In recent years, vanadium oxides have gained immense attention in the field of energy storage devices due to their low-cost, layered structure and multi-valency despite their limited electrical conductivity and lower structural stability. In this brief review, we have focused on electrochemical properties of the stoichiometric vanadium oxides along with VOx composites. The morphology engineering, doping with heteroatom and formation of composites with carbon-based materials and/or conducting polymers in enhancing the supercapacitive performances of the vanadium oxides are discussed in detail. Finally, the potentiality and challenges of vanadium oxides nanocomposites for supercapacitor applications are discussed.
Supercapacitor, Vanadium oxide, Specific capacitance, Pseudocapacitance, Phase transition
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 , however, they come with narrow charge storage capacity and relatively low energy density . On the other hand, the conducting polymers are a good choice as a pseudocapacitor . 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 . 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 , 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 VOx 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.
Electrochemical Properties of Vanadium Dioxide
Vanadium dioxide (VO2) is known to be stabilized in different polymorphs, including VO2(A), VO2(B), VO2(C), VO2(D), among others. . 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 . 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 . 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° . The approximate crystallographic relationship between M1 and R phase is bR- cR . In the M1 phase, there are significant differences in the arrangement of V along cR axis. The V atoms form pair, and the pairs tilt along the cR 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 .
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.  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.  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 . 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.  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  to produce a specific capacitance of ~486 mF·cm−2 at the scan rate of 10 mV·s−1. Nie et al.  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 .
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 .
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. . Ren et al.  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.  exhibited a specific capacitance of 203 F·g−1 at the current density of 0.2 A·g−1. Lv et al.  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.  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 . The comparison of various VO2 based supercapacitors and their synthesis procedures are shown in Table 1.
|Nanostructures (Growth Technique)||Electrolyte||Specific
|Current density||Cycling stability (%)||Ref.|
|VO2 (B) hollow spheres (Solvothermal)||1 M Na2SO4/
|68% (10000 cycles)|||
|VO2 nanosheets (Solvothermal)||6 M KOH||663 F·g−1||10 A·g−1||99.4% (9000 cycles)|||
|VO2 nanosheets (Solvothermal)||6 M KOH||47 mAh·g−1||1 A·g−1||89% (10 000 cycles)|||
|VO2 nanosheet (Solution Reduction of
hydrothermally exfoliated bulk V2O5)
|1 M LiClO4/PPC||405 F·g−1||1 A·g−1||82% (6000 cycles)|||
|VO2 nanorod thin films
(RF magnetron sputtering)
|0.1 M NaOH||486 mF·cm−2||10 mV·s−1||100% (5000 cycles)|||
(Reactive templated organic layer on solvothermally grown VO2nanobelt)
|0.5 M Na2SO4||246 F. g−1||0.5 A. g-1||(28.6%) (1000 cycles)|||
(Vapour transport of bulk V2O5 on C-paper)
|93.7% (5000 cycles)|||
(Hydrothermally grown VO2 on 3DG)
|0.5 M K2SO4||507 F·g−1||3
|63.5% (3000 cycles)|||
|VO2 nanoparticle/EOGF (Hydrothermally grown VO2 on EOGF)||5 M LiCl||119 mF·cm−2||2 mV·s−1||70% (1500 cycles)|||
|VO2(B)/C core-shell (Single pot hydrothermal)||1 M Na2SO4||203 F·g−1||0.2 A·g−1||10.4% (100 cycles)|||
|VO2(B) nanobelts/rGO (Hydrothermally grown VO2 on rGO)||0.5 M K2SO4||353 F·g−1||1 A·g−1||78% (10 000 cycles)|||
Table 1: Comparison of various VO2 based supercapacitors.
Electrochemical Properties of Vanadium Trioxide
Vanadium trioxide (V2O3) revels a rhombohedral corundum structure at room temperature, (space group R3?c ) , 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) .
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) .
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 . Hu et al.  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.  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.
|Nanostructures (Growth Technique)||Electrolyte||Specific
|Current density||Cycling stability (%)||Ref.|
(500 °C NH3 reduction of V2O5 gel/ GO films)
|1 M Na2SO4||216 mF·cm−2||1 mA·cm−2||81% (10000 cycles)|||
|V2O3/C nanocomposites (Calcination of hydrothermally grown (NH4)2V3O8)||5 M LiCl||458.6 F·g−1||0.5 A·g−1||86% (1000 cycles)|||
|V2O3@C core-shell nanorods (Single pot hydrothermal process using V2O5 nanorod)||5 M LiCl||228 F·g−1||0.5 A·g−1||86% (1000 cycles)|||
|V2O3 nanofoam@activated carbon (Calcination of NH4VO3 solution and activated C)||1 M NaNO3||185 F·g−1||0.05 A·g−1||49% (100 cycles)|||
Table 2:Comparison of various V2O3 based supercapacitors.
Electrochemical Properties of Vanadium Pentoxide
Vanadium pentoxide (V2O5) stabilizes in various phases including α-V2O5, β-V2O5, δ-V2O5, γ′-V2O5, ζ-V2O5, and ε′- V2O5 . 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 . 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) .
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.  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. . Apart from supercapacitor performance, the change in electrolytes in case of V2O5 also controls its mechanical stability and chemical dissolution. Pandit et al.  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. . 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 . Choudhury et al.  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.  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.  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.  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).
However, the insertion of nitrogen atoms into the carbon network enhances the electrochemical performance by restraining the hydrophobicity. Sun et al.  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.  reported 3D V2O5/PPy nanostructures, which exhibited a high specific capacitance of 448 F·g−1. However, Bi et al.  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 , 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.  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 . 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.
|Nanostructures (Growth Technique)||Electrolyte||Specific
|Current density||Cycling stability (%)||Ref.|
|Hollow spherical V2O5 (Solvothermal)||5 M LiNO3||479 F·g−1||5 mV·s−1||43% (100 cycles)|||
(Chemical bath deposition)
|2 M LiClO4||735 F·g−1||1 mV·s−1||71% (1000 cycles)|||
|rGO/V2O5 hybrid aerogel
(One-pot Hydrothermally grown)
|1 M LiClO4/PPC||384 F·g−1||0.1 A·g−1||82.2% (10 000
(Mixing rGO with hydrothermally
|1 M KCl||653 F·g−1||1 A·g−1||94% (3000
|VNF/graphene nanohybrid (Hydrothermally grown VNF mixed with exfoliated graphene)||1 M LiTFSI in acetonitrile||218 F·g−1||1 A·g−1||87% (700 cycles)|||
|Carbon coated flower V2O5
(Co-precipitation method followed by
annealing at 400 oC)
|1 M K2SO4||417 F·g−1||0.5 A·g−1||100% (2000
(One-pot hydrothermal process of
V2O5 and hydrophilic CNTs)
|1 M LiClO4/PPC||228 C·g−1||20
|V2O5/MWCNT core/shell hybrid aerogels (One-step sol-gel process)||1 M Na2SO4||625 F·g−1||0.5 A·g−1||120% (20 000
|3D N-CNFs/V2O5 aerogels
(Self-assembly of nanostructured
V2O5 onto CNF aerogels with N)
|1 M Na2SO4||595.1 F·g−1||0.5 A·g−1||97% (12000
|3D V2O5/PPy core/shell nanostructures (V2O5 by ion exchange attached with PPy)||5 M LiNO3||448 F·g−1||0.5 A·g−1||81% (1000
|V2O5-Conductive polymer nanocables (V2O5 sol attached to functionalized polymers)||1 M Na2SO4||614 F·g−1||0.5 A·g−1||111% (15 000
(Two-step hydrothermal process
using Ni foam)
|1 M LiNO3||587 F·g−1||0.5 A·g−1||92% (1000
|V2O5-α-Fe2O3 composite nanotubes (One-step electrospinning)||3 M KOH||183 F·g−1||1 A·g−1||81.5% (200
|0.1 M KCl||121.39 F·g−1||100
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  may be adopted for the growth of various V2O5.
Electrochemical Properties of VOx
Other than the stoichiometric oxides of V, there are also reports for electrochemical applications of multi-valent vanadium oxides (VOx). The V-O phase diagram comprises mixed-valence oxides comprehending two oxidation states, namely, V3O7, V6O13, V8O15, V7O13, V6O11, among others . It permits conversion between oxides of different stoichiometry. Huang et al.  reported a high areal capacitance of 1.31 F·cm−2 from VOx 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.  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 , which increases the electronic conductivity of the material. Zhai et al.  reported V6O13 as well as sulfurdoped, oxygen-deficient V6O13-x as an anode electrode.
V6O13-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.  reported the high electrochemical performance of 3D microflower structure of V4O9 supercapacitors (~392 F·g−1).
Conclusion and Outlook
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.
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