- Email: [email protected]
Mesoporous silica: Polymer composite membrane for direct methanol fuel cell
3036
Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.
M E S O P O R O U S SILICA: P O L Y M E R C O M P O S I T E M E M B R A N E FOR DIRECT M E T H A N O L FUEL CELL Kim, H.J., Lim, J.E., Shul, Y.G. and Han, H. Department of Chemical Engineering, Yonsei University, Seoul, 120-749 Korea. E-mail: [email protected]
ABSTRACT The mesoporous s i l i c a - poly (vinyl alcohol) hybrid polymer electrolyte membranes were successfully synthesized with 140ktm thickness. The membrane has multiple layered structures, in which the both the top and bottom layers were mainly PVA and middle layer was composed of mesoporous silica hydrogel with PVA. The mesoporous s i l i c a - PVA hybrid membranes possessed well-resolved hexagonal mesoporous channel. We found that as increasing the acidity ofheteropolyacids, the proton conductivity of the membrane was also increased. The proton conductivities of the hybrid membranes with PW, PMo and SiW at high temperature (70~ -~) were higher than that of Nation | membrane. Applications of the mesoporous silica PVA composite membranes to DMFC seems to be promising at high temperature (>70~
INTRODUCTION The fuel cell technology has been considered as a promising alternative for future energy needs and cleaner environment. The fuel cell is the electric chemical reaction device which converts the chemical energy of the fuel to direct electrical energy. Among the several kinds of fuel cells, direct methanol fuel cells (DMFCs) is one of the most attractive power sources for a variety of wide applications from vehicles to portable electrical equipment[l]. Presently, Nation | membrane, a perfluorosulfonate ionomer, is the predominanting membrane used in polymer electrolyte membrane fuel cells. However, Nation| membrane have some drawbacks limiting the practical realization of fuel cells, such as high cost, poor thermal stability, and methanol crossover[2]. Therefore, it is beneficial to find alternative proton conductive membranes. Recently, attention for new polymer electrolyte has been focused on the preparation of organic-inorganic composite membranes through sol-gel process. Sol-gel hybrid organic - inorganic materials have gained considerable interest in the view of new composite compounds with unique properties. The material properties of thermal, electrical conductivities, mechanical strength, flexibility, optical density and corrosion toughness can be widely controlled by adjusting compositions, nanophase, and chemical bonds between organics and inorganics[3]. Many researchers have already reported o r g a n i c - inorganic composite membranes such as PEO (polyethylene o x i d e ) - SIO213,4], PEG (polyethylene g l y c o l ) - SIO212] and PPG (polypropylene glycol)- SIO215] and PTMO (polytetraethylene oxide)- SIO214]. These membranes showed good proton conductivity and was found to be flexible as well as thermally stable up to 150~ However, the conductivities of all hybrid membranes were lower than commercial Nation | membrane. Furthermore, the cell performance of organic- inorganic membrane was too low to compete with Nation | membrane[4]. For the practical applications of o r g a n i c - inorganic hybrid membranes, the cell performances should be improved by increasing the conductivity and modifying the interfaces between electrode and membrane. As mentioned above, SiO2 was generally used for the inorganic matrix to improve the mechanical and thermal stability of polymer electrolyte membrane. Ozin et al. indicated that ordered mesoporous silica might be beneficial for their use as possible solid-state electrolyte alternatives[6]. Mesoporous inorganic materials have a uniform pore size range of 2-50 nm. They include high specific surface area, open channels of large dimension, and a huge structural variability. The channel structure can give some routes for the transport of proton. However, applications of mesoporous silica for polymer electrolyte are quite limited because of low ionic conductivities and poor flexibility. If the mesoporous silica zeolite should be homogeneously mixed with polymer matrix, the membrane could overcome these drawbacks. However, it has never tried to make mesoporous silica- polymer composite membrane for fuel cells. For the mixing of mesoporous silica with polymer, polymer should be soluble in water and alcohols which are reagents for synthesis of mesoporous structured silica. On the other hand, polymer electrolyte should be insoluble in water and/or alcohol because the fuel in PEMFC or DMFC contains water and/or
3037 alcohol. To overcome this contradiction, poly (vinyl alcohol) could be a good candidate. PVA is very soluble in water and could be easily crosslinked. Crosslinked PVA is water-insoluble and maintains its hydrophilicity. Therefore, PVA could be appropriate polymer matrix in the hybrid system of mesoporous silica- polymer. In many studies of organic - inorganic composite polymer electrolyte, the proton conductivity could be promoted by heteropolyacid. Heteropolyacids are one of the best proton conductive materials among the inorganic solid electrolytes[7]. The heteropolyacids have three protons in the unit structure, which dissociates in the humid conditions. They have been already used in PEMFCs or DMFCs with polymer matrix[8]. However, the heteropolyacid electrolyte dissolved in the water fuelled with hydrogen or methanol[8,5]. To overcome the problems of electrolyte dissolution and the consequent short lifetime of the fuel cell, heteropolyacid should be incorporated in a host material such as silica. The organic - inorganic composite membranes doped with heteropolyacid showed high proton conductivity and thermal stability[3-5]. In this study, we have synthesized the flexible and homogeneous mesoporous silica- poly (vinyl alcohol) membranes. The membrane becomes proton conducting electrolyte by doping with various kinds of heteropolyacids immobilized on the mesoporous silica phase. The effect of heteropolyacids on the proton conductivity was explored. Finally, methanol crossover rate was measured for the application of the hybrid membranes to DMFC. EXPERIMENTAL
Membrane preparation Mesoporous s i l i c a - poly (vinyl alcohol) membrane have been prepared by the homogeneous mixing of mesoporous silica sol and poly (vinyl alcohol) solution. The protonic conductivity of the hybrid membrane was provided by incorporating the various kinds of heteropolyacids in the mesoporous silica phase. The heteropolyacids used in this study were phosphotungstic acid (H3PW12040, PW), silicotungstic acid (H4SiW12040, SiW), phosphomolibdic acid (H3PMo12040, PMo), silicomolibdic acid (H4SiMo12040, SiMo). Poly (vinyl alcohol) was crosslinked by sulphur succinic acid to prevent the dissolution of PVA in water. Mesoporous silica sol was prepared by the sol-gel process as reported in our previous studies[9,10]. To prepare the silica precursor, 9.71g of tetramethoxysilane (TMOS) was partially hydrolyzed by a substoichiometric amount of water of 2.25 lg under acid conditions for 2 hours at room temperature. The silica precursor was dropped into 20g of cetyltrimethyl-ammoniumchloride solution with a vigorous stirring. After the aging for 7 hours at room temperature, the mesoporous silica sol could be obtained. Four types of heteropolyacids were dissolved in de-ionized water to be 30% heteropolyacid solution respectively. The heteropolyacids solutions wet added to the mesoporous silica sol. The ratio of SiO2 / heteropolyacid was fixed to 10%. Poly vinyl alcohol (PVA) with a molecular weight of 80,000 was used as the polymer matrix. Aqeous 10% PVA solutions were prepared by dissolving dry PVA in de-ionized water and heating at 90~ for 6 hours. The PVA solutions were mixed with the sulfur succinic acid(SSA) as a crosslinking agent for 1 day at room temperature[11 ]. Figure 1 shows the postulated reaction mechanism between PVA and SSA[11 ]. The PVA/SSA solutions were added into the mesoporous silica sol with vigorous stirring for 1 hour. The mixture became homogeneous and viscous. Before the preparation of the mesoporous s i l i c a - PVA membranes, PVA/SSA solution was cast onto a glass plate. After drying the PVA/SSA membranes at 40~ for 1 hour, the given amounts of mesoporous silica s o l - PVA solution was cast onto the previously prepared PVA/SSA membranes and it was dried at 40~ Finally, PVA/SSA solutions were cast on the mesoporous silica- PVA layers. The composite film was dried in air at 40~ for 12 hours. For the crosslinking of PVA, the dried membranes were heated at 100~ for 4 hours.
Characterizations X-ray diffraction (XRD) patterns of mesoporous silica were obtained with a Cu Kot X-ray source using a Rigaku instrument at room temperature. Scanning electron microscopy (SEM) images of the membranes were taken show the surfaces and the cross-sections with JEOL instrument.
3038 The proton conductivity was measured by using AC impedance spectroscopy. The samples were equilibrated in 0.1M HC1 solution at room temperature overnight. The proton conductivity of the membrane was calculated by the following equation: L RS
(1)
S represents the effective membrane area of the cell, 1.5crux 1.5cm, which is the same size as platinum electrode, p, L and R denote the proton conductivity, membrane thickness, and the resistance of the membrane, respectively
OH .--.>
I OH
SO3H
0 0 I C-O I OH2 I HO3S--CH I c=o
OH
I
0
OH
OH
Figure 1. Postulated reaction mechanism of PVA and sulfur succinic acid[ 11].
Immobilization of heteropolyacid in the hybrid membrane The mesoporous s i l i c a - PVA membrane was immersed in water to confirm the immobilization of heteropolyacid in the hybrid membrane. The amounts of the extracted heteropolyacid from the membrane was measured by pH change of water with immersion time.
Methanol crossover measurement The membrane is embedded in a single cell and one side of the membrane is fed with a methanol 2M solution. The solution is injected continuously by a pump (l.8ml/min). The other side of the membrane is purified with a continuous flow of oxygen at a fixed (300sccm/min) flow rate. To avoid condensation of water, cathode outlet line was heated. The outlet flow rate was monitored by using bubble flow-meter. The rate of methanol crossover across the membrane is determined by gas chromatography equipped with FID.
Direct methanol fuel cell operation To prepare the electrode, a thin layer(diffusion layer) of active carbon fiber and electrocatalyst were spread on the teflonised carbon cloth support(E-Tek). The anode catalyst was platinum - ruthenium(Pt : Ru: C = 2 : l : 2) and the cathode catalyst was platinum( Pt : C = 3 : 2). The catalysts were loaded by 4.0mg/cm 2 Membrane electrode assembly (MEA) was fabricated by a hot pressing method for 10min at 125~ 3 Metric ton (2000psi). The performance tests were carried out using a small-scale laboratory DMFC with an external electrode area of lcm 2. The cell was electrically heated to enable isothermal operation at a fixed temperature. The anode feed was a methanol/water solution under ambient pressure, preheated to the desired operating temperature of the cell. The cathode feed was oxygen at 2kgf/cm 2. RESULTS AND DISCUSSIONS The mesoporous s i l i c a - PVA hybrid membranes with various kinds of heteropolyacids were successfully prepared with about 140pm thickness. After the preparation of mesoporous silica, XRD patterns were taken to confirm the ordering of mesostructure. Figure 2 shows the XRD patterns of mesoporous silica - PVA hybrid membranes. All patterns are similar and exhibit a typical low angle diffraction associated with the nature of mesoporous silica[12]. Analogous diffraction patterns have been observed for hexagonal mesoporous structures electrostatic or neutral templating pathways[12]. The peak centred at 20 = 1.46 ~
3039 which are indexed to the 110 reflections based on the the hexagonal structure system. Unit cell constant was calculated according to the formula ao = 2dl00/root(3), where dlo0 is the interplanar spacing corresponding to the Bragg reflection. As a dl00 value is 5.7 nm, a0 is 5.53 nm. This result indicated that the mesoporous silica - PVA hybrid membranes possessed well-resolved hexagonal mesoporous channel with uniform sized pore structure. It is noteworthy that addition of PVA made the membrane flexible without the modification of mesoporous structure.
:3500
:3OOO :::)
2see Itl
1000
. . . . . . . . . 2
-"
6
~1
10
2 theta
Figure 2. X-ray diffraction patterns of mesoporous silica - PVA membranes with phosphotungstic acid (a) and silicotungstic acid (b). Figure 3 and 4 shows the SEM images of mesoporous s i l i c a - PVA membrane containing the phosphotungstic acid. Figure 3 shows the dense and smooth surface of m e s o p o r o u s - PVA hybrid membrane. The fractured surface of the membrane in figure 4 indicates that the membrane possessed three layers with 140 gm. The top and bottom layers were mainly composed of PVA and middle layer consisted of mesoporous silica and PVA. The inorganic hybrid membranes have been homogeneous structures physically or chemically as reported in some literatures[3-5]. In the case of mesoporous silica- PVA hybrid membrane prepared in this study, PVA layers on the top and bottom of membrane enable us to make flexible hybrid membrane. The mechanical strength and thermal stability of membrane could be also improved by the mesoporous silica layer in the middle. In addition, good contact with electrode in membrane/electrode assembly (MEA) is possible with PVA layers.
(a)
(b)
Figure 3. SEM images of surface of mesoporous silica - PVA membrane with phosphotungstic acid (a: top surface and b: bottom surface).
3040
(a)
(b)
(c)
Figure 4. SEM images of fractured surface of mesoporous silica - PVA membrane with phosphottmgstic aicd (a: whole of membrane, b: PVA side, c: mesoporous silica side).
Figure 5 shows the result of elution test of heteropolyacid from the mesoporous silica- PVA membrane containing the phosphotungstic acid. When the heteropolyacid was immobilized in the PVA membrane, the pH of water was drastically decreased with immersion time. The decrease of pH in figure 5 was mainly due to the dissolution of heteropolyacid from the PVA membrane. On the other hand, mesoporous silica- PVA membrane containing the phosphotungstic acid showed no change of pH with time. It indicates that heteropolyacid could be immobilized in the mesoporous silica matrix of the hybrid membrane.
kAAAAA 3 o -r r
,4
Immersedtime (hour) Figure 5. pH change of water containing the mesoporous s i l i c a - P V A membrane and PVA membrane.
9 9Mo o
9
.... ....-~
...... .
~W
Nailo n
---._ . . . .
o
|
2.6
2.7
|
2.8
|
2.9
|
3.0
T
|
|
3.1
3.2
3.3
10EKI/T(K'I) Figure 6. Proton conductivity of mesoporous s i l i c a - PVA hybrid membranes with phosphotungstic acid(PW),
silicotungstic acid(SiW) and silicomolibdic acid(SiMo).
3041 Figure 6 shows the proton conductivity of mesoporous s i l i c a - PVA membranes with various kinds of heteropolyacid. At the room temperature, the proton conductivity of the hybrid membrane is about 3.7 x 10.2 S/cm for the membrane containing PW and 3.1 x 10-2 S/cm for the membrane containing PMo. These values are much higher than those reported[3-5]. The organic - inorganic hybrid membranes showed the conductivity level of 10.3 -~ 10.2 S/cm. Honma et al. reported that the highest conductivity of PTMO / SiO2 membrane was below 10.2 S/cm. In the hydrated PVA membrane, its proton conductivity can only reach 3.65 x l0 2 S/cm[13]. When the heteropolyacids were incorporated in mesoporous s i l i c a - PVA membranes, it could act as the proton carrier and enhance the proton conductivity of hybrid membranes. Heteropolyacids are complex proton acids that incorporate polyoxometalate anions (heteropoly anions) having metal-oxygen octahedral as the basic structural units. The heteropoly anions have various types of outer oxygen atoms as potential protonation centres. They make the heteropolyacids strong Br6nsted acids. The acid strength of crystalline heteropolyacids decreases in the series PW > SiW > PMo > SiMo which is identical to that in solutions[14]. The order of acid strength is quite same with that of proton conductivity of mesoporous s i l i c a - PAV membranes in figure 6. As increasing the acidity of heteropolyacids, the proton conductivity of the membrane was also increased. This result suggests that the acidity of solid heteropolacid affected on the conduction of proton. Namely, the mobility of proton could be enhanced by the increasing acid strength of proton carrier. The proton conducting mechanism generally involves an activation barrier and thus the relationship between the ionic conductivity and temperature can be expressed by the Arrhenius law[ 15]. cr = A exp (- EA / RT )
(2)
where or, A, EA, R, and T denote the proton conductivity, frequency factor, activation energy for conduction, gas constant and temperature, respectively. The proton conductivity of mesoporous silica- PVA hybrid membranes were well correlated with the temperature. As increasing the temperature, the proton conductivity of the mesoporous - PVA membrane was enhanced. Strikingly, the proton conductivities of the hybrid membranes with PW, PMo and SiW at high temperature (70~ ---) were higher than that of Nation | membrane. The slope of proton conductivity in the figure 6 was also higher than that of Nation | membrane. It means that the activation energy for proton conduction in hybrid membrane is lower than that of Nation | membrane. While the Nation | membrane has activation energy of 9.34 kJ/mol, mesoporous silica - PVA hybrid membranes with PW have 0.91 kJ/mol. Although the proton conductivities of hybrid membranes are varied with the types of heteropolyacids, the membranes showed similar activation energy. The differences in activation energy might be due to the difference in the mechanism of the conduction of proton. The mesoporous structure could give an easy route for transportation of proton by heteropolyacid in the membrane. In addition, the hydrophilic properties of PVA and the crosslinking by sulfonic acid group should also contribute to the high proton conductivity of hybrid membranes. _.--------0 9{:
E
7O
t,,i
Ir
~50
9- t - -
Nation n ' B r t r a n e M e s o p o r o u s silica - PVA m e r t ) m n e
--~ 20
30
i
i
i
i
i
40
50
60
70
80
Terr~rature
90
(~
Figure 7. Methanol crossover rate of Nation | and mesoporous silica - PVA with PW hybrid membranes.
3042 Figure 7 illustrates the relationship between the methanol crossover rates of the mesoporous silica- PVA and Nation | membranes. The methanol crossover rate is the crucial factor in direct methanol fuel cell. In general, the transport of methanol and water through conducting polymer membrane is a complex process. At a whole experimental temperature, the methanol crossover of the mesoporous silica - PVA membrane was quite lower than that of Nation | membrane. The mesoporous silica matrix in the middle layer of the hybrid membrane could efficiently block the methanol crossover. The mesoporous s i l i c a - PVA hybrid membranes were applied in a direct methanol fuel cell. Figure 8 shows the I-V characteristics of the single cell using the hybrid membrane. The performance of the single cell was measured at 80~ by using the methanol/oxygen fuel cell. The power output was approximately equal to 9 mA/cm 2 at 0.2V for the mesoporous silica- PVA hybrid membrane containing the PW. Although the hybrid membrane has high proton conductivity, the DMFC performance is quite low. OreI et al. also reported 2.5 mA/cm 2 at 0.4V for the PPG - SiO2 membranes[5]. The low performances of hybrid membranes should be due to the lack of optimization of electrodes/membrane interfaces. The interfacial resistance between the membrane and electrode in hybrid membrane must be higher than that in Nation | membrane. The output would be increased by improving the interfaces of MEA. 500
400
~" 300
g g :~
200
100
0
i
i
i
,
i
I
i
2
4
6
8
10
12
14
16
Current Density ( m N c m 2)
Figure 8. Cell voltage-current density relation of the single cell.
CONCLUSION The heteropolyacid incorporated mesoporous s i l i c a - PVA hybrid polymer electrolyte membranes were successfully prepared. The mesoporous silica- PVA hybrid membranes possessed well-resolved hexagonal mesoporous channel with uniform sized pore structure. The proton conductivities of the hybrid membranes incorporated with PW, PMo and SiW at high temperature (70~ ---) were higher than that of Nation | membrane. The activation energy for proton conduction in hybrid membrane is lower than that of Nation | membrane. Application of the m e s o p o r o u s - PVA hybrid membrane to DMFC or PEMFC seems to be feasible by further improving the MEA interface. ACKNOWLEDGEMENTS This work was supported by Ministry of Science and Technology of Korea through the National Research Laboratory Program. And, authors also thank Center for Ultramicrochemical Process Systems(CUPS) for their collaboration in this work.
3043 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Chu, Y.H., Shul, Y.G., Choi, W.C., Woo, S.I., Han, H.S., J. Power Sources, 118 (2003), 334-341. Chang, H.Y., Lin, C.W., J. Membr. Sci., 218 (2003), 295-306. Honma, I., Takeda, Y., Bae, J.M., Solid State Ionics, 120 (1999), 255-264. Honma, I., Nakajima, H., Nomura, S., Solid State lonics, 154-155 (2002), 707-712. Stangar, U.L., Groselj, N., Orel, B., Schmitz, A., Colomban, P., Solid State Ionics, 145 (2001), 109-118. Halla, J.D., Mamak, M., Williams, D.E., Ozin, G.A., Adv. Func. Mater., 13 (2003), 133-138. Kreuer, K.D., Chem. Mater., 49 (1994) 610-? Park, M.W., Yang, J.C., Han, H.S., Shul, Y.G., Denki Kagaku, 64 (1996) 743-748. Jung, K.T., Chu, Y.H., Haam, S.J., Shul, Y.G., J. Non-Cryst. Solids., 298 (2002), 193-201. Chu, Y.H., Kim, H.J., Song, K.Y., Shul, Y.G., Jung, K.T., Lee, K.T., Han, M.H., Catal. Today, 74 (2002), 249-256. Rhim, J.W., Yeom, C.K., Kim, S.W., J. Appl. Polym. Sci., 68 (1998), 1717-1723. Chen, C.Y., Li, H.Y., Burkett, S.L., Davis, M.E., Micropor. Mater., 2 (1993), 27-? Li, L., Xu, L., Wang, Y., Mater. Lett., 57 (2003) 1406-1410. Kozhevnikov, I.V., Chem. Rev., 98 (1998), 171-198. Costamagna, P., Yang, C., Bocarsly, A.B., Srinivasan, S., Electrochimica Acta 47 (2002) 1023-1033.
Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.
M E S O P O R O U S SILICA: P O L Y M E R C O M P O S I T E M E M B R A N E FOR DIRECT M E T H A N O L FUEL CELL Kim, H.J., Lim, J.E., Shul, Y.G. and Han, H. Department of Chemical Engineering, Yonsei University, Seoul, 120-749 Korea. E-mail: [email protected]
ABSTRACT The mesoporous s i l i c a - poly (vinyl alcohol) hybrid polymer electrolyte membranes were successfully synthesized with 140ktm thickness. The membrane has multiple layered structures, in which the both the top and bottom layers were mainly PVA and middle layer was composed of mesoporous silica hydrogel with PVA. The mesoporous s i l i c a - PVA hybrid membranes possessed well-resolved hexagonal mesoporous channel. We found that as increasing the acidity ofheteropolyacids, the proton conductivity of the membrane was also increased. The proton conductivities of the hybrid membranes with PW, PMo and SiW at high temperature (70~ -~) were higher than that of Nation | membrane. Applications of the mesoporous silica PVA composite membranes to DMFC seems to be promising at high temperature (>70~
INTRODUCTION The fuel cell technology has been considered as a promising alternative for future energy needs and cleaner environment. The fuel cell is the electric chemical reaction device which converts the chemical energy of the fuel to direct electrical energy. Among the several kinds of fuel cells, direct methanol fuel cells (DMFCs) is one of the most attractive power sources for a variety of wide applications from vehicles to portable electrical equipment[l]. Presently, Nation | membrane, a perfluorosulfonate ionomer, is the predominanting membrane used in polymer electrolyte membrane fuel cells. However, Nation| membrane have some drawbacks limiting the practical realization of fuel cells, such as high cost, poor thermal stability, and methanol crossover[2]. Therefore, it is beneficial to find alternative proton conductive membranes. Recently, attention for new polymer electrolyte has been focused on the preparation of organic-inorganic composite membranes through sol-gel process. Sol-gel hybrid organic - inorganic materials have gained considerable interest in the view of new composite compounds with unique properties. The material properties of thermal, electrical conductivities, mechanical strength, flexibility, optical density and corrosion toughness can be widely controlled by adjusting compositions, nanophase, and chemical bonds between organics and inorganics[3]. Many researchers have already reported o r g a n i c - inorganic composite membranes such as PEO (polyethylene o x i d e ) - SIO213,4], PEG (polyethylene g l y c o l ) - SIO212] and PPG (polypropylene glycol)- SIO215] and PTMO (polytetraethylene oxide)- SIO214]. These membranes showed good proton conductivity and was found to be flexible as well as thermally stable up to 150~ However, the conductivities of all hybrid membranes were lower than commercial Nation | membrane. Furthermore, the cell performance of organic- inorganic membrane was too low to compete with Nation | membrane[4]. For the practical applications of o r g a n i c - inorganic hybrid membranes, the cell performances should be improved by increasing the conductivity and modifying the interfaces between electrode and membrane. As mentioned above, SiO2 was generally used for the inorganic matrix to improve the mechanical and thermal stability of polymer electrolyte membrane. Ozin et al. indicated that ordered mesoporous silica might be beneficial for their use as possible solid-state electrolyte alternatives[6]. Mesoporous inorganic materials have a uniform pore size range of 2-50 nm. They include high specific surface area, open channels of large dimension, and a huge structural variability. The channel structure can give some routes for the transport of proton. However, applications of mesoporous silica for polymer electrolyte are quite limited because of low ionic conductivities and poor flexibility. If the mesoporous silica zeolite should be homogeneously mixed with polymer matrix, the membrane could overcome these drawbacks. However, it has never tried to make mesoporous silica- polymer composite membrane for fuel cells. For the mixing of mesoporous silica with polymer, polymer should be soluble in water and alcohols which are reagents for synthesis of mesoporous structured silica. On the other hand, polymer electrolyte should be insoluble in water and/or alcohol because the fuel in PEMFC or DMFC contains water and/or
3037 alcohol. To overcome this contradiction, poly (vinyl alcohol) could be a good candidate. PVA is very soluble in water and could be easily crosslinked. Crosslinked PVA is water-insoluble and maintains its hydrophilicity. Therefore, PVA could be appropriate polymer matrix in the hybrid system of mesoporous silica- polymer. In many studies of organic - inorganic composite polymer electrolyte, the proton conductivity could be promoted by heteropolyacid. Heteropolyacids are one of the best proton conductive materials among the inorganic solid electrolytes[7]. The heteropolyacids have three protons in the unit structure, which dissociates in the humid conditions. They have been already used in PEMFCs or DMFCs with polymer matrix[8]. However, the heteropolyacid electrolyte dissolved in the water fuelled with hydrogen or methanol[8,5]. To overcome the problems of electrolyte dissolution and the consequent short lifetime of the fuel cell, heteropolyacid should be incorporated in a host material such as silica. The organic - inorganic composite membranes doped with heteropolyacid showed high proton conductivity and thermal stability[3-5]. In this study, we have synthesized the flexible and homogeneous mesoporous silica- poly (vinyl alcohol) membranes. The membrane becomes proton conducting electrolyte by doping with various kinds of heteropolyacids immobilized on the mesoporous silica phase. The effect of heteropolyacids on the proton conductivity was explored. Finally, methanol crossover rate was measured for the application of the hybrid membranes to DMFC. EXPERIMENTAL
Membrane preparation Mesoporous s i l i c a - poly (vinyl alcohol) membrane have been prepared by the homogeneous mixing of mesoporous silica sol and poly (vinyl alcohol) solution. The protonic conductivity of the hybrid membrane was provided by incorporating the various kinds of heteropolyacids in the mesoporous silica phase. The heteropolyacids used in this study were phosphotungstic acid (H3PW12040, PW), silicotungstic acid (H4SiW12040, SiW), phosphomolibdic acid (H3PMo12040, PMo), silicomolibdic acid (H4SiMo12040, SiMo). Poly (vinyl alcohol) was crosslinked by sulphur succinic acid to prevent the dissolution of PVA in water. Mesoporous silica sol was prepared by the sol-gel process as reported in our previous studies[9,10]. To prepare the silica precursor, 9.71g of tetramethoxysilane (TMOS) was partially hydrolyzed by a substoichiometric amount of water of 2.25 lg under acid conditions for 2 hours at room temperature. The silica precursor was dropped into 20g of cetyltrimethyl-ammoniumchloride solution with a vigorous stirring. After the aging for 7 hours at room temperature, the mesoporous silica sol could be obtained. Four types of heteropolyacids were dissolved in de-ionized water to be 30% heteropolyacid solution respectively. The heteropolyacids solutions wet added to the mesoporous silica sol. The ratio of SiO2 / heteropolyacid was fixed to 10%. Poly vinyl alcohol (PVA) with a molecular weight of 80,000 was used as the polymer matrix. Aqeous 10% PVA solutions were prepared by dissolving dry PVA in de-ionized water and heating at 90~ for 6 hours. The PVA solutions were mixed with the sulfur succinic acid(SSA) as a crosslinking agent for 1 day at room temperature[11 ]. Figure 1 shows the postulated reaction mechanism between PVA and SSA[11 ]. The PVA/SSA solutions were added into the mesoporous silica sol with vigorous stirring for 1 hour. The mixture became homogeneous and viscous. Before the preparation of the mesoporous s i l i c a - PVA membranes, PVA/SSA solution was cast onto a glass plate. After drying the PVA/SSA membranes at 40~ for 1 hour, the given amounts of mesoporous silica s o l - PVA solution was cast onto the previously prepared PVA/SSA membranes and it was dried at 40~ Finally, PVA/SSA solutions were cast on the mesoporous silica- PVA layers. The composite film was dried in air at 40~ for 12 hours. For the crosslinking of PVA, the dried membranes were heated at 100~ for 4 hours.
Characterizations X-ray diffraction (XRD) patterns of mesoporous silica were obtained with a Cu Kot X-ray source using a Rigaku instrument at room temperature. Scanning electron microscopy (SEM) images of the membranes were taken show the surfaces and the cross-sections with JEOL instrument.
3038 The proton conductivity was measured by using AC impedance spectroscopy. The samples were equilibrated in 0.1M HC1 solution at room temperature overnight. The proton conductivity of the membrane was calculated by the following equation: L RS
(1)
S represents the effective membrane area of the cell, 1.5crux 1.5cm, which is the same size as platinum electrode, p, L and R denote the proton conductivity, membrane thickness, and the resistance of the membrane, respectively
OH .--.>
I OH
SO3H
0 0 I C-O I OH2 I HO3S--CH I c=o
OH
I
0
OH
OH
Figure 1. Postulated reaction mechanism of PVA and sulfur succinic acid[ 11].
Immobilization of heteropolyacid in the hybrid membrane The mesoporous s i l i c a - PVA membrane was immersed in water to confirm the immobilization of heteropolyacid in the hybrid membrane. The amounts of the extracted heteropolyacid from the membrane was measured by pH change of water with immersion time.
Methanol crossover measurement The membrane is embedded in a single cell and one side of the membrane is fed with a methanol 2M solution. The solution is injected continuously by a pump (l.8ml/min). The other side of the membrane is purified with a continuous flow of oxygen at a fixed (300sccm/min) flow rate. To avoid condensation of water, cathode outlet line was heated. The outlet flow rate was monitored by using bubble flow-meter. The rate of methanol crossover across the membrane is determined by gas chromatography equipped with FID.
Direct methanol fuel cell operation To prepare the electrode, a thin layer(diffusion layer) of active carbon fiber and electrocatalyst were spread on the teflonised carbon cloth support(E-Tek). The anode catalyst was platinum - ruthenium(Pt : Ru: C = 2 : l : 2) and the cathode catalyst was platinum( Pt : C = 3 : 2). The catalysts were loaded by 4.0mg/cm 2 Membrane electrode assembly (MEA) was fabricated by a hot pressing method for 10min at 125~ 3 Metric ton (2000psi). The performance tests were carried out using a small-scale laboratory DMFC with an external electrode area of lcm 2. The cell was electrically heated to enable isothermal operation at a fixed temperature. The anode feed was a methanol/water solution under ambient pressure, preheated to the desired operating temperature of the cell. The cathode feed was oxygen at 2kgf/cm 2. RESULTS AND DISCUSSIONS The mesoporous s i l i c a - PVA hybrid membranes with various kinds of heteropolyacids were successfully prepared with about 140pm thickness. After the preparation of mesoporous silica, XRD patterns were taken to confirm the ordering of mesostructure. Figure 2 shows the XRD patterns of mesoporous silica - PVA hybrid membranes. All patterns are similar and exhibit a typical low angle diffraction associated with the nature of mesoporous silica[12]. Analogous diffraction patterns have been observed for hexagonal mesoporous structures electrostatic or neutral templating pathways[12]. The peak centred at 20 = 1.46 ~
3039 which are indexed to the 110 reflections based on the the hexagonal structure system. Unit cell constant was calculated according to the formula ao = 2dl00/root(3), where dlo0 is the interplanar spacing corresponding to the Bragg reflection. As a dl00 value is 5.7 nm, a0 is 5.53 nm. This result indicated that the mesoporous silica - PVA hybrid membranes possessed well-resolved hexagonal mesoporous channel with uniform sized pore structure. It is noteworthy that addition of PVA made the membrane flexible without the modification of mesoporous structure.
:3500
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2see Itl
1000
. . . . . . . . . 2
-"
6
~1
10
2 theta
Figure 2. X-ray diffraction patterns of mesoporous silica - PVA membranes with phosphotungstic acid (a) and silicotungstic acid (b). Figure 3 and 4 shows the SEM images of mesoporous s i l i c a - PVA membrane containing the phosphotungstic acid. Figure 3 shows the dense and smooth surface of m e s o p o r o u s - PVA hybrid membrane. The fractured surface of the membrane in figure 4 indicates that the membrane possessed three layers with 140 gm. The top and bottom layers were mainly composed of PVA and middle layer consisted of mesoporous silica and PVA. The inorganic hybrid membranes have been homogeneous structures physically or chemically as reported in some literatures[3-5]. In the case of mesoporous silica- PVA hybrid membrane prepared in this study, PVA layers on the top and bottom of membrane enable us to make flexible hybrid membrane. The mechanical strength and thermal stability of membrane could be also improved by the mesoporous silica layer in the middle. In addition, good contact with electrode in membrane/electrode assembly (MEA) is possible with PVA layers.
(a)
(b)
Figure 3. SEM images of surface of mesoporous silica - PVA membrane with phosphotungstic acid (a: top surface and b: bottom surface).
3040
(a)
(b)
(c)
Figure 4. SEM images of fractured surface of mesoporous silica - PVA membrane with phosphottmgstic aicd (a: whole of membrane, b: PVA side, c: mesoporous silica side).
Figure 5 shows the result of elution test of heteropolyacid from the mesoporous silica- PVA membrane containing the phosphotungstic acid. When the heteropolyacid was immobilized in the PVA membrane, the pH of water was drastically decreased with immersion time. The decrease of pH in figure 5 was mainly due to the dissolution of heteropolyacid from the PVA membrane. On the other hand, mesoporous silica- PVA membrane containing the phosphotungstic acid showed no change of pH with time. It indicates that heteropolyacid could be immobilized in the mesoporous silica matrix of the hybrid membrane.
kAAAAA 3 o -r r
,4
Immersedtime (hour) Figure 5. pH change of water containing the mesoporous s i l i c a - P V A membrane and PVA membrane.
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9
.... ....-~
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o
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2.6
2.7
|
2.8
|
2.9
|
3.0
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|
|
3.1
3.2
3.3
10EKI/T(K'I) Figure 6. Proton conductivity of mesoporous s i l i c a - PVA hybrid membranes with phosphotungstic acid(PW),
silicotungstic acid(SiW) and silicomolibdic acid(SiMo).
3041 Figure 6 shows the proton conductivity of mesoporous s i l i c a - PVA membranes with various kinds of heteropolyacid. At the room temperature, the proton conductivity of the hybrid membrane is about 3.7 x 10.2 S/cm for the membrane containing PW and 3.1 x 10-2 S/cm for the membrane containing PMo. These values are much higher than those reported[3-5]. The organic - inorganic hybrid membranes showed the conductivity level of 10.3 -~ 10.2 S/cm. Honma et al. reported that the highest conductivity of PTMO / SiO2 membrane was below 10.2 S/cm. In the hydrated PVA membrane, its proton conductivity can only reach 3.65 x l0 2 S/cm[13]. When the heteropolyacids were incorporated in mesoporous s i l i c a - PVA membranes, it could act as the proton carrier and enhance the proton conductivity of hybrid membranes. Heteropolyacids are complex proton acids that incorporate polyoxometalate anions (heteropoly anions) having metal-oxygen octahedral as the basic structural units. The heteropoly anions have various types of outer oxygen atoms as potential protonation centres. They make the heteropolyacids strong Br6nsted acids. The acid strength of crystalline heteropolyacids decreases in the series PW > SiW > PMo > SiMo which is identical to that in solutions[14]. The order of acid strength is quite same with that of proton conductivity of mesoporous s i l i c a - PAV membranes in figure 6. As increasing the acidity of heteropolyacids, the proton conductivity of the membrane was also increased. This result suggests that the acidity of solid heteropolacid affected on the conduction of proton. Namely, the mobility of proton could be enhanced by the increasing acid strength of proton carrier. The proton conducting mechanism generally involves an activation barrier and thus the relationship between the ionic conductivity and temperature can be expressed by the Arrhenius law[ 15]. cr = A exp (- EA / RT )
(2)
where or, A, EA, R, and T denote the proton conductivity, frequency factor, activation energy for conduction, gas constant and temperature, respectively. The proton conductivity of mesoporous silica- PVA hybrid membranes were well correlated with the temperature. As increasing the temperature, the proton conductivity of the mesoporous - PVA membrane was enhanced. Strikingly, the proton conductivities of the hybrid membranes with PW, PMo and SiW at high temperature (70~ ---) were higher than that of Nation | membrane. The slope of proton conductivity in the figure 6 was also higher than that of Nation | membrane. It means that the activation energy for proton conduction in hybrid membrane is lower than that of Nation | membrane. While the Nation | membrane has activation energy of 9.34 kJ/mol, mesoporous silica - PVA hybrid membranes with PW have 0.91 kJ/mol. Although the proton conductivities of hybrid membranes are varied with the types of heteropolyacids, the membranes showed similar activation energy. The differences in activation energy might be due to the difference in the mechanism of the conduction of proton. The mesoporous structure could give an easy route for transportation of proton by heteropolyacid in the membrane. In addition, the hydrophilic properties of PVA and the crosslinking by sulfonic acid group should also contribute to the high proton conductivity of hybrid membranes. _.--------0 9{:
E
7O
t,,i
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~50
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Nation n ' B r t r a n e M e s o p o r o u s silica - PVA m e r t ) m n e
--~ 20
30
i
i
i
i
i
40
50
60
70
80
Terr~rature
90
(~
Figure 7. Methanol crossover rate of Nation | and mesoporous silica - PVA with PW hybrid membranes.
3042 Figure 7 illustrates the relationship between the methanol crossover rates of the mesoporous silica- PVA and Nation | membranes. The methanol crossover rate is the crucial factor in direct methanol fuel cell. In general, the transport of methanol and water through conducting polymer membrane is a complex process. At a whole experimental temperature, the methanol crossover of the mesoporous silica - PVA membrane was quite lower than that of Nation | membrane. The mesoporous silica matrix in the middle layer of the hybrid membrane could efficiently block the methanol crossover. The mesoporous s i l i c a - PVA hybrid membranes were applied in a direct methanol fuel cell. Figure 8 shows the I-V characteristics of the single cell using the hybrid membrane. The performance of the single cell was measured at 80~ by using the methanol/oxygen fuel cell. The power output was approximately equal to 9 mA/cm 2 at 0.2V for the mesoporous silica- PVA hybrid membrane containing the PW. Although the hybrid membrane has high proton conductivity, the DMFC performance is quite low. OreI et al. also reported 2.5 mA/cm 2 at 0.4V for the PPG - SiO2 membranes[5]. The low performances of hybrid membranes should be due to the lack of optimization of electrodes/membrane interfaces. The interfacial resistance between the membrane and electrode in hybrid membrane must be higher than that in Nation | membrane. The output would be increased by improving the interfaces of MEA. 500
400
~" 300
g g :~
200
100
0
i
i
i
,
i
I
i
2
4
6
8
10
12
14
16
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Figure 8. Cell voltage-current density relation of the single cell.
CONCLUSION The heteropolyacid incorporated mesoporous s i l i c a - PVA hybrid polymer electrolyte membranes were successfully prepared. The mesoporous silica- PVA hybrid membranes possessed well-resolved hexagonal mesoporous channel with uniform sized pore structure. The proton conductivities of the hybrid membranes incorporated with PW, PMo and SiW at high temperature (70~ ---) were higher than that of Nation | membrane. The activation energy for proton conduction in hybrid membrane is lower than that of Nation | membrane. Application of the m e s o p o r o u s - PVA hybrid membrane to DMFC or PEMFC seems to be feasible by further improving the MEA interface. ACKNOWLEDGEMENTS This work was supported by Ministry of Science and Technology of Korea through the National Research Laboratory Program. And, authors also thank Center for Ultramicrochemical Process Systems(CUPS) for their collaboration in this work.
3043 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
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