Regulating Humidity-Dependent Catalyst-Layer Transport by an Ultrathin Carbon Interface for Low-RH PEMFCs

Proton exchange membrane fuel cells (PEMFCs) suffer substantial performance losses under low relative humidity (RH) because catalyst-layer dehydration compromises proton conduction and interfacial reactant transport. Here, commercial Pt/C was modified through polydopamine deposition followed by thermal treatment to form an ultrathin nitrogen-doped carbon coating (NCS-Pt/C), aiming to regulate hydration and the transport environment within the catalyst layer. The modification largely preserved the porous architecture of the parent Pt/C while increasing water vapor uptake and improving surface wettability. When incorporated into membrane electrode assemblies at a cathode Pt loading of 0.10 mg_Pt cm^-2, NCS-Pt/C delivered higher performance across 30-90% RH. The peak power density increased from 0.35 to 0.50 W cm^-2 at 30% RH, from 0.57 to 0.67 W cm^-2 at 60% RH, and from 0.67 to 0.77 W cm^-2 at 90% RH. Under low-RH conditions, NCS-Pt/C exhibited improved dry proton accessibility and lower apparent catalyst-layer proton-conduction resistance. Substantially lower catalyst-layer oxygen transport resistance was also observed at 90% RH. These results show that the carbon interfacial modification is associated with reduced humidity sensitivity, accompanied by coupled changes in catalyst-layer hydration, interfacial transport, and reaction kinetics, with the benefit becoming particularly pronounced under dry operating conditions.