Current Applied Physics 11 (2011) S229eS237
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Preparation of poly(vinyl alcohol)/montmorillonite/poly(styrene sulfonic acid) composite membranes for hydrogeneoxygen polymer electrolyte fuel cells Chun-Chen Yang*, Shwu-Jer Chiu, Shih-Chen Kuo Department of Chemical Engineering, Ming Chi University of Technology, 84 Gungjuan Rd., Taipei Hsien 243, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 September 2010 Received in revised form 3 November 2010 Accepted 15 November 2010 Available online 23 November 2010
The good performance poly(vinyl alcohol)/montmorillonite/poly(styrene sulfonic acid) (PVA/MMT/PSSA) composite membrane is prepared by a solution casting method. The characteristic properties of the composite membranes are investigated by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), X-ray diffraction (XRD), scanning electron microscopy (SEM), micro-Raman spectroscopy and AC impedance method. The ionic conductivities of the PVA/MMT/PSSA composite membranes in water at ambient temperature are of the order of 103 S cm1, i.e., at 2.07 103e6.69 103 S cm1. The ionic conductivity of the novel composite membrane is greatly enhanced due to two proton ionic sources used, i.e., the modified MMT fillers and PSSA polymer. Under atmospheric pressure, the peak power densities of the polymer electrolyte membrane fuel cell (PEMFC) at 25 and 50 C are 65.23 and 90.70 mW cm2, respectively. The results indicate that the PEMFC comprised of the PVA/MMT/PSSA composite membrane has a good electrochemical performance. This composite membrane is a potential candidate for the future PEMFC application. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Poly(vinyl alcohol) (PVA) Montmorillonite (MMT) Poly(styrene sulfonic acid) (PSSA) Proton conducting Polymer electrolyte membrane fuel cell (PEMFC)
1. Introduction Polymer electrolyte membrane fuel cells (PEMFCs) [1e10] are considered as the most promising alternative green power sources for the future EV transportation use, because they have high energy and power densities, good efficiency, low pollutant emission, and high utilization. During the last decades, progress has been achieved for the PEMFC in materials, system design, manufacturing, and application. At present, a Nafion membrane is widely used in PEMFC, due to its excellent chemical, mechanical stability, and high proton conductivity. However, the high cost of the Nafion membrane and the lack of suitable hydrogen storage media are two major problems for PEMFC applications. Developing a cheap and high performance polymer electrolyte membrane that facilitates the transport of proton at different operating conditions can overcome the first major problem on the commercializing fuel cell devices. Some aromatic hydrocarbon polymers based PEMFCs [2e7] are prepared to replace using the Nafion membrane. These polymer membranes, unfortunately, exhibit poor proton conductivity in a low humidity environment and restrict their further applications in PEMFCs. Many efforts have been made by researchers to modify the polymer electrolyte membrane. Inorganic-organic composite
* Corresponding author. Tel.: þ88629089899; fax: þ886 29041914. E-mail address:
[email protected] (C.-C. Yang). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.11.043
membrane is one of the effective methods, which has been studied to enhance the water retention and to decrease the fuel crossover. Silica, zirconia, and titania are three major inorganic fillers. Watanabe et al. [4] first reported their work for the addition of hydroscopic oxides into perfluorosulfonic acid membranes. Tang et al. [5] studied the Nafion/silica polymer membrane for fuel cell at elevated temperature. Chen et al. [6] investigated the Nafion/ Zeolite nanocomposite polymer membrane for a direct methanol fuel cell application. The addition of nonconductive inorganic fillers into the membrane normally results in a decrease of the proton conductivity. In order to avoid this phenomenon, some modified polymer membranes with functionalized silica have been proposed [7e10] and several of them report about sulfonic acid functionalized Nafion/silica composite membranes. Pereira et al. [7] examined the mesostructured hybrid Nafion/silica composite membrane for high temperature fuel cell applications. Nicolic et al. [9] prepared a cross-linked poly(vinyl alcohol) (PVA) polymer membrane for a hydrogen fuel cell by a gamma irradiation technique. Recently, Yang et al. [11,12] synthesized a cross-linked PVA/MMT composite polymer membrane and applied the membrane to an acidic DMFC. However, a challenge for the PVAbased polymer membrane is that they show a poor proton conductivity and the acidic electrolyte may leak out from the polymer membranes. The main reason for this problem is that the PVA polymer itself does not contain any negatively charged ions or the negative organic functional groups, such as carboxylic (eCOOH) or
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sulfonic acid (eSO3H) groups. Some negatively charged ionic groups must be grafted or blended into the PVA polymer host before using PVA polymer membranes in a H2eO2 fuel cell. From this point of view, the poly(styrene sulfonic acid) (PSSA) was chosen as a proton donor, because it can achieve reasonable proton conductivity for the PVA/PSSA blend composite polymer membrane. In addition, Rhim et al. [13] synthesized a PVA/sulfosuccinic acid (SSA) proton-conducting polymer membrane: the SSA containing both eSO3H and eCOOH groups served as the cross-linking agent and a proton donor with varying from 5 to 30wt.%. Previous authors [14,15] have studied a sulfonated poly(ether ether ketone)/poly(vinyl alcohol) (SPEEK/PVA blend), called double-layer membrane, for a fuel cell. A sub-layer of PVA polymer clung to the anode and provided a good barrier for the alcohol crossover. On the other hand, the SPEEK sub-layer maintained the mechanical stability and a low swelling ratio. Kumar et al. [16] reported an interesting result about the poly(vinyl alcohol)/para toluene sulfonic acid (PVA/PTSA) polymer membranes. The authors introduced a suitable amount of proton charge carriers (namely, eSO3H group), this hydrophilic PTSA greatly enhanced the ionic conductivity of PVA/PTSA polymer membrane. Moreover, Sahu et al. [17] investigated the effects of poly(styrene sulfonic acid) content on a PVA/PSSA blend polymer membrane and its application in a hydrogeneoxygen PEMFC. They revealed that a maximum proton conductivity of the PVA/PSSA blend membrane was appeared when the weight percentage of PSSA appeared at 35wt.%. With the optimized PVA/PSSA polymer membrane, the PEMFC achieved a peak power density of 210 mW cm2 at 500 mA cm2 at 75 C, compared to a peak power density of only 38 mW cm2 at 80 mA cm2 for the PEMFC with a pristine PVA membrane. In this work, the constant amount of 10wt.% PSSA, i.e., PVA:PSSA ¼ 1:0.10, was added in the PVA matrix. As we know, montmorillonite (MMT) is a well-known silicate material with layered structure. Therefore, the addition of these hydrophilic MMT ceramic fillers into the polymer matrix not only reduces the crystallinity of the PVA polymer but also increases the amorphous phases of PVA polymer matrix, resulting in an increase of its ionic conductivity [18,19]. A variety of ceramic fillers have been used in PVA membrane, such as TiO2 (PVA/TiO2) [20], SiO2 (PEG/SiO2 and PVA/SiO2) [21], and hydroxyapatite (PVA/HAP) [22]. A suitable amount of the MMT fillers was first blended with the PSSA polymer in a weight ratio of 1:1 under continuous stirring condition for 24 h. These PSSA-blended MMT fillers were designated as the modified MMT fillers in this work. We attempted to disperse the modified MMT fillers into the PVA and PSSA polymer matrix as the solid plasticizers and ionic solid conductors, which were capable of enhancing the ionic conductivity. TGA and DSC experiments analyzed the thermal stability properties of the composite membrane. DMA and SEM were applied to study the mechanical properties and surface morphology of the composite polymer membranes, respectively. XRD was used to examine the crystallinity of PVA/MMT/PSSA solid polymer electrolytes (SPEs) and microRaman spectroscopy was used to investigate the chemical composition information within each composite membrane. The ionic conductivity of proton-conducting composite polymer electrolyte was measured by AC impedance spectroscopy. The characteristic properties of PVA/MMT/PSSA composite membranes with different amounts (5,10,15, and 20wt.%) of the modified MMT fillers (function as the second ionic source) and 10wt.% PSSA polymer (act as the major ionic source) were examined and discussed in detail. Furthermore, a PEMFC with the air cathode, the hydrogen anode, and the PVA/MMT/PSSA composite polymer membrane was assembled and compared the performance at operating temperatures of 25 and 50 C, respectively. The electrochemical characteristics of the PEMFC employing the PVA/MMT/PSSA composite
membranes were investigated by the linear polarization, potentiostatic and galvanostatic methods; especially for the peak power density of the PEMFC. 2. Experimental 2.1. Preparation of the PVA/MMT/PSSA composite membrane PVA (Aldrich), PSSA (Aldrich, 18wt.% solution), nano-sized MMT fillers (Aldrich) and H2SO4 (Merck) were used as received without further purification. Degree of polymerization and saponification of PVA were 1700 and 98e99%, respectively. The PVA/MMT/PSSA composite membrane was prepared by a solution casting method. The inert ceramic MMT clays were first blended with PSSA polymer solution in a weight ratio of 1:1 at room temperature for 24 h and followed by drying at 100 C overnight, the obtained solid mixture powders were further ground to fine particles. The ionic MMT fillers were further dried at 120 C for 12 h before use. The impregnation pre-treatment process can transform the MMT from an inert filler to an ionic conducting filler (the MMT impregnating with eSO3H groups). Therefore, the proton conductivity of the composite membranes with these modified MMT fillers was greatly improved. Several different percentages (0e20wt.%) of the modified MMT fillers were added slowly into the viscous solution under stirring condition. The obtained solution became homogeneous and viscous by stirring continuously at 85 C for 3 h. The contents of the modified MMT fillers in the PVA and PSSA polymers were well controlled. About 5wt.% glutaraldehyde (GA, 25wt.% content in distilled water, Merck) was added into the viscous mixture polymer solution to carry out the cross-linking reaction with PVA. The resulting viscous blend polymer solution was coated onto a glass plate. It was found that the viscous mixture polymer solution became gel when the content of the PSSA polymer (it also acted as a catalyst for the crosslinking reaction) was more than 10wt.%. The maximum content of the PSSA polymer, therefore, was controlled at 10wt.% in order to avoid the rapid formation high viscosity gel. The thickness of the wet composite membrane is between 0.030 and 0.050 cm. The sample of glass plate with viscous PVA/MMT/PSSA composite polymer was weighed again and then the excess water was allowed to evaporate slowly at 60 C in a relative humidity of 30%. The glass plate with the composite polymer membrane was weighed again after water solvent was completely evaporated. The composition of PVA/MMT/PSSA composite membrane was determined from the mass balance. The thickness of the dried composite membrane was controlled between 0.020 and 0.030 cm. The detailed preparation methods of the composite membranes based on PVA by a solution casting method have been reported in literature [11,12]. 2.2. Crystal structure, surface morphology, thermal and mechanical properties The surface morphology of the PVA/MMT/PSSA composite membranes was investigated using a Hitachi S-2600H scanning electron microscopy (SEM). Differential scanning calorimetry (DSC) (a Perkin Elmer Pyris 7 DSC system) and TGA (a Mettler Toledo TGA/ SDT 851e system) were used to examine the thermal properties of the PVA/MMT/PSSA composite membrane. DSC measurements were carried out in a dry N2 atmosphere from 25 to 250 C with a heating rate of 10 C min1. TGA measurements were carried out by heating from 25 to 600 C under a N2 atmosphere at a heating rate of 10 C min1 with a sample about 5e10 mg. Derivative thermogravimetry (DTG) curve is the first order differential of TGA curve with respect to temperature. DMA were conducted using an RSA-III Instrument DMA-Thermal analyzer (TA) at a frequency of
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1 Hz and oscillation amplitude of 0.15 mm. DMA measurements were carried out by heating from 25 to 150 C under an air atmosphere at a heating rate of 5 C min1. The crystal structures of the PVA/MMT/PSSA composite membranes were examined using a Philips X’Pert XRD with Cu Ka radiation of wavelength (l) of 1.54056 Å for 2q angles between 10 and 80 .
PEMFC with H2 gas feed at a rate of 46 cm3 s1 and O2 gas feed at a rate of 100 cm3 s1 were examined in ambient pressure and at 25 and 50 C, respectively.
2.3. Ionic conductivity measurements
3.1. Thermal and mechanical properties
The ionic conductivity of the PVA/MMT/PSSA composite membrane was measured by an AC impedance method. The PVA/ MMT/PSSA composite membranes were first immersed in a 2 M H2SO4 solution for at least 24 h, and then washed with D.I. water several times before test. The composite membranes were clamped between stainless steel (SS304), ion-blocking electrodes, each of surface area 1.32 cm2, in a spring-loaded glass holder. In order to measure correct temperature, a thermocouple was put closely to the composite polymer membrane for temperature measurement. Each sample was kept at the experimental temperature at least for 30 min to reach equilibrium before measurement. AC impedance measurements were carried out using an Autolab PGSTAT-30 equipment (Eco Chemie B.V., Netherlands). The AC spectra in the range of 100 kHz to 10 Hz at an excitation signal of 5 mV were recorded. AC impedance spectra of the composite polymer membrane were recorded at a temperature range between 30 and 70 C. The experimental temperatures were maintained constant within the variation of 0.5 C by a convection oven. Each PVA/MMT/PSSA composite membrane was examined at least three times.
Fig. 1(a) shows TGA thermographs for pure PSSA film, the PVA/ 5wt.%GA film, and the PVA/10wt.%PSSA/5e20wt.% MMT/5wt.%GA composite membranes, respectively. The TGA curve of pure PSSA film also shows three major weight loss regions, which appear as three major peaks in the DTG curves, as shown in Fig. 1(b). The TGA
3. Results and discussion
2.4. Micro-Raman analyses The micro-Raman spectroscopy analysis was carried out using a Renishaw confocal microscopy Raman spectroscopy system with a microscope equipped with a 50 objective and a charge coupled device (CCD) detector. Raman excitation source was provided by a 632.8 nm HeeNe laser beam, which had the beam power of 17 mW and was focused on the sample with a spot size of about 1 mm in diameter. 2.5. Preparation of the anode and the cathode The catalyst slurry ink for the anode was prepared by using Pt/C catalyst (20wt.% of Pt), 15wt.% Nafion binder solution (Aldrich), and a suitable amount of distilled water and isopropyl alcohol (IPA). The resulting Pt/C inks were first ultrasonicated for 2 h. The Pt/C inks were loaded onto the carbon paper (GDL 10BB, SIGRACET, Germany) by an air spray method to achieve a loading of 1 mg cm2. The electrode area was 5 cm2. The as-prepared anode was dried in a vacuum oven at 110 C for 2 h. The air cathode was prepared using similar procedure as the anode, but the cathode with the Pt black (Alfa) inks of 1.0 mg cm2. 2.6. Electrochemical property measurements The PVA/MMT/PSSA composite membrane was sandwiched between the sheets of the anode and cathode and then pressed at 25 C under 100 kgf cm2 for 5 min to obtain a membrane electrode assembly (MEA). The electrode area of the MEA was about 5 cm2. The electrochemical measurements were carried out in a twoelectrode system. The current densityevoltage (IeV) and the power density (P.D.) curves for the PEMFC comprised of the PVA/MMT/ PSSA composite membranes were recorded, respectively. All of the electrochemical measurements were performed on an Autolab PGSTAT-30 electrochemical system with GPES 4.8 package software (Eco Chemie, Netherland). The electrochemical performances of the
Fig. 1. (a) TGA and (b) DTG thermographs for the PVA/PSSA/xwt.%MMT composite membranes.
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curve of PVA/5wt.%GA film (without any MMT fillers) shows three major weight loss regions, which appear as three corresponding major peaks in the DTG curves. The TGA curve of the PVA/10wt.% PSSA/xwt.%MMT polymer membranes shows a similar result as PVA/5wt.%GA film. The first region at a temperature of 50e90 C (Tp,1 ¼72 C) is due to the evaporation of weakly physical and strongly chemical bounded H2O; the weight loss of the membrane is approximately 7w8wt.%. The second transition region at around 300e400 C (Tp,2 ¼ 334 C) is due to the degradation of eSO 3 group (or called desulfonation) on the PSSA polymer and the total weight loss corresponding to this phase is about 27e28wt.%. The peak of the third transition at around 400e450 C (Tp,3 ¼ 421 C) is as a result of the cleavage of the sidechain of the PSSA polymer, the total weight loss corresponding to this phase is about 27e28wt.%. The total weight loss is about 58wt.% when the temperature reaches at 600 C. For cross-linked PVA/5wt.%GA film, the first region at a temperature of 90e150 C (Tp,1 ¼ 113 C) is because of the evaporation of physical and strongly chemical bounded H2O and the weight loss of the membrane is approximately 4w5wt.%. The second transition region at around 250e400 C (Tp,2 ¼ 340 C) is due to the cleavage of the side-chain of PVA polymer and the total weight loss corresponding to this phase is about 78e79wt.%. In addition, it is also found that there is a shoulder peak at 288 C (PVA degradation peak) at this thermal oxidation stage. The third transition region at around 400e470 C (Tp,3 ¼ 421 C) is caused by the cleavage of the backbone of PVA polymer, which reflects the total weight loss is approximately 94e95wt.% at 600 C. For the as-synthesized PVA/MMT/PSSA composite membranes, the weight loss due to the evaporation of physical and strongly chemical bounded H2O at a temperature of 80e140 C is very small, only about 1w2wt.%. The first transition region at around 150e220 C (Tp,1 ¼ 183 C) is due to the degradation of eSO 3 group on the PSSA polymer, the total weight loss in this phase is about 29e30wt.%. The second transition region at around 370e500 C (Tp,2 ¼ 433 C) is owning to the cleavage of the side-chain of PVA polymer and the total weight loss is approximately 78e79wt.%. The peak of the third transition at around 500e540 C (Tp,3 ¼ 520 C) is also caused by the cleavage of the backbone of PVA polymer, the total weight loss is about 81e82wt.% at 600 C. The results for weight loss of the PVA/MMT/PSSA composite membranes are summarized and listed in Table 1. Accordingly, the degradation peaks of the cross-linked PVA/MMT/PSSA composite membrane samples are less intense and shift towards higher temperature. It can be concluded that the thermal stability is improved probably due to the effect of the addition of modified MMT fillers and the chemical cross-linked reaction between PVA and glutaraldehyde. The DSC measurements were carried out by a heatingecoolingeheating cycle, called the HeCeH procedure. The purpose of the first heating cycle was to remove any thermal history of the PVA composite polymer membrane. The DSC (2nd
heating) curves for the pure PVA film, the PVA/GA film, and the PVA/MMT/PSSA composite polymer membranes with various MMT compositions (0e20wt.%) are shown in Fig. 2. An endothermic peak was appeared at 225 C, which corresponded to the melting temperature (Tm) of the pure PVA film. It had been reported that the Tm of the pure PVA polymer with 98e99% degree of hydrolysis was at 226 C [11,12]. Moreover, an endothermic peak was presented at 187 C, which corresponded to the Tm of the PVA/GA film. It was found that the melting temperature, Tm, of the cross-linked PVA/GA film shifted toward lower temperature when the PVA film was cross-linked with GA. In addition, two melting temperatures, Tm2 ¼ 187 C and Tm1 ¼ 174 C, for the cross-linked PVA/10wt.%MMT/10wt.%PSSA composite membrane were found. In contrast, the two melting temperatures, Tm2 and Tm1, for the cross-linked PVA/20%MMT/10% PSSA composite membrane were at 178 C and 167 C, respectively. Both melting temperatures were shifted toward lower temperature region when the amount of added modified MMT fillers was added increasingly. The above result indicates that the degree of crystallinity of the PVA/MMT/PSSA membrane gradually decreased when the amount of the modified MMT fillers increased. It was also found that the re-crystallization peak temperature (Tc) was shifted to a lower temperature direction and the Tc peak became broader from the DSC cooling curve (not shown here). This also indicated a change between a semi-crystalline phase and an amorphous phase. The storage moduli (E0 ) vs. temperature curves of pure PVA film, the PVA/10wt.%PSSA film, and the PVA/0e20wt.%MMT/10wt.%PSSA composite membranes were measured (figure not shown here). The storage modulus of pure PVA film (E0 ¼6.60 108 Pa) was lower than those of PVA/xwt.%MMT/10wt.%PSSA composite membranes (E0 ¼ 1.16 w 1.87 109 Pa) at 30 C. It was found that the storage modulus of PVA/MMT/PSSA composite membranes was slightly increased with increasing the loading of the MMT fillers (up to 20wt.%). The storage modulus started to decrease when the content of the modified MMT fillers was over 10wt.%, as listed in Table 2. As a matter of fact, the poor physicochemical properties, the mechanical properties of the PVA/ MMT/PSSA composite membrane were obtained when the quantity
(1) (2) (3) (4)
P V A film P V A /5w t.% G A P V A /10w t.% P S S A /10w t.% M M T /5w t.% G A P V A /10w t.% P S S A /20w t.% M M T /5w t.% G A Tm1 Tm2
Heat Flow Endo up/ mW
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(4)
(3)
(2) Tm
Table 1 TGA results for weight loss of PVA/PSSA/MMT composite membranes with different amounts of modified MMT fillers. Loss(%)
Temperatures 100 C 200 C 300 C 400 C 500 C 600 C
(1)
Types Pure PSSA 13.0 PVA/10%GA 1.29 PVA//10%PSSA/5wt.%MMT/5%GA 1.18 PVA/10%PSSA/10wt.%MMT/5%GA 1.43 PVA/10%PSSA/20wt.%MMT/5%GA 1.48
16.5 23.97 19.71 17.74 18.82
18.8 36.13 34.73 33.94 32.61
40.1 44.51 44.12 44.46 41.63
54.3 82.66 78.42 76.71 67.98
57.5 85.00 81.03 79.88 72.29
50
75
100
125
150
175
200
225
250
T em perature/ o C Fig. 2. DSC thermographs of pure PVA, the PVA/GA and the PVA/MMT/PSSA composite membranes.
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Table 2 DMA results (storage modulus, tand vs. T) of the cross-linked PVA/PSSA/MMT composite membranes with different quantities of the modified MMT fillers. PVA film
E’/Pa
PVA/10%PSSA
PVA/10%PSSA/5wt.%MMT/5wt.%GA
PVA/10%PSSA/10wt.%MMT/5wt.%GA
PVA/10%PSSA/20wt.%MMT/5wt.%GA
Temp. 30 C 60 C 100 C 150 C tan(d), oC
6.60 1.22 1.17 8.38 27.01
108 108 108 107
2.23 1.33 1.57 1.02 54.83
109 108 108 108
1.87 8.78 8.02 4.70 52.18
109 107 107 107
1.72 9.12 7.99 6.34 52.33
of the modified MMT fillers was 25wt.% (data not shown here). Actually, the amount of modified MMT fillers added in the PVA/MMT/ PSSA composite membrane needs to be well controlled. It can be concluded that the maximum content of the modified MMT fillers in the PVA/MMT/PSSA composite membrane was limited to 20wt.%. Interestingly, it was also found that the mechanical properties were improved when the thermal treatment was carried out at 120 C. This may be due to the annealing effect on the cross-linked PVA-based composite membrane. In general, the annealing treatment will enhance the degree of cross-linking between the eOH group of the PVA polymer and the eCHO group of GA. Fig. 3 shows the tan(d) vs. temperature curves of the PVA/MMT/ PSSA composite membranes. The glass transition temperatures (Tg) can be taken at a peak (tan(d)1) of the tan(d) curve. The glass transition temperatures of pure PVA film (Tg, PVA) and all cross-linked PVA/x% MMT/10wt.%PSSA composite membranes are listed in Table 2. The results indicated that the glass transition temperatures of pure PVA film (considered as a Tg, PVA) and the PVA/10wt.%PSSA membranes were at 27.01 and 54.83 C, respectively. Apparently, it was observed that there is only one tan(d) peak for the PVA/xwt.%MMT/10wt.%PSSA composite membranes. The tan(d) peaks for the composite membranes, they were taken as the Tg, PVA, were located between 52 and 54 C. Interestingly, these tan(d) peaks were much broaden for the PVA/xwt.%MMT/10wt.%PSSA composite membranes, as compared with those of pure PVA film and the PVA/PSSA blend membrane. The broader and lowering intensities for tan(d) peaks for
109 107 107 107
1.16 1.13 1.08 1.00 54.47
109 108 108 108
the PVA/xwt.%MMT/10wt.%PSSA composite membranes are strong evidences for decreasing of the crystallinity of the composite membranes.
3.2. XRD and SEM analyses X-ray diffraction measurement was performed to examine the crystallinity of the PVA/MMT/PSSA composite membrane. Fig. 4(a) shows the XRD pattern for the as-received MMT fillers and the modified MMT fillers containing PSSA polymer, respectively. It was observed that there were two XRD characteristic peaks for these MMT materials at 2q angles of 1.75 and 3.30 . It was found that the diffraction peak positions are only slightly changed; however, the peak intensity is greatly reduced and becomes broad. Fig. 4(b) illustrates the diffraction pattern of the PVA/MMT/PSSA composite
(1). Pure PVA (2). PVA/10wt.%PSSA/5wt.GA (3). PVA/10wt.%PSSA/5wt.%MMT/5wt.%GA (4). PVA/10wt.%PSSA/10wt.%MMT/5wt.%GA (5). PVA/10wt.%PSSA/20wt.%MMT/5wt.%GA
tan( )/ a.u.
(5) (4) (3) (2)
-0 Tg, PVA 20
40
(1) 60
80
100
120
140
160
o
Temperature/ C Fig. 3. DMA curves for the pure PVA and the PVA/10wt.%PSSA/xwt.%MMT composite membranes (the tan(d) vs. T curves).
Fig. 4. XRD patterns for (a) the as-received MMT fillers and the modified MMT fillers; (b) the pure PVA film, the PVA/GA film, and the PVA/MMT/GA, PVA/10wt.%PSSA/ 20wt.%MMT/GA composite membranes.
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membranes that were prepared by adding different amounts of the modified MMT fillers. The PVA polymer is well-known exhibiting a semi-crystalline structure with a large crystalline peak at a 2q angle of 19e20 and a small amorphous peak at 39e40 . The peak at 19.5 is corresponded to the (101) plane for the pure PVA film. The peak intensity at 2q ¼ 19 became weaker for the PVA/ 5e20wt.%MMT composite polymer membranes, which could be seen clearly in Fig. 4(b). But, the intensities of the (101) peak for PVA/MMT/PSSA composite membranes were greatly reduced when the amount of added modified MMT fillers was increased. Under such circumstances, it was clear that the amorphous domain in the PVA/MMT/PSSA composite polymer membranes was markedly augmented (i.e. the degree of crystallinity decreased). Table 3 demonstrates the values of relative crystallinity (%) for the PVA/MMT/PSSA composite polymer membranes with different compositions of MMT fillers. The relative crystallinity values of the PVA/MMT/PSSA composite membrane decreased from 100 to 46.03%. Note that the domain of amorphous phases increases with an increase in the contents of the modified MMT fillers. There is a significant motion of the polymer chain in the amorphous phase or some defects existing at an interface between the polymer chains and the MMT fillers. Therefore, the PVA/MMT/PSSA composite membranes show good ionic transport property (data shown later section). This is due to the more amorphous phase and flexibility of local PVA chain segmental motion in the PVA/MMT/PSSA composite membranes. The top and cross-sectional views of SEM photographs for the PVA/20wt.%MMT/10wt.%PSSA composite membranes can be found in Fig. 5(a), (b), and (c), respectively. Furthermore, it was found that these hydrophilic MMT fillers are dispersed well into the PVA polymer matrix. The dimension for the modified MMT fillers embedded in PVA matrix is about 0.5 mm 1w2 mm with long thin oval shape, MMT fillers as shown clearly in Fig. 5(c). As we can see, these modified MMT fillers are completely mixed with PVA polymer. As a whole, the compatibility of the PVA polymer and the modified MMT fillers shows a uniform surface texture when the content of the modified MMT fillers is not over 20wt.%. In short, both hydrophilic PVA polymer and the modified MMT fillers were homogeneous and fully compatible without any phase separation occurring while a suitable amount of the modified MMT fillers was added. It is well accepted that the suitable amount of the modified MMT fillers (also as the ionic conductors containing eSO3H groups) [18,19] can greatly increase the ionic conductivity of the composite membrane.
3.3. Micro-Raman analyses 3.3.1. Micro-Raman spectroscopy is a powerful tool to characterize the PVA/MMT/PSSA composite membrane Fig. 6(a) shows the micro-Raman spectra of pure PVA and PVA/ xwt.%MMT/10wt.%PSSA composite membranes, respectively. The spectra showed some strong characteristic scattering peaks for PVA
Table 3 XRD results of PVA/10wt.%PSSA/xwt.%MMT composite membranes. Types
XRD peaks position Position/deg.
Intensity/a.u.
Relative intensity/%
19.53 19.41 19.47 19.41 19.49
18600 11100 11271 8947 8562
100.00 59.68 60.60 48.10 46.03
Membranes Pure PVA film 0wt.%MMT 5wt.%MMT 10wt.%MMT 20wt.%MMT
Fig. 5. SEM photographs for PVA/10wt.%PSSA/20wt.%MMT composite membranes: (a) top view; (b) and (c) cross-sectional views.
polymer at 1440, 1258, 1146, 919, and 860 cm1 [11,12,22]. A very strong peak for the PVA polymer at 1440 cm1 was observed due to the CeH bending and OeH bending. In addition, two additional vibrational peaks for PVA polymer at 919 and 860 cm1 were due to the CeC stretching and several weak scattering peaks at 1258, 1146, 1093, and 1066 cm1 due to the CeC stretching and CeO stretching were shown in Fig. 6(a). For the PSSA, three characteristic scattering peaks for the PSSA polymer were at 1599, 1125, and 1041 cm1, as seen in Fig. 6(a). More specifically, the peaks of 1599 cm1 were due to the C]C stretching and the peaks of 1125 and 1041 cm1 were due to the eSO 3 stretching.
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14 0 (1) (2) (3) (4) (5)
12 0
3 0 oC 4 0 oC 5 0 oC o 60 C 7 0 oC
(1)
(2)
10 0
-Z''/ ohm
(3 ) 80
(4 ) (5 )
60
40
20
0 0
10
20
30
40
50
60
70
Z '/ ohm
Fig. 7. Nyquist plot for the PVA/20wt.%MMT/10wt.%PSSA composite membrane.
The most important outcome with respect to the micro-Raman analysis, it can be clearly seen that the intensities of these SO3vibrational peaks (at 1041 and 1126 cm1) for the PVA/MMT/PSSA composite membrane were slightly varied, as shown in Fig. 6(b). A 1 is an indicator for the vibrational peak of the eSO 3 at 1041 cm PSSA polymer. In other words, it indicated that the ionic conductivity is increased; it is due to the existing negative ionic groups (-SO 3 ) in the PVA/MMT/PSSA composite membrane. The addition of the PSSA polymer and the modified MMT (both containing the -SO 3 ionic sources) results in enhancing the proton conductivity.
1200
120 o
(1). I-V at 25 C o (2). I-V at 50 C o (3). P.D. at 25 C o (4). P.D. at 50 C
1100
100
1000
(4)
The typical AC impedance spectra of the PVA/MMT/PSSA composite membrane, synthesized by blending PVA and PSSA polymers with 20wt.% modified MMT fillers, could be obtained at different temperatures, as shown in Fig. 7 (Nyquist plot). The AC spectra are typically non-vertical spikes for stainless steel (SS) blocking electrodes, i.e., an SSrPVA/MMT/PSSA SPErSS cell. Analysis of the spectra yields information about the properties of the PVA/MMT/ PSSA composite membrane, such as bulk resistance, Rb. The bulk resistance associated with the membrane conductivity was determined from the high-frequency intercept of the impedance with real axis. Taking into account the thickness of the composite membranes, the ionic conductivity (s) was calculated from the Rb value, according to the equation: s ¼ L=Rb ,A, where s is the proton conductivity of the composite membrane (S cm1), L is the thickness (cm) of the PVA/ MMT/PSSA composite membrane, A is the cross-sectional area of the
80 -2
3.4. Ionic conductivity measurements
Cell voltage/ mV
900 (3)
800
60 700 600
P.D./ mW cm
Fig. 6. (a) micro-Raman spectra for the pure PVA film, and the PVA/10wt.%PSSA/ 20wt.%MMT composite membranes; (b) micro-Raman spectra for the PVA/10wt.% PSSA/Xwt.%MMT composite membranes.
blocking electrode (cm2), and Rb is the bulk resistance (ohm) of a proton-conducting composite polymer membrane. Typically, the Rb values of the PVA/20wt.%MMT/10wt.%PSSA composite membranes were on the order of 4e16 U and are highly dependent on the contents of the modified MMT fillers [18,19]. Note that these composite polymer membranes were immersed in D.I. water for 24 h before measurement. Table 4 shows the ionic conductivities of PVA/0e20wt.%MMT/ 10wt.%PSSA composite membranes at different temperatures. As listed in Table 4, the ionic conductivity value of the PVA/10wt.% PSSA composite membrane (without any MMT fillers) in water is 2.07 103 S cm1 at 30 C. In contrast, the ionic conductivity values for PVA/xwt.%MMT/10wt.%PSSA composite membranes with 5, 10, 15 and 20wt.% MMT fillers are 3.11 103, 3.62 103,
40
500 20 400 (2)
(1) 300
0 0
25
50
75
100 125 150 175 200 225 250
Current density/ mA cm
-2
Fig. 8. The IeV and P.D. curves for the PEMFC composed of the PVA/10wt.%PSSA/ 20wt.%MMT composite membrane with H2 and O2 gas fuels at 25 and 50 C and at atmospheric pressure.
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Table 4 The ionic conductivities (S cm1) of the PVA/10wt.%PSSA/xwt.%MMT composite membranes.
s(S cm1) 0%MMT
5%MMT
10%MMT
15%MMT
20%MMT
Temperature
Temp. 30 40 50 60 70
C C C C C
Table 5 Electrochemical performances of the PEMFC comprised of the PVA/10wt.%PSSA/ 20wt.%MMT composite membrane with H2 and O2 feeds at 25 and 50 C and at ambient pressure.
2.07 2.47 3.36 4.27 4.71
103 103 103 103 103
3.11 3.82 4.55 5.19 5.84
103 103 103 103 103
3.62 4.64 5.17 5.78 6.33
103 103 103 103 103
3.69 4.88 5.82 6.97 8.19
103 103 103 103 103
6.69 7.72 9.01 10.4 12.0
103 103 103 103 103
3.69 103 and 6.69 103 S cm1 at 30 C, respectively. It was found that the PVA/20wt.%MMT/10wt.%PSSA composite membrane has the highest ionic conductivity, s ¼ 6.69 103 S cm1, at ambient temperature. By contrast, Sahu et al. [17] showed an ionic conductivity of 1.30 103 S cm1 for the PVA/PSSA blend membrane in fully humidified condition at 30 C. Moreover, they also showed that the ionic conductivity of a pristine PVA membrane was only 1.0 105 S cm1. According to the above results, it is seen clearly that the ionic conductivity of the PVA/MMT/PSSA membrane decreases when the content of the modified MMT fillers is higher than 20wt.%. Furthermore, Huang et al. [23] also studied the proton-conducting membrane based on PVA and poly (vinyl pyrrolidone) (PVP) with SSA. Their experimental results showed the proton conductivity of the order of 103 S cm1 for the PVA/PVP/SSA composite membranes. According to the above results, it was observed that the ionic conductivity of the PVA/xwt.%MMT/10wt.%PSSA composite electrolytes in water was of the order of 103 S cm1 at ambient temperature. The temperature dependence of the ionic conducEa Þ, where so is a pretivity is of the Arrhenius type: s ¼ so expðRT exponential factor, Ea is the activation energy, and T is the temperature in Kelvins. From the log10 (s) vs. 1/T plots, the activation energy (Ea) can be obtained for the PVA/20wt.%MMT/10wt.% PSSA composite electrolytes, which is highly dependent on the loading of modified MMT fillers [11,12]. The Ea values of the PVA/ 10wt.%PSSA/xwt.%MMT composite membrane are approximately 8e12 kJ mol1, which are lower than the general Nafion 117 membrane having Ea value of 15e20 kJ mol1. The proton transport follows two mechanisms: one is the Grotthus mechanism, which can be explained as a proton jump from one solvent molecule to the next through hydrogen bonds; the other is the vehicle mechanism, in which the proton diffuses together with solvent molecules by forming a complex (i.e., H3Oþ) and subsequently diffusing intact. Both the Grotthus and vehicle mechanisms may be responsible for the composite polymer membrane proton transfer. 3.5. Electrochemical performance measurements Fig. 8 shows the potentialecurrent density (IeV) and the power density-current density curves for the PEMFC with H2 fuel and O2 oxidant at 25 and 50 C, respectively. At 25 C, the peak power density of 65.23 mW cm2 was achieved for the PEMFC using PVA/20wt.% MMT/10wt.%PSSA composite membrane at Ep,max ¼ 0.410 V with a peak current density (ip,max) of 159.1 mA cm2, as displayed clearly in Table 5. At 50 C, the peak power density of 90.70 mW cm2 was achieved for the PEMFC with PVA/20wt.%MMT/10wt.%PSSA composite membrane at Ep,max ¼ 0.41 0 V with a peak current density (ip,max) of 221.23 mA cm2. Only few studies based on the PVA-based composite polymer membrane for the PEMFC could be found in the literature. By comparison, the peak power density of 210 mW cm2 for the PEMFC with gaseous H2 and O2 feed at 75 C (w100%RH) and atmospheric pressure is achieved with the PVA/PSSA (25wt.%)
25 C
50 C
0.930 90.70 221.23 0.410
0.941 65.23 159.10 0.410
Parameters Eocp/V Peak P.D./mW cm2 ip,max/mA cm2 Ep,max/V
blend membrane [17]. However, the peak power density of 38 mW cm2 for the PEMFC is obtained with pure PVA membrane under identical operating condition. By comparison, the difference performance of the peak power density in PEMFC is due to different compositions of the proton-conducting composite membrane and the operating conditions. The performance characteristics of the PEMFC with different PVA/PSSA composite membranes are summarized in detail and shown in Table 5. The above results clearly manifest that the PEMFC comprised of the PVA/MMT/PSSA composite membrane showed comparable electrochemical performance under ambient conditions. The merit is clear that the PVA/MMT/PSSA composite membrane is a cheap non-perfluorosulfonated polymer membrane, as compared with the perfluorosulfonated Nafion 117 membrane; which is an expensive polymer membrane. 4. Conclusions The proton-conducting composite membranes based on the PVA, PSSA polymers, and the modified MMT fillers were prepared by a solution casting method. The ionic conductivities of the composite membranes were of the order of 103 S cm1 in water at ambient temperature. Before the addition of MMT fillers, the MMT fillers were pre-blended with the PSSA polymer in a weight ratio of 1:1. These modified MMT fillers were well dispersed into the mixture of PVA and PSSA polymer matrix. The modified MMT filler containing PSSA polymer was used as a second ionic source, which is capable of enhancing the ionic conductivity, thermal properties, and dimensional stability on the PVA/MMT/PSSA composite membrane. The polymer electrolyte membrane fuel cell (PEMFC) comprised of the acidic PVA/MMT/PSSA composite membrane was assembled and examined. The performance of the PEMFC using H2 and O2 feeds was examined at ambient temperature and atmospheric pressure. Moreover, it was found that the peak power densities at ambient pressure are 65.23 and 90.70 mW cm2 at 25 and 50 C, respectively. In conclusion, the PVA/MMT/PSSA composite membranes are a good candidate for the future PEMFC applications. Acknowledgements Financial support from the National Science Council, Taiwan (Project No: NSC-96-2221-E131-009-MY2) is gratefully acknowledged. References [1] S. Sambandam, V. Ramani, J. Power Sources 170 (2007) 259e267. [2] E.P. Ambrosio, C. Francia, M. Manzoli, N. Penazzi, Int. J. Hydrogen. Energy. 33 (2008) 3142e3145. [3] A.K. Sahu, K.G. Nishanth, G. Selvarani, P. Sridhar, S. Pitchumani, A.K. Shukla, Carbon 47 (2009) 102e108. [4] M. Watanabe, H. Uchida, Y. Seki, M. Emori, Proceeding of the Electrochemical Society Meeting PV94-2, Abstract No. 606. Electrochemical Society, Pennington, NJ, 1994, pp. 946e947. [5] H.L. Tang, M. Pan, J. Phys. Chem. C 112 (2008) 11556e11568.
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