Free volume and gas permeation in ion-exchanged forms of the Nafion membrane

Abstract

Variations of free volume and gas permeability of the Nafion membrane upon ion-exchange of H+ with Cs+ or Pt2+ was studied as a function of temperature. Free volume was quantified using the positron annihilation lifetime technique. Our results showed that the free volume (V FV,Ps) of the dried membrane is enlarged by thermal expansion. It was found that the ion-exchange significantly expands the free volume and at the same time decreases the permeabilities of O2 and H2. Good linear correlations between the logarithm of permeabilities of O2 and H2 at different temperatures and 1/VFV,Ps for the ion-exchanged forms of Nafion in the dried state suggest an important role played by the free volume in gas permeation. Considerable downward deviation of the correlations for the ion-exchanged ionomers from the H+-form suggested the importance of polymer stiffening in gas permeation. © 2010 IOP Publishing Ltd.