Composition effect of Co/Ni on the morphology and electrochemical properties of NH4Co1−xNixPO4·H2O nanocrystallites prepared by a facile hydrothermal method
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Co/Ni ammonium phosphate hydrates (NH4Co1−xNixPO4·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 NH4Co1−xNixPO4·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 NH4Co0.50Ni0.50PO4·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 NH4Co1−xNixPO4·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.
This work was financially supported by the Nanotec–KKU Center of Excellence on Advanced Nanomaterials for Energy Production and Storage, Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand. The Institute of Nanomaterials Research and Innovation for Energy (IN-RIE), Khon Kaen University is also acknowledged for their co-financial support. The authors express their appreciation to the Synchrotron Light Research Institute (SLRI), Nakhon Ratchasima, Thailand for the XPS measurements and analysis.
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