<figure><img src="image_53.png" /><figcaption>**Figure 39.** Commonly employed insulating (top) and conjugated/conducting (bottom) polymers, used in blended polymer–CNT TE composites.</figcaption></figure>

the other components.<sup>330–332</sup> A challenge in nanocomposites is exerting control over the energy barrier(s) that limit charge transport between the two phases or between conductive segments of the primary transport material. These barriers can be large and force the charge carriers to pass only through the percolation transport network of the most conductive component. However, these energy barriers may also provide a means to exploit quantum effects for carrier filtering to enhance the Seebeck coefficient.<sup>333–337</sup>

Several combinations of materials, with varying levels of complexity for the final composite, have been proposed and explored. Such composites have produced interesting results and some high TE metrics, but charge carrier transport within such systems is still poorly understood. More recent work has also begun exploring the use of CNTs or graphene to enhance the TE properties of composites with inorganic materials.

Several open questions remain in the context of nanocarbon composites. These include whether the composite system can be considered as (i) a single conductive material with hybrid electronic states and conductive paths determined by both components or (ii) as a “simple” mixture whose properties can be described using typical “composite models”. Additionally, as outlined in Lin et al., it is yet to be determined conclusively whether the phenomenon of carrier energy filtering by electronic barriers at interfaces is a viable approach to decouple the thermopower and electrical conductivity in nanocarbon composites.<sup>338</sup> While the answers to these questions likely depend on the precise nature of the composite system, fundamental studies focused on addressing these topics may enable further improvements in the thermoelectric performance of nanocomposite materials and devices. Within this review, we note the studies that ascribed the observed improvements in thermoelectric performance to carrier energy filtering, but we

encourage the reader to evaluate these conclusions within the context of the analyses outlined by Lin et al.<sup>338</sup>

4.2.5.1. *Blended Polymer–Carbon Nanotube Nanocomposites.* The unique electronic and mechanical properties of CNTs described earlier has motivated significant effort toward the fabrication and characterization of TE composites with CNT inclusions. Much of the effort has focused on blended TE composites with CNT conductive fillers incorporated into either insulating or conducting polymeric host matrices (Figure 39),<sup>339–358</sup> although there have been some recent efforts toward the use of molecular host materials.<sup>359–366</sup> Over the past decade, TE composites with CNT conductive fillers have regularly achieved room-temperature TE power factors exceeding 100 µW m<sup>-1</sup> K<sup>-2</sup>.<sup>339–355,357,358,361–364,366</sup> For a thorough review of polymer-based blended TE composites with carbon nanotube fillers, we direct the reader to recent reviews by Blackburn et al.<sup>208</sup> and Nandihalli et al.<sup>367</sup> Here we will focus on key advances in blended TE composites comprising organic host matrices and CNT inclusions.

Despite their possible detrimental influence on charge-carrier transport, there continues to be some effort related to the inclusion of insulating polymers into CNT-based composites.<sup>350,354,358</sup> As described below, several insulating polymers (e.g., arabic gum, polyvinyl acetate, etc.) have been employed as a stabilizing binder to improve the morphology and charge transport in blended composites of CNTs and conducting polymers.<sup>327,339,341</sup> However, there have been some impressive results recently where the composite is comprised of an interconnected CNT network stabilized by an insulating polymer binder. Mo et al. prepared ca. 30 µm thick cellulose acetate/SWCNT composites by bar-coating from an acetone paste, which exhibited a TE power factor of ca. 140 µW m<sup>-1</sup> K<sup>-2</sup> for p-type transport and could be easily doped n-type via
<figure><img src="image_54.png" /><figcaption>**Figure 40.** Thermoelectric performance of blended composites of poly(γ-cyclodextrin) (PγCyD) with CNT inclusions. Cartoons illustrating (a) the PγCyD structure, showing “pockets” in the γ-cyclodextrin macrocycles, and (b) the different PγCyD-solvent interactions with either H₂O or NMP that result in the observed (c) p-type or (d) n-type TE transport behavior. Adapted with permission from ref 354. Copyright 2020 The Japan Society of Applied Physics.</figcaption></figure>

painting the freestanding films with a solution of 6 wt % polyethylenimine (PEI) in ethanol.358 The authors attributed the impressive performance to the presence of “secondary” aggregates (i.e., bundles of bundles) and a corresponding reduction in the interbundle resistance, in addition to possible carrier energy filtering enhancement of the Seebeck coefficient

at the CNT–cellulose acetate junction. More recently, Hata et al. demonstrated composites of CNTs embedded in a γ-cyclodextrin polymer cross-linked with epichlorohydrin (PγCyD) that exhibit TE power factors >200 µW m⁻¹ K⁻² for both p-type and n-type transport (Figure 40).354 The difference in the majority carrier type was attributed to different
interactions between the processing solvent, either H₂O or N-
methyl-2-pyrrolidone (NMP), where the “pocket” in the γCyD
macrocycle is capable of hosting the NMP molecule (Figure
40b), leading to enhanced n-type behavior than for an equivalent
composite prepared from polyvinylpyrrolidone (PVP).

In recognition that the presence of the insulating polymer can
introduce detrimental energetic barriers to charge carrier
transport, Ha et al. performed an esterification of polyvinyl
alcohol (PVA), and a subsequent condensation reaction
between the resulting PVA-COOH and amine-functionalized
SWCNTs, to produce PVA-grafted nanotubes that exhibited
peak TE power factor of ca. 85 µW m⁻¹ K⁻² for the composite
with ca. 30 wt % SWCNTs.³⁵⁰ With the addition of a small
amount (3 wt %) of the commercial nonionic surfactant Triton
X-100 during the condensation step, the Seebeck coefficient of
the composite was more than doubled (to ca. 85 µV K⁻¹) with
only a small decrease in the electrical conductivity, resulting in
an enhancement of the TE power factor to >200 µW m⁻¹ K⁻².

The majority of recent research on CNT-based TE
composites has focused on several prototypical conjugated/
conducting polymers, including PANI,³⁵⁵ PEDOT,³⁴⁹,³⁵¹,³⁵²,³⁵⁶
and P3HT.³⁵⁷ PANI, primarily in the emeraldine form, has been
one of the most widely studied polymer host matrices for TE
composites with CNT inclusions, partly because of its
amenability to tuning of the polymer redox and electrical
transport properties through protonation of the imine nitrogen
atoms. Early work by Meng et al. demonstrated a rather low TE
power factor (ca. 5 µW m⁻¹ K⁻²) for composites where the
PANI was polymerized after formation of a MWCNT
network.³²⁸ This peak TE power factor was observed for
composites with ca. 15 wt % PANI, corresponding to a thin
coating of the PANI on the MWCNT bundle surface. The
Seebeck coefficient more than doubled compared to the PANI-
free MWCNT network, with only a small decrease in the
electrical conductivity. Replacement of the MWCNTs with
SWCNTs saw an improvement in both the Seebeck coefficient
(ca. 40 µV K⁻¹) and electrical conductivity (<12 500 S m⁻¹) for
the PANI/SWCNT composite, resulting in a 4-fold increase of
the TE power factor to ca. 20 µW m⁻¹ K⁻².³²⁹ The authors
attributed these observations to strong interactions between the
conjugated π-orbitals of the CNTs and PANI, which induced
ordering in the polymer chains on the nanotube surface,
resulting in enhancement of the electrical conductivity and
Seebeck coefficient. An increase in polymer ordering is expected
to reduce the bandgap. Both this reduced bandgap and the
measured increase of the carrier density would be expected to
reduce the Seebeck coefficient. Therefore, the observation of a
simultaneous increase in both the electrical conductivity and
Seebeck coefficient suggests that either (i) the thermoelectric
properties of the composite are dominated by conduction
pathways restricted to the SWCNTs or (ii) the strong
interactions between the polymer chains and nanotubes may
result in a hybrid material with enhanced electronic properties
relative to the individual components.

The choice of solvent is known to influence the PANI polymer
chain conformation in solution, with *m*-cresol yielding expanded
polymer coils that favor *π*-polaron delocalization along the
polymer chain.³⁶⁸ This favorable solvent–polymer interaction
was exploited by Wang et al. to in situ polymerize PANI in a
SWCNT dispersion in *m*-cresol, resulting in a more uniform
PANI coating on the nanotube surface and exceptional electrical
conductivities exceeding 140 000 S m⁻¹ and peak TE power
factors of ca. 220 µW m⁻¹ K⁻².³⁴⁷ Perhaps the most impressive

TE performance was recently achieved for composites where PANI was polymerized in dispersions containing amine-functionalized single- and double-walled CNTs, where careful control of the charge-carrier density by camphor sulfonic acid (CSA) resulted in thin-film composite TE power factors of ca. 400 µW m⁻¹ K⁻² (Figure 41).³⁵⁵ The enhanced performance

<figure><img src="image_55.png" /><figcaption>**Figure 41.** Thermoelectric composites of polyaniline (PANI) and amine-functionalized carbon nanotubes (a-CNTs). SEM images of (a) a-CNT and (b) 94 wt % a-CNT/PANI films, illustrating that the PANI serves to coat the a-CNT bundles and generate a more densely packed network. (c) Electrical conductivity and Seebeck and (d) corresponding TE power factor of 94 wt % a-CNT/PANI composites with varying mole ratios of PANI/CSA. Adapted with permission from ref 355. Copyright 2018 Elsevier Ltd.</figcaption></figure>

relative to unfunctionalized CNTs was attributed to better
dispersion of the amine-functionalized CNTs in solution and a
resulting improved coating by, and interaction with, the PANI
chains (Figure 41).

For prepolymerized PANI, Yao et al. exploited the favorable
solvent interaction with *m*-cresol, in addition to the suscepti-
bility of PANI to doping with CSA, to prepare composites with
significantly enhanced electrical conductivity and TE perform-
ance (PF ca. 175 µW m⁻¹ K⁻²).³⁴² Despite the rather large
SWCNT content (64 wt %), the thermal conductivity was still as
low as 0.43 W m⁻¹ K⁻¹, more than an order of magnitude lower
<figure><img src="image_56.png" /><figcaption>**Figure 42.** Impact of the loading of PEDOT NWs (PEDOT NW to PEDOT:PSS weight ratio) on the thermoelectric properties of ternary composite films of SWCNT/PEDOT:PSS/PEDOT NW: (a) electrical conductivity and Seebeck coefficient, (b) thermoelectric power factor, and (c) carrier mobility and concentration from Hall effect measurements. (d) Schematic illustration of carrier energy filtering effect proposed at NW and PEDOT:PSS interfaces. Reproduced with permission from ref 352. Copyright 2019 American Chemical Society.</figcaption></figure>

than expected from simple mixing of the two components. A subsequent study by Weng et al. found that replacing the SWCNTs with DWCNTs and preparation of the dispersions using a probe-tip sonicator, resulted in peak TE power factors exceeding 200 µW m⁻¹ K⁻² for composite prepared with CSA-doped PANI.³⁴⁴ A recent study of the effect of polymer molecular weight and nanotube content in CSA-doped PANI/SWCNT composites found that extremely high electrical conductivities (>400 000 S m⁻¹) could be achieved, albeit at the expense of severely reduced Seebeck coefficient and TE power factors <100 µW m⁻¹ K⁻².³⁶⁹

Early work by the groups of Yu and Grunlan on PEDOT:PSS/CNT composites focused on the use of either polymeric³³⁹ or dual polymeric-molecular³⁴¹ CNT stabilizers aimed at improving the solid-state microstructure and carrier transport in the composites. These efforts achieved electrical conductivities around or exceeding 100 000 S m⁻¹, resulting in power factors of ca. 160 µW m⁻¹ K⁻² for PEDOT:PSS/SWCNT composites stabilized by polyvinyl acetate (PVAc) and ca. 500 µW m⁻¹ K⁻² for PEDOT:PSS/DWCNT composites stabilized by both PVAc and tetrakis(4-carboxyphenyl)porphyrin (TCPP). Inspired by the observation that solvent treatments (e.g., ethylene glycol, EG) can remove excess insulating PSS from the PEDOT:PSS matrix to enhance both the electrical conductivity and Seebeck coefficient,⁷⁹ Lee et al. used EG to remove excess PSS from PEDOT:PSS/DWCNT composites and enhance the electrical conductivity for all DWCNT loadings, culminating in a peak power factor of ca. 150 µW m⁻¹ K⁻².³⁴⁶

An alternative strategy to eliminate the insulating PSS is the formation of composites of CNTs with PEDOT, where the ethylenedioxythiophene (EDOT) monomer is polymerized immediately prior to composite formation. A recent approach,

where the EDOT monomer is chemically polymerized in situ in a preprepared aqueous SWCNT suspension resulted in a composite with strong interactions between the PEDOT chains and the SWCNT sidewall and an interconnected 3D network of PEDOT-wrapped SWCNT bundles, leading to a TE power factor of ca. 150 µW m⁻¹ K⁻².³⁵¹ The factor of 4 improvement over previous attempts at in situ chemical polymerization enhancement was attributed to the improved formation of a thin, uniform PEDOT coating on the carbon nanotubes. An even larger enhancement in the TE performance was obtained for the samples prepared from EDOT monomers electro-polymerized in a 3-phase process that were subsequently mixed with acid-doped, amine-functionalized nanotubes (a-SWCNTs).³⁴⁹ In this case, the acid doping of the a-SWCNTs resulted in a large increase of the electrical conductivity within the composite and TE power factors approaching 350 µW m⁻¹ K⁻². The precise balance between the electrical conductivity and Seebeck coefficient in the blended PEDOT/a-SWCNTs was found to depend on the chemical identity of the counterion employed in the 3-phase interfacial EDOT electropolymerization procedure. The perchlorate (ClO₄⁻) anion afforded the largest electrical conductivity, at the expense of a reduction in the Seebeck coefficient, but the hexafluorophosphate (PF₆⁻) anion gave the highest TE power factor.³⁴⁹

Templated chemical polymerization of EDOT in a solution of sodium dodecyl sulfate (SDS) and iron (III) chloride (FeCl₃) has also been used to prepare PEDOT nanowires that were dispersed with SWCNTs and subsequently combined with a PEDOT:PSS suspension.³⁵² The resulting blended ternary PEDOT NW/SWCNT/PEDOT:PSS composite with 0.5% by weight PEDOT NW to PEDOT:PSS yielded a TE power factor of ca. 350 µW m⁻¹ K⁻² (Figure 42a,b), which the authors
<figure><img src="image_57.png" /><figcaption>**Figure 43.** Thermoelectric composites prepared using donor–acceptor (D–A) conjugated polymers. (a) Chemical structures of P3HT and three D–A polymers used in the study. (b) Photographs showing vials containing polymer/FWCNT composite solutions: (upper panel) as-prepared and (lower panel) after 10 days of storage. The vials were vigorously shaken just before being photographed. (c) SEM images of drop-cast polymer/FWCNT composite films prepared using different conjugated polymers. All images have the same scale bar. (d, top) Bundle size distributions estimated from SEM images in (c) for the various polymer/FWCNT composite films. The inset image shows schematic illustrations of probable FWCNT bundles in P3HT/FWCNT and D–A polymer/FWCNT composites. (d, bottom) Schematic comparisons of FWCNT bundle structures and their interconnections in drop-cast films of P3HT/FWCNT and D–A polymer/FWCNT composites. (e, left) Photograph showing PCDTPT/FWCNT composite-based flexible p-type TEG device and (e, right) magnified schematic illustration showing specific configuration of several legs and electrodes in the TEG device. (f) Plots of output power and voltage vs current of the flexible TEG device at various temperature gradients ($\Delta T = 5, 10, 15$, and $20$ K). Adapted with permission from ref 353. Copyright 2019 American Chemical Society.</figcaption></figure>

attributed to an enhanced Seebeck coefficient due to carrier energy filtering at the PEDOT NW/PEDOT:PSS interface (Figure 42d).352

In contrast to these observations for PEDOT, initial attempts at in situ chemical polymerization of 3-hexylthiophene in a dispersion of MWCNTs in chloroform yielded P3HT/MWCNT composites with very low electrical conductivities (<15 S m⁻¹) and Seebeck coefficients (<12 µV K⁻¹).370 The ability of poly(3-hexylthiophene), in combination with ultrasonic agitation, to exfoliate and solubilize carbon nanotubes in organic solvents,371–373 was subsequently exploited by Bounioux et al. to prepare TE composites of P3HT with both SWCNTs and MWCNTs.374 The undoped composites exhibited quite low electrical conductivities, with the peak TE power factor for the P3HT:SWCNT composite of ca. 18 µW m⁻¹ K⁻², about 2 orders of magnitude larger than that of the P3HT:MWCNT composite. To enhance the TE performance, the authors employed anhydrous iron (III) chloride (FeCl₃) in nitromethane as a p-type dopant. The increased carrier density gave an order of magnitude increase in the peak electrical conductivity to ca. 150 000 S m⁻¹ for the P3HT/SWCNT composite at lower SWCNT content than for the undoped samples, resulting in a peak TE power factor of ca. 95 µW m⁻¹ K⁻² despite a drop in the Seebeck coefficient to ca. 25 µV K⁻¹.374

Subsequently, Lee et al. were able to achieve slightly higher TE power factors for undoped P3HT/SWCNT composite

samples prepared via wire-bar coating,375 due primarily to an increase in the Seebeck coefficient (ca. 38 µV K⁻¹) but also an enhanced electrical conductivity (ca. 50 000 S m⁻¹) over drop-cast films. This improvement was attributed to an improved composite morphology, with the formation an interconnected network of fairly small SWCNT bundles (6–23 nm in diameter), which is known to enhance the TE performance.252 This performance was improved through a spin-casting doping process, using FeCl₃ in nitromethane, primarily due to a factor of 5 increase in the electrical conductivity (>275 000 S m⁻¹) that overcame a small decrease (<20%) in the Seebeck coefficient to yield a TE power factor of ca. 270 µW m⁻¹ K⁻².345 Here, the FeCl₃ doping of P3HT/SWCNT composite results in significant charge-carrier injection into P3HT, suggesting that the increased electrical conductivity likely arises from improved carrier transport through both the polymer matrix and interconnected SWCNT network. Hong et al. demonstrated a further increase in the TE power factor to ca. 325 µW m⁻¹ K⁻² via spray deposition of the P3HT/CNT composite, this time containing inclusions of few-walled carbon nanotubes (FWCNTs).376 Despite a significant reduction of the electrical conductivity (<35 000 S m⁻¹), these spray-coated films exhibited Seebeck coefficients approaching 100 µV K⁻¹, even with the inevitable presence of metallic species in the FWCNTs. More recently, Jung et al. demonstrated that drop-cast P3HT/SWCNT composites can achieve TE power factors of ca. 230
<figure><img src="image_58.png" /><figcaption>**Figure 44.** Controlling n-type doping in composites of molecular organic semiconductors (mOSCs) and CNTs. (a) Chemical structures of the 4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene (IDT) core (left) and the terminal groups (right). Cartoons illustrating (b) direct electron transfer and (c) hydride abstraction-mediated electron transfer from two different n-type dopants. The composites doped with an alkyl diamine derivative exhibit (d) n-type Seebeck coefficients and (e) large TE power factors. Adapted with permission from ref 366. Copyright 2019 Elsevier BV.</figcaption></figure>

$\mu$W m$^{-1}$ K$^{-2}$, despite rather large bundle sizes of ca. 33 nm in diameter.353

Poly(alkylthiophenes) are well-known to exhibit optical and electronic properties that strongly depend on the regioregularity of the side chains attached to the polythiophene backbone, which subsequently determines the extent of ordering within the solid-state microstructure. Tonga et al. demonstrated that the impact of this regioregularity extends to the TE performance of composites with SWCNT inclusions, with iodine-doped regioregular-P3HT/SWCNT composites displaying a superior electrical conductivity and Seebeck coefficient to their regiorandom counterparts, resulting in a factor of 50 increase in the TE power factor to ca. 150 $\mu$W m$^{-1}$ K$^{-2}$.357 These observations suggest that the conformation of the polymer, and the impact that this might have on the resulting microstructure of the composite, should be considered when choosing the host in polymer-based CNT composites.

In 2019, Jung et al. demonstrated that “donor–acceptor” polymers with alternating electron-rich and electron-poor conjugated moieties (Figure 43a), similar to those employed in high-performance organic photovoltaics, were capable of dispersing FWCNTs more effectively than P3HT (Figure 43b), resulting in about a factor of 2 reduction in the size of the bundles in the resulting network (Figure 43c,d).353 The improved electrical transport in these networks ($\sigma$ > 70 000 S m$^{-1}$) resulted in peak TE power factors exceeding 300 $\mu$W m$^{-1}$ K$^{-2}$, with the best performance demonstrated by the PCDTPT/ FWCNT networks (ca. 460 $\mu$W m$^{-1}$ K$^{-2}$), despite a slight reduction in the Seebeck coefficient compared to the P3HT composite. The authors were able to fabricate an all p-type TEG (Figure 43e) using the PCDTPT/FWCNT networks that was capable of generating >200 nW for a temperature difference of 20 K (Figure 43f).353

#### 4.2.5.2. Blended Molecule–Carbon Nanotube Nanocomposites.
In contrast to their polymer counterparts, molecular organic semiconductors (mOSCs) have received far less attention as a component in CNT-based TE composites. The

recent emergence of high-performance molecular electron acceptors for organic photovoltaics provides an excellent example of the rich chemical diversity available for small molecule-based TE materials. The precise chemical structure, chemical tunability, and potential for unrivaled purity, coupled to the recent demonstration of exceptionally large Seebeck coefficients,191,377–379 suggests mOSCs as high-performance TE composite materials with a huge potential for carefully designed structure–function studies. It is important to note here that the distinction between a composite, which we define as having a significant volume fraction loading of multiple components that are mixed together prior to network formation, and small molecule-doping of a preformed porous CNT network becomes a little blurred. The following discussion focuses on the former, where we expect the molecular species to play additional roles beyond being a simple surface-adsorbed redox (charge-transfer) dopant. Even here this definition can be open to interpretation, and any omission on the part of the authors is unintentional.

Motivated by the potential advantages of mOSCs, Wu et al. investigated composites of amino-substituted rylene diimides, previously explored as self-doped TE molecular crystals,191 and SWCNTs prepared via vacuum filtration of an ultrasonically prepared dispersion of the two components.359 The amino-substituted rylene diimides appear to be effective n-type dopants for the SWCNT network, resulting in an n-type Seebeck coefficient comparable to the best-performing n-type dopants explored to date. In this context, the SWCNT inclusions enable far superior electrical conductivities (>40 000 S m$^{-1}$) to thin films of the substituted rylene diimides. The composites prepared from both derivatives achieve peak TE power factors exceeding 100 $\mu$W m$^{-1}$ K$^{-2}$, with the optimum SWCNT to rylene diimide weight ratio lying between 2:1 and 1:1.359 While the TE performance of the composites appears to be dominated by the SWCNT network, there is a clear influence from the electronic properties of the rylene diimide. The composite containing the naphthalene diimide derivative slightly outperforms the perylene diimide derivative, primarily due to a slightly
larger Seebeck coefficient that might arise from the wider bandgap of the naphthalene diimide. This observation points to a potential modulation of the carbon nanotube density of states by the rylene diimide, which would require more detailed investigations of the electronic structure of the composites. However, the morphology in the two composites varies dramatically, suggesting that significant changes in the charge carrier transport (mobility) could play a dominant role in the observed changes in the Seebeck coefficient.

Shortly thereafter, and inspired by the substitutional chemistry of the rylene diimides described above, Gao et al. explored a series of acridone derivatives with different terminal tertiary amine groups (ADTA) as n-type components in SWCNT-based TE composites.³⁶¹ Irrespective of the chemical nature of the terminal tertiary amine group, the samples all exhibited n-type behavior at elevated temperature, although the room-temperature behavior appeared to be rather complex and suggestive of incomplete n-type doping. For instance, both the piperidine- and 4-hydroxypiperidine-terminated acridones exhibited p-type TE power factors of >150 µW m⁻¹ K⁻² at room temperature, whereas the diethyl amine-terminated acridone exhibited a n-type TE power factor of ca. 125 µW m⁻¹ K⁻² at room temperature.³⁶¹ The same group explored a similar series of compounds, this time based on acridine derivatives where the nature of the terminal tertiary amine group, the length of the connecting alkyl chain, and the anion chemistry were systematically varied.³⁶³ Once again, the TE transport behavior was rather complex, with small changes in the chemical structure of the acridine derivative resulting in dramatic switches between p-type and n-type behavior.

Diphenylamine and carbazole moieties are common in hole (p-type) transport materials for organic electronic and optoelectronic applications.³⁸⁰,³⁸¹ Inspired by their potential for effective p-type transport, Yin et al. prepared a series of compounds to compare the impact of both the central core and the peripheral moieties on the TE performance of the corresponding SWCNT composites.³⁶² When the diphenyl-amine units were connected to a biphenyl core the composite exhibited the largest Seebeck coefficient, consistent with the larger bandgap of the mOSC and lowest electrical conductivity. Replacing the biphenyl core with pyrene resulted in a reduction of the Seebeck coefficient but a slight increase in the electrical conductivity. However, the highest TE performance was exhibited by the mOSC with carbazole moieties attached to a pyrene core, primarily due to the improved electrical conductivity (ca. 190 000 S m⁻¹) that enabled a peak TE power factor of ca. 110 µW m⁻¹ K⁻².³⁶²

Recently, Yin et al. explored the TE performance of composites for a series of mOSCs based on the 4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene (IDT) core (Figure 44a),³⁶⁶ which is similar to one of the highest performing nonfullerene electron acceptors in organic photovoltaics. To control the charge-carrier density and improve the electrical transport, the authors also investigated n-type doping using two dopants that function via different electron-transfer mechanisms (Figure 44b,c). For all combinations of the SWCNTs with the four ITD molecules and two dopants, the composites exhibited n-type transport and TE power factors exceeding 50 µW m⁻¹ K⁻² (Figure 44d,e). For the 1,3-dimethyl-2-(2,4,6-trimethoxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole (TM-DMBI) dopant, the electron injection is mediated by hydride transfer, apparently resulting in more effective charge transfer (cf., lower Seebeck coefficients and larger electrical conductivities) that may push

the carrier density of the composite beyond the peak TE performance. In contrast, the highest performance was observed for direct electron transfer from the alkyl diamine-based electron dopant to the composites possessing the two largest bandgap IDT derivatives (PF > 150 µW m⁻¹ K⁻²).³⁶⁶

The most impressive TE performance achieved to date in mOSC/CNT composites was for samples prepared with the high charge mobility semiconductor 2,7-diphenyl[1]-benzothieno[3,2-b][1]benzothiophene (DPh-BTBT).³⁶⁴ Even in the undoped DPh-BTBT/SWCNT composites, the TE power factor was quite impressive (>160 µW m⁻¹ K⁻²), but the authors also explored the impact that the p-type dopant chemistry and the doping process had on the TE performance. For both 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ), solution phase doping, by direct mixing of the dopant into the DPh-BTBT/SWCNT casting solution, resulted in a near-universal drop of the TE power factor irrespective of the doping concentration. In contrast, vapor deposition of thin TCNQ or F₄TCNQ films (<40 nm) onto a preformed DPh-BTBT/SWCNT composite enhanced the TE power factor to (>200 µW m⁻¹ K⁻²), with the 40 nm thick vapor deposited TCNQ sample reaching an optimum value of ca. 280 µW m⁻¹ K⁻².³⁶⁴

#### 4.2.5.3. Blended Nanocomposites of Polymers and Graphitic Nanocarbons.
Much like their polymer-carbon nanotube counterparts, blended composites of polymers and two-dimensional (2D) graphitic nanocarbons have seen significant improvement in their TE performance over the past decade, with many of the notable achievements highlighted in a recent review by Nadihalli et al.³⁶⁷ Although a number of polymer matrices have been explored, including polythiophenes and polypyrroles,³⁸²⁻³⁸⁶ the most significant advances have been achieved with PANI and PEDOT:PSS.

Early studies of nanocomposites of conducting/conjugated polymers and graphitic nanocarbons focused on blends of PANI with graphite or graphene nanosheets, where the TE power factor and zT measured at room temperature were low (<10 µW m⁻¹ K⁻² and <0.001, respectively).³⁸⁷⁻³⁸⁹ Improvements in the TE power factor (14 µW m⁻¹ K⁻²) were obtained for composites prepared by cold pressing powders of mechanically mixed PANI and graphene nanoplatelets (GNPs) at 10 tons for 3 min.³⁹⁰ More recently, drop-cast films prepared following solution-phase ultrasonic mixing of CSA-doped PANI and GNPs achieved TE power factors exceeding 15 µW m⁻¹ K⁻², due primarily to an enhanced electrical conductivity.³⁹¹ The authors demonstrated that disruption of the electronic structure of the sp²-hybridized hexagonal carbon lattice, through the incorporation of oxygenic functional groups, weakened the interaction with the PANI chains and reduced the electrical conductivity and TE power factor. Motivated by this observation, Wang et al. polymerized aniline in situ on the surface of CVD-grown GNPs, which were subsequently dispersed and doped in a solution of CSA in m-cresol before drop-casting to form films that exhibited enhanced Seebeck coefficients (ca. 26 µV K⁻¹) and TE power factors (ca. 55 µW m⁻¹ K⁻²).³⁹² To date, the most impressive TE performance was achieved for spin-cast films of CSA-doped PANI and thermally reduced GO nanoplatelets prepared by solution-phase ultrasonic mixing, reaching TE power factors exceeding ca. 210 µW m⁻¹ K⁻².³⁹³ An interesting alternative approach was recently demonstrated by Hsieh et al., who demonstrated TE power factors exceeding ca. 80 µW m⁻¹ K⁻² for a three-dimensional
<figure><img src="image_59.png" /><figcaption>**Figure 45.** (a) Schematic illustration showing the template synthesis of a 3D graphene–PANI composite. (b) Digital photographs of the 3D graphene–PANI composite containing 80 wt % PANI supported by a Kapton (polyimide) film (upper images) and as a free-standing flexible sample (lower images). Reproduced with permission from ref 394. Copyright 2019 The Royal Society of Chemistry.</figcaption></figure>

(3D) graphene matrix infiltrated with PANI via in situ polymerization of aniline in the porous 3D graphene matrix (Figure 45a).³⁹⁴ One advantage of this approach is that the 3D graphene matrix, produced via CVD synthesis of graphene in a polymer-bound nickel powder catalyst template, can be cast into various form factors, including onto a supporting substrate or as free-standing films (Figure 45b). This versatility could enable fabrication of TE composites that can be tailored for the waste heat source.

Similar to PANI-based nanocomposites, early composites based on PEDOT and graphitic nanocarbons exhibited relatively low room-temperature power factor and $zT$ (<15 µW m⁻¹ K⁻² and <0.02, respectively). Motivated by previous demonstrations that >35 wt % CNT loadings were typically required to improve the electrical conductivity in PEDOT:PSS-CNT composites, Kim et al. prepared composites with low loadings (<3 wt %) of reduced graphene sheets, which afforded an increase in both the carrier concentration and mobility relative to the pristine PEDOT:PSS sample.³⁹⁵ The increased carrier concentration resulted in a decrease of the Seebeck coefficient by a factor of ca. 3, but this was more than offset by a

> 20 times enhancement in the electrical conductivity, enabling TE power factors of ca. 11 µW m⁻¹ K⁻². Surprisingly, the thermal conductivity of the composites was actually reduced by the inclusion of graphene, which the authors attributed to a reduction in the phonon (lattice) thermal conductivity due to the low graphene loading and phonon scattering at the interfaces between the polymer matrix and graphene fillers. Several subsequent studies demonstrated that similarly low loadings (<5 wt %) of graphene or reduced-GO improved both the electrical conduction and TE power factor (exceeding >25 µW m⁻¹ K⁻²) of the pristine PEDOT:PSS but had little impact on the magnitude of the Seebeck coefficient.³⁹⁶⁻³⁹⁸ For samples prepared by in situ polymerization of EDOT in an ultrasonically mixed aqueous PSS-graphene dispersion, a peak TE power factor of ca. 45 µW m⁻¹ K⁻² was achieved for the composite with 3 wt % graphene.³⁹⁶ Recently, flexible p-type TEGs with 5 legs were fabricated from vacuum-filtered PEDOT:PSS-graphene dispersions (Figure 46), exhibiting reasonable TE open-circuit voltages of ca. 4 mV for a temperature difference of 56 K (corresponding to a Seebeck coefficient of ca. 14 µV K⁻¹) and
<figure><img src="image_60.png" /><figcaption>**Figure 46.** (a) Schematic illustration showing preparation of a flexible p-type TEG from vacuum-filtered PEDOT:PSS-graphene composite. (b) Temperature-dependent open-circuit voltage generated by the TEGs and (c) Output voltage and power as a function of current for the prepared TEGs as a function of the number of p-type legs. (d,e) Digital photos illustrating the TEG flexibility. Reproduced with permission from ref 398. Copyright 2019 Springer Science+Business Media, LLC.</figcaption></figure>

<figure><img src="image_61.png" /><figcaption>**Figure 47.** (a) Schematic representation of the layer-by-layer self-assembly procedure and (b) images of aqueous suspensions of PANI, graphene (stabilized by PEDOT:PSS), and DWCNT (stabilized by PEDOT:PSS). AFM topographical images of the corresponding suspensions cast onto silicon wafers are shown next to graphene and DWNT suspensions. The corresponding thermoelectric performance of the multilayer PCNs: (a) Seebeck coefficient, (d) electrical conductivity, and (e) thermoelectric power factor. Adapted with permission from ref 335. Copyright 2015 Wiley-VCH.</figcaption></figure>

less than 3% reduction in device resistance the after 100 bending cycles.³⁹⁸

Several other nanocarbon fillers have been explored for PEDOT-based composites, including fullerene-decorated reduced GO³⁹⁹ and graphene quantum dots (GQDs).⁴⁰⁰ In the former study, noncovalent attachment of a heavily fluorinated C₆₀ fullerene (C₆₀F₃₆) to the surface of reduced GO and subsequent loading (<3 wt %) into PEDOT:PSS led to an enhanced TE power factor relative to both the pristine PEDOT:PSS sample and PEDOT:PSS-rGO composite.³⁹⁹ The enhanced performance was primarily associated with an

improved Seebeck coefficient, which the authors attributed to carrier filtering of low-energy carriers by the Schottky barrier at the interface between the PEDOT:PSS and rGO-C₆₀F₃₆ fillers. In the latter study, although the baseline TE performance of the pristine PEDOT:PSS sample was low compared to most previously reported values, the inclusion of GQDs prepared via chemical cutting of GO resulted in an enhancement of the electrical conductivity, Seebeck coefficient, and TE power factor.⁴⁰⁰

Inspired by the demonstration that replacement of the PSS polyanion with the molecular toluenesulfonate, or tosylate
(Tos), anion can lead to huge enhancements in the TE performance of PEDOT,⁴⁰¹ Maity et al. explored the TE properties of composites prepared via polymerization of EDOT in the presence of Tos and graphene.⁴⁰² Despite the rather high content of graphene (>10 wt %), the composites exhibited large Seebeck coefficients (>130 µV K⁻¹) and reasonable electrical conductivities (>500 S m⁻¹), resulting in a peak TE power factor of ca. 25 µW m⁻¹ K⁻² for 25 wt % graphene loading.

#### 4.2.5.4. Multilayer Nanocomposites.
Layer-by-layer (LbL) assembly, through sequential exposure of a substrate to solutions containing complementarily functionalized materials, is a simple and versatile method for creating conformal multifunctional films, particularly for composites with a large number of constituent components. The LbL approach has been used to explore the thermoelectric performance of hierarchical assemblies generated from (i) polycation/polyanion multilayers,⁴⁰³ (ii) CNTs sequentially wrapped by polycation/polyanion layers,⁴⁰⁴ and (iii) ligand-exchanged Cu₂ₓSe nanocrystal arrays.⁴⁰⁵

While there has been significant progress in the TE performance of blended nanocomposites with different nanocarbon inclusions, the best-performing systems remain the multilayered nanocomposites prepared by LbL deposition of the various constituents, which can address some of the difficulties associated with the creation of uniform, high-performance bulk composites. Cho et al. provided the first example of multilayered polymer carbon nanocomposites (PCNs) fabricated from PANI, graphene, and DWCNTs using the LbL approach.⁴⁰⁶ This study demonstrated that PCNs comprised of PANI/DWCNT bilayers or PANI/graphene/PANI/DWCNT quadlayers both exhibited increasing electrical conductivity and Seebeck with the number of deposition cycles, up to an optimum around 40 cycles. The PANI/DWCNT bilayers exhibited TE power factors >750 µW m⁻¹ K⁻², more than a factor of 4 larger than those for previously published blended PANI/CNT composites. However, the PANI/graphene/PANI/DWCNT quadlayers exhibited impressive Seebeck coefficients (ca. 130 µV K⁻¹) and TE power factors exceeding 1800 µW m⁻¹ K⁻², which approached those for state-of-the-art inorganic telluride semiconductors. The authors attributed the impressive transport properties to the good electrical transport of the graphene and DWCNT network and the large Seebeck coefficients to carrier energy filtering at the ubiquitous nanoscale interfaces within the composite, both enabled by the deposition of thin (few nm) layers during each step in the LbL process.⁴⁰⁶

Through the stabilization of the graphene and DWCNT suspensions with PEDOT:PSS (Figure 47a), Cho et al. were able to induce almost a factor of 2 enhancement in the electrical conductivity (ca. 190 000 S m⁻¹) with smaller than a 10% decrease in the Seebeck coefficient compared to the non-PEDOT:PSS sample (Figure 47c,d).³³⁵ These observations were attributed primarily to an enhanced charge-carrier mobility in the PEDOT:PSS stabilized composites, reaching ca. 19 cm² V⁻¹ s⁻¹, for similar charge-carrier densities. The use of PEDOT:PSS also appeared to allow for the fabrication of thicker PCNs (80 cycles) with continued improvement in the performance.³³⁵ The PEDOT:PSS stabilized 80 quadlayer PCN achieved a peak TE power factor exceeding 2700 µW m⁻¹ K⁻² (Figure 47e), which represents one of the highest performing all-organic thin-film TE materials to date.

In an effort to produce the n-type PCNs, required to build an all-PCN thermoelectric generator, Cho et al. employed the same LbL approach, this time stabilizing the DWNTs and graphene

with the n-type polymers polyethyleneimine (PEI) and polyvinylpyrrolidone (PVP), respectively.³³⁴ The DWCNT-PEI/graphene-PVP bilayer PCNs exhibit n-type transport behavior with electrical conductivities of ca. 30 000 S m⁻¹ and Seebeck coefficients of ca. −80 µV K⁻¹. However, the magnitude of the Seebeck coefficient appears to be constrained by the large electron concentration, which limits the TE performance (PF ~ 190 µW m⁻¹ K⁻²) and potentially also reduces the charge-carrier mobility. These observations point to (i) improved PCN nanomorphology and (ii) carefully controlled electron density as strategies to improve the TE performance of n-type multilayered PCNs. Interestingly, the PCN nanomorphology appears to provide some resistance to the loss of n-type TE performance via compensation of the n-type dopants by oxygen, attributed to the tortuosity for gas diffusion offered by the exfoliated graphene layers.³³⁴

While the performance of the p-type multilayer composites with PEDOT:PSS-stabilized graphene and DWNTs has not been surpassed, the LbL fabrication approach has been used to demonstrate impressive p-type and n-type behavior in multilayer composites containing reduced graphene oxide (rGO).³³⁷,⁴⁰⁷ For the p-type composite, which is comprised of multiple bilayers formed from PANI and DWNTs stabilized by graphene oxide (GO), the composite initially yielded reasonable performance, with a TE power factor of ca. 135 µW m⁻¹ K⁻².³³⁷ However, reduction of GO by L-ascorbic acid enhances both the electrical conductivity and Seebeck coefficient, resulting in an order of magnitude improvement in the TE power factor to ca. 1250 µW m⁻¹ K⁻². Consistent with the observations for n-type multilayer composites based on DWNTs and graphene, the n-type multilayer composites based on PEI-stabilized DWNTs and GO underperform their p-type counterparts.⁴⁰⁷ However, thermal reduction of the composite, at 150 °C for 30 min, to yield rGO results in a similar enhancement of the TE performance to the p-type rGO composite, resulting in a power factor of ca. 400 µW m⁻¹ K⁻². These observations for both the p-type and n-type composites are attributed to a reduction in the number of sp³ hybridized carbon atoms in the graphene oxide layer through removal of some of the surface oxide species, as determined from Raman and X-ray photoelectron spectroscopy measurements, and a corresponding increase in the charge-carrier mobility.³³⁷,⁴⁰⁷

In the last couple of years, poly(diallyldimethylammonium chloride) (PDDA) has been explored as an alternative to PANI in multilayer PCNs in composites with PEDOT:PSS stabilized graphene and/or DWNTs.⁴⁰⁸,⁴⁰⁹ Improved TE performance in these composites could be induced by thermal treatment at temperatures above 300 °C⁴⁰⁸ or solution-phase predoping of the PEDOT:PSS with potassium bromide (KBr).⁴⁰⁹ In both cases, an enhancement in the TE performance was attributed to removal of insulating PSS (as determined by X-ray photoelectron spectroscopy of the sulfur 2p peaks) from the multilayer composite, with an improved charge-carrier density and electrical conductivity but little deterioration in the Seebeck coefficient. In their first study, Stevens et al. showed that the thermal treatment of a PDDA/graphene-PEDOT:PSS/PDDA/DWCNT-PEDOT:PSS quadlayer PCN induced degradation of the PDDA and PSS components, resulting in an increase in the electrical conductivity that improved the peak TE power factor by an order of magnitude to ca. 170 µW m⁻¹ K⁻².⁴⁰⁸ Subsequently, Stevens et al. fabricated a PDDA/DWCNT-PEDOT:PSS bilayer PCN where KBr doping of the PEDOT:PSS eliminated the need for the thermal treatment.⁴⁰⁹
The KBr doping approach yielded increased film thickness for the same number of LbL deposition cycles and composites that exhibited an impressive peak TE power factor of ca. 625 µW m⁻¹ K⁻².

The excellent TE performance and relative ease of fabrication for the multilayer all-organic TE composites prepared by LbL fabrication processes mark them out as a one of the most promising candidates for incorporation into commercially viable flexible TEGs. Further advances in this area will require the fabrication and optimization of TEGs, where both the n-type and p-type materials that have been demonstrated, as well as a more fundamental understanding of the underlying physical mechanisms behind the impressive TE performance introduced by the nanoscale interfaces and transport pathways in these multilayer composite materials.

A transparent, flexible hybrid conductive film, prepared by spin-coating a PEDOT:PSS film onto a graphene film prepared by rapid thermal chemical vapor deposition (RTCVD), recently demonstrated exceptional electrical conductivity (>100 000 S m⁻¹) and TE power factors exceeding 55 µW m⁻¹ K⁻².⁴¹⁰ This hybrid exhibited >85% transmittance across the visible spectrum, and the TE properties of the flexible films on PET substrates were robust to repeated bending cycles (<15% increase in the sheet resistance and negligible change in the Seebeck coefficient).

### 4.2.5.5. Hybrid Nanocarbon–Inorganic Composites.
Inorganic chalcogenides, including elemental Te and compound chalcogenide semiconductors, represent one of the most well-studied and highest-performing TE material classes.⁴¹¹,⁴¹² However, the approach to reduce the thermal conductivity of the inorganic chalcogenides, via micro- and nanostructuring, often limits the electrical conductivity, requiring a carefully tuned balance between optimization of the two transport processes for peak TE performance.⁴¹¹ Nanostructured carbon networks, and particularly carbon nanotube networks, have great potential for serving as high surface area conductive scaffolds (vide supra) in a number of technologies, including photo- and electrocatalytic architectures, fuel cell electrodes, batteries, and photovoltaics.²⁰,²¹⁹ The introduction of nanoscale inclusions into inorganic TE materials has also been suggested as a method to yield interfaces that facilitate carrier energy filtering and phonon scattering. These interfaces may facilitate decoupling of the Seebeck coefficient, electrical conductivity, and thermal conductivity, allowing for enhancement of the TE performance,⁴¹³ although a recent study advocates for more thorough experimental and theoretical treatment of composites, including explicit consideration of the interfaces present in inhomogeneous materials.³³⁸ These predictions have inspired the incorporation of nanocarbon scaffolds for hybrid nanocarbon–inorganic TE composites, while the questions that remain to be answered provide motivation for rigorous fundamental studies to better understand the role interfaces play in the TE properties of nanocomposite systems.

The majority carrier type in these hybrid nanocarbon–metal chalcogenide nanocomposites is typically dependent on the nature of the chalcogenide semiconductor, with bismuth⁴¹⁴⁻⁴¹⁷ and silver⁴¹⁸ chalcogenides being n-type and antimony,⁴¹⁹⁻⁴²³ lead,⁴²⁴ and tin⁴²⁵⁻⁴²⁷ chalcogenides p-type.

The bulk of research in the past decade has focused on mechanically mixed and fabricated composites of bismuth- and antimony-based chalcogenides and FWCNTs or MWCNTs,⁴¹⁴⁻⁴¹⁷,⁴¹⁹,⁴²¹ with examples of bismuth-antimony telluride composites achieving a material zT of >1.0.⁴¹⁹,⁴²¹

Several studies have demonstrated that the incorporation of small quantities of MWCNT “nanoinclusions” (<0.25 wt %) in Bi₂(Te₀.₉Se₀.₁)₃⁴¹⁴ or (Bi₀.₂Sb₀.₈)₂Te₃⁴¹⁹,⁴²¹ (see Figure 48) can

<figure><img src="image_62.png" /><figcaption>**Figure 48.** Thermoelectric performance of p-type (Bi₀.₂Sb₀.₈)₂Te₃ nanocomposites. (a) Electrical resistivity, (b) Seebeck coefficient, (c) electronic and phonon thermal conductivity, and (d) dimensionless figure of merit, ZT. Small loadings of MWCNTs result in a slight decrease in the electrical conductivity, which is compensated for by a significant enhancement of the Seebeck coefficient and reduction of the phonon thermal conductivity, resulting in a 50% increase in the ZT. Reproduced with permission from ref 419. Copyright 2014 Elsevier Ltd.</figcaption></figure>

reduce the thermal conductivity to less that 1 W m⁻¹ K⁻¹ at room temperature due to a phonon mismatch between the two components and a concomitant reduction in the phonon thermal conductivity. The introduction of interfaces in nanocomposite appears to have little to no impact on the electrical conductivity and induces either an enhancement or only a small decrease in the Seebeck coefficient, resulting in peak zT values that approach or exceed 1.0 (Figure 48).⁴¹⁹,⁴²¹ Similar observations have been made recently for mechanically mixed composites of graphite and (Bi₀.₂Sb₀.₇₅)₂Te₃, where a graphite loading of <0.2 wt % resulted in a zT > 1.0 at room temperature.⁴²² In contrast, other studies have suggested that the incorporation of MWCNTs into nanocomposites of Bi₂(Te₀.₈Se₀.₂)₃ have a dramatic deleterious effect on the TE transport properties and performance,⁴¹⁶ suggested to arise from (i) the reduced grain size of the Bi₂(Te₀.₈Se₀.₂)₃ grains, (ii) a reduction in the number of tellurium vacancies, and (iii) apparent n-type donor effects due to the MWCNTs.

Several chemical approaches have also been explored for the generation of CNT/bismuth chalcogenide nanocomposites. A solvothermal approach to the growth of Bi₂Te₃ on the MWCNT surface, via nucleation at MWCNT surface oxygen functional groups generated prior to the reaction with Bi(CH₃COO)₃ and TeCl₄, generated “nanopowders” that were subsequently ball-milled and subjected to spark plasma sintering to form the bulk nanocomposite.⁴²⁸ The resulting nanocomposites exhibited
<figure><img src="image_63.png" /><figcaption>**Figure 49.** (a) Schematic illustration of the fabrication of a free-standing Bi₂Te₃/SWCNT hybrid thermoelectric material. (b) Bright-field TEM image of Bi₂Te₃ nanocrystals anchored on a SWCNT bundle. Inset is a schematic of the Bi₂Te₃-SWCNT orientation alignment. (c) zT for 600 nm thick Bi₂Te₃/SWCNT hybrid thermoelectric. (a–c) Reproduced with permission from ref 431. Copyright 2019 Springer Nature. (d) FESEM image of the surface of a Sb₂Te₃/SWCNT hybrid film after annealing. (e–f) Photos of a flexible TE generator formed from 10 p/n legs where p-type legs are Sb₂Te₃/SWCNT films and n-type legs are Bi₂Te₃/RGO films. (g) Power output of the TE generator as a function of temperature difference ΔT. (d–g) reprinted with permission from ref 423. Copyright 2019 Wiley.</figcaption></figure>

more than an order of magnitude enhancement in the carrier mobility, but with almost 2 orders of magnitude decrease in carrier density that led to a significantly enhanced Seebeck coefficient (ca. 35% increase). This increase in the thermopower, coupled with a ca. 40% reduction in the thermal conductivity to ca. 0.65 W m⁻¹ K⁻¹, gave an enhanced zT of ca. 0.5 at room temperature and ca. 0.85 at 473 K.⁴²⁸ More recently, composites have been prepared via vacuum filtration of a ca. 50:50 mix of SWCNTs and DWCNTs and pregrown Bi₂Te₃ nanowires (prepared by a NW templating process, involving reaction of Te NWs with a Bi precursor solution).⁴²⁹ The as-prepared nanocomposites exhibited rather low Seebeck coefficients and electrical conductivities, but a thermal annealing process (300 °C for two hours in flowing argon at ambient conditions) resulted in enhanced charge-carrier transport and a factor of 2 enhancement in the Seebeck coefficient, yielding peak TE power factors of. ca. 225 µW m⁻¹ K⁻² for composites with <2 vol % CNTs.⁴²⁹

There have also been a number of recent studies focused on SWCNT:Bi₂Te₃ nanocomposites. One of the earliest reports found that the majority carrier was dependent on the SWCNT loading, with n-type transport for compositions with <1 wt % SWCNT and p-type transport for higher loadings.⁴³⁰ For a SWCNT loading of 0.5 wt %, there was a slight reduction in the thermal conductivity and a significantly enhanced Seebeck coefficient, yielding a ca. 25% improvement in the zT close to room temperature.⁴³⁰ More recently, Jin et al. demonstrated the growth of layered Bi₂Te₃ on a SWCNT scaffold by magnetron sputtering (Figure 49a).⁴³¹ Crystallographic alignment between the Bi₂Te₃(1210) axis, and the SWCNT bundle axis leads to a highly ordered Bi₂Te₃ nanocrystals on the SWCNT bundles (Figure 49b). The authors suggest that high in-plane electrical conductivity is favored by two factors. First, amorphous material is not observed between Bi₂Te₃ nanocrystals or between Bi₂Te₃ and the SWCNT bundles, suggesting very clean interfaces and

minimized carrier scattering. Second, crystallographic Bi₂Te₃ orientation on the bundle axis, leading to (000l) texturing, capitalizes upon high in-plane conductivity of the anisotropic Bi₂Te₃. The resulting hybrid composite achieves PF values in the range of ca. 1600–1100 µW m⁻¹ K⁻² in the range of 300–473 K. A low in-plane thermal conductivity of ca. 0.26 W m⁻¹ K⁻¹, one of the lowest reported to date for this type of hybrid composite, results in a room temperature zT of 0.89 (Figure 49c). This low in-plane *k* arises primarily from a reduction in the lattice contribution and is attributed to (i) phonon mismatch between Bi₂Te₃ and the SWCNT scaffold, (ii) a high number of multiscale defects in the hybrid material, and (iii) interface roughness on the length scale of the primary Bi₂Te₃ phonons.

This hybrid organic–inorganic TE material appears to have good potential for flexible TE devices, with exceptional flexibility and resilience to bending⁴³¹ and promising power conversion factors.⁴³² The resistance of the hybrid material changed negligibly down to a bend radius of 5 mm and was also unaffected after 100 bending cycles at a bend radius of 10 mm. In contrast, the resistance of a Bi₂Te₃ film on a polyimide substrate doubled upon bending at a 5 mm radius and increased by 400% after 100 cycles at 10 mm bend radius.⁴³¹

Wu et al. recently prepared thin-film TE composites from SWCNTs and Sb₂Te₃ by a solution-based filtration process (Figure 49d).⁴²³ They obtained an optimal p-type PF of ca. 55 µW m⁻¹ K⁻² for a SWCNT loading of 16%. These p-type SWCNT/Sb₂Te₃ showed good performance stability with respect to repeated bending and could be combined with n-type composites containing Bi₂Te₃ and reduced graphene oxide (rGO) to form TE generators (Figure 49e,f). Flexible and wearable generators formed from 10 legs of p/n couples provided a power output of ca. 25 µW for a temperature difference of 70 K (Figure 49g).⁴²³

One early example of CNT composites with inorganic materials was the fabrication of n-type CNT/Ag₂Te hybrid
buckypapers reported by Yan et al.⁴¹⁸ Well-dispersed CNTs were added to silver precursor solution, followed by the injection of TOP-Te solution to form a hybrid of Ag₂Te nanoparticles bound to the surface of the CNT bundles.⁴¹⁸ The CNT/Ag₂Te nanocomposite with a loading of ca. 5 wt % CNTs exhibited a PF of >200 µW m⁻¹ K⁻² close to room temperature and 579 µW m⁻¹ K⁻² at 525 K, both superior to the pure CNT and Ag₂Te samples. Combined with a slightly lower thermal conductivity of ~0.7 W m⁻¹ K⁻¹, a high zT value of 0.44 at 525 K was estimated for n-type CNT/Ag₂Te hybrid buckypaper with a weight ratio of 1:17.

MWCNTs have also been explored as conductive fillers in TE composites with tellurium nanorods synthesized from a Te precursor solution via a polyvinylpyrrolidone-assisted solution-phase process.⁴³,⁴³⁴ Although the MWCNTs result in a decrease of the Seebeck coefficient of the composite, low loadings (<2 wt %) enhance the electrical conductivity and suppress the lattice thermal conductivity, presumably due to phonon scattering at the interface between the two components, resulting in an enhanced TE power factor a peak zT for the composite with 2 wt % MWCNT. Composites with chemically synthesized Te nanorods outperform those prepared from ball-milled Te beads, primarily due to the improved electrical conductivity and reduced thermal conductivity.⁴³⁴

Several studies have begun exploring the incorporation of graphene into composites with inorganic TE materials. Composites of Bi₂Te₃ with 4–5 nm thick sheets of graphene oxide (GO) showed slight improvement of zT, relative to the Bi₂Te₃ material alone.⁴³⁵ The peak zT for composites reached ca. 0.4 for Bi₂Te₃ NWs composited with 2 wt % GO, compared to zT = 0.3 for the Bi₂Te₃ NWs on their own. Bi₀.₅Sb₁.₅Te₃ composites with 0.1 vol % expanded graphene showed a 45% improvement to zT, relative to the neat Bi₀.₅Sb₁.₅Te₃ material, with peak zT values in the range of 1.13–1.24 for the composites.⁴³⁶ Ding et al. fabricated flexible TEGs out of rGO sheets and Bi₂Te₃ nanoplates.⁴³⁷ Six-element TEGs with 1% mass ratio of rGO produced ~1.72 µW at a ΔT of 20 K and retained its performance over 1000 bending cycles.

Zong et al. noted that few-layer rGO sheets increased the grain boundary (GB) thermal resistivity by a factor of 3–5 (relative to GBs without graphene) in skutterudite thermo-electric composites.⁴³⁸ This modification led to peak zT values of 1.5 for n-type Yb₃Co₄Sb₁₂ and 1.06 for p-type Ce₃Fe₃CoSb₁₂, allowing the authors to produce 4–8% efficient 16-leg TE modules using n- and p-type skutterudite/graphene composites.

Bhardwaj et al. prepared ball-milled composites of p-type Mg₃Sb₂ and Mg₃Sb₁.₈Bi₀.₂ with varying ratios of few-layer graphene nanosheets (GNS), where the GNS were derived from thermally expanded graphite.⁴³⁹ Champion zT values were obtained for ca. 1.25 wt % loading of GNS into each composite, with zT = 0.71 at 773 K for Mg₃Sb₂/GNS and zT = 1.35 at 773 K for Mg₃Sb₁.₈Bi₀.₂/GNS (Figure 50a). The improvements of ca. 170% and 125%, relative to the uncomposited Mg₃Sb₂ and Mg₃Sb₁.₈Bi₀.₂ materials, respectively, derived from an increase in conductivity (due to an increase in both carrier concentration and mobility). The Seebeck coefficient decreased with GNS addition, as expected for the increase in carrier concentration, and the thermal conductivity increased.

Lin et al. recently explored the potential energy filtering role of graphene nanoplatelets (GNP) at the grain boundaries of n-type Te-doped Mg₃Sb₂ (Figure 50b,c).³³⁸ In this study, the GNPs were produced by a surfactant exfoliation process⁴⁴⁰ and composites were fabricated by sintering and annealing. Nano-

<figure><img src="image_64.png" /><figcaption>**Figure 50.** (a) Temperature-dependent zT of p-type Mg₃Sb₂/GNS composites with varying mass ratios and for Mg₃Sb₁.₈Bi₀.₂/GNS composites with mass ratio of 80:1. Reproduced with permission from ref 439. Copyright 2015 Royal Society of Chemistry. (b) Temperature-dependent zT of n-type Mg₃.₂Sb₁.₉₉Te₀.₀₁, either prepared with large (ca. 13 µm) or small (ca. 0.6) grains and Mg₃.₂Sb₁.₉₉Te₀.₀₁/GNP composite with ca. 1 µm grains. (c) Linear correlation between Seebeck coefficient and thermal conductivity in Mg₃.₂Sb₁.₉₉Te₀.₀₁ and Mg₃.₂Sb₁.₉₉Te₀.₀₁/GNP composites, demonstrating that the Seebeck coefficient scales with thermal conductivity and not carrier concentration (inset). (b,c) Reproduced with permission from ref 338. Copyright 2020 Royal Society of Chemistry.</figcaption></figure>

structuring and the inclusion of GNPs lead to a 40% increase in zT, up to 1.73 near 773 K (Figure 50b). The authors note that the inclusion of the GNPs leads to an increase of the materials' Seebeck coefficients without a change in the carrier concentration (Figure 50c inset), a classic effect that is theoretically expected for energy filtering but rarely experimentally observed. They introduce a new two-phase (grain + grain boundary) heterogeneous transport model to analyze their finding, with an important outcome of the model being the introduction of an interfacial Seebeck coefficient term. This term arises from the large temperature drop associated with the grain boundary
<figure><img src="image_65.png" /><figcaption>**Figure 51.** Hybrid GDTe thermoelectric papers: (a) Schematic illustration of the preparation of the ternary hybrid TE papers from reduced GO and PEDOT:PSS-coated Te NWs. (b) Impact of the number of bending cycles on the electrical conductivity (top), thermopower (middle), and TE power factor (bottom) of a $G_{0.1}DTe_{0.9}$ paper, illustrating only small negative impacts on the TE performance up to 1000 bending cycles. (c) Improved stability of the electrical conductivity (top), thermopower (middle), and TE power factor (bottom) for the $G_{0.1}DTe_{0.9}$ paper relative to the Te NWs and PEDOT:PSS-coated Te NWs. Reproduced with permission from ref 261. Copyright 2016 Wiley-VCH.</figcaption></figure>

thermal resistance, which in turn creates a large associated grain boundary voltage. Thus, the authors posit that the key to realizing the large energy filtering effect is by maximizing the temperature drop across the grain boundary region and is thus anticorrelated with thermal conductivity (Figure 50c). They conclude that GNP and associated 2D materials can play such a role in materials with charged grain boundaries and grain size dependent transport properties.

#### 4.2.5.6. Multinary Nanocomposites.
In an effort to exploit the prevalent interfaces formed in multinary nanocomposites, which may enhance carrier filtering effect and introduce phonon scattering due to material mismatch, several studies have explored composites formed with either two different nano-carbon fillers,⁴⁴¹ two different nanoscale inorganic fillers,⁴⁴²⁻⁴⁴⁴ or some combination of the two filler types.⁴²⁶,⁴²⁷,⁴⁵⁻⁴⁷

In one of the first examples, Kim et al. prepared a three-component nanocomposite made via vacuum filtration of a dispersion of inorganic Te nanowires coated with conducting polymer PEDOT:PSS combined with reduced-GO (Figure 51a).²⁶¹ The free-standing and flexible composite possessed a power factor of ca. 140 µW m⁻¹ K⁻² at 300 K, allowing fabrication of flexible devices that were robust to repeated bending cycles and retained the majority of their TE

performance even after stressing at 85 °C and 85% humidity for 7 days (Figure 51b,c).

Recently, Erden et al. demonstrated ternary composites prepared from anatase $TiO_2$ nanocrystals with PANI-coated amine-functionalized CNTs that exhibited impressive room temperature TE power factors exceeding of 100 µW m⁻¹ K⁻².⁴⁴⁶ The addition of the $TiO_2$ nanocrystals caused a significant decrease in the electrical conductivity due to a reduction in both the carrier mobility and concentration which led to an increase in the Seebeck coefficient and an optimum TE power factor for 30 wt % $TiO_2$.

Multinary TE composites have also been demonstrated for tin chalcogenide nanosheets combined with carbon nanotubes and conducting polymers.⁴²⁶,⁴²⁷ The addition of a small loading of MWCNTs (<1.0 wt %) to a nanocomposite of PVDF and PANI-coated $SnSe_{0.8}S_{0.2}$ nanosheets resulted in an enhanced electrical conductivity and a room temperature TE power factor of ca. 260 µW m⁻¹ K⁻², which increased to ca. 300 µW m⁻¹ K⁻² at 400 K.⁴²⁶ More recently, Liu et al. fabricated flexible, free-standing TE composites by vacuum filtration of dispersions of SWCNTs, PEDOT:PSS, and $SnSe$ nanosheets that were prepared via chemical intercalation and exfoliation.⁴²⁷ The TE properties of these composites were optimized for a SWCNT loading of 60 wt %, reaching ca. 160 µW m⁻¹ K⁻² at room
<figure><img src="image_66.png" /><figcaption>**Figure 52.** (a) Photograph of the 11-leg TEG before and after series connection using silver paint. (b) Open-circuit voltage of the TEG as a function of temperature difference. (c) Output voltage and power versus current for different temperature differences. (d) The power density of the device as a function of load resistance. Reproduced with permission from ref 444. Copyright 2020 Elsevier Ltd.</figcaption></figure>

temperature, mainly due to an enhancement in the Seebeck coefficient.

In the past few years, metal halide perovskites have received some attention as potential TE materials,⁴⁴⁸ partly because their “soft” crystal structure should lead to exceptionally low thermal conductivity, although their typically low electrical conductivity and issues associated with the chemical and compositional stability need to be overcome. Kishan et al. recently demonstrated PANI-methylammonium lead iodide (MAPI)-MWCNTs TE composites, where the PANI and MWCNTs appeared to synergistically enhance the electrical conductivity and Seebeck coefficient, resulting in a peak TE power factor of ca. 10 µW m⁻¹ K⁻².⁴⁴⁷

Over the last several years, several studies have explored ternary composites of PEDOT:PSS and PVP with two different nanoscale inorganic fillers, with those based on silver chalcogenides exhibiting particularly impressive performance.⁴⁴³,⁴⁴⁴ In all cases, alloy metal chalcogenide nanowires (NWs) were prepared by chemical alloying of polymer-coated chalcogen NWs. Chemical conversion of polymer-coated Te NWs was achieved via reaction with either copper chloride or silver nitrate to form Cu₇Te₄ or Ag₂Te NWs, respectively. For ternary composites prepared from a mixture of PEDOT:PSS-coated Te NWs and PEDOT:PSS-coated Cu₇Te₄ NWs, the peak TE power factor exceeded ca. 60 µW m⁻¹ K⁻² for >90 wt % PEDOT:PSS-coated Te NWs content.⁴⁴² The performance of a PVP/Ag nanoparticle/Ag₂Te NW ternary composite, prepared via vacuum filtration and subsequent heat treatment, delivered even more impressive performance, with a room temperature TE power factor of ca. 215 µW m⁻¹ K⁻² and ca. 370 µW m⁻¹ K⁻² at 393 K.⁴⁴³ In 2020, Lu et al. demonstrated exceptional TE performance for an n-type PEDOT:PSS/Ag₂Se NW/CuAgSe NW composite, with a room temperature TE power factor of ca. 1600 µW m⁻¹ K⁻² and an estimated zT of as much as 1.⁴⁴⁴ Flexible TEGs comprising 11 n-type legs were fabricated from the PEDOT:PSS/Ag₂Se NW/CuAgSe NW composite (Figure 52), and power densities of >6 W m⁻² were achieved for a temperature difference of ca. 30 K (Figure 52d).

### 4.3. Ionic Thermoelectric Materials

Thus far, this review has presented results concerning voltage generation through the thermal diffusion of electronic charge carriers. However, ionic charge carriers can also drift under an external thermal gradient, the so-called Soret effect. This effect is particularly pronounced at high humidity due to faster ion transport in liquid/electrolyte media. Contrary to the electronic Seebeck effect, the thermodiffused ions remain confined in the film instead of being extracted by the electrodes, making these materials more suitable for capacitor-like applications instead of for continuous power generation. For a more in-depth explanation of the role of humidity, we refer the reader to a recently published review by Sohn et al.⁴⁴⁹

In 2015, Wang et al. reported an ionic Seebeck effect in the mixed ion-electron conducting polymer PEDOT:PSS.⁴⁵⁰ It was found that the Seebeck coefficient of PEDOT:PSS increased from 10 µV K⁻¹ to 215 µV K⁻¹ with increasing ambient humidity from 10 to 80% RH, accompanied by a dramatic enhancement in the ionic conductivity. The resulting instantaneous power factor could then be increased by 4 orders of magnitude, but the fact that the thermogenerated current dropped with time indicated that the ionic contribution was not constant. Later, Ail et al. studied the mechanism underlying this phenomenon and observed that the thermodiffusion of protons in PEDOT:PSS, at high humidity, promoted an internal electrochemical reaction within the mixed ion-electron conductor film.⁴⁵¹ The time dependence of the open-circuit voltage V<sub>oc</sub>(t), after applying a thermal gradient, showed a peak that corresponded to the ionic Seebeck contribution, which then decreased until reaching a constant value at equilibrium; this behavior was ascribed to the electronic Seebeck effect. Therefore, the ionic Seebeck effect in mixed polymer conductors could not be used to produce much extra power in a TE module because its character vanishes with time. Instead, the ionic Seebeck effect was used to independently sense the humidity and temperature gradient with a three-parameter sensor (pressure, humidity, temperature) made of the polymer aerogel,⁴⁵² which exploits the ion thermodiffusion in high humidity environment to expand the sensing capability of
<figure><img src="image_67.png" /><figcaption>**Figure 53.** (a) Sketch of charge transport (ions and electrons) inside the PEDOT:PSS films at high (left) and low (right) humidity levels. (b) Ionic conductivity as a function of PSSH content and for different RH values (from 50% to 90%). (c) Voltage output vs time for different TE modules having different number of thermocouples, with the 20-leg device photo on the right. Reproduced with permission from ref 456. Copyright 2019 Wiley.</figcaption></figure>

<figure><img src="image_68.png" /><figcaption>**Figure 54.** (a) Schematic ion transport and morphology of PEDOT:PSS CuCl₂ films. (b) Seebeck coefficient vs CuCl₂ content (wt %) for 60% (black), 70% (red), and 80% (blue) of relative humidity. (c) Picture of the thermionic generator together with voltage output and temperature difference over time. Reproduced with permission from ref 457. Copyright 2019 Royal Society of Chemistry.</figcaption></figure>

previously published results.⁴⁵³,⁴⁵⁴ Other examples of temperature sensors based on the thermodiffusion of ions were presented by Zhao et al., using PEO-NaOH electrolyte to modulate the current of an electrolyte-gated organic transistor (with P3HT as the active material), enhancing the device sensitivity to small temperature variations.⁴⁵⁵ The high Seebeck values connected to ion transport in polymers represented interesting results and offered a new path toward the development of a new generation of thermal harvesters.

The first example of a stable thermionic effect in PEDOT:PSS films was reported in 2016, with the addition of polystyrene sulfonic acid (PSSH), showing an ionic conductivity of 9 S m⁻¹ and an ionic Seebeck coefficient of 7.9 mV K⁻¹ at 70% humidity.⁶⁶ Later on, Wang et al. studied the effect of humidity on the ionic mobility and ionic Seebeck coefficient of polystyrene sulfonate sodium (PSS⁻Na⁺), in which PSSNa served as a polyanionic membrane because the negative charges

(sulfonate) were attached covalently to the polymer chain and effectively immobile, while the cations were the only mobile species. When the humidity was increased from 50% to 100%, the ionic Seebeck coefficient increased from 0.26 mV K⁻¹ to 4 mV K⁻¹.⁶⁴ In 2018, Kim et al. expanded these results by reporting a mixed polymer film of PEDOT:PSS doped with PSSH, using a PEDOT:PSS formulation with higher PSS content (1:6) and PSSH possessing a similar molecular weight (M~w~) to that of the PSS. In this case, the Seebeck coefficient reached 16 mV K⁻¹ and remained constant over time, with a conductivity of 29 S m⁻¹ observed at 90% relative humidity.⁴⁵⁶

The addition of PSSH acted as a proton dopant by creating PSS-rich domains, where H⁺ ions could move freely under high humidity conditions (Figure 53a,b). Finally, they demonstrated that the high thermionic properties remained constant under ambient conditions by using a hydrogel bilayer architecture for the thermoelectric generator to maintain high humidity levels,
thus allowing the exploitation of the maximum harvesting capacity of such material.²⁹⁰

In 2019, the same group reported the conversion of thermionic properties of PEDOT:PSS from p- to n-type when in the presence of copper chloride (CuCl₂) salts.⁴⁵⁷ Through metal binding with the PSS domains, a Cl⁻ channel could be observed inside the film, leading to an S of ~-18 mV K⁻¹ and a PF of ~1.7 mW m⁻¹ K⁻² at 80% RH (Figure 54a,b). In addition, they integrated their p- and n-type thermionic materials in a generator composed of 10 thermocouples, achieving 1.55 V at an external temperature difference of 4.5 K and with an RH of 80% (Figure 54c).

However, the above method was not the only route followed to increase the time response of the ionic contribution, thus harvesting constant power. Chang et al. added Ag cations (using Ag-based salts) in Nafion (Ag-Nafion) and polystyrenesulfonate (Ag-PSS),⁴⁵⁸ which, coupled with Ag electrodes, enabled a thermogalvanic effect, producing an additional constant thermogenerated current on top of the nonconstant ionic Seebeck effect over time. Figure 55b shows the values of the Seebeck coefficient corresponding to two different blends: a positive Seebeck coefficient of approximately 5 mV K⁻¹ for the 100% Ag⁺/SO₃⁻ molar ratio in PSS and a maximum negative S of -1.5 mV K⁻¹ for silver nitrate (AgNO₃) in Nafion. Regarding the ion conductivity (Figure 55b), a maximum was instead obtained for low Ag⁺ concentrations (33%), corresponding to 2.7 mS cm⁻¹ for Ag-PSS and 4.3 mS cm⁻¹ for Ag-Nafion. Because the ionic conductivity of Nafion is extremely humidity dependent due to water-assisted ion transport, Chang et al. investigated the effects of humidity on the conductivity and Seebeck coefficient of these materials (Figure 55d,e). The Ag-Nafion ionic conductivity increased exponentially by more than 3 orders of magnitude, while the Seebeck coefficient switched from being positive to negative and reached a plateau at -2 mV K⁻¹ for RH >50%. On the other hand, for Ag-PSS, S decreased but remained positive up to a value of 1 mV K⁻¹. Finally, they constructed a flexible device (Figure 55f) with six thermionic legs connected by silver paste, showing a maximum of 7 mV at a ΔT = 4 K and an RH of 50%.

Another important feature of these materials, apart from the Seebeck coefficient and electrical/thermal conductivity, is the stability of such properties after mechanical stress; indeed, brittle materials are not compatible for use in flexible and conformable electronics. Therefore, the development of composites can offer an effective way to combine the properties of different components in a single material to further broaden their applications in flexible and wearable electronics. Jiao et al. recently prepared robust and flexible ionic thermoelectric paper by a simple and cost-effective method.⁴⁵⁹ In contrast to the extremely brittle pure polystyrene sulfonate sodium (PSSNa) film, the nanofibrillated cellulose (NFC)-PSSNa composite paper (Figure 56a) exhibited a tensile strength of 16.6 ± 1.5 MPa and a Young's modulus of 0.9 ± 0.1 GPa. Remarkably, the NFC-PSSNa paper retained high ionic thermoelectric properties (Figure 56c), with an overall figure-of-merit of 0.025 at 100 RH % and room temperature, which was slightly higher than that of pure PSSNa.

While polyelectrolytes display a large ionic Seebeck coefficient in humid environments, the drawback of using water to enable ionic thermodiffusion is that water evaporates, thus, there is a small temperature window for operation. Instead, polymer electrolytes have also been investigated for their ionic thermoelectric properties. Zhao et al. showed a low molecular

<figure><img src="image_69.png" /><figcaption>**Figure 55.** (a) Chemical structures of Ag-PSS (black box) and AgNafion (red box). (b) Ionic conductivity and (c) Seebeck coefficient of PSS and Nafion with the addition of different molar ratios of Ag salts. (d) Ionic conductivity and (e) Seebeck coefficient of Ag-PSS and Ag-Nafion with increasing RH values. (f) Picture (left) and voltage output (middle) of a flexible p-n device as a function of different external temperature differences, and the module thermopower (right) as a function of the number of pairs of p-n components. Reproduced with permission from ref 458. Copyright 2016 American Chemical Society.</figcaption></figure>

weight polyethylene glycol (PEG) with NaOH possessing a positive Seebeck coefficient of 10 mV K⁻¹.²⁴ Recently, Li et al. impregnated porous aligned cellulose fibers with PEG-NaOH and reported an even higher Seebeck coefficient reaching 24 mV K⁻¹.⁴⁶⁰ It was also found that poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) blended with the ionic liquid 1-ethyl-3-methylimidazolium ([EMIM]) bis(trifluoromethylsulfonyl)imide ([TFSI]) (Figure 56d) had a negative Seebeck coefficient of -4 mV K⁻¹ (Figure 56e), while the addition of low molecular weight PEG switched the sign of the Seebeck coefficient to +12 mV K⁻¹, which enabled the authors to screen print the first ionic thermopile on a flexible polymide substrate (Figure 56e,f).⁴⁶¹

The impressive values of S linked to ionic charge carries are unfortunately counterbalanced by the low charge conductivity, usually in the order of mS cm⁻¹, which severely limits the power
<figure><img src="image_70.png" /><figcaption>**Figure 56.** (a) Photo and structure of the NFC-PSSNa composite films. (b) AFM images of the NFC-PSSNa film. (c) Seebeck coefficient of NFC-PSSNa at various humidity levels and a comparison with pure PSSNa. (a–c) Reproduced with permission from ref 459. Copyright 2017 The Royal Society of Chemistry. (d) Composition of [EMIM][TFSI]/PVDFHFP polymer gels. Organic thermionic generator printed on a flexible substrate (e) and the voltage output for small temperature differences (f). (d–f) Reproduced with permission from ref 461. Copyright 2019 Nature.</figcaption></figure>

that can be extracted. Recently, Jiang et al. tried to circumvent this issue by proposing a mixed ion-electron n-type organic conductor with a highly crystalline and reduced perylene bisimide.⁴⁶² This material presented a $S > 3$ mV K⁻¹ due to the presence of quasi-frozen ionic charges, while the charge transport is mediated by electrons yielding a conductivity as high as 0.18 S cm⁻¹, an impressive value for an n-type organic semiconductor. The efficient combination of ion-electron transport allowed to yield a PF as high as 165 µW m⁻¹ K⁻² and a zT = 0.23 at room temperature. Note that the high PF values were calculated using S (generated by the thermodiffusion of ion) and the electronic component of σ. While this approach might look unusual because of the mixed ion-electron contribution, the authors reported stable power output in a condition of matched resistance, compatible with the conductivity of electronic charge carriers. At the same time, the thermal voltage was generated by ionic contribution, as verified by the authors. Therefore, because PF is a parameter that indicates the capability of a material to generate continuous power output thermally, we believe this approach to be suitable to calculate PF.

## 4.4. Materials for Thermogalvanic Cells (TGCs)

**4.4.1. Redox Couples.** Another thermodiffusion phenomenon is the thermogalvanic effect. Here, the Seebeck coefficient is proportional to the entropy change of the redox reaction. This reaction has an entropic contribution from the redox molecule itself but also from the solvation shell, which is sensitive to the interactions between the redox molecule and the solvent. Redox couples considered for TGC applications are comprised of inorganic, organic, and radioactive redox couples in aqueous or nonaqueous electrolytes, and they display either positive or negative values of the Seebeck coefficient. Regarding aqueous electrolytes, the highest values of $S_{\text{redox}}$ have been reported as -1.4 to -1.5 mV K⁻¹ for $\text{Fe(CN)}_6^{3-}/\text{Fe(CN)}_6^{4-}$, 54,62,463 0.5-0.7 mV K⁻¹ for $\text{Fe}^{3+}/\text{Fe}^{2+}$, 464,465 0.55 mV K⁻¹ for $\text{I}_3^-/\text{I}^-$, 466 1.87 mV K⁻¹ for $\text{Np}^{4+}/\text{Np}^{3+}$, and -1.53 mV K⁻¹ for $\text{Pu}^{4+}/\text{Pu}^{3+}$. In

nonaqueous electrolytes, the largest Seebeck coefficient recorded is for cobalt(II/III) tris(bipyridyl) (-2.19 mV K⁻¹).

Among redox-active electrolytes, ferro/ferrocyanide [Fe(CN)₆³⁻/Fe(CN)₆⁴⁻] has been extensively studied for TGCs due to its high solubility and stability in water, its high exchange current density, and relatively high Seebeck coefficient of 1.4 mV K⁻¹. However, the Seebeck coefficient of ferro/ferrocyanide also depends weakly on the concentration of the redox couple, with maximum values as high as 1.72 mV K⁻¹ reported for 5 mM solutions.⁴⁶⁷ Unfortunately, power generation is too small at these low concentrations. Studies have shown that the Seebeck coefficient remains constant for redox couple concentrations above 0.1 M and a concentration of up to 0.4 M can be used for stable TGC applications.⁴⁶⁷ A high concentration (0.9 M) of ferro/ferricyanide in aqueous solution can be reached by replacing the conventional $\text{K}_4\text{Fe(CN)}_6/\text{K}_3\text{Fe(CN)}_6$ with $(\text{NH}_4)_4\text{Fe(CN)}_6/\text{K}_3\text{Fe(CN)}_6$.⁴⁶⁸ While the Seebeck coefficients are not significantly modified (1.43 mV K⁻¹), the increase in concentration from 0.4 to 0.9 M is accompanied by a 66% enhancement in the ionic conductivity and a minor decrease in the thermal conductivity of the electrolyte. The combination of these effects results in a 50% improvement in the generated power.

Chaotropic agents, molecules that disrupt the hydrogen bonding network of water, can have a significant impact on the entropy (and thus thermopower) of thermogalvanic reactions. Duan et al. introduced strong chaotropic cations (guanidinium) and highly soluble amide derivatives (urea) into aqueous [Fe(CN)₆³⁻/Fe(CN)₆⁴⁻] electrolytes to significantly boost the thermopowers from 1.4 to 4.2 mV K⁻¹.⁴⁶⁹ [Fe(CN)₆³⁻/Fe(CN)₆⁴⁻] are categorized as chaotropic anions in the Hofmeister series and can bond with chaotropic cations based on chaotrope-chaotrope ion specificity.⁴⁷⁰ Therefore, strong chaotropic cations can be expected to rearrange the solvation shells of the redox couple and change the entropy of the redox reaction, thereby affecting the Seebeck coefficient. By introducing guanidinium chloride (GdmCl) into the ferro/
<figure><img src="image_71.png" /><figcaption>**Figure 57.** (a) Seebeck coefficients of the solidified electrolytes and aqueous solution. (b) Diffusion coefficients of $[Fe(CN)6]^{4-}$ and $[Fe(CN)6]^{3-}$ in the solid electrolytes and the solution at 22 °C. Reproduced with permission from ref 476. Copyright 2016 American Chemical Society.</figcaption></figure>

ferricyanide aqueous electrolyte, the Seebeck coefficient increased up to 2.7 mV K⁻¹. Urea is known to strongly impact the hydrogen-bonding network and hydration shell of ions in water.⁴⁶⁹ An extremely high Seebeck coefficient value of ~4.2 mV K⁻¹ is achieved when adding urea (24 M) and GdmCl (2.6 M) simultaneously.

Recently, a few groups have demonstrated TGC applications of ferro/ferricyanide in a mixture of aqueous/nonaqueous solvents. When ferro/ferricyanide is used in a mixture of an organic solvent, such as methanol or DMSO, with water, it shows a significant improvement in the Seebeck coefficient (2.9 mV K⁻¹).⁴⁷¹ However, the solubility is affected, and the ionic conductivity decreases from approximately 180 mS cm⁻¹ to 50 mS cm⁻¹. Overall, there is a positive effect of the mixing of solvent, with an area power density of 0.64 mW m⁻² K⁻² for the TGC, which is higher than the 0.35 mW m⁻² K⁻² with an aqueous electrolyte at a ΔT of 4.1 °C.

The iron(II)/(III) system is yet another well-known redox couple for thermogalvanic applications and was tested at the early stage of thermogalvanic research. Iron(II)/(III) systems have not been investigated in detail due to their low Seebeck coefficient (0.54 mV K⁻¹) at high ionic concentrations compared to the $[Fe(CN)_6^{3-}/Fe(CN)_6^{4-}]$ system. However, while high Seebeck values of 1.8 mV K⁻¹ have been reported for aqueous iron (II)/(III) systems at very low ionic concentrations, above 0.2 M these values decrease rapidly. A high-power TGC requires more concentrated redox molecules. Among the studied iron(II)/(III) systems, aqueous chlorides and sulfate systems have been studied the most. Indeed, regarding aqueous iron(II/III) chloride systems, Burrows reported a Seebeck value of +0.6 mV K⁻¹ for aqueous $FeCl_2/FeCl_3$ in the presence of HCl.⁴⁷² On the other hand, with regard to iron(II/III) sulfate systems, Al Maimani et al. reported Seebeck values in the range of +0.24 to +0.40 mV K⁻¹ at different concentrations (1.4 M Fe(II)/Fe(III) to 0.0014 M Fe(II)/Fe(III), respectively), which could be further increased to +0.54 mV K⁻¹ with the addition of $H_2SO_4$⁴⁷³ clearly showing the influence of electrolyte pH on the Seebeck coefficient of the iron(II)/(III) systems.

This dependence was further proven by Buckingham et al., who reported a Seebeck coefficient of +1.46 mV K⁻¹ for acidified iron(II/III) trifluoromethanesulfonate at ΔT = 20 K.⁴⁷⁴ In this report, they studied four aqueous Fe(II)/Fe(III) salt systems that were based on 0.2 M ammonium iron sulfate, iron sulfate, iron trifluoromethanesulfonate, and iron nitrate, and the Seebeck coefficient varied from +0.13, +0.29, +1.35, and +1.34 mV K⁻¹, respectively. The value of the Seebeck coefficient was further enhanced in the presence of 1 M conjugate acid, reaching values of +0.84, +0.90, +1.46, and +1.38 mV K⁻¹ for acidified

ammonium iron sulfate, iron sulfate, iron trifluoromethanesulfonate, and iron nitrate, respectively. The addition of 1 M conjugate acid to the systems resulted in significant increases in the Seebeck coefficients of both $Fe[SO_4]^{-}$ and $[NH_4]FeSO_4$ based systems, while only slightly increasing the Seebeck coefficients of the $FeNO_3$ and $FeCF_3SO_3$ systems. In this report, this observation was rationalized in terms of acid-base equilibrium and ion pairing.

Because of the liquid nature of the electrolytes, the application of TGCs in flexible and wearable devices is limited, mainly due to the potential leakage of the electrolyte. Recently, there have been a few attempts to fabricate TGCs using ferro/ferrocyanide redox couples in polymer gels to eliminate electrolyte leakage and to support the development of flexible TGCs.²³,⁴⁷⁵ Jin et al. reported a ferric/ferrocyanide redox couple within a cellulose matrix.⁴⁷⁶ Figure 57 displays the effect of the addition of cellulose on the Seebeck coefficient and the diffusion coefficient of the redox couples in gel. The introduction of 5% cellulose in the electrolyte barely affected the Seebeck coefficient (−1.38 mV K⁻¹, Figure 57a) but provided decent mechanical properties and a maximum power output of 14 mW m⁻² (compared to 18 mW m⁻² without cellulose). Thus, modules of TGCs based on legs composed of two types of gel redox electrolytes with positive and negative Seebeck coefficients were envisioned to power wearable electronics.²³ Poly(vinyl alcohol) (PVA) was used as the polymer gel matrix, yielding Seebeck coefficients as high as 1.02 mV K⁻¹ and −1.21 mV K⁻¹ when used in combination with ferric/ferrous chloride and ferro/ferricyanide, respectively.

The second most studied redox couple in thermogalvanics is iodide/triiodide. Its Seebeck coefficient depends on the nature of the solvent (aqueous/organic), the concentration of the electrolyte, and the presence of additives. As an additive, Zhou et al. proposed the addition of α-cyclodextrin in an aqueous iodide/triiodide redox electrolyte and reported a maximum Seebeck coefficient of 2 mV K⁻¹.⁴⁷⁷ The proposed hypothesis was that α-cyclodextrin encapsulated triiodide ions and affected their solvation entropy. Abraham et al. studied the thermogalvanic properties of the iodide/triiodide redox couple in various ionic liquids.⁴⁷⁸ Typically, when switching from water to ionic liquids, the Seebeck coefficient decreased. The main advantage of ionic liquids compared to aqueous electrolytes was that a larger temperature gradient could be utilized in thermogalvanic cells with temperatures at the hot side reaching 400 °C. Interestingly, changing the anionic component of the ionic liquid, e.g., from 1-butyl-3-methylimidazolium tetrafluoroborate to 1-butyl-3-methylimidazolium iodide, led to a change in the sign of the Seebeck coefficient.⁴⁶⁶,⁴⁷⁷,⁴⁷⁹ TGCs based on
iodide/triiodide in ionic liquids could reach a power output of 29 mW m⁻² when the hot side was at 150 °C.⁴⁸⁰

A cobalt redox couple is promising for TGC applications, as it displays a relatively high Seebeck coefficient in nonaqueous electrolytes compared to iodide/triiodide.⁵⁵,⁴⁷⁸,⁴⁸¹,⁴⁸² The first demonstration of cobalt redox couple-based TGCs made use of a Co²⁺/³⁺(bpy)₃(NTf₂)₂/₃ couple in combination with a series of ionic liquids and 3-methoxypropionitrile (MPN).⁴⁷⁸ These electrolytes showed high Seebeck coefficients ranging from 1.5 to 2.2 mV K⁻¹ in the temperature range of 20–140 °C. The highest Seebeck coefficient value of 2.19 mV K⁻¹ was obtained with the 0.01 M Co²⁺/³⁺(bpy)₃(NTf₂)₂/₃ couple in MPN and 1.88 mV K⁻¹ with the ionic liquid ([C2mim][eFAP]). These values of the cobalt redox couple were attributed to the additional electronic entropy component of the total entropy change.⁴⁸³ The power output of TGCs reached 522 mW m⁻² for the 0.1 M Co²⁺/³⁺ (bpy)₃(NTf₂)₂/₃ redox couple in MPN and 880 mW m⁻² at ΔT = 70 °C with an electrolyte composition of 3:1 (v/v) MPN-[C2mim][B(CN)₄].⁴⁷⁸

Regarding the development of solid or quasi-solid-state electrolytes, Taheri et al. reported gelation of an organic solvent-based electrolyte system containing a cobalt bipyridyl redox couple for application in thermogalvanic cells.⁴⁸⁴ Polyvinylidene difluoride (PVDF) and PVDF-HFP were used for gelation of the liquid electrolyte (Figure 58a,b), which was consistent with a cobalt bipyridyl redox couple dissolved in 3-methoxypropionitrile (MPN). Cobalt bipyridyl redox couples

<figure><img src="image_72.png" /><figcaption>**Figure 58.** Quasi-solid-state electrolytes containing 0.05 M [Co(bpy)₃]²⁺/³⁺[NTf₂]₂/₃ in MPN gelled with (a) 5 wt% PVDF and (b) 5 wt% PVDF-HFP. (c) Electrochemical behavior of the redox electrolytes before and after gelation measured using a three-electrode cell equipped with a platinum working electrode and two platinum wires as the counter and pseudoreference electrodes at a scan rate of 50 mV s⁻¹. Reproduced with permission from ref 484. Copyright 2018 European Chemical Society.</figcaption></figure>

are known to display high Seebeck coefficients; thus, the use of cobalt redox couples in a quasi-solid-state electrolyte could allow access to higher Seebeck coefficients. Interestingly, the addition of PVDF or PVDF-HFP to the cobalt bipyridyl redox electrolyte did not significantly affect the Seebeck coefficient. Indeed, the presence of solvent interactions with redox molecules affected the Seebeck coefficient because of changes in the entropy and free energy.⁴⁸³,⁴⁸⁵,⁴⁸⁶ In this report, the absence of any strong interaction between the polymer chains and redox molecules resulted in a negligible effect on the entropy change in the redox reaction, thus not affecting the Seebeck coefficient.⁴⁸⁴ The diffusivity of the redox couple in the gel electrolyte was the key to TGC operation, and in this report, the study on the effect of gelation, investigated using chronoamperometry, revealed that gelation did not cause a significant decrease in the diffusion coefficient (Table 3). Furthermore, this observation was

<table><caption>**Table 3.** Effects of the Gelation of the Electrolyte on the Seebeck Coefficient (S) and Diffusion Coefficient (D) at 25 °C</caption><thead><tr><th rowspan="2">electrolyte</th><th rowspan="2">S (mV K<sup>-1</sup>)</th><th colspan="2">D (10<sup>6</sup>cm<sup>2</sup>s<sup>-1</sup>)</th></tr><tr><th>Co(bpy)<sub>3</sub><sup>2+</sup></th><th>Co(bpy)<sub>3</sub><sup>3+</sup></th></tr></thead><tbody><tr><td>MPN</td><td>1.81 ± 0.03</td><td>5.00 ± 0.39</td><td>4.90 ± 0.27</td></tr><tr><td>5 wt % PVDF-MPN</td><td>1.80 ± 0.02</td><td>4.63 ± 0.13</td><td>4.21 ± 0.09</td></tr><tr><td>5 wt % PVDF-HFP-MPN</td><td>1.84 ± 0.01</td><td>4.48 ± 0.15</td><td>3.55 ± 0.25</td></tr></tbody></table>

supported by cyclic voltammetry (Figure 58c), which showed that the redox couple in the gelled electrolytes maintained quasi-reversible electrochemical behavior.

Maximum power densities of 6 and 4.5 mW m⁻² have been obtained for PVDF and PVDF-HFP gel electrolytes, respectively, compared to 48 mW m⁻² for a liquid MPN-based electrolyte at the same temperature difference. The decrease in thermogalvanic cell power with the gel electrolyte is mainly due to the limited convection in the device, which decreases mass transport. A free-standing cellulose-based electrolyte, prepared by immersing a cellulose membrane (Figure 59) in a liquid electrolyte, was also reported for use in thermogalvanic cells.⁴⁸⁷

Three different redox systems were studied: [Co(bpy)₃]Cl₂/₃ in water, [Co(bpy)₃][NTf₂]₂/₃ in MPN, and K₃/₄Fe(CN)₆ in water. The Seebeck coefficient of the water-soluble cobalt redox couple decreased by one-third (1/3) compared to that of the cobalt redox couple in the organic electrolyte. The Seebeck coefficient of the three different redox systems was not significantly affected in the free-standing cellulose electrolytes; however, the diffusion coefficient decreased by ~20% and ~50% for the K₃/₄Fe(CN)₆ and [Co(bpy)₃] redox systems, respectively (see Table 4). As a result, the performance of the thermocells decreased after solidification, but the performance was improved by increasing the redox couple concentration. The highest power densities of 168 and 24 mW m⁻² were produced using cellulose/0.4 M aqueous K₃/₄Fe(CN)₆³⁻/₄⁻ or cellulose/0.1 M [Co(bpy)₃][NTf₂]₂/₃ in MPN, respectively (Table 4).

Al-Masri et al. studied the influence of the nature of the redox couple on the Seebeck coefficient and further investigated the diffusion coefficient and charge transfer resistance of different redox couples.⁴⁸⁸ In their paper, they chose four different cobalt complexes containing the ligands 2-(1H-pyrazol-1-yl)pyridine) (Co²⁺/³⁺(py-pz)₃), 2-(1H-pyrazol-1-yl)-4-tert-butylpyridine (Co²⁺/³⁺(bupy-pz)₃), 2,6-di(1H-pyrazol-1-yl)pyridine (Co²⁺/³⁺(pz-py-pz)₂), and 1,10-phenanthroline (Co²⁺/³⁺(phen)₃) in a 3:1 dimethyl sulfoxide:1-ethyl-3-methyl-
<figure><img src="image_73.png" /><figcaption>**Figure 59.** Cellulose-based quasi-solid-state electrolytes: (a) before immersion in an electrolyte (containing water without a redox couple) and after immersion in (b) 0.05 M [Co(bpy)₃]Cl₂/₃ in water, (c) 0.05 M [Co(bpy)₃][NTf₂]₂/₃ in MPN, and (d) 0.05 M K₃/₄Fe(CN)₆ in water. Reproduced with permission from ref 487. Copyright 2019 Elsevier Ltd.</figcaption></figure>

<table><caption>**Table 4.** Effect of the Solidification of the Electrolyte on the Seebeck Coefficient and Diffusion Coefficient of [Co(bpy)₃]²⁺/³⁺ and [Fe(CN)₆]³⁻/₄⁻ Ions</caption><thead><tr><th rowspan="2">electrolyte</th><th rowspan="2">Seebeck coefficient<br>(mV K<sup>-1</sup>)</th><th rowspan="2">CV peak-to-peak potential<br>separation (mV)</th><th colspan="2">diffusion coefficient at 25 °C (D × 10<sup>6</sup> cm<sup>2</sup> s<sup>-1</sup>)</th></tr><tr><th>[Co(bpy)₃]<sup>2+</sup> or<br>[Fe(CN)₆]<sup>4-</sup></th><th>[Co(bpy)₃]<sup>3+</sup> or<br>[Fe(CN)₆]<sup>3-</sup></th></tr></thead><tbody><tr><td>0.05 M [Co(bpy)₃]Cl<sup>2/3</sup> in water</td><td>1.21 ± 0.08</td><td>159</td><td>3.18 ± 0.08</td><td>5.90 ± 0.07</td></tr><tr><td>cellulose/0.05 M [Co(bpy)₃]Cl<sup>2/3</sup> in<br>water</td><td>1.15 ± 0.06</td><td>114</td><td>1.86 ± 0.10</td><td>2.95 ± 0.20</td></tr><tr><td>0.05 M [Co(bpy)₃][NTf₂]<sup>2/3</sup> in MPN</td><td>1.81 ± 0.03</td><td>380</td><td>5.00 ± 0.39</td><td>54.90 ± 0.27</td></tr><tr><td>cellulose/0.05 M [Co(bpy)₃][NTf₂]<sup>2/3</sup> in<br>MPN</td><td>1.83 ± 0.01</td><td>254</td><td>3.65 ± 0.04</td><td>3.43 ± 0.21</td></tr><tr><td>0.05 M K₃/₄Fe(CN)₆ in water</td><td>-1.55 ± 0.04</td><td>153</td><td>10.51 ± 0.31</td><td>10.32 ± 0.13</td></tr><tr><td>cellulose/0.05 M K₃/₄Fe(CN)₆ in water</td><td>-1.44 ± 0.03</td><td>142</td><td>8.52 ± 0.02</td><td>8.89 ± 0.08</td></tr></tbody></table>

<table><caption>**Table 5.** Seebeck Coefficients (S) of the Redox Couples Measured in 3:1 DMSO:[C₂mim][eFAP] or 3:1 PC:[C₂mim][eFAP] at 0.1 M, Plus the Calculated Radius of Each Complex</caption><thead><tr><th>redox couple</th><th>solvent mixture</th><th>S (mV K<sup>-1</sup>) ± 0.02</th><th>Co<sup>2+</sup> complex radius ± 0.03 (Å)</th></tr></thead><tbody><tr><td>Co<sup>2+/3+</sup>(py-pz)<sub>3</sub></td><td>3:1 DMSO:[C<sub>2</sub>mim][eFAP]</td><td>2.36</td><td>5.76</td></tr><tr><td>Co<sup>2+/3+</sup>(bupy-pz)<sub>3</sub></td><td>3:1 DMSO:[C<sub>2</sub>mim][eFAP]</td><td>2.23</td><td>6.73</td></tr><tr><td>Co<sup>2+/3+</sup>(pz-py-pz)<sub>3</sub></td><td>3:1 PC:[C<sub>2</sub>mim][eFAP]</td><td>1.78</td><td>6.31</td></tr><tr><td>Co<sup>2+/3+</sup>(phen)<sub>3</sub></td><td>3:1 DMSO:[C<sub>2</sub>mim][eFAP]</td><td>1.70</td><td>6.10</td></tr><tr><td>Co<sup>2+/3+</sup>(bpy)<sub>3</sub></td><td>3:1 DMSO:[C<sub>2</sub>mim][eFAP]</td><td>2.00</td><td>5.75</td></tr><tr><td>Co<sup>2+/3+</sup>(bpy)<sub>3</sub></td><td>3:1 PC:[C<sub>2</sub>mim][eFAP]</td><td>1.84</td><td>5.75</td></tr></tbody></table>

<figure><img src="image_74.png" /><figcaption>**Figure 60.** (a) Schematic of an acetone–2-propanol thermocell. (b) Open-circuit voltage (V<sub>oc</sub>) between the hot-side electrode and cold-side electrode in the acetone–2-propanol thermocell. The cold-side temperature was maintained at 20–23 °C. The heating of the gas bridge allows acetone vapor to transfer from the hot side to the cold side and increases the V<sub>oc</sub>. Without heating the gas bridge, acetone vapor condenses at the neck of the hot-side cell and continues to reflux inside. Average heating rate = 1 °C/min. Reproduced with permission from ref 489. Copyright 2018 American Chemical Society.</figcaption></figure>

imidazolium tris(pentafluoroethyl) trifluorophosphate mixture. The highest Seebeck coefficient was measured for Co²⁺/³⁺(py-pz)₃ (2.36 mV K⁻¹), which was related to a combination of its small radius, bidenticity, and lower degree of aromaticity compared to the other listed ligands in Table 5.

Recently, Zhou et al. were the first to demonstrate a TGC based on the redox couple of acetone and 2-propanol, reporting the highest Seebeck coefficient of –9.9 mV K⁻¹ when the hot side was above the boiling point of acetone.⁴⁸⁹ However, in the temperature range (20–60 °C) below the boiling point of
<figure><img src="image_75.png" /><figcaption>**Figure 61.** (a) Comparison of the thermopower among the as-fabricated i-TE materials of gelatin-x KCl-m/n FeCN<sup>4-/-3-</sup> (x and m/n are the KCl and K<sub>4</sub>Fe(CN)<sub>6</sub>/K<sub>3</sub>Fe(CN)<sub>6</sub> molar concentrations, respectively) in this work as gelatin (x = 0 M, m/n = 0 M), gelatin-FeCN<sup>4-/-3-</sup> (x = 0 M, m/n = 0.42/0.25 M), gelatin-KCl (x = 0.8 M, m/n = 0 M), and gelatin-KCl-FeCN<sup>4-/-3-</sup> (x = 0.8 M, m/n = 0.42/0.25 M, volume ratio of water to gelatin r<sub>v</sub> = 2.0 and 3.0). (b) Absolute thermopower of the i-TE materials containing the thermodiffusion effect or the thermogalvanic effect. The filled and unfilled columns represent p-type and n-type thermopower, respectively. (c) Thermopower of the i-TE materials of gelatin-x KCl, gelatin-x KNO<sub>3</sub>, and gelatin-x NaCl at varying concentrations of KCl, KNO<sub>3</sub>, and NaCl. (d) Thermopower of the i-TE materials of gelatin-x KCl, at various pH values tuned by HCl and KOH. (e) Thermopower of the i-TE materials of gelatin-x KCl-m/n FeCN<sup>4-/-3-</sup> at x = 0.8 M. (f) Thermopower dependence of the volume ratio of water to gelatin for gelatin-0.8 M KCl-0.42/0.25 M FeCN<sup>4-/-3-</sup>, r<sub>v</sub> = 2.0 was maintained in (c–e). Reproduced with permission from ref 491. Copyright 2020 American Association for the Advancement of Science.</figcaption></figure>

acetone, the reported Seebeck coefficient was -3.4 mV K⁻¹. Above 60 °C, the Seebeck coefficient increased by a factor of 3 and reached its highest reported value due to the phase change of acetone from the liquid to the gas state. In addition, heating of the gas bridge that connected the hot-side and cold-side cells promoted an increase in the Seebeck coefficient due to the increase in the mass transfer of acetone vapor (see Figure 60). In that report, a Pt-Sn catalyst electrodeposited on a Pt substrate was used as the electrode for two reasons: (1) the reduction of acetone to 2-propanol on platinum catalysts had not yet been reported⁴⁸⁹ and (2) the oxidation of isopropanol on a Pt-Sn catalyst was usually conducted in strongly acidic solutions. However, when acetone was heated under strongly acidic conditions, self-condensation readily occurred to produce phorone and acetone-derived polymers.⁴⁹⁰

Han et al. recently demonstrated a giant positive Seebeck coefficient in ionic gelatin-based thermoelectric materials in which they synergistically combined the thermodiffusion of ions (KCl, NaCl, KNO₃) and the thermogalvanic effect of the ferro

cyanide redox couple, as shown in Figure 61.⁴⁹¹ A high Seebeck coefficient of 12.7 mV K⁻¹ was obtained for a flexible, quasi-solid-state, ionic thermoelectric material of gelatin-x KCl-m/n Fe(CN)⁴⁻/³⁻ (x = 0.8 M and m/n = 0.42/0.25), where x and m/n are the molar concentrations of KCl and K₄Fe(CN)₆/K₃Fe(CN)₆, respectively. Finally, they further increased the Seebeck coefficient from 12.7 to 17 mV K⁻¹ by optimizing the volume ratio of water to gelatin.

### 4.4.2. Electrode Materials.
Platinum electrodes have been studied for TGC applications due to their absence of an oxide layer and their high electrochemical activity.⁴⁹²⁻⁴⁹⁴ However, the wide application of TGCs with platinum electrodes is limited simply due to the cost of the material, which promotes investigating alternative electrode materials. Metal-free materials, such as carbon and conducting polymers, have recently emerged as efficient electrode materials for TGCs.⁶²,⁴⁶³,⁴⁹⁵,⁴⁹⁶ Among carbon-based materials, carbon nanostructures, such as carbon fibers, CNTs, graphite, and graphene, have displayed higher performance than platinum electrodes because of their
<table><caption>Table 6. Performance of TGCs with Different Combinations of Electrodes and Electrolytes</caption><thead><tr><th>electrode</th><th>electrolyte</th><th>S (mV K<sup>-1</sup>)</th><th>ΔT (K)</th><th>P<sub>max</sub> (W m<sup>-2</sup>)</th><th>η</th><th>η<sub>c</sub></th></tr></thead><tbody><tr><td>Pt</td><td>0.26 M Fe(CN)<sub>6</sub><sup>3-</sup>/Fe(CN)<sub>6</sub><sup>4-</sup> 0.8 M KC</td><td>-1.50</td><td>20</td><td></td><td>0.031</td><td>0.500</td></tr><tr><td>MWCNT buckypaper</td><td>0.4 M Fe(CN)<sub>6</sub><sup>3-</sup>/Fe(CN)<sub>6</sub><sup>4-</sup></td><td>-1.40</td><td>60</td><td>1.800</td><td></td><td>1.400</td></tr><tr><td>SWCNT</td><td>0.2 M Fe(CN)<sub>6</sub><sup>3-</sup>/Fe(CN)<sub>6</sub><sup>4-</sup></td><td></td><td>20</td><td></td><td></td><td></td></tr><tr><td>Pt</td><td>0.4 M I<sup>-</sup>/I<sup>3-</sup> in [C2mim][BF4]</td><td>0.23</td><td>70</td><td>0.029</td><td></td><td></td></tr><tr><td>Pt</td><td>0.1 M Co<sup>2+</sup>/3+ (bpy)<sub>3</sub>(NTf<sub>2</sub>)<sub>2/3</sub></td><td>2.19</td><td>70</td><td>0.499</td><td></td><td></td></tr><tr><td>CNT/RGO composites</td><td>0.4 M Fe(CN)<sub>6</sub><sup>3-</sup>/Fe(CN)<sub>6</sub><sup>4-</sup></td><td>-1.40</td><td>31</td><td>1.850</td><td>0.218</td><td>2.630</td></tr><tr><td>Pt Black</td><td>0.1 M Con(bpy)<sub>3</sub>(NTf<sub>2</sub>)<sub>n</sub> in [C2mim][B(CN)<sub>4</sub>]</td><td>1.80</td><td>70</td><td>0.240</td><td></td><td></td></tr><tr><td>CNT/carbon fabric</td><td>0.4 M Fe(CN)<sub>6</sub><sup>3-</sup>/Fe(CN)<sub>6</sub><sup>4-</sup></td><td>-1.40</td><td>31</td><td>0.460</td><td></td><td></td></tr><tr><td>Pt</td><td>30 mM [Fc]/[I<sup>3-</sup>]</td><td>0.81</td><td>50</td><td>2.4 × 10<sup>-7</sup></td><td></td><td></td></tr><tr><td>Pt/CNT</td><td>0.4 M Fe(CN)<sub>6</sub><sup>3-</sup>/Fe(CN)<sub>6</sub><sup>4-</sup></td><td>-1.43</td><td>51</td><td>6.600</td><td>0.560</td><td>3.950</td></tr><tr><td>Pt</td><td>0.4 M Fe(CN)<sub>6</sub><sup>3-</sup>/Fe(CN)<sub>6</sub><sup>4-</sup>/5% cellulose</td><td>-1.38</td><td>15</td><td>0.014</td><td></td><td></td></tr><tr><td>Pt</td><td>0.1 M Co<sup>2+</sup>/3+tris(bipyridyl)/MPN-[C2mim][B(CN)<sub>4</sub>]</td><td>2.01</td><td>70</td><td>0.880</td><td></td><td></td></tr><tr><td>Pt</td><td>4 mM α-CD-2,5 mM I<sup>-</sup>/10 mM I<sup>-</sup> in 200 mM KCl</td><td>1.90</td><td>34</td><td></td><td></td><td></td></tr><tr><td>Pt</td><td>0.4 M Fe(CN)<sub>6</sub><sup>3-</sup>/Fe(CN)<sub>6</sub><sup>4-</sup>/methanol</td><td>-2.90</td><td>4.1</td><td>0.011</td><td>0.003</td><td>0.024</td></tr><tr><td>PEDOT-Tos</td><td>0.4 M Fe(CN)<sub>6</sub><sup>3-</sup>/Fe(CN)<sub>6</sub><sup>4-</sup></td><td>-1.43</td><td>30</td><td>0.417</td><td>0.024</td><td>0.260</td></tr><tr><td>activated carbon cloth</td><td>0.9 M Fe(CN)<sub>6</sub><sup>3-</sup>/Fe(CN)<sub>6</sub><sup>4-</sup></td><td>-1.50</td><td>84</td><td>12.000</td><td>0.065</td><td>0.300</td></tr><tr><td>Au/Cr</td><td>0.1 M Fe(CN)<sub>6</sub><sup>3-</sup>/Fe(CN)<sub>6</sub><sup>4-</sup> and 0.1 M Fe<sup>3+</sup>/Fe<sup>2+</sup></td><td>-1.21 and 1.02</td><td>10</td><td>90 × 10<sup>-9</sup></td><td></td><td></td></tr><tr><td>PEDOT-PSS</td><td>0.4 M Fe(CN)<sub>6</sub><sup>3-</sup>/Fe(CN)<sub>6</sub><sup>4-</sup></td><td>-1.43</td><td>30</td><td>0.410</td><td>0.023</td><td>0.256</td></tr></tbody></table>

<figure><img src="image_76.png" /><figcaption>**Figure 62.** Performance of thermogalvanic cells with various PEDOT-Tos and platinum thicknesses. (a) Power output and current density (normalized with the geometric surface area) as a function of the device voltage. (b) Comparison of the maximum power for a cell with platinum electrode to cells with PEDOT-Tos electrodes of different thicknesses. Reproduced with permission from ref 496. Copyright 2017 Royal Society of Chemistry.</figcaption></figure>

high surface area.⁶¹,⁶²,⁴⁶⁸,⁴⁹⁷,⁴⁹⁸ A wide range of manufacturing techniques are available to fabricate different varieties of high surface area carbon materials. Most of the recent research on carbon-based electrode TGCs has focused on composite carbon materials, such as nanostructures and graphene. In addition, there are some demonstrations with additives and doping on carbon materials that show promising performance.⁵⁴,⁴⁹⁹

The first demonstration of TGCs comprised of CNT-based electrodes was reported in 2010.⁶¹ Well-ordered MWCNTs, prepared by chemical vapor deposition with acetylene as the carbon source, were used as the electrodes of a TGC composed of [Fe(CN)₆]⁴⁻ and [Fe(CN)₆]³⁻ as the redox couple, and this TGC showed an efficiency as high as 1.4% of the Carnot efficiency, which was 3 times that of the equivalent platinum-based TGC measured under the same conditions.

Romano et al. enhanced the electrode porosity by fabricating a composite of CNTs and reduced graphene oxide.⁶² On the one hand, the porous nature of these materials provided a larger active surface area in comparison to flat electrodes, allowing for a higher exchange current density. On the other hand, the accessibility of the redox couples in these tortuous electrodes could limit the transport of the reaction product, thus limiting the TGC power output as well. They optimized the composite electrode by varying the composition of the two materials. It was

found that the electrode comprised of 90% SWCNTs and 10% rGO had the best performance, with a power output as high as 337 mW m⁻² (ΔT = 30 K). The power output could be enhanced by increasing the electrode thickness, reaching up to 460 mW m⁻² with 4.5 µm thick electrodes or even 1800 mW m⁻² for electrodes consisting of 10 stacked layers (45 µm) at the same ΔT, with an efficiency as high as 2.6% of the Carnot efficiency.

Table 6 summarizes the performance of some TGCs based on a typical combination of electrodes and electrolytes. The highest power output achieved so far for a TGC is 12 W m⁻² (ΔT = 84 K) by using an activated carbon cloth electrode and an aqueous ferro/ferricyanide electrolyte.⁴⁶⁸ A high power density could be achieved by using an electrolyte-filled thermal separator and by using the highly soluble redox electrolyte (NH₄)₄Fe(CN)₆/ (NH₄)₃Fe(CN)₆ at a concentration of 0.9 M. This strategy increased the ionic conductivity by 66% and reduced the thermal conductivity by 5.7%, thereby improving the thermogalvanic power efficiency.

Another type of low-cost electrode without an insulating oxide on the surface is a conducting polymer. Abraham et al. considered a coating of conducting polymer on a metal electrode for use in TGCs.⁵⁵ To avoid any electron transfer with the metal electrode and to reveal the true potential of polymer electrodes
alone, a highly conducting polymer electrode made of PEDOT-Tos (conductivity ~1000 S cm⁻¹) was used.⁷⁵,⁵⁰⁰⁻⁵⁰³ Thin films of the PEDOT-Tos electrodes were fabricated on glass by chemical polymerization, and the performance of the TGCs was studied by using a 0.4 M K₄Fe(CN)₆/K₃Fe(CN)₆ redox electrolyte. As shown in Figure 62a, the maximum power generation of the TGC with a 223 nm thick electrode reached 178 mW m⁻² (ΔT = 30 K), which was only three times lower than that of flat platinum electrodes (438 mW m⁻²) under the same conditions. Increasing the thickness of the PEDOT-Tos electrodes led to the same power as that of flat Pt electrodes (Figure 62b). The same authors further investigated the interplay between the electrical conductivity of the polymer electrode and the rate of electron transfer with ferro/ferricyanide, as well as its impact on the performance of the TGCs.⁵⁰⁴ PEDOT-PSS (PH1000) was modified with various amounts of DMSO (up to 6 wt %) to tune its morphology by inducing the demixing of excess insulating PSS, which resulted in an increase in conductivity by more than 3 orders of magnitude (Figure 63a). The rate of electron transfer from the

<figure><img src="image_77.png" /><figcaption>**Figure 63.** (a) Conductivity of PEDOT:PSS-DMSO electrodes (the dashed red line demonstrates the trend in the electrical conductivity with respect to the DMSO wt %). (b) Maximum power output of the TGC as a function of the DMSO ratio and the maximum power output of TGC as a function of the different thicknesses of PEDOT:PSS-5% DMSO. The dashed black lines are guides for the eyes to show the trend in those graphs. All measurements are taken at a ΔT of 30 °C. Reproduced with permission from ref 504. Copyright 2018 National Academy of Sciences.</figcaption></figure>

PEDOT:PSS electrode to the ferro/ferricyanide couple was related to the percolation transport of the polymer electrode. The power produced by the TGC increased by 4 orders of magnitude after changing the morphology and electrical conductivity of the PEDOT electrode (Figure 63b).

PEDOT:PSS films represent an attractive alternative to platinum electrodes. Recently, Wang et al. demonstrated a PEDOT:PSS-based thermo-electrochemical cell using ferricyanide/ferrocyanide as the electrolyte. This device delivered a maximum power output of 300 µW, with a 1 Ω loading at a temperature difference of 30 K; additionally, this device powered an array of light-emitting diodes and Bluetooth humidity/temperature sensors for wireless communication.⁵⁰⁵ Furthermore, they investigated the electrochemical properties of insoluble redox couples such as Prussian blue (PB) analogue compounds by hybridizing them with PEDOT:PSS via ball milling, drop-casting on a polystyrene mold, and heating at 70 °C for 24 h. In this work, they combined nickel ferrocyanide (NiHFC) with Fe²⁺/Fe³⁺ using a cation separator (Figure 64a) and PB with K₃Fe(CN)₆/K₄Fe(CN)₆ using an anion separator

(Figure 64b). This electrochemical cell generated a maximum power of 4 µW over a temperature range of 5–60 °C.

<figure><img src="image_78.png" /><figcaption>**Figure 64.** Schematic of the charge-free TREC system combining (a) PB-PEDOT:PSS with Fe(CN)₆$^{3-}$ and (b) NiHFC-PEDOT:PSS with Fe²⁺/Fe³⁺. Reproduced with permission from ref 505. Copyright 2020 Wiley.</figcaption></figure>

Li et al. demonstrated highly efficient cylindrical TGCs, working in a wide operation window of cold temperatures down to -40 °C.⁵⁰⁶ In parallel, they developed a eutectic electrolyte composed of formamide and water, which exhibited high ionic conductivities (between 10 and 100 mS cm⁻¹ in the temperature range of -40 to 90 °C, as shown in Figure 65a), and an electrode material based on an anisotropic porous graphene aerogel, which had aligned sheets and pores (Figure 65b) and demonstrated high ionic conductivity. These two materials were then employed to create a device composed of 15 thermocells capable of delivering 2 V and 350 µW at an external ΔT = 106 K (Figure 65c,d).

Recently, Ding et al. reported a hybrid photothermal generator consisting of a pyroelectric generator and a TGC for multiple situations, namely structured/unstructured, static/dynamic, and day/night waste heat harnessing for continuous operation.⁵⁰⁷ In this device, CNT-based nanocomposites served as: (1) broadband solar absorbers, (2) good thermally/electrically conductive electrodes, and (3) electrodes with a high electrochemical specific surface area. The high photothermal properties and electrical conductivity of the carbon nanotube/cellulose nanocrystal nanocomposite, together with the heat localization effect, resulted from the sponge-confined electrolyte; thus, the simultaneously high thermogalvanic and pyroelectric performances were achieved with maximum outputs of 1.86 and 0.9 mW m⁻², respectively (Figure 66). In addition, a scaled-up hybrid photothermal generator was successfully tested on both land and water surfaces under natural sunlight, and the feasibility of nighttime operation was verified.

Shin et al. demonstrated a hybrid photovoltaic thermogalvanic cell module (PV/T) by integrating a TGC directly into the back of a solar panel (Figure 67), exploring the feasibility of the module for practical implementation.⁵⁰⁸ Here, a fabricated thermogalvanic cell consisting of an electrolyte with a ferric/ferrous redox couple ([Fe(CN)₆]⁴⁻ and [Fe(CN)₆]³⁻) and SWCNT electrodes was integrated directly into the back of a commercial solar panel array. The TGC in the fabricated PV/T hybrid system performed both as a cooling system and produced additional power output by converting the thermal energy absorbed by the solar panel into useful electrical energy. Under illumination with air mass (AM) of 1.5 G, using a solar simulator,
<figure><img src="image_79.png" /><figcaption>**Figure 65.** (a) Conductivity of the 0.4 M electrolyte at different temperatures. The inset is a photo measurement of the electrolyte temperature with a thermocouple. (b) Illustration showing the ionic transport model in the ordered porous graphene channels. (c) Plots of the output voltage and power versus the current of the 15-cylinder device for different temperature differences. (d) Output voltage versus time obtained from $\Delta T = 106$ °C. The inset in (d) is the photo of LED beads powered by the device. Reproduced with permission from ref 506. Copyright 2019 Wiley.</figcaption></figure>

<figure><img src="image_80.png" /><figcaption>**Figure 66.** Schematic of the hybrid photothermal generator. The mechanism of the photothermal pyroelectric generator (Pyro) and thermocell (Thermo) are depicted in the upper and lower figures on the right, respectively. Reproduced with permission from ref 507. Copyright 2019 Wiley.</figcaption></figure>

the conversion efficiency of the solar cell improved from 13.2% to 15% by cooling the solar cell from 61 to 34 °C due to the thermogalvanic cell and the additional harvested power of 3.53 µW cm⁻².

Kim et al. combined n-type Fe-TECs with p-type FeCN-TECs in series to produce output powers sufficient for large-scale applications.⁵⁰⁹ They fabricated a TEC module consisting of 32 pairs of Fe-TEC $ClO_4$[0.5 M Fe($ClO_4$)$_2$/Fe($ClO_4$)$_3$] and FeCN-TEC-NH₄ [0.9 M K₃Fe(CN)₆/(NH₄)₄Fe(CN)₆] (Figure 68c). The TEC module generated a $V_{oc}$ of 2.05 V and an $I_{sc}$ of 0.2 A m⁻², respectively, at a $\Delta T$ of 25 °C, which powered a commercial light-emitting diode (Figure 68d).

Duan et al.⁶⁹ demonstrated potential applications of their optimized systems (previously discussed) for harvesting low-grade thermal energy and fabricated a prototype module containing 50 urea/guanidine chloride (UGdmCl) units (1 cm² area, 0.5 cm thickness) connected by Cu wires in series (Figure 69). This device generated a voltage of 3.4 V and a

current of 1.2 mA at an applied $\Delta T$ = 18 °C and directly powered an LED array. Furthermore, the module could harvest heat from the human body, and a stable voltage of more than 0.3 V was generated by a small temperature difference of approximately 1.3 K, as shown in Figure 69d. In addition, this module could harvest waste heat in a cold environment.

An array of series-connected n-type and p-type TGCs that provided an output voltage of 2 V were fabricated by Zhang et al.⁶⁸ Figure 70b–d describes this TGC array, which consists of a K₃Fe(CN)₆/(NH₄)₄Fe(CN)₆ couple for the p-type element and a FeSO₄/Fe₂(SO₄)₃ couple for the n-type element. The low voltage of the iron sulfate TGC limited the maximum output current for the array. A TGC array containing 28 alternating, series-connected half-cells is shown in Figure 70c,d, where the cross-sectional areas of both legs are the same. The voltage generated by the 28 half-cells was ~27 mV K⁻¹ (1.93 mV K⁻¹ × 14 cells), and four identical TGC arrays were connected in series to obtain an open-circuit voltage of 2.18 V for a $\Delta T$ of 21 °C. This TGC output was used to charge commercial capacitors with capacitances of 47 to 1000 µF to 2 V during times ranging from 1 to 20 min.

Yang et al. fabricated a wearable TGC system to harvest low-grade body heat.²³ The system consisted of two types of thermogalvanic gel electrolytes with positive and negative thermo-electrochemical Seebeck coefficients, corresponding to the n-type and p-type elements of a conventional thermoelectric generator, respectively. Poly(vinyl alcohol) (PVA) was used as the gel solution, with ferric/ferrous chloride or potassium ferricyanide/ferrocyanide couples to obtain the thermogalvanic gel electrolytes exhibiting positive and negative thermo-electrochemical Seebeck coefficients. These gel electrolytes exhibited both moderate thermoelectric performance and good mechanical properties. The integrated wearable device contained 59 n-type and 59 p-type TGC gel units (a height of 1 mm and a diameter of 3 mm), with bridging Au/Cr interconnections
<figure><img src="image_81.png" /><figcaption>**Figure 67.** Fabrication and operation of the hybrid PV/T module: (a) schematic of the hybrid PV/T module and thermocell operation, (b) cell components and their assembly in a thermocell, and (c) optical image of the fabricated PV/T module. Reproduced with permission from ref 508. Copyright 2020 Multidisciplinary Digital Publishing Institute.</figcaption></figure>

<figure><img src="image_82.png" /><figcaption>**Figure 68.** (a) Long-term performance of Fe-TEC-ClO₄ using 0.1 M HCl or 0.1 M HClO₄ for T_cold and T_hot of 25 and 50 °C, respectively. (b) Thermoelectrochemical performances of Fe-TEC-ClO₄, FeCN-TEC-NH₄, and TECs connected in series for T_cold and T_hot of 25 and 50 °C, respectively. (c) Schematic diagram of the series module and the (d) current vs voltage of the series module containing 64 units. The inset in (d) is an image of LED powered by the series module. Reproduced with permission from ref 509. Copyright 2020 Elsevier Ltd.</figcaption></figure>

on a flexible PI substrate, as illustrated in Figure 71; this device could generate a maximum output power of approximately 0.3 mW by utilizing body heat (Figure 71).

Finally, because of the thermocell voltage's direct dependence on the electrode's surface temperature, the lack of thermal contact resistance between the specimen and TGC makes the thermal response of thermogalvanic cells fast (<300 µs).⁵¹⁰ The fast response to thermal variation, combined with the high sensitivity linked to the high values of S, makes such devices an attractive solution for sensing applications.

## 5. CONCLUSIONS AND OUTLOOKS

Solid-state inorganic semiconductors currently dominate research on thermal-to-electrical energy conversion due to their high performances and efficiencies. In this case, the concept of electron-crystal/phonon-glass behavior has been the driving force for developing new materials. However, despite the impressive results that have been obtained, this technology has been limited to niche applications such as space (e.g., powering satellites) and scientific instrumentation (e.g., cooling detectors) due to the high costs of materials and processing. In the last 20 years, carbon-based semiconductors have been scrutinized as potential thermoelectric materials for low-temperature energy harvesting because of the high abundance of their atomic
<figure><img src="image_83.png" /><figcaption>**Figure 69.** Demonstration of low-grade heat harvesting by a prototype module. (a) Schematic of the structure and a photograph of the module. (b) Voltage and current curves of the module corresponding to the applied temperature difference. (c) Image of the module directly powering an LED array containing 29 red LEDs. The module was sandwiched by two aluminum heat exchangers with water as the heat transfer fluid. The temperatures of the cold and hot water were controlled at approximately 278 and 333 K, respectively. Because of inevitable thermal conduction in the module and thermal contact resistance between the module and heat exchangers, the ΔT of 18 K shown by the module at the steady state was smaller than the nominal temperature difference between the two heat exchangers. (d) Harvesting heat energy from the human body. The module was placed on a steel platform as a heat sink with a temperature of approximately 298 K and then covered by one hand. After several minutes, a stable temperature difference of 1.3 K was observed, which induced an output of approximately 0.3 V by the module. The temperature and voltage were measured by a thermocouple (left) and multimeter (right), respectively. Reproduced with permission from ref 469. Copyright 2018 Nature.</figcaption></figure>

<figure><img src="image_84.png" /><figcaption>**Figure 70.** Enhanced output voltage using interconnected thermocell modules. (a) Illustration of two series-connected p-type half-cells that both use aqueous Fe(CN)₆^{4-}/Fe(CN)₆^{3-} as the redox mediator. (b) Illustration of a p-type half-cell (using aqueous Fe(CN)₆^{4-}/Fe(CN)₆^{3-} as the redox mediator) and an n-type half-cell (using aqueous Fe²⁺/Fe³⁺ as the redox mediator) that are connected in series. (c) Photograph of alternating n-type and p-type mediators for a thermocell array with 14 n-p cells. (d) Photograph of the sealed thermocell array for the redox couple array in (c), showing the top plate that completes the electrical connections. (e) Voltage-time curves of different capacitors when charged by four series-connected thermocell arrays. Reproduced with permission from ref 468. Copyright 2017 Wiley.</figcaption></figure>

Other advantages are linked to their low-costs and large-area (low T) processing, using printing techniques. Furthermore, their intrinsically lower thermal conductivity represents a significant advantage in harvesting energy from small temperature differences.

These materials can be divided into two main classes: carbon nanostructures (e.g., carbon nanotubes and graphene) and organic polymers/small molecules. Semiconducting single-

walled carbon nanotubes can be separated from their metallic counterparts, using scalable solution processes through specific complexation with specific polymers. While the thermal conductivity is large along single nanostructures, in thin films composed of a network of these elements, the thermal conductance is significantly reduced by phonon scattering at interfaces, thus making them viable candidates for thermoelectric applications. Their Seebeck coefficients can be
<figure><img src="image_85.png" /><figcaption>**Figure 71.** (top) Schematic and image of the integrated gel-based thermocell. Both the PFC and PPF gels are sandwiched between two flexible substrates (polyimide, PI). With alternating top and bottom interconnections, the PFC and PPF gels are connected in series. (bottom) Applications of the wearable thermocell. (left) Voltage and current generated by the thermocell from body heat. The inset shows a photograph of the wearable thermocell on a forearm. (right) Voltage–time curves of different capacitors charged by the thermocell utilizing human body heat. The inset shows the energy stored in the capacitors, which is also the electrical energy harvested from body heat. Reproduced with the permission from ref 23. Copyright 2016 Wiley.</figcaption></figure>

controlled by tuning their structure (e.g., the nanotube diameter) and doping level. One unique feature is that they can be reduced by chemical means to display a stable negative Seebeck coefficient in air and a power factor higher than many n-doped conducting polymers. Conducting polymers are stable in air in their oxidized form (p-doped) and reach impressive values of power factor and zT (e.g., 0.52 for PEDOT:PSS), much higher than that of their n-doped counterparts. The power factor of conducting polymers can be controlled not only through their doping level but also by tuning their morphology at various scales because it affects the electronic properties and transport regime of the charge carriers.

For both classes of materials, emerging processing techniques enable the fabrication of thick layers necessary to utilize large thermal gradients in a traditional vertical thermoelectric module. Unfortunately, stable high thermoelectric performance is challenging to achieve with thick layers. Thus, technical solutions have been proposed to overcome this problem, for example, employing new geometries based on printing lateral thermocouples on flexible and foldable substrates. However, even with this strategy, thick layers will be beneficial, and such a structure prevents the actual integration of thermocouples in a monolithic device.

Recently, the blending of high *PF* inorganic nanostructures with conducting polymers, CNTs, and graphene matrices has shown promising results, representing an innovative way to process these novel materials. However, the thermoelectric properties of the individual parts of the composite are not additive, and they typically lead to decreased thermoelectric performance because the nanocomposites cannot have higher performance than the individual parts unless quantum effects dominate. In this regard, one of the challenges is to avoid the percolation of one phase of the composite because the phase with the highest conductivity will then govern most of the thermoelectric properties of the composite. In addition, the transport of charge carriers across both phases is typically made difficult by the presence of a potential barrier at the interface, which can stem from an oxide layer on the inorganic filler or equilibration of the Fermi levels of the two phases.

A less explored area of research on thermoelectricity is that of the thermodiffusion of matter. The thermodiffusion of ions in electrolytes yields large ionic Seebeck coefficients, in the range of tens of mV K⁻¹, opening new avenues for energy storage, ultrasensitive heat sensors, and thermoelectronics. Unfortunately, ionic thermoelectric energy storage devices are restricted to discontinuous heat sources or need to be coupled with a supercapacitor to work as energy harvesters.
Another type of thermoelectric effect is based on the entropy of electron transfer during an electrochemical reaction, which leads to thermogalvanic cells. Despite the low efficiency (only a tiny fraction of the Carnot efficiency), several breakthroughs have motivated research on these energy conversion devices, e.g., redox couples with high Seebeck coefficients, high surface area electrodes, and heat transport control in electrolytes and complementary p-/n- thermoelectric modules. The use of gel electrolytes is attractive because thermogalvanic cells can be flexible and stretchable or elastic. These properties will open a new unique research path compared to solid-state thermo-electric materials for powering low-power electronics from body heat. While the solvent's chaotropic effect around charged species is mainly understood for aqueous electrolytes in biochemistry, these mechanisms are less understood for other media. Therefore, through studies on thermogalvanic cell optimization, new knowledge will be acquired for various other organic, ionic liquid, and polymer gel electrolytes.

In conclusion, carbon-based materials represent a novel path in the field of thermoelectricity and allow for the development of a new generation of energy harvesters and sensors. This new generation of thermoelectric devices will be available for some niche applications and everyday life scenarios, from the IoT to wearable and biomedical devices. In this regard, it is of paramount importance to evaluate material toxicity when placed in contact with biological media (tissues, cells, skin, and others). Some materials, such as PEDOT:PSS, have already proven to be biocompatible.⁵¹¹ However, for many other materials presented in this review, not enough data are currently available. The use of robust, biocompatible, and nondegradable capping/protection layers (e.g., PDMS or parylene) could represent a solution to material toxicity and enable the integration of TE devices with biological tissues. In addition, the inherent flexibility of additive manufacturing techniques has led to the development of new geometries and devices that can adapt to many kinds of irregular flexible and soft substrates (e.g., plastic or cloth) while still maintaining most of the thermoelectric properties. Because of this, organic materials represent the best choice for inexpensive low-power, room-temperature TE devices. Unfortunately, the development of scalable devices and fabrication processes has lagged far behind materials development and is mainly limited to simple demonstrators. As new carbon-based thermoelectric materials continue to be discovered and the material performance continues to improve, additional focus will need to be directed toward the integration of these materials into functional devices. This will require consideration of the various materials interfaces and the device/system architectures to successfully translate lab-scale demonstrators into commercially relevant products. Recently, however, more and more examples of integrated, printed, and woven devices have been presented in the literature, showing the possibility of commercializing OTEG in a not-so-far future.

## AUTHOR INFORMATION

### Corresponding Authors

**Simone Fabiano** — Department of Science and Technology, Linköping University, SE-60174 Norrköping, Sweden;

● orcid.org/0000-0001-7016-6514;

Email: simone.fabiano@liu.se

**Jeffrey L. Blackburn** — National Renewable Energy Laboratory, Golden, Colorado 80401, United States; ● orcid.org/0000-0002-9237-5891; Email: jeffrey.blackburn@nrel.gov

**Xavier Crispin** — Department of Science and Technology, Linköping University, SE-60174 Norrköping, Sweden;

● orcid.org/0000-0001-8845-6296; Email: xavier.crispin@

liu.se

### Authors

**Matteo Massetti** — Department of Science and Technology, Linköping University, SE-60174 Norrköping, Sweden

**Fei Jiao** — Department of Science and Technology, Linköping University, SE-60174 Norrköping, Sweden; Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Sciences, Tianjin University & Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China; ● orcid.org/0000-0002-9801-6283

**Andrew J. Ferguson** — National Renewable Energy Laboratory, Golden, Colorado 80401, United States

**Dan Zhao** — Department of Science and Technology, Linköping University, SE-60174 Norrköping, Sweden

**Kosala Wijeratne** — Department of Science and Technology, Linköping University, SE-60174 Norrköping, Sweden

**Alois Würger** — Laboratoire Ondes et Matière d'Aquitaine, Université de Bordeaux, F-33405 Talence Cedex, France

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.chemrev.1c00218

### Author Contributions

M.M. and F.J. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

### Notes

The authors declare the following competing financial interest(s): S.F. is the founder and Chief Scientific Officer of n-Ink AB.

### Biographies

Matteo Massetti obtained his Ph.D. in Physics in 2019 at "Politecnico di Milano" (Italy) with a thesis on the study of the thermoelectric properties of organic semiconductors and on the development of printed integrated thermoelectric generators. He then joined Prof. Fabiano's group at the Laboratory of Organic Electronics (Linköping University, Sweden) as a postdoc, where he is currently working on neuromorphic devices and continuing his activity in the field of printed electronics with a focus on organic electrochemical transistors, thermoelectric/thermoionic generators.

Fei Jiao is an associate professor at the School of Science in Tianjin University. He received his B.S. degree in Chemical Engineering and Technology from Jilin University in 2009 and Ph.D. degree in Organic Chemistry from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 2014, under the supervision of Prof. Daoben Zhu. From 2014 to 2018, he worked as a postdoctoral researcher in Prof. Xavier Crispin's group in the Laboratory of Organic Electronics in Linköping University, and currently his research interests focus on organic composites for thermoelectrics and field-effect transistors.

Andrew Ferguson received his Ph.D. in Physical Chemistry from Imperial College, United Kingdom. Following a postdoctoral appointment at the National Renewable Energy Laboratory (NREL), he became a senior scientist and then group manager in NREL's Chemistry & Nanoscience Center. His primary research interests focus on light-matter interactions and developing an understanding of exciton, carrier, and heat transport in organic, inorganic, and nanoscale material
systems. He is also working on the integration of these materials into
novel energy harvesting and conversion technologies.

Dan Zhao is an Assistant professor at the Laboratory of Organic
Electronics (LOE) at Linköping University. She received her B.S. from
the Chemistry Department in Nankai University in 2008. After
finishing her Ph.D. in Renmin University in China, she joined LOE to
continue postdoctoral research in 2013. Her research interest includes
ionic thermoelectrics and electrolyte-based and dielectric materials.

Kosala Wijeratne received his Ph.D. in Applied Physics from the Linköping University, Sweden, in 2018. After that, he joined the Research Institute of Sweden as a development engineer. He is currently an Electrochemical Engineer at Battery Materials, Johnson Matthey, United Kingdom. His primary research interests focus on lithium-ion batteries.

Alois Würger is Professor in the Condensed Matter Theory Group of LOMA, at the Université de Bordeaux. He received his Ph.D. from the University of Erlangen, and his venia legendi from University Heidelberg and was a Research Associate at the Institut Laue-Langevin in Grenoble, where he worked on low-temperature physics and quantum dissipation. His present research interests include non-equilibrium complex fluids, colloidal thermophoresis, nanofluidics, and thermoelectric effects.

Jeff Blackburn received his Ph.D in Chemistry from the University of Colorado, Boulder, in 2004 and performed his postdoctoral studies at the National Renewable Energy Laboratory (NREL). He is currently a senior scientist, group manager, and Distinguished Member of the Research Staff at NREL. His research interests include the fundamental understanding of optical and electrical properties and processes in low-dimensional materials (SWCNTs, graphene, two-dimensional semiconductors, and hybrid organic/inorganic semiconductors) and the integration of these materials into sustainable energy technologies.

Xavier Crispin is a Professor in the Laboratory of Organic Electronics (LOE) at Linköping University, Sweden. In 2000, He obtained his Ph.D. in the University of Mons (Belgium) and developed his own research group at Linköping University as an ERC fellow. He was awarded the Tage Erlander and Göran Gustafsson prizes by the Swedish Academy of Science for his work on organic thermoelectrics. His research interest is the use of organic materials in energy devices: batteries, thermoelectrics, and fuel cells/electrolysers.

Simone Fabiano received his Ph.D. in Physical Chemistry from the University of Palermo (Italy) in 2012. From 2010 to 2012, he worked as a visiting researcher in the Zernike Institute for Advanced Materials at the University of Groningen (The Netherlands). He then carried out postdoctoral research at Linköping University (Sweden) in 2012 and Northwestern University (USA) in 2016. Since 2018, he is an Associate Professor at the Department of Science and Technology at Linköping University, guiding the Organic Nanoelectronics Group. He is a Docent in Applied Physics and founder and Chief Scientific Officer of n-Ink AB, a company developing n-type conductive inks. His research interests include developing organic (electronic and mixed ionic-electronic) conductors and integrating them into printed electronics and novel energy technologies.

ACKNOWLEDGMENTS

We authors at Linköping University thank the Knut and Alice Wallenberg foundation, the Swedish Research Council (2016-03979 and 2020-03243), ÅForsk (18-313 and 19-310), Olle Engkvists Stiftelse (204-0256), the European Commission through the Marie Sklodowska-Curie project HORATES (GA-955837), and the Swedish Government Strategic Research

Area in Materials Science on Functional Materials at Linköping University (faculty grant SFO-Mat-LiU 2009-00971). This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under contract no. DE-AC36-08GO28308. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government.

REFERENCES

(1) *Data and Statistics*; International Energy Agency, 2021; https://www.iea.org/data-and-statistics?country=WORLD&fuel=Energy%20supply&indicator=TPESbySource (accessed 2021-06-30).

(2) *Renewable Energy*; Center for Climate and Energy Solutions, 2021; https://www.c2es.org/content/renewable-energy (accessed 2021-06-30).

(3) Chen, H. J.; Goswami, D. Y.; Stefanakos, E. K. A review of thermodynamic cycles and working fluids for the conversion of low-grade heat. *Renewable Sustainable Energy Rev.* **2010**, *14*, 3059.

(4) Pathak, S.; Shukla, S. A Review on the Performance of Organic Rankine Cycle with Different Heat Sources and Absorption Chillers. *Distrib. Gener. Altern. Energy J.* **2018**, *33*, 6.

(5) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y. C.; Minnich, A.; Yu, B.; Yan, X. A.; Wang, D. Z.; Muto, A.; Vashaee, D.; et al. High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys. *Science* **2008**, *320*, 634.

(6) Kraemer, D.; Poudel, B.; Feng, H. P.; Caylor, J. C.; Yu, B.; Yan, X.; Ma, Y.; Wang, X. W.; Wang, D. Z.; Muto, A.; et al. High-performance flat-panel solar thermoelectric generators with high thermal concentration. *Nat. Mater.* **2011**, *10*, 532.

(7) Beretta, D.; Neophytou, N.; Hodges, J. M.; Kanatzidis, M. G.; Narducci, D.; Martin-Gonzalez, M.; Beekman, M.; Balke, B.; Cerretti, G.; Tremel, W.; et al. Thermoelectrics: From history, a window to the future. *Mater. Sci. Eng., R* **2019**, *138*, 100501.

(8) Vining, C. B. An inconvenient truth about thermoelectrics. *Nat. Mater.* **2009**, *8*, 83.

(9) Avery, A. D.; Zhou, B. H.; Lee, J.; Lee, E. S.; Miller, E. M.; Ihly, R.; Wesenberg, D.; Mistry, K. S.; Guillot, S. L.; Zink, B. L.; et al. Tailored semiconducting carbon nanotube networks with enhanced thermoelectric properties. *Nat. Energy* **2016**, *1*, 16033.

(10) Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X. Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4-ethylenedioxythiophene). *Nat. Mater.* **2011**, *10*, 429.

(11) Yao, Q.; Chen, L. D.; Zhang, W. Q.; Liufu, S. C.; Chen, X. H. Enhanced Thermoelectric Performance of Single-Walled Carbon Nanotubes/Polyaniline Hybrid Nanocomposites. *ACS Nano* **2010**, *4*, 2445.

(12) Sher, I.; Levinzon-Sher, D.; Sher, E. Miniaturization limitations of HCCI internal combustion engines. *Appl. Therm. Eng.* **2009**, *29*, 400.

(13) Bottner, H.; Nurnus, J.; Gavrikov, A.; Kuhner, G.; Jagle, M.; Kunzel, C.; Eberhard, D.; Plescher, G.; Schubert, A.; Schlereth, K. H. New thermoelectric components using microsystem technologies. *J. Microelectromech. Syst.* **2004**, *13*, 414.

(14) Kim, Y. J.; Gu, H. M.; Kim, C. S.; Choi, H.; Lee, G.; Kim, S.; Yi, K. K.; Lee, S. G.; Cho, B. J. High-performance self-powered wireless sensor node driven by a flexible thermoelectric generator. *Energy* **2018**, *162*, 526.

(15) vom Boegel, G.; Meyer, F.; Kemmerling, M. Wireless Sensor System for Industrial Applications Powered by Thermoelectric Generator. In *Smart SysTech 2014, European Conference on Smart Objects, Systems and Technologies*, 2014; p 1.

(16) Yang, Y.; Wei, X. J.; Liu, J. Suitability of a thermoelectric power generator for implantable medical electronic devices. *J. Phys. D: Appl. Phys.* **2007**, *40*, 5790.

(17) Weber, J.; Potje-Kamloth, K.; Haase, F.; Detemple, P.; Völklein, F.; Doll, T. Coin-size coiled-up polymer foil thermoelectric power generator for wearable electronics. *Sens. Actuators, A* **2006**, *132*, 325.
(18) Kim, S. J.; We, J. H.; Cho, B. J. A wearable thermoelectric generator fabricated on a glass fabric. *Energy Environ. Sci.* **2014**, *7*, 1959.

(19) Jo, S. E.; Kim, M. K.; Kim, M. S.; Kim, Y. J. Flexible thermoelectric generator for human body heat energy harvesting. *Electron. Lett.* **2012**, *48*, 1015.

(20) Zhang, F. J.; Zang, Y. P.; Huang, D. Z.; Di, C. A.; Zhu, D. B. Flexible and self-powered temperature-pressure dual-parameter sensors using microstructure-frame-supported organic thermoelectric materials. *Nat. Commun.* **2015**, *6*, 8356.

(21) Wu, W.; Du, Z.; Cui, J.; Shi, Z.; Deng, Y. Thermoelectric Generator Used in Fire-Alarm Temperature Sensing. *J. Electron. Mater.* **2015**, *44*, 1851.

(22) Shi, Y.; Wang, Y.; Deng, Y.; Gao, H.; Lin, Z.; Zhu, W.; Ye, H. A novel self-powered wireless temperature sensor based on thermoelectric generators. *Energy Convers. Manage.* **2014**, *80*, 110.

(23) Yang, P.; Liu, K.; Chen, Q.; Mo, X.; Zhou, Y.; Li, S.; Feng, G.; Zhou, J. Wearable Thermocells Based on Gel Electrolytes for the Utilization of Body Heat. *Angew. Chem., Int. Ed.* **2016**, *55*, 12050.

(24) Zhao, D.; Wang, H.; Khan, Z. U.; Chen, J.; Gabrielsson, R.; Jonsson, M. P.; Berggren, M.; Crispin, X. Ionic thermoelectric supercapacitors. *Energy Environ. Sci.* **2016**, *9*, 1450.

(25) Wood, C. Materials for thermoelectric energy conversion. *Rep. Prog. Phys.* **1988**, *51*, 459.

(26) De Groot, S. R.; Mazur, P. *Non-equilibrium Thermodynamics*; Dover Publication Inc: New York, 1953.

(27) Simpkins, P. G.; Greenberg-Kosinski, S.; MacChesney, J. B. Thermophoresis: the mass transfer mechanism in modified chemical vapor deposition. *J. Appl. Phys.* **1979**, *50*, 5676.

(28) Stejskal, J.; Trchova, M.; Ananieva, I. A.; Janca, J.; Prokes, J.; Fedorova, S.; Sapurina, I. Poly(aniline-co-pyrole): powders, films, and colloids. Thermophoretic mobility of colloidal particles. *Synth. Met.* **2004**, *146*, 29.

(29) Thamdrup, L. H.; Larsen, N. B.; Kristensen, A. Light-Induced Local Heating for Thermophoretic Manipulation of DNA in Polymer Micro- and Nanochannels. *Nano Lett.* **2010**, *10*, 826.

(30) Chapman, S.; Cowling, T. G. *The Mathematical Theory of Non-uniform Gases: An Account of the Kinetic Theory of Viscosity, Thermal Conduction and Diffusion in Gases*, 3rd ed.; Cambridge University Press, 1970.

(31) Wurger, A. Thermal non-equilibrium transport in colloids. *Rep. Prog. Phys.* **2010**, *73*, 126601.

(32) Ashcroft, N. W.; Mermin, N. D. *Solid State Physics*; Academic Press: New York, 1976.

(33) Goupil, C.; Seifert, W.; Zabrocki, K.; Müller, E.; Snyder, G. J. Thermodynamics of Thermoelectric Phenomena and Applications. *Entropy* **2011**, *13*, 1481.

(34) Zhu, T.; Liu, Y.; Fu, C.; Heremans, J. P.; Snyder, G. J.; Zhao, X. Compromise and Synergy in High-Efficiency Thermoelectric Materials. *Adv. Mater.* **2017**, *29*, 1605884.

(35) He, J.; Tritt, T. M. Advances in thermoelectric materials research: Looking back and moving forward. *Science* **2017**, *357*, eaak9997.

(36) Lu, N. D.; Li, L.; Liu, M. A review of carrier thermoelectric-transport theory in organic semiconductors. *Phys. Chem. Chem. Phys.* **2016**, *18*, 19503.

(37) Assael, M. J.; Goodwin, A. R.; Vesovic, V.; Wakeham, W. A. *Experimental Thermodynamics, Volume IX: Advances in Transport Properties of Fluids*; Royal Society of Chemistry, 2014.

(38) Agar, J. N.; Mou, C. Y.; Lin, J. L. Single-Ion Heat of Transport in Electrolyte-Solutions-a Hydrodynamic Theory. *J. Phys. Chem.* **1989**, *93*, 2079.

(39) Wurger, A. Transport in charged colloids driven by thermoelectricity. *Phys. Rev. Lett.* **2008**, *101*, 108302.

(40) Vigolo, D.; Buzzacaro, S.; Piazza, R. Thermophoresis and Thermoelectricity in Surfactant Solutions. *Langmuir* **2010**, *26*, 7792.

(41) Eslahian, K. A.; Majee, A.; Maskos, M.; Wurger, A. Specific salt effects on thermophoresis of charged colloids. *Soft Matter* **2014**, *10*, 1931.

(42) Majee, A.; Würger, A. Thermocharge of a hot spot in an electrolyte solution. *Soft Matter* **2013**, *9*, 2145.

(43) Di Lecce, S.; Albrecht, T.; Bresme, F. The role of ion-water interactions in determining the Soret coefficient of LiCl aqueous solutions. *Phys. Chem. Chem. Phys.* **2017**, *19*, 9575.

(44) Di Lecce, S.; Bresme, F. Thermal Polarization of Water Influences the Thermoelectric Response of Aqueous Solutions. *J. Phys. Chem. B* **2018**, *122*, 1662.

(45) Würger, A. Thermopower of ionic conductors and ionic capacitors. *Phys. Rev. Res.* **2020**, *2*, No. 042030(R).

(46) Heikes, R. R.; Ure, R. W. *Thermoelectricity: Science and Engineering*; Interscience Publishers, University of Michigan, 1961.

(47) Liu, L.; Sun, Y.; Li, W.; Zhang, J.; Huang, X.; Chen, Z.; Sun, Y.; Di, C.; Xu, W.; Zhu, D. Flexible unipolar thermoelectric devices based on patterned poly[Kx(Ni-ethylenetetrathiolate)] thin films. *Mater. Chem. Front.* **2017**, *1*, 2111.

(48) Zheng, C.; Xiang, L.; Jin, W.; Shen, H.; Zhao, W.; Zhang, F.; Di, C.-A.; Zhu, D. A Flexible Self-Powered Sensing Element with Integrated Organic Thermoelectric Generator. *Adv. Mater. Technol.* **2019**, *4*, 1900247.

(49) Bubnova, O.; Crispin, X. Towards polymer-based organic thermoelectric generators. *Energy Environ. Sci.* **2012**, *5*, 9345.

(50) Sun, T.; Zhou, B.; Zheng, Q.; Wang, L.; Jiang, W.; Snyder, G. J. Stretchable fabric generates electric power from woven thermoelectric fibers. *Nat. Commun.* **2020**, *11*, 572.

(51) Chen, A.; Madan, D.; Wright, P. K.; Evans, J. W. Dispenser-printed planar thick-film thermoelectric energy generators. *J. Micro-mech. Microeng.* **2011**, *21*, 104006.

(52) Wijeratne, K. Conducting Polymer Electrodes for Thermogalvanic Cells. Doctoral dissertation. Linköping University Electronic Press: Linköping, Sweden, 2018.

(53) Mua, Y.; Quickenden, T. Power conversion efficiency, electrode separation, and overpotential in the ferricyanide/ferrocyanide thermogalvanic cell. *J. Electrochem. Soc.* **1996**, *143*, 2558.

(54) Salazar, P. F.; Kumar, S.; Cola, B. A. Nitrogen-and boron-doped carbon nanotube electrodes in a thermo-electrochemical cell. *J. Electrochem. Soc.* **2012**, *159*, B483.

(55) Abraham, T. J.; Tachikawa, N.; MacFarlane, D. R.; Pringle, J. M. Investigation of the kinetic and mass transport limitations in thermoelectrochemical cells with different electrode materials. *Phys. Chem. Chem. Phys.* **2014**, *16*, 2527.

(56) Dupont, M. F.; MacFarlane, D. R.; Pringle, J. M. Thermoelectrochemical cells for waste heat harvesting - progress and perspectives. *Chem. Commun.* **2017**, *53*, 6288.

(57) Romano, M. S.; Razal, J. M.; Antiohos, D.; Wallace, G.; Chen, J. Nano-Carbon Electrodes for Thermal Energy Harvesting. *J. Nanosci. Nanotechnol.* **2015**, *15*, 1.

(58) Romano, M. S.; Gambhir, S.; Razal, J. M.; Gestos, A.; Wallace, G. G.; Chen, J. Novel carbon materials for thermal energy harvesting. *J. Therm. Anal. Calorim.* **2012**, *109*, 1229.

(59) Quickenden, T.; Mua, Y. A review of power generation in aqueous thermogalvanic cells. *J. Electrochem. Soc.* **1995**, *142*, 3985.

(60) Quickenden, T.; Mua, Y. The power conversion efficiencies of a thermogalvanic cell operated in three different orientations. *J. Electrochem. Soc.* **1995**, *142*, 3652.

(61) Hu, R.; Cola, B. A.; Haram, N.; Barisci, J. N.; Lee, S.; Stoughton, S.; Wallace, G.; Too, C.; Thomas, M.; Gestos, A.; et al. Harvesting Waste Thermal Energy Using a Carbon-Nanotube-Based Thermoelectrochemical Cell. *Nano Lett.* **2010**, *10*, 838.

(62) Romano, M. S.; Li, N.; Antiohos, D.; Razal, J. M.; Nattestad, A.; Beirne, S.; Fang, S.; Chen, Y.; Jalili, R.; Wallace, G. G.; et al. Carbon nanotube-reduced graphene oxide composites for thermal energy harvesting applications. *Adv. Mater.* **2013**, *25*, 6602.

(63) Im, H.; Kim, T.; Song, H.; Choi, J.; Park, J. S.; Ovalle-Robles, R.; Yang, H. D.; Kihm, K. D.; Baughman, R. H.; Lee, H. H.; et al. High-efficiency electrochemical thermal energy harvester using carbon nanotube aerogel sheet electrodes. *Nat. Commun.* **2016**, *7*, 10600.

(64) Wang, H.; Zhao, D.; Khan, Z. U.; Puzinas, S.; Jonsson, M. P.; Berggren, M.; Crispin, X. Ionic Thermoelectric Figure of Merit for Charging of Supercapacitors. *Adv. Electron. Mater.* **2017**, *3*, 1700013.
(65) Bonetti, M.; Nakamae, S.; Huang, B. T.; Salez, T. J.; Wiertel-Gasquet, C.; Roger, M. Thermoelectric energy recovery at ionic-liquid/ electrode interface. *J. Chem. Phys.* **2015**, *142*, 244708.

(66) Kim, S. L.; Lin, H. T.; Yu, C. Thermally Chargeable Solid-State Supercapacitor. *Adv. Energy Mater.* **2016**, *6*, 1600546.

(67) Zhao, D.; Fabiano, S.; Berggren, M.; Crispin, X. Ionic thermoelectric gating organic transistors. *Nat. Commun.* **2017**, *8*, 14214.

(68) Lim, H.; Shi, Y.; Wang, M.; Qiao, Y. Effects of work function on thermal sensitivity of electrode potential. *Appl. Phys. Lett.* **2015**, *106*, 223901.

(69) Lim, H.; Lu, W. Y.; Chen, X.; Qiao, Y. Effects of ion concentration on thermally-chargeable double-layer supercapacitors. *Nanotechnology* **2013**, *24*, 465401.

(70) Lim, H.; Lu, W. Y.; Qiao, Y. Dependence on cation size of thermally induced capacitive effect of a nanoporous carbon. *Appl. Phys. Lett.* **2012**, *101*, 063902.

(71) Fujii, S.; Suzuki, Y.; Kawamata, J.; Tsunashima, R. Large in-plane/out-of-plane anisotropic conduction in PEDOT-based hybrid films: lamellar assemblies structured by mono-layered nanosheets. *J. Mater. Chem. C* **2015**, *3*, 7153.

(72) Lee, J. A.; Aliev, A. E.; Bykova, J. S.; de Andrade, M. J.; Kim, D.; Sim, H. J.; Lepro, X.; Zakhidov, A. A.; Lee, J. B.; Spinks, G. M.; et al. Woven-Yarn Thermoelectric Textiles. *Adv. Mater.* **2016**, *28*, 5038.

(73) Rojas, J. P.; Singh, D.; Inayat, S. B.; Sevilla, G. A. T.; Fahad, H. M.; Hussain, M. M. Micro and Nano-Engineering Enabled New Generation of Thermoelectric Generator Devices and Applications. *ECS J. Solid State Sci. Technol.* **2017**, *6*, N3036.

(74) Park, Y. W. Structure and Morphology-Relation to Thermopower Properties of Conductive Polymers. *Synth. Met.* **1991**, *45*, 173.

(75) Bubnova, O.; Khan, Z. U.; Wang, H.; Braun, S.; Evans, D. R.; Fabretto, M.; Hojati-Talemi, P.; Dagnelund, D.; Arlin, J. B.; Geerts, Y. H.; et al. Semi-metallic polymers. *Nat. Mater.* **2014**, *13*, 190.

(76) Park, T.; Park, C.; Kim, B.; Shin, H.; Kim, E. Flexible PEDOT electrodes with large thermoelectric power factors to generate electricity by the touch of fingertips. *Energy Environ. Sci.* **2013**, *6*, 788.

(77) Culebras, M.; Gomez, C. M.; Cantarero, A. Enhanced thermoelectric performance of PEDOT with different counter-ions optimized by chemical reduction. *J. Mater. Chem. A* **2014**, *2*, 10109.

(78) Kim, B.; Shin, H.; Park, T.; Lim, H.; Kim, E. NIR-Sensitive Poly(3,4-ethylenedioxyselenophene) Derivatives for Transparent Photo-Thermo-Electric Converters. *Adv. Mater.* **2013**, *25*, 5483.

(79) Kim, G. H.; Shao, L.; Zhang, K.; Pipe, K. P. Engineered doping of organic semiconductors for enhanced thermoelectric efficiency. *Nat. Mater.* **2013**, *12*, 719.

(80) Lee, S. H.; Park, H.; Kim, S.; Son, W.; Cheong, I. W.; Kim, J. H. Transparent and flexible organic semiconductor nanofilms with enhanced thermoelectric efficiency. *J. Mater. Chem. A* **2014**, *2*, 7288.

(81) See, K. C.; Feser, J. P.; Chen, C. E.; Majumdar, A.; Urban, J. J.; Segalman, R. A. Water-Processable Polymer-Nanocrystal Hybrids for Thermoelectrics. *Nano Lett.* **2010**, *10*, 4664.

(82) Villalva, D. R.; Haque, M. A.; Nugraha, M. I.; Baran, D. Enhanced Thermoelectric Performance and Lifetime in Acid-Doped PEDOT:PSS Films Via Work Function Modification. *ACS Appl. Energy Mater.* **2020**, *3*, 9126.

(83) Wang, L.; Zhang, Z.; Liu, Y.; Wang, B.; Fang, L.; Qiu, J.; Zhang, K.; Wang, S. Exceptional thermoelectric properties of flexible organic-inorganic hybrids with monodispersed and periodic nanophase. *Nat. Commun.* **2018**, *9*, 3817.

(84) Qu, S. Y.; Yao, Q.; Wang, L. M.; Chen, Z. H.; Xu, K. Q.; Zeng, H. R.; Shi, W.; Zhang, T. S.; Uher, C.; Chen, L. D. Highly anisotropic P3HT films with enhanced thermoelectric performance via organic small molecule epitaxy. *NPG Asia Mater.* **2016**, *8*, No. e292.

(85) Untilova, V.; Hynynen, J.; Hofmann, A. I.; Scheunemann, D.; Zhang, Y.; Barlow, S.; Kemerink, M.; Marder, S. R.; Biniek, L.; Müller, C.; et al. High Thermoelectric Power Factor of Poly(3-hexylthiophene) through In-Plane Alignment and Doping with a Molybdenum Dithiolene Complex. *Macromolecules* **2020**, *53*, 6314.

(86) Hiroshige, Y.; Ookawa, M.; Toshima, N. Thermoelectric figure-of-merit of iodine-doped copolymer of phenylenevinylene with dialkoxyphenylenevinylene. *Synth. Met.* **2007**, *157*, 467.

(87) Kiyota, Y.; Kadoya, T.; Yamamoto, K.; Iijima, K.; Higashino, T.; Kawamoto, T.; Takimiya, K.; Mori, T. Benzothienobenzothiophene-Based Molecular Conductors: High Conductivity, Large Thermoelectric Power Factor, and One-Dimensional Instability. *J. Am. Chem. Soc.* **2016**, *138*, 3920.

(88) Patel, S. N.; Glaudell, A. M.; Peterson, K. A.; Thomas, E. M.; O'Hara, K. A.; Lim, E.; Chabinyc, M. L. Morphology controls the thermoelectric power factor of a doped semiconducting polymer. *Sci. Adv.* **2017**, *3*, No. e1700434.

(89) Vijayakumar, V.; Zhong, Y.; Untilova, V.; Bahri, M.; Herrmann, L.; Biniek, L.; Leclerc, N.; Brinkmann, M. Bringing Conducting Polymers to High Order: Toward Conductivities beyond 105 S cm-1 and Thermoelectric Power Factors of 2 mW m-1 K-2. *Adv. Energy Mater.* **2019**, *9*, 1900266.

(90) Wang, Y.; Zhang, S. M.; Deng, Y. Flexible low-grade energy utilization devices based on high-performance thermoelectric polyaniline/tellurium nanorod hybrid films. *J. Mater. Chem. A* **2016**, *4*, 3554.

(91) Li, H.; Song, J.; Xiao, J.; Wu, L.; Katz, H. E.; Chen, L. Synergestically Improved Molecular Doping and Carrier Mobility by Copolymerization of Donor-Acceptor and Donor-Donor Building Blocks for Thermoelectric Application. *Adv. Funct. Mater.* **2020**, *30*, 2004378.

(92) Sun, Y. M.; Sheng, P.; Di, C. A.; Jiao, F.; Xu, W.; Qiu, D.; Zhu, D. B. Organic Thermoelectric Materials and Devices Based on p- and n-Type Poly(metal 1,1,2,2-ethenetetrathiolate)s. *Adv. Mater.* **2012**, *24*, 932.

(93) Sun, Y. H.; Qiu, L.; Tang, L. P.; Geng, H.; Wang, H. F.; Zhang, F. J.; Huang, D. Z.; Xu, W.; Yue, P.; Guan, Y. S.; et al. Flexible n-Type High-Performance Thermoelectric Thin Films of Poly(nickel-ethyl-enetetrathiolate) Prepared by an Electrochemical Method. *Adv. Mater.* **2016**, *28*, 3351.

(94) Chen, Y. N.; He, M. H.; Liu, B.; Bazan, G. C.; Zhou, J.; Liang, Z. Q. Bendable n-Type Metallic Nanocomposites with Large Thermoelectric Power Factor. *Adv. Mater.* **2017**, *29*, 1604752.

(95) Liu, J.; van der Zee, B.; Alessandri, R.; Sami, S.; Dong, J.; Nugraha, M. I.; Barker, A. J.; Rousseva, S.; Qiu, L.; Qiu, X.; et al. N-type organic thermoelectrics: demonstration of ZT > 0.3. *Nat. Commun.* **2020**, *11*, 5694.

(96) Lu, Y.; Yu, Z.-D.; Liu, Y.; Ding, Y.-F.; Yang, C.-Y.; Yao, Z.-F.; Wang, Z.-Y.; You, H.-Y.; Cheng, X.-F.; Tang, B.; et al. The Critical Role of Dopant Cations in Electrical Conductivity and Thermoelectric Performance of n-Doped Polymers. *J. Am. Chem. Soc.* **2020**, *142*, 15340.

(97) Yang, C.-Y.; Ding, Y.-F.; Huang, D.; Wang, J.; Yao, Z.-F.; Huang, C.-X.; Lu, Y.; Un, H.-I.; Zhuang, F.-D.; Dou, J.-H.; et al. A thermally activated and highly miscible dopant for n-type organic thermoelectrics. *Nat. Commun.* **2020**, *11*, 3292.

(98) Huang, D. Z.; Wang, C.; Zou, Y.; Shen, X. X.; Zang, Y. P.; Shen, H. G.; Gao, X. K.; Yi, Y. P.; Xu, W.; Di, C. A.; et al. Bismuth Interfacial Doping of Organic Small Molecules for High Performance n-type Thermoelectric Materials. *Angew. Chem., Int. Ed.* **2016**, *55*, 10672.

(99) Huang, D.; Yao, H.; Cui, Y.; Zou, Y.; Zhang, F.; Wang, C.; Shen, H.; Jin, W.; Zhu, J.; Diao, Y.; et al. Conjugated-Backbone Effect of Organic Small Molecules for n-Type Thermoelectric Materials with ZT over 0.2. *J. Am. Chem. Soc.* **2017**, *139*, 13013.

(100) McCullough, R. D. The chemistry of conducting polythiophenes. *Adv. Mater.* **1998**, *10*, 93.

(101) Groenendaal, B. L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. Poly(3,4-ethylenedioxythiophene) and its derivatives: Past, present, and future. *Adv. Mater.* **2000**, *12*, 481.

(102) Zaumseil, J.; Sirringhaus, H. Electron and ambipolar transport in organic field-effect transistors. *Chem. Rev.* **2007**, *107*, 1296.

(103) Wu, W. P.; Liu, Y. Q.; Zhu, D. B. pi-Conjugated molecules with fused rings for organic field-effect transistors: design, synthesis and applications. *Chem. Soc. Rev.* **2010**, *39*, 1489.

(104) Sekitani, T.; Nakajima, H.; Maeda, H.; Fukushima, T.; Aida, T.; Hata, K.; Someya, T. Stretchable active-matrix organic light-emitting
diode display using printable elastic conductors. Nat. Mater. 2009, 8, 494.

(105) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated polymer-based organic solar cells. Chem. Rev. 2007, 107, 1324.

(106) Dong, H. L.; Wang, C. L.; Hu, W. P. High performance organic semiconductors for field-effect transistors. Chem. Commun. 2010, 46, 5211.

(107) Su, W. P.; Schrieffer, J. R.; Heeger, A. J. Soliton excitations in polyacetylene. Phys. Rev. B: Condens. Matter Mater. Phys. 1980, 22, 2099.

(108) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. Synthesis of electrically conducting organic polymers: halogen derivatives of polyacetylene, (CH). J. Chem. Soc., Chem. Commun. 1977, 1977, 578.

(109) Chiang, C. K.; Fincher, C. R.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.; Louis, E. J.; Gau, S. C.; MacDiarmid, A. G. Electrical Conductivity in Doped Polyacetylene. Phys. Rev. Lett. 1977, 39, 1098.

(110) Diaz, A. F.; Castillo, J. I.; Logan, J. A.; Lee, W.-Y. Electrochemistry of conducting polypyrrole films. J. Electroanal. Chem. Interfacial Electrochem. 1981, 129, 115.

(111) MacDiarmid, A. G.; Epstein, A. J. Polyanilines: a novel class of conducting polymers. Faraday Discuss. Chem. Soc. 1989, 8, 317.

(112) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; et al. Two-dimensional charge transport in self-organized, high-mobility conjugated polymers. Nature 1999, 401, 685.

(113) Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F. P. V.; Stingelin, N.; Smith, P.; Toney, M. F.; Salleo, A. A general relationship between disorder, aggregation and charge transport in conjugated polymers. Nat. Mater. 2013, 12, 1038.

(114) Park, Y. W.; Heeger, A. J.; Druy, M. A.; Macdiarmid, A. G. Electrical Transport in Doped Polyacetylene. J. Chem. Phys. 1980, 73, 946.

(115) Park, Y. W.; Yoon, C. O.; Lee, C. H.; Shirakawa, H.; Suezaki, Y.; Akagi, K. Conductivity and Thermoelectric-Power of the Newly Processed Polyacetylene. Synth. Met. 1989, 28, D27.

(116) Kaneko, H.; Ishiguro, T.; Takahashi, A.; Tsukamoto, J. Magnetoresistance and Thermoelectric-Power Studies of Metal-Nonmetal Transition in Iodine-Doped Polyacetylene. Synth. Met. 1993, 57, 4900.

(117) Bubnova, O.; Berggren, M.; Crispin, X. Tuning the Thermoelectric Properties of Conducting Polymers in an Electrochemical Transistor. J. Am. Chem. Soc. 2012, 134, 16456.

(118) Shi, W.; Zhao, T. Q.; Xi, J. Y.; Wang, D.; Shuai, Z. G. Unravelling Doping Effects on PEDOT at the Molecular Level: From Geometry to Thermoelectric Transport Properties. J. Am. Chem. Soc. 2015, 137, 12929.

(119) Yue, R. R.; Xu, J. K. Poly(3,4-ethylenedioxythiophene) as promising organic thermoelectric materials: A mini-review. Synth. Met. 2012, 162, 912.

(120) Shi, H.; Liu, C. C.; Jiang, Q. L.; Xu, J. K. Effective Approaches to Improve the Electrical Conductivity of PEDOT:PSS: A Review. Adv. Electron. Mater. 2015, 1, 1500017.

(121) Sun, K.; Zhang, S. P.; Li, P. C.; Xia, Y. J.; Zhang, X.; Du, D. H.; Isikgor, F. H.; Ouyang, J. Y. Review on application of PEDOTs and PEDOT: PSS in energy conversion and storage devices. J. Mater. Sci.: Mater. Electron. 2015, 26, 4438.

(122) Wei, Q. S.; Mukaida, M.; Kirihara, K.; Naitoh, Y.; Ishida, T. Recent Progress on PEDOT-Based Thermoelectric Materials. Materials 2015, 8, 732.

(123) Li, Y. Y.; Du, Y.; Dou, Y. C.; Cai, K. F.; Xu, J. Y. PEDOT-based thermoelectric nanocomposites - A mini-review. Synth. Met. 2017, 226, 119.

(124) Scheunemann, D.; Vijayakumar, V.; Zeng, H.; Durand, P.; Leclerc, N.; Brinkmann, M.; Kemerink, M. Rubbing and Drawing: Generic Ways to Improve the Thermoelectric Power Factor of Organic Semiconductors? Adv. Electron. Mater. 2020, 6, 2000218.

(125) Lim, E.; Glaudell, A. M.; Miller, R.; Chabinyc, M. L. The Role of Ordering on the Thermoelectric Properties of Blends of Regioregular and Regiorandom Poly(3-hexylthiophene). Adv. Electron. Mater. 2019, 5, 1800915.

(126) Hynneny, J.; Kiefer, D.; Müller, C. Influence of crystallinity on the thermoelectric power factor of P3HT vapour-doped with F4TCNQ. RSC Adv. 2018, 8, 1593.

(127) Vijayakumar, V.; Durand, P.; Zeng, H.; Untilova, V.; Herrmann, L.; Algayer, P.; Leclerc, N.; Brinkmann, M. Influence of dopant size and doping method on the structure and thermoelectric properties of PBTTT films doped with F6TCNNQ and F4TCNQ. J. Mater. Chem. C 2020, 8, 16470.

(128) Patel, S. N.; Glaudell, A. M.; Peterson, K. A.; Thomas, E. M.; O'Hara, K. A.; Lim, E.; Chabinyc, M. L. Morphology controls the thermoelectric power factor of a doped semiconducting polymer. Sci. Adv. 2017, 3, No. e1700434.

(129) Wang, L. M.; Yao, Q.; Xiao, J. X.; Zeng, K. Y.; Shi, W.; Qu, S. Y.; Chen, L. D. Enhanced Thermoelectric Properties of Polyaniline Nanofilms Induced by Self-Assembled Supramolecules. Chem. - Asian J. 2016, 11, 1955.

(130) Kee, S.; Haque, M. A.; Corzo, D.; Alshareef, H. N.; Baran, D. Self-Healing and Stretchable 3D-Printed Organic Thermoelectrics. Adv. Funct. Mater. 2019, 29, 1905426.

(131) Kee, S.; Kim, H.; Paleti, S. H. K.; El Labban, A.; Neophytou, M.; Emwas, A.-H.; Alshareef, H. N.; Baran, D. Highly Stretchable and Air-Stable PEDOT:PSS/Ionic Liquid Composites for Efficient Organic Thermoelectrics. Chem. Mater. 2019, 31, 3519.

(132) Kim, N.; Lienemann, S.; Petsagkourakis, I.; Alemu Mengistie, D.; Kee, S.; Ederth, T.; Gueskine, V.; Leclère, P.; Lazzaroni, R.; Crispin, X.; Tybrandt, K. Elastic conducting polymer composites in thermoelectric modules. Nat. Commun. 2020, 11, 1424.

(133) Sun, Y. M.; Wei, Z. M.; Xu, W.; Zhu, D. B. A three-in-one improvement in thermoelectric properties of polyaniline brought by nanostructures. Synth. Met. 2010, 160, 2371.

(134) Wu, J. S.; Sun, Y. M.; Pei, W. B.; Huang, L.; Xu, W.; Zhang, Q. C. Polypyrrole nanotube film for flexible thermoelectric application. Synth. Met. 2014, 196, 173.

(135) Lee, H. J.; Anoop, G.; Lee, H. J.; Kim, C.; Park, J. W.; Choi, J.; Kim, H.; Kim, Y. J.; Lee, E.; Lee, S. G.; et al. Enhanced thermoelectric performance of PEDOT: PSS/PANI-CSA polymer multilayer structures. Energy Environ. Sci. 2016, 9, 2806.

(136) Lu, G. H.; Bu, L. J.; Li, S. J.; Yang, X. N. Bulk Interpenetration Network of Thermoelectric Polymer in Insulating Supporting Matrix. Adv. Mater. 2014, 26, 2359.

(137) Kim, Y.; Lund, A.; Noh, H.; Hofmann, A. I.; Craighero, M.; Darabi, S.; Zokaei, S.; Park, J. I.; Yoon, M.-H.; Müller, C. Robust PEDOT:PSS Wet-Spun Fibers for Thermoelectric Textiles. Macromol. Mater. Eng. 2020, 305, 1900749.

(138) Lund, A.; Tian, Y.; Darabi, S.; Müller, C. A polymer-based textile thermoelectric generator for wearable energy harvesting. J. Power Sources 2020, 480, 228836.

(139) Coates, N. E.; Yee, S. K.; McCulloch, B.; See, K. C.; Majumdar, A.; Segalman, R. A.; Urban, J. J. Effect of Interfacial Properties on Polymer-Nanocrystal Thermoelectric Transport. Adv. Mater. 2013, 25, 1629.

(140) Zaia, E. W.; Sahu, A.; Zhou, P.; Gordon, M. P.; Forster, J. D.; Aloni, S.; Liu, Y. S.; Guo, J. H.; Urban, J. J. Carrier Scattering at Alloy Nanointerfaces Enhances Power Factor in PEDOT:PSS Hybrid Thermoelectrics. Nano Lett. 2016, 16, 3352.

(141) Zhang, B.; Sun, J.; Katz, H. E.; Fang, F.; Opila, R. L. Promising Thermoelectric Properties of Commercial PEDOT:PSS Materials and Their Bi2Te3 Powder Composites. ACS Appl. Mater. Interfaces 2010, 2, 3170.

(142) He, M.; Ge, J.; Lin, Z. Q.; Feng, X. H.; Wang, X. W.; Lu, H. B.; Yang, Y. L.; Qiu, F. Thermopower enhancement in conducting polymer nanocomposites via carrier energy scattering at the organic-inorganic semiconductor interface. Energy Environ. Sci. 2012, 5, 8351.

(143) Levesque, I.; Bertrand, P. O.; Blouin, N.; Leclerc, M.; Zecchin, S.; Zotti, G.; Ratcliffe, C. I.; Klug, D. D.; Gao, X.; Gao, F. M.; et al.
Synthesis and thermoelectric properties of polycarbazole, polyindolocarbazole, and polydiindolocarbazole derivatives. *Chem. Mater.* **2007**, *19*, 2128.

(144) Aich, R. B.; Blouin, N.; Bouchard, A.; Leclerc, M. Electrical and Thermoelectric Properties of Poly(2,7-Carbazole) Derivatives. *Chem. Mater.* **2009**, *21*, 751.

(145) Murase, I.; Ohnishi, T.; Noguchi, T.; Hirooka, M. Alkoxy-substituent effect of poly (p-phenylene vinylene) conductivity. *Polym. Comm.* **1985**, *26*, 362.

(146) Hiroshige, Y.; Ookawa, M.; Toshima, N. High thermoelectric performance of poly(2,5-dimethoxyphenylenevinylene) and its derivatives. *Synth. Met.* **2006**, *156*, 1341.

(147) Li, H.; Liu, F.; Wang, X.; Gu, C.; Wang, P.; Fu, H. Diketopyrrolopyrrole-Thiophene-Benzothiadiazole Random Copolymers: An Effective Strategy To Adjust Thin-Film Crystallinity for Transistor and Photovoltaic Properties. *Macromolecules* **2013**, *46*, 9211.

(148) Scholes, D. T.; Yee, P. Y.; McKeown, G. R.; Li, S.; Kang, H.; Lindemuth, J. R.; Xia, X.; King, S. C.; Seferos, D. S.; Tolbert, S. H.; et al. Designing Conjugated Polymers for Molecular Doping: The Roles of Crystallinity, Swelling, and Conductivity in Sequentially-Doped Selenophene-Based Copolymers. *Chem. Mater.* **2019**, *31*, 73.

(149) Zuo, G. Z.; Liu, X. J.; Fahlman, M.; Kemerink, M. High Seebeck Coefficient in Mixtures of Conjugated Polymers. *Adv. Funct. Mater.* **2018**, *28*, 1703280.

(150) Mahan, G.; Sofo, J. The best thermoelectric. *Proc. Natl. Acad. Sci. U. S. A.* **1996**, *93*, 7436.

(151) Sun, J.; Yeh, M. L.; Jung, B. J.; Zhang, B.; Feser, J.; Majumdar, A.; Katz, H. E. Simultaneous Increase in Seebeck Coefficient and Conductivity in a Doped Poly(alkylthiophene) Blend with Defined Density of States. *Macromolecules* **2010**, *43*, 2897.

(152) Li, H.; Plunkett, E.; Cai, Z.; Qiu, B.; Wei, T.; Chen, H.; Thon, S. M.; Reich, D. H.; Chen, L.; Katz, H. E. Dopant-Dependent Increase in Seebeck Coefficient and Electrical Conductivity in Blended Polymers with Offset Carrier Energies. *Adv. Electron. Mater.* **2019**, *5*, 1800618.

(153) Zuo, G.; Abdalla, H.; Kemerink, M. Conjugated Polymer Blends for Organic Thermoelectrics. *Adv. Electron. Mater.* **2019**, *5*, 1800821.

(154) Kim, J.-Y.; Kim, J.-T.; Kwon, M.-H.; Han, D.-K.; Kwon, S.-J. ATR-infrared spectroscopic study of n-doped polyacetylene films. *Macromol. Res.* **2007**, *15*, 5.

(155) Hua, M. Y.; Hwang, G. W.; Chuang, Y. H.; Chen, S. A.; Tsai, R. Y. Soluble n-doped polyaniline: Synthesis and characterization. *Macromolecules* **2000**, *33*, 6235.

(156) Schlitz, R. A.; Brunetti, F. G.; Glaudell, A. M.; Miller, P. L.; Brady, M. A.; Takacs, C. J.; Hawker, C. J.; Chabinyc, M. L. Solubility-Limited Extrinsic n-Type Doping of a High Electron Mobility Polymer for Thermoelectric Applications. *Adv. Mater.* **2014**, *26*, 2825.

(157) Wei, P.; Oh, J. H.; Dong, G. F.; Bao, Z. N. Use of a 1H-Benzoimidazole Derivative as an n-Type Dopant and To Enable Air-Stable Solution-Processed n-Channel Organic Thin-Film Transistors. *J. Am. Chem. Soc.* **2010**, *132*, 8852.

(158) Kiefer, D.; Giovannitti, A.; Sun, H. D.; Biskup, T.; Hofmann, A.; Koopmans, M.; Cendra, C.; Weber, S.; Koster, L. J. A.; Olsson, E.; et al. Enhanced n-Doping Efficiency of a Naphthalenediimide-Based Copolymer through Polar Side Chains for Organic Thermoelectrics. *ACS Energy Lett.* **2018**, *3*, 278.

(159) Liu, J.; Qiu, L.; Alessandri, R.; Qiu, X. K.; Portale, G.; Dong, J. J.; Talsma, W.; Ye, G.; Sengrian, A. A.; Souza, P. C. T.; et al. Enhancing Molecular n-Type Doping of Donor-Acceptor Copolymers by Tailoring Side Chains. *Adv. Mater.* **2018**, *30*, 1704630.

(160) Wang, S. H.; Sun, H. D.; Ail, U.; Vagin, M.; Persson, P. O. A.; Andreasen, J. W.; Thiel, W.; Berggren, M.; Crispin, X.; Fazzi, D.; et al. Thermoelectric Properties of Solution-Processed n-Doped Ladder-Type Conducting Polymers. *Adv. Mater.* **2016**, *28*, 10764.

(161) Wang, S.; Ruoko, T.-P.; Wang, G.; Riera-Galindo, S.; Hultmark, S.; Puttisong, Y.; Moro, F.; Yan, H.; Chen, W. M.; Berggren, M.; et al. Sequential Doping of Ladder-Type Conjugated Polymers for Thermally Stable n-Type Organic Conductors. *ACS Appl. Mater. Interfaces* **2020**, *12*, S3003.

(162) Nava, D.; Shin, Y.; Massetti, M.; Jiao, X. C.; Biskup, T.; Jagadeesh, M. S.; Calloni, A.; Duo, L.; Lanzani, G.; McNeill, C. R.; et al. Drastic Improvement of Air Stability in an n-Type Doped Naphthalene-Diimide Polymer by Thionation. *ACS Appl. Energy Mater.* **2018**, *1*, 4626.

(163) Zhan, X.; Facchetti, A.; Barlow, S.; Marks, T. J.; Ratner, M. A.; Wasielewski, M. R.; Marder, S. R. Rylene and Related Diimides for Organic Electronics. *Adv. Mater.* **2011**, *23*, 268.

(164) Shin, Y.; Massetti, M.; Komber, H.; Biskup, T.; Nava, D.; Lanzani, G.; Caironi, M.; Sommer, M. Improving Miscibility of a Naphthalene Diimide-Bithiophene Copolymer with n-Type Dopants through the Incorporation of “Kinked” Monomers. *Adv. Electron. Mater.* **2018**, *4*, 1700581.

(165) Naab, B. D.; Gu, X. D.; Kurosawa, T.; To, J. W. F.; Salleo, A.; Bao, Z. A. Role of Polymer Structure on the Conductivity of N-Doped Polymers. *Adv. Electron. Mater.* **2016**, *2*, 1600004.

(166) Shi, K.; Zhang, F. J.; Di, C. A.; Yan, T. W.; Zou, Y.; Zhou, X.; Zhu, D. B.; Wang, J. Y.; Pei, J. Toward High Performance n-Type Thermoelectric Materials by Rational Modification of BDPPV Backbones. *J. Am. Chem. Soc.* **2015**, *137*, 6979.

(167) Zhao, X.; Madan, D.; Cheng, Y.; Zhou, J.; Li, H.; Thon, S. M.; Bragg, A. E.; DeCoster, M. E.; Hopkins, P. E.; Katz, H. E. High Conductivity and Electron-Transfer Validation in an n-Type Fluoride-Anion-Doped Polymer for Thermoelectrics in Air. *Adv. Mater.* **2017**, *29*, 1606928.

(168) Han, J.; Fan, H.; Zhang, Q.; Hu, Q.; Russell, T. P.; Katz, H. E. Dichlorinated Dithienylethene-Based Copolymers for Air-Stable n-Type Conductivity and Thermoelectricity. *Adv. Funct. Mater.* **2021**, *31*, 2005901.

(169) Yang, C.-Y.; Stoeckel, M.-A.; Ruoko, T.-P.; Wu, H.-Y.; Liu, X.; Kolhe, N. B.; Wu, Z.; Puttisong, Y.; Musumeci, C.; Massetti, M.; et al. A high-conductivity n-type polymeric ink for printed electronics. *Nat. Commun.* **2021**, *12*, 2354.

(170) Jiao, F.; Di, C.-A.; Sun, Y.; Sheng, P.; Xu, W.; Zhu, D. Inkjet-printed flexible organic thin-film thermoelectric devices based on p- and n-type poly(metal 1,1,2,2-ethenetetrathiolate)s/polymer composites through ball-milling. *Philos. Trans. R. Soc., A* **2014**, *372*, 20130008.

(171) Menon, A. K.; Wolfe, R. M. W.; Marder, S. R.; Reynolds, J. R.; Yee, S. K. Systematic Power Factor Enhancement in n-Type NiETT/PVDF Composite Films. *Adv. Funct. Mater.* **2018**, *28*, 1801620.

(172) Wolfe, R. M. W.; Menon, A. K.; Fletcher, T. R.; Marder, S. R.; Reynolds, J. R.; Yee, S. K. Simultaneous Enhancement in Electrical Conductivity and Thermopower of n-Type NiETT/PVDF Composite Films by Annealing. *Adv. Funct. Mater.* **2018**, *28*, 1803275.

(173) Saglio, B.; Mura, M.; Massetti, M.; Scuratti, F.; Beretta, D.; Jiao, X.; McNeill, C. R.; Sommer, M.; Famulari, A.; Lanzani, G.; et al. N-Alkyl substituted 1H-benzimidazoles as improved n-type dopants for a naphthalene-diimide based copolymer. *J. Mater. Chem. A* **2018**, *6*, 15294.

(174) Chen, J. M.; Wang, D.; Shuai, Z. G. First-Principles Predictions of Thermoelectric Figure of Merit for Organic Materials: Deformation Potential Approximation. *J. Chem. Theory Comput.* **2012**, *8*, 3338.

(175) Kwak, J. F.; Chaikin, P. M.; Russel, A. A.; Garito, A. F.; Heeger, A. J. Anisotropic Thermoelectric-Power of TTF-TCNQ. *Solid State Commun.* **1975**, *16*, 729.

(176) Zhang, Q.; Sun, Y. M.; Xu, W.; Zhu, D. B. Organic Thermoelectric Materials: Emerging Green Energy Materials Converting Heat to Electricity Directly and Efficiently. *Adv. Mater.* **2014**, *26*, 6829.

(177) Itahara, H.; Maesato, M.; Asahi, R.; Yamochi, H.; Saito, G. Thermoelectric Properties of Organic Charge-Transfer Compounds. *J. Electron. Mater.* **2009**, *38*, 1171.

(178) Oyama, M.; Wei, Q. S.; Mukaida, M.; Kawanishi, Y.; Ishida, T. Electrical conduction and thermoelectric properties of tetrathaifulvalene-tetracyanoquinodimethane cast films prepared with N,N-dimethylacetamide. *Synth. Met.* **2017**, *230*, 12.

(179) Sato, R.; Kiyota, Y.; Kadoya, T.; Kawamoto, T.; Mori, T. Thermoelectric power of oriented thin-film organic conductors. *RSC Adv.* **2016**, *6*, 41040.
(180) Pudzis, K.; Vembris, A.; Rutkis, M.; Woodward, S. Thin Film Organic Thermoelectric Generator Based on Tetrathiotetracene. *Adv. Electron. Mater.* **2017**, *3*, 1600429.

(181) Sun, Y. M.; Xu, W.; Di, C. A.; Zhu, D. B. Metal-organic complexes-towards promising organic thermoelectric materials. *Synth. Met.* **2017**, *225*, 22.

(182) Rincon-Garcia, L.; Ismael, A. K.; Evangelici, C.; Grace, I.; Rubio-Bollinger, G.; Porfyrakis, K.; Agrait, N.; Lambert, C. J. Molecular design and control of fullerene-based bi-thermoelectric materials. *Nat. Mater.* **2016**, *15*, 289.

(183) Kojima, H.; Abe, R.; Ito, M.; Tomatsu, Y.; Fujiwara, F.; Matsubara, R.; Yoshimoto, N.; Nakamura, M. Giant Seebeck effect in pure fullerene thin films. *Appl. Phys. Express* **2015**, *8*, 121301.

(184) Park, E. B.; Shim, J. W.; Choi, H. J.; Park, Y. W. Anomalous Thermoelectric-Power of the Alkali-Metal Doped and the Pyrolyzed Fullerene. *Synth. Met.* **1993**, *56*, 3258.

(185) Tang, J. K.; Chepko, C.; Wang, W. D.; Wang, X. J.; Zhao, G. L. Crossover Behavior of Variable Range Hopping in Bi-Doped C-60. *J. Supercond. Novel Magn.* **2010**, *23*, 873.

(186) Barbot, A.; Di Bin, C.; Lucas, B.; Ratier, B.; Aldissi, M. N-type doping and thermoelectric properties of co-sublimed cesium-carbonate-doped fullerene. *J. Mater. Sci.* **2013**, *48*, 2785.

(187) Almutlaq, N.; Al-Galiby, Q.; Bailey, S.; Lambert, C. J. Identification of a positive-Seebeck-coefficient exohedral fullerene. *Nanoscale* **2016**, *8*, 13597.

(188) Hayashi, K.; Shinano, T.; Miyazaki, Y.; Kajitani, T. Fabrication of iodine-doped pentacene thin films for organic thermoelectric devices. *J. Appl. Phys.* **2011**, *109*, 023712.

(189) Liu, J.; Qiu, L.; Portale, G.; Koopmans, M.; Ten Brink, G.; Hummelen, J. C.; Koster, L. J. A. N-Type Organic Thermoelectrics: Improved Power Factor by Tailoring Host-Dopant Miscibility. *Adv. Mater.* **2017**, *29*, 1701641.

(190) Massetti, M.; Bonfadini, S.; Nava, D.; Butti, M.; Criante, L.; Lanzani, G.; Qiu, L.; Hummelen, J. C.; Liu, J.; Koster, L. J. A.; et al. Fully direct written organic micro-thermoelectric generators embedded in a plastic foil. *Nano Energy* **2020**, *75*, 104983.

(191) Russ, B.; Robb, M. J.; Brunetti, F. G.; Miller, P. L.; Perry, E. E.; Patel, S. N.; Ho, V.; Chang, W. B.; Urban, J. J.; Chabinyc, M. L.; et al. Power Factor Enhancement in Solution-Processed Organic n-Type Thermoelectrics Through Molecular Design. *Adv. Mater.* **2014**, *26*, 3473.

(192) Zuo, G. Z.; Li, Z. J.; Wang, E. G.; Kemerink, M. High Seebeck Coefficient and Power Factor in n-Type Organic Thermoelectrics. *Adv. Electron. Mater.* **2018**, *4*, 1700501.

(193) Russ, B.; Glaudell, A.; Urban, J. J.; Chabinyc, M. L.; Segalman, R. A. Organic thermoelectric materials for energy harvesting and temperature control. *Nat. Rev. Mater.* **2016**, *1*, 16050.

(194) Ushirokita, H.; Tada, H. In-plane Thermal Conductivity Measurement of Conjugated Polymer Films by Membrane-based AC Calorimetry. *Chem. Lett.* **2016**, *45*, 735.

(195) Choy, C. L.; Wong, Y. W.; Yang, G. W.; Kanamoto, T. Elastic modulus and thermal conductivity of ultradrawn polyethylene. *J. Polym. Sci., Part B: Polym. Phys.* **1999**, *37*, 3359.

(196) Shen, S.; Henry, A.; Tong, J.; Zheng, R.; Chen, G. Polyethylene nanofibres with very high thermal conductivities. *Nat. Nanotechnol.* **2010**, *5*, 251.

(197) Singh, V.; Bougher, T. L.; Weathers, A.; Cai, Y.; Bi, K.; Pettes, M. T.; McMenamin, S. A.; Lv, W.; Resler, D. P.; Gattuso, T. R.; et al. High thermal conductivity of chain-oriented amorphous polythiophene. *Nat. Nanotechnol.* **2014**, *9*, 384.

(198) Zhang, T.; Wu, X.; Luo, T. Polymer Nanofibers with Outstanding Thermal Conductivity and Thermal Stability: Fundamental Linkage between Molecular Characteristics and Macroscopic Thermal Properties. *J. Phys. Chem. C* **2014**, *118*, 21148.

(199) Liu, J.; Wang, X.; Li, D.; Coates, N. E.; Segalman, R. A.; Cahill, D. G. Thermal Conductivity and Elastic Constants of PEDOT:PSS with High Electrical Conductivity. *Macromolecules* **2015**, *48*, 585.

(200) Weathers, A.; Khan, Z. U.; Brooke, R.; Evans, D.; Pettes, M. T.; Andreasen, J. W.; Crispin, X.; Shi, L. Significant Electronic Thermal

Transport in the Conducting Polymer Poly(3,4-ethylenedioxythiophene). *Adv. Mater.* **2015**, *27*, 2101.

(201) Scheunemann, D.; Kemerink, M. Non-Wiedemann-Franz behavior of the thermal conductivity of organic semiconductors. *Phys. Rev. B: Condens. Matter Mater. Phys.* **2020**, *101*, 075206.

(202) Zapata-Arteaga, O.; Perevedentsev, A.; Marina, S.; Martin, J.; Reparaz, J. S.; Campoy-Quiles, M. Reduction of the Lattice Thermal Conductivity of Polymer Semiconductors by Molecular Doping. *ACS Energy Lett.* **2020**, *5*, 2972.

(203) Patel, S. N.; Chabinyc, M. L. Anisotropies and the thermoelectric properties of semiconducting polymers. *J. Appl. Polym. Sci.* **2017**, *134*, 44403.

(204) Kommandur, S.; Yee, S. K. An empirical model to predict temperature-dependent thermal conductivity of amorphous polymers. *J. Polym. Sci., Part B: Polym. Phys.* **2017**, *55*, 1160.

(205) Paulsen, B. D.; Fabiano, S.; Rivnay, J. Mixed Ionic-Electronic Transport in Polymers. *Annu. Rev. Mater. Res.* **2021**, *51*, 73.

(206) Peterson, K. A.; Thomas, E. M.; Chabinyc, M. L. Thermoelectric Properties of Semiconducting Polymers. *Annu. Rev. Mater. Res.* **2020**, *50*, 551.

(207) Dai, L. M.; Chang, D. W.; Baek, J. B.; Lu, W. Carbon Nanomaterials for Advanced Energy Conversion and Storage. *Small* **2012**, *8*, 1130.

(208) Blackburn, J. L.; Ferguson, A. J.; Cho, C.; Grunlan, J. C. Carbon-Nanotube-Based Thermoelectric Materials and Devices. *Adv. Mater.* **2018**, *30*, 1704386.

(209) Iijima, S. Helical microtubules of graphitic carbon. *Nature* **1991**, *354*, 56.

(210) Wilder, J. W. G.; Venema, L. C.; Rinzler, A. G.; Smalley, R. E.; Dekker, C. Electronic structure of atomically resolved carbon nanotubes. *Nature* **1998**, *391*, 59.

(211) Prasek, J.; Drbohlavova, J.; Chomoucka, J.; Hubalek, J.; Jasek, O.; Adam, V.; Kizek, R. Methods for carbon nanotubes synthesis—review. *J. Mater. Chem.* **2011**, *21*, 15872.

(212) Janas, D. Towards monochiral carbon nanotubes: a review of progress in the sorting of single-walled carbon nanotubes. *Mater. Chem. Front.* **2018**, *2*, 36.

(213) Weisman, R. B.; Bachilo, S. M. Dependence of Optical Transition Energies on Structure for Single-Walled Carbon Nanotubes in Aqueous Suspension: An Empirical Kataura Plot. *Nano Lett.* **2003**, *3*, 1235.

(214) Dukovic, G.; Wang, F.; Song, D.; Sfeir, M. Y.; Heinz, T. F.; Brus, L. E. Structural Dependence of Excitonic Optical Transitions and Band-Gap Energies in Carbon Nanotubes. *Nano Lett.* **2005**, *5*, 2314.

(215) Bergin, S. D.; Sun, Z.; Rickard, D.; Streich, P. V.; Hamilton, J. P.; Coleman, J. N. Multicomponent Solubility Parameters for Single-Walled Carbon Nanotube-Solvent Mixtures. *ACS Nano* **2009**, *3*, 2340.

(216) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Sorting carbon nanotubes by electronic structure using density differentiation. *Nat. Nanotechnol.* **2006**, *1*, 60.

(217) Tu, X.; Manohar, S.; Jagota, A.; Zheng, M. DNA sequence motifs for structure-specific recognition and separation of carbon nanotubes. *Nature* **2009**, *460*, 250.

(218) Wang, H.; Bao, Z. Conjugated polymer sorting of semiconducting carbon nanotubes and their electronic applications. *Nano Today* **2015**, *10*, 737.

(219) Hirotani, J.; Ohno, Y. Carbon Nanotube Thin Films for High-Performance Flexible Electronics Applications. *Top. Curr. Chem.* **2019**, *377*, 3.

(220) Ghosh, S.; Bachilo, S. M.; Weisman, R. B. Advanced sorting of single-walled carbon nanotubes by nonlinear density-gradient ultra-centrifugation. *Nat. Nanotechnol.* **2010**, *5*, 443.

(221) Liu, H.; Nishide, D.; Tanaka, T.; Kataura, H. Large-scale single-chirality separation of single-wall carbon nanotubes by simple gel chromatography. *Nat. Commun.* **2011**, *2*, 309.

(222) Fagan, J. A.; Khripin, C. Y.; Silvera Batista, C. A.; Simpson, J. R.; Hároz, E. H.; Hight Walker, A. R.; Zheng, M. Isolation of Specific Small-Diameter Single-Wall Carbon Nanotube Species via Aqueous Two-Phase Extraction. *Adv. Mater.* **2014**, *26*, 2800.
(223) Fagan, J. A.; Hároz, E. H.; Ihly, R.; Gui, H.; Blackburn, J. L.; Simpson, J. R.; Lam, S.; Hight Walker, A. R.; Doorn, S. K.; Zheng, M. Isolation of > 1 nm Diameter Single-Wall Carbon Nanotube Species Using Aqueous Two-Phase Extraction. ACS Nano 2015, 9, 5377.

(224) Nish, A.; Hwang, J.-Y.; Doig, J.; Nicholas, R. J. Highly selective dispersion of single-walled carbon nanotubes using aromatic polymers. Nat. Nanotechnol. 2007, 2, 640.

(225) Mistry, K. S.; Larsen, B. A.; Blackburn, J. L. High-Yield Dispersions of Large-Diameter Semiconducting Single-Walled Carbon Nanotubes with Tunable Narrow Chirality Distributions. ACS Nano 2013, 7, 2231.

(226) Tenent, R. C.; Barnes, T. M.; Bergeson, J. D.; Ferguson, A. J.; To, B.; Gedvilas, L. M.; Heben, M. J.; Blackburn, J. L. Ultrasmooth, Large-Area, High-Uniformity, Conductive Transparent Single-Walled-Carbon-Nanotube Films for Photovoltaics Produced by Ultrasonic Spraying. Adv. Mater. 2009, 21, 3210.

(227) Graf, A.; Zakharko, Y.; Schießl, S. P.; Backes, C.; Pfohl, M.; Flavel, B. S.; Zaumseil, J. Large scale, selective dispersion of long single-walled carbon nanotubes with high photoluminescence quantum yield by shear force mixing. Carbon 2016, 105, 593.

(228) An, C. J.; Kang, Y. H.; Lee, A. Y.; Jang, K.-S.; Jeong, Y.; Cho, S. Y. Foldable Thermoelectric Materials: Improvement of the Thermoelectric Performance of Directly Spun CNT Webs by Individual Control of Electrical and Thermal Conductivity. ACS Appl. Mater. Interfaces 2016, 8, 22142.

(229) Zhou, W.; Fan, Q.; Zhang, Q.; Cai, L.; Li, K.; Gu, X.; Yang, F.; Zhang, N.; Wang, Y.; Liu, H.; et al. High-performance and compact-designed flexible thermoelectric modules enabled by a reticulate carbon nanotube architecture. Nat. Commun. 2017, 8, 14886.

(230) Choi, J.; Jung, Y.; Yang, S. J.; Oh, J. Y.; Oh, J.; Jo, K.; Son, J. G.; Moon, S. E.; Park, C. R.; Kim, H. Flexible and Robust Thermoelectric Generators Based on All-Carbon Nanotube Yarn without Metal Electrodes. ACS Nano 2017, 11, 7608.

(231) An, C. J.; Kang, Y. H.; Song, H.; Jeong, Y.; Cho, S. Y. Highly Integrated and Flexible Thermoelectric Module Fabricated by Brush-Cast Doping of a Highly Aligned Carbon Nanotube Web. ACS Appl. Energy Mater. 2019, 2, 1093.

(232) Hung, N. T.; Nugraha, A. R.; Hasdeo, E. H.; Dresselhaus, M. S.; Saito, R. Diameter dependence of thermoelectric power of semiconducting carbon nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 165426.

(233) Hayashi, D.; Nakai, Y.; Kyakuno, H.; Yamamoto, T.; Miyata, Y.; Yanagi, K.; Maniwa, Y. Improvement of thermoelectric performance of single-wall carbon nanotubes by heavy doping: Effect of one-dimensional band multiplicity. Appl. Phys. Express 2016, 9, 125103.

(234) Kim, S. M.; Jang, J. H.; Kim, K. K.; Park, H. K.; Bae, J. J.; Yu, W. J.; Lee, I. H.; Kim, G.; Loc, D. D.; Kim, U. J.; et al. Reduction-Controlled Viologen in Bisolvent as an Environmentally Stable n-Type Dopant for Carbon Nanotubes. J. Am. Chem. Soc. 2009, 131, 327.

(235) Mistry, K. S.; Larsen, B. A.; Bergeson, J. D.; Barnes, T. M.; Teeter, G.; Engtrakul, C.; Blackburn, J. L. n-Type Transparent Conducting Films of Small Molecule and Polymer Amine Doped Single-Walled Carbon Nanotubes. ACS Nano 2011, 5, 3714.

(236) Nonoguchi, Y.; Ohashi, K.; Kanazawa, R.; Ashiba, K.; Hata, K.; Nakagawa, T.; Adachi, C.; Tanase, T.; Kawai, T. Systematic Conversion of Single Walled Carbon Nanotubes into n-type Thermoelectric Materials by Molecular Dopants. Sci. Rep. 2013, 3, 3344.

(237) Nonoguchi, Y.; Nakano, M.; Murayama, T.; Hagino, H.; Hama, S.; Miyazaki, K.; Matsubara, R.; Nakamura, M.; Kawai, T. Simple Salt-Coordinated n-Type Nanocarbon Materials Stable in Air. Adv. Funct. Mater. 2016, 26, 3021.

(238) Li, L.-J.; Khlobystov, A. N.; Wiltshire, J. G.; Briggs, G. A. D.; Nicholas, R. J. Diameter-selective encapsulation of metalloenzymes in single-walled carbon nanotubes. Nat. Mater. 2005, 4, 481.

(239) Sauer, M.; Shiozawa, H.; Ayala, P.; Ruiz-Soria, G.; Kataura, H.; Yanagi, K.; Krause, S.; Pichler, T. In situ filling of metallic single-walled carbon nanotubes with ferrocene molecules. Phys. Status Solidi B 2012, 249, 2408.

(240) Sauer, M.; Shiozawa, H.; Ayala, P.; Ruiz-Soria, G.; Liu, X.; Chernov, A.; Krause, S.; Yanagi, K.; Kataura, H.; Pichler, T. Internal charge transfer in metallicity sorted ferrocene filled carbon nanotube hybrids. Carbon 2013, 59, 237.

(241) Fukumaru, T.; Fujigaya, T.; Nakashima, N. Development of n-type cobaltocene-encapsulated carbon nanotubes with remarkable thermoelectric property. Sci. Rep. 2015, 5, 7951.

(242) Kharlamova, M. V.; Kramberger, C.; Domanov, O.; Mittelberger, A.; Saito, T.; Yanagi, K.; Pichler, T.; Eder, D. Comparison of Doping Levels of Single-Walled Carbon Nanotubes Synthesized by Arc-Discharge and Chemical Vapor Deposition Methods by Encapsulated Silver Chloride. Phys. Status Solidi B 2018, 255, 1800178.

(243) Shimotani, H.; Tsuda, S.; Yuan, H.; Yomogida, Y.; Moriya, R.; Takenobu, T.; Yanagi, K.; Iwasa, Y. Continuous Band-Filling Control and One-Dimensional Transport in Metallic and Semiconducting Carbon Nanotube Tangled Films. Adv. Funct. Mater. 2014, 24, 3305.

(244) Yanagi, K.; Kanda, S.; Oshima, Y.; Kitamura, Y.; Kawai, H.; Yamamoto, T.; Takenobu, T.; Nakai, Y.; Maniwa, Y. Tuning of the Thermoelectric Properties of One-Dimensional Material Networks by Electric Double Layer Techniques Using Ionic Liquids. Nano Lett. 2014, 14, 6437.

(245) Shimizu, S.; Iizuka, T.; Kanahashi, K.; Pu, J.; Yanagi, K.; Takenobu, T.; Iwasa, Y. Thermoelectric Detection of Multi-Subband Density of States in Semiconducting and Metallic Single-Walled Carbon Nanotubes. Small 2016, 12, 3388.

(246) Fukuhara, K.; Ichinose, Y.; Nishidome, H.; Yomogida, Y.; Katsutani, F.; Komatsu, N.; Gao, W.; Kono, J.; Yanagi, K. Isotropic Seebeck coefficient of aligned single-wall carbon nanotube films. Appl. Phys. Lett. 2018, 113, 243105.

(247) Gao, W.; Komatsu, N.; Taylor, L. W.; Naik, G. V.; Yanagi, K.; Pasquali, M.; Kono, J. Macroscopically aligned carbon nanotubes for flexible and high-temperature electronics, optoelectronics, and thermoelectrics. J. Phys. D: Appl. Phys. 2020, 53, 063001.

(248) Ichinose, Y.; Yoshida, A.; Horiuchi, K.; Fukuhara, K.; Komatsu, N.; Gao, W.; Yomogida, Y.; Matsubara, M.; Yamamoto, T.; Kono, J.; et al. Solving the Thermoelectric Trade-Off Problem with Metallic Carbon Nanotubes. Nano Lett. 2019, 19, 7370.

(249) Blackburn, J. L.; Ferguson, A. J.; Reid, O. G. In Handbook of Carbon Nanomaterials; World Scientific, 2017; Vols. 9/10.

(250) Hertel, T. In Handbook of Carbon Nanomaterials; World Scientific, 2017; Vols. 9/10.

(251) Eckstein, K. H.; Oberndorfer, F.; Achsnich, M. M.; Schöppler, F.; Hertel, T. Quantifying Doping Levels in Carbon Nanotubes by Optical Spectroscopy. J. Phys. Chem. C 2019, 123, 30001.

(252) MacLeod, B. A.; Stanton, N. J.; Gould, I. E.; Wesenberg, D.; Ihly, R.; Owczarczyk, Z. R.; Hurst, K. E.; Fewox, C. S.; Folmar, C. N.; Holman Hughes, K.; et al. Large n- and p-type thermoelectric power factors from doped semiconducting single-walled carbon nanotube thin films. Energy Environ. Sci. 2017, 10, 2168.

(253) Blackburn, J. L.; Kang, S. D.; Roos, M. J.; Norton-Baker, B.; Miller, E. M.; Ferguson, A. J. Intrinsic and Extrinsically Limited Thermoelectric Transport within Semiconducting Single-Walled Carbon Nanotube Networks. Adv. Electron. Mater. 2019, 5, 1800910.

(254) Nonoguchi, Y.; Takata, A.; Goto, C.; Kitano, T.; Kawai, T. Thickness-dependent thermoelectric power factor of polymer-functionalized semiconducting carbon nanotube thin films. Sci. Technol. Adv. Mater. 2018, 19, 581.

(255) Blackburn, J. L.; Barnes, T. M.; Beard, M. C.; Kim, Y.-H.; Tenent, R. C.; McDonald, T. J.; To, B.; Coutts, T. J.; Heben, M. J. Transparent Conductive Single-Walled Carbon Nanotube Networks with Precisely Tunable Ratios of Semiconducting and Metallic Nanotubes. ACS Nano 2008, 2, 1266.

(256) Zhang, Q.; Hároz, E. H.; Jin, Z.; Ren, L.; Wang, X.; Arvidson, R. S.; Lüttge, A.; Kono, J. Plasmonic Nature of the Terahertz Conductivity Peak in Single-Wall Carbon Nanotubes. Nano Lett. 2013, 13, 5991.

(257) Graupner, R.; Abraham, J.; Vencelová, A.; Seyller, T.; Hennrich, F.; Kappes, M. M.; Hirsch, A.; Ley, L. Doping of single-walled carbon nanotube bundles by Brønsted acids. Phys. Chem. Chem. Phys. 2003, 5, S472.