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Characterization of cation-exchange membranes prepared from poly(vinyl alcohol) and poly(vinyl alcohol-b-styrene sulfonic acid)
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Characterization of cation-exchange membranes prepared from poly(vinyl alcohol) and poly(vinyl alcohol-b-styrene sulfonic acid) Mitsuru Higa*, Megumi Nishimura, Kota Kinoshita, Atsushi Jikihara Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube-city, Yamaguchi 755-8611, Japan
article info
abstract
Article history:
Block-type cation-exchange membranes (CEMs) have been prepared by blending poly(vinyl
Received 7 March 2011
alcohol) (PVA) and the polyanion poly(vinyl alcohol-b-styrene sulfonic acid) at various
Received in revised form
molar percentages of cation-exchange groups to vinyl alcohol groups, Cpa, and by cross-
20 May 2011
linking the PVA chains with glutaraldehyde (GA) solution at various GA concentrations,
Accepted 1 June 2011
CGA. The characteristics of the block-type CEMs were compared with random-type CEMs
Available online 16 July 2011
prepared in a previous study from the random copolymer, poly(vinyl alcohol-co-2acrylamido-2-methylpropane sulfonic acid). At equal molar percentages of the cation-
Keywords:
exchange groups, the water content of the block-type CEMs was less than that of the
Poly(vinyl alcohol)
random-type CEMs. The charge density of the block-type CEMs increased with increasing
Cation-exchange membrane
Cpa and reached a maximum value. Further, the maximum value of the charge density
Cross-linking conditions
increased with increasing CGA. The maximum charge density value of 1.3 mol/dm3 was
Ionic transport property
obtained for the block-type CEM with Cpa ¼ 3.1 mol% and CGA ¼ 0.10 vol.%, which is almost
Block copolymer
two thirds of the value of a commercially available CEM [CMX: ASTOM Corp. Japan]. A comparison of the block-type and random-type CEMs with almost the same membrane resistance showed that the block-type CEMs had higher dynamic transport numbers than the random-type ones. The dynamic transport number and membrane resistance of the block-type CEM with Cpa ¼ 4.0 mol% and CGA ¼ 0.10 vol.% were 0.96 and 4.9 U cm2, respectively. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Cation-exchange membranes (CEMs) have been used for various industrial purposes [1] such as separation of environmental polluting metal ions from hard water [2], chlor-alkali electrolysis [3], fuel cells [4e8], electrodialytic concentration or desalination of electrolyte solutions [9], and continuous hydrogen production in electro-electrodialysis systems [10e12]. At present, the vast majority of commercially available
CEMs for electrodialysis include styrene-co-divinylbenzene matrices. The drawbacks of this type of membrane are as follows: it is difficult to control the membrane structure because it undergoes copolymerization and cross-linking processes simultaneously, and the cost of membrane preparation is high. Recently, many novel ion exchange membranes have been developed to overcome these problems [13,14]. Ion exchange membranes can be prepared by mixing watersoluble base polymers and a polyelectrolyte and then cross-
* Corresponding author. Tel.: þ81 836 85 9203; fax: þ81 836 85 9201. E-mail address: [email protected] (M. Higa). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.06.003
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linking the base polymer. The membrane thus obtained consists of a semi-interpenetrating network (semi-IPN) structure in which the polyelectrolyte chains are immobilized in the cross-linked network polymer matrix. The ion exchange capacity of the membrane can be easily controlled by changing the ratio of water-swollen base polymer to polyelectrolyte. Poly(vinyl alcohol) (PVA) is one of the most popular watersoluble base polymers. PVA is a polyhydroxy polymer that has been studied intensively because of its good film forming and physical properties, high hydrophilicity, processability, biocompatibility, and good chemical resistance [15e18]. Many PVA-based CEMs have been developed for use in direct methanol fuel cells because of the excellent methanol barrier properties of PVA [19e26]. Various kinds of PVA-based CEMs with a semi-IPN structure have been prepared by blending PVA with a polyanion such as poly(styrene sulfonic acid) [27], poly(acrylic acid) [19], poly(styrene sulfonic acid-co-maleic acid) [20], sulfosuccinic acid [21], sulfonated poly(ether ether ketone) [22], sulfated b-cyclodextrin [23], titanium oxide nanotubes and poly(styrene sulfonic acid) [24], montmorillonite and poly(styrene sulfonic acid) [25]. One of the potential disadvantages of CEMs with a semi-IPN structure is that they show low longterm stability in aqueous solutions owing to the dissolution of the un-crosslinked water-soluble polyelectrolytes from the network into the solution. To overcome this disadvantage, CEMs with an IPN structure have been prepared by blending PVA and PVA-based polyanions such as the random copolymer of vinyl alcohol and 2-methyl-1-propanesulfonic acid (AMPS) groups [26,28e30]. In CEMs with an IPN structure, the chains of PVA and the PVA-based polyanion are cross-linked with each other; hence, these CEMs show higher long-term stability in water than CEMs with a semi-IPN structure. However, the counter-ion selectivity of CEMs with an IPN structure in previous study [30] is much lower than that of commercially available CEMs. The aim of this study is to obtain a PVA-based CEM with higher counter-ion selectivity than that of previously reported PVA-based CEMs. To this end, CEMs with an IPN structure (hereafter referred to as PVA-b-PSS) have been prepared by blending PVA with the PVA-based polyanion, poly(vinyl alcohol-b-styrene sulfonic acid), which is a block copolymer of vinyl alcohol and sodium p-styrenesulfonate groups. The chemical structure of PVA-b-PSS is shown in Fig. 1. Because PVA is a semi-crystalline polymer, the crystalline region acts as a physical cross-link point, and hence, the water content of the CEM, which considerably influences the counter-ion selectivity, electrical resistance, and mechanical properties of the membrane, depends on the degree of crystallinity. The crystallinity of the PVA region within a CEM prepared from the
Fig. 1 e Chemical structure of polyanion, PVA-b-PSS: poly(vinyl alcohol-b-styrene sulfonic acid).
block copolymer is higher than that of a CEM prepared from the random copolymer, because the cation-exchange groups of the random copolymer chains disturb the formation of the crystal. Thus, the former has a higher counter-ion permselectivity than the latter. We have investigated the differences in the following ionic transport properties between PVA-based CEMs prepared from the two different copolymers: charge density, membrane resistance, and transport number.
2.
Experimental
2.1.
Materials
Poly(vinyl alcohol) (PVA, 100% hydrolyzed, average Mw ¼ 198,000) and poly(vinyl alcohol-b-styrene sulfonic acid) (PVA-b-PSS, 100% hydrolyzed) were obtained from Kuraray Co., Ltd. Glutaraldehyde (GA) (25 wt.% solution in water) was of analytical grade and was obtained from Wako Pure Chemical Industries. Sulfuric acid and sodium sulfate were of analytical grade and were obtained from Nakarai Tesque.
2.2.
Preparation of block-type CEMs
Self-standing base membranes, BP-X (X ¼ 1e6), for block-type CEMs were prepared by casting an aqueous solution of a mixture of PVA and PVA-b-PSS on a plastic plate and then drying the mixture over a hot stage (NISSIN, NH-45N) overnight at 50 C. The thickness of the base membranes was ca. 0.1 mm. The weight percentage of PVA-b-PSS to PVA in the mixture solution was varied to control the molar percentage of cation-exchange groups to vinyl alcohol groups (Cpa) in BPX, as shown in Table 1. The block-type CEMs were prepared by annealing BP-X at 180 C for 30 min under vacuum to induce physical cross-linking between the PVA chains. Chemical cross-linking was induced by immersing the membranes in an aqueous solution of various concentrations of GA, 0.05 mol/ dm3 of H2SO4 (pH ¼ 1), and 2.0 mol/dm3 of Na2SO4 at 25 C for
Table 1 e Molar percentage of cation-exchange groups, Cpa, and ion-exchange capacity, IEC, of base membrane: membranes prepared from block copolymers, BP-X (X [ 1e6); and membrane prepared from random copolymer, RP-1. Sample BP-1 BP-2 BP-3 BP-4 BP-5 BP-6 RP-1b CMXc
Cpa (mol%)
IEC (meq/g-dry CEM)
1.8 2.2 2.6 3.1 3.5 4.0 3.2 e
0.28 (0.36a) 0.39 (0.44) 0.48 (0.51) 0.54 (0.58) 0.58(0.65) 0.59 (0.65) 0.62 (0.78) 2.0
a Results calculated from the molar percentage of the cationexchange groups. b One of the random copolymers used in Ref. [30]. c A commercially available CEM [Neosepta CMX, ASTOM Corp. Japan].
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24 h to control the water content of the membranes in the equilibrium swelling state in deionized water. Fig. 2 shows the chemical cross-linking mechanism of PVA and GA.
2.3.
Measurement of membrane water content
The water content, H, of the membranes was measured as follows. Base membranes were weighed in the dry state after annealing and chemical cross-linking. The cross-linked membranes were immersed in deionized water at 25 C for 7 days. The membranes were removed from water, dabbed with a filter paper to remove excess water on the membrane surfaces, and weighed in the wet state. For the sake of simplicity, we assume the volume between the water phase and the polymer phase of a swollen membrane to be additive. The volumetric water content is calculated from the weights in the wet state, Ww, and in the dry state, Wd, as shown below: H¼
ðWw Wd Þ=1:0 Wd ðWw Wd Þ=1:0 þ 1:3
(1)
where 1.0 and 1.3 are the densities of water and PVA [31], respectively.
2.4.
Measurement of ion exchange capacity (IEC)
The IEC of a CEM is an important parameter because the ionic transport properties of the CEM depend on the amount and species of the ion exchange groups. IEC is expressed as milliequivalent per gram of membrane (meq/g-dry CEM) and is determined as follows: a sample membrane was immersed in 0.10 M KCl solution for 3 h before measuring the IEC. The membrane was rinsed with deionized water to remove nonexchange KCl electrolyte adsorbed on the membranes and was then immersed in 0.20 mol/dm3 of NaNO3 with a volume of 50 cm3 under stirring for 12 h to achieve the complete exchange of Kþ ions in the membrane with Naþ ions in the solution. The concentration of Kþ ions in the solution, CKþ , was determined by using an ion chromatograph (Dionex ICS1500). The membrane was dried under vacuum for 24 h and was weighed in the dry state, Wd. The IEC of the base membrane was calculated using the following equation: IEC ¼
2.5.
CKþ 100 Wd 1000
(2)
Determination of membrane charge density
Membrane charge density is one of the most important factors for estimating the permselectivity between ions with different
charge and valence in an ion exchange membrane. In order to estimate the charge density, the membrane potentials, Dɸ, were measured as a function of the KCl concentration in the two chambers of the apparatus described elsewhere [32]. One chamber of the cell was filled with KCl solution of various concentrations, Co, while the other chamber was filled with KCl solutions with concentration, Cd, which is five times higher than that in the former (r ¼ Cd/Co ¼ 5). The membrane potential between the solutions in the two chambers was measured at 25.0 0.5 C using Ag/AgCl electrodes (TOA HS205C) with a salt bridge (3.0 mol/dm3 KCl) and a voltmeter [TOA HM-20E]. The membrane charge density, Cx, was calculated from the measured membrane potential using the following equation [33,34]: 1 0 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi C2x þ ð2Co Þ2 Cx C RT B Df ¼ ln@r$qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi A F C2x þ ð2rCo Þ2 Cx 1 0qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 þ ð2rC Þ2 C W C o x RT x C B W ln@ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi A F C2x þ ð2Co Þ2 Cx W
(3)
where W ¼ (uK uCl)/(uK þ uCl); uK and uCl are the Kþ and Cl ion mobilities in the membrane, respectively; F, R, and T are the Faraday constant, gas constant, and absolute temperature, respectively. Parameters W and Cx are adjusted such that the left-hand side of Eq. (3) fits the experimental data at various KCl concentrations.
2.6.
Measurement of membrane resistance
The electrical resistance of the membranes was measured by using a hand-made acrylic plastic cell composed of two parts separated by a membrane, as described elsewhere [32], with an LCR meter operated at 10 KHz AC (A&D Corp. Ltd. AD-5827). 0.5 mol/dm3 of NaCl solution was poured into the two cell compartments, after which the electrical resistance, Ro, was measured at 25.0 0.5 C in a water bath. Subsequently, a sample membrane was set in the cell and the resistance, Rs, was measured again under the same conditions. The difference between Rs and Ro gives the membrane resistance, Rm.
2.7.
Measurement of dynamic state transport number
The dynamic state transport number, tdþ, of the membranes was determined by electrodialysis carried out using the handmade acrylic plastic cell with two parts separated by a membrane as shown in Fig. 3. The measurements were performed at a current density of 10 mA cm2 and temperature of 25 C for 75 min. The amount of ions transported through the membrane during electrodialysis was measured using a conductivity meter (HORIBA 3552-10D). The dynamic state transport number was obtained using the following equation: tdþ ¼
Fig. 2 e Cross-linking mechanism of PVA using GA.
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DCVF Q
(4)
where DC, V, and Q are the concentration change of the transported ions, volume of the measurement solution, and
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Fig. 3 e Apparatus for measuring dynamic state transport number. A, power supply; B, ampere meter; C, coulomb meter; D, voltmeter; E, motor; F, stirrer; G, cathode electrode; H, anode electrode; I, 0.5 mol/dm3 NaCl solution; J, sample membrane. The effective area of the cell is 8.0 cm2.
amount of electricity passing through the membrane during electrodialysis, respectively.
3.
Results and discussion
3.1.
Water content as a function of polyanion content
The water content of an ion exchange membrane is an important property because both the charge density and electric resistance of the membrane depend on the water content. Fig. 4 shows the water content of the CEMs prepared in this study as a function of Cpa. Data obtained for the random-type CEMs in a previous study [30] are also shown in order to compare the characteristics of the block-type CEMs with the random-type ones. The water content increased with increasing Cpa, which is attributed to the fact that the osmotic pressure in the membranes increases with an increase in the number of charged groups in the membranes. Further, the water content decreased with increasing GA concentration, CGA, because of the increase in the number of chemical crosslinking points with increasing CGA. A comparison of the water content of the block-type CEMs with that of the random-type CEMs at the same molar percentage of the cation-exchange groups shows that the water content of the block-type CEMs is lower. For example, in CEMs with Cpa ¼ 3.1 mol% and CGA ¼ 0.01 vol.%, the water content of the block-type and the random-type CEMs are 0.45 and 0.75, respectively. This difference can be attributed to the fact that the crystallinity of the PVA region within the CEM prepared from the block copolymer is higher than that of the CEM prepared from the random copolymer.
Fig. 4 e Water content, H, of CEMs as a function of molar percentage of cation-exchange groups to vinyl alcohol groups, Cpa. Polyanion: open symbols, random copolymer; closed symbols, block copolymers. Concentration of GA solution: squares, 0.010 vol.%; triangles, 0.050 vol.%; closed circles, 0.10 vol.%; and open circles, 0.15 vol.%.
IEC of CEMs
3.2.
Table 1 shows the molar percentage of the cation-exchange groups, Cpa, and IEC of the base membranes (BP-X ) along with the values for a commercially available CEM, Neosepta CMX (ASTOM Corp., Japan). The experimental IEC values are about 10% less than the calculated results. This may be attributed to the fact that all the monomers with cationexchange groups in the reaction mixture did not react to form copolymers. The IEC of commercial CMX is 2.0 meq/g-dry CEM. Hence, the IEC of BP-6 is about one third of that of CMX.
Charge density of membranes as a function of Cpa
3.3.
The charge density can be used as an indicator of the permselectivity of an ion exchange membrane, because the higher the charge density of a membrane, the higher is its counterion permselectivity. Fig. 5 shows the charge density of the membranes as a function of Cpa. The charge density increased with Cpa and reached a maximum value. The charge density is proportional to the IEC divided by the water content, H: Cx f
IEC H
(5)
IEC is proportional to Cpa, as shown in Table 1. The water content also increases with Cpa. Hence, in the first stage of the charge density-Cpa curves, the charge density increases with increasing Cpa because of the increase in IEC. At high Cpa, the effect of the increase in the water content on the charge density is larger than that of the increase in IEC. As a result, the charge density decreases after reaching a maximum value. The maximum value of the charge density increases
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Fig. 5 e Charge density, Cx, of CEMs as a function of molar percentage of cation-exchange groups, Cpa. The definition of symbols in this figure is the same as that in Fig. 4.
with increasing CGA because the water content decreases with increasing CGA. The maximum value of the charge density is 1.27 mol/dm3 for the block-type CEM having Cpa ¼ 3.1 mol% and CGA ¼ 0.10 vol.%. The charge density of CMX measured under the same conditions is 1.8 mol/dm3. Thus, the charge density of the block-type CEM is almost two thirds of that of the commercially available CEM. For CEMs with Cpa ¼ 3.1 mol % and CGA ¼ 0.05 vol.%, the charge densities of the block-type CEM (with base membrane BP-5) and the random-type CEM (with base membrane RP-1) are 1.1 mol/dm3 and 0.33 mol/ dm3, respectively, although the base membranes of the two CEMs have almost the same IEC as shown in Table 1. The higher charge density of the block-type CEM relative to the random-type CEM is attributed to the fact that the former has lower water content than the latter, as shown in Fig. 4.
3.4.
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Fig. 6 e Membrane resistance, Rm, of CEMs as a function of molar percentage of cation-exchange groups, Cpa. The definition of symbols in this figure is the same as that in Fig. 4.
allow the permeation of only the counter ions in an electrodialysis system. Fig. 7 shows the dynamic state transport number of the CEMs as a function of Cpa. The transport number of the block-type CEMs increased with increasing Cpa, and then decreased after reaching a maximum value at Cpa z 3.1 mol%. As mentioned above, this trend is attributed to the fact that the charge density has a maximum value at Cpa ¼ 3.1 mol%. The transport number increased with increasing CGA. These results indicate that the transport number of the membranes can be controlled by changing the
Membrane resistance as a function of Cpa
The membrane resistance is important for energy-saving electrodialysis. Fig. 6 shows the membrane resistance as a function of Cpa. The membrane resistance decreases with increasing Cpa because of the increase in the water content and IEC with increasing Cpa. The resistance increases with increasing GA concentration because of the decrease in the water content. This implies that, in this study, the membrane resistance can be controlled by changing the water content. The lowest membrane resistance of 2.1 U cm2 was observed for the block-type CEMs at Cpa ¼ 4.0 mol% and CGA ¼ 0.01 vol.%.
3.5. Dynamic state transport number of CEMs as a function of Cpa The dynamic state transport number relates to the counterion permselectivity of an ion exchange membrane in electrodialysis. A membrane with a transport number of 1.0 can
Fig. 7 e Dynamic state transport number, tdD, of CEMs as a function of molar percentage of cation-exchange groups, Cpa. The definition of symbols in this figure is the same as that in Fig. 4.
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cross-linking conditions. The block-type CEMs with Cpa and CGA values similar to those of the random-type CEMs have larger dynamic state transport numbers. The dynamic transport number of the block-type CEM with Cpa ¼ 3.0 mol% and CGA ¼ 0.10 vol.% is 0.97, while that of CMX is 0.98 under the same conditions, indicating that the block-type CEM has almost the same dynamic transport number as the commercially available CEM.
3.6. Relationship between membrane resistance and dynamic state transport number An ion exchange membrane having both high ion permselectivity and low membrane resistance is desirable in electrodialysis processes. Fig. 8 shows the relationship between the membrane resistance and dynamic state transport number of the membranes. The membrane shown on the upper left-hand side of the figure has high performance for electrodialysis processes. The membrane resistance decreases and the dynamic state transport number increases with increasing Cpa. The transport number of the membrane increases with increasing GA concentration. A comparison of the block-type and random-type CEMs with almost the same membrane resistance shows that the block-type CEMs have higher transport numbers than the random-type CEMs. For example, the transport number of the block-type CEM with Cpa ¼ 4.0 mol % and CGA ¼ 0.10 vol.% is 0.96 whereas that of the random-type CEM with Cpa ¼ 3.5 mol% and CGA ¼ 0.10 vol.% is 0.80. Thus, the transport number of the former is about 20% higher than the latter although both CEMs have almost the same membrane resistances, 4.0 and 4.1 U cm2, respectively. The dynamic transport number and membrane resistance of the block-type CEM at Cpa ¼ 4.0 mol% and CGA ¼ 0.10 vol.% is 0.96 and 4.1 U cm2, respectively, while those of CMX is 0.98 and
2.3 U cm2, respectively. The block-type CEM prepared herein has almost the same dynamic transport number and a membrane resistance that is roughly two times higher than that of a commercially available CEM.
4.
Conclusions
In this study, CEMs with an interpenetrating network structure were prepared by blending PVA with the PVA-based polyanion, poly(vinyl alcohol-b-styrene sulfonic acid), at various molar percentages of cation-exchange groups, Cpa, and by cross-linking the polymers under various GA concentrations. The water content of the block-type CEMs was lower than that of the random-type CEMs when the molar percentage of the cation-exchange groups was almost the same for both the CEMs. The charge density of the obtained membranes increased with an increase in Cpa of the membranes and reached a maximum value. The maximum value of the charge density increased with increasing crosslinker concentration. The maximum charge density value of 1.3 mol/dm3 was obtained for the block-type CEM with Cpa ¼ 3.1 mol% and CGA ¼ 0.10 vol.%, which was almost two thirds of the value of the commercially available CEM, CMX. A comparison of the block-type and random-type CEMs having almost the same membrane resistance showed that the block-type CEMs had higher transport numbers than the random-type ones. The dynamic transport number and membrane resistance of the block-type CEM with Cpa ¼ 4.0 mol% and CGA ¼ 0.10 vol.% were 0.96 and 4.1 U cm2, respectively, while those of CMX were 0.98 and 2.3 U cm2, respectively. The membrane resistance of PVA-based CEMs can be reduced by optimizing the molar percentage of the cationexchange groups to vinyl alcohol groups and the crosslinking conditions, namely, the annealing temperature and GA concentration. The PVA-based CEMs showed almost the same performance for electrodialysisdhigh counter-ion permselectivity and low membrane resistancedas the commercially available CEMs and were prepared at a lower cost than the commercially available ones. Thus, PVA-based CEMs are potentially applicable to electrodialysis systems.
Acknowledgments This work was partly supported by the Electric Technology Research Foundation of Chugoku, Mazda Foundation, UBE Foundation, Kurita Water and Environmental Foundation (No. 20320), and Salt Science Research Foundation (Nos. 0612, 0709, and 0810).
Fig. 8 e Dynamic state transport number, tdD, of CEMs as a function of membrane resistance, Rm. The definition of symbols in this figure is the same as that in Fig. 4. Open diamond: Commercially available CEM, Neosepta CMX (ASTOM Corp. Japan).
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containing sulfonic acid groups for direct methanol fuel cell applications. J Membr Sci 2004;240:37e48. Yang T. Preliminary study of SPEEK/PVA blend membranes for DMFC applications. Int J Hydrogen Energy 2008;33: 6772e9. Yang T. Poly(vinyl alcohol)/sulfated b-cyclodextrin for direct methanol fuel cell applications. Int J Hydrogen Energy 2009; 34:6917e24. Yang C-C, Chien W-C, Li YJ. Direct methanol fuel cell based on poly(vinyl alcohol)/titanium oxide nanotubes/ poly(styrene sulfonic acid) (PVA/nt-TiO2/PSSA) composite polymer membrane. J Power Sources 2010;195:3407e15. Yang C-C. Fabrication and characterization of poly(vinyl alcohol)/montmorillonite/poly(styrene sulfonic acid) protonconducting composite membranes for direct methanol fuel cells. Int J Hydrogen Energy 2011;36:4419e31. Higa M, Sugita M, Maesowa S, Endo N. Poly(vinyl alcohol)based polymer electrolyte membranes for direct methanol fuel cells. Electrochim Acta 2010;55:1445e9. Uragami T, Nakamua R, Sugihara M. Studies on syntheses and permeabilities of special polymer membranes, 45. Active and selective transport of alkali metal ions through crosslinked poly(styrene sulfonic acid) membrane. Makromol Chem Rapid Commun 1982;3:467e70. Higa M, Yamakawa T. Design and preparation of a novel temperature-responsive ionic gel. 1. A fast and reversible temperature response in the charge density. J Phys Chem B 2004;108:16703e7. Yamakawa T, Ishida S, Higa M. Transport properties of ions through temperature-responsive charged membranes prepared using poly(vinyl alcohol)/ poly(Nisopropylacrylamide)/poly(vinyl alcohol-co-2-acrylamido-2methylpropane sulfonic acid). J Membr Sci 2005;250:61e8. Nishimura M, Shimizu E, Higa M. Electrodialytic transport properties of cation exchange membranes prepared from poly(vinyl alcohol) and poly(vinyl alcohol-co-2-acrylamido2-methylpropane sulfonic acid). Desalination Water Treat 2010;17:255e61. Brandrup J, Immergeut EH, Grulke EA. Polymer handbook. 4th ed. Wiley-Interscience Publ.; 1999. VI-14. Nishimura M, Higa M, Akamine K, Masudaya S. Preparation and characterization of anion-exchange membranes with a semi-interpenetrating network structure of poly(vinyl alcohol) and poly(allyl amine). Desalination 2008;233: 157e65. Teorell T. Transport process in ionic membranes. Prog Biophys Biophys Chem 1953;3:305e69. Meyer KH, Sievers JF. Permeability of membranes I. Theory of ionic permeability. Helv Chim Acta 1936;19:649e64.
Symbols C: Ionic concentration in solution, mol/dm3 CGA: Concentration of GA solution, vol.% C0: KCl concentration in low-concentration chamber, mol/dm3 Cpc: Polyanion content in membrane, wt.% Cx: Membrane charge density, mol/dm3 DC: Concentration of ions transported through membrane during electrodialysis, mol/dm3 F: Faraday’s constant: F ¼ 96484.6, C mol1 H:Water content IEC Ion exchange capacity, meq/g-dry membrane T: Temperature, K Q: Amount of electricity, C R: Gas constant: R ¼ 8.31442, J/K/mol Rm: Membrane resistance, U cm2: Rm ¼ Rs Ro, U cm2 Ro: Measured resistance without a sample membrane, U cm2Greek letters
6168 3: Porosity q: Tortuosity Dɸ: Membrane potential u: Ionic mobilitySubscripts
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i: ith ion K: Potassium ion Cl: Chloride ion x: Fixed charge in membrane
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Characterization of cation-exchange membranes prepared from poly(vinyl alcohol) and poly(vinyl alcohol-b-styrene sulfonic acid) Mitsuru Higa*, Megumi Nishimura, Kota Kinoshita, Atsushi Jikihara Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube-city, Yamaguchi 755-8611, Japan
article info
abstract
Article history:
Block-type cation-exchange membranes (CEMs) have been prepared by blending poly(vinyl
Received 7 March 2011
alcohol) (PVA) and the polyanion poly(vinyl alcohol-b-styrene sulfonic acid) at various
Received in revised form
molar percentages of cation-exchange groups to vinyl alcohol groups, Cpa, and by cross-
20 May 2011
linking the PVA chains with glutaraldehyde (GA) solution at various GA concentrations,
Accepted 1 June 2011
CGA. The characteristics of the block-type CEMs were compared with random-type CEMs
Available online 16 July 2011
prepared in a previous study from the random copolymer, poly(vinyl alcohol-co-2acrylamido-2-methylpropane sulfonic acid). At equal molar percentages of the cation-
Keywords:
exchange groups, the water content of the block-type CEMs was less than that of the
Poly(vinyl alcohol)
random-type CEMs. The charge density of the block-type CEMs increased with increasing
Cation-exchange membrane
Cpa and reached a maximum value. Further, the maximum value of the charge density
Cross-linking conditions
increased with increasing CGA. The maximum charge density value of 1.3 mol/dm3 was
Ionic transport property
obtained for the block-type CEM with Cpa ¼ 3.1 mol% and CGA ¼ 0.10 vol.%, which is almost
Block copolymer
two thirds of the value of a commercially available CEM [CMX: ASTOM Corp. Japan]. A comparison of the block-type and random-type CEMs with almost the same membrane resistance showed that the block-type CEMs had higher dynamic transport numbers than the random-type ones. The dynamic transport number and membrane resistance of the block-type CEM with Cpa ¼ 4.0 mol% and CGA ¼ 0.10 vol.% were 0.96 and 4.9 U cm2, respectively. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Cation-exchange membranes (CEMs) have been used for various industrial purposes [1] such as separation of environmental polluting metal ions from hard water [2], chlor-alkali electrolysis [3], fuel cells [4e8], electrodialytic concentration or desalination of electrolyte solutions [9], and continuous hydrogen production in electro-electrodialysis systems [10e12]. At present, the vast majority of commercially available
CEMs for electrodialysis include styrene-co-divinylbenzene matrices. The drawbacks of this type of membrane are as follows: it is difficult to control the membrane structure because it undergoes copolymerization and cross-linking processes simultaneously, and the cost of membrane preparation is high. Recently, many novel ion exchange membranes have been developed to overcome these problems [13,14]. Ion exchange membranes can be prepared by mixing watersoluble base polymers and a polyelectrolyte and then cross-
* Corresponding author. Tel.: þ81 836 85 9203; fax: þ81 836 85 9201. E-mail address: [email protected] (M. Higa). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.06.003
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linking the base polymer. The membrane thus obtained consists of a semi-interpenetrating network (semi-IPN) structure in which the polyelectrolyte chains are immobilized in the cross-linked network polymer matrix. The ion exchange capacity of the membrane can be easily controlled by changing the ratio of water-swollen base polymer to polyelectrolyte. Poly(vinyl alcohol) (PVA) is one of the most popular watersoluble base polymers. PVA is a polyhydroxy polymer that has been studied intensively because of its good film forming and physical properties, high hydrophilicity, processability, biocompatibility, and good chemical resistance [15e18]. Many PVA-based CEMs have been developed for use in direct methanol fuel cells because of the excellent methanol barrier properties of PVA [19e26]. Various kinds of PVA-based CEMs with a semi-IPN structure have been prepared by blending PVA with a polyanion such as poly(styrene sulfonic acid) [27], poly(acrylic acid) [19], poly(styrene sulfonic acid-co-maleic acid) [20], sulfosuccinic acid [21], sulfonated poly(ether ether ketone) [22], sulfated b-cyclodextrin [23], titanium oxide nanotubes and poly(styrene sulfonic acid) [24], montmorillonite and poly(styrene sulfonic acid) [25]. One of the potential disadvantages of CEMs with a semi-IPN structure is that they show low longterm stability in aqueous solutions owing to the dissolution of the un-crosslinked water-soluble polyelectrolytes from the network into the solution. To overcome this disadvantage, CEMs with an IPN structure have been prepared by blending PVA and PVA-based polyanions such as the random copolymer of vinyl alcohol and 2-methyl-1-propanesulfonic acid (AMPS) groups [26,28e30]. In CEMs with an IPN structure, the chains of PVA and the PVA-based polyanion are cross-linked with each other; hence, these CEMs show higher long-term stability in water than CEMs with a semi-IPN structure. However, the counter-ion selectivity of CEMs with an IPN structure in previous study [30] is much lower than that of commercially available CEMs. The aim of this study is to obtain a PVA-based CEM with higher counter-ion selectivity than that of previously reported PVA-based CEMs. To this end, CEMs with an IPN structure (hereafter referred to as PVA-b-PSS) have been prepared by blending PVA with the PVA-based polyanion, poly(vinyl alcohol-b-styrene sulfonic acid), which is a block copolymer of vinyl alcohol and sodium p-styrenesulfonate groups. The chemical structure of PVA-b-PSS is shown in Fig. 1. Because PVA is a semi-crystalline polymer, the crystalline region acts as a physical cross-link point, and hence, the water content of the CEM, which considerably influences the counter-ion selectivity, electrical resistance, and mechanical properties of the membrane, depends on the degree of crystallinity. The crystallinity of the PVA region within a CEM prepared from the
Fig. 1 e Chemical structure of polyanion, PVA-b-PSS: poly(vinyl alcohol-b-styrene sulfonic acid).
block copolymer is higher than that of a CEM prepared from the random copolymer, because the cation-exchange groups of the random copolymer chains disturb the formation of the crystal. Thus, the former has a higher counter-ion permselectivity than the latter. We have investigated the differences in the following ionic transport properties between PVA-based CEMs prepared from the two different copolymers: charge density, membrane resistance, and transport number.
2.
Experimental
2.1.
Materials
Poly(vinyl alcohol) (PVA, 100% hydrolyzed, average Mw ¼ 198,000) and poly(vinyl alcohol-b-styrene sulfonic acid) (PVA-b-PSS, 100% hydrolyzed) were obtained from Kuraray Co., Ltd. Glutaraldehyde (GA) (25 wt.% solution in water) was of analytical grade and was obtained from Wako Pure Chemical Industries. Sulfuric acid and sodium sulfate were of analytical grade and were obtained from Nakarai Tesque.
2.2.
Preparation of block-type CEMs
Self-standing base membranes, BP-X (X ¼ 1e6), for block-type CEMs were prepared by casting an aqueous solution of a mixture of PVA and PVA-b-PSS on a plastic plate and then drying the mixture over a hot stage (NISSIN, NH-45N) overnight at 50 C. The thickness of the base membranes was ca. 0.1 mm. The weight percentage of PVA-b-PSS to PVA in the mixture solution was varied to control the molar percentage of cation-exchange groups to vinyl alcohol groups (Cpa) in BPX, as shown in Table 1. The block-type CEMs were prepared by annealing BP-X at 180 C for 30 min under vacuum to induce physical cross-linking between the PVA chains. Chemical cross-linking was induced by immersing the membranes in an aqueous solution of various concentrations of GA, 0.05 mol/ dm3 of H2SO4 (pH ¼ 1), and 2.0 mol/dm3 of Na2SO4 at 25 C for
Table 1 e Molar percentage of cation-exchange groups, Cpa, and ion-exchange capacity, IEC, of base membrane: membranes prepared from block copolymers, BP-X (X [ 1e6); and membrane prepared from random copolymer, RP-1. Sample BP-1 BP-2 BP-3 BP-4 BP-5 BP-6 RP-1b CMXc
Cpa (mol%)
IEC (meq/g-dry CEM)
1.8 2.2 2.6 3.1 3.5 4.0 3.2 e
0.28 (0.36a) 0.39 (0.44) 0.48 (0.51) 0.54 (0.58) 0.58(0.65) 0.59 (0.65) 0.62 (0.78) 2.0
a Results calculated from the molar percentage of the cationexchange groups. b One of the random copolymers used in Ref. [30]. c A commercially available CEM [Neosepta CMX, ASTOM Corp. Japan].
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24 h to control the water content of the membranes in the equilibrium swelling state in deionized water. Fig. 2 shows the chemical cross-linking mechanism of PVA and GA.
2.3.
Measurement of membrane water content
The water content, H, of the membranes was measured as follows. Base membranes were weighed in the dry state after annealing and chemical cross-linking. The cross-linked membranes were immersed in deionized water at 25 C for 7 days. The membranes were removed from water, dabbed with a filter paper to remove excess water on the membrane surfaces, and weighed in the wet state. For the sake of simplicity, we assume the volume between the water phase and the polymer phase of a swollen membrane to be additive. The volumetric water content is calculated from the weights in the wet state, Ww, and in the dry state, Wd, as shown below: H¼
ðWw Wd Þ=1:0 Wd ðWw Wd Þ=1:0 þ 1:3
(1)
where 1.0 and 1.3 are the densities of water and PVA [31], respectively.
2.4.
Measurement of ion exchange capacity (IEC)
The IEC of a CEM is an important parameter because the ionic transport properties of the CEM depend on the amount and species of the ion exchange groups. IEC is expressed as milliequivalent per gram of membrane (meq/g-dry CEM) and is determined as follows: a sample membrane was immersed in 0.10 M KCl solution for 3 h before measuring the IEC. The membrane was rinsed with deionized water to remove nonexchange KCl electrolyte adsorbed on the membranes and was then immersed in 0.20 mol/dm3 of NaNO3 with a volume of 50 cm3 under stirring for 12 h to achieve the complete exchange of Kþ ions in the membrane with Naþ ions in the solution. The concentration of Kþ ions in the solution, CKþ , was determined by using an ion chromatograph (Dionex ICS1500). The membrane was dried under vacuum for 24 h and was weighed in the dry state, Wd. The IEC of the base membrane was calculated using the following equation: IEC ¼
2.5.
CKþ 100 Wd 1000
(2)
Determination of membrane charge density
Membrane charge density is one of the most important factors for estimating the permselectivity between ions with different
charge and valence in an ion exchange membrane. In order to estimate the charge density, the membrane potentials, Dɸ, were measured as a function of the KCl concentration in the two chambers of the apparatus described elsewhere [32]. One chamber of the cell was filled with KCl solution of various concentrations, Co, while the other chamber was filled with KCl solutions with concentration, Cd, which is five times higher than that in the former (r ¼ Cd/Co ¼ 5). The membrane potential between the solutions in the two chambers was measured at 25.0 0.5 C using Ag/AgCl electrodes (TOA HS205C) with a salt bridge (3.0 mol/dm3 KCl) and a voltmeter [TOA HM-20E]. The membrane charge density, Cx, was calculated from the measured membrane potential using the following equation [33,34]: 1 0 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi C2x þ ð2Co Þ2 Cx C RT B Df ¼ ln@r$qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi A F C2x þ ð2rCo Þ2 Cx 1 0qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 þ ð2rC Þ2 C W C o x RT x C B W ln@ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi A F C2x þ ð2Co Þ2 Cx W
(3)
where W ¼ (uK uCl)/(uK þ uCl); uK and uCl are the Kþ and Cl ion mobilities in the membrane, respectively; F, R, and T are the Faraday constant, gas constant, and absolute temperature, respectively. Parameters W and Cx are adjusted such that the left-hand side of Eq. (3) fits the experimental data at various KCl concentrations.
2.6.
Measurement of membrane resistance
The electrical resistance of the membranes was measured by using a hand-made acrylic plastic cell composed of two parts separated by a membrane, as described elsewhere [32], with an LCR meter operated at 10 KHz AC (A&D Corp. Ltd. AD-5827). 0.5 mol/dm3 of NaCl solution was poured into the two cell compartments, after which the electrical resistance, Ro, was measured at 25.0 0.5 C in a water bath. Subsequently, a sample membrane was set in the cell and the resistance, Rs, was measured again under the same conditions. The difference between Rs and Ro gives the membrane resistance, Rm.
2.7.
Measurement of dynamic state transport number
The dynamic state transport number, tdþ, of the membranes was determined by electrodialysis carried out using the handmade acrylic plastic cell with two parts separated by a membrane as shown in Fig. 3. The measurements were performed at a current density of 10 mA cm2 and temperature of 25 C for 75 min. The amount of ions transported through the membrane during electrodialysis was measured using a conductivity meter (HORIBA 3552-10D). The dynamic state transport number was obtained using the following equation: tdþ ¼
Fig. 2 e Cross-linking mechanism of PVA using GA.
6163
DCVF Q
(4)
where DC, V, and Q are the concentration change of the transported ions, volume of the measurement solution, and
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Fig. 3 e Apparatus for measuring dynamic state transport number. A, power supply; B, ampere meter; C, coulomb meter; D, voltmeter; E, motor; F, stirrer; G, cathode electrode; H, anode electrode; I, 0.5 mol/dm3 NaCl solution; J, sample membrane. The effective area of the cell is 8.0 cm2.
amount of electricity passing through the membrane during electrodialysis, respectively.
3.
Results and discussion
3.1.
Water content as a function of polyanion content
The water content of an ion exchange membrane is an important property because both the charge density and electric resistance of the membrane depend on the water content. Fig. 4 shows the water content of the CEMs prepared in this study as a function of Cpa. Data obtained for the random-type CEMs in a previous study [30] are also shown in order to compare the characteristics of the block-type CEMs with the random-type ones. The water content increased with increasing Cpa, which is attributed to the fact that the osmotic pressure in the membranes increases with an increase in the number of charged groups in the membranes. Further, the water content decreased with increasing GA concentration, CGA, because of the increase in the number of chemical crosslinking points with increasing CGA. A comparison of the water content of the block-type CEMs with that of the random-type CEMs at the same molar percentage of the cation-exchange groups shows that the water content of the block-type CEMs is lower. For example, in CEMs with Cpa ¼ 3.1 mol% and CGA ¼ 0.01 vol.%, the water content of the block-type and the random-type CEMs are 0.45 and 0.75, respectively. This difference can be attributed to the fact that the crystallinity of the PVA region within the CEM prepared from the block copolymer is higher than that of the CEM prepared from the random copolymer.
Fig. 4 e Water content, H, of CEMs as a function of molar percentage of cation-exchange groups to vinyl alcohol groups, Cpa. Polyanion: open symbols, random copolymer; closed symbols, block copolymers. Concentration of GA solution: squares, 0.010 vol.%; triangles, 0.050 vol.%; closed circles, 0.10 vol.%; and open circles, 0.15 vol.%.
IEC of CEMs
3.2.
Table 1 shows the molar percentage of the cation-exchange groups, Cpa, and IEC of the base membranes (BP-X ) along with the values for a commercially available CEM, Neosepta CMX (ASTOM Corp., Japan). The experimental IEC values are about 10% less than the calculated results. This may be attributed to the fact that all the monomers with cationexchange groups in the reaction mixture did not react to form copolymers. The IEC of commercial CMX is 2.0 meq/g-dry CEM. Hence, the IEC of BP-6 is about one third of that of CMX.
Charge density of membranes as a function of Cpa
3.3.
The charge density can be used as an indicator of the permselectivity of an ion exchange membrane, because the higher the charge density of a membrane, the higher is its counterion permselectivity. Fig. 5 shows the charge density of the membranes as a function of Cpa. The charge density increased with Cpa and reached a maximum value. The charge density is proportional to the IEC divided by the water content, H: Cx f
IEC H
(5)
IEC is proportional to Cpa, as shown in Table 1. The water content also increases with Cpa. Hence, in the first stage of the charge density-Cpa curves, the charge density increases with increasing Cpa because of the increase in IEC. At high Cpa, the effect of the increase in the water content on the charge density is larger than that of the increase in IEC. As a result, the charge density decreases after reaching a maximum value. The maximum value of the charge density increases
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Fig. 5 e Charge density, Cx, of CEMs as a function of molar percentage of cation-exchange groups, Cpa. The definition of symbols in this figure is the same as that in Fig. 4.
with increasing CGA because the water content decreases with increasing CGA. The maximum value of the charge density is 1.27 mol/dm3 for the block-type CEM having Cpa ¼ 3.1 mol% and CGA ¼ 0.10 vol.%. The charge density of CMX measured under the same conditions is 1.8 mol/dm3. Thus, the charge density of the block-type CEM is almost two thirds of that of the commercially available CEM. For CEMs with Cpa ¼ 3.1 mol % and CGA ¼ 0.05 vol.%, the charge densities of the block-type CEM (with base membrane BP-5) and the random-type CEM (with base membrane RP-1) are 1.1 mol/dm3 and 0.33 mol/ dm3, respectively, although the base membranes of the two CEMs have almost the same IEC as shown in Table 1. The higher charge density of the block-type CEM relative to the random-type CEM is attributed to the fact that the former has lower water content than the latter, as shown in Fig. 4.
3.4.
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Fig. 6 e Membrane resistance, Rm, of CEMs as a function of molar percentage of cation-exchange groups, Cpa. The definition of symbols in this figure is the same as that in Fig. 4.
allow the permeation of only the counter ions in an electrodialysis system. Fig. 7 shows the dynamic state transport number of the CEMs as a function of Cpa. The transport number of the block-type CEMs increased with increasing Cpa, and then decreased after reaching a maximum value at Cpa z 3.1 mol%. As mentioned above, this trend is attributed to the fact that the charge density has a maximum value at Cpa ¼ 3.1 mol%. The transport number increased with increasing CGA. These results indicate that the transport number of the membranes can be controlled by changing the
Membrane resistance as a function of Cpa
The membrane resistance is important for energy-saving electrodialysis. Fig. 6 shows the membrane resistance as a function of Cpa. The membrane resistance decreases with increasing Cpa because of the increase in the water content and IEC with increasing Cpa. The resistance increases with increasing GA concentration because of the decrease in the water content. This implies that, in this study, the membrane resistance can be controlled by changing the water content. The lowest membrane resistance of 2.1 U cm2 was observed for the block-type CEMs at Cpa ¼ 4.0 mol% and CGA ¼ 0.01 vol.%.
3.5. Dynamic state transport number of CEMs as a function of Cpa The dynamic state transport number relates to the counterion permselectivity of an ion exchange membrane in electrodialysis. A membrane with a transport number of 1.0 can
Fig. 7 e Dynamic state transport number, tdD, of CEMs as a function of molar percentage of cation-exchange groups, Cpa. The definition of symbols in this figure is the same as that in Fig. 4.
6166
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cross-linking conditions. The block-type CEMs with Cpa and CGA values similar to those of the random-type CEMs have larger dynamic state transport numbers. The dynamic transport number of the block-type CEM with Cpa ¼ 3.0 mol% and CGA ¼ 0.10 vol.% is 0.97, while that of CMX is 0.98 under the same conditions, indicating that the block-type CEM has almost the same dynamic transport number as the commercially available CEM.
3.6. Relationship between membrane resistance and dynamic state transport number An ion exchange membrane having both high ion permselectivity and low membrane resistance is desirable in electrodialysis processes. Fig. 8 shows the relationship between the membrane resistance and dynamic state transport number of the membranes. The membrane shown on the upper left-hand side of the figure has high performance for electrodialysis processes. The membrane resistance decreases and the dynamic state transport number increases with increasing Cpa. The transport number of the membrane increases with increasing GA concentration. A comparison of the block-type and random-type CEMs with almost the same membrane resistance shows that the block-type CEMs have higher transport numbers than the random-type CEMs. For example, the transport number of the block-type CEM with Cpa ¼ 4.0 mol % and CGA ¼ 0.10 vol.% is 0.96 whereas that of the random-type CEM with Cpa ¼ 3.5 mol% and CGA ¼ 0.10 vol.% is 0.80. Thus, the transport number of the former is about 20% higher than the latter although both CEMs have almost the same membrane resistances, 4.0 and 4.1 U cm2, respectively. The dynamic transport number and membrane resistance of the block-type CEM at Cpa ¼ 4.0 mol% and CGA ¼ 0.10 vol.% is 0.96 and 4.1 U cm2, respectively, while those of CMX is 0.98 and
2.3 U cm2, respectively. The block-type CEM prepared herein has almost the same dynamic transport number and a membrane resistance that is roughly two times higher than that of a commercially available CEM.
4.
Conclusions
In this study, CEMs with an interpenetrating network structure were prepared by blending PVA with the PVA-based polyanion, poly(vinyl alcohol-b-styrene sulfonic acid), at various molar percentages of cation-exchange groups, Cpa, and by cross-linking the polymers under various GA concentrations. The water content of the block-type CEMs was lower than that of the random-type CEMs when the molar percentage of the cation-exchange groups was almost the same for both the CEMs. The charge density of the obtained membranes increased with an increase in Cpa of the membranes and reached a maximum value. The maximum value of the charge density increased with increasing crosslinker concentration. The maximum charge density value of 1.3 mol/dm3 was obtained for the block-type CEM with Cpa ¼ 3.1 mol% and CGA ¼ 0.10 vol.%, which was almost two thirds of the value of the commercially available CEM, CMX. A comparison of the block-type and random-type CEMs having almost the same membrane resistance showed that the block-type CEMs had higher transport numbers than the random-type ones. The dynamic transport number and membrane resistance of the block-type CEM with Cpa ¼ 4.0 mol% and CGA ¼ 0.10 vol.% were 0.96 and 4.1 U cm2, respectively, while those of CMX were 0.98 and 2.3 U cm2, respectively. The membrane resistance of PVA-based CEMs can be reduced by optimizing the molar percentage of the cationexchange groups to vinyl alcohol groups and the crosslinking conditions, namely, the annealing temperature and GA concentration. The PVA-based CEMs showed almost the same performance for electrodialysisdhigh counter-ion permselectivity and low membrane resistancedas the commercially available CEMs and were prepared at a lower cost than the commercially available ones. Thus, PVA-based CEMs are potentially applicable to electrodialysis systems.
Acknowledgments This work was partly supported by the Electric Technology Research Foundation of Chugoku, Mazda Foundation, UBE Foundation, Kurita Water and Environmental Foundation (No. 20320), and Salt Science Research Foundation (Nos. 0612, 0709, and 0810).
Fig. 8 e Dynamic state transport number, tdD, of CEMs as a function of membrane resistance, Rm. The definition of symbols in this figure is the same as that in Fig. 4. Open diamond: Commercially available CEM, Neosepta CMX (ASTOM Corp. Japan).
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Symbols C: Ionic concentration in solution, mol/dm3 CGA: Concentration of GA solution, vol.% C0: KCl concentration in low-concentration chamber, mol/dm3 Cpc: Polyanion content in membrane, wt.% Cx: Membrane charge density, mol/dm3 DC: Concentration of ions transported through membrane during electrodialysis, mol/dm3 F: Faraday’s constant: F ¼ 96484.6, C mol1 H:Water content IEC Ion exchange capacity, meq/g-dry membrane T: Temperature, K Q: Amount of electricity, C R: Gas constant: R ¼ 8.31442, J/K/mol Rm: Membrane resistance, U cm2: Rm ¼ Rs Ro, U cm2 Ro: Measured resistance without a sample membrane, U cm2Greek letters
6168 3: Porosity q: Tortuosity Dɸ: Membrane potential u: Ionic mobilitySubscripts
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 6 1 6 1 e6 1 6 8
i: ith ion K: Potassium ion Cl: Chloride ion x: Fixed charge in membrane