Influence of Headgroups in Ethylene-Tetrafluoroethylene-Based Radiation-Grafted Anion Exchange Membranes for CO₂ Electrolysis

The performance of zero-gap CO2 electrolysis (CO2E) is significantly influenced by the membrane’s chemical structure and physical properties due to its effects on the local reaction environment and water/ion transport. Radiation-grafted anion-exchange membranes (RG-AEM) have demonstrated high ionic conductivity and durability, making them a promising alternative for CO2E. These membranes were fabricated using two different thicknesses of ethylene-tetrafluoroethylene polymer substrates (25 and 50 μm) and three different headgroup chemistries: benzyl-trimethylammonium, benzyl-N-methylpyrrolidinium, and benzyl-N-methylpiperidinium (MPIP). Our membrane characterization and testing in zero-gap cells over Ag electrocatalysts under commercially relevant conditions showed correlations between the water uptake, ionic conductivity, hydration, and cationic-head groups with the CO2E efficiency. The thinner 25 μm-based AEM with the MPIP-headgroup (ion-exchange capacities of 2.1 ± 0.1 mmol g–1) provided balanced in situ test characteristics with lower cell potentials, high CO selectivity, reduced liquid product crossover, and enhanced water management while maintaining stable operation compared to the commercial AEMs. The CO2 electrolyzer with an MPIP-AEM operated for over 200 h at 150 mA cm–2 with CO selectivities up to 80% and low cell potentials (around 3.1 V) while also demonstrating high conductivities and chemical stability during performance at elevated temperatures (above 60 °C)

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 reveal 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+ quickly to 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 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