Molecular Architecture of Proton Donors Governs Selectivity in Nonaqueous Electrochemical CO ₂ Reduction

ABSTRACT

In electrochemical conversions, electron transfers are mostly coupled with proton transfers, necessitating active and abundant proton donors to achieve optimal reaction rates. However, high proton activity often also compromises the selectivity of the target reactions by competing H ₂ formation. Therefore, there is an urgent need for strategies that decouple the chemical activity of proton donors from their electrochemical activity. In this study, using nonaqueous electrochemical CO ₂ reduction as a model, we demonstrate that the molecular architecture of active proton donors can be leveraged to direct them to the proton‐coupled electron transfer (PCET) pathway to the target reaction, rather than H ₂ formation. Using a series of tertiary ammonium proton donors with near‐identical pK _a but varied structures, we show that increasing the steric hindrance around the proton center suppresses H ₂ formation kinetics and shifts proton transfer towards CO ₂ reduction. We further identify structural rigidity as an independent, complementary factor by examining cyclic tertiary ammonium proton donors, which exhibit a distinct enhancement of CO selectivity at more cathodic potentials, attributed to restricted reorientation of the N─H bond at the electrode interface. Our findings establish proton donor molecular architecture as a design lever for selective electrochemical conversions involving PCET.