Fabrication Of Self-supported Nickel(hydr)oxide Electrodes And Their Performance For Supercapacitors

Supercapacitor is considered as a very important and new-type element of energy storage applications which fill the gap between batteries and conventional electrostatic capacitors in terms of electrochemical properties. The electrode material is the key factor which directly affects the electrochemical performance of supercapacitor. Transition metal oxide and hydroxide materials have broad application prospects in supercapacitors owing to their rich morphologies and structures. As a typical transition metal element, nickel hydroxide(Ni(OH)2) and nickel oxide(Ni O) have become the research focus of supercapacitor electrode materials with the advantagements of high specific capacitance and energy density, low cost and easy preparation.According to the unsatisfactory of the fabrication method of Ni(OH)2 electrode, in the present work, a facile, cost-effective and green method is developed to in situ fabricate the self-supported Ni(OH)2 electrode. The electrochemical performance of Ni(OH)2 electrode and the effect of electrolyte concentration on electrochemical performance of Ni(OH)2 electrode were investigated in this study. Furthermore, the Ni O electrode was obtained by calcined Ni(OH)2 precursor in various gas atmosphere and the affectation of calcined atmosphere on morphologies, crystal structures and electrochemical performance of Ni O electrode were disscussed. The crystal structures, chemical elements and morphologies of Ni(OH)2 and Ni O were characterized by X-ray diffraction(XRD), X-ray photoelectron spectroscopy(XPS), emission scanning electron microscopy(SEM) and transmission electron microscopy(TEM). The electrochemical performance and cycle stability of self-sopported Ni(OH)2 electrode and Ni O electrode were evaluated by cyclic voltammetry(CV), galvanostatic charge-discharge test and electrochemical impedance spectroscopy(EIS).The self-supported Ni(OH)2 electrode was obtained by a one-step hydrothermal treatment of Ni foam in a 15 wt% H2O2 aqueous solution at 180 ℃ for 24 h without the use of nickel salts, acids, bases, or any post-treatment. XRD analysis reveals Ni(OH)2 in this study is pure hexagonal phase and SEM observation suggests Ni(OH)2 hexagonal platelets with the side lengths of 900 nm and thicknesses of 200 nm are obliquely and firmly grown on the surface of Ni foam substrate. The self-surrported Ni(OH)2 electrode can be used directly as a supercapacitor electrode material and which demonstrate high capacitance(1703 F g-1 at a scan rate of 1 m V s-1) and excellent cycling stability(97% capacitance retention after 2000 cycles at a scan rate of 50 m V s-1) in 1M KOH electrolyte, which is mainly due to the in situ grown Ni(OH)2 hexagonal platelets on substrate increase inherent active nature, shorten the diffusion path of current carriers and further boost the overall electrochemical activity.The affectation of KOH electrolyte solution concentration(1M, 2M and 6M) on the electrochemical performance of self-supported Ni(OH)2 electrode was investigated. The concentration of reaction agent(OH-) is too low in 1M KOH to satisfy the need of electrochemical reaction, and so the speci?c capacitance is worse than that in 2M KOH solution. However, the exceeded high-concentration electrolyte leads to electrolyte resistance increase which may hinder the transfer rates of electronics and finally decrease the speci?c capacitance. As to cycle stability, the higher electrolyte concentration, the worse cycle stability. Take speci?c capacitance and cycle stability into account, 2M KOH is the suitable electrolyte for the Ni(OH)2 electrode in this study.Based on above studies, the Ni(OH)2 was further transformed to Ni O in various gas atmosphere including air, oxygen(O2), nitrogen(N2) and argon(Ar). In order to shorten the preparation time of Ni(OH)2 precursors, the precursors were prepared in 30 wt% H2O2 at 200 ℃ for 6 h. And then the precursors were calnined in four different atmospheres at 400 ℃ for 1 h at a ramping rate of 5 ℃ min-1 to transform into Ni O. Results show that the atmosphere has not any influence on the crystal structures and morphologies of Ni O electrode, while it does affect the concentration of oxygen vacancies existing in Ni O crystals. Obviously, concentration of oxygen vacancies of Ni O calnined in N2 and Ar are higher than that in air or O2. Oxygen vacancy is well known to play an important role in determining the electrochemical properties. Synchronously, the speci?c capacitance of Ni O electrode calnined in oxygen-free atmosphere(N2 or Ar) is superior than that in oxygen-containing atmosphere(air or O2). This is because the existence of oxygen vacancies in Ni O crystal could signi?cantly improve the electrical conductivity and accelerate the kinetics of the surface redox reactions, and thus enhanc their electrochemical performances. As to cycle stability, there is no significant difference among various Ni O electrode and the agglomeration of Ni O platelets reduce the active material area, resulting in a reduction of specific capacitance. The electrochemical performance of Ni O electrode can be enhanced by improving the concentration of oxygen vacancies calnined in oxygen-free atmosphere.