(220b) Fuel Cell Membranes with Enhanced Durability and Performance Based on Fluoroelastomers Functionalized with Heteropoly Acids

Authors: 
Herring, A. M., Colorado School of Mines
Motz, A. R., Colorado School of Mines
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 such as Nafion, i.e. >100°C and <50%RH. One approach pursued by us and independently others is the use of inorganic super acids, 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 in crystaline HPA. The key to making these systems work, thefore is make them amorphous.

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 at T >80°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. This is acheived by dehydrofluorinating the based perflourinated polymer and functioalizing it with phosphonate groups taht serve as covalnent attcahmnet points to the HPA. Very recently we have fully perfected this chemistry and can now produce large area thin, 10 μm, high loaded HPA films for fuel cell cell operation. Not only do the materials have very low ASRs, <0.02 Ω 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.