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. Electrocatalytic CO_2 reduction (eCO_2R) 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 eCO_2R is driven by growing concerns about global CO_2 emissions and the quest for valorization of captured CO_2. However, product selectivity and electrocatalyst longevity persist as obstacles to broad implementation of eCO_2R. One possible solution to address this challenge is to apply a pulsed potential during eCO_2R, which creates a stable reduction environment and tunable product selectivity. We leveraged this long-term product stability of pulsed potential eCO_2R 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 CO_2 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 eCO_2R. In the case of constant potential eCO_2R, increasing KHCO_3 concentration is known to favor the formation of H_2 and CH_4. In contrast, for pulsed potential eCO_2R, H_2 formation is suppressed due to the periodic adsorption of surface hydroxides, while CH_4 is still favored. In the case of KCl, increasing the concentration during constant potential eCO_2R does not affect product distribution, mainly producing H_2 and CO. However, during pulsed potential eCO_2R, increasing KCl concentration suppresses H_2 evolution and greatly favors C_2 products, reaching 71% Faradaic efficiency. Collectively, these results provide new mechanistic insights into pulsed potential eCO_2R 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.
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