(Invited) Highly Proton Conducting Membranes for Electrochemical Energy Conversion Applications

Abstract

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 is the use of inorganic super acids as the heteropoly acids (HPAs) r zirconium phosphonates. 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 have doped both PFSA and PBI polymers with HPA and shown that both approaches lead to membrnes with much improved properties for proton cnduction under higher an drier conditions. The HPA/PBI/phosphorics acid system that we have developed may be used to investigate direct fuel cells with a number of fuels beyond hydrogen as the membranes can opearte at temperatues >160°C. Our most ambitious 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 that have demonstrated high performance fuel cell operation. We will show performance data in both hot and dry fuel cell operation and also durability data that shows the materials pass the DOE tests. These membranes are currently being investigated as seperators in redox flow batteries using a large number of redox couples.