- Email: [email protected]
High temperature proton conducting hybrid polymer electrolyte membranes
Solid State Ionics 154 – 155 (2002) 707 – 712 www.elsevier.com/locate/ssi
High temperature proton conducting hybrid polymer electrolyte membranes Itaru Honma a,*, Hitoshi Nakajima a, Shigeki Nomura b a
Energy Electronic Institute, National Institute of Advanced Industrial Science and Technology, Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan b Tsukuba Research Laboratories, Sekisui Chemical Co., Ltd., Tsukuba, Ibaraki 300-4292, Japan Accepted 2 April 2002
Abstract Hybrid materials with nano-size interfaces between organic and inorganic offer exceptional opportunities to create entirely unique properties. Bridged polyethers are a family of hybrid organic/inorganic materials prepared by sol – gel processing of monomers that contain a variable polyether bridging group and functional silyl groups. The materials possess thermal as well as chemical tolerance at high temperatures due to the inorganic interfaces in the macromolecules. In the present paper, organic/ inorganic nanohybrid membrane has been prepared for proton conducting polymer electrolyte by incorporating heteropolyacids such as 12-phosphotungstic acid (PWA) clusters at the interfaces and high temperature conductivity has been examined. While the intermediate temperature polymer electrolyte membrane fuel cell (PEMFC) can be a candidate for overcoming major problems in the current PEMFC such as CO poisoning on the Pt electrodes surfaces and heat managements, new proton conducting polymer electrolytes with more temperature tolerance has been investigated on functional hybrids. The unique hybrid macromolecules with fast ionic conduction at higher temperature have been synthesized in the present work. D 2002 Published by Elsevier Science B.V. Keywords: Organic/inorganic nanohybrids; Bridged polyethers; Hybrid macromolecules; Sol – gel process; Protonic conducting membrane; Polymer electrolyte membrane; 12-Phosphotungstic acid (PWA)
1. Introduction Polymer electrolyte membrane fuel cells (PEMFC) are an extremely attractive energy conversion system for use in many industrial applications including electric vehicles (EV) and on-site power plant due to
*
Corresponding author. Tel.: +81-298-61-5648; fax: +81-29861-5829. E-mail address: [email protected] (I. Honma).
their inherently higher efficiency and lower emission when compared to that of internal combustion engines. There is an increasing interest in using hydrogen-rich gases produced by reforming methanol or even gasoline. Those gases contain traces of different condensable species such as CO, which drastically reduces the activity of platinum or platinum alloys at the anodes. As the CO tolerance increases with increasing temperature, higher temperature operated PEMFC has been desired to industrialize the technology for market, compatible to the reformable fossil oil
0167-2738/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 4 3 1 - 9
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delivery. Additionally, direct electrochemical methanol oxidation at anode as well as oxygen electroreduction at cathode can be significantly promoted with increasing temperatures. However, water and methanol crossover remarkably increases with temperature, which reduces the fuel efficiency and requires an expensive water management system. Proton conducting membranes such as Nafion have been designed for a low temperature operation of the PEMFC below 100 jC, which limits the fuel only within hydrogen gas contaminated by CO species down to 5– 10 ppm levels. Different approaches are based on the complexation of basic polymers, such as polybenzoimidazol with oxo-acids including phosphoric acid, or inorganic inclusion to the temperature tolerant polymers [1 – 23]. In particular, new approaches are developed by elaborating organic/inorganic macromolecules with nano-sized interfaces synthesized by sol – gel processing [9 – 14,19 – 25]. The materials’ thermal, chemical and electrochemical properties have been controlled by preparing hydrophilic/hydrophobic interfaces with highly conductive networks. In this work, a flexible, homogeneous and largesized hybrid membrane has been synthesized by crosslinking nano-sized silicate species to polyethers polymers (PEO: polyethylene oxides; PPO: polypropylene oxide; PTMO: polytetramethylene oxide). The membrane becomes proton conducting electrolyte by doping with heteropolyacids such as 12-phosphotungstic acid (PWA) supported on the condensed silicate species [26,27]. Hydrated PWA is known to
present a large proton conductivity at around 0.2 S/ cm. The protonic conductivities at intermediate temperatures up to 160 jC were examined for the hybrid electrolyte membrane. The effect of the polyether molecular structure, PWA doping concentration and binary blend hybrid polymers of PEO and PTMO have been studied for fast protonic conduction at high temperatures.
2. Experiment Organically modified alkoxysilane precursors developed in our previous studies [10 –14] have been used to form organic/inorganic macromolecules to become proton conducting nanohybrid membranes. Sol – gel processing provides versatile processing route to change hydrophobic/hydrophilic interfaces. Fig. 1 shows the molecular structure of organic/ inorganic hybrid precursors where polyethylene oxide (PEO) as well as polytetramethylene oxide (PTMO) have been crosslinked with alkoxysilanes through isocyanato coupling. The hybrid macromolecular structure can be varied through systematic change of the molecular length of PEO and PTMO. The hybrid precursors hydrolyze and condense to form macromolecules of flexible, transparent, glassy hybrid polymer materials. The membrane’s polymer structures are controlled by the sol –gel process conditions such as gelation temperatures or humidities. Initially, polytetramethyleneglycols with molecular weight of 250, 650, 1000
Fig. 1. A molecular structure of bridged organic/inorganic hybrid macromolecules.
I. Honma et al. / Solid State Ionics 154 – 155 (2002) 707–712
and 2000 were derivatized with isocyanatopropyltriethoxysilane to make bridged molecular precursors. A similar process has been taken for PEO hybrid polymers. This hybrid precursor was mixed with heteropolyacids of PWA and casted on the petri dish and aged for 24 h at 60 jC under saturated humidities. The structure of the hybrid membrane has been characterized by 1H-NMR to confirm the bridged molecular structure in Fig. 1 [28]. Fundamental vibrations of the macromolecular structure has been characterized by infrared spectra (IR). A nano-sized structure of the hybrid membrane has been characterized by a transmission electron micrograph (TEM) on a sliced specimen at a magnification of 200,000 with accelerating voltage of 120 kV (JEOL 1200EXII). Thermal stability was also studied by TG-DTA analysis and the difference of water evolution can be measured between PTMO and PEO polymers. Proton conductivity of organic/inorganic nanohybrid membrane was measured by a two terminal impedance spectroscopic method (Solartron 1260) over the frequency range of 1 Hz – 2 MHz. Cell temperatures, pressures and humidities were widely changed, for example, when the conductivity of the membrane was measured at higher temperatures, the apparatus cell was pressurized to maximum 6 atm (160 jC) in order to keep saturated water vapor pressures. A typical impedance response (cole – cole plots) of the membrane shows a feature similar to that of highly ionic conducting membrane such as Nafion.
3. Results and discussions SEM photograph shows the membrane was completely transparent, homogeneous, isotropic materials. A flat membrane of 8 cm diameter and a thickness range from 100 to 300 Am can be easily obtained, as shown in Fig. 2. The mechanical stability was very good for polyether hybrid materials in particular, for example, it can be rolled up by easily without breaking the membrane although the membrane contains a maximum of about 50% inorganic composition. Also, a TEM photograph of the specimen was studied and no discernible microstructure has been observed at the magnification scale, which indicates the hybrid materials is completely homogeneous at nano-scale. The hybrid materials are considered to
709
Fig. 2. The SEM photograph of the organic/inorganic hybrid membrane.
form nano-phase separated structure between hydrophobic and hydrophilic domains. Various polyethers such as polyethyleneoxide PEO, polypropyleneoxide (PPO) and polytetramethyleneoxide (PTMO) have been used for hybrid materials. Also, binary blend hybrid polymers have been synthesized between PEO and PTMO polymers. The thermal analysis was measured for PEO and PTMO hybrid membrane and the difference of water evolution properties were studied. It is known that PEO is a hydrophilic polymer and PTMO is a hydrophobic polymer, so that the interaction between the polymers and water might be different between the two. Fig. 3(a) shows the TG-DTA analysis of hybrid membrane of PEO and PTMO in a temperature range from RT to 280 jC with a heating rate of 10 jC/min. The molecular weight of the polymer has been changed for PEO (200 to 600) and PTMO (250 to 1000), respectively. As the organic as well as inorganic compositions in the hybrid are not volatile materials, the large weight decrease is an evolution of water from the membrane. As a common tendency to the both polymers, the water content in the membrane is proportional to the molecular weight of the polymers, i.e., the organic chain length in the hybrid macromolecules. Also, it is very interesting that the water evolution temperature at the TG weight loss position is about 50 to 80 jC for PEO and 100 to 150 jC for PTMO although the hydrophilic PEO hybrid polymer incorporate larger amount of water than PTMO
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Fig. 3. (a) TG-DTA data for hybrid polymer membranes with PEO (Mw = 200, 400, 600) and PTMO (Mw = 250, 650, 1000), respectively. (b) Water concentration in PEO and PTMO hybrid membrane determined by TG-DTA analysis as a function of polyether polymer’s molecular weight.
hybrid. It is comprehensible that the PEO has hydrophilic interaction with water, so that it contains more water inside its macromolecular matrix. However, the hydrophobic PTMO does not seem to strongly interact with water. The TG-DTA data indicates the presence of water inside hybrid macromolecules might be quite different between PEO and PTMO. Fig. 3(b) shows a dependence of the water content as a function of polymer molecular weight for PEO and PTMO, respectively. In both cases, the water content increases with the polymer weight almost linearly, while, the water weight concentration in the PEO hybrid is almost twice larger than PTMO polymers. The data indicate that the water content is strongly correlated with a polymer chain length; in other words, water domain size in the hybrid macromolecules is primarily
determined by the polymer’s unit length for both materials. If assumed that there is a conductive channel structure of hybrid macromolecules assembled along acidic PWA clusters, the volume fraction of water can be controlled by the polymer chain length. Additionally, a shift of conductivity by doping concentration of PWA has been reported elsewhere [10 –14,28,29]. Fig. 4 shows a proton conductivity of the hybrid membranes of PEO, PTMO and PEO/PTMO binary hybrids in the temperature range 80 –140 jC. The proton conductivity measurements were performed at each temperature after the membranes were equilibrated for more than 1 h with saturated humidities at each temperature in order to incorporate saturated water content in the hybrid membrane. It is very interesting that the large proton conductivity has been achieved up to the 140 jC. The conductivity level of 10 3 to 10 2 S/cm can be obtained for three hybrid polymers. The conductivity at 140 jC is larger for PTMO as well as PTMO/PEO hybrid polymers than PEO polymer, which is presumably originated from the difference of the water interaction with the polymer matrix. The TG-DTA results indicate that the PEO can keep more water than PTMO at a same molecular weight but release them at much lower temperatures. The hydrophobic PTMO hybrid polymer provides large conductivities and water content above 100 jC, whose unusual behaviour must be induced by the original macromolecular
Fig. 4. Proton conductivities of the hybrid membranes of PEO, PTMO and PEO/PTMO binary hybrids in the temperature range 80 – 140 jC.
I. Honma et al. / Solid State Ionics 154 – 155 (2002) 707–712
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the hybrid membrane and the electrodes are not optimized; just pressed and sandwiched MEA was tested here, the power output of approximately 15 mW/cm2 was obtained. The output will be increased by optimizing the interfaces to reduce electrode resistivity by increasing surface areas of three phase boundary. The sol – gel processes will be directly applied to form interpenetrated network between Pt/ C electrodes and organic/inorganic hybrid membrane with interfacial resistivity as low as possible. 4. Conclusion
Fig. 5. I – V characteristics of the MEA made from the organic/ inorganic hybrid membrane.
structures. The conductivity level close to 10 2 S/cm is an application level. Also, in the present experiments, all membrane made from PTMO hybrids are found to be very stable for high temperature proton conductivity measurements. No discernible change of the structure has been found by higher temperature annealing (140 jC) at humidified condition, which indicates that decomposition of silicate species did not occur even in the acidic membranes. Membrane electrode assembly (MEA) was made for PTMO hybrid polymers and fuel cells performance has been tested. The Pt/C electrodes (E-TEK) were just placed on both sides of the hybrid membrane and an optimization of electrodes/membrane interfaces was not intentionally executed. The MEA was mounted on the fuel cells measurement cell with O-ring sealing between anode and cathode. So, the polarization can be measured even at high pressure conditions up to 6 atm. Pure hydrogen and oxygen gases were used for the fuel cell’s power output measurements. Fig. 5 shows I –V characteristic of the MEA at different operational temperatures. The open circuit voltage of approximately 0.8 V is a conventional potential for hydrogen/oxygen cell and it indicates there is negligible gas permeability through the membrane. The permeability of this membrane was studied for hydrogen gas and it was almost similar to that of Nafion membrane. Although the interfaces between
Proton conducting organic/inorganic hybrids polymer electrolyte membranes have been synthesized through the sol – gel processing of bridged polyethers with alkoxysilanes. The binary blend hybrids of PTMO and PEO polymers are also investigated. The hybrid membrane doped with heteropolyacids (PWA) shows enhanced protonic conductivities at high temperatures up to 140 jC, where the temperature tolerance of the polyethers have been enormously enhanced by crosslinking with the inorganic moieties. Protonic conductivities of 10 3 –10 2 S/cm in the temperature range from RT to 140 jC have been achieved at saturated humidity conditions. The MEA of the hybrid membrane was tested in advance, and interfaces should be studied before the membrane can be used for high temperature operated PEMFC, which is considered to potentially provide the technological option for low temperature reformable (CO tolerant) fuel cell and direct methanol fuel cells (DMFC). Acknowledgements We thank Dr. Doyama of Sekisui Chemical and Mr. Iwasa of Chiba Institute of Technology for their collaboration in this work.
References [1] K.D. Kreuer, Chem. Mater. 8 (1996) 610 – 641. [2] H. Iwahara, H. Uchida, K. Morimoto, J. Electrochem. Soc. 137 (1990) 462 – 465. [3] N. Giordano, P. Staiti, A.S. Arico, E. Passalacqua, L. Abete, S. Hocevar, Electrochim. Acta 42 (1997) 1645 – 1652.
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[4] P. Donoso, W. Gorecki, C. Berthier, T. Defendini, C. Poinsignon, M.B. Armand, Solid State Ionics 28/30 (1988) 969 – 974. [5] A.M. Grillone, S. Panero, B.A. Retamel, B. Scrosati, J. Electrochem. Soc. 146 (1999) 27 – 31. [6] W.A. England, M.G. Cross, A. Hamnett, P.J. Wiseman, J.B. Goodenough, Solid State Ionics 1 (1980) 231 – 249. [7] K.D. Kreuer, M. Hampele, K. Dolde, A. Rabenau, Solid State Ionics 28/30 (1988) 589 – 593. [8] Y. Sone, A. Kishimoto, T. Kudo, Solid State Ionics 66 (1993) 53 – 59. [9] M. Tatsumisago, H. Honjo, Y. Sakai, T. Minami, Solid State Ionics 74 (1994) 105. [10] I. Honma, S. Hirakawa, K. Yamada, J.M. Bae, Solid State Ionics 118 (1998) 29 – 36. [11] I. Honma, Y. Takeda, J.M. Bae, Solid State Ionics 120 (1999) 255 – 264. [12] I. Honma, S. Nomura, H. Nakajima, J. Membr. Sci. 185 (2001) 83. [13] H. Nakajima, I. Honma, Proceedings of International Symposium on Fuel Cells for Vehicles, November 20 – 22, Nagoya, Japan, p. 190. [14] J.M. Bae, S. Hirakawa, I. Honma, J. Korean Phys. Soc. 35 (1999) 315. [15] J.-T. Wang, R.F. Savinell, J. Wainright, M. Litt, H. Yu, Electrochim. Acta 41 (1996) 193 – 197.
[16] J.J. Fontanella, M.C. Wintersgill, J.S. Wainright, R.F. Savinell, M. Litt, Electrochim. Acta 43 (1998) 1289 – 1296. [17] T. Kobayashi, M. Rikukawa, K. Sanui, N. Ogata, Solid State Ionics 106 (1998) 219 – 225. [18] S. Haufe, U. Stimming, J. Membr. Sci. 185 (2001) 95. [19] S.M. Haile, D.A. Boysen, C.R.I. Chisholm, R.B. Merle, Nature 410 (2001) 910. [20] L. Depre, J. Kappel, M. Popall, Electrochim. Acta 43 (1998) 1301 – 1306. [21] I. Gautier-Lineau, A. Denoyelle, J.Y. Sanchez, C. Poinsignon, Electrochim. Acta 37 (1992) 1615 – 1618. [22] B. Scrosati, F. Croce, Polym. Adv. Technol. 4 (1992) 198. [23] A.T. Kohl, R. Mimna, R. Shick, L. Rhodes, Z.L. Wang, P.A. Kohl, Electrochem. Solid-State Lett. 2 (1999) 77. [24] K.J. Shea, D.A. Loy, MRS Bull. 26 (2001) 368. [25] Y. Chujo, R. Tamaki, MRS Bull. 26 (2001) 389. [26] M.W. Park, J.C. Yang, H.S. Han, Y.G. Shul, T.H. Lee, Y.I. Cho, Denki Kagaku 64 (1996) 743. [27] S.M.J. Zaidi, S.D. Mikhailenko, G.P. Robertson, M.D. Guiver, S. Kaliaguine, J. Membr. Sci. 173 (2000) 17. [28] H. Nakajima, S. Nomura, T. Sugimoto, S. Nishikawa, I. Honma, J. Electrochem. Soc. 148 (8) (2002) A953. [29] I. Honma, H. Nakajima, S. Nishikawa, T. Sugimoto, S. Nomura, Electrochem. Solid-State Lett. (2002), in press.
High temperature proton conducting hybrid polymer electrolyte membranes Itaru Honma a,*, Hitoshi Nakajima a, Shigeki Nomura b a
Energy Electronic Institute, National Institute of Advanced Industrial Science and Technology, Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan b Tsukuba Research Laboratories, Sekisui Chemical Co., Ltd., Tsukuba, Ibaraki 300-4292, Japan Accepted 2 April 2002
Abstract Hybrid materials with nano-size interfaces between organic and inorganic offer exceptional opportunities to create entirely unique properties. Bridged polyethers are a family of hybrid organic/inorganic materials prepared by sol – gel processing of monomers that contain a variable polyether bridging group and functional silyl groups. The materials possess thermal as well as chemical tolerance at high temperatures due to the inorganic interfaces in the macromolecules. In the present paper, organic/ inorganic nanohybrid membrane has been prepared for proton conducting polymer electrolyte by incorporating heteropolyacids such as 12-phosphotungstic acid (PWA) clusters at the interfaces and high temperature conductivity has been examined. While the intermediate temperature polymer electrolyte membrane fuel cell (PEMFC) can be a candidate for overcoming major problems in the current PEMFC such as CO poisoning on the Pt electrodes surfaces and heat managements, new proton conducting polymer electrolytes with more temperature tolerance has been investigated on functional hybrids. The unique hybrid macromolecules with fast ionic conduction at higher temperature have been synthesized in the present work. D 2002 Published by Elsevier Science B.V. Keywords: Organic/inorganic nanohybrids; Bridged polyethers; Hybrid macromolecules; Sol – gel process; Protonic conducting membrane; Polymer electrolyte membrane; 12-Phosphotungstic acid (PWA)
1. Introduction Polymer electrolyte membrane fuel cells (PEMFC) are an extremely attractive energy conversion system for use in many industrial applications including electric vehicles (EV) and on-site power plant due to
*
Corresponding author. Tel.: +81-298-61-5648; fax: +81-29861-5829. E-mail address: [email protected] (I. Honma).
their inherently higher efficiency and lower emission when compared to that of internal combustion engines. There is an increasing interest in using hydrogen-rich gases produced by reforming methanol or even gasoline. Those gases contain traces of different condensable species such as CO, which drastically reduces the activity of platinum or platinum alloys at the anodes. As the CO tolerance increases with increasing temperature, higher temperature operated PEMFC has been desired to industrialize the technology for market, compatible to the reformable fossil oil
0167-2738/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 4 3 1 - 9
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I. Honma et al. / Solid State Ionics 154 – 155 (2002) 707–712
delivery. Additionally, direct electrochemical methanol oxidation at anode as well as oxygen electroreduction at cathode can be significantly promoted with increasing temperatures. However, water and methanol crossover remarkably increases with temperature, which reduces the fuel efficiency and requires an expensive water management system. Proton conducting membranes such as Nafion have been designed for a low temperature operation of the PEMFC below 100 jC, which limits the fuel only within hydrogen gas contaminated by CO species down to 5– 10 ppm levels. Different approaches are based on the complexation of basic polymers, such as polybenzoimidazol with oxo-acids including phosphoric acid, or inorganic inclusion to the temperature tolerant polymers [1 – 23]. In particular, new approaches are developed by elaborating organic/inorganic macromolecules with nano-sized interfaces synthesized by sol – gel processing [9 – 14,19 – 25]. The materials’ thermal, chemical and electrochemical properties have been controlled by preparing hydrophilic/hydrophobic interfaces with highly conductive networks. In this work, a flexible, homogeneous and largesized hybrid membrane has been synthesized by crosslinking nano-sized silicate species to polyethers polymers (PEO: polyethylene oxides; PPO: polypropylene oxide; PTMO: polytetramethylene oxide). The membrane becomes proton conducting electrolyte by doping with heteropolyacids such as 12-phosphotungstic acid (PWA) supported on the condensed silicate species [26,27]. Hydrated PWA is known to
present a large proton conductivity at around 0.2 S/ cm. The protonic conductivities at intermediate temperatures up to 160 jC were examined for the hybrid electrolyte membrane. The effect of the polyether molecular structure, PWA doping concentration and binary blend hybrid polymers of PEO and PTMO have been studied for fast protonic conduction at high temperatures.
2. Experiment Organically modified alkoxysilane precursors developed in our previous studies [10 –14] have been used to form organic/inorganic macromolecules to become proton conducting nanohybrid membranes. Sol – gel processing provides versatile processing route to change hydrophobic/hydrophilic interfaces. Fig. 1 shows the molecular structure of organic/ inorganic hybrid precursors where polyethylene oxide (PEO) as well as polytetramethylene oxide (PTMO) have been crosslinked with alkoxysilanes through isocyanato coupling. The hybrid macromolecular structure can be varied through systematic change of the molecular length of PEO and PTMO. The hybrid precursors hydrolyze and condense to form macromolecules of flexible, transparent, glassy hybrid polymer materials. The membrane’s polymer structures are controlled by the sol –gel process conditions such as gelation temperatures or humidities. Initially, polytetramethyleneglycols with molecular weight of 250, 650, 1000
Fig. 1. A molecular structure of bridged organic/inorganic hybrid macromolecules.
I. Honma et al. / Solid State Ionics 154 – 155 (2002) 707–712
and 2000 were derivatized with isocyanatopropyltriethoxysilane to make bridged molecular precursors. A similar process has been taken for PEO hybrid polymers. This hybrid precursor was mixed with heteropolyacids of PWA and casted on the petri dish and aged for 24 h at 60 jC under saturated humidities. The structure of the hybrid membrane has been characterized by 1H-NMR to confirm the bridged molecular structure in Fig. 1 [28]. Fundamental vibrations of the macromolecular structure has been characterized by infrared spectra (IR). A nano-sized structure of the hybrid membrane has been characterized by a transmission electron micrograph (TEM) on a sliced specimen at a magnification of 200,000 with accelerating voltage of 120 kV (JEOL 1200EXII). Thermal stability was also studied by TG-DTA analysis and the difference of water evolution can be measured between PTMO and PEO polymers. Proton conductivity of organic/inorganic nanohybrid membrane was measured by a two terminal impedance spectroscopic method (Solartron 1260) over the frequency range of 1 Hz – 2 MHz. Cell temperatures, pressures and humidities were widely changed, for example, when the conductivity of the membrane was measured at higher temperatures, the apparatus cell was pressurized to maximum 6 atm (160 jC) in order to keep saturated water vapor pressures. A typical impedance response (cole – cole plots) of the membrane shows a feature similar to that of highly ionic conducting membrane such as Nafion.
3. Results and discussions SEM photograph shows the membrane was completely transparent, homogeneous, isotropic materials. A flat membrane of 8 cm diameter and a thickness range from 100 to 300 Am can be easily obtained, as shown in Fig. 2. The mechanical stability was very good for polyether hybrid materials in particular, for example, it can be rolled up by easily without breaking the membrane although the membrane contains a maximum of about 50% inorganic composition. Also, a TEM photograph of the specimen was studied and no discernible microstructure has been observed at the magnification scale, which indicates the hybrid materials is completely homogeneous at nano-scale. The hybrid materials are considered to
709
Fig. 2. The SEM photograph of the organic/inorganic hybrid membrane.
form nano-phase separated structure between hydrophobic and hydrophilic domains. Various polyethers such as polyethyleneoxide PEO, polypropyleneoxide (PPO) and polytetramethyleneoxide (PTMO) have been used for hybrid materials. Also, binary blend hybrid polymers have been synthesized between PEO and PTMO polymers. The thermal analysis was measured for PEO and PTMO hybrid membrane and the difference of water evolution properties were studied. It is known that PEO is a hydrophilic polymer and PTMO is a hydrophobic polymer, so that the interaction between the polymers and water might be different between the two. Fig. 3(a) shows the TG-DTA analysis of hybrid membrane of PEO and PTMO in a temperature range from RT to 280 jC with a heating rate of 10 jC/min. The molecular weight of the polymer has been changed for PEO (200 to 600) and PTMO (250 to 1000), respectively. As the organic as well as inorganic compositions in the hybrid are not volatile materials, the large weight decrease is an evolution of water from the membrane. As a common tendency to the both polymers, the water content in the membrane is proportional to the molecular weight of the polymers, i.e., the organic chain length in the hybrid macromolecules. Also, it is very interesting that the water evolution temperature at the TG weight loss position is about 50 to 80 jC for PEO and 100 to 150 jC for PTMO although the hydrophilic PEO hybrid polymer incorporate larger amount of water than PTMO
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Fig. 3. (a) TG-DTA data for hybrid polymer membranes with PEO (Mw = 200, 400, 600) and PTMO (Mw = 250, 650, 1000), respectively. (b) Water concentration in PEO and PTMO hybrid membrane determined by TG-DTA analysis as a function of polyether polymer’s molecular weight.
hybrid. It is comprehensible that the PEO has hydrophilic interaction with water, so that it contains more water inside its macromolecular matrix. However, the hydrophobic PTMO does not seem to strongly interact with water. The TG-DTA data indicates the presence of water inside hybrid macromolecules might be quite different between PEO and PTMO. Fig. 3(b) shows a dependence of the water content as a function of polymer molecular weight for PEO and PTMO, respectively. In both cases, the water content increases with the polymer weight almost linearly, while, the water weight concentration in the PEO hybrid is almost twice larger than PTMO polymers. The data indicate that the water content is strongly correlated with a polymer chain length; in other words, water domain size in the hybrid macromolecules is primarily
determined by the polymer’s unit length for both materials. If assumed that there is a conductive channel structure of hybrid macromolecules assembled along acidic PWA clusters, the volume fraction of water can be controlled by the polymer chain length. Additionally, a shift of conductivity by doping concentration of PWA has been reported elsewhere [10 –14,28,29]. Fig. 4 shows a proton conductivity of the hybrid membranes of PEO, PTMO and PEO/PTMO binary hybrids in the temperature range 80 –140 jC. The proton conductivity measurements were performed at each temperature after the membranes were equilibrated for more than 1 h with saturated humidities at each temperature in order to incorporate saturated water content in the hybrid membrane. It is very interesting that the large proton conductivity has been achieved up to the 140 jC. The conductivity level of 10 3 to 10 2 S/cm can be obtained for three hybrid polymers. The conductivity at 140 jC is larger for PTMO as well as PTMO/PEO hybrid polymers than PEO polymer, which is presumably originated from the difference of the water interaction with the polymer matrix. The TG-DTA results indicate that the PEO can keep more water than PTMO at a same molecular weight but release them at much lower temperatures. The hydrophobic PTMO hybrid polymer provides large conductivities and water content above 100 jC, whose unusual behaviour must be induced by the original macromolecular
Fig. 4. Proton conductivities of the hybrid membranes of PEO, PTMO and PEO/PTMO binary hybrids in the temperature range 80 – 140 jC.
I. Honma et al. / Solid State Ionics 154 – 155 (2002) 707–712
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the hybrid membrane and the electrodes are not optimized; just pressed and sandwiched MEA was tested here, the power output of approximately 15 mW/cm2 was obtained. The output will be increased by optimizing the interfaces to reduce electrode resistivity by increasing surface areas of three phase boundary. The sol – gel processes will be directly applied to form interpenetrated network between Pt/ C electrodes and organic/inorganic hybrid membrane with interfacial resistivity as low as possible. 4. Conclusion
Fig. 5. I – V characteristics of the MEA made from the organic/ inorganic hybrid membrane.
structures. The conductivity level close to 10 2 S/cm is an application level. Also, in the present experiments, all membrane made from PTMO hybrids are found to be very stable for high temperature proton conductivity measurements. No discernible change of the structure has been found by higher temperature annealing (140 jC) at humidified condition, which indicates that decomposition of silicate species did not occur even in the acidic membranes. Membrane electrode assembly (MEA) was made for PTMO hybrid polymers and fuel cells performance has been tested. The Pt/C electrodes (E-TEK) were just placed on both sides of the hybrid membrane and an optimization of electrodes/membrane interfaces was not intentionally executed. The MEA was mounted on the fuel cells measurement cell with O-ring sealing between anode and cathode. So, the polarization can be measured even at high pressure conditions up to 6 atm. Pure hydrogen and oxygen gases were used for the fuel cell’s power output measurements. Fig. 5 shows I –V characteristic of the MEA at different operational temperatures. The open circuit voltage of approximately 0.8 V is a conventional potential for hydrogen/oxygen cell and it indicates there is negligible gas permeability through the membrane. The permeability of this membrane was studied for hydrogen gas and it was almost similar to that of Nafion membrane. Although the interfaces between
Proton conducting organic/inorganic hybrids polymer electrolyte membranes have been synthesized through the sol – gel processing of bridged polyethers with alkoxysilanes. The binary blend hybrids of PTMO and PEO polymers are also investigated. The hybrid membrane doped with heteropolyacids (PWA) shows enhanced protonic conductivities at high temperatures up to 140 jC, where the temperature tolerance of the polyethers have been enormously enhanced by crosslinking with the inorganic moieties. Protonic conductivities of 10 3 –10 2 S/cm in the temperature range from RT to 140 jC have been achieved at saturated humidity conditions. The MEA of the hybrid membrane was tested in advance, and interfaces should be studied before the membrane can be used for high temperature operated PEMFC, which is considered to potentially provide the technological option for low temperature reformable (CO tolerant) fuel cell and direct methanol fuel cells (DMFC). Acknowledgements We thank Dr. Doyama of Sekisui Chemical and Mr. Iwasa of Chiba Institute of Technology for their collaboration in this work.
References [1] K.D. Kreuer, Chem. Mater. 8 (1996) 610 – 641. [2] H. Iwahara, H. Uchida, K. Morimoto, J. Electrochem. Soc. 137 (1990) 462 – 465. [3] N. Giordano, P. Staiti, A.S. Arico, E. Passalacqua, L. Abete, S. Hocevar, Electrochim. Acta 42 (1997) 1645 – 1652.
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