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Studies on Ionic Conduction in Ce<sub>0.95</sub>Eu<sub>0.05</sub>P<sub>2</sub>O<sub>7</sub> at Intermediate Temperatures
Studies on Ionic Conduction in Ce0.95Eu0.05P2O7 at Intermediate Temperatures
Bulletin of the Korean Chemical Society. 2014. May, 35(5): 1465-1468
Copyright © 2014, Korea Chemical Society
  • Received : December 16, 2013
  • Accepted : January 23, 2014
  • Published : May 20, 2014
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About the Authors
Hongtao Wang
Lin Sun
Chunhua Luo
Suhua Fan

Abstract
In this study, an intermediate temperature ionic conductor, Ce 0.95 Eu 0.05 P 2 O 7 , was prepared by solid state reaction. The variation of conductivities with the pressure p H 2 O or time were studied. The highest conductivity of Ce 0.95 Eu 0.05 P 2 O 7 sample was observed in dry air atmosphere at 300 ℃ to be 1.1 × 10 -4 S·cm -1 and in wet air atmosphere ( p H 2 O=7.4 × 10 3 Pa) at 100 ℃ to be 1.4 × 10 -3 S·cm -1 , respectively. The log σ ~ log ( p O 2 ) plot result indicated that Ce 0.95 Eu 0.05 P 2 O 7 was almost a pure ionic conductor under high oxygen partial pressure and a mixed conductor of ion and electron under low oxygen partial pressure.
Keywords
Introduction
Fuel cells operated at intermediate temperatures (100-600 ℃) have been paid much attention since they can increase the operating life of the cells and decrease the risk of delamination of the cell components. 1 - 4 For operation at intermediate temperature, two routes have been adopted. One is to develop the electrolyte membrane. The other more effective route is to search new electrolytes with higher ionic conductivities at intermediate temperature.
Pyrophosphate salts were known to exhibit high protonic conductivities in the 100-400 ℃ temperature range. 5 - 9 Hibino and co-workers have demonstrated that Al 3+ , In 3+ and Mg 2+ doped SnP 2 O 7 have high protonic conductivities of above 10 −2 S·cm −1 under unhumidified conditions in this temperature range. 10 , 11 Ma et al . reported that the highest conductivities were observed for the sample of Sn 0.91 Ga 0.09 P 2 O 7 to be 4.6 × 10 −2 S·cm −1 at 175 ℃ 12 and Sn 0.94 Sc 0.06 P 2 O 7 to be 2.8 × 10 −2 S·cm −1 at 200 ℃ 13 in wet H 2 , respectively. Wang et al . 14 and Tsai et al . 15 reported the conductivities of CeP 2 O 7 kept above 10 −2 S·cm −1 under moist conditions in the intermediate temperature range.
However, the ionic conduction of the reported CeP 2 O 7 -based materials were yet not too clear. The effective ionic radius of Eu 3+ is 0.095 nm, which is close to that of Ce 4+ (0.087 nm) in 6-fold coordination. 16 Therefore, it is interesting to investigate the CeP 2 O 7 with Eu 3+ as dopant. In this paper, Ce 0.95 Eu 0.05 P 2 O 7 was prepared by solid state reaction. The structural characteristics of the sample was analyzed using XRD and SEM. Electrochemical properties were also investigated using some electrochemical methods at intermediate temperatures (100-300 ℃).
Experimental
The initial molar ratios of phosphorus vs. metal ions, P ini /(Ce + Eu), should be controlled to be 2.3 due to a fraction of phosphorus loss in the process by vaporization. 6 12 The Ce 0.95 Eu 0.05 P 2 O 7 was synthesized via the conventional solidstate reaction method. The required amounts of CeO 2 , Eu 2 O 3 (99.99%) and 85% H 3 PO 4 reagents were fully mixed for 0.5 h with an alumina crucible and then held at 300 ℃ while stirring from slurry to a dry paste until a solid mixture was formed. In order to promote the following reaction 7 CeO 2 + 2H 3 PO 4 = CeP 2 O 7 + 3H 2 O ↑ carried on, the resulting mixture heat-treated at 350 ℃ for 1 h until it became yellowish powder. The calcined powder was reground and uniaxially pressed into pellet (diameter 20 mm, thickness 2 mm) under 120 MPa. The pellet annealed at 350 ℃ for 4 h in a muffle furnace to yield the desired sample. The phase structure of the Ce 0.95 Eu 0.05 P 2 O 7 was detected by the X–ray diffraction (XRD) with a Panalytical X′ Pert Pro MPD diffractometer. The microscopic feature of the Ce 0.95 Eu 0.05 P 2 O 7 was performed by a scanning electron microscope (SEM).
For the electrochemical determinations, round plate roughly 19.3 mm in diameter with a thickness of about 1.0 mm was produced. 20%Pd-80%Ag paste was smeared on both sides (area: 0.5 cm 2 ) of the sample which acted as electrodes. The impedance spectra were recorded over the frequency range from 0.1 Hz to 1 MHz with an electrochemical analyzer (CHI660E made in China). The conductivity measurements in dry and wet air atmospheres as a function of temperature or time were performed. For comparison, the CeP 2 O 7 sample was also prepared. The conductivity as the function of oxygen partial pressure ( p O 2 ) was measured in the p O 2 range of 1-10 −20 atm. The p O 2 was accommodated by commixing O 2 , air, N 2 , H 2 O and H 2 in proper ratio, and measured using an oxygen sensor on line.
Results and Discussion
Structure of the Sample. For the ideal cubic structure, the tolerance factor t should equal one. The more t deviates from unity, the less stable the cubic structure. The incorporation of Eu into the CeP 2 O 7 lattice was conducted systematically because the large ionic radius of trivalent Eu is 0.095 nm, which is close to that of Ce 4+ (0.087 nm) in 6-fold coordination. 16 Figure 1 showed XRD pattern at room temperature of the Ce 0.95 Eu 0.05 P 2 O 7 sample. The XRD angles at 17.93°, 20.77°, 23.24°, 25.48°, 29.49°, 34.74°, 36.34°, 37.88°, 42.19°, 46.20°, 47.48°, 52.36°, 55.79°, 61.24°, 64.36° and 65.37° belonged to the (111), (200), (210), (211), (220), (311), (222), (320), (400), (331), (420), (422), (511), (440), (531) and (600) crystal planes of CeP 2 O 7 , respectively.15 As shown, the Ce 0.95 Eu 0.05 P 2 O 7 was in agreement with the cubic phase structure of CeP 2 O 7 in JCPDS 16-0584. Besides, there were some additional peaks of CeO 2 impurity, although the initial P ini /(Ce + Eu) molar ratio was kept at 2.3 to avoid the impurity of CeO 2 . The sample sintered at 350 ℃ in order to minimize the surface-adsorbed or intergranular water species during the conductivity measurements. 6
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XRD pattern of the Ce0.95Eu0.05P2O7 sample.
SEM Image. Typical SEM image of the cross section of the Ce 0.95 Eu 0.05 P 2 O 7 heat-treated at 350 ℃ for 4 h was displayed in Figure 2 . From the SEM image, the sample exhibited a fine uniform microstructure, though there were some pores occur in sample. The relative density of the Ce 0.95 Eu 0.05 P 2 O 7 sample prepared in this study was estimated to be ~82.4%, somewhat lower than those reported for Sr 2+ and Mg 2+ -doped CeP 2 O 7 prepared through similar heat treatments. 6 15
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The cross-sectional SEM image of the Ce0.95Eu0.05P2O7 sample.
Ionic Conductivities in Dry and Wet Air Atmospheres. Figure 3 showed temperature dependence of the conductivities of the Ce 0.95 Eu 0.05 P 2 O 7 and CeP 2 O 7 samples in dry air atmosphere at 100-300 ℃. As can be seen from Figure 3 , one to two orders conductivities of undoped CeP 2 O 7 lower than that of 5 mol % Eu 3+ doped CeP 2 O 7 . This may be explained by the reason described previously in Ref.. 13 Ma et al . 13 reported that the higher Sc 3+ doping level resulted in the higher oxygen vacancy concentration and the effective concentration of oxygen vacancy may reach its maximum value in Sn 0.94 Sc 0.06 P 2 O 7 sample. It is clear that the conductivities of the Sc 3+ doped samples are higher than the conductivities of undoped SnP 2 O 7 . 13 Therefore, the higher conductivities of the Ce 0.95 Eu 0.05 P 2 O 7 are resulted from the substitution of Eu 3+ for Ce 4+ -site which increased the concentration of mobile charge carriers. The highest conductivities were observed for the Ce 0.95 Eu 0.05 P 2 O 7 and CeP 2 O 7 to be 1.1 × 10 −4 S·cm −1 and 4.2 × 10 −6 S·cm −1 at 300 ℃, respectively. However, the conductivities of Ce 0.95 Eu 0.05 P 2 O 7 were much lower than the reported values of CeP 2 O 7 (~10 −2 S·cm −1 ). 14 15 It can be related to large difference in particle size and morphology of SEM image and the synthetic history. 17
Figure 4 showed the variation of conductivities of Ce 0.95 Eu 0.05 P 2 O 7 and CeP 2 O 7 with time during humidification in air at 100 ℃ ( p H 2 O = 3.2 × 10 3 Pa). The humidification was a slow process. For the interpretation of our data, we consider the introduction of hydroxyd ion
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into the samples by the following two reactions. Therefore, the contribution of hole conduction decreased and proton conduction appeared.
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Temperature dependence of conductivities of the Ce0.95Eu0.05P2O7 and CeP2O7 samples in dry air atmosphere at 100-300 ℃.
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The variation of conductivities of Ce0.95Eu0.05P2O7 and CeP2O7 samples with time during humidification in air at 100 ℃ (pH2O = 3.2 × 103 Pa).
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As can be seen from Figure 4 , there was about four orders of magnitude increased in conductivities and higher conductivity level could be achieved after about 50 h to reach a steady state. From this recovery it is concluded that the humidification process not only led to the formation of
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in the samples, as expressed by Eqs. (1) and (2), but also introduced some water species (H 2 O or H 3 O + ) into the samples, which can act as charge carriers and increase in conductivities. 6
Figure 5 showed the variation of conductivities of the Ce 0.95 Eu 0.05 P 2 O 7 sample with time while the p H 2 O pressure changing from 3.2 × 10 3 Pa to 7.4 × 10 3 Pa in air at 100 ℃. Correspondingly, the conductivities were observed from 9.3 × 10 −4 S·cm −1 to 1.4 × 10 −3 S·cm −1 with saturating water vapor increased. The observation can be explained using Eqs. (1) and (2). The increasing p H 2 O pressure shifted the Eqs. (1) and (2) toward the right side, resulting in an increase of the proton concentration in the Ce 0.95 Eu 0.05 P 2 O 7 sample.
In order to investigate the ionic conductivities of the Ce 0.95 Eu 0.05 P 2 O 7 sample, the relationship between the conductivities and the oxygen partial pressure ( p O 2 = 10 −20 ~ 1 atm) was measured. Figure 6 showed the plot of log σ ~ log ( p O 2 ) in wet air atmosphere ( p H 2 O = 3.2 × 10 3 Pa) at 100 ℃. It is clear that the conductivities were almost independent of p O 2 , confirming that Ce 0.95 Eu 0.05 P 2 O 7 was almost a pure ionic conductor at high oxygen partial pressure range. While in hydrogen-containing atmosphere, the protonic conduction becomes dominant as can be seen from Eq. (3). The total conductivities increased with the decreasing oxygen partial pressure indicated that it is a mixed conductor of ion and electron in hydrogen-containing atmosphere. This may be due to the reduction of Ce 4+ to Ce 3+ to a certain extent at low oxygen partial pressure range. In conclusion, Ce 0.95 Eu 0.05 P 2 O 7 is a good intermediate temperature solid ion conductor although its conductivity is low. It has been reported that excess H 3 PO 4 or PmOn layer can increase the sample conductivity by providing additional pathways for proton transport. 10 18 Therefore, it could apply CeP 2 O 7 -based material as electrolyte to the fuel cells by forming composite electrolyte. 18
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Time dependence of conductivities of the Ce0.95Eu0.05P2O7 sample while the pH2O pressure changing from 3.2 × 103 Pa to 7.4 × 103 Pa in air at 100 ℃.
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The conductivities of the Ce0.95Eu0.05P2O7 sample as a function of pO2 in air (pH2O = 3.2 × 103 Pa) at 100 ℃.
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Conclusion
In this paper, an intermediate temperature ionic conductor, Ce 0.95 Eu 0.05 P 2 O 7 , was prepared. The XRD result confirmed that the Ce 0.95 Eu 0.05 P 2 O 7 exhibited cubic phase structure of cerium pyrophosphate. The variation of conductivities with time during humidification process revealed the humidification was a slow process and there was about four orders of magnitude increased in conductivities. The conductivities of Ce 0.95 Eu 0.05 P 2 O 7 increased from 9.3 × 10 −4 S·cm −1 to 1.4 × 10 −3 S·cm −1 while the p H 2 O pressure changing from 3.2 × 10 3 Pa to 7.4 × 10 3 Pa in air at 100 ℃. The log σ ~ log ( p O 2 ) plot result indicated that Ce 0.95 Eu 0.05 P 2 O 7 was almost a pure ionic conductor under high oxygen partial pressure and the protonic conduction became dominant under low oxygen partial pressure.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 21201037), the Natural Science Foundation of the Anhui Higher Education Institutions of China (No.KJ2013B193), and the Specialized Research Fund for the Doctoral Program of Fuyang Teachers College 2012.
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