Abstract: High/medium entropy alloys (HEAs/MEAs) with high electrocatalytic activity have attracted great attention in water electrolysis applications. However, facile synthesis of self-supporting high/medium entropy alloys electrocatalysts with rich active sites through classical metallurgical methods is still a challenge. Here, a self-supporting porous FeCoNi MEA electrocatalyst with nanosheets-shaped surface for oxygen evolution reaction (OER) was prepared by a one-step electrochemical process from the metal oxides in molten CaCl₂. The formation of the FeCoNi MEA is attributed to the oxides electro-reduction, high-temperature diffusion and solid solution. Additionally, the morphology and structure of the FeCoNi MEA can be precisely controlled by adjusting the electrolysis time and temperature. The electronic structure regulation and the reduced energy barrier of OER from the “cocktail effect”, the abundant exposed active sites brought by surface ultrathin nanosheets, the good electronic conductivity and electrochemical stability from the self-supporting structure enable the FeCoNi MEA electrode shows high-performance OER electrocatalysis, exhibiting a low overpotential of 233 mV at a current density of 10 mA cm^-2, a low Tafel slope of 29.8 mV dec^-1, and an excellent stability for over 500 h without any obvious structural destruction. This work demonstrates a facile one-step electrochemical metallurgical approach for fabricating self-supporting HEAs/MEAs electrocatalysts with nanosized surface for the application in water electrolysis.

Key words: Molten salt electrosynthesis, FeCoNi medium entropy alloy, Self-supporting electrocatalyst, Nanosheets-shaped surface, Oxygen evolution reactions