(Invited) Anion Exchange Membrane and Ionomer Development for Electrochemical CO2 Reduction

Recent developments have proven the economic potential of electrochemical reduction of CO2 to value added chemicals, where single cells are now capable of achieving high energetic efficiencies at industrially relevant current densities. 1 These advances are in no small part due to the increased ionic conductivity, hydroxide stability and commercial availability of anion exchange membranes (AEMs). However, there currently exists little understanding as to how these materials affect the efficiency of CO2 conversion devices because the research community is only now beginning to understand the variety and complexity of the transport processes involved. 2 In collaboration with Ionomr Innovations Inc. and the National Research Council of Canada, and as part of the Energy for Clean Materials Challenge Program, we have made advances in the understanding of how AEM properties affect device performance and how we can develop materials tailor-made for CO2 electrolyser technology. Here, we demonstrate the development of a zero-gap single cell design, utilizing first generation Aemion® materials for the conversion of CO2 to CO with an energetic efficiency of 40% at 200 mA cm-2. 3 Despite the initially high energetic efficiency, we demonstrate how the crossover of carbonate dianions results in the reduction of anolyte pH and deconvolute how this results in a diminished cell efficiency over extended operation. From this, we show how functionalization of the polymer electrolyte structure can reduce this degradation mechanism while retaining high energetic efficiencies. In addition, we demonstrate how under milder electrolysis conditions, the total cell efficiency has a significant dependency on the flux of alkali metal cationic species from the supporting anolyte to the cathode. We show that due to the large promotion effect of cations for the electrochemical CO2 reduction, AEM design not only influences ohmic resistances in the cell, but also greatly affects the charge transfer resistance (RCT) of the cathode to a much greater extent than other electrochemical conversion devices. We thus make correlations between water permeability and perm-selectivity of AEMs to the overall CO2 conversion efficiency. We then discuss the incorporation of anion exchange ionomers in the cathode catalyst layer of CO2 electrolysis cells and how the ionomer parameters define the efficiency and selectivity of Ag catalysts towards electrochemical CO2 reduction. Through this work, we demonstrate the influencing factors of AEM and ionomer materials on the efficiency of electrochemical CO2 conversion and conclude that further advances are paramount for the adoption of this promising technology, which is integral in closing the carbon loop of the petrochemical industry and meeting our wider climate change targets. References: P. De Luna et al., Science, 364, eaav3506 (2019). D. Salvatore, C. Gabardo, A. Reyes, and S. Holdcroft, Nat. Energy, 6, 339–348 (2021). P. Mardle, S. Cassegrain, F. Habibzadeh, Z. Shi, and S. Holdcroft, J. Phys. Chem. C, 125, 25446–25454 (2021). Figure 1