Adjusting the conduction properties of La0.995Ca0.005NbO4 - δ by doping for proton conducting fuel cells electrode operation

Structural characterization and transport properties of A- (Ce and Pr) and B-site (Mn) doped La0.995Ca0.005- NbO4 (-) (delta) (LCN) have been investigated. Conductivity data have been analyzed in the temperature range from 400 to 800 degrees C as a function of the oxygen partial pressure under dry atmospheres and under different wet atmospheres. All compounds present changes in the activation energy due to a phase transformation at ca. 550 degrees C. Under wet conditions the conductivity is predominantly protonic in all compounds except for the Ce doped sample under oxidizing atmospheres. The conductivity of Ce doped sample is principally electronic at high pO(2) and high temperatures while it becomes more ionic at low pO(2) and temperatures. The combination of n-type electronic and protonic conductivity for Ce doped LCN under reducing atmospheres suggests its potential application as an anode component for LCN-based proton conducting fuel cells (PCFC). Moreover, Pr doped LCN shows remarkable p-type electronic conductivity at high pO(2) in combination with protonic conductivity, which are interesting properties for LCN-based PCFC cathode components. Furthermore, Ce and Pr addition have allowed increasing the sintering activity with respect to LCN powders. The activation energies and the preexponential factors under wet atmospheres have been studied and correlated. It was found that they obey the Meyer-Neldel law and an estimation of the activation energy of protons in both crystal structures has been derived. (C) 2011 Elsevier B.V. All rights reserved.