Methanol-equilibrated Nafion

Membrane Diffusion in Nonaqueous Solvent Environments. Self-diffusion coefficients of Na+ and Cs+ for 1200 EW Nafion membranes in dilute methanol and acetonitrile solutions have been measured (5). Arrhenius plots of these results are shown in Figure 7 along with corresponding results for aqueous experiments activation energies of diffusion are listed in Table IV. Diffusion coefficients of Na+ in methanol and water-equilibrated membranes are very similar, and the activation energy of diffusion for the methanol system is only slightly higher than the respective value for Na+ in pure methanol solvent, 12.9 kJ mol 1 (27). Thus a solution-like diffusion mechanism is inferred for both solvent systems. Cesium ion diffusion in the methanol equilibrated membrane is much slower than sodium ion diffusion in fact the... 

A variation of the lamellar model was recently proposed by Haubold et al., in which synchrotron SAXS studies were performed on acid form Nafion 117 samples. The molecular weight of these samples was reported to be 250 000 g mol and the experiments were conducted on dry samples in air and samples equilibrated with water, methanol, and a range of water/methanol mixtures using an in situ flow cell. The most fundamental result of this study is that the data show the usual ionomer peak at = 1.4 nm which gives a Bragg spacing of d= Zjtlq ... 

Bauminger et al. 0,40 find a spectrum at 4 K composed of doublets A and B for Nafion 110 equilibrated with a methanol solution of FeCl3. Parameters were the same as for the aqueous exchanges, but the intensity ratio was somewhat different. These authors have also examined Redcat poly(ethylene sulphonic acid) and sulphonated polysulphone membranes where they again find two doublets, but A has QS = 0.60(3) mm/s. 

Figure 8. Mossbauer spectra of iron in Nafion equilibrated in methanol solution...

Other membranes, such as sulfonated polyimide [102] or cross-linked sulfonated PVA [103] exhibit a major amoimt, although no quantified, of non-freezable water as compared to Nafion. An interesting behavior was observed in the case of PBl and ABPBI membranes doped with phosphoric acid [104], where the amount of frozen water is 2.2 % for PBI and between 1.1 % and 9.1 % at 100 % relative humidity, much lower than that observed for Nafion under similar conditions. When equilibrated with aqueous methanol, PBI membranes exhibit a maximum of 10 wt% of frozen water, while ABPBI membranes, particularly those prepared by high temperature casting, presented very low percentages of frozen water in all methanol concentrations studied, with a maximum value of 0.6 wt% in methanol 25 w/w%. This result is relevant for the application of ABPBI in DMFC because, the fuel cell start up at low temperatures would not be affected. 

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