Modulating D ‐Band Electron Occupancy in Ru‐Based Heterostructures for Durable Acidic Oxygen Evolution

The rational design of acid‐stable, iridium‐free electrocatalysts for the oxygen evolution reaction (OER) is critical for advancing proton exchange membrane water electrolysis (PEMWE), yet balancing activity and durability remains a formidable challenge. Herein, we report a RuO 2 /Mn 3 O 4 heterojunction with engineered oxygen vacancies (O v ) as a durable, high‐performance iridium alternative. Engineering triggers substantial electron transfer from Mn 3 O 4 to RuO 2 , lowering the average Ru oxidation state from +3.69 to +3.34 and increasing d ‐band occupancy from 4.23 to 4.50. This enhanced occupancy strengthens Ru–O covalency via intensified coupling with O 2 p orbitals, corroborated by density functional theory calculations showing a reduced energy barrier of potential‐determining step by 0.62 eV. In situ spectroscopy further reveals a distinctive dual H 2 O adsorption configuration at adjacent Ru–O v sites, enabling direct O–O coupling and promoting a more efficient OER pathway. Consequently, the optimized RuO 2 /Mn 3 O 4 ‐O v catalyst achieves an exceptionally low overpotential of 185 mV at 10 mA cm − 2 and a turnover frequency of 4.33 s − 1 at 185 mV—188‐fold higher than commercial RuO 2 . Notably, it maintains stable operation for over 200 h at 100 mA cm − 2 in 0.1 M HClO 4 , highlighting its promise for replacing iridium catalysts in PEMWE applications.