Porous Bi 2 S 3 Bulk With Excellent Thermoelectric Performance by Solid States Replacement and Low Melting‐Point Metal Volatilization

Abstract Bismuth sulfide (Bi 2 S 3 ) exhibits potentials in thermoelectric field, due to their environmental friendliness, high Seebeck coefficients, and low thermal conductivity. However, the peak ZT for binary Bi 2 S 3 does not exceed 1.0, inhibiting its practical applications. Starting from the precipitation smelting of bismuth concentrate process, this study constructs multi‐type, multi‐scale in‐situ secondary phases and porous structures through FeCoNi (FCN) medium‐entropy alloy addition, significantly enhancing the ZT value of Bi 2 S 3 ‐based thermoelectric materials. The introduced FCN reacts with pre‐synthesized Bi 2 S 3 nanorod matrix during spark plasma sintering and forms precipitate complex with FCN‐S core and Bi shell microstructures. FCN doping improves the carrier concentration of Bi 2 S 3 and the reduced Bi from Bi 2 S 3 acts as carrier transport channels for mobility optimization. Due to the stacking effect of Bi 2 S 3 nanorods and the volatile nature of metallic Bi, porous Bi 2 S 3 structure is formed, characterized by randomly‐distributed and micro‐to‐nanoscale pores. The coexistence of various lattice defects effectively scatter phonons and suppress the lattice thermal conductivity, thus an excellent peak ZT of 1.1 is achieved at 773 K in a 0.25 wt.% FCN‐doped Bi 2 S 3 sample. This study, drawing on the process of ore smelting, proposes a convenient method for preparing high‐performance chalcogenide thermoelectric materials with porous structures.