# Porous Bi2S3 Bulk With Excellent Thermoelectric Performance by Solid States Replacement and Low Melting‐Point Metal Volatilization

## TL;DR

A new method using FeCoNi alloy improves the thermoelectric performance of bismuth sulfide, achieving a ZT of 1.1 at 773 K.

## Contribution

A novel method using FeCoNi alloy and low melting-point metal volatilization to enhance Bi2S3 thermoelectric performance.

## Key findings

- FeCoNi alloy addition creates porous structures and secondary phases in Bi2S3.
- A peak ZT of 1.1 is achieved at 773 K with 0.25 wt.% FCN doping.
- The method is applicable to other sulfur-based thermoelectric materials.

## Abstract

Bismuth sulfide (Bi2S3) exhibits potentials in thermoelectric field, due to their environmental friendliness, high Seebeck coefficients, and low thermal conductivity. However, the peak ZT for binary Bi2S3 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 Bi2S3‐based thermoelectric materials. The introduced FCN reacts with pre‐synthesized Bi2S3 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 Bi2S3 and the reduced Bi from Bi2S3 acts as carrier transport channels for mobility optimization. Due to the stacking effect of Bi2S3 nanorods and the volatile nature of metallic Bi, porous Bi2S3 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 Bi2S3 sample. This study, drawing on the process of ore smelting, proposes a convenient method for preparing high‐performance chalcogenide thermoelectric materials with porous structures.

By introducing FeCoNi medium‐entropy alloy, the bismuth sulfide (Bi2S3) material achieves a record‐high ZT of 1.1 at 773 K, owing to the solid‐states replacement reaction and the volatilization of low melting‐point metal. This strategy is also applicable to other sulfur‐based thermoelectric materials.