Thin Film Complex Oxide Proton Conductors: Synthesis and Applications

Abstract

The performance of ultra-thin film solid oxide fuel cells (μ-SOFC) is highly dependent on the structural, microstructural and transport properties of the electrolyte. The focus of this thesis is on understanding the effect of synthesis and processing parameters of BaY0.2Zr0.8O3 (BYZ), a complex oxide proton-conducting electrolyte, on thin-film solid oxide fuel cell (SOFC) performance.

The properties of BYZ thin films are highly dependent on film growth techniques and parameters. The relationship between electrolyte thickness and fuel cell performance is investigated in the ultra-thin film thickness range of ~ 70 nm to ~ 200 nm for BYZ films grown by RF sputtering. The microstructure, crystal structure, and electrical behavior of BYZ films were examined as a function of thickness to attain high power density in SOFCs. The optimal thickness that allows for a balance between the leakage current and Ohmic resistance for these devices was determined to be t0 ~150 nm. XRD examination showed a thickness dependent stress behavior in BYZ thin films, with the most compressive state occurring for films of thickness t0. A Volmer-Weber thin film growth mode is proposed for the observed thickness dependent evolution in film properties. The findings of this examination can allow for an increase in the limits of SOFC power density in the ultra-thin regime for proton conducting electrolytes.

The presence of a large number of grain boundaries in BYZ films processed at intermediate temperatures leads to diminished conductivity. To mitigate this reduced conductivity while maintaining reasonable processing temperatures, it is essential to increase the effective surface area or TPB of the device. A study of the insertion of ion-selective interfacial layers between the electrode-electrolyte interfaces in μ-SOFCs performance is presented. A nearly two-fold increase in power density of μ-SOFCs in the intermediate temperature range is demonstrated by the addition of ultra-thin palladium interlayers. In addition to enhancing performance, this approach may yield important insight into the proton conduction behavior of BYZ and other proton conducting materials.

Finally, to address some of the shortcomings in the current synthesis techniques for BYZ, a novel intermediate temperature thin film synthesis route is demonstrated. This new technique (SP-GNP) is a combination of a thin film deposition technique, Spray Pyrolysis (SP), with a low temperature oxide powder synthesis technique, Glycine Nitrate Process (GNP). A proposed working mechanism and a discussion of the principal parameters that dictate film properties is presented. By using this technique, single-phase perovskite BYZ films were successfully grown at a temperature of 200 °C followed by annealing at 750 °C. The compositional and microstructural evolution of BYZ thin films obtained by SP-GNP is investigated as a function of several technique parameters such as precursor concentration, solvent properties and substrate properties. A microstructural evolution from porous to dense in BYZ thin films by changing precursor composition is demonstrated. This intermediate temperature technique may allow for a deeper insight into the properties of refractory complex oxides through incorporation of novel dopants and may lead to the emergence of new applications for these materials.