Electroreduction of CO2 to valuable chemical products has been investigated as an attractive strategy for mitigating climate change. Although recent research efforts have improved the performance of CO2 electrolysis (i.e., low cell voltage, high product selectivity, high current density, and high durability), it is still insufficient for commercial processes. To reduce ionic resistance, the gap between the anode and cathode should be minimized. Therefore, a zero-gap electrolyzer, in which two electrodes are pressed against a separator, is a promising design for low-voltage operation.

Currently, polymer electrolyte membranes such as anion exchange membranes (AEMs), cation exchange membranes (CEMs), and bipolar membranes (BPMs) are used as separators in zero-gap CO2 electrolyzers. These membranes separate the anode and cathode, mediate ion conduction, suppress gas crossover, and prevent flooding of the gas diffusion cathode, which is critical for delivery of CO2 to active sites efficiently. Among polymer electrolyte membranes, AEM-based cells exhibit high selectivity at high current densities by suppressing the hydrogen evolution reaction (HER). However, the AEM technology is still immature. Even state-of-the-art AEMs should be improved for their ionic conductivity, chemical and mechanical stability, and costs.

Herein, we present a novel zero-gap electrolyzer with a porous diaphragm instead of a polymer electrolyte membrane. While porous diaphragms are prone to more gas crossover and flooding than polymer electrolyte membranes, we found that these problems can be prevented by balancing the bubble point of the diaphragm, CO2 pressure, and electrolyte pressure. The zero-gap electrolyzer with a porous diaphragm exhibited lower cell voltage than the conventional AEM, mainly because of the high mobility of ions in liquid phase. Furthermore, the product selectivity and current density of the diaphragm electrolyzer are comparable to those of the AEM electrolyzer.