Activity-Stability Balance Enabled by Orbital Splitting Energy in Ruthenium-Based Oxide Catalysts.

Ruthenium oxides have been the model catalyst for the acidic oxygen evolution reaction. It has been a challenge to achieve both stability and activity and establish a comprehensive descriptor for Ru-based catalysts. Herein, guided by the ligand field theory, we introduce the Ru orbital splitting energy (ΔRu) as a quantitative parameter to describe the interaction between Ru 4d and O 2p orbitals to govern the overall catalytic performance. A series of two-dimensional (2D) multielement metal oxides are synthesized via a self-assembly method to modulate ΔRu. The optimized RuCoFeNiO catalyst for water oxidation delivers a current density of 10 mA cm-2 at a low overpotential of 180 mV and shows high activity for more than 2500 h at 1 A cm-2 in acidic condition. In situ measurements and theoretical analysis reveal that the surface hydrogen-bond network balances the amount of surficial two-hydrogen-bonded water and free water, boosting the proton-transfer process. Moreover, the optimized ΔRu (2.405 eV) modifies the adsorption of *OOH on Ru sites, switching the dominant reaction pathway from the lattice oxygen mediated mechanism to absorbate evolution mechanism. This work highlights the pivotal role of ΔRu in tailoring both stability and activity and may offer insights into high-entropy 2D Ru-based oxides as high-performance oxygen evolution electrocatalysts.