(Keynote) The Use of Polyoxometallates in Proton Conducting Membranes for Electrochemical Energy Conversion

Abstract

Heteropoly acids (HPAs), a subclass of the polyoxometalates, are a class of proton conducting radical activating or decomposing molecules. Several types of HPAs have demonstrated the ability to improve membrane chemical stability and proton conductivity through polymer blends, but they still suffer from migration of the HPA species and only result in marginal gains. A method of covalent bonding HPAs to carbon in the catalyst layer has also shown some improvements in chemical stability, but chemical degradation mitigation within the membrane is needed. Our group has developed two membrane platforms with HPAs covalently attached and immobilized within a polymer membrane, serving as the proton conducting acid. More recently, the outstanding chemical stability of one of these platforms has been demonstrated, however, the stability demonstrated in this study was criticized for using rather thick, 80 µm membranes in sub-scale fuel cells. In a recent study a 50 cm2 fuel cell was fabricated using a thin, 25 µm membrane with covalently attached silicotungstic acid, which was subjected to an accelerated stress test for chemical degradation and displayed an OCV decay rate of 520 µV h-1. To the authors knowledge, this is the first reported fuel cell of a larger practical area containing a hybrid HPA film and represents a significant step towards demonstrating this technology on a commercially relevant scale. The resulting data was analyzed to show the loss in OCV is mainly due to an electrical short and not increased reactant gas crossover. This study further analyzes the chemical stability observed in these membranes and proposes a mechanism for radical decomposition. A reaction mechanism is proposed utilizing reactions found in literature as well as density functional theory (DFT) calculations. The main conclusion from this work is that covalently attached HPAs could be more efficient radical scavengers with less susceptibility to migration, accumulation, and leaching when compared to the use of Ce(III) cations. We have recently realized a method for cleaning these membrane materials, removing many of the impurities formed during synthesis and have also begun to cross-link these material to eliminate swelling and achieve a dimensionally stable films. These much-improved materials show even higher performance in fuel cells and could potentially solve many of the issues associated with the PFSA materials. In addition the HPA functionalized films have advantages in selective low area spec resistant membranes for redox flow batteries with a variety of redox couples. We all shoe that these membranes can facilitate redox flow batteries with labile vanadium substituted HPAs.