Chemical Stability via Radical Decomposition Using Silicotungstic Acid Moieties for Polymer Electrolyte Fuel Cells

Abstract

Chemical degradation of perfluorinated sulfonic acid membranes, such as Nafion, via radical attack, represents one of the current challenges of fuel cell durability. Here we report on a recent breakthrough in chemical durability that has been achieved through using covalently attached heteropoly acid (HPA) moieties as both the proton conducting acid and the radical decomposition catalyst. Exceptional chemical durability is reported for a thin (25 μm) film in an accelerated stress test that eventually had an open circuit voltage decay rate of 520 μV h−1, which was shown to be a result of the formation of an electrical short after thinning due to mechanical stresses. A mechanism is proposed using density functional theory in which the W atoms in the HPA reversibly change oxidation state from W(VI) to W(V) while decomposing radical species. Using rate constants found in the literature and realistic concentrations of scavenging species, it is hypothesized that the rate of radical decomposition can be >35x faster for HPA containing membranes than it is for Ce doped films. It is concluded that covalently tethered HPA should be considered as a next generation chemical stabilization strategy for polymer electrolyte fuel cells.