Species Transport in Hydrophilic Nafion-Coated Cu Catalysts for CO2 Electroreduction

Abstract

Ionomer coated catalysts have increasingly been utilized in the electrocatalytic reaction of CO2 to multi-carbon products on Cu-based GDEs1,2. Their proton conductivity coupled with the Donnan exclusion due to the charged immobilized ions have served as a means of tailoring the microenvironment of the catalyst to desired products. Whilst there are experimental efforts geared towards the tuning of the local environment to desired products, there is a lack of supporting computational models that elucidate ionic species profile within the Nafion coated catalyst in the presence of fixed negative sulfonate ion charges from Nafion and the negative charges from the electrocatalysts.On the other hand, hydrated Nafion ionomer in the presence of a supporting electrolyte forms a biphasic polymer matrix which allows the transport of CO2 in both the hydrophilic and hydrophobic regions of Nafion ionomer3. Recent literature has highlighted the transport of CO2 in hydrophobic domains but in not in hydrophilic domains4. In this work, we look a closer look at CO2 transport in hydrophilic domains. We combine mass transport generalized modified Poisson-Nernst-Planck (GMPNP) equations and molecular dynamics to elucidate the transport of reactant/product species in the local environment of Nafion coating on planar Cu catalysts.We find that the thickness of the Nafion coating, the presence of immobilized sulfonic charges, ionic concentration of electrolyte impacts the availability of CO2, pH and the concentration of cations which are important parameters for CO2 reduction reaction. We also provide a relationship between the pKa of Nafion and the local dielectric constant of the reaction’s microenvironment using molecular modelling and the corresponding impact of Nafion’s ability to effectively minimize carbonate reactions. Our results underscore the need to manage the catalyst-ionomer microenvironments for CO2 reduction on copper electrodes. This model in conjunction with further experimental endeavours can serve as a benchmark for other intricate porous catalyst models that are significant for optimizing catalyst performance for large scale CO2 reduction operations. Bibliography Bui, J. C. et al. Engineering Catalyst–Electrolyte Microenvironments to Optimize the Activity and Selectivity for the Electrochemical Reduction of CO2 on Cu and Ag. Acc. Chem. Res. 17, acs.accounts.1c00650 (2022).Kim, C. et al. Properties of the ionomer under CO 2 R-related environments Tailored catalyst microenvironments for CO 2 electroreduction to multicarbon products on copper using bilayer ionomer coatings. doi:10.1038/s41560-021-00920-8 (2022).Ren, X., Myles, T. D., Grew, K. N. & Chiu, W. K. S. Carbon Dioxide Transport in Nafion 1100 EW Membrane and in a Direct Methanol Fuel Cell. J. Electrochem. Soc. 162, F1221–F1230 (2015).García de Arquer, F. P. et al. CO2 electrolysis to multicarbon products at activities greater than 1 A cm−2. Science (80-. ). 367, 661–666 (2020). Figure 1