Effect of cation doping on ionic and electronic properties for lanthanum silicate-based solid electrolytes

摘要

Electronic as well as ionic conducting properties for oxyapatite-type solid electrolytes based on lanthanum silicate, La__(9.333 + x)Si_ 6O_(26 + 1.5x) (LSO) were investigated in the oxygen-excess region (x >ca. 0.3). We have found that the oxygen excess-type LSO (OE-LSO), namely La_(10)Si_6O_(27) on weighted basis, exhibited high conductivity, and substitution of the Si-site of LSO with some dopants (Mn+) had a positive effect toward the conducting property. Furthermore, it was also found that addition of a very small amount of iron ions into the M-doped OE-LSO, La_(10)(Si_(6-y)M~(n+)_y)O27-(2-0.5n)y, improved its conductivity. On the other hand, replacement of the La-site with various ions for La_(10)(Si _(6-y)M~(n+)_y)O27-(2-0.5n)y did little to improve conductivity. The electronic transport numbers for Al-doped OE-LSO with Fe-addition, (1-α){La_(10)(Si_(5.8)Al_(0.2))O _(26.9)}-α(FeOγ), evaluated with the Hebb-Wagner polarization method were very low: i.e., 1.1 × 10~(- 3) and 2.9 × 10~(- 3) under P(O2) = 1.1 × 104 Pa at 1073 K for α = 0.00 and 0.005, respectively. Conductivity for each sample was unchanged under humidified atmosphere at 1073 K sustained for over 50 h, revealing that both compositions were chemically stable. It was concluded that 0.995{La10(Si5.8Al0.2)O 26.9}-0.005(FeOγ) is suitable for the fuel cell electrolytes because of its high and almost pure ionic conductivity, and its good chemical stability under humidified as well as reducing conditions.