Deciphering <i> E _g </i> Occupancy Descriptor in Oxygen Electrocatalysis

Abstract Single variable descriptors serve as pivotal tools for evaluating catalyst performance, elucidating reaction mechanisms and guiding catalyst design in electrocatalysis. Among the existing oxygen electrocatalysis descriptors, e g occupancy stands out for integrating multidimensional electronic factors into a quantitative framework, enabling precise prediction of activity/selectivity trends. However, a comprehensive model for translating the mechanistic insights of e g occupancy into rational catalyst design remains underdeveloped. This review bridges this gap through a systematic analysis of e g occupancy across five dimensions: 1) fundamental principles of e g occupancy tuning via transition metal coordination environments, 2) characterization techniques for e g occupancy determination, 3) its governing role in oxygen evolution/reduction reaction (OER/ORR) mechanisms and kinetics, 4) precise e g occupancy tuning strategies for material design, and 5) emerging challenges and future perspectives in rational catalyst design. By integrating these perspectives, this work not only deciphers the mechanistic link between e g occupancy and catalytic activities but also establishes design principles for e g ‐optimized electrocatalysts, thereby advancing next‐generation oxygen electrocatalysts beyond noble‐metal benchmarks for sustainable energy applications (e.g., fuel cells, water electrolyzers, and zinc‐air batteries).