Studies on Ionic Conduction in Ce_0.95Eu_0.05P₂O₇ at Intermediate Temperatures

Abstract

In this study, an intermediate temperature ionic conductor, $Ce_{0.95}Eu_{0.05}P_2O_7$, was prepared by solid state reaction. The variation of conductivities with the pressure $pH_2O$ or time were studied. The highest conductivity of $Ce_{0.95}Eu_{0.05}P_2O_7$ sample was observed in dry air atmosphere at $300^{\circ}C$ to be $1.1{\times}10^{-4}S{\cdot}cm^{-1}$ and in wet air atmosphere ($pH_2O=7.4{\times}10^3Pa$) at $100^{\circ}C$ to be $1.4{\times}10^{-3}S{\cdot}cm^{-1}$, respectively. The log ${\sigma}$ ~ log ($pO_2$) plot result indicated that $Ce_{0.95}Eu_{0.05}P_2O_7$ was almost a pure ionic conductor under high oxygen partial pressure and a mixed conductor of ion and electron under low oxygen partial pressure.

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 Al3+, In3+ and Mg2+ doped SnP2O7 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 Sn0.91Ga0.09P2O7 to be 4.6 × 10−2 S·cm−1 at 175 ℃12 and Sn0.94Sc0.06P2O7 to be 2.8 × 10−2 S·cm−1 at 200 ℃13 in wet H2, respectively. Wang et al.14 and Tsai et al.15 reported the conductivities of CeP2O7 kept above 10−2 S·cm−1 under moist conditions in the intermediate temperature range.

However, the ionic conduction of the reported CeP2O7-based materials were yet not too clear. The effective ionic radius of Eu3+ is 0.095 nm, which is close to that of Ce4+ (0.087 nm) in 6-fold coordination.16 Therefore, it is interesting to investigate the CeP2O7 with Eu3+ as dopant. In this paper, Ce0.95Eu0.05P2O7 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, Pini/(Ce + Eu), should be controlled to be 2.3 due to a fraction of phosphorus loss in the process by vaporization.6,12 The Ce0.95Eu0.05P2O7 was synthesized via the conventional solidstate reaction method. The required amounts of CeO2, Eu2O3 (99.99%) and 85% H3PO4 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 reaction7 CeO2 + 2H3PO4 = CeP2O7 + 3H2O ↑ 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 Ce0.95Eu0.05P2O7 was detected by the X–ray diffraction (XRD) with a Panalytical X′ Pert Pro MPD diffractometer. The microscopic feature of the Ce0.95Eu0.05P2O7 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 cm2) 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 CeP2O7 sample was also prepared. The conductivity as the function of oxygen partial pressure (pO2) was measured in the pO2 range of 1-10−20 atm. The pO2 was accommodated by commixing O2, air, N2, H2O and H2 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 CeP2O7 lattice was conducted systematically because the large ionic radius of trivalent Eu is 0.095 nm, which is close to that of Ce4+ (0.087 nm) in 6-fold coordination.16 Figure 1 showed XRD pattern at room temperature of the Ce0.95Eu0.05P2O7 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 CeP2O7, respectively.15 As shown, the Ce0.95Eu0.05P2O7 was in agreement with the cubic phase structure of CeP2O7 in JCPDS 16-0584. Besides, there were some additional peaks of CeO2 impurity, although the initial Pini/(Ce + Eu) molar ratio was kept at 2.3 to avoid the impurity of CeO2. The sample sintered at 350 ℃ in order to minimize the surface-adsorbed or intergranular water species during the conductivity measurements.6

Figure 1.XRD pattern of the Ce0.95Eu0.05P2O7 sample.

SEM Image. Typical SEM image of the cross section of the Ce0.95Eu0.05P2O7 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 Ce0.95Eu0.05P2O7 sample prepared in this study was estimated to be ~82.4%, somewhat lower than those reported for Sr2+ and Mg2+-doped CeP2O7 prepared through similar heat treatments.6,15

Figure 2.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 Ce0.95Eu0.05P2O7 and CeP2O7 samples in dry air atmosphere at 100-300 ℃. As can be seen from Figure 3, one to two orders conductivities of undoped CeP2O7 lower than that of 5 mol % Eu3+ doped CeP2O7. This may be explained by the reason described previously in Ref..13 Ma et al.13 reported that the higher Sc3+ doping level resulted in the higher oxygen vacancy concentration and the effective concentration of oxygen vacancy may reach its maximum value in Sn0.94Sc0.06P2O7 sample. It is clear that the conductivities of the Sc3+ doped samples are higher than the conductivities of undoped SnP2O7.13 Therefore, the higher conductivities of the Ce0.95Eu0.05P2O7 are resulted from the substitution of Eu3+ for Ce4+-site which increased the concentration of mobile charge carriers. The highest conductivities were observed for the Ce0.95Eu0.05P2O7 and CeP2O7 to be 1.1 × 10−4 S·cm−1 and 4.2 × 10−6 S·cm−1 at 300 ℃, respectively. However, the conductivities of Ce0.95Eu0.05P2O7 were much lower than the reported values of CeP2O7 (~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 Ce0.95Eu0.05P2O7 and CeP2O7 with time during humidification in air at 100 ℃ (pH2O = 3.2 × 103 Pa). The humidification was a slow process. For the interpretation of our data, we consider the introduction of hydroxyd ion into the samples by the following two reactions. Therefore, the contribution of hole conduction decreased and proton conduction appeared.

Figure 3.Temperature dependence of conductivities of the Ce0.95Eu0.05P2O7 and CeP2O7 samples in dry air atmosphere at 100-300 ℃.

Figure 4.The variation of conductivities of Ce0.95Eu0.05P2O7 and CeP2O7 samples with time during humidification in air at 100 ℃ (pH2O = 3.2 × 103 Pa).

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 in the samples, as expressed by Eqs. (1) and (2), but also introduced some water species (H2O or H3O+) into the samples, which can act as charge carriers and increase in conductivities.6

Figure 5 showed the variation of conductivities of the Ce0.95Eu0.05P2O7 sample with time while the pH2O pressure changing from 3.2 × 103 Pa to 7.4 × 103 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 pH2O pressure shifted the Eqs. (1) and (2) toward the right side, resulting in an increase of the proton concentration in the Ce0.95Eu0.05P2O7 sample.

In order to investigate the ionic conductivities of the Ce0.95Eu0.05P2O7 sample, the relationship between the conductivities and the oxygen partial pressure (pO2 = 10−20 ~ 1 atm) was measured. Figure 6 showed the plot of log σ ~ log (pO2) in wet air atmosphere (pH2O = 3.2 × 103 Pa) at 100 ℃. It is clear that the conductivities were almost independent of pO2, confirming that Ce0.95Eu0.05P2O7 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 Ce4+ to Ce3+ to a certain extent at low oxygen partial pressure range. In conclusion, Ce0.95Eu0.05P2O7 is a good intermediate temperature solid ion conductor although its conductivity is low. It has been reported that excess H3PO4 or PmOn layer can increase the sample conductivity by providing additional pathways for proton transport.10,18 Therefore, it could apply CeP2O7-based material as electrolyte to the fuel cells by forming composite electrolyte.18

Figure 5.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 ℃.

Figure 6.The conductivities of the Ce0.95Eu0.05P2O7 sample as a function of pO2 in air (pH2O = 3.2 × 103 Pa) at 100 ℃.

 

Conclusion

In this paper, an intermediate temperature ionic conductor, Ce0.95Eu0.05P2O7, was prepared. The XRD result confirmed that the Ce0.95Eu0.05P2O7 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 Ce0.95Eu0.05P2O7 increased from 9.3 × 10−4 S·cm−1 to 1.4 × 10−3 S·cm−1 while the pH2O pressure changing from 3.2 × 103 Pa to 7.4 × 103 Pa in air at 100 ℃. The log σ ~ log (pO2) plot result indicated that Ce0.95Eu0.05P2O7 was almost a pure ionic conductor under high oxygen partial pressure and the protonic conduction became dominant under low oxygen partial pressure.