Spatial reactant distribution in CO₂ electrolysis: balancing CO₂ utilization and faradaic efficiency

The production of value added C1 and C2 compounds within CO₂ electrolyzers has reached sufficient catalytic performance that system and process performance - such as CO₂ utilization - have come more into consideration. Efforts to assess the limitations of CO₂ conversion and crossover within electrochemical systems have been performed, providing valuable information to position CO₂ electrolyzers within a larger process. Currently missing, however, is a clear elucidation of the inevitable trade-offs that exist between CO₂ utilization and electrolyzer performance, specifically how the faradaic efficiency of a system varies with CO₂ availability. Such information is needed to properly assess the viability of the technology. In this work, we provide a combined experimental and 3D modelling assessment of the trade-offs between CO₂ utilization and selectivity at 200 mA cm^-2 within a membrane-electrode assembly CO₂ electrolyzer. Using varying inlet flow rates we demonstrate that the variation in spatial concentration of CO₂ leads to spatial variations in faradaic efficiency that cannot be captured using common 'black box' measurement procedures. Specifically, losses of faradaic efficiency are observed to occur even at incomplete CO₂ consumption (80%). Modelling of the gas channel and diffusion layers indicated that at least a portion of the H₂ generated is considered as avoidable by proper flow field design and modification. The combined work allows for a spatially resolved interpretation of product selectivity occurring inside the reactor, providing the foundation for design rules in balancing CO₂ utilization and device performance in both lab and scaled applications.