Cation Competition Experiments during Electrochemical CO ₂ Reduction As a Probe of the Film-Modified Copper Microenvironment

Alkali metal electrolyte cations affect product activity and selectivity in electrochemical CO₂ reduction (CO₂R); however, the precise mechanisms of action are not fully understood. By performing cation competition experiments with LiHCO₃ and CsHCO₃ during CO₂R with bare and an organic film-modified Cu electrode, we deconvolute cation-dependent electric-field and nonelectric-field effects and their impact on CO₂R performance. Attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) experiments reveal that organic additive films do not change cation accumulation at the electrochemical interface. Instead, through time-resolved ATR-SEIRAS experiments, we find that additive films alter the transport and interfacial concentrations of CO₂ and CO, a CO₂R intermediate. This lowers the required [Cs⁺] to achieve improved CO₂R without changing the intrinsic reaction kinetics. Additionally, we find that even small amounts of Cs⁺ significantly disrupt the interfacial water structure, which we infer is key to the promotion of CO₂R to CO and/or CO reduction to C-C coupled products. Together, this study yields spectroscopic evidence for the mechanism of improved CO₂R selectivity with organic film-modified electrodes and decouples electric-field and nonelectric-field cation effects in CO₂R.