Factors Governing the Performance and Stability of Solid Oxide Fuel Cell Cathodes Prepared by Infiltration

Infiltration method, developed at the University of Pennsylvania, is a unique analytical platform for investigating the effect of material properties and electrode microstructure on the performance of solid oxide fuel cell (SOFC) electrodes. During cell fabrication by infiltration, the ion-conducting electrolyte phase is sintered first, followed by the addition of the catalytically active perovskite phase into the pores of the electrolyte. The use of separate sintering steps for the electrolyte and the active phase gives one a high degree of control over the microstructure of both phases, unattainable with traditional fabrication methods. In this thesis, the infiltration approach has been used to conduct a systematic investigation into the factors that govern the performance and stability of solid oxide fuel cell cathodes. As a result, a number of microstructural and material properties, crucial for obtaining high electrode activity, were identified. In particular, the effect of varying the ionic conductivity of the porous electrolyte, the specific surface area of the perovskite as well as the specific surface area of the porous electrolyte, and the effect of solid-state reactions between the two phases were studied and were found to significantly affect performance. The experimental findings agreed well with the predictions of a mathematical model that was developed to describe the electrochemical characteristics of SOFC composite cathodes. Both theoretical and experimental evidence suggests the performance of SOFC cathodes prepared by infiltration is limited by slow oxygen adsorption on the perovskite surface. The chemical composition of the perovskite surface therefore plays an important role in determining the overall performance of the electrode. The last chapter of this thesis introduces a novel method that may allow one to characterize the active sites on the perovskite surface under SOFC cathode operating conditions (600-700°C, ambient air atmosphere, polarization), unattainable with traditional surface characterization techniques

Effect of the Ionic Conductivity of the Electrolyte in Composite SOFC Cathodes

Solid oxide fuel cell (SOFC) cathodes were prepared by infiltration of 35 wt % La0.8Sr0.2FeO3 (LSF) into porous scaffolds of three, zirconia-based electrolytes in order to determine the effect of the ionic conductivity of the electrolyte material on cathode impedances. The electrolyte scaffolds were 10 mol % Sc2O3-stabilized zirconia (ScSZ), 8 mol % Y2O3-stabilized zirconia (YSZ), and 3 mol % Y2O3- 20 mol % Al2O3-doped zirconia (YAZ), prepared by tape casting with graphite pore formers. Each electrolyte scaffold was 65% porous, with identical pore structures as determined by scanning electron microscopy (SEM). Both symmetric cells and fuel cells were prepared and tested between 873 and 1073 K, using LSF composites that had been calcined to 1123 or 1373 K. Literature values for the electrolyte conductivities were confirmed using the ohmic losses from the impedance spectra. The electrode impedances decreased with increasing electrolyte conductivity, with the dependence being between to the power of 0.5 and 1.0, depending on the operating temperature and LSF calcination temperature

Systematic Studies of the Cathode-Electrolyte Interface in SOFC Cathodes Prepared by Infiltration

In this study, the effect of the morphology and ionic conductivity of the electrolyte material in SOFC composite cathodes is systematically studied. The specific surface area of prous yttria-stabilized zirconia (YSZ) scaffolds was varied by almost two orders of magnitude using different pore formers and surface treatment with hydrofluoric acid (HF). The effect of ionic conductivity on the performance of SOFC cathodes was studied for electrodes prepared by infiltration of 35 wt % LSF into 65% porous scandia-stabilized zirconia (ScSZ), YSZ, or yttria-alumina co-stabilized zirconia (YAZ) scaffolds of identical microstructure cathodes

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

To stabilize solid oxide fuel cells cathodes prepared by infiltration of La0.8Sr0.2CoO3 (LSCo) into porous yttria-stabilized zirconia (YSZ), a coating of Sm-doped ceria (SDC) was first deposited onto the YSZ scaffold. The dense SDC coating was prepared by infiltration with aqueous solutions of SM(NO3)3 and Ce(NO3)3, followed by calcination to 1473 K. The SDC coating prevented ~ 20 mΩ cm2, at 973 K, with acceptable degradation after heating to 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 La0.8Sr0.2FeO3 (LSF) and La0.8Sr0.2MnO3 (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 O2 at the perovskite surface. Adsorption is treated as a reaction between gas-phase O2 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

Atomic Layer Deposition on Porous Materials: Problems with Conventional Approaches to Catalyst and Fuel Cell Electrode Preparation

Atomic layer deposition (ALD) offers exciting possibilities for controlling the structure and composition of surfaces on the atomic scale in heterogeneous catalysts and solid oxide fuel cell (SOFC) electrodes. However, while ALD procedures and equipment are well developed for applications involving flat surfaces, the conditions required for ALD in porous materials with a large surface area need to be very different. The materials (e.g., rare earths and other functional oxides) that are of interest for catalytic applications will also be different. For flat surfaces, rapid cycling, enabled by high carrier-gas flow rates, is necessary in order to rapidly grow thicker films. By contrast, ALD films in porous materials rarely need to be more than 1 nm thick. The elimination of diffusion gradients, efficient use of precursors, and ligand removal with less reactive precursors are the major factors that need to be controlled. In this review, criteria will be outlined for the successful use of ALD in porous materials. Examples of opportunities for using ALD to modify heterogeneous catalysts and SOFC electrodes will be given

An Investigation of Oxygen Reduction Kinetics in LSF Electrodes

The characteristics of solid oxide fuel cell (SOFC) cathodes, prepared by infiltration of La0.8Sr0.2FeO3−δ (LSF) into porous yttria-stabilized zirconia (YSZ) scaffolds, were evaluated by studying the effect of p(O2) and of Al2O3overlayers deposited by Atomic Layer Deposition (ALD) on impedance spectra at 873 and 973 K. The electrode resistance of LSF-YSZ composites calcined at 1123 K was dominated by high-frequency processes that show a relatively weak p(O2) dependence of −0.2 at 973 K. Composites calcined to 1373 K exhibited additional, low-frequency features in their impedance spectra that were more strongly dependent on p(O2), −0.43. These low-frequency processes are due to O2 adsorption limitations caused by the lower surface area of the LSF phase. Decreases in the exposed LSF surface caused by ALD films caused similar changes in the impedance spectra. The ALD overlayers were disrupted by heating to 1073 K and electrode polarization at 873 K. The implications of these results for understanding O2 adsorption limitations on SOFC cathodes are discussed

Atomic Layer Deposition on Porous Materials: Problems with Conventional Approaches to Catalyst and Fuel Cell Electrode Preparation

Atomic layer deposition (ALD) offers exciting possibilities for controlling the structure and composition of surfaces on the atomic scale in heterogeneous catalysts and solid oxide fuel cell (SOFC) electrodes. However, while ALD procedures and equipment are well developed for applications involving flat surfaces, the conditions required for ALD in porous materials with a large surface area need to be very different. The materials (e.g., rare earths and other functional oxides) that are of interest for catalytic applications will also be different. For flat surfaces, rapid cycling, enabled by high carrier-gas flow rates, is necessary in order to rapidly grow thicker films. By contrast, ALD films in porous materials rarely need to be more than 1 nm thick. The elimination of diffusion gradients, efficient use of precursors, and ligand removal with less reactive precursors are the major factors that need to be controlled. In this review, criteria will be outlined for the successful use of ALD in porous materials. Examples of opportunities for using ALD to modify heterogeneous catalysts and SOFC electrodes will be given