Unveiling transport mechanisms of cesium and water in operando zero-gap CO₂ electrolyzers

In zero-gap CO₂ electrolyzers, maintaining the balance of water and cations is crucial. Excessive accumulation at the cathode causes performance degradation, leading to flooding and salt precipitation. Using operando wide-angle X-ray scattering and X-ray fluorescence techniques, we observed the dynamic evolution of H₂O and Cs⁺ inside a membrane electrode assembly. Our findings indicate that Cs⁺ movement across the membrane from the anode to the cathode is governed by migration and drags H₂O via electroosmosis. H₂O diffusion then allows Cs⁺ diffusion further within the gas diffusion electrode. When decreasing the applied voltage, the concentration gradient causes Cs⁺ to quickly diffuse back to the anode. The H₂O content in the macro-porous layer remains at the same level, thus showcasing an origin of gas diffusion electrode (GDE) flooding. By regulating the electrolyte concentration, we deconvolute the correlation of water and cations for selectivity changes. Our work underscores the significance of water/cation management strategies in zero-gap electrolyzers.