Nickel Sulfate as an Influential Precursor of Amorphous High-Valent Ni(III) Oxides for Efficient Water Oxidation in Preparation via a Mixed Metal-Imidazole Casting Method

Nickel-based materials are promising electrocatalysts for an alkaline oxygen evolution reaction (OER), which is indispensable for developing water-splitting systems to produce hydrogen as a carbon-neutral fuel. Different NiOx films adhered rigidly on nickel foam (NF) and fluorine-doped tin oxide substrates were prepared via a mixed metal-imidazole casting method from precursor solutions containing Ni salts with four different counter anions of SO42–, NO3–, Cl–, and CH3COO– (Ac) in methanol/1-methyimidazole mixed solvents (3:1) followed by calcination at 450 °C. The counter ions of Ni salts are significantly influential on the structure and characteristics of the formed NiOx films. In the case of the film (SO4-NiOx) prepared from the NiSO4 precursor, an amorphous NiIIIOx nanosheet film was formed in contrast to face-centered cubic NiIIO crystalline nanoparticle films formed using the other precursors, under the same conditions. The special characteristics of the SO4-NiOx film are caused by the inhibition effect of the SO42– ions on the thermochemical formation of NiIIO from the precursor complex, leading to the formation of a NiIIIOx intermediate. The SO4-NiOx/NF electrode demonstrated superior OER performance with an overpotential, η10 = 228 mV at a current density of 10 mA cm–2 in alkaline KOH solutions, which is lower by 145∼165 mV than those (η10 = 373∼393 mV) of the other NiOx/NF electrodes. The OER performance of the SO4-NiOx/NF electrode compares advantageously with that of state-of-the-art Ni-based anodes reported to date.