Composite fuel cell membranes based on an inert polymer matrix and proton-conducting hybrid silica particles

The present contribution evaluates a novel route to low-cost proton-exchange membranes. The composite membranes were prepared by inserting polystyrenesulfonic acid-grafted silica particles into an inert polymer matrix of poly(vinylidene fluoride-co-hexafluoropropylene), abbreviated PVDF-HFP. The first step consisted in using atom transfer radical polymerization techniques in order to obtain modified silica particles grafted with sodium 4-styrenesulfonate, referred to as A390-g-PSSNa. Ion-exchange capacities up to 3.0 mequiv./g were readily obtained for these modified silica particles. In a second step, homogeneous PVDF-HFP-based composite membranes charged with various amounts of A390-g-PSSNa were prepared by solvent casting after which they were characterized with regard to their inherent physical properties, as well as to their water uptake and proton conductivity when immersed in water. A percolation threshold of the particle-rich phase was obtained at approx. 30 wt.%. Notably, membranes with loadings ranging from 30 to 60 wt.% exhibited proton conductivities from 15 to 95 mS/cm at room temperature in water. In addition, composite membranes with a loading of 50 wt.% exhibited power densities above 1 W/cm2 at 70 °C in single-cell fuel cell tests with non-hydrated gas feeds.