While perfluorinated sulfonic acids (PFSA)s have many desirable qualities for use as a membrane separator in a polymer electrolyte membrane fuel cell (PEMFC), they suffer from decomposition via radical attack and excessive swelling. These two properties reduce the chemical and mechanical durability, respectively, of PFSA materials in the operating environment of a fuel cell. Through using heteropoly acids (HPA)s as an ion-conducting, crosslinking moiety, these short comings of PFSA membranes can be overcome. HPAs are known to be highly conductive [1] and are able to react with radicals. Our approach is to use a commercially available fluoroelastomer and functionalizes using HPA crosslinks. First, FC-2178 (polyvinylidene-co-hexafluoropropylene) is functionalized with phosphonic acid containing sidechains. Next, lacunary silicotungstic acid is covalently bonded to acidic sicechains. This material is stable in aqueous environments, unlike blends of HPA and PFSA. The results of in-situ evaluation of these membranes show impressive durability in accelerated stress test (AST). The loss of open circuit voltage (OCV) in the chemical degradation test is much less than traditional films, resulting in >0.8V OCV after 500 hrs. Also, the minimal swelling of this material leads to greater durability in humidity cycling ASTs with negligible H₂ crossover after 20,000 dry wet cycles. Overall, this material shows great promise in outperforming PFSAs in chemical and mechanical ASTs.

References:

[1] Nakamura, O.; Kodama, T.; Ogino, I.; Miyake, Y., High-Conductivity Solid Proton Conductors - Dodecamolybdophosphoric Acid and Dodecatungstophosphoric Acid Crystals. Chemistry Letters 1979, 17-18.