Elucidating the Pre-Oxygen Evolution Surface Chemistry on Ruthenium Dioxide Surfaces

Abstract

The unique interaction between water and rutile Ruthenium Dioxide (RuO2) affords high pseudocapacitance and catalytic activities for a number of reactions such as the oxygen evolution reaction (OER)1,2,3. While the low energy, RuO2 (110) and (100) surfaces have been studied as model systems for gas phase catalysis and ultra high vacuum surface science studies4,5, the nature of adsorbed species in aqueous solutions remains to be understood. In this work, we examine the structural and chemical changes occurring on oriented RuO2 single crystal surfaces as a function of potential, in acidic electrolyte, using in situ synchrotron-based surface X-ray diffraction (crystal truncation rod) measurements. We find that the positions of the surface Ru and O atoms are largely unchanged from 0.5 V to 1.5 V versus the reversible hydrogen electrode (RHE) scale while adsorbed water molecules on the co-ordinatively unsaturated site (CUS) are deprotonated gradually with increasing potential. At oxygen evolution potentials, we observe the formation of an –OO like group on the co-ordinatively unsaturated site, which is the probable precursor of the evolved oxygen. In order to validate experimentally observed changes in the nature of adsorbed oxygen, we use density functional theory to compute surface Pourbaix diagrams that show the most stable surface termination at any given potential. The experimental and computational results are in strong agreement and provide an atomistic understanding of the surface structural changes associated with the redox transitions prior to oxygen evolution and its implications on the oxygen evolution pathway on RuO2.