TRANSPORT AND EQUILIBRIUM PROPERTIES OF NAFION(R) MEMBRANES WITH H+ AND NA+ IONS

Transference coefficients, ionic conductivities and equilibrium proper ties are reported for Nafion(R) 117 membranes containing a mixture of two cations and water. The membranes were equilibrated in 11 kinds of aqueous solutions of HCl and NaCl, with mole fractions of HCl between 0 and 1, all having c(Cl-)= 0.03 kmol m(-3). Some experiments were rep eated for Nafion(R) 112 and 115 membranes. By electron probe microanal ysis (EPMA), it was found that Nafion(R) 117 membranes have a slightly higher affinity for Na+ than for H+. The water content of the membran e was determined gravimetrically. It decreased from 20.6 +/- 0.1 to 18 .4 +/- 0.2 molecules water per cationic site, as the membrane changed from the pure H-form (HM) to the pure Na-form (NaM). The water transfe rence coefficient, t(H2O), obtained by streaming potential measurement s, increased in Nafion(R) 117 from 2.5 +/- 0.1 to 9.2 +/- 0.1 as the m ole fraction of Na+ in the membrane, X-NaM, increased from 0 to 1. Mos t of the increase occurred for X-NaM > 0.5. The transference number of H+ in the membrane, t(H+)(m), which was determined by an improved emf -method, showed a rapid decrease for X-HM < 0.5. The plot of t(H2O) vs . t(H+) was almost linear, indicating that each ion promotes transfer of a characteristic number of water molecules. The membrane conductivi ty, measured by an ac impedance method, changed almost linearly with X -HM, With the same results for Nafion(R) 115 and 117. These results im ply that possible sodium ion contamination in the fuel cell membranes will promote membrane drying. The significance of the new methodology established in this work for mixed form ion exchange membranes is stre ssed, with relevance to the membrane contamination problems. (C) 1998 Elsevier Science S.A.