Redox engineering of strontium titanate-based thermoelectrics
The development of thermoelectrics for high-temperature applications imposes several essential requirements on the material properties. In some energy-conversion scenarios, the cost and thermal stability requirements may dominate over efficiency issues, making abundant, high-temperature-stable and low-toxic oxides attractive alternative thermoelectric materials. As compared to “traditional” thermoelectrics, oxides possess unique redox flexibility and defect chemistry, which can be precisely set-up by external temperature and oxygen partial pressure conditions. This work aims to demonstrate how, by redox-sensitive substitutions, the thermoelectric properties of oxides can be tuned and enhanced. The proposed strategy is exemplified by considering molybdenum-containing strontium titanate within nominally single-substituted and nanocomposite concepts. The involved materials design allows us to proceed from an in-depth understanding of the redox-promoted effects to the demonstration of the enhanced thermoelectric performance attained by redox engineering. Synergistic enhancement of the Seebeck coefficient and suppressing the thermal conductivity due to combined carrier filtering effects and efficient phonon scattering at redox-induced interfaces provided up to 25% increase in the thermoelectric performance. The results demonstrate extraordinary flexibility of the perovskite lattice towards retaining a rich combination of the molybdenum redox states and shifting their ratio by tuning the A-site stoichiometry, underlying the prospects for developing new materials joining thermoelectric and (electro)catalytic functionalities.