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Noncovalent interactions at electrified interfaces are key to improving activity for the oxygen evolution reaction (OER).Here, we showed that on RuO 2 (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.

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Molten salt electrolytes and nickel oxide–based electrodes enable four-electron transfer