- Email: [email protected]
SPEEK proton exchange membranes modified with silica sulfuric acid nanoparticles
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 ) 1 1 8 5 3 e1 1 8 6 1
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
SPEEK proton exchange membranes modified with silica sulfuric acid nanoparticles Lin Du, Xiaoming Yan, Gaohong He*, Xuemei Wu, Zhengwen Hu, Yongdong Wang State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China
article info
abstract
Article history:
In order to increase both of the water uptake and conductivity, the proton exchange
Received 31 October 2011
membranes were fabricated by sulfonated poly(ether ether ketone) (SPEEK) doped with
Received in revised form
varied contents of silica sulfuric acid (SSA) which is obtained by treating SiO2 nanoparticles
5 May 2012
with more volatile SO2Cl2. SEM images of the composite membranes show that SSA
Accepted 8 May 2012
nanoparticles are dispersed within the membranes uniformly, indicating the good organic
Available online 7 June 2012
compatibility of SSA particles. TEM images show that the composite membranes have improved ionic clusters distribution. The water uptakes of the composite membranes in
Keywords:
water and under low relative humidities are all higher than that of the pristine SPEEK
Proton exchange membrane
membrane. The addition of SSA enhances the conductivity obviously. The composite
Silica sulfuric acid
membrane containing 5wt.% SSA exhibits the highest conductivity of 0.13 Scm1 at 80 C,
SPEEK
approximately 18.6% higher than that of the pristine SPEEK membrane and 8.6% higher
Composite membrane
than that of Nafion117. The composite membranes also show good thermal stability. These results imply the potential application of the SPEEK/SSA composite membranes as improved PEMs in PEMFC. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Proton exchange membrane fuel cells (PEMFCs), as promising clean energy sources with high energy conversion efficiency and zero harmful emissions, have drawn wide attention [1]. Proton exchange membrane（PEM） is one of the key components of the PEMFCs, serving as oxidant/fuel separator and proton conductor simultaneously. Nafion has been viewed as the bench mark for PEMs, due to its good conductivity and chemical stability [2]. However, its sharp decline in proton conductivity above 80 C, difficulty in the synthesis, high cost and harm to the environment have hindered its large-scale application as PEM [3e5]. As alternatives, hydrocarbon-based ionomers, such as sulfonated polyimide (SPI) [6,7], sulfonated poly(ether ether ketone) (SPEEK) [8,9],
and sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK) [10,11] have been successfully prepared and extensively investigated as PEMs. Though more economical and environment-friendly, these membranes are still restricted by their poor water retention at high temperature [12]. Incorporation of hygroscopic inorganic fillers is an important approach to improve the water retention at high temperature of PEMs, with manifold extra advantages of (1) improving the thermal, mechanical and chemical stabilities by the inorganic-organic interfacial interactions [13], (2) enhancing proton conductivity by the extra acid sites on the surface of inorganic fillers [14,15], (3) increasing reactants permeation resistance by blocking the permeation pathways [16]. SiO2 is an effective and commonly used inorganic filler mainly serving to improve the water retention of PEMs.
* Corresponding author. Tel.: þ86 411 84707892; fax: þ86 411 84707700. E-mail address: [email protected] (G. He). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.05.024
11854
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 ) 1 1 8 5 3 e1 1 8 6 1
However, with poor organic compatibility and nonconductive property, SiO2 particles not only easily aggregate in polymeric matrix but also lower the conductivity of PEMs [17]. To overcome this problem, SiO2 can be modified by sulfonic acids to produce silica sulfuric acid (SSA) with improved interaction to the polymer matrix. Nafion based composite membranes containing SSA particles show both higher water retention and proton conductivity [2,18]. To the best of our knowledge, researches of SSA doped composite membranes based on low cost non-fluorinate PEMs have not yet been reported. This prompts us to utilize SSA, an effective bifunctional additive, to improve both of the water retention and the proton conductivity of non-fluorinated PEMs. SPEEK is a typical non-fluorinated polymer with low cost, good chemical and thermal stability and fairly high conductivity [19,20], and thus was chosen as the polymeric matrix in this work. SSA was reportedly prepared by treating SiO2 particles with H2SO4 aqueous solution [21], concentrated H2SO4 [18] and chlorosulfonic acid [2]. SO2Cl2, another commonly used sulfonating reagent, with a lower boiling point of 69.1 C, is easy to evacuate from the system after sulfonation reaction. Thus, SO2Cl2 was chosen here as the sulfonating reagent for the synthesis of SSA. A series of SPEEK-based composite membranes with varied doping contents of SSA was fabricated and properties like morphology, water uptake, dimensional stability, conductivity and thermal stability were investigated.
2.
Experimental
2.1.
Chemicals
Poly (ether ether ketone) (PEEK) was kindly provided by Degussa (China) Co, Ltd. and was dried overnight at 120 C before using for reaction. Silica particles with a size of approximate 25 nm were prepared according to the literature [22]. Sulfuric acid (98%), sulfuryl chloride, N-methyl-2-pyrrolidone (NMP) were obtained commercially and used as received. All the reagents are analytical grade.
2.2.
Preparation of SPEEK
SPEEK was prepared according to the procedure described in literature [8]. Specifically, 10 g PEEK was added into the sulfuric acid (98 wt.%) slowly under magnetic stirring at room temperature. After complete dissolution, the temperature was raised to 50 C and then the reaction was kept for 1 h. Subsequently, the polymer solution was poured into ice-cold deionized (DI) water under mechanic stirring, to separate the product SPEEK from the solution. Then, the white fiber products were washed by DI water until neutral pH, and were continuously immersed in DI water overnight to remove H2SO4 completely. Finally, the resulted polymer was dried at 60 C for 12 h, and then 120 C for another 12 h.
2.3.
Preparation of SSA nanoparticles
SiO2 particles were exposed to excess amount of SO2Cl2 in a round flask for 30 min at room temperature. Then, the
evolved HCl and unreacted SO2Cl2 in the reaction mixture were evacuated in a vacuum. The obtained particles were then hydrolyzed in water at room temperature for 1 h, centrifugally separated from the water and finally dried in an oven at temperature 60 C to obtain the SSA nanoparticles. To assess the hydrolysis stability of SSA, some SSA particles were boiled in water for 3 h and then dried in an oven at a temperature of 150 C. The resulted particles were labeled as H-SSA.
2.4.
Preparation of composite membranes
The SPEEK/SSA composite membranes with varied doping contents were prepared by the solvent-casting method. The SPEEK and SSA particles were mixed in varied mass ratios in 50 mL flask, and subsequently a certain amount of NMP was added, followed by magnetic stirring until the SPEEK was completely dissolved in the solvent. To make sure the SSA particles well dispersed in the casting solution, the mixture was then treated in an ultrasonic bath for 30 min. Finally, the mixture was poured onto a glass plate and the solvent was evaporated at 40 C for 48 h followed by 120 C for 12 h. After that, the membranes were peeled off from the glass plate, immersed in H2SO4 (2 M) for 12 h to achieve complete protonation of the polymer, and then washed with DI water until neutral. Finally the membranes were deposited in DI water.
2.5.
Characterization
2.5.1.
Energy dispersive X-ray (EDX)
The EDX spectra of SiO2, SSA and H-SSA particles were obtained by EDX microanalysis performed at 10 KeV using an INCA Energy 300 spectrometer.
2.5.2.
TEM and SEM
The morphology and size of the particles were viewed on a Tecnai-20 transmission electron microscope (TEM) operated at an accelerating voltage of 200 kV. The morphology of the cross sections of the composite membranes was studied using an S-4800 field emission scanning electron microscope (Hitachi, Japan). All the samples were sputtered with a thin layer of gold in vacuum prior to the analysis. Ionic clusters in the membrane samples were investigated by the TEM images of the stained membrane samples as reported in reference [23]. The samples were stained in a saturated lead acetate solution overnight, rinsed with water and dried under room temperature for 4 h. Then, the samples were embedded in Spurr’s epoxy resin, cured overnight and sectioned to obtain slices approximate 70 nm thick by an ultramicrotome. The slices were then viewed on a Tecnai-20 TEM with an accelerating voltage of 120 kV.
2.5.3.
Ion-exchange capacity (IEC)
The IEC of the samples were determined by the classical titration method. Initially, the samples were immersed in saturated NaCl aqueous solution for 24 h, followed by being titrated with 0.01 mol L1 NaOH solution with phenolphthalein as an end point indicator. The IEC was calculated in the following equation:
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 ) 1 1 8 5 3 e1 1 8 6 1
11855
Fig. 1 e TEM images of (a) SiO2 and (b) SSA.
IEC ¼
VNaOH CNaOH 1000 mmol g1 weight of dried sample
where VNaOH (L) and CNaOH (mol L1) are the volume and molar concentration of the NaOH solution, respectively.
2.5.4.
Thermal property
A thermogravimetric analyzer (Mettler Toledo TGA/SDTA851e) was employed to assess the thermal stability of the SSA particles and composite membranes. The particles were heated from 20 to 900 C, and the membranes from 60 to 550 C, both at a heating rate of 10 C/min, and under a nitrogen atmosphere.
2.5.5.
Water uptake and swelling ratio
Membrane samples were equilibrated at desired relative humidity (or water for the evaluation of water uptake in water environment) and temperature for 12 h. The residual water on the surface of wet membranes was removed with filter paper; then their weights and the dimensional lengths were measured rapidly. After the wet membranes were dried at 120 C for 12 h in vacuum, the weights and dimensional sizes of the corresponding dry membranes were obtained. The water uptake and swelling ratio were calculated by the following equations: Water uptake ¼
Wwet Wdry 100% Wdry
Fig. 2 e EDX spectra of (a) SiO2, (b) SSA and (c) H-SSA.
11856
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 ) 1 1 8 5 3 e1 1 8 6 1
Swelling ratio ¼
lwet ldry 100% ldry
where Wwet and Wdry are the weights of the wet and dry membranes, respectively, lwet and ldry are the geometric averages (ldry¼ldry,widthldry,length,lwet¼lwet,widthlwet,length) of the widths and lengths of the wet and dry membrane samples, respectively.
2.5.6.
Proton conductivity
Proton conductivity of the membrane samples was measured by the four-electrode AC impedance spectroscopy method [24], over the frequency range from 1 Hz to 100 kHz (Ivium Technologies A08001). The measuring apparatus contains two Pt foils as the current carrier electrodes and two Pt wires as the potential sensor electrodes. During the measurement, the home-made measurement apparatus together with the membrane samples was kept in a Constant Temperature & Humidity Incubator at the desired temperature and relative humidity. Proton conductivity was calculated as followed: s ¼ L=RA S cm1 : where L (cm) is the distance between the two potential electrodes, R (U) is the membrane resistance, derived from the impedance value at zero phase angle and A (cm2) is the crosssectional area of the membrane.
3.
Results and discussion
3.1.
Characterization of SSA particles Fig. 3 e (a) TGA and (b) DTG curves of SiO2 and SSA particles.
It is generally recognized that the filler size has great influence on the filled system, for small size favors the fillers’ uniform dispersion in the polymeric matrix. The pristine SiO2 nanoparticles have a uniform diameter of approximate 25 nm, as confirmed in Fig. 1a. After sulfonation reaction, TEM image of SSA particles, as shown in Fig. 1b, has no obvious difference from that of SiO2 nanoparticles, which indicates the treating process with sulfuryl chloride did not alter the morphological characteristics of the fillers. The existence of sulfonic acid groups on the surface of SSA can be partially proved by the presence of sulfur. Thus the EDX analysis was made and the spectra are shown in Fig. 2. The corresponding atomic ratios obtained by EDX analysis are given in Table 1. While the EDX spectrum of SiO2 (Fig. 2a) displays only the peaks due to silicon and oxygen, the spectrum of SSA (Fig. 2b) shows a further peak due to the sulfur presence. The sulfur peak can also be found in the spectrum of H-SSA
(Fig. 2c). The S/Si atomic ratio in H-SSA is 0.69:15.84, only a little smaller than that of SSA whose S/Si atomic ratio is 0.78: 14.68. This result shows that the SSA is stable to hydrolysis. To evaluate the effect of the functionalization on the surface of silica and to further confirm the hydrolysis stability of SSA, IEC value of SiO2, SSA and H-SSA were measured and they were found to be 0.02 mmol/g, 0.42 mmol/g and 0.40 mmol/g, respectively as shown in Table 1. The increased IEC of SSA reveals the enhanced acidity provided by the sulfonic acid groups on the surface of the particles. The IEC of H-SSA decreases very slightly compared to that of SSA, which further confirms the good hydrolysis stability of SSA. TGA analysis was made to evaluate the thermal stability of SSA particles. Traces of TGA and DTG are shown in Fig. 3, and
Table 1 e IEC values and atomic ratios of SiO2, SSA and HSSA particles.
Table 2 e Thermal characteristic temperatures (TODa and TFDb) of SiO2 and SSA particles.
Sample
Sample
SiO2 SSA H-SSA
IEC(mmol/g)
S/Si/O atomic ratio
0.02 0.42 0.40
0:22.92:77.08 0.78:14.68:84.53 0.69:15.84:83.48
SiO2 SSA
TOD1 ( C)
TFD1 ( C)
TOD2 ( C)
TFD2 ( C)
262 308
301 352
479 480
543 547
a The onset decomposition temperature. b The fastest decomposition temperature.
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 ) 1 1 8 5 3 e1 1 8 6 1
Table 3 e Description of the membrane samples. Nomenclature SPEEK SPEEK-2.5SSA SPEEK-5SSA SPEEK-7.5SSA SPEEK-10SSA SPEEK-5SiO2
Weight ratio of SPEEK to SSA or SiO2 100:0 100:2.5 100:5 100:7.5 100:10 100:5
IEC(mmol/g) 1.28 1.27 1.25 1.24 1.19 1.16
the thermal characteristic temperatures e the onset decomposition temperatures (TODs) and the fastest decomposition temperatures (TFDs) e are listed in Table 2. For both SSA and SiO2, the first weight-losing stage before 200 C is not caused by decomposition but by moisture evaporation. SiO2 particles’ second weight-losing stage (TOD: 262 C, TFD: 301 C) is attributed to the condensation of silanol groups [22]. Unlike SiO2,
11857
SSA does not experience any weight loss in the temperature range of 262e301 C, indicating a complete consumption of silanol groups during the sulfonation process. Instead, its weight-losing stage (TOD: 308 C, TFD: 352 C) is in accordance with the reported degradation of eSO3H group [2]. Combined with the evidence in the EDX spectra and TGA, it is safe to conclude that SSA has been successfully prepared. Base on the comparison of the EDX spectra and IEC values of SSA and H-SSA, a conclusion can be drawn that SSA is stable to hydrolysis.
3.2. Morphological properties and IEC values of the composite membranes To investigate the effect of SSA doping content on the composite membranes, a series of composite membranes with various doping contents were prepared. The description of the prepared membrane samples was listed in Table 3.
Fig. 4 e SEM images of the composite membranes cross section: (a) SPEEK-2.5SSA (b) SPEEK-5SSA (c) SPEEK-7.5SSA (d) SPEEK-10SSA (e) SPEEK-5SiO2.
11858
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 ) 1 1 8 5 3 e1 1 8 6 1
Fig. 5 e TEM micrographs of stained (a) SPEEK and (b) SPEEK-5SSA.
SPEEK-5SiO2 was also prepared for comparison. As directly adding SiO2 particles rendered serious aggregation in the SPEEK matrix and severe phase separation, shown in Fig. 4e, further investigation of SPEEK/SiO2 composite membranes was quitted. The dispersion of SSA particles within the
membranes was investigated by SEM. As shown in Fig. 4, SSA particles are well dispersed in the membranes when the filler contents are no more than 7.5wt.%, while in the sample of SPEEK-10SSA slight particle aggregation is observed. The better dispersion of SSA than SiO2 can be explained by (1) the decrement of the silanol groups on SiO2 particles e consumed by the reaction with sulfuryl chloride e which rendered the SiO2 particles less likely to condensate with each other, and (2) the polar interaction between the acid sites on the surface of the fillers and the eHSO3 groups on the polymer matrix, which restricted the motion of the SSA in the membranes during the solvent volatilization process. TEM images for stained SPEEK and SPEEK-5SSA membranes are shown in Fig. 5. The ionic domains are represented by the dark regions on the graph. As shown in Fig. 5a, the micro morphology of SPEEK is characterized by many isolated ionic clusters, which is in agreement with the reference [25]. With SSA particles incorporated into the SPEEK, as shown in Fig. 5b, more ionic clusters are observed and the distances between them become shorter, which is benefit for the formation of hydrophilic cluster network in a hydrated state. IEC value is one of the key factors determining the proton conductivity of PEMs. The IEC values of the samples, obtained by classical titration, are listed in Table 2. As shown, the IEC value goes down slightly as the augmentation of the fillers. This suggests SSA particles have a lower ion exchange capacity than SPEEK.
3.3.
Thermal stability
In order to investigate the influence of SSA on the thermal stability of the membranes, the TGA and DTG traces of SPEEK and SPEEK-5SSA are compared in Fig. 6. The thermal
Table 4 e Thermal characteristic temperatures of SPEEK and SPEEK-5SSA. Sample Fig. 6 e (a) TGA and (b) DTG traces of the SPEEK and SPEEK5SSA membranes.
SPEEK SPEEK-5SSA
TOD1 ( C)
TFD1 ( C)
TOD2 ( C)
TFD2 ( C)
303 306
350 348
484 482
516 522
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 ) 1 1 8 5 3 e1 1 8 6 1
characteristic temperatures are listed in Table 4. Generally, three distinct weight loss stages are observed in Fig. 6b: the first stage before 150 C is not induced by decomposition of the membrane samples, but by the evaporation of the sorbed water, while the second stage (TOD: 303 C, TFD: 350 C) is related to the decomposition of the sulfonic acid groups, and the third stage (TOD: 484 C, TFD: 516 C) to the PEEK-backbone decomposition [26]. Compared with the SPEEK membrane, the composite membrane also shows good thermal stability (TOD: 306 C, TFD: 348 C) at the second stage. In this stage, the weight loss of SPEEK-5SSA is larger than that of SPEEK, which is attributed to the decomposition of the extra eSO3H groups of the fillers in the composite membrane. At the third stage (TOD: 482 C, TFD: 522 C), the weight loss of the composite membrane is not as rapid as that of the SPEEK membrane, probably by virtue of the thermally stable silica particles. The TGA results indicate that the introduction of SSA particles does not weaken the thermal stability of the membrane, and the composite membrane can be applied to fuel cell.
3.4.
11859
Fig. 8 e Water uptake of the membrane samples at 80 C and varied relative humidities.
Water uptake and swelling ratio in water, and the trend is in accordance with that of water uptake in Fig. 7. Composite membranes show a small increase in swelling ratio compared with the plain membrane.
Besides the value of IEC, water uptake is another essential factor affecting the transport of the protons within electrolyte membranes. Water uptake of the membrane samples are compared in Fig. 7 (samples tested in water at different temperatures) and Fig. 8 (samples tested at 80 C but different relative humidities). Shown both in Figs. 7 and 8, the composite membranes generally exhibit higher water uptake than the pristine SPEEK membrane. This is due to the strong polar interaction between water molecules and the acid sites on the surface of the fillers. However, when the content of the fillers is too big, severe aggregation of the fillers will decrease the effective surface area of the particles, leading to the decrease of the water uptake. This is why the SPEEK-10SSA has a water uptake lower than that of other composite membranes. As shown in Figs. 7 and 8, the optimum doping content of SSA is between 5wt% and 7.5wt%. Fig. 9 shows the swelling ratios of the samples at varied testing temperatures
The proton conductivity of the membrane samples tested in water in the temperature range of 20e80 C is shown in Fig. 10, in which all of the composite membranes exhibit higher conductivity than the SPEEK membrane. SPEEK-5SSA performs best probably because it has a best balance among IEC, water uptake and fillers’ dispersion state. Compare SPEEK-5SSA with the plain SPEEK membrane, the conductivity increases from 0.04 S cm1 to 0.05 S cm1 at 20 C, and from 0.11 S cm1 to 0.13 S cm1 at 80 C. The SPEEK-5SSA has a higher conductivity than Nafion117 whose conductivity is 0.12 S cm1 at 80 C as reported [2]. The proton conductivity of the membrane samples at 80 C and varied relative humidities are compared in Fig. 11.
Fig. 7 e Water uptake of SPEEK and the composite membranes in water.
Fig. 9 e Swelling ratio of SPEEK and the composite membranes in water.
3.5.
Proton conductivity
11860
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 ) 1 1 8 5 3 e1 1 8 6 1
channels within the membrane, enhance the water retention capacity, provide additional conductive groups, as a consequence, improve the conductivity.
4.
Fig. 10 e Proton conductivities of SPEEK and the composite membranes in water.
Though all of the membranes have small proton conductivity under low relative humidity, the membranes modified with SSA still show great increase in conductivity compared with the pristine SPEEK membrane. The conductivity enhancement can be explained by two predominant proton transport mechanisms: vehicle mechanism and Grotthus mechanism. Vehicle mechanism needs good hydrophilic cluster network and larger water uptake in the membrane. As shown in Fig. 5, the addition of SSA particles will make the clusters in the membranes distribute much denser and more closely with each other, which will improve the formation of transport channels. The hygroscopic inorganic particles can hold more water more tightly, as shown in Figs. 7 and 8. Therefore, the SSA will facilitate the transport of proton as vehicle mechanism (movement of hydrated protons). Additionally, SSA can provide acid groups on its surface, bridge the neighboring sulfonic acid groups on the polymer matrix, shorten the distance of the proton hoping transport, thus can enhance the conductivity of the membrane as the Grotthus mechanism (proton hoping). Generally, SSA particles serve to improve the formation of proton transport
Fig. 11 e Proton conductivities of the membrane samples at 80 C and varied relative humidities.
Conclusion
With the objective of improving the water retention ability and conductivity of SPEEK membrane, SSA nanoparticles were successfully prepared by sulfonating SiO2 nanoparticles with volatile SO2Cl2 and a series of SPEEK-based composite membranes with variant contents of SSA were fabricated by solution-casting method. SSA particles have increased IEC of 0.42 mmol/g and good hydrolysis stability. SSA also has better organic compatibility than SiO2, thus can be dispersed in SPEEK matrix more uniformly, as confirmed by the SEM images. The presence of SSA helps the ionic clusters distribute in the membrane much denser as confirmed by the TEM images of stained samples. The IEC values of the composite membranes are slightly lower than that of the plain SPEEK membrane. The SSA particles do not affect the membranes’ thermal property; the composite membranes have comparable thermal stability with the plain membrane. The hygroscopic and conductive properties of SSA favor both the water uptake and conductivity. All of the composite membrane samples show an improved water retention and conductivity in water and under low relative humidity. At 80 C in water, the conductivity of SPEEK5SSA is 18.6% higher than that of the plain SPEEK membrane and 8.6% higher than that of Nafion117.
Acknowledgement The authors thank the financial support of National Science Fund for Distinguished Young Scholars of China (Grant No. 21125628) and National Natural Science Foundation of China (Grant No. 20976027 and 21176044).
references
[1] Bose S, Kuila T, Nguyen TXH, Kim NH, Lau K-t, Lee JH. Polymer membranes for high temperature proton exchange membrane fuel cell: recent advances and challenges. Progress in Polymer Science 2011;36:813e43. [2] Kumar GG, Kim AR, Nahm KS, Elizabeth R. Nafion membranes modified with silica sulfuric acid for the elevated temperature and lower humidity operation of PEMFC. International Journal of Hydrogen Energy 2009;34:9788e94. [3] Hou HY, Sun GQ, Wu ZM, Jin W, Xin Q. Zirconium phosphate/ Nafion115 composite membrane for high-concentration DMFC. International Journal of Hydrogen Energy 2008;33:3402e9. [4] Jung EH, Jung UH, Yang TH, Peak DH, Jung DH, Kim SH. Methanol crossover through PtRu/Nafion composite membrane for a direct methanol fuel cell. International Journal of Hydrogen Energy 2007;32:903e7. [5] Wu X, He G, Gu S, Hu Z, Yao P. Novel interpenetrating polymer network sulfonated poly (phthalazinone ether sulfone ketone)/polyacrylic acid proton exchange membranes for fuel cell. Journal of Membrane Science 2007; 295:80e7.
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 ) 1 1 8 5 3 e1 1 8 6 1
[6] Yin Y, Du Q, Qin YZ, Zhou YB, Okamoto K. Sulfonated polyimides with flexible aliphatic side chains for polymer electrolyte fuel cells. Journal of Membrane Science 2011;367: 211e9. [7] Zhai FX, Guo XX, Fang JH, Xu HJ. Synthesis and properties of novel sulfonated polyimide membranes for direct methanol fuel cell application. Journal of Membrane Science 2007;296: 102e9. [8] Smitha B, Sridhar S, Khan AA. Synthesis and characterization of sulfonated PEEK membranes for fuel cell application. Journal of Polymer Materials 2004;21:99e106. [9] Yang B, Manthiram A. Sulfonated poly(ether ether ketone) membranes for direct methanol fuel cells. Electrochemical and Solid State Letters 2003;6:A229e31. [10] Gaohong H, Shuang G, Xuemei W, Li C, Hongjing L, Chang L, et al. Synthesis and characteristics of sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK) for direct methanol fuel cell (DMFC). Journal of Membrane Science 2006;281:121e9. [11] Xuemei W, Gaohong H, Shuang G, Wan C, Pingjing Y. Sulfonation of poly(phthalazinone ether sulfone ketone) by heterogeneous method and its potential application on proton exchange membrane (PEM). Journal of Applied Polymer Science; 2007:1002e9. [12] Zhang Y, Zhang HM, Zhu XB, Bi C. Promotion of PEM selfhumidifying effect by nanometer-sized sulfated zirconiasupported Pt catalyst hybrid with sulfonated poly(ether ether ketone). Journal of Physical Chemistry B 2007;111: 6391e9. [13] Wang JT, Zhang YM, Wu H, Xiao LL, Jiang ZY. Fabrication and performances of solid superacid embedded chitosan hybrid membranes for direct methanol fuel cell. Journal of Power Sources 2010;195:2526e33. [14] Kim YS, Wang F, Hickner M, Zawodzinski TA, McGrath JE. Fabrication and characterization of heteropolyacid (H3PW12O40)/directly polymerized sulfonated poly(arylene ether sulfone) copolymer composite membranes for higher temperature fuel cell applications. Journal of Membrane Science 2003;212:263e82. [15] Navarra MA, Croce F, Scrosati B. New, high temperature superacid zirconia-doped Nafion composite membranes. Journal of Materials Chemistry 2007;17:3210.
11861
[16] Wu Z, Sun G, Jin W, Hou H, Wang S, Xin Q. Nafion and nanosize TiO2eSO2 4 solid superacid composite membrane for direct methanol fuel cell. Journal of Membrane Science 2008; 313:336e43. [17] Rodgers MP, Shi Z, Holdcroft S. Transport properties of composite membranes containing silicon dioxide and Nafion. Journal of Membrane Science 2008;325:346e56. [18] Ke C-C, Li X-J, Shen Q, Qu S-G, Shao Z-G, Yi B-L. Investigation on sulfuric acid sulfonation of in-situ solegel derived Nafion/ SiO2 composite membrane. International Journal of Hydrogen Energy 2011;36:3606e13. [19] Celso F, Mikhailenko SD, Kaliaguine S, Duarte UL, Mauler RS, Gomes AS. SPEEK based composite PEMs containing tungstophosphoric acid and modified with benzimidazole derivatives. Journal of Membrane Science 2009;336:118e27. [20] Hande VR, Rath SK, Rao S, Patri M. Cross-linked sulfonated poly (ether ether ketone) (SPEEK)/reactive organoclay nanocomposite proton exchange membranes (PEM). Journal of Membrane Science 2011;372:40e8. [21] Wang L, Zhao D, Zhang HM, Xing DM, Yi BL. Water-Retention effect of composite membranes with different types of nanometer silicon dioxide. Electrochemical and Solid-State Letters 2008;11:B201. [22] Gnana Kumar G, Senthilarasu S, Dae Nyung L, Ae Rhan K, Pil K, Kee Suk N, et al. Synthesis and characterization of aligned SiO2 nanosphere arrays: spray method. Synthetic Metals 2008;684e7. [23] Peron J, Mani A, Zhao X, Edwards D, Adachi M, Soboleva T, et al. Properties of Nafion NR-211 membranes for PEMFCs. Journal of Membrane Science 2010;356:44e51. [24] Yan X, He G, Gu S, Wu X, Du L, Zhang H. Quaternized poly(ether ether ketone) hydroxide exchange membranes for fuel cells. Journal of Membrane Science 2011;375:204e11. [25] Mahajan CV, Ganesan V. Atomistic simulations of structure of Solvated sulfonated Poly(ether ether ketone) membranes and their comparisons to Nafion: I. Nanophase Segregation and hydrophilic domains. Journal of Physical Chemistry B 2009;114:8357e66. [26] Wu J, Cui Z, Zhao C, Li H, Zhang Y, Fu T, et al. High proton conductive advanced hybrid membrane based on sulfonated Si-SBA-15. International Journal of Hydrogen Energy 2009;34: 6740e8.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
SPEEK proton exchange membranes modified with silica sulfuric acid nanoparticles Lin Du, Xiaoming Yan, Gaohong He*, Xuemei Wu, Zhengwen Hu, Yongdong Wang State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China
article info
abstract
Article history:
In order to increase both of the water uptake and conductivity, the proton exchange
Received 31 October 2011
membranes were fabricated by sulfonated poly(ether ether ketone) (SPEEK) doped with
Received in revised form
varied contents of silica sulfuric acid (SSA) which is obtained by treating SiO2 nanoparticles
5 May 2012
with more volatile SO2Cl2. SEM images of the composite membranes show that SSA
Accepted 8 May 2012
nanoparticles are dispersed within the membranes uniformly, indicating the good organic
Available online 7 June 2012
compatibility of SSA particles. TEM images show that the composite membranes have improved ionic clusters distribution. The water uptakes of the composite membranes in
Keywords:
water and under low relative humidities are all higher than that of the pristine SPEEK
Proton exchange membrane
membrane. The addition of SSA enhances the conductivity obviously. The composite
Silica sulfuric acid
membrane containing 5wt.% SSA exhibits the highest conductivity of 0.13 Scm1 at 80 C,
SPEEK
approximately 18.6% higher than that of the pristine SPEEK membrane and 8.6% higher
Composite membrane
than that of Nafion117. The composite membranes also show good thermal stability. These results imply the potential application of the SPEEK/SSA composite membranes as improved PEMs in PEMFC. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Proton exchange membrane fuel cells (PEMFCs), as promising clean energy sources with high energy conversion efficiency and zero harmful emissions, have drawn wide attention [1]. Proton exchange membrane（PEM） is one of the key components of the PEMFCs, serving as oxidant/fuel separator and proton conductor simultaneously. Nafion has been viewed as the bench mark for PEMs, due to its good conductivity and chemical stability [2]. However, its sharp decline in proton conductivity above 80 C, difficulty in the synthesis, high cost and harm to the environment have hindered its large-scale application as PEM [3e5]. As alternatives, hydrocarbon-based ionomers, such as sulfonated polyimide (SPI) [6,7], sulfonated poly(ether ether ketone) (SPEEK) [8,9],
and sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK) [10,11] have been successfully prepared and extensively investigated as PEMs. Though more economical and environment-friendly, these membranes are still restricted by their poor water retention at high temperature [12]. Incorporation of hygroscopic inorganic fillers is an important approach to improve the water retention at high temperature of PEMs, with manifold extra advantages of (1) improving the thermal, mechanical and chemical stabilities by the inorganic-organic interfacial interactions [13], (2) enhancing proton conductivity by the extra acid sites on the surface of inorganic fillers [14,15], (3) increasing reactants permeation resistance by blocking the permeation pathways [16]. SiO2 is an effective and commonly used inorganic filler mainly serving to improve the water retention of PEMs.
* Corresponding author. Tel.: þ86 411 84707892; fax: þ86 411 84707700. E-mail address: [email protected] (G. He). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.05.024
11854
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 ) 1 1 8 5 3 e1 1 8 6 1
However, with poor organic compatibility and nonconductive property, SiO2 particles not only easily aggregate in polymeric matrix but also lower the conductivity of PEMs [17]. To overcome this problem, SiO2 can be modified by sulfonic acids to produce silica sulfuric acid (SSA) with improved interaction to the polymer matrix. Nafion based composite membranes containing SSA particles show both higher water retention and proton conductivity [2,18]. To the best of our knowledge, researches of SSA doped composite membranes based on low cost non-fluorinate PEMs have not yet been reported. This prompts us to utilize SSA, an effective bifunctional additive, to improve both of the water retention and the proton conductivity of non-fluorinated PEMs. SPEEK is a typical non-fluorinated polymer with low cost, good chemical and thermal stability and fairly high conductivity [19,20], and thus was chosen as the polymeric matrix in this work. SSA was reportedly prepared by treating SiO2 particles with H2SO4 aqueous solution [21], concentrated H2SO4 [18] and chlorosulfonic acid [2]. SO2Cl2, another commonly used sulfonating reagent, with a lower boiling point of 69.1 C, is easy to evacuate from the system after sulfonation reaction. Thus, SO2Cl2 was chosen here as the sulfonating reagent for the synthesis of SSA. A series of SPEEK-based composite membranes with varied doping contents of SSA was fabricated and properties like morphology, water uptake, dimensional stability, conductivity and thermal stability were investigated.
2.
Experimental
2.1.
Chemicals
Poly (ether ether ketone) (PEEK) was kindly provided by Degussa (China) Co, Ltd. and was dried overnight at 120 C before using for reaction. Silica particles with a size of approximate 25 nm were prepared according to the literature [22]. Sulfuric acid (98%), sulfuryl chloride, N-methyl-2-pyrrolidone (NMP) were obtained commercially and used as received. All the reagents are analytical grade.
2.2.
Preparation of SPEEK
SPEEK was prepared according to the procedure described in literature [8]. Specifically, 10 g PEEK was added into the sulfuric acid (98 wt.%) slowly under magnetic stirring at room temperature. After complete dissolution, the temperature was raised to 50 C and then the reaction was kept for 1 h. Subsequently, the polymer solution was poured into ice-cold deionized (DI) water under mechanic stirring, to separate the product SPEEK from the solution. Then, the white fiber products were washed by DI water until neutral pH, and were continuously immersed in DI water overnight to remove H2SO4 completely. Finally, the resulted polymer was dried at 60 C for 12 h, and then 120 C for another 12 h.
2.3.
Preparation of SSA nanoparticles
SiO2 particles were exposed to excess amount of SO2Cl2 in a round flask for 30 min at room temperature. Then, the
evolved HCl and unreacted SO2Cl2 in the reaction mixture were evacuated in a vacuum. The obtained particles were then hydrolyzed in water at room temperature for 1 h, centrifugally separated from the water and finally dried in an oven at temperature 60 C to obtain the SSA nanoparticles. To assess the hydrolysis stability of SSA, some SSA particles were boiled in water for 3 h and then dried in an oven at a temperature of 150 C. The resulted particles were labeled as H-SSA.
2.4.
Preparation of composite membranes
The SPEEK/SSA composite membranes with varied doping contents were prepared by the solvent-casting method. The SPEEK and SSA particles were mixed in varied mass ratios in 50 mL flask, and subsequently a certain amount of NMP was added, followed by magnetic stirring until the SPEEK was completely dissolved in the solvent. To make sure the SSA particles well dispersed in the casting solution, the mixture was then treated in an ultrasonic bath for 30 min. Finally, the mixture was poured onto a glass plate and the solvent was evaporated at 40 C for 48 h followed by 120 C for 12 h. After that, the membranes were peeled off from the glass plate, immersed in H2SO4 (2 M) for 12 h to achieve complete protonation of the polymer, and then washed with DI water until neutral. Finally the membranes were deposited in DI water.
2.5.
Characterization
2.5.1.
Energy dispersive X-ray (EDX)
The EDX spectra of SiO2, SSA and H-SSA particles were obtained by EDX microanalysis performed at 10 KeV using an INCA Energy 300 spectrometer.
2.5.2.
TEM and SEM
The morphology and size of the particles were viewed on a Tecnai-20 transmission electron microscope (TEM) operated at an accelerating voltage of 200 kV. The morphology of the cross sections of the composite membranes was studied using an S-4800 field emission scanning electron microscope (Hitachi, Japan). All the samples were sputtered with a thin layer of gold in vacuum prior to the analysis. Ionic clusters in the membrane samples were investigated by the TEM images of the stained membrane samples as reported in reference [23]. The samples were stained in a saturated lead acetate solution overnight, rinsed with water and dried under room temperature for 4 h. Then, the samples were embedded in Spurr’s epoxy resin, cured overnight and sectioned to obtain slices approximate 70 nm thick by an ultramicrotome. The slices were then viewed on a Tecnai-20 TEM with an accelerating voltage of 120 kV.
2.5.3.
Ion-exchange capacity (IEC)
The IEC of the samples were determined by the classical titration method. Initially, the samples were immersed in saturated NaCl aqueous solution for 24 h, followed by being titrated with 0.01 mol L1 NaOH solution with phenolphthalein as an end point indicator. The IEC was calculated in the following equation:
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 ) 1 1 8 5 3 e1 1 8 6 1
11855
Fig. 1 e TEM images of (a) SiO2 and (b) SSA.
IEC ¼
VNaOH CNaOH 1000 mmol g1 weight of dried sample
where VNaOH (L) and CNaOH (mol L1) are the volume and molar concentration of the NaOH solution, respectively.
2.5.4.
Thermal property
A thermogravimetric analyzer (Mettler Toledo TGA/SDTA851e) was employed to assess the thermal stability of the SSA particles and composite membranes. The particles were heated from 20 to 900 C, and the membranes from 60 to 550 C, both at a heating rate of 10 C/min, and under a nitrogen atmosphere.
2.5.5.
Water uptake and swelling ratio
Membrane samples were equilibrated at desired relative humidity (or water for the evaluation of water uptake in water environment) and temperature for 12 h. The residual water on the surface of wet membranes was removed with filter paper; then their weights and the dimensional lengths were measured rapidly. After the wet membranes were dried at 120 C for 12 h in vacuum, the weights and dimensional sizes of the corresponding dry membranes were obtained. The water uptake and swelling ratio were calculated by the following equations: Water uptake ¼
Wwet Wdry 100% Wdry
Fig. 2 e EDX spectra of (a) SiO2, (b) SSA and (c) H-SSA.
11856
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 ) 1 1 8 5 3 e1 1 8 6 1
Swelling ratio ¼
lwet ldry 100% ldry
where Wwet and Wdry are the weights of the wet and dry membranes, respectively, lwet and ldry are the geometric averages (ldry¼ldry,widthldry,length,lwet¼lwet,widthlwet,length) of the widths and lengths of the wet and dry membrane samples, respectively.
2.5.6.
Proton conductivity
Proton conductivity of the membrane samples was measured by the four-electrode AC impedance spectroscopy method [24], over the frequency range from 1 Hz to 100 kHz (Ivium Technologies A08001). The measuring apparatus contains two Pt foils as the current carrier electrodes and two Pt wires as the potential sensor electrodes. During the measurement, the home-made measurement apparatus together with the membrane samples was kept in a Constant Temperature & Humidity Incubator at the desired temperature and relative humidity. Proton conductivity was calculated as followed: s ¼ L=RA S cm1 : where L (cm) is the distance between the two potential electrodes, R (U) is the membrane resistance, derived from the impedance value at zero phase angle and A (cm2) is the crosssectional area of the membrane.
3.
Results and discussion
3.1.
Characterization of SSA particles Fig. 3 e (a) TGA and (b) DTG curves of SiO2 and SSA particles.
It is generally recognized that the filler size has great influence on the filled system, for small size favors the fillers’ uniform dispersion in the polymeric matrix. The pristine SiO2 nanoparticles have a uniform diameter of approximate 25 nm, as confirmed in Fig. 1a. After sulfonation reaction, TEM image of SSA particles, as shown in Fig. 1b, has no obvious difference from that of SiO2 nanoparticles, which indicates the treating process with sulfuryl chloride did not alter the morphological characteristics of the fillers. The existence of sulfonic acid groups on the surface of SSA can be partially proved by the presence of sulfur. Thus the EDX analysis was made and the spectra are shown in Fig. 2. The corresponding atomic ratios obtained by EDX analysis are given in Table 1. While the EDX spectrum of SiO2 (Fig. 2a) displays only the peaks due to silicon and oxygen, the spectrum of SSA (Fig. 2b) shows a further peak due to the sulfur presence. The sulfur peak can also be found in the spectrum of H-SSA
(Fig. 2c). The S/Si atomic ratio in H-SSA is 0.69:15.84, only a little smaller than that of SSA whose S/Si atomic ratio is 0.78: 14.68. This result shows that the SSA is stable to hydrolysis. To evaluate the effect of the functionalization on the surface of silica and to further confirm the hydrolysis stability of SSA, IEC value of SiO2, SSA and H-SSA were measured and they were found to be 0.02 mmol/g, 0.42 mmol/g and 0.40 mmol/g, respectively as shown in Table 1. The increased IEC of SSA reveals the enhanced acidity provided by the sulfonic acid groups on the surface of the particles. The IEC of H-SSA decreases very slightly compared to that of SSA, which further confirms the good hydrolysis stability of SSA. TGA analysis was made to evaluate the thermal stability of SSA particles. Traces of TGA and DTG are shown in Fig. 3, and
Table 1 e IEC values and atomic ratios of SiO2, SSA and HSSA particles.
Table 2 e Thermal characteristic temperatures (TODa and TFDb) of SiO2 and SSA particles.
Sample
Sample
SiO2 SSA H-SSA
IEC(mmol/g)
S/Si/O atomic ratio
0.02 0.42 0.40
0:22.92:77.08 0.78:14.68:84.53 0.69:15.84:83.48
SiO2 SSA
TOD1 ( C)
TFD1 ( C)
TOD2 ( C)
TFD2 ( C)
262 308
301 352
479 480
543 547
a The onset decomposition temperature. b The fastest decomposition temperature.
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 ) 1 1 8 5 3 e1 1 8 6 1
Table 3 e Description of the membrane samples. Nomenclature SPEEK SPEEK-2.5SSA SPEEK-5SSA SPEEK-7.5SSA SPEEK-10SSA SPEEK-5SiO2
Weight ratio of SPEEK to SSA or SiO2 100:0 100:2.5 100:5 100:7.5 100:10 100:5
IEC(mmol/g) 1.28 1.27 1.25 1.24 1.19 1.16
the thermal characteristic temperatures e the onset decomposition temperatures (TODs) and the fastest decomposition temperatures (TFDs) e are listed in Table 2. For both SSA and SiO2, the first weight-losing stage before 200 C is not caused by decomposition but by moisture evaporation. SiO2 particles’ second weight-losing stage (TOD: 262 C, TFD: 301 C) is attributed to the condensation of silanol groups [22]. Unlike SiO2,
11857
SSA does not experience any weight loss in the temperature range of 262e301 C, indicating a complete consumption of silanol groups during the sulfonation process. Instead, its weight-losing stage (TOD: 308 C, TFD: 352 C) is in accordance with the reported degradation of eSO3H group [2]. Combined with the evidence in the EDX spectra and TGA, it is safe to conclude that SSA has been successfully prepared. Base on the comparison of the EDX spectra and IEC values of SSA and H-SSA, a conclusion can be drawn that SSA is stable to hydrolysis.
3.2. Morphological properties and IEC values of the composite membranes To investigate the effect of SSA doping content on the composite membranes, a series of composite membranes with various doping contents were prepared. The description of the prepared membrane samples was listed in Table 3.
Fig. 4 e SEM images of the composite membranes cross section: (a) SPEEK-2.5SSA (b) SPEEK-5SSA (c) SPEEK-7.5SSA (d) SPEEK-10SSA (e) SPEEK-5SiO2.
11858
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 ) 1 1 8 5 3 e1 1 8 6 1
Fig. 5 e TEM micrographs of stained (a) SPEEK and (b) SPEEK-5SSA.
SPEEK-5SiO2 was also prepared for comparison. As directly adding SiO2 particles rendered serious aggregation in the SPEEK matrix and severe phase separation, shown in Fig. 4e, further investigation of SPEEK/SiO2 composite membranes was quitted. The dispersion of SSA particles within the
membranes was investigated by SEM. As shown in Fig. 4, SSA particles are well dispersed in the membranes when the filler contents are no more than 7.5wt.%, while in the sample of SPEEK-10SSA slight particle aggregation is observed. The better dispersion of SSA than SiO2 can be explained by (1) the decrement of the silanol groups on SiO2 particles e consumed by the reaction with sulfuryl chloride e which rendered the SiO2 particles less likely to condensate with each other, and (2) the polar interaction between the acid sites on the surface of the fillers and the eHSO3 groups on the polymer matrix, which restricted the motion of the SSA in the membranes during the solvent volatilization process. TEM images for stained SPEEK and SPEEK-5SSA membranes are shown in Fig. 5. The ionic domains are represented by the dark regions on the graph. As shown in Fig. 5a, the micro morphology of SPEEK is characterized by many isolated ionic clusters, which is in agreement with the reference [25]. With SSA particles incorporated into the SPEEK, as shown in Fig. 5b, more ionic clusters are observed and the distances between them become shorter, which is benefit for the formation of hydrophilic cluster network in a hydrated state. IEC value is one of the key factors determining the proton conductivity of PEMs. The IEC values of the samples, obtained by classical titration, are listed in Table 2. As shown, the IEC value goes down slightly as the augmentation of the fillers. This suggests SSA particles have a lower ion exchange capacity than SPEEK.
3.3.
Thermal stability
In order to investigate the influence of SSA on the thermal stability of the membranes, the TGA and DTG traces of SPEEK and SPEEK-5SSA are compared in Fig. 6. The thermal
Table 4 e Thermal characteristic temperatures of SPEEK and SPEEK-5SSA. Sample Fig. 6 e (a) TGA and (b) DTG traces of the SPEEK and SPEEK5SSA membranes.
SPEEK SPEEK-5SSA
TOD1 ( C)
TFD1 ( C)
TOD2 ( C)
TFD2 ( C)
303 306
350 348
484 482
516 522
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 ) 1 1 8 5 3 e1 1 8 6 1
characteristic temperatures are listed in Table 4. Generally, three distinct weight loss stages are observed in Fig. 6b: the first stage before 150 C is not induced by decomposition of the membrane samples, but by the evaporation of the sorbed water, while the second stage (TOD: 303 C, TFD: 350 C) is related to the decomposition of the sulfonic acid groups, and the third stage (TOD: 484 C, TFD: 516 C) to the PEEK-backbone decomposition [26]. Compared with the SPEEK membrane, the composite membrane also shows good thermal stability (TOD: 306 C, TFD: 348 C) at the second stage. In this stage, the weight loss of SPEEK-5SSA is larger than that of SPEEK, which is attributed to the decomposition of the extra eSO3H groups of the fillers in the composite membrane. At the third stage (TOD: 482 C, TFD: 522 C), the weight loss of the composite membrane is not as rapid as that of the SPEEK membrane, probably by virtue of the thermally stable silica particles. The TGA results indicate that the introduction of SSA particles does not weaken the thermal stability of the membrane, and the composite membrane can be applied to fuel cell.
3.4.
11859
Fig. 8 e Water uptake of the membrane samples at 80 C and varied relative humidities.
Water uptake and swelling ratio in water, and the trend is in accordance with that of water uptake in Fig. 7. Composite membranes show a small increase in swelling ratio compared with the plain membrane.
Besides the value of IEC, water uptake is another essential factor affecting the transport of the protons within electrolyte membranes. Water uptake of the membrane samples are compared in Fig. 7 (samples tested in water at different temperatures) and Fig. 8 (samples tested at 80 C but different relative humidities). Shown both in Figs. 7 and 8, the composite membranes generally exhibit higher water uptake than the pristine SPEEK membrane. This is due to the strong polar interaction between water molecules and the acid sites on the surface of the fillers. However, when the content of the fillers is too big, severe aggregation of the fillers will decrease the effective surface area of the particles, leading to the decrease of the water uptake. This is why the SPEEK-10SSA has a water uptake lower than that of other composite membranes. As shown in Figs. 7 and 8, the optimum doping content of SSA is between 5wt% and 7.5wt%. Fig. 9 shows the swelling ratios of the samples at varied testing temperatures
The proton conductivity of the membrane samples tested in water in the temperature range of 20e80 C is shown in Fig. 10, in which all of the composite membranes exhibit higher conductivity than the SPEEK membrane. SPEEK-5SSA performs best probably because it has a best balance among IEC, water uptake and fillers’ dispersion state. Compare SPEEK-5SSA with the plain SPEEK membrane, the conductivity increases from 0.04 S cm1 to 0.05 S cm1 at 20 C, and from 0.11 S cm1 to 0.13 S cm1 at 80 C. The SPEEK-5SSA has a higher conductivity than Nafion117 whose conductivity is 0.12 S cm1 at 80 C as reported [2]. The proton conductivity of the membrane samples at 80 C and varied relative humidities are compared in Fig. 11.
Fig. 7 e Water uptake of SPEEK and the composite membranes in water.
Fig. 9 e Swelling ratio of SPEEK and the composite membranes in water.
3.5.
Proton conductivity
11860
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 ) 1 1 8 5 3 e1 1 8 6 1
channels within the membrane, enhance the water retention capacity, provide additional conductive groups, as a consequence, improve the conductivity.
4.
Fig. 10 e Proton conductivities of SPEEK and the composite membranes in water.
Though all of the membranes have small proton conductivity under low relative humidity, the membranes modified with SSA still show great increase in conductivity compared with the pristine SPEEK membrane. The conductivity enhancement can be explained by two predominant proton transport mechanisms: vehicle mechanism and Grotthus mechanism. Vehicle mechanism needs good hydrophilic cluster network and larger water uptake in the membrane. As shown in Fig. 5, the addition of SSA particles will make the clusters in the membranes distribute much denser and more closely with each other, which will improve the formation of transport channels. The hygroscopic inorganic particles can hold more water more tightly, as shown in Figs. 7 and 8. Therefore, the SSA will facilitate the transport of proton as vehicle mechanism (movement of hydrated protons). Additionally, SSA can provide acid groups on its surface, bridge the neighboring sulfonic acid groups on the polymer matrix, shorten the distance of the proton hoping transport, thus can enhance the conductivity of the membrane as the Grotthus mechanism (proton hoping). Generally, SSA particles serve to improve the formation of proton transport
Fig. 11 e Proton conductivities of the membrane samples at 80 C and varied relative humidities.
Conclusion
With the objective of improving the water retention ability and conductivity of SPEEK membrane, SSA nanoparticles were successfully prepared by sulfonating SiO2 nanoparticles with volatile SO2Cl2 and a series of SPEEK-based composite membranes with variant contents of SSA were fabricated by solution-casting method. SSA particles have increased IEC of 0.42 mmol/g and good hydrolysis stability. SSA also has better organic compatibility than SiO2, thus can be dispersed in SPEEK matrix more uniformly, as confirmed by the SEM images. The presence of SSA helps the ionic clusters distribute in the membrane much denser as confirmed by the TEM images of stained samples. The IEC values of the composite membranes are slightly lower than that of the plain SPEEK membrane. The SSA particles do not affect the membranes’ thermal property; the composite membranes have comparable thermal stability with the plain membrane. The hygroscopic and conductive properties of SSA favor both the water uptake and conductivity. All of the composite membrane samples show an improved water retention and conductivity in water and under low relative humidity. At 80 C in water, the conductivity of SPEEK5SSA is 18.6% higher than that of the plain SPEEK membrane and 8.6% higher than that of Nafion117.
Acknowledgement The authors thank the financial support of National Science Fund for Distinguished Young Scholars of China (Grant No. 21125628) and National Natural Science Foundation of China (Grant No. 20976027 and 21176044).
references
[1] Bose S, Kuila T, Nguyen TXH, Kim NH, Lau K-t, Lee JH. Polymer membranes for high temperature proton exchange membrane fuel cell: recent advances and challenges. Progress in Polymer Science 2011;36:813e43. [2] Kumar GG, Kim AR, Nahm KS, Elizabeth R. Nafion membranes modified with silica sulfuric acid for the elevated temperature and lower humidity operation of PEMFC. International Journal of Hydrogen Energy 2009;34:9788e94. [3] Hou HY, Sun GQ, Wu ZM, Jin W, Xin Q. Zirconium phosphate/ Nafion115 composite membrane for high-concentration DMFC. International Journal of Hydrogen Energy 2008;33:3402e9. [4] Jung EH, Jung UH, Yang TH, Peak DH, Jung DH, Kim SH. Methanol crossover through PtRu/Nafion composite membrane for a direct methanol fuel cell. International Journal of Hydrogen Energy 2007;32:903e7. [5] Wu X, He G, Gu S, Hu Z, Yao P. Novel interpenetrating polymer network sulfonated poly (phthalazinone ether sulfone ketone)/polyacrylic acid proton exchange membranes for fuel cell. Journal of Membrane Science 2007; 295:80e7.
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 ) 1 1 8 5 3 e1 1 8 6 1
[6] Yin Y, Du Q, Qin YZ, Zhou YB, Okamoto K. Sulfonated polyimides with flexible aliphatic side chains for polymer electrolyte fuel cells. Journal of Membrane Science 2011;367: 211e9. [7] Zhai FX, Guo XX, Fang JH, Xu HJ. Synthesis and properties of novel sulfonated polyimide membranes for direct methanol fuel cell application. Journal of Membrane Science 2007;296: 102e9. [8] Smitha B, Sridhar S, Khan AA. Synthesis and characterization of sulfonated PEEK membranes for fuel cell application. Journal of Polymer Materials 2004;21:99e106. [9] Yang B, Manthiram A. Sulfonated poly(ether ether ketone) membranes for direct methanol fuel cells. Electrochemical and Solid State Letters 2003;6:A229e31. [10] Gaohong H, Shuang G, Xuemei W, Li C, Hongjing L, Chang L, et al. Synthesis and characteristics of sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK) for direct methanol fuel cell (DMFC). Journal of Membrane Science 2006;281:121e9. [11] Xuemei W, Gaohong H, Shuang G, Wan C, Pingjing Y. Sulfonation of poly(phthalazinone ether sulfone ketone) by heterogeneous method and its potential application on proton exchange membrane (PEM). Journal of Applied Polymer Science; 2007:1002e9. [12] Zhang Y, Zhang HM, Zhu XB, Bi C. Promotion of PEM selfhumidifying effect by nanometer-sized sulfated zirconiasupported Pt catalyst hybrid with sulfonated poly(ether ether ketone). Journal of Physical Chemistry B 2007;111: 6391e9. [13] Wang JT, Zhang YM, Wu H, Xiao LL, Jiang ZY. Fabrication and performances of solid superacid embedded chitosan hybrid membranes for direct methanol fuel cell. Journal of Power Sources 2010;195:2526e33. [14] Kim YS, Wang F, Hickner M, Zawodzinski TA, McGrath JE. Fabrication and characterization of heteropolyacid (H3PW12O40)/directly polymerized sulfonated poly(arylene ether sulfone) copolymer composite membranes for higher temperature fuel cell applications. Journal of Membrane Science 2003;212:263e82. [15] Navarra MA, Croce F, Scrosati B. New, high temperature superacid zirconia-doped Nafion composite membranes. Journal of Materials Chemistry 2007;17:3210.
11861
[16] Wu Z, Sun G, Jin W, Hou H, Wang S, Xin Q. Nafion and nanosize TiO2eSO2 4 solid superacid composite membrane for direct methanol fuel cell. Journal of Membrane Science 2008; 313:336e43. [17] Rodgers MP, Shi Z, Holdcroft S. Transport properties of composite membranes containing silicon dioxide and Nafion. Journal of Membrane Science 2008;325:346e56. [18] Ke C-C, Li X-J, Shen Q, Qu S-G, Shao Z-G, Yi B-L. Investigation on sulfuric acid sulfonation of in-situ solegel derived Nafion/ SiO2 composite membrane. International Journal of Hydrogen Energy 2011;36:3606e13. [19] Celso F, Mikhailenko SD, Kaliaguine S, Duarte UL, Mauler RS, Gomes AS. SPEEK based composite PEMs containing tungstophosphoric acid and modified with benzimidazole derivatives. Journal of Membrane Science 2009;336:118e27. [20] Hande VR, Rath SK, Rao S, Patri M. Cross-linked sulfonated poly (ether ether ketone) (SPEEK)/reactive organoclay nanocomposite proton exchange membranes (PEM). Journal of Membrane Science 2011;372:40e8. [21] Wang L, Zhao D, Zhang HM, Xing DM, Yi BL. Water-Retention effect of composite membranes with different types of nanometer silicon dioxide. Electrochemical and Solid-State Letters 2008;11:B201. [22] Gnana Kumar G, Senthilarasu S, Dae Nyung L, Ae Rhan K, Pil K, Kee Suk N, et al. Synthesis and characterization of aligned SiO2 nanosphere arrays: spray method. Synthetic Metals 2008;684e7. [23] Peron J, Mani A, Zhao X, Edwards D, Adachi M, Soboleva T, et al. Properties of Nafion NR-211 membranes for PEMFCs. Journal of Membrane Science 2010;356:44e51. [24] Yan X, He G, Gu S, Wu X, Du L, Zhang H. Quaternized poly(ether ether ketone) hydroxide exchange membranes for fuel cells. Journal of Membrane Science 2011;375:204e11. [25] Mahajan CV, Ganesan V. Atomistic simulations of structure of Solvated sulfonated Poly(ether ether ketone) membranes and their comparisons to Nafion: I. Nanophase Segregation and hydrophilic domains. Journal of Physical Chemistry B 2009;114:8357e66. [26] Wu J, Cui Z, Zhao C, Li H, Zhang Y, Fu T, et al. High proton conductive advanced hybrid membrane based on sulfonated Si-SBA-15. International Journal of Hydrogen Energy 2009;34: 6740e8.