Flexible Thermoelectric Generator Research Articles

Experimental design using the Taguchi method for the development of conductive textiles used in flexible thermoelectric generators

This paper presents several aspects of the robust experimental design methodology using the Taguchi method to develop electrically conductive textiles. These conductive textiles will be used to make thermoelectric generators based on the Seebeck effect. Since the experimental development involves the use of the magnetron sputtering method with more than three variables, the Taguchi method was selected to observe how different parameters (5 independent variables such as argon flow, power, pressure, deposition surface area and metal type used for solid targets) influence the mean and variance of the process performance defined by the thickness of the metal film deposited (dependent variable). In the experimental design framework, the analysis using the Taguchi method, followed by optimisation, helped select the optimal experiments from the set of possible experiments. This methodology reduced the number of experiments by 21–42% and minimised resource consumption (e.g., metal targets, argon, energy).

Touch-activated information interaction system based on body-heat-powered flexible thermoelectric generator for food spoilage monitoring

With the rapid advancement of the Internet of Things, a wide range of information interaction platforms have been developed to enhance the quality of life and work. Nevertheless, rigid batteries greatly limit its development, which requires frequent replacement or recharge. Thermoelectric generators (TEG) have emerged as an attractive form for energy suppliers owing to their ability to convert low-grade energy heat into sustainable electricity. Herein, we have implemented a flexible TEG-based multifunctional information interaction system, which utilizes body heat to enable information interaction through finger touch. To increase the efficiency of energy conversion, the flexible TEG achieves high performance by optimizing the filling factor and material thermal conductivity, leading to a normalized power density of 1.43 μW/cm2 K2. As a proof-of-concept demonstration, the system has been applied to food spoilage monitoring with an MXene-based sensor to achieve stable and sensitive ammonia detection. The TEG-based information interaction system provides an attractive solution for intelligent packaging and will act as an exciting platform for human body energy harvesting and human–machine interaction for point of care testing.

High figure-of-merit of single-walled carbon nanotubes films with metallic type conduction

Carbon nanotubes are promising candidates for thermoelectric power generation because of their one-dimensionality mediated high Seebeck coefficient, high electrical conductivity with added advantages of flexibility, light weight, and scalability. We report the temperature-dependent thermoelectric properties of single-walled carbon nanotube (SWCNTs) films. The SWCNTs films exhibit p-type metallic conduction with high Seebeck coefficient (∼69.5 μVK−1) and moderate electrical conductivity (∼76 Scm−1). The films exhibit low thermal conductivity (∼0.1 Wm−1 K−1) due to phonon scattering at the interjunction region. The synergetic combination of thermoelectric properties resulted in a high figure-of-merit of ∼0.11 at 305 K. A flexible thermoelectric generator based on SWCNTs films mounted on a curved hot surface exhibited an output of 17 mV and 54 μA under a small temperature gradient of 10 K. The present work provides possible avenues for developing wearable SWCNTs based thermoelectric power generation modules for harvesting body heat.

Nanoengineering approach toward ultrahigh power factor Ag2Se/polyvinylpyrrolidone composite film for flexible thermoelectric generator

Flexible thermoelectric (TE) generators (F-TEGs), able to harvest heat energy from the human body, are considered to be competitive energy supply devices for wearable and implantable electronics. Herein, we fabricate Ag2Se/polyvinylpyrrolidone (PVP) films on nylon membranes via a novel nanoengineering approach: First, small amount Ag nanoparticles decorated Ag2Se nanowires (NWs) coated with a nanolayer of PVP are synthesized via a Se NW template method, and the Ag nanoparticles react with small amount Se residues at the core of the Ag2Se NWs during hot-pressing and form Ag2Se nanograins rich in defects embedded in big Ag2Se grains. The Ag2Se nanograin has a different orientation from the big Ag2Se grain; hence, the interface between the Ag2Se nanograin and the big Ag2Se grain enhances the phonon scattering but almost does not affect the transport of carriers. An optimized film exhibits a maximum power factor of 2478 ± 67 μW m−1 K−2 (corresponding ZT ∼ 1.05) at 300 K, which is one of the highest values reported for flexible Ag2Se films. The excellent TE properties of the film are attributed to its unique microstructure: highly densified, coherent/semi-coherent Ag2Se grains, embedded Ag2Se nanograins, and a very small amount of PVP located at nanopores. Besides, the film shows good flexibility: after 1500 times bending along a rod with a radius of 4.0 mm, the electrical conductivity still maintains 90.9 %. Furthermore, a 6-leg F-TEG assembled with the optimal film generates a maximum power of 4.58 μW (corresponding power density of ∼ 31.2 W m−2) at a temperature gradient of 38.7 K. This work provides an efficient strategy for the fabrication of ultrahigh-performance Ag2Se-based flexible TE films.

Fabrication of low-cost flexible thermoelectric generator using conductive HB graphite paint operated by solar radiation

Paper-based flexible thin film thermoelectric generators have emerged as a promising and feasible alternative to organic and inorganic conductors due to their ability to operate at room temperature within a limited temperature range. Here, a flexible solar thermoelectric generator (STEG) designed from a single material has been introduced, prepared by a simple painting method. We prepared HB graphite-based conductive paint and demonstrated a very unique method to design flexible STEG devices. The graphite paint shows p-type semiconductive behaviour, while, in conjunction with the polyethyleneimine (PEI) polymer, it acts as an n-type material. Based on the transport properties, the paint appears to be a good candidate for designing STEG devices. At room temperature, the power factor of 378 nW m−1 K−2 for p-type paint and 1.51 nW m−1 K−2 for n-type paint is obtained. In order to examine flexibility over the long term, the performance of the material was inspected through 300 repeated cycles, and transport properties (conductivity) were found to increase from 21 700 S m−1 to 73 500 S m−1 due to the excellent emulsifying properties of gum Arabic, which were confirmed by field emission scanning electron microscope analysis. To evaluate the performance of the thermoelectric (TE) generator, eight pairs of p–n legs are fabricated on normal copier (80 GSM) paper, and an output voltage of ∼0.1 mV K−1 (∼5.5 mV) for a temperature gradient T of up to ∼60 K was achieved. Further, the performance of flexible TE devices can be improved by increasing the number of thermoelectric legs and by sandwiching the device between Kapton tapes. Our work suggests a promising and simple approach to achieving cost-effective conversion of solar energy into electricity and highlights the potential of flexible STEGs for low-power applications.

Improved Heat Dissipation of Dip-Coated Single-Walled Carbon Nanotube/Mesh Sheets with High Flexibility and Free-Standing Strength for Thermoelectric Generators

Single-walled carbon nanotubes (SWCNTs) are promising thermoelectric materials used in thermoelectric generators (TEGs) to power sensors. However, the limitation of SWCNTs is their high thermal conductivity, which makes it difficult to create a sufficient temperature difference. In this study, we fabricated dip-coated SWCNT/mesh sheets using an SWCNT dispersion. Several types of mesh materials were tested, and the most suitable material was polyphenylene sulfide (PPS). SWCNTs were uniformly deposited on the PPS mesh surface without filling the mesh openings. The SWCNT/PPS mesh sheets exhibited flexibility and free-standing strength. When the edge of the SWCNT/PPS mesh sheets were heated, a higher temperature gradient was produced compared with that of the conventional SWCNT film owing to the increase in heat dissipation. A flexible and free-standing TEG with an area of 1200 mm2, fabricated using SWCNT/PPS mesh sheets, exhibited an output voltage of 31.5 mV and maximum power of 631 nW at a temperature difference of 60 K (Tlow: 320 K). When the TEG was exposed to wind at 3 m/s, temperature difference further increased, and the performance of the TEG increased by a factor of 1.3 for output voltage and 1.6 for maximum power. Therefore, we demonstrated that the TEG’s performance could be improved using SWCNT/PPS mesh sheets.

Optimizing Waste Heat Conversion: Integrating Phase-Change Material Heatsinks and Wind Speed Dynamics to Enhance Flexible Thermoelectric Generator Efficiency.

Flexible thermoelectric generators (FTEGs) have garnered significant attention for their potential in harnessing waste heat energy from various sources. To optimize their efficiency, FTEGs require efficient and adaptable heatsinks. In this study, we propose a cost-effective solution by integrating phase-change materials into FTEG heatsinks. We developed and tested three flexible phase-change material thicknesses (4 mm, 7 mm, and 10 mm), focusing on preventing leaks during operation. Additionally, we investigated the impact of wind speed on the output performance of FTEGs with a flexible phase-change material heatsink. The results indicate that the appropriate flexible phase-change material thickness, when integrated with considerations for wind speed, demonstrates remarkable heat-absorbing capabilities at phase-change temperatures. This integration enables substantial temperature differentials across the FTEG modules. Specifically, the FTEG equipped with a 10 mm thick flexible phase-change material heatsink achieved a power density more than four times higher when the wind speed was at 1 m/s compared to no wind speed. This outcome suggests that integrating phase-change material heatsinks with relatively low wind speeds can significantly enhance flexible thermoelectric generator efficiency. Finally, we present a practical application wherein the FTEG, integrated with the flexible phase-change material heatsink, efficiently converts waste heat from a circular hot pipe into electricity, serving as a viable power source for smartphone devices. This work opens exciting possibilities for the future integration of flexible thermoelectric modules with flexible phase-change material heatsinks, offering a promising avenue for converting thermal waste heat into usable electricity.

High-Performance Ag 2 Se/Methyl Cellulose Thermoelectric Composites for Flexible Power Generators

Flexible thermoelectric generators (FTEGs) offer a promising solution for powering wearable electronics, while their practical applications are mainly obstructed by the moderate properties of flexible thermoelectric (TE) materials. Here, flexible Ag 2 Se nanowire (NW)/methyl cellulose (MC) composite films were developed via facile screen-printing technology combined with cold pressing and annealing treatment, and a highest power factor of 1,641.58 μW m −1 K −2 at 360 K was achieved. The reasons for the high TE performance of the Ag 2 Se NW/MC composite films were because, after the annealing treatment, the Ag 2 Se NWs were sintered to form conductive network structures, the crystallinity of Ag 2 Se was markedly enhanced, and the content of insulating phase MC in the composite film was decreased. The Ag 2 Se NW/MC composite film held appreciable flexibility, as its room-temperature power factor (1,312.08 μW m −1 K −2 ) can retain ~93% after bending for 1,000 cycles at a radius of 4 mm. Furthermore, the assembled FTEG consisting of 4 strips can generate a maximal power density of 3.51 W m −2 at a temperature difference of 14.1 K. Our results open an effective and large-scale strategy for fabricating high-performance flexible TE materials and energy-harvesting devices.

Thermoelectric properties of low-temperature-grown polycrystalline InAs1−xSbx films

The development of thin-film thermoelectric generators for micro-energy harvesting is highly anticipated. In this study, we have investigated the synthesis and thermoelectric applications of ternary alloy InAs1−xSbx thin films, which are narrow-gap III–V compound semiconductors. Polycrystalline InAs1−xSbx thin films with sub-micrometer grain size were synthesized on glass using molecular-beam deposition at 400 °C with all composition x. The InAs1−xSbx thin films exhibited n-type conduction, and their electrical and thermoelectric properties were strongly dependent on x and the amount of Sn doping. The ternary alloying reduced the power factor and contributed to a reduction in thermal conductivity. The InAs0.2Sb0.8 thin film on a glass substrate exhibited a power factor of 100 μW m−1 K−2 and a thermal conductivity of 2.0 W m−1 K−1 at room temperature. Furthermore, a comparable performance was demonstrated for an InAs0.2Sb0.8 thin film grown on a plastic film. These achievements will pave the way for the application of III–V compound semiconductors in flexible thermoelectric generators.

Giant Seebeck effect over 0.1V K-1 - is this an intrinsic phenomenon in organic semiconductors?

A good solution for energy harvesting is to generate electricity using waste heat from our bodies or living environment. Therefore, the development of flexible and lightweight thermoelectric generators (TEGs) is urgently necessary, and studies on organic thermoelectric materials have become increasingly intensive. This article will present ongoing studies about a mysterious phenomenon in organic semiconductors, the giant Seebeck effect (GSE). The GSE was first discovered with pure C60 thin films and eventually confirmed to occur in various organic semiconductors. In the thin films or single crystals of organic small-molecule semiconductors with high purity, i.e., small carrier density, huge Seebeck coefficients, >0.1 V K-1, were reproducibly observed in the temperature range near 300-400 K. The facts revealed by the experiments to date will be presented, and unresolved mysteries will be discussed.

Enhanced Thermoelectric Performance of Poly(3,4-ethylenedioxythiophene) and Graphene/Poly(3,4-ethylenedioxythiophene) Composites via Vapor Phase Polymerization

Organic thermoelectric (TE) materials are interesting candidates for wearable energy harvesting systems because they are flexible and convert thermoelectric energy. Thermally conductive flexible thermoelectric generators (TEGs) built of these materials produce power from body and environmental temperature differences, enabling devices to run without recharging. The exceptional electrical conductivity, mechanical flexibility, and environmental robustness of organic transition metal (TE) materials like PEDOT and graphene composites are becoming more known. Due to their synergistic link, PEDOT and graphene layers are appealing thermoelectric possibilities because they increase See beck coefficient and electrical conductivity. Vapor phase polymerization (VPP) may replace chemical or electrochemical polymerization for PEDOT manufacture. This method allows one to accurately regulate PEDOT layer electrical characteristics by altering thickness and structure. These PEDOT and graphene/PEDOT composites were made using iron (III) to sylate as the VPP oxidizing agent. VPP technique produces homogeneous films with better electrical conductivity than existing approaches. We tested the electrical resistance and Linseis LSR-3/800 Seebeck coefficient of PEDOT and graphene/PEDOT composites to determine their thermoelectric characteristics. VPP’s graphene/PEDOT composites have better electrical conductivity and See beck coefficient than conventional PEDOT, improving their thermoelectric power factor. These results show that VPP may be utilized to develop organic thermoelectric materials for lightweight energy collectors.

Additive manufacturing of highly conductive carbon nanotube architectures towards carbon-based flexible thermoelectric generators

SWCNT/epoxy-based p-type 3D-printed TE material with power factor 102 μW mK−2. SWCNT/epoxy-based n-type 3D-printed TE material with power factor 75 μW mK−2. Open circuit voltage of 3D-CTEG: VOC = 13.6 mV. Power output of 3D-CTEG: Pmax = 4.1 μW.

Novel Thermoelectric Fabric Structure with Switched Thermal Gradient Direction toward Wearable In-Plane Thermoelectric Generators.

Wearable thermoelectric generators (TEGs) have exhibited great potential to convert the temperature gradient between the human body and the environment into electrical energy for maintenance-free wearable applications. A 2D planar device structure is widely employed for fabricating flexible TEGs due to its simple structure and facile fabrication properties. However, this device configuration is more appropriate for utilizing in-plane temperature differences than the out-of-plane direction, which limits their application in wearable cases since the temperature difference between the human body and the environment is in the out-of-plane direction. To solve this problem, a novel fabric-based TEG structure that can utilize the out-of-plane temperature gradient is proposed in this work. By introducing thermally conductive components in the generator, the out-of-plane temperature difference can be switched to the in-plane direction, which can be further utilized for 2D planar devices in wearable applications. The prepared thermoelectric fabric prototype with only 12 p-type TE legs exhibits a maximum open-circuit voltage of 4.69mV and an output power of 39.7 nW at a temperature difference of 30 K. This strategy exhibits a high degree of versatility and can be readily applied to other 2D planar TEGs, thus expanding their potential application in wearable technology.

Enhancing thermoelectric performance through acid-treated NbFeSb: A versatile approach for sustainable energy conversion

Drop-casting has emerged as a promising and scalable technique for fabricating thermoelectric generators, suitable for energy harvesting and waste heat recovery applications. This study delves into the potential of thermoelectric drop-casting utilizing acid-treated NbFeSb, a half-Heusler material. The acid treatment notably reduces the particle dimensions of NbFeSb from a micro-scale to a nano-scale, thereby enhancing phonon scattering and diminishing thermal conductivity from 16 to 12 W/mK at room temperature. The formulation of the thermoelectric ink involves ethylene glycol as the solvent and polyvinylpyrrolidone (PVP) as an additive, meticulously optimized to enhance dispersion and rheological attributes. To systematically analyze the performances of the printed films, an investigation was conducted, varying the PVP concentration. Our findings indicate that a 20% PVP content yields the highest power factor, concurrently enhancing the Seebeck coefficient and electrical conductivity, while preserving reduction in thermal conductivity. The ink-printed films of acid-treated NbFeSb exhibited superior thermoelectric performance compared to pristine NbFeSb films, owing to the uniform distribution of nanoparticles and the absence of particle agglomeration. Furthermore, this study successfully fabricates a flexible thermoelectric generator (TEG) using acid-treated NbFeSb ink, demonstrating its potential integration into wearable devices and power generation systems. The TEG attains an open-circuit voltage of 88 mV across a temperature gradient of 40 K, with a peak output power of 8.1 mW. This study highlights the significant impact of thermoelectric drop-casting as an adaptable and efficient approach to enhance the thermoelectric performance of high-thermal-conductivity half-Heusler materials, opening new avenues for sustainable energy conversion technologies.

Development of the Photolithography Process for the Fabrication of a Flexible Thin Film Thermoelectric Generator

Development of the Photolithography Process for the Fabrication of a Flexible Thin Film Thermoelectric Generator

Improvement of thermoelectric performance of flexible compound Ag2S0.55Se0.45 by means of partial V-substitution for Ag

The effects of the partial V-substitution for Ag on the thermoelectric (TE) properties are investigated for a flexible semiconducting compound Ag2S0.55Se0.45. Density functional theory calculations predict that such a partial V-substitution constructively modifies the electronic structure near the bottom of the conduction band to improve the TE performance. The synthesized Ag1.97V0.03S0.55Se0.45 is found to possess a TE dimensionless figure-of-merit (ZT) of 0.71 at 350 K with maintaining its flexible nature. This ZT value is relatively high in comparison with those reported for flexible TE materials below 360 K. The increase in the ZT value is caused by the enhanced absolute value of the Seebeck coefficient with less significant variation in electrical resistivity. The high ZT value with the flexible nature naturally allows us to employ the Ag1.97V0.03S0.55Se0.45 as a component of flexible TE generators.

High-Performance Paper-Based Thermoelectric Generator from Cu2SnS3 Nanocubes and Bulk-Traced Bismuth.

Flexible paper-based thermoelectric generators (PTEGs) have drawn significant interest in recent years due to their various advantages such as flexibility, adaptability, environment friendliness, low cost, and easy fabrication process. However, the reported PTEG's output performance still lags behind the performance of other flexible devices as it is not so easy to obtain a compact film on a paper-based substrate with desirable power output with the standard thermoelectric (TE) materials that have been previously utilized. In this direction, Cu2SnS3 (CTS), an earth-abundant, ternary sulfide, can be a good choice p-type semiconductor, when paired with a suitable n-type TE material. In this article, CTS nanocubes are synthesized via a simple hot injection method and a thick film device on emery paper was prepared and optimized. Furthermore, a flexible, 20-pair PTEG is fabricated with p-type CTS legs and traced and pressed n-type bismuth legs assembled using Kapton tape that produced a significantly high output power of 2.18 μW (output power density ∼0.85 nW cm-2 K-1) for a temperature gradient of ΔT = 80 K. The TE properties are also supported by finite element simulation. The bending test conducted for the PTEG suggests device stability for up to 800 cycles with <0.05% change in the internal resistance. A proof-of-concept field-based demonstration for energy harvesting from waste heat of a motorbike exhaust is shown recovering an output power of ∼42 nW for ΔT = 20 K, corroborating the experimental and theoretical results.

Tunable Thermoelectric Performance of the Nanocomposites Formed by Diketopyrrolopyrrole/Isoindigo-Based Donor-Acceptor Random Conjugated Copolymers and Carbon Nanotubes.

This paper presents the development of thermoelectric properties in nanocomposites comprising donor-acceptor random conjugated copolymers and single-walled carbon nanotubes (SWCNTs). The composition of the conjugated polymers, specifically the ratio of diketopyrrolopyrrole (DPP) to isoindigo (IID), is manipulated to design a series of random conjugated copolymers (DPP0, DPP5, DPP10, DPP30, DPP50, DPP90, DPP95, and DPP100). The objective is to improve the dispersion of SWCNTs into smaller bundles, leading to enhanced thermoelectric properties of the polymer/SWCNT nanocomposite. This dispersion strategy promotes an interconnected conducting network, which plays a critical role in optimizing the thermoelectric performance. Accordingly, the effects of morphologies on the thermoelectric properties of the nanocomposites are systematically investigated. The DPP95/SWCNT nanocomposite exhibits the strongest interaction, resulting in the highest power factor (PF) of 711.1 μW m-1 K-2, derived from the high electrical conductivity of 1690 S cm-1 and Seebeck coefficient of 64.8 μV K-1. The prototype flexible thermoelectric generators assembled with a DPP95/SWCNT film achieve a maximum power output of 20.4 μW m-2 at a temperature difference of 29.3 K. These findings highlight the potential of manipulating the composition of random conjugated copolymers and incorporating SWCNTs to efficiently harvest low-grade waste heat in wearable thermoelectric devices.

Largely Enhanced Thermoelectric Power Factor of Flexible Cu2-xS Film by Doping Mn.

Copper-sulfide-based materials have attracted noteworthy attention as thermoelectric materials due to rich elemental reserves, non-toxicity, low thermal conductivity, and adjustable electrical properties. However, research on the flexible thermoelectrics of copper sulfide has not yet been reported. In this work, we developed a facile method to prepare flexible Mn-doped Cu2-xS films on nylon membranes. First, nano to submicron powders with nominal compositions of Cu2-xMnyS (y = 0, 0.01, 0.03, 0.05, 0.07) were synthesized by a hydrothermal method. Then, the powders were vacuum-filtrated on nylon membranes and finally hot-pressed. Phase composition and microstructure analysis revealed that the films contained both Cu2S and Cu1.96S, and the size of the grains was ~20-300 nm. By Mn doping, there was an increase in carrier concentration and mobility, and ultimately, the electrical properties of Cu2-xS were improved. Eventually, the Cu2-xMn0.05S film showed a maximum power factor of 113.3 μW m-1 K-2 and good flexibility at room temperature. Moreover, an assembled four-leg flexible thermoelectric generator produced a maximum power of 249.48 nW (corresponding power density ~1.23 W m-2) at a temperature difference of 30.1 K, and had good potential for powering low-power-consumption wearable electronics.

Encapsulated Ag2Se-based flexible thermoelectric generator with remarkable performance

Flexible thermoelectric generators hold great potential for powering wearable electronics. Herein, we employ a novel encapsulation technology to fabricate a Ag2Se-based composite film, which possesses a remarkable room-temperature power factor (PF) of 25.6 μW cm−1 K−2 and a peak PF of ∼27 μW cm−1 K−2 at 380 K. The microscopic origin of the above performance is attributed to the appearance of CuAgSe second phase by our in-situ transmission electron microscopy evidences. Moreover, the encapsulation guarantees the Ag2Se layer in a zero-strain state, contributing to a negligible relative resistance change (<0.2%) even after 2000 bending cycles under a bending radius of 3 mm. The encapsulated flexible thermoelectric generator assembled with six Ag2Se legs demonstrates a maximum power density of 130 W m−2 at a temperature difference of 60 K and is compatible with harsh working environment.