Styrene grafted ethylene chlorotrifluoroethylene (ECTFE-g-PSSA) protonic membranes: Preparation, characterization, and transport mechanism

The main goal of the present work is the development of partially fluorinated, low-cost proton exchange membranes. The styrene grafted onto commercial ethylene chlorotrifluoroethylene (ECTFE) membranes using solution grafting technique, and after that the membranes were sulfonated. Diluting styrene on ECTFE with a solvent mixture of methanol plus methylene chloride (1:1) was highly effective in promoting the grafting reaction as indicated by the increase in the degree of grafting (DG) to 21.3% compared to other solvents. The DG in ECTFE membranes increased with an increase in the monomer concentration up to 60% and then declined. Fourier transform infrared spectroscopic analysis confirmed grafting and sulfonation onto ECTFE films. The maximum value of proton conductivity for ECTFE-g-PSSA film with DG=21.3% was observed to be 141mScm(-1), which is also higher than those of Nafion 212 membrane. Furthermore, the activation energy of ECTFE-g-PSSA membranes was obtained ranging from 8.27 to 9.726kJmol(-1). So both proton transport mechanisms (hopping and vehicle) have been commonly accepted. The mobility of the charge carriers calculated from proton conductivity data has robust dependence on the grafting yield and temperature. Moreover, the tensile strength and elongation at break ratio decreases with the increase in DG. The water and methanol uptakes increase up to 0.97% and 30%, respectively, for the highest DG value. Finally, the ECTFE-g-PSSA has lower cost and higher conductivity they could be better used instead of Nafion in direct methanol fuel cells.