We and Kunz and coauthors independntly showed that HPAs could dramtically enhance the proton conductivity of perfluoro sulfonic acid (PFSA) PEM under hotter and drier operating conditions. Despite their desirable properties HPAs are highly water soluble. Kunz and co-workers took the approach of immobilizing the HPA as the insoluble Cs salt and very ingeniously dispersed the Cs HPA clusters as nano-sized moieties, thereby generating a composite Nafion® film with superior properties. HPAs are known to decompose peroxides and oxygen based radicals and so have been shown to enhance oxidative stability of PEMs. A furher advantage advantage of the Si centered HPA is that the parent H4SiW12O40 HPA showed enhanced oxidative stability and performance of perfluoro sulfonic acid polymers (PFSAs) when it was used as a dopant in fuel cell testing.
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-1 T >80°C and 50% RH. This model system contained ester linkages that we think may be hydrolysed under the harsh conditions of fuel cell operation and so we attached HPA via phosphonate linkages to perfluorinated polymers. Very recently we have fully perfected this chemistry and can now produce large area thin high loaded HPA films for both fuel cell and redox flow cell operation. We will show performance data in both hot and dry fuel cell operation and for redox flow batteries.
This work is sponsored by DOE EERE and ARPA-E.