(114a) Understanding the Impact of Mass Transport on Electrochemical Carbon Dioxide Reduction Reaction

Electrochemical CO₂ reduction reaction (CO₂RR) represents a promising strategy for converting CO₂ into valuable chemicals under ambient condition. In early studies, CO₂RR was conducted in batch cells, where the reaction rates were limited due to the low solubility of CO₂ in aqueous electrolytes. The introduction of gas diffusion electrodes (GDEs) can alleviate these limitations, promoting CO₂RR rates to industrial levels. Based on GDEs, flow cells and membrane electrode assemblies (MEAs) are two representative configurations for electrochemical CO₂RR. Despite significant advancements in CO₂RR research based on these systems, a fundamental understanding of the differences and connections between them remains limited, impeding the transferability of conclusions from one configuration to another.

In this work, we discovered that flow cells exhibit high selectivity for ethylene, while MEAs show excellent selectivity for CO at low current density (< 200 mA cm^-2) when Cu was used as the catalyst. Increasing the current density to 400 mA cm^-2 will lead to an increase in Faraday efficiency of H₂ in flow cells and ethylene in MEAs, highlighting the distinct microenvironment of catalysts in different configurations. The performance disparity between these two configurations largely diminished upon the introduction of a non-conductive protective layer in the flow cell. We proposed that the thickness of the electrolyte layer covering the catalyst, which corresponding to the CO₂ diffusion length, is a crucial factor governing both CO₂RR activity and selectivity. Furthermore, while improving CO₂ mass transport did not necessarily enhance the selectivity of multi-carbon product, we found that it facilitated CO₂RR under low CO₂ partial pressures, offering a promising strategy for upgrading waste carbon emissions. These findings provide valuable insights into optimizing GDE structures and highlight the importance of mass transport in achieving efficient CO₂ conversion.