Sulfonated poly(ether ether ketone)-functionalised silica composite membranes for applications in proton exchange membrane fuel cells

Sulfonated Poly(ether ether ketone) (SPEEK) and its composites are explored as potential alternatives for Nafion in fuel cell applications. In this work, we investigate the effects of the silica functionalisation route on the thermal, mechanical and structural properties of SPEEK-Silica composite membranes. We compare two different methods - direct sulfonation and co-condensation (sol-gel) - to add sulfonic acid groups to silica nanoparticles, which are used as fillers in SPEEK membranes. The degree of sulfonation (DS) of the cast membranes determined by H-1 NMR ranged from 40% to 86%. TGA-DSC measurement shows higher desulfonation temperatures for the composite membranes, indicating better thermal stability compared to pure SPEEK membranes. DMA results show that the sol-gel membranes are mechanically more stable with a wider operational temperature range than the directly sulfonated and pristine SPEEK membranes. Water uptake of the membranes increases with silica content, indicating the formation of additional water-mediated pathways for proton conduction.