pH- and Cation-Dependent Water Oxidation on Rutile RuO2(110)

Noncovalent interactions at electrified interfaces are key to improving activity for the oxygen evolution reaction (OER). Here, we showed that on RuO₂(110) in alkaline solutions, OER activity is cation-dependent, being largest in 0.1 M KOH compared to LiOH and NaOH. Using crystal truncation rod analysis, -O is detected on the coordinatively unsaturated site at 1.5 V-RHE in 0.1 M KOH, suggesting that the rate-determining step is -O + OH^- → -OOH + e^-, which is different from that in acid involving the final deprotonation of -OOH. The ordering of interfacial water in base was found to decrease with increasing potential and independent of cations. Using surface-enhanced infrared spectroscopy, the density of isolated water molecules (zero H-bonds) was found to increase, and the density of icelike water molecules (four H-bonds) decreases from Li⁺ to K⁺ at OER potentials. The higher activity of more isolated interfacial OH^- ions in the case of K⁺ and the lesser stabilization of -O intermediates by hydration water of K⁺ compared to Na⁺ and Li⁺ can result in higher OER activity for KOH. This work provides molecular details of the interface as a function of potential and electrolyte and enables the design of more active electrochemical interfaces.