Mixed conductivity and oxygen surface exchange kinetics of lanthanum-praseodymium doped cerium dioxide

Abstract

The effects of lanthanum doping on the transport properties and oxygen exchange kinetics of Lax(Ce0.9Pr0.1)1−xO2−δ (x = 0, 0.1 and 0.2) have been investigated by electrical conductivity on bulk samples and optical relaxation on thin films respectively. Lanthanum substitution was observed to increase the magnitude of ionic conductivity to values comparable to that of single substituted LaxCe1−xO2−δ, with a maximum for a La content of x = 0.1, reaching σ = 9.5 × 10−3 S·cm−1 at 600 °C and a decrease in activation energy from ≈1 eV for Ce0.9Pr0.1O2−δ down to ≈0.75 eV for the lanthanum substituted compositions. At the same time, a significant electronic contribution to the total conductivity, estimated to be on the order of 10%, supports significant mixed ionic-electronic conductivity.Substitution with lanthanum increases the absolute values of the oxygen surface exchange coefficient from kchem = 6 × 10−7 cm·s−1 for Ce0.9Pr0.1O2−δ to kchem = 1 × 10−6 cm·s−1 for La0.2(Ce0.9Pr0.1)0.8O2−δ at 550 °C. Moreover, one observes significant increases in activation energy from 0.7 eV for Ce0.9Pr0.1O2−δ to ≈1 eV for La substituted compositions, suggesting a change in the rate-limiting step. Possible sources for the change in the rate-limiting step are suggested. The composition with the highest La content of x = 0.2, although not exhibiting the highest ionic conductivity, shows the highest surface exchange rates, is suggested to result from enhanced segregation of catalytically active lanthanum to the surface.