N-aryl pyridinium additives electrodeposited in situ onto Cu foils (Cu/add) during CO2 reduction (CO2R) were shown to suppress the hydrogen evolution reaction (HER) to as low as ~15% FEHER and boost the selectivity to C≥2 products to as high as &gt;70% FEC≥2 at −1.4 V vs. RHE in pH = 2 electrolyte (H3PO4/KH2PO4, [K+] = 0.1 M). We attribute the 10× decrease in HER partial current density to limited proton mass transport to the electrode surface. Rotating disk electrode experiments support this claim, showing a 27% decrease in the H+ diffusion coefficient upon the incorporation of an additive film on Cu. Interestingly, the C≥2 product partial current density is not influenced by varying the additive concentration. Furthermore, high FEC≥2 (&gt; 60%) is maintained on Cu/add under even decreased CO2 partial pressures (0.8 ~ 0.2 atm) in Ar/CO2 fed gas in pH = 2 electrolyte, suggesting that the film can also stabilize and accumulate CO2R intermediates on the Cu surface to facilitate the reduction to C≥2 products. In more acidic 1.0 M H3PO4 (pH ~ 1.1, [K+] = 0.1 M), the Cu/add electrode still achieves ~45% FEC≥2 and suppresses HER (~40% FEHER) at −1.4 V, which is an outstanding result among the rare examples of CO2R in strongly acidic electrolytes.

Cu electrodes decorated with in situ electrodeposited films from N-aryl pyridinium additives boost the selectivity of electrochemical CO2 reduction to C≥2 products in strongly acidic electrolytes

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