Durability and Performance Study of Chemically Anchored Heteropoly Acid with Perfluorinated Sulfonic Acid-Expanded Polytetrafluoroethylene Composite Membrane for Proton Exchange Membrane Fuel Cells

Abstract

Chemical degradation and mechanical degradation are the major challenges for heavy-duty vehicle fuel cell commercialization. Perfluorinated sulfonic acid is the benchmark material that has high proton conductivity and robust mechanical properties. However, chemical degradation can occur through radical formation during the fuel cell operation. Chemical degradation can also have a synergetic effect with mechanical degradation. Recent studies have used cerium and manganese additives to suppress the radical formation or chemical degradation caused by radicals. The limitation of the metal and metal oxide additives was the migration and agglomeration of the additives. Both migration and clustering can lead to changes in membrane morphology, resulting in a loss in proton conductivity. Our group has previously reported that immobilization of heteropoly acid to a fluoroelastomer can be used to both enhance proton conductivity and chemical degradation. The durability test has shown that the chemical durability was significantly enhanced, but the mechanical durability remained the challenge. In this study, we hypothesized that when a heteropoly acid can be chemically bound to the perfluorinated polymer and cast on a composite membrane with expanded polytetrafluoroethylene (e-PTFE) will enhance the chemical and mechanical durability without migration. Proton conductivity was measured using impedance spectroscopy. The structure-property relationship was studied using multi-scale morphology analysis methods such as scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), and small-angle x-ray scattering (SAXS). The chemical degradation will be tested under the highly accelerated standard test (HAST) condition, a more severe fuel cell operation condition than the accelerated standard condition (AST). The mechanical durability of the composite membrane will also be tested on the HAST condition with humidity cycling.