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

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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.