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.