Long Period Voltage Oscillations Associated with Reaction Changes between CO 2 Reduction and H 2 Formation in Zero-Gap-Type CO 2 Electrochemical Reactor.

Zero-gap-type reactors with gas diffusion electrodes (GDE) that facilitate the CO 2 reduction reaction (CO 2 RR) 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 CO 2 RR; decreases in cell voltage occur with the switch from the CO 2 RR to HER. Further, real time visual observations show that salt precipitation occurs during the CO 2 RR, whereas salt dissolution occurs during the HER. Slow flooding triggers the transition from the CO 2 RR to HER. A number of processes combine to slowly reduce the water content in the microporous layer, which triggers the transition back to the CO 2 RR.