Impact of Pulsed Electrochemical Reduction of CO₂ on the Formation of C₂₊ Products over Cu

We report the results of experimental and theoretical studies aimed at developing a detailed understanding of how pulsed electrolysis alters the production of the temporal evolution of products over Cu and in particular increases the formation of C2+ products. The catalyst is a Cu film sputtered onto the surface of a PTFE membrane, through which the products of CO2 reduction are sampled for analysis by differential electrochemical mass spectroscopy (DEMS). To avoid changes in the catalyst morphology, the cathode potential is set at −0.8 V vs RHE and −1.15 V vs RHE. We find that the faradaic efficiency (FE) for hydrogen evolution reaction (HER) minimizes and that for the carbon dioxide reduction reaction (CO2RR) maximizes when the durations at each potential are 10 s. Under these conditions, the FE for the HER decreases to 11%, relative to 22% for static electrolysis, at −1.15 V vs RHE, and the FE for the CO2RR increases to 89%, relative to 78% for static electrolysis. Pulsed electrolysis also increases the FE for C2+ products from 68% for static electrolysis to 81%. Temporal analysis of the products by DEMS reveals that while the variation in product concentrations near the cathode begins in synchrony at the start of pulsed electrolysis, the concentration of C2H4 increases and those of CO and H2 decrease with extended time. We attribute these trends to an increase in the ratio of adsorbed CO to H on the catalyst surface. Simulation of pulsed electrolysis also shows that during the period when the cathode is at −0.8 V vs RHE, the local concentration of CO2 in the electrolyte near the cathode builds up. This inventory then allows electrolysis during the period at −1.15 V vs RHE to occur with a higher CO2 concentration than could be achieved for static electrolysis. The net effect of alternating cathode potentials is to enhance the local concentration of CO2, which favors the progress of the CO2RR relative to the HER and in particular the formation of C2+ products.