Flexible Quasi‐Solid State Ionogels with Remarkable Seebeck Coefficient and High Thermoelectric Properties

Thermoelectric materials can be used to harvest low‐grade heat that is otherwise dissipated to the environment. But the conventional thermoelectric materials that are semiconductors or semimetals, usually exhibit a Seebeck coefficient of much less than 1 mV K^?1. They are expensive and consist of toxic elements as well. Here, it is demonstrated environmental benign flexible quasi‐solid state ionogels with giant Seebeck coefficient and ultrahigh thermoelectric properties. The ionogels made of ionic liquids and poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) can exhibit a giant Seebeck coefficient up to 26.1 mV K^?1, the highest for electronic and ionic conductors. In addition, they have a high ionic conductivity of 6.7 mS cm^?1 and a low thermal conductivity of 0.176 W m^?1 K^?1. Their thermoelectric figure of merit (ZT) is thus 0.75. The giant Seebeck coefficient is related to the ion‐dipole interaction between PVDF‐HFP and ionic liquids. Their application in ionic thermoelectric capacitors is also demonstrated for the conversion of intermittent heat into electricity. They are especially important to harvest the low‐grade thermal energy that is abundant on earth.     

High‐Performance GeTe‐Based Thermoelectrics: from Materials to Devices

High‐performance GeTe‐based thermoelectrics have been recently attracting growing research interest. Here, an overview is presented of the structural and electronic band characteristics of GeTe. Intrinsically, compared to low‐temperature rhombohedral GeTe, the high‐symmetry and high‐temperature cubic GeTe has a low energy offset between L and Σ points of the valence band, the reduced direct bandgap and phonon group velocity, and as a result, high thermoelectric performance. Moreover, their thermoelectric performance can be effectively enhanced through either carrier concentration optimization, band structure engineering (bandgap reduction, band degeneracy, and resonant state engineering), or restrained lattice thermal conductivity (phonon velocity reduction or phonon scattering). Consequently, the dimensionless figure of merit, ZT values, of GeTe‐based thermoelectric materials can be higher than 2. The mechanical and thermal stabilities of GeTe‐based thermoelectrics are highlighted, and it is found that they are suitable for practical thermoelectric applications except for their high cost. Finally, it is recognized that the performance of GeTe‐based materials can be further enhanced through synergistic effects. Additionally, proper material selection and module design can further boost the energy conversion efficiency of GeTe‐based thermoelectrics.     

Sodium‐Doped Tin Sulfide Single Crystal: A Nontoxic Earth‐Abundant Material with High Thermoelectric Performance

Lead‐free tin sulfide (SnS), with an analogous structure to SnSe, has attracted increasing attention because of its theoretically predicted high thermoelectric performance. In practice, however, polycrystalline SnS performs rather poorly as a result of its low power factor. In this work, bulk sodium (Na)‐doped SnS single crystals are synthesized using a modified Bridgman method and a detailed transport evaluation is conducted. The highest zT value of ≈1.1 is reached at 870 K in a 2 at% Na‐doped SnS single crystal along the b‐axis direction, in which high power factors (2.0 mW m^?1 K^?2 at room temperature) are realized. These high power factors are attributed to the high mobility associated with the single crystalline nature of the samples as well as to the enhanced carrier concentration achieved through Na doping. An effective single parabolic band model coupled with first‐principles calculations is used to provide theoretical insight into the electronic transport properties. This work demonstrates that SnS‐based single crystals composed of earth‐abundant, low‐cost, and nontoxic chemical elements can exhibit high thermoelectric performance and thus hold potential for application in the area of waste heat recovery.     

Highly Passivated n‐Type Colloidal Quantum Dots for Solution‐Processed Thermoelectric Generators with Large Output Voltage

Colloidal quantum dots (CQDs) are attractive materials for thermoelectric applications due to their simple and low‐cost processing; advantageously, they also offer low thermal conductivity and high Seebeck coefficient. To date, the majority of CQD thermoelectric films reported upon have been p‐type, while only a few reports are available on n‐type films. High‐performing n‐ and p‐type films are essential for thermoelectric generators (TEGs) with large output voltage and power. Here, high‐thermoelectric‐performance n‐type CQD films are reported and showcased in high‐performance all‐CQD TEGs. By engineering the electronic coupling in the films, a thorough removal of insulating ligands is achieved and this is combined with excellent surface trap passivation. This enables a high thermoelectric power factor of 24 µW m^?1 K^?2, superior to previously reported n‐type lead chalcogenide CQD films operating near room temperature (<1 µW m^?1 K^?2). As a result, an all‐CQD film TEG with a large output voltage of 0.25 V and a power density of 0.63 W m^?2 at ?T = 50 K is demonstrated, which represents an over fourfold enhancement to previously reported p‐type only CQD TEGs.     

Thermally Chargeable Solid‐State Supercapacitor

Ubiquitous low‐grade thermal energy, which is typically wasted without use, can be extremely valuable for continuously powering electronic devices such as sensors and wearable electronics. A popular choice for waste heat recovery has been thermoelectric energy conversion, but small output voltage without energy‐storing capability necessitates additional components such as a voltage booster and a capacitor. Here, a novel method of simultaneously generating a large voltage from a temperature gradient and storing electrical energy without losing the benefit of solid‐state no‐moving part devices like conventional thermoelectrics is reported. Thermally driven ion diffusion is used to greatly increase the output voltage (8 mV K^?1) with polystyrene sulfonic acid (PSSH) film. Polyaniline‐coated electrodes containing graphene and carbon nanotube sandwich the PSSH film where thermally induced voltage‐enabled electrochemical reactions, resulting in a charging behavior without an external power supply. With a small temperature difference (5 K) possibly created over wearable energy harvesting devices, the thermally chargeable supercapacitor produce 38 mV with a large areal capacitance (1200 F m^?2). It is anticipated that the attempt with thermally driven ion diffusion behaviors initiates a new research direction in thermal energy harvesting.     

High Performance Thermoelectricity in Earth‐Abundant Compounds Based on Natural Mineral Tetrahedrites

Thermoelectric materials can convert waste heat into electricity, potentially improving the efficiency of energy usage in both industry and everyday life. Unfortunately, known good thermoelectric materials often are comprised of elements that are in low abundance and require careful doping and complex synthesis procedures. Here, we report dimensionless thermoelectric figure of merit near unity in compounds of the form Cu_12‐xM_xSb₄S₁₃, where M is a transition metal such as Zn or Fe, for wide ranges of x. The compounds investigated here span the range of compositions of the natural mineral family of tetrahedrites, the most widespread sulfosalts on Earth, and we further show that the natural mineral itself can be used directly as an inexpensive source thermoelectric material. Thermoelectrics comprised of earth‐abundant elements will pave the way to many new, low cost thermoelectric energy generation opportunities.     

Synergistically Optimizing Electrical and Thermal Transport Properties of BiCuSeO via a Dual‐Doping Approach

The layered oxyselenide BiCuSeO system is known as one of the high‐performance thermoelectric materials with intrinsically low thermal conductivity. By employing atomic, nano‐ to mesoscale structural optimizations, low thermal conductivity coupled with enhanced electrical transport properties can be readily achieved. Upon partial substitution of Bi³⁺ by Ca²⁺ and Pb²⁺, the thermal conductivity can be reduced to as low as 0.5 W m^?1 K^?1 at 873 K through dual‐atomic point‐defect scattering, while a high power factor of ≈1 × 10^?3 W cm^?1 K^?2 is realized over a broad temperature range from 300 to 873 K. The synergistically optimized power factor and intrinsically low thermal conductivity result in a high ZT value of ≈1.5 at 873 K for Bi_0.88Ca_0.06Pb_0.06CuSeO, a promising candidate for high‐temperature thermoelectric applications. It is envisioned that the all‐scale structural optimization is critical for optimizing the thermoelectricity of quaternary compounds.     

Lead‐Free Thermoelectrics: High Figure of Merit in p‐type AgSnmSbTem+2

Thermoelectric materials based on Pb‐free compositions are of considerable current interest in environmentally friendly power‐generation applications derived from waste‐heat sources. Here, a new study of the thermoelectric properties of the tin‐based compositions with the general formula AgSn_mSbTe_m+2 (m = 2, 4, 5, 7, 10, 14, 18) is presented, where the m value is used as the tuning parameter of the thermoelectric properties. The electrical conductivity, Seebeck coefficient, and thermal conductivity are measured from 300 K to 723 K and the resulting thermoelectric figure of merit is determined as a function of the SnTe/AgSbTe₂ ratio. A thermoelectric figure of merit ZT ≈1 is obtained at 710 K for m = 4, indicating that the system AgSn_mSbTe_m+2 holds great promise as an alternative p‐type, lead‐free, thermoelectric material.     

High Thermoelectric Efficiency of n‐type PbS

PbS shares several features with the other lead chalcogenides PbX (X: Te, Se), which are good thermoelectric materials. PbS has a potential advantage in that it is quite earth abundant and inexpensive. In this work we tune the transport properties in n‐type, single‐phase polycrystalline PbS_1‐xCl_x (x ≤ 0.008) with different carrier densities. Lead chloride provides a nearly 100% efficient doping control up to 1.2 × 10²⁰ cm^?3. The maximum zT achieved at 850 K is 0.7 with a predicted zT ～ 1 at 1000 K. This is about twice as high as what was previously reported (～0.4) for binary PbS. Compared with the other lead chalcogenides the higher effective mass and higher lattice thermal conductivity makes binary PbS an inferior thermoelectric material. However this study also predicts greater potential of zT improvement in PbS by material engineering such as alloying or nanostructuring compared to PbSe or PbTe. Considering their abundance and low cost, PbS based materials are quite competitive among the lead chalcogenides for thermoelectric applications.     

Optimum Carrier Concentration in n‐Type PbTe Thermoelectrics

Taking La‐ and I‐doped PbTe as an example, the current work shows the effects of optimizing the thermoelectric figure of merit, zT, by controlling the doping level. The high doping effectiveness allows the carrier concentration to be precisely designed and prepared to control the Fermi level. In addition to the Fermi energy tuning, La‐doping modifies the conduction band, leading to an increase in the density of states effective mass that is confirmed by transport, infrared reflectance and hard X‐ray photoelectron spectroscopy measurements. Taking such a band structure modification effect into account, the electrical transport properties can then be well‐described by a self‐consistent single non‐parabolic Kane band model that yields an approximate (m*T)^1.5 dependence of the optimal carrier concentration for a peak power factor in both doping cases. Such a simple temperature dependence also provides an effective approximation of carrier concentration for a peak zT and helps to explain, the effects of other strategies such as lowering the lattice thermal conductivity by nanostructuring or alloying in n‐PbTe, which demonstrates a practical guide for fully optimizing thermoelectric materials in the entire temperature range. The principles used here should be equally applicable to other thermoelectric materials.     

Biomimetic MXene Textures with Enhanced Light‐to‐Heat Conversion for Solar Steam Generation and Wearable Thermal Management

2D materials are of particular interest in light‐to‐heat conversion, yet challenges remain in developing a facile method to suppress their light reflection. Herein, inspired by the black scales of Bitis rhinoceros, a generalized approach via sequential thermal actuations to construct biomimetic 2D‐material nanocoatings, including Ti₃C₂T_x MXene, reduced graphene oxide (rGO), and molybdenum disulfide (MoS₂) is designed. The hierarchical MXene nanocoatings result in broadband light absorption (up to 93.2%), theoretically validated by optical modeling and simulations, and realize improved light‐to‐heat performance (equilibrium temperature of 65.4 °C under one‐sun illumination). With efficient light‐to‐heat conversion, the bioinspired MXene nanocoatings are next incorporated into solar steam‐generation devices and stretchable solar/electric dual‐heaters. The MXene steam‐generation devices require much lower solar‐thermal material loading (0.32 mg cm^?2) and still guarantee high steam‐generation performance (1.33 kg m^?2 h^?1) compared with other state‐of‐the‐art devices. Additionally, the mechanically deformed MXene structures enable the fabrication of stretchable and wearable heaters dual‐powered by sunlight and electricity, which are reversibly stretched and heated above 100 °C. This simple fabrication process with effective utilization of active materials promises its practical application value for multiple solar–thermal technologies.     

Functional Graded Germanium–Lead Chalcogenide‐Based Thermoelectric Module for Renewable Energy Applications

High thermoelectric conversion efficiencies can be achieved by making use of materials with, as high as possible, figure of merit, ZT, values. Moreover, even higher performance is possible with appropriate geometrical optimization including the use of functionally graded materials (FGM) technology. Here, an advanced n‐type functionally graded thermoelectric material based on a phase‐separated (PbSn_0.05Te)_0.92(PbS)_0.08 matrix is reported. For assessment of the thermoelectric potential of this material, combined with the previously reported p‐type Ge_0.87Pb_0.13Te showing a remarkable dimensionless figure of merit of 2.2, a finite‐element thermoelectric model is developed. The results predict, for the investigated thermoelectric couple, a very impressive thermoelectric efficiency of 14%, which is more than 20% higher than previously reported values for operating under cold and hot junction temperatures of 50 °C and 500 °C, respectively. Validation of the model prediction is done by a thermoelectric couple fabricated according to the model's geometrical optimization conditions, showing a good agreement to the theoretically calculated results, hence approaching a higher technology readiness level.     

Low‐Cost N,N′‐Bicarbazole‐Based Dopant‐Free Hole‐Transporting Materials for Large‐Area Perovskite Solar Cells

Despite the recent unprecedented development of efficient dopant‐free hole transporting materials (HTMs) for high‐performance perovskite solar cells (PSCs) on small‐area devices (≤0.1 cm²), low‐cost dopant‐free HTMs for large‐area PSCs (≥1 cm²) with high power conversion efficiencies (PCEs) have rarely been reported. Herein, two novel HTMs, 3,3′,6,6′ (or 2,2′,7,7′)‐tetrakis(N,N′‐di‐p‐methoxyphenylamine)‐N,N′‐bicarbazole (3,6 BCz‐OMeTAD or 2,7 BCz‐OMeTAD), are synthesized via an extremely simple route from very cheap raw materials. Owing to their excellent film‐forming abilities and matching energy levels, 3,6 BCz‐OMeTAD and 2,7 BCz‐OMeTAD can be successfully employed as a perfect ultrathin (≈30 nm) hole transporting layer in large‐area PSCs up to 1 cm². The 3,6 BCz‐OMeTAD and 2,7 BCz‐OMeTAD based large‐area PSCs show highest PCEs up to 17.0% and 17.6%, respectively. More importantly, high performance large‐area PSCs based on 2,7 BCz‐OMeTAD retain 90% of the initial efficiency after 2000 h storage in an ambient environment without encapsulation.     

High Thermoelectric zT in n‐Type Silver Selenide films at Room Temperature

In this work, a zT value as high as 1.2 at room temperature for n‐type Ag₂Se films is reported grown by pulsed hybrid reactive magnetron sputtering (PHRMS). PHRMS is a novel technique developed in the lab that allows to grow film of selenides with different compositions in a few minutes with great quality. The improved zT value reported for room temperature results from the combination of the high power factors, similar to the best values reported for bulk Ag₂Se (2440 ± 192 µW m^?1 K^?2), along with a reduced thermoelectric conductivity as low as 0.64 ± 0.1 W m^?1 K^?1. The maximum power factor for these films is of 4655 ± 407 µW m^?1 K^?2 at 103 °C. This material shows promise to work for room temperature applications. Obtaining high zT or, in other words, high power factor and low thermal conductivity values close to room temperature for thin films is of high importance to develop a new generation of wearable devices based on thermoelectric heat recovery.     

Pathway Engineering of Bacillus subtilis for Enhanced N‐Acetylneuraminic Acid Production via Whole‐Cell Biocatalysis

N‐acetylneuraminic acid (NeuAc) is a common sialic acid that has a wide range of applications in nutraceuticals and pharmaceuticals. However, low production efficiency and high environmental pollution associated with traditional extraction and chemical synthesis methods constrain the supply of NeuAc. Here, a biological approach is developed for food‐grade NeuAc production via whole‐cell biocatalysis by the generally regarded as safe (GRAS) bacterium Bacillus subtilis (B. subtilis). Promoters for controlling N‐acetylglucosamine 2‐epimerase (AGE) and NeuAc adolase (NanA) are optimized, yielding 32.84 g L^?1 NeuAc production with a molar conversion rate of 26.55% from N‐acetylglucosamine (GlcNAc). Next, NeuAc production is further enhanced to 46.04 g L^?1, which is 40.2% higher than that of the strain with promoter optimization, by expressing NanA from Staphylococcus hominis instead of NanA from Escherichia coli. To enhance the expression level of ShNanA, the N‐terminal coding sequences of genes with high expression levels are fused to the 5′‐end of the ShNanA gene, resulting in 56.82 g L^?1 NeuAc production. Finally, formation of the by‐product acetoin from pyruvate is blocked by deleting the alsS and alsD genes, resulting in 68.75 g L^?1 NeuAc production with a molar conversion rate of 55.57% from GlcNAc. Overall, a GRAS B. subtilis strain is demonstrated as a whole‐cell biocatalyst for efficient NeuAc production.     

All‐Printed MnHCF‐MnOx‐Based High‐Performance Flexible Supercapacitors

Here, a simple active materials synthesis method is presented that boosts electrode performance and utilizes a facile screen‐printing technique to prepare scalable patterned flexible supercapacitors based on manganese hexacyanoferrate‐manganese oxide and electrochemically reduced graphene oxide electrode materials (MnHCF‐MnO_x/ErGO). A very simple in situ self‐reaction method is developed to introduce MnO_x pseudocapacitor material into the MnHCF system by using NH₄F. This MnHCF‐MnO_x electrode materials can deliver excellent capacitance of 467 F g^?1 at a current density of 1 A g^?1, which is a 2.4 times capacitance increase compared to MnHCF. In addition a printed, patterned, flexible MnHCF‐MnO_x/ErGO supercapacitor is fabricated, showing a remarkable areal capacitance of 16.8 mF cm^?2 and considerable energy and power density of 0.5 mWh cm^?2 and 0.0023 mW cm^?2, respectively. Furthermore, the printed patterned flexible supercapacitors also exhibit exceptional flexibility, and the capacitance remains stable, even while bending to various angles (60°, 90°, and 180°) and for 100 cycles. The flexible supercapacitor arrays integrated by multiple prepared single supercapacitors can power various LEDs even in the bent states. This approach offers promising opportunities for the development of printable energy storage materials and devices with high energy density, large scalability, and excellent flexibility.     

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.     

Improved Bulk Materials with Thermoelectric Figure‐of‐Merit Greater than 1: Tl10–xSnxTe6 and Tl10–xPbxTe6

Noting the steadily worsening problem of depleted fossil fuel sources, alternate energy sources have become increasingly important; these include thermoelectrics, which may use waste heat to generate electricity. To be economically viable, the thermoelectric figure‐of‐merit, zT, which is related to the energy conversion efficiency, needs to be in excess of unity (zT > 1). Tl₄SnTe₃ and Tl₄PbTe₃ were reported to attain a thermoelectric figure‐of‐merit zT _max = 0.74 and 0.71, respectively, at 673 K. Here, the thermoelectric properties of both materials are presented as a function of x in Tl_10–x Sn _x Te₆ and Tl_10–x Pb _x Te₆, with x varying between 1.9 and 2.05, culminating in zT values in excess of 1.2. These materials are charge balanced when x = 2, according to (Tl⁺)₈(Sn²⁺)₂(Te^2?)₆ and (Tl⁺)₈(Pb²⁺)₂(Te^2?)₆ (or: (Tl⁺)₄Pb²⁺(Te^2?)₃). Increasing x causes an increase in valence electrons, and thus a decrease in the dominating p‐type charge carriers. Larger x values occur with a smaller electrical conductivity and a larger Seebeck coefficient. In each case, the lattice thermal conductivity remains under 0.5 W m^?1 K^?1, resulting in several samples attaining the desired zT _max > 1. The highest values thus far are exhibited by Tl_8.05Sn_1.95Te₆ with zT = 1.26 and Tl_8.10Pb_1.90Te₆ with zT = 1.46 around 685 K.     

Low‐Temperature‐Processed Colloidal Quantum Dots as Building Blocks for Thermoelectrics

Colloidal quantum dots (CQDs) are demonstrated to be promising materials to realize high‐performance thermoelectrics owing to their low thermal conductivity. The most studied CQD films, however, are using long ligands that require high processing and operation temperature (>400 °C) to achieve optimum thermoelectric performance. Here the thermoelectric properties of CQD films cross‐linked using short ligands that allow strong inter‐QD coupling are reported. Using the ligands, p‐type thermoelectric solids are demonstrated with a high Seebeck coefficient and power factor of 400 μV K^?1 and 30 µW m^?1 K^?2, respectively, leading to maximum ZT of 0.02 at a lower measurement temperature (<400 K) and lower processing temperature (<300 °C). These ligands further reduce the annealing temperature to 175 °C, significantly increasing the Seebeck coefficient of the CQD films to 580 μV K^?1. This high Seebeck coefficient with a superior ZT near room temperature compared to previously reported high temperature‐annealed CQD films is ascribed to the smaller grain size, which enables the retainment of quantum confinement and significantly increases the hole effective mass in the films. This study provides a pathway to approach quantum confinement for achieving a high Seebeck coefficient yet strong inter‐QD coupling, which offers a step toward low‐temperature‐processed high‐performance thermoelectric generators.     

Subtle Roles of Sb and S in Regulating the Thermoelectric Properties of N‐Type PbTe to High Performance

A high ZT (thermoelectric figure of merit) of ≈1.4 at 900 K for n‐type PbTe is reported, through modifying its electrical and thermal properties by incorporating Sb and S, respectively. Sb is confirmed to be an amphoteric dopant in PbTe, filling Te vacancies at low doping levels (<1%), exceeding which it enters into Pb sites. It is found that Sb‐doped PbTe exhibits much higher carrier mobility than similar Bi‐doped materials, and accordingly, delivers higher power factors and superior ZT . The enhanced electronic transport is attributed to the elimination of Te vacancies, which appear to strongly scatter n‐type charge carriers. Building on this result, the ZT of Pb_0.9875Sb_0.0125Te is further enhanced by alloying S into the Te sublattice. The introduction of S opens the bandgap of PbTe, which suppresses bipolar conduction while simultaneously increasing the electron concentration and electrical conductivity. Furthermore, it introduces point defects and induces second phase nanostructuring, which lowers the lattice thermal conductivity to ≈0.5 W m^?1 K^?1 at 900 K, making this material a robust candidate for high‐temperature (500–900 K) thermoelectric applications. It is anticipated that the insights provided here will be an important addition to the growing arsenal of strategies for optimizing the performance of thermoelectric materials.