Spatial reactant distribution in CO2 electrolysis: Balancing CO2 utilization and Faradaic Efficiency

27 September 2021, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

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/cm2 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 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.

Keywords

CO2 electroreduction
membrane electrode assembly
CO2 utilization
Faradaic efficiency
Spatial variations

Supplementary materials

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Title
Spatial reactant distribution in CO2 electrolysis- Balancing CO2 utilization and Faradaic efficiency
Description
Electrochemical technologies remain a promising technological avenue for the reduction of ‘non-electrifiable’ carbon emissions associated with dense chemicals and fuels such as kerosene and plastics that are currently derived from fossil fuels. To measurably reduce emissions, however, electrochemical technologies are needed at global scales producing 100’s of megatons of products per year. As research matures, greater emphasis will then be required on assessing the scalability of larger area electrochemical systems to ensure that the promising performance achieved at smaller scales translates to larger systems. Research into maximizing CO2 utilization, understanding spatial performance, and local operando measurements techniques will then be required in the research community to assess such systems. The work presented here provides a first step towards understanding the spatial Faradaic efficiencies towards CO2 reduction products within a membrane electrode assembly cell at higher CO2 utilization rates.
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