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 CO2 reduction reaction (CO2RR). 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 CO2RR 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 CO2 feed humidity, which are attributed to concentration gradients of water and CO2 across the CL.