Intrinsically stretchable thermoelectric (TE) materials are strongly urged for constituting mechanically sustainable heat-electricity conversion system in dynamic and transforming environments for energy-efficient heat harvesting/regulation1,2. As the diversifying thermal energy sources growingly assume more complex exteriors and the targets of interest are rather dynamic than static, large shape deformations and mechanical loadings are expected to exert highly localized stress/strain on thermoelectric generators (TEGs)3,4. For instance, human bodies offer a thermally reliable platform for body heat harvesting, but their surfaces are arbitrarily shaped and multidirectional movements and extension/contraction of joints, muscles, and skins can incur repeated tensile strains as large as 80%5–7. Therefore, in addition to material flexibility that ensures seamless TEG conformability/adaptability to heat sources, sufficient inherent stretchability is a prerequisite to accommodating such highly concentrated strains without compromising TE performance in mechanically harsh environments.
Despite the escalating need for simultaneously energy-efficient and stretchable TE materials, an empirical trade-off between the two has been reached. TE efficiency is evaluated by the dimensionless figure of merit (zT = σS2T/kthermal), where σ, S, and kthermal indicate electrical conductivity, Seebeck coefficient, and thermal conductivity, respectively8. As conventional high-zT (³10-1) inorganic materials of rigid/brittle nature are chronically plagued by extremely low fracture thresholds, material-oriented endeavours to impart stretchability have been centred around intrinsically flexible organic TE polymers (poly(3,4-ethylenedioxythiophene), polyaniline, etc.) of relatively lower TE efficiency7,9,10. Embedding such organic TE materials into elastomers/soft polymers (polyurethane, polyisoprene, etc.) as composites can complement their insufficient elasticity, but at the apparent expense of considerably undermined electrical conductivity by these electrically insulating matrices11–14. While plasticizing the polymeric TE network with surfactants15–17, polyols/polymers18–20, or ionic liquids11,21,22 may improve electrical conductivity by molecular rearrangements or secondary doping effects, both the stretchability enhancement and TE efficiency (zT ≤10-2) are still insufficient to lift their trade-off. Low dimensional carbon nanomaterials, such as graphene oxides and carbon nanotubes (CNTs), have also been extensively investigated to leverage their broad engineerable window for simultaneously facilitating TE performance and mechanical stability23,24. Nevertheless, their extremely high kthermal (>101 W m-1 K-1) severely diminishes zT25 and embedding into elastomer-based composites for stretchability compromises TE power factor (σS2)26–33. Consequently, the immense hurdle of simultaneously achieving outstanding stretchability (³100%) with high TE efficiency has not been surmounted to date, necessitating the material-level breakthrough.
Herein, we address such trade-off between TE efficiency and intrinsic stretchability via ingenious restructuring of TE network predominantly comprising single-walled carbon nanotubes (SWCNTs). To tackle this intricate challenge, we rationally redesign SWCNT network with small amounts of polymeric dopants and ionic liquid of high dielectric constants, which concurrently alleviate the intertube entanglement and regulate charge/thermal transport (Fig. 1a-d). Low-molecular-weight poly(acrylic acid) (PAA) of high chain mobility allocates greater free volume to SWCNT matrix and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) alleviates the aggregation among the π-π interacting nanotubes, restructuring the SWCNTs to flawlessly absorb tensile strains (≥180%) while preserving the power factor (Fig. 1a,b). Importantly, PAA and [EMIM][TFSI] not only hole-dope SWCNTs (Fig. 1c), but also improve carrier mobility by establishing a higher degree of debundled nanotube interconnections, thereby promoting the power factor by ³50%. Further assisted by intensified phonon scattering at SWCNT-polymer heterointerfaces, considerable thermal conductivity suppression (~70%) achieves nearly four-fold enhancement in zT (≥10-1) (Fig. 1d). Overall, our restructured SWCNT exhibits an unparalleled pair of the highest TE efficiency and all-around electromechanical reliability, outperforming any reported intrinsically stretchable TE material (Fig. 1e). To demonstrate the possibility of harnessing our innovative restructured SWCNT in thermal energy management system enduring intense mechanical deformation (Fig. 1f), we showcase Peltier-induced temperature regulation and a stretchable TEG exhibiting minimally compromised TE performance under large strains.
Enhanced thermoelectric properties of restructured SWCNT films
To develop intrinsically stretchable materials of high TE efficiency, SWCNTs are homogeneously restructured with agents that simultaneously regulate charge/heat transport and nanotube network interconnectivity with tenable free volume. Specifically, SWCNTs are homogeneously restructured with PAA of differing molecular weight (MW) and [EMIM][TFSI] by dispersing in diethylene glycol (DEG) via high-energy ball-milling (see Methods). The resulting viscoelastic TE ink can be directly blade-cast or printed on any substrate and micrometre-thick thin films are obtained by vacuum drying to characterize TE performance and electromechanical reliability.
As compared to the undoped pristine SWCNT, hybridization with 3,000 kDa PAA (SP3,000k, in short ) exhibits 17% rise in electrical conductivity (σ), owing to the p-type charge carrier (hole) transfer from oxygen and moisture adsorbed on hygroscopic PAA34, despite its electrically insulating nature. (Fig. 2a). Such p-type doping effect is more pronounced for SWCNT restructured with 1.8 kDa PAA (SP1.8k), suggesting that the lower-MW polymer dopants of greater chain mobility can be more homogeneously distributed within the SWCNT network. [EMIM]+ can adsorb on π-electron-rich SWCNTs to provide additional holes from its cationic imidazolium rings35 and the SWCNT restructured with both PAA1.8k and IL (SP1.8kIL) experiences nearly two-fold improvements in σ (812.3 S cm-1). In contrast to the remarkable electrical conductivity enhancement, the compromise in Seebeck coefficient (S), which is inversely proportional to the carrier concentration, is not as significant. This may suggest that the σ improvement is partly attributable to facilitated carrier mobility from nanotube junction rearrangement36,37 or heterointerface-induced bandgap adjustment. Overall, each hybridization step gradually promotes the power factor (σS2) to ~300.0 μW m-1 K-2 for the SP1.8kIL films (Supplementary Fig. 1).
To further elucidate the origin of the electrical conductivity encouragement, restructured SWCNT film work functions were estimated by ultraviolet photoelectron spectroscopy (UPS). While pristine SWCNT displays a characteristic work function (Φ) of ~4.70 eV, incorporation of PAA and [EMIM][TFSI] continues to increase Φ (Fig. 2b and Supplementary Table 1). As additional holes are transferred from the polymeric dopant and ionic liquid, effective SWCNT Fermi levels can be downshifted to induce p-type doping and heighten Φ. Charge carrier mobility can be approximated by weighted mobility (μw) from the measured Seebeck coefficient and electrical conductivity as
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)
where me and kB denote electron mass and Boltzmann constant, respectively38. The calculated weighted mobility interestingly escalates as the SWCNT network is restructured with PAA of lowering MW and [EMIM][TFSI], amounting to ~67% improvement for the SP1.8kIL. Alleviation of bundled SWCNT entanglement can establish a higher density of nanotube junctions to proliferate electrical conduction pathways, as previously evidenced elsewhere on dense films of debundled CNTs36,37,39. Therefore, hybridization can further restructure the conductive network to promote carrier mobility, thereby independently increasing the electrical conductivity while minimally affecting the Seebeck coefficient, and overcome their inversely coupled dependence on carrier concentration.
An elemental composition analysis from X-ray photoelectron spectroscopy (XPS) survey spectra confirms that the atomic concentration of oxygen is increased by 55% with the introduction of PAA1.8k, indicating higher concentrations of oxygen and water molecules adsorbed on the SWCNT (Fig. 2c and Supplementary Fig. 2). In addition, as compared to the pristine SWCNT film showing no appreciable signal for any target element in high-resolution scans, the SP1.8kIL film exhibits distinct N 1s, S 2p, and F 1s fingerprints of [EMIM][TFSI]40 (Fig. 2d and Supplementary Fig. 3). The N 1s peak has collaborative contributions from the two nitrogen atoms in [EMIM]+ rings and one in [TFSI]-, occurring at disparate binding energies due to their cationic and anionic characteristics, respectively. Particularly, the [EMIM]+ signal is further deconvolved into two distinguishable peaks as some of the imidazolium rings preferentially adsorb on the SWCNT surfaces via cation-π/π-π interaction35,40 (see Fig. 1c). As compared to the bulk cations (~403.2 eV), the lowered binding energy (~401.6 eV) of the adsorbed [EMIM]+ demonstrates interfacial π-electron transfer from SWCNTs, accounting for the p-type doping effect observed in the TE measurements.
Electromechanical reliability of restructured SWCNT thermoelectric performance
Intrinsic electromechanical stability of restructured SWCNT was investigated by stretching or bending the TE films deposited on polydimethylsiloxane (PDMS) substrates (see Methods). Electrical conduction is governed by charge transport along/across the individual SWCNTs, whose interconnectivity is highly susceptible to mechanical deformation. As compared to the pristine SWCNT film showing nearly 710% rise in electrical resistance (R/R0 ~8.1) when stretched by 100%, the restructured SP3,000k experiences DR/R0 ~350% at the same strain (Fig. 3a). As hypothesized, high-dielectric PAA can alleviate the π-π inter-nanotube interactions11,41 and allocate supplementary free volume to allow for individual SWCNTs to accommodate the applied strains without rupturing. Such effect is more prevalent in the SP1.8k film, where the lower-MW PAA of higher chain mobility can provide greater free volume, mitigating DR/R0 to a similar extent (180%) as that arising solely from dimensional changes after 100% stretching (Supplementary Figs. 4 and 5). Considering that PAA is a crystalline polymer with brittleness (Supplementary Fig. 5), such stretchability promotion is fundamentally distinguishable from blending into inherently stretchable elastomers/polymers, which also significantly undermines electrical conductivity.
Furthermore, when [EMIM][TFSI] is additionally incorporated, the ionic liquid of high dielectric constant can shield strong π-π stacking among the nanotubes and prevent their aggregation7,11, immensely promoting the SP1.8kIL stretchability to DR/R0 of astonishing 60% even at ε = 100%. Such SWCNT network restructuring is also well reflected in the film surface morphology, where successive hybridization with PAA1.8k and [EMIM][TFSI] systematically reduces the generated microcrack average dimensions and remarkably suppresses their propagation upon stretching until ε = 180% (Inset of Fig. 3a, Supplementary Fig. 6, and Supplementary Video 1). The augmented electromechanical integrity allows the SP1.8kIL film to flawlessly endure rigorous 5,000 stretch-release cycles at 50% strain (Fig. 3b) and 5,000 cycles of 3-mm-bending (Fig. 3c and Supplementary Fig. 7), while maintaining DR/R0 below 27% and 1%, respectively, after which R0 is mostly reacquired without noticeable film surface degradation. Furthermore, the restructured film exhibits unwavering TE performance even after extended exposure to extreme vibration (100 W/40 kHz for 24 h) (Supplementary Fig. 8).
Notably, the facilitated material stretchability is more prominent in the exceptionally stable Seebeck coefficient (S), which was measured by imposing temperature differences with Peltier modules attached at fixed ends of TE films during stretching (Fig. 3d). The Seebeck voltage (VSeebeck) curve measured from an SP1.8kIL film at ΔT = 0-2.5 K surprisingly remains uncompromised up to 60% strain (Fig. 3e). Intrinsic SWCNT Seebeck coefficient heavily depends on its band gap dictated by the tube diameter and chirality42,43, which are unlikely to be altered by applying strains on the random network. Rather, the network contracts in the direction orthogonal to stretching and the individual nanotubes can be closely packed so that a larger fraction of SWCNTs may more effectively leverage the applied ΔT to generate moderately higher VSeebeck. Only after the film was stretched by 80% does the measured Seebeck voltage begin to slightly downshift with additional strains, which eventually instigate loss of inter-nanotube contacts and network rupturing.
While the Seebeck coefficients of all the measured TE films remain nearly consistent up to 50% stretching, the structure-regulated SP1.8kIL exhibits incrementally better reliability, when stretched further, with DS/S0 remarkably restrained at -12% even at 100% strain (Fig. 3f). This again implies that the applied strain is absorbed predominantly by the debundled SWCNT network with greater free volume and thus the junctions among the individual SWCNTs are reasonably preserved. Overall, such unwavering TE properties prepare the ground for evaluating the power factor (σS2) of our SP1.8kIL film during stretching, where its electrical conductivity was calculated from sheet resistance directly measured from the film surface with a four-point probe. The acquired power factor of 296.0 μW m-1 K-2 remains exceptionally sustainable up to 100% strain, outshining any reported intrinsically stretchable TE material (Fig. 3g). Therefore, our SWCNT film restructured with PAA and [EMIM][TFSI] not only complements nearly inappreciable mechanical deformability of rigid inorganic materials, but also outperforms organic materials of inadequate TE properties, to offer energy-efficient platforms to harvest and regulate heat with outstanding stretchability.
Restructured SWCNT ink rheology and film morphology characterization
To further clarify the basis of such stretchability enhancement, the mechanical properties and surface morphologies of the restructured SWCNT ink and films were investigated. It was hypothesized that SWCNT restructuring would modulate viscoelasticity of the ball-milled TE ink, prior to film fabrication, to display disparate rheological properties. As compared to the pristine SWCNT dispersion in DEG, restructuring with PAA3,000k lowers the ink dynamic viscosity (η) at all tested shear rates (10-1-102 s-1), which can be ascribed to the free volume allocated to SWCNT network and attenuated inter-nanotube interactions, as further corroborated by even greater viscosity mitigation with lower MW-PAA (SP1.8k ink) (Fig. 4a and Supplementary Fig. 9). Considering that dispersing polymers in organic solvents typically produces more viscous solution, such viscosity reduction by the PAA implies that the ink rheological properties are dominantly governed by the SWCNT interactions44. As the lubricating [EMIM][TFSI] is sequentially incorporated, the π-π stacking SWCNT bundling/entanglement is moderated to further reduce the ink viscosity. Such rheological modulation is also observed in the ink storage modulus (G’) over the entire shear stress range, suggesting that PAA and [EMIM][TFSI] progressively deprive the TE ink of solid-like characteristics by homogeneously alleviating SWCNT aggregation (Fig. 4b). The restructuring progressively tunes the TE ink rheological properties to even enable direct printing of complex patterns with micrometre-scale resolution (Fig. 4c and Supplementary Fig. 10).
A series of structural and mechanical analyses were further carried out to quantify the network restructuring effects on the fabricated SWCNT films. X-ray diffraction (XRD) analysis was performed to estimate the overall SWCNT intertube spacing regulated by PAA and [EMIM][TFSI]. As predicted by Bragg’s law (λ = 2dsinθ), the SWCNT characteristic (002) graphitic peak is expected to shift to lower angles, as intercalating molecules shield the π-π interaction within the SWCNT bundles, widening their inter-nanotube distances (d) 45,46. When compared to the pristine SWCNT, the (002) peak of the SP1.8k film transitions from 2θ = 22.9 to 22.1°, which can be attributed to the broadening inter-nanotube distances with greater network free volume allocated by PAA1.8k (Fig. 4d). The continuous peak downshift to ~21.7° for the SP1.8kIL and SP1.8kIL3x (with three-times IL content) is highly indicative of the effective SWCNT debundling by [EMIM][TFSI]. These observations suggest that PAA and [EMIM][TFSI] can alleviate the aggregation of bundled nanotubes with greater free volume, accounting for such appreciably advanced reliability against stretching. The network restructuring effects are also clearly demonstrated in the declining modulus of the hybridized SWCNT films in nanoindentation measurements. As typically observed in polymeric composites, additional network free volume provided by PAA reduces elastic modulus (Er) from 0.176 to 0.130 GPa for SP3,000k film (Fig. 4e). Er declines further down to 0.104 GPa for SP1.8k, originating from the higher chain mobility of lower-MW PAA. Alleviation of the bundled/aggregated SWCNTs with [EMIM][TFSI] induces additional network relaxation and Er eventually dwindles to 0.050 GPa for the SP1.8kIL film.
Such SWCNT restructuring consequently enables regulation of microcracks upon stretching for homogeneous strain distributions throughout the entire film. Sequentially acquired optical microscope images of elongating SWCNT films were analysed by digital image correlation (DIC) algorithm to convert the displacement fields into Lagrangian strain values (εxx)47 (Supplementary Fig. 11). DIC-mapped strain distributions of a pristine SWCNT film show that disproportionately sized microcracks sporadically occur and absorb most of the applied strains (εapplied), leaving the irregularly-sized islands of SWCNT patches bearing εxx ≤0.1 (or, 10%) (Fig. 4f and Supplementary Fig. 12). At εapplied = 0.5 (or, 50%), the SWCNT islands cannot accommodate any additional strains and the broadening cracks eventually trigger local network rupturing, as also clearly represented in the line profile evolution with spacious white blank areas below. On the other hand, PAA1.8k (Fig. 4g) and [EMIM][TFSI] (Fig. 4h) can progressively homogenize the nanotube network debundling and restrain the microcrack generation/propagation even at large applied strains. With the elongating restructured SWCNT network predominantly accommodating the applied strains, the calculated εxx is almost homogeneous throughout the entire surface and nearly matches the εapplied.
TE figure of merit facilitation by thermal conductivity attenuation
To evaluate the figure of merit zT of the TE films, in-plane thermal conductivity (kthermal) was measured via a 3ω method (see Methods). While the reference tip-sonicated SWCNT sample shows thermal conductivity of ~10.7 W m-1 K-1 at 298.15 K, kthermal of the ball-milled SWCNT film is highly suppressed to 2.2 W m-1 K-1 (Fig. 5a). Such remarkable 80% reduction is indicative of the shortened SWCNT lengths and higher number of inter-nanotube junctions established by ball-milling, both of which discourage lattice thermal conductivity by truncating phonon mean free path and intensifying phonon scattering at the junctions, respectively48. Furthermore, hybridization with PAA constitutes SWCNT-polymer heterointerfaces, where the mismatched phonon spectra between the two phases instigate additional phonon scattering49. Therefore, the heightened interfacial barrier to energy-carrying phonon transport further attenuates kthermal by ~70% to 0.66-0.82 W m-1 K-1, which is even comparable to that of neat PAA films50 (~0.37 W m-1 K-1).
To further delineate the origin of diminished thermal conduction, electronic (kelectronic) and lattice (klattice) contributions to total thermal conductivity (kthermal = kelectronic + klattice) were evaluated. The Wiedemann–Franz relation quantifies the dependence of electronic thermal conductivity on material electrical conductivity (σ) by kelectronic = σLT, where L denotes the Lorenz number51. As conventionally conceived, collective lattice vibrations (phonon transport) predominates the thermal conduction in SWCNT network51,52 and the calculated lattice thermal conductivity indeed constitutes ~88% of kthermal (Fig. 5b). Nevertheless, restructuring with PAA intensifies phonon scattering at SWCNT-PAA interfaces49,53,54 and substantially depreciates klattice by 79-82%. [EMIM][TFSI] alleviates SWCNT aggregation to further impose considerable intertube thermal impedance across nanotube junctions, thereby restricting klattice to only ~30% of kthermal. Meanwhile, the relatively inappreciable electronic thermal conductivity continues to escalate, as PAA and IL promote electrical conductivity, and thus occupies increasingly higher fraction of the total kthermal. Nonetheless, the restructured SWCNT still exhibits remarkably low kthermal, on the order of 10-1 W m-1 K-1, suggesting that controlling phonon transport can resolve the interdependence between thermal conductivity and electrical conductivity, relying on relatively longer phonon mean free paths in SWCNTs than those of charge carriers55,56.
Restructuring with the polymer dopants and ionic liquid simultaneously encourages power factor and attenuates thermal conductivity, both of which are conducive to facilitating thermoelectric efficiency. Indeed, the calculated zT (0.11-0.12) of the restructured SWCNT corresponds to nearly four-fold enhancements from that of the pristine network (~0.03) (Fig. 5c). Owing to such advances, our TE film notches an unprecedented set of the highest zT and stretchability, outperforming any reported intrinsically stretchable TE material by orders of magnitude (Fig. 5d and Supplementary Table 2). The results qualify the exceptional capability of our system to regulate thermal energy at high efficiency, combined with outstanding mechanical reliability. Therefore, restructuring of carbon nanotubes to engineer their internal network and regulate charge/phonon transport can help address the intricate material trade-off between TE efficiency and stretchability.
Applications in Peltier-induced temperature regulation and highly stretchable TEGs
Such facilitated TE efficiency qualifies us to regulate thermal transport by exploiting the Peltier effect to generate temperature differences. As an electric current (I) is applied across our restructured TE system, charge carriers absorb/release thermal energy from/to the connected metal electrodes to cool/heat the junctions (Fig. 6a). Here, the suppressed thermal conductivity (kthermal) can reduce the internal heat diffusion down the thermal gradient to maximize the heat absorbed at the cold side (qc) or generated temperature gradient (DTPeltier) as1,57
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)
As I through an SP1.8kIL film was systematically increased from 0 to 300 mA, the temperature evolution at each terminal was recorded. At the terminal where the current flows from the SP1.8kIL film to the electrode, the collaborative effects of Peltier and Joule heating induce the measured temperature (THot) to immediately and rapidly surge with the applied current (Fig. 6b). On the contrary, at the opposite terminal, the Joule heating is counteracted by the Peltier cooling, incurred from carrier heat absorption (see equation (2)), and the resulting temperature (TCold) inappreciably increases up to 100 mA and only moderately grows from then on. Consequently, the temperature difference generated by the Peltier heating and cooling effects (DTPeltier = THot – TCold) across the SP1.8kIL film consistently rises with I, as further evidenced by the infrared thermal images capturing the temperature distribution evolution (Fig. 6c). Originating from the reduced kthermal of the restructured SP1.8kIL, high DTPeltier of 20.5 K is achieved at the current density of 5 A mm-2. In order to further amplify the cooling effect or generated temperature difference, it is essential to control heat dissipation into the surroundings by employing thermally insulating substrates or alleviate Joule heating by reducing material electrical resistivity.
Finally, to showcase sustainable thermal energy harvesting under large deformation for practical applications, a TEG whose entire components are intrinsically stretchable was fabricated. Stretchable composite electrodes were deposited on a glass and embedded by casting a PDMS substrate (Fig. 6d) (see Methods for materials and details). After peeling off the electrode-embedded substrate, SP1.8kIL films were deposited, connecting the exposed electrodes. Peltier modules were employed to impose temperature gradients and TE performance was evaluated while stretching the constructed TEG.
The open-circuit voltage (VOC) acquired at DT = 10 K is remarkably retained during stretching, with negligible 13% reduction at 70% strain, suggesting that the restructured SP1.8kIL can flawlessly extend its stretchability to the device level (Fig. 6e). Moreover, the total electrical resistance (Rtotal) also remains uncompromised until 50% stretching and only slightly increases (~20%) at εapplied = 70%. When distinguishing the contribution to Rtotal from each component, the stretchable electrodes continuously undergo escalating electrical resistance that more than doubles (~137%) when stretched by 70% (Fig. 6f and Supplementary Fig. 13). Meanwhile, the TE legs seamlessly accommodate the applied strains via the restructured SP1.8kIL network and DR/R0 is substantially restrained to only 16%. Owing to its prominent performance stability against stretching, our 10-pair TEG can reliably maintain its normalized output power density of 1.10 μW m-2 K-2 at DT = 10 K up to 50% stretching (Fig. 6g). Although the power begins to gradually dwindle with further stretching, this penalty is likely to be resolved by employing electrodes with greater stretchability. Still, such application-level TE performance reliability against stretching demonstrates that our highly stretchable TEG can provide versatile platforms for efficient heat harvesting/management in the environments with an intense level of deformation.