Interfacial electric field regulates surface water environment in RuO₂ coated Co₃O₄ for proton exchange membrane water electrolysis

Designing Ru-based anodic catalysts to substitute Ir in proton exchange membrane water electrolysis (PEMWE) is pivotal to slashing hydrogen generation expenses. Herein, a conformal coating of RuO₂ has been uniformly deposited over Co₃O₄ nanocrystals (RuO₂-Co₃O₄) to enhance the utilization efficiency and durability of ruthenium for acidic oxygen evolution reaction (OER). Structural characterizations and density functional theory (DFT) calculations reveal that Co₃O₄ regulates the electronic structure of the surface RuO₂ through Ru-O-Co motifs, activating the Ru sites while inhibiting the participation of lattice oxygen by lowering the energy level of O 2p band. As expected, unlike commercial RuO₂, the RuO₂-Co₃O₄ catalyst switches to a robust adsorbate evolution mechanism pathway, as evidenced by electrochemical and in situ differential electrochemical mass spectrometry (DEMS) results. Remarkably, benefiting from the interfacial electric field between RuO₂ and Co₃O₄, the RuO₂-Co₃O₄ displays an enhanced *OH coverage and improved proportion of free-H₂O in the electric-double-layer region, which are responsible for the improved acidic OER kinetics. Consequently, the obtained RuO₂-Co₃O₄ displays a high mass activity of 376.6 A g^-1, which is 27-fold higher than that of the commercial RuO₂. When implemented in PEMWE devices, a current density of 500 mA cm^-2 is achieved with only 1.75 V under an extremely low catalyst loading of 0.15 mg_Ru cm^-2.