Alkali Cation Inhibition of Imidazolium-Mediated Electrochemical CO2 Reduction on Silver

Abstract

Imidazolium-based ionic liquids have led to enhanced CO₂ electroreduction activity due to cation effects at the cathode surface, stabilizing the reaction intermediates and decreasing the activation energy. In aqueous media, alkali cations are also known to improve CO₂ reduction activity on metals such as Ag, with the enhancement attributed to electrical double layer effects and trending with the size of the alkali cation. However, the effect of a mixed catholyte solution of alkali cations in the presence of an imidazolium-based ionic liquid has not been well-explored. Herein, 1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM][BF₄], in water was investigated with alkali salts to unravel the interaction effects for CO₂ electroreduction on Ag. Although both [EMIM]⁺ and alkali cations have individually improved CO₂ to CO conversion on Ag in water, electrochemical results showed that alkali cations hindered imidazolium-mediated CO₂ electroreduction in most conditions. Li⁺, in particular, was sharply inhibitory compared to other alkali cations and strongly redirected the selectivity to hydrogen evolution. The nature of the alkali cation inhibition was investigated with spectroscopic techniques, including in situ surface-enhanced Raman spectroscopy (SERS) and dynamic electrochemical impedance spectroscopy (DEIS). Along with computational insights from density functional theory (DFT), the electrochemical and spectroscopic data suggest that alkali cations inhibit [EMIM]-mediated CO₂ reduction by competing for surface adsorption sites, preventing the potential-dependent structural reorientation of imidazolium, and promoting hydrogen evolution by bringing solvated water to the cathode surface.