1. Introduction

Energy is essential in modern society, and research on energy storage devices is gradually increased in the requirement of energy. Energy storage devices such as batteries and supercapacitors have been used to satisfy the demand for electric vehicles, various electronic devices and supplies.^1,2,3,4,5,6) Supercapacitors, also called electrochemical capacitors, have outstanding electrochemical performance including high power density, fast charge/discharge kinetics, stable cycle performance and large specific capacitance etc.^7,8,9) Furthermore, supercapacitors comprise electrodes, electrolyte, separator and current collectors, which might relate to the electrochemical performance. Transition metal oxide-based electrode materials for supercapacitors, such as ruthenium oxide (RuO₂), manganese oxide (MnO₂), cobalt oxide (Co₃O₄), nickel oxide (NiO), copper oxide (CuO), molybdenum oxide (MoO₃), ferric oxide (Fe₂O₃) and vanadium oxide (V₂O₅), exhibit excellent electrochemical performance for attractive supercapacitive behaviors as high theoretical capacitance and high cycling stability.^{10,11,12,13,14,15,16,17)}

Among transition metal oxide-based electrode materials, vanadium oxide-based materials (V₂O₅, VO₂, V₂O₃, V₆O₁₃ etc.) has been frequently investigated as electrodes of energy storage devices for several years and has mostly synthesized by adding acid to vanadium precursor.^17,18,19,20) Above all, the advantages of vanadium pentoxide (V₂O₅) are a layered structure, high theoretical specific capacitance, multiple oxidation valence state, low cost and natural abundance.^21,22) In addition, hydrated vanadium pentoxide (V₂O₅ ‧ nH₂O) that H₂O inserted into V₂O₅ is expanded interlayer spacing of V₂O₅ material to improve electrochemical performance due to the expanded layered structure.²³⁾ However, V₂O₅ ‧ nH₂O and V₂O₅ have low electrical conductivity and structural instability which might restrict implementation of energy storage devices.^24,25,26) To solve these problems, V₂O₅ ‧ nH₂O has been used along with carbon-based materials that substantial surface area and high electrical conductivity such as carbon nanotube (CNT), reduced graphene oxide (rGO) and graphene oxide (GO).^27,28,29) Geng and Wang²⁸⁾ developed V₂O₅ ‧ nH₂O/graphene composite for supercapacitors that was greater electrochemical performance than V₂O₅ ‧ nH₂O xerogel by adding GO. Fan et al.²⁹⁾ reported V₂O₅ ‧ nH₂O/rGO composite film that V₂O₅ ‧ nH₂O was sandwiched with rGO by chemical bonding to ensure good mechanical properties, which exhibited high specific capacitance and good cycling stability for ammonium-ion energy storage. In particular, abundant oxygen functional groups and high specific surface area of GO increase electrochemical performance.^30,31) Dai et al.³⁰⁾ developed MnO₂ nanowires/GO nanosheets composite that possessed high specific capacitance and high cycle performance due to the oxygen functional groups and high specific surface area of GO, and the composite was prevented aggregation of GO and MnO₂ nanowires and accordingly provided high specific surface area. Additionally, electrochemical performance might vary according to various drying processes at pressure and temperature.³²⁾ Azadvari et al.³²⁾ reported MXene of a three-layered structure that had better electrochemical performance and higher capacitance retention under vacuum-drying condition at 80 °C than under air-drying condition at 25 °C.

Herein, we demonstrate that increase electrochemical performance of V₂O₅ ‧ nH₂O/GO nanobelts added with GO through a simple microwave-assisted hydrothermal synthesis. Moreover, we manufacture V₂O₅ ‧ nH₂O/GO nanobelts based on water without using acid. V₂O₅ ‧ nH₂O/GO nanobelts exhibit high specific capacitance of 206 F/g at 0.1 A/g and enhanced cycle stability, which is better than V₂O₅ ‧ nH₂O that shows specific capacitance of 108 F/g at 0.1 A/g and low cycle stability. Also, we investigate differences in electrochemical performance of V₂O₅ ‧ nH₂O/GO nanobelts depending on drying processes of carbon paper electrode at different pressure and temperature. These electrochemical properties of V₂O₅ ‧ nH₂O/GO nanobelts become a potential candidate as an electrode for supercapacitors.

2. Experimental Procedure

V₂O₅ ‧ nH₂O/GO, the result of V₂O₅ and GO in water, was manufactured by mixing V₂O₅ powder (Sigma-Aldrich, item No. 223794), GO (CHARMGRAPHENE, 2 mg/mL), and DI water and then using a microwave-assisted hydrothermal method. 7 mg V₂O₅ was mixed in 15 mL water and stirred for 12 h using a magnetic bar, then stirred for 3 h at 50 °C. This solution was filtered using PVDF filter paper with a pore size of 0.22 µm. The filtered solution of 12 mL was added GO 0.1 mL and sonicated for 10 min. This solution was transferred to glass vial and reacted microwave-assisted hydrothermal process with stirring at 150 °C for 1 h. The product was vacuum filtered using the filter paper and washed with water. Additionally, V₂O₅ ‧ nH₂O was prepared in the same process by excluding GO.

A mixture of completely reduced VO₂ and partially reduced V₁₀O₂₄ ‧ nH₂O, a result of V₂O₅ in oxalic acid (Sigma-Aldrich, item No. 194131), was manufactured by mixing V₂O₅, water, and oxalic acid and then using a microwave-assisted hydrothermal method. 0.727 g V₂O₅ and 1.080 g oxalic acid were mixed in 72 mL water and stirred for 12 h using a magnetic bar. This solution was transferred to glass vial and reacted microwave-assisted hydrothermal process with stirring at 150 °C for 1 h.

The product was vacuum filtered using the filter paper and washed several times with water and ethanol.

The structural characterization of the results created by the above process was measured. The X-ray diffraction (XRD) patterns were obtained by an X-ray diffractometer (Rigaku, MiniFlex 600) using Cu-Kα radiation in the range of 3° to 60° (2θ) with scanning rate of 2°/min. The Raman spectra were measured by a Raman spectrometer (Thermo Fisher Scientific, DXR3xi) with laser excitation of 532 nm at power of 1 mW. The morphology was evaluated by a field emission scanning electron microscopy (FE-SEM; Tescan, CZ/MIRA I LMH).

A process was carried out to measure electrochemical properties as follows. 1 mg active material, 20 µL nafion, 200 µL ethanol and 800 µL water were mixed using ultrasonicator for 20 min. This solution of 30 µL was dropped on carbon paper and dried at different processes then the dried carbon papers were used the electrode. A drying process of the carbon paper, low-vacuum (500~760 torr) room temperature (RT) process, the carbon paper was dried in oven at 60 °C for 2 h and in low vacuum at RT for 12 h. Another drying process of the carbon paper, high-vacuum (10^-3~10^-4 torr) 100 °C process, the carbon paper was dried in oven at 60 °C for 15 min and then dried in high vacuum at 100 °C for 2 h.

The electrochemical measurement was assessed by a three-electrode system using electrochemical workstation (Ivium Technologies, IviumStat.h). an Ag/AgCl electrode, a graphite rod and the active material on carbon paper were used as the reference electrode, the counter electrode and the working electrode. The electrolyte was used by 2 M KCl solution. The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were executed. The voltage window for CV and GCD measurements was 0 to 0.8 V. The frequency range for EIS was from 100 kHz to 0.1 Hz with an amplitude of 5 mV. The specific capacitance (C_S, F/g) was calculated from CV curve and GCD curve by the following Eqs. (1) and (2).^33,34)

where, ∮I(V)dV is the integrated area of the CV curve, m (g) is the mass of active material on carbon paper, v (mV/s) is the scan rate, ΔV (V) is the potential window.

where, I (A) is the discharge current, Δt (s) is the discharge time, m (g) is the mass of active material on carbon paper, ΔV (V) is the potential change during discharge process.

3. Results and Discussion

The synthesis process of V₂O₅ ‧ nH₂O/GO nanobelts is shown in Fig. 1. V₂O₅ powder was dissolved in deionized water and filtered to make V₂O₅ solution with uniformly diffused V⁵⁺ ions. The V₂O₅ solution was mixed with GO and sonicated that completely dispersed. This solution formed V₂O₅ ‧ nH₂O/GO through microwave-assisted hydrothermal reaction. The result became a flexible paper using vacuum filtration. Vanadium oxide-materials including V₂O₅ ‧ nH₂O were chiefly generated by adding acids (hydrogen peroxide, oxalic acid, citric acid, sulfuric acid etc.) to vanadium precursor.^{35,36,37,38,39,40,41)} The resultant existed as a mixture in Fig. 1, VO₂ and V₁₀O₂₄ ‧ nH₂O, when oxalic acid was added to V₂O₅ powder and water. However, inset digital image (Fig. 1) and Fig. S1 show that GO cannot be dispersed in oxalic acid (pH 1). Because of the relationship between GO and pH, it was not completely dispersed in acid (pH 1), therefore it was likely that the effect of GO would not be applied uniformly.⁴²⁾

Fig. 1.

Illustration on the preparation of V₂O₅ ‧ nH₂O/GO nanobelts. Image (left) of black dotted box is a mixture of completely reduced VO₂ and partially reduced V₁₀O₂₄ ‧ nH₂O after microwave-assisted hydrothermal reaction process of V₂O₅ in oxalic acid. Image (right) of black dotted box is not uniform diffusion of GO when V₂O₅ + GO in oxalic acid. In image of red dotted box, V⁵⁺ ions and GO are uniformly mixed by sonication and formed V₂O₅ ‧ nH₂O/GO nanobelts after microwave-assisted hydrothermal reaction process of V₂O₅ + GO in water.

Fig. 2(a) shows characteristic diffraction peaks that V₂O₅ has different results in water and oxalic acid after microwave-assisted hydrothermal reaction. Si/SiO₂ substrate was used to support the results and the substrate peak is observed in the XRD pattern. The XRD pattern of V₂O₅ in oxalic acid shows as the mixture that exists completely reduced VO₂ and partially reduced V₁₀O₂₄ ‧ nH₂O. VO₂ is completely reduced from V⁵⁺ to V⁴⁺. The reduction reaction of VO₂ using V₂O₅ and oxalic acid is described by following Eqs. (3) and (4).⁴³⁾

However, due to a low temperature process condition of 150 °C or soft-reducing condition, V⁵⁺ is partially reduced to produce V₁₀O₂₄ ‧ nH₂O.^44,45) The XRD pattern confirms that is included V₁₀O₂₄ ‧ nH₂O (JCPDS No. 00-025-1006) among the peaks of VO₂ (JCPDS No. 01-084-3056). The XRD pattern of V₂O₅ in water shows which is not reduced from V⁵⁺. Compared with XRD analysis of V₂O₅ powder (Fig. S2), H₂O is inserted into V₂O₅ to become V₂O₅ ‧ nH₂O (JCPDS No. 00-040-1296) after microwave-assisted hydrothermal reaction. Fig. 2(b) displays Raman analysis to identify structures of vanadium oxide. V₂O₅ ‧ nH₂O has vibration modes of chemical bond related to vanadium and oxide in 50~1,100 cm^-1. The bands at 89 cm^-1 and 156 cm^-1 in low-frequency mode are assigned to the lattice vibration with layered structure.^46,47) The V=O bending mode is observed at 266 cm^-1 and 407 cm^-1.⁴⁸⁾ The bands at 316 cm^-1 corresponds to the V₃-O bending modes.⁴⁹⁾ The band at 503 cm^-1 is assigned to the stretching mode of the V₃-O mode.⁴⁸⁾ The stretching mode of V₂-O is observed at 692 cm^-1.⁴⁶⁾ The stretching mode of V=O is located at 1,012 cm^-1.⁴⁸⁾ During microwave-assisted hydrothermal reaction, water molecules are inserted between V₂O₅ layers to intense peak of 503 cm^-1 and 692 cm^-1.⁵⁰⁾ The Raman analysis of V₂O₅ ‧ nH₂O/GO also shows chemical bonds related to V-O type and is similar to V₂O₅ ‧ nH₂O in 50~1,100 cm^-1. In the inset image of Fig. 2(b), V₂O₅ ‧ nH₂O/GO has D peak and G peak that demonstrate the existence of carbon materials in 1,200~1,800 cm^-1. The G peak at 1,600 cm^-1, representing the E_2g mode, is shifted due to oxygenation.⁵¹⁾ In addition, the reduction in size of in plane sp² domains in graphite after oxidation, resulting in a broadened D peak at 1,353 cm^-1.⁵²⁾Fig. 2(c) shows V₂O₅ ‧ nH₂O/GO and V₂O₅ ‧ nH₂O which correspond to (001), (003), (004), (005) lattice plane. These planes confirm a layered structure of V₂O₅ ‧ nH₂O (JCPDS No. 00-040-1296). The characteristic peaks of V₂O₅ ‧ nH₂O/GO and V₂O₅ ‧ nH₂O comprise similar peak positions, which means that GO not affect the lattice position. Si/SiO₂ substrate peak is observed in the XRD pattern. As displayed in Fig. 2(d) and Fig. S3, V₂O₅ ‧ nH₂O/GO and V₂O₅ ‧ nH₂O exhibit a nanobelt-like morphology. However, unlike V₂O₅ ‧ nH₂O in Fig. S3, V₂O₅ ‧ nH₂O/GO in Fig. 2(d) shows a clear nanobelt morphology due to likely prevented aggregation of GO and V₂O₅ ‧ nH₂O.^30,31)

Fig. 2.

(a) XRD patterns of V₂O₅ in water and oxalic acid on substrate after microwave-assisted hydrothermal reaction. (b) Raman spectra of V₂O₅ ‧ nH₂O and V₂O₅ ‧ nH₂O/GO on substrate. (c) XRD patterns of V₂O₅ ‧ nH₂O and V₂O₅ ‧ nH₂O/GO on substrate. (d) SEM image of V₂O₅ ‧ nH₂O/GO.

Fig. 3(a) displays the CV curves of V₂O₅ ‧ nH₂O/GO and V₂O₅ ‧ nH₂O between 0 and 0.8 V at a scan rate of 100 mV/s. Fig. 3(a) shows the scan rate of 100 mV/s among the CV curves at different scan rates which measured in order from 5 mV/s to 100 mV/s in Fig. S4(a, b). When comparing the integrated area under CV curves, specific capacitance is larger in V₂O₅ ‧ nH₂O/GO. Therefore, the calculated specific capacitance of V₂O₅ ‧ nH₂O and V₂O₅ ‧ nH₂O/GO at a scan rate of 100 mV/s is 127.58 F/g and 350.86 F/g. Fig. 3(b, c) indicate the GCD curves at various current densities of V₂O₅ ‧ nH₂O/GO and V₂O₅ ‧ nH₂O. As shown in Fig. 3(d), the GCD curves at various current densities of V₂O₅ ‧ nH₂O/GO and V₂O₅ ‧ nH₂O are manifested as the calculated specific capacitance. The specific capacitance of V₂O₅ ‧ nH₂O/GO is 206, 182, 170, 160 F/g under current density of 0.1, 0.5, 1, 2 A/g. The specific capacitance of V₂O₅ ‧ nH₂O is 108, 86, 75, 65 F/g under current density of 0.1, 0.5, 1, 2 A/g. Accordingly, V₂O₅ ‧ nH₂O/GO under the same measuring conditions has greater specific capacitance than V₂O₅ ‧ nH₂O due to abundant oxygen functional groups and high specific surface area of GO.^30,31)

Fig. 3.

(a) CV curves of V₂O₅ ‧ nH₂O/GO and V₂O₅ ‧ nH₂O at scan rate of 100 mV/s. (b) GCD curves of V₂O₅ ‧ nH₂O/GO at different current densities. (c) GCD curves of V₂O₅ ‧ nH₂O at different current densities. (d) Specific capacitance versus current density of V₂O₅ ‧ nH₂O/GO and V₂O₅ ‧ nH₂O.

The Nyquist plots of V₂O₅ ‧ nH₂O/GO and V₂O₅ ‧ nH₂O from 100 kHz to 0.1 Hz frequency region are presented in Fig. 4(a) and based on the equivalent circuit of Fig. S5. V₂O₅ ‧ nH₂O/GO in Fig. 4(a) shows larger slope of the sloping line in low frequency region than V₂O₅ ‧ nH₂O that indicates the fast diffusion rate of ions.⁵³⁾ Moreover, Fig. 4(b) shows the Nyquist plots at high frequency region. The diameter of semicircle in Fig. 4(b) implies charge transfer resistance (R_ct), the electrode resistance, which is associated to the surface area and conductivity of the electrode.⁵⁴⁾ The R_ct values of V₂O₅ ‧ nH₂O/GO and V₂O₅ ‧ nH₂O are 6.0 Ω and 20.2 Ω. The low resistance value of V₂O₅ ‧ nH₂O/GO is due to the inclusion of GO that features high conductivity and high surface area.⁵⁵⁾ Additionally, the low resistance value of V₂O₅ ‧ nH₂O/GO exhibits a fast electrochemical reaction.⁵⁶⁾Fig. 4(c) shows the cycling performances of V₂O₅ ‧ nH₂O/GO and V₂O₅ ‧ nH₂O during 200 cycles and 100 cycles at scan rate of 100 mV/s. Cycling performance of V₂O₅ ‧ nH₂O has specific capacitance retention rate of 57.26 % in 100 cycles. Cycling performance of V₂O₅ ‧ nH₂O/GO has specific capacitance retention rate of 85.91 % in 100 cycles and 73.84 % in 200 cycles. V₂O₅ ‧ nH₂O/GO indicates better cycling performance than V₂O₅ ‧ nH₂O, which is due to GO possesses high specific surface area and abundant oxygen functional group.^30,31)Fig. 4(d) shows cycling performances depending on the drying processes. The high-vacuum 100 °C process and the low-vacuum RT process have capacitance retention of 81.65 % and 73.84 % at 200 cycles. The high-vacuum 100 °C process has better cycling performance than the low-vacuum RT process due to the drying process at high vacuum and high temperature.³²⁾ In addition, Fig. S6, the initial cycle curves of cycling performance in Fig. 4(d) at different drying processes, has specific capacitance of 388.4 F/g in the high-vacuum 100 °C process and the specific capacitance (388.4 F/g) greater than specific capacitance of 385.08 F/g in the low-vacuum RT process.

Fig. 4.

(a) Nyquist plots of V₂O₅ ‧ nH₂O/GO and V₂O₅ ‧ nH₂O from 0.1 Hz to 100 MHz region. (b) Nyquist plots of V₂O₅ ‧ nH₂O/GO and V₂O₅ ‧ nH₂O in high-frequency region. (c) Cycling performances of V₂O₅ ‧ nH₂O/GO and V₂O₅ ‧ nH₂O during 200 cycles at scan rate of 100 mV/s. (d) Cycling performances of V₂O₅ ‧ nH₂O/GO at different vacuum and temperature conditions in scan rate of 100 mV/s.

4. Conclusion

V₂O₅ ‧ nH₂O/GO nanobelts were fabricated through microwave-assisted hydrothermal reaction by utilizing V₂O₅, water and GO. The structural properties of V₂O₅ ‧ nH₂O/GO nanobelts that produced after the reaction were confirmed through XRD analysis, Raman analysis and SEM image. The electrochemical properties of V₂O₅ ‧ nH₂O/GO nanobelts present high specific capacitance of 206 F/g at 0.1 A/g and stable cycling performance. V₂O₅ ‧ nH₂O/GO nanobelts exhibit higher specific capacitance, cycling performance, lower charge transfer resistance and faster ion diffusion rate than V₂O₅ ‧ nH₂O. In addition to using GO, V₂O₅ ‧ nH₂O/GO nanobelts indicate great specific capacitance and cycling performance by drying at high vacuum and high temperature. These good electrochemical performances of V₂O₅ ‧ nH₂O/GO nanobelts exhibit a great potential as an energy storage device electrode.