Synergistic Effects of Mixing and Strain in High Entropy Spinel Oxides for Oxygen Evolution Reaction

Abstract Designing electrochemically active, stable, and earth-abundant electrocatalysts is essential for boosting oxygen evolution reaction (OER) rates for sustainable hydrogen production. High entropy oxides (HEOs) consist of five or more metal cations, offering ample opportunities to tune their catalytic properties toward high OER efficiency. In this work, we combine theoretical and experimental studies to scrutinize the OER activity and stability for spinel-type HEOs. Our density functional theory calculations unequivocally confirm that randomly mixed metal sites are more thermodynamically stable, while we also show that adsorption energies of the intermediates display widening distributions accompanied by the equatorial strain of active metal-oxygen bonds under mixing. The rapid sol-flame method was successfully employed to synthesize spinel oxides up to five third-row metal cations (Co, Fe, Ni, Cr, and Mn) with high degree of entropy. Such catalyst exhibits the highest OER activity of 309 mV at 10 mA cm− 2 and prolonged durability up to 168 hours under alkaline conditions over less complex mixtures despite partial formation of surface oxyhydroxide. The high-entropy model developed in this work identifies mixed Co3+ active site as the most active center of this system. Overall, this work reveals that a key feature controlling the enhanced activity of high-entropy oxides is related to mixing and resulting strain near the active site.