Optimizing cation synergy in high entropy oxides for superior bifunctional oxygen electrocatalysis

Abstract

High entropy oxide materials are new-generation materials with diverse possible applications in various fields, including electrocatalyst research. However, the effectiveness of non-stoichiometric high-entropy oxides remains unexplored. Using the well-established sol-gel method, this study synthesized five high-entropy oxides with varying deviations from stoichiometry. Variations in metal-oxygen bond lengths and work functions are observed by deviating from stoichiometric ratios without showing significant changes in oxygen vacancy content. Among the high-entropy oxides (HEOs), the Co-rich (Co_0.3Fe_0.175Cr_0.175Mn_0.175Zn_0.175)₃O₄ (Co0.3) and (Co_0.225Fe_0.1Cr_0.225Mn_0.225Zn_0.225)₃O₄ (Fe0.1) compositions demonstrate a better performance in OER, ORR, and battery applications attributed to their enhanced charge transfer rates and catalytic activity. Co0.3 exhibited the lowest overpotential of 428 mV at a current density of 10 mA cm⁻² and the smallest Bifunctional Index (BI) of 0.95 V. Analysis reveals a non-linear correlation between Co and Fe content, work function, metal-oxygen bond length, and electrocatalytic activity. Our findings propose a better understanding of the effect of non-stoichiometry and Co and Fe synergy on high-entropy oxide for advancing electrocatalyst design.