Switching Reaction Pathways of CO₂ Electroreduction by Modulating Cations in the Electrochemical Double Layer

Tuning the selectivity of CO₂ electroreduction reaction (CO₂RR) solely by changing electrolyte is a very attractive topic. In this study, we conducted CO₂RR in different aqueous electrolytes over bulk metal electrodes. It was discovered that controlled CO₂RR could be achieved by modulating cations in the electrochemical double layer. Specifically, ionic liquid cations in the electrolyte significantly inhibits the hydrogen evolution reaction (HER), while yielding high Faraday efficiencies toward CO (FE_CO) or formate (FE_formate) depending on the alkali metal cations. For example, the product could be switched from CO (FE_CO=97.3 %) to formate (FE_formate=93.5 %) by changing the electrolyte from 0.1 M KBr-0.5 M 1-octyl-3-methylimidazolium bromide (OmimBr) to 0.1 M CsBr-0.5 M OmimBr aqueous solutions over pristine Cu foil electrode. In situ spectroscopy and theoretical calculations reveal that the ordered structure generated by the assembly of Omim⁺ under an applied negative potential alters the hydrogen bonding structure of the interfacial water, thereby inhibiting the HER. The difference in selectivity in the presence of different cations is attributed to the hydrogen bonding effect caused by Omim⁺, which alters the solvated structure of the alkali metal cations and thus affects the stabilization of intermediates of different pathways.