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.

The primary impact of the OxICFM CDT will be the highly-trained world-class scientists that it delivers. This impact will encompass both the short term (during their doctoral studies), the medium term (subsequent employment) and ultimately the longer timescale defined by their future careers and consequent impact on science, engineering and policy in the UK.

The impact of OxICFM students during their doctoral studies will be measured by the culture change in graduate training that the Centre brings about - in working at the interface between inorganic synthesis and manufacturing, and fostering cross-sector industry/academia working practices. By embedding not only from larger companies, but also SMEs, we have developed a training regime that has broader relevance across the sector, and the potential for building bridges by fostering new collaborations spanning enormous diversity in scientific focus and scale. Moreover, at a broader level, OxICFM offers to play a unique role as a major focus (and advocate) for manufacturing engagement with academic inorganic synthetic science in the UK.

From a scientific perspective, OxICFM will be uniquely able to offer a broad training programme incorporating innovative and challenging collaborative projects spanning all aspects of fundamental and applied inorganic synthesis, both molecular and materials based (40+ faculty). These will address key challenges in areas such as energy provision/storage, catalysis, and resource provision/renewal necessary to enhance the capability and durability of UK plc in the medium term. To give some idea of perspective, the output from previous CDTs in Oxford's MPLS Division include two start-up companies and in excess of 30 patents.

It is not only in the industrial and scientific realms that students will have impact during their timeframe of their doctorate. Part of the training programme will be in public engagement: team-based challenges in resource development/training and outreach exercises/implementation will form part of the annual summer school. These in turn will constitute a key part of the impact derived from the CDT by its engagement with the public - both face-to-face and through electronic/web-based media. As the centre matures, our aspiration is that our students - from diverse backgrounds - will act as ambassadors for the programme and promote even higher levels of inclusion from all parts of society.

For our partners, and businesses both large and small in the manufacturing sector, it will be our students who are considered the ultimate output of the OxICFM CDT. Our programme has been shaped by the need of such companies (frequently expressed in preliminary discussions) to recruit doctoral graduates who can apply themselves to a broad spectrum of multi-disciplinary challenges in manufacturing-related synthesis. OxICFM's cohort-based training programme integrates significant industry-led training components and has been designed to deliver a much broader skill set than standard PhD schemes. The current lack of CDT training at the interface of inorganic chemistry and manufacturing (and the relevance of inorganic molecules/materials to numerous industrial sectors) heightens the need for - and the potential impact of - the OxICFM CDT. Our students will represent a tangible and valuable asset to meet the long-term skills demand for scientists to develop new materials and nanotechnology identified in the UK Government's 2013 Foresight report.

In the longer term, the broad and relevant training delivered by OxICFM, and the uniquely wide perspective of the manufacturing sector it will deliver, will allow our graduates to obtain (and thrive in) positions of significant responsibility in industry and in research facilities/institutes. Ultimately we believe that many will go on to be future research leaders, driving innovation and changing research culture, and thereby making a lasting contribution to the UK economy.