Solvation-Induced Onsager Reaction Field Rather than Double Layer Field Controls CO2 Reduction on Gold

The selectivity and activity of the carbon dioxide reduction reaction (CO2R) are sensitive functions of the electrolyte cation. By measuring the vibrational Stark shift of in-situ generated CO on Au in the presence of alkali cations, we quantify the total electric field present during turnover and deconvolute this field into contributions from 1) the electrochemical Stern layer and 2) the Onsager, or solvation-induced, reaction field. The magnitude of the Onsager field is shown to be on the same order as the Stern layer field (∼10 MV/cm) but follows an opposite trend with cation, increasing from Li⁺< Na⁺< K⁺< Rb⁺≈Cs⁺. Contrary to theoretical reports,CO₂R kinetics are not correlated with the Stern field but instead are controlled by the strength of the Onsager reaction field with Cs⁺ as an exception. Spectra of interfacial water as a function of cation show that Cs⁺ induces a change in the interfacial water structure correlated with a dramatic drop in CO₂R activity, highlighting the importance of cation-dependent interfacial water structure on reaction kinetics. These findings show that both the Onsager reaction field and interfacial solvation structure must be explicitly considered for accurate modeling of CO₂R reaction kinetics.