Synthesis and characterization of hollow V₂O₅ microspheres for supercapacitor electrode with pseudocapacitance

Supercapacitors (SCs) bridge the gap between batteries and conventional solid-state and electrolytic capacitors because of their high power density, rapid charging/discharging rate, superior rate capability, long operating lifetimes and low maintenance cost [1–6]. These remarkable properties make SCs to be considered as one of the most promising candidates for the next-generation energy storage devices. The performance of SCs is strongly dependent on the properties of electrode materials. Carbon materials, transition metals oxides, hydroxides, sulfides and conductive polymers are widely used in SCs [2]. Two types of SCs can be distinguished based on the charge storage mechanism: double-layer capacitance (non-faradaic process) with carbon electrodes and pseudocapacitors (faradaic process) with transition metal oxides electrodes and higher specific capacitance than double-layer capacitance [2, 4, 7].

Vanadium oxides and their related compounds have received increasing attention as functional materials owing to their layered structures, novel chemical and physical properties, which make them promising in a wide range of potential applications, such as cathode materials for reversible lithium batteries, catalysts, SCs, intelligent thermochromic windows, electrical and optical devices, laser shields and so on [8–21]. Recently, V₂O₅ as one of the promising candidates for electrode materials in SCs has attracted increasing attention because of its low cost, unique layered structure, abundance and high potential pseudocapacitive characteristics [3, 22, 23]. To date, lots of V₂O₅ with different structures and morphologies have been synthesized for SCs electrodes [3, 23–28], including microspheres, nanobelts, nanowires, nanoflowers, nanofibers, 3D-architectures, nanoporous networks, nanochains, etc. The performed investigations demonstrated that the performance of V₂O₅ SCs is strongly dependent on their morphology and structure. In the past decades, hollow V₂O₅ microspheres (HVOM) have attracted great interest owing to their excellent applications as cathode materials for Li- and Na-ion batteries [ 29–32]. For examples, Pan et al. [29] reported HVOM synthesized from VO₂ hollow spheres and exhibited a high initial reversible capacity of 256 mAh·g^–1 at a current density of 300 mAh·g^–1. Su et al. [31] reported HVOM prepared from vanadyl ethylene glycolate (VEG) hollow nanospheres which demonstrated a good high-rate capability and cycling stability. However, HVOM used as the cathode material for SCs has rarely been reported. Yang et al. [22], based on the results obtained by the cyclic voltammetry (CV) method, reported that hollow V₂O₅ spheres exhibited the maximum capacitance of 479 F·g^–1 at 5 mV·s^–1 when used as SCs electrodes in 5 M LiNO₃.

In this contribution, HVOM were synthesized by a facile solvothermal approach and subsequent calcination, using NH₄VO₃, ethylene glycol (EG) and carbon spheres as the starting materials. Furthermore, hollow V₂O₅ microspheres, used as SCs electrodes, exhibited an excellent pseudocapacitance.

Fig. 1 shows the FE-SEM images of the carbon spheres, which reveal that the uniform carbon spheres are obtained in agreement with the literature [33]. Fig. 2 depicts the FE-SEM images of the prepared sample. The overall view (Fig. 2a and Fig. 2b) show that the sample consists of microspheres with the diameters of 5 µm to 6 µm. From the broken microspheres (Fig. 2c and Fig. 2d), the spheres are hollow structures. The hollow microspheres were constructed from nanoparticles to form rough surfaces and enormous cavities, which would provide promising applications of the material in Li-ion intercalation and extraction [3].

Fig. 3 shows the XRD patterns of the hollow microspheres, which reveal that all the diffraction peaks can be indexed as the orthorhombic crystalline phase (space group: Pmmn 59) of V₂O₅ with a lattice constants values of a = 1.1516 nm, b = 0.35656 nm, and c = 0.4372 nm (JCPDS Card No. 41-1426), whose plots are shown in Fig. 3a. No obvious peaks of any other impurities are detected, suggesting that HVOM with a high purity were synthesized.

Fig. 4 depicts IR spectrum of the as-obtained HVOM. The absorption peaks ranging from 1200 cm^–1 to 400 cm^–1 are assigned to the characteristic V–O vibration band [18, 34, 35]. The peak at 1018 cm^–1 is attributed to the symmetric stretching vibration of V⁵⁺ = O bond, which is the characteristic structure of the layered orthorhombic V₂O₅ [35]. The absorption band at 840 cm^–1 is assigned to the vibration of O–(V)₃. The absorption peaks at 639 and 477 cm^–1 correspond to the asymmetric and the symmetric stretching modes of V–O–V bridging bonds. Besides, the absorption peaks at 3439 and 1630 cm^–1 are attributed to the stretching modes of water molecules adsorbed on the samples and can be disregarded. The above analysis is in agreement with the result of XRD analysis.

Fig. 5a shows the N₂ adsorption-desorption isotherms of the as-obtained HVOM. The isotherms depict a sharp capillary condensation step at high relative pressure, which belongs to type IV isotherm according to IUPAC classification. The specific surface area of HVOM obtained by BET method is 7.3 m²·g^–1, as depicted in Fig. 5b.

To exhibit the merits of the as-obtained HVOM, the electrochemical investigation of their pseudocapacitance was evaluated by CV and GCD tests in a three electrode cell. Fig. 6a shows CV curves of HVOM on various potential limits in 1 mol·dm^–3 LiNO₃ electrolyte at a scan rate 20 mV·s^–1, which indicate that the electrode in LiNO₃ solution has been proven to offer the best reversibility and reactivity in the –0.4 V to 0.6 V range. Fig. 6b displays the CV curves of HVOM and foamed Ni. The foamed Ni has no redox peak. HVOM show two pairs of redox peaks: two anodic peaks at 0.025 V and 0.213 V at positive current density and two cathodic peaks at 0.056 V and –0.112 V at negative current density. The above peaks are associated with the intercalation/deintercalation of Liion into/from HVOM, indicating that they have pseudocapacitance property [3, 22, 36]. The electrochemical reaction can be expressed as follows: V2O5+xLi+xe−dischargechargeLixV2O50⩽x⩽1$$\begin{equation}{V_2}{O_5} + xLi + x{e^ - }\frac{{discharge}}{{charge}}L{i_x}{V_2}{O_5}\left(0\, { \leqslant x \leqslant 1} \right) \end{equation}$$(4)

The changes of crystal phases which occurred during the whole process were generally designated as α (0 < x < 0.1), ϵ (0.35< x < 0.5) and δ (0.9 < x < 1) [22, 36].

To calculate the specific capacitance and understand the rate capability of HVOM, the charge and discharge tests were performed at various current densities (Fig. 7). The specific capacitance of HVOM shown in Fig. 7a, calculated from equation 1, is 488 F·g^–1, 455 F·g^–1, 434 F·g^–1 and 396 F·g^–1 at the current density of 0.5 A·g^–1, 1 A·g^–1, 2 A·g^–1 and 5 A·g^–1, respectively. The specific capacitance gradually decreases at higher current densities owing to the incremental voltage drop and insufficient active material being involved in the redox reaction at a higher current density [4, 25]. In addition, the process of charge-discharge at a high current density may lead to a low utilization rate of active material [2]. The SCs performance of HVOM is much higher than the literature reports, as summarized in Table 1. Fig. 7b shows a Ragone plot of HVOM. Based on equation 2 and equation 3, the calculated E, at the current densities of 0.5 A·g^–1, 1 A·g^–1, 2 A ·g^–1 and 5 A·g^–1 are 8.784 × 10⁵J·kg^–1, 8.208 × 10⁵ J·kg^–1, 7.812 × 10⁵ J·kg^–1 and 7.128 × 10⁵ J·kg^–1 and the power densities are 900 W·kg^–1,1800 W·kg^–1, 3600 W·kg^–1 and 9000 W·kg^–1, respectively. The above results exhibit the outstanding SCs performance of HVOM.

Table 1

Comparison of specific capacitance of the HVOM with V₂O₅ materials reported in the previous literature.

Cycling stability is a critical parameter for high-performance SCs. The CV and GCD tests were used to observe and quantitatively describe the cycling stability of HVOM. Fig. 8 that shows the CV curves of HVOM at different cycles, intuitively reveals that the specific capacitance quickly fades with the cycles increasing. Fig. 9a exhibits the change and discharge of the specific capacitance. A high initial specific capacitance of 455 F·g^–1 was achieved. The specific capacitance after 10, 30, 50, 100 cycles was 314 F·g^–1, 170 F·g^–1, 103 F·g^–1, 33 F·g^–1 which means that 69.0 %, 37.4 %, 22.6 %, 7.3 % of its initial capacitance was remained (Fig. 9b). The results intuitively reveal that the specific capacitance of HVOM quickly fades over 100 cycles. The reason is related with phase conversion and dissolution of HVOM. The redox peaks in the 100^th cycle shown in Fig. 8 can be obviously detected in the CV curves. Fig. 9a displays the characteristic plateaus which exist at almost identical potential during the whole cycling process. The above results indicate that the phase transition is completely reversible with increasing the numbers of cycles. Thus, the phase transformation of HVOM cannot be the reason for the fast fading of specific capacitance. The reason may be the dissolution of electrode material because the electrolyte solution gradually turns yellow during the constant charge/discharge process, which is a common phenomenon for vanadium oxides used as electrodes for Li-ion batteries and SCs [3, 4, 41]. Moreover, the strong stress introduced by the structure expansion during Li-ion insertion into HVOM may easily cause the structure to collapse which promotes dissolution. The coulombic efficiency is almost 100 % as depicted in Fig. 9b, which suggests using HVOM as the SCs electrode with high efficiency. This result also supports the view that the dissolution of electrode material of HVOM is the reason for the fast fading of specific capacitance. To further improve cycling stability, we may choose a suitable electrolyte or coat a conductive polymer layer on their surface [42].

Hollow V₂O₅ microspheres with rough surface constructed from nanoparticles were synthesized by a template solvothermal approach and subsequent calcination. The samples were characterized by FE-SEM, XRD, IR, BET, CV and GCD. Hollow V₂O₅ microspheres displayed excellent pseudocapacitance property and their specific capacitances were 488 F·g^–1, 455 F·g^–1, 434 F·g^–1 and 396 F·g^–1 at the current density of 0.5 A·g^–1, 1 A·g^–1, 2 A·g^–1 and 5 A·g^–1, respectively. They also exhibited an excellent energy density of 8.784 × 10⁵ J·kg^–1 at a power density of 900 W· kg^–1. The good electrochemical properties of hollow V₂O₅ microspheres make them a promising candidate as a cathode material for SCs.