Thermal Stability of LaP3O9 and LaP5O14
Proton Conductors
Naoyuki Hatada, Kazuaki Toyoura, Yoshitaro Nose, Tetsuya Uda
Department of Materials Science and Engineering, Kyoto University
Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
[email protected]
In recent years, lanthanum polyphosphate (LaP3O9) and ultraphosphate (LaP5O14)
have received attention because of their potential application as solid electrolytes in
intermediate-temperature fuel cells [1,2]. But it has been reported that they thermally
decompose at high temperatures by the following reactions [3]:
(1)
LaP3O9 → LaPO4 + 1/2 P4O10 (g);
(2)
LaP5O14 → LaP3O9 + 1/2 P4O10 (g).
This makes sintering of these compounds difficult and limits the maximum operating
temperatures for the electrolytes. However, the thermodynamic properties of the
decomposition reactions have not been well characterized. In this work, the thermal
stability of LaP3O9 and LaP5O14 was evaluated
Quartz separator
with a narrow hole
Quartz tube
by means of vapor pressure measurements.
Sample
Vapor pressure measurements were carried
O inlet
O outlet
out using a transpiration apparatus (Fig. 1). A
Pt boat
pellet of LaP3O9 or LaP5O14 (~ 0.3 g) was heated
Heater
Fig.
1
Schematic
illustration of
to a predetermined temperature in the quartz tube
under O2 flow (25 sccm). The vapor pressure of the transpiration apparatus.
P4O10 was calculated from the weight loss of the
T '/ ° C
pellet and the volume of O2 flowed in the tube.
1000
800
600
-3
La
O 25 sccm
P
Fig. 2 shows the temperature dependence of
-4
La O
PO →
-5 L L aP O
+1
the vapor pressure of P4O10. The least-squares fit
aP
/2
-6
O →
PO
+1
parameters are given in Table 1. When pP4O10
-7
/2
−6
PO
was ~10 atm, a significant weight loss was
-8
-9
detected in 50 h; For long-term stability (~40000
0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4
1000T / K
h), the maximum operating temperatures for the
LaP3O9 and LaP5O14 electrolytes would be Fig. 2 Temperature dependence
of the vapor pressure of P4O10.
~600 °C and ~450 °C, respectively.
2
2
5
3
3
14
9
9
4
4
4
10
log (pP O / atm )
2
4
10
10
-1
-1
Table 1 Fit parameters for the temperature dependence of the vapor pressure of P4O10.
log (pP4O10 / atm) = A + B T −1 Temperature
Reaction
Range / K
A
B
(1)
5.30
−1.25×104
1123 – 1323
(2)
5.37
−1.08×104
923 – 1173
References
[1] K. Amezawa et al., Electrochem. Solid-State Lett. 7 (2004) A511-A514.
[2] I. Hammas et al., J. Rare Earths 28 (2010) 321-328.
[3] H. D. Park and E. R. Kreidler, J. Am. Ceram. Soc. 67 (1984) 23-26.