<div><p>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<sup>+</sup><
Na<sup>+</sup>< K<sup>+</sup>< Rb<sup>+</sup>≈Cs<sup>+</sup>. Contrary to
theoretical reports,CO<sub>2</sub>R kinetics are not correlated with the Stern
field but instead are controlled by the strength of the Onsager reaction field
with Cs<sup>+</sup> as an exception. Spectra of interfacial water as a function
of cation show that Cs<sup>+</sup> induces a change in the interfacial water
structure correlated with a dramatic drop in CO<sub>2</sub>R 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 CO<sub>2</sub>R reaction kinetics.</p><br></div>