Flexible Thermoelectric Material Research Articles

Solvent-assisted synthesis of Ag2Se and Ag2S nanoparticles on carbon fabric for enhanced thermoelectric performance

The challenge of developing low-cost, highly flexible, and high-performance thermoelectric (TE) materials persists due to the low thermoelectric efficiency of conducting polymers and the inflexibility of inorganic materials. In this study, we successfully integrated Ag2Se and Ag2S with highly conductive carbon fabric (CF) to produce a flexible thermoelectric material. A facile one-step solvothermal method was employed to synthesize the Ag2Se-CF and Ag2S-CF, which were then subjected to X-ray analysis to confine the phase formation of Ag2Se and Ag2S on the carbon fabric. The analysis revealed that Ag2Se and Ag2S nanoparticles were tightly packed on the surface of carbon fabric, and compositional analysis confirmed the interaction between the material and carbon fabric. The thermoelectric properties of Ag2Se-CF and Ag2S-CF were significantly altered due to carrier concentration and mobility variations, resulting in a low power factor of 6.7 μW/mK2 for Ag2Se-CF and a high-power factor of 24 μW/mK2 at 373 K for Ag2S-CF. The growth of Ag2Se-CF and Ag2S-CF on carbon fabric led to an enhancement in their thermoelectric properties. Further, TE legs were fabricated using the Ag2Se-CF (p-type) and Ag2S-CF (n-type), and the fabricated legs exhibited an output voltage of ∼20 mV to ∼86.65 mV at a temperature gradient (ΔT) of 3–8 K. This work represents a cutting-edge approach to the fabrication of high-performance, wearable thermoelectric devices.

Flexible Ag-S-Te System with Promising Room-Temperature Thermoelectric Performance.

Silver chalcogenides demonstrate great potential as flexible thermoelectric materials due to their excellent ductility and tunable electrical and thermal transport properties. In this work, we report that the amorphous/crystalline phase ratio and thermoelectric properties of the Ag2SxTe1-x (x = 0.55-0.75) samples can be modified by altering the S content. The room-temperature power factor of the Ag2S0.55Te0.45 sample is 4.9 μW cm-1 K-2, and a higher power factor can be achieved by decreasing the carrier concentration as predicted by the single parabolic band model. The addition of a small amount of excessive Te into Ag2S0.55Te0.45 (Ag2S0.55Te0.45+y) not only enhances the power factor by decreasing the carrier concentration but also reduces the total thermal conductivity due to decreased electronic thermal conductivity. Owing to the effectively optimized carrier concentration, the thermoelectric power factor and dimensionless figure of merit zT of the sample with y = 0.007 reaches, respectively, 6.2 μW cm-1 K-2 and 0.39, while the excellent plastic deformability is well maintained, demonstrating its promising potential as a flexible thermoelectric material at room temperature.

Development of polypyrrole-coated cotton thermoelectric fabrics

Polypyrrole (PPy) is most widely used conductive polymer for conductive textile applications owing to its excellent electrical conductivity, good electromagnetic shielding, good anti-bacterial properties, etc. However, it has not been explored much as thermoelectric material. PPy can be coated on cotton fabric in aqueous media from its monomer to prepare electro-conductive fabrics, which will be a flexible organic thermoelectric material. In this study, PPy-coated cotton fabrics are prepared by in situ chemical polymerisations of pyrrole in presence of ferric chloride. The polymerisation process is optimised by employing Box–Behnken response surface design and method of steepest ascent to achieve minimum electrical resistivity. The lowest resistivity of PPy-coated cotton fabric is achieved to be 1.23 Ω/cm after the optimisation. The PPy-coated cotton fabric with copper thermocouple shows Seebeck coefficient as high as 11.95 µV/K, power factor 0.139 µW m−1 K−2, and thermoelectric figure of merit 1.22 × 10−4 at an absolute temperature of 300K. The thermoelectric performance of this coated fabric is found to be comparable to other costly conductive polymers and inorganic semiconductors. The surface morphology of PPy-coated cotton fabric is observed by field-emission scanning electron microscopy. Chemical interaction between cotton and PPy is investigated by FTIR spectroscopy. Analysis of semi-crystalline structure is done by X-ray diffraction and thermo-gravimetric analyser is used for analysis of thermal degradation.

Enhancing hydrovoltaic power generation through heat conduction effects

Restricted ambient temperature and slow heat replenishment in the phase transition of water molecules severely limit the performance of the evaporation-induced hydrovoltaic generators. Here we demonstrate a heat conduction effect enhanced hydrovoltaic power generator by integrating a flexible ionic thermoelectric gelatin material with a porous dual-size Al2O3 hydrovoltaic generator. In the hybrid heat conduction effect enhanced hydrovoltaic power generator, the ionic thermoelectric gelatin material can effectively improve the heat conduction between hydrovoltaic generator and near environment, thus increasing the water evaporation rate to improve the output voltage. Synergistically, hydrovoltaic generator part with continuous water evaporation can induce a constant temperature difference for the thermoelectric generator. Moreover, the system can efficiently achieve solar-to-thermal conversion to raise the temperature difference, accompanied by a stable open circuit voltage of 6.4 V for the hydrovoltaic generator module, the highest value yet.

Flexible thermoelectric and photosensitive thin-film material based on nanostructured ZnO:In layer covered by nanocellulose

In this study, to create new flexible thermoelectric and photosensitive thin-film material for in-plane thermoelectric generator (TEG) and broadband photodetector (PD) we deposited nanostructured indium-doped zinc oxide (ZnO:In) thin films on polyimide (PI) substrates by using the inexpensive and scalable method Successive Ionic Layer Adsorption and Reaction (SILAR). To reduce the resistivity of the ZnO:In film in the ZnO:In/PI composition to about 0.02 Ω∙m, it was annealed in a vacuum at 300 °C. To protect ZnO:In from the influence of gases and environmental moisture, the composition ZnO:In/PI was coated with nanocellulose (NC) manufactured by TEMPO-mediated oxidation of cheap and abundant herbaceous plant. An analysis of the surface morphology using scanning electron microscopy (SEM) using secondary electron (SE) and backscattered electron (BSE) images combined with energy dispersive X-ray spectrometry (EDS) made it possible to select the optimal mode for the formation of a continuous nanocellulose coating in the prepared in this way NC/ZnO:In/PI sample. By using ZnO:In/PI and NC/ZnO:In/PI test samples with ohmic aluminum contacts we determined output thermoelectric characteristics and time-dependent photoresponses depending on the illumination wavelength λ, bias voltage U and irradiance intensity Pλ. It has been experimentally established that both samples are suitable for use as broadband photodetectors. Their spectral responsivity Rλ data are from 2 to 0.1 A/W in the UV–visible-NIR detection range. The external quantum efficiencies (EQEs) for UV and blue light are especially high, at the level of hundreds of percent. But even under red and NIR illuminations, the obtained ZnO:In/PI and NC/ZnO:In/PI test samples demonstrate Rλ and EQE values, which exceed those of modern photodetectors based on zinc oxide and its composites. The specific detectivity D* is in the level of 1010-1012 Jones (cm∙s−1/2∙W−1) for all U, λ and Pλ, which is similar to D* values obtained for the known broadband photodetectors based on ZnO-contained compositions.

An Approach toward the Realization of a Through-Thickness Glass Fiber/Epoxy Thermoelectric Generator.

The present study demonstrates, for the first time, the ability of a 10-ply glass fiber-reinforced polymer composite laminate to operate as a structural through-thickness thermoelectric generator. For this purpose, inorganic tellurium nanowires were mixed with single-wall carbon nanotubes in a wet chemical approach, capable of resulting in a flexible p-type thermoelectric material with a power factor value of 58.88 μW/m·K2. This material was used to prepare an aqueous thermoelectric ink, which was then deposited onto a glass fiber substrate via a simple dip-coating process. The coated glass fiber ply was laminated as top lamina with uncoated glass fiber plies underneath to manufacture a thermoelectric composite capable of generating 54.22 nW power output at a through-thickness temperature difference οf 100 K. The mechanical properties of the proposed through-thickness thermoelectric laminate were tested and compared with those of the plain laminates. A minor reduction of approximately 11.5% was displayed in both the flexural modulus and strength after the integration of the thermoelectric ply. Spectroscopic and morphological analyses were also employed to characterize the obtained thermoelectric nanomaterials and the respective coated glass fiber ply.

Optimized Thermoelectric Performance of Carbon Nanoparticle-Carbon Nanotube Heterostructures by Tuning Interface Barrier Energy.

Herein, thermoelectric carbon nanoparticle (CNP)-carbon nanotube (CNT) heterostructures are introduced as a promising flexible thermoelectric material. The optimal barrier energy between the CNP and CNT increases the Seebeck coefficient (S) of the heterostructures through the energy filtering effect. For optimized thermoelectric performance, the CNP-CNT barrier energy can be effectively tuned by controlling the work function of the CNPs. The optimized p-type CNP-CNT heterostructures exhibited S and power factor (PF) of 50.6 ± 1.4 μV K-1 and 400 ± 26 μW m-1 K-2, respectively. The n-type CNP-CNT heterostructures, optimized for another work function of the CNPs, exhibited S and PF of up to -37.5 ± 3.4 μV K-1 and 214 ± 42 μW m-1 K-2, respectively. The energy harvesting capability of a thermoelectric generator prepared using p- and n-type CNP-CNT heterostructures with optimized barrier energies is demonstrated. The thermoelectric generator with 10 p-type and 9 n-type thermoelectric elements exhibited a maximum output power of 0.12 μW from a ΔT of 5 K. This work shows a facile strategy for synthesizing thermoelectric CNP-CNT heterostructures with optimized energy filtering effects. Application to the thermoelectric device on a paper substrate is also discussed.

Pencil painting like preparation for flexible thermoelectric material of high-performance p-type Na1.4Co2O4 and novel n-type NaxCo2O4

Flexible thermoelectric materials are presented with potential applications in electronic devices and energy conversion due to their convenient preparation, good flexibility, and various forms. However, as ductility is rarely observed in inorganic semiconductors and ceramic insulators, reports on applications of inorganic oxide materials in flexible thermoelectric materials are sparse. Here, we report a new method for the synthesis of a flexible Na1.4Co2O4 thermoelectric material based on Na1.4Co2O4 bulk materials, which are prepared by a self-flux method and painted on print paper. Seebeck coefficient and power factor of the obtained thermoelectric material are 78–102 μVK-1 and 159–223 μWm−1K−2, respectively, in a temperature range of 303–522 K, which are superior to those values of other conductive polymers and their compounds. More interestingly, the n-type Na1.4Co2O4 flexible material is obtained in the painting process at higher pressure with Seebeck coefficients of −109 to −183 μVK−1 in a temperature range of 303–522 K. The convenient preparation method of these novel flexible thermoelectric materials may be expanded to the synthesis of other flexible thermoelectric materials, which will be the focus of future work.

Enhanced thermoelectric properties exhibited by unreduced freestanding graphene oxide/carbon nanotube membranes

The considerable thermoelectric power factor of 5.33 × 10−2 μW mK−2 observed for unreduced GO/CNT (1 : 2 ratio) implies the suitability of unreduced GO/CNT as a flexible thermoelectric material.

Mass-produced metallic multiwalled carbon nanotube hybrids exhibiting high N-type thermoelectric performances

Finding a stable n-type flexible thermoelectric (TE) material with a high power factor and mass production is a challenge for wearable TE devices.

Efficient biodegradable flexible hydrophobic thermoelectric material based on biomass-derived nanocellulose film and copper iodide thin nanostructured layer

Here we applied solar energy converted into biomass to produce efficient biodegradable flexible hydrophobic thermoelectric (TE) material with nanocellulose (NC) film as environmentally friendly functional substrate. We used fast-growing perennial herb Miscanthus × giganteus to manufacture flexible 12 µm thick NC film with stable monoclinic cellulose structure (Iβ), high crystallinity index (CI = 78%) and average crystallite size 3 – 4 nm. Through the low-temperature cheap and scalable method Successive Ionic Layer Adsorption and Reaction (SILAR) we deposited copper iodide (CuI) films on NC substrates and thus obtained non-toxic TE materials CuI/NC, which can be water-repellent, as their contact angles reach 140°. In the most efficient TE sample CuI/NC, the obtained via SILAR 0.39 µm thick nanostructured CuI film consists of cubic (111)-oriented γ-CuI crystals with faceted surfaces of ~200–300 nm. The high electrical conductivity (σ) and shape of the σ vs. temperature (T) curve of this CuI/NC sample is realized through suppression of grain boundary scattering due to tunneling currents in CuI. The CuI/NC material has large thermoelectric power factor (PF) that grows with increasing temperature and reaches value 140 μW·m−1·K−2 at T = 333 K. This PF is the record for biodegradable flexible thermoelectric materials. At ΔT = 40 K the CuI/NC-based single p-CuI thermoelectric leg generates open circuit voltage 3.5 mV, short circuit current 4 µA, and power 3.8 nW, and these output parameters can be further improved through a thickening the CuI film in CuI/NC by increasing the number of SILAR cycles.

Worm-Like PEDOT:Tos coated polypropylene fabrics via low-temperature interfacial polymerization for high-efficiency thermoelectric textile

Inspired by worm morphology, this study report a low-cost and facile synthesis approach to construct a worm-like P-type flexible thermoelectric material—poly(3,4-ethylene dioxythiophene):p-toluenesulfonic acid (PEDOT:Tos) coated polypropylene (PP) nonwoven fabrics via low-temperature interfacial polymerization. This as-prepared thermoelectric fabric made of fibers with a core-shell structure has an electrical conductivity of 2.19 S/cm and the Seebeck coefficient of 16.15 μV K−1 at a temperature difference of 30 °C. Furthermore, it presents good mechanical and washing stability by bending and ultrasonic test. A thermoelectric device designed of five pairs of thermoelectric material (p-type) and copper wire (n-type) generates 0.512 mV voltage at a temperature difference of 10 °C. This thermoelectric device has a prospect in the application of portable and interactive devices. This study proposes an easy way to manufacture bioinspired thermoelectric fabric with high thermoelectric performance and high potential applications, such as in wearable and flexible sensors.

Decoupling phonon and carrier scattering at carbon nanotube/Bi2Te3 interfaces for improved thermoelectric performance

Inorganic semiconductor-carbon nanotube (CNT) hybrid is a new type of promising flexible thermoelectric (TE) material for efficient thermal energy recovery and positive cooling. However, the influence of CNTs, that typically have a high thermal conductivity (κ), high electrical conductivity (σ) and low figure-of-merit (ZT) value, on the hybrid properties remains unclear, and this is vital for the rational design of high-performance CNT-containing TEs. Here, we report a flexible TE material of Bi2Te3 nanocrystals attached to a CNT network with tuned κ and σ. By plasma pretreating the CNT scaffold, a high density of defects was generated at the hybrid interface, which introduced strong phonon scattering without much sacrifice of σ, due to the different attenuations of the wavelength spectra of phonons and carriers. Consequently, the hybrid lattice κ was significantly reduced to ∼0.19 W m−1 K−1, and a ZT value of ∼0.25 was obtained at room temperature, ∼50% higher than that obtained using pristine CNTs. Our results provide a general approach to the decoupling of phonon and carrier transport properties by tailoring the hybrid interface, which is significant for developing high-performance flexible CNT-based TE hybrids.

Use of biomass for a development of nanocellulose-based biodegradable flexible thin film thermoelectric material

In this work, we used solar energy converted via photosynthesis into chemical energy of the biomass of the fast-growing perennial herb Miscanthus × giganteus for the manufacture of nanocellulose (NC) films, which are biodegradable alternative to common petroleum-based polymer substrates used in flexible electronics. To create the NC substrates, we applied an environmentally friendly method of organosolv delignification of plant raw materials carried out at a low temperature and in a relatively short time. Then by means of the low-temperature cheap and scalable method Successive Ionic Layer Adsorption and Reaction (SILAR) we deposited copper iodide (CuI) film of 0.72 µm thickness on both sides of the 12 µm thick NC substrate, and thus obtained light-weight and flexible biodegradable nontoxic thermoelectric material CuI/NC. Crystal structure, morphology, chemical composition, and optical, electrical and thermoelectric properties of the CuI/NC have been researched. Studies have shown that nanostructured p-type semiconductor CuI film in the CuI/NC TE material is quite dense and completely covers the NC surface. It has typical optical direct band gap ≈ 3.0 eV, is single-phase γ-CuI with crystallite sizes in the 19–25 nm range, with moderate dislocation density of (1.6–2.8) × 1015 lines/m2, and tolerable microstrains ε of (4–9) × 10−3 a.u. The determined value of the Seebeck coefficient S is ~228 μV K−1, at that, S is constant in the temperature range 290–335 K. Together with the thermoelectric power factor ≈ 36 μW·m−1·K−2it is favorable for the use of CuI/NC as new thermoelectric material for an in-plane design of biodegradable flexible thin film thermoelectric generator (TEG). At temperature gradient of 50 K, the single p-CuI thermoelectric leg made from CuI/NC strip of 3 cm long and 0.5 cm wide generates open circuit voltage 8.4 mV, short circuit current 0.7 µA and maximum output power 1.5 nW. It corresponds to the output power density 10 µW/m2, and thus confirms the suitability of CuI/NC to obtain electricity by the harvesting the waste environmental heat.

Tunneling-Limited Thermoelectric Transport in Carbon Nanotube Networks Embedded in Poly(dimethylsiloxane) Elastomer

As an emerging flexible thermoelectric material, polymer composites with embedded carbon nanotube (CNT) networks have shown promising properties, but their thermoelectric transport has not been fully understood. Herein, we embedded CNT networks in poly(dimethylsiloxane) (PDMS) elastomers and analyzed their thermoelectric properties using Landauer theory. We find that the simultaneous increase in Seebeck coefficient and electrical conductivity with increasing CNT content up to ∼40% can be attributed to the tunneling transport at CNT junctions with the gap distance decreasing with increasing CNT content. Beyond 40% CNTs, both properties are reduced and saturated due to the reduced PDMS content and increased material nonuniformity, which effectively replaced the PDMS gap with an air gap at the junction with a higher barrier to reduce the properties. Our results and analysis provide important insights into the material optimization of hybrid thermoelectric composites based on CNT networks and many other nanos...

Noncovalent Modification of Single-Walled Carbon Nanotubes Using Thermally Cleavable Polythiophenes for Solution-Processed Thermoelectric Films.

Four thermally cleavable polythiophene derivatives containing carbonate and solubilizing groups were synthesized for noncovalent modification of single-walled carbon nanotubes (SWCNTs). A well-dispersed polythiophene/SWCNTs composite was obtained by adsorption of the polymer at the SWCNT surface. The solution-processed composite film exhibited solid-state thermal cleavage of the insulating solubilizing group through decarboxylation, producing an insoluble composite film. The thermally cleavable composite film was evaluated for potential application as a thermoelectric (TE) material. The electrical conductivity (σ) of the thermally treated composite film was up to 250 times higher than that of the as-prepared composite film. The increased σ contributed to an increase in the power factor (PF). The ethanol-processed composite film could be applicable for green processing of a TE material using the less-toxic solvent. The substrate-free polythiophene/SWCNTs composite film prepared by simple solvent evaporation yielded a figure-of-merit of 3.1 × 10-2 with a PF of 28.8 μW m-1 K-2 at 25 °C. This solution-processed methodology is beneficial for the development of a flexible TE material.

Flexible layer-structured Bi2Te3 thermoelectric on a carbon nanotube scaffold.

Inorganic chalcogenides are traditional high-performance thermoelectric materials. However, they suffer from intrinsic brittleness and it is very difficult to obtain materials with both high thermoelectric ability and good flexibility. Here, we report a flexible thermoelectric material comprising highly ordered Bi2Te3 nanocrystals anchored on a single-walled carbon nanotube (SWCNT) network, where a crystallographic relationship exists between the Bi2Te3 <[Formula: see text]> orientation and SWCNT bundle axis. This material has a power factor of ~1,600 μW m-1 K-2 at room temperature, decreasing to 1,100 μW m-1 K-2 at 473 K. With a low in-plane lattice thermal conductivity of 0.26 ± 0.03 W m-1 K-1, a maximum thermoelectric figure of merit (ZT) of 0.89 at room temperature is achieved, originating from a strong phonon scattering effect. The origin of the excellent flexibility and thermoelectric performance of the Bi2Te3-SWCNT material is attributed, by experimental and computational evidence, to its crystal orientation, interface and nanopore structure. Our results provide insight into the design and fabrication of high-performance flexible thermoelectric materials.

Ultrahigh thermoelectric power factor in flexible hybrid inorganic-organic superlattice

Hybrid inorganic–organic superlattice with an electron-transmitting but phonon-blocking structure has emerged as a promising flexible thin film thermoelectric material. However, the substantial challenge in optimizing carrier concentration without disrupting the superlattice structure prevents further improvement of the thermoelectric performance. Here we demonstrate a strategy for carrier optimization in a hybrid inorganic–organic superlattice of TiS2[tetrabutylammonium]x[hexylammonium]y, where the organic layers are composed of a random mixture of tetrabutylammonium and hexylammonium molecules. By vacuum heating the hybrid materials at an intermediate temperature, the hexylammonium molecules with a lower boiling point are selectively de-intercalated, which reduces the electron density due to the requirement of electroneutrality. The tetrabutylammonium molecules with a higher boiling point remain to support and stabilize the superlattice structure. The carrier concentration can thus be effectively reduced, resulting in a remarkably high power factor of 904 µW m−1 K−2 at 300 K for flexible thermoelectrics, approaching the values achieved in conventional inorganic semiconductors.

Transparent flexible thermoelectric material based on non-toxic earth-abundant p-type copper iodide thin film

Thermoelectric devices that are flexible and optically transparent hold unique promise for future electronics. However, development of invisible thermoelectric elements is hindered by the lack of p-type transparent thermoelectric materials. Here we present the superior room-temperature thermoelectric performance of p-type transparent copper iodide (CuI) thin films. Large Seebeck coefficients and power factors of the obtained CuI thin films are analysed based on a single-band model. The low-thermal conductivity of the CuI films is attributed to a combined effect of the heavy element iodine and strong phonon scattering. Accordingly, we achieve a large thermoelectric figure of merit of ZT=0.21 at 300 K for the CuI films, which is three orders of magnitude higher compared with state-of-the-art p-type transparent materials. A transparent and flexible CuI-based thermoelectric element is demonstrated. Our findings open a path for multifunctional technologies combing transparent electronics, flexible electronics and thermoelectricity.

ウェアラブル発電用 ZnO ナノ構造/綿布材料の熱電特性

For high-efficiency wearable power generator, we have investigated the ZnO nanostructures grown on cotton fabric (CF) as a novel flexible thermoelectric material. We have fabricated ZnO nanostructures on CF by two-step growth method consisting of a seed creation process and a nanostructure growth process. It has been confirmed from ultraviolet transmittance spectra that the ZnO nanostructures are strongly fixed on the CF even after laundering them ten times. In the present study, ZnO nanopillar-nanosheet composites were formed on CF by the two-step growth method. By means of an ac calorimetric method, the thermal diffusivity of the ZnO nanostructure/CF sample was evaluated to be 1.4x10-4m2/s.