Doped lanthanum ferrite cathode development for use in single-step co-fired solid oxide fuel cells

A major obstacle to the commercialization of solid oxide fuel cells (SOFCs) is the high operating temperature range (800 to 1000°C). Lowering the operating temperature to approximately 600°C allows for cost reduction through the use of inexpensive stack housing and sealing materials, but conventional SOFC cathode materials have high charge transfer resistance at those temperatures which results in poor performance.;This research focused on developing an SOFC cathode material with low charge transfer resistance at low operating temperatures and a porous microstructure that would not impede mass transfer when synthesized using the single-step co-firing process. Towards this goal, mixed ionic and electronic conducting lanthanum ferrite perovskite cathode materials were synthesized using calcium and cerium as dopants. A specific stoichiometry of calcium doped lanthanum ferrite, La0.78Ca0.16FeO3+/-delta (LCF), proved to be a superior cathode compared to state-of-the-art conventional cathode materials across a range of measures.;In order to understand the LCF cathode performance, the defect model structure was determined using thermogravimetric (TGA) measurements, oxygen-ion permeability and four-probe conductivity measurements as a function of temperature and oxygen partial pressure (pO2). The results were analyzed to determine defect concentrations and mobility. The electrochemical performance of LCF was characterized using electrochemical impedance spectroscopy measurements on symmetrical cells which compared favorably to conventional lanthanum manganite cathode materials. Reactivity of LCF with yttria-stabilized zirconia (YSZ) electrolyte was confirmed and prevented using a gadolinium doped ceria (GDC) barrier layer. Microstructural analysis showed evidence of a small amount (2--5 wt%) of secondary phase that precipitated from LCF as a liquid during sintering at approximately 1220°C. The secondary phase was a poor n-type oxide (Ca-Fe-O), present within both the LCF cathode and GDC barrier layer microstructures. In spite of the presence of the liquid phase, LCF symmetrical cells yielded adequate microstructures and satisfactory electrochemical performance.;To understand the reasons for the superior electrochemical performance of LCF, the chemical oxygen ion diffusivity and surface exchange coefficient were determined using conductivity relaxation measurements. Both of these parameters in LCF were found to be an order of magnitude greater than conventional cathode materials.