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 VRHE 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.