Adjusting the Composition of Novel Earth‐Abundant Transition Metal‐Based Oxide Nanoparticles for Electrocatalytic Oxygen Evolution Reaction

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Abstract
To produce green hydrogen through water electrolysis, highly efficient catalysts that use inexpensive, earth‐abundant materials are required. In search of novel and inexpensive electrocatalysts, we investigate the adjustment of the composition of transition metal‐based catalysts for the alkaline oxygen evolution reaction (OER) and evaluate the effects of both redox‐active and ‐inactive elements. Based on the known synthesis of AFe 2 O 4 ‐type spinels (A = transition metal cation), we adjusted the composition of the A and B positions (B = Fe position), achieving a significant improvement in catalytic activity. Additionally, we are investigating the high‐entropy effect, which is expected to enhance the activity and stability of electrocatalysts. Introducing cobalt and manganese and combining these elements with nickel and zinc formed a NiO/(NiZn)(MnCoFe) 2 O 4 composite that reduced the overpotential for OER to 360 mV. However, increasing the number of A‐cations in the spinel beyond six to reach a high‐entropy spinel oxide had no positive effect on the overpotential. In terms of composition, the amount of nickel, and thus NiO, could be reduced to 5%, maintaining low overpotentials. The same applies to the proportion of cobalt, which does not scale linearly with activity.

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