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 RuO₂/Mn₃O₄ heterojunction with engineered oxygen vacancies (O_v) as a durable, high-performance iridium alternative. Engineering triggers substantial electron transfer from Mn₃O₄ to RuO₂, 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 2p 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 H₂O adsorption configuration at adjacent Ru-O_v sites, enabling direct O-O coupling and promoting a more efficient OER pathway. Consequently, the optimized RuO₂/Mn₃O₄-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 RuO₂. Notably, it maintains stable operation for over 200 h at 100 mA cm^{- 2} in 0.1 M HClO₄, highlighting its promise for replacing iridium catalysts in PEMWE applications.