Quantitative Analysis of the Influence of Cation Crossover on CO 2 Reduction Faradaic Efficiency Based on Perfluorosulfonic Acid Membrane

Electrochemical CO 2 reduction using membrane electrode assemblies (MEAs) based on cation exchange membranes (CEMs) offers a promising route for carbon conversion. Owing to the intrinsic anion-blocking properties of CEMs, this configuration effectively mitigates the low CO 2 utilization problem commonly observed in anion exchange membrane-based systems. However, the acidic cathode environment in CEM-based MEAs intensifies the competing hydrogen evolution reaction, severely limiting CO 2 reduction selectivity. As a result, this system remains underexplored, with limited mechanistic understanding of how ion transport governs reaction selectivity. This study reveals that the proportion of alkali metal cations in charge carriers is positively correlated with the faradaic efficiency of CO 2 reduction, by quantitatively measuring the transport of cations in the membrane. Furthermore, factors such as the concentration and type of anolyte, current density, and anode structure all influence the transport of cations, thereby affecting the cathode local environment and the electrochemical CO 2 reduction selectivity. Leveraging these principles, we further introduced an immobilized cation layer between the cathode and the membrane, which achieved a CO faradaic efficiency exceeding 90% over a current density range of 50 to 300 mA cm −2 .