The Effect of Ca, Sr, and Ba Doping on the Ionic Conductivity and Cathode Performance of LaFeO3

The influence of ionic conductivity on the performance of solid oxide fuel cell cathodes was studied for electrodes prepared by infiltration of 40 wt % La0.8Ca0.2FeO3 (LCF) La0.8Sr0.2FeO3 (LSF) and La0.8Ba0.2FeO3 (LBF) into 65% porous yttria-stabilized zirconia (LSZ). The ionic conductivities of LCF, LSF, and LBF, measured between 923 and 1073 K using permeation rates in a membrane reactor, showed that LSF exhibited the highest ionic conductivities, followed by LBF and LCF. When electrodes were calcined to 1123 K, the performance characteristics of each composite were essentially identical, exhibiting current-independent impedances of 0.2 Ω cm2 at 973 K. When the composites were calcined to 1373 K, the open-circuit impedances were much larger and showed a strong dependence on current density. The open-circuit impedances followed the ionic conductivities, with LSF– YSZ electrodes showing the lowest impedance and LCF–YSZ electrodes the highest. Scanning electron microscopy images and Brunauer–Emmett–Teller surface areas indicate that calcination at 1373 K causes the perovskites to form dense layers over the YSZ pores. A model is proposed in which diffusion of ions through the perovskite film limits the performance of the composite electrodes calcined at 1373 K.