The Conventional Gas Diffusion Electrode May Not Be Resistant to Flooding during CO2/CO Reduction

The electrochemical CO2 or CO reduction to chemicals and fuels using renewable energy is a promising way to reduce anthropogenic carbon emissions. The gas diffusion electrode (GDE) design enables low-carbon manufacturing of target products at a current density (e.g., 500 mA cm−2) relevant to industrial requirements. However, the long-term stability of the GDE is restricted by poor water management and flooding, resulting in a significant hydrogen evolution reaction (HER) within almost an hour. Therefore, the optimization of water management in the GDE demands a thorough understanding of the role of the gas diffusion layer (GDL) and the catalyst layer (CL) distinctively. In a recent paper published in J. Electrochem. Soc. (J. Electrochem. Soc. 169, 104506, 2022, DOI 10.1149/1945-7111/ac9b96), Wu and co-workers tried to address the issues and performed a systematic study to investigate the role of GDL in water management and to reveal the water transport mechanism in porous GDE during CO2RR/CORR.

To conduct the investigation, four GDEs with independently adjusted hydrophobicity for CL and GDL were fabricated and tested in the flow cell for CORR to decouple their effect on water management. Then, a CL-free GDE with enhanced GDL hydrophobicity through polydimethylsiloxane (PDMS)-coating was designed to directly observe the time-dependent variation in GDL hydrophobicity as CORR proceeds. The authors also implied in-situ optical microscope and ex-situ micro-computed tomography (micro-CT) to qualitatively and quantitively characterize the water distribution in the GDL, respectively.

According to the experimental observations, increasing the GDL and CL hydrophobicity can alter the water flow pattern inside the GDE, but it cannot prevent the GDE from flooding. The conventional GDE structure has shown excellent stability in other reactions like water electrolysis and fuel cell. However, the electrochemical CO2 or CO reduction possesses more sophisticated reaction scenarios. First, the electrochemical CO2 or CO reduction involves reactants in both gas and liquid phases, rather than the simple liquid phase reactant in water electrolysis. Thus, the GDE must compromise the activity of gas and liquid phase reactants in the CL to ensure sufficient reaction kinetics. Second, the electrochemical CO or CO2 reduction involves electrolytes in various concentrations rather than pure water in the fuel cell. Hence, electro-osmosis will speed up the flooding and thereby restrict gas availability. The conventional GDE structure is intrinsically prone to flooding, resulting in poor stability of CORR and CO2RR.

Based on the obtained results, the authors suggest that future research on solving flooding should focus on changing the reaction conditions to neutral electrolytes like pure water or transforming the structure of GDE for long-term stable operation of CO2RR and CORR.