Engineering the Thermoelectric Transport in Half‐Heusler Materials through a Bottom‐Up Nanostructure Synthesis

Half‐Heusler (HH) alloys are among the best promising thermoelectric (TE) materials applicable for the middle‐to‐high temperature power generation. Despite of the large thermoelectric power factor and decent figure‐of‐merit ZT (≈1), their broad applications and enhancement on TE performance are limited by the high intrinsic lattice thermal conductivity (κ_L) due to insufficiencies of phonon scattering mechanisms, and the fewer powerful strategies associated with the microstructural engineering for HH materials. This study reports a bottom‐up nanostructure synthesis approach for these HH materials based on the displacement reaction between metal chlorides/bromides and magnesium (or lithium), followed by vacuum‐assisted spark plasma sintering process. The samples are featured with dense dislocation arrays at the grain boundaries, leading to a minimum κ_L of ≈1 W m^?1 K^?1 at 900 K and one of the highest ZT (≈1) and predicted η (≈11%) for n‐type Hf_0.25Zr_0.75NiSn_0.97Sb_0.03. Further manipulation on the dislocation defects at the grain boundaries of p‐type Nb_0.8Ti_0.2FeSb leads to enhanced maximum power factor of 47 × 10^?4 W m^?1 K^?2 and the predicted η of ≈7.5%. Moreover, vanadium substitution in FeNb_0.56V_0.24Ti_0.2Sb significantly promotes the η to ≈11%. This strategy can be extended to a broad range of advanced alloys and compounds for improved properties.     

High Band Degeneracy Contributes to High Thermoelectric Performance in p‐Type Half‐Heusler Compounds

Half‐Heusler (HH) compounds are important high temperature thermoelectric (TE) materials and have attracted considerable attention in the recent years. High figure of merit zT values of 0.8 to 1.0 have been obtained in n‐type ZrNiSn‐based HH compounds. However, developing high performance p‐type HH compounds are still a big challenge. Here, it is shown that a new p‐type HH alloy with a high band degeneracy of 8, Ti‐doped FeV_0.6Nb_0.4Sb, can achieve a high zT of 0.8, which is one of the highest reported values in the p‐type HH compounds. Although the band effective mass of this system is found to be high, which may lead to a low mobility, its low deformation potential and low alloy scattering potential both contribute to a reasonably high mobility. The enhanced phonon scattering by alloying leads to a reduced lattice thermal conductivity. The achieved high zT demonstrates that the p‐type Ti doped FeV_0.6Nb_0.4Sb HH alloys are promising as TE materials and offer an excellent TE performance match with n‐type ones for high temperature power generation.     

Thermoelectric Property Study of Nanostructured p‐Type Half‐Heuslers (Hf,Zr, Ti)CoSb0.8Sn0.2

Based on the best peak theromoelectric figure‐of‐merit value (ZT) of ca. 0.8 in Hf_0.5Zr_0.5CoSb_0.8Sn_0.2 and ca. 1 in Hf_0.8Ti_0.2CoSb_0.8Sn_0.2, the effect of Ti on thermoelectric properties is studied in (Hf, Zr, Ti)CoSb_0.8Sn_0.2, with the aim of further improving the ZT and reducing the usage of Hf. By either partial replacement of Hf and Zr with Ti in Hf_0.5Zr_0.5CoSb_0.8Sn_0.2 or partial replacement of Hf and Ti with Zr in Hf_0.8Ti_0.2CoSb_0.8Sn_0.2, a peak ZT of ≥1 is achieved at 800°C in Hf_0.44Zr_0.44Ti_0.12CoSb_0.8Sn_0.2. This composition has two advantages over the previous two best compositions: higher ZT than Hf_0.5Zr_0.5CoSb_0.8Sn_0.2 and less Hf than in Hf_0.8Ti_0.2CoSb_0.8Sn_0.2. A higher ZT with less Hf is very much desired since Hf is much more expensive than other constituent elements. The ZT improvement is the result of thermal conductivity reduction due to phonon scattering by both alloy and the nanostructure effect.     

Effect of Hf Concentration on Thermoelectric Properties of Nanostructured N‐Type Half‐Heusler Materials HfxZr1–xNiSn0.99Sb0.01

Half‐Heusler n‐type thermoelectric materials MNiSn (M = Hf, Zr) have been shown to exhibit peak thermoelectric dimensionless figure‐of‐merit (ZT) of ～1.0 at 600–700 °C with a composition of Hf_0.75Zr_0.25NiSn_0.99Sb_0.01. This work demonstrates that it is possible to achieve the same ZT by reducing the concentration of the most expensive component Hf to one third of the previously reported best composition, i.e., Hf_0.25Zr_0.75NiSn_0.99Sb_0.01, which corresponds to an overall 50% reduction on material cost. The samples are prepared by ball milling the arc melted ingot and hot pressing the finely ground powders. The reduction of Hf concentration is crucial for such materials to be used in large‐scale waste heat recovery.     

Unique Role of Refractory Ta Alloying in Enhancing the Figure of Merit of NbFeSb Thermoelectric Materials

NbFeSb‐based half‐Heusler alloys have been recently identified as promising high‐temperature thermoelectric materials with a figure of merit zT > 1, but their thermal conductivity is still relatively high. Alloying Ta at the Nb site would be highly desirable because the large mass fluctuation between them could effectively scatter phonons and reduce the lattice thermal conductivity. However, practically it is a great challenge due to the high melting point of refractory Ta. Here, the successful synthesis of Ta‐alloyed (Nb_1?xTa_x)_0.8Ti_0.2FeSb (x = 0 – 0.4) solid solutions with significantly reduced thermal conductivity by levitation melting is reported. Because of the similar atomic sizes and chemistry of Nb and Ta, the solid solutions exhibit almost unaltered electrical properties. As a result, an overall zT enhancement from 300 to 1200 K is realized in the single‐phase Ta‐alloyed solid solutions, and the compounds with x = 0.36 and 0.4 reach a maximum zT of 1.6 at 1200 K. This work also highlights that the isoelectronic substitution by atoms with similar size and chemical nature but large mass difference should reduce the lattice thermal conductivity but maintain good electrical properties in thermoelectric materials, which can be a guide for optimizing the figure of merit by alloying.     

High Efficiency Half‐Heusler Thermoelectric Materials for Energy Harvesting

Half‐Heusler (HH) compounds have gained ever‐increasing popularity as promising high temperature thermoelectric materials. High figure of merit zT of ≈1.0 above 1000 K has recently been realized for both n‐type and p‐type HH compounds, demonstrating the realistic prospect of these high temperature compounds for high efficiency power generation. Here, recent progress in advanced fabrication techniques and the intrinsic atomic disorders in HH compounds, which are linked to the understanding of the electrical transport, is discussed. Thermoelectric transport features of n‐type ZrNiSn‐based HH alloys are particularly emphasized, which is beneficial to further improving thermoelectric performance and comprehensively understanding the underlying mechanisms in HH thermoelectric materials. The rational design and realization of new high performance p‐type Fe(V,Nb)Sb‐based HH compounds are also demonstrated. The outlook for future research directions of HH thermoelectric materials is also discussed.     

Enhancement in Thermoelectric Figure‐Of‐Merit of an N‐Type Half‐Heusler Compound by the Nanocomposite Approach

An enhancement in the dimensionless thermoelectric figure‐of‐merit (ZT) of an n‐type half‐Heusler material is reported using a nanocomposite approach. A peak ZT value of 1.0 was achieved at 600 °C–700 °C, which is about 25% higher than the previously reported highest value. The samples were made by ball‐milling ingots of composition Hf_0.75Zr_0.25NiSn_0.99Sb_0.01 into nanopowders and hot‐pressing the powders into dense bulk samples. The ingots were formed by arc‐melting the elements. The ZT enhancement mainly comes from reduction of thermal conductivity due to increased phonon scattering at grain boundaries and crystal defects, and optimization of antimony doping.     

Grain Boundary Scattering of Charge Transport in n‐Type (Hf,Zr)CoSb Half‐Heusler Thermoelectric Materials

High thermoelectric figure of merit zT of ≈1.0 has been reported in both n‐ and p‐type (Hf,Zr)CoSb‐based half‐Heusler compounds, and further improvement of thermoelectric performance relies on the insightful understanding of electron and phonon transport mechanisms. In this work, the thermoelectric transport features are analyzed for (Hf_0.3Zr_0.7)_1?xNb_xCoSb (x = 0.02–0.3) with a wide range of carrier concentration. It is found that, although both temperature and energy dependencies of charge transport resemble ionized impurity scattering, the grain boundary scattering is the dominant scattering mechanism near room temperature. With increasing carrier concentration and grain size, the influence of the grain boundary scattering on electron transport weakens. The dominant scattering mechanism changes from grain boundary scattering to acoustic phonon scattering as temperature rises. The lattice thermal conductivity decreases with increasing Nb doping content due to the increased strain field fluctuations. These results provide an in‐depth understanding of the transport mechanisms and guidance for further optimizing thermoelectric properties of half‐Heusler alloys and other thermoelectric systems.     

A Dual Role by Incorporation of Magnesium in YbZn2Sb2 Zintl Phase for Enhanced Thermoelectric Performance

1‐2‐2‐type Zintl phase compounds have promising thermoelectric properties because of their complex crystal structures and multiple valence‐band structures. In this work, a series of single phase (Yb_0.9Mg_0.1)Mg_xZn_2?xSb₂ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) compounds are prepared by alloying YbZn₂Sb₂ with 10 at% MgZn₂Sb₂ and different amounts of YbMg₂Sb₂. The incorporation of Mg at the Yb site, as well as at the Zn site, not only leads to an effective orbital alignment confirmed by the dramatically enhanced density of states effective mass and Seebeck coefficients, but also increases the point defect scattering, contributing to a low lattice thermal conductivity ≈0.54 W m^?1 K^?1 at 773 K. Combined with the optimization of the carrier concentration by Ag doping at the Zn site, a highest ZT value ≈1.5 at 773 K is achieved in (Yb_0.9Mg_0.1)Mg_0.8Zn_1.2Ag_0.002Sb₂, which is higher than that of all the previously reported 1‐2‐2‐type Zintl phase compounds.     

High‐Performance Thermoelectric Paper Based on Double Carrier‐Filtering Processes at Nanowire Heterojunctions

As commercial interest in flexible power‐conversion devices increases, the demand for high‐performance alternatives to brittle inorganic thermoelectric (TE) materials is growing. As an alternative, we propose a rationally designed graphene/polymer/inorganic nanocrystal free‐standing paper with high TE performance, high flexibility, and mechanical/chemical durability. The ternary hybrid system of the graphene/polymer/inorganic nanocrystal includes two hetero­junctions that induce double‐carrier filtering, which significantly increases the electrical conductivity without a major decrease in the thermopower. The ternary hybrid shows a power factor of 143 μW m^?1 K^?1 at 300 K, which is one to two orders of magnitude higher than those of single‐ or binary‐component materials. In addition, with five hybrid papers and polyethyleneimine (PEI)‐doped single‐walled carbon nanotubes (SWCNTs) as the p‐type and n‐type TE units, respectively, a maximum power density of 650 nW cm^?2 at a temperature difference of 50 K can be obtained. The strategy proposed here can improve the performance of flexible TE materials by introducing more heterojunctions and optimizing carrier transfer at those junctions, and shows great potential for the preparation of flexible or wearable power‐conversion devices.     

Tuning Multiscale Microstructures to Enhance Thermoelectric Performance of n‐Type Bismuth‐Telluride‐Based Solid Solutions

Microstructure manipulation plays an important role in enhancing physical and mechanical properties of materials. Here a high figure of merit zT of 1.2 at 357 K for n‐type bismuth‐telluride‐based thermoelectric (TE) materials through directly hot deforming the commercial zone melted (ZM) ingots is reported. The high TE performance is attributed to a synergistic combination of reduced lattice thermal conductivity and maintained high power factor. The lattice thermal conductivity is substantially decreased by broad wavelength phonon scattering via tuning multiscale microstructures, which includes microscale grain size reduction and texture loss, nanoscale distorted regions, and atomic scale lattice distotions and point defects. The high power factor of ZM ingots is maintained by the offset between weak donor‐like effect and texture loss during the hot deformation. The resulted high zT highlights the role of multiscale microstructures in improving Bi₂Te₃‐based materials and demonstrates the effective strategy in enhancing TE properties.     

Skutterudite Unicouple Characterization for Energy Harvesting Applications

Skutterudites are promising thermoelectric materials because of their high figure of merit, ZT, and good thermomechanical properties. This work reports the effective figure of merit, ZT_eff, and the efficiency of skutterudite legs and a unicouple working under a large temperature difference. The p‐ and n‐type legs are fabricated with electrodes sintered directly to the skutterudite during a hot pressing process. CoSi₂ is used as the electrode for the n‐type skutterudite (Yb_0.35Co₄Sb₁₂) and Co₂Si for the p‐type skutterudite (NdFe_3.5Co_0.5Sb₁₂). A technique is developed to measure the ZT_eff of individual legs and the efficiency of a unicouple. An ZT_eff of 0.74 is determined for the n‐type legs operating between 52 and 595 °C, and an ZT_eff of 0.51 for the p‐type legs operating between 77 and 600 °C. The efficiency of the p–n unicouple is determined to be 9.1% operating between ～70 and 550 °C.     

Mechanically Robust BiSbTe Alloys with Superior Thermoelectric Performance: A Case Study of Stable Hierarchical Nanostructured Thermoelectric Materials

Bismuth telluride based thermoelectric materials have been commercialized for a wide range of applications in power generation and refrigeration. However, the poor machinability and susceptibility to brittle fracturing of commercial ingots often impose significant limitations on the manufacturing process and durability of thermoelectric devices. In this study, melt spinning combined with a plasma‐activated sintering (MS‐PAS) method is employed for commercial p‐type zone‐melted (ZM) ingots of Bi_0.5Sb_1.5Te₃. This fast synthesis approach achieves hierarchical structures and in‐situ nanoscale precipitates, resulting in the simultaneous improvement of the thermoelectric performance and the mechanical properties. Benefitting from a strong suppression of the lattice thermal conductivity, a peak ZT of 1.22 is achieved at 340 K in MS‐PAS synthesized structures, representing about a 40% enhancement over that of ZM ingots. Moreover, MS‐PAS specimens with hierarchical structures exhibit superior machinability and mechanical properties with an almost 30% enhancement in their fracture toughness, combined with an eightfold and a factor of six increase in the compressive and flexural strength, respectively. Accompanied by an excellent thermal stability up to 200 °C for the MS‐PAS synthesized samples, the MS‐PAS technique demonstrates great potential for mass production and large‐scale applications of Bi₂Te₃ related thermoelectrics.     

High‐Efficiency Thermoelectric Power Generation Enabled by Homogeneous Incorporation of MXene in (Bi,Sb)2Te3 Matrix

The (Bi,Sb)₂Te₃ (BST) compounds have long been considered as the benchmark of thermoelectric (TE) materials near room temperature especially for refrigeration. However, their unsatisfactory TE performances in wide‐temperature range severely restrict the large‐scale applications for power generation. Here, using a self‐assembly protocol to deliver a homogeneous dispersion of 2D inclusion in matrix, the first evidence is shown that incorporation of MXene (Ti₃C₂T_x) into BST can simultaneously achieve the improved power factor and greatly reduced thermal conductivity. The oxygen‐terminated Ti₃C₂T_x with proper work function leads to highly increased electrical conductivity via hole injection and retained Seebeck coefficient due to the energy barrier scattering. Meanwhile, the alignment of Ti₃C₂T_x with the layered structure significantly suppresses the phonon transport, resulting in higher interfacial thermal resistance. Accordingly, a peak ZT of up to 1.3 and an average ZT value of 1.23 from 300 to 475 K are realized for the 1 vol% Ti₃C₂T_x/BST composite. Combined with the high‐performance composite and rational device design, a record‐high thermoelectric conversion efficiency of up to 7.8% is obtained under a temperature gradient of 237 K. These findings provide a robust and scalable protocol to incorporate MXene as a versatile 2D inclusion for improving the overall performance of TE materials toward high energy‐conversion efficiency.     

New Insight on Tuning Electrical Transport Properties via Chalcogen Doping in n‐type Mg3Sb2‐Based Thermoelectric Materials

n‐type Mg₃Sb_1.5Bi_0.5 has recently been discovered to be a promising thermoelectric material, yet the effective n‐type dopants are mainly limited to the chalcogens. This may be attributed to the limited chemical insight into the effects from different n‐type dopants. By comparing the effects of different chalcogen dopants Q (Q = S, Se, and Te) on thermoelectric properties, it is found that the chalcogen dopants Q become more efficient with decreasing electronegativity difference between Q and Mg, which is mainly due to the increasing carrier concentration and mobility. Using density functional theory calculations, it is shown that the improving carrier concentration originates from the increasing doping limit induced by the stabilizing extrinsic defect. Moreover, the increasing electron mobility with decreasing electronegativity difference between Q and Mg is attributed to the smaller effective mass resulting from the enhancing chemical bond covalency, which is supported by the decreasing theoretical density of states. According to the above trends, a simple guiding principle based on electronegativity is proposed to shed new light on n‐type doping in Zintl antimonides.     

Synergistical Enhancement of Thermoelectric Properties in n‐Type Bi2O2Se by Carrier Engineering and Hierarchical Microstructure

Oxygen‐containing compounds are promising thermoelectric (TE) materials for their chemical and thermal stability. As compared with the high‐performance p‐type counterparts (e.g., ZT ≈1.5 for BiCuSeO), the enhancement of the TE performance of n‐type oxygen‐containing materials remains challenging due to their mediocre electrical conductivity and high thermal conductivity. Here, n‐type layered Bi₂O₂Se is reported as a potential TE material, of which the thermal conductivity and electrical transport properties can be effectively tuned via carrier engineering and hierarchical microstructure. By selective modification of insulating [Bi₂O₂]²⁺ layers with Ta dopant, carrier concentration can be increased by four orders of magnitude (from 10¹⁵ to 10¹⁹ cm^?3) while relatively high carrier mobility can be maintained, thus greatly enhancing the power factors (≈451.5 µW K^?2 m^?1). Meanwhile, the hierarchical microstructure can be induced by Ta doping, and the phonon scattering can be strengthened by atomic point defects, nanodots of 5–10 nm and grains of sub‐micrometer level, which progressively suppresses the lattice thermal conductivity. Accordingly, the ZT value of Bi_1.90Ta_0.10O₂Se reaches 0.36 at 773 K, a ≈350% improvement in comparison with that of the pristine Bi₂O₂Se. The average ZT value of 0.30 from 500 to 823 K is outstanding among n‐type oxygen‐containing TE materials. This work provides a desirable way for enhancing the ZT values in oxygen‐containing compounds.     

Achieving High Pseudocapacitance of 2D Titanium Carbide (MXene) by Cation Intercalation and Surface Modification

Supercapacitors attract great interest because of the increasing and urgent demand for environment‐friendly high‐power energy sources. Ti₃C₂, a member of MXene family, is a promising electrode material for supercapacitors owing to its excellent chemical and physical properties. However, the highest gravimetric capacitance of the MXene‐based electrodes is still relatively low (245 F g^?1) and the key challenge to improve this is to exploit more pseudocapacitance by increasing the active site concentration. Here, a method to significantly improve the gravimetric capacitance of Ti₃C₂T_x MXenes by cation intercalation and surface modification is reported. After K⁺ intercalation and terminal groups (OH^?/F^?) removing , the intercalation pseudocapacitance is three times higher than the pristine MXene, and MXene sheets exhibit a significant enhancement (about 211% of the origin) in the gravimetric capacitance (517 F g^?1 at a discharge rate of 1 A g^?1). Moreover, the as‐prepared electrodes show above 99% retention over 10 000 cycles. This improved electrochemical performance is attributed to the large interlayer voids of Ti₃C₂ and lowest terminated surface group concentration. This study demonstrates a new strategy applicable to other MXenes (Ti₂CT_x , Nb₂CT_x , etc.) in maximizing their potential applications in energy storage.     

High Thermoelectric Performance in PbSe–NaSbSe2 Alloys from Valence Band Convergence and Low Thermal Conductivity

PbSe is an attractive thermoelectric material due to its favorable electronic structure, high melting point, and lower cost compared to PbTe. Herein, the hitherto unexplored alloys of PbSe with NaSbSe₂ (NaPb_mSbSe_m+2) are described and the most promising p‐type PbSe‐based thermoelectrics are found among them. Surprisingly, it is observed that below 500 K, NaPb_mSbSe_m+2 exhibits unorthodox semiconducting‐like electrical conductivity, despite possessing degenerate carrier densities of ≈10²⁰ cm^?3. It is shown that the peculiar behavior derives from carrier scattering by the grain boundaries. It is further demonstrated that the high solubility of NaSbSe₂ in PbSe augments both the thermoelectric properties while maintaining a rock salt structure. Namely, density functional theory calculations and photoemission spectroscopy demonstrate that introduction of NaSbSe₂ lowers the energy separation between the L‐ and Σ‐valence bands and enhances the power factors under 700 K. The crystallographic disorder of Na⁺, Pb²⁺, and Sb³⁺ moreover provides exceptionally strong point defect phonon scattering yielding low lattice thermal conductivities of 1–0.55 W m^‐1 K^‐1 between 400 and 873 K without nanostructures. As a consequence, NaPb₁₀SbSe₁₂ achieves maximum ZT ≈1.4 near 900 K when optimally doped. More importantly, NaPb₁₀SbSe₁₂ maintains high ZT across a broad temperature range, giving an estimated record ZT_avg of ≈0.64 between 400 and 873 K, a significant improvement over existing p‐type PbSe thermoelectrics.     

High‐Performance Ta2O5/Al‐Doped Ag Electrode for Resonant Light Harvesting in Efficient Organic Solar Cells

A efficient indium tin oxide (ITO)‐free transparent electrode based on an improved Ag film is designed by introducing small amount of Al during co‐deposition, producing ultrathin and smooth Ag film with low loss. A transparent electrode as thin as 4 nm is achieved by depositing the film on top of Ta₂O₅ layer, and organic solar cells based on such ultrathin electrode are built, producing power conversion efficiency over 7%. The device efficiency can be optimized by simply tuning Ta₂O₅ layer thickness external to the organic photovoltaic (OPV) structure to create an optical cavity resonance inside the photoactive layer. Therefore Ta₂O₅/Al‐doped Ag films function as a high‐performance electrode with high transparency, low resistance, improved photon management capability and mechanical flexibility.     

Enhanced Thermoelectric and Mechanical Properties in Yb0.3Co4Sb12 with In Situ Formed CoSi Nanoprecipitates

Filled Skutterudites are one group of the most promising thermoelectric materials in real power generation applications. Herein, homogeneously dispersed multiscale CoSi nanostructures are successfully embedded into grains of the classic skutterudite system, Yb_0.3Co₄Sb₁₂, by the in situ precipitation method. Such nanoprecipitates contribute much to the synergistic enhancement of thermoelectric and mechanical properties. On one hand, by the fine deployment of multiscale CoSi nanoparticles, the lattice thermal conductivity is significantly depressed almost to the theoretical limit because of the disrupted propagation of the heat‐carrying phonons at phase boundaries. On the other hand, low‐energy electrons are effectively screened due to the energy filtering effect by the interfacial potential barrier between the CoSi nanoprecipitate and the matrix, resulting in an enhanced power factor. Taken together, an enhanced peak ZT value of ≈1.5 at 873 K for the Yb_0.3Co₄Sb₁₂/0.05CoSi composite is obtained with a high average ZT ≈0.95 between 300 and 873 K through decoupling the electrical and thermal transport parameters. Moreover, such a microstructure with multiscale CoSi nanoparticles shows significantly improved mechanical properties owing to particle hardening, making it more competitive for practical applications.