Synthesis and electrochemical performance of V₂O₅ nanosheets for supercapacitor

Energy storage devices play a crucial role in the development of electronic products. Supercapacitors have recently received a multitude of attention owing to rapid charge and discharge, high power density, and good cycle stability.^1–5 It is proverbial that an electrode material has a decisive influence on the electrochemical performance of a supercapacitor.⁶ Advanced electrode materials can provide supercapacitors with high specific capacity, high rate capability, and good cycle stability. In general, RuO₂ and IrO₂ are excellent electrode materials for supercapacitors; due to the high cost of RuO₂ and IrO₂, it is urgent to find more cost-effective transition metal oxides.^7–9 Cheap transition metal oxides (TMOs) are a potential alternative to the supercapacitor electrode material.^10–14 

Among the multifarious compounds, TMOs V₂O₅ has emerged as an interesting candidate because of its high theoretical specific capacity.^15–19 Moreover, the easy synthesis, stable crystal structure, low cost, and high faradaic activity endow V₂O₅ diffusely used in energy storage.²⁰ Many researchers reported V₂O₅ with different morphologies synthesized by the hydrothermal method, such as V₂O₅ nanobelt,²¹ V₂O₅ nanoflower, and V₂O₅ nanosheet.²² However, the low electrical conductivity and cycling property limit the wide application of V₂O_5.

Song et al. took NH₄VO₃ as the vanadium source and added oxalic acid in the hydrothermal synthesis to obtain V₂O₅ nanosheets. The nanosheets were 50–70 nm thick and about 250 nm long. The capacity retention rate could reach 91% after 1000 times of constant current charge and discharge.²³ Liang et al. synthesized V₂O₅ nanosheets of thickness only 2–5 nm by the solvothermal method. Moreover, the specific capacity of the V₂O₅ nanosheet electrode could still maintain 92.6% after 500 discharge/charge cycles.²⁴ Therefore, we can infer that the size of V₂O₅ can affect the electrochemical performance of its supercapacitor.

In this paper, two-dimensional V₂O₅ nanosheets with different thicknesses and structures were synthesized by the hydrothermal method by changing the morphology and improving the crystallinity and crystal structure. V₂O₅ material prepared with water as the solvent is used as the coating on a nickel foam substrate, and the capacitance retention rate is 20% at 2 A g⁻¹. V₂O₅ material prepared with water and ethylene glycol as solvent is used for coating the nickel foam substrate with a capacitance retention of 94.6% at 0.5 A g⁻¹. These performances will make V₂O₅ have a broad prospect in the field of supercapacitors.

All the samples involved in this experiment are analytically pure. 0.05 mol ammonium metavanadate (Aladdin Reagents Co., Ltd.) was dissolved in 50 ml ethanol glycol (EG) and deionized water solution with a volume ratio of V (H₂O):V (EG) = 4:1 and stirred for 30 min to get a clear bright yellow solution. Afterward, hydrochloric acid was added dropwise to the above solution until a pH of 3 is reached. Subsequently, the solution was poured into a polytetrafluoroethylene-lined reactor for reaction at 180 °C for 24 h. After cooling down to room temperature, the precipitate was filtered and washed with distilled water and ethanol. Finally, the resulting product is calcined in a muffle furnace at 350 °C for 2 h in air atmosphere. By changing different hydrothermal solvents, pure water, and V (H₂O):V (EG) = 1:0.25, we obtained V₂O₅ nanosheets with 119 nm thickness and 107 μm length. In this paper, sample V₂O₅-A is named pure water as the solvent, and sample V₂O₅-B is named V (H₂O):V (EG) = 4:1 as the solvent.

X-ray diffraction (XRD) patterns were obtained using a Rigaku D/MAX 2200 PC x-ray diffractometer with a Cu K_α radiation (λ = 0.15406 nm) to characterize the crystal structure of the as-prepared products. The morphology of V₂O₅ was detected by a scanning electron microscope (SEM) (FESEM, JSM-7610F).

Vanadium pentoxide, acetylene black, and polyvinylidene fluoride (PVDF) were mixed with the mass ratio of 16:3:1 to make a homogeneous material, and the homogeneous material was coated on the nickel foam substrate to make a working electrode. Saturated calomel electrode and platinum plate were used as a reference and auxiliary electrodes, respectively. 0.5M K₂SO₄ was used as the electrolyte. About 3 mg of V₂O₅ powder was coated on the working electrode. The electrochemical performance of the V₂O₅ was evaluated using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) utilizing electrical chemistry workstation (CHI 660E, CH Instruments). Specific capacitance can be obtained by the following equation:

The XRD of V₂O₅ prepared by the hydrothermal method with different solvents is shown in Fig. 1. All diffraction peaks in Fig. 1 are corresponding to the crystallographic planes of V₂O₅ (PDF#41–1426). When 2θ is 20.2°, 21.7°, 31.0°, 37.3°, and 45.4°, it corresponds to the crystal faces of (001), (101), (301), (401), and (411), respectively, of standard PDF cards (JCPDS 41–1426, space group Pmmn).²⁵ Figure 1 shows that the diffraction peak strength of V₂O₅-B is higher than V₂O₅-A, and the crystallinity increases. It shows that the so-obtained characteristic peak of V₂O₅-B is the strongest and the crystal growth is the best. In an acidic aqueous solution, NH₄VO₃ exists in the form of (NH₄)₄V₄O₁₂. The hydrolysis reaction of (NH₄)₄V₄O₁₂ is represented as formula (2). We speculated that the nanosheets with better crystallinity have better electrochemical performance, which will be verified in the following sections. The chemical reactions to produce vanadium pentoxide are shown in the following equations:

As can be seen from Fig. 2, when V (H₂O):V (EG) = 4:1, the average grain size of V₂O₅ is smaller than V₂O₅-A. This may be because the surface tension of ethylene glycol is smaller than that of water. At 20 °C, the surface tension of water is 72.8 N/m, while the surface tension of ethylene glycol is only 46.49 Mn/m. When ethylene glycol is added into the water, the surface tension of the solution is less than that of pure water solvent, so that the crystal growth resistance is small, and the crystal is easier to grow, which is conducive to increase the specific capacitance of the supercapacitor.

When the solvent is pure water, the prepared powder contains a large number of white sequins. This is because water is a polar solvent, the inorganic particles synthesized in the process of hydrothermal reaction may have groups on the surface and form hydrogen bond adsorption with the solvent. In the subsequent drying and calcining processes, secondary agglomeration is likely to occur, resulting in white sequins. When ethylene glycol is added to the solution, the contact area between the reactant and the mixed solvent of water and ethylene glycol can be increased, and the reaction rate of the whole hydrothermal reaction can be improved. Moreover, the viscosity of ethylene glycol is higher than that of water, and so the viscosity of the mixture of the two is higher than that of water, which effectively controls the growth rate of the crystal and limits the size of the particles. Using particle size distribution calculation software, the maximum thickness of 119 nm, minimum thickness of 45.28 nm, and average thickness of 74.07 nm are calculated. The particle size distribution is shown in Fig. 2(e).

The CV curves of the first three cycles from −1 to 1 V of V₂O₅ prepared by the hydrothermal method with different solvents are shown in Figs. 3(a) and 3(b). All the curves are symmetric, and it shows good reversibility. The current response occurs when the direction of the sweep direction is changed. The transient change of current indicates that the electrode has been electrically cycled. It has good reversibility in process dynamics.^{8, 9}

As can be seen from Fig. 3(a), when the solvent is V (H₂O):V (EG) = 4:1, the strong peaks at −0.13 and 0.8 V are oxidation peaks, and the strong peaks at −0.35 V are reduction peaks. Each CV curve exhibits an asymmetric reduction peak, indicating a reversible insertion and extraction of K⁺ on the electrode surface. As shown in the equation,

In addition, it can be seen that the curves of the first three cycles are the same, which indicates that V₂O₅ is highly electrochemical reversible between −1 and 1 V. It can be seen from Fig. 3(a) that when the solvent is V (H₂O):V (EG) = 1:0.25, the graph area enclosed by the curve is larger, indicating that the electrode has the better capacitance performance. Figure 4(a) shows the CV curve at scan rates from 10, 20, 40, 60, 80, and 100 mV/s and current density from −20 to 20 A g⁻¹. As can be seen from the graph, the CV curves are well maintained at different scanning rates, revealing superior redox reversibility.

The EIS spectra of V₂O₅ prepared by hydrothermal method with different solvents are shown in Fig. 4(b). It is well known that an ultracapacitor can be regarded as an assembly circuit composed of many small capacitors and resistors. As the test frequency increases, the effect of resistor R increases. As the frequency decreases, the effect of capacitor C increases. Finally, it is concluded that the impedance Z in the high-frequency region (HF) is characterized by the resistance (R), and the impedance Z in the low-frequency region (LF) is characterized by the series characteristics of the resistance R and capacitance C. As can be seen from Fig. 4(b), the high-frequency resistance of the EIS image obtained under the sample V₂O₅-B is 2 Ω, and the slope is higher in LF, indicating a faster diffusion rate of ions, thus improving the charge and discharge rate of the supercapacitor. The high-frequency resistance of the EIS image obtained under the sample V₂O₅-A is 5 Ω higher than that of V₂O₅-B, and the curve slope in the low-frequency region is relatively small, indicating that the charge and discharge rate is slow. As can be seen from the EIS spectra of V₂O₅-B, its high-frequency semicircular radius is smaller than that of pure water, indicating lower charge transfer resistance and faster electrochemical reaction. In addition, the slope of V₂O₅-B at LF is higher than V₂O₅-A, indicating that the ion diffusion rate is faster. Therefore, when the volume ratio of water to ethanol is 4:1, the transfer of charge and ions is greatly improved.

Figure 5(a) shows the GCD curves at different current densities. The voltage range is −0.2–0.6 V, the current density range is 0.5, 1.0, 1.5, and 2.0 A g⁻¹, and the constant current charge–discharge curves obtained are symmetrical, which indicate that the supercapacitor has a good reversibility. As the current density continues to rise, the charge–discharge time becomes shorter and shorter, and the specific capacity becomes smaller and smaller through calculation. Therefore, the current density of 0.5 A g⁻¹ is used for testing in our experiment. When the volume ratio of water to ethanol is 4:1, the GCD curve of the V₂O₅ electrode presents a nearly symmetric shape, indicating that it has a good reversible redox reaction and rate capacity. From the results of GCD, we can calculate C_m, as shown in Fig. 5(b). It can be seen from the figure that when the current density is 0.5 A g⁻¹, the specific capacity obtained is 375 F g⁻¹; when the current density is 1.0 A g⁻¹, the specific capacity obtained is 175 F g⁻¹; when the current density is 1.5 A g⁻¹, the specific capacity obtained is 100 F g⁻¹; and when the current density is 2.0 A g⁻¹, the specific capacity obtained is 75 F g⁻¹. As can be seen from Fig. 5(b), the specific capacity decreases with the increase of current density. The increase in current density leads to the electrode interface to capture a large amount of vanadium electrolyte ions, the interface of vanadium ion concentration drops rapidly, concentration polarization is bound to increase, and maintaining a high current density will need higher excitation voltage, but did not increase the interface charge number, so will cause decreases in specific capacitance with the increase in current density.

As can be seen from Fig. 6, the specific capacitance of the V₂O₅ nanosheet electrode is about 350 F g⁻¹. With the increase in the number of cycles, the specific capacitance of V₂O₅-A decreases continuously, and the retention rate is poor. The specific capacitance of V₂O₅-B is the same, which indicates that this material has a high capacitance holding rate and good cycling performance. When the solvent is V (H₂O):V (EG) = 4:1, the specific capacitance increases from 350 to 375 F g⁻¹ after 500 cycles. After 1000 cycles, the capacitance retention rate of the V₂O₅-B electrode reaches 96.8%, and the capacitance retention rate of the V₂O₅-A electrode is only 20%, indicating that the cycling performance of V₂O₅ material has been improved when the solvent is water and ethanol glycol volume ratio is 4:1. Supercapacitors mainly rely on Faraday reaction for energy storage, and the fast reversible redox reaction is not 100% reversible in actual reaction, so specific capacitor descend often occurs in the cycle test. The structural integrity and electrochemical reaction of the electrode material are important factors to determine the cycling performance. The structural instability of V₂O₅ can be solved by constructing unique nanostructures. Figure 2 shows that V₂O₅-B has a unique nanosheet structure, and the nanosheet structure can effectively alleviate the volume change in redox reaction, and so it has a better cycling performance. Therefore, the cyclic stability of V₂O₅-B is better than that of V₂O₅-A.

To sum up, the main aim of this study is to prepare V₂O₅ nanosheets, hoping to improve the electrochemical properties of V₂O₅ powders by adjusting the structure. V₂O₅ nanosheets were prepared by the hydrothermal method. The morphology and size of the V₂O₅ nanosheets were controlled by adjusting the type of the solvent, and the size of the V₂O₅ nanosheet was uniform. The optimum process condition is that the volume ratio of water to ethanol is 4:1. Electrochemical test results show that the V₂O₅ nanosheet has a good performance in 0.5M K₂SO₄ electrolyte. The specific capacitance is up to 375 F g⁻¹ at the current density of 0.5 A g⁻¹, and the specific capacitance retention rate is 96.8% after 1000 cycles. Because of its good cycling performance, it has great application prospects in supercapacitors.

This work was supported by the Natural Science Foundation of Shaanxi Province (Grant Nos. 2021JQ-762 and 2022 JQ-462), the Natural Science Foundation of Shaanxi Provincial Department of Education (Grant No. 21JK0558), and the Scientific Research Startup Program for Introduced Talents of Shaanxi University of Technology (Grant Nos. SLGRCQD2025 and SLGRCQD2133).

The authors have no conflicts to disclose.

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy.