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 CO 2 reduction (CO 2 R); however, the precise mechanisms of action are not fully understood. By performing cation competition experiments with LiHCO 3 and CsHCO 3 during CO 2 R with bare and an organic film-modified Cu electrode, we deconvolute cation-dependent electric-field and nonelectric-field effects and their impact on CO 2 R 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 CO 2 and CO, a CO 2 R intermediate. This lowers the required [Cs + ] to achieve improved CO 2 R 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 CO 2 R to CO and/or CO reduction to C–C coupled products. Together, this study yields spectroscopic evidence for the mechanism of improved CO 2 R selectivity with organic film-modified electrodes and decouples electric-field and nonelectric-field cation effects in CO 2 R.