Membrane-Electrode-Assembly Design Parameters and Their Impact on CO2 Reduction

Abstract

CO2 electrolyzers that operate at high current densities and selectivities are essential for commercializing CO2 electroreduction and for a sustainable, CO2-neutral green energy paradigm. As part of this, membrane-electrode assemblies (MEAs) are critical components that enable industrial-scale electrochemical CO2 conversion to value-added fuels and chemicals. They consist of humidified gaseous feeds at one or both electrodes and a solid, ion-conducting polymer (ionomer) as the electrolyte. Such assemblies can overcome the experimentally observed ohmic and mass-transfer limitations characteristic of aqueous gas-diffusion electrode (GDE) and planar systems, respectively, which limit their maximum achievable current densities and render these systems impractical for industrial implementation.Foundational design parameters that characterize the ionomer-based catalyst layers unique to MEA systems include the ionomer-to-catalyst (I:Cat) ratio, the catalyst loading, and the catalyst layer thickness. In this talk, the results of a comprehensive experimental study on a Ag-cathode CO2 reduction (CO2R) MEA will be reported, where we systematically analyzed the effects of these design parameters on CO2R MEA performance and selectivity. An optimum, intermediate I:Cat ratio was uncovered and rationalized based on electrochemically-active surface area and ionomer distribution arguments. The effect of decreasing the catalyst loading and/or thickness corroborates previous modeling work, resulting in an optimum range for catalyst layer utilization for CO2R. The effect of MEA anolyte/exchange solution concentration on system performance is also explored, yielding an efficient Ag-cathode CO2R MEA that operates at 200 to 1000 mA/cm2, at CO FEs of 78 to 91%, and with an area-specific resistance of 1 Ω*cm2.The scientific and experimental design learnings from our Ag-cathode MEA studies are subsequently applied to a Cu-cathode MEA, which has farther reaching and impactful applications due to its ability to produce value-added chemical precursors like ethylene and ethanol. The Cu-MEA cathodic I:Cat ratio and catalyst loading are fixed based on settings from the optimized Ag-MEA and additional catalyst layer parameters and operating conditions are examined. This systematic exploration provides deeper insights into the underlying interrelationships between physical phenomena in the Cu-MEA system. Overall, these studies provide a fundamental understanding of the relationships between critical design parameters and inherent operating and materials tradeoffs for practical CO2 reduction devices. Acknowledgements The authors gratefully acknowledge Lawrence Berkeley National Laboratory’s Laboratory Directed Research and Development (LDRD) Grant for funding.