Probing Inside the Catalyst Layer on Gas Diffusion Electrodes in Electrochemical Reduction of CO and CO₂

Abstract Gas diffusion electrodes (GDEs) are widely used in electrochemical CO and CO 2 reduction reactions (CO (2) RR) in flow cells due to their ability to alleviate mass transport limitations of gaseous reactants. The flow cell configuration makes uniform distribution of reactants, intermediates, products, and speciation within the catalyst layer (CL) unlikely. In this work, a first‐of‐its‐kind in situ characterization technique capable of probing the cross section of the CL with confocal Raman spectroscopy was developed to investigate the speciation distribution across the Cu CL in CO (2) RR with a spatial resolution of ∼4 µm. In both CORR in alkaline medium and CO 2 RR in acidic electrolyte, the active region of CL was identified as that with the presence of the Raman band for adsorbed CO (CO ad ). The strong correlation of CO ad and CO 3 2− bands provides the first spectroscopic evidence that CO 2 RR only occurs in an alkaline microenvironment.