Mechanical reliability of proton exchange membranes in fuel cells [electronic resource]
Keywords: Mechanical Engineering

Proton exchange membrane (PEM) fuel cells have gained extensive attention as a promising power source for a wide range of applications. The essential function of a unit fuel cell occurs in the PEM, which is coated with two platinum-based catalyst layers (CLs) on the both sides forming a catalyst coated membrane (CCM). Despite significant improvements in fuel cell technologies in recent years, the durability of current PEM fuel cells is still a challenging issue in terms of long term operational use, as their lifetime is primarily limited by the chemical and mechanical reliability of the membranes. Accordingly, we develop a number of novel thin film experimental techniques in this research program to quantitatively assess the mechanical and fracture property of PEMs and CCMs under simulated fuel cell operational environments. This study also demonstrates the deleterious effect of some dominant factors on the mechanical reliability of the membranes in fuel cells and provides a basis for improving the durability of the current PEM fuel cells. The bulge testing method is developed to characterize the biaxial stress in PEMs under hydrated pressure loading on the surface. The effect of foreign cation contamination on the mechanical property of PEMs is investigated, showing increasing stiffness of membranes with the increase of cation radius. We also describe the application of the essential work of fracture analysis and out-of-plane tearing test method to assess the fracture behavior of PEMs under different loading modes in simulated fuel cell operational environments. It is found that the fracture resistance of PEMs contaminated with foreign cations is remarkably reduced in different environments, which should be related to the cation interaction with the molecular structure of perfluorosulfonic acid (PFSA) polymer in PEMs. In addition, we investigate the effects of catalyst Pt dispersions intended to simulate the re-deposited catalyst on the mechanical durability of the PEM. The study shows that with increasing Pt dispersion concentration the stiffness of the PEMs increases, and the membranes become less ductile and inclined to fracture at lower stresses under pressure loading. Deterioration in fracture resistance is explained in terms of the Pt distribution and aggregation as defects inside the membranes characterized by electron microscopy. Fracture is shown to initiate preferentially at the interface of Pt particles and the polymer matrix, and propagate through the defect regions in polymer with lower energy, thus reducing the overall fracture resistance of the PEM. Finally, the double cantilever beam test is applied to characterize the fracture property of CCMs. We initially reveal that cohesive fracture occurs preferentially in the CLs, and investigate the effects of foreign cations and chloride contamination and moisture absorption on the fracture behavior of CCMs. The fracture resistance of contaminated CCMs is significantly reduced and the time dependent crack growth in the CLs in moist air environments occurs at lower crack driving force thresholds.