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