Scalable-produced 3D elastic thermoelectric network for body heat harvesting

Defect Engineering for Flexible n-Type Mo2TiC2Tx o-MXene Thermoelectric Efficiency Enhancement

With the rapid advancement of wearable electronics, the demand for efficient portable power supplies has become increasingly urgent. Thermoelectric materials, which can directly convert heat, such as body heat, into electricity, offer a promising avenue for sustainable energy supplementation. However, achieving a high thermoelectric performance in flexible materials suitable for body heat harvesting remains a significant challenge. Here, we introduce a strategy for synergistically tuning surface oxygen defects and optimizing microstructures in low-dimensional semiconductor materials, resulting in flexible, ammoniated dual-transition metal carbide o-MXene N-Mo2TiC2Tx with enhanced properties. Theoretical and experimental analyses reveal that high-temperature ammoniation produces a low-oxygen-functionalized surface, reduces interlayer spacing, and minimizes defect density, thereby significantly increasing the electrical conductivity. Nitrogen atoms incorporated at the nanosheet terminals further increase the electron density near the Fermi level, resulting in an enhanced Seebeck coefficient. Consequently, N-Mo2TiC2Tx films treated at 900 K achieve an electrical conductivity of 1.03 × 104 S m-1, a Seebeck coefficient of -27.8 μV K-1, and a power factor of 7.99 μW m-1 K-2 at room temperature, nearly 1.2-fold higher than that of untreated materials, while retaining excellent flexibility. Moreover, a wearable thermoelectric generator constructed from these N-Mo2TiC2Tx films produces a voltage of 1.4 mV under a temperature gradient of approximately 12 K between human skin and ambient air, underscoring its excellent capacity for harvesting low-grade thermal energy. These findings establish a paradigm for the development of flexible, high-performance thermoelectric materials, paving the way for next-generation wearable and industrial energy applications.

N-Type Silver Selenide Thermoelectric Cotton Thread for Antibacterial and Versatile Textile Electronics

Thermoelectric textiles have garnered significant attention in energy harvesting and temperature sensing due to their comfort and reliable long-term power generation capabilities. However, existing thermoelectric textiles rarely realize antibacterial, high output performance, and sensing capabilities simultaneously. Here, we present a facile and scalable method for fabricating n-type silver selenide (Ag2Se) cotton threads with exceptional antibacterial, high power output, and advanced sensing capabilities. The Ag-Ag2Se segmented structures are prepared using the segmented selenization method. Subsequently, a thermoelectric textile consisting of 50 pairs of p-n legs is fabricated, which can generate a power density of 500 μW m-2 at a temperature difference of 30 K, and it can provide an output voltage of 24.7 mV when worn on the arm at room temperature. The textile-based sensor exhibits temperature detection (0.7 K) and a response time (2.49 s). Integrating Ag2Se cotton threads onto textiles enables the utilization of multipixel touchpads for writing and communication. Additionally, these sensors can be incorporated into gloves to accurately detect the surrounding objects' temperatures. This thermoelectric cotton thread not only facilitates energy harvesting but also establishes a solid foundation for widespread application in multifunctional textile electronics.

Strategies and Prospects for High-Performance Te-Free Thermoelectric Materials

Thermoelectric materials hold great potential for direct conversion of ubiquitous waste heat into electricity. However, their commercialization is hindered by low efficiency, reliance on rare and expensive Te, and limited stability under operating conditions. This review explores recent advances in novel strategies for achieving high thermoelectric performance and stability in Te-free inorganic bulk materials. First, we discuss diverse innovative techniques aimed at substantially enhancing electrical transport properties. These methods encompass strategies such as charge carrier engineering, band convergence, band inversion, valley anisotropy, multiband synglisis, and the incorporation of resonant levels or midgap states. Then we focus on strategies to reduce lattice thermal conductivity, including phonon scattering induced by multidimensional defects, off-center doping, resonance scattering, and lattice softening. Additionally, this review presents strategies for decoupling electron and phonon transport to enhance the thermoelectric performance of materials further. The strategies include interface engineering, crystal symmetry manipulation, high-entropy engineering and nanostructuring, high-pressure technology, and magnetically enhanced thermoelectrics. Moreover, we highlight novel strategies for improving the chemical and thermal stability of materials under operating conditions. Last, we discuss current controversies and challenges and suggest future directions for further research to improve the thermoelectric performance of Te-free bulk materials.

Graphene-based thermoelectric materials: toward sustainable energy-harvesting systems

Among sustainable energy-harvesting systems, thermoelectric technology has attracted considerable attention because of its ability to directly convert heat into electricity and diverse applications. Graphene, with its exceptional electrical conductivity and mechanical properties, is a promising candidate for thermoelectric materials. However, efficient thermoelectric applications require materials with a high Seebeck coefficient and low thermal conductivity-criteria that graphene does not inherently satisfy, owing to its gapless energy band structure and ballistic thermal conduction. This review examines the thermoelectric properties of graphene, optimization strategies, and the potential of graphene hybridization for thermoelectric applications. To overcome the intrinsic limitations of graphene for thermoelectric utilization, nanostructuring strategies based on its synthesis methods are discussed. Furthermore, strategies for graphene hybridization are introduced, with a focus on maximizing thermoelectric efficiency through interactions with nanostructured materials of various dimensions. Finally, the potential of graphene-based thermoelectric materials and future research directions are discussed.

Thermoelectric Conversion Eutectogels for Highly Sensitive Self-Powered Sensors and Machine Learning-Assisted Temperature Identification

Endowing flexible sensors with self-powering capabilities is of significant importance. However, the thermoelectric conversion gels reported so far suffer from the limitations of insufficient flexibility, signal distortion under repetitive deformation, and insufficient comprehensive performance, which seriously hinder their wide application. In this work, we designed and prepared eutectogels by an ionic liquid and a polymerizable deep eutectic solvent (PDES), which exhibit good mechanical properties, adhesion, and excellent thermoelectric conversion and thermoelectric response performance. The Seebeck coefficient (Si) can reach 30.38 mV K-1 at a temperature difference of 10 K. To amplify the self-powered performance of individual gel units, we assembled them into arrays and further prepared temperature sensors. The combination of the K-means clustering algorithm of machine learning can filter out the noise of traditional thermoelectric sensors and improve the consistency of signals, thereby enabling the prediction of absolute temperature under the conditions of 10 or 20 K temperature difference. This study also demonstrates potential application of these eutectogels in thermoelectric self-powered sensing.

Three-dimensional flexible thermoelectric fabrics for smart wearables

Wearable thermoelectric devices, capable of converting body heat into electrical energy, provide the potential driving power for the Internet of Things, artificial intelligence, and soft robotics. However, critical parameters have long been overlooked for these practical applications. Here, we report a three-dimensional flexible thermoelectric device with a structure featuring an inner rigid and outer flexible woven design. Such a structure includes numerous small static air pockets that create a stable out-of-plane temperature difference, enabling precise temperature signal detection (accuracy up to 0.02 K). Particularly, this structure exhibits excellent multi-signal decoupling capability, excellent elasticity (>10,000 compression cycles), ultra-fast compression response (20 ms), stable output signal under 50% compressive strain, high breathability (1300 mm s-1), and washability. All these metrics achieve the highest values currently reported, fully meeting the requirements for body heat and moisture exchange, as demonstrated in our designed integrated smart mask and smart glove systems based on vector machine learning technology. This work shows that our three-dimensional flexible thermoelectric device has broad applicability in wearable electronics.

Laser-assisted thermoelectric-enhanced hydrogen peroxide biosensors based on Ag2Se nanofilms for sensitive detection of bacterial pathogens

Thermoelectric (TE) materials can convert the heat produced during biochemical reactions into electrical signals, enabling the self-powered detection of biomarkers. In this work, we design and fabricate a simple Ag2Se nanofilm-based TE biosensor to precisely quantify hydrogen peroxide (H2O2) levels in liquid samples. A chemical reaction involving horseradish peroxidase, ABTS and H2O2 in the specimens produces a photothermal agent-ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) free radical, which triggers the heat fluctuations at the TE sensor through the photo-thermal effect, eventually enabling the sensing of H2O2. Consequently, the constructed sensor can achieve a detection limit of 0.26 μM by a three-leg TE device design. Further investigations suggest that the application of our TE sensor can be extended in testing H2O2 in beverages (including milk, soda water, and lemonade) and evaluating the load of bacterial pathogens relevant to dental diseases and infections including Streptococcus sanguinis and Methicillin-resistant Staphylococcus aureus with high analytical accuracy. This strategy utilizes the combination of high thermoelectric performance with chemical reactions to realize a straightforward and accurate biomarker detection method, making it suitable for applications in medical diagnostics, personalized health monitoring, and the food industry.

Thermoelectric porous laser-induced graphene-based strain-temperature decoupling and self-powered sensing

Despite rapid developments of wearable self-powered sensors, it is still elusive to decouple the simultaneously applied multiple input signals. Herein, we report the design and demonstration of stretchable thermoelectric porous graphene foam-based materials via facile laser scribing for self-powered decoupled strain and temperature sensing. The resulting sensor can accurately detect temperature with a resolution of 0.5°C and strain with a gauge factor of 1401.5. The design of the nanocomposites also explores the synergistic effect between the porous graphene and thermoelectric components to greatly enhance the Seebeck coefficient by almost four times (from 9.703 to 37.33 μV/°C). Combined with the stretchability of 45%, the self-powered sensor platform allows for early fire detection in remote settings and accurate and decoupled monitoring of temperature and strain during the wound healing process in situ. The design concepts from this study could also be leveraged to prepare multimodal sensors with decoupled sensing capability for accurate multi-parameter detection towards health monitoring.

Integration of Flexible Thermoelectric Energy Harvesting System for Self-Powered Sensor Applications

Flexible thermoelectric generators (FTEGs) can continuously harvest energy from the environment or the human body to supply wearable electronic devices, which should be a clean energy solution and provide an opportunity to satisfy the increasing power consumption of multimodal sensing and data transmission in wearable electronic devices. Here, the 64-pair FTEG was fabricated by introducing the plated through-hole and heterotypic electrode structures to optimize the thermal transport, showing the largely improved output power of 4.1 mW and record-high power density of 312 μW cm-2 at a given ambient temperature of 15 °C inside a measurement equipment. And a high power density of 79.8 μW cm-2 was also obtained in a FTEG worn on the wrist during working at a relative high atmosphere temperature of 16.5 °C. In addition, an intelligent real-time healthcare system is designed to continuously track various physiological parameters and transmit the processed data to a smart terminal, whose power consumption was around 0.1 mW can be solely supplied by body heat even at the static state of the human body. Overall, this work provides a viable method to increase the power density of FTEG and a global optimization scheme for wearable electronics.

High-Performance Thermoelectric Composite of Bi2Te3 Nanosheets and Carbon Aerogel for Harvesting of Environmental Electromagnetic Energy

Intensifying the severity of electromagnetic (EM) pollution in the environment represents a significant threat to human health and results in considerable energy wastage. Here, we provide a strategy for electricity generation from heat generated by electromagnetic wave radiation captured from the surrounding environment that can reduce the level of electromagnetic pollution while alleviating the energy crisis. We prepared a porous, elastomeric, and lightweight Bi2Te3/carbon aerogel (CN@Bi2Te3) by a simple strategy of induced in situ growth of Bi2Te3 nanosheets with three-dimensional (3D) carbon structure, realizing the coupling of electromagnetic wave absorption (EMA) and thermoelectric (TE) properties. With ultra-low thermal conductivity (0.07 W m-1 K-1), the CN@Bi2Te3 composites achieved a minimum reflection loss (RL) of 51.30 dB and an effective absorption bandwidth (EAB) of 6.20 GHz at an optimal thickness of 2.8 mm. Additionally, under a temperature gradient of 80 K, the flexible thermoelectric generator (FTEG) system generates a maximum output power of 42.2 μW. By absorbing 2.45 GHz microwaves to build the temperature difference, the EMA-TE-coupled device achieves an optimal Uoc of 38.4 mV, a short-circuit current of 1.03 mA, and an output power of 9.87 μW upon a radiation time of 50 s. This work establishes a potential pathway for further recycling electromagnetic energy in the environment, which is also promising for the preparation of large-area flexible EM to electrical energy conversion devices.

Helical dislocation-driven plasticity and flexible high-performance thermoelectric generator in α-Mg3Bi2 single crystals

Inorganic plastic semiconductors play a crucial role in the realm of flexible electronics. In this study, we present a cost-effective plastic thermoelectric semimetal magnesium bismuthide (α-Mg3Bi2), exhibiting remarkable thermoelectric performance. Bulk single-crystalline α-Mg3Bi2 exhibits considerable plastic deformation at room temperature, allowing for the fabrication of intricate shapes such as the letters "SUSTECH" and a flexible chain. Transmission electron microscopy, time-of-flight neutron diffraction, and chemical bonding theoretic analyses elucidate that the plasticity of α-Mg3Bi2 stems from the helical dislocation-driven interlayer slip, small-sized Mg atoms induced weak interlayer Mg-Bi bonds, and low modulus of intralayer Mg2Bi22- networks. Moreover, we achieve a power factor value of up to 26.2 µW cm-1 K-2 along the c-axis at room temperature in an n-type α-Mg3Bi2 crystal. Our out-of-plane flexible thermoelectric generator exhibit a normalized power density of 8.1 μW cm-2 K-2 with a temperature difference of 7.3 K. This high-performance plastic thermoelectric semimetal promises to advance the field of flexible and deformable electronics.

Nanobinders advance screen-printed flexible thermoelectrics

Limited flexibility, complex manufacturing processes, high costs, and insufficient performance are major factors restricting the scalability and commercialization of flexible inorganic thermoelectrics for wearable electronics and other high-end cooling applications. We developed an innovative, cost-effective technology that integrates solvothermal, screen-printing, and sintering techniques to produce an inorganic flexible thermoelectric film. Our printable film, comprising Bi2Te3-based nanoplates as highly orientated grains and Te nanorods as "nanobinders," shows excellent thermoelectric performance for printable films, good flexibility, large-scale manufacturability, and low cost. We constructed a flexible thermoelectric device assembled by printable n-type Bi2Te3-based and p-type Bi0.4Sb1.6Te3 films, which achieved a normalized power density of >3 μW cm-2 K-2, ranking among the highest in screen-printed devices. Moreover, this technology can be extended to other inorganic thermoelectric film systems, such as Ag2Se, showing broad applicability.

Synergistically Improved Crystallinity and Molecular Doping Ability of Polythiophene-Diketopyrropyrrole Derivatives by m-Trifluoromethylbenzene Containing Side Chains for Improved Thermoelectric Materials

m-Trifluoromethylbenzene (FB) groups have been widely employed in various fields; however, no studies have reported the use of FB in side chains to enhance the carrier mobility and molecular doping of conjugated polymers. In this study, based on density functional theory (DFT) calculations, we discovered that FB groups can effectively bind to [FeCl4]-, the counterion of the p-type dopant FeCl3, thereby increasing doping ability. Consequently, FB groups were incorporated into the side chains of thiophene-diketopyrrolopyrrole-based donor-acceptor (D-A)-conjugated polymers, and a series of random conjugated polymers were synthesized (denoted as PDPPFB-x, where x represents the molar ratio of the FB side chain). The findings revealed that an appropriate number of FB groups can decrease the π-π stacking distances, enhance the films' crystallinity, and consequently improve the charge transfer ability. Furthermore, after doping with FeCl3, the UV-vis-NIR spectra indicated that the doping efficiency was augmented by increasing the molar fraction of the FB side chain. Among these polymers, PDPPFB-10 exhibited the highest conductivity and power factor, which were 2.0 and 1.5 times higher than those of PDPPFB-0, respectively. These results illustrated a straightforward molecular design strategy for enhancing the crystallinity and conductivity of conjugated polymers, thereby expanding the way to optimize their thermoelectric performance.

Efficient Permeable Monolithic Hybrid Tribo-Piezo-Electromagnetic Nanogenerator Based on Topological-Insulator-Composite

Escalating energy demands of self-independent on-skin/wearable electronics impose challenges on corresponding power sources to offer greater power density, permeability, and stretchability. Here, a high-efficient breathable and stretchable monolithic hybrid triboelectric-piezoelectric-electromagnetic nanogenerator-based electronic skin (TPEG-skin) is reported via sandwiching a liquid metal mesh with two-layer topological insulator-piezoelectric polymer composite nanofibers. TPEG-skin concurrently extracts biomechanical energy (from body motions) and electromagnetic radiations (from adjacent appliances), operating as epidermal power sources and whole-body self-powered sensors. Topological insulators with conductive surface states supply notably enhanced triboelectric and piezoelectric effects, endowing TPEG-skin with a 288 V output voltage (10 N, 4 Hz), ∼3 times that of state-of-the-art devices. Liquid metal meshes serve as breathable electrodes and extract ambient electromagnetic pollution (±60 V, ±1.6 µA cm-2). TPEG-skin implements self-powered physiological and body motion monitoring and system-level human-machine interactions. This study provides compatible energy strategies for on-skin/wearable electronics with high power density, monolithic device integration, and multifunctionality.

High-Performance n-Type Organic Thermoelectrics with Exceptional Conductivity by Polymer-Dopant Matching

Achieving high electrical conductivity (σ) and power factor (PF) simultaneously remains a significant challenge for n-type organic themoelectrics (OTEs). Herein, we demonstrate the state-of-the-art OTEs performance through blending a fused bithiophene imide dimer-based polymer f-BTI2g-SVSCN and its selenophene-substituted analogue f-BSeI2g-SVSCN with a julolidine-functionalized benzimidazoline n-dopant JLBI, vis-à-vis when blended with commercially available n-dopants TAM and N-DMBI. The advantages of introducing a more lipophilic julolidine group into the dopant structure of JLBI are evidenced by the enhanced OTEs performance that JLBI-doped films show when compared to those doped with N-DMBI or TAM. In fact, thanks to the enhanced intermolecular interactions and the lower-lying LUMO level enabled by the increase of selenophene content in polymer backbone, JLBI-doped films of f-BSeI2g-SVSCN exhibit a unprecedent σ of 206 S cm-1 and a PF of 114 μW m-1 K-2. Interestingly, σ can be further enhanced up to 326 S cm-1 by using TAM dopant as a consequence of its favorable diffusion behavior into densely packed crystalline domains. These values are the highest to date for solution-processed molecularly n-doped polymers, demonstrating the effectiveness of the polymer-dopant matching approach carried out in this work.

Advancing flexible thermoelectrics for integrated electronics

With the increasing demand for energy and the climate challenges caused by the consumption of traditional fuels, there is an urgent need to accelerate the adoption of green and sustainable energy conversion and storage technologies. The integration of flexible thermoelectrics with other various energy conversion technologies plays a crucial role, enabling the conversion of multiple forms of energy such as temperature differentials, solar energy, mechanical force, and humidity into electricity. The development of these technologies lays the foundation for sustainable power solutions and promotes research progress in energy conversion. Given the complexity and rapid development of this field, this review provides a detailed overview of the progress of multifunctional integrated energy conversion and storage technologies based on thermoelectric conversion. The focus is on improving material performance, optimizing the design of integrated device structures, and achieving device flexibility to expand their application scenarios, particularly the integration and multi-functionalization of wearable energy conversion technologies. Additionally, we discuss the current development bottlenecks and future directions to facilitate the continuous advancement of this field.

Enhanced Thermoelectric Performance in Flexible Sulfur-Alloyed Ag2Se Thin Films

Flexible thermoelectric generators can directly convert thermal energy harvested from the human body into electricity. The Ag2Se flexible film, a promising material for wearable thermoelectric generators, normally demonstrates an inferior electrical transport property due to its weakened in-plane mobility. In this study, the in-plane electrical transport properties of flexible Ag2Se films were optimized by alloying with additional sulfur. This optimization is achieved by leveraging the differences in elemental electronegativity and the preferred orientation of the Ag2Se films. The sulfur-alloyed Ag2Se thin film, with a nominal ratio of 3 atom %, can reach a maximum mobility of 1150 cm-2 V-1 s-1 at 300 K. So, the optimized room-temperature power factor increases to 1935 μW m-1 K-2. Furthermore, the Ag2Se film alloyed with 3 atom % sulfur exhibits excellent flexibility even after 1000 bending cycles with a radius of 5 mm, characterized by a relative resistance increment of less than 3%. In addition, the corresponding π-type flexible thermoelectric generator possesses a maximum power density of 51 W m-2 at a temperature difference of 50 K.

Flexible Ag2Se Thermoelectric Films Enable the Multifunctional Thermal Perception in Electronic Skins

Skin is critical for shaping our interactions with the environment. The electronic skin (E-skin) has emerged as a promising interface for medical devices to replicate the functions of damaged skin. However, exploration of thermal perception, which is crucial for physiological sensing, has been limited. In this work, a multifunctional E-skin based on flexible thermoelectric Ag2Se films is proposed, which utilizes the Seebeck effect to replicate the sensory functions of natural skin. The E-skin can enable capabilities including temperature perception, tactile perception, contactless perception, and material recognition by analyzing the thermal conduction behaviors of various materials. To further validate the capabilities of constructed E-skins, a wearable device with multiple sensory channels was fabricated and tested for gesture recognition. This work highlights the potential for using flexible thermoelectric materials in advanced biomedical applications including health monitoring and smart prosthetics.

Enhanced Electrical Conductivity and Mechanical Properties of Stretchable Thermoelectric Generators Formed by Doped Semiconducting Polymer/Elastomer Blends

Recent research efforts have concentrated on the development of flexible and stretchable thermoelectric (TE) materials. However, significant challenges have emerged, including increased resistance and reduced electrical conductivity when subjected to strain. To address these issues, rigid semiconducting polymers and elastic insulating polymers have been incorporated and nanoconfinement effects have been exploited to enhance the charge mobility. Herein, a feasible approach is presented for fabricating stretchable TE materials by using a doped semiconducting polymer blend consisting of either poly(3-hexylthiophene) (P3HT) or poly(3,6-dithiophen-2-yl-2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-thienylenevinylene-2,5-yl) (PDVT-10) as the rigid polymer with styrene-ethylene-butylene-styrene (SEBS) as the elastic polymer. In particular, the blend composition is optimized to achieve a continuous network structure with SEBS, thereby improving the stretchability. The optimized polymer films exhibit well-ordered microstructural aggregates, indicative of good miscibility with FeCl3 and enhanced doping efficiency. Notably, a lower activation energy and higher charge-carrier concentration contribute to an improved electrical conductivity under high tensile strain, with a maximum output power of 1.39 nW at a ΔT of 22.4 K. These findings offer valuable insights and serve as guidelines for the development of stretchable p-n junction thermoelectric generators based on doped semiconducting polymer blends with potential applications in wearable electronics and energy harvesting.