While the surface atomic structure of RuO2 has been well studied in ultra high vacuum, much less is known about the
interaction between water and RuO2 in aqueous solution. In this work, in situ surface X-ray scattering measurements
combined with density functional theory (DFT) was used to determine the surface structural changes on single-crystal
RuO2 (110) as a function of potential in acidic electrolyte. The redox peaks at 0.7, 1.1 and 1.4 V vs. reversible hydrogen
electrode (RHE) could be attributed to surface transitions associated with the successive deprotonation of -H2O on the
coordinatively unsaturated Ru sites (CUS) and hydrogen adsorbed to the bridging oxygen sites. At potentials relevant to
the oxygen evolution reaction (OER), an –OO species on the Ru CUS sites was detected, which was stabilized by a
neighboring -OH group on the Ru CUS or bridge site. Combining potential-dependent surface structures with their
energetics from DFT led to a new OER pathway, where the deprotonation of the -OH group used to stabilize –OO was
found to be rate-limiting.