The Solvation-Induced Onsager Reaction Field Rather than the Double-Layer Field Controls CO₂Reduction on Gold

Abstract

The selectivity and activity of the carbon dioxide reduction (CO 2 R) reaction 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 at catalytic active sites and deconvolute this field into contributions from (1) the electrochemical Stern layer and (2) the Onsager (or solvation-induced) reaction field. Contrary to recent theoretical reports, the CO 2 R kinetics does not depend on the Stern field but instead is closely correlated with the strength of the Onsager reaction field. These results show that in the presence of adsorbed (bent) CO 2 , the Onsager field greatly exceeds the Stern field and is primarily responsible for CO 2 activation. Additional measurements of the cation-dependent water spectra using vibrational sum frequency generation spectroscopy show that interfacial solvation strongly influences the CO 2 R activity. These combined results confirm that the cation-dependent interfacial water structure and its associated electric field must be explicitly considered for accurate understanding of CO 2 R reaction kinetics.

A unifying mechanism for cation effect modulating C1 and C2 productions from CO2 electroreduction

“…This enables us to conclude that the cation effect on CO and C 2 H 4 formations to be controlled by the | σ |. However, the atomic origin of its cation-species dependence needs to be further unraveled; cation-specific interfacial solvation could play a significant role 13 , 25 – 29 , 51 , suggesting a future research direction.…”

“…Also, using the scanning electrochemical microscopic technique, the Koper group demonstrated an absence of CO 2 RR activity for CO formation without M + , which led them to propose a mechanism based on a M + -complexation (or coupling) to the CO 2 − intermediate in conjunction with their ab initio molecular dynamics (AIMD) simulation results 13 . Moreover, cation-dependent interfacial water structure has been exploited to understand the cation effect on the CO 2 RR, which yields different electric field strengths 25 , 26 , adsorption rate 27 , or surface-dependent solvation structure 28 . There further exist other general discussions on the cation effect to the electrocatalytic activity 29 , e.g ., site-blocking of reactants on the electrode or surface reconstruction, albeit it has not been directly linked with the cation effect on the CO 2 RR.…”

Comparing interfacial cation hydration at catalytic active sites and spectator sites on gold electrodes: understanding structure sensitive CO2 reduction kinetics

“…9,61 Alternatively, it could strengthen the electric eld present between the Au surface and the OHP because of the compressed Stern layer. 10,13,21 A single hydration layer around active sites also suggests strong alignment of interfacial water molecules, which could explain the strong Onsager reaction elds, which were previously reported to correlate with cation-dependent solvation structure. 13 Last but not least, the larger solvation shell at spectator sites can also undermine the proton transfer process as a result of more disordered Hbonding network.…”

“…13 Last but not least, the larger solvation shell at spectator sites can also undermine the proton transfer process as a result of more disordered Hbonding network. 13,62,63 This is consistent with the nding that the proton is involved in the rate determining step of CO 2 R at high overpotentials. 10,21 A number of studies have indicated that specic cation effects are a result of electric eld activation of electron transfer due to coupling with the dipole of bent CO 2 , facilitating the formation of CO 2 À .…”

“…Surface adsorbed CO has been widely used as a vibrational Stark reporter to probe the interfacial solvation structure and electric eld inside the EDL. 11,13,14,[30][31][32] Unlike other metals such as Cu and Pt, in situ generated CO on Au produces low spectroscopic signals due to both a low density of surface active sites as well as weak CO adsorption resulting in a low steady state coverage. 22,23 For this reason, many studies opt to increase the CO surface concentration by directly purging CO into the electrolyte.…”