Spatial reactant distribution in CO 2 electrolysis: balancing CO 2 utilization and faradaic efficiency.

The production of value added C1 and C2 compounds within CO 2 electrolyzers has reached sufficient catalytic performance that system and process performance - such as CO 2 utilization - have come more into consideration. Efforts to assess the limitations of CO 2 conversion and crossover within electrochemical systems have been performed, providing valuable information to position CO 2 electrolyzers within a larger process. Currently missing, however, is a clear elucidation of the inevitable trade-offs that exist between CO 2 utilization and electrolyzer performance, specifically how the faradaic efficiency of a system varies with CO 2 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 CO 2 utilization and selectivity at 200 mA cm -2 within a membrane-electrode assembly CO 2 electrolyzer. Using varying inlet flow rates we demonstrate that the variation in spatial concentration of CO 2 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 CO 2 consumption (80%). Modelling of the gas channel and diffusion layers indicated that at least a portion of the H 2 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 CO 2 utilization and device performance in both lab and scaled applications.