Suppressing H₂ Evolution with Sterically Encumbered Proton Sources to Improve the Faradaic Efficiency for CO₂ Reduction to Formate

Electrochemical reduction is an ambient temperature and pressure method of accessing hydrogenated products, but suppressing the hydrogen evolution reaction remains a challenge. We present a strategy that leverages steric interactions to modify the protonation rate around a catalyst active site to reduce the rate of hydrogen evolution in favor of the CO2 reduction product. Stoichiometric studies found that protonation from sterically large acids to form the H–H bond was 35× slower compared to that from less hindered acids. The Faradaic efficiency for CO2 reduction to formate was improved from 40 to 70% by modifying the steric properties of the acid source despite a greater thermodynamic driving force for hydrogen evolution. Our findings highlight the manipulation of steric bulk in catalytic design to modulate selectivity in electrocatalysis.