A simple approach for the synthesis of NiO nanoparticles with both improved OER performance and superparamagnetic behaviour

Synthesis of oxygen evolution reaction (OER) catalysts with comparable or better properties than expensive IrO2 and RuO2 is crucial for large scale applications of alkaline water electrolysis. In this regard, NiO is one of the candidates for such applications and provides unique magnetic properties as well. In this study, NiO nanoparticles were synthesised using a simple co-precipitation + calcination approach to obtain particles with sizes suitable for both improved OER activity and superparamagnetic behaviour. The sample prepared with NaOH was single phase and had smaller crystallite size in comparison to the use of oxalic acid, which led to the formation of minor amount of Ni secondary phase. Electrodes prepared using nickel plated steel substrates with NiO catalyst exhibited an OER overpotential of 309.5 mV at 10 mA cm- 2 current density, and the Tafel slope was calculated to be 131.2 mV dec- 1. These results were comparable to the best OER performance for NiO. However, the presence of minor secondary nickel phase had a detrimental effect on the OER performance for the sample prepared with oxalic acid. Moreover, NiO electrocatalyst exhibited exceptional stability for 24 h of OER operation at 310 mV overpotential with minimal change in the current density. In addition, nanoparticles prepared using NaOH had smaller sizes and exhibited superparamagnetic behaviour while the use of oxalic acid led to a larger crystallite size and combined with the Ni secondary phase this led to a ferromagnetic behaviour.