Current progress in vanadium oxide nanostructures and its composites as supercapacitor electrodes

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 tried to focus on electrochemical properties of the stoichiometric vanadium oxides along with VO_x 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 details. Finally, the potentiality and challenges of vanadium oxides nanocomposites for supercapacitor applications are discussed.


Introduction:
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][2][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 is poor. Towards this end, transition metal oxides (TMOs) are alternative candidates due to their multiple oxidation states and rapid redox kinetics [2]. Amongst other TMOs [5][6][7], vanadium oxides have received recent attention owing to their low-cost, variety of valence states, and abundant sources [8][9][10]. V is a transition metal ([Ar]3d 3 4s 2 ) with valences in the range of +2 to +5 with major oxides as VO, V2O3, VO2, and V2O5 [11]. 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 charging-discharging 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 results in poor electrochemical and cycle performance. Issues related to the presence of multiple valence states of V [10], 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.

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. [24]. Among the VO2 polymorphs, VO2(B) attracted much attention for its well known MIT at a technologically important temperature of 340K, which is very close to room temperature [25]. 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 [26,27]. In the high-temperature R phase, V atoms are equally spaced, forming linear chains along the cR axis with each V atom surrounded by an oxygen octahedron [28]. The lattice parameters are cR = 2.85 Å, and aR =bR = 4.55 Å. Whereas in the low-temperature 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° [29]. The approximate crystallographic relationship between M1 and R phase is aM1 ↔ 2cR, bM1 ↔ aR, and cM1 ↔ bR -cR [30]. 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 [31].
There are several reports on the supercapacitive performance of VO2(B) in the M1 phase.
However, its low rate capability and cycling instability become the obstacles to serve as a commercial supercapacitor. The modification in structure designing has been adopted to overcome the barrier. Zhang et al. [32] prepared a 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 enhanced stability, and 68% of the capacitance was retained after 10000 cycles. However, 2D nanosheet of VO2 is reported to be a more eligible candidate for electrochemical performance than that of 3D counterpart because of large specific surface area shortening the diffusion path of ion and enhance the redox reaction. Ndiaye et al. [33] 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-d), 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 [34]. 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. [35] reported a specific capacitance of ~ 405 F·g −1 at the current density of 1 A·g −1 of 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 [36] to produce a specific capacitance of ~486 mF·cm −2 at the scan rate of 10 mV·s −1 . Nie et al. [19] 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-f) exhibit a specific capacitance of 20.7 (e) (f) Porous mF·cm −2 at the current density of 0.3 mA·cm −2 [37]. 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 [37]. 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 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. [38]. Ren [20] 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 mixed-valence and structural stability because of the increased edge sharing and the consequent resistance to lattice shearing during cycling [42]. The comparison of various VO2 based supercapacitors and their synthesis procedures are shown in table 1.

Electrochemical properties of vanadium trioxide:
Vanadium trioxide (V2O3) revels a rhombohedral corundum structure at room temperature, (space group R3̅ c) [43], 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 antiferromagnetic insulating transition happens along with the structural transition to the monoclinic phase (space group I2/a) [44].
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 10000 cycles (figure 3c) [45].
One of them is a strong and short V-OI bond with a length of 1.577 Å (d1 showing specific capacitance of 417 F·g −1 at a current density of 0.5 A·g −1 . Chen et al. [59] synthesized V2O5 nanocomposites with carbon nanotubes (CNT) which provided a capacity of 228   (b) (a) 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 [60,63,67] and electrospinning [53] may be adopted for the growth of various V2O5.

Electrochemical properties of VOx:
Other than the stoichiometric oxides of V, there are also reports of electrochemical applications of multi-valent vanadium oxides (VOx 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. [70] prepared NC-coated 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 V 4+ and V 5+ oxidation states with 2:1 ratio [71], which increases the electronic conductivity of the material. Zhai et al. [72] reported V6O13 as well as sulfur-doped, 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 10000 cycles. Pang et al. [73] 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

(a) (b)
increases the specific surface area and offer more active sites which intern 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.