Conversion of waste heat to voltage has the potential to significantly reduce the carbon footprint of a number of critical energy sectors, such as the transportation and electricity-generation sectors, and manufacturing processes. Thermal energy is also an abundant low-flux source that can be harnessed to power portable/wearable electronic devices and critical components in remote off-grid locations. As such, a number of different inorganic and organic materials are being explored for their potential in thermoelectric-energy-harvesting devices. Carbon-based thermoelectric materials are particularly attractive due to their use of nontoxic, abundant source-materials, their amenability to high-throughput solution-phase fabrication routes, and the high specific energy (i.e., W g ) enabled by their low mass. Single-walled carbon nanotubes (SWCNTs) represent a unique 1D carbon allotrope with structural, electrical, and thermal properties that enable efficient thermoelectric-energy conversion. Here, the progress made toward understanding the fundamental thermoelectric properties of SWCNTs, nanotube-based composites, and thermoelectric devices prepared from these materials is reviewed in detail. This progress illuminates the tremendous potential that carbon-nanotube-based materials and composites have for producing high-performance next-generation devices for thermoelectric-energy harvesting.

between 2010 and 2040.[ 1 ] Sustainability is critical because current affordable energy, mainly from fossil fuels, is being rapidly depleted, while world demand increases. Even if the supply were unlimited, the use of fossil fuels is accompanied by environmental problems, such as pollution and the greenhouse effect. [ 2 ] Efforts to harness sustainable energy from solar, wind, nuclear, and other sources have shown promise, but cost and effi ciency remain signifi cant challenges to widespread adoption. Approximately 60% of all energy produced is wasted as heat, [ 3 ] which has driven the development of thermoelectric (TE) materials over the past two decades. [ 4 ] Heat is an abundant energy supply that can be harvested from a multitude of sources (engines, human body, etc.) with no moving parts. The TE performance is directly related to a dimensionless fi gureof-merit ( ZT = S 2 σ T κ −1 ), where S is the Seebeck coeffi cient, σ is the electrical conductivity, and κ is the thermal conductivity at a given temperature T . It is clear that a large σ and S , with a small κ , are desired to achieve high effi ciency ( ZT ≈ 1 corresponds to 4%-5% conversion effi ciency), [ 5 ] but there is a well-known confl ict among the three parameters that imposes limitations on traditional thermoelectric semiconductor development. [ 6 ] The best inorganic TE materials have achieved a ZT > 2, [ 7 ] but it should be noted that this value is measured at temperatures >600 K. The ZT of these materials at room temperature is <0.5, with a power factor (PF) <400 µW m −1 K −2 . These best commercially available materials are bismuth telluride-based alloys that are expensive and plagued by scarcity and toxicity concerns. Additionally, TE materials would need a ZT ≥ 3 to be effi cient enough to enter the power generation fi eld, making them commercially viable for more than niche applications. [ 8 ] Alternatively, low cost and lightweight materials that are printable (or paintable) could be useful even with relatively low conversion effi ciency.More recently, size effects in nanostructured systems such as nanowires, [ 9 ] quantum dots, [ 10 ] superlattices, [ 11 ] and a wide variety of composites with irregular nanosized inclusions have led to signifi cant improvements in thermoelectric efficiency over traditional bulk semiconductors. [ 12,13 ] For instance,In an effort to create a paintable/printable thermoelectric material, comprised exclusively of organic components, polyaniline (PANi), graphene, and double-walled nanotube (DWNT) are alternately deposited from aqueous solutions using the layer-by-layer assembly technique. Graphene and DWNT are stabilized with an intrinsically conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). An 80 quadlayer thin fi lm (≈1 µm thick), comprised of a PANi/graphene-PEDOT:PSS/PANi/DWNT-PEDOT:PSS repeating sequence, exhibits unprecedented electrical conductivity ( σ ≈ 1.9 × 10 5 S m −1 ) and Seebeck coeffi cient ( S ≈ 120 µV K −1 ) for a completely organic material. These t...

Composed exclusively of organic components, polyaniline (PANi), graphene, and double-walled nanotubes (DWNTs) are alternately deposited from aqueous solutions using a layer-by-layer assembly. The 40 quadlayer thin film (470 nm thick) exhibits electrical conductivity of 1.08 × 10(5) S m(-1) and a Seebeck coefficient of 130 μV K(-1) , producing a thermoelectric power factor of 1825 μW m(-1) K(-2) .

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In an effort to reduce the flammability of polyurethane foam, a thin film of renewable inorganic nanoparticles (i.e., anionic vermiculite [VMT] and cationic boehmite [BMT]) was deposited on polyurethane foam via layer-by-layer (LbL) assembly. One, two, and three bilayers (BL) of BMT-VMT resulted in foam with retained shape after being exposed to a butane flame for 10 s, while uncoated foam was completely consumed. Cone calorimetry confirmed that the coated foam exhibited a 55% reduction in peak heat release rate with only a single bilayer deposited. Moreover, this protective nanocoating reduced total smoke release by 50% relative to untreated foam. This study revealed that 1 BL, adding just 4.5 wt % to PU foam, is an effective and conformal flame retardant coating. These results demonstrate one of the most efficient and renewable nanocoatings prepared using LbL assembly, taking this technology another step closer to commercial viability.

Air‐stable n‐type organic thermoelectric (TE) materials with high power factor are needed to produce efficient, lightweight devices that could be self‐powered by harnessing waste heat. Here, a completely organic n‐type TE nanocomposite is achieved by depositing layers of double‐walled carbon nanotubes (DWNT) stabilized with polyethylenimine (PEI) and graphene oxide (GO) in a layer‐by‐layer fashion from aqueous solutions. A 30 bilayer (BL) film (≈610 nm thick), comprised of this DWNT‐PEI/GO sequence, exhibits electrical conductivity of 27.3 S cm−1 and a Seebeck coefficient of −30 µV K−1, producing a power factor of 2.5 µW m−1 K−2. Low temperature thermal reduction (150 °C for 30 min) of this composite thin film significantly improves its thermoelectric performance. An electrical conductivity of 460 S cm−1 and Seebeck coefficient of −93 µV K−1 are achieved. A 30 BL DWNT‐PEI/reduced graphene oxide (rGO) film (≈480 nm thick) exhibits a power factor as large as 400 µW m−1 K−2, which is one of the highest values reported for an organic n‐type material. By depositing layers containing montmorillonite clay on top, these n‐type nanocomposites exhibit excellent air stability. This combination of air stability and high power factor could enable efficient thermoelectric devices on flexible substrates (e.g., clothing).

In an effort to produce effective thermoelectric nanocomposites with multiwalled carbon nanotubes (MWCNT), layer-by-layer assembly was combined with electrochemical polymerization to create synergy that would produce a high power factor. Nanolayers of MWCNT stabilized with poly(diallyldimethylammonium chloride) or sodium deoxycholate were alternately deposited from water. Poly(3,4-ethylene dioxythiophene) [PEDOT] was then synthesized electrochemically by using this MWCNT-based multilayer thin film as the working electrode. Microscopic images show a homogeneous distribution of PEDOT around the MWCNT. The electrical resistance, conductivity (σ) and Seebeck coefficient (S) were measured before and after the PEDOT polymerization. A 30 bilayer MWCNT film (<1 μm thick) infused with PEDOT is shown to achieve a power factor (PF = Sσ) of 155 μW/m K, which is the highest value ever reported for a completely organic MWCNT-based material and competitive with lead telluride at room temperature. The ability of this MWCNT-PEDOT film to generate power was demonstrated with a cylindrical thermoelectric generator that produced 5.5 μW with a 30 K temperature differential. This unique nanocomposite, prepared from water with relatively inexpensive ingredients, should open up new opportunities to recycle waste heat in portable/wearable electronics and other applications where low weight and mechanical flexibility are needed.

Carbon‐based thermoelectric materials are attractive due to their use of nontoxic materials, their solution‐based processing, and their high specific energy (i.e., W g−1). In article number https://doi.org/10.1002/adma.201704386, Jeffrey L. Blackburn, Andrew J. Ferguson, Chungyeon Cho, and Jaime C. Grunlan review these organic thermoelectric materials, which offer the opportunity to produce wearable devices that can recycle body heat. The image, created by photographer Igor Kraguljac, shows a real nanotube‐based thermoelectric device using body heat as the source to generate the voltage needed to power a lightbulb. Although this is a real working device, the image shown is a dramatization.