Pyrite High-Entropy Sulfides for Bifunctional Oxygen Electrocatalysis and High-Performance Zinc-Air Batteries

Zinc-air batteries are limited by sluggish, reversible oxygen electrocatalysis at the air cathode. High-entropy sulfides (HESs) are introduced as bifunctional oxygen catalysts, leveraging multication disorder to create robust, tunable active sites. Three pyrite-type, single-phase compositions: HES-TM [(FeNiCoCrMn)-S₂], HES-CuTi [(FeNiCoCuTi)-S₂], and HES-Co0.4 [(FeNiCo_0.4CrMn)-S₂], exhibit homogeneous elemental distributions and nanoscale particles. Among them, HES-TM delivers the strongest overall bifunctionality (0.94 V), combining lower OER overpotential with more favorable ORR polarization. Koutecký-Levich analysis indicates mixed pathways consistent with partially four-electron ORR. XPS reveals predominantly Co³⁺ with mixed Ni and Fe valence, while HES-TM shows a higher sulfide (S₂ ^2-) fraction with a thin SO _x surface layer, signatures of stronger metal-sulfur coordination, and optimized *OH/*OOH binding. Despite higher conductivity in HES-CuTi, activity trends confirm that active-site chemistry, not conductivity alone, governs performance. In full zinc-air cells, HES-TM reduces the charge-discharge voltage gap and enhances power and energy delivery, consistent with its low bifunctional index. These results position disorder-engineered pyrite HESs as cost-effective, scalable cathodes and provide design rules that link surface sulfur chemistry and mixed metal valence to reversible oxygen electrocatalysis.