Carbonate anions and radicals induce interfacial water ordering in CO2 electroreduction on gold

Interfacial hydration layers critically determine energy and chemical conversion processes, notably influencing the kinetics of electrocatalytic reactions. Fundamental mechanisms of reactions such as CO₂ electroreduction and hydrogen evolution remain controversial due to the challenge of in situ deciphering of hydration structures alongside reaction intermediates and products. Here, by using vibrational and electrochemical spectroscopy paired with theory we reveal how carbonates structure interfacial water, affecting CO₂ electroreduction and hydrogen evolution reactions on gold electrocatalysts in bicarbonate electrolytes. High cathodic potentials accelerate hydrogen evolution reactions by rapid proton delivery from ordered interfacial hydration networks, induced by carbonate molecules in equilibrium with their anion radicals. These radicals can serve, in addition to CO₂, as a carbon source for CO and aldehyde production. Moreover we show water to be the primary proton donor for CO₂ electroreduction and hydrogen evolution reactions, with bicarbonate mostly participating in the Heyrovsky step. Our molecular-level insights are relevant to rationalizing and optimizing electrochemical interfaces.