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Composite effects of silicon pyrophosphate as a supporting matrix for CsH5(PO4)2 electrolytes at intermediate temperatures
Electrochimica Acta 51 (2006) 3719–3723
Composite effects of silicon pyrophosphate as a supporting matrix for CsH5(PO4)2 electrolytes at intermediate temperatures Toshiaki Matsui ∗ , Tomokazu Kukino, Ryuji Kikuchi, Koichi Eguchi ∗ Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Received 2 September 2005; received in revised form 14 October 2005; accepted 18 October 2005 Available online 21 November 2005
Abstract The proton-conductive electrolytes of CsH5 (PO4 )2 /SiP2 O7 composites were synthesized, and composite effects of silicon pyrophosphate as a supporting matrix at intermediate temperatures were investigated by comparing the properties of CsH5 (PO4 )2 /SiO2 composite. Although both composites showed similar thermal stability, the temperature dependence of the conductivity was quite different each other; the conductivity of the composite electrolyte of CsH5 (PO4 )2 /SiP2 O7 was about one-order magnitude higher at every temperature investigated and the maximum conductivity achieved was 116 mS cm−1 at 230 ◦ C. These results suggested that the interfacial interaction between the proton-conductor phase of CsH5 (PO4 )2 and the matrix of SiP2 O7 played an important role in the proton conduction mechanism. © 2005 Elsevier Ltd. All rights reserved. Keywords: Intermediate-temperature fuel cells; Proton conductor; Oxo-acid salt; Phosphate; Composite
1. Introduction Fuel cells are attractive energy conversion devices because of the high efficiencies and low emissions, and many studies have been conducted. Among them, solid state fuel cells operative at 200–300 ◦ C are promising technology since they combine many advantages of high and low temperature fuel cells. However, they have not been developed sufficiently because of the absence of good ionic-conductors with high thermal stability at intermediate temperatures. The proton-conductive solid acid salts, such as CsH2 PO4 and CsHSO4 , are well known as one of the promising materials for intermediate temperature applications [1–7]. It is noted that these materials undergo the phase transition to the superionic phase and exhibit high conductivity of ∼10−2 S cm−1 . However, their conductivity is somewhat lower than that requested for the practical use and the operating temperature range is narrow because of the phase transition. The addition of oxides as matrices has been studied as an effective method for the improvement in the ionic conductivity [8–12], and it was clarified that the low con-
∗
Corresponding authors. Tel.: +81 75 383 2523; fax: +81 75 383 2520. E-mail addresses: [email protected] (T. Matsui), [email protected] (K. Eguchi). 0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.10.026
ductivity phase was remarkably affected by the surface area, pore sizes, and the molar ratio of oxides. However, the notable results at intermediate temperatures have not been achieved so far. Recently, good compatibility of pyrophosphates as the matrix for ammonium polyphosphate electrolytes has been reported [13]. The composite electrolytes of NH4 PO3 /TiP2 O7 showed high proton conductivity up to 300 ◦ C as a result of interaction at the proton-conductor phase/pyrophosphate interface. In the previous paper, we synthesized a novel proton-conductive electrolyte of CsH2 PO4 /SiP2 O7 -based composite and reported their electrochemical properties [14,15]. In this composite, during heat-treatment at intermediate temperatures, the composition of CsH2 PO4 chemically reacted with a part of SiP2 O7 matrix, and then the new ionic-conduction phase of CsH5 (PO4 )2 was formed. The resulting composite was in the solid states at elevated temperature despite the melting of CsH5 (PO4 )2 and showed high proton conductivity of 44 mS cm−1 at 266 ◦ C under 30% H2 O/Ar atmosphere. However, the proton-conduction mechanism has not been elucidated sufficiently. Then, in this study, we aimed to investigate composite effects of silicon pyrophosphate as a supporting matrix for CsH5 (PO4 )2 electrolytes. The composite electrolytes of CsH5 (PO4 )2 /SiP2 O7 and CsH5 (PO4 )2 /SiO2 were synthesized, and their thermal stability and electrochemical properties were compared at intermediate temperatures.
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2. Experimental Cesium pentahydrogen diphosphate, CsH5 (PO4 )2 , was synthesized from Cs2 CO3 (Wako Pure Chemical Industries, guaranteed reagent) and H3 PO4 (Wako Pure Chemical Industries, guaranteed reagent) as starting materials. An aqueous solution of H3 PO4 –Cs2 CO3 was dried overnight at 100 ◦ C. The X-ray diffraction pattern of the resulting CsH5 (PO4 )2 agreed well with the literature data [16], and by-products were not observed. The matrix of silicon pyrophosphate, SiP2 O7 , was prepared from SiO2 (Nihon Silica, hydrophilic) and H3 PO4 [14], and the formation with the mixed phase of Forms II and III was clarified by XRD [17–19]. Composite electrolytes of CsH5 (PO4 )2 /SiP2 O7 and CsH5 (PO4 )2 /SiO2 were prepared from the resulting powders and SiO2 (Nihon Silica, hydrophilic) in various molar ratios. The mixtures were ball-milled for 24 h and pressed into pellets (diameter: 10 mm, thickness: 1–2 mm). The structures of composite electrolytes were examined using in situ high-temperature X-ray diffraction (Rigaku RINT 1400 X-ray diffractometer). Thermogravimetric and differential thermal analysis were conducted to investigate the thermal stability (SII Nano Technology Inc., EXSTAR6000 TG/DTA 6300). The density of components was measured by Archimedes’ method (Quntachrome, Ultrapycnometer 1000). For electrochemical measurements, pelletized samples with Pt/C electrodes (E-TEK, polymer electrolyte fuel cell, 1.0 mg cm−2 ) were used. Proton conductivity was measured at atmospheric pressure using AC impedance spectroscopy (Solartron 1260 frequency response analyzer and Solartron 1287 potentiostat). The applied frequency was in the range of 0.1 Hz–1 MHz with a voltage amplitude of 30 mV. The measurement was conducted at 110–270 ◦ C in heating process under dry Ar and Ar–H2 O atmospheres. Gaseous mixtures of 10 and 30% H2 O/Ar were supplied by bubbling Ar through water kept at 46 and 70 ◦ C, respectively. The sample was kept for 30 min at each temperature until the steady state was achieved.
Fig. 1. High-temperature XRD patterns of as-prepared CsH5 (PO4 )2 /SiO2 composite (molar ratio: 1/2) in air.
changes above 160 ◦ C regardless of the difference in the matrix. The TG and DTA profiles of both composites are shown in Fig. 3. Each composite exhibited similar DTA profile with two large endothermic peaks at ca. 150 and 200 ◦ C. Taking into account the results of XRD analysis and the melting point of the pure CsH5 (PO4 )2 (at ca. 150 ◦ C), the former peak should be attributed to the melting process of CsH5 (PO4 )2 in the composite. For each composite, the peak broadening and the decrease in a melting point of few 10-degrees were also observed compared to the pure CsH5 (PO4 )2 . On the other hand, the latter peak accompanied with relatively large weight loss should be
3. Results and discussion The high-temperature X-ray diffraction patterns of asprepared CsH5 (PO4 )2 /SiO2 (molar ratio: 1/2) and CsH5 (PO4 )2 / SiP2 O7 (1/2) in air are shown in Figs. 1 and 2, respectively. Each composite was heated at a rate of 5 ◦ C min−1 and then kept for 5 min before the measurement. At 30 ◦ C, the XRD pattern of CsH5 (PO4 )2 /SiO2 composite was quite identical to that of CsH5 (PO4 )2 . Therefore, the matrix of SiO2 should be in amorphous state, and no by-products were formed during ball-milling. At elevated temperature up to 130 ◦ C, the same diffraction lines were confirmed. Above 160 ◦ C, however, a broad halo pattern was observed accompanied with the disappearance of CsH5 (PO4 )2 . On the other hand, the as-prepared CsH5 (PO4 )2 /SiP2 O7 composite consisted of CsH5 (PO4 )2 and SiP2 O7 phases, and the same diffraction lines were observed up to 130 ◦ C. However, as is the case with CsH5 (PO4 )2 /SiO2 composite, the lines assigned to CsH5 (PO4 )2 disappeared with no change of SiP2 O7 above 160 ◦ C. Thus, the state of CsH5 (PO4 )2 in the composite
Fig. 2. High-temperature XRD patterns of as-prepared CsH5 (PO4 )2 /SiP2 O7 composite (molar ratio: 1/2) in air.
T. Matsui et al. / Electrochimica Acta 51 (2006) 3719–3723
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Fig. 3. TG and DTA profiles of CsH5 (PO4 )2 /SiO2 (molar ratio: 1/2) and CsH5 (PO4 )2 /SiP2 O7 (molar ratio: 1/2) composite in N2 flow at a heating rate of 10 ◦ C min−1 .
attributed to the dehydration and condensation of phosphates. However, it was noted that these two composites showed different TG profiles, especially at intermediate temperatures. The weight of CsH5 (PO4 )2 /SiO2 composite drastically decreased above 120 ◦ C, and the total weight loss up to 390 ◦ C was about twice larger than that of CsH5 (PO4 )2 /SiP2 O7 composite. Then, to investigate the thermal stability of phosphates in the composite, the weight change per total weight of CsH5 (PO4 )2 was calculated using the data above 120 ◦ C, assuming that the dehydration and condensation take place only in CsH5 (PO4 )2 . The calculated values were ca. 11 and 8.6% for CsH5 (PO4 )2 /SiO2 composite and CsH5 (PO4 )2 /SiP2 O7 composite, respectively. Consequently, it was suggested that the state of CsH5 (PO4 )2 in the composite was almost independent on the matrix at intermediate temperatures. Temperature dependence of the conductivity for composites under 30% H2 O/Ar atmosphere is shown in Fig. 4. For comparison, the result of pure SiP2 O7 is also shown. In the case of CsH5 (PO4 )2 /SiO2 composite, the conductivity showed no temperature-dependence above 150 ◦ C. This anomalous behavior implies that conductivity depends on the humidity since the relative humidity decreases with an increase in the temperature. On the other hand, the composite electrolytes of CsH5 (PO4 )2 /SiP2 O7 showed quite different behavior. Their conductivity was about one-order of magnitude higher than that of CsH5 (PO4 )2 /SiO2 composite at every temperature investigated, and the maximum conductivity attained was about 116 mS cm−1 at 230 ◦ C. Moreover, the conductivity showed non-linear behavior against the reciprocal temperature and slightly decreased at elevated temperature depending on the molar ratio of CsH5 (PO4 )2 . These results indicate that CsH5 (PO4 )2 is responsible for the high conductivity and SiP2 O7 serves as a supporting matrix. Accordingly, the decrease in the conductivity should be attributed to the dehydration and the condensation of
Fig. 4. Temperature dependence of the conductivity for CsH5 (PO4 )2 /SiO2 and CsH5 (PO4 )2 /SiP2 O7 composites under 30% H2 O/Ar atmosphere.
CsH5 (PO4 )2 , resulting in a decrease in the carrier concentration. What makes the remarkable difference in the conductivity between CsH5 (PO4 )2 /SiP2 O7 composite and CsH5 (PO4 )2 /SiO2 composite? Weight and volume ratio of each component in the composite (molar ratio: 1/2) are summarized in Table 1. The calculated molar concentration of CsH5 (PO4 )2 was almost the same value for each composite. Assuming that only the conduction phase of CsH5 (PO4 )2 serves as a proton carrier, the conductivity should be almost the same value regardless of the difference in the matrix. In this case, however, the result conflicted with this prediction. The conductivity of SiP2 O7 itself was a few mS cm−1 and decreased drastically above 200 ◦ C. The XRD patterns of CsH5 (PO4 )2 /SiP2 O7 composites after measurements were consistent with that of as-prepared composite at room temperature. Accordingly, the matrix of SiP2 O7 is stable in this experimental condition and should not serve as a proton carrier at intermediate temperatures. It would be expected that the difference in the conductivity originated from the interfacial interaction between the proton-conductor phase and the matrix, resulting in the difference in the proton mobility. This may be a useful guide for the realization of intermediate-temperature fuel cells. Throughout these experiments, the conductor phase of CsH5 (PO4 )2 showed good compatibility with the matrix of SiP2 O7 . Then, the thermal stability and water resistance of Table 1 Weight and volume ratio of each component in the composite
CsH5 (PO4 )2 /SiP2 O7 CsH5 (PO4 )2 /SiO2
Molar ratio
Weight ratio
Volume ratio
0.50 0.50
0.81 2.7
1.7 2.5
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conduction mechanism. On the other hand, under a dry atmosphere, the gradual weight loss with time was observed because of the dehydration and condensation of phosphates, resulting in the continuous decrease in the conductivity. Accordingly, at intermediate temperatures, humidity conditions are required to maintain the proton conductivity. Since the conductivity and the mechanical strength are the trade-off relationship in this system, controlling the composition ratio of ionic-conductor phase to matrix and the humidity is very important for the development of the intermediate-temperature fuel cells employing CsH5 (PO4 )2 /SiP2 O7 composite electrolytes. 4. Conclusions
Fig. 5. Time course of weight change for CsH5 (PO4 )2 /SiP2 O7 composite (molar ratio: 1/2) at 220 ◦ C under dry and wet atmospheres.
CsH5 (PO4 )2 /SiP2 O7 composite (molar ratio: 1/2) were studied. Time course of weight change and conductivity were measured at 220 ◦ C under dry and wet atmospheres, and the results are shown in Figs. 5 and 6, respectively. The relative humidity of 10 and 30% H2 O/Ar gaseous mixture at 220 ◦ C is 0.4 and 1.3%, respectively. Under 10% H2 O/Ar atmosphere, the weight of the composite was gradually increased up to ca. 0.7%, although the conductivity remained unchanged for 10 h. Furthermore, the conductivity strongly depended on the humidity. It was clarified that the composite electrolyte absorbs water in equilibrium with the ambient humidity even under low relative humidity atmosphere, which also plays an important role in the proton-
The proton-conductive electrolytes of CsH5 (PO4 )2 /SiP2 O7 composites were synthesized, and composite effects of silicon pyrophosphate as a supporting matrix at intermediate temperatures were investigated by comparing the properties of CsH5 (PO4 )2 /SiO2 composite. The proton-conductor phase of CsH5 (PO4 )2 showed a good compatibility with a matrix of SiP2 O7 , resulting in high proton-conductivity. It was suggested that these unique properties originated from the interfacial interaction between the conductor phase and the matrix, which may be the useful guide for the development of intermediatetemperature fuel cells. Moreover, it was clarified that the absorbed water also played an important role in the proton conduction even under low relative humidity atmosphere because of the hygroscopicity. However, this property has the possibility of bringing about the lowering of mechanical strength. Therefore, controlling the composition and humidity is required in the actual operation of fuel cells. These results indicate that the CsH5 (PO4 )2 /SiP2 O7 composites are promising electrolytes for use in intermediate-temperature fuel cells. Acknowledgement This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan and also by a Grant-in-Aid for the 21st COE program—COE for a United Approach to New Materials Science—from the Ministry of Education, Culture, Sports, Science, and Technology. References
Fig. 6. Time dependence of the conductivity for CsH5 (PO4 )2 /SiP2 O7 composite (molar ratio: 1/2) at 220 ◦ C under dry and wet atmospheres.
[1] S.M. Haile, D.A. Boysen, C.R.I. Chisholm, R.B. Merle, Nature 410 (2001) 910. [2] A.I. Baranov, B.V. Merinov, A.V. Tregubchenko, V.P. Khiznichenko, L.A. Shuvalov, N.M. Schagina, Solid State Ionics 36 (1989) 279. [3] W. Bronowska, J. Chem. Phys. 114 (2001) 611. [4] D.A. Boysen, T. Uda, C.R.I. Chisholm, S.M. Haile, Science 303 (2004) 68. [5] K.-S. Lee, J. Phys. Chem. Solid 57 (1996) 333. [6] B.M. Nirsha, E.N.G. Gudinitsa, A.A. Fakeev, V.A. Efremov, B.V. Zhadanov, V.A. Olikova, Russ. J. Inorg. Chem. 27 (1982) 770. [7] T. Norby, M. Friesel, B.E. Mellander, Solid State Ionics 77 (1995) 105. [8] J. Otomo, N. Minagawa, C.-J. Wen, K. Eguchi, H. Takahashi, Solid State Ionics 156 (2003) 357. [9] V.G. Ponomareva, G.V. Lavrova, L.G. Simonova, Solid State Ionics 118 (1999) 317.
T. Matsui et al. / Electrochimica Acta 51 (2006) 3719–3723 [10] V.G. Ponomareva, G.V. Lavroba, Solid State Ionics 145 (2001) 197. [11] H. Shigeoka, J. Otomo, C. Wen, M. Ogura, H. Takahashi, J. Electrochem. Soc. 151 (2004) J76. [12] S. Wang, J. Otomo, M. Ogura, C. Wen, H. Nagamoto, H. Takahashi, Solid State Ionics 176 (2005) 755. [13] T. Matsui, S. Takeshita, Y. Iriyama, T. Abe, M. Inaba, Z. Ogumi, J. Electrochem. Soc. 152 (2005) A167.
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[14] T. Matsui, T. Kukino, R. Kikuchi, K. Eguchi, Electrochem. Solid State Lett. 8 (2005) A256. [15] T. Matsui, T. Kukino, R. Kikuchi, K. Eguchi, J. Electrochem. Soc., in press. [16] JCPDS 24-0252. [17] H. Makart, Helv. Chim. Acta 50 (1967) 399. [18] JCPDS 25-0755. [19] JCPDS 22-1320.
Composite effects of silicon pyrophosphate as a supporting matrix for CsH5(PO4)2 electrolytes at intermediate temperatures Toshiaki Matsui ∗ , Tomokazu Kukino, Ryuji Kikuchi, Koichi Eguchi ∗ Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Received 2 September 2005; received in revised form 14 October 2005; accepted 18 October 2005 Available online 21 November 2005
Abstract The proton-conductive electrolytes of CsH5 (PO4 )2 /SiP2 O7 composites were synthesized, and composite effects of silicon pyrophosphate as a supporting matrix at intermediate temperatures were investigated by comparing the properties of CsH5 (PO4 )2 /SiO2 composite. Although both composites showed similar thermal stability, the temperature dependence of the conductivity was quite different each other; the conductivity of the composite electrolyte of CsH5 (PO4 )2 /SiP2 O7 was about one-order magnitude higher at every temperature investigated and the maximum conductivity achieved was 116 mS cm−1 at 230 ◦ C. These results suggested that the interfacial interaction between the proton-conductor phase of CsH5 (PO4 )2 and the matrix of SiP2 O7 played an important role in the proton conduction mechanism. © 2005 Elsevier Ltd. All rights reserved. Keywords: Intermediate-temperature fuel cells; Proton conductor; Oxo-acid salt; Phosphate; Composite
1. Introduction Fuel cells are attractive energy conversion devices because of the high efficiencies and low emissions, and many studies have been conducted. Among them, solid state fuel cells operative at 200–300 ◦ C are promising technology since they combine many advantages of high and low temperature fuel cells. However, they have not been developed sufficiently because of the absence of good ionic-conductors with high thermal stability at intermediate temperatures. The proton-conductive solid acid salts, such as CsH2 PO4 and CsHSO4 , are well known as one of the promising materials for intermediate temperature applications [1–7]. It is noted that these materials undergo the phase transition to the superionic phase and exhibit high conductivity of ∼10−2 S cm−1 . However, their conductivity is somewhat lower than that requested for the practical use and the operating temperature range is narrow because of the phase transition. The addition of oxides as matrices has been studied as an effective method for the improvement in the ionic conductivity [8–12], and it was clarified that the low con-
∗
Corresponding authors. Tel.: +81 75 383 2523; fax: +81 75 383 2520. E-mail addresses: [email protected] (T. Matsui), [email protected] (K. Eguchi). 0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.10.026
ductivity phase was remarkably affected by the surface area, pore sizes, and the molar ratio of oxides. However, the notable results at intermediate temperatures have not been achieved so far. Recently, good compatibility of pyrophosphates as the matrix for ammonium polyphosphate electrolytes has been reported [13]. The composite electrolytes of NH4 PO3 /TiP2 O7 showed high proton conductivity up to 300 ◦ C as a result of interaction at the proton-conductor phase/pyrophosphate interface. In the previous paper, we synthesized a novel proton-conductive electrolyte of CsH2 PO4 /SiP2 O7 -based composite and reported their electrochemical properties [14,15]. In this composite, during heat-treatment at intermediate temperatures, the composition of CsH2 PO4 chemically reacted with a part of SiP2 O7 matrix, and then the new ionic-conduction phase of CsH5 (PO4 )2 was formed. The resulting composite was in the solid states at elevated temperature despite the melting of CsH5 (PO4 )2 and showed high proton conductivity of 44 mS cm−1 at 266 ◦ C under 30% H2 O/Ar atmosphere. However, the proton-conduction mechanism has not been elucidated sufficiently. Then, in this study, we aimed to investigate composite effects of silicon pyrophosphate as a supporting matrix for CsH5 (PO4 )2 electrolytes. The composite electrolytes of CsH5 (PO4 )2 /SiP2 O7 and CsH5 (PO4 )2 /SiO2 were synthesized, and their thermal stability and electrochemical properties were compared at intermediate temperatures.
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2. Experimental Cesium pentahydrogen diphosphate, CsH5 (PO4 )2 , was synthesized from Cs2 CO3 (Wako Pure Chemical Industries, guaranteed reagent) and H3 PO4 (Wako Pure Chemical Industries, guaranteed reagent) as starting materials. An aqueous solution of H3 PO4 –Cs2 CO3 was dried overnight at 100 ◦ C. The X-ray diffraction pattern of the resulting CsH5 (PO4 )2 agreed well with the literature data [16], and by-products were not observed. The matrix of silicon pyrophosphate, SiP2 O7 , was prepared from SiO2 (Nihon Silica, hydrophilic) and H3 PO4 [14], and the formation with the mixed phase of Forms II and III was clarified by XRD [17–19]. Composite electrolytes of CsH5 (PO4 )2 /SiP2 O7 and CsH5 (PO4 )2 /SiO2 were prepared from the resulting powders and SiO2 (Nihon Silica, hydrophilic) in various molar ratios. The mixtures were ball-milled for 24 h and pressed into pellets (diameter: 10 mm, thickness: 1–2 mm). The structures of composite electrolytes were examined using in situ high-temperature X-ray diffraction (Rigaku RINT 1400 X-ray diffractometer). Thermogravimetric and differential thermal analysis were conducted to investigate the thermal stability (SII Nano Technology Inc., EXSTAR6000 TG/DTA 6300). The density of components was measured by Archimedes’ method (Quntachrome, Ultrapycnometer 1000). For electrochemical measurements, pelletized samples with Pt/C electrodes (E-TEK, polymer electrolyte fuel cell, 1.0 mg cm−2 ) were used. Proton conductivity was measured at atmospheric pressure using AC impedance spectroscopy (Solartron 1260 frequency response analyzer and Solartron 1287 potentiostat). The applied frequency was in the range of 0.1 Hz–1 MHz with a voltage amplitude of 30 mV. The measurement was conducted at 110–270 ◦ C in heating process under dry Ar and Ar–H2 O atmospheres. Gaseous mixtures of 10 and 30% H2 O/Ar were supplied by bubbling Ar through water kept at 46 and 70 ◦ C, respectively. The sample was kept for 30 min at each temperature until the steady state was achieved.
Fig. 1. High-temperature XRD patterns of as-prepared CsH5 (PO4 )2 /SiO2 composite (molar ratio: 1/2) in air.
changes above 160 ◦ C regardless of the difference in the matrix. The TG and DTA profiles of both composites are shown in Fig. 3. Each composite exhibited similar DTA profile with two large endothermic peaks at ca. 150 and 200 ◦ C. Taking into account the results of XRD analysis and the melting point of the pure CsH5 (PO4 )2 (at ca. 150 ◦ C), the former peak should be attributed to the melting process of CsH5 (PO4 )2 in the composite. For each composite, the peak broadening and the decrease in a melting point of few 10-degrees were also observed compared to the pure CsH5 (PO4 )2 . On the other hand, the latter peak accompanied with relatively large weight loss should be
3. Results and discussion The high-temperature X-ray diffraction patterns of asprepared CsH5 (PO4 )2 /SiO2 (molar ratio: 1/2) and CsH5 (PO4 )2 / SiP2 O7 (1/2) in air are shown in Figs. 1 and 2, respectively. Each composite was heated at a rate of 5 ◦ C min−1 and then kept for 5 min before the measurement. At 30 ◦ C, the XRD pattern of CsH5 (PO4 )2 /SiO2 composite was quite identical to that of CsH5 (PO4 )2 . Therefore, the matrix of SiO2 should be in amorphous state, and no by-products were formed during ball-milling. At elevated temperature up to 130 ◦ C, the same diffraction lines were confirmed. Above 160 ◦ C, however, a broad halo pattern was observed accompanied with the disappearance of CsH5 (PO4 )2 . On the other hand, the as-prepared CsH5 (PO4 )2 /SiP2 O7 composite consisted of CsH5 (PO4 )2 and SiP2 O7 phases, and the same diffraction lines were observed up to 130 ◦ C. However, as is the case with CsH5 (PO4 )2 /SiO2 composite, the lines assigned to CsH5 (PO4 )2 disappeared with no change of SiP2 O7 above 160 ◦ C. Thus, the state of CsH5 (PO4 )2 in the composite
Fig. 2. High-temperature XRD patterns of as-prepared CsH5 (PO4 )2 /SiP2 O7 composite (molar ratio: 1/2) in air.
T. Matsui et al. / Electrochimica Acta 51 (2006) 3719–3723
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Fig. 3. TG and DTA profiles of CsH5 (PO4 )2 /SiO2 (molar ratio: 1/2) and CsH5 (PO4 )2 /SiP2 O7 (molar ratio: 1/2) composite in N2 flow at a heating rate of 10 ◦ C min−1 .
attributed to the dehydration and condensation of phosphates. However, it was noted that these two composites showed different TG profiles, especially at intermediate temperatures. The weight of CsH5 (PO4 )2 /SiO2 composite drastically decreased above 120 ◦ C, and the total weight loss up to 390 ◦ C was about twice larger than that of CsH5 (PO4 )2 /SiP2 O7 composite. Then, to investigate the thermal stability of phosphates in the composite, the weight change per total weight of CsH5 (PO4 )2 was calculated using the data above 120 ◦ C, assuming that the dehydration and condensation take place only in CsH5 (PO4 )2 . The calculated values were ca. 11 and 8.6% for CsH5 (PO4 )2 /SiO2 composite and CsH5 (PO4 )2 /SiP2 O7 composite, respectively. Consequently, it was suggested that the state of CsH5 (PO4 )2 in the composite was almost independent on the matrix at intermediate temperatures. Temperature dependence of the conductivity for composites under 30% H2 O/Ar atmosphere is shown in Fig. 4. For comparison, the result of pure SiP2 O7 is also shown. In the case of CsH5 (PO4 )2 /SiO2 composite, the conductivity showed no temperature-dependence above 150 ◦ C. This anomalous behavior implies that conductivity depends on the humidity since the relative humidity decreases with an increase in the temperature. On the other hand, the composite electrolytes of CsH5 (PO4 )2 /SiP2 O7 showed quite different behavior. Their conductivity was about one-order of magnitude higher than that of CsH5 (PO4 )2 /SiO2 composite at every temperature investigated, and the maximum conductivity attained was about 116 mS cm−1 at 230 ◦ C. Moreover, the conductivity showed non-linear behavior against the reciprocal temperature and slightly decreased at elevated temperature depending on the molar ratio of CsH5 (PO4 )2 . These results indicate that CsH5 (PO4 )2 is responsible for the high conductivity and SiP2 O7 serves as a supporting matrix. Accordingly, the decrease in the conductivity should be attributed to the dehydration and the condensation of
Fig. 4. Temperature dependence of the conductivity for CsH5 (PO4 )2 /SiO2 and CsH5 (PO4 )2 /SiP2 O7 composites under 30% H2 O/Ar atmosphere.
CsH5 (PO4 )2 , resulting in a decrease in the carrier concentration. What makes the remarkable difference in the conductivity between CsH5 (PO4 )2 /SiP2 O7 composite and CsH5 (PO4 )2 /SiO2 composite? Weight and volume ratio of each component in the composite (molar ratio: 1/2) are summarized in Table 1. The calculated molar concentration of CsH5 (PO4 )2 was almost the same value for each composite. Assuming that only the conduction phase of CsH5 (PO4 )2 serves as a proton carrier, the conductivity should be almost the same value regardless of the difference in the matrix. In this case, however, the result conflicted with this prediction. The conductivity of SiP2 O7 itself was a few mS cm−1 and decreased drastically above 200 ◦ C. The XRD patterns of CsH5 (PO4 )2 /SiP2 O7 composites after measurements were consistent with that of as-prepared composite at room temperature. Accordingly, the matrix of SiP2 O7 is stable in this experimental condition and should not serve as a proton carrier at intermediate temperatures. It would be expected that the difference in the conductivity originated from the interfacial interaction between the proton-conductor phase and the matrix, resulting in the difference in the proton mobility. This may be a useful guide for the realization of intermediate-temperature fuel cells. Throughout these experiments, the conductor phase of CsH5 (PO4 )2 showed good compatibility with the matrix of SiP2 O7 . Then, the thermal stability and water resistance of Table 1 Weight and volume ratio of each component in the composite
CsH5 (PO4 )2 /SiP2 O7 CsH5 (PO4 )2 /SiO2
Molar ratio
Weight ratio
Volume ratio
0.50 0.50
0.81 2.7
1.7 2.5
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conduction mechanism. On the other hand, under a dry atmosphere, the gradual weight loss with time was observed because of the dehydration and condensation of phosphates, resulting in the continuous decrease in the conductivity. Accordingly, at intermediate temperatures, humidity conditions are required to maintain the proton conductivity. Since the conductivity and the mechanical strength are the trade-off relationship in this system, controlling the composition ratio of ionic-conductor phase to matrix and the humidity is very important for the development of the intermediate-temperature fuel cells employing CsH5 (PO4 )2 /SiP2 O7 composite electrolytes. 4. Conclusions
Fig. 5. Time course of weight change for CsH5 (PO4 )2 /SiP2 O7 composite (molar ratio: 1/2) at 220 ◦ C under dry and wet atmospheres.
CsH5 (PO4 )2 /SiP2 O7 composite (molar ratio: 1/2) were studied. Time course of weight change and conductivity were measured at 220 ◦ C under dry and wet atmospheres, and the results are shown in Figs. 5 and 6, respectively. The relative humidity of 10 and 30% H2 O/Ar gaseous mixture at 220 ◦ C is 0.4 and 1.3%, respectively. Under 10% H2 O/Ar atmosphere, the weight of the composite was gradually increased up to ca. 0.7%, although the conductivity remained unchanged for 10 h. Furthermore, the conductivity strongly depended on the humidity. It was clarified that the composite electrolyte absorbs water in equilibrium with the ambient humidity even under low relative humidity atmosphere, which also plays an important role in the proton-
The proton-conductive electrolytes of CsH5 (PO4 )2 /SiP2 O7 composites were synthesized, and composite effects of silicon pyrophosphate as a supporting matrix at intermediate temperatures were investigated by comparing the properties of CsH5 (PO4 )2 /SiO2 composite. The proton-conductor phase of CsH5 (PO4 )2 showed a good compatibility with a matrix of SiP2 O7 , resulting in high proton-conductivity. It was suggested that these unique properties originated from the interfacial interaction between the conductor phase and the matrix, which may be the useful guide for the development of intermediatetemperature fuel cells. Moreover, it was clarified that the absorbed water also played an important role in the proton conduction even under low relative humidity atmosphere because of the hygroscopicity. However, this property has the possibility of bringing about the lowering of mechanical strength. Therefore, controlling the composition and humidity is required in the actual operation of fuel cells. These results indicate that the CsH5 (PO4 )2 /SiP2 O7 composites are promising electrolytes for use in intermediate-temperature fuel cells. Acknowledgement This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan and also by a Grant-in-Aid for the 21st COE program—COE for a United Approach to New Materials Science—from the Ministry of Education, Culture, Sports, Science, and Technology. References
Fig. 6. Time dependence of the conductivity for CsH5 (PO4 )2 /SiP2 O7 composite (molar ratio: 1/2) at 220 ◦ C under dry and wet atmospheres.
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