Phase-transition temperature suppression to achieve cubic GeTe and high thermoelectric performance by Bi and Mn codoping.

Germanium telluride (GeTe)-based materials, which display intriguing functionalities, have been intensively studied from both fundamental and technological perspectives. As a thermoelectric material, though, the phase transition in GeTe from a rhombohedral structure to a cubic structure at ∼700 K is a major obstacle impeding applications for energy harvesting. In this work, we discovered that the phase-transition temperature can be suppressed to below 300 K by a simple Bi and Mn codoping, resulting in the high performance of cubic GeTe from 300 to 773 K. Bi doping on the Ge site was found to reduce the hole concentration and thus to enhance the thermoelectric properties. Mn alloying on the Ge site simultaneously increased the hole effective mass and the Seebeck coefficient through modification of the valence bands. With the Bi and Mn codoping, the lattice thermal conductivity was also largely reduced due to the strong point-defect scattering for phonons, resulting in a peak thermoelectric figure of merit (ZT) of ∼1.5 at 773 K and an average ZT of ∼1.1 from 300 to 773 K in cubic Ge0.81Mn0.15Bi0.04Te. Our results open the door for further studies of this exciting material for thermoelectric and other applications.

Native defects and impurity band behavior in half-Heusler thermoelectric NbFeSb.

To investigate the electronic behavior and magnetic properties of NbFeSb, we have performed 93Nb NMR, specific heat and magnetic measurements on NbFeSb samples heat treated at high temperatures. Magnetic measurements combined with an observed Schottky anomaly and changes in the NMR line width indicate the presence of a 0.2% concentrated native magnetic defect in stoichiometric NbFeSb samples. The origin of these native defects is believed to be due to Fe antisites on Nb sites. In addition, NMR shift and spin-lattice relaxation results below 200 K reveal a Korringa-like response indicating heavily-doped p-type behavior due to native defects. Above 280 K, this converts to an activated behavior, indicating the presence of an impurity band, empty at low temperatures, which is located around 0.03 eV above the valence band maximum.

Highly Efficient Hydrogen Evolution from Edge-Oriented WS2(1-x)Se2x Particles on Three-Dimensional Porous NiSe2 Foam.

The large consumption of natural fossil fuels and accompanying environmental problems are driving the exploration of cost-effective and robust catalysts for hydrogen evolution reaction (HER) in water splitting. Tungsten dichalcogenides (WS2, WSe2, etc.) are promising candidates for such purpose, but their HER performances are inherently limited by the sparse catalytic edge sites and poor electrical conductivity. Here we demonstrate a highly active and stable HER catalyst by integrating ternary tungsten sulfoselenide WS2(1-x)Se2x particles with a 3D porous metallic NiSe2 foam, in which good electrical conductivity, good contact, large surface area, and high-density active edge sites are simultaneously obtained, thus contributing to outstanding catalytic performance: large cathode current density (-10 mA/cm2 at -88 mV), low Tafel slope (46.7 mV/dec), large exchange current density (214.7 µA/cm2), and good stability, which is better than most reports on WS2 and NiSe2 catalysts. This work paves an interesting route for boosting HER efficiency of transition metal dichalcogenide catalysts.

Solvothermal synthesis of ternary Cu2 MoS4 nanosheets: structural characterization at the atomic level.

Cu2 MoS4 nanosheets are synthesized by a solvothermal method in which the Cu2 O starting material acts as a sacrificial template. The microstructure of the Cu2 MoS4 nanosheets is characterized at the atomic level, and the growth mechanism is monitored at the nanoscale through systematic time-dependent experiments. As a result, the unprecedented observation of the allotropic phase change in Cu2 MoS4 that occurs during the solvothermal process is possible.

Half-Heusler alloys as emerging high power density thermoelectric cooling materials.

To achieve optimal thermoelectric performance, it is crucial to manipulate the scattering processes within materials to decouple the transport of phonons and electrons. In half-Heusler (hH) compounds, selective defect reduction can significantly improve performance due to the weak electron-acoustic phonon interaction. This study utilized Sb-pressure controlled annealing process to modulate the microstructure and point defects of Nb0.55Ta0.40Ti0.05FeSb compound, resulting in a 100% increase in carrier mobility and a maximum power factor of 78 µW cm-1 K-2, approaching the theoretical prediction for NbFeSb single crystal. This approach yielded the highest average zT of ~0.86 among hH in the temperature range of 300-873 K. The use of this material led to a 210% enhancement in cooling power density compared to Bi2Te3-based devices and a conversion efficiency of 12%. These results demonstrate a promising strategy for optimizing hH materials for near-room-temperature thermoelectric applications.

Improved figure of merit (z) at low temperatures for superior thermoelectric cooling in Mg3(Bi,Sb)2.

The low-temperature thermoelectric performance of Bi-rich n-type Mg3(Bi,Sb)2 was limited by the electron transport scattering at grain boundaries, while removing grain boundaries and bulk crystal growth of Mg-based Zintl phases are challenging due to the volatilities of elemental reactants and their severe corrosions to crucibles at elevated temperatures. Herein, for the first time, we reported a facile growth of coarse-grained Mg3Bi2-xSbx crystals with an average grain size of ~800 µm, leading to a high carrier mobility of 210 cm2 · V-1 · s-1 and a high z of 2.9 × 10-3 K-1 at 300 K. A [Formula: see text]T of 68 K at Th of 300 K, and a power generation efficiency of 5.8% below 450 K have been demonstrated for Mg3Bi1.5Sb0.5- and Mg3Bi1.25Sb0.75-based thermoelectric modules, respectively, which represent the cutting-edge advances in the near-room temperature thermoelectrics. In addition, the developed grain growth approach can be potentially extended to broad Zintl phases and other Mg-based alloys and compounds.

Highly efficient thermoelectric air conditioner with kilowatt capacity realized by ground source heat-exchanging system.

The enormous need for refrigeration of modern human life has inevitably aggravated the environmental crisis worldwide. To date, there are very few refrigeration technologies available beholding both harmless refrigerants and high efficiency. Here, we proposed a geothermal-thermoelectric air conditioning system (GeoTEAC) with refrigerant-free and high energy efficiency through synergistically combining the merits of thermoelectric effect and ground source heat exchanging system. The system showed competitive cooling and heating COPs of 5.83 and 2.92, respectively, with kilowatt capacity, which are 3-4 times higher than that of previously reported thermoelectric air-conditioning setups. For a conceptual scenario, we demonstrated the lowest TEWI values for the GeoTEAC system among different air-conditioning types. Our work provides sustainable and climate-friendly solutions to realize worldwide emission peaks and carbon neutralization.

Mobility enhancement in heavily doped semiconductors via electron cloaking.

Doping is central for solid-state devices from transistors to thermoelectric energy converters. The interaction between electrons and dopants plays a pivotal role in carrier transport. Conventional theory suggests that the Coulomb field of the ionized dopants limits the charge mobility at high carrier densities, and that either the atomic details of the dopants are unimportant or the mobility can only be further degraded, while experimental results often show that dopant choice affects mobility. In practice, the selection of dopants is still mostly a trial-and-error process. Here we demonstrate, via first-principles simulation and comparison with experiments, that a large short-range perturbation created by selected dopants can in fact counteract the long-range Coulomb field, leading to electron transport that is nearly immune to the presence of dopants. Such "cloaking" of dopants leads to enhanced mobilities at high carrier concentrations close to the intrinsic electron-phonon scattering limit. We show that the ionic radius can be used to guide dopant selection in order to achieve such an electron-cloaking effect. Our finding provides guidance to the selection of dopants for solid-state conductors to achieve high mobility for electronic, photonic, and energy conversion applications.

Conformal High-Power-Density Half-Heusler Thermoelectric Modules: A Pathway toward Practical Power Generators.

Thermoelectric generators (TEGs) exploiting the Seebeck effect provide a promising solution for waste heat recovery. Among the large number of thermoelectric (TE) materials, half-Heusler (hH) alloys are leading candidates for medium- to high-temperature power generation applications. However, the fundamental challenge in this field has been inhomogeneous material properties at large wafer diameters, insufficient power output from the modules, and rigid form factors of TE modules. This has restricted the transition of TEGs in practical applications for over three decades. Here, we successfully demonstrate large diameter wafers with uniform TE properties, high-power conformal hH TE modules for high-temperature application, and their direct integration on flue gas platforms, such as cylindrical tubes, to form large area flexible TEGs. This new conformal architecture design provides a breakthrough toward medium-/high-temperature TEGs over the conventional BiTe- and polymer-based flexible TEG design. A variable fill factor and greater flexibility due to the conformal design result in higher device performance as compared to conventional rigid TEG devices. Modules with 72-couple hH legs exhibit a device high-power-density of 3.13 W cm-2 and a total output power of 56.6 W under a temperature difference of 570 °C. These results provide a promising pathway toward widespread utilization of thermoelectric technology into the waste heat recovery application and will have a significant impact on the development of practical thermal to electrical converters.

Preparation of Room Temperature Vulcanized Silicone Rubber Foam with Excellent Flame Retardancy.

To retard the spread of fire in many cases with sealing materials is significant. A series of silicone rubber foam materials were prepared with room temperature vulcanization and foaming reactions. The morphology, chemical structure, cell structure, and thermal stability were investigated and results proved that the synthesis of silicone rubber was successful in a wide range of feed ratios. The fire-retardant tests were carried out to study the fire-proof property of the composite materials, and the excellent performance showed a promising prospect for wide application in sealing materials.

Bismuth Telluride Thermoelectrics with 8% Module Efficiency for Waste Heat Recovery Application.

Thermoelectric generators (TEGs) offer cost-effective and sustainable solid-state energy conversion mechanism from wasted heat into useful electrical power. Thermoelectric (TE) materials based upon bismuth telluride (BiTe) systems are widely utilized in applications ranging from energy generation to sensing to cooling. There is demand for BiTe materials with high figure of merit (zT) and TEG modules with high conversion efficiency over intermediate temperatures (25°C-250°C). Here we provide fundamental breakthrough in design of BiTe-based TE materials and utilize them to demonstrate modules with outstanding conversion efficiency of 8%, which is 40% higher compared with state-of-the-art commercial modules. The average zT of 1.08 for p-type and 0.84 for n-type bismuth telluride alloys is obtained between 25 and 250°C. The significant enhancement in zT is achieved through compositional and defect engineering in both p- and n-type materials. The high conversion efficiency accelerates the transition of TEGs for waste heat recovery.

Understanding Oxidation Resistance of Half-Heusler Alloys for in-Air High Temperature Sustainable Thermoelectric Generators.

High temperature waste heat recovery has gained tremendous interest to generate useful electricity while reducing the harmful impact on the environment. Thermoelectric (TE) solid-state materials enable direct conversion of heat into electricity with high efficiency, thereby offering a practical solution for waste heat recovery. Half-Heusler (hH) alloys are the leading TE materials for medium to high temperature applications, as they exhibit a high figure of merit and mechanical strength at temperatures as high as 973 K. Here we investigate the most promising hH alloys represented as MNiSn, MCoSb, and NbFeSb systems (M = Hf, Zr, and Ti) and provide fundamental understanding of their in-air thermal stability at high temperatures under realistic operating conditions required for energy generation. The understanding of oxidation resistance of TE materials is crucial for their practical deployment in extreme environments without vacuum sealing. The n-type MNiSn and p-type NbFeSb compounds are found to exhibit excellent oxidation resistance at a high temperature of 873 K. The oxidation resistance is enhanced through the presence of an intermetallic Ni-Sn layer for MNiSn and Nb-TiO2 double layer for (Nb,Ti)FeSb. A unicouple thermoelectric generator (TEG) fabricated from thermally stable materials demonstrated consistent performance for more than 150 h at 873 K in air. These results demonstrate the significance of TE materials in waste heat recovery systems.

Understanding the asymmetrical thermoelectric performance for discovering promising thermoelectric materials.

Thermoelectric modules, consisting of multiple pairs of n- and p-type legs, enable converting heat into electricity and vice versa. However, the thermoelectric performance is often asymmetrical, in that one type outperforms the other. In this paper, we identified the relationship between the asymmetrical thermoelectric performance and the weighted mobility ratio, a correlation that can help predict the thermoelectric performance of unreported materials. Here, a reasonably high ZT for the n-type ZrCoBi-based half-Heuslers is first predicted and then experimentally verified. A high peak ZT of ~1 at 973 K can be realized by ZrCo0.9Ni0.1Bi0.85Sb0.15. The measured heat-to-electricity conversion efficiency for the unicouple of ZrCoBi-based materials can be as high as ~10% at the cold-side temperature of ~303 K and at the hot-side temperature of ~983 K. Our work demonstrates that the ZrCoBi-based half-Heuslers are highly promising for the application of mid- and high-temperature thermoelectric power generation.

High thermoelectric cooling performance of n-type Mg3Bi2-based materials.

Thermoelectric materials have a large Peltier effect, making them attractive for solid-state cooling applications. Bismuth telluride (Bi2Te3)-based alloys have remained the state-of-the-art room-temperature materials for many decades. However, cost partially limited wider use of thermoelectric cooling devices because of the large amounts of expensive tellurium required. We report n-type magnesium bismuthide (Mg3Bi2)-based materials with a peak figure of merit (ZT) of ~0.9 at 350 kelvin, which is comparable to the commercial bismuth telluride selenide (Bi2Te3- x Se x ) but much cheaper. A cooling device made of our material and p-type bismuth antimony telluride (Bi0.5Sb1.5Te3) has produced a large temperature difference of ~91 kelvin at the hot-side temperature of 350 kelvin. n-type Mg3Bi2-based materials are promising for thermoelectric cooling applications.

Improved Thermoelectric Performance of Tellurium by Alloying with a Small Concentration of Selenium to Decrease Lattice Thermal Conductivity.

Phonon scattering through alloying is a highly effective way to reduce lattice thermal conductivity due to the mass difference between the host and alloyed atoms and strains caused by the different atoms. In this work we investigate the thermoelectric properties of Te between 323 and 623 K. By varying the alloying concentration of Se, a minimum lattice thermal conductivity was achieved with ∼10% (by stoichiometry) alloying of Te by Se. Additionally, Sb has been used as a dopant to increase the carrier concentration of the system. With reduced lattice thermal conductivity by Se alloying and increased carrier concentration by Sb doping, the room-temperature figure of merit ( ZT) increased by 60%, leading to an average ZT of ∼0.8 in Te0.88Se0.10Sb0.02, which corresponds to an engineering figure of merit ( ZT)eng ∼ 0.5 between 323 and 623 K and an efficiency of ∼8% in the same temperature range. The results indicate that the combination of Se alloying and Sb doping is successful in improving the thermoelectric properties of Te.

n-Type TaCoSn-Based Half-Heuslers as Promising Thermoelectric Materials.

Half-Heusler compounds are recognized as promising thermoelectric materials for high-temperature power generation, but their relatively high lattice thermal conductivity impedes further improvement of ZT. Here, we report the synthesis of a new half-Heusler compound TaCoSn with a low thermal conductivity. Experimentally, the pristine TaCoSn exhibits a low lattice thermal conductivity of ∼5.7 W m-1 K-1 at 300 K, which is lower than that of most of the other half-Heusler compounds. Phonon calculations by density functional theory indicate that the low phonon velocity, small Debye temperature, and large Grüneisen parameter are the contributors to the low thermal conductivity of TaCoSn. Importantly, intense point-defect scattering can be induced by alloying Nb at the Ta site, which further suppresses the thermal conductivity of TaCoSn. Additionally, Sb has been identified as an efficient dopant for supplying high electron concentration. Finally, the optimized n-type Ta0.6Nb0.4CoSn0.94Sb0.06 demonstrates a peak ZT above 0.7 at 973 K. Our work demonstrates that the TaCoSn-based half-Heuslers are promising thermoelectric materials.

Discovery of TaFeSb-based half-Heuslers with high thermoelectric performance.

Discovery of thermoelectric materials has long been realized by the Edisonian trial and error approach. However, recent progress in theoretical calculations, including the ability to predict structures of unknown phases along with their thermodynamic stability and functional properties, has enabled the so-called inverse design approach. Compared to the traditional materials discovery, the inverse design approach has the potential to substantially reduce the experimental efforts needed to identify promising compounds with target functionalities. By adopting this approach, here we have discovered several unreported half-Heusler compounds. Among them, the p-type TaFeSb-based half-Heusler demonstrates a record high ZT of ~1.52 at 973 K. Additionally, an ultrahigh average ZT of ~0.93 between 300 and 973 K is achieved. Such an extraordinary thermoelectric performance is further verified by the heat-to-electricity conversion efficiency measurement and a high efficiency of ~11.4% is obtained. Our work demonstrates that the TaFeSb-based half-Heuslers are highly promising for thermoelectric power generation.

Ultrahigh Power Factor in Thermoelectric System Nb0.95M0.05FeSb (M = Hf, Zr, and Ti).

Conversion efficiency and output power are crucial parameters for thermoelectric power generation that highly rely on figure of merit ZT and power factor (PF), respectively. Therefore, the synergistic optimization of electrical and thermal properties is imperative instead of optimizing just ZT by thermal conductivity reduction or just PF by electron transport enhancement. Here, it is demonstrated that Nb0.95Hf0.05FeSb has not only ultrahigh PF over ≈100 µW cm-1 K-2 at room temperature but also the highest ZT in a material system Nb0.95M0.05FeSb (M = Hf, Zr, Ti). It is found that Hf dopant is capable to simultaneously supply carriers for mobility optimization and introduce atomic disorder for reducing lattice thermal conductivity. As a result, Nb0.95Hf0.05FeSb distinguishes itself from other outstanding NbFeSb-based materials in both the PF and ZT. Additionally, a large output power density of ≈21.6 W cm-2 is achieved based on a single-leg device under a temperature difference of ≈560 K, showing the realistic prospect of the ultrahigh PF for power generation.

Large thermoelectric power factor from crystal symmetry-protected non-bonding orbital in half-Heuslers.

Modern society relies on high charge mobility for efficient energy production and fast information technologies. The power factor of a material-the combination of electrical conductivity and Seebeck coefficient-measures its ability to extract electrical power from temperature differences. Recent advancements in thermoelectric materials have achieved enhanced Seebeck coefficient by manipulating the electronic band structure. However, this approach generally applies at relatively low conductivities, preventing the realization of exceptionally high-power factors. In contrast, half-Heusler semiconductors have been shown to break through that barrier in a way that could not be explained. Here, we show that symmetry-protected orbital interactions can steer electron-acoustic phonon interactions towards high mobility. This high-mobility regime enables large power factors in half-Heuslers, well above the maximum measured values. We anticipate that our understanding will spark new routes to search for better thermoelectric materials, and to discover high electron mobility semiconductors for electronic and photonic applications.