Flexible Thermoelectric Research Articles

High thermoelectric performance of flexible Ag/Ag2Se composite film on nylon for low-grade energy harvesting

In this work, Ag/Ag2Se composite films with excellent thermoelectric (TE) properties and flexibility are prepared based on a simple one-pot method. By adjusting the nominal ratios of Ag/Se, an optimal Ag/Ag2Se composite film shows a large power factor of ∼2275 μW m−1 K−2 at 300 K. Such an outstanding TE performance of the composite film is due to the unique microstructure and the synergistic effect between the Ag and Ag2Se. Meanwhile, the composite film also shows outstanding flexibility (∼91.8% of the initial electrical conductivity is maintained, and the S is unchanged after 1500 bending cycles with a bending radius of 4 mm). Furthermore, a 4-leg flexible TE generator assembled with the optimal film produces a voltage of 14.06 mV and 4.96 μW at a temperature difference of 30.4 K. This work provides a new inspiration for the preparation of flexible Ag2Se-based films with excellent TE performance near room temperature.

Vacuum filtration method towards flexible thermoelectric films

Thermoelectric (TE) conversion technology can directly exploit the temperature difference of several Kelvin between the human body and the environment to generate electricity, which provides a self-powered solution for wearable electronics. Flexible TE materials are increasingly being developed through various methods, among which the vacuum filtration method stands out for its unique advantages, attracting the favor of researchers. It has been proven to construct flexible TE thin films with excellent performance effectively. This paper presents a comprehensive overview and survey of the advances of the vacuum filtration method in producing flexible TE thin films. The materials covered in this study include conducting polymer-based materials, carbon nanoparticle-based materials, inorganic materials, two-dimensional materials, and ternary composites. Finally, we explore potential research outlooks and the significance of flexible films, which are at the forefront of research in TE materials science.

Synthesis, magnetoresistance, and thermoelectrical properties of environmentally stable n-type nitrogen-doped multiwalled carbon nanotubes

Nitrogen-doped multiwalled carbon nanotubes (N-MWCNTs) are known as a perspective material for a variety of applications in nanoelectronic devices, sensors, catalysts for carbon dioxide reduction, and flexible thermoelectrics. However, up to date most of the reports on the properties of N-MWCNTs are focused on a narrow niche of research, for example, a study of low-temperature magnetoresistance or room-temperature thermoelectrical properties. In this work, N-MWCNTs were synthesized using benzene:pyridine precursor in different ratios, and both magnetoresistance and thermoelectrical properties of the synthesized N-MWCNTs were systematically investigated in the temperature range 2-300 K and compared with the properties of undoped MWCNTs. Unexpected switching of the magnetoresistance of the N-MWCNTs at low temperatures from negative to positive values was observed, and the processes underlying this effect are discussed. The study of the thermoelectrical properties revealed n-type conductance in the N-MWCNTs, which was attributed to the impact of nitrogen defects incorporated in the MWCNT structure. Performed for the first-time investigations of the thermal stability of the Seebeck coefficient of N-MWCNTs in air revealed that the Seebeck coefficient retains its negative values and even increases after annealing of the N-MWCNTs in air at 500 °C. These findings illustrate the high potential of the presented in this work N-MWCNTs for applications in different devices in a wide range of temperatures.

Optimized Thermoelectric Performance and Plasticity of Ductile Semiconductor Ag2S0.5Se0.5 Via Dual‐Phase Engineering

AbstractInorganic semiconductor Ag2S with excellent plasticity is highly desired in flexible and wearable thermoelectrics. However, the compromise between plasticity and thermoelectric performance limits the advances in Ag2S‐based thermoelectric materials and their devices. Here, a 0.5 mol.% Ag2Te‐alloyed Ag2S0.5Se0.5 bulk material is designed, which has a competitively high near‐room‐temperature figure of merit of ≈0.43 at 323 K and an ultra‐high bending strain of ≈32.5% without cracks. Introducing Ag2Te can optimize the carrier concentration and mobility of the Ag2S0.5Se0.5 matrix due to its metal‐like conducting features, leading to a maximum power factor of ≈6 µW cm−1 K−2. Simultaneously, Ag2Te induces Ag‐poor amorphous phase boundaries, serving as buffer layers to enhance the overall plasticity. Moreover, such amorphous phase boundaries combined with multiscale phonon scattering sources can significantly suppress the lattice thermal conductivity to ≈0.28 W m−1 K−1 at 323 K, leading to a high figure of merit. This study demonstrates an innovative route to simultaneously boost the thermoelectric performance and plasticity of ductile semiconductors.

Magnetic ordering boost excellent thermoelectric performance of flexible films

The poor electrical transport performance of flexible thermoelectric (TE) films is a major bottleneck for the development of in-plane heat dissipation technology based on Peltier effect. Herein, ordered Fe nanoparticles (Fe-NPs) and in-situ Fe3O4 nanoparticles (Fe3O4-NPs) as magnetic functional elements (MFEs) were embedded into Bi0·5Sb1·5Te3 (BST)/epoxy flexible films in order to enhance the TE performance. The remarkable increases in electrical conductivity of BST/MFEs/BST/epoxy flexible films have been confirmed to originate from the carrier mobility increases. The retention of large Seebeck coefficient is attributed to the negative magnetoresistance (MR−) induced by spin-dependent scattering and weak localization effect from MFEs. As a result, the maximum power factor of BST/MFEs/BST/epoxy flexible films increased by 45%, and the maximum cooling temperature difference enhanced by 1.6 times. This work reveals that the ordered MFEs can significantly boost the electro-thermal conversion performance of flexible TE films and the magnetoresistance theory can reveal the scattering mechanism of charge carriers in thermo-electromagnetic flexible films.

High-Performance Ag2Se Film by a Microwave-Assisted Synthesis Method for Flexible Thermoelectric Generators.

Flexible Ag2Se thermoelectric (TE) films are promising for wearable applications near room temperature (RT). Herein, a Ag2Se film on a nylon membrane with high TE performance was fabricated by a facile method. First, Ag2Se powders were prepared by a microwave-assisted synthesis method using Ag nanowires as a template. Second, the Ag2Se powders were deposited onto nylon via vacuum filtration followed by hot pressing. Through modulating the Ag/Se molar ratio for synthesizing the Ag2Se powders, an optimized Ag2Se film demonstrates a high power factor of 1577.1 μW m-1 K-2 and good flexibility at RT. The flexibility of the Ag2Se film is mainly attributed to the flexible nylon membrane. In addition, a six-leg flexible TE generator (f-TEG) fabricated with the optimized Ag2Se film exhibits a maximum power density of 18.4 W m-2 at a temperature difference of 29 K near RT. This work provides a new solution to prepare high-TE-performance flexible Ag2Se films for f-TEGs.

Effective Thermoelectric Switch of Hollow Weakly-Coupled Molecular Junction Based on Twist Angle Effect with Boron-Doping

ABSTRACT Rational design and adjustment of flexible thermoelectric devices are key points for sustainable and effective thermoelectric conversion, which remains a fundamental challenge due to inherent high thermal conductivity and uncontrolled carrier concentration induced by non-uniform dispersion. Under ingenious combination of weakly-coupled hollow interface and nanotube structure, thermoelectric performance of a dumbbell-like molecular junction comprised of a phenyl-terminated polyyne as central molecule and two semi-infinite 1D single-walled carbon nanotube (SWCNT) as electrodes has been investigated at certain twisted angles (θ). The results indicate that molecule twisting can be reviewed as an effective thermoelectric switch to coordinatingly control electronic and phononic transmission properties simultaneously. Resonance of molecular discrete state and electrode continuous state leads to low thermal conductance, which is sensitively affected by twist angle. Meanwhile, cyclic transformation between p-type and n-type flexible thermoelectrics can be realized by manipulating twist angle in a certain period of rotation. Thermoelectric performance of such a molecular junction can be further improved by boron atom doping at head-to-tail positions, and an excellent figure of merit (ZT = 1.75) is observed near Fermi level under 25° twisted angle. This result inspires an effective strategy to modulate and control thermoelectric conversion, which will greatly broaden applications in thermoelectric twistronics.

Flexible, durable, green thermoelectric composite fabrics for textile-based wearable energy harvesting and self-powered sensing

Textile-based wearable thermoelectric (TE) devices which could realize electricity harvesting and self-powered sensing using body heat attract great attention as they combine flexibility and skin conformality, yet do not require an external power supply. However, the challenge remains in developing durable TE fabrics which could generate sufficient power. Herein, a novel and scalable fabrication strategy that realizes simultaneous carbon nanotube (CNT) electrospray on poly (lactic acid) (PLA) fabric electrospinning is proposed to fabricate flexible TE composite fabrics. The optimal TE fabrics demonstrate a high Seebeck coefficient of 62.9 ± 6.2 μV K−1 and a maximum power density of 37.3 nW cm−2 is achieved at a temperature difference of 17 K with flexible textile TE generator assembled with CNT/PLA TE composite fabrics on neat PLA textile. The device further demonstrated superior capability in electricity harvesting from the human body and self-powered temperature and/or breath sensing. Moreover, the device exhibited excellent durability after 500 times bending and 100 times water dip-wash, which is ascribed to the uniform CNT electrospray among the PLA nanofibers and strong adhesion between the CNT and PLA. This work provides a novel fabrication strategy for green, durable and wearable TE composites and textiles in emerging power generation and smart sensing applications.

Ultra-flexible self-supporting Ag2Se/nylon composite films for wearable thermoelectric devices

Recently, flexible thermoelectric (TE) generators (f-TEGs) have received a lot of attention. Compared with TE films on flexible substrates, self-supporting composite films prepared by combining TE materials with soft fabrics and thus retaining the inherent advantages of fabrics, such as stretchability, breathability, and flexibility, are more practical and may significantly expand the application of f-TEGs in smart wearable devices. Hence, in this work, a facile and rapid method for preparing ultra-flexible self-supporting Ag2Se/nylon composite films is developed. The composite films with both TE properties and flexibility were obtained by fast selenization of commercial Ag/nylon fiber fabrics in a short time (60 s) at room temperature and then hot pressing at 230 °C in a vacuum. The maximum power factor of an optimized composite film at room temperature can reach 73.7 μW m−1 K−2. Meanwhile, the film exhibits excellent flexibility with the Seebeck coefficient and electrical conductivity decreased by only 7.37 and 8.48% after being bent 4000 repetitions around a round bar with a radius of 4 mm, which exceeds most reported flexible TE films. The ultra-high flexibility is mainly attributed to its high content of the nylon matrix (∼85.05 wt%), the tight bonding between Ag2Se grains, and the good combination between the Ag2Se and nylon matrix. In addition, a 3-leg f-TEG assembled with the optimal film produces a maximum power of ∼1.524 μW (corresponding to a power density of ∼0.47 W m−2) at a temperature gradient of 40.6 K, verifying the good TE performance of the composite film. This f-TEG can also be used as a self-powered temperature sensor, showing potential applications in daily life.

Flexible free-standing BixSey@C/PEDOT:PSS thermoelectric composite film with high-performance electromagnetic interference shielding

Due to the component advantages and structural features, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) based inorganic–organic composite films shows huge application potential in various fields. However, there are few reports on the bi-functions of thermoelectric (TE) and electromagnetic interference (EMI) shielding for the PEDOT:PSS-based functional films. Herein, metal–organic frameworks (MOFs)-derived BixSey@C sheets with average size about ∼ 7.32 μm are obtained via the solvothermal reaction and selenization pyrolysis treatment, and then flexible free-standing BixSey@C/PEDOT: PSS (BSCP) composite films are obtained by a feasible vacuum filtration combined with cold pressing treatment, using different contents of BixSey@C content as inorganic component. Micrometer BixSey@C sheets are uniformly dispersed in the PEDOT:PSS matrix constructing bi-functional composite films with unique layered structure. When the MOFs-derived BixSey@C content is 70%, BSCP-70 composite film gets a power factor of 11.23 μW m−1 K−2 at 340 K. Meanwhile, the BSCP-70 composite film reaches the highest average total EMI shielding effectiveness (SET) value of 45.0 dB at only 41.9 μm thickness at X-band. Based on multi-physical properties of MOF-derived BixSey@C and PEDOT:PSS, flexible BixSey@C/PEDOT:PSS composite films display good application potentiality in the flexible TE devices and EMI shielding protection.

Organic-SnSe2 Hybrid Superlattice toward Synergistic Electrical Transport Optimization and Thermal Conductance Suppression.

Recently, layered SnSe2 has drawn broad research interest as a promising thermoelectric material that possesses great potential for application in energy conversion. However, extensive efforts have been devoted to optimizing the thermoelectric performance of SnSe2, but the ZT value is still far from satisfactory. Therefore, we developed an organic-inorganic superlattice hybrid by intercalating organic cations into SnSe2 interlayers in an attempt to enhance the thermoelectric properties. Organic intercalants can enlarge the basal spacing and decouple the SnSe2 layers, bringing about synergistic electrical transport modification and phonon softening. Thus, by simultaneously improving the electrical conductivity and reducing the thermal conductivity, a ZT value of 0.34 is achieved at 342 K in tetrabutylammonium-intercalated SnSe2, approximately two orders of magnitude higher than that of pristine SnSe2 single crystals. In addition, by opening van der Waals gaps via organic cations, outstanding flexibility of organic-intercalated SnSe2 is realized, with a superior figure of merit for flexibility of approximately 0.068. This work demonstrates a general and facile strategy to fabricate organic-inorganic superlattice hybrids with a considerable improvement in the thermoelectric performance via organic cation intercalation, which is promising for flexible thermoelectrics.

High-performance Ag2Se film by a template method for flexible thermoelectric generator

Herein, we developed a facile method to prepare a high-performance Ag2Se thermoelectric (TE) film. First, Ag2Se powders were synthesized by the reaction between Ag nanowires (NWs) with Se precursor at room temperature (RT), then vacuum filtered on a nylon membrane, and finally hot pressed. The film exhibits a high power factor of 1636.9 μW m−1 K−2 at RT via optimizing the Ag/Se ratio. The film exhibits good flexibility because of the nylon substrate and the nylon well combined with the Ag2Se film. Moreover, a flexible TE generator (f-TEG) consisting of six optimized film legs was assembled, and it generates a maximum output power of ∼3.42 μW (corresponding power density of ∼13.5 W m−2) under a temperature gradient of 31.4 K. This work paves a simple, fast, and feasible route to prepare high-performance flexible Ag2Se film.

Evaluation of Performance and Power Consumption of a Thermoelectric Module-Based Personal Cooling System—A Case Study

Thermoelectric (TE) technology is promising for reducing thermal discomfort of workers during their routine professional activities. In this manuscript, a preliminary evaluation of a newly developed personal cooling system (PCS) with flexible TE modules is presented based on an analysis of cooling efficiency and power consumption. For this purpose, tests with human participation were performed involving the monitoring of local skin temperature changes and electrical parameters of the controller. Thanks to TE cooling, a significant reduction of local skin temperature was observed at the beginning of the experiment, reaching as much as 6 °C. However, the observed effect systematically became weaker with time, with the temperature difference decreasing to about 3 °C. Cooling efficiency stayed at the same level over the ambient temperature range from 25 °C to 35 °C. The obtained results showed that a proper fitting of the PCS to the human body is a crucial factor influencing the PCS cooling efficiency.

Flexible Thermoelectric Devices with Flexible Heatsinks of Phase-Change Materials and Stretchable Interconnectors of Semi-Liquid Metals.

Flexible thermoelectric (TE) devices offer great potential for wearable thermal management and self-powered systems, but heat dissipation and electrical interconnection remain key challenges. In this study, we address these issues by integrating flexible TE devices with phase-change material (PCM) heatsinks and stretchable semi-liquid metal (semi-LM) interconnectors. The effectiveness of PCMs with varying melting points for temperature regulation in different environmental conditions is demonstrated, delivering cooling effects exceeding 10 °C. Furthermore, the utilization of semi-LMs instead of LMs enables excellent stretchability and efficient heat dissipation. Moreover, the TE devices generate power with a density of 7.3 μW/cm2 at an ambient temperature of 22 °C, making it an ideal power source for a wearable self-powered sensing system. Successful integration into garments and armbands confirms the practicality and adaptability of these flexible thermoelectric devices, establishing them as critical components for future wearables with superior resilience to daily wear and tear.

Enhanced thermoelectric properties of SWCNT by new nitrogen chains doping

Single walled carbon nanotubes (SWCNT) are attractive materials for thermoelectric (TE) devices because of their unique one-dimensional structure and provide us for developing a flexible TE devices due to their mechanical properties. In this paper, the TE properties of pristine small SWCNT (3,3) and three new nitrogen (N) doping models namely CN, C2N2 (doping by two N-chains) and C4N2 (doping by three N-chains) are calculated using the density functional theory (DFT) combined with Boltzmann's semi-classical transport theory. Dynamical and energetic stability of the compounds are confirmed from the phonon density of states curve and formation energy calculations. The values of Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (κ), power factor (PF) and figure of merit (ZT) are reported at different temperature and discussed in detail. The results show that N-doping significantly improves the TE properties of the pristine nanotube and show a reasonable agreement with previous experiments and ab-initio calculations. It was found that the C2N2 configuration presents the largest Seebeck coefficient of 70.53 μV/K and the figure of merit ZT of 0.25 compared to other doping models. N-chains doping is therefore a new suitable way to improve the TE properties of small SWCNT in order to making them a good candidate for flexible TE devices.

High‐Sensitivity Flexible Thermocouple Sensor Arrays Via Printing and Photonic Curing

AbstractDespite remarkable advances, high‐performance thermoelectric (TE) materials‐based thermocouples (TCs) lack mechanical robustness and flexibility. Hence, conventional low Seebeck coefficient metals (Ni, Cu, Fe) wire‐junction‐based TCs are used for temperature sensor applications. However, not only the sensitivity of the metal‐based TC sensor is low, but also it is very difficult to fabricate a pixelated sensor using the conventional approach. In this study, high‐performance (SbBi)2(TeSe)3‐based printed flexible TE materials are employed to fabricate two types of shape‐conformable TC‐based temperature sensor arrays with 25 pixels (TSA I and TSA II). Bi0.5Sb1.5Te3‐based p‐type (p‐BST) printed TE film and copper are used to fabricate TSA I, and the p‐type film together with the Bi2Te2.7Se0.3‐based n‐type (n‐BT) film is used to fabricate TSA II. A high average sensitivity (Sexp) value of ≈124 µV °C−1 for TSA I and ≈319 µV °C−1 for TSA II is attained with high linearity.

Screen-printed, flexible, and eco-friendly thermoelectric touch sensors based on ethyl cellulose and graphite flakes inks

Despite the undoubtable interest in energy conversion, thermoelectric (TE) materials can be approached from a temperature-sensitive perspective, as they can detect small thermal stimuli, such as a human touch or contact with cold/hot objects. This feature offers possibilities for different applications one of them being the integration with scalable and cost-effective, biocompatible, flexible, and lightweight thermal sensing solutions, exploring the combination of sustainable Seebeck coefficient-holding materials with printing techniques and flexible substrates. In this work, ethyl cellulose and graphite flakes inks were optimized to be used as functional material for flexible thermal touch sensors produced by screen-printing. Graphite concentrations of 10, 20 and 30 wt% were tested, with 1, 2 and 3 printed layers on four different substrates—office paper, sticker label paper, standard cotton, and organic cotton. The conjugation of these variables was assessed in terms of printability, sheet resistance and TE response. The best electrical-TE output combination is achieved by printing two layers of the ink with 20 wt% of graphite on an office paper substrate. Subsequently, thermal touch sensors with up to 48 TE elements were produced to increase the output voltage response (>4.5 mV) promoted by a gloved finger touch. Fast and repeatable touch recognition were obtained in optimized devices with a signal-to-noise ratio up to 340 and rise times bellow 0.5 s. The results evidence that the screen-printed graphite-based inks are highly suitable for flexible TE sensing applications.

Nonwoven Polymeric Nanofiber Sheets: Path Forward Toward a New Class of Hybrid Flexible Thermoelectrics

Hybrid flexible thermoelectric (TE) materials are widely being explored for wearable TEs, micro power generators, and self‐powered electronics. The commonly used methods for developing hybrid flexible TE materials are organic–inorganic composite which exhibits low TE performance and second is inorganic material deposition over organic substrate making it a two‐sheet structure that has sustainability concerns. In view of this, herein, a different class of highly flexible hybrid TE materials are designed using a network of electrospun 1D nanofibers (NFs) as a substrate which provides high surface area, better connectivity, and high flexibility. For the proof of concept, the NFs sheets of two polymers polyacrylonitrile (PAN) and cellulose acetate are used to deposit the well‐known Bi2Te3 as a case study. Herein, the directionally aligned PAN‐based NFs sheet exhibits good TE properties leading to the maximum PF ≈51.4 μW m−1 K−2 at 384 K for the P‐400 sample. Herein, the concept of a new class of hybrid TEs is successfully demonstrated by utilizing the flexible NF sheet as a substrate for inorganic TE materials, which will pave the way for sustainable flexible TE. Further, the TE performance can be enhanced by modulating certain factors such as the synthesis conditions of NFs and inorganic material, the thickness of deposition, and so on.

Flexible Thermoelectrics Based on Plastic Inorganic Semiconductors

AbstractFlexible thermoelectrics, including flexible thermoelectric materials and devices, can generate electricity by utilizing the small temperature difference between the human body and surrounding environment, exhibiting great potential for the continuous powering of wearable devices. It has long been assumed that inorganic thermoelectric materials are usually brittle at room temperature except for size‐induced flexibility. Until recently, this perception has been overturned by the discovery of inorganic semiconductors with intrinsic plastic deformability. Herein, this review provides a comprehensive summary of the recently burgeoning plastic inorganic semiconductors in thermoelectrics. First the requirements to thermoelectric materials are introduced by flexible thermoelectrics. Then, the mechanical properties and potential plastic deformation mechanisms at the atomic level of plastic inorganic semiconductors are systematically summarized. Subsequently, the optimization strategies for the mechanical and thermoelectric properties of plastic inorganic semiconductors, such as doping and alloying, are summarized. Furthermore, the advantages of plastic inorganic semiconductors in flexible thermoelectric devices and their potential applications in wearable electronic devices are also highlighted. Finally, the current challenges are presented and future directions are predicted, including high‐throughput screening methods for plastic inorganic semiconductor materials and the development and application of flexible inorganic thermoelectric materials.

Inkjet Printing Flexible Thermoelectric Devices Using Metal Chalcogenide Nanowires

AbstractDevelopment of flexible thermoelectric devices offers exciting opportunities for wearable applications in consumer electronics, healthcare, human–machine interface, etc. Despite the increased interests and efforts in nanotechnology‐enabled flexible thermoelectrics, translating the superior properties of thermoelectric materials from nanoscale to macroscale and reducing the manufacturing costs at the device level remain a major challenge. Here, an economic and scalable inkjet printing method is reported to fabricate high‐performance flexible thermoelectric devices. A general templated‐directed chemical transformation process is employed to synthesize several types of 1D metal chalcogenide nanowires (e.g., Ag2Te, Cu7Te4, and Bi2Te2.7Se0.3). These nanowires are made into inks suitable for inkjet printing by dispersing them in ethanol without any additives. As a showcase for thermoelectric applications, fully inkjet‐printed Ag2Te‐based flexible films and devices are prepared. The printed films exhibit a power factor of 493.8 µW m−1 K−2 at 400 K and the printed devices demonstrate a maximum power density of 0.9 µW cm−2 K−2, both of which are significantly higher than those reported in state‐of‐the‐art inkjet‐printed thermoelectrics. The protocols of metal chalcogenide ink formulations, as well as printing are general and extendable to a wider range of material systems, suggesting the great potential of this printing platform for scalable manufacturing of next‐generation, high‐performance flexible thermoelectric devices.