Membrane-free CO2 hydrogenation electrolyzer for salt precipitation management in acidic electrochemical CO2 reduction

Electrochemical CO2 reduction (ECR) in acidic electrolytes minimizes CO2 loss and carbonate formation issues, allowing for high CO2 utilization efficiency and showing good potential for practical CO2 upgrading applications. However, in the membrane-based electrolyzer, the proton transfer efficiency across the membrane from the anolyte to the catholyte is crucial for the stability of the catholyte pH in acidic ECR, especially at high current density and during long-term electrolysis. Here, we investigate the effects of proton transfer efficiency and salt precipitation in different acidic ECR electrolyzer and propose a membrane-free CO2 hydrogenation electrolyzer, which couple CO2 reduction and hydrogen oxidation. This electrolyzer design effectively maintains a stable electrolyte pH during long-term electrolysis, and simultaneously achieves high Faradaic efficiency for HCOOH production, high single-pass carbon utilization efficiency, and a lower cell voltage. At a current density of 100 mA cm−2, the system requires only 1.7 V to achieve a 90% HCOOH Faradaic efficiency and demonstrates stable operation for 208 hours. Acidic electrochemical CO2 reduction promises efficient carbon use, but maintaining stable acidity at high current remains a major challenge. Here, the authors demonstrate a membrane-free CO2 hydrogenation electrolyzer that stabilizes pH and enables efficient and stable formic acid production.