The interface between the catalyst layer (CL) and the micro porous layer (MPL) in proton exchange membrane fuel cells (PEMFCs) has been studied in ex-situ experiments. The interfacial morphology, specifically the area, origin and dimensions of interfacial gaps in between compressed CLs and MPLs were investigated with high-resolution X-ray micro computed tomography. In a separate experiment, the electric contact resistance (CR) was evaluated using a custom four-point-probe setup for CLs with different compositions as a function of compression pressure and relative humidity (RH).

The interfacial gap area (fraction of the interface separated by gaps) was higher for gas diffusion layers (GDL, with MPL) – catalyst coated membrane (CCM) assemblies with large differences in the surface roughness of the CL and MPL. The interfacial gap area decreased with increasing compression and with increased similarity in roughness. Relatively large continuous gaps were found in proximity of specific cracks in the MPL. These are hypothesized to form due to the presence of large pores on the surface of the GDL, in which the MPL sags and cracks. Relatively small gaps form by means of the regular surface roughness features throughout the CL-MPL interface. Smaller pores on the GDL surface serving as substrate for the MPL could reduce the number of MPL crack-induced gaps. Moreover, adjusting the CL and MPL surface roughness parameters to achieve similar orders of roughness can result in fewer enclosed gaps, and therefore, enhance the mating characteristics.

The electric CR followed a similar trend for all the CL compositions, featuring a non-linear decrease in resistance with the increase in the compression pressure. Moreover, the CR was also found to increase with the ionomer content in the CL and with the increase in RH. Physical characterization of the CL surfaces revealed that this increase in the ionomer content enhances the surface roughness features and the surface coverage by the ionomer, both of which affecting the electrical CR towards the MPL. With increasing RH, the CR values doubled for all CL compositions as a result of humidity induced ionomer swelling with the uptake of water.