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Strontium cross–linked sPEEK proton exchange membranes for fuel cell
Solid State Ionics 192 (2011) 627–631
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Strontium cross–linked sPEEK proton exchange membranes for fuel cell Dinh Xuan Luu, Dukjoon Kim ⁎ School of Chemical Engineering, Sunkyunkwan University, Suwon, Kyunggi 440-746, Republic of Korea
a r t i c l e
i n f o
Article history: Received 28 September 2009 Received in revised form 2 February 2010 Accepted 3 May 2010 Available online 23 May 2010 Keywords: PEEK Fuel cell Membrane Crosslinking Proton conductivity SAXs
a b s t r a c t In order to alleviate the high water uptake and dissolution properties at elevated temperatures, the major drawbacks of the highly sulfonated polyetheretherketone (sPEEK) electrolyte membranes (above 60%), crosslinks are created by accommodation of the strontium (Sr) earth metal via ionic bonding with sulfonic groups of the sPEEK. The effect of crosslinking on water uptake, thermal and mechanical stability, methanol permeability, and proton conductivity, etc. of membranes is investigated. Addition of a small amount of Sr up to 10 wt.% considerably decreases water uptake and thus increases the mechanical strength in water at 80 °C. Although the crosslinking decreases not only the methanol permeability but proton conductivity, incorporation of 2–6% Sr results in higher selectivity than Nafion® with much less water uptake. Those crosslinking effects on membrane properties are closely related with the ionic cluster dimension of membrane revealed by small angle X-ray (SAXs) pattern. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nafion® is one of the most widely used commercial polymer electrolyte membranes, as it has high chemical and physical stabilities as well as high proton conductivity at ambient condition. However, a few drawbacks associated with high cost, high methanol permeability, and low proton conductivity at high temperature, have drawn a great attention to the development of new electrolyte polymers in substitute for Nafion®. One of the promising candidates is the sulfonated poly (ether ether ketone) (sPEEK) owing to its low methanol permeability, high chemical, mechanical, and thermal resistances, and more importantly, low cost and good processability [1–3]. However, a disadvantage of the sPEEK membrane is in its mechanical weakness in hot water (over 80 °C) associated with high swelling, especially when the degree of sulfonation (DS) is high (greater than 65%). Although several other methods have ever been introduced; grafting [4], blending [5–9], and crosslinking may be an effective way to cure this problem. It may also improve the tensile strength and toughness of solvent absorbed polymer at elevated temperatures [10,11]. For PEEK, it is very difficult to create crosslinks because of its rigid and stable molecular backbone structure. Although a few chemical crosslinking schemes for PEEK were applied, the resulting membrane performance was not sufficiently satisfied. sPEEK is generally crosslinked by the following chemical methods — crosslinking of the backbond
⁎ Corresponding author. Tel.: + 82 31 290 7250; fax: + 82 31 299 4700. E-mail address: [email protected] (D. Kim). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.05.007
where PEEK is irradiated with a high energy to create the reactive radical to initiate crosslinking [12] or synthesis of the monomer or precursor of PEEK possessing unsaturated bonds for further crosslinking reactions [13]. The other method involves the physical crosslinking where the ionic complex is formed by organic/organic or organic/inorganic phases. As the sPEEK is often simply prepared by sulfonation of the commercial PEEK to provide the proton transport properties, the physical crosslinking is much more feasible than the chemical (covalent) crosslinking. Introduction of a small amount of the earth metal may create the physical crosslinking of the sPEEK. This metal–polymer crosslink is based on the interaction between earth metal ions and sulfonic groups where the cationic earth metal ions make very strong bonds with the anionic sulfate or sulfonic groups of the sPEEK. Among a number of earth metals, the solubility product constant (Ksp) at 25 °C of their sulfates decreased in the order of barium, strontium and calcium with respective Ksp values of 1.1 × 10− 10, 3.5 × 10− 7 and 1.7 × 10− 5. As the barium provides too strong bonding to sPEEK, the bariumcrosslinked system resulted in too inflexible polymer chains and thus made the proton transport infeasible in our preliminary study. On the contrary to barium, the calcium creates too weak a bonding to sPEEK, and thus the crosslinked membrane was mechanically unstable in hot water from our preliminary experiments. For these reasons, the choice of earth metal in this study was strontium (Sr), as its bonding strength lays between the two mentioned above. In this study, the crosslinked sPEEK was prepared using the strontium ion as a crosslinking agent. Several important characteristics of the crosslinked sPEEK electrolyte membrane synthesized were investigated including proton conductivity, thermal and mechanical stabilities, methanol permeability, and water uptake, etc.
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D.X. Luu, D. Kim / Solid State Ionics 192 (2011) 627–631
2. Experimental 2.1. Synthesis of sPEEK The PEEK (Victrex® 450PF) was purchased from ICI Company (Rotherham, UK) with a molecular weight of 100,000 g/mol and a particle size of 100 μm. Sulfonation of PEEK was conducted as follows. PEEK (10.0 g) was dissolved in 50 mL of methylsulfonic acid under vigorous stirring for 4 h. The resulting mixture was then placed in 400 mL of 97% sulfuric acid in a three neck flask under nitrogen atmosphere. The sulfonation reaction was conducted at 30 °C for different periods of time depending on the desired degree of sulfonation. After completion of the sulfonation reaction, sPEEK was washed with de-ionized water several times, followed by drying in a vacuum oven for storage. The degree of sulfonation (DS) was determined using a back-titration method. The amount of 0.1 g of sPEEK particles was placed in 20 ml of 0.05 M NaOH aqueous solution. After 3 days, the solution was titrated with 0.05 M HCl aqueous solution using a pH meter (Orion 420+, Waltham, MA, USA). The calculation of DS has been reported elsewhere [1]. 2.2. Preparation of Sr-sPEEK membrane The sPEEK membrane was prepared using dimethylacetamide (DMAc), a cast solvent. sPEEK, 0.3 g, was dissolved in 9.7 g DMAc with stirring until a homogeneous phase was achieved. The sPEEK solution was cast on a dish glass and then dried at 100 °C for 10 h in a convection oven, followed by at 120 °C for 2 h in a vacuum oven. The strontium acetate solution, 0.1 M, was prepared dissolving 1.0355 g Sr (CH3COO)2 in 50 mL de-ionized water. After diluting the original solution in 50 mL de-ionized water to a predetermined concentration, sPEEK membranes were immersed in it. After soaking the membrane for 5 h at room temperature, the membrane was washed with deionized water to remove trace of free strontium. The degree of strontium exchange was determined by analyzing the concentration of strontium ion that remained in solution by the atomic absorption spectroscopy (AAS, Z-6100, Hitachi, Japan). As two sulfonic groups of polymeric molecules are bonded with a strontium ion as shown in Fig. 1, crosslinks are created. 2.3. Characterization of membrane 2.3.1. Water uptake The membrane was dried at 80 °C for 24 h under vacuum before measuring the dry membrane weight. The membrane was soaked in water for 12 h to promote the swelling. When the sample was swollen
to equilibrium, the weight of the membrane was measured after its surface was wiped to remove any trace of liquid by a clean tissue. The liquid uptake was defined by: W% =
Wwet −Wdry × 100% Wdry
ð1Þ
where, Wwet and Wdry are the weight of swollen and dry membranes, respectively. 2.3.2. Proton conductivity The proton conductivity of membrane was measured using an Impedance Measurement Unit (IM6eX, Zahner, Germany) with a Bekktech cell (BT-115, USA). The ionic transport resistance was measured at various temperatures from 40 to 90 °C at the fixed humidity of 95%. The conductivity cell has a nominal internal volume of 55 cm3. The flow rate of hydrogen gas was 990 SCCM, which avoids the water precipitation on the membrane. The ion conductivity of membrane was determined by the following equation: σ=
L R×W ×T
ð2Þ
where W and T are the membrane width and thickness, respectively; R ohmic resistance of membrane; L distance between the voltage measurement probes in the direction of ionic flow. 2.3.3. Methanol permeability Methanol permeability of the samples was measured using a diaphragm cell [14]. The glass cell consisted of two identical compartments containing 2 M methanol on one side and de-ionized water on the other side, separated by the test membranes. Both compartments were magnetically stirred during the permeation experiments. Concentration of the permeated methanol was measured by a RI detector (RI750F Younglin Instrument, Korea). The methanol permeability was calculated from the slope of the linear plot of the methanol concentration versus permeation time. More detailed measurement methods and procedures have been described elsewhere [14]. 2.3.4. Thermal behaviors Differential scanning calorimetry (DSC 2910, PerkinElmer, USA) was employed to measure the thermal transition temperatures of sPEEK samples. The samples were heated from 30 to 300 °C at a scanning rate of 10 °C/min. The thermogravimetric analyzer (TGA7, PerkinElmer, USA) was used to investigate the thermal degradation behavior of sPEEK samples. The samples were heated from 25 to 650 °C at a scanning rate of 10 °C/min in nitrogen atmosphere. 2.3.5. Mechanical property The tensile strength of both non-crosslinked and crosslinked membranes were measured using a universal tensile machine (UTM model 5565, Lloyd, Fareham, UK). The samples with 20 mm in width and 50 mm in length were stretched with a 250 N load cell pulled at 20 mm/min within 21 cm of Gauge length. The measurements were conducted at least three times and the average value was taken for its determination. 3. Results and discussion 3.1. Water uptake
Fig. 1. Schematic of ionic interaction between strontium and sulfonic acid groups of polymer.
Fig. 2(a) shows the temperature dependence of water uptake for strontium crosslinked membranes with the DS of 75%. The water uptake of sPEEK membranes increased with temperature and its effect is more prominent at high temperatures. While the uncrosslinked membrane with DS of 75% was even dissolved in water at 80 °C,
D.X. Luu, D. Kim / Solid State Ionics 192 (2011) 627–631
Fig. 2. (a) Effect of crosslinking density on the temperature dependence of water uptake for strontium crosslinked sPEEK membranes with 75% DS and (b) effect of sulfonation degree on crosslinking dependence of water uptake for strontium crosslinked sPEEK membranes at 80 °C.
addition of a small amount of strontium at 2% degree of strontium exchange protected its dissolution, resulting in high water uptake of 110%. At higher degrees of strontium exchanges of 4 and 6%, the water uptake reduced to 90 and 70%, respectively. This high temperature water swelling resistance increased with increasing strontium concentration. Fig. 2(b) shows the effect of strontium exchange degree on the water uptake for the sPEEK membrane systems with different sulfonation degrees of 65, 70, and 75% at 80 °C. Water uptake decreased considerably with increasing strontium content for all sulfonation degrees. The lower water uptake is observed at lower sulfonation degree. 3.2. Proton conductivity Fig. 3(a) shows the temperature dependence of proton conductivity of sPEEK membranes with different sulfonation degrees. Proton conductivity increased with increasing temperature or increasing degree of sulfonation. Although the conductivity of 75% DS sPEEK is almost the same as that of Nafion® at 80 °C, too high water uptake at this temperature limits the extensive application of sPEEK membranes. As a part of proton is exchanged with Sr ion in crosslinked membrane, the proton conductivity is lowered by the introduction of Sr ions. It is associated with reduced number of mobile proton ions and less flexible molecular structure. As shown in Fig. 3(b), the proton
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Fig. 3. Effects of (a) sulfonation degree and (b) crosslinking density on the temperature dependence of proton conductivity of sPEEK membranes, respectively.
conductivity of 75% DS crosslinked membranes considerably decreased with the Sr exchange degree. However, the proton conductivity of the crosslinked membranes with the strontium concentration 2, 4 and 6% strontium, 0.068, 0.065 and 0.06 S/cm, respectively, were still higher than that of sPEEK with 70% DS, 0.042 S/cm. The fact that the proton conductivity of crosslinked membranes decreases very gradually with increasing crosslinking density up to 6%, implies that the membranes prepared at these crosslinking densities maintain high proton conductivity without mechanical deterioration in high temperature water. 3.3. Methanol permeability The sPEEK membrane is known as a better methanol barrier than Nafion® [15]. As the formation of crosslink reduced water uptake, it may reduce the methanol permeability further. Fig. 4 shows the methanol permeability of strontium exchange membranes as a function of Sr concentration. The methanol permeability reduces with strontium exchange degree, as expected. The methanol permeability of crosslink membranes was about one-third that of Nafion® 117 membrane. 3.4. Selectivity The ratio of proton conductivity to methanol permeability is defined as the selectivity of membrane. The effect of strontium content on the selectivity of strontium crosslinked sPEEK membranes (DS of 75%)
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Fig. 4. Effect of Sr exchange % on methanol permeability of strontium crosslinked sPEEK membrane with DS of 75%. Fig. 6. Sr exchange degree effect on SAXs pattern of sPEEK membranes with DS of 75%.
normalized with respect to that of the Nafion® 117 membrane is shown in Fig. 5. This selectivity ratio decreases with Sr exchange %. As the selectivity of sPEEK membranes with 2 and 4% strontium is still higher than that of the Nafion® 117 membrane, better cell performance than Nafion is expected for sPEEK membranes at Sr exchange % up to 5% with less water uptake in high temperature water. 3.5. SAXs analysis In Fig. 6, the scattering vector (q) increases with increasing strontium content. It indicates that the ionic cluster size reduces from 2.65 to 2.54 nm by increasing Sr exchange degree from 0 to 10% (refer to Table 1). The decreasing ionic cluster size was due to the restricted ion mobility to form ion clusters by the bonds created between strontium and sulfonic groups. This decreasing ionic cluster size affected the whole membrane properties such as water uptake, proton conductivity, methanol permeability, illustrated in Figs. 2–5. 3.6. Thermal behavior Fig. 7(a) shows the TGA curves of both uncrosslinked and strontium crosslinked sPEEK membranes with the Sr exchange degree of 6 and
10%. The weight of membranes was lost by about 10% around 100 °C due to water evaporation. The uncrosslinked sPEEK membrane with DS 70% was de-sulfonated at 325 °C, slightly lower than that of crosslinked membranes, 336 and 347 °C with Sr concentration of 6 and 10%, correspondingly. This result is caused by the tight network structure associated with the ionic crosslinking, and as such the desulfonation is more difficult than the uncrosslinked system. Desulfonation temperature is enhanced by increased Sr content. Degradation of PEEK backbone began around 500 °C. Fig. 7(b) shows the DSC thermograms of sPEEK 75% membranes with different Sr exchange %. The endothermic peaks were observed around 260 to 280 °C. The Tm of crosslinked sPEEK membranes increased with increasing Sr exchange degree by formation of tighter network structure. 3.7. Mechanical stability The tensile strength of crosslinked sPEEK (DS was 75%) membranes with different strontium exchanges degree is illustrated in Fig. 8. The tensile strength of all crosslinked membranes was higher than that of uncrosslinked one and increased with increasing Sr exchange %. This increase in the tensile strength for crosslinked membranes is due to the reduced water uptake as well as the inflexible polymer chains associated with a tight network structure. 4. Conclusions The crosslinked sPEEK membranes were prepared using strontium earth metal, and the degree of the crosslink was controlled by its concentration. The membranes with even a small amount of Sr led to a considerable increase in thermal and mechanical stabilities accompanied by a huge reduction in water uptake, associated with the reduced ion cluster size revealed by SAXs. While the crosslink developed by ionic interaction between anionic sulfonic groups and cationic Sr reduced both the methanol crossover and proton conductivity, the selectivity of the crosslinked membranes including
Table 1 Ionic cluster size of crosslinked membranes and non-crosslink.
Fig. 5. Effect of strontium exchange % on the selectivity of sPEEK membranes with DS of 75% normalized with respect to that of Nafion® 117 membrane.
%Sr
0
2
4
6
8
10
Scattering vector Cluster size (nm)
0.237 2.65
0.239 2.63
0.241 2.60
0.243 2.58
0.245 2.56
0.247 2.54
D.X. Luu, D. Kim / Solid State Ionics 192 (2011) 627–631
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Fig. 8. Tensile strength of crosslinked sPEEK 75% membranes with different strontium exchange %.
whole relevant properties such as water uptake, proton conductivity, and methanol permeability.
Acknowledgement This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant (R0A-2007-000-10029-0) and the National Research Foundation (NRF-2009-0093033) of Korea Grant funded by the Korean Government (MEST).
References
Fig. 7. (a) TGA and (b) DSC thermograms of sPEEK 75% with different strontium exchange %, respectively.
the strontium contents from 2 to 5%, was still higher than that of the Nafion® 117 membrane. As the ionic metal crosslinked membranes formed an ion-bridge due to interaction between the metal and sulfonic groups, suitable levels of crosslink may have the polymer electrolyte membrane operating at high temperature with high performance. From those results, it is expected that suitable levels of crosslink between 2 and 5% Sr may give very positive effects on sPEEK membrane performance at elevated temperature, considering
[1] D.T. Ngan Khanh, D. Kim, J. Power Sour. 185 (2008) 63. [2] P. Xinh, G.P. Robertson, M.D. Guiver, S.D. Mikhailenko, K. Wang, S. Koliaguine, J. Membr. Sci. 229 (2004) 95. [3] C. Zhao, X. Li, Zh. Wang, Zh. Dou, Sh. Zhong, H. Na, J. Membr. Sci. 280 (2006) 643. [4] S. Zhong, X. Cui, H. Cai, T. Fu, K. Shao, H. Na, J. Power Sour. 168 (2007) 154. [5] H. Cai, K. Shao, S. Zhong, C. Zhao, G. Zhang, X. Li, H. Na, J. Membr. Sci. 297 (2007) 162. [6] H.-Y. Jung, J.-K. Park, Electrochim. Acta 52 (2007) 7464. [7] C. Zhao, Z. Wang, D. Bi, H. Lin, K. Shao, T. Fu, S. Zhong, H. Na, Polymer 48 (2007) 3090. [8] Y. Fu, A. Mathiram, M.D. Guiver, Electrochem. Commun. 8 (2006) 1386. [9] J. Roeder, H. Silva, S.P. Nunes, A.T.N. Pires, Solid State Ionics 176 (2005) 1411. [10] J.V. Gasa, R.A. Weiss, M.T. Shaw, J. Membr. Sci. 304 (2007) 173. [11] A. Radoslaw, R.A. Wach, H. Mitomo, N. Nagasawa, F. Yoshii, Phys. Chem. 68 (2003) 771. [12] J. Chen, Y. Maekawa, M. Asano, M. Yoshida, Polymer 48 (2007) 6002. [13] S. Zhong, X. Cui, H. Cai, T. Fu, C. Zhao, H. Na, J. Membr. Sci. 164 (2007) 65. [14] M. Hwang, H.-Y. Ha, D. Kim, J. Membr. Sci. 325 (2008) 647. [15] K.D. Kreuer, J. Membr. Sci. 185 (2001) 29.
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Strontium cross–linked sPEEK proton exchange membranes for fuel cell Dinh Xuan Luu, Dukjoon Kim ⁎ School of Chemical Engineering, Sunkyunkwan University, Suwon, Kyunggi 440-746, Republic of Korea
a r t i c l e
i n f o
Article history: Received 28 September 2009 Received in revised form 2 February 2010 Accepted 3 May 2010 Available online 23 May 2010 Keywords: PEEK Fuel cell Membrane Crosslinking Proton conductivity SAXs
a b s t r a c t In order to alleviate the high water uptake and dissolution properties at elevated temperatures, the major drawbacks of the highly sulfonated polyetheretherketone (sPEEK) electrolyte membranes (above 60%), crosslinks are created by accommodation of the strontium (Sr) earth metal via ionic bonding with sulfonic groups of the sPEEK. The effect of crosslinking on water uptake, thermal and mechanical stability, methanol permeability, and proton conductivity, etc. of membranes is investigated. Addition of a small amount of Sr up to 10 wt.% considerably decreases water uptake and thus increases the mechanical strength in water at 80 °C. Although the crosslinking decreases not only the methanol permeability but proton conductivity, incorporation of 2–6% Sr results in higher selectivity than Nafion® with much less water uptake. Those crosslinking effects on membrane properties are closely related with the ionic cluster dimension of membrane revealed by small angle X-ray (SAXs) pattern. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nafion® is one of the most widely used commercial polymer electrolyte membranes, as it has high chemical and physical stabilities as well as high proton conductivity at ambient condition. However, a few drawbacks associated with high cost, high methanol permeability, and low proton conductivity at high temperature, have drawn a great attention to the development of new electrolyte polymers in substitute for Nafion®. One of the promising candidates is the sulfonated poly (ether ether ketone) (sPEEK) owing to its low methanol permeability, high chemical, mechanical, and thermal resistances, and more importantly, low cost and good processability [1–3]. However, a disadvantage of the sPEEK membrane is in its mechanical weakness in hot water (over 80 °C) associated with high swelling, especially when the degree of sulfonation (DS) is high (greater than 65%). Although several other methods have ever been introduced; grafting [4], blending [5–9], and crosslinking may be an effective way to cure this problem. It may also improve the tensile strength and toughness of solvent absorbed polymer at elevated temperatures [10,11]. For PEEK, it is very difficult to create crosslinks because of its rigid and stable molecular backbone structure. Although a few chemical crosslinking schemes for PEEK were applied, the resulting membrane performance was not sufficiently satisfied. sPEEK is generally crosslinked by the following chemical methods — crosslinking of the backbond
⁎ Corresponding author. Tel.: + 82 31 290 7250; fax: + 82 31 299 4700. E-mail address: [email protected] (D. Kim). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.05.007
where PEEK is irradiated with a high energy to create the reactive radical to initiate crosslinking [12] or synthesis of the monomer or precursor of PEEK possessing unsaturated bonds for further crosslinking reactions [13]. The other method involves the physical crosslinking where the ionic complex is formed by organic/organic or organic/inorganic phases. As the sPEEK is often simply prepared by sulfonation of the commercial PEEK to provide the proton transport properties, the physical crosslinking is much more feasible than the chemical (covalent) crosslinking. Introduction of a small amount of the earth metal may create the physical crosslinking of the sPEEK. This metal–polymer crosslink is based on the interaction between earth metal ions and sulfonic groups where the cationic earth metal ions make very strong bonds with the anionic sulfate or sulfonic groups of the sPEEK. Among a number of earth metals, the solubility product constant (Ksp) at 25 °C of their sulfates decreased in the order of barium, strontium and calcium with respective Ksp values of 1.1 × 10− 10, 3.5 × 10− 7 and 1.7 × 10− 5. As the barium provides too strong bonding to sPEEK, the bariumcrosslinked system resulted in too inflexible polymer chains and thus made the proton transport infeasible in our preliminary study. On the contrary to barium, the calcium creates too weak a bonding to sPEEK, and thus the crosslinked membrane was mechanically unstable in hot water from our preliminary experiments. For these reasons, the choice of earth metal in this study was strontium (Sr), as its bonding strength lays between the two mentioned above. In this study, the crosslinked sPEEK was prepared using the strontium ion as a crosslinking agent. Several important characteristics of the crosslinked sPEEK electrolyte membrane synthesized were investigated including proton conductivity, thermal and mechanical stabilities, methanol permeability, and water uptake, etc.
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D.X. Luu, D. Kim / Solid State Ionics 192 (2011) 627–631
2. Experimental 2.1. Synthesis of sPEEK The PEEK (Victrex® 450PF) was purchased from ICI Company (Rotherham, UK) with a molecular weight of 100,000 g/mol and a particle size of 100 μm. Sulfonation of PEEK was conducted as follows. PEEK (10.0 g) was dissolved in 50 mL of methylsulfonic acid under vigorous stirring for 4 h. The resulting mixture was then placed in 400 mL of 97% sulfuric acid in a three neck flask under nitrogen atmosphere. The sulfonation reaction was conducted at 30 °C for different periods of time depending on the desired degree of sulfonation. After completion of the sulfonation reaction, sPEEK was washed with de-ionized water several times, followed by drying in a vacuum oven for storage. The degree of sulfonation (DS) was determined using a back-titration method. The amount of 0.1 g of sPEEK particles was placed in 20 ml of 0.05 M NaOH aqueous solution. After 3 days, the solution was titrated with 0.05 M HCl aqueous solution using a pH meter (Orion 420+, Waltham, MA, USA). The calculation of DS has been reported elsewhere [1]. 2.2. Preparation of Sr-sPEEK membrane The sPEEK membrane was prepared using dimethylacetamide (DMAc), a cast solvent. sPEEK, 0.3 g, was dissolved in 9.7 g DMAc with stirring until a homogeneous phase was achieved. The sPEEK solution was cast on a dish glass and then dried at 100 °C for 10 h in a convection oven, followed by at 120 °C for 2 h in a vacuum oven. The strontium acetate solution, 0.1 M, was prepared dissolving 1.0355 g Sr (CH3COO)2 in 50 mL de-ionized water. After diluting the original solution in 50 mL de-ionized water to a predetermined concentration, sPEEK membranes were immersed in it. After soaking the membrane for 5 h at room temperature, the membrane was washed with deionized water to remove trace of free strontium. The degree of strontium exchange was determined by analyzing the concentration of strontium ion that remained in solution by the atomic absorption spectroscopy (AAS, Z-6100, Hitachi, Japan). As two sulfonic groups of polymeric molecules are bonded with a strontium ion as shown in Fig. 1, crosslinks are created. 2.3. Characterization of membrane 2.3.1. Water uptake The membrane was dried at 80 °C for 24 h under vacuum before measuring the dry membrane weight. The membrane was soaked in water for 12 h to promote the swelling. When the sample was swollen
to equilibrium, the weight of the membrane was measured after its surface was wiped to remove any trace of liquid by a clean tissue. The liquid uptake was defined by: W% =
Wwet −Wdry × 100% Wdry
ð1Þ
where, Wwet and Wdry are the weight of swollen and dry membranes, respectively. 2.3.2. Proton conductivity The proton conductivity of membrane was measured using an Impedance Measurement Unit (IM6eX, Zahner, Germany) with a Bekktech cell (BT-115, USA). The ionic transport resistance was measured at various temperatures from 40 to 90 °C at the fixed humidity of 95%. The conductivity cell has a nominal internal volume of 55 cm3. The flow rate of hydrogen gas was 990 SCCM, which avoids the water precipitation on the membrane. The ion conductivity of membrane was determined by the following equation: σ=
L R×W ×T
ð2Þ
where W and T are the membrane width and thickness, respectively; R ohmic resistance of membrane; L distance between the voltage measurement probes in the direction of ionic flow. 2.3.3. Methanol permeability Methanol permeability of the samples was measured using a diaphragm cell [14]. The glass cell consisted of two identical compartments containing 2 M methanol on one side and de-ionized water on the other side, separated by the test membranes. Both compartments were magnetically stirred during the permeation experiments. Concentration of the permeated methanol was measured by a RI detector (RI750F Younglin Instrument, Korea). The methanol permeability was calculated from the slope of the linear plot of the methanol concentration versus permeation time. More detailed measurement methods and procedures have been described elsewhere [14]. 2.3.4. Thermal behaviors Differential scanning calorimetry (DSC 2910, PerkinElmer, USA) was employed to measure the thermal transition temperatures of sPEEK samples. The samples were heated from 30 to 300 °C at a scanning rate of 10 °C/min. The thermogravimetric analyzer (TGA7, PerkinElmer, USA) was used to investigate the thermal degradation behavior of sPEEK samples. The samples were heated from 25 to 650 °C at a scanning rate of 10 °C/min in nitrogen atmosphere. 2.3.5. Mechanical property The tensile strength of both non-crosslinked and crosslinked membranes were measured using a universal tensile machine (UTM model 5565, Lloyd, Fareham, UK). The samples with 20 mm in width and 50 mm in length were stretched with a 250 N load cell pulled at 20 mm/min within 21 cm of Gauge length. The measurements were conducted at least three times and the average value was taken for its determination. 3. Results and discussion 3.1. Water uptake
Fig. 1. Schematic of ionic interaction between strontium and sulfonic acid groups of polymer.
Fig. 2(a) shows the temperature dependence of water uptake for strontium crosslinked membranes with the DS of 75%. The water uptake of sPEEK membranes increased with temperature and its effect is more prominent at high temperatures. While the uncrosslinked membrane with DS of 75% was even dissolved in water at 80 °C,
D.X. Luu, D. Kim / Solid State Ionics 192 (2011) 627–631
Fig. 2. (a) Effect of crosslinking density on the temperature dependence of water uptake for strontium crosslinked sPEEK membranes with 75% DS and (b) effect of sulfonation degree on crosslinking dependence of water uptake for strontium crosslinked sPEEK membranes at 80 °C.
addition of a small amount of strontium at 2% degree of strontium exchange protected its dissolution, resulting in high water uptake of 110%. At higher degrees of strontium exchanges of 4 and 6%, the water uptake reduced to 90 and 70%, respectively. This high temperature water swelling resistance increased with increasing strontium concentration. Fig. 2(b) shows the effect of strontium exchange degree on the water uptake for the sPEEK membrane systems with different sulfonation degrees of 65, 70, and 75% at 80 °C. Water uptake decreased considerably with increasing strontium content for all sulfonation degrees. The lower water uptake is observed at lower sulfonation degree. 3.2. Proton conductivity Fig. 3(a) shows the temperature dependence of proton conductivity of sPEEK membranes with different sulfonation degrees. Proton conductivity increased with increasing temperature or increasing degree of sulfonation. Although the conductivity of 75% DS sPEEK is almost the same as that of Nafion® at 80 °C, too high water uptake at this temperature limits the extensive application of sPEEK membranes. As a part of proton is exchanged with Sr ion in crosslinked membrane, the proton conductivity is lowered by the introduction of Sr ions. It is associated with reduced number of mobile proton ions and less flexible molecular structure. As shown in Fig. 3(b), the proton
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Fig. 3. Effects of (a) sulfonation degree and (b) crosslinking density on the temperature dependence of proton conductivity of sPEEK membranes, respectively.
conductivity of 75% DS crosslinked membranes considerably decreased with the Sr exchange degree. However, the proton conductivity of the crosslinked membranes with the strontium concentration 2, 4 and 6% strontium, 0.068, 0.065 and 0.06 S/cm, respectively, were still higher than that of sPEEK with 70% DS, 0.042 S/cm. The fact that the proton conductivity of crosslinked membranes decreases very gradually with increasing crosslinking density up to 6%, implies that the membranes prepared at these crosslinking densities maintain high proton conductivity without mechanical deterioration in high temperature water. 3.3. Methanol permeability The sPEEK membrane is known as a better methanol barrier than Nafion® [15]. As the formation of crosslink reduced water uptake, it may reduce the methanol permeability further. Fig. 4 shows the methanol permeability of strontium exchange membranes as a function of Sr concentration. The methanol permeability reduces with strontium exchange degree, as expected. The methanol permeability of crosslink membranes was about one-third that of Nafion® 117 membrane. 3.4. Selectivity The ratio of proton conductivity to methanol permeability is defined as the selectivity of membrane. The effect of strontium content on the selectivity of strontium crosslinked sPEEK membranes (DS of 75%)
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Fig. 4. Effect of Sr exchange % on methanol permeability of strontium crosslinked sPEEK membrane with DS of 75%. Fig. 6. Sr exchange degree effect on SAXs pattern of sPEEK membranes with DS of 75%.
normalized with respect to that of the Nafion® 117 membrane is shown in Fig. 5. This selectivity ratio decreases with Sr exchange %. As the selectivity of sPEEK membranes with 2 and 4% strontium is still higher than that of the Nafion® 117 membrane, better cell performance than Nafion is expected for sPEEK membranes at Sr exchange % up to 5% with less water uptake in high temperature water. 3.5. SAXs analysis In Fig. 6, the scattering vector (q) increases with increasing strontium content. It indicates that the ionic cluster size reduces from 2.65 to 2.54 nm by increasing Sr exchange degree from 0 to 10% (refer to Table 1). The decreasing ionic cluster size was due to the restricted ion mobility to form ion clusters by the bonds created between strontium and sulfonic groups. This decreasing ionic cluster size affected the whole membrane properties such as water uptake, proton conductivity, methanol permeability, illustrated in Figs. 2–5. 3.6. Thermal behavior Fig. 7(a) shows the TGA curves of both uncrosslinked and strontium crosslinked sPEEK membranes with the Sr exchange degree of 6 and
10%. The weight of membranes was lost by about 10% around 100 °C due to water evaporation. The uncrosslinked sPEEK membrane with DS 70% was de-sulfonated at 325 °C, slightly lower than that of crosslinked membranes, 336 and 347 °C with Sr concentration of 6 and 10%, correspondingly. This result is caused by the tight network structure associated with the ionic crosslinking, and as such the desulfonation is more difficult than the uncrosslinked system. Desulfonation temperature is enhanced by increased Sr content. Degradation of PEEK backbone began around 500 °C. Fig. 7(b) shows the DSC thermograms of sPEEK 75% membranes with different Sr exchange %. The endothermic peaks were observed around 260 to 280 °C. The Tm of crosslinked sPEEK membranes increased with increasing Sr exchange degree by formation of tighter network structure. 3.7. Mechanical stability The tensile strength of crosslinked sPEEK (DS was 75%) membranes with different strontium exchanges degree is illustrated in Fig. 8. The tensile strength of all crosslinked membranes was higher than that of uncrosslinked one and increased with increasing Sr exchange %. This increase in the tensile strength for crosslinked membranes is due to the reduced water uptake as well as the inflexible polymer chains associated with a tight network structure. 4. Conclusions The crosslinked sPEEK membranes were prepared using strontium earth metal, and the degree of the crosslink was controlled by its concentration. The membranes with even a small amount of Sr led to a considerable increase in thermal and mechanical stabilities accompanied by a huge reduction in water uptake, associated with the reduced ion cluster size revealed by SAXs. While the crosslink developed by ionic interaction between anionic sulfonic groups and cationic Sr reduced both the methanol crossover and proton conductivity, the selectivity of the crosslinked membranes including
Table 1 Ionic cluster size of crosslinked membranes and non-crosslink.
Fig. 5. Effect of strontium exchange % on the selectivity of sPEEK membranes with DS of 75% normalized with respect to that of Nafion® 117 membrane.
%Sr
0
2
4
6
8
10
Scattering vector Cluster size (nm)
0.237 2.65
0.239 2.63
0.241 2.60
0.243 2.58
0.245 2.56
0.247 2.54
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Fig. 8. Tensile strength of crosslinked sPEEK 75% membranes with different strontium exchange %.
whole relevant properties such as water uptake, proton conductivity, and methanol permeability.
Acknowledgement This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant (R0A-2007-000-10029-0) and the National Research Foundation (NRF-2009-0093033) of Korea Grant funded by the Korean Government (MEST).
References
Fig. 7. (a) TGA and (b) DSC thermograms of sPEEK 75% with different strontium exchange %, respectively.
the strontium contents from 2 to 5%, was still higher than that of the Nafion® 117 membrane. As the ionic metal crosslinked membranes formed an ion-bridge due to interaction between the metal and sulfonic groups, suitable levels of crosslink may have the polymer electrolyte membrane operating at high temperature with high performance. From those results, it is expected that suitable levels of crosslink between 2 and 5% Sr may give very positive effects on sPEEK membrane performance at elevated temperature, considering
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