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Structural change and proton conductivity of phosphosilicate gel–polyimide composite membrane for a fuel cell operated at 180 °C
Journal of Membrane Science 324 (2008) 188–191
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Structural change and proton conductivity of phosphosilicate gel–polyimide composite membrane for a fuel cell operated at 180 ◦ C Kiyoharu Tadanaga ∗ , Yoshiki Michiwaki, Teruaki Tezuka, Akitoshi Hayashi, Masahiro Tatsumisago Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
a r t i c l e
i n f o
Article history: Received 20 December 2007 Received in revised form 1 July 2008 Accepted 2 July 2008 Available online 11 July 2008 Keywords: Phosphosilicate gel Fuel cell Polyimide Inorganic–organic composite
a b s t r a c t Proton-conductive composite membranes were prepared from phosphosilicate gel powders and a commercially available polyimide precursor. The conductivity of a composite membrane containing 75 wt.% of gel powder was about 2.5 × 10−3 S cm−1 at 180 ◦ C under more than 0.4%RH. However, under dry N2 atmosphere at 180 ◦ C, the conductivity decreased with time because of the formation of Si5 O(PO4 )6 crystals. The open circuit voltage (OCV) of a fuel cell in which the phosphosilicate gel–polyimide composite sheet was used as electrolyte and Pt-loaded carbon paper sheets were used as electrodes in the membrane electrode assembly, was about 0.85 V at 80 ◦ C with a flow of hydrogen and air, and the OCV decreased to about 0.70 V at temperatures higher than 100 ◦ C. At 180 ◦ C under 2%RH, the cell OCV was 0.67 V; power density of about 14 mW cm−2 was obtained with current density of 40 mA cm−2 . Consequently, the fuel cell using the composite sheet as an electrolyte was verified to operate at temperatures from room temperature to 180 ◦ C. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Operation conditions of the polymer electrolyte fuel cells (PEFCs) are generally restricted to be lower than 100 ◦ C and under high humidity because the proton conductive membranes used for the fuel cells lose their conductivity at temperatures higher than 100 ◦ C and under low humidity. Solid state proton-conductive membranes with high conductivity in the medium temperature range (100–200 ◦ C), even under low humidity, are useful as electrolytes for PEFC [1–4] because operation of PEFCs at a medium temperature range provides many advantages including improvement of the utilization of total electric power generated in the cells and suppression of the poisoning of Pt catalysts with CO in the fuel gases [5–8]. For an electrolyte intended for use in the medium temperature range, various proton conducting inorganic materials [9–15] and organic–inorganic hybrid membranes [4,5,16–18] have been reported. We have reported the preparation of protonconductive phosphosilicate gels [19–20] and inorganic–organic hybrid films [21], which showed high proton conductivity at temperatures higher than 100 ◦ C with low humidity. We have also reported thermally stable proton-conducting composite membranes prepared from phosphosilicate gel powder and polyimide precursor, and demonstrated that those membranes showed con-
∗ Corresponding author. Tel.: +81 72 254 9333; fax: +81 72 254 9910. E-mail address: [email protected] (K. Tadanaga). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.07.009
ductivity of 2 × 10−4 S cm−1 at 150 ◦ C and 0.4%RH [22]. Furthermore, we have reported that fuel cells using the composite sheet as an electrolyte can operate at 30 ◦ C [22] or at 150 ◦ C under 4%RH [23]. However, fuel cell operation at higher temperatures is required. For the present work, structural changes of phosphosilicate gel–polyimide composite membranes heated to temperatures as high as 180 ◦ C were examined. The effects of structural changes on the proton conductivity are discussed in this report. Moreover, operation and electrochemical behavior of the fuel cell using the composite membranes was investigated, from room temperature to 180 ◦ C. 2. Experimental 2.1. Preparation of composite membranes Proton-conductive composite membranes were prepared from phosphosilicate gel powders and a commercially available polyimide precursor (U-imide-varnish type C; Unitika Ltd.). Phosphosilicate gel powders were obtained from tetraethoxysilane and H3 PO4 using the sol–gel method. The procedures were fundamentally identical to those reported in previous papers [19,20]. The mole ratio of P/Si was fixed to 1.0. The dry gel powders heat-treated at 150 ◦ C were further ground into fine powders of a few micrometers’ diameter using a planetary ball mill (Pulverisette 7; Fritsch GmbH). Furthermore, the phosphosilicate gel powders were sieved with a 145 mesh to control the particle diameter of the powder
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to less than about 5 m [23]. The sieved gel powder was then mixed with the N,N-dimethylacetoamide solution of a polyimide precursor with stirring for 30 min; the mixture was stirred under ultrasonic wave irradiation for an additional 30 min. The weight ratio gel powder:polyimide used in this study was 75:25. The resultant yellowish composite slurries were cast on a glass plate and developed using a spacer to control their thickness. The developed composite membranes on glass plates were heat-treated at 150 ◦ C for 3 h and then at 180 ◦ C for 3 h [23]. It is known that polyimides are known to have low hydrolytic stability. However, no degradation of the composite films was observed during the present study. 2.2. Characterization of composite membranes X-ray diffraction (XRD) patterns of the membranes were obtained using an X-ray diffractometer (RINT1100; Rigaku Corp.). Electric conductivities of the composite membranes were determined with AC impedance data obtained using an impedance analyzer (SI1260; Solartron Analytical) at frequencies of 10 Hz to 8 MHz. Water vapor pressure was controlled by changing the bubbler temperature; conductivity measurements were carried out in humidified N2 with a flow rate of 100 cm3 min−1 . 2.3. Preparation and evaluation of MEAs and fuel cells The single fuel cell tests were carried out for the membrane/electrode assemblies (MEAs), which consisted of the composite sheet as an electrolyte and commercially available Pt-loaded (1 mg cm−2 ) carbon paper sheets (EC-20-10-10; Electrochem Inc.) as electrodes. The MEAs were obtained by hot-pressing the composite sheet between the Pt-loaded carbon paper sheets at 130 ◦ C under about 11 MPa in vacuo for 10 min. Current–voltage profiles in the fuel cells were obtained using an electrochemical measurement system (HZ-3000; Hokuto Denko). The H2 and air that had been humidified by passage through the bubbler were provided to the cells when current–voltage profiles of the fuel cell were measured. The cell temperature was controlled by holding the cell in an oven. Flow rates of pure hydrogen for anode and of air for cathode were respectively, 20 and 100 cm3 min−1 .
Fig. 1. XRD patterns for composites without any pre-treatment, and pre-treated at 30 ◦ C under 60%RH or pre-treatment and kept at 180 ◦ C with either 3% or 0.2%RH, or with dry N2 .
3. Results and discussion
under the ambient atmosphere. Thus, the crystallization behavior in this study suggests that the heat-treatment under dry conditions enhances the crystallization of Si5 O(PO4 )6 crystals, and the introduction of water vapor suppresses the crystallization. Fig. 2 portrays variations in conductivities with time for the composite membranes kept at 180 ◦ C under dry N2 , 0.13, 0.26 and 0.42%RH The composite membranes were pre-treated at 30 ◦ C under 60%RH for 3 h before the conductivity measurements. With dry N2 and 0.13%RH, the conductivity of the composite membranes decreases with time. At 0.26 or 0.42%RH, the conductivity is almost constant with retention of temperature at 180 ◦ C. As shown in Fig. 1, Si5 O(PO4 )6 crystals were observed in the composite membranes kept at 180 ◦ C under 0.2% and dry N2 . For that reason, the decrease in conductivity with time under dry N2 and 0.13%RH displayed in Fig. 2 is attributable to formation of Si5 O(PO4 )6 crystals.
Fig. 1 shows XRD patterns for the composites without any pretreatment and pre-treated at 30 ◦ C under 60%RH Data are also shown for the composite with the pre-treatment and kept at 180 ◦ C under 3%, 0.2%RH and dry N2 . In the XRD pattern of as-prepared composite film, diffraction peaks attributable to Si5 O(PO4 )6 are visible. In the preparation of the membranes, they were heat-treated at 150 ◦ C for 3 h and then 180 ◦ C for 3 h. Because it was confirmed that phosphosilicate gel heat-treated at 150 ◦ C was amorphous state, Si5 O(PO4 )6 were presumably crystallized with heat treatment at 180 ◦ C. Keeping the as-prepared composite membranes at 30 ◦ C under 60%RH, the halo pattern is observed. This pattern is attributable to hydrolysis of Si5 O(PO4 )6 crystals with the moisture in the atmosphere. 31 P MAS NMR spectra of the sample showed the formation of H3 PO4 with keeping the as-prepared composite membranes at 30 ◦ C under 60%RH. When the pre-treated composite membranes were kept at 180 ◦ C under 3%RH, the halo pattern was observed, although peaks attributable to Si5 O(PO4 )6 crystals were observed in the composite membranes kept at 180 ◦ C under 0.2% and dry N2 . Consequently, crystallization of Si5 O(PO4 )6 was observed at 180 ◦ C under dry conditions. Crystallization of Si5 O(PO4 )6 from sol–gel derived phosphosilicate gel has been reported by some authors [24,25], but Si5 O(PO4 )6 crystals were only formed with a heat-treatment at temperatures higher than 400 ◦ C
Fig. 2. Variation in conductivities with time for the composite membranes, kept at 180 ◦ C under dry N2 , and with either 0.13, 0.26 or 0.42% RH.
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Fig. 3. Relative-humidity dependence of the ionic conductivity for the composite membranes at 180 ◦ C.
Fig. 3 presents the relative-humidity dependence of conductivity for a composite membrane containing 75 wt.% phosphosilicate gel powders at 180 ◦ C. The membrane was kept at 30 ◦ C under 60%RH before measurements. Conductivity of the composite membrane increases with increased relative humidity up to about 0.4%RH; it is almost constant up to 5%RH. The XRD and conductivity results indicate that formation of Si5 O(PO4 )6 crystals decreases the conductivity, whereas the hydrolysis of Si5 O(PO4 )6 leads to high conductivity. The high conductivity must be brought by the formation of H3 PO4 through hydrolysis. Even under low relative humidity,
re-crystallization of Si5 O(PO4 )6 was inhibited, causing high proton conductivity, as depicted in Fig. 3. As reported previously [23], conductivity for the composite sheet containing 75 wt.% phosphosilicate gel powders under a constant water vapor pressure of 150 mmHg increased with increasing temperatures of 80–150 ◦ C; it was as high as 3.0 × 10−3 S cm−1 at 150 ◦ C [23]. At 180 ◦ C, the water vapor pressure of 150 mmHg corresponds to about 2.0%RH; conductivity is about 3 × 10−3 S cm−1 , as portrayed in Fig. 3. Consequently, water vapor pressure of 150 mmHg is probably sufficient to maintain constant proton conductivity in a wide temperature range up to 180 ◦ C. Fig. 4 shows the cell voltage versus current density with the resulting power density plots for a fuel cell using an MEA which consists of the composite sheet containing 75 wt.% phosphosilicate gel as an electrolyte and Pt-loaded carbon paper sheets as electrodes. Open circles, squares and triangles, respectively represent the current–voltage characteristics of the fuel cells at 30 ◦ C under 75%RH, at 80 ◦ C under 42%RH and at 180 ◦ C under 2.0%RH. Solid circles, squares and triangles, respectively signify the calculated current–power density plots of the fuel cells at 30, 80 and 180 ◦ C. The conductivity of the membrane at 30 ◦ C under 75%RH, and at 80 ◦ C under 42%RH was 3 × 10−3 S cm−1 and 1 × 10−3 S cm−1 , respectively. The open circuit voltage of the cell at 30 and 80 ◦ C is about 0.85 V, whereas the voltage of the cell decreases to about 0.67 V at 180 ◦ C. At temperatures lower than 100 ◦ C, the pores in the composite membranes are closed because of absorbed water, but at temperatures higher than 100 ◦ C, the physically absorbed water evaporates, and the evaporation of water must causes the H2 crossover. At 180 ◦ C under 2%RH, the cell OCV was 0.67 V; power of about 14 mW cm−2 was obtained with current density of 40 mA cm−2 . Impedance spectroscopy measurements of the cells suggest that the small power density in the cell performance measurements is due to the large polarization resistance [26]. Preparation of MEA must be optimized to obtain high cell performance. Nevertheless, the operation at temperatures from 30 to 180 ◦ C in the present study proved that the fuel cells using the MEA consisting of the composite sheet as an electrolyte can be used in the wide temperature range. 4. Conclusions Proton-conductive composite membranes were prepared from phosphosilicate gel powders and a commercially available polyimide precursor. The conductivity of the composite membranes containing 75 wt.% of gel powder was about 2.5 × 10−3 S cm−1 at 180 ◦ C under more than 0.4%RH However, under dry N2 atmosphere at 180 ◦ C, the conductivity decreased with time because of the formation of Si5 O(PO4 )6 crystals. Introduction of water vapor suppressed the crystallization. The OCV of the fuel cell, in which the phosphosilicate gel–polyimide composite sheet was used as electrolyte, at 30 and 80 ◦ C was about 0.85 V. At 180 ◦ C under 2%RH, the cell OCV was 0.67 V, and power density of about 14 mW cm−2 was obtained with current density of 40 mA cm−2 . Therefore, a fuel cell using the composite sheet as an electrolyte was confirmed to operate at temperatures from room temperature to 180 ◦ C. Acknowledgement The present work was supported by a Grant-in-aid for Scientific Research on Priority Area, “Nanoionics (439)” by the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
Fig. 4. Cell voltage versus current density for a test cell at 30 ◦ C under 75%RH, at 80 ◦ C under 42%RH and at 180 ◦ C under 2.0%RH The composite sheet containing 75 wt.% phosphosilicate gel was used as an electrolyte and Pt-loaded carbon paper sheets were used as electrodes.
References [1] P. Jannasch, Recent developments in high-temperature proton conducting polymer electrolyte membranes, Curr. Opin. Colloid Interf. Sci. 8 (2003) 96.
K. Tadanaga et al. / Journal of Membrane Science 324 (2008) 188–191 [2] A.S. Arico, V. Baglio, A. Di Blasi, P. Creti’, P.L. Antonucci, V. Antonucci, Influence of the acid–base characteristics of inorganic fillers on the high temperature performance of composite membranes in direct methanol fuel cells, Solid State Ionics 161 (2003) 251. [3] T. Norby, Solid-state protonic conductors: principles, properties, progress and prospects, Solid State Ionics 125 (1) (1999). [4] I. Honma, H. Nakajima, O. Nishikawa, T. Sugimoto, S. Nomura, Amphiphilic organic/inorganic nanohybrid macromolecules for intermediate-temperature proton conducting electrolyte membranes, J. Electrochem. Soc. 149 (2002) A1389. [5] K.T. Adjemian, S.J. Lee, S. Srinivasan, J. Benziger, A.B. Bocarsly, Silicon oxide Nafion composite membranes for proton-exchange membrane fuel cell operation at 80–140 degrees C, J. Electrochem. Soc. 149 (2002) 256. [6] S. Malhotra, R. Datta, Membrane-supported nonvolatile acidic electrolytes allow higher temperature operation of proton-exchange membrane fuel cells, J. Electrochem. Soc. 144 (1997) 23. [7] K.D. Kreuer, On the development of proton conducting materials for technological applications, Solid State Ionics 97 (1997) 1. [8] P. Colomban, Latest developments in proton conductors, Ann. Chim. 24 (1999) 1. [9] T. Matsui, S. Takeshita, Y. Iriyama, T. Abe, M. Inaba, Z. Ogumi, Proton conductivity of (NH4 )2 TiP4 O13 -based material for intermediate temperature fuel cells, Electrochem. Commun. 6 (2004) 180. [10] T. Matsui, T. Kukino, R. Kikuchi, K. Eguchi, Intermediate-temperature fuel cell employing CsH2 PO4 /SiP2 O7 -based composite electrolytes, J. Electrochem. Soc. 153 (2006) A339. [11] M. Nagao, A. Takeuchi, P. Heo, T. Hibino, M. Sano, A. Tomita, A proton-conducting In3+ -doped SnP2 O7 electrolyte for intermediate-temperature fuel cells, Electrochem. Solid-State Lett. 9 (2006) A105. [12] J. Otomo, T. Tamaki, S. Nishida, S. Wang, M. Ogura, T. Kobayashi, C.J. Wen, H. Nagamoto, H. Takahashi, Effect of water vapor on proton conduction of cesium dihydrogen phosphate and application to intermediate temperature fuel cells, J. Appl. Electrochem. 35 (2005) 865. [13] D. Lu, W. Lu, C. Li, J. Liu, J. Xu, Proton-conducting composite membranes derived from poly(2,6-dimethyl-1,4-phenylene oxide) doped with phosphosilicate gels, Solid State Ionics 177 (2006) 1111. [14] V.G. Ponomareva, E.S. Suhtova, Composite electrolytes Cs3 (H2 PO4 )(HSO4 )2 /SiO2 with high proton conductivity, Solid State Ionics 176 (2005) 2905.
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[15] M. Tatsumisago, T. Tezuka, A. Hayashi, K. Tadanaga, Preparation of proton conductive composites with cesium hydrogen sulfate and phosphosilicate gel, Solid State Ionics 176 (2005) 2909. [16] Z.G. Shao, P. Joghee, I.-M. Hsing, Preparation and characterization of hybrid Nafion–silica membrane doped with phosphotungstic acid for high temperature operation of proton exchange membrane fuel cells, J. Membr. Sci. 229 (2004) 43. [17] P. Heo, M. Nagao, T. Kamiya, M. Sano, A. Tomita, T. Hibino, Sn0.9 In0.1 P2 O7 based organic/inorganic composite membranes application to intermediatetemperature fuel cells, J. Electrochem. Soc. 154 (2007) B63. [18] M. Kato, W. Sakamoto, T. Yogo, Synthesis of proton-conductive sol–gel membranes from trimethoxysilylmethylstyrene and phenylvinylphosphonic acid, J. Membr. Sci. 303 (2007) 43. [19] A. Matsuda, T. Kanzaki, K. Tadanaga, M. Tatsumisago, T. Minami, Medium temperature range characterization as a proton conductor for phosphosilicate dry gels containing large amounts of phosphorus, Electrochim. Acta 47 (2001) 939. [20] A. Matsuda, T. Kanzaki, K. Tadanaga, M. Tatsumisago, T. Minami, Phosphosilicate gels as a solid state proton conductor at medium temperature and low humidity, J. Ceram. Soc. Jpn. 110 (2002) 131. [21] K. Tadanaga, H. Yoshida, A. Matsuda, T. Minami, M. Tatsumisago, Inorganic–organic hybrid films from 3-glycidoxypropyltrimethoxysilane and orthophosphoric acid for medium temperature fuel cells, Electrochem. Commun. 5 (2003) 644. [22] A. Matsuda, N. Nakamoto, K. Tadanaga, M. Tatsumisago, T. Minami, Preparation and characterization of thermally stable proton-conducting composite sheets composed of phosphosilicate gel and polyimide, Solid State Ionics 162/163 (2003) 247. [23] N. Nakamoto, A. Matsuda, K. Tadanaga, T. Minami, M. Tatsumisago, Medium temperature operation of fuel cells using thermally stable proton-conducting composite sheets composed of phosphosilicate gel and polyimide, J. Power Sources 138 (2004) 51. [24] M. D’Apuzzo, A. Arronne, S. Esposito, P. Pernice, Sol–gel synthesis of humiditysensitive P2 O5 –SiO2 amorphous films, J. Sol–Gel Sci. Technol. 17 (2000) 247. [25] A. Aronne, M. Turco, G. Bagnasco, P. Pernice, M. Di Serio, N.J. Clayden, E. Marenna, E. Fanelli, Chem. Mater. 17 (2005) 2081. [26] J. Lobato, P. Canizares, M.A. Rodrigo, J.J. Linares, PBI-based polymer electrolyte membranes fuel cells temperature effects on cell performance and catalyst stability, Electrochim. Acta 52 (2007) 3910.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Structural change and proton conductivity of phosphosilicate gel–polyimide composite membrane for a fuel cell operated at 180 ◦ C Kiyoharu Tadanaga ∗ , Yoshiki Michiwaki, Teruaki Tezuka, Akitoshi Hayashi, Masahiro Tatsumisago Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
a r t i c l e
i n f o
Article history: Received 20 December 2007 Received in revised form 1 July 2008 Accepted 2 July 2008 Available online 11 July 2008 Keywords: Phosphosilicate gel Fuel cell Polyimide Inorganic–organic composite
a b s t r a c t Proton-conductive composite membranes were prepared from phosphosilicate gel powders and a commercially available polyimide precursor. The conductivity of a composite membrane containing 75 wt.% of gel powder was about 2.5 × 10−3 S cm−1 at 180 ◦ C under more than 0.4%RH. However, under dry N2 atmosphere at 180 ◦ C, the conductivity decreased with time because of the formation of Si5 O(PO4 )6 crystals. The open circuit voltage (OCV) of a fuel cell in which the phosphosilicate gel–polyimide composite sheet was used as electrolyte and Pt-loaded carbon paper sheets were used as electrodes in the membrane electrode assembly, was about 0.85 V at 80 ◦ C with a flow of hydrogen and air, and the OCV decreased to about 0.70 V at temperatures higher than 100 ◦ C. At 180 ◦ C under 2%RH, the cell OCV was 0.67 V; power density of about 14 mW cm−2 was obtained with current density of 40 mA cm−2 . Consequently, the fuel cell using the composite sheet as an electrolyte was verified to operate at temperatures from room temperature to 180 ◦ C. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Operation conditions of the polymer electrolyte fuel cells (PEFCs) are generally restricted to be lower than 100 ◦ C and under high humidity because the proton conductive membranes used for the fuel cells lose their conductivity at temperatures higher than 100 ◦ C and under low humidity. Solid state proton-conductive membranes with high conductivity in the medium temperature range (100–200 ◦ C), even under low humidity, are useful as electrolytes for PEFC [1–4] because operation of PEFCs at a medium temperature range provides many advantages including improvement of the utilization of total electric power generated in the cells and suppression of the poisoning of Pt catalysts with CO in the fuel gases [5–8]. For an electrolyte intended for use in the medium temperature range, various proton conducting inorganic materials [9–15] and organic–inorganic hybrid membranes [4,5,16–18] have been reported. We have reported the preparation of protonconductive phosphosilicate gels [19–20] and inorganic–organic hybrid films [21], which showed high proton conductivity at temperatures higher than 100 ◦ C with low humidity. We have also reported thermally stable proton-conducting composite membranes prepared from phosphosilicate gel powder and polyimide precursor, and demonstrated that those membranes showed con-
∗ Corresponding author. Tel.: +81 72 254 9333; fax: +81 72 254 9910. E-mail address: [email protected] (K. Tadanaga). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.07.009
ductivity of 2 × 10−4 S cm−1 at 150 ◦ C and 0.4%RH [22]. Furthermore, we have reported that fuel cells using the composite sheet as an electrolyte can operate at 30 ◦ C [22] or at 150 ◦ C under 4%RH [23]. However, fuel cell operation at higher temperatures is required. For the present work, structural changes of phosphosilicate gel–polyimide composite membranes heated to temperatures as high as 180 ◦ C were examined. The effects of structural changes on the proton conductivity are discussed in this report. Moreover, operation and electrochemical behavior of the fuel cell using the composite membranes was investigated, from room temperature to 180 ◦ C. 2. Experimental 2.1. Preparation of composite membranes Proton-conductive composite membranes were prepared from phosphosilicate gel powders and a commercially available polyimide precursor (U-imide-varnish type C; Unitika Ltd.). Phosphosilicate gel powders were obtained from tetraethoxysilane and H3 PO4 using the sol–gel method. The procedures were fundamentally identical to those reported in previous papers [19,20]. The mole ratio of P/Si was fixed to 1.0. The dry gel powders heat-treated at 150 ◦ C were further ground into fine powders of a few micrometers’ diameter using a planetary ball mill (Pulverisette 7; Fritsch GmbH). Furthermore, the phosphosilicate gel powders were sieved with a 145 mesh to control the particle diameter of the powder
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to less than about 5 m [23]. The sieved gel powder was then mixed with the N,N-dimethylacetoamide solution of a polyimide precursor with stirring for 30 min; the mixture was stirred under ultrasonic wave irradiation for an additional 30 min. The weight ratio gel powder:polyimide used in this study was 75:25. The resultant yellowish composite slurries were cast on a glass plate and developed using a spacer to control their thickness. The developed composite membranes on glass plates were heat-treated at 150 ◦ C for 3 h and then at 180 ◦ C for 3 h [23]. It is known that polyimides are known to have low hydrolytic stability. However, no degradation of the composite films was observed during the present study. 2.2. Characterization of composite membranes X-ray diffraction (XRD) patterns of the membranes were obtained using an X-ray diffractometer (RINT1100; Rigaku Corp.). Electric conductivities of the composite membranes were determined with AC impedance data obtained using an impedance analyzer (SI1260; Solartron Analytical) at frequencies of 10 Hz to 8 MHz. Water vapor pressure was controlled by changing the bubbler temperature; conductivity measurements were carried out in humidified N2 with a flow rate of 100 cm3 min−1 . 2.3. Preparation and evaluation of MEAs and fuel cells The single fuel cell tests were carried out for the membrane/electrode assemblies (MEAs), which consisted of the composite sheet as an electrolyte and commercially available Pt-loaded (1 mg cm−2 ) carbon paper sheets (EC-20-10-10; Electrochem Inc.) as electrodes. The MEAs were obtained by hot-pressing the composite sheet between the Pt-loaded carbon paper sheets at 130 ◦ C under about 11 MPa in vacuo for 10 min. Current–voltage profiles in the fuel cells were obtained using an electrochemical measurement system (HZ-3000; Hokuto Denko). The H2 and air that had been humidified by passage through the bubbler were provided to the cells when current–voltage profiles of the fuel cell were measured. The cell temperature was controlled by holding the cell in an oven. Flow rates of pure hydrogen for anode and of air for cathode were respectively, 20 and 100 cm3 min−1 .
Fig. 1. XRD patterns for composites without any pre-treatment, and pre-treated at 30 ◦ C under 60%RH or pre-treatment and kept at 180 ◦ C with either 3% or 0.2%RH, or with dry N2 .
3. Results and discussion
under the ambient atmosphere. Thus, the crystallization behavior in this study suggests that the heat-treatment under dry conditions enhances the crystallization of Si5 O(PO4 )6 crystals, and the introduction of water vapor suppresses the crystallization. Fig. 2 portrays variations in conductivities with time for the composite membranes kept at 180 ◦ C under dry N2 , 0.13, 0.26 and 0.42%RH The composite membranes were pre-treated at 30 ◦ C under 60%RH for 3 h before the conductivity measurements. With dry N2 and 0.13%RH, the conductivity of the composite membranes decreases with time. At 0.26 or 0.42%RH, the conductivity is almost constant with retention of temperature at 180 ◦ C. As shown in Fig. 1, Si5 O(PO4 )6 crystals were observed in the composite membranes kept at 180 ◦ C under 0.2% and dry N2 . For that reason, the decrease in conductivity with time under dry N2 and 0.13%RH displayed in Fig. 2 is attributable to formation of Si5 O(PO4 )6 crystals.
Fig. 1 shows XRD patterns for the composites without any pretreatment and pre-treated at 30 ◦ C under 60%RH Data are also shown for the composite with the pre-treatment and kept at 180 ◦ C under 3%, 0.2%RH and dry N2 . In the XRD pattern of as-prepared composite film, diffraction peaks attributable to Si5 O(PO4 )6 are visible. In the preparation of the membranes, they were heat-treated at 150 ◦ C for 3 h and then 180 ◦ C for 3 h. Because it was confirmed that phosphosilicate gel heat-treated at 150 ◦ C was amorphous state, Si5 O(PO4 )6 were presumably crystallized with heat treatment at 180 ◦ C. Keeping the as-prepared composite membranes at 30 ◦ C under 60%RH, the halo pattern is observed. This pattern is attributable to hydrolysis of Si5 O(PO4 )6 crystals with the moisture in the atmosphere. 31 P MAS NMR spectra of the sample showed the formation of H3 PO4 with keeping the as-prepared composite membranes at 30 ◦ C under 60%RH. When the pre-treated composite membranes were kept at 180 ◦ C under 3%RH, the halo pattern was observed, although peaks attributable to Si5 O(PO4 )6 crystals were observed in the composite membranes kept at 180 ◦ C under 0.2% and dry N2 . Consequently, crystallization of Si5 O(PO4 )6 was observed at 180 ◦ C under dry conditions. Crystallization of Si5 O(PO4 )6 from sol–gel derived phosphosilicate gel has been reported by some authors [24,25], but Si5 O(PO4 )6 crystals were only formed with a heat-treatment at temperatures higher than 400 ◦ C
Fig. 2. Variation in conductivities with time for the composite membranes, kept at 180 ◦ C under dry N2 , and with either 0.13, 0.26 or 0.42% RH.
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Fig. 3. Relative-humidity dependence of the ionic conductivity for the composite membranes at 180 ◦ C.
Fig. 3 presents the relative-humidity dependence of conductivity for a composite membrane containing 75 wt.% phosphosilicate gel powders at 180 ◦ C. The membrane was kept at 30 ◦ C under 60%RH before measurements. Conductivity of the composite membrane increases with increased relative humidity up to about 0.4%RH; it is almost constant up to 5%RH. The XRD and conductivity results indicate that formation of Si5 O(PO4 )6 crystals decreases the conductivity, whereas the hydrolysis of Si5 O(PO4 )6 leads to high conductivity. The high conductivity must be brought by the formation of H3 PO4 through hydrolysis. Even under low relative humidity,
re-crystallization of Si5 O(PO4 )6 was inhibited, causing high proton conductivity, as depicted in Fig. 3. As reported previously [23], conductivity for the composite sheet containing 75 wt.% phosphosilicate gel powders under a constant water vapor pressure of 150 mmHg increased with increasing temperatures of 80–150 ◦ C; it was as high as 3.0 × 10−3 S cm−1 at 150 ◦ C [23]. At 180 ◦ C, the water vapor pressure of 150 mmHg corresponds to about 2.0%RH; conductivity is about 3 × 10−3 S cm−1 , as portrayed in Fig. 3. Consequently, water vapor pressure of 150 mmHg is probably sufficient to maintain constant proton conductivity in a wide temperature range up to 180 ◦ C. Fig. 4 shows the cell voltage versus current density with the resulting power density plots for a fuel cell using an MEA which consists of the composite sheet containing 75 wt.% phosphosilicate gel as an electrolyte and Pt-loaded carbon paper sheets as electrodes. Open circles, squares and triangles, respectively represent the current–voltage characteristics of the fuel cells at 30 ◦ C under 75%RH, at 80 ◦ C under 42%RH and at 180 ◦ C under 2.0%RH. Solid circles, squares and triangles, respectively signify the calculated current–power density plots of the fuel cells at 30, 80 and 180 ◦ C. The conductivity of the membrane at 30 ◦ C under 75%RH, and at 80 ◦ C under 42%RH was 3 × 10−3 S cm−1 and 1 × 10−3 S cm−1 , respectively. The open circuit voltage of the cell at 30 and 80 ◦ C is about 0.85 V, whereas the voltage of the cell decreases to about 0.67 V at 180 ◦ C. At temperatures lower than 100 ◦ C, the pores in the composite membranes are closed because of absorbed water, but at temperatures higher than 100 ◦ C, the physically absorbed water evaporates, and the evaporation of water must causes the H2 crossover. At 180 ◦ C under 2%RH, the cell OCV was 0.67 V; power of about 14 mW cm−2 was obtained with current density of 40 mA cm−2 . Impedance spectroscopy measurements of the cells suggest that the small power density in the cell performance measurements is due to the large polarization resistance [26]. Preparation of MEA must be optimized to obtain high cell performance. Nevertheless, the operation at temperatures from 30 to 180 ◦ C in the present study proved that the fuel cells using the MEA consisting of the composite sheet as an electrolyte can be used in the wide temperature range. 4. Conclusions Proton-conductive composite membranes were prepared from phosphosilicate gel powders and a commercially available polyimide precursor. The conductivity of the composite membranes containing 75 wt.% of gel powder was about 2.5 × 10−3 S cm−1 at 180 ◦ C under more than 0.4%RH However, under dry N2 atmosphere at 180 ◦ C, the conductivity decreased with time because of the formation of Si5 O(PO4 )6 crystals. Introduction of water vapor suppressed the crystallization. The OCV of the fuel cell, in which the phosphosilicate gel–polyimide composite sheet was used as electrolyte, at 30 and 80 ◦ C was about 0.85 V. At 180 ◦ C under 2%RH, the cell OCV was 0.67 V, and power density of about 14 mW cm−2 was obtained with current density of 40 mA cm−2 . Therefore, a fuel cell using the composite sheet as an electrolyte was confirmed to operate at temperatures from room temperature to 180 ◦ C. Acknowledgement The present work was supported by a Grant-in-aid for Scientific Research on Priority Area, “Nanoionics (439)” by the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
Fig. 4. Cell voltage versus current density for a test cell at 30 ◦ C under 75%RH, at 80 ◦ C under 42%RH and at 180 ◦ C under 2.0%RH The composite sheet containing 75 wt.% phosphosilicate gel was used as an electrolyte and Pt-loaded carbon paper sheets were used as electrodes.
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