ZHU Mei-Hua WU Xue-Min NIU Bai-Tong GUO Di GUO Hong-Xu

?

Facile Mechanochemical Green Fabricationof CuBr(tu)3Nanowires as ElectrochemicalSupercapacitor Electrodes①

ZHU Mei-Hua WU Xue-Min NIU Bai-Tong GUO Di GUO Hong-Xu②

(363000)

CuBr(tu)3nanowires (tu: thiourea) was prepared by a facile mechanochemical green method for the first time and their application as electrode for supercapacitors is also studied. The as-prepared CuBr(tu)3nanowires show length of around 4 μm and width of 50～60 nm. Those nanowires exhibited high specific capacitance of 1408 F·g-1at a scan rate of 5 mV·s-1and 1683 F·g-1at current density of 10 A·g-1in a 2 M KOH electrolyte. This study provides a simple, efficient, and environmental strategy to fabricate novel nanoscale electrode materials constructing metal-organic frameworks.

CuBr(tu)3nanowires, supercapacitor, mechanochemical;

1 INTRODUCTION

With the rapid growth in energy demand due to the fast growing global economy, developing clean, sustainable and efficient energy storage devices has attracted more and more attention for researchers in the recent years[1]. Supercapacitors as an important class of electrochemical energy storage devices remain an important and unique position because of their higher power density and longer cycle life than batteries[2].

Nanoscale metal-organic materials (NMOMs), as an interesting class of hybrid materials built from metal ions and organic bridging ligands, have aroused significant research attentionin various applications[3]. Among the multitudinous available pseudocapacitive materials, the latest generation of NMOMs acts as an outstanding candidate for pseudocapacitors[4].But the capacitance values for the reported electrode materials for pseudocapacitors still exhibit some issues as the low of specific capacitance. Therefore, developing a novel electrode material with a combination of low cost, larger capacitance and excellent electrochemical stability remains a great challenge.

The interest in NMOMs demands the development of rapid and high-yielding methods for their synthesis and discovery[5]. Mechanosynthesis has a significant potential in that sense, as it offers short reaction times and avoids large quantities of solvent and high temperatures[6]. Herein we report the development of CuBr(tu)3nanowires by a facile mechanochemical method for the first time and their application as electrode for supercapacitors was also studied. The high specific capacitance ofpseudo- capacitorbased on the as-prepared CuBr(tu)3nano- wires reached 1408 F·g-1at a scan rate of 5 mV·s-1and 1683 F·g-1at current density of 10 A·g-1in a 2 M KOH electrolyte. The good electrochemical pro- perties indicate that the CuBr(tu)3is a promising flexible electrode for supercapacitors.

2 EXPERIMENTAL

All reagents in the experiment were analytical reagent and used without further purification. The CuBr(tu)3nanowires were prepared by a facile green mechanochemical synthesis as follows: First, CuBr2(1.1168 g, 1.5 mmol) and thiourea (0.3806 g, 6 mmol) were mixed and ground for 10 min at room temperature. Then, 0.5 mL of ethanol was added, and the mixture was ground for an additional 20 min again. At last, the resulting solid was thoroughly washed with doubly deionized water for several times to remove excess raw materials and dried at 60 ℃ for 8 h.

X-ray diffraction studies (XRD) were carried out on a Bruker D8 Advance diffractometer with Curadiation. Fourier transform infrared spectroscopy (FT-IR) was recorded on a Nicolet 6700 FTIR spec- trometer using KBr pellets. Scanning electron microscopy (SEM) was conducted with a Quant- achrome NOVA 4000-E microscope. X-ray photo- electron spectroscopy (XPS) was performed on a Thermo ESCALAB 250Xi spectrometer with Alradiation.

Electrochemical properties of the samples were evaluated using a potentiostat (CS350, Wuhan Kesite, China) based on a conventional three-electrode system consisting of the working electrode, a pla- tinum as the counter electrode, and a saturated calo- mel electrode as the reference electrode in a 2 M KOH electrolyte. For the working electrode, a mixture containing 80 wt% active material, 10 wt% acetylene black, and 10 wt% poly (tetrafluoro- ethylene) binder was mixed well. Then, the mixture with several drops of ethanol was coated onto a piece of nickel foam (1.0cm × 1.0cm) using a spatula. Finally, the fabricated electrode was pressed at 2 MPa to a thin foil and then dried under vacuum at 60 ℃ for 10 h. The mass loading of the sample was about 2 mg. The EIS data were collected with an AC voltage of 5 mV amplitude in the frequency range from 0.1 to 100 kHz at open-circuit potential.

3 RESULTS AND DISCUSSION

X-ray diffraction patterns of the as-prepared CuBr(tu)3complex are recorded as shown in Fig. 1(a). It can be seen that the diffraction pattern for the as-prepared complex can be indexed to be the phase of CuBr(tu)3crystal (CCDC No. 665785) from the reference document[7]. The strong and sharp diffraction peaks in the XRD pattern indicated that the product was well crystallized.

Fig. 1. XRD spectrum (a) and FT-IR spectrum (b) of the as-prepared CuBr(tu)3sample

The FT-IR spectrum of the as-prepared CuBr(tu)3nanowires is shown in Fig. 1(b). The main charac- teristic peaks at 3500～3000, 1600, 1400 and 600 cm-1in thiourea were also observed in the CuBr(tu)3nanowires, which were ascribed to the stretching vibration of N?H, the bending vibration of N?H, and the stretching vibration of C?N and C=S, respec- tively[8]. And the peaks disappeared at 1475, 1080 and 720 cm-1of CuBr(tu)3nanowires, which suggests that the reduced C=S bond resulting from C–S–(Cu) formed in the compound[9].

The morphology of the as-prepared CuBr(tu)3sample was analyzed using SEM,as shown in Fig. 2. From the low magnification SEM image (Fig. 2a), the sample was composed of uniform slender wire-like with 1.5～5.0 μm in length. Fig. 2b shows a high-magnification SEM image of the intertwined nanowires with approximate 60 nm in width with relatively slender and uniform fibers.

Fig. 2. SEM images of the as-prepared CuBr(tu)3sample

Fig. 3. XPS spectra of CuBr(tu)3nanowires. (a) Survey scan; (b) C 1; (c) N 1; (d) Br 3; (e) S 2; and (f) Cu 2

In order to better characterize the surface chemical composition and chemical states of the CuBr(tu)3, XPS measurements were performed as shown in Fig. 3. In Fig. 3a, the survey scan XPS spectra provided Cu, N, C, Sand Br peaks for the CuBr(tu)3nano- wires.

The C 1XPS spectrum (Fig. 3b) can be deconvo- luted into three different signals with binding energies of 284.8, 288.6, and 290.0 eV, correspon- ding to carbon in C–C bonds and C–N, C–S groups, respectively.The N 1XPS spectrum (Fig. 3c) consisted of two types of nitrogen atoms,, N–H at 399.9 eV and N–C at 400.5 eV. The Br 3XPS spectrum (Fig. 3d) exhibited two doublets, where the binding energies of Br 35/2and Br33/2are located at 68.7 and 69.7 eV, respectively. The S2XPS spec- trum (Fig. 3e) has a signature with two bands. One (162～167 eV) with three peaks is attributed to the S21/2and S23/2orbitals of S2-, and the other (167～172 eV) with doublet peaks is attributed to the formation of Cu–S bonds in a CuBr(tu)3complex.In the XPS Cu 2spectra (Fig. 3f), the deconvoluted doublet peaks located at the binding energies of 932.3 eV (Cu 23/2) and 952.3 eV (Cu 21/2) are characteristic of the Cu(I) species, which conform the valence state of the CuBr(tu)3complex[10].

The CV curves of CuBr(tu)3complex at various scan rates are shown in Fig. 4a, which displayed a couple of redox peaks on each CV curve, indicating that the pseudocapacitive behaviour is mainly based on redox reactions. These redox peaks may corre- spond to the redox reactions of the cations in electrode materials during operations. At the scan rates of 5, 10, 20, 50 and 100 mV·s-1, the corre- sponding specific capacitances of CuBr(tu)3were 1408, 1097, 937, 756 and 619 F·g-1, respectively. With the increase of scan rate, the redox current increased, the anodic peak become more positive and the cathodic peaks become more negative due to the faster charge and discharge rates.

The charge discharge curves of CuBr(tu)3at various current densities are plotted in Fig. 4b. The electrode exhibited gravimetric specific capacitances of 1642, 1593, 1528, 1503, 1446 and 1401 F·g-1at current densities of 10, 14, 18, 22, 26 and 30 A·g-1, respectively. Such high specific capacitances and symmetric electrochemical characteristics proved the outstanding advantages of the CuBr(tu)3electrode.

Fig. 4. CV curves at different scan rates (a), galvanostatic charge discharge curves under different constant densities (b), the EIS spectra (c), and cycling performance at a current density of 10 A·g-1(d) of the as-prepared CuBr(tu)3electrode

Electrochemical impedance spectroscopy (EIS) was also studied. Fig. 4c shows Nyquist plots of CuBr(tu)3nanowires. The depressed semicircle appears at high-to-medium frequencies, correspon- ding to the charge transfer process. The small semicircle diameter indicates a low charge transfer resistance. The straight line at low frequencies originates from diffusive resistance of OH–ions from the bulk solution to the CuBr(tu)3nanowires sur- face[11]. The phase angle of the Nyquist impedance plot was observed to be higher than 45° in the low-frequency region, which indicated good acces- sibility of the ions, owing to the interlayer space of CuBr(tu)3sample.

Fig. 4d displays the cycling performance of CuBr(tu)3at a current density of 10 A·g-1. After 1000 cycles, the CuBr(tu)3electrodepossessed 78% capacitance retention (1281 F·g?1), which is higher than the similar complex [Cu(tu)]Cl·1/2H2O nano- belts reported in our group[12]. The result indicates that the CuBr(tu)3electrode has good cycling stabi- lity, which is important for practical applications.

4 CONCLUSION

In summary, a facile one-step mechanochemical process has been developed for the interwoven nanowire architecture of the CuBr(tu)3pseudo- capacitor electrode material with outstanding electro- chemical performance. The specific capacitance of CuBr(tu)3was up to 1642 F·g-1at current density of 10 A·g-1in a 2 M KOH electrolyte, and possessed 78% capacitance retention after 1000 cycles. This study provides a simple, efficient, and environmental strategy to fabricate novel nanoscale electrode materials constructing metal-organic framework.

(1) Mai, L. Q.; Tian, X. C.; Xu, X.; Chang, L.; Xu, L. Nanowire electrodes for electrochemical energy storage devices.2014, 114, 11828–11862.

(2) Huo, H.; Zhao, Y.; Xu, C. 3D Ni3S2nanosheet arrays supported on Ni foam for high-performance supercapacitor and non-enzymatic glucose detection.. 2014, 2, 15111–15117.

(3) Zhuang, J.; Kuo, C.; Chou, L.; Liu, D.; Weerapana E.;Tsung C. Optimized metal-organic–framework nanospheres for drug delivery: evaluation of small-molecule encapsulation.2014, 8, 2812–2819.

(4) Gholipour-Ranjbar, H.;Soleimania, M.;Naderi, H. R. Application of Ni/Co-based metal-organic frameworks (MOFs) as an advanced electrode material for supercapacitors.2016, 40, 9187–9193.

(5) Xu, J.; Yang, C.; Xue, Y. F.; Wang, C.; Cao, J. Y.; Chen, Z. D. Facile synthesis of novel metal-organic nickel hydroxide nanorods for high performance supercapacitor.2016, 211, 595–602.

(6) Das, G.; Shinde, D. B.; Kandambeth, S.; Biswa,l B. P.;Banerjee, R. Mechanosynthesis of imine,-ketoenamine, and hydrogen-bonded imine-linked covalent organic frameworks using liquid-assisted grinding... 2014, 50, 12615–12618.

(7) Bowmaker,G. A.; Hanna,J. V.; Pakawatchai,C.; Skelton,B. W.; Thanyasirikul,Y.; White,A. H. Crystal structures and vibrational spectroscopy of copper(I) thiourea complexes.2009, 48, 350–368.

(8) Mao,J. F.; Shu,Q.; Wen,Y. Q.; Yuan,H. Y.; Xiao,D.; Choi,M. M. F. Facile fabrication of porous CuS nanotubes using well–aligned [Cu(tu)]Cl·1/2H2O nanowire precursors as self-sacrificial templates.. 2009, 9, 2546–2548.

(9) Sarkar, S.; Dutta, S.; Chakrabarti, S.; Bairi, P.; Tarasankar, P. Redox-switchable copper(I) metallogel: a metal-organic material for selective and naked-eye sensing of picric acid.2014, 6, 6308–6316.

(10) Mao, P.; Qi, L. Y.; Liu, X. D.; Liu, Y.; Jiao, Y.; Chen, S. W.; Yang, Y. Synthesis of Cu/Cu2O hydrides for enhanced removal of iodide from water.2017, 328, 21–28.

(11) Meher, S. K.; Justin, P.; Rao, G. R. Microwave-mediated synthesis for improved morphology and pseudocapacitance performance of nickel oxide.. 2011, 3, 2063–2073.

(12) Guo, D.; Guo, H. X.; Ke, Y. C.; Wang, D. F.; Chen, J. H.; Wang, Q. X.; Weng, W. Facile one-step mechanochemical synthesis of [Cu(tu)]Cl?1/2H2O nanobelts for high-performance supercapacitor.2015, 5, 38527–38532.

24 November 2017;

16 March 2018

the Natural Science Foundation of Fujian Province (No. 2017J01420) and the undergraduate innovation program from Fujian province

. Guo Hong-Xu. E-mail: guohx@mnnu.edu.cn

10.14102/j.cnki.0254-5861.2011-1898