Fabrication And Capacitance Properties Of Sandwich-Type Nanoporous Nickel/Nickel Oxide Composite Electrode

In this paper,the nanoporous nickel/nickel oxide?np-Ni@NiO?composite electrode with a sandwich structure?NiO@Ni-np?amorphous alloy?np-Ni@NiO?and good flexibility was synthesized by dealloying the ductile Ni-based amorphous alloy coupled with natural oxidation.The effects of the dealloying process on the pore structure,crack-control and capacitance properties of np-Ni@NiO were studied.In addition,the ralationship between the dealloying process,microstructure and energy storage characteristics was systematically discussed.Meanwhile,the uniformity of the porous and the electrochemical performance of np-Ni@NiO are further improved by optimizing the composition of the precursor.This study aims to provide an important theoretical basis for the controllable fabrication of nanoporous nickel/nickel oxide composite electrode with good flexibility.With the Ni₄₀Zr₆₀ as a dealloying precursor,a np-Ni@NiO composite with a sandwich structure was prepared in 0.05 M HF solution by chemical dealloy.From the SEM results,it can be observed that a large number of star-shaped cracks were uniformly distributed on the surface of as-dealloyed Ni₄₀Zr₆₀0 ribbon.By adjusting the dealloying process,the crack-free np-Ni@NiO composite electrode are obtained as follows:?1?By reducing the concentration of the etching solution from 0.05 M HF to 0.02 M or 0.01 M HF,the cracks are effectively eliminated by chemical dealloying;?2?Using electrochemical dealloying?-0.2 V or 5 mA/cm² in 0.05 M HF?instead of free chemical dealloying,it is proven that electrochemical dealloying is an efficent way for eliminating the surface cracks of np-Ni@NiO;?3?By alloying Ti to the Ni₄₀Zr₆₀ for replacing a portion of Zr,the np-Ni@NiO prepared by chemical dealloying Ni₄₀Zr₂₀Ti₄₀ in 0.05 M HF solution performs a homogerous nanoporous structure.The np-Ni@NiO composites prepared by the above three methods show an excellent flexibility due to their continuous defect-free microstructure.Ni₄₀Zr₆₀ and Ni₄₀Zr₂₀Ti₄₀ were chemical dealloyed in 0.02 M and 0.05 M HF solution,respectively.The microstructures of the both are almost the same with the change of dealloying time.With the extension of the dealloying time,the number of nano pores gradually increase and the ligaments gradually evolve into a fine network structure.Meanwhile,the thickness of dealloying layer increases significantly,indicating that the amount of NiO increases.The electrochemical performance of the np-Ni@NiO composite was examined.The results showed that the electrodes prepared by different precursors exhibited good pseudocapacitance characteristics at 1 M KOH.At the charge-discharge current density of 0.5 A/cm³,the maximum specific capacitance of the np-Ni@NiO composites prepared by dealloying the Ni₄₀Zr₆₀ and Ni₄₀Zr₂₀Ti₄₀ are 640 F/cm³ and 749F/cm³,respectively.As the dealloying time extends,the specific capacitance of np-Ni@NiO composite increases.However,the electrode's rate performance reduced due to the increase of the charge-transfer resistance R_ct.The np-Ni@NiO composites prepared from the Ni₄₀Zr₂₀Ti₄₀ exhibit a more uniform and continuous nano-porous structure and perform higher capacitance and better flexibility as compared to those made from the the Ni₄₀Zr₆₀.Meanwhile,the composites demonstrate a good cycle stability,outstanding energy density as well as power density in the simulation test of symmetrical energy storage devices.