The Influence of ppm-Level Divalent Ionic Impurities from Real-World Feedstocks on Electrochemical Cu-Based CO ₂ Reduction

Large-scale operation of electrochemical CO2 reduction (CO2R) reactors can be promoted by utilizing abundant feedstocks, such as tap water, to reduce the electrolyte costs in CO2R systems, but trace concentrations of ionic impurities can significantly affect CO2R performance. We investigated the consequences of trace concentrations (e.g., ≤20 ppm) of Mg2+ and Ca2+, non-electrodepositing, divalent cations commonly found in tap water, on Cu-based CO2R. We probed the influences of individual ions, and their mixture, on CO2R activity and selectivity. We also explored the influence of these ions across two time domains of CO2R: the electrocatalyst break-in period and pseudo-steady-state operation. Our results indicate that Mg2+ significantly influences CO2R selectivity, favoring the competing H2 evolution reaction, while Ca2+ decreases total activity without significantly influencing selectivity. This work highlights the importance of considering trace concentrations of ionic impurities as they can severely impact electrochemical performance.