Distribution of Speciation and Activity Across the Catalyst Layer during CO₂ Electroreduction in Membrane Electrode Assembly

Membrane electrode assembly (MEA) configuration offers a promising reactor architecture for scaling up electrochemical CO 2 reduction reaction (CO 2 RR). Yet the spatial heterogeneity of speciation and activity within the catalyst layer (CL) remains inadequately understood owing to the compacted layered structure. Here, we develop an innovative in situ cross-sectional Raman MEA cell capable of spatially mapping intermediates across Cu-based CLs under practical current densities (up to 200 mA cm −2 ). Raman spectra clearly reveal that regions with linearly adsorbed CO exhibit higher CO 2 RR activity than those with bridge-bonded CO. The distribution of surface species is significantly heterogeneous across the CL and depends sensitively on current density and CO 2 feed humidity, which are attributed to concentration gradients of water and CO 2 across the CL.