Abstract
Co/Ni ammonium phosphate hydrates (NH4Co1-xNixPO4H2O, where x=0.00, 0.25, 0.50, 0.75, and 1.00) nanocrystallites were synthesized by a facile hydrothermal method. XRD results indicated an orthorhombic structure in all the obtained products within the space group, Pmn2(1). SEM images revealed a microsized morphology of quadrilateral-plates, platelets and flower-like particles in samples with x=0.00, 0.25-0.75 and 1.00, respectively. The measured average diagonal size of NH4Co1-xNixPO4H2O decreased from the largest value 14.08 mu m in a sample with x=0.00 to the smallest of 5.60 mu m in a sample with x=0.50. This is supported by BET results showing the largest specific surface area, 8.39m(2)g(-1), and total pore volume, 0.069cm(3)g(-1), in the x=0.50 sample. A very dense and regular distribution with the largest specific surface area and total pore volumes of nanocrystalline NH4Co0.50Ni0.50PO4H2O might be attributed to improvement of the electro-active sites in the electrode, resulting in an enhanced redox reaction. The electrochemical properties of the mesoporous NH4Co1-xNixPO4H2O investigated by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectrum (EIS) were performed with a three-electrode system in a 3M KOH electrolyte. The results displayed the highest specific capacitance, 540Fg(-1), at a current density of 0.5Ag(-1) with a low charge transfer resistance of 0.72 in a sample where x=0.50. This was about 5 time higher than that of the NH4CoPO4H2O (x=0.00) sample. Furthermore, the capacitance retention of this sample was 84.5% after a 1000 cycle test at a current density of 5A g(-1).