Controllable Preparation of Ni-Co Nanosheets Covered Nanocages Via Acid Etching with Enhanced Electrochemical Properties

Controllable preparation of Ni/Co nanosheets covered nanocages via acid etching with enhanced electrochemical properties

Zijian Lv, Qin Zhong^*, Yunfei Bu

School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, PR China

Email: lzj_0321@163.com; zq304@mail.njust.edu.com.cn; jpu441@yahoo.com.

Abstract

It has been well accepted that the performance of supercapacitor material highly depends on its morphology, thus the control on morphology is significant for acquiring advanced materials.^[1] Herein, a facile method for controllable synthesis of nickel-cobalt layered double hydroxide (Ni/Co-LDH) with a unique hollow structure is presented. In which zeolitic imidazolate framework-67 (ZIF-67) nanocrystals are used as templates and etched in a nitrate solution. The morphology of Ni/Co-LDH shows an interesting variation with a different dosage of Ni(NO₃)₂ as depicted in Fig. 1(a-f). After reaction with Ni(NO₃)₂ in ethanol solution, the particles still retain a polyhedral shape under a low concentration of Ni(NO₃)₂ but lose the structural integrity with an excessive dosage of Ni(NO₃)₂. Specifically, Ni/Co-LDH-3 (Ni/Co-LDH-x, in which x indicates the mass ratio of Ni(NO₃)₂/ZIF-67) can maintain the polyhedral structure and obtain a hollow structure covered by interlaced nanosheets. Furthermore, it can be observed from the TEM images (Fig. 1(g-h)) that Ni/Co-LDH-3 exhibits a hollow structure with nanosheets forming the shell.

Fig. 1 SEM images of ZIF-67 (a) and Ni/Co-LDH-x (b-f); TEM images of Ni/Co-LDH-3 (g, h).

Fig. 2(a-b) shows the electrochemical properties for as-prepared Ni/Co-LDH-x as supercapacitor electrodes. As depicted in Fig. 2(a), a pair of redox current peaks can be clearly observed in the cyclic voltammetry (CV) curves, indicating that the pseudocapacitive performance resulted from the surface faradaic redox reactions related to M-O/M-O-OH (M= Ni or Co).^[2] It can be observed form Fig. 2(b) that Ni/Co-LDH-3 manifests an excellent electrochemical performance in terms of high specific capacitance (1580 F g^-1 at 2 A g^-1). It might be attributed to the hollow structure which can provide interconnected pathways for both electrons and ions.^[3] Furthermore, the interlaced nanosheets can serve as ion reservoirs, which might shorten the diffusion distance to the interior surfaces and accelerate the ion diffusion process in the electrode, so as to enhance the electrochemical property.^[4] Moreover, the rate capability and cycling performance of Ni/Co-LDH-3 are investigated and the corresponding results are displayed in Fig. 2(c-d). Obviously, the Ni/Co-LDH-3 exhibits a favourable rate capability and displays high capacitance retention of 82.9% after 1000 charge/discharge cycles. It is attributed to the special hollow structure covered by interlaced nanosheets which can maintain the morphology and prevent agglomeration and deactivation during the iterative charge/discharge process.

Fig. 2 (a) The CV curves of Ni/Co-LDH-x at 5 mV s^-1; (b) comparison of the galvanostatic charge-discharge results at 2 A g^-1; (c) specific capacitance of Ni/Co-LDH-3 electrode at different current densities; (d) Cycling performance of the Ni/Co-LDH-3 electrode at a current density of 8 A g^-1, the inset shows the charge/discharge curves of 20 cycles.

In summary, the variation of morphology indeed affects the electrochemical performance, and the Ni/Co-LDH-3 stand out from the as-prepared materials due to its unique hollow structure composed of interlaced nanosheets. This study provides an opportunity to explore the optimization of electrochemical properties by controlling the morphology in improving the electrochemical performances, which could provide further insights in the material design.

References

[1] H. Hu, B. Guan, B. Xia, X.W. Lou, J. Am. Chem. Soc. 137 (2015) 5590-5595.

[2] G.Q. Zhang, H.B. Wu, H.E. Hoster, M.B. Chan-Park, X.W. Lou, Energ. Environ. Sci. 5 (2012) 9453.

[3] C. Sun, J. Yang, X. Rui, W. Zhang, Q. Yan, P. Chen, F. Huo, W. Huang, X. Dong, J. Mater. Chem. A. 3 (2015) 8483-8848.

[4] Z. Jiang, Z. Li, Z. Qin, H. Sun, X. Jiao, D. Chen, Nanoscale. 5 (2013) 11770-11775.