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.     

Ultrahigh Thermoelectric Performance Realized in Black Phosphorus System by Favorable Band Engineering through Group VA Doping

Black phosphorus (BP) has emerged as a promising thermoelectric candidate because of its strong electronic and thermal anisotropy, suggesting a large σ/κ ratio can be realized by controlling carrier transport orientation for a potentially high ZT. Nevertheless, to date, low conversion efficiency (ZT ≈0.08, 300 K) and poor stability of BP remain the major issues that have hampered its practical applications. This work reports a material family in simple composition XP₇, XP₃, and XP (X = N, As, Sb, Bi) with high‐performance thermoelectric properties by first‐principles calculations. Strikingly, an ultrahigh ZT up to 1.21 at 300 K is achieved in p‐type BiP₇ with an optimal carrier concentration of 5.48 × 10¹⁹ cm^?3 and ZT in n‐type NP₃ can reach up to ≈0.87 at the electron concentration of 3.67 × 10¹⁹ cm^?3 along the zigzag direction, owing to their enhanced density of states and multivalley band structures around the Fermi level through the resonant effects of VA guest and host atoms. Additionally, the calculations demonstrate further improvement in thermoelectric performance of pristine BP by ≈4.8 and 4.5 times at 800 K in p‐type NP and n‐type NP₃, respectively. Considering the high stability, current results indicate that N–P based systems are highly promising for novel metal‐free, nontoxic, and ultralight thermoelectrics.     

Atomic Disorders Induced by Silver and Magnesium Ion Migrations Favor High Thermoelectric Performance in α‐MgAgSb‐Based Materials

Thermoelectric devices can directly convert thermal energy to electricity or vice versa with the efficiency being determined by the materials’ dimensionless figure of merit (ZT). Since the revival of interests in the last decades, substantial achievements have been reached in search of high‐performance thermoelectric materials, especially in the high temperature regime. In the near‐room‐temperature regime, MgAgSb‐based materials are recently obtained with ZT ≈ 0.9 at 300 K and ≈1.4 at 525 K, as well as a record high energy conversion efficiency of 8.5%. However, the underlying mechanism responsible for the performance in this family of materials has been poorly understood. Here, based on structure refinements, scanning transmission electron microscopy (STEM), NMR experiments, and density function theory (DFT) calculations, unique silver and magnesium ion migrations in α‐MgAg_0.97Sb_0.99 are disclosed. It is revealed that the local atomic disorders induced by concurrent ion migrations are the major origin of the low thermal conductivity and play an important role in the good ZT in MgAgSb‐based materials.