Flexible Thermoelectric Devices Research Articles

Enhanced thermoelectric performance by constructing PEDOT:PSS/graphene quantum dots/single-walled carbon nanotube multilayer films

Flexible thermoelectric materials are important as self-powered systems in wearable and flexible thermoelectric devices; to this end, using single-walled carbon nanotubes (SWCNTs) is advantageous for enhancing the overall thermoelectric performance. In this study, to further enhance the thermoelectric properties of SWCNTs and overcome the weak interfacial interaction between hydrophilic poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and hydrophobic pristine SWCNTs, a graphene quantum dots (GQDs) intermediate layer is used to couple PEDOT:PSS and SWCNTs. Multilayer films comprising PEDOT:PSS, GQDs, and SWCNTs are constructed and prepared by successively drip-coating the GQDs and PEDOT:PSS on the SWCNTs film surface. Analysis of the interfacial morphology reveals that the hydrophilic GQDs improve interfacial adhesion between PEDOT:PSS and the SWCNTs. Compared to a PEDOT:PSS/SWCNT bilayer film, the electrical conductivity of the PEDOT:PSS/GQDs/SWCNT multilayer films is enhanced, while the Seebeck coefficient is slightly lower. Meanwhile, the thermal conductivity is significantly decreased due to the increased interfacial phonon scattering. The minimum in-plane thermal conductivity for the PEDOT:PSS/GQDs/SWCNT films is 7.4 W‧m−1‧K−1, resulting in an enhanced figure of merit of 1.96 × 10−3 at 298 K. Thus, the GQDs intermediate layer is beneficial for coupling PEDOT:PSS and SWCNTs layers. The strategy employed herein offers novel insights into the preparation of high-performance SWCNTs-based thermoelectric materials.

Dual Post‐Treatments Boost Thermoelectric Performance of PEDOT:PSS Films and Their Devices

AbstractOwing to intrinsically high electrical conductivity and low thermoelectric conductivity, poly(3,4‐ethylenedioxithiophene):poly(styrenesulfonate) (PEDOT:PSS) shows promising thermoelectric properties. However, its relatively low power factor limits the practical applications of PEDOT:PSS. Here, unique dual post‐treatments by sodium sulfite (Na2SO3) and formamide (CH3NO) to boost the thermoelectric performance of flexible PEDOT:PSS films with an optimized power factor of 74.09 µW m–1 K–2 are used. Comprehensive characterizations confirm that CH3NO reduces the excessive insulating PSS and thereby increases the electrical conductivity, while Na2SO3 lowers the reduction of the doping level of PEDOT, leading to an increased Seebeck coefficient. Furthermore, the rationally post‐treated PEDOT:PSS films are assembled into a flexible thermoelectric device that exhibits an open‐circuit voltage of 2.8 mV using the heat from the human arm and an output power density of 2.56 µW cm–2 by a temperature difference of 25 K, indicating great potential for practical applications on sustainably charging low‐grade wearable electronics.

Flexible thermoelectric device with excellent durability towards self-powered light intensity detection

Thermoelectric technology can generate thermoelectric electromotive force under the condition of temperature difference, which have unlimited prospects in the emerging fields of sensing, wearable electronic devices, self-powered arbitrary doors, electronic skin. These application scenarios also put forward higher requirements for thermoelectric devices, excellent flexibility and self-supporting characteristics. Herein, a novel flexible self-supported thermoelectric device based on carbon nanotubes (CNTs) was designed and constructed with simple laminated construction of electrical series and thermal parallel. CNTs/PEDOT:PSS and polyethylenimine (PEI)-doped CNTs/polyvinylidene fluoride (PVDF) flexible self-supported films were used as p- and n-type films respectively. The obtained flexible self-supported thermoelectric device can generate a voltage of 3.31 mV under temperature difference of 35 K. The excellent light absorption and photothermal conversion of CNTs, together with the thermoelectric characteristics of CNTs and PEDOT:PSS, enables its promising applications in self-powered light intensity detection. Furthermore, its excellent flexibility and self-supporting characteristics give it outstanding durability.

Wide‐temperature range thermoregulating e‐skin design through a hybrid structure of flexible thermoelectric devices and phase change materials heat sink

AbstractThe realization of the thermoregulating function of electronic skin (e‐skin) by simulating the human temperature perception system can greatly improve the intelligence of the e‐skin. Here, we report a thermoregulating e‐skin that fits on the surface of a prosthetic limb based on a hybrid structure consisting of a flexible thermoelectric device and a phase‐change heat sink. The hybrid e‐skin possesses outstanding temperature adaptability similar to that of the human body; it can maintain the surface temperature at 35°C in environmental temperatures ranging from 10 to 45°C. The power expenditure of the e‐skin is essentially the same as the energy required by the human body to regulate temperature and is only 14.22 mW cm−2 in the thermoneutral zone. Thermoregulation based on this e‐skin can greatly improve the temperature distribution of the target surface, providing a promising solution for the biomimetic thermoregulation of robots and the next generation of intelligent prostheses.image

Near room-temperature in situ interfacial polymerization for PEDOT-based thermoelectric textile

Fiber-based organic thermoelectric composites have attracted increasing attention on smart wearable devices. Herein, a near room-temperature in situ interfacial polymerization method is proposed to fabricate a thermoelectric textile with core-shell structures. To optimize the parameters of fabrication process, the response surface methodology is applied to obtain the optimal experimental parameters. The thermoelectric textiles fabricated by the optimal parameters (0.30 mol/L Na2S2O8, 0.31 mol/L TsOH, 225 μl/10 ml EDOT, and −13 ℃ reaction temperature) show a high electrical conductivity (2.19 S·cm−1) and Seebeck coefficient (14 μV·K−1). Further, a thermopile composed of thermoelectric textile strips and copper wires is constructed to verify the heat-to-electricity conversion of thermoelectric textile. At a temperature difference of 30 °C, the thermoelectric device can continuously produce voltage and power factor to 0.72 mV and 5.52 × 10−2 nW respectively. Thus, the development of the thermoelectric fabric provides a conception to fabricate flexible thermoelectric devices.

Recent Advances in 2D Material/Conducting Polymer Composites for Thermoelectric Energy Conversion

AbstractTwo‐dimensional material (2DM)/conducting polymer (CP) composites with excellent electrical transport behavior, functional multidimensional structure, and good flexibility have attracted widespread attention as potential composite thermoelectric (TE) materials in the field of energy conversion. Appropriate preparation strategies determine the interface microstructure of 2DM/CP composites, which would influence the optimization degree of TE performance and practical applications. This review describes the recent progress in preparing advanced 2DM/CP composites by different design strategies for improving TE performance. In addition, some typical 2DM/CP composites applied to TE generators are also presented. Finally, other challenges and opportunities toward high‐performance 2DM/CP composites are discussed, including precise structural regulation, an in‐depth understanding of the interfacial transport mechanism, and the development of practical 2DM/CP‐based flexible TE devices.

Highly Efficient Flexible n‐Type Thermoelectric Films Formed by Encapsulation of Bi2Se3‐MWCNT Hybrid Networks in Polyvinyl Alcohol

AbstractThe use of organic–inorganic nanocomposites has shown the greatest potential for engineering efficient flexible thermoelectric devices. In this work, a novel approach of encapsulation of as‐grown bismuth selenide‐multiwalled carbon nanotubes (Bi2Se3‐MWCNT) hybrid network in polyvinyl alcohol for fabrication of n‐type flexible thermoelectric films is demonstrated as a successful alternative to the mechanically mixed counterparts. The developed stable flexible n‐type thermoelectric material has a Seebeck coefficient and power factor at room temperature as high as −85 μV K−1 and 0.4 μW m−1 K−2, and figure‐of‐merit, exceeding the value shown by the mixed counterpart by ≈2 orders of magnitude, while requiring 3–4 times less inorganic material in comparison with mixed composites. Charge carrier transport mechanisms and contribution of Bi2Se3 and MWCNT components of not encapsulated and encapsulated hybrid networks to the total Seebeck coefficient, electrical conductance, and power factor are studied. In addition, the fabricated flexible thermoelectric films show good environmental stability at relative humidity levels up to 60%, as well as great mechanical and electrical stability with the increase of resistance within 0.5% and deviations of the Seebeck coefficient within 2% from the initial value during the 100 repetitive bending cycles.

A first-principles study on the electrical conductivity of Ag2S1−xSex (x = 0, 0.25, 0.5): Electron–phonon coupling

The development of flexible thermoelectric devices is gradually attracting increasing attention, particularly in the field of material design. In this study, we use first-principles calculations combined with Boltzmann equations to study the electronic and transport properties of Ag2S1−xSex, a key material with many important properties and extraordinary ductility, as well as a wide range of thermoelectric applications. The effect of Se alloying on the electronic structure of Ag2S and defect formation is investigated, and the role of alloying in increasing the n-type carrier concentration is discussed. The electron–phonon coupling approximation is used to reproduce the experimentally observed transport properties reasonably well, which shows that this scattering model is suitable for predicting the transport properties of semiconductors in thermoelectric applications.

A flexible and smart shape memory alloy composite sheet based on efficient and bidirectional thermal management

ABSTRACT Leveraging advances in shape memory alloys (SMAs) and flexible thermoelectric devices (f-TEDs), this paper presents a structural and functional integrity composite sheet to address the inefficient and bulky activation of 2-D SMAs. A series of experimental tests were performed to reveal the generation, change, and transfer mechanisms of different types of electrically-induced heat in the f-TED, as well as the temperature-induced SMA phase transformation behaviors. The results show that the composite sheet exhibited good bidirectional thermal management capacity. The austenite deformation can be completed within 10s at an operating current of 2 A. The cooling recovery, in particular, performs much better than in conventional modes (the temperature declines exponentially with time). Finally, through two functional prototypes, a light switch and a flexible gripper, the application potential of the proposed composite was further experimentally demonstrated.

Realizing high thermoelectric performance in highly (0l0)-textured flexible Cu2Se thin film for wearable energy harvesting

Searching for eco-friendly and earth-abundant materials to supersede traditional high-cost bismuth telluride for fabricating wearable devices is of great significance in thermoelectrics. In this work, promising flexible Cu2Se based thin films with high thermoelectric performance is successfully fabricated via a facile co-sputtering method. Experimental results indicate that excess Cu in Cu2Se films leads to the decrease of carrier concentration by suppressing the formation of Cu vacancies and donating electrons, benefiting to achieve high Seebeck coefficient. Moreover, Cu-excess Cu2Se films have highly (0l0) preferred orientation and extra high carrier mobility, maintaining the decent electrical conductivity in the whole measurement temperature range. Combined with the low thermal conductivity, a maximum ZT of 0.42 is obtained at 275 °C from the Cu-excess Cu2Se due to the simultaneous optimization both of electrical and heat transport. Subsequently, a flexible thermoelectric device assembled with high performance Cu2Se films exhibits a maximum power density of 4.28 Wm-2 at a temperature difference of 50 °C, which thermal stability is greatly improved after introducing a molybdenum buffer layer into electrode layer. Therefore, this work demonstrates that rational microstructure manipulations and connection technology improvement can achieve high performance in the flexible thermoelectric device, which possess potential in wearable applications.

Simultaneous enhancement of conductivity and Seebeck coefficient of PEDOT:PSS by triflic acid treatment for flexible thermoelectric generator

Organic thermoelectric materials are of great interest for development of flexible thermoelectric devices. Two crucial parameters namely conductivity and Seebeck coefficient apparently display a trade-off effect towards optimization of thermoelectric (TE) properties. Dopants normally increase the conductivity of PEDOT:PSS but Seebeck coefficient remains unaffected at around 15–18 µV K−1. In this work, pre-treatment using triflic acid to PEDOT:PSS solution is employed displaying simultaneous increase in conductivity of 1686 S cm−1 and Seebeck coefficient of 23.6 µV K−1. Mechanism of improvements are attributed to the increased order of long-range crystal packing as well as hopping between newly generated DOS. HRTEM and XRD data substantiated the improved crystallinity. Acid treatment induced change of conformation from coil like benzoid to linear quinoid structure as observed by Raman spectra, with increasing level of oxidation based on Raman and UV–vis–NIR spectra. The carrier concentration and mobility increased leading to the power factor of 94 µW m−1 K−2. Flexible thermoelectric generator (TEG) device is developed on PET film which generate thermovoltage of 2.0 mV at temperature difference of 15 K.

A simple method for fabricating flexible thermoelectric nanocomposites based on bacterial cellulose nanofiber and Ag2Se

Flexible thermoelectric (FTE) devices have become attractive in recent years since they can be utilized as a power generator for wearable and portable electronics. This work fabricated FTE nanocomposites from bacterial cellulose (BC) and Ag2Se via an easy and inexpensive method. The blended BC was thoroughly mixed with Ag2Se powders before casting onto a filter paper via vacuum filtration, followed by oven-drying and hot-pressing. Phase formation of Ag2Se in the BC nanofiber network was confirmed by x-ray diffraction and energy dispersive spectroscopy. SEM images revealed the distribution of Ag2Se particles in the BC matrix. The Ag2Se particles were densely packed for large Ag2Se concentrations in the BC/Ag2Se nanocomposite. Thermoelectric measurements found that the electrical conductivity (σ) and Seebeck coefficient (S) varied with the Ag2Se proportion due to the changes in the carrier concentration and carrier mobility. The maximum σ of 5.7 × 104 S/m and S of −80 μV/K were observed at room temperature (RT), yielding the power factor (PF) of ∼300 μW/mK2. This PF value is comparable to other FTE materials, but the process used in this research is much simpler. The thermal conductivity was 0.56 W/mK at RT. Moreover, the BC/Ag2Se nanocomposites were highly flexible and could be attached to curved surfaces. In addition, the FTE module was constructed from BC/Ag2Se uni-leg elements, which could generate an output power of 0.28 μW. In addition, the simple fabrication process makes the BC/Ag2Se nanocomposite readily expandable to an industrial scale for modern FTE devices.

Flexible pCu2Se-nAg2Se thermoelectric devices via in situ conversion from printed Cu patterns

Flexible thermoelectric (TE) devices have great potential in wearable electronics, but there is great challenge to realize robust TE device with high performance via feasible integration process. Herein, based on precisely printed copper patterns, a flexible pCu2Se-nAg2Se TE device is initially realized by in situ ion exchange reaction. The as-prepared Cu2Se and Ag2Se films possess typical hierarchical defects including atomic scale vacancies, the nanosheet fragments and microscale porous structure, which could significantly scatter phonons in wide frequency range. Accordingly, ultra-low thermal conductivity (Cu2Se: 0.13 W m−1 K−1; Ag2Se: 0.15 W m−1 K−1) and optimal ZTs (Cu2Se: 0.5; Ag2Se: 0.7) are achieved. Meanwhile, the as-fabricated pCu2Se-nAg2Se TE device exhibits an excellent power density of 13.4 W m−2 at a temperature gradient of 40 K, which is among the highest values of printed in-plane film devices. Furthermore, the good adhesion between TE films and porous PI substrate endowed excellent flexibility and stability of pCu2Se-nAg2Se devices.

Cheap, Large-Scale, and High-Performance Graphite-Based Flexible Thermoelectric Materials and Devices with Supernormal Industry Feasibility.

Flexible thermoelectric materials and devices show great potential to solve the energy crisis but still face great challenges of high cost, complex fabrication, and tedious postprocessing. Searching for abnormal thermoelectric materials with rapid and scale-up production can significantly accelerate their applications. Here, we develop superlarge 25 × 20 cm2 commercial graphite-produced composite films in batches, achieved by a standard 10 min industrial process. The high cost effectiveness (S2σ/cost) of 7250 μW g m-1 K-2 $-1 is absolutely ahead of that of the existing thermoelectric materials. The optimized composite film shows a high power factor of 94 μW m-1 K-2 at 150 °C, representing the optimal value of normal carbon materials so far. Furthermore, we design two types of flexible thermoelectric devices fabricated based on such a novel composite, which achieve an output open-circuit voltage of 3.70 mV using the human wrist as the heat source and 1.33 mV soaking in river water as the cold source. Our study provides distinguished inspiration to enrich flexible and cost-effective thermoelectric materials with industrial production.

Flexible Bi2Te3-based thermoelectric generator with an ultra-high power density

• A flexible thermoelectric generator with newly structure design was fabricated. • The generator consists of rigid Bi 2 Te 3 legs sandwiched by two flexible substrates. • The upper substrate is diced into blocks to ensure the generator some flexibility. • A specific power density of 13.1 mW/cm 2 is achieved for the generator. • The output power of the generator remains almost unchanged after 7400 bends. Environmental energy harvesting based on flexible thermoelectric generator (f-TEG) provides an ideal micro-power source to drive the node sensors for the Internet of Things (IoT) and wearable electronics. However, the existing flexible thermoelectric devices have some disadvantages, such as low strength, large thermal resistance, complex manufacturing processes, and low reliability. Here, we report a high-performance f-TEG with a dimension of 2 × 16 × 0.6 mm 3 , encapsulating 18 pairs of thermoelectric legs with the size of 0.38 × 0.38 × 0.38 mm 3 . The flexible polyimide (PI) films with patterned electrodes act as the substrates, of which the upper one is cut into blocks to ensure the generator a minimum bending radius of 4.5 mm. The f-TEG can produce an open circuit voltage of 236 mV and output power of 4.19 mW under a temperature difference of 50 K, which remain almost unchanged even after 7400 times of bending tests. The power density reaches up to 13.1 mW/cm 2 at ΔT = 50 K, and the normalized output power density is 5.26 μW/cm 2 ·K 2 . These results are among the best performances reported for f-TEGs, which opens a new avenue for flexible micro-power system design and promotes the development of the next generation self-powered sensors and charge-free electronic devices.

Soft multi-modal thermoelectric skin for dual functionality of underwater energy harvesting and thermoregulation

Sustaining a long-term but viable power source for underwater electronics has been an engineering conundrum due to a lack of feasible physical mechanisms to harvest energy in the underwater environment. In this regard, thermoelectricity, which converts heat into electricity, can suggest a potent solution, since the ocean and other water bodies maintain the constant water temperature and therefore serve as a permanent thermal differential. In addition to energy harvesting, the same thermoelectric device can be also utilized to both heat and cool, thereby providing an effective means of controlling the temperature of the arbitrary subject in the underwater environment. In this light, we present a soft and stretchable multi-modal thermoelectric skin (TES) that can both (i) generate electricity across the temperature differential between the ocean water and the human body and (ii) thermoregulate the body temperature in the underwater environment. The soft and elastic nature of TES enables an intimate and thorough contact with the deformable and irregular surfaces of the human skin, therefore maximizing the heat conduction at the human-device interface that the rigid or flexible thermoelectric devices can not fully attain. To the authors’ best knowledge, TES produces the highest electrical power density when compared to the stretchable thermoelectric devices reported so far, mainly owing to its optimum design factors. Along with the outstanding device performance, the underwater environment further boosts the thermoelectric efficiency of TES both in energy harvesting and thermoregulatory perspectives due to the much more favorable thermal properties of water than those of air. Furthermore, to verify the practical usage of TES and demonstrate its high wearability, we incorporated multiple TES units into the neoprene dry-suit. The TES units can self-power multiple embedded sensors that wirelessly monitor the physiological condition and further provide the spatial information of the tactical diver, such that the TES units and embedded system can function as a wearable underwater rescue platform. Lastly, the temperature feedback loop algorithm embedded in the thermoregulatory system allows the TES units to constantly regulate the temperature of the human body and thus prevent underwater hypo-/hyperthermia.

Design of a scalable, flexible, and durable thermoelectric cooling device for soft electronics using Kirigami cut patterns

A flexible, soft thermoelectric cooling device is presented that shows potential for human cooling applications in wearable technologies and close-to-body applications. Current developments lack integration feasibility due to non-scalable assembly procedures and unsuitable materials for comfortable and durable integration into products. Our devices have been created and tested around the need to conform to the human body which we have quantified through the creation of a repeatable drape testing procedure, a metric used in the textile industry. Inspired by mass manufacturing constraints, our flexible thermoelectric devices are created using commercially available materials and scalable processing techniques. Thermoelectric legs are embedded in a foam substrate to provide flexibility, while Kirigami-inspired cuts are patterned on the foam to provide the drape necessary for mimicking the performance of textile and close to body materials. In total, nine different configurations, three different fill factors and three different Kirigami cut patterns were fabricated and inspected for thermal characterization, mechanical testing, flexibility and drape. Our studies show that adding Kirigami patterns can increase the durability of the device, improve the flexibility, decrease the drape coefficient, and have <1% of impact on cooling performance at higher fill factors (>1.5%), reaching temperature differences up to 4.39 °C ± 0.17 °C between the hot and cold faces of the device. These thermoelectric cooling devices show great flexibility, durability, and cooling for integration into soft cooling products.

Ultraflexible and High-Thermoelectric-Performance Sulfur-Doped Ag2Se Film on Nylon for Power Generators.

In this work, we developed a facile method to fabricate low-cost, flexible, and high-thermoelectric-performance n-type Ag2Se1-xSx@(Ag2S1-ySey/S) composite film on a nylon membrane. The composite film was prepared by first performing wet-chemical synthesis of the S-doped Ag2Se powder, then vacuum-assisted filtration of the powder on a nylon membrane, and finally hot-pressing. Transmission electron microscopy (TEM) observation and energy-dispersive system (EDS) analysis of the film revealed that the film had a porous network-like microstructure, in which Ag2Se1-xSx sub-micron grains formed the skeleton and are coated by a ∼15 nm thick layer of S-rich Ag2S1-ySey nanograins mixed with an S amorphous phase. The film showed a power factor of ∼954.7 μW·m-1·K-2 at 300 K and superior flexibility (94.4% of the original electrical conductivity was preserved after bending 2000 times around a rod with a radius of 4 mm). Moreover, a six-leg flexible thermoelectric generator was assembled with the film and produced a maximum power of 6.67 μW (corresponding power density ∼14.8 W/m2) at a temperature difference of 38.7 K. This work reveals a novel approach to explore high-performance and low-cost flexible thermoelectric devices suitable for room-temperature applications.

Microstructurally Tailored Thin β-Ag2 Se Films toward Commercial Flexible Thermoelectrics.

Aiming to overcome both the structural and commercial limitations of flexible thermoelectric power generators, an efficient room-temperature aqueous selenization reaction that can be completed in air within less than 1min, to directly fabricate thin β-Ag2 Se films consisting of perfectly crystalline and large columnar grains with both in-plane randomness and out-of-plane [201] preferred orientation, is designed. A high power factor (PF) of 2590± 414 µW m-1 K-2 and a figure-of-merit (zT) of 1.2± 0.42 are obtained from a sample with a thickness of ≈1µm. The maximum output power density of the best 4-leg thermoelectric generator sample reach 27.6± 1.95 and 124± 8.78 W m-2 at room temperature with 30 and 60 K temperature differences, respectively, which may be useful in future flexible thermoelectric devices.

Thermoelectric characteristics of flexible reduced graphene oxide/silver selenide nanowire composites prepared by a facile vacuum filtration process

The reduced graphene oxide/silver selenide nanowire (rGO/Ag2Se NW) composite powders were fabricated via a wet chemical approach, and then flexible rGO/Ag2Se NW composite film was prepared by a facile vacuum filtration method combined with cold-pressing treatment. A highest power factor of 228.88 μW·m−1·K−2 was obtained at 331 K for the cold-pressed rGO/Ag2Se NW composite film with 0.01 wt% rGO. The rGO/Ag2Se NW composite film revealed superior flexibility as the power factor retained 94.62% after bending for 500 times with a bending radius of 4 mm, which might be due to the interwoven network structures of Ag2Se NWs and pliability of rGO as well as nylon membrane. These results demonstrated that the GO/Ag2Se NW composite film has a potential for preparation of flexible thermoelectric devices.