Influence of redox engineering on the trade-off relationship between thermopower and electrical conductivity in lanthanum titanium based transition metal oxides†

Discovery of new materials plays a critical role in developing advanced high-temperature thermoelectric (TE) applications. Transition metal oxides (TMOs) are one of the attractive candidates for high-temperature TE applications due to their thermal and chemical stability. However, the trade-off relationship between thermopower (S) and electrical conductivity (σ) limits the maximum attainable power factor (PF), thereby hindering improvements in TE conversion efficiency. To overcome this trade-off relationship, the emerging approach of the redox-driven metal exsolution in TMOs shows promise in improving both S and σ. However, the effect of metal exsolution with different particle sizes and densities on S and σ is still largely unexplored. This study demonstrates an unusually large enhancement in PF through the exsolution of Ni nanoparticles in epitaxial La_0.7Ca_0.2Ni_0.25Ti_0.75O₃ (LCNTO) thin films. Metal exsolution leads to a decrease in the carrier concentration while increasing the carrier mobility due to energy filtering effects. In addition, the exsolved metal particles introduce high-mobility electron carriers into the low-mobility LCNTO matrix. Consequently, the exsolution of metal particles results in a significant enhancement in S along with a substantial increase in σ, compared to the pristine film. Overall, the TE power factor of LCNTO is dramatically enhanced by up to 8 orders of magnitude owing to the presence of exsolved metal particles. This enhancement is attributed to the selective filtering of carriers caused by energy band bending at the metal–oxide interfaces and the high-mobility carriers from the exsolved Ni particles with a high Ni⁰ fraction. This study unequivocally demonstrates the impact of metal exsolution on oxide TE properties and provides a novel route to tailor the interconnected physical and chemical properties of oxides, leading to enhanced TE power output.