Oxygen Exchange on a Highly Oriented Thin Film Electrode
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Oxygen Exchange on a Highly Oriented Thin Film Electrode Y. L. Yang1, C. L. Chen2, G. P. Luo2, C. W. Chu2, and A. J. Jacobson1 1 Materials Research Science and Engineering Center University of Houston Houston, TX 77204-5500, USA 2 Texas Center for Superconductivity University of Houston Houston, TX 77204-5932, USA ABSTRACT The cathodic kinetic processes on a highly oriented LSCO thin film electrode supported on YSZ(100) surface were studied with a 3-probe ac impedance method under varying bias potential and annealing temperatures. Three distinctive features observed in the impedance spectra were assigned to contributions from the ionic conduction of the YSZ electrolyte, the ionic transfer at the LSCO/YSZ interface, and the oxygen exchange on the LSCO electrode surface. The changes of the three features with respect to the annealing history and bias potential were measured. The impedance data were analyzed using an equivalent circuit model: (RelCel)(RinterfaceQinterface)(RsurfCsurf). INTRODUCTION Thin films of mixed conducting oxides have been used to investigate transport processes at gas-solid and solid-solid interfaces [1-3]. Electrodes of these thin films have a well-defined and simplified structure that confines gas-phase oxygen exchange to the electrode external surface. The simplified electrode structure eliminates many complex structural and kinetic issues commonly found in porous electrodes. In a porous electrode, triple phase boundary (TPB) and its extensions, total surface area, porosity, and tortuosity are difficult to characterize experimentally. Therefore, interpretation of the porous electrode impedance and polarization results is often a subject of debate [4,5]. In the present study, we employed a 3-probe ac impedance technique to study a highly oriented La0.5Sr0.5CoO3−δ (LSCO) thin film electrode. The thickness of the electrode was tailored so that the ionic diffusion did not limit the overall electrode kinetics. Therefore, the relevant transport steps are reduced to: (a) the exchange of the gas-phase oxygen molecules at the electrode surface, and (b) the oxide ions transfer at the electrode/electrolyte interface. We have reported part of our studies elsewhere [6]. Our emphasis in this work is on how the surface and interface exchange kinetics vary as a function of the biased electrical potential and annealing history.
EXPERIMENTAL Two (100) surfaces of a YSZ single crystal disk (8%mol, diameter: 0.5 cm, and thickness: 0.2 cm) was coated with highly oriented LSCO thin films of equal thickness by the pulsed laser deposition technique. The substrate was maintained at 825±25 °C and pO2 = 33 Pa during the AA3.3.1
deposition. After deposition, the sample was annealed in 6.7×104 Pa of oxygen at 825 °C for 15 min and then cooled to room temperature in the same ambient. The thin film electrode was characterized with X-ray diffraction (XRD) (Siemens GADDS) and field emission scanning electron microscopy (FE-SEM) (JEOL JSM-6330F) before and after the impedance measurements. Determined by SEM, the thickness
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