Long Period Voltage Oscillations Associated with Reaction Changes between CO2 Reduction and H2 Formation in Zero-Gap-Type CO2 Electrochemical Reactor

Zero-gap-type reactors with gas diffusion electrodes (GDE) that facilitate the CO₂ reduction reaction (CO₂RR) are attractive due to their high current density and low applied voltage. These reactors, however, suffer from salt precipitation and anolyte flooding of the cathode, leading to a short lifetime. Here, using a zero-gap reactor with a transparent cathode end plate, we report periodic voltage oscillations under constant current operation. Increases in cell voltages occur at the same time as the reactor switches from the hydrogen evolution reaction (HER) to predominant CO₂RR; decreases in cell voltage occur with the switch from the CO₂RR to HER. Further, real time visual observations show that salt precipitation occurs during the CO₂RR, whereas salt dissolution occurs during the HER. Slow flooding triggers the transition from the CO₂RR to HER. A number of processes combine to slowly reduce the water content in the microporous layer, which triggers the transition back to the CO₂RR.