Built-in Fields at RuO ₂ /TiO ₂ p−n Interfaces Promote Water Dissociation and Durable Acidic Oxygen Evolution

Developing durable and active catalysts for the acidic oxygen evolution reaction (OER) remains a central challenge for water electrolysis. Here, we demonstrate that constructing supported p−n junctions provides an effective strategy to simultaneously couple interfacial water activation with active-site stabilization. RuO 2 nanoparticles immobilized on oxygen vacancy-rich TiO 2 nanoribbons (RuO 2 /TiO 2 NRs) form a robust p−n heterojunction whose built-in field directs charge separation and tunes interfacial energetics. TiO 2 acts as a water-activation co-catalyst, facilitating H 2 O dissociation at the junction, while structural confinement and charge redistribution suppress Ru over-oxidation and dissolution. A transient potential scanning technique directly visualizes the interfacial charge storage and release behaviors, providing dynamic evidence for the built-in electric field of the p−n junction in regulating charge separation and water-dissociation kinetics. Theoretical calculations reveal electron enrichment on Ru, weakened *OH/*O adsorption, and a reduced barrier for the potential-determining *O → *OOH step, while differential electrochemical mass spectrometry confirms an adsorbate-evolution mechanism without lattice oxygen participation. Consequently, RuO 2 /TiO 2 NRs deliver an overpotential of 192 mV at 10 mA cm −2 in 0.5 M H 2 SO 4 and sustain a ∼650 h durability. This heterojunction-guided loading strategy unifies activity and acid durability by co-engineering charge transfer and water activation at oxide/oxide interfaces.