The Advanced Electron Microscopy Characterisation of Oxide Materials at the Atomic Level for Energy Storage and Conversion Technologies

Abstract

At the forefront of the challenges faced by modern-day society is the growing demand for clean and renewable energy sources. This challenge has been met by a surge in research activities on the development of electrochemical energy storage and conversion technologies, that can be deemed competitive with both the cost and performance of fossil fuels. The success of such research endeavours is heavily reliant on a thorough understanding of the materials involved, which goes hand in hand with the desire to control materials at the atomic level, in order to successfully achieve the tailoring of their physical, chemical, and electronic properties. Advanced characterisation methods based on electron microscopy possess the ability to link across multiple length and complexity scales, bridging the gap between material properties and their atomic structure.

The aim of this work is to develop a deep understanding of the role that electron microscopy and related spectroscopic techniques play in the design, development, and optimisation of materials for energy storage and conversion technologies. In doing so, this thesis focuses on the synthesis and characterisation of three different metal oxide systems; BaMnO3, BaTixMn1-xO3, and WO3. The affordable cost and versatility of metal oxide materials make them ideal for the innovative materials engineering required for real-world energy applications.

By targeting the replacement of platinum (Pt)-based electrocatalysts in fuel cell technologies, whose high cost limit their widespread use, phase pure hexagonal BaMnO3 rods are synthesised via a simple low-temperature molten salt method. Atomic resolution imaging, in combination with electron energy loss spectroscopy (EELS), reveals the presence of an amorphous layer at the surface of BaMnO3 rods, consisting of reduced Mn states. Such states are identified as Mn3+, and found to be the source of activity in the BaMnO3 perovskite system. As an extension of this, this thesis presents the dielectric engineering of BaMnO3, enabling the otherwise inaccessible heterogeneous nucleation of Pt nanoparticles at its surface. Using Ti-doping of the B-site cation, Pt-BaTi0.5Mn0.5O3 hybrid electrocatalysts are formed. BaTi0.5Mn0.5O3 loaded with 1% Pt displays superior electrocatalytic performance when compared with higher weight loading regimes, due to the nucleation of smaller and better dispersed Pt nanoparticles on BaTi0.5Mn0.5O3.

Switching from energy conversion to energy storage technologies, this thesis presents the formation of several WO3 phases for comparison as supercapacitor electrodes. By controlling the annealing temperature, phase selection is demonstrated. Further to this, atomic resolution imaging, in combination with electrochemical measurements, relates the structural motifs exhibited by the WO3 system to the corresponding electrochemical performance and charge storage mechanism.

Through the materials characterised in this thesis, I hope that the concepts demonstrated will pave the way for the role advanced electron microscopy techniques can, and must, play in the development of high-performing and practically-relevant energy storage and conversion technologies.