Composition effect of Co/Ni on the morphology and electrochemical properties of NH₄Co_1−xNi_xPO₄·H₂O nanocrystallites prepared by a facile hydrothermal method

Co/Ni ammonium phosphate hydrates (NH₄Co_1−xNi_xPO₄·H₂O, 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, Pmn2₁. 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 NH₄Co_1−xNi_xPO₄·H₂O decreased from the largest value 14.08 µm in a sample with x = 0.00 to the smallest of 5.60 µm in a sample with x = 0.50. This is supported by BET results showing the largest specific surface area, 8.39 m² g⁻¹, and total pore volume, 0.069 cm³ 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 NH₄Co_0.50Ni_0.50PO₄·H₂O 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 NH₄Co_1−xNi_xPO₄·H₂O investigated by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectrum (EIS) were performed with a three–electrode system in a 3 M KOH electrolyte. The results displayed the highest specific capacitance, 540 F g⁻¹, at a current density of 0.5 A g⁻¹ with a low charge transfer resistance of 0.72 Ω in a sample where x = 0.50. This was about 5 time higher than that of the NH₄CoPO₄·H₂O (x = 0.00) sample. Furthermore, the capacitance retention of this sample was 84.5% after a 1000 cycle test at a current density of 5 A g⁻¹.