Elucidating key mechanistic processes during acidic CO2 electroreduction on gas diffusion electrodes towards stable production of formic acid

The electrochemical reduction of CO2 using renewable electricity could be a sustainable alternative to produce formic acid. Although using bulk acidic electrolyte offers system and downstream processing benefits, neutral to alkaline electrolyte systems prevail due to challenge suppressing hydrogen evolution in acidic environment. In recent years, it has been shown that HER could be mitigated by confining the OH- being generated and constraining the movement of H3O+ towards reaction surface, thereby producing an alkaline reaction environment within the electrode. Herein, we have demonstrated how a porous and highly hydrophobic Bi-based gas diffusion electrode enables remarkably efficient bulk acidic electrolysis. Importantly, the local alkaline reaction environment has been proven via electrochemical impedance spectroscopy and pH visualization. Furthermore, optical monitoring of electrolyte conditions through a transparent cell sheds light on the CO2 recovery process in the acidic system, promoting a better technical practice in handling with electrochemical cells and CO2 recirculation. Finally, HCOOH accumulation as the key degradation reason is unveiled through investigation into the impact of operating modes on the stability at high current density, where earlier studies failed to adequately address. It is believed that this valuable insight could facilitate stable and efficient formic acid production in future work.