Abstract
We compare approaches to measure oxygen surface exchange kinetics, by simultaneous optical transmission relaxation (OTR) and AC-impedance spectroscopy (AC-IS), on the same mixed conducting SrTi0.65Fe0.35O3-x film. Surface exchange coefficients were evaluated as a function of oxygen activity in the film, controlled by gas partial pressure and/or DC bias applied across the ionically conducting yttria-stabilized zirconia substrate. Changes in measured light transmission through the film over time (relaxations) resulted from optical absorption changes in the film corresponding to changes in its oxygen and oxidized Fe (~Fe4+) concentrations; such relaxation profiles were successfully described by the equation for surface exchange-limited kinetics appropriate for the film geometry. The kchem values obtained by OTR were significantly lower than the AC-IS derived kchem values and kq values multiplied by the thermodynamic factor (bulk or thin film), suggesting a possible enhancement in k by the metal current collectors (Pt, Au). Long-term degradation in kchem and kq values obtained by AC-IS was also attributed to deterioration of the porous Pt current collector, while no significant degradation was observed in the optically derived kchem values. The results suggest that, while the current collector might influence measurements by AC-IS, the OTR method offers a continuous, in situ, and contact-free method to measure oxygen exchange kinetics at the native surfaces of thin films.
Highlights
Oxide thin films have been undergoing widespread development for both advancement of device performance and fundamental scientific studies
As mentioned in the experimental section, the oxygen partial pressure was measured in a larger, separate furnace with a much slower gas flush time than the optical transmission relaxation (OTR) furnace, and the gas change times shown in Figure 1 are much slower than those experienced by the sample
The OTR technique was applied to thin films of the mixed conductor SrTi0.65Fe0.35O3-x, with a focus on evaluating its oxygen surface exchange kinetics at 600 °C as a function of oxygen partial pressure
Summary
Oxide thin films have been undergoing widespread development for both advancement of device performance and fundamental scientific studies. A number of useful functionalities arise from their well-defined and low-dimensional geometries, compared to their bulk counterparts, including lower (cross-plane) electrical resistance, higher optical transmittance, and surface exchange- rather than bulk diffusion-dominated kinetics. Some of these effects scale with the geometry (trivial size effects) while other nonlinear changes can emerge at the limit of nanoscale and confined thin films (true size effects) [1]. Thin films have enabled a number of novel components and studies: low resistance electrolytes [2] and active functional layers [3] in solid oxide fuel cells (SOFCs), sensitive chemi-resistive.
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