Enhanced π‐orbital Contribution at RuO 2 ‐Cluster/CeO 2 Interfaces Promotes the Oxide‐Path Mechanism for Acidic Water Oxidation

ABSTRACT Different interfacial configurations in ruthenium oxide (RuO x ) composites lead to variations in orbital structures and catalytic performance. Therefore, understanding the role of these interfaces in the acidic oxygen evolution reaction (OER) is crucial for designing more stable Ru‐based catalysts. Here, single‐atom RuO x (Ru SA O x ), sub‐nanoclusters (Ru SNC O x ), and heterostructures (RuO x ) were incorporated into cerium oxide (CeO x ) to construct three distinct interfaces (i.e., Ce–Ce‐Ce‐Ru, Ce‐Ru‐Ru‐Ce, and Ce‐Ce‐Ru‐Ru), enabling a systematic evaluation of their effects on acidic OER performance. Combining in situ and ex situ spectroscopies, including attenuated total reflection surface‐enhanced infrared absorption spectroscopy (ATR‐SEIRAS), ultraviolet–visible spectroscopy (UV–vis), Raman spectroscopy, differential electrochemical mass spectrometry (DEMS), and other characterization techniques with density functional theory (DFT) calculations reveals that the Ru SNC O x ‐CeO x interface induces additional π orbitals. Electron delocalization between these π orbitals and *O intermediates increases *O coverage and suppresses *O‐*O repulsion, which promotes the OPM pathway. The electron delocalization by π orbitals also optimizes interfacial water and hydrogen‐bond networks, as well as accelerates deprotonation, sustains *O supply, and mitigates Ru over‐oxidation, collectively enhancing OER kinetics and durability. As a result, Ru SNC O x ‐CeO x requires only 178 mV to reach 10 mA cm −2 in 0.5 M H 2 SO 4 and remains stable for over 1000 h. In proton exchange membrane (PEM) electrolyzers, it outperforms conventional RuO 2 , sustaining 1 A cm −2 for more than 1000 h.