(258e) 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.