Characterization of interfacial morphology in polymer electrolyte fuel cells: Micro-porous layer and catalyst layer surfaces

The interface between the micro-porous layer (MPL) and the catalyst layer (CL) can have an impact on thermal, electrical and two-phase mass transport in a polymer electrolyte fuel cell (PEFC). However, there is scant information available regarding the true morphology of the MPL and CL surfaces. In this work, optical profilometry is used to characterize the MPL and CL surfaces at the sub-micron level scale to gain a better understanding of the surface morphology. Selected MPL and CL surfaces were sputtered with a thin layer of gold to enhance the surface reflectivity for improved data acquisition. The results show that, for the materials tested, the MPL surface has a relatively higher roughness than the CL surface, indicating the potential dominance of the MPL surface morphology on the local transport and interfacial contact across the MPL|CL interface. The level of roughness can be on the order of 10 μm peak height, which is significant in comparison to other length scales involved in transport, and can result in significant interfacial water storage capacity (approximately 6-18% of the total water content in a PEFC [37]) along this interface. Another surface characteristic that can have a profound influence on multi-phase transport is the existence of deep cracks along the MPL and CL surfaces. The cracks on MPL and CL surfaces are observed to differ significantly in terms of their orientation, size, shape, depth and density. The areal crack density of the CL tested is calculated to be 3.4 ± 0.2%, while the areal crack density of the MPL is found to vary from 2.8% to 8.9%. The results of this study can be useful to understand the true nature of the interfacial transport in PEFCs.