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The ability to control reaction kinetics and double layer species during an electrocatalytic process is highly desirable, especially for electrochemical CO2 reduction (CO2R) — a complex process in which multiple reaction steps are competing on the electrode surface. Here we show evidence suggesting the double layer can be disrupted with the application of a pulsed potential during CO2R. Pulsing the potential during CO2R 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 constant applied potential.1 However, the underlying mechanism and contribution of interfacial/surface phenomena behind the pulsed potential application remain largely unknown. To uncover this 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 a highly oxidative limit (> 0.6 V vs. reversible hydrogen electrode (RHE)). Our results suggest that the pulsed anodic potential influences the double layer 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 electro-adsorption of hydroxides, which outcompetes protons for surface sites. As shown in a recent in-situ infrared study2, 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 surface-pH environment, promoting CO adsorption. We can describe this pulsed potential dynamic double layer mechanism in a competing quaternary Langmuir isotherm model. We conclude that the active disruption of the double layer can be leveraged to tune the surface reaction environment during CO2R. Furthermore, the insights from this investigation have wide-ranging implications for applying pulsed potential profiles to improve electrocatalytic processes in general by dynamically disrupting double layer species. [1] Kimura, K. W.; Fritz, K. E.; Kim, J.; Suntivich, J.; Abruña, H. D.; Hanrath, T. Controlled Selectivity of CO2 Reduction on Copper by Pulsing the Electrochemical Potential. ChemSusChem 2018, 11 (11), 1781–1786. https://doi.org/10.1002/cssc.201800318. [2] Iijima, G.; Inomata, T.; Yamaguchi, H.; Ito, M.; Masuda, H. Role of a Hydroxide Layer on Cu Electrodes in Electrochemical CO2 Reduction. ACS Catal. 2019, 9 (7), 6305–6319. https://doi.org/10.1021/acscatal.9b00896. |
