Tokyo Tech scientists have developed environmentally benign inverse-perovskites with high energy conversion efficiency with potential for practical application as thermoelectric materials (TEMs). Addressing the limitations typically faced with TEMs, such as insufficient energy conversion efficiency and environmental toxicity due to heavy elements, the new TEMs provide a suitable alternative to TEMs based on toxic elements with better thermoelectric properties than conventional eco-friendly TEMs.

An open-access paper on their work is published in Advanced Science.

The energy conversion efficiency of TEMs depends on a dimensionless figure of merit (ZT), which is a product of two different factors: the inverse of thermal conductivity (κ) and the power factor (PF).

A high-performance TEM exhibits a high ZT if it possesses low κ and high PF. Over the years, scientists developed several high-performance heavy metal chalcogenide-based TEMs, such as Bi₂Te₃ and PbTe, that fulfill these criteria.

While these materials were ideal for energy conversion, they were toxic to the environment and the health of living organisms—they contained toxic heavy elements, such as lead (Pb) and tellurium (Te), which limited their practical applications. On the other hand, although oxide-based TEMs, such as SrTiO₃, have several advantages of non-toxicity and abundant natural resources, their ZT has been limited due to their high κ.

To address this, the research team led by Associate Professor Takayoshi Katase from Tokyo Institute of Technology explored efficient yet environmentally benign toxic-element-free TEMs. In the study published in Advanced Science, the researchers presented “inverse”-perovskite-based high ZT TEMs with the chemical formula Ba₃BO, where B refers to silicon (Si) and germanium (Ge).

Unlike normal perovskites, such as SrTiO₃, the positions of cation and anion sites are inverted in inverse-perovskites Ba₃BO. So, they contain a large amount of the heavy element, Ba, and their crystal structure is formed by a soft flamework made up of weak O-Ba bonds. These characteristics realize the low κ in inverse-perovskites.

—Dr. Katase

The research team clarified the synthesized bulk polycrystals of Ba₃BO possess extremely low κ of 1.0–0.4 W/mK at a T of 300-600 K, which is lower than those of Bi₂Te₃ and PbTe bulks.

As a result, the Ba₃BO bulks exhibit rather high ZT of 0.16-0.84 at T = 300-623 K. Additionally to the promising experimental results, the team carried out theoretical calculations which predicted a potential maximum ZT of 2.14 for Ba₃SiO and 1.21 for Ba₃GeO at T = 600 K by optimizing hole concentration. The maximum ZT of these non-toxic TEMs is much higher than that of other eco-friendly TEMs and comparable to the toxic heavy element ones in the same temperature range.

In addition, the team clarified that the high ZT of Ba₃BO is due not only to its low κ but also its high PF: B ion, which usually behaves as a positively charged cation but as a negatively charged anion in Ba₃BO. The B anions are responsible for the carrier transport, which achieves high PF.

The study validates the potential of the newly designed Ba3BOas a high-performing and eco-friendly alternative to conventional toxic, heavy element-based TEMs. The results establish inverse-perovskites as a promising option for developing advanced environmentally benign TEMs.

Resources

• X. He, S. Kimura, T. Katase, T. Tadano, S. Matsuishi, M. Minohara, H. Hiramatsu, H. Kumigashira, H. Hosono, T. Kamiya (2023) “Inverse-Perovskite Ba₃BO (B = Si and Ge) as a High Performance Environmentally Benign Thermoelectric Material with Low Lattice Thermal Conductivity.” Adv. Sci. doi: 10.1002/advs.202307058