Beyond Intrinsic Size Effects: Regulating Multiphase Transport and Interfacial Microenvironments within Catalyst Layers for CO2 Electrolysis

Electrochemical CO2 reduction to multicarbon (C2+) products under industrial current densities in membrane electrode assemblies (MEA) is heavily governed by the coupling of mass transport and interfacial microenvironments. Here, we identify catalyst particle size as a key structural descriptor that simultaneously regulates gas/water transport and ionic conduction. Using cubic Cu2O particles with different edge lengths as model platforms, we reveal a volcano-shaped relationship between particle size and C2+ selectivity, with Cu2O-3/GDE achieving a peak C2+ selectivity of 70%. Combined structural, electrochemical, spectroscopic, and time-resolved analyses indicate that Cu2O-3/GDE establishes a more balanced multiphase transport environment. This architecture maintains continuous ionic conduction while mitigating the high-rate gas-transport limitations associated with densely packed small-particle layers and the interfacial diffusion resistance associated with thick ionomer regions in large-particle layers. This work bridges catalyst particle architecture, dynamic transport, and interfacial environments for high-rate CO2 electrolysis.