Impact of Compressive Stress on MEA Pore Structure and Its Consequence on PEMFC Performance

One of the most important factors in the assembly of polymer electrolyte membrane fuel cells (PEMFCs) is to set the appropriate normal compressive stress to the cell to balance the conflicting demands of mitigating gas leaks and decreasing contact resistance without damaging the porous components so that optimal performance is obtained. Herein we systematically evaluate the influence of compressive stress (0 to 1.4 MPa) on membrane electrode assemblies (MEAs) composed of spray-coated catalyst layers (CLs) and gas diffusion media (GDM) containing both a gas diffusion layer (GDL) and a microporous layer (MPL) by changing the gasket thickness. The CL structure is unaltered at the levels of compressive stress evaluated in this study, as confirmed by postmortem scanning electron microscopy and electrochemical evaluation. However, the pore structure of the MPL of the GDM is significantly altered at compressive stresses ≥0.96 MPa, resulting in a loss of up to 95% of the cumulative pore volume in the MPL. This collapse of the MPL structure results in up to a 19% decrease in current density in the mass transport region of the polarization curve. Oxygen gain voltage analysis confirms that the mass transport limitation occurs in the GDM. We conclude that compressive stress must be optimized around the MPL and that cross-sectional SEM is an effective tool for observing changes in the overall porosity in MEA components.