Nanoscale Organic Thermoelectric Materials: Measurement,Theoretical Models,and Optimization Strategies

The demands for waste heat energy recovery from industrial production, solar energy, and electronic devices have resulted in increasing attention being focused on thermoelectric materials. Over the past two decades, significant progress is achieved in inorganic thermoelectric materials. In addition, with the proliferation of wireless mobile devices, economical, efficient, lightweight, and bio‐friendly organic thermoelectric (OTE) materials have gradually become promising candidates for thermoelectric devices used in room‐temperature environments. With the development of experimental measurement techniques, the manufacturing for nanoscale thermoelectric devices has become possible. A large number of studies have demonstrated the excellent performance of nanoscale thermoelectric devices, and further improvement of their thermoelectric conversion efficiency is expected to have a significant impact on global energy consumption. Here, the development of experimental measurement methods, theoretical models, and performance modulation for nanoscale OTE materials are summarized. Suggestions and prospects for the future development of these devices are also provided.     

Addressing the Challenges of Commercializing New Thermoelectric Materials

The time it takes for new thermoelectric materials to make the transition from first announcement in peer-reviewed publications to commercialization is undesirably long. As a result, universities, laboratories, government agencies, commercial users, and venture funding providers throughout the world have not supported research in the field to the level that would be expected for such an otherwise promising technology. This delay also has led to some misdirection of research efforts and a lack of availability of dependable long-term sponsorship commitments to research in the field. From the perspective of commercial users, this presentation discusses the challenges that the thermoelectric material research community faces in creating materials of commercial value. These challenges are broken down into objectives for both the traditional research activities related to improving ZT and those efforts needed to satisfy other, less recognized requirements which, if unaddressed, can significantly impede or even prevent commercialization. The ZT thresholds that enable much larger markets are presented for power generation, cooling, heating, and temperature control materials. Other important considerations, including semiconductor to metal interface (metallization) properties, material stability and constituent requirements, and costs and environmental-impact-related requirements are discussed. At the system level, factors that impede material development are identified, including challenges arising from a lack of property measurement repeatability among different organizations. Approaches and results are compared with that of the more heavily funded and rapidly developing photovoltaic field. The presentation concludes with recommendations for measures to accelerate thermoelectric material commercialization. International Conference on Thermoelectrics (August 3–7, 2008, Corvallis, Oregon, USA).     

Evaluation of Half‐Heusler Compounds as Thermoelectric Materials Based on the Calculated Electrical Transport Properties

A theoretical evaluation of the thermoelectric‐related electrical transport properties of 36 half‐Heusler (HH) compounds, selected from more than 100 HHs, is carried out in this paper. The electronic structures and electrical transport properties are studied using ab initio calculations and the Boltzmann transport equation under the constant relaxation time approximation for charge carriers. The electronic structure results predict the band gaps of these HH compounds, and show that many HHs are narrow‐band‐gap semiconductors and, therefore, are potentially good thermoelectric materials. The dependence of Seebeck coefficient, electrical conductivity, and power factor on the Fermi level is investigated. Maximum power factors and the corresponding optimal p‐ or n‐type doping levels, related to the thermoelectric performance of materials, are calculated for all HH compounds investigated, which certainly provide guidance to experimental work. The estimated optimal doping levels and Seebeck coefficients show reasonable agreement with the measured results for some HH systems. A few HHs are recommended to be potentially good thermoelectric materials based on our calculations.     

Organic Thermoelectric Materials: Niche Harvester of Thermal Energy

Organic thermoelectric (OTE) materials promise convenient energy conversion between heat gradients and voltage with flexible and wearable power-supplying devices at a low price. Although a variety of OTE materials are investigated, the TE performance is still far from practical application. To achieve high TE performance, a thorough understanding of the structure–property relationship in OTE materials is necessary. In this comprehensive review, the fundamentals of OTEs are summarized, the recent achievements of OTE materials are reviewed, and the relationship between structure and properties in high-performance OTE materials is discussed. Furthermore, how the molecular backbones, side chains, energy levels, molecular packing, and heteroatom effect all play vital roles in thermoelectric properties is addressed. Finally, the future direction of research on OTE materials is envisaged.     

Effect of Nanoparticles on Electron and Thermoelectric Transport

Recent experimental results have shown that adding nanoparticles inside a bulk material can enhance the thermoelectric performance by reducing the thermal conductivity and increasing the Seebeck coefficient. In this paper we investigate electron scattering from nanoparticles using different models. We compare the results of the Born approximation to that of the partial-wave method for a single nanoparticle scattering. The partial-wave method is more accurate for particle sizes in the 1 nm to 5 nm range where the point scattering approximation is not valid. The two methods can have different predictions for the thermoelectric properties such as the electrical conductivity and the Seebeck coefficient. To include a random distribution of nanoparticles, we consider an effective medium for the electron scattering using the coherent potential approximation. We compare various theoretical results with the experimental data obtained with ErAs nanoparticles in an InGaAlAs matrix. Reasonably good agreement is found between the measured and theoretical electrical conductivity and Seebeck data in the 300 K to 850 K temperature range.     

Advances on Emerging Materials for Flexible Supercapacitors: Current Trends and Beyond

The progressive size reduction of electronic components is experiencing bottlenecks in shrinking charge storage devices like batteries and supercapacitors, limiting their development into wearable and flexible zero‐pollution technologies. The inherent long cycle life, rapid charge–discharge patterns, and power density of supercapacitors rank them superior over other energy storage devices. In the modern market of zero‐pollution energy devices, currently the lightweight formula and shape adaptability are trending to meet the current requirement of wearables. Carbon nanomaterials have the potential to meet this demand, as they are the core of active electrode materials for supercapacitors and texturally tailored to demonstrate flexible and stretchable properties. With this perspective, the latest progress in novel materials from conventional carbons to recently developed and emerging nanomaterials toward lightweight stretchable active compounds for flexi‐wearable supercapacitors is presented. In addition, the limitations and challenges in realizing wearable energy storage systems and integrating the future of nanomaterials for efficient wearable technology are provided. Moreover, future perspectives on economically viable materials for wearables are also discussed, which could motivate researchers to pursue fabrication of cheap and efficient flexible nanomaterials for energy storage and pave the way for enabling a wide‐range of material‐based applications.     

High-Density Frenkel Defects as Origin of N-Type Thermoelectric Performance and Low Thermal Conductivity in Mg3Sb2-Based Materials

Mg₃Sb₂-based intermetallic compounds with exceptionally high thermoelectric performance exhibit unconventional n-type dopability and anomalously low thermal conductivity, attracting much attention to the underlying mechanisms. To date, investigations have been limited to first-principle calculations and thermodynamic analysis of defect formation, and detailed experimental analysis on crystal structure and phonon modes has not been achieved. Here, a synchrotron X-ray diffraction study clarifies that, against a previous view of a simple crystal structure with a small unit cell, Mg₃Sb₂ is inherently a heavily disordered material with Frenkel defects, charge-neutral defect complexes of cation vacancies and interstitials. Ionic charge neutrality preserved in Mg₃Sb₂ is responsible for exotic n-type dopability, which is unachievable for other Zintl phase materials. The thermal conductivity of Mg₃Sb₂ exhibits deviation from the standard T⁻¹ temperature dependency with strongly limited phonon transport due to a strain field. Inelastic X-ray scattering measurement reveals enhanced phonon scattering induced by disorder. The results will draw renewed attention to crystal defects and disorder as means to explore new high-performance thermoelectric materials.     

Enhancement of Thermopower of TAGS‐85 High‐Performance Thermoelectric Material by Doping with the Rare Earth Dy

Enhancement of thermopower is achieved by doping the narrow‐band semiconductor Ag_6.52Sb_6.52Ge_36.96Te₅₀ (acronym TAGS‐85), one of the best p‐type thermoelectric materials, with 1 or 2% of the rare earth dysprosium (Dy). Evidence for the incorporation of Dy into the lattice is provided by X‐ray diffraction and increased orientation‐dependent local fields detected by ¹²⁵Te NMR spectroscopy. Since Dy has a stable electronic configuration, the enhancement cannot be attributed to 4f‐electron states formed near the Fermi level. It is likely that the enhancement is due to a small reduction in the carrier concentration, detected by ¹²⁵Te NMR spectroscopy, but mostly due to energy filtering of the carriers by potential barriers formed in the lattice by Dy, which has large both atomic size and localized magnetic moment. The interplay between the thermopower, the electrical resistivity, and the thermal conductivity of TAGS‐85 doped with Dy results in an enhancement of the power factor (PF) and the thermoelectric figure of merit (ZT) at 730 K, from PF = 28 μW cm^?1 K^?2 and ZT ≤ 1.3 in TAGS‐85 to PF = 35 μW cm^?1 K^?2 and ZT ≥ 1.5 in TAGS‐85 doped with 1 or 2% Dy for Ge. This makes TAGS‐85 doped with Dy a promising material for thermoelectric power generation.     

High Thermoelectric Performance of ZnO by Coherent Phonon Scattering and Optimized Charge Transport

ZnO is identified as a potentially attractive n-type oxide thermoelectric material due to its abundance, nontoxicity, and a high degree of stability. However, working with ZnO is challenging due to its high thermal conductivity from its strong ionic bonds and low electrical conductivity due to its low charge concentrations. Here, it is demonstrated that the electrical and thermal transport properties of ZnO can be simultaneously improved via the successful doping of Al and ZnS coating. The ZnS coating in Al-doped ZnO is observed and analyzed through microstructure and spectroscopic studies. The power factor for 1% ZnS-coated Zn_0.98Al_0.02O is increased to ≈0.75 mW m⁻¹ K⁻² at 1073 K, 161% higher than pure ZnO. This enhancement in the power factor can be explained by the aliovalent Al³⁺ doping and modifications in intrinsic defects, leading to an increased carrier concentration. Interestingly, ZnS coating significantly reduces lattice thermal conductivity to ≈2.31 W m⁻¹ K⁻¹ at 1073 K for 2% ZnS-coated Zn_0.98Al_0.02O, a 62% decrease over pure ZnO. This large reduction in lattice thermal conductivity can be elucidated based on coherent phonon scattering via Callaway's model. Overall, the figure of merit, zT, increases to 0.2 in 2% ZnS-coated Zn_0.98Al_0.02O, which is 272% higher than pure ZnO at 1073 K.     

SnSe的制备方法、热电性能及潜在应用

硒化锡(SnSe)具有极低热导率、高功率因子和高热电优值,是一种性能优异的热电材料。SnSe的热电性能与其结构和掺杂状态密切相关。从SnSe的基本特性出发,介绍了SnSe的典型制备方法,综述了SnSe的热电性能及光(热)电性能,讨论了SnSe在光伏、锂离子电池、柔性热电器件及相变存储器等领域的潜在应用,总结了目前SnSe研究中存在的问题,并对提高其低温热电性能的方法进行了展望。     

Electronic Interpretation of Interlayer Energy Landscape in Layered Materials

The interlayer energy landscape of layered materials is essential to disassemble their structure–property relationships. However, a clear definition of interlayer electronic coupling that generally rules the interlayer energy landscape for their outstanding electronic and tribological properties, remains a matter of debate. Herein, diverse methods for electron coupling are evaluated to discriminate their feasibility to interpret interlayer sliding energy landscape for frictional sliding or stacking faults, by using density functional theory calculation of the layered models in the case of transition metal dichalcogenides (TMDs). It is discovered that the charge density evolution in dynamic stacking configurations dictates the interlayer energy landscape along the sliding pathway, challenging the prevailing belief that the energy corrugation arises from the nonuniform distribution of charge density or the charge density in the interface region. The present studies may open the way to disassemble the electron coupling principle underlying interlayer energy landscape for structure–property relationships as stacking faults, registry effects, even superlubric behavior in layered structures.     

Electronic Structure and Thermoelectric Properties of Si-Based Clathrate Compounds

Thermoelectric properties of Au-substituted Si-based clathrates, Ba₈AuGa₁₃Si₃₂ and Ba₆A₂AuGa _x Si_45−x (A = Sr, Eu, x = 13, 14), were experimentally and theoretically investigated. The polycrystalline samples of the Au-substituted Si-based clathrates were prepared by using the spark plasma sintering technique. The electronic structure of Ba₆A₂AuGa₁₃Si₃₂ was theoretically calculated by ab initio calculations, and the thermoelectric properties of Ba₆A₂AuGa _x Si_45−x were estimated through the calculated electronic structure. The effective mass of Ba₆A₂AuGa _x Si_45−x was experimentally estimated to be greater than that of Ba₈AuGa₁₃Si₃₂. Experimentally observed electronic properties agree with the calculations for Ba₆A₂AuGa _x Si_45−x . The maximum ZT value of Ba₆Sr₂AuGa₁₄Ge₃₁ is about 0.5 at 850 K. The calculated thermoelectric properties agree very well with the experimental results in the range from room temperature to 900 K.     

Nanomorphology and Charge Generation in Bulk Heterojunctions Based on Low‐Bandgap Dithiophene Polymers with Different Bridging Atoms

Carbon bridged (C‐PCPDTBT) and silicon‐bridged (Si‐PCPDTBT) dithiophene donor–acceptor copolymers belong to a promising class of low bandgap materials. Their higher field‐effect mobility, as high as 10^?2 cm² V^?1 s^?1 in pristine films, and their more balanced charge transport in blends with fullerenes make silicon‐bridged materials better candidates for use in photovoltaic devices. Striking morphological changes are observed in polymer:fullerene bulk heterojunctions upon the substitution of the bridging atom. XRD investigation indicates increased π–π stacking in Si‐PCPDTBT compared to the carbon‐bridged analogue. The fluorescence of this polymer and that of its counterpart C‐PCPDTBT indicates that the higher photogeneration achieved in Si‐PCPDTBT:fullerene films (with either [C60]PCBM or [C70]PCBM) can be correlated to the inactivation of a charge‐transfer complex and to a favorable length of the donor–acceptor phase separation. TEM studies of Si‐PCPDTBT:fullerene blended films suggest the formation of an interpenetrating network whose phase distribution is comparable to the one achieved in C‐PCPDTBT:fullerene using 1,8‐octanedithiol as an additive. In order to achieve a balanced hole and electron transport, Si‐PCPDTBT requires a lower fullerene content (between 50 to 60 wt%) than C‐PCPDTBT (more than 70 wt%). The Si‐PCPDTBT:[C70]PCBM OBHJ solar cells deliver power conversion efficiencies of over 5%.     

Valleytronics,Carrier Filtering and Thermoelectricity in Bismuth: Magnetic Field Polarization Effects

Valley polarization of multi‐valleyed materials is of significant interest for potential applications in electronic devices. The main challenge is removing the valley degeneracy in some controllable way. The unique properties of bismuth, including its anisotropic electronic structure and Dirac valley degeneracy, make this material an excellent system for valleytronics. It is demonstrated theoretically that the direction of an externally applied magnetic field in the binary‐bisectrix plane has a profound effect not only on the charge, but also on the thermal transport along the trigonal direction. The rotating field probes the electronic mass anisotropy and tunes the contribution from a particular Dirac valley in the electrical resistivity, Seebeck coefficient, and thermal conductivity at moderate temperatures and field strengths. It is further shown that the field polarization of the transport properties is accompanied by selective filtering of the carriers type providing further opportunities for thermoelectric transport control.     

Achieving High-Efficiency Wind Energy Harvesting Triboelectric Nanogenerator by Coupling Soft Contact,Charge Space Accumulation,and Charge Dissipation Design

To enhance the durability of a triboelectric nanogenerator (TENG), soft contact is an effective approach due to its flexible and elastic contact mode. However, soft contact is hard to obtain with a large charge density by triboelectrification, resulting in low power output. Herein, a novel blade soft contact TENG (BSC-TENG), coupling flexible functional blades with shielded electrodes on the rotor, charge accumulation, and charge dissipation design on the stator, is proposed. Extra polishing blades and debris storage grooves are adopted in the BSC-TENG to further ensure high durability. A remarkable charge density of 328 µC m⁻² is achieved, setting a new record for soft contact TENGs. Besides, the output charge remains at 100% even after 200 000 cycles. The wind-driven BSC-TENG not only can power 3840 green LEDs and 80 parallel hygrothermometers but also can drive electronic devices for smart farms, establishing self-powered sensing systems. This work provides a novel strategy for enhancing soft contact TENG output and durability.     

基于压电材料的振动能量回收技术现状综述

随着微机电系统(MEMS)技术的迅猛发展,基于压电振动的能量回收技术可以为MEMS提供电能,受到国内外众多学者的关注。该文介绍了压电式振动能量回收装置的工作机理;分别从能量回收装置的结构和材料、能量转化的接口电路、能量的存储技术、能量回收的应用实例等方面系统的介绍国内外的主要研究成果和研究进展;并对压电振动能量回收技术的发展方向进行了预测。     

Peering into Alloy Anodes for Sodium‐Ion Batteries: Current Trends,Challenges, and Opportunities

Sodium‐ion batteries (SIBs) are regarded as a complementary technology to lithium‐ion batteries (LIBs) in the effort of searching for alternative energy solutions that are cost‐effective and sustainable. The identification of suitable alternative anode materials is essential to close the gap in energy density between SIBs and LIBs. Solid‐state alloying reactions that work beyond intercalation mechanism are able to provide a significant improvement in specific capacity. This review describes key advances in SIBs with a primary emphasis on alloy anodes. Recent information and results published in the literatures are stressed to provide an overview of their development in SIBs. With the discussion of some of the remaining challenges and possible solutions, the authors hope to sketch out the scope for future studies in this field.     

Tailoring Interfacial Charge Transfer for Optimizing Thermoelectric Performances of MnTe-Sb2Te3 Superlattice-Like Films

Interfacial charge transfer has a vital role in tailoring the thermoelectric performance of superlattices (SLs), which, however, is rarely clarified by experiments. Herein, based on epitaxially grown p-type (MnTe)_x(Sb₂Te₃)_y superlattice-like films, synergistically optimized thermoelectric parameters of carrier density, carrier mobility, and Seebeck coefficient are achieved by introducing interfacial charge transfer, in which effects of hole injection, modulation doping, and energy filtering are involved. Carrier transport measurements and angle-resolved photoemission spectroscopy (ARPES) characterizations reveal a strong hole injection from the MnTe layer to the Sb₂Te₃ layer in the SLs, originating from the work function difference between MnTe and Sb₂Te₃. By reducing the thickness of MnTe less than one monolayer, all electronic transport parameters are synergistically optimized in the quantum-dots (MnTe)_x(Sb₂Te₃)₁₂ superlattice-like films, leading to much improved thermoelectric power factors (PFs). The (MnTe)_0.1(Sb₂Te₃)₁₂ obtains the highest room-temperature PF of 2.50 mWm⁻¹K⁻², while the (MnTe)_0.25(Sb₂Te₃)₁₂ possesses the highest PF of 2.79 mWm⁻¹K⁻² at 381 K, remarkably superior to the values acquired in binary MnTe and Sb₂Te₃ films. This research provides valuable guidance on understanding and rationally tailoring the interfacial charge transfer of thermoelectric SLs to further enhance thermoelectric performances.     

A Materials Perspective on Direct Recycling of Lithium-Ion Batteries: Principles,Challenges and Opportunities

As the dominant means of energy storage technology today, the widespread deployment of lithium-ion batteries (LIBs) would inevitably generate countless spent batteries at their end of life. From the perspectives of environmental protection and resource sustainability, recycling is a necessary strategy to manage end-of-life LIBs. Compared with traditional hydrometallurgical and pyrometallurgical recycling methods, the emerging direct recycling technology, rejuvenating spent electrode materials via a non-destructive way, has attracted rising attention due to its energy efficient processes along with increased economic return and reduced CO₂ footprint. This review investigates the state-of-the-art direct recycling technologies based on effective relithiation through solid-state, aqueous, eutectic solution and ionic liquid mediums and thoroughly discusses the underlying regeneration mechanism of each method regarding different battery chemistries. It is concluded that direct regeneration can be a more energy-efficient, cost-effective, and sustainable way to recycle spent LIBs compared with traditional approaches. Additionally, it is also identified that the direct recycling technology is still in its infancy with several fundamental and technological hurdles such as efficient separation, binder removal and electrolyte recovery. In addressing these remaining challenges, this review proposes an outlook on potential technical avenues to accelerate the development of direct recycling toward industrial applications.