Water and methanol uptake in proton conducting Nafion® membranes

Water and methanol uptake in proton conducting Nafion® membranes

Solid State Ionics 97 (1997) 333–337 Water and methanol uptake in proton conducting Nafion  membranes 1 E. Skou*, P. Kauranen , J. Hentschel 2 De...

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Solid State Ionics 97 (1997) 333–337

Water and methanol uptake in proton conducting Nafion  membranes 1

E. Skou*, P. Kauranen , J. Hentschel

2

Department of Chemistry, Odense University, DK-5230 Odense, Denmark

Abstract Methanol uptake from methanol–water mixtures has been measured at ambient temperature in Nafion  117 membranes initially saturated with water. After equilibration for 18 h the membranes were dried at 708C in vacuum with a cold trap cooled with liquid nitrogen and the total amount of absorbent was determined from the weight loss. The methanol–water ratio was determined by 1 H NMR. A plot of the mole fraction of methanol in the membrane vs. the mole fraction in the water–methanol mixture was linear in the whole range from pure water to pure methanol. The slope indicated that Nafion  does not preferentially take up either water or methanol. The number of solvent molecules was found to be 23 per sulphonic group for both methanol and water. Keywords: Proton conductor; Methanol; Polymer electrolyte; Nafion Materials: Nafion

1. Introduction There has been renewed interest in the direct oxidation methanol fuel cell concept during the last few years [1]. This has mainly been due to the improvement of the methanol oxidation catalyst performance and the use of perfluorosulphonate proton exchange membranes, e.g., Nafion  117 from DuPont, which have made it possible to raise the operating temperature to 80–1108C. The per*Corresponding author. Tel: 145 66 158600; fax: 145 66 158780; e-mail: [email protected] 1 Present address: Oy Hydrocell Ltd, Minkkikatu 1-3, Fin-04430 ¨ ¨ ¨ Finland. Jarvenpaa, 2 Present address: Department of Chemistry, University of Kaiserslautern, D-67663 Kaiserslautern, Germany. 0167-2738 / 97 / $17.00  1997 Elsevier Science B.V. All rights reserved PII S0167-2738( 97 )00033-7

fluorosulphonate membranes are, however, quite permeable to methanol [2–5] leading to methanol cross-over to the oxygen cathode. This cross-over causes losses in terms of lost fuel and cathode depolarization. At present little is known about the behavior of perfluorosulphonate membranes towards mixtures of methanol and water, in particular concerning the distribution of methanol between the water and the membrane phases. Verbrugge [2] has used a radioactive tracer method to measure the methanol diffusivity in Nafion  equilibrated with sulphuric acid at room temperature. From the measurements he was able to extract a partition coefficient for methanol of 0.8. Ren et al. [4] have investigated the methanol uptake of Nafion  117 membranes equilibrated with methanol solutions up to 8 M at 308C using 1 H NMR to

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determine the methanol water ratio in the membranes. They found a distribution coefficient of one, i.e., the membrane shows no preference for either water or methanol. They also found that methanol goes into the membrane in addition to the water already present. Nandan et al. [6] have investigated the methanol uptake in a series of cationic forms of Nafion  117 membranes equilibrated with pure methanol and a 50 mole% water–methanol mixture at room temperature. In contrast to the findings of Ren et al. [4] they found a preferential uptake of water also for the protonic form. They also found an increased total solvent uptake from the mixed solvent compared to the uptake from pure solvents. In contrast to the findings of Ren et al. [4] they even found an additional water uptake compared to the uptake from pure water. The methanol content, on the other hand, was found to be less than the uptake from pure methanol. There are thus some discrepancies in the literature concerning the solvent fractionation properties of Nafion  117. The origin of these is unclear, but they may be related to the pretreatment of the membrane. Also the concentration range to which the additive uptake found of Ren et al. [4] can be applied has not yet been clarified. The distribution of methanol between methanol– water mixtures and Nafion  117 membranes in a well controlled reference state is investigated at room temperature, in the present work, covering the total concentration range from pure water to pure methanol.

2. Theory The equilibrium condition for a compound which is distributed between two phases, e.g., a liquid phase (l) and a polymer phase (p) will be that the chemical potential ( mi ) of the compound is identical in the two phases:

m 0i,l 1 RT ln a i,l 5 m 0i,p 1 RT ln a i,p where m 0 is the standard chemical potential and a is the activity of the component. The expression can be rearranged to:

a i,p ] 5 Ki a i,l where Ki is the partition coefficient. For liquid mixtures it is customary to use the mole fraction of the component as a measure of its activity. This leads to the following expression: Xi,p ] 5 gi ? Ki Xi,l

(1)

where gi is an activity coefficient taking care of deviations from ideality. The Nafion  polymer is a two phase system consisting of an organic phase with liquid filled pores. The mole fraction of, e.g., methanol in the polymer may then be calculated as the mole fraction of methanol in the water–methanol mixture filling the pores, i.e., the polymer itself is not included in the calculation of the mole fraction. Interactions between water–methanol and the polymer are expressed through the activity coefficient. The partition coefficient of a component can thus be determined from a plot of the mole fractions in the liquid phase and in the polymer phase.

3. Experimental The membrane material used was Nafion  117 from DuPont. Pieces of 5310 cm 2 were cut out of the foil. In order to convert the membrane to their protonic forms the pieces were boiled in 3% hydrogen peroxide, distilled water, 1 M sulphuric acid and distilled water again, one hour for each step. Two sets of three membranes were used. The three membranes in a set were equilibrated for 18 h in a large excess (200 ml) of different water–methanol mixtures covering the range from pure water to pure methanol. After equilibration the membranes were dried in vacuum (0.1 mbar) at 708C for 30 min. The evaporated liquid was collected in a cold trap cooled with liquid nitrogen. By comparing the weight loss of the membranes with the amount of liquid collected in the trap it was found that more than 90% of the liquid was collected in the trap. To recover the membranes after drying they were reboiled in dis-

E. Skou et al. / Solid State Ionics 97 (1997) 333 – 337

tilled water for one hour. All experiments were run in duplicate and for some mixtures more than one membrane was used. Before weighing the wet membranes were wiped gently with tissue paper in order to remove adhering liquid. The composition of the liquid taken up by each membrane was determined by use of the liquid from the cold trap. A sample of 300 ml was diluted with 300 ml deuterium oxide and an 1 H NMR spectrum ¨ was recorded (250 MHz, Bruker AC FT instrument). The dilution was necessary in order to avoid saturation of the NMR signal. The peaks were integrated and used for the calculation of the mole fractions. The intensity of the CH protons was taken to be proportional to three times the methanol concentration. The intensity of the OH protons included contributions from both water and methanol. This eliminates the uncertainty coming from proton exchange between D 2 O and OH protons in methanol. The mole fraction of methanol in the mixture can thus be found from the following expression: 1 ] ? Integral CH XCH 3 OH 3 3 ]]]] 5 ]]]]. 2 2 XCH 3 OH Integral OH

(2)

4. Results and discussion

4.1. Calibration of the NMR method

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Fig. 1. The ratio between ]13 of the integrated intensity of the CH 3 protons and the integrated intensity of the OH protons as a function of the mole fraction of methanol in water–methanol mixtures. The solid line represents the function 0.9653XMeOH / (220.9653XMeOH ) and is used for the determination of the methanol content in unknown water–methanol mixtures.

of weight as shown in Fig. 2. This means that the membranes do not only swell in the methanol–water mixtures but that a gradual dissolution also takes place. As will be shown later, this dissolution does not influence the equilibrium between methanol and water in the membranes. It does, however, impose an uncertainty in the determinations of the total uptake of solvents.

In Fig. 1, the 1 H NMR calibration curve is shown. The ratio between ]13 of the integrated intensity of the CH 3 protons and the integrated intensity of the OH protons is plotted as function of the mole fraction of methanol in the analyzed methanol–water mixtures. The smooth curve corresponds to the function A3 XMeOH /(22 A3XMeOH ) with A equal to 0.965. The value of A was determined by least square fitting and is in good agreement with the value of one predicted in Eq. (2). The smooth curve was subsequently used for the determination of methanol in the equilibrated membranes.

4.2. Membrane history During the equilibration experiments the weights of the dried membranes were monitored as described in Section 3. All membranes showed a gradual loss

Fig. 2. Weight loss of the three membrane samples as a function of time. The membranes were saturated with water, equilibrated with water–methanol mixtures, dried and saturated with water again in a series of experiments. The values in the figure show the weight of the dried samples.

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4.3. Distribution coefficients The mole fraction of methanol in the membrane as a function of the mole fraction of methanol in the equilibrating liquid is shown in Fig. 3. The points are seen to fall on a straight line. The slope of the line is 0.945, i.e., the partition coefficient in Eq. (1) is close to one, indicating that saturated Nafion  membranes shows no preference for either methanol or water. This is in close agreement with the findings of Ren et al. [4] based on a similar method and in fair agreement with the findings of Verbrugge [2], who found the distribution coefficient to be 0.8 based on methanol permeation measurements. The results show some disagreement with the findings of Nandan et al. [6] who used membrane samples ‘as received’. The samples were not in a swollen state before equilibration with the solvents and the results may thus not be comparable.

4.4. Total solvent uptake The total solvent uptake was determined from the weight loss upon drying of the membranes as described in Section 3. Based on this weight loss and the ratio between methanol and water the individual and total solvent uptake per gram of dry membrane were calculated. The results showed some scatter (15–20%) particularly at methanol mole fractions below 0.3. At mole fractions above 0.3 all measurements fell on a smooth curve. The scatter is probably

Fig. 3. Mole fraction of methanol in Nafion  117 at room temperature as a function of the mole fraction in the equilibrating liquid water–methanol mixture.

connected to changes in the swelling properties accompanying the gradual dissolution described in Section 4.2. Also in the region below X50.3 the values for the total uptake of solvent were distributed around a smooth curve. In order to average the measurements the measured points were fitted with a polynomial of fourth degree and the values from this curve were subsequently used in the normalization, i.e., the total uptake per gram of dry membrane was taken from the fitted curve and the ratio between methanol and water in the membrane was taken from the 1 H NMR measurements. The result of this procedure is shown in Fig. 4. The total solvent uptake is seen to pass through a maximum value at X50.6. At low methanol concentrations the water content is seen to be little affected by the uptake of methanol, i.e., the methanol is taken up in addition to the water already present. At higher methanol concentrations water starts to become excluded. This result is also in agreement with the finding of Ren et al. [4], who found that methanol goes into the membrane in addition to water at methanol concentrations below 8 M at 308C. An 8 M methanol concentration corresponds to a mole fraction of 0.17 which is within the region where the additivity is observed. The curve expressing the total weight uptake has the parameters: mol /(g of dry membrane)50.02071 0.0152(XMeOH )20.0050(XMeOH )2 20.010(XMeOH )4 . This gives a water saturation value of 0.0207 and a methanol saturation value of 0.0209. The equivalent weight of Nafion  117 is 1100 per sulphonic group.

Fig. 4. Total solvent uptake and uptake of water and methanol in Nafion  117 membranes equilibrated with liquid water–methanol mixtures at room temperature.

E. Skou et al. / Solid State Ionics 97 (1997) 333 – 337

This gives a water content of 23 per sulphonic group in good agreement with values in the literature [7]. This also shows that the drying procedure employed (708C and an extremely low partial pressure of solvent due to the liquid nitrogen trap) is sufficient to remove all solvent from the membranes. The value for the methanol saturated membrane also corresponds to 23 mol per sulphonate group. Water and methanol thus show very similar behavior in the Nafion  membrane. The reason for the additional solvent uptake in either the water saturated or the methanol saturated membrane is unclear, but Nafion  membranes are known to change their internal structure, when solvent concentrations are changed [8]. The solvent uptakes found in this work are higher than those reported by Nandan et al. [6], but as mentioned earlier, the pretreatments of the samples were different.

5. Conclusion Nafion  membranes in their protonic form and completely solvent saturated are found to behave similarly towards water and methanol, i.e., the membranes do not prefer the one to the other. The solvation of the protonic groups are also identical for the two solvents. For mixtures of methanol and water an additional solvent uptake is observed. At low concentrations for one of the solvents the uptake takes place in addition to the solvent already present but at intermediate concentrations mutual exclusion takes place.

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Upon repeated treatments with liquid water– methanol mixtures a gradual weight loss is observed, indicating a limited stability of Nafion  towards these mixtures.

Acknowledgments Dr. P. Stein, Department of Chemistry, Odense University is gratefully acknowledged for assistance with the NMR measurements. Financial supports from the Nordic Energy Research Program for P. Kauranen and from the ERASMUS program for J. Hentschel are also gratefully acknowledged.

References [1] X. Ren, M.S. Wilson, S. Gottesfeld, J. Electrochem. Soc. 143 (1996) L12. [2] M.W. Verbrugge, J. Electrochem. Soc. 136 (1989) 417. [3] R.S. Chen, J.R.P. Jayakody, S.G. Greenbaum, Mat. Res. Soc. Symp. Proc. Vol. 293 (1993) 99. [4] X. Ren, T.A. Zawodzinski Jr., F. Uribe, H. Dai, S. Gottesfeld, in: Proc. First Int. Symp. Proton Conducting Membrane Fuel Cells I, eds. S. Gottesfeld, G. Halpert and A. Landgrebe (Electrochemical Society, 1995). [5] P.S. Kauranen, E. Skou, J. Appl. Electrochem. 26 (1996) 909. [6] D. Nandan, H. Mohan, R.M. Iyer, J. Mem. Sci. 71 (1992) 69. [7] T.A. Zawodzinski Jr., T.E. Springer, F. Uribe, S. Gottesfeld, Solid State Ionics 60 (1993) 199. [8] G. Pourcelly and C. Gavach, in: Perfluorinated Membranes in Proton Conductors, ed. P. Colomban (Cambridge University Press, 1992) p. 294.