Comparative investigations of ion-exchange membranes

Journal of Membrane Science 153 (1999) 83±90

Comparative investigations of ion-exchange membranes S. Kotera,*, P. Piotrowskia, J. Kerresb a

Faculty of Chemistry, N. Copernicus University, 7 Gagarin Street, PL-87100 TorunÂ, Poland b Institute for Chemical Engineering, University of Stuttgart, Stuttgart, Germany Received 5 May 1998; accepted 5 August 1998

Abstract The equilibrium and transport properties (conductivity, transport number, diffusion) of crosslinked ionomer membranes based on sul®nated and sulfonated PSU in aqueous solutions of HCl, NaCl and KCl have been investigated and compared with a Na®on 117 membrane. It has been found that these membranes are more compact and their conducting paths are of smaller dimension than that of the Na®on 117. The in¯uence of length of crosslinking chain, changing from ±(CH2)4± to ±(CH2)12±, is particularly indicated by the diffusion coef®cients; the conductivity and transport numbers of counterions are in¯uenced only slightly. Practically no dependence of this effect on the transport number of H‡ has been found. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Cation-exchange membrane; Sulfonated-crosslinked polysulfone; Swelling; Conductivity; Transport number; Diffusion

1. Introduction The ion-exchange membranes ®nd application in diverse processes (electrodialysis, diffusion dialysis, reverse osmosis, membrane electrolysis, membrane fuel cells) which are energy-, resource- and environment saving. The development of ion-exchange membranes of high chemical, mechanical and thermal stability, which meet growing demands of the aforementioned processes, is of great importance. Currently, most of the commercial ion-exchange membranes are composed of a sulfonated styrenedivinylbenzene copolymer showing suf®cient stability in many processes. In harsh environments, however, *Corresponding author. Tel.: +48-56-6114525; fax: +48-566542477; e-mail: [email protected]

the chemical stability of these membranes is unsatisfactory, and thus there is a great demand for new developments. For applications like membrane fuel cells, where the usage of sulfonated polystyrene± divinylbenzene copolymer membranes fails due to unsatisfactory oxidation stability, the per¯uorinated highly stable ionomer Na®on is the only commercial polymer available [1,2]. The high cost of this membrane ($800 per m2), however, has prevented a broad commercial application so far. As substitute for Na®on, sulfonated arylene-main-chain polymers like the poly(ethersulfone)s (PSU) or poly(etherketone)s (PEEK) are the materials of choice because they show the best chemical stability next to Na®on. Many publications deal with the synthesis of these sulfonated polymers and membranes. In some previous papers crosslinking procedures for these sulfonated

0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00242-7

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polymers and membranes have been reported, e.g. crosslinking of sulfonated PEEK via reaction of SO2Cl groups with dioxy-1,4-phenylene moieties of the PEEK chain [3], crosslinking of PSU-sul®nate± PSU-sulfonate blends and sul®nated and sulfonated PSU homopolymers by disproportionation of the sul®nic acid groups or alkylation of the sul®nate groups with a,w-dihalogenoalkanes [4±6]. However, little is known about the structure of these sulfonated blend membranes so far, and thus it is important to investigate their morphology. Many analytical tools can be applied for the structure elucidation of ionomers (e.g. small angle X-ray scattering, transmission electron microscopy, Raman spectroscopy, thermal analysis [7±12]). Alternate methods for characterization of membranes include the sorption, permeability, conductivity and diffusion measurements [13±16]. In this paper the basic characteristics (sorption, swelling, transport number, conductivity and permeability) of newly developed PSU membranes in three electrolytes (HCl, NaCl and KCl) is presented and discussed. As a reference for these membranes two cation-exchange membranes of different composition and structure have been chosen: the per¯uorinated membrane Na®on 117 (du Pont de Nemours, USA) and the interpolymer polyethylene [polystyrenesulfonic acid)-co-divinylbenzene] membrane KESD (Galena, Poland). 2. Experimental All the measurements were performed at 298 K. 2.1. Membranes The membranes chosen for the investigation are listed in Table 1. The preparation of PSU membranes has been described in [17]. The measurements of membranes in different ionic forms were performed with the same samples. 2.2. Measurements The swelling of membranes and sorption of electrolytes were determined by the standard procedures [18]. The geometric dimensions of membranes and their density were determined using a planimeter and a pycnometer, as described in [19].

The conductivity of membranes was determined by the ac method [20]. The apparent transport number of counterions, t1;app , was determined by the emf method using the silver±silver chloride electrodes. The transport number was calculated from the following equation taking the activity coef®cients from [21]: E ˆ ÿ2

RT a00 t1;app ln  : a0 F

(1)

In Eq. (1)a0 and a00 are the activities of the electrolyte solutions separated by a membrane. The permeability measurements were performed by monitoring conductometrically the changes of concentration on the dilute side of a membrane. The area of membrane was 3.14 cm2. The permeability coef®cient, Ps, was calculated according to Eq. (2)[22], using the least square method.  00  V 0 dm c ÿ c0 …0† ln 00 (2) ˆ Ps t: Yˆ c ÿ c0 …t† A…1 ÿ vs c00 † In Eq. (2) V0, c0 are the volume and the concentration of the dilute solution (at the beginning of experiment pure water), respectively, A the area of membrane, vs the partial molar volume of electrolyte, c00 the concentration of concentrated solution, and t is the time. The measurements were performed for the concentration c00 ˆ0.3 M. 3. Results and discussion 3.1. Ion-exchange capacity and swelling The non-transport properties of the membranes are summarized in Table 2. The main feature of an ion-exchange membrane is its ion exchange capacity. As the densities of membranes differ, the ion exchange capacity expressed in moles per dm3 of polymer network is used (Fig. 1) for discussion. The necessary volume of polymer network has been calculated as the difference of a volume of swollen membrane and a volume of sorbed water, assuming its density the same as for bulk water. According to the chemical composition of PSU membranes their IEC is similar. The exception is the 1,4BrM40 membrane in which the weight fraction of sulfonated polysulfon is two times smaller than in

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Table 1 Membranes Membrane

Polymer

1,4BrM40

PSU 1 group ±SO2Li/mer 60 40 1,4-Dibromobutane PSU 0.8 group ±SO3Li/mer PSU 1 group ±SO2Li/mer 20 80 1,6-Dibromohexane PSU 0.75 group ±SO3Li/mer PSU 1 group ±SO2Li/mer 20 80 1,8-Dibromooctane PSU 0.8 group ±SO3Li/mer PSU 0.5 group ±SO2Li/mer 20 80 1,8-Dibromooctane PSU 0.75 group ±SO3Li/mer 20 80 1,12-Dibromodecane PSU 1 group ±SO2Li/mer PSU 0.75 group ±SO3Li/mer Saponified copolymer of sulfonyl fluoride vinyl ether and tetrafluoroethylene Sulfonated polyethylene/poly(styrene-co-divinylbenzene) interpenetrating polymer network [31]

1,6BrM80 1,8BrM80 1,8BrM80 0.5 G/E 1,12BrM80 Nafion 117 KESD

PSU-SO2Li (wt%)

PSU-SO3Li (wt%)

Crosslinking agent

Table 2 Nontransport properties of membranes Membrane

1,4BrM40 1,6BrM80 1,8BrM80 1,8BrM80 0,5 G/E 1,12BrM80 Nafion 117 KESD

Thickness (cm)

Density (g/cm3)

IEC (mol/kg dry mb)

IEC (mol/dm3 swollen mb)

Water content (wt%)

Na‡, H2O

Na‡, H2O

Na‡

Na‡, H2O

H‡

Na‡

K‡

0.011 0.012 0.015 0.012 0.008 0.021 0.022

1.23 1.24 1.25 1.23 1.24 1.74 1.05

0.91 1.15 1.23 1.19 1.28 0.83 1.29

0.88 1.09 1.13 1.04 1.16 1.09 0.75

22.5 25.2 26.9 29.4 26.7 22.1 46.3

22.0 23.7 26.4 29.2 26.6 18.9 44.6

19.6 21.6 24.2 24.7 23.6 13.8 40.9

other PSU membranes. However, it should be noted that its IEC is only about one-fourth smaller. The IEC (per volume of the polymer matrix) of Na®on 117 is similar to the PSU membranes crosslinked with longer dibromoalkanes, whereas the IEC of KESD is between that of the 1,4BrM40 and 1,6BrM80 membranes. Immersed in water the membranes take up water to different degrees (Fig. 2). The 1,4BrM40 membrane having the shortest crosslinking chains (±(CH2)4±) absorbs the smallest amount of water. The 1,8BrM80 0.5 G/E, which is less dense crosslinked (one ±SO2Li group per two mers, Table 1), absorbs even more water than the 1,12BrM80 crosslinked with the longest chains (±(CH2)12±). The KESD of low IEC absorbs the greatest amount of water which shows the loose structure of that membrane compared to the PSU membranes. The Na®on 117 absorbs similar amount of water as the PSU membranes. Generally, according

to the cation hydration [23] the swelling of membranes increases from the K‡- to the H‡-form. It is characteristic that the water content in the K‡-form of Na®on 117 with respect to the Na‡-form (ca. 73%) is lower than for other membranes (ca. 90%). It is in agreement with the previous observations for Na®on 120 [24] and for polystyrenesulfonate resins of different crosslinking [25]. Such a difference can be caused by a better access of the K‡-ions to the sulfonate groups attached to the ends of pendant chains in Na®on compared to the PSU and KESD membranes, where the sulfonate groups are attached to the main chains. 3.2. Sorption of electrolytes The concentration of sorbed electrolytes into the membranes equilibrated with 1 M electrolyte solu-

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Fig. 2. Water content (in moles per dm3 of swollen membrane) in membranes equilibrated with water.

Fig. 1. Concentration of fixed charges in membranes equilibrated with water: (A) moles per dm3 of polymer matrix, (B) in moles per kg of sorbed water.

tions is shown in Fig. 3. It increases in the order Na®on 117Na‡>K‡ [23]. The lower the hydration energy, the stronger the electrostatic interactions of counterions with charged groups, which result in a higher sorption. It should be noted that according to the

Fig. 3. Molality of electrolytes inside membranes equilibrated with 1 M solution.

conclusion by Xue et al. [26], based on the IR studies [27], the association of the K‡ ions with the sulfonic groups should not take place. A difference in the association degree of K‡±Clÿ and Na‡±Clÿ [28] can also be taken into account. 3.3. Conductivity Similarly as for aqueous solutions the conductivity of membranes changes in the order NaCl
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clusters is ca. 4 nm, that of channels connecting them ca. 1 nm [29]. As the dimension of clusters is higher than the distance between the chains in the PSU membranes, the tortuosity and hindrance factor in the Na®on should be smaller, which manifests itself in the high conductivity. For better comparison of the in¯uence of charged polymer matrix on the ionic mobilities, the ratio of conductivities of electrolytes X and Y (Fig. 5) has been calculated according to the formula  ratio ˆ Fig. 4. Specific conductivity of membranes equilibrated with 1 M solution of electrolyte; HCl ± left Y-axis, NaCl and KCl ± right Yaxis.

membranes a very high (more than two times) increase of conductivity is observed coming from the 1,4BrM40 (a cross-linking bridge ±(CH2)4±) to the 1,6BrM80 (a cross-linking bridge ±(CH2)6±), although the total concentration of ions in both membranes is similar (Figs. 1 and 3). The calculation of the distance between the polysulfon chains at the points of crosslinking yields: ±(CH2)4± ± 0.625, ±(CH2)6± ± 0.875 nm). Further increase of the length of crosslinking chain (±(CH2)8± ± 1.125, ±(CH2)12± ± 1.625 nm) does not result in such an increase in conductivity ± when comparing the 1,6BrM80, 1,8BrM80 and 1,12BrM80 (Fig. 4). Also the decrease of crosslinking density along the polysulfon chain does not result in a noteworthy increase of conductivity ± when comparing the 1,8BrM80 and the 1,8BrM80 0.5 G/E, for which the distances between the crosslinking points along the folded polysulfon chain are 1.4 and 2.8 nm, respectively. Certainly, the above-mentioned distances concern only the net of neutral chains which is penetrated by the charged chains. The distance between the latter chains can be different. The conductivity of Na®on 117 is much higher than that of the PSU membranes, although the IEC (Fig. 1), the water content (Fig. 2) and the sorption of electrolytes (Fig. 3) in these membranes are comparable. This fact evidently indicates the different morphology of these membranes. For the Na®on the cluster-network model is generally accepted. The dimension of

…c1 01

m;X =m;Y ;  ‡ c2 02 †X =…c1 01 ‡ c2 02 †Y

(3)

where ci is the concentration of ion i in moles per m3 of swollen membrane, and 0i is the limiting molar conductivity of ion i, taken from [21]. In the ideal case the kappa ratio should be 1. The deviation from this value indicates different interactions of counterions with ®xed charges and water. Close to the ideal case is the ratio KCl/NaCl, except for Na®on 117. Generally, the Na‡ ions are slightly more favourable than the K‡ ions. Comparing the metal cations with the H‡ ions it is clearly seen that the latter ones are less hampered by the membranes. The protons hop between neighbouring molecules of water, this mechanism of transport known as the Grotthuss mechanism was lastly discussed in [30]. Comparing HCl and NaCl it is seen that only for the Na®on 117 and KESD the kappa ratio is close to 1. In the KESD membrane the water content is higher than in the PSU

Fig. 5. Conductivity ratio (Eq. (3)) for membranes in 1 M solution of electrolytes.

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membranes (Fig. 2), which makes the migration of Na‡ and H‡ more similar to that in a solution. In the Na®on 117 the internal solution occupies larger domains than in the PSU, which makes the migration of Na‡ and H‡ more similar to that in a solution. One should think that in the PSU ionomers the type of aggregation of ions is rather multiplets than clusters. 3.4. Apparent transport number of counterions The transport number of ions in the membrane is a function of mobility and of concentration ratios of coions and counterions. According to the highest mobility of hydrogen ion its apparent transport number in all the membranes exceeds that of other cations (Fig. 5). Having in mind that in a solution the mobility of K‡ is about 50% higher than Na‡, it could be surprising that in the membranes tK‡ ;app < tNa‡ ;app . This fact is consistent with the previous statement about stronger interactions of less hydrated potassium ions with ®xed charges, which reduces the mobility of these ions and increases the sorption. Both these factors reduce the transport number. It is also con®rmed by the kappa ratio (KCl/NaCl) <1 (Fig. 5). Taking into account the PSU membranes ± the longer the crosslinking chain the lower the t1;app . The only exception is the 1,4BrM40 because of a low content of charged polymer. This effect is more pronounced for metal cations than for hydrogen ions. The PSU membranes seem to be more H‡-selective than the Na®on 117. The polystyrenesulfonic membrane KESD shows the smallest selectivity (Fig. 6). This is the only membrane for which tK‡ ;app > tNa‡ ;app like in aqueous solutions. 3.5. Diffusion of electrolytes The diffusion coef®cients of electrolytes inside the membranes have been calculated according to  S ˆ PS cS =cS ; D

(4)

where cS is the concentration of the sorbed electrolyte. The PSU membranes are much less permeable to electrolytes than the Na®on 117 and the KESD (Fig. 7) similarly as in the case of the conductivity (Fig. 4). The correlation between the permeability and length of crosslinking chain is clearly visible. Passing from the 1,8BrM80 to 1,12BrM80 the increase of

Fig. 6. Apparent transport number of counterions in membranes for the mean concentration 1 M.

Fig. 7. Diffusion coefficient of electrolytes in membranes, c00 ˆ0.3 M.

diffusion coef®cient is more pronounced than that of conductivity. This fact suggests that these two transport processes proceed in different places, i.e. more and less remote from the charged polymer chains. For all the membranes the diffusion coef®cient of HCl is higher than DKCl and DNaCl. The inequality DKCl>DNaCl is observed for the PSU membranes. Only in the Na®on DKCl
S. Koter et al. / Journal of Membrane Science 153 (1999) 83±90

per¯uorinated Na®on 117 membrane in the H‡- and Na‡-forms and less than the interpolymer polyethylene [polystyrenesulfonic acid)-co-divinylbenzene] membrane KESD. The analysis of transport data leads to the conclusion that the PSU membranes are characterized by more narrow conducting paths than the Na®on membrane. Their type of aggregation of ions is rather multiplets than clusters. In comparison to the sodium ions the transport of protons is more favourable in the PSU membranes. The increase of the length of crosslinking chains (from ±(CH2)6± to ±(CH2)12 in¯uences swelling and sorption (<25%) and the apparent transport numbers of sodium and potassium ions (<6%) rather slightly. On the other hand the conductivity (70±80%) and especially the diffusion (>300%) strongly increases. The transport number of hydrogen ions is practically unchanged. References [1] W.G. Grot, Perfluorinated ion-exchange polymers and their use in research and industry, Macromol. Symp. 82 (1994) 161. [2] K. Ledjeff, A. Heinzel, F. Mahlendorf, V. Peinecke, Die reversible Membran-Brennstoffzelle Dechema-Monographien Band 128, VCH Verlagsgesellschaft, 1993, p. 103. [3] F. Helmer-Metzmann, K. Ledjeff, R. Nolte et al., Polymerelektrolyt-Membran und Verfahren zu ihrer Herstellung, EP 0 574 791 A2 Offenlegungsschrift, Hoechst AG, 1993. [4] J. Kerres, W. Cui, R. Disson, W. Neubrand, Development and characterization of crosslinked ionomer membranes based upon sulfinated and sulfonated PSU. 1. Crosslinked PSU blend membranes by disproportionation of sulfinic acid groups, J. Membr. Sci., in press. [5] J. Kerres, W. Cui, M. Junginger, Development and characterization of crosslinked ionomer membranes based upon sulfinated and sulfonated PSU. 2. Crosslinked PSU blend membranes by alkylation of sulfinate groups with dihalogenoalkanes, J. Membr. Sci., in press. [6] J. Kerres, W. Zhang, W. Cui, New sulfonated engineering polymers via the metalation route. 2. Sulfinated±sulfonated poly(ethersulfone) PSU Udel1 and its crosslinking, J. Polym. Sci. A, 36 (1998) 1441. [7] H.W. Starkweather Jr., , Crystallinity in perfluorosulfonic acid ionomers and related polymers, Macromolecules 15 (1982) 320. [8] G. Gebel, P. Aldebert, M. Pineri, Structure and related properties of solution-cast perfluorosulfonated ionomer films, Macromolecules 20 (1987) 1425. [9] J. Halim, F.N. Buchi, O. Haas, M. Stamm, G.G. Scherer, Characterization of perfluorosulfonic acid membranes by

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