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PVP-assisted hydrothermal synthesis of VO(OH)2 nanorods for supercapacitor electrode with excellent pseudocapacitance
Materials Letters 227 (2018) 217–220
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
PVP-assisted hydrothermal synthesis of VO(OH)2 nanorods for supercapacitor electrode with excellent pseudocapacitance Meng Chen a, Yifu Zhang a,⇑, Jiqi Zheng a, Yanyan Liu a, Zhanming Gao b,⇑, Zhihui Yu c, Changgong Meng a a
School of Chemistry, Dalian University of Technology, Dalian 116024, PR China Chemistry Analysis & Research Center, Faculty of Chemical, Environmental & Biological Science and Technology, Dalian University of Technology, Dalian 116024, PR China c School of Light Industry and Chemical Engineering, Dalian Polytechnic University, Dalian 116034, China b
a r t i c l e
i n f o
Article history: Received 16 March 2018 Received in revised form 18 April 2018 Accepted 18 May 2018 Available online 18 May 2018 Keywords: VO(OH)2 Microstructure Nanorods Electrical properties Energy storage and conversion
a b s t r a c t Vanadium oxyhydroxide VO(OH)2, which may possess specific chemical and physical properties, has been paid less attention comparing with vanadium oxides. Herein, with the assistance of polyvinyl pyrrolidone (PVP) and by adjusting pH about 4.7, VO(OH)2 was successfully synthesized by a facile hydrothermal method for the first time, which were short nanorods with widths of 50–130 nm and lengths of 250– 500 nm. Electrochemical performance of VO(OH)2 nanorods was firstly investigated as supercapacitor electrodes, which were studied by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). VO(OH)2 nanorods showed capacitive behavior based on pseudocapacitance and superior rate capability. Specific capacitance of 198 Fg 1 was achieved at 0.5 Ag 1. These findings suggested that VO(OH)2 nanorods can be promising candidate as potential material for supercapacitor electrode. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction The rapid consumption of fossil fuels and increasing environmental pollution have led to an urgent demand for efficient, clean, sustainable energy conversion and storage. Compared with batteries, supercapacitors (SCs) receive tremendous attention due to their superiorities of excellent power output, exceptional cycling life, lightweight, and ease of handing, etc. [1–5]. SCs’ performance mainly relies on the properties of electrode materials, therefore it’s of decisive significance that how to develop novel materials for SCs’ electrodes [6–9]. VO(OH)2 is a novel V-based materials, which is scarcely reported in the literature [10]. Julie et al. [10] first studied three vanadium oxyhydroxides formation mechanisms especially for Haggite V2O3(OH)2 and Gain’s hydrate V2O4(H2O)2. And they first studied the electrochemical behavior of Duttonite VO(OH)2 and Haggite applied for Li and Na batteries, confirming V4+ oxyhydroxides can bear scientific importance and promising chemical and physical properties in electrochemistry. Thus, VO(OH)2 may possess scientific importance and specific chemical and physical properties compared with other V-based materials [11–13]. However, the electrochemical properties of VO(OH)2 applied to SC’s electrodes for energy storage have been little studied in the
⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Zhang), [email protected] (Z. Gao). https://doi.org/10.1016/j.matlet.2018.05.086 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
literature to the best of our knowledge. In this work, we focused on synthesis and electrochemical properties of VO(OH)2 as SC’s electrodes.
2. Experimental All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd and used without any further purification. In detail, 0.22 g PVP was dispersed into 25 ml distilled water under ultrasonication to form a transparent colorless solution after 20 min. Subsequently, 0.82 g VOSO4 was added to obtain a transparent blue VOSO4 solution (2 M). Then NaOH solution (0.5 M) was slowly dropped into above VOSO4 solution to adjust pH about 4.7, and brown precipitates were formed and suspended in the solution. Last, the suspension was transferred to a Teflon-lined stainless steel autoclave, sealed, and was heated at 100 °C for 48 h. After the reaction, the resulted precipitate was filtered off, washed and dried in vacuum. Phase and composition were identified by X-ray powder diffraction (XRD), energy-dispersive X-ray spectrometer (EDS), elemental mapping, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Morphology was observed by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Electrochemical tests were performed in 1 molL 1 Na2SO4 electrolyte. All detail information was seen in Supplementary data.
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M. Chen et al. / Materials Letters 227 (2018) 217–220
3. Results and discussion The composition of the as-obtained samples were characterized by EDS, elemental mappings, XRD, XPS and FTIR. Fig. S1 (Supplementary data) shows EDS spectrum and elemental mappings, which demonstrates that the sample contains two elements V and O with homogeneous distribution in the view. Fig. 1a shows the XRD pattern of the sample. Almost all the peaks are assigned to VO(OH)2 (JCPDS, No. 11-0209) except that some small peaks are indexed to VO2 (JCPDS, No. 09-0142), suggesting the successful synthesis of VO(OH)2. The full XPS spectrum (Fig. 1b) also confirms that the sample comprises V and O (C1s is used as charge reference [14]), in agreement with the observations of EDS and elemental mappings. The core-level spectrum of V2p-O1s is depicted in Fig. 1c. The peak of V2p splits two binding energies of V2p3/2 and V2p1/2, which are respectively centered at 516.2 eV and 523.6 eV, in good accordance with the characteristic binding energies of V4+ [15]. O1s peak locates at 530.5 eV in agreement with V–O [15]. Fig. 1d shows the FTIR spectrum, which can well explain the successful synthesis of VO(OH)2 [16]. The bands at 3565 and 3516 cm 1 are attributed to the stretching vibrations of V–OH. Furthermore, the peaks at 867 and 800 cm 1 are assigned to the inplane V–OH deformations. The strong peak at 963 cm 1 is ascribed to the typical VO2+ stretching vibration, which is very consistent with the molecular formula of VO(OH)2. Moreover, three peaks at 619, 538 and 449 cm 1 are probably related to torsional modes of –OH groups [16]. The above results confirm that VO(OH)2 is synthesized by the PVP-assisted hydrothermal route. Fig. 2 shows FE-SEM and TEM images of the as-obtained VO (OH)2. It can be observed that short nanorods with a diameter of 50–130 nm and lengths from 250 to 500 nm are obtained. HRTEM image (insert in Fig. 2d) displays that the lattice fringes of VO(OH)2
can be seen, and the distance between neighboring planes is about 0.224 nm, which is consistent with 2h = 40.2° (Fig. 1a) of VO(OH)2. To explore the merits of the as-prepared VO(OH)2 nanorods, their electrochemical properties as SC’s electrodes were investigated by CV, GCD and EIS in a three electrode cell. Fig. S2 depicts CV curves of VO(OH)2 nanorods at various potential limits, which suggests that it exhibits good symmetry and high capacity at the potential window of 0.2 V to 0.8 V. Therefore, CV and GCD of VO(OH)2 nanorods were tested in the potential range from 0.2 V to 0.8 V. Fig. 3a presents CV curves at the scan rates from 5 mVs 1 to 100 mVs 1. All curves show the quasi-rectangular shape, which demonstrates that VO(OH)2 has a capacitive behavior with pseudocapacitance. Furthermore, these curves maintain their original shape with the scan rate increasing, indicating good ionic and electron conduction of VO(OH)2 nanorods [7]. To calculate the value of specific capacitance and understand the rate capability, GCD curves of VO(OH)2 nanorods at various current densities are shown in Fig. 3b and their corresponding results are summarized in Fig. 3c. The discharge curves and the corresponding charge curves (Fig. 3b) almost displays line-like and symmetrical over the whole potential region, suggesting the good capacitive behavior of VO(OH)2 in agreement with CV observation (Fig. 3a). Specific capacitances are 198, 165, 149, 134 and 120 Fg 1 at discharge current densities of 0.5, 1, 2, 5 and 10 Ag 1, respectively. Specific capacitances decrease as current densities increasing, and about 61% of specific capacitance at 0.5 Ag 1 is remained at 10 Ag 1. This decline is caused by the incremental voltage drop and involvement of insufficient active materials [17]. Besides, GCD process leading to a low utilization rate of active materials at high current densities is another reason [18]. Maximum specific capacitance of VO(OH)2 nanorods as SC’s electrodes reach to 198 Fg 1 at 0.5 Ag 1 in this study, which has not been reported. This value is even
Fig. 1. Characterization of VO(OH)2 nanorods: (a) XRD; (b) Full XPS; (c) V2p-O1s XPS; (d) FTIR.
M. Chen et al. / Materials Letters 227 (2018) 217–220
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Fig. 2. FE-SEM (a-b) and TEM (c-d) images of VO(OH)2 nanorods.
Fig. 3. Electrochemical properties of VO(OH)2 nanorods: (a) CV curves at different scan rates; (b) GCD curves at different current densities; (c) Specific capacitance vs. current density; (d) Nyquist plots over the frequency range of 100 kHz to 0.01 Hz.
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M. Chen et al. / Materials Letters 227 (2018) 217–220
higher than the reported VOOH (102 Fg 1 at 0.2 Ag 1) [7] and some VO2-based materials (Table S1). Cycling performance is an another important parameter for electrode materials. The capacitance retention as a function of cycling numbers at a scan rate of 50 mVs 1 is shown in Fig. S3. The capacitance decreases with the cycle increasing and it retains around 62%, 47% and 14% of its initial value after 50, 100 and 600 cycles, respectively. The capacitance drop is due to dissolution of VO(OH)2 during charging/discharging process, which is a common phenomenon of V-based materials as electrode in aqueous electrolyte [19]. During electrochemical redox reaction, soluble substances may be generated because the electrolyte had become yellow green after cycling. And the dissolution of vanadium may be avoided by choosing a suitable electrolyte or protecting the nanoparticles surface with conductive carbon coating or polymer layer on its surface [11]. Fig. 3d shows the Nyquist plots of VO (OH)2 nanorods, which is composed of an arc at the higher frequency region and a straight line at the lower frequency region. The intercept of the plots with the real impedance in the range of high frequency represents the electrolyte resistance, the intrinsic resistance of the active material VO(OH)2, and the contact resistance at the interface of the active material and the collector. The intrinsic resistance is estimated to be around 0.29 X, which suggests a good ion respond and high-quality contact between nickel foam and VO(OH)2 [20]. The semicircle is nearly invisible, which depends on the conductivity at the interface between the active material and the electrolyte, indicating a significantly superior interfacial charge-transfer behavior in the aqueous electrolyte system. The sloped portion of plot at the end in the low-frequency region is long because of more paths for ion diffusion in VO(OH)2 nanorods aqueous electrolyte system, which is a result of the frequency of ion diffusion in the electrolyte to the electrode interface [21]. To study the durability of VO(OH)2, the sample which placed for two months was characterized by XRD. By comparing the VO(OH)2 before and after two months in Fig. S4, it is obvious that most of the characteristic diffraction peaks disappear, which implying VO (OH)2 may be deteriorated. Therefore, it can be inferred that VO (OH)2 has poor durability. 4. Conclusion In summary, VO(OH)2 nanorods were firstly synthesized via a facile hydrothermal method with the assistant of PVP by adjusting
pH about 4.7. Their composition and structure were investigated by EDS, elemental mapping, XRD, XPS, FTIR, FE-SEM and TEM. VO (OH)2 nanorods were explored to apply to a SC’s electrode and exhibited the capacitive behavior based on pseudocapacitance. Specific capacitance of 198 Fg 1 was achieved at 0.5 Ag 1. This work provided insights into exploration of novel materials as electrodes for SC applications. The cycle performance of VO(OH)2 nanorods is poor owing to their dissolution in aqueous electrolyte. As a novel material for SC’s electrode, VO(OH)2 shows great potential and may draw more attention in the future. Acknowledgement This work was partially supported by National Natural Science Foundation of China (21601026, 21771030) and the Natural Science Foundation of Liaoning Province (No. 20170520427). References [1] C. Zhong, Y. Deng, W. Hu, J. Qiao, L. Zhang, J. Zhang, Chem. Soc. Rev. 44 (2015) 7484. [2] M.-S. Balogun, W. Qiu, W. Wang, P. Fang, X. Lu, Y. Tong, J. Mater. Chem. A 3 (2015) 1364. [3] Y. Zhang, X. Jing, Q. Wang, J. Zheng, H. Jiang, C. Meng, Dalton Trans. 46 (2017) 15048. [4] M. Li, S. Magdassi, Y. Gao, Y. Long, Small 13 (2017) 1701147. [5] J. Zheng, Y. Zhang, Q. Wang, H. Jiang, Y. Liu, T. Lv, et al., Dalton Trans. 47 (2018) 452. [6] W. Luo, J.-J. Gaumet, L. Mai, MRS Commun. 7 (2017) 152. [7] Y. Zhang, X. Jing, Mater. Lett. 205 (2017) 1. [8] Y. Zhang, X. Jing, Q. Wang, J. Zheng, S. Zhang, T. Hu, et al., Microporous Mesoporous Mater. 249 (2017) 137. [9] Y. Zhang, J. Zheng, Q. Wang, T. Hu, F. Tian, C. Meng, Appl. Surf. Sci. 399 (2017) 151. [10] J. Besnardiere, X. Petrissans, F. Ribot, V. Briois, C. Surcin, M. Morcrette, et al., Inorg. Chem. 55 (2016) 11502. [11] Y. Yue, H. Liang, Adv. Energy Mater. 7 (2017) 1602545. [12] C. Wu, F. Feng, Y. Xie, Chem. Soc. Rev. 42 (2013) 5157. [13] Y. Zhang, J. Zheng, Q. Wang, S. Zhang, T. Hu, C. Meng, Appl. Surf. Sci. 423 (2017) 728. [14] Y. Zhang, Y. Huang, Mater. Lett. 182 (2016) 285. [15] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, Handbook of X-Ray Photoelectrom Spectroscopy, Perkin-Elmer Corporation, Minnesota, 1979. [16] E.G. Ferrer, E.J. Baran, Spectrochim. Acta Part A: Mol. Spectrosc. 50 (1994) 375. [17] H.-Y. Li, C. Wei, L. Wang, Q.-S. Zuo, X. Li, B. Xie, J. Mater. Chem. A 3 (2015) 22892. [18] Z. Yu, L. Tetard, L. Zhai, J. Thomas, Energ. Environ. Sci. 8 (2015) 702. [19] Y. Zhang, J. Zheng, Q. Wang, T. Hu, C. Meng, RSC Adv. 6 (2016) 93741. [20] C. Zhao, W. Zheng, X. Wang, H. Zhang, X. Cui, H. Wang, Sci. Rep. 3 (2013) 2986. [21] J.L. Qi, X. Wang, J.H. Lin, F. Zhang, J.C. Feng, W.-D. Fei, J. Mater. Chem. A 3 (2015) 12396.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
PVP-assisted hydrothermal synthesis of VO(OH)2 nanorods for supercapacitor electrode with excellent pseudocapacitance Meng Chen a, Yifu Zhang a,⇑, Jiqi Zheng a, Yanyan Liu a, Zhanming Gao b,⇑, Zhihui Yu c, Changgong Meng a a
School of Chemistry, Dalian University of Technology, Dalian 116024, PR China Chemistry Analysis & Research Center, Faculty of Chemical, Environmental & Biological Science and Technology, Dalian University of Technology, Dalian 116024, PR China c School of Light Industry and Chemical Engineering, Dalian Polytechnic University, Dalian 116034, China b
a r t i c l e
i n f o
Article history: Received 16 March 2018 Received in revised form 18 April 2018 Accepted 18 May 2018 Available online 18 May 2018 Keywords: VO(OH)2 Microstructure Nanorods Electrical properties Energy storage and conversion
a b s t r a c t Vanadium oxyhydroxide VO(OH)2, which may possess specific chemical and physical properties, has been paid less attention comparing with vanadium oxides. Herein, with the assistance of polyvinyl pyrrolidone (PVP) and by adjusting pH about 4.7, VO(OH)2 was successfully synthesized by a facile hydrothermal method for the first time, which were short nanorods with widths of 50–130 nm and lengths of 250– 500 nm. Electrochemical performance of VO(OH)2 nanorods was firstly investigated as supercapacitor electrodes, which were studied by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). VO(OH)2 nanorods showed capacitive behavior based on pseudocapacitance and superior rate capability. Specific capacitance of 198 Fg 1 was achieved at 0.5 Ag 1. These findings suggested that VO(OH)2 nanorods can be promising candidate as potential material for supercapacitor electrode. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction The rapid consumption of fossil fuels and increasing environmental pollution have led to an urgent demand for efficient, clean, sustainable energy conversion and storage. Compared with batteries, supercapacitors (SCs) receive tremendous attention due to their superiorities of excellent power output, exceptional cycling life, lightweight, and ease of handing, etc. [1–5]. SCs’ performance mainly relies on the properties of electrode materials, therefore it’s of decisive significance that how to develop novel materials for SCs’ electrodes [6–9]. VO(OH)2 is a novel V-based materials, which is scarcely reported in the literature [10]. Julie et al. [10] first studied three vanadium oxyhydroxides formation mechanisms especially for Haggite V2O3(OH)2 and Gain’s hydrate V2O4(H2O)2. And they first studied the electrochemical behavior of Duttonite VO(OH)2 and Haggite applied for Li and Na batteries, confirming V4+ oxyhydroxides can bear scientific importance and promising chemical and physical properties in electrochemistry. Thus, VO(OH)2 may possess scientific importance and specific chemical and physical properties compared with other V-based materials [11–13]. However, the electrochemical properties of VO(OH)2 applied to SC’s electrodes for energy storage have been little studied in the
⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Zhang), [email protected] (Z. Gao). https://doi.org/10.1016/j.matlet.2018.05.086 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
literature to the best of our knowledge. In this work, we focused on synthesis and electrochemical properties of VO(OH)2 as SC’s electrodes.
2. Experimental All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd and used without any further purification. In detail, 0.22 g PVP was dispersed into 25 ml distilled water under ultrasonication to form a transparent colorless solution after 20 min. Subsequently, 0.82 g VOSO4 was added to obtain a transparent blue VOSO4 solution (2 M). Then NaOH solution (0.5 M) was slowly dropped into above VOSO4 solution to adjust pH about 4.7, and brown precipitates were formed and suspended in the solution. Last, the suspension was transferred to a Teflon-lined stainless steel autoclave, sealed, and was heated at 100 °C for 48 h. After the reaction, the resulted precipitate was filtered off, washed and dried in vacuum. Phase and composition were identified by X-ray powder diffraction (XRD), energy-dispersive X-ray spectrometer (EDS), elemental mapping, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Morphology was observed by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Electrochemical tests were performed in 1 molL 1 Na2SO4 electrolyte. All detail information was seen in Supplementary data.
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M. Chen et al. / Materials Letters 227 (2018) 217–220
3. Results and discussion The composition of the as-obtained samples were characterized by EDS, elemental mappings, XRD, XPS and FTIR. Fig. S1 (Supplementary data) shows EDS spectrum and elemental mappings, which demonstrates that the sample contains two elements V and O with homogeneous distribution in the view. Fig. 1a shows the XRD pattern of the sample. Almost all the peaks are assigned to VO(OH)2 (JCPDS, No. 11-0209) except that some small peaks are indexed to VO2 (JCPDS, No. 09-0142), suggesting the successful synthesis of VO(OH)2. The full XPS spectrum (Fig. 1b) also confirms that the sample comprises V and O (C1s is used as charge reference [14]), in agreement with the observations of EDS and elemental mappings. The core-level spectrum of V2p-O1s is depicted in Fig. 1c. The peak of V2p splits two binding energies of V2p3/2 and V2p1/2, which are respectively centered at 516.2 eV and 523.6 eV, in good accordance with the characteristic binding energies of V4+ [15]. O1s peak locates at 530.5 eV in agreement with V–O [15]. Fig. 1d shows the FTIR spectrum, which can well explain the successful synthesis of VO(OH)2 [16]. The bands at 3565 and 3516 cm 1 are attributed to the stretching vibrations of V–OH. Furthermore, the peaks at 867 and 800 cm 1 are assigned to the inplane V–OH deformations. The strong peak at 963 cm 1 is ascribed to the typical VO2+ stretching vibration, which is very consistent with the molecular formula of VO(OH)2. Moreover, three peaks at 619, 538 and 449 cm 1 are probably related to torsional modes of –OH groups [16]. The above results confirm that VO(OH)2 is synthesized by the PVP-assisted hydrothermal route. Fig. 2 shows FE-SEM and TEM images of the as-obtained VO (OH)2. It can be observed that short nanorods with a diameter of 50–130 nm and lengths from 250 to 500 nm are obtained. HRTEM image (insert in Fig. 2d) displays that the lattice fringes of VO(OH)2
can be seen, and the distance between neighboring planes is about 0.224 nm, which is consistent with 2h = 40.2° (Fig. 1a) of VO(OH)2. To explore the merits of the as-prepared VO(OH)2 nanorods, their electrochemical properties as SC’s electrodes were investigated by CV, GCD and EIS in a three electrode cell. Fig. S2 depicts CV curves of VO(OH)2 nanorods at various potential limits, which suggests that it exhibits good symmetry and high capacity at the potential window of 0.2 V to 0.8 V. Therefore, CV and GCD of VO(OH)2 nanorods were tested in the potential range from 0.2 V to 0.8 V. Fig. 3a presents CV curves at the scan rates from 5 mVs 1 to 100 mVs 1. All curves show the quasi-rectangular shape, which demonstrates that VO(OH)2 has a capacitive behavior with pseudocapacitance. Furthermore, these curves maintain their original shape with the scan rate increasing, indicating good ionic and electron conduction of VO(OH)2 nanorods [7]. To calculate the value of specific capacitance and understand the rate capability, GCD curves of VO(OH)2 nanorods at various current densities are shown in Fig. 3b and their corresponding results are summarized in Fig. 3c. The discharge curves and the corresponding charge curves (Fig. 3b) almost displays line-like and symmetrical over the whole potential region, suggesting the good capacitive behavior of VO(OH)2 in agreement with CV observation (Fig. 3a). Specific capacitances are 198, 165, 149, 134 and 120 Fg 1 at discharge current densities of 0.5, 1, 2, 5 and 10 Ag 1, respectively. Specific capacitances decrease as current densities increasing, and about 61% of specific capacitance at 0.5 Ag 1 is remained at 10 Ag 1. This decline is caused by the incremental voltage drop and involvement of insufficient active materials [17]. Besides, GCD process leading to a low utilization rate of active materials at high current densities is another reason [18]. Maximum specific capacitance of VO(OH)2 nanorods as SC’s electrodes reach to 198 Fg 1 at 0.5 Ag 1 in this study, which has not been reported. This value is even
Fig. 1. Characterization of VO(OH)2 nanorods: (a) XRD; (b) Full XPS; (c) V2p-O1s XPS; (d) FTIR.
M. Chen et al. / Materials Letters 227 (2018) 217–220
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Fig. 2. FE-SEM (a-b) and TEM (c-d) images of VO(OH)2 nanorods.
Fig. 3. Electrochemical properties of VO(OH)2 nanorods: (a) CV curves at different scan rates; (b) GCD curves at different current densities; (c) Specific capacitance vs. current density; (d) Nyquist plots over the frequency range of 100 kHz to 0.01 Hz.
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higher than the reported VOOH (102 Fg 1 at 0.2 Ag 1) [7] and some VO2-based materials (Table S1). Cycling performance is an another important parameter for electrode materials. The capacitance retention as a function of cycling numbers at a scan rate of 50 mVs 1 is shown in Fig. S3. The capacitance decreases with the cycle increasing and it retains around 62%, 47% and 14% of its initial value after 50, 100 and 600 cycles, respectively. The capacitance drop is due to dissolution of VO(OH)2 during charging/discharging process, which is a common phenomenon of V-based materials as electrode in aqueous electrolyte [19]. During electrochemical redox reaction, soluble substances may be generated because the electrolyte had become yellow green after cycling. And the dissolution of vanadium may be avoided by choosing a suitable electrolyte or protecting the nanoparticles surface with conductive carbon coating or polymer layer on its surface [11]. Fig. 3d shows the Nyquist plots of VO (OH)2 nanorods, which is composed of an arc at the higher frequency region and a straight line at the lower frequency region. The intercept of the plots with the real impedance in the range of high frequency represents the electrolyte resistance, the intrinsic resistance of the active material VO(OH)2, and the contact resistance at the interface of the active material and the collector. The intrinsic resistance is estimated to be around 0.29 X, which suggests a good ion respond and high-quality contact between nickel foam and VO(OH)2 [20]. The semicircle is nearly invisible, which depends on the conductivity at the interface between the active material and the electrolyte, indicating a significantly superior interfacial charge-transfer behavior in the aqueous electrolyte system. The sloped portion of plot at the end in the low-frequency region is long because of more paths for ion diffusion in VO(OH)2 nanorods aqueous electrolyte system, which is a result of the frequency of ion diffusion in the electrolyte to the electrode interface [21]. To study the durability of VO(OH)2, the sample which placed for two months was characterized by XRD. By comparing the VO(OH)2 before and after two months in Fig. S4, it is obvious that most of the characteristic diffraction peaks disappear, which implying VO (OH)2 may be deteriorated. Therefore, it can be inferred that VO (OH)2 has poor durability. 4. Conclusion In summary, VO(OH)2 nanorods were firstly synthesized via a facile hydrothermal method with the assistant of PVP by adjusting
pH about 4.7. Their composition and structure were investigated by EDS, elemental mapping, XRD, XPS, FTIR, FE-SEM and TEM. VO (OH)2 nanorods were explored to apply to a SC’s electrode and exhibited the capacitive behavior based on pseudocapacitance. Specific capacitance of 198 Fg 1 was achieved at 0.5 Ag 1. This work provided insights into exploration of novel materials as electrodes for SC applications. The cycle performance of VO(OH)2 nanorods is poor owing to their dissolution in aqueous electrolyte. As a novel material for SC’s electrode, VO(OH)2 shows great potential and may draw more attention in the future. Acknowledgement This work was partially supported by National Natural Science Foundation of China (21601026, 21771030) and the Natural Science Foundation of Liaoning Province (No. 20170520427). References [1] C. Zhong, Y. Deng, W. Hu, J. Qiao, L. Zhang, J. Zhang, Chem. Soc. Rev. 44 (2015) 7484. [2] M.-S. Balogun, W. Qiu, W. Wang, P. Fang, X. Lu, Y. Tong, J. Mater. Chem. A 3 (2015) 1364. [3] Y. Zhang, X. Jing, Q. Wang, J. Zheng, H. Jiang, C. Meng, Dalton Trans. 46 (2017) 15048. [4] M. Li, S. Magdassi, Y. Gao, Y. Long, Small 13 (2017) 1701147. [5] J. Zheng, Y. Zhang, Q. Wang, H. Jiang, Y. Liu, T. Lv, et al., Dalton Trans. 47 (2018) 452. [6] W. Luo, J.-J. Gaumet, L. Mai, MRS Commun. 7 (2017) 152. [7] Y. Zhang, X. Jing, Mater. Lett. 205 (2017) 1. [8] Y. Zhang, X. Jing, Q. Wang, J. Zheng, S. Zhang, T. Hu, et al., Microporous Mesoporous Mater. 249 (2017) 137. [9] Y. Zhang, J. Zheng, Q. Wang, T. Hu, F. Tian, C. Meng, Appl. Surf. Sci. 399 (2017) 151. [10] J. Besnardiere, X. Petrissans, F. Ribot, V. Briois, C. Surcin, M. Morcrette, et al., Inorg. Chem. 55 (2016) 11502. [11] Y. Yue, H. Liang, Adv. Energy Mater. 7 (2017) 1602545. [12] C. Wu, F. Feng, Y. Xie, Chem. Soc. Rev. 42 (2013) 5157. [13] Y. Zhang, J. Zheng, Q. Wang, S. Zhang, T. Hu, C. Meng, Appl. Surf. Sci. 423 (2017) 728. [14] Y. Zhang, Y. Huang, Mater. Lett. 182 (2016) 285. [15] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, Handbook of X-Ray Photoelectrom Spectroscopy, Perkin-Elmer Corporation, Minnesota, 1979. [16] E.G. Ferrer, E.J. Baran, Spectrochim. Acta Part A: Mol. Spectrosc. 50 (1994) 375. [17] H.-Y. Li, C. Wei, L. Wang, Q.-S. Zuo, X. Li, B. Xie, J. Mater. Chem. A 3 (2015) 22892. [18] Z. Yu, L. Tetard, L. Zhai, J. Thomas, Energ. Environ. Sci. 8 (2015) 702. [19] Y. Zhang, J. Zheng, Q. Wang, T. Hu, C. Meng, RSC Adv. 6 (2016) 93741. [20] C. Zhao, W. Zheng, X. Wang, H. Zhang, X. Cui, H. Wang, Sci. Rep. 3 (2013) 2986. [21] J.L. Qi, X. Wang, J.H. Lin, F. Zhang, J.C. Feng, W.-D. Fei, J. Mater. Chem. A 3 (2015) 12396.