Electrolyte Concentration Variation during Pulsed Potential Electrochemical CO2 reduction to Control Product Selectivity

Abstract

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.