Fred Bidrawn

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

bKorea Institute of Energy Research, Daejeon 305-343, Korea 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 ,L a0.8Sr0.2FeO3 LSF, and La0.8Ba0.2FeO3 LBF into 65% porous yttria-stabilized zirconia YSZ. 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 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.

Modeling Impedance Response of SOFC Cathodes Prepared by Infiltration

A mathematical model has been developed to understand the performance of electrodes prepared by infiltration of La 0.8 Sr 0.2 FeO 3 (LSF) and La 0.8 Sr 0.2 MnO 3 (LSM) into yttria-stabilized zirconia (YSZ). The model calculates the resistances for the case where perovskite-coated, YSZ fins extend from the electrolyte. Two rate-limiting cases are considered: oxygen ion diffusion through the perovskite film or reactive adsorption of O 2 at the perovskite surface. Adsorption is treated as a reaction between gas-phase O 2 and oxygen vacancies, using equilibrium data. With the exception of the sticking probability, all parameters in the model are experimentally determined. Resistances and capacitances are calculated for LSF-YSZ and there is good agreement with experimental values at 973 K, assuming adsorption is rate limiting, with a sticking probability between 10 -3 and 10 -4 on vacancy sites. According to the model, perovskite ionic conductivity does not limit performance so long as it is above ~10 -7 S/cm. However, the structure of the YSZ scaffold, the ionic conductivity of the scaffold, and the slope of the perovskite redox isotherm significantly impact electrode impedance. Finally, it is shown that characteristic frequencies of the electrode cannot be used to distinguish when diffusion or adsorption is rate-limiting.

Doped-Ceria Diffusion Barriers Prepared by Infiltration for Solid Oxide Fuel Cells

The most commonly used material for cathodes in solid oxide fuel cells is a composite of Sr-doped LaMnO3 LSM and yttriastabilized zirconia YSZ, with the LSM in the composite providing electronic conductivity and catalytic activity for oxygen reduction. The addition of YSZ to the electrode provides ionic conductivity to increase the length of the three-phase boundary by providing ionconducting channels from the electrolyte into the electrode. 1 In most cases, LSM‐YSZ composites are prepared by sintering a mixture of LSM and YSZ powders onto the YSZ electrolyte. Relatively high temperatures 1300 K 2 are required to sinter the YSZ particles in the electrode to the electrolyte. Significantly improved performance can be achieved by replacing LSM with mixed conducting perovskites, such as Sr-doped LaCoO3 LSCo, 3-7 LaFeO3 LSF, 8-12

Fabrication of LSM-YSZ Composite Electrodes by Electrodeposition

Composites of Sr-doped LaMnO 3 (LSM) and yttria-stabilized zirconia (YSZ) were prepared by sequential electrodeposition of La and Mn species from nitrate salts in Dimethyl Sulfoxide (DMSO) into a porous, carbon-coated YSZ substrate, followed by the infiltration of Sr(NO 3 ) 2 . The La and Mn species were uniformly deposited throughout the depth of a 50 μm porous layer to the interface with a dense YSZ electrolyte. The LSM perovskite phase was formed after heating to 1373 K. Solid oxide fuel cell cathodes prepared by single-step electrodeposition showed similar performance to LSM-YSZ electrodes prepared by wet impregnation using many steps.