Enhancing proton conductivity via sub-micron structures in proton conducting membranes originating from sulfonated PVDF powder by radiation-induced grafting

We report here submicron-structured proton conducting poly(vinylidene fluoride)-graft-poly(styrene sulfonic acid) (PVDF-g-PSSA) membranes for polymer electrolyte membrane fuel cells (PEMFC). Highly conductive proton exchange membranes were obtained by single-step radiation grafting of sodium styrene sulfonate (SSS) to powder-form PVDF, followed by casting and subsequent solvent evaporation. The obtained submicron structure of membrane through solvent evaporation led to the arrangement of ionic channels proving increasing proton conductivity with the increase in graft level. In addition, a temperature above melting point of PVDF was used for solvent evaporation to allow melted PVDF to fill the formed pores, providing denser structure resulting in improved mechanical properties of the membranes. SSS grafting to PVDF powder was verified by NMR spectroscopy, and resultant membranes were characterized for proton conductivity, water up-take, morphology, mechanical and thermal properties, and fuel cell performance. According to preliminary tests, proton conductivities which were observed to increase with graft level were found to be around 70 mS cm2 at 35% graft level. Thus, this led to a promising power density of 250 mW/cm2 at 650 mA/cm2.