The influence of ionic conductivity on the performance of solid oxide fuel cell cathodes was studied for electrodes prepared by infiltration of
40wt%
normalLa0.8normalCa0.2FenormalO3
(LCF),
normalLa0.8normalSr0.2FenormalO3
(LSF), and
normalLa0.8normalBa0.2FenormalO3
(LBF) into 65% porous yttria-stabilized zirconia (YSZ). The ionic conductivities of LCF, LSF, and LBF, measured between 923 and
1073K
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
1123K
, the performance characteristics of each composite were essentially identical, exhibiting current-independent impedances of
0.2Ωcm2
at
973K
. When the composites were calcined to
1373K
, 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
1373K
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
1373K
.
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 À3 and 10 À4 on vacancy sites. According to the model, perovskite ionic conductivity does not limit performance so long as it is above $10 À7 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.
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