Energy security and the scale of change needed to meet net-zero targets are top of the global political agenda. Up to 70% of worldwide energy is currently lost as heat and this is expected to improve to just 50% by 2030, representing vast quantities of wasted energy and unnecessary greenhouse gas emissions. This creates an urgent need for technologies to improve energy efficiency, which would make low-carbon energy go further and provide "breathing space" to develop and scale-up other net-zero technologies such as electric vehicles, grid storage and carbon capture. Thermoelectric generators (TEGs) harness the Seebeck effect in a thermoelectric material to recover waste heat as electricity, and are a front-running technology for improving the efficiency of energy-intensive processes. TEGs are flexible and can, with the right materials, be used at scales from powering wearable devices to recovering waste heat from industrial furnaces. However, despite established applications in the aerospace industry and an estimated $1.2bn global market by 2027, large-scale thermoelectric power is currently not feasible due to limited efficiency and the scarcity and toxicity of the materials used. High-performance thermoelectric materials must balance a large Seebeck coefficient and electrical conductivity with a low thermal conductivity. This makes optimising the thermoelectric figure of merit, ZT, a complex interdisciplinary materials-design challenge. Furthermore, a high-performance thermoelectric not only requires high efficiency (large ZT), but must also meet cost and sustainability requirements for the intended application. With this in mind, progress in thermoelectrics has historically been limited by a poor understanding of thermal conductivity and how to engineer it, which has led to unsustainable materials made from rare and/or toxic elements such as the current industry standards Bi2Te3 and PbTe. The initial part of the Future Leaders Fellowship has made use of state-of-the-art computational materials modelling to address two key questions: What are the key microscopic processes that suppress or enhance heat transport in materials, and how can we use materials design and engineering to control them? In doing so, the Fellow and team have established a cutting-edge research programme on thermal conductivity and thermoelectrics, which has provided new fundamental insight into the heat transport in thermoelectrics together with modelling tools to make accurate predictions of the thermal conductivity, electrical properties and ZT of a wide range of thermoelectric materials. Over the three years of the FLF Renewal period, we will exploit these developments to design efficient, cost-effective and sustainable materials for thermoelectric power and related renewable-energy applications including photovoltaics (solar cells). By working with experimental collaborators and industrial partners, we will further seek to progress these materials from predictions to prototype devices. In parallel, we will also consolidate the developments from across the FLF to make them as widely accessible as possible to the project beneficiaries, and thus maximise the impact of the research programme. The continuation of the research programme will deliver thermoelectric materials suitable for widespread commercialisation and enable the UK to reap the economic benefits of the $560bn global market for energy efficiency while contributing to the UN Sustainable Development Goals of affordable and clean energy, climate action, and reducing poverty. The improved ability to control heat transport enabled by this programme will also benefit other technologies, for example more efficient solar cells, better thermal management in batteries and improved power electronics and silicon chips, providing considerable scope to maintain and grow our research beyond the Fellowship.