Advancement in half-Heusler thermoelectric materials and strategies to enhance the thermoelectric performance

The large carbon footprint created during the production of conventional power by using coal and the exhaust of gasoline engines marks climate change as one of the world's most urgent concerns today. Moreover, half of the total heat energy produced globally is wasted due to thermodynamic limitations. Thermoelectric (TE) materialsbased devices convert heat into clean/green energy. Additionally, TE devices are environment-safe, have no moving components, and require no maintenance. However, these devices exhibit poor TE efficiency due to poor material performance and thermal stability. We have critically reviewed the Half-Heusler (HH)-based thermoelectric materials in this work. Among the numerous state-of-the-art TE materials, the HH compounds have been deliberated as potential candidate materials worldwide due to their narrow band gap, high substitutability, improved contact engineering, and high thermal stability. However, these materials encounter high thermal conductivity. Thus, in the past, Full-Heusler (FH) and HH phase mixing led to an impressive thermoelectric performance by dramatically lowering thermal conductivity, enabling the realization of the high thermoelectric performance of HH compounds using both in-situ and ex-situ composite techniques. Conversely, the nanostructuring (Phonon-Glass and Electron-Crystal concepts) of the materials enhances the grain boundary/interface density, further increasing the phonon scattering and decreasing the thermal conductivity of the TE materials. The current report also critically reviewed the HH materials' thermal properties and zT of various preliminary and cutting-edge TE materials. Also, the concept of double-half-Heusler has been incorporated. Moreover, the challenges associated with HH-based TE materials have been discussed extensively. Lastly, strategies for making HH materials with high thermoelectric performance have been suggested in this review. These include isoelectronic doping, aliovalent doping, modulation doping, energy filtering, resonant levels, and nanostructuring.