Abstract
Practical proton exchange membrane (PEM) fuel cells still use PFSA materials for durability and performance reasons and are limited operating temperatures of 80°C at reasonable pressures to ensure they have sufficient hydration for high proton conductivity. PFSA materials have become undesirable due to the use of forever chemicals in their synthesis and low temperature PEM limits heat transfer and the use of these devices as direct fuel cells for fuels beyond hydrogen. We were inspired to create membranes using polyoxometlates (POM) by John Turner over 25 years ago when he was inspired by a paper he attended where aqueous solutions of POMs showed promise as an alternate electrolyte to PFSA membranes. In collaboration with NREL students we showed that POM membranes in the solid state had high performance in low temperature fuel cells and could even function as fuel cell membranes at >200°C. The POMs are extremely soluble in water and so we use lacunary POMs functionalized with chemistry that can lead to the functionalized POM being a monomer in a variety of insoluble polymer systems. Initially we used a polyacrylate system that produced fuel cell ready membranes with very high proton conductivity's measured to 120°C under relatively dry conditions. Inevitably we came to the conclusion that the ester linkage in these interesting conditions might not be fuel cell stable. So we next turned our attention to functionalizing dehydroflourinated PVDF based materials. This perflourinated solution gave materials with superior performance under standard operating conditions and exceptional durability results in accelerated stress tests. Currently we are investigating a polyamide solution that continued research will result in a fluorine free membrane that has promise to operate at hotter and drier conditions than are currently accessible enabling heavy duty fuel cell applications. One chance encounter with John has led to a continuing 25 year effort that is challenging what is possible in PEM membranes.