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.