Isolated chemical degradation induced decay of mechanical membrane properties in fuel cells

Abstract

Membrane durability is a major consideration for the operational lifetime of polymer electrolyte membrane fuel cells. During fuel cell operation, the membrane is exposed to combined chemical and mechanical degradation that could ultimately lead to hydrogen leaks and cell failure. In this work, a direct link between chemical degradation and membrane mechanics is established by using a steady-state open-circuit voltage accelerated stress test (AST) to induce isolated chemical membrane degradation. Cross-sectional microscopy reveals gradual membrane thinning with AST time without crack and hole formation. Partially degraded catalyst coated membranes are subjected to tensile tests at room and fuel cell conditions in order to quantify changes in mechanical properties during the chemical degradation process. Reductions in ultimate tensile stress, fracture strain, and elastic modulus are progressively observed during the AST and correlated to membrane thinning and fluoride release. Additionally, hygrothermal expansion experiments reveal 33% decay in hygral expansion at 70 °C and 40% decay in thermal expansion at 90% relative humidity due to chemical degradation. Given the absence of mechanical degradation and associated physical damage such as cracks, the gradual decay in ductility is likely induced by polymer chain disentanglement as a precursor for mechanical failure.