Perfluorosulfonate membranes water sorption

E.E. Boakye and H.L. Yeager. Water sorption and ionic-diffusion in short side-chain perfluorosulfonate ionomer membranes. Journal of Membrane Science 69, 155-167 1992. 

Kreuer et al. [25] investigated the membrane properties, including water sorption, transport (proton conductivity, electro-osmotic water drag and water diffusion), microstructure and viscoelasticity of the short-side-chain (SSC) perfluorosulfonic acid ionomers (PFSA, Dow 840 and Dow 1150) with different lEC-values. The data were compared to those for Nafion 117, and the implications for using such ionomers as separator materials in direct methanol and hydrogen fuel cells discussed. Tire major advantages of PFSA membranes were seen to be (i) a high proton conductivity. 

Further anomalies are seen in the diffusional behavior of cesium ion for the perfluorosulfonate membrane. Figure 10 represents the plot of the logarithm of the diffusion coefficient vs. the function Vp/(l-Vp), where Vp is the volume fraction of polymer in the water-swollen material and 1 - Vp is the volume fraction of water, as calculated from sorption measurements. This plot corresponds to a test of the equation... 

The results of ion and water sorption measurements for the two polymers under these solution conditions help to explain this difference. Table 5 " lists the concentrations of various sorbed species and the mole ratio of water to cation/anion in the polymer phase for NaCl and NaOH solution environments. This ratio decreases both in the polymers and in solution with increasing concentration. In solution, the ratio varies from 10.8 to 4.0 over the concentration range of 5-12.5 M NaOH, so that ions in the polymer phase exist in a significantly less aqueous environment compared to the solution phase. As noted by Mauritz and co-workers for perfluorosulfonate membranes, these water contents are insufficient to provide even primary hydration spheres for sodium ions, sorbed anions, and exchange sites, and the likelihood... 

R. Schlogl and F. Helfferich, Theory of exchange membrane potentials, Z. Elektro-chem, 1952, 56, 644-647 N. Ishibashi, T. Seiyama and W Sakai, Electrochemical studies on ion exchangers (Part 10) Mobilities of Ca+ and Cl- in the cation exchange membrane, Denki Kagaku (J. Electrochem. Soc. Jpn.), 1955, 23, 182-186 E.E. Boakye and H.L. Yeager, Water sorption and ionic diffusion in short side chain perfluorosulfonate ionomer membranes, J. Membr. Sci., 1992, 69, 155-167. 

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