Disentangling Elemental Roles in an Amorphous Medium-Entropy Electrocatalyst for Oxygen Evolution

Understanding elemental cooperation in high/medium-entropy electrocatalysts is essential for rational design but remains challenging due to their compositional complexity. Herein, an amorphous medium-entropy catalyst, FeCoNiMo-a, with a nanoring architecture, is developed as a highly active and durable oxygen evolution catalyst. Operando spectroscopic investigations uncover a clear division of elemental functions within the disordered framework. Cobalt is identified as the dominant active centre, forming high-valence oxyhydroxide species during operation, while iron and molybdenum synergistically regulate the electronic structure and stabilize these oxidized intermediates. In contrast, nickel remains largely metallic, ensuring efficient charge transport and structural robustness. Through these cooperative effects, FeCoNiMo-a achieves only 190 mV overpotential at 10 mA cm^-2 in 1 m KOH. When integrated into an anion exchange membrane (AEM) water electrolyser, FeCoNiMo-a delivers current densities of 1 and 5 A cm^-2 at 1.6 and 1.9 V, respectively, and operates stably for over 600 h at 1 A cm^-2. Further theoretical investigations confirm that Mo incorporation optimizes *OH adsorption and deprotonation, while the amorphous configuration enhances orbital hybridization, facilitating O─O bond cleavage and lattice oxygen participation. This study reveals element-specific cooperation in medium-entropy catalysts, guiding the design of non-precious OER catalysts for practical alkaline water electrolysis.