 mV·s⁻¹. Nie et al. [22] reported VO₂@Polyaniline coaxial nanobelts exhibiting a higher specific capacitance of 246 F·g^-1 at 0.5 A g-1 than that of VO₂ nanobelts (160.9 F·g^-1). The specific capacitance was almost constant at around 118 mF·cm⁻² after 5000 cycles at the scan rate of 100 mV·s⁻¹. VO₂ nanoporous structures on carbon fiber in the M1 phase (Figures 1e and 1f) exhibit a specific capacitance of 20.7 mF·cm⁻² at the current density of 0.3 mA·cm⁻² [40].

It also demonstrates capacitance retention of 93.7% and coulombic efficiency of 98.2% for 5000 charge-discharge cycles. However, the similar nanoporous structures in M2 and T phases of VO₂ show poor specific capacitance (Figure 2a) as well as cyclic stability (Figure 2b) because of mixed valency [40].

Another way to enhance the electrochemical performance of VO₂ is by combining with carbon materials, which improve the electrical conductivity. VO₂ nanoflowers on 3D graphene (3DG) networks were reported to exhibit a large specific capacitance of 466 mF·cm⁻², capacitance retention of 63.5% after 3000 cycles by Wang et al. [41]. Ren et al. [42] synthesized VO₂ nanoparticles on edge-oriented graphene foam (EOGF) which exhibit a capacitance of 119 mF·cm⁻² at the scan rate of 2 mV·s⁻¹. The VO₂(B)/carbon core-shell composites prepared by Zhang et al. [43] exhibited a specific capacitance of 203 F·g⁻¹ at the current density of 0.2 A·g⁻¹. Lv et al. [44] prepared VO₂(B) nanobelts/rGO composites with a porous framework, which showed an excellent power density of 7152 W·kg⁻¹ at the energy density of 3.13 Wh·kg⁻¹. Shao et al. [23] reported VO₂ demonstrating superior properties as supercapacitor compared to that for the V₂O₅, which was well known for its SC performance. It is due to higher electronic conductivity in VO₂, as compared to V₂O₅, originating from a mixedvalence and structural stability because of the increased edge sharing and the consequent resistance to lattice shearing during cycling [45]. The comparison of various VO₂ based supercapacitors and their synthesis procedures are shown in Table 1.

Table 1: Comparison of various VO₂  based supercapacitors.

Vanadium trioxide (V₂O₃) revels a rhombohedral corundum structure at room temperature, (space group R3?c ) [46], where the V atoms pair along the crystal c-axis and form honeycomb lattices in the ab-plane. Whereas, below the temperature ~150 K, a paramagnetic metallic to an antiferro-magnetic insulating transition happens along with the structural transition to the monoclinic phase (space group I2/a) [47].

There are very few reports on the electrochemical studies on V₂O₃, mostly because of the poor stability of this material. A binder-free electrode of V₂O₃ nanoflakes on N-doped rGO (Figure 3a) was reported to have an areal capacitance of 216 mF·cm⁻² at a current density of 1 mA·cm⁻² (Figure 3b). It also exhibits cycling stability with retention of ~81% of the initial capacitance value after 10,000 cycles (Figure 3c) [48].

However, V₂O₃ combined with carbon composites are reported to serve as superior electrode material. Zheng et al. reported V₂O₃/C composites exhibiting high pseudocapacitance of 458.6 F·g⁻¹ at 0.5 A·g⁻¹. The composite also shows a retention rate of 86% after 1000 cyclesin aqueous electrolyte [49]. Hu et al. [50] synthesized V₂O₃@C core-shell nanorods with porous structures which exhibited 228, 221, 207, 158, and 127 F·g⁻¹ specific capacitances at current densities of 0.5, 1, 2, 5, and 10 A·g⁻¹, respectively. Zhang et al. [51] reported a V₂O₃ nanofoam@ activated carbon composite, which showed a specific capacitance of 185 F·g⁻¹ at 0.05 A·g⁻¹. The comparison of various V₂O₃ based supercapacitors, fabricated following different process steps, are shown in Table 2.

Table 2:Comparison of various V₂O₃  based supercapacitors.

Vanadium pentoxide (V₂O₅) stabilizes in various phases including α-V₂O₅, β-V₂O₅, δ-V₂O₅, γ′-V₂O₅, ζ-V₂O₅, and ε′- V₂O₅ [52]. The most well-known phase is α-V₂O₅, which crystallizes into an orthorhombic structure composed of weakly Van der Walls bonded layers of VO5 pyramids sharing their vertices and corners [53,54]. The unit-cell parameters are a = 11.51 Å, b = 3.56 Å, and c = 4.37 Å [53]. It has space group with distorted squarepyramidal coordination symmetry around each V atom. There are three non-equivalent oxygen atoms in each unit cell (denoted as OI, OII, and OIII). OI is the terminal (vanadyl) oxygen with two different bond lengths. One of them is a strong and short V-OI bond with a length of 1.577 Å (d₁). Another one is large and weak Van der Waals type connecting two adjacent layers in the V₂O₅ structure, with a bond length of 2.793 Å. Both of these OI atoms orient almost along the c-axis. The two-fold coordinated bridging oxygen (OII) connects two adjacent V atoms with V-OII bond length of 1.78 Å (d₂). The ladder-shaped OIII atoms are the three-fold coordinated oxygen with three different V-OIII bond lengths of 1.88 (d₃), 1.88 (d₃), and 2.02 Å (d₄) [53].

The SC properties in V₂O₅ is reported to be superior to other vanadium oxides because of its stability and layered structure [19,21-23]. Yang et al. [55] prepared hollow V₂O₅ spheres which showed an excellent capacitance of 479 F·g⁻¹ at 5 mV·s⁻¹. V₂O₅ nanofibers showed specific capacitance of 190 F·g⁻¹ in aqueous electrolyte (KCl) and 250 F·g⁻¹ in the organic electrolyte (LiClO₄ in PPC) as reported by Wee et al. [56]. Apart from supercapacitor performance, the change in electrolytes in case of V₂O₅ also controls its mechanical stability and chemical dissolution. Pandit et al. [57] synthesized V₂O₅ thin film on a pliable stainless steel substrate which was reported to exhibit a high specific capacitance of 735 F·g⁻¹ at 1 mV·s⁻¹ with capacitors retention of 71% after 1000 cycles.

The rGO/V₂O₅ composites showed specific capacitance of 386, 338, 294, 241, and 197 F·g⁻¹ at current density of 0.1, 0.2, 0.5, 1, and 2 A·g⁻¹, respectively, as reported by Liu et al. [58]. However, 2D heterostructures of V₂O₅ nanosheets growing on rGO flakes showed relatively high specific capacitance of 653 F·g⁻¹ at 1 A·g⁻¹ and cyclic stability of 94% after 3000 cycles [59]. Choudhury et al. [60] prepared V₂O₅ nanofiber (VNF)/exfoliated graphene nanohybrid with the mass ratio of 1:0.25 and 1:0.5 with a superior capacitance value of 218 F·g⁻¹ at 1 A·g⁻¹ for 1:0.5 mass ratio. Balasubramanian et al. [61] reported flowery V₂O₅ structures coated with carbon showing specific capacitance of 417 F·g⁻¹ at a current density of 0.5 A·g⁻¹. Chen et al. [62] synthesized V₂O₅ nanocomposites with carbon nanotubes (CNT) which provided a capacity of 228 C·g⁻¹ between 1.8 and 4.0 V. Wu et al. [63] reported V₂O₅/ multi-walled CNT core/shell hybrid aerogel (Figure 4a), which demonstrated the maximum specific capacitance of 625 F·g⁻¹ with outstanding cycle performance (>20000 cycles). The hybrid aerogel showed better performance than that of raw V₂O₅ powder, MWCNTs, and V₂O₅ aerogel (Figure 4b).

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However, the insertion of nitrogen atoms into the carbon network enhances the electrochemical performance by restraining the hydrophobicity. Sun et al. [64] reported self-assembled 3D N-carbon nanofibers (CNFs)/V₂O₅ aerogels showing the specific capacitance of 575.6 F·g⁻¹ even after 12,000 cycles (97% of the initial value). V₂O₅ also have been combined with conducting polymers e.g., polypyrrole (PPy), poly (3, 4-ethylenedioxythiophene) (PEDOT), and polyaniline (PANI), to enhance the electrical conductivity and prevent the V from dissolving. Qian et al. [65] reported 3D V₂O₅/PPy nanostructures, which exhibited a high specific capacitance of 448 F·g⁻¹. However, Bi et al. [66] showed a comparative study with oxygen vacancy (Ö) resulting with the specific capacitance of 614 F·g⁻¹ for VÖ-V₂O₅/PEDOT higher than that of 523 F·g⁻¹ for VÖ-V₂O₅/PANI and 437 F·g⁻¹ for VÖ-V₂O₅/PPy (Figures 5a and 5b).

The association of V₂O₅ with other metal oxides is also reported to enhance the electrochemical properties. Xu et al. prepared V₂O₅ nanobelts/TiO₂ nanoflakes composites [67], which exhibited high specific capacitance of 587 F·g⁻¹ at 0.5 A·g⁻¹ with good cyclic stability of 97% after 5000 cycles. The V₂O₅-doped α-Fe₂O₃ composites by Nie et al. [68] showed a capacitance value of 150 F·g⁻¹ over 200 cycles at 6 A·g⁻¹. CNT-SnO₂-V₂O₅ composites exhibited higher specific capacitance compared to CNT, V₂O₅, and CNT-V₂O₅ [69]. V₂O₅ shows higher performance combined with carbon and other metal oxides than that of the other two phases of vanadium oxides discussed before. The comparison of various V₂O₅ based supercapacitors with involved synthesis techniques are shown in Table 3.

Table 3: Comparison of various V₂O₅  based supercapacitors.

Large scale production, however, can perhaps be considered for the most stable phase of V₂O₅ with its optimum performance as a supercapacitor. Thus, looking into the economic prospect, materials involving sol-gel route synthesis [63,66,70] and electrospinning [56] may be adopted for the growth of various V₂O₅.

Other than the stoichiometric oxides of V, there are also reports for electrochemical applications of multi-valent vanadium oxides (VO_x). The V-O phase diagram comprises mixed-valence oxides comprehending two oxidation states, namely, V₃O₇, V₆O₁₃, V₈O₁₅, V₇O₁₃, V₆O₁₁, among others [71]. It permits conversion between oxides of different stoichiometry. Huang et al. [72] reported a high areal capacitance of 1.31 F·cm⁻² from VO_x functionalized by a carbon nanowire array. V₃O₇ was reported to get converted to V₆O₁₃ at the lowest potential of -0.6 V and V₂O₅ at the highest potential of 0.2 V. Zhao et al. [73] prepared NCcoated nest-like V₃O₇ which showed the specific capacity of 660.63 F·g^-1 at 0.5 A·g^-1 (Figure 6a), a significantly higher than that of V₃O₇ (362.63 F·g^-1 ). NC-V₃O₇ exhibited 80.47% of the initial capacitance at 10 A·g⁻¹ after 4000 cycles, which is 23.16% higher than that of V₃O₇ (Figure 6b).

V₆O₁₃ has stimulated extensive attention owing to its high specific capacitance and decent cycle ability for Li batteries. However, there are very few reports on its SC performance. V₆O₁₃ is known as a mixed-valence oxide as it exists between the V⁴⁺ and V⁵⁺ oxidation states with 2:1 ratio [74], which increases the electronic conductivity of the material. Zhai et al. [75] reported V₆O₁₃ as well as sulfurdoped, oxygen-deficient V₆O_13-x as an anode electrode.

V₆O_13-x provides a capacitance of 1353 F·g⁻¹ at a current density of 1.9 A·g⁻¹ and outstanding capacitance retention of 92.3% after 10,000 cycles. Pang et al. [76] reported the high electrochemical performance of 3D microflower structure of V₄O₉ supercapacitors (~392 F·g⁻¹).

This brief review deals with the electrochemical performances of vanadium oxides and its composites as supercapacitors. Vanadium oxides have attracted tremendous attention in electrochemistry due to their multi-valency, low cost and abundant sources on earth. However, poor electrical conductivity, structural instability, poor specific capacitance and low energy density limit their practical applications. The scientific community is working towards removing the obstacles by morphology engineering (increase the specific surface area), doping with a heteroatom (reduce hydrophobicity and structural stability), combined with carbon-based materials and/ or conducting polymers (increasing conductivity) and so on. The structure designing increases the specific surface area and offer more active sites which in turn increases the contact between the material and electrolyte, generates more redox reactions, and enhances the electrochemical performance of the materials. It has been observed from the previous studies that nanostructures with porosity are the best choice for increasing supercapacitive performance. On the other hand, combining with composites increases the specific capacity, cyclic stability, and finally energy and power density. Comments are also made for the commercial viability in terms of large area synthesis of a stable phase of vanadium oxide with optimized supercapacitance properties. Finding a proper composite material for a specific vanadium oxide is still a challenge for future development. Finally, as vanadium oxides are prone to change the oxidization state, insitu characterization techniques are likely to be carried out during the electrochemical processes. Most of the vanadium oxides also undergo electrical, magnetic, and/ or structural transition with minimal change in the electric field, temperature, or pressure. Therefore, to understand the change in phases and its role in the electrochemical process, advanced in-situ characterization techniques should be incorporated.