Oxide thermoelectric materials: A review of emerging strategies for efficient waste heat recovery

In response to the escalating global energy demand and the environmental impact of fossil fuels, oxide thermoelectric materials have emerged as promising candidates for waste heat recovery due to their stability, non-toxicity, and capacity for high-temperature applications. This comprehensive review evaluates critical advances in oxide-based thermoelectrics, focusing on p-type (Ca₃Co₄O₉, BiCuSeO) and n-type (ZnO, SrTiO₃, CaMnO₃, In₂O₃) materials. Key challenges in achieving high thermoelectric performance, such as balancing the Seebeck coefficient, electrical conductivity, and thermal conductivity, are addressed through diverse strategies including doping, compositional tuning, nanostructuring, and interface engineering. Notable achievements include a ZT of 1.42 at 1050 K in Nb-doped SrTiO₃ with graphite inclusions, a ZT of 0.9 in nonstoichiometric Ca₃Co₄O₉, and a ZT of 0.47 at 1223 K in Ce-doped In₂O₃ through defect engineering. BiCuSeO stands out with a ZT of 1.5 at 923 K, achieved via three-step texturation and dual-doping, demonstrating its potential for mid-to-high-temperature applications. Innovations such as high-entropy oxide compositions, two-dimensional electron gas systems, and advanced synthesis techniques like spark plasma sintering and solvothermal synthesis have shown remarkable potential for ZT improvement across these materials. This review highlights a multi-faceted approach to improving thermoelectric efficiency and outlines strategic pathways for scalable, eco-friendly thermoelectric applications in energy harvesting and industrial heat recovery.