Impacts of Unintended Cation Crossover through Anion Exchange Membranes on the Operation of Zero-gap Cu-based CO2 Electrolysers

El-Nagar, Gumaa A. and Haun, Flora and Gupta, Siddharth and Stojkovikj, Sasho and Mayer, Matthew T. (2022) Impacts of Unintended Cation Crossover through Anion Exchange Membranes on the Operation of Zero-gap Cu-based CO2 Electrolysers. In: 2nd Symposium on Insights into Gas Diffusion Electrodes: From Fundamentals to Industrial Applications, 5-7 Sept 2022, Magdeburg, Germany.

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Abstract

CO2 electroreduction (CO2ER) in gas-fed membrane electrode assemblies enable CO2ER at industrially relevant rates, and hence provide a promising strategy to scale-up this technology for future practical applications. Electrolysers based on anion exchange membranes (AEMs) have been introduced as a strategy for preventing undesired H2 evolution at the cathode during CO2ER under zero-gap, catholyte-free conditions. One expects that the anolyte concentration should not significantly impact the performance of AEM-based CO2 cathodes, since the (an)ion conduction direction is presumed to be from cathode to anode, maintained mainly by CO32- and HCO3- derived from CO2. In contrast to this expectation, we observed that varied anolyte concentrations have substantial effects on the behaviors of zero-gap Cu-based cathodes. When using pure water or dilute alkaline anolytes, CO was produced as dominant product, while concentrated anolytes (>0.1M) directed the selectivity towards C2+ products. The parasitic H2 evolution reaction was also influenced, being more suppressed for the dilute anolytes. Typically, selectivity trends for Cu-based catalysts are correlated to surface speciation (e.g. Cu oxidation states) and/or morphological changes during CO2ER. To control for these possibilities, we assessed both aspects as a function of anolyte employed. Using an adapted electrochemical reactor with X-ray transparent window, we conducted operando X-ray absorption spectroscopy (XAS) to track the catalyst in the zero-gap cell under true operation conditions. The results indicate complete reduction of the oxidized Cu surface occurs for all the investigated anolytes. Additionally, ex-situ XRD and SEM characterization revealed no obvious structural or morphological changes between samples tested with different anolytes. These observations suggest the varied selectivities we see are not simply explainable by composition or morphological/structural effects. Interestingly, under certain conditions, the unintended but significant migration of K+ cations from the anolyte to the cathode surface was observed. The amount of K+ detected at the cathode depends on the anolyte concentration, with a threshold concentration of approx. 0.1 M KOH required before significant amounts of K+ crossover occurs, correlating with the selectivity switching from CO to C2+ products. This strongly suggests the absence or presence of cations can be a major influencer of CO2ER selectivity, even in catholyte- free configurations using AEMs. Moreover, cations were found to crossover even under open circuit, and therefore may be diffusing spontaneously along with water uptake by the membrane.

Item Type:	Conference or Workshop Item (Speech)
Subjects:	Engineering and Technology > Chemical engineering Natural sciences > Chemical sciences Natural sciences > Earth and related environmental sciences Engineering and Technology > Environmental engineering Engineering and Technology > Materials engineering
Divisions:	Faculty of Technology
Depositing User:	Saso Stojkovik
Date Deposited:	11 Mar 2024 08:36
Last Modified:	11 Mar 2024 08:36
URI:	https://eprints.ugd.edu.mk/id/eprint/33841

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