High-Performance Polyaniline-Coated Carbon Nanotube Yarns for Wearable Thermoelectric Generators

Carbon nanotubes/polyaniline (CNTs/PANI) composites have attracted significant attention in thermoelectric (TE) conversion due to their excellent stability and easy synthesis. However, their TE performance is far from practical demands, and few flexible yarns/fibers have been developed for wearable electronics. Herein, we developed flexible CNTs/PANI yarns with outstanding TE properties via facile soaking of CNT yarns in a PANI solution, in which the PANI layer was coated on the CNT surface and served as a bridge to interconnect adjacent CNT filaments. With optimizing PANI concentration, immersing duration, and doping level of PANI, the power factor reached 1294 μW m-1 K-2 with a high electrical conductivity of 3651 S cm-1, which is superior to that of most of the reported CNTs/PANI composites and organic yarns. Combining outstanding TE performance with excellent bending stability, a highly integrated and flexible TE generator was assembled consisting of 40 pairs of interval p-n segments, which generate a high power of 377 nW at a temperature gradient of 10 K along the out-of-plane direction. These results indicate the promising application of CNTs/PANI yarns in wearable energy harvesting.

Advanced Thermoelectric Textiles for Power Generation: Principles, Design, and Manufacturing

Self-powered wearable thermoelectric (TE) devices significantly reduce the inconvenience caused to users, especially in daily use of portable devices and monitoring personal health. The textile-based TE devices (TETs) exhibit the excellent flexibility, deformability, and light weight, which fulfill demands of long-term wearing for the human body. In comparison to traditional TE devices with their longstanding research history, TETs are still in an initial stage of growth. In recent years, TETs to provide electricity for low-power wearable electronics have attracted increasing attention. This review summarizes the recent progress of TETs from the points of selecting TE materials, scalable fabrication methods of TE fibers/yarns and TETs, structure design of TETs and reported high-performance TETs. The key points to develop TETs with outstanding TE properties and mechanical performance and better than available optimization strategies are discussed. Furthermore, remaining challenges and perspectives of TETs are also proposed to suggest practical applications for heat harvesting from human body.

Acid enhanced zipping effect to densify MWCNT packing for multifunctional MWCNT films with ultra-high electrical conductivity

The outstanding electrical and mechanical properties remain elusive on macroscopic carbon nanotube (CNT) films because of the difficult material process, which limits their wide practical applications. Herein, we report high-performance multifunctional MWCNT films that possess the specific electrical conductivity of metals as well as high strength. These MWCNT films were synthesized by a floating chemical vapor deposition method, purified at high temperature and treated with concentrated HCl, and then densified due to the developed chlorosulfonic acid-enhanced zipping effect. These large scalable films exhibit high electromagnetic interference shielding efficiency, high thermoelectric power factor, and high ampacity because of the densely packed crystalline structure of MWCNTs, which are promising for practical applications.

Doping Engineering of Single-Walled Carbon Nanotubes by Nitrogen Compounds Using Basicity and Alignment

Charge transport properties in single-walled carbon nanotubes (SWCNTs) can be significantly modified through doping, tuning their electrical and thermoelectric properties. In our study, we used more than 40 nitrogen-bearing compounds as dopants and determined their impact on the material's electrical conductivity. The application of nitrogen compounds of diverse structures and electronic configurations enabled us to determine how the dopant nature affects the SWCNTs. The results reveal that the impact of these dopants can often be anticipated by considering their Hammett's constants and pK_a values. Furthermore, the empirical observations supported by first-principles calculations indicate that the doping level can be tuned not only by changing the type and the concentration of dopants but also by varying the orientation of nitrogen compounds around SWCNTs.

Operation of Wearable Thermoelectric Generators Using Dual Sources of Heat and Light

A wearable thermoelectric generator (WTEG) that utilizes human body heat can be a promising candidate for the wearable power generators. The temperature difference (ΔT) between the body and the environment is a stable source driving the WTEG, but this driving force is limited by the ambient temperature itself at the same time. Here, a novel WTEG that can be operated using the dual source of body heat and light with exceptionally high driving force is fabricated. The printable solar absorbing layer attached to the bottom of the WTEG absorbs ≈95% of the light from ultraviolet to far infrared and converts it into heat. To optimize the power density of WTEGs, the fill factor of the thermoelectric (TE) leg/electrode is considered through finite-difference time-domain (FDTD) simulation. When operated by the dual sources, the WTEG exhibits a power density of 15.33 µW cm^-2 , which is the highest under "actual operating conditions" among all kinds of WTEGs. In addition, unlike conventional WTEGs, the WTEG retains 83.1% of its output power at an ambient temperature of 35 °C compared to its output power at room temperature. This study will accelerate the commercialization of WTEGs by introducing a novel method to overcome their limitations.

A Meta-Analysis of Conductive and Strong Carbon Nanotube Materials

A study of 1304 data points collated over 266 papers statistically evaluates the relationships between carbon nanotube (CNT) material characteristics, including: electrical, mechanical, and thermal properties; ampacity; density; purity; microstructure alignment; molecular dimensions and graphitic perfection; and doping. Compared to conductive polymers and graphitic intercalation compounds, which have exceeded the electrical conductivity of copper, CNT materials are currently one-sixth of copper's conductivity, mechanically on-par with synthetic or carbon fibers, and exceed all the other materials in terms of a multifunctional metric. Doped, aligned few-wall CNTs (FWCNTs) are the most superior CNT category; from this, the acid-spun fiber subset are the most conductive, and the subset of fibers directly spun from floating catalyst chemical vapor deposition are strongest on a weight basis. The thermal conductivity of multiwall CNT material rivals that of FWCNT materials. Ampacity follows a diameter-dependent power-law from nanometer to millimeter scales. Undoped, aligned FWCNT material reaches the intrinsic conductivity of CNT bundles and single-crystal graphite, illustrating an intrinsic limit requiring doping for copper-level conductivities. Comparing an assembly of CNTs (forming mesoscopic bundles, then macroscopic material) to an assembly of graphene (forming single-crystal graphite crystallites, then carbon fiber), the ≈1 µm room-temperature, phonon-limited mean-free-path shared between graphene, metallic CNTs, and activated semiconducting CNTs is highlighted, deemphasizing all metallic helicities for CNT power transmission applications.

Chlorosulfonic Acid Stretched Carbon Nanotube Sheet for Flexible and Low-Voltage Heating Applications

The carbon nanotube (CNT) is celebrated for its electrothermal property, which indicates the capability of a material to transform electrical energy into heat due to the Joule effect. The CNT nanostructure itself, as a one-dimensional material, limits the electron conduction path, thereby creating a unique heating phenomenon. In this work, we explore the possible correlation between CNT alignment in sheets and heating performance. The alignment of carbon nanotubes is induced by immersion and stretching in chlorosulfonic acid (CSA) solution. The developed CSA-stretched CNT sheet demonstrated excellent heating performance with a fast response rate of 6.5 °C/s and reached 180 °C in less than 30 s under a low voltage of 2.5 V. The heating profile of the stretched CNT sheet remained stable after bending and twisting movements, making it a suitable heating material for wearable devices, heatable smart windows, and in de-icing or defogging applications. The specific strength and specific conductance of the CSA-stretched CNT sheet also increased five- and two-fold, respectively, in comparison to the pristine CNT sheet.

Enhancing thermoelectric properties of single-walled carbon nanotubes using halide compounds at room temperature and above

Carbon nanotubes (CNTs) are materials with exceptional electrical, thermal, mechanical, and optical properties. Ever since it was demonstrated that they also possess interesting thermoelectric properties, they have been considered a promising solution for thermal energy harvesting. In this study, we present a simple method to enhance their performance. For this purpose, thin films obtained from high-quality single-walled CNTs (SWCNTs) were doped with a spectrum of inorganic and organic halide compounds. We studied how incorporating various halide species affects the electrical conductivity, the Seebeck coefficient, and the Power Factor. Since thermoelectric devices operate under non-ambient conditions, we also evaluated these materials' performance at elevated temperatures. Our research shows that appropriate dopant selection can result in almost fivefold improvement to the Power Factor compared to the pristine material. We also demonstrate that the chemical potential of the starting CNT network determines its properties, which is important for deciphering the true impact of chemical and physical functionalization of such ensembles.

Thermal treatment using microwave irradiation for the phytosanitation of Xylella fastidiosa in pecan graftwood

Pecan bacterial leaf scorch caused by Xylella fastidiosa is an emerging disease for the U.S. and international pecan industries and can be transmitted from scion to rootstock via grafting. With the expanse of global transportation and trade networks, phytosanitation is critical for reducing the spread of economically significant pathogens, such as X. fastidiosa. We developed and evaluated thermal treatments using microwave irradiation and microwave absorbers [sterile deionized water (dH₂O) and carbon nanotubes (CNTs)] as novel disinfectant methods for remediating X. fastidiosa in pecan scions. Partial submergence of scions in dH₂O or CNT dispersions resulted in the transport of microwave absorbers in the xylem tissue via transpiration but did not compromise plant health. The microwave absorbers effectively transferred heat to the scion wood to reach an average temperature range of 55–65°C. Microwave radiation exposure for 6 sec (3 sec for two iterations) of CNT- or dH₂O-treated scions reduced the frequency of X. fastidiosa-positive in pecan scions without negatively affecting plant viability when compared to the control group (dH₂O-treated with no microwave). The efficacy of the new thermal treatments based on microwave irradiation was comparable to the conventional hot-water treatment (HWT) method, in which scions were submerged in 46°C water for 30 min. Microwave irradiation can be employed to treat X. fastidiosa-infected scions where the conventional HWT treatment is not feasible. This study is the first report to demonstrate novel thermal treatment methods based on the microwave irradiation and microwave absorbers of dH₂O and CNT as an application for the phytosanitation of xylem-inhabiting bacteria in graftwood.

Supramolecular Functionalization for Improving Thermoelectric Properties of Single-Walled Carbon Nanotubes-Small Organic Molecule Hybrids

Single-walled carbon nanotube (SWCNTs-P)-small organic molecule hybrid materials are promising candidates for achieving high thermoelectric (TE) performance. In this study, we synthesized rod-coil amphiphilic molecules, that is, tri(ethylene oxide) chain-attached bis(bithiophenyl)-terphenyl derivatives (1 and 2). Supramolecular functionalization of SWCNTs-P with 1 or 2 induced charge-transfer interactions between them. Improved TE properties of the supramolecular hybrids (SWCNTs-1 and SWCNTs-2) are attributed to increased charge-carrier concentration (electrical conductivity), interfacial phonon scattering (thermal conductivity), and energy difference between the transport and Fermi levels (E_Tr - E_F; Seebeck coefficient). SWCNTs-2 exhibited a ZT of 0.42 × 10^-2 at 300 K, which is 350% larger than that of SWCNTs-P. Furthermore, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ)-doped SWCNTs-2 showed the highest ZT value of 1.96 × 10^-2 at 300 K among SWCNTs-P/small organic molecule hybrids known until now. These results demonstrated that the supramolecular functionalization of SWCNTs-P with small organic molecules could be useful for enhancement of TE performance and applications in wearable/flexible thermoelectrics.

Efficient Debundling of Few-Walled Carbon Nanotubes by Wrapping with Donor-Acceptor Polymers for Improving Thermoelectric Properties

Organic thermoelectric (TE) materials have great potential as sustainable energy sources for powering flexible and wearable electronic devices via harvesting of human body heat. Recent advances in soluble conjugated polymer/carbon nanotube (CNT) composites have facilitated achievement of high TE power factors. However, the effects of conjugated polymers on the debundling and electrical percolation of CNTs and on the TE properties of their composites are not yet fully understood. Herein, we introduce a novel type of polymer/CNT composite composed of a donor-acceptor (D-A)-type polymer and few-walled CNTs (FWCNTs). Three kinds of D-A polymers are employed to disperse FWCNTs, and the photophysical, morphological, and TE properties of the resulting polymer/FWCNT composites are compared with those of composites composed of FWCNTs dispersed with conventional donor-only poly(3-hexylthiophene). The results reveal that the strong intermolecular interaction forces and high backbone planarity of the D-A polymers facilitate effective debundling of FWCNTs, which results in much smaller bundle sizes. Consequently, the D-A polymer/FWCNT composite films show superior electrical percolation and TE performances with improved power factors of up to 459 μW/mK². Finally, we demonstrate the feasibility of the D-A polymer/FWCNT composites for use in the fabrication of a flexible TE generator, which shows a maximum power output of 210 nW at a temperature gradient of 20 K.

Nonuniform functional group distribution of carbon nanotubes studied by energy dispersive X-ray spectrometry imaging in SEM

Functionalization is a key technique to improving the dispersibility of carbon nanotubes (CNTs) in solvents and polymer matrices for producing versatile CNT-based materials. Therefore, a robust and efficient characterization method is required to confirm that the functionalization on the CNT surface is spatially uniform. Although several imaging techniques for transmission electron microscopes can characterize the spatial localization of elements chemically bound to an isolated CNT surface, they are unsuitable for examinations on a practical scale because of their limited scanning area. Here, we present high spatially resolved energy dispersive X-ray spectrometry (EDS) imaging of functionalized single-walled CNTs (SWCNTs) in scanning electron microscopy (SEM). Highly sensitive EDS detection and drift-free operation enables our technique to image the light elements of SWCNTs with sufficient spatial resolution (<10 nm). We describe an experimental visualization of the spatial distribution of the functionalization on individual SWCNT bundle structures and discuss the CNT de-bundling mechanism via surface modification and the uniformity of the CNT dispersion state.

Seamless Tube-Type Heater with Uniform Thickness and Temperature Distribution Based on Carbon Nanotubes Aligned by Circumferential Shearing

The uniform temperature distribution, one of the requirements for long-term durability, is essential for composite heaters. An analytical model for temperature distribution of a tube-type heater was derived, and it revealed that thickness uniformity is one order more important than intrinsic material properties such as density, heat capacity, and electrical conductivity of the heating tube. We introduced a circumferential shearing process to fabricate a flexible, seamless tube-type heating layer of carbon nanotube/silicone rubber composite with outstanding uniform distribution of thickness and temperature, which may be attributed to a shorter characteristic dimension in the circumferential direction than in the axial direction. The temperature uniformity was experimentally verified at various temperatures under heating. The difference in measured thickness and temperature in circumferential direction was within ±1.3~3.0% (for t_avg = 352.7 μm) and ±1.1% (for T_avg = 138.8 °C), respectively, all over the heating tube. Therefore, the circumferential shearing process can be effective for cylindrical heaters, like a heating layer of a laser printer, which fuse toners onto papers during printing.

Thermoelectric properties of sorted semiconducting single-walled carbon nanotube sheets

Single-walled carbon nanotubes (SWNTs), especially their semiconducting type, are promising thermoelectric (TE) materials due to their high Seebeck coefficient. In this study, the in-plane Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) of sorted semiconducting SWNT (s-SWNT) free-standing sheets with different s-SWNT purities are measured to determine the figure of merit ZT. We find that the ZT value of the sheets increases with increasing s-SWNT purity, mainly due to an increase in Seebeck coefficient while the thermal conductivity remaining constant, which experimentally proved the superiority of the high purity s-SWNT as TE materials for the first time. In addition, from the comparison between sorted and unsorted SWNT sheets, it is recognized that the difference of ZT between unsorted SWNT and high-purity s-SWNT sheet is not remarkable, which suggests the control of carrier density is necessary to further clarify the superiority of SWNT sorting for TE applications.

Carbon Nanotube-Based Organic Thermoelectric Materials for Energy Harvesting

Carbon nanotubes (CNTs) have attracted much attention in developing high-performance, low-cost, flexible thermoelectric (TE) materials because of their great electrical and mechanical properties. Theory predicts that one-dimensional semiconductors have natural advantages in TE fields. During the past few decades, remarkable progress has been achieved in both theory and experiments. What is more important is that CNTs have shown desirable features for either n-type or p-type TE properties through specific strategies. Up to now, CNT‒polymer hybrids have held the record for TE performance in organic materials, which means they can potentially be used in high-performance TE applications and flexible electronic devices. In this review, we intend to focus on the intrinsic TE properties of both n-type and p-type CNTs and effective TE enhanced strategies. Furthermore, the current trends for developing CNT-based and CNT‒polymer-based high TE performance organic materials are discussed, followed by an overview of the relevant electronic structure‒TE property relationship. Finally, models for evaluating the TE properties are provided and a few representative samples of CNT‒polymer composites with high TE performance are highlighted.

Exploring High-Performance n-Type Thermoelectric Composites Using Amino-Substituted Rylene Dimides and Carbon Nanotubes

Taking advantage of the high electrical conductivity of a single-walled carbon nanotube (SWCNT) and the large Seebeck coefficient of rylene diimide, a convenient strategy is proposed to achieve high-performance n-type thermoelectric (TE) composites containing a SWCNT and amino-substituted perylene diimide (PDINE) or naphthalene diimide (NDINE). The obtained n-type composites display greatly enhanced TE performance with maximum power factors of 112 ± 8 (PDINE/SWCNT) and 135 ± 14 (NDINE/SWCNT) μW m^-1 K^-2. A short doping time of 0.5 h can ensure high TE performance. The corresponding TE module consisting of five p-n junctions reaches a large output power of 3.3 μW under a 50 °C temperature gradient. In addition, the n-type composites exhibit high air stability and excellent thermal stability. This design strategy benefits the future fabricating of high-performance n-type TE materials and devices.

Carbon nanotube fibers and films: synthesis, applications and perspectives of the direct-spinning method

The direct-spinning method of creation of CNT macroassemblies has received a lot of attention because of its simplicity to produce high-performance material without apparent limits to its size. CNT fibers or films have shown unparalleled properties and opened new areas of research and commercial development. The process designed more than a decade ago has already given interesting information about the basic science of nanomaterials, which in parallel led to the creation of the first prototypes with high potential of implementation in everyday life. Because of this, there has been growing interest in this technique with research articles coming into view from all around the world on a frequent basis. This review aims to summarize all the progress made in the direct-spinning process on a spectrum of fronts ranging from the study of complex synthesis parameters, material properties to its viable applications. The strong and weak points of the "Cambridge process" are carefully evaluated to put forward what challenges are most pressing. The future overlook puts the state of the art into perspective and suggests the prospective research directions.

Enlightening the ultrahigh electrical conductivities of doped double-wall carbon nanotube fibers by Raman spectroscopy and first-principles calculations

Highly aligned, packed, and doped carbon nanotube (CNT) fibers with electrical conductivities approaching that of copper have recently become available. These fibers are promising for high-power electrical applications that require light-weight, high current-carrying capacity cables. However, a microscopic understanding of how doping affects the electrical conductance of such CNT fibers in a quantitative manner has been lacking. Here, we performed Raman spectroscopy measurements combined with first-principles calculations to determine the position of the average Fermi energy and to obtain the temperature of chlorosulfonic-acid-doped double-wall CNT fibers under high current. Due to the unique way in which double-wall CNT Raman spectra depend on doping, it is possible to use Raman data to determine the doping level quantitatively. The correspondence between the Fermi level shift and the carbon charge transfer is derived from a tight-binding model and validated by several calculations. For the doped fiber, we were able to associate an average Fermi energy shift of ∼-0.7 eV with a conductance increase by a factor of ∼5. Furthermore, since current induces heating, local temperature determination is possible. Through the Stokes-to-anti-Stokes intensity ratio of the G-band peaks, we estimated a temperature rise at the fiber surface of ∼135 K at a current density of 2.27 × 10⁸ A m^-2 identical to that from the G-band shift, suggesting that thermalization between CNTs is well achieved.

Advanced catalyst design induced enhancement of multi-walled nanotube debundling and electrical conductivity of multi-walled nanotube/silicone composites

Density of MWNT bundles can be controlled by synthetic process of metal catalysts. Direct correlation between morphology of MWNT catalysts and electrical conductivity of MWNT/polymer composite was experimentally demonstrated.

Thermally Driven Large N-Type Voltage Responses from Hybrids of Carbon Nanotubes and Poly(3,4-ethylenedioxythiophene) with Tetrakis(dimethylamino)ethylene

Hybrids of carbon nanotubes (CNTs) and poly(3,4-ethylenedioxythiophene) (PEDOT) treated by tetrakis(dimethylamino)ethylene (TDAE) have large n-type voltages in response to temperature differences. The reduced carrier concentration by TDAE reduction and partially percolated CNT networks embedded in the PEDOT matrix result in high thermopower and low thermal conductivity. The high electron mobility in the CNTs helps to minimally reduce the electrical conductivity of the hybrid, resulting in a large figure-of-merit.

Degradation of the Common Aqueous Antibiotic Tetracycline using a Carbon Nanotube Electrochemical Filter

In this work, a carbon nanotube (CNT) electrochemical filter was investigated for treatment of aqueous antibiotics using tetracycline (TC) as a model compound. Electrochemical filtration of 0.2 mM TC at a total cell potential of 2.5 V and a flow rate of 1.5 mL min(-1) (hydraulic residence time <2 s) resulted in an oxidative flux of 0.025 ± 0.001 mol h(-1) m(-2). Replacement of the perforated Ti cathode with a CNT cathode increased the TC oxidative flux by 2.3-fold to 0.020 ± 0.001 mol h(-1) m(-2) at a total cell potential of 1.0 V. Effluent analysis by liquid chromatography-mass spectrometry and disk agar biocidal diffusion tests indicate that the electrochemical filtration process can degrade the TC molecular structure and significantly decrease its antimicrobial activity, respectively. Addition of dissolved natural organic matter (NOM) negatively affected the TC electrooxidation because of competition for CNT sorption and electrooxidation sites. At 2.0 V total cell potential, TC spiked (0.2 mM) into drinking water reservoir and wastewater treatment plant effluent samples had an oxidative flux of 0.015 ± 0.001 and 0.022 ± 0.001 mol h(-1) m(-2), respectively, and an energy requirement of 0.7 kWh kgCOD(-1) or 0.084 kWh m(-3). These results indicate a CNT electrochemical filter may have potential to effectively and efficiently treat antibiotics in water and wastewater effluent.

Simultaneously improving electrical conductivity and thermopower of polyaniline composites by utilizing carbon nanotubes as high mobility conduits

Electrical conductivity and thermopower of isotropic materials typically have inversely proportional correlation because both are strongly affected in the opposite way by the electronic carrier concentration. This behavior has been one of the major hurdles in developing high-performance thermoelectrics whose figure-of-merit enhances with large thermopower and high electrical conductivity. Here we report a promising method of simultaneously improving both properties with polyaniline (PANI) composites filled by carbon nanotubes (CNTs). With addition of double-wall CNTs (DWCNTs), the electronic mobility of PANI doped with camphorsulfonic acid (PANI-CSA) was raised from ∼0.15 to ∼7.3 cm(2)/(V s) (∼50 time improvement) while the carrier concentration was decreased from ∼2.1 × 10(21) to ∼5.6 × 10(20) cm(-3) (∼4 time reduction). The larger increase of mobility increased electrical conductivity despite the carrier concentration reduction that enlarges thermopower. The improvement in the carrier mobility could be attributed to the band alignment that attracts hole carriers to CNTs whose mobility is much higher than that of PANI-CSA. The electrical conductivity of the PANI-CSA composites with 30-wt % DWCNTs was measured to be ∼610 S/cm with a thermopower value of ∼61 μV/K at room temperature, resulting in a power factor value of ∼220 μW/(m K(2)), which is more than two orders higher than that of PANI-CSA as well as the highest among those of the previously reported PANI composites. Further study may result in high performance thermoelectric organic composites uniquely offering mechanical flexibility, light weight, low toxicity, and easy manufacturing. unlike conventional inorganic semiconductors.

Smart conducting polymer composites having zero temperature coefficient of resistance

Zero temperature coefficient of resistance (TCR) is essential for the precise control of temperature in heating element and sensor applications. Many studies have focused on developing zero-TCR systems with inorganic compounds; however, very few have dealt with developing zero-TCR systems with polymeric materials. Composite systems with a polymer matrix and a conducting filler show either a negative (NTC) or a positive temperature coefficient (PTC) of resistance, depending on several factors, e.g., the polymer nature and the filler shape. In this study, we developed a hybrid conducting zero-TCR composite having self-heating properties for thermal stability and reliable temperature control. The bi-layer composites consisted of a carbon nanotube (CNT)-based layer having an NTC of resistance and a carbon black (CB)-based layer having a PTC of resistance which was in direct contact with electrodes to stabilize the electrical resistance change during electric Joule heating. The composite showed nearly constant resistance values with less than 2% deviation of the normalized resistance until 200 °C. The CB layer worked both as a buffer and as a distributor layer against the current flow from an applied voltage. This behavior, which was confirmed both experimentally and theoretically, has been rarely reported for polymer-based composite systems.

Flexible power fabrics made of carbon nanotubes for harvesting thermoelectricity

Thermoelectric energy conversion is very effective in capturing low-grade waste heat to supply electricity particularly to small devices such as sensors, wireless communication units, and wearable electronics. Conventional thermoelectric materials, however, are often inadequately brittle, expensive, toxic, and heavy. We developed both p- and n-type fabric-like flexible lightweight materials by functionalizing the large surfaces and junctions in carbon nanotube (CNT) mats. The poor thermopower and only p-type characteristics of typical CNTs have been converted into both p- and n-type with high thermopower. The changes in the electronic band diagrams of the CNTs were experimentally investigated, elucidating the carrier type and relatively large thermopower values. With our optimized device design to maximally utilize temperature gradients, an electrochromic glucose sensor was successfully operated without batteries or external power supplies, demonstrating self-powering capability. While our fundamental study provides a method of tailoring electronic transport properties, our device-level integration shows the feasibility of harvesting electrical energy by attaching the device to even curved surfaces like human bodies.

N-type thermoelectric performance of functionalized carbon nanotube-filled polymer composites

Carbon nanotubes (CNTs) were functionalized with polyethyleneimine (PEI) and made into composites with polyvinyl acetate (PVAc). CNTs were dispersed with different amounts of sodium dodecylbenzenesulfonate (SDBS) prior to the PEI functionalization. The resulting samples exhibit air-stable n-type characteristics with electrical conductivities as great as 1500 S/m and thermopowers as large as −100 µV/K. Electrical conductivity and thermopower were strongly affected by CNT dispersion, improving the properties with better dispersion with high concentrations of SDBS. This improvement is believed to be due to the increase in the number of tubes that are evenly coated with PEI in a better-dispersed sample. Increasing the amount of PEI relative to the other constituents positively affects thermopower but not conductivity. Air exposure reduces both thermopower and conductivity presumably due to oxygen doping (which makes CNTs p-type), but stable values were reached within seven days following sample fabrication.