Layered Oxides, Chalcogenides and Pnictides as Thermoelectric Materials

The world has been meeting ever-growing demands for electricity through the consumption of non-renewable resources such as fossil fuels. Our crucial reliance on such sources has continues to have alarming environmental impacts, putting both us and our natural systems at risk. As the UK aims to achieve net zero carbon emissions by 2050, the search for cleaner, more sustainable energy sources has become vital.

In the UK, over 80% of wasted energy is in the form of heat. Among the viable avenues for more sustainable technologies, direct waste heat-to-electricity energy conversion represents a promising route to make our electricity base more sustainable as heat can be considered a renewable resource. Waste heat is abundant and ubiquitous.

Instead of going to waste, heat from sources such as homes, automotive exhausts and industrial processes could be scavenged and converted into electricity using thermoelectric generators; silent, dependable, and scalable solid-state devices which do not rely on chemical reactions or produce toxic by-products. Electricity can also be used to provide cooling for refrigeration or cooling.

Thermoelectrics work based on the Seebeck and Peltier effects discovered in the 19th century. The process can be understood as a heat engine with a hot side and a cold side that uses electric charge carriers as the working medium to generate electrical current. For comparison, in an internal combustion engine, the gas produced from the combustion of fuel is the working medium, generating mechanical motion.

Currently, the efficiency of thermoelectrics, characterised by the figure of merit zT, lags that of other waste heat energy-conversion technology. Broad and affordable application of thermoelectrics is reliant on developing higher performing materials with higher values of zT.

For this, a variety of conflicting transport properties must be optimized: high electrical conductivity, a strong Seebeck effect (high Seebeck coefficient), and low thermal conductivity. The synthesis and characterisation of novel materials is critical to develop the theoretical understanding of the mechanisms that improve thermoelectric performance.

Recent work has highlighted layered structures as promising thermoelectric materials. Multi- anion solids contain one or more metallic elements in the periodic table in combination with two or more anion-forming elements and tend to form layered structures. Examples include oxide sulfides and oxide nitrides.

Such compounds give rise to interesting electronic and magnetic functionalities which can be utilised in catalysis, batteries, and superconductors. These properties are specified by their composition. The tuning of the composition and thus the properties of multi-anion solids will be a useful platform to achieve higher efficiency thermoelectrics.

The proposed aim of this project is to synthesise novel layered oxide chalcogenides and oxide nitrides for thermoelectric applications. Existing literature on such phases in the context of thermoelectric applications is scarce. The compositions of these solids will be tuned chemically to develop a fundamental understanding of how composition affects structure, electronic and magnetic properties, and ultimately the thermoelectric performance in these multi-anion compounds.

Soft synthetic routes will also be employed to obtain compositions that would not be accessible using traditional solid-state synthetic methods. A wide range of analytical techniques will be used to characterise these novel materials including x-ray and neutron spectroscopy and magnetometry. This project falls within the EPSRC Materials for Energy Applications research area, under the theme of Energy.