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Cell designs that integrate membrane-electrode assemblies (MEAs) with highly selective catalysts are a promising route to reduce ohmic losses and achieve high energy efficiency in CO2 reduction at industrially relevant current densities. In this work, porous silver filtration membranes are demonstrated as simple and efficient gas-diffusion electrodes for CO2 reduction to CO at high current densities in an MEA-type device. A partial current density for CO of up to ca. 200 mA cm-2 was achieved at a cell voltage of ca. 3.3 V, in tandem with minimal H2 production. However, the analysis of cathodic and anodic outlet streams revealed that CO2 cross-over across the anion-exchange membranes, mostly in the form of CO32- but partially as HCOO- generated over the cathode, actually exceeds the amount of CO2 converted to the target product, resulting in a poor utilization of the reactant and in the early onset of mass transfer limitations. In addition, CO2 cross-over leads to a nonstoichiometric decrease of the outlet flow rate from the cathodic compartment. This effect can lead to a substantial overestimation of catalytic performance if the inlet flow rate of CO2 is used as reference for calculating partial current densities and Faradaic efficiencies. The results of this work highlight the importance of carrying out a carbon balance, in addition to traditional measurements of activity and selectivity, to adequately assess the performance of CO2 reduction devices at high current densities, and inform future efforts aimed at mitigating membrane cross-over in MEA-type electrolyzers for CO2 reduction.
Larrazábal et al. (Fri,) studied this question.