In Situ Investigation of Cation Transport across Anion Exchange Membranes in Membrane Electrode Assembly Electrolyzers

Alkali metal cations play a critical role in determining the performance of various electrocatalytic systems by modulating the local reaction environment. In membrane electrode assembly (MEA) CO₂ electrolysis systems, excessive cation crossover through ion-exchange membranes can result in salt precipitation. However, the mechanisms governing cation transport across these membranes under different operating current densities, as well as their accumulation at the catalytic interface and within the electrolyzer flow fields, are not fully understood. In this work, we investigate these phenomena using in situ X-ray fluorescence spectroscopy. Employing Cs⁺ as a model cation, we examine its transport across a representative anion exchange membrane (AEM, Sustainion) and cation exchange membrane (CEM, Nafion) under various operating regimes, including potential step-up, potential step-down, and pulsed potential conditions. We observe distinct variations in cation concentrations within the cathodic flow fields, depending on the applied operation mode, as well as clear differences in transport behavior between the AEM and CEM. Our results reveal that during pulsed potential operation with an AEM, cations accumulated at the catalytic interface can transiently diffuse into the cathodic flow field rather than crossing over to the anode. This behavior suggests that pulsed potential operation may, in fact, accelerate salt precipitation, underscoring the need for careful consideration when it is employed as a mitigation strategy.