Realizing High Thermoelectric Performance in Earth-Abundant Bi₂S₃ Bulk Materials via Halogen Acid Modulation

Bi₂S₃-based thermoelectric materials without toxic and expensive elements have a high Seebeck coefficient and intrinsic low thermal conductivity. However, Bi₂S₃ suffers from low electrical conductivity, which makes it a less-than-perfect thermoelectric material. In this work, halogen elements F, Cl, and Br from halogen acid are successfully introduced into the Bi₂S₃ lattice using a hydrothermal procedure to efficiently improve the carrier concentration. Compared with the pure sample, the electron concentration of the Bi₂S₃ sample treated with HCl is increased by two orders of magnitude. An optimal power factor of 470 µW m⁻¹ K⁻² for the Bi₂S_2.96Cl_0.04 sample at 673 K is obtained. Density functional theory calculations reveal that an effective delocalized electron conductive network forms after Cl doping, which raises the Fermi level into the conduction bands, thus generating more free electrons and improving the conductivity of the Bi₂S₃-based materials. Ultimately, an excellent ZT of ≈0.8 is achieved at 673 K for the Bi₂S_2.96Cl_0.04 sample, which is one of the highest values reported for a state-of-the-art Bi₂S₃ system. The energy conversion efficiency of the module reaches 2.3% at 673 K with a temperature difference of 373 K. This study offers a new method for enhancing the thermoelectric properties of Bi₂S₃ by adding halogen acid in the hydrothermal process for powder synthesis.