Suppressing H<sub>2</sub> Evolution with Sterically\nEncumbered Proton Sources to Improve the Faradaic Efficiency for CO<sub>2</sub> Reduction to Formate

Electrochemical reduction is an ambient temperature and\npressure\nmethod of accessing hydrogenated products, but suppressing the hydrogen\nevolution reaction remains a challenge. We present a strategy that\nleverages steric interactions to modify the protonation rate around\na catalyst active site to reduce the rate of hydrogen evolution in\nfavor of the CO<sub>2</sub> reduction product. Stoichiometric studies\nfound that protonation from sterically large acids to form the H–H\nbond was 35× slower compared to that from less hindered acids.\nThe Faradaic efficiency for CO<sub>2</sub> reduction to formate was\nimproved from 40 to 70% by modifying the steric properties of the\nacid source despite a greater thermodynamic driving force for hydrogen\nevolution. Our findings highlight the manipulation of steric bulk\nin catalytic design to modulate selectivity in electrocatalysis.