(220b) Fuel Cell Membranes with Enhanced Durability and Performance Based on Fluoroelastomers Functionalized with Heteropoly Acids
Our approach is to make monomers from HPA and immobilize the HPA by polymerization into hybrid systems. In order to functionalize the Keggin anion one W oxygen octahedra is removed and a Si or P based organic functionality introduced that may be a monomer or a tether to a functionalized polymer backbone. Our first generation materials based on divinyl functionalized HPA and acrylate chemistry produced films with impressive conductivities, >100 mS cm-1Â at T >80Â°C and 50% RH. This model system contained ester linkages that we think would be hydrolysed under the harsh conditions of fuel cell operation and so we attached HPA via di-phosphonate linkages to perfluorinated polymers.Â This is acheived by dehydrofluorinating the based perflourinated polymer and functioalizing it with phosphonate groups taht serve as covalnent attcahmnet points to the HPA.Â Very recently we have fully perfected this chemistry and can now produce large area thin, 10Â Î¼m, high loaded HPA films for fuel cell cell operation. Not only do the materials have very low ASRs, <0.02 Î© cm2, under all operating conditions (except freeze), but there is very little cross-over of H2 and O2. The materials also survive the DOE mechanical stability test and exceed the chemical stability test. The chemical stability is proof a theory; published by us using PFSA/HPA composite films, the silicotungstic HPA moieties catalytically decompose peroxy radicals. The measured water flux is also superior to PFSA materials allowing back diffusion of water during fuel cell operation. Unfortunately, we do not yet have a suitable ionomer for the electrodes to operate in an hot and dry environment and so fuel cell data using PFSA ionomers will be presented under more standard operating conditions.
This work is sponsored by DOE EERE and ARPA-E.