Mass transport-dependent in situ Raman detection in CO/CO₂ electrolysis

In situ Raman spectroscopy has been widely employed in the CO₂/CO electroreduction field to extract mechanistic insights into reaction pathways toward product formation. However, most of the previous in situ Raman studies are based on H-type spectroelectrochemical cells, which constrain the mass transport to the catalyst surface, potentially affecting the coverage of key intermediates relevant to the Raman signals and even distorting the mechanistic understanding. Here, we present a systematic comparison for the in situ Raman detection of intermediates during CO₂/CO reduction between an H-type spectroelectrochemical cell and a GDE-type spectroelectrochemical flow cell. We found that cell configurations exert a minimal influence on the in situ Raman detection and analysis of surface-adsorbed intermediates during CO₂ reduction, which is likely linked to the high solubility of CO₂ in aqueous media. In contrast, during CO reduction, the severe CO mass transport limitations in H-type spectroelectrochemical cells significantly lower the formation and coverage of key intermediates, resulting in in situ Raman detection and analysis results that are distinct from those obtained using GDE-type spectroelectrochemical flow cells. Thereby, circumventing mass transport limitation is crucial for in situ Raman tests to unveil the underlying mechanism of electrolysis.