Nafion®-graft-polystyrene sulfonic acid membranes for direct methanol fuel cells

Polystyrene (PS) molecules were grafted onto the surface of a Nafion® 117 membrane via the plasma-induced polymerization technique in order to reduce its methanol permeability in applications involving direct methanol fuel cells (DMFCs). The PS grafting reaction and degree of sulfonation were measured using FT-IR–ATR spectroscopy and XPS, respectively. The kinetics and equilibrium yield of the grafting reaction were analyzed by measuring the changes in the weight and thickness of the membranes during the grafting reactions. Crosslinking affected not only the grafting rate, but also the size and distribution of the ionic clusters in the membrane in the water swollen state. The equilibrium water uptake, proton conductivity and methanol permeability were affected by both the extent of the grafting reaction and the crosslinking density. The crosslinked polystyrene sulfonic acid (PSSA) layers that were grafted reduced the methanol permeability of the membranes considerably.