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Proton-conducting hybrid solid electrolytes for intermediate temperature fuel cells
Solid State Ionics 148 (2002) 607 – 610 www.elsevier.com/locate/ssi
Proton-conducting hybrid solid electrolytes for intermediate temperature fuel cells Hitoshi Nakajima, Itaru Honma * Energy Electronics Institute, National Institute of Advanced Industrial Science and Technology and Japan Science and Technology Corporation, Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan
Abstract A new family of proton-conducting/inorganic hybrid polymer electrolyte membrane was prepared by sol – gel processes. The method involves stabilization of metastable acidic tungsten oxide cluster in the polymer composite membrane. A freestanding, flexible and pale-yellowish transparent membrane was formed from tungsten acidic homogeneous solution by sol – gel condensation with alkoxysilylated polyethylene oxide (PEO). The membrane showed very high protonic conductivity of 10 2 – 10 3 S cm 1 from R.T. to 120 jC, especially, 1.4 10 2 S cm 1 at 80 jC. The structure of tungsten cluster moieties was neither peroxo complex nor WO32H2O. It is suggested that nano-size clusters are dispersed homogeneously in polymer matrix without long-range ordering. Tungsten complexes with carboxylate ligands such as malonic acid could form similar homogeneous membrane. Thermal and ionic transport properties could be controlled by this family of organic derivatives. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Fuel cell; Proton; Polyethylene oxide
1. Introduction A protonic conductive membrane is a very important element for polymer electrolyte fuel cells, while it is also applicable for proton sensor, separation, acidic catalyst, and so on. Recently, incorporation of inorganic solid acids in polymer has been attempted for ionic conductive materials [1 –4]. For example, hydrated acidic oxides, such as WO32H2O and Sb2O5nH2O, are well-known for their high protonic conduction. However, as these kinds of oxides have little solubility for water or * Corresponding author. Electrochemical Laboratory, Energy Division, AIST, MITI, Tsukuba, Ibaraki 305, Japan. Tel.: +81-29854-5797; fax: +81-298-54-5805. E-mail address: [email protected] (I. Honma).
organic solvents, homogeneous mixture into organic polymer is difficult, and the preparation of homogeneous membrane also becomes difficult. Twelve phosphotungstic acid is known as a solid oxide cluster which has both high protonic conductivity and high solubility for water and alcohol, and many groups are used as doping inorganic solid acids. However, not many of these kinds of crystals are obtained in protonic form until now. Many solid acid complex or oxide clusters are crystallized only in alkali – metal or (alkyl) ammonium salt and not crystallized in protonic form because of its charge balance, ionic radius, and so on, so that they are stable only in the solution. In this work, we investigated the preparation of solid protonic conductive membrane by stabilizing metastable inorganic acidic clusters in the polymer
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 1 2 7 - 3
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H. Nakajima, I. Honma / Solid State Ionics 148 (2002) 607–610
membrane and investigated the organic/inorganic hybrid structure and protonic conductivities in the temperature range from R.T. to 160 jC, which potentially provides more efficient high-temperature operation of PEM fuel cells.
2. Experimental Tungstic acid (1 g) was dissolved in 30 wt.% hydrogen peroxide (2 g) at 40 jC. After 2 h, hydrogen peroxide was decomposed by Pt net (Pt black was supported) and this solution was filtered (A). Then, 30 wt.% hydrogen peroxide was added again to adjust the concentration of hydrogen peroxide to 5 wt.%, and a pale yellow clear solution was obtained (B). The whole species in this solution were not completely identified, though one of the species was tungsten peroxo complex, [W2O3(O2)4(H2O)2]2 , estimated by 183W NMR [5]. For example, this cluster was crystallized only as Na salt and none was obtained by H form crystal until now. Tungstic acidic complex solution with carboxylate ligands was also prepared as follows: oxalic acid dihydrate was dissolved in solution (A). Tungsten oxalate chelate complex was suggested to be formed in this solution by Raman spectroscopy [6]. Malonic acid, dimethylmalonic acid, and citric acid were also used to prepare chelate ligand metal complex. Organic polymers of polyethylene oxide (PEO) have been cross-linked with alkoxysilanes through isocyanato coupling. These precursors, hereafter designated as ‘‘hybrid precursors’’, hydrolyzed and condensed form macromolecules of flexible, glassy hybrid materials. In this study, PEO600 was dissolved in 10 ml of methanol. Solution (B) was added dropwise with icecooled bath. Adequate amount of water was added to the solution and casted into a polystylene substrate and annealed at 60 jC for 18 –36 h. Proton conductivity of the membranes were evaluated by the a.c. impedance method between 1 Hz and 1 MHz using an impedance analyzer (Solartron SI1260 and Solartron SI-1287) in a stainless pressure resistant vessel from R.T. to 160 jC. Water was previously introduced in the vessel and measurement was operated always under equilibrium water pressure at desired temperature.
3. Results and discussion 3.1. Conductivity of membrane from tungsten acidic homogeneous solution First, we prepared a membrane with following conditions: PEO600 was used as hybrid polymer, molar ratio of [W]/[PEO] = 0.25 and [H 2 O]/ [PEO] = 2.6 102. Organic/inorganic hybrid membranes of a freestanding, flexible and pale-yellowish transparent membrane (1) was formed from tungsten acidic homogeneous solution (B). Thickness of the membrane was about 0.3 mm. On the other hand, for comparison, the membrane prepared from the precursor WO32H2O was directly added into methanol solution of hybrid precursor (2) was heterogeneous and not transparent. Conductivity of (1) and (2) from R.T. to 160 jC were shown in Fig. 1. The hybrid membrane (1) showed very high protonic conductivity of 10 2 – 10 3 S cm 1 from R.T. to 120 jC, especially, 1.4 10 2 S cm 1 at 80 jC. On the other hand, (2) did not show a conductivity excess of 10 3 S cm 1. It was shown that the tungstic acidic cluster has been stabilized in the polymer matrix and the resulting membrane has good protonic conductivity, even at 140 jC it has 10 4 S cm 1. 3.2. Structure of tungsten acidic clusters in membrane The infrared red spectra of (1) and (2) were shown in Fig. 2. The bands at 586, 816 and 976 cm 1 were observed only in (1) and not observed in the other membrane. These three bands are not assigned to the polymer or silica matrix. The bands at 586 and 816 cm 1 are assigned to v(WOW) due to corner-sharing WO6 units and those at 976 cm 1 are assigned to v(WO) [7]. No bands except weak bands due to polymer or silica matrix were observed in the region from 870 to 900 cm 1. Bands assigned to v(OO) due to peroxo group coordinated to metal and to v(WOW) due to edge-sharing WO6 units both to be strongly appeared in this wave number region. Peroxo complexes, formed in solution (B) as mentioned above, were transformed to non-peroxo tungsten cluster moieties during the synthesis process. The WO6 octahedral units in these tungsten cluster moieties were not bonded by edge-sharing but by corner-shared. Nevertheless,
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Fig. 2. IR spectra of (a) membrane prepared from tungsten homogeneous complex (1), (b) membrane prepared from WO32H2O powder (2), (c) membrane without tungsten species, and (d) WO32H2O powder.
Fig. 1. Conductivity of membranes (R.H. = 100%), (a) prepared from tungsten homogeneous solution (1), (b) prepared from WO32H2O powder (2).
tungsten acidic clusters and choosing the combination of polymer to acidic clusters. The thermal stability of the clusters was suggested to be improved by organic derivatives such as carboxylato ligands. So in the present study, we have checked the cluster stability
WO32H2O, known by corner-shared protonic tungsten oxide, showed IR bands at 1007, 945, 918 and 700 cm 1, different from those of (1), indicating that these tungsten cluster moieties were not WO32H2O. In addition, the bands at 586, 816 and 976 cm 1 were sharp, suggesting that these cluster were molecularitic nano-size clusters rather than crystal. The powder XRD pattern for (1) and (2) were shown in Fig. 3. No sharp diffraction peaks were observed in (1), suggesting those tungsten cluster moieties were fixed in polymer matrix without longrange ordering. 3.3. Effect of carboxylate ligands Next, we attempted some improvement of the membrane by producing organic derivative of the
Fig. 3. XRD spectra of (a) membrane prepared from tungsten homogeneous complex, (b) membrane prepared from WO32H2O powder, and (c) WO32H2O powder.
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H. Nakajima, I. Honma / Solid State Ionics 148 (2002) 607–610
conductivity at 140 jC was about three times higher than that of the sample without ligands, indicating positive effects of malonic acid to the conductivity in higher temperature range. The sample with a malonic acid to tungsten ratio of 4 showed a maximum conductivity in the experimental temperature range. The XRD pattern of these membrane showed that all samples can be assigned to the amorphous phase. It indicated that even if malonic acid to tungsten ratio is 4, neither tungsten trioxide nor malonic acid is crystallized in the membrane.
4. Conclusion
Fig. 4. Effect of carboxylic ligand on conductivity of membrane.
and conductivity by systematic variation among carboxylic acid, oxalic acid, malonic acid, and so on. We have checked the effect of different carboxylato ligands for protonic conduction among oxalic acid, citric acid, malonic acid, and dimethylmalonic acid. The result of conductivity is shown in Fig. 4. Malonic acid sample showed higher conductivity than the other, suggesting these kinds of ligand were effective for protonic conduction. The dependence of conductivity on the ratio of malonic acid to tungsten was examined. In the case where the ratio of malonic acid to tungsten was less than or equal to 0.5, no significant increase of conductivity was observed. However, in the case where the ratio of malonic acid to tungsten was one, the
A freestanding, flexible and pale-yellowish transparent membrane was formed from tungsten acidic homogeneous solution. The membrane showed very high protonic conductivity of 10 2 – 10 3 S cm 1 from R.T. to 120 jC, especially, 1.4 10 2 S cm 1 at 80 jC. The structure of tungsten cluster moieties was neither peroxo complex nor WO32H2O. It is suggested that nano-size clusters are dispersed homogeneously in polymer matrix without long-range ordering. Thermal and ionic transport properties could be controlled by organic derivatives, such as malonic acid, to clusters.
References [1] [2] [3] [4] [5] [6] [7]
S. Malhotra, et al., J. Electrochem. Soc. 144 (1997) L23. J.M. Amarilla, et al., Solid State Ionics 127 (2000) 133. N. Groselj, et al., Solid State Ionics 125 (1999) 125. I. Honma, et al., J. Membr. Sci. 185 (2001) 83. H. Nakajima, et al., Chem. Lett. (1997) 693. H. Nakajima, et al., Chem. Mater. 11 (1999) 691. M.F. Daniel, et al., J. Solid State Chem. 67 (1987) 235.
Proton-conducting hybrid solid electrolytes for intermediate temperature fuel cells Hitoshi Nakajima, Itaru Honma * Energy Electronics Institute, National Institute of Advanced Industrial Science and Technology and Japan Science and Technology Corporation, Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan
Abstract A new family of proton-conducting/inorganic hybrid polymer electrolyte membrane was prepared by sol – gel processes. The method involves stabilization of metastable acidic tungsten oxide cluster in the polymer composite membrane. A freestanding, flexible and pale-yellowish transparent membrane was formed from tungsten acidic homogeneous solution by sol – gel condensation with alkoxysilylated polyethylene oxide (PEO). The membrane showed very high protonic conductivity of 10 2 – 10 3 S cm 1 from R.T. to 120 jC, especially, 1.4 10 2 S cm 1 at 80 jC. The structure of tungsten cluster moieties was neither peroxo complex nor WO32H2O. It is suggested that nano-size clusters are dispersed homogeneously in polymer matrix without long-range ordering. Tungsten complexes with carboxylate ligands such as malonic acid could form similar homogeneous membrane. Thermal and ionic transport properties could be controlled by this family of organic derivatives. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Fuel cell; Proton; Polyethylene oxide
1. Introduction A protonic conductive membrane is a very important element for polymer electrolyte fuel cells, while it is also applicable for proton sensor, separation, acidic catalyst, and so on. Recently, incorporation of inorganic solid acids in polymer has been attempted for ionic conductive materials [1 –4]. For example, hydrated acidic oxides, such as WO32H2O and Sb2O5nH2O, are well-known for their high protonic conduction. However, as these kinds of oxides have little solubility for water or * Corresponding author. Electrochemical Laboratory, Energy Division, AIST, MITI, Tsukuba, Ibaraki 305, Japan. Tel.: +81-29854-5797; fax: +81-298-54-5805. E-mail address: [email protected] (I. Honma).
organic solvents, homogeneous mixture into organic polymer is difficult, and the preparation of homogeneous membrane also becomes difficult. Twelve phosphotungstic acid is known as a solid oxide cluster which has both high protonic conductivity and high solubility for water and alcohol, and many groups are used as doping inorganic solid acids. However, not many of these kinds of crystals are obtained in protonic form until now. Many solid acid complex or oxide clusters are crystallized only in alkali – metal or (alkyl) ammonium salt and not crystallized in protonic form because of its charge balance, ionic radius, and so on, so that they are stable only in the solution. In this work, we investigated the preparation of solid protonic conductive membrane by stabilizing metastable inorganic acidic clusters in the polymer
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 1 2 7 - 3
608
H. Nakajima, I. Honma / Solid State Ionics 148 (2002) 607–610
membrane and investigated the organic/inorganic hybrid structure and protonic conductivities in the temperature range from R.T. to 160 jC, which potentially provides more efficient high-temperature operation of PEM fuel cells.
2. Experimental Tungstic acid (1 g) was dissolved in 30 wt.% hydrogen peroxide (2 g) at 40 jC. After 2 h, hydrogen peroxide was decomposed by Pt net (Pt black was supported) and this solution was filtered (A). Then, 30 wt.% hydrogen peroxide was added again to adjust the concentration of hydrogen peroxide to 5 wt.%, and a pale yellow clear solution was obtained (B). The whole species in this solution were not completely identified, though one of the species was tungsten peroxo complex, [W2O3(O2)4(H2O)2]2 , estimated by 183W NMR [5]. For example, this cluster was crystallized only as Na salt and none was obtained by H form crystal until now. Tungstic acidic complex solution with carboxylate ligands was also prepared as follows: oxalic acid dihydrate was dissolved in solution (A). Tungsten oxalate chelate complex was suggested to be formed in this solution by Raman spectroscopy [6]. Malonic acid, dimethylmalonic acid, and citric acid were also used to prepare chelate ligand metal complex. Organic polymers of polyethylene oxide (PEO) have been cross-linked with alkoxysilanes through isocyanato coupling. These precursors, hereafter designated as ‘‘hybrid precursors’’, hydrolyzed and condensed form macromolecules of flexible, glassy hybrid materials. In this study, PEO600 was dissolved in 10 ml of methanol. Solution (B) was added dropwise with icecooled bath. Adequate amount of water was added to the solution and casted into a polystylene substrate and annealed at 60 jC for 18 –36 h. Proton conductivity of the membranes were evaluated by the a.c. impedance method between 1 Hz and 1 MHz using an impedance analyzer (Solartron SI1260 and Solartron SI-1287) in a stainless pressure resistant vessel from R.T. to 160 jC. Water was previously introduced in the vessel and measurement was operated always under equilibrium water pressure at desired temperature.
3. Results and discussion 3.1. Conductivity of membrane from tungsten acidic homogeneous solution First, we prepared a membrane with following conditions: PEO600 was used as hybrid polymer, molar ratio of [W]/[PEO] = 0.25 and [H 2 O]/ [PEO] = 2.6 102. Organic/inorganic hybrid membranes of a freestanding, flexible and pale-yellowish transparent membrane (1) was formed from tungsten acidic homogeneous solution (B). Thickness of the membrane was about 0.3 mm. On the other hand, for comparison, the membrane prepared from the precursor WO32H2O was directly added into methanol solution of hybrid precursor (2) was heterogeneous and not transparent. Conductivity of (1) and (2) from R.T. to 160 jC were shown in Fig. 1. The hybrid membrane (1) showed very high protonic conductivity of 10 2 – 10 3 S cm 1 from R.T. to 120 jC, especially, 1.4 10 2 S cm 1 at 80 jC. On the other hand, (2) did not show a conductivity excess of 10 3 S cm 1. It was shown that the tungstic acidic cluster has been stabilized in the polymer matrix and the resulting membrane has good protonic conductivity, even at 140 jC it has 10 4 S cm 1. 3.2. Structure of tungsten acidic clusters in membrane The infrared red spectra of (1) and (2) were shown in Fig. 2. The bands at 586, 816 and 976 cm 1 were observed only in (1) and not observed in the other membrane. These three bands are not assigned to the polymer or silica matrix. The bands at 586 and 816 cm 1 are assigned to v(WOW) due to corner-sharing WO6 units and those at 976 cm 1 are assigned to v(WO) [7]. No bands except weak bands due to polymer or silica matrix were observed in the region from 870 to 900 cm 1. Bands assigned to v(OO) due to peroxo group coordinated to metal and to v(WOW) due to edge-sharing WO6 units both to be strongly appeared in this wave number region. Peroxo complexes, formed in solution (B) as mentioned above, were transformed to non-peroxo tungsten cluster moieties during the synthesis process. The WO6 octahedral units in these tungsten cluster moieties were not bonded by edge-sharing but by corner-shared. Nevertheless,
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Fig. 2. IR spectra of (a) membrane prepared from tungsten homogeneous complex (1), (b) membrane prepared from WO32H2O powder (2), (c) membrane without tungsten species, and (d) WO32H2O powder.
Fig. 1. Conductivity of membranes (R.H. = 100%), (a) prepared from tungsten homogeneous solution (1), (b) prepared from WO32H2O powder (2).
tungsten acidic clusters and choosing the combination of polymer to acidic clusters. The thermal stability of the clusters was suggested to be improved by organic derivatives such as carboxylato ligands. So in the present study, we have checked the cluster stability
WO32H2O, known by corner-shared protonic tungsten oxide, showed IR bands at 1007, 945, 918 and 700 cm 1, different from those of (1), indicating that these tungsten cluster moieties were not WO32H2O. In addition, the bands at 586, 816 and 976 cm 1 were sharp, suggesting that these cluster were molecularitic nano-size clusters rather than crystal. The powder XRD pattern for (1) and (2) were shown in Fig. 3. No sharp diffraction peaks were observed in (1), suggesting those tungsten cluster moieties were fixed in polymer matrix without longrange ordering. 3.3. Effect of carboxylate ligands Next, we attempted some improvement of the membrane by producing organic derivative of the
Fig. 3. XRD spectra of (a) membrane prepared from tungsten homogeneous complex, (b) membrane prepared from WO32H2O powder, and (c) WO32H2O powder.
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H. Nakajima, I. Honma / Solid State Ionics 148 (2002) 607–610
conductivity at 140 jC was about three times higher than that of the sample without ligands, indicating positive effects of malonic acid to the conductivity in higher temperature range. The sample with a malonic acid to tungsten ratio of 4 showed a maximum conductivity in the experimental temperature range. The XRD pattern of these membrane showed that all samples can be assigned to the amorphous phase. It indicated that even if malonic acid to tungsten ratio is 4, neither tungsten trioxide nor malonic acid is crystallized in the membrane.
4. Conclusion
Fig. 4. Effect of carboxylic ligand on conductivity of membrane.
and conductivity by systematic variation among carboxylic acid, oxalic acid, malonic acid, and so on. We have checked the effect of different carboxylato ligands for protonic conduction among oxalic acid, citric acid, malonic acid, and dimethylmalonic acid. The result of conductivity is shown in Fig. 4. Malonic acid sample showed higher conductivity than the other, suggesting these kinds of ligand were effective for protonic conduction. The dependence of conductivity on the ratio of malonic acid to tungsten was examined. In the case where the ratio of malonic acid to tungsten was less than or equal to 0.5, no significant increase of conductivity was observed. However, in the case where the ratio of malonic acid to tungsten was one, the
A freestanding, flexible and pale-yellowish transparent membrane was formed from tungsten acidic homogeneous solution. The membrane showed very high protonic conductivity of 10 2 – 10 3 S cm 1 from R.T. to 120 jC, especially, 1.4 10 2 S cm 1 at 80 jC. The structure of tungsten cluster moieties was neither peroxo complex nor WO32H2O. It is suggested that nano-size clusters are dispersed homogeneously in polymer matrix without long-range ordering. Thermal and ionic transport properties could be controlled by organic derivatives, such as malonic acid, to clusters.
References [1] [2] [3] [4] [5] [6] [7]
S. Malhotra, et al., J. Electrochem. Soc. 144 (1997) L23. J.M. Amarilla, et al., Solid State Ionics 127 (2000) 133. N. Groselj, et al., Solid State Ionics 125 (1999) 125. I. Honma, et al., J. Membr. Sci. 185 (2001) 83. H. Nakajima, et al., Chem. Lett. (1997) 693. H. Nakajima, et al., Chem. Mater. 11 (1999) 691. M.F. Daniel, et al., J. Solid State Chem. 67 (1987) 235.