| Currently, in order to enhance the performance of solid oxide fuel cells (SOFCs), it is necessary to achieve better understanding and improvement on the sluggish oxygen reduction reaction (ORR) on the cathode side of the cell. Owing to the complexity of the ORR, the exact mechanisms are not clear yet. Therefore, this study adopts a thin film approach to isolate the surface properties from bulk to gain a better understanding of the surface activities of cathode materials.;The major observations made in this study could be summarized from three aspects. Firstly, surface anisotropy was found to have a significant influence on the surface exchange rate of La0.7Sr0.3MnO3 . An up to 70% difference is observed in kchem among surfaces of (100), (110), (111) and (621) orientations. k chem on (100), (110), (111) and (621) surfaces of La0.7 Sr0.3MnO3 films were determined and vary from ≈ 1 to 65 x 10-6 cm/s, depending on temperature and orientation. In each orientation, kchem has a strong temperature dependency with activation energy about ≈ 1.0 +/- 0.2 eV, indicative of a surface process. Secondly, strain states and dislocation population related film thickness and substrates effects were demonstrated to strongly influence the surface exchange rate and activation energy of kchem. There were two distinct activation energies (Ea) for k chem, indicating two parallel mechanisms related to surface exchange. Ea for surface regions that are influenced by the presence of extended defects intersecting the surface is ≈ 0.75 eV / atom for (100) oriented films and ≈ 1.16 eV / atom for (110) oriented films. For surface regions free of dislocations, it was found that Ea ≈ 1.5 eV / atom for (100) oriented films and ≈ 1.9 eV / atom for (110) oriented films. kchem of films on SrTiO3 are faster than on NdGaO3, for fully strained films, an order of magnitude difference were observed. Lastly, (110) and (100) oriented textured La 0.7Sr0.3MnO3 films with controlled variant structures were prepared on YSZ (111) and MgO (100) substrates, respectively. Two activation energies (Ea) for kchem were found, which indicative of two parallel surface exchange mechanisms. The first (native grain surface) process has E(110) ≈ 1.67 eV, E(100) ≈ 1.11 eV and the second (grain boundaries) process has E(110) ≈ 0.87 eV, E(100) ≈ 0.78 eV. Moreover, under the same temperature, films with different grain sizes showed different A values, representing the relative contribution from two mechanisms to the total transient response varies with grain sizes.;In this thesis, the results of oxygen surface exchange rates for La 0.7Sr0.3MnO3 concerning the effects of crystallographic orientation, substrate related strain / dislocation, and grain boundaries will be presented and discussed relative to the state of knowledge in SOFCs, aiming to improve the total cell performance.;In this work, pulsed laser deposition (PLD) is used to synthesize highly epitaxial, low surface roughness La0.7Sr0.3MnO 3 (LSM) films for applications in the SOFC cathodes. Electrical conductivity relaxation (ECR) method is used to study oxygen chemical exchange coefficient (kchem) of LSM films in SOFC-like conditions (of temperature and oxygen pressures). Firstly, we will compare values and activation energies of kchem for different crystallographic oriented surfaces by depositing films on various SrTiO3 substrates. Secondly, we will determine the kchem values on both strained surfaces and surfaces intersecting with dislocations, and further determine the relationship between kchem and film thickness, temperature and underlying substrates. Lastly, textured La 0.7Sr0.3MnO3 films will be synthesized to study the relative contribution of the native surface and of the grain boundaries intersecting the surface to the chemical surface exchange properties. |
