Mixed ionic-electronic conduction in Ni doped lanthanum gallate perovskites

Lanthanum gallate is a promising material for "monolithic" fuel cells or oxygen pumps, i.e. one in which the electrolyte and electrodes are formed from a common phase. We have investigated La1-xSrxGa1-yNiO3 (LSGN(x-y)) with x=0.1 and y=0.2 and 0.5 as a potential cathode material for such an electrochemical device. The sigma(PO2,T) for LSGN(10-20) points to a p-type electronic conductivity at high PO2 and predominantly ionic conductivity at low PO2. LSGN(10-50) has an electronic conductivity suitable for SOFC applications of approximately 50 S/cm in air at high temperature. AC impedance spectroscopy on an electron blocking cell of the form M/LSG/LSGN/LSG/M was used to isolate the ionic conductivity in the LSGN(10-20) material. The ionic conductivity was found to have a similar magnitude and activation energy to that of undoped LSG material with sigma(i)= 0.12 S/cm at 800 degrees C and E-A= 1.0 +/- 0.1 eV. Thermal expansion measurements on the LSGN materials were characterized as a function of temperature and dopant level and were found to match that of the electrolyte under opeating conditions.