Improving Gas Diffusion Electrodes for Electrochemical CO2 Reduction: Influence of the Ionomer

Electrochemical CO2 reduction is a promising way of harnessing greenhouse gas as an abundant resource for the production of value-added hydrocarbons. By employing copper as the catalyst, CO2 can be converted to multi-carbon compounds such as ethylene. Highly porous gas diffusion electrodes (GDEs) enable efficient contacting of gaseous CO2 and the electrolyte. Copper-coated GDEs are commonly manufactured by depositing a mixture of copper nanoparticles and ionomer on a gas diffusion layer. The ionomer not only acts as a binder, but also influences the GDE’s wetting behavior and can improve mass transport within the catalyst layer due to the combination of a hydrophobic PTFE-backbone and hydrophilic ion groups. Thus, optimizing the type and content of ionomer in the catalyst layer is of utmost importance. In this study, we investigate how the composition of catalyst layers affects the faradaic efficiency (FE) toward ethylene during electrochemical CO2 reduction. For this purpose, we fabricate different GDEs with constant copper loading but varied ionomer content and type (Nafion, Sustainion). To assess the performance of the GDEs, we conduct electrolysis experiments in a flow cell (Flex-E-Cell) setup with KHCO3 as the catholyte. Preliminary screening experiments at up to 50 mA cm-2 reveal a strong effect of the Nafion/copper mass ratio (NCR) on the FE toward ethylene. Within the investigated range (NCRs of 0.1, 0.2, 0.3, 0.4 and 0.5), we identify a sweet spot at an NCR of approximately 0.4. We hypothesize that at higher NCRs negative effects, such as decreasing porosity and an increasingly acidic pH, outweigh the improved mass transport. To assess the performance of the different GDEs at industrially relevant conditions, we perform constant current experiments at ≥ 100 mA cm-2. Here, we corroborate the previously observed trend and find that increasing the NCR from 0.2 to 0.4 doubles the maximum observed FE toward ethylene. When replacing Nafion with an anion exchange ionomer such as Sustainion, the total FE toward carbon products remains nearly unchanged. However, the selectivity shifts from ethylene toward single-carbon species such as CO and formic acid. These results highlight that ionomers play a decisive role in hydrogen evolution mitigation and tuning of the product spectrum during electrochemical CO2 reduction. Figure 1