Solvent-mediated outer-sphere CO 2 electro-reduction mechanism over the Ag111 surface.

The electrocatalytic CO ₂ reduction reaction (CO ₂ RR) is one of the key technologies of the clean energy economy. Molecular-level understanding of the CO ₂ RR process is instrumental for the better design of electrodes operable at low overpotentials with high current density. The catalytic mechanism underlying the turnover and selectivity of the CO ₂ RR is modulated by the nature of the electrocatalyst, as well as the electrolyte liquid, and its ionic components that form the electrical double layer (EDL). Herein we demonstrate the critical non-innocent role of the EDL for the activation and conversion of CO ₂ at a high cathodic bias for electrocatalytic conversion over a silver surface as a representative low-cost model cathode. By using a multiscale modeling approach we demonstrate that under such conditions a dense EDL is formed, which hinders the diffusion of CO ₂ towards the Ag111 electrocatalyst surface. By combining DFT calculations and ab initio molecular dynamics simulations we identify favorable pathways for CO ₂ reduction directly over the EDL without the need for adsorption to the catalyst surface. The dense EDL promotes homogeneous phase reduction of CO ₂ via electron transfer from the surface to the electrolyte. Such an outer-sphere mechanism favors the formation of formate as the CO ₂ RR product. The formate can undergo dehydration to CO via a transition state stabilized by solvated alkali cations in the EDL.
Competing Interests: There are no conflicts to declare.
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