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 [IijimaG. Iijima, G. ACS Catalysis201996305, 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.

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With rising CO2 emissions and growing interests towards CO2 valorization, electrochemical CO2 reduction (eCO2R) has emerged as a promising prospect for carbon recycling and chemical energy storage. Yet, product selectivity and electrocatalyst longevity persist as obstacles to the broad implementation of eCO2R. A possible solution to ameliorate this challenge is to pulse the applied potential. However, it is currently unclear whether and how the trends and lessons obtained from the more conventional constant potential eCO2R translate to pulsed potential eCO2R. In this work, we report that the relationship between electrolyte concentration/composition and product distribution for pulsed potential eCO2R is different from constant potential eCO2R. In the case of constant potential eCO2R, increasing KHCO3 concentration favors the formation of H2 and CH4. In contrast, for pulsed potential eCO2R, H2 formation is suppressed due to the periodic desorption of surface protons, while CH4 is still favored. In the case of KCl, increasing the concentration during constant potential eCO2R does not affect product distribution, mainly producing H2 and CO. However, increasing KCl concentration during pulsed potential eCO2R persistently suppresses H2 formation and greatly favors C2 products, reaching 71 % Faradaic efficiency. Collectively, these results provide new mechanistic insights into the pulsed eCO2R mechanism within the context of proton‐donator ability and ionic conductivity.

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.

One of the grand challenges in electrocatalysis is to better understand the factors that determine activity and selectivity to control the precision of electrochemical reactions.1 Electrocatalytic CO2 reduction (eCO2R) is a prototypical example of such a reaction, where control over product selectivity would completely transform electrosynthesis processes. Beyond the pursuit of fundamentally understanding electrochemical catalysis, development of eCO2R is driven by growing concerns about global CO2 emissions and the quest for valorization of captured CO2. However, product selectivity and electrocatalyst longevity persist as obstacles to broad implementation of eCO2R. One possible solution to address this challenge is to apply a pulsed potential during eCO2R, which creates a stable reduction environment and tunable product selectivity.2 We leveraged this long-term product stability of pulsed potential eCO2R to examine the relationship between electrolyte concentration and composition with product selectivity for a copper electrode. Whereas constant potential experiments suffer from quick degradation as selectivity towards CO2 reduction products lasts only on the order of one hour, pulsing the potential maintains robust selectivity over 24 hours. This stability presents a unique opportunity to vary the electrolyte parameters while keeping experimental conditions consistent thereby eliminating electrode variability. We find the relation of electrolyte concentration and composition differs greatly for constant and pulsed potential eCO2R. In the case of constant potential eCO2R, increasing KHCO3 concentration is known to favor the formation of H2 and CH4. In contrast, for pulsed potential eCO2R, H2 formation is suppressed due to the periodic adsorption of surface hydroxides, while CH4 is still favored. In the case of KCl, increasing the concentration during constant potential eCO2R does not affect product distribution, mainly producing H2 and CO. However, during pulsed potential eCO2R, increasing KCl concentration suppresses H2 evolution and greatly favors C2 products, reaching 71% Faradaic efficiency. Collectively, these results provide new mechanistic insights into pulsed potential eCO2R in context of the ionic conductivity and higher presence of surface hydroxides which promote C-C bonding. More broadly, the techniques employed here can be used to understand and optimize other electrosynthesis processes. [1] Bell, A. T.; Gates, B. C.; Ray, D.; Thompson, M. R. Basic research needs: catalysis for energy; Pacific Northwest National Lab.(PNNL), Richland, WA (United States): 2008. [2] 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.