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Abstract In this study, we employed electrochemical‐mass spectrometry (EC‐MS) to elucidate the role of halide anions in electrochemical CO₂and CO reduction. We found that the undesired hydrogen evolution reaction (HER) was significantly suppressed by the anion used. Specifically, the rates of H₂production decreased in the order KF > KCl > KI, meaning that I⁻most strongly suppressed HER. Interestingly, CO reduction products showed an inverse relationship to HER, with KI leading to the highest rate of CO reduction. By pairing our experimental findings with classical molecular dynamics simulations, we propose a mechanism wherein halide anions influence the dynamic interplay between CO reduction and HER by modulating the competition of H* and CO* for active sites on the Cu surface. We propose that this interaction is enabled by the interfacial concentration of K⁺being greater in the presence of F⁻than in I⁻. Our results highlight the need to more broadly consider the properties of ions at electrocatalytic interfaces and they point to thus far underappreciated avenues to optimize hydrocarbon production while suppressing hydrogen evolution.