Anolyte Enhances Catalyst Utilization and Ion Transport Inside a CO₂ Electrolyzer Cathode

Abstract

Electrochemical CO2 reduction is a promising technology to capture and convert CO2 to valuable chemicals. High Faradaic efficiencies of CO2 reduction products are achieved with zero-gap alkaline CO2 electrolyzers with a supporting electrolyte at the anode (anolyte). Herein, we investigate the effect of the anolyte on the electrode properties such as catalyst utilization, ionic accessibility etc. of a CO2 reduction cathode using electrochemical techniques and cell configurations that avoid the complexities related to co-electrolysis. Using 1 M KOH as the anolyte and a Cu gas-diffusion-electrode with low Nafion content as the model CO2 reduction electrode, we find that electrode capacitance (proxy for electrochemically active surface area) and ionic conductivity inside the cathode increase approximately 4 and 447 times, respectively, in the presence of KOH. Liquid anolyte wets the electrode’s pore structure more efficiently than capillary condensation of the feed water vapor. The ionomer coverage is very low, and its distribution inside the electrode is highly fragmented. Surface ion conduction mechanisms inside the electrode are orders of magnitude lower than the bulk ion conduction in presence of anolyte. This study shows that when an anolyte (e.g., KOH) is used, catalyst utilization and ionic accessibility inside the electrode increase significantly.

Elucidation of Critical Catalyst Layer Phenomena toward High Production Rates for the Electrochemical Conversion of CO to Ethylene

“…This contrasts with previous experiments using anodes supported on Toray diffusion media, where we did not observe a needed break-in period for the same performance. Leveraging the understanding revealed both from our prior work, which highlighted the impact of anolyte crossover on catalyst utilization and R CL , OH – inside the cathode as well as the insight gleaned here, showcasing the relationship between R CL , OH – and ethylene FE, it is apparent that the lower porosity Ti PTLs (53–56%) prevents fast KOH crossover across the membrane to the cathode and slows down the wetting of cathode pores. This decreases the OH – conductivity and catalyst utilization inside the electrode.…”

“…This is exemplified by the fact that the 0% Nafion electrodes maintain high ethylene FE at high current densities (Figure ). We have previously published a method to measure electrode-scale properties of a Cu cathode that utilizes EIS . These properties include, but are not limited to, capacitance (a qualitative measure of catalyst utilization), ionic conductivity ( R CL , OH – ), and catalyst–ionomer interactions (gleaned from the ratio of capacitance at low and high RH).…”

“…31 While this is true for the abundance of configurations that employ a flowing catholyte, we recently demonstrated that even for pure gas phase cathodes, anolyte crossover dictates ionic accessibility of the cathode catalyst. 32 Here, we extend that prior diagnostic development effort and understanding to examine the impact of catalyst ink dispersion and deposition techniques on electrode level OH − conduction and catalyst/ionomer interactions in pressed MEA systems. Using a combination of techniques, we describe fundamental characteristics that COR electrodes should possess to promote catalyst utilization and increase product selectivity for MEA style COR devices.…”

“…We have previously published a method to measure electrode-scale properties of a Cu cathode that utilizes EIS. 32 These properties include, but are not limited to, capacitance (a qualitative measure of catalyst utilization), ionic conductivity (R CL , OH − ), and catalyst−ionomer interactions (gleaned from the ratio of capacitance at low and high RH). Here, we extend the original technique development to elucidate the impact of ink mixing and deposition on these properties.…”

“…In contrast, when the electrode is in contact with an electrolyte, as in many COR and CO 2 R configurations, ionic accessibility to catalyst sites can be dictated by the electrolyte and the ionomer’s role is not as vital . While this is true for the abundance of configurations that employ a flowing catholyte, we recently demonstrated that even for pure gas phase cathodes, anolyte crossover dictates ionic accessibility of the cathode catalyst . Here, we extend that prior diagnostic development effort and understanding to examine the impact of catalyst ink dispersion and deposition techniques on electrode level OH – conduction and catalyst/ionomer interactions in pressed MEA systems.…”