Preparation And Electrochemical Properties Of Nickel Hydroxide

Supercapacitors, also termed as electrochemical capacitors or ultracapacitors, have attracted tremendous attention due to their high power density, long cycling life and no harm to the environment. They can instantaneously provide higher power density than batteries, and higher energy density than conventional dielectric capacitors. They are of great interest for their potential applications in portable electronics, back-up power devices and hybrid electronic vehicles. Ni(OH)2 is one of potential electrode materials for supercapacitors in view of its high theoretical specific capacitance, inexpensive cost and environmental friendly. In this paper, Ni(OH)2 and its composites have been prepared by a facile coprecipiation rout. In order to obtain high electrochemical performance electrode materials for supercapacitors,additional metal ions were doped to Ni(OH)2, and integrate them with graphene (GNS) or carbon nantobes (CNTs). The morphology and microstructure of samples were examined by X-ray diffraction(XRD), scanning electron microscopy (SEM), transition electron microscopy(TEM), Fourier transform infrared (FTIR) spectroscopy, Barrett-Emmett-Teller (BET) and X-ray photoelectron spectroscopy (XPS). Electrochemical properties were characterized by cyclicvoltammetry (CV), galvanostatic charge/discharge (CP) and electrochemical impedance spectroscopy (EIS).Initially,pure Ni(OH)2 was prepared by a coprecipiation method. The effect of the value of pH, the addition rate of ammonia and reaction time on the microstructure and electrochemistry propertis of Ni(OH)2 was deeply investigated. The results indicate that Ni(OH)2 transformed from sphered-shape to layered-shape with the increase of pH value and the layered structure disappeared when the pH exceeded 9.5. Moreover, the grain size gradually enlarged with the decrease of addition rate of ammonia. Additionally, Ni(OH)2 transformed from a layered structure to a loosely packed turbostratic structure with the increase of reaction time. The loosely packed turbostratic Ni(OH)2 showed excellent electrochemical property due to is 2D-layer structure, which is benefit to the fast tansfer of elctrolyte ion.Additional metal ions (La3+, Al3+ and Co2+) doped composits have also been synthesized by a coprecipitaion rout. The XRD patterns and electrochemical results reveal that the interlayer spacing is enlarged after doping 1% (wt. %) La3+, which enhances the ion transportation and proton diffusion. Consequently resistance decreases and the specific capacitance increase. However, the unbalance of internal charge distribution leads to the slight decrease of specific capacitance with the continuous adding of La3+. More importantly, with the addition of Co2+, the electron transportation and the conductivity were further improved,resulting in the enhanced specific capacitance. Notably, the successful phase transmission from β-Ni(OH)2 to α-Ni(OH)2 after Al3+ doping was observed and the enlarged interlayer spacing for a-Ni(OH)2 is favorable for redox reactions for Ni(OH)2. The superior electrochemical performance of Al-Co-Ni(OH)2 is ascribed to the synergistic effect of Co2+and A13+ for doped Ni(OH)2, enhancing both the interlayer spacing and conductivity of overall electrode material..Finally,conductive carbon materials (CNTs and GNS) was added to promote the conductivity of Al-Co-Ni(OH)2. The results indicate that both the rate capability and specific capacitance of composite were further enhanced after incorporation with proper amount of conductive CNTs or GNS due to the good conductivity of carbon materials. The composite with the addition of GNS shows better electrochemical propertis due its 2D lyered structure GNS not only can prohibite the aggregation of doped Ni(OH)2, but also inhibit the crystal expansion of doped Ni(OH)2, enhancing the conductivity for overall electrode material and providng electric doule layer capacitance.