Heteropoly Acid Functionalized Membranes for High Performance and Chemically Stable Fuel Cell Operation

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 and independently by Kunz and co-workers was 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. In addition the Si based HPA are known to decompose oxygenated radicals and so offer a path to the development of a PEM that is oxidatively stable under fuel cell operating conditions. 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. The key is to distribute the HPA on a perfluorinated polymer backbone in such a way that the natural tendency of the HPA to cluster and crystalize is suppressed. Using dehydroflourination chemistry we have learnt to functionalism PVDF type polymers with molecules that can act as anchor points for lacunary HPA, where a lacunary HPA has 1,2 or 3 tungsten oxygen clusters removed to give bonding points. After much research we now can produce these materials in large area thin film membranes. The base material outperforms the state of the art PEM under standard conditions and has the highest chemical stability of any PEM proposed to date. We are now varying the HPA structure to try and avoid the HPA clustering issue, these membranes are being sprayed on gas diffusion electrodes which act as the support to the tin films. This is allowing us to run these as fuel cells at higher temperatures to asses the potential of HPA functionalized membranes in these applications.