Water-methanol uptake

Diaz et al. [280] have recently study the behaviour of the water-methanol uptake from the vapour phase by ultra-thin (thickness < 100 nm) Nafion cast membranes and they observed similarities with the water-methanol uptake from the liquid of thick commercial membranes. As illustrated in Fig. 6.17, the total uptake also exhibits a maximum close to x = 0.5. Moreover, the apparent partition constant increase smoothly isK = 1.20 0.06 for methanol fraction in the vapor between 0.12 and 0.69, indicating a preference for methanol uptake. 

Water-methanol uptake from the vapor phase by PBI was determined in thin membranes (100 nm thick) using the quartz crystal microbalance method [280]. The results, expressed as moles of water plus methanol sorbed per imidazole ring, are shown in Fig. 6.31. It was assumed that the partition constant of methanol is Kx = 1.29, as determined by NMR analysis of a PBI membrane in equilibria with a methanol aqueous solution (20 wt%) [280]. 

The behavior observed in Fig. 6.32 is different from that observed Nafion in Fig. 6.17. In the case of PBI, and also for the modified ABPBI membrane, the pme water sorption almost doubles pure methanol sorption, that is, these membranes have a clear preference for water uptake over methanol, while no differences in the uptake of water and methanol was observed in Nafion. The water-methanol uptake from the liquid phase (methanol 20 wt%) of a thick PBI membrane (50-100 pm) shows the same behavior, that is water uptake (A = 3.48) is much higher than methanol uptake (2 = 0.63), which is desirable behaviour for a membrane which intend to be a good barrier for methanol crossover. 

Fig. 6.32 Water-methanol uptake in FBI and ABPBI membranes at room temperature as a function of the vapor composition...

From these shortcomings, the tasks for the designation of improved fuel cell membranes, compared with the state of the art, can be defined. The property profile of improved ionomer fuel cell membranes includes high -conductivity, low water/ methanol uptake, low methanol (and other liquid fuel) permeability, and fuel cell-applicability also at T > 100°C, because the higher the fuel cell operation temperature, the higher the fuel utilization, and applicability also in other (electro)membrane processes. Last but not least, the membranes should have a low price. 

Ren X, Springer TE, Gottesfeld S (2000) Water and methanol uptakes in Nafion membranes and membrane effects on direct methanol cell performance. J Electrochem Soc 147 92-8... 

The cluster-expanded solids exhibit increased capacities for uptake of water, methanol and ethanol. Even better uptake results were obtained with cluster solids... 

The trimethylamine complex is stable to water, methanol, and hydrogen chloride at room temperature. Heating with HCl in a sealed tube leads to the uptake of two molecules of the hydrogen halide, suggesting formation of an ionic product (53). 

X. Ren, M.S. Wilson and S. Gottesfeld, High performance direct methanol polymer electrolyte fuel cells, J. Electrochem. Soc., 1996,143, L12-L15 X. Ren, T.E. Springer and S. Gottesfeld, Water and methanol uptakes in Nafion membranes and membrane effect on direct methanol cell performance, J. Electrochem. Soc., 2000,147, 92-98. 

Some fundamental studies relating to methanol transport in membranes has also been reported. A study of the transport of formaldehyde and ethylene glycol through ion permeable membranes in electrolytic ceils has also been investigated. The water and methanol uptake from methanol-water... 

The swelling and ion-exchange properties of diethylaminoethyl- and benzoyl-ated diethylaminoethyl-cellulose in water-methanol-lM-hydrochloric acid mixtures have been compared.Benzoylation changes these properties of diethylaminoethyl-cellulose in aqueous solution the water uptake of the benzoylated polysaccharide is much less than that of diethylaminoethyl-cellulose, while its ability to adsorb metal ions (Cd ", Hg ", Bi ) is much better. At high... 

The uptake of alcohols by Naflon membranes has been much less studied than the water uptake. Nevertheless, several authors determined the sorption of pure methanol from the liquid and the vapor phase in commercial and recast Naflon membranes [268-282] and the reported results are summarized in Table 6.2, expressed as A, that is, moles of methanol sorbed by sulphonic group. The treatment of the membrane previous to the methanol uptake determination is also indicated. 

The observed scatter in the data is in part due to differences in measurement temperature, which was not well controlled in some of the cases, but it is mainly due to different membrane treatment previous to the alcohol uptake. As in the case of water uptake, methanol uptake from the liquid phase is higher than from the vapor phase when compared for the same membrane under the same treatment. This phenomenon, known as Schroeder s paradox, is related to the thermal history of the adsorbing polymer [90]. 

The expanded form of the Nafion membrane, obtained by boiling the membrane in 3 % H2O2, followed by boiling in water, sulphuric acid and finally in water again (treatment A), tends to uptake more methanol than the non-expanded or cast membrane. At temperatures close to ambient, typical liquid methanol uptake (/I) range from 17 to 27, except for a few studies. These values are similar to that observed for the uptake of water by Nafion. 

In the case of Nafion/zirconium phosphate membranes an increase of the water and a decrease of the methanol uptakes is observed with increasing content of the inorganic phase. 

Park et al. [102] reported a noticeable reduction of the methanol uptake in Nafion/polypyrrole composite membranes, which is accompanied by a similar decrease in the water sorption. A Nafion/polyaniline membrane containing polyanilines with different oxidation states is reported [127] to reduce around 10 % the methanol uptake, particularly the oxidized state, along with a very small reduction of the water uptake. A similar study on Nafion/polyaniline membrane reported a larger reduction (40 %) of the methanol uptake, which reached 50 % when Si02 was added to the composite membrane, without any significant effect on the water uptake [125]. 

The methanol uptake of a grafted Nafion/poly(glycidyl methacrylate) membrane was studied by Mohy Eldin et al. [162]. The membrane, its amine derivate, and the phosphoric acid -doped membrane exhibit an important enhance of the methanol and water uptake with increasing grafting percentages. 

Nandan et al. [268] have determined the sorption of an equimolar water-methanol mixture in Nafion 117 and observed that the total uptake of the mixture (A + = 30.8) was larger than the uptake of the corresponding pure... 

Saarinen et al. [282] observed that the total uptake by Nafion 117 from a water-methanol solution reaches a maximum at x = 0.8, that is, shifted toward more concentrated methanol solutions as compared with former results. However the treatment of the membrane before measurements was not reported in this case. These authors performed uptake measurements for ethanol, 2-propanol and tert-butanol aqueous solutions. The maximum total sorption decrease inversely with the alcohol size (x 0.7 for ethanol, x = 0.25 for 2-propanol, and x = 0.10 for t-butanol), as can be observed in Fig. 6.16. The total mass up taken from the ethanol-water mixture is similar to that reported by Song et al. [286] for dilute solutions (x < 0.2). Godino et al. [287] have studied the liquid uptake of methanol, ethanol, 1-propanol, and 2-propanol aqueous mixtures and up to molar fractions 0.36 for methanol, 0.28 for ethanol, and 0.23 for propanol. The results for methanol and ethanol agree with the previous ones, while in the case of 2-propanol they observed a maximum total uptake at x w 0.15, and for 1-propanol the maximum seems to be close to x = 0.23. 

The preference of Nafion for up taking methanol over water is certainly an undesirable property for DMFC using this proton conducting membrane. It would be worth to review the effect of inorganic or organic Nafion composite membranes on the sorption of water-methanol from the liquid phase and on the partition constant. The few reported studies include Nafion/sulfonated organosilica [47] and Nafion/zirconium phosphate [78], where a reduction of the total liquid uptake is observed for the composites in methanol solutions up to 10 M, attributed to a reduction of the free volume in the ionic clusters. 

Skou E, Kauranen P, Hentschel J (1997) Water and methanol uptake in proton conducting Nafion membranes. Solid State Ion 97 333-337... 

The methanol permeation (P) was found to decrease with the reduction in the porosity and crystallinity accompanying the increase in the content of PSSA. This was despite the increase in the methanol uptake and the lEC. The reduction in the porosity marginalizes the effect of the free methanol trapping, which limits the uptake to the PSSA domains, which have preferential selectivity for water over methanol. On the other hand, the reduction in X seems to play no significant role in methanol permeation as PVDF has a low glass transition temperature, which should enhance the PSSA chain mobility, allowing more methanol uptake. 

Nylon 12 has been studied by solution casting from a phenol-ethanol mixture and shown to isomerize from the y to the a form. The permeation character istics of nylon 12 monbranes have also been evaluated and shown to be related to structural features associated with uptake of water by the casting solution, and the composition of the water-methanol mixed solvent used as the gelation medium. 

The ion-exchange capacity (lEQ as well as their water and methanol uptake/sweUing, measured at 25 and 80 °C, for these hydrocarbon membranes are reported in Table 2.1. 

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