Cathode Materials for Low Temperature SOFCs
Committee Members: Advisor: Uday Pal, MSE/ME; Srikanth Gopalan, MSE/ME; Soumendra N. Basu, MSE/ME; Vinod Sarin, MSE/ME; Appointed Chair: Adam C. Powell, IV, ME
Abstract: 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 that would have low charge transfer resistance at low operating temperatures and still be able to maintain 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±δ) 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 calcium doped lanthanum ferrite (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 the defect concentrations and mobility.
The overall electrochemical performance of LCF was characterized using electrochemical impedance spectroscopy measurements on symmetrical cells which compared favorably to conventional lanthanum manganite cathode materials. During these measurements, undesirable reactivity of LCF with yttria–stabilized zirconia (YSZ) electrolyte was confirmed and later prevented using a gadolinium doped ceria (GDC) barrier layer. Dilatometry, electron probe microanalysis (EPMA) and transmission electron microscopy (TEM) 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) and was present within both the LCF cathode and GDC barrier layer microstructures. Diffusion of cerium from the GDC barrier layer into the LCF cathode microstructure was also detected and the incorporation of Ce was seen to decrease the electrical conductivity of LCF. However, symmetrical cells with LCF cathode 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 of LCF 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.