Selective Electrochemical CO2 Reduction during Pulsed Potential Stems from Dynamic Interface

Pulsing the potential during the electrochemical CO2 reduction (CO2R) reaction using copper has been shown to influence product selectivity (i.e., to suppress the undesired hydrogen evolution reaction (HER)) and to improve electrocatalyst stability compared to the constant applied potential. However, the underlying mechanism and contribution of interfacial/surface phenomena behind the pulsed potential application remain largely unknown. We investigated the state of the copper surface during the pulsed potential electrochemical CO2R using in situ X-ray adsorption near-edge spectroscopy (XANES). We probed the surface valence of the metallic electrode and found that the Cu electrode remains metallic over a broad pulsed potential range and only oxidizes to form Cu(OH)(2) in the bulk when the pulsed potential reaches the highly oxidative limit (greater than 0.6 V vs reversible hydrogen electrode (RHE)). Our results suggest that the pulsed anodic potential influences the interfacial species on the electrode surface, i.e., the dynamic competition between protons and hydroxide adsorbates instead of bulk copper oxidation. We attribute the suppressed HER to the electroadsorption of hydroxides, which outcompetes protons for surface sites. As shown in a recent in situ infrared study [Iijima, G. et al.; ACS Catalysis 2019, 9, 6305], adsorbed hydroxides promote CO adsorption, a crucial CO2 reduction intermediate, by preventing CO from becoming inert through a near-neighbor effect. We corroborate this interpretation by demonstrating that the pulsed potential application can suppress the HER during the CO reduction just as the CO2R. Our results suggest that the pulsed potential mechanism favors CO2R over the HER due to two effects: (1) proton desorption/displacement during the anodic potential and (2) the accumulation of OHads, creating a higher pH-surface environment, promoting CO adsorption. We can describe this pulsed potential dynamic interfacial mechanism in a competing quaternary Langmuir isotherm model. The insights from this investigation have wide-ranging implications for applying pulsed potential profiles to improve other electrochemical reactions.