DEVELOPMENT OF FUNCTIONAL METAL OXIDE THIN FILMS VIA HIGHTHROUGHPUT PULSED LASER DEPOSITION FOR ADVANCED ENERGY APPLICATIONS

Abstract

High-throughput methodologies are effective for rapid exploration of new materials with enhanced physical properties. In this thesis, we combine highthroughput pulsed laser deposition (HT-PLD) synthesis with rapid characterization techniques (X-Ray Diffraction, Atomic Force Microscopy, Electrochemical Impedance Spectroscopy, etc.) to quickly optimize metal oxide materials for energy conversion devices. The solid oxide fuel cell (SOFC) is one of the most promising energy conversion technologies. Despite years of concerted efforts by the research community, widespread commercialization of SOFCs is hindered by their high operating temperature requirements (>800 °C). Currently, there are limitations on the performance of electrolyte and cathode materials, which prevent a significant reduction in this operating temperature. To this end, we developed all-thin-film SOFC structures to probe fundamental transport properties via out-of-plane measurements in epitaxial electrolyte films with idealized interfaces. A highly conducting and thermally stable bottom electrode is combined with a library of top microelectrodes (30𝜇𝑚 ≤ 𝑑 ≤500𝜇𝑚), in a Cox and Strack-like geometry, which enables a direct and highspatial- resolution investigation of the intrinsic transport properties of the model electrolyte Sm0.2Ce0.8O2-δ (SDC20). This work demonstrated the utility of prototypical out-of-plane all-thin-film heteroepitaxial electrochemical devices as a model platform which can be extended to high-throughput investigations. We have used the high-throughput thin film formalism to develop a fundamental understanding of surface oxygen reduction reaction (ORR) mechanisms in mixed-conducting cathode materials by fabricating thin-film microelectrode arrays of La0.6Sr0.4Co1-xFexO3-δ (0≤x≤1) on a YSZ (100) substrate. The electrochemical properties of these microelectrode stacks are investigated via scanning impedance spectroscopy, and reveal that electrochemical resistance is dominated by surface oxygen exchange reactions on the electrode through a two-phase boundary pathway. A monotonic increase in electrochemical resistance is observed in La0.6Sr0.4Co1-xFexO3-δ from x =0 to x =1 along with a decrease in chemical capacitance corresponding to a decrease in oxygen vacancy concentration. A 𝑝𝑂( dependence of 𝑅* and 𝑘, for the whole spread film with the 𝑚 in a range of 0.5 to 0.75 is observed, indicating that the oxygen vacancy transport to surface-adsorbed oxygen intermediates is the ratedetermining step for mixed conducting cathodes. This study demonstrates the rich insights obtained via high-throughput methodologies and the promise of applying such techniques to discover highly active solid-state cathode materials. We have also looked at PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) as a doubleperovskite cathode material, which exhibits the combined conduction of e-, O2-, and H+. The high capacity of PBSCF to adsorb H2O at high temperature (Proton concentration: 1.7 mol% at 600 °C) and its excellent ORR performance can facilitate the cathodic electrochemical reaction in proton conducting SOFCs (p-SOFCs). A thinfilm library was used to investigate the ORR mechanism for PBSCF by systematically varying the size of the microelectrode arrays. By combining a chemically stable electrolyte, BaZr0.4Ce0.4Y0.1Yb0.1O3 (BZCYYb4411) with a thin dense PLD PBSCF interface layer between the cathode material and the electrolyte, we have demonstrated breakthrough performance in p-SOFCs with a peak power density of 548 mW/cm2 at 500 °C and an unprecedented stability under CO2. The behavior of this p-SOFC can compete with that of high performance oxide-ion-conducting SOFCs. Such performance can create new avenues for incorporating fuel cells into a sustainable energy future. We have further developed a high-throughput pulsed laser deposition approach to grow phase pure and high quality crystalline V1-xWxO2 (0 ≤ x < 4%) thin films on different substrates, which is challenging because of the complex phase diagram of vanadium oxides where there are many polymorphs of VO2. We systematically study how tungsten doping affects the poorly-understood phase transition hysteresis via a composition-spread approach. We have demonstrated for the first time that a composition of V1-xWxO2 (x ≈ 2.4%) satisfies unique ‘cofactor conditions’ based on geometric nonlinear theory. Our findings inform a strategy for developing more reliable vanadium dioxide materials. In addition, the potential application of V1-xWxO2 thin films in lithium-ion rechargeable batteries were systematically studied based on the tungsten concentration dependence of electrical properties of V1-xWxO2.