Hydrothermal synthesis and supercapacitor electrode of low crystallinity VOOH hollow spheres with pseudocapacitance in aqueous solution

Accepted Manuscript Hydrothermal synthesis and supercapacitor electrode of low crystallinity VOOH hollow spheres with pseudocapacitance in aqueous solution Yifu Zhang, Xuyang Jing PII: DOI: Reference:

S0167-577X(17)30936-9 http://dx.doi.org/10.1016/j.matlet.2017.06.044 MLBLUE 22756

To appear in:

Materials Letters

Received Date: Accepted Date:

5 May 2017 10 June 2017

Please cite this article as: Y. Zhang, X. Jing, Hydrothermal synthesis and supercapacitor electrode of low crystallinity VOOH hollow spheres with pseudocapacitance in aqueous solution, Materials Letters (2017), doi: http://dx.doi.org/ 10.1016/j.matlet.2017.06.044

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hydrothermal synthesis and supercapacitor electrode of low crystallinity VOOH hollow spheres with pseudocapacitance in aqueous solution Yifu Zhang *, Xuyang Jing School of Chemistry, Dalian University of Technology, Dalian 116024, PR China *Corresponding author. E-mail address: [email protected] Abstract: Vanadyl hydroxide (VOOH) may bear scientific importance and novel chemical and physical properties but they are uncommonly reported comparing with vanadium oxides. In the present study, lepidocrocite VOOH hollow spheres with low crystallinity were synthesized through a template-free hydrothermal route. Diameter of VOOH hollow spheres ranged from 300 nm to 500 nm, and VOOH shell thickness reached 20 nm on average. Electrochemical properties of VOOH hollow spheres as supercapacitor electrodes in aqueous electrolyte were developed and studied by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD). Results revealed that VOOH hollow spheres featured capacitive behavior based on pseudocapacitance. Specific capacitance of 102 F·g−1 at current density of 0.2 A· g−1 was achieved. At current densities of 0.5, 1, 2.5 and 10 A·g−1, specific capacitances measured 87, 68, 56 and 29 F·g−1, respectively. Findings in present study proved that VOOH hollow spheres could be promising candidate as ideal material for supercapacitor electrodes. Keywords: Nanoparticles; Microstructure; VOOH; Electrical properties; Energy storage and conversion 1. Introduction Recently, supercapacitors (SCs) have drawn tremendous interest because they can complement Li-ion batteries due to their superiorities of excellent power output, exceptional cycling life, lightweight, ease of handing, and so on [1-3]. SCs’ performance is strongly dependent on properties of electrode materials [1]. Development of novel materials for SCs electrodes is significant and challenging [4]. VOOH, as family of V-based materials, was delivered increasing attention [5-11]. Xie et al. [5] first prepared VOOH hollow dandelions and applied to Li-ion batteries. Next, her group studied shape evolution of lepidocrocite VOOH from single-shelled to double-shelled hollow nanospheres [6], hollandite-type VOOH quadrangular nanorods [7] and montroseite VOOH hollow nanourchins [8]. These literatures mainly investigated synthesis of VOOH with different shapes, 1

application in Li-ion batteries, conversion to VO2 [8, 9] and electrical switch material. Shao et al. [10] applied VOOH to Na-ion storage which exhibited outstanding rate behavior and long life. Ruan et al. [11] synthesized 1D hierarchical layered of groove-like VOOH nanostructures by in-suit Kirkendall effect and oriented attachment process and studied their application in Li-ion batteries. Just recently, Wang et al. [12] reported applications of lepidocrocite VOOH in electrocatalytic water splitting. Thus, VOOH may bear scientific importance and novel chemical and physical properties but they are uncommonly reported as compared with other V-based materials [13-15]. However, to the best of our knowledge, no report studied electrochemical properties of VOOH applied to SCs electrodes for energy storage in aqueous solution. In present study, we focused on synthesis and electrochemical properties of VOOH as SCs electrodes. 2. Experimental All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd and used as received. In a typical procedure, 0.234 g NH4VO3 and 45 mL distilled water were mixed in a 100 mL beaker and then strongly stirred. Subsequently, 1 mL HCl (1.0 mol· L-1 ) solution was dropped to above solution. After solution turned transparent yellow, 3 mL N2 H4· H2O was dropped. After resulting mixture was stirred for 30 min at room temperature, this suspension was transferred to a Teflon-lined stainless steel autoclave, sealed, and maintained at 120 °C for given time. Products were filtered off, washed and dried in vacuum. Morphology of products were observed by field emission scanning electron microscopy (FE-SEM, NOVA NanoSEM 450, FEI) and transmission electron microscopy (TEM, FEITecnai F30). Phase and composition of products were identified by X-ray powder diffraction (XRD, Panalytical X’Pert powder diffractometer), X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, Thermo Fisher Scientific) and Fourier transform infrared spectroscopy (FTIR, Nicolet 6700). Electrochemical tests were performed in 1 mol·L−1 LiNO3 aqueous electrolyte, and detail process was seen in Supplementary materials. 3. Results and discussion

2

Fig. 1. Compositions of as-obtained VOOH: (a-b) XPS; (c) XRD; (d) FTIR; (e) crystal structure. The composition of products was studied by XPS (Fig. 1a-b), which revealed that sample comprised V and O (C1s was used as charge reference [16]). Binding energies corresponding to V2p3/2 and V2p1/2 (Fig. 1b) could be assigned to V3+ [5]. XRD pattern (Fig. 1c) exhibited several weak diffraction peaks, suggesting low crystalline degree of sample. These diffraction peaks matched with (020), (021), (130) and (150) crystalline planes of VOOH [5-7], suggesting a structure analogous to γ-FeOOH (JCPDS, No. 74-1877). Fig. 1e established structure of VOOH by atomic replacement and geometry optimization process, showing maintenance of structural symmetry and atomic ratio constant, 3

due to high resemblance of atomic crystal structure of as-obtained VOOH with that of lepidocrocite γ-FeOOH [6]. VOOH allowed intercalation of guest molecules or cations into layers owing to its open-layered structures by edge sharing of VO6 octahedra. FTIR (Fig. 1d) provided chemical bonding information in lepidocrocite VOOH. Symmetric stretching vibration of V3+=O bond [17], V-O-V bending and stretching vibrations [8, 10, 18], O-H stretching and bending vibrations were observed. All results confirmed that lepidocrocite VOOH with low crystallinity was synthesized.

Fig. 2. Morphologies of as-obtained VOOH hollow spheres: (a-c) FE-SEM images; (d-i) TEM images. Fig. 2 showed morphology of product VOOH. Hollow spheres with hollow cores and VOOH shells were clearly observed. VOOH hollow spheres had rough surface consisted of nanoparticles. Diameter of as-synthesized VOOH hollow spheres ranged from 300 nm to 500 nm, and their shell thickness reached 20 nm in average. HRTEM image (Fig. 2i) of shell of VOOH hollow sphere showed that its lattice fringes could be partly observed. Distance between neighboring planes was ca. 0.247 nm. This value was consistent with (130) plane of VOOH corresponding with XRD results (Fig. 1c). 4

Amorphous structures could also be seen in HRTEM of shell, suggesting low crystallinity of VOOH hollow spheres in agreement with XRD observations. Besides, some pores (marked by red circle) were occasionally observed in shell (Fig. 2i). Based on the above analyses, lepidocrocite VOOH hollow spheres with low crystallinity of VOOH shells consisted of nanoparticles were synthesized and their formation underwent an inside-out Ostwald ripening [6]. Such structures suggested that as-obtained VOOH hollow spheres probably had excellent electrochemical properties used as the electrode materials [19]. To explore merits of as-synthesized VOOH hollow spheres, electrochemical properties as SCs electrodes were studied in 1 mol· L−1 LiNO3 aqueous electrolyte. CV curves at different scan rates from 2 mV·s−1 to 100 mV· s−1 (Fig. 3a) showed a quasi-rectangular shape, revealing that VOOH hollow spheres had capacitive behavior with pseudocapacitance [20]. GCD curves at various current densities in Fig. 3b were used to calculate value of specific capacitance and to understand rate capability of VOOH hollow spheres. Specific capacitances calculated from discharge times were 102, 87, 68, 56 and 29 F·g−1 at discharge current densities of 0.2, 0.5, 1, 2.5 and 10 A·g−1, respectively. As current densities increasing, specific capacitances decreased. About 67% of specific capacitance at 0.2 A·g−1 was remained at 1 A·g−1. This results was caused by incremental voltage drop and involvement of insufficient active materials [21]. At high current densities, charge-discharge process leading to a low utilization rate of active materials was another reason [1]. Maximum specific capacitance of VOOH hollow spheres using SCs electrodes was 102 F· g−1 at 0.2 A· g−1 in present study, which was not reported. Fig. 3c depicted Ragone plots of VOOH hollow spheres. Calculated energy density (E) values at current densities of 0.2, 0.5, 1, 2.5 and 10 A·g−1 measured 14.1, 12.0, 9.5, 7.8 and 4.0 W· h· kg−1, respectively. Corresponding power density (P) totaled 100, 250, 500, 1250 and 5000 W· kg−1, respectively.

5

Fig. 3. Electrical properties of as-prepared VOOH hollow spheres in aqueous electrolyte: (a) CV curves at different scan rates; (b) GCD curves at different current densities; (c) Ragone plots; (d) Specific capacitances and coulombic efficiency versus cycle numbers at 0.5 A·g−1. Cycling stability is an important parameter for electrode materials used as SCs. CV curves (Fig. S1, Supplementary materials) and GCD curves (Fig. S2) of VOOH hollow spheres at different cycles showed that specific capacitances faded with increasing cycles in aqueous electrolyte. Specific capacitances of as-prepared VOOH hollow spheres were 87, 69, 59, 34 and 16 F·g-1 in 1st, 5th, 10th, 40th and 100th cycle, respectively. Corresponding retentions were 79, 68, 39 and 18 % of initial specific capacitance. Decrease of specific capacitance was attributed to dissolution of VOOH during charging/discharging process, which was a common phenomenon of vanadium materials as electrode materials for SCs or Li-ion batteries in aqueous electrolyte [22, 23], because multiple soluble vanadium species were produced during electrochemical redox reaction of VOOH hollow spheres. There were three issues supporting above explanation: 1) Color of electrolyte turned yellowish after cycles, revealing dissolution of V-containing species, which is proved by our previous reports [24]. 2) Coulombic efficiency remained close to 100% during cycles (Fig. 3d), indicating excellent reversibility of electrodes. 3) FE-SEM images of working electrodes before and after cycles (Fig. S3) showed VOOH hollow spheres were far less after the cycles. 6

4. Conclusion In summary, lepidocrocite VOOH hollow spheres with hollow interior and VOOH shell were prepared and their composition, morphology and structure were evaluated by XPS, XRD, IR, FE-SEM and TEM. Diameter of as-obtained VOOH hollow spheres ranged from 300 nm to 500 nm, and their shell thickness reached 20 nm in average. VOOH hollow spheres were developed to apply to SCs electrodes and exhibited a capacitive behavior based on pseudocapacitance in aqueous solution. Specific capacitance of 102 F· g−1 at current density of 0.2 A·g−1 was achieved. Present study provided insights into exploration of new materials used as electrodes for SC applications. Specific capacitances of VOOH hollow spheres faded with increasing cycles in aqueous electrolyte. Future work will focus on how to improve cycle performance, for examples, designing composites or using organic electrolyte. Acknowledgement This work was supported by National Natural Science Foundation of China (Grant No. 21601026). Supplementary materials Supplementary materials associated with this article can be found, in the online version, at http:// References [1] Yu Z, Tetard L, Zhai L, Thomas J. Energ. Environ. Sci. 2015;8:702. [2] Yu M, Qiu W, Wang F, Zhai T, Fang P, Lu X, et al. J. Mater. Chem. A 2015;3:15792. [3] Zhong C, Deng Y, Hu W, Qiao J, Zhang L, Zhang J. Chem. Soc. Rev. 2015;44:7484. [4] Zheng J, Zhang Y, Wang N, Zhao Y, Tian F, Meng C. Mater. Lett. 2016;171:240. [5] Wu CZ, Xie Y, Lei LY, Hu SQ, OuYang CZ. Adv. Mater. 2006;18:1727. [6] Wu C, Zhang X, Ning B, Yang J, Xie Y. Inorg. Chem. 2009;48:6044. [7] Wu C, Wei H, Ning B, Yang J, Xie Y. Chem. Commun. 2010;46:1845. [8] Xu Y, Zheng L, Xie Y. Dalton Trans. 2010;39:10729. [9] Wu C, Feng F, Feng J, Dai J, Yang J, Xie Y. J. Phys. Chem. C 2011;115:791. [10] Shao J, Ding Y, Li X, Wan Z, Wu C, Yang J, et al. J. Mater. Chem. A 2013;1:12404. [11] Zhu H, Ruan S. Mater. Lett. 2016;184:134. [12] Shi H, Liang H, Ming F, Wang Z. Angew. Chem., Int. Ed. 2017;56:573. 7

[13] Zhang Y, Zheng J, Wang Q, Hu T, Tian F, Meng C. Appl. Surf. Sci. 2017;399:151. [14] Yilmaz G, Guo CX, Lu X. ChemElectroChem 2016;3:158. [15] Zhou X, Chen Q, Wang A, Xu J, Wu S, Shen J. ACS Appl. Mater. Inter. 2016;8:3776. [16] Zhang Y, Huang Y. Mater. Lett. 2016;182:285. [17] Zhang Y, Fan M, Liu X, Huang C, Li H. Eur. J. Inorg. Chem. 2012;2012:1650. [18] Wang Q, Zhang Y, Zheng J, Hu T, Meng C. Microporous and Mesoporous Materials 2017;244:264. [19] Zhang C, Chen Z, Guo Z, Lou XW. Energ. Environ. Sci. 2013;6:974. [20] Brousse T, Bélanger D, Long JW. Journal of The Electrochemical Society 2015;162:A5185. [21] Deng L, Zhang G, Kang L, Lei Z, Liu C, Liu Z-H. Electrochim. Acta 2013;112:448. [22] Cao L, Zhu J, Li Y, Xiao P, Zhang Y, Zhang S, et al. J. Mater. Chem. A 2014;2:13136. [23] Zhang Y, Zheng J, Wang Q, Hu T, Meng C. RSC Adv. 2016;6:93741. [24] Zhang Y, Zheng J, Zhao Y, Hu T, Gao Z, Meng C. Appl. Surf. Sci. 2016;377:385.

8

Graphical Abstract

9

Highlights


Lepidocrocite VOOH hollow spheres with low crystallinity were synthesized.
Electrochemical properties of VOOH hollow spheres as supercapacitor electrodes in aqueous electrolyte were developed.
VOOH hollow spheres featured capacitive behavior based on pseudocapacitance. −1


Specific capacitance of 102 F·g

at current density of 0.2 A·g−1 was achieved.

10