(Invited) Using Heteropoly Acids As a Proton Conductor in High Temperature Proton Exchange Membrane Fuel Cells
Authors: Herring Andrew M , Motz Andrew R , Kuo Mei-Chen 
Published: February 2020

There is still a need for membranes that operate in proton exchange membrane (PEM) fuel cells at hotter and drier conditions than can be achieved with current materials, >100°C and <50%RH. One approach pursued by us and independently by Kunz and co-workers was the use of inorganic super acids as proton conducting entities, such as the heteropoly acids (HPAs). HPAs are a sub group of the large class of metal oxygen clusters known as polyoxometalates in which a central heteroatom is surrounded by a number of W or Mo oxygen octahedra. For proton conductivity it is desirable that a strong negative charge be delocalized across the whole anion so that the proton will be as dissociated as possible. This limits the choice of HPA to the spherical tungsten based Keggin anion with as light as possible a heteroatom. This limit is reached with Si as the P based HPA is known to decompose in the presence of peroxide and the electron deficient nature of B renders the spherical Keggin anion unstable. Many fundamental studies have been undertaken on solid state HPA systems. These studies indicated that despite the original report in the early 80’s that HPA had the highest proton conductivity reported at that time, that when dry there proton conductivity was disappointingly low at moderate temperatures.
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.