The power of pores: review on porous thermoelectric materials

Waste heat can be utilized to generate electric power via thermoelectric solid-state materials in a more reliable and noise-free way than conventional heat power systems. However, the primary concern arises from its low efficiency compared to conventional conversion technologies. Thermoelectric efficiency is measured in terms of a figure of merit (ZT), which is proportional to electrical conductivity (sigma) and Seebeck coefficient (S) and is inversely proportional to the thermal conductivity (kappa) of a material system. A material that has high electrical conductivity usually has low Seebeck coefficient and high thermal conductivity and vice versa and, therefore, a low ZT value. Hence, semiconductors are a vital choice for achieving a high ZT value compared to ceramics and metals, where all the above-mentioned parameter values are in an intermediate range. Researchers have been working for decades to independently control these thermoelectric parameters via various routes, such as nanostructuring, doping, compositional tuning, and band gap engineering. One significant approach in this direction is introducing controlled porosity into thermoelectric materials to achieve low thermal conductivity (kappa) while minimizing any detrimental impact on electrical conductivity (sigma). This review highlights the importance of porosity and its effect on porous thermoelectric materials and summarizes recent progress in developing advanced porous structures for high-performance thermoelectric materials systems. It also offers some prospects and strategies to independently control kappa with a very low synergistic effect on sigma to achieve a high ZT value. The review explores porous thermoelectric materials, emphasizing controlled porosity's impact on phonon scattering and thermal conductivity reduction. Multiple studies were discussed, highlighting prospects and limitations, alongside the latest trends.