J. Phys. Chem. B 2001, 105, 4151-4154
4151
Electrochemical and Raman Spectroscopy Study of a Nafion Perfluorosulfonic Membrane in Organic Solvent-Water Mixtures Jocelyne Ethe` ve, Patrice Huguet, Christophe Innocent, Jean Luc Bribes, and Ge´ rald Pourcelly* Institut Europe´ en des Membranes, UniVersite´ Montpellier II, CC 047, 34095 Montpellier Cedex 5, France ReceiVed: October 4, 2000; In Final Form: February 7, 2001
The presence of an organic solvent often modifies the transport properties of an ion exchange membrane. Compared to ethylene glycol (EG) and N-methylformamide (NMF), tetrahydrofuran (THF) has a particular behavior in a Nafion perfluorosulfonic membrane. This solvent induces a higher dimensional expansion but without significant modification of the transport properties in a 20% v/v THF-water mixture. Raman spectroscopy enables the location of the THF molecules in the membrane material. The hydrophobic part of the solvent molecules is in the perfluorinated chains (inducing a high dimensional expansion) while the hydrophilic part interacts with the aqueous clusters. Three water molecules in the range of 20-80% v/v THF-water surround each THF molecule.
1. Introduction
2. Experimental Section
Ion exchange membranes (IEMs) are used as separators in electrolytic cells because they allow the selective transport of cation or anion. Most of these applications are in an aqueous medium.1 The electrochemical applications of IEMs in organic solvent-water mixtures is a relative new domain of R&D,2-7 which requires the basic research into ion behavior across these charged membranes, such as membrane potential, ionic mobility, or electrical resistance.8-10 The presence of an organic solvent often decreases the performance as for example in a methanol fuel cell where methanol diffuses through the ion-conducting polymer. To achieve a better understanding of the behavior of an IEM equilibrated in an organic solvent mixture, it is necessary to investigate not only the basic electrochemical properties but also the sorption properties such as the repartition of the solvent in the different parts of the membrane material. From the point of view of industrial applications and for an understanding of the relationship of the structural properties, the swelling properties of IEMs are of crucial importance.11,12 Up to now, most of the structural investigations were essentially restricted to the study of dry membranes and of membranes equilibrated either with water or with aqueous electrolytes.13 However, some studies of the swelling of perflorosulfonic membranes in different nonaqueous solvents have been published.14-16 This swelling depends on the solvent, the counterion, and the temperature, and the swollen membranes are described as a twophase model where the swelling agent is contained in the clusters. To go deeper into the knowledge of the swelling properties of these membranes, it was interesting to apply Raman spectroscopy, which already appears as a powerful tool for investigating IEMs. This technique allows easy measurements of fundamental vibrational spectra of the membrane constituents when IEMs are immersed in various aqueous solutions17-20 or even in organic solvent-water mixtures.21
2.1. Materials. Measurements were made using the Nafion 117 membrane from Du Pont de Nemours (equivalent weight 1100 g, dry thickness 178 µm). Prior to any measurements, a classical acid-base pretreatment was carried out on 5 × 5 cm samples to remove impurities from the membrane phase.22 In addition, to carry out electrodialysis, the AMH anion exchange membrane was associated with the Nafion membrane. This membrane (equivalent weight 700, dry thickness 260 µm) from Tokuyama Soda Co. is a strong basic exchanger and has a high chemical resistance. Tetrahydrofuran (THF), ethylene glycol (EG), and N-methylformamide (NMF) of purum quality were purchased from Fluka. All the other reagents were of analytical grade. 2.2. Water Content Measurements. The amount of water contained in the polymeric membrane was determined by the Karl Fischer method. The Nafion membrane in the proton form was first equilibrated in organic solvent-water mixtures for a duration of 24 h. Then its faces were carefully swept using blotting paper, and a sample (1.5 × 1.5 cm) was weighed and introduced into the Karl Fischer titration cell (Tacussel, France) with 50 mL of solvent (Hydranal solvent from Riedel de Haen). The Karl Fischer reactant (Hydranal titrant from Riedel de Hean) was then introduced into the cell with an automatic buret, EBX 3 (from Tacussel, France), and the equivalent point was determined with a platinum indicator electrode (XM 190, Radiometer) connected to a processor (TT processor 2, Tacussel). The amount of water in the membrane is related to the weight of this membrane measured just before the Karl Fischer titration. Desiccation measurements were carried out with a halogen moisture analyzer, HR 73, from Mettler Toledo. A piece of membrane (about 1 cm2) was placed in the sample analyzer. A halogen lamp was used to increase temperature, and the weight of the sample was recorded as a function of time. The loss of weight was automatically calculated (wt %). Optimal conditions have been determined: a temperature of 60 °C for 5 min was necessary to obtain a constant mass, which corresponds to a complete dehydration.
* To whom correspondence should be addressed. Phone: +33 (0)467149110. Fax: +33 (0)467149119. E-mail:
[email protected].
10.1021/jp003642h CCC: $20.00 © 2001 American Chemical Society Published on Web 04/17/2001
4152 J. Phys. Chem. B, Vol. 105, No. 19, 2001
Figure 1. Scheme of the laboratory cell for the electrodialysis experiment. CEM, Nafion 117 membrane; AEM, AMH from Tokuyama Soda; 30 mA cm-2 compartments, (1) 1 L of K2SO4, 0.1 M, (2 and 2′) 0.5 L of H2SO4, 0.5 M, (3) 0.250 L of NaCl, 0.1 M.
2.3. Raman Spectroscopy Measurements. Raman spectra were obtained by excitation with 632.8 nm radiation from a He-Ne laser operated at about 17 mW (∼12 mW on the sample). The spectra were recorded, at 25 °C, with a LABRAM 1B confocal Raman spectrometer (Jobin-Yvon S.A., Horiba, France). The detector had a two-dimensional array of MPPCCD (1024 × 256 pixels) TE cooled at a temperature of about -65 °C. This spectrometer can be used in a macroscopic or microscopic configuration. Solution Spectra. Raman spectra were obtained in the macroscopic mode. An optical system mounted on the turret of the microscope was used to work with the laser beam perpendicular to the microscope optical axis. This device allows the entire microscopic and macroscopic objectives to work horizontally. In the macroscopic mode, the laser beam is focused on the sample by a 50 mm focal lens, and in this case the volume of the illuminated solution is constituted by a cylinder (diameter 250 µm, height 10 mm). The solutions are contained in optical parallepipedic cells filled with 0.2 mL of liquid. Membrane Spectra. Membrane spectra were obtained in microscopic mode using the device described above. Pieces of membranes are maintained vertically between a coverglass and an optical cell face. This cell (5 × 5 × 2 cm) contains the membrane immersed in the solution. We used a long work distance (10.6 mm), objective 50× (NA ) 0.50), to record Raman spectra through the cell. Bearing in mind that our membranes are about 150 µm thick, the microscope confocality allows us to record only the light scattered by the membrane sample, because in our experimental conditions, the analyzed membrane depth is about 50 µm. 2.4. Electrodialysis. The electrodialysis stack (ED) was composed of two electrode compartments between which two identical compartments of 10 mm thickness were inserted (Figure 1). The cell was built in PTCFE (Kel’F), which has an excellent chemical stability toward organic solvents. Viton seals performed the electrolyte tightness between the membranes and the PTCFE. The cell operated in a batch mode, with a uniform linear velocity of 0.05 m s-1. Each platinum-coated electrode had a useful working area of 40 cm2. The applied current density was 30 mA cm-2, and the operating temperature was 25 °C. All the solutions were acids (H2SO4) or salts (K2SO4 or NaCl) dissolved in water for the first set of experiments and then in 20% v/v THF-water mixtures. This protocol enabled the comparison of fluxes of sodium and chloride ions through the
Ethe`ve et al.
Figure 2. Expansion area of a Nafion membrane versus the amount of solvent in the equilibrating solution: (2) THF, ([) NMF, (9) EG.
membranes (CEM and AEM, respectively) in water and in THF-water mixtures. The anode compartment (no. 1 in Figure 1) was added to the cell to prevent chloride ions coming from compartment no. 3 from being oxidized at the anode. The outlet of the acidic compartment (no. 2) was recycled again into the cathode compartment (no. 2′). Compartment no. 3 was filled with 0.5 M NaCl. Sodium and chloride ions were then titrated versus time in compartment no. 2 or 2′ by atomic absorption spectroscopy (Varian Spectra AA-20) and chloride analyzer (CA 926 from Corning). 3. Results and Discussion 3.1. Swelling Measurements. To characterize membrane materials in an organic solvent-water mixture, dimensional expansion and water content have been determined in three solvents of different dielectric constants, THF (7.4), EG (37.7), and NMF (183.4). The expansion area of the Nafion membrane under the proton form versus the composition of the equilibrating solution is depicted in Figure 2. Expansion of an ion exchange membrane is usually measured by the change in length of the membrane with swelling. However, due to the fact that swelling of the Nafion membrane changes according to the direction, it is the area which has been plotted versus the amount of solvent instead of the length. The behavior of the membrane in a THFwater mixture is completely different from that in the two other solvents. The amount of water versus the composition of the equilibrating solution is reported in Figure 3. Swelling decreases for the EG- and NMF-water mixtures while it presents a maximum for THF. When the amount of THF exceeds 50 vol % in the equilibrating solution, the water content of the membrane not only does not increase any more but is slightly reduced. 3.2. Raman Spectroscopy and Electrodialysis. Swelling measurements have shown the specific behavior of the Nafion membrane in THF-water mixtures. We have already emphasized the important advantages of Raman spectroscopy in the study of ion exchange membranes to obtain valuable information.23 First, we have recorded Raman spectra of aqueous THF solutions with different compositions. The most intense band at 913 cm-1, in the pure liquid at room temperature, is disymmetric. In aqueous solutions at low water content a shoulder appears. When the water composition of the solution is increased, a splitting into two bands occurs (see Figure 4a). Moreover, the ∆ν˜ hl between the two bands [ν˜ h(highest) ν˜ l(lowest)] varies linearly versus the volume percentage in water
A Nafion Membrane in Solvent-Water Mixtures
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4153
Figure 3. Amount of water in the Nafion membrane versus the amount of solvent in the equilibrating solution: (2) THF, ([) NMF, (9) EG.
Figure 5. (a) Band doublet splitting evolution in THF-water mixtures of different percentages. (b) Band doublet splitting evolution inside the Nafion membrane equlibrated with different THF-water mixtures.
Figure 4. (a) Raman spectra of solutions (% v/v): (A) pure THF, (B) THF-water (70/30), (C) THF-water (40/60). (b) Raman spectra of the Nafion membrane equilibrated with solutions (% v/v): (A) pure THF, (B) THF-water (80/20), (C) THF-water (50/50), (D) THFwater (20/80), (E) pure water.
(see Figure 5a). Taking into account the important work published by Cadioli et al.,24 these experimental results can be explained. Actually, the accidental degeneracy of several bands occurs at 913 cm-1 in pure THF. Six lines are neatly resolved between 840 and 955 cm-1 in the liquid state at 165 K, but at room temperature only one line is present24 at 913 cm-1. Among these bands, three of them, ν13 (919 cm-1), ν14 (895 cm-1),
and ν30 (910 cm-1), are due to ring stretching vibrations. According to their calculations, ν13 and ν30 are essentially C-C ring stretching vibrations whereas ν14 is essentially a C-O-C ring stretching vibration. Deconvolution of all the recorded spectra shows that the position of the lowest frequency band (ν14) is strongly affected by the surroundings of water molecules while this is not the case for the highest ones (ν13 and ν30). Our results, concerning ∆ν˜ hl, are not in agreement with the work of Strajbl et al.,25 where some confusion has been made on the data (solid for liquid) given by Cadioli et al.24 By confocal Raman microspectrometry we can record spectra inside the Nafion membrane immersed and in equilibrium with various aqueous THF solutions (Figure 4b). In this case the ∆ν˜ hl remains constant versus the percentage volume of water within the precision of our technique (Figure 5b). Besides, if we record the Raman spectra of the Nafion membrane equilibrated with pure THF (some water content is still present in the membrane), the ∆ν˜ hl clearly decreases in this case. It is to be noted that we have completely dried a piece of Nafion membrane and placed it in pure THF. In this case the membrane was solubilized, forming a sort of gel with pure THF. Then, as the ∆ν˜ hl is due to the interactions between water and THF, we can use the ∆ν˜ hl as a probe to determine the number of water molecules interacting with a THF molecule inside the Nafion membrane. Taking into account the ∆ν˜ hl value measured in the Nafion membrane, we use the linear plot obtained in solution to determine the corresponding THF/water percentage and then the number of water molecules interacting
4154 J. Phys. Chem. B, Vol. 105, No. 19, 2001 TABLE 1: Sodium and Chloride Ionic Fluxes (mol cm-2 s-1) during a 3 h Electrodialysis in Water and in a 20 % v/v THF-Water Mixturea
0.5 M NaCl in water 0.5 M NaCl in 20 vol % THF
Na+ ions (through the CEM)
Cl- ions (through the AEM)
2.15 × 10-7 2.15 × 10-7
2.16 × 10-7 2.16 × 10-7
a Calculated values of transport numbers of sodium and chloride ions: tNa+ ) 0.69 through the Nafion membrane. tCl- ) 0.69 through the AEM (AMH membrane).
Ethe`ve et al. Finally, we have measured the kinetic desorption of the THF. At time t ) 0 a Nafion membrane equilibrated with pure THF is immersed in water during some given interval of time. Then at time t1 it is removed, its faces are dried, and Raman spectra are recorded. The ratio R ) ITHF/IREF is determined. ITHF is the intensity of a chosen band of the THF molecule whereas IREF is the intensity of a chosen band of the polymer matrix of the membrane. Here, we have chosen the band at 725 cm-1, involving essentially CF2 symmetric stretching vibrations.17 The operation is repeated again until all the THF has been desorbed. As can be seen in Figure 6 a very rapid desorption of the THF is observed, pointing out that no strong interaction is involved between THF and the Nafion membrane as is expected if this material interacts with the polymer chains only. References and Notes
Figure 6. Kinetic desorption in water of the THF contained within the Nafion membrane.
with one THF molecule. Our results give a value of three water molecules for one THF molecule. A reasonable hypothesis is that the hydrophobic part of the THF molecules is located in the perfluorinated chains while the hydrophilic part interacts with the water molecules contained in the clusters. This hypothesis can explain the important swelling of the Nafion membrane where THF separates the hydrophobic polymeric chains, consequently allowing a higher water uptake. According to our assumption, hydrophilic clusters, which constitute the conducting region of the membrane, are not affected by the presence of THF. In this case, the values of the transport number of sodium ions should not be affected by the presence of THF in electrodialysis tests. Effectively, Table 1 shows that 3 h electrodialysis tests carried out in 20% v/v THF or in water did not show any difference in the flux of sodium ions toward the cathode (and chloride ions toward the anode). With an applied current density of 30 mA cm-2, the transport number of sodium ions through the Nafion membrane is 0.69 in both solutions. The sulfate species coming from the cathode compartment carry the other part of the current. THF does not therefore interact with the fixed charged sites of the membranes as previously established by ESR measurements.26
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