Stress relaxation of PBI based membrane electrode assemblies

Stress relaxation in the membrane electrode assemblies (MEA) in PEM fuel cells subjected to compressive loads is analyzed. This behavior is important because nonzero contact stress is required to maintain low electric resistivity in the fuel cell stack. Experimental results are used to guide the choice of the viscoelastic properties of the constituents of the MEA, the membrane and the gas diffusion layer (GDL), needed for the model. These properties are incorporated into the model that treats the membrane as a porous-viscoelastic solid, and the gas diffusion layer as a nonlinear elastic solid. Using numerical simulations (finite element method), the stress relaxation curves for the MEA are obtained for different fluid flow boundary conditions, variations in the material properties of the membrane and the GDL. The results are compared to experimental stress relaxation curves. Most of the experimental data were obtained at a temperature of 180 °C, corresponding to operating conditions, so in the model the temperature was considered fixed and equal to this value.