Identification of Active Regions in a Catalyst Layer on a Gas Diffusion Electrode in the Electroreduction of CO₂

Although a gas diffusion electrode (GDE) has been widely employed in the electrochemical CO₂ reduction reaction (CO₂RR) to effectively mitigate mass transport limitations, knowledge of the active region within the catalyst layer on the GDE remains incomplete. In this work, we employed a thin layer of Bi as a reporter to identify the active region within the Ag catalyst layer by taking advantage of the distinct selectivity of these two metals in the CO₂RR. Through manipulating the position of the Bi layer within the Ag layer, we showed that at 100 mA cm^-2, the active region was located between 150 and 600 nm from the PTFE substrate. Current density, feed gas humidity, and the hydrophobicity of the substrate were found to impact both the location and thickness of the active region. Further, operando Raman spectroscopy capable of characterizing the cross-section of the catalyst layer was developed to locate the active region inside the catalyst layer.