Tracking Water Dissociation on RuO₂(110) Using Atomic Force Microscopy and First-Principles Simulations

The interaction between interfacial water and transition metal oxides is a primary enabling step for the oxygen evolution reaction (OER). RuO₂ is a prototypical OER electrocatalyst whose ability to activate interfacial water molecules is essential to its OER activity. We image the dissociation of surface water into OH* and O* on RuO₂(110), where * denotes adsorbed species, using atomic force microscopy. Starting from the surface-bound water molecules, which form a one-dimensional network along the rows of Ru surface sites, increasing the oxidative potential strips hydrogen away and transforms the water molecules into OH* and O*. This oxidative step changes the pattern of the adsorbates from one- to two-dimensional. First-principles calculations with interfacial polarization, capacitive charging, and adsorbate interactions attribute this evolution to the cooperative dehydrogenation of adsorbed water and OH* on RuO₂. We use these results to map the surface phase diagram of RuO₂(110) and provide a quantitative interpretation of its cyclic voltammetry. Our result provides the visualization of the water dissociation on a conductive oxide surface, a critical step in the OER, and demonstrates that the water activation is a collective phenomenon at RuO₂(110) electrodes.