Flexible Thermoelectric Device Based on Poly(ether-b-amide12) and

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Flexible Thermoelectric Device Based on Poly(ether-b-amide12) and High-purity Carbon Nanotubes Mixed Bilayer Heterogeneous Films Ting Luo, and Kai Pan ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00190 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Applied Energy Materials

Flexible Thermoelectric Device Based on Poly(ether-b-amide12) and High-purity Carbon Nanotubes Mixed Bilayer Heterogeneous Films Ting Luo,† Kai Pan,*,† †

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and

Engineering, Beijing University of Chemical Technology, 15 North 3rd Ring East Road, Chaoyang District, Beijing, 100029, China. *E-mail: [email protected]

ABSTRACT: Thermoelectric (TE) generators are an appealing, eco-friendly energy harvesting technology capable of converting temperature gradients into electricity. Carbon nanotube (CNT) filled organic composites typically offer the best potential for making cheaper TE materials. In this study, a block copolymer poly(ether-b-amide12) (PEBA) with excellent flexibility and mechanical properties was used to fabricate TE materials via blending CNTs. After doping PEBA with Lithium chloride (PEBA-Li+), two types (p-type and n-type) of CNT filled PEBA-Li+ (CNT-PEBA-Li+) TE composites were obtained via a simple one-step casting method. Here, the n-type CNT is prepared by diethylenetriamine (DETA) doping treatment of the pristine CNT (p-type). The resultant TE materials exhibited a bilayer heterogeneous (BH) structure comprised of a thermally nonconductive layer and a conductive layer. The Seebeck coefficient (S) and electrical conductivity (σ) (under the application of temperature gradient 20 K) for the obtained p-type CNT-PEBA-Li+ bilayer heterogeneous film (BHF) are 36.88 µV/K and 456 S/m, for the

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n-type CNT-PEBA-Li+ BHF are -33.25 µV/K and 492 S/m, respectively. We constructed this novel TE device by integrating p- and n-type elements. The results showed that generated TE voltages for the module made of 3 TE couples was able to reach 120 mV for a temperature gradient of 60 K. The TE device construction was more efficient and simpler due to the BH structure, as only two BHF types were needed and no thermal insulating layer was necessary. In addition, our TE device is flexible throughout the entire structure. These novel flexible TE composites may benefit future organic TE material research and promote the application of flexible and wearable power-conversion devices for next-generation power generators and waste-heat-recovery systems. KEYWORDS:

Poly(ether-b-amide12),

carbon

nanotube,

bilayer

heterogeneous,

thermoelectric materials, flexible thermoelectric device

INTRODUCTION Thermoelectric (TE) materials are very useful for the conversion of heat into electricity and have been utilized to harvest electrical energy from industrial waste heat or unused heat.1-3 The basic principles of electricity harvesting via temperature gradient were first explored nearly 200 years ago.1 The efficiency of TE devices is denoted by a dimensionless parameter known as a figure of merit, ZT,4,5 which is expressed as follows: ܼܶ = ߪܵ ଶ ܶ/݇ where T is the absolute temperature, σ is electrical conductivity, S is the Seebeck coefficient (S > 0 for p-type and S < 0 for n-type semiconductors), and k is thermal


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conductivity.6 Due to the intrinsic properties of low k in polymer materials (0.1-0.5 Wm-1K-1),7 the TE power factor (PF = σS2) is adopted to replace ZT. The PF of most polymer TE materials is in the range of 10-6-10-10 Wm-1K-2.8 Additionally, the output voltage (VTE) and the maximum power output (PTE) of TE modules are generally employed to improve TE performance judgment.9 Every state-of-the-art TE material thus far has been inorganic semiconductors such as several alloys and intermetallics based on Bi, Te, Ge, Pb, etc.10-13 with high σ. Extensive research on these materials has been done over the last half century. However, there has recently been an advancement in a number of promising alternative organic TE materials including

poly(3,4-ethylenedioxythiophene)

(PEDOT),14-16

polyaniline

(PANI),17-19

polyalkyl thiophenes (PTH),20,21 and polypyrrole (PPy).22,23 These organic materials have the advantages of being abundant, light-weight, flexible, solution-processable, and, in most cases, low-cost. These organic TE materials are still inferior to traditional inorganic TEs due to their lower levels of σ and S, however.24-26 Several studies have sought to address this problem by studying a combination of inorganic materials and polymers to fabricate TE composites. Of particular interest to organic TE development are carbon nanotube (CNT)/polymer thin films.27-29 These CNTs are most frequently used as nanofillers due to their unique shape, superconductivity, light weight, high stiffness, and axial strength. These properties make them superior to previously reported hybrid alloys.30 Introducing CNTs into polymer blends can modulate the σ and S to optimize ZT. For example, a CNT/ polyvinyl acetate (PVAc) composite showed an enhanced σ of 48 S/cm with a CNT



concentration of 20% at 300 K.31 When PVAc is replaced by PEDOT: PSS, it enhances σ from 48 S/cm in PVAc/CNT matrix to 1,350 S/cm in PEDOT:PSS/CNT composite. A reported CNT/PANI composite showed a maximum S of 28.6 mV/K at 350 K.32 Du et al.33 synthesized a CNT/poly(3-hexylthiophene) (P3HT) composite and found that the sample containing 30% CNT showed reduced σ from 0.13 S/cm to 0.11 S/cm and increased S from 9.7 to 11.3 mV/K within a temperature gradient (∆T) from 293 to 493 K. In short: CNT filled organic composites can improve the TE effect. TE materials must be combined into devices or modules relying on alternative connection of p- and n-type units in series to produce sufficient power,34 and temperature gradients can be further optimized via rational device structure design. Hewitt et al.35 synthesized CNT filled polyvinylidene fluoride (PVDF) (CNT/PVDF) composite films layered into multiple element modules resembling a felt fabric. These CNT/PVDF conduction layers alternate between PVDF insulation layers as p-type CNT and n-type CNT, with the cumulative effect of each layer providing a higher thermoelectric voltage. Wu et al.9 developed a novel strategy to prepare n-type CNT and fabricated flexible modules via a multilayered stacked structure with polyimide insulated films inserted between the p-type and the n-type films, resulting in excellent TE performance. However, large thermal gradients are essential to produce practical voltage and required power. To address this issue, a thermal insulating layer made up of PVDF35 or polyimide9 must be inserted between the p- and the n-type elements to partially cover the device bottom, thus allowing the thermal gradient to be established between the Th and Tc ends or through a layering of multiple modules to ensure


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thermal conductivity in parallel. The poor plug compatibility between the TE elements and extra thermal insulating layers weakens the flexibility, amenity, and aesthetics of wearable TE devices. In

this

work,

we

focused

on

a

semi-crystalline

block

copolymer,

poly(ether-b-amide12) (PEBA), a commercially available thermoplastic elastomer containing hard block polyamide-12 (PA12), and soft block poly(tetramethylene oxide) (PTMO). PEBA has excellent mechanical strength and elastic properties over a wide temperature range, and has shown extensive application in various fields.36-38 Salt doped PEBA showed excellent antistatic properties in decreasing surface resistivity (Rs), for example.39 Lithium is the lightest, most electropositive metal that works to facilitate storage system design with high energy density. We first doped PEBA with LiClO4 to enhance its electrical properties, then blended it with p-type and n-type CNTs to prepare new flexible TE materials. We developed a simple and efficient TE device using this special bilayer heterogeneous (BH) structure of the obtained CNT-PEBA-Li+ TE materials. The new TE material is flexible, environmentally friendly, and may greatly benefit wearable power-conversion devices for next-generation power generators.

RESULTS AND DISCUSSION The detailed fabrication process of the two BHF types is illustrated in Figure 1. The homogeneous solution was first prepared by dissolving PEBA granules in a formic acid solvent at 60 °C under vigorous agitation. The chemical structure and Fourier transform



infrared (FTIR) spectrum of PEBA consists of dodecyl amide (hard segment at 1640 cm-1) and ether components ([EO], soft segment at 1100 cm-1), as shown in Figure S1. The PEBA copolymer composition is calculated at 60 wt% ether components, which has both high dielectric constant and strong lithium ion solvating ability40 to obtain a lithium doped PEBA semiconductor. LiClO4 power with [EO] unit to [Li+] mass ratio of 12 was added to obtain a PEBA-Li+ homogeneous solution. TGA was carried out to analyze the thermal decomposition behavior of the PEBA-Li+ films as shown in Figure S2a. The initial degradation temperature of PEBA-Li+ (doped LiClO4 of 5 wt%) severely decreased by 190 °C compared to pure PEBA, thus contributing to the poor thermostability of the inorganic salt molecule. The second scan during heating is shown in Figure S2b. The crystallization (melting peak at 145 °C) disappeared after the addition of LiClO4, implying that the doped salt not only decreases the mobility of soft segments, but also restrains the crystallinity of soft regions.39 The surface and fracture morphology of pure PEBA and PEBA-Li+ films were observed via SEM, as shown in Figure S2c-f. The surface resistance (ρs) and thermal conductivity (k) are shown in Table S1. The PEBA-Li+ film had lower ρs (5.43×106 Ω) and k (0.103 mm2/S), suggesting favorable potential as templates and scaffolds in flexible and wearable power-conversion device fabrication and application. These PEBA-Li+ homogeneous solutions were mixed with p- and n-type CNTs, respectively. The schematic illustration for the preparation of the n-type CNT by DETA (diethylenetriamine) doping as shown in Figure S3. Figure 2a shows that S and σ increased as CNTs content increased. However, further increases resulted in the destruction of film


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integrity. Hence, We controlled the CNT content at 20 wt% in our experiments. The calculated thermoelectric power factor as shown in Figure 2b. These mixed solutions were cast in a glass petri dish to prepare BHFs via a simple one-step method. After a slow drying process, the BH structure with a thermally nonconductive layer (T) and an electrically connected layer (B) was established. The obtained BHF (Figure S4a) was flexible enough to be folded into a variety of shapes, as shown in Figure S4b, with the enlarged figure showing a black layer (T layer) and a silver gray layer (B layer) (Figure S4e). Figure S4c,d shows that the B layer (d) can light the LED while the T layer (c) is electrically nonconductive. Figure 3a-d shows the surface SEM and TEM images for the p-type (a, c) and n-type CNTs (b, d), respectively. The pristine p-type CNT bundles are relatively straight, while the n-type DETA-CNTs are curled and more disorderly. Furthermore, the CNTs doped by DETA is demonstrated by FTIR spectrum (Figure 3e). Compared with the spectrum of pristine CNTs, a series of novel absorption bands resulting from DETA occur in the DETA-CNT spectrum. The band at 2,340 cm-1 with medium intensity is ascribed to the primary and the secondary N-H stretching vibrations. The strong band at 1,653 cm-1 results from the C-N stretching vibrations. TGA analysis, XRD spectrum, and Raman spectroscopy were performed to demonstrate the conversion of p- to n-type CNTs (Figure 3f-h, respectively). The TGA curve of the pristine p-type CNTs does not appear in the weightlessness peak, however, the DETA-CNT curve shows that the weight loss caused by PVP and DETA decomposition is about 75 wt%. The corresponding XRD spectra show the



diffraction peak intensity of DETA-CNTs is much lower than the pristine CNTs (002) plane (2θ = 26º). It means that pristine CNTs are more organized and have fewer structural defects than the DETA-CNTs,41 since the DETA doping causes imperfections in the graphitic layers. Figure 3h presents the Raman spectra of the pure CNTs and DETA-CNTs. The typical peak of pure CNTs at 1,582 cm-1 is attributed to the graphite wall. The band at 1,352 cm-1 is assigned to the slightly disordered graphite.42 The corresponding Raman characterization clearly shows the D band (the peak around 1,352 cm-1, is the disorderly mode or SP3 mode) of the DETA-CNTs exhibits higher peak intensity than pristine CNTs, which can be attributed to increased disorder due to the modification by DETA. These results indicate that n-type CNTs can be successfully obtained via DETA doping and become more disorderly than the pristine p-type CNTs. Figure 4a,b shows resultant BHF flexibility. The rectangular strips (called legs, 80×10) can be rolled into a knot; this flexibility enables its application in wearable power-conversion devices. The two sides also have different colors: The grey side is the B layer and the black side is the T layer. Figure 4c-g shows the heterostructure of the p-type BHF. The mini LED in the T layer circuit cannot be lit, as shown in Figure 4c. However, the mini LED was lit when changed to B layer circuit (Figure 4d). SEM was used to observe the micromorphology of the BHFs. Figure 4e and 4g show the surface SEM images of the T and B layers, respectively. There are a few scattered CNTs on the T layer (Figure 4e), and many aggregated CNTs on the B layer (Figure 4g). The inset is a magnified SEM image of the B layer which shows the aggregated CNTs


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connected to each other. CNT intensity is the most important factor contributing to the different conductivity of the two sides of BHF. Figure 4f shows the cross-sectional image of the p-type BHF with a thickness of about 280 µm. This BH structure consists of the T layer (about 230 µm) and B layer (about 50 µm). The same characterizations were performed on the n-type BHF and similar results were obtained, including flexibility, heterostructure, and conductivity for the two layers (Figure 4h-l). The aggregated DETA-CNTs showed different morphology resulting from the disordered n-type DETA-CNTs (inset, Figure 4l). The cross-section image in Figure 4k shows a relatively thinner T layer (about 100 µm) and thicker B layer (about 180 µm). The two BHF types also exhibit lower k values and significantly higher S and σ values (Table 1). The results of the seebeck coefficient indicate the transition from p-type to n-type had been successfully completed, since thermoelectric materials of p- and n-types possess positive (36.88 µV/K) and negative (-33.25 µV/K) Seebeck coefficients, respectively. These results demonstrate that our BHFs are a reasonable contender for making light weight flexible polymer composites for next-generation TE applications. A series of characterizations were performed to further analysis the heterostructure of p- and n-type BHFs as shown in Figure 5. The ATR-FTIR spectra of the T and B layers of the two BHF types are shown in Figure 5a. The p-type BHF-T (T layer of the p-type BHF) (red curve) spectrum is similar to the PEBA-Li+ film (black curve), while the p-type BHF-B (blue curve) spectrum is similar to that of pure CNTs (Figure 3e). A series of absorption peaks became stronger in the n-type BHF-T (green curve) spectrum compared



to the p-type, including characteristic peaks at 1,102 and 1,641 cm-1 assigned to the ether peak and carbonyl peak, respectively, due to the presence of DETA. The peak at 2,340 cm-1 represents the stretching vibration of the N-H group from DETA. The n-type BHF-B (pink curve) spectrum is similar to that of DETA-CNTs (Figure 3e). Analysis of the FTIR spectra indicates that the T layers of both BHFs show polymer matrix properties, while the B layers equipped with CNT properties are consistent with the above conclusions. Figure 5b gives the XRD analysis of the two BHF types. The p-type BHF-T and -B exhibited a typical diffraction peak at 2θ = 26°. However, a dramatic shift of the peak (2θ = 20°) for n-type was observed due to the presence of DETA-CNTs by considerably disrupting the inter chain hydrogen bonding between the amide blocks, thus changing the structure. We also conducted Raman spectra for further BHF characterization (Figure 5c) and observed no extra peak, but only the characteristic peaks of CNTs and DETA-CNTs (Figure 3h). These results suggest that there no new chemical bond formed between the CNTs and PEBA in the BHFs. However, both of the BHF-T types exhibited obvious baseline drift. To explain this, fluorescence emission spectra (excitation wavelength 365 nm) was gathered for the PEBA-Li+ film as shown in Figure 5d. In this emission spectrum, we found a relatively strong blue emission peak with a corresponding CIE coordinate of 0.17, 0.18 (inset, Figure 5d). The PEBA-Li+ film was blue under UV light (excitation wavelength 360 nm).


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The thermal stability and decomposition temperature of BHFs were obtained using a TGA instrument. As shown in Figure 5e, PEBA-Li+ film decomposition began above 200 °C while the decomposition began at a higher temperature with the addition of CNTs into the polymer matrix up to 350 °C for BHFs. The thermal stability improvement is conducive to using BHFs as TE materials. The typical strain-stress curves of the BHFs are shown in Figure 5f. Adding an appropriate amount of CNTs or DETA-CNTs can greatly improve the mechanical properties of BHFs. Compared to the PEBA-Li+ film, the tensile modulus of the p-type BHF increased due to the relatively straight and orderly p-type CNTs, which caused an increase in modulus. The tensile strain of the n-type BHF increased due to the DETA modified n-type CNTs being more disorderly, thus providing more space to increase tensile strain. We propose a possible mechanism for the formation of the BHFs based on the obtained results and observations in this study. Figure 6a shows the states of the CNTs in the T and B layers of the BHFs, respectively, where the section above the dotted line represents the T layer, and a few CNTs (black lines) dispersed in the polymer matrix (green part). The CNTs formed links among themselves in a net-like three-dimensional CNTs protrusion structure in the B layer (section below the dotted line). Panels (b) and (c) in Figure 6 illustrate the heterostructure formation during the drying process, as the nanotubes in the PEBA matrix are short and the Van der Waal (VDW) force (gray arrows) between the CNTs encouraged their aggregation. During drying (solution evaporation),



however, the polymer matrix diffused into the nanotubes interstitial spaces (blue arrows) and filled the narrow spaces, thus allowing the formation of a much more compact layer. CNTs are wrapped around the PEBA to form the CNTs balls (Figure 6c) and a steady link was established among them that effectively locked the CNTs in the B layer. In addition, the n-type B layer was much thicker due to the n-type CNTs providing greater space for diffusion among the polymer matrix. Further illustration of the above mechanism is provided in Figure 6d. A sample of part of the T layer shows a clear edge in the surface SEM image. The top side of the image is smooth while the bottom side of image is very rough. We also used energy dispersive spectra (EDS) to investigate the element diffusion inside BHFs and SEM/EDS on the fracture surface (Figure S5). The EDS elemental mapping shown in Figure 6e revealed that the BHF was mainly composed of C, N, and O elements. The C/N/O ratio from B to T layer confirms the existence of the BH structure, where the results obey the mechanism described above. To generate a sufficient power output, p- and n-type TE units must be assembled into modules. We built a TE generator according to the sketch map shown in Figure S6 including several couples of legs connected with copper. The completed prototype of the TE generator is shown in Figure 7a. The substrate (polyimide) is the limitation of the maximum working temperature of our TE generator (about 450 K). If alternative substrate materials were used (e.g., ceramic fiber papers, mats), the working temperature of the device could be much higher since the BHFs would begin to degrade at 625 K (Figure 5e).


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Most of the previously reported flexible TE devices are unable to work at higher temperatures due to the low melting point of the polymers.43 To characterize the TE property of the device, we heated one end using a micro-heater while leaving the cold sides at room temperature (15 °C). We observed the temperature distribution of one side of the generator with an infrared camera as shown in Figure 7b. There was a temperature lapse from the hot end to the cold end, with an 80 K temperature difference in total. We measured the open circuit voltage as a function of temperature gradient (∆T) to evaluate the TE performance as a module. Figure 7c shows the TE performance of a module consisting of 1 to 3 couples without any external resistance loading. With increase in ∆T, the open circuit voltages increased almost linearly for all TE couples. Increase in the number of TE couples also caused increase in voltage at the same ∆T. These results are in accordance with previous reports. Importantly, when ∆T was 60 K, the generated voltages for the 3 TE couple module reached 120 mV, which is comparable to the highest performance for organic TE modules currently available.

CONCLUSIONS In summary, we successfully prepared a series of p-type and n-type PEBA-CNT BHFs as TE materials by a simple one-step casting method. The Seebeck coefficient for the p- and n-type BHFs were 36.88 µV/K and -33.25 µV/K, respectively. The unique heterostructure

with

thermally

insulated

(T

layer)


and

electrically

connected


nano-structured network (B layer) of the BHFs efficiently ensured high-efficiency thermopower of the films and prevented rapid heat dissemination. A possible mechanism for the formation of the BHFs is proposed to explain the existence of the BH structure. We fabricated a flexible, thermostable TE device by alternatively connecting n- and p-type BHFs mounted over a polyimide substrate. The TE device, including 3 pairs of legs, reached around 120 mV TE power at a temperature gradient of about 60 K. Apart from the excellent TE properties, this flexible, lightweight, and stretchable TE generator may represent a new strategy for the development of energy harvesting devices.

METHODS Materials. Poly(ether-b-amide12) (PEBA) (Arkema Company, France). High-purity carbon nanotubes (CNTs) fine powders were purchased from Shenzhen SUSN Sinotech New Materials Co.,Ltd. (China). The CNTs have an average diameter of 10 nm and an average length of 10 mm. Lithium chloride (LiClO4, battery grade, dry, 99.99% trace metals basis) was purchased from Shanghai Aladdin Company (China). Diethylenetriamine (DETA, C.P. pure grade with a purity of > 97.0%) was purchased from Shanghai Aladdin Company (China). Polyvinylpyrrolidone (PVP, Mw=1300,000 g/mol) were purchased from Sigma-Aldrich Co. Ltd. (China). Formic acid was of analytical grade from Tianjin fine chemical industry research institute (China). All the chemicals are of reagent grade and were used without any further purification.


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Preparation of p- or n-type PEBA/CNTs bilayer heterogeneous films. We used PEBA and CNTs to prepare a PEBA/CNTs mixed bilayer heterogeneous film (BHF) via a one-step solution casting method. We first added 1 g PEBA granules to formic acid to make a 10 wt% solution. The polymer was dissolved at 80 ºC under vigorous stirring for 2 h, and then LiClO4 power with [EO] unit to [Li+] mass ratio of 12 was added and the mixture was stirred at 45 ºC for 2 h to obtain a homogeneous solution. The typical preparation procedure of n-type CNT can be summarized as follows. First, CNT fine powders were added to an ethanol solution containing 15 wt% PVP. The mixture was ultrasonically treated with a bath type sonicator for 1 h and 60 wt% DETA was titrated into the system and stirred for 5 h. Next, the mixture was vacuum filtered and washed with anhydrous ethanol for several times, then the n-type DETA-CNT was dried in a vacuum oven at 45 ºC for 24 h to remove the residual solvent. The 20 wt% p-type CNTs fine powders and n-type DETA-CNT were weighed and added into 30 mL formic acid and ultrasonically dispersed for 2 h. The PEBA-Li+ homogenous polymer solution and CNT dispersed solution was mixed and stirred at 45 ºC for 24 h. A three-dimensional network developed in the insulating PEBA matrix after long-term drying to ensure heterogeneous structure formation. Next, the dried n- and p-type BHFs were peeled from the glass plate and further dried in a vacuum oven at 50 °C for 48 h to remove the residue solvent. Pure PEBA-Li+ film was prepared in a similar manner but without adding any CNTs.



Preparation and measurement of thermoelectric (TE) devices. The p- and n-type BHFs were cut into 30 mm x 10 mm rectangles. The electrical conductivities and Seebeck coefficients at room temperature were measured with a Thin-Film Thermoelectric Parameter Test System (MRS-3RT, Wuhan Joule Yacht Science & Technology Co., Ltd.). A quasi-steady-state mode was adopted. At least 50 tests were run and averaged as the final results reported here. To produce sufficient power, TE materials need to be combined into a module containing many alternating p- and n-type legs connected electrically in a series and thermally in parallel. As-prepared p- and n-type BHFs were cut into several strips (80 mm x 10 mm). TE modules were made by alternatively connecting p- and n-type mounted over a polyimide substrate. Each strip was electrically connected in the series via a copper foil (diameter 5 mm). The output open circuit voltages were measured directly without any load resistance. Characterization. The surface morphology of the PEBA/CNTs BHFs were examined by scanning electron microscopy (SEM, Hitachi S-4700, Japan). X-ray diffraction (XRD) patterns were acquired with a single crystal X-ray diffraction (XRD, D8 ADVANCE, Germany) using Cu Kα (1.5406 Å) radiation. Raman spectrum and mapping were recorded using a WITEC confocal spectrometer with 600 lines/mm grating, and 514.5 nm excitation. The chemical structure of PEBA/CNTs BHF were characterized by attenuated total reflectance-Fourier transform infrared (ATR-FTIR, Perkin-Elmer, USA). Energy dispersive X-ray spectrometer (EDS, GENESIE 2000, Japan) was used to confirm the


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elemental composition of the sample surface. The electrical conductivities and the Seebeck coefficients at room temperature were measured by a commercial instrument, Thin-Film Thermoelectric Parameter Test System (MRS-3RT, Wuhan, China). The Fluorescence emission spectrum of PEBA-Li+ film was obtained using F-7000 Fluorescence Spectrophotometer (Hitachi F-7000, Japan). Thermogravimetry analysis (TGA, TG209C, USA) under nitrogen atmospheres was performed to investigate thermal pyrolysis of the two type CNTs and BHFs. The typical strain-stress curves of the PEBA-Li+ film and the two type BHFs were tested by thermomechanical analysis (TMA, Q800, USA).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Supporting Information Available: The chemical structure and Fourier transform infrared (FTIR) spectrum of PEBA consists of dodecyl amide, materials characterization of the PEBA and PEBA-Li+ films, Schematic illustration for the preparation of the N-type CNT by DETA doping, The photographs of the BHF which demonstrate the heterogeneous structure, The fracture surface by ways of SEM and Energy Dispersion Spectrum (SEM/EDS), The schematic illustration of the TE device fabrication.

ACKNOWLEDGEMENTS



This work was financially supported by the opening foundation of State Key Laboratory of Organic-Inorganic Composites 201701012.

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Kim, D.; Kim, Y.; Choi, K.; Grunlan, J. C.; Yu, C. H. Improved Thermoelectric

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(22) Zhang, Z.; Chen, G. M.; Wang, H. F.; Zhai, W. T. Enhanced Thermoelectric Property by the Construction of a Nanocomposite 3D Interconnected Architecture Consisting of Graphene Nanolayers Sandwiched by Polypyrrole Nanowires. J. Mater. Chem. C 2015, 3, 1649-1654. (23) Han, S. B.; Zhai, W. T.; Chen, G. M.; Wang, X. Morphology and Thermoelectric Properties of Graphene Nanosheets Enwrapped with Polypyrrole. RSC Adv. 2014, 4, 29281-29285. (24) Chen, Y. N.; Zhao, Y.; Liang, Z. Q. Solution Processed Organic Thermoelectrics: Towards Flexible Thermoelectric Modules. Energy Environ. Sci. 2015, 8, 401-422. (25) Cho, C.; Stevens, B.; Hsu, J. H.; Bureau, R.; Hagen, D. A.; Regev, O.; Yu, C.; Grunlan, J. C. Completely Organic Multilayer Thin Film with Thermoelectric Power Factor Rivaling Inorganic Tellurides. Adv. Mater. 2015, 27, 2996-3001. (26) Wang, M. C.; Lin, S. C. Anisotropic and Ultralow Phonon Thermal Transport in Organic-Inorganic Hybrid Perovskites: Atomistic Insights into Solar Cell Thermal Management and Thermoelectric energy Conversion Efficiency. Adv. Funct. Mater. 2016, 26, 5297-5306. (27) Zhao, W. Y.; Fan, S. F.; Xiao, N.; Liu, D. Y.; Tay, Y. Y.; Yu, C.; Sim, D. H.; Hng, H. H.; Zhang, Q. C.; Boey, F.; Ma, J.; Zhao, X. B.; Zhang, H.; Yan, Q. Y. Flexible Carbon Nanotube Papers with Improved Thermoelectric Properties. Energy Environ. Sci. 2012, 5, 5364-5369.



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Figure 1. Schematic diagram of p- and n-type PEBA/CNT BHF preparation. Figure 2. (a) The Seebeck coefficient and electrical conductivity were measured of BHFs with different CNTs content at room temperature along the in-plane direction. (b) The calculated thermoelectric power factor. Figure 3. (a), (b) SEM images and (c), (d) TEM images of p-type and n-type CNTs; (e) FTIR spectra; (f) TGA curves; (g) XRD; and (h) Raman spectra of p-type and n-type CNTs. Figure 4. (a) Diagram of forming procedure. (b) Photo of folded BHF stripe (80 mm × 10 mm); (c) The mini LED in the T layer circuit of the P-type cannot be lit. However, when changed to (d) B layer circuit, the mini LED was lit. (e) Top SEM image; (T) SEM layer; (f) Fractured cross sections; (g) Bottom (B) layer of p-type BHF; (h-l) n-type BHF with reference to corresponding relationship. Figure 5. (a) ATR-FTIR spectra; (b) XRD of PEBA-Li+ film and T/B layers of p- and n-type BHFs; (c) Raman spectra for T/B layers of p- and n-type BHFs (d) Fluorescence spectrum of PEBA, insets are chromaticity coordinates (0.17, 0.18) and electronic photos in UV light state; (e) TGA curves of p- and n-type BHFs; (f) Typical stress-strain curves of PEBA-Li+ film and p- and n-type BHFs. Figure 6. (a) Illustration of proposed formation of BHFs, where green part and black lines denote PEBA matrix and CNTs, respectively; (b) Schematic diagram of interaction force (gray arrows) and its


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diffusion path (blue arrows); (c) Magnified of CNT ball in the B layer; (d) SEM image of BHF cut off of the T layer along the dotted line (inset); (e) EDS scanning trace profile curves of BHF. Figure 7. (a) Completed TE device consists of p-type and n-type (80 mm long ×10 mm wide) BHFs; (b) Infrared thermal image of the TE device and the distribution of temperature at the area on the right side of the solid line box; (c) TE output voltage as a function of temperature gradient (∆T) for TE modules consisting of 1 to 3 couples of p- and n-type BHFs. Table 1. The measurement results of p- and n-type BHFs.

Figure 1. Schematic diagram of p- and n-type PEBA/CNT BHF preparation.



Figure 2. (a) The Seebeck coefficient and electrical conductivity were measured of BHFs with different CNTs content at room temperature along the in-plane direction. (b) The calculated thermoelectric power factor.

Figure 3. (a), (b) SEM images and (c), (d) TEM images of p-type and n-type CNTs; (e) FTIR spectra; (f) TGA curves; (g) XRD; and (h) Raman spectra of p-type and n-type CNTs.


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Figure 4. (a) Diagram of forming procedure. (b) Photo of folded BHF stripe (80 mm × 10 mm); (c) The mini LED in the T layer circuit of the P-type cannot be lit. However, when changed to (d) B layer circuit, the mini LED was lit. (e) Top SEM image; (T) SEM layer; (f) Fractured cross sections; (g) Bottom (B) layer of p-type BHF; (h-l) n-type BHF with reference to corresponding relationship.



Figure 5. (a) ATR-FTIR spectra; (b) XRD of PEBA-Li+ film and T/B layers of p- and n-type BHFs; (c) Raman spectra for T/B layers of p- and n-type BHFs (d) Fluorescence spectrum of PEBA, insets are chromaticity coordinates (0.17, 0.18) and electronic photos in UV light state; (e) TGA curves of p- and n-type BHFs; (f) Typical stress-strain curves of PEBA-Li+ film and p- and n-type BHFs.


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Figure 6. (a) Illustration of proposed formation of BHFs, where green part and black lines denote PEBA matrix and CNTs, respectively; (b) Schematic diagram of interaction force (gray arrows) and its diffusion path (blue arrows); (c) Magnified of CNT ball in the B layer; (d) SEM image of BHF cut off of the T layer along the dotted line (inset); (e) EDS scanning trace profile curves of BHF.



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Figure 7. (a) Completed TE device consists of p-type and n-type (80 mm long ×10 mm wide) BHFs; (b) Infrared thermal image of the TE device and the distribution of temperature at the area on the right side of the solid line box; (c) TE output voltage as a function of temperature gradient (∆T) for TE modules consisting of 1 to 3 couples of p- and n-type BHFs.

Table 1. The measurement results of p- and n-type BHFs. Majority Type carrier

Thermal diffusivity (mm2/s)

Resistivity

Conductivity

Seebeck coefficient

Power Factor

(µΩ·m)

(S/m)

(µV/K)

(nWm-1K-2)

P-type

hole

0.153

2194.99

456

36.88

620.22

N-type

electron

0.119

2032.53

492

-33.25

543.94


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Figure 1. Schematic diagram of p- and n-type PEBA/CNT BHF preparation. 292x165mm (300 x 300 DPI)



Figure 2. (a) The Seebeck coefficient and electrical conductivity were measured of BHFs with different CNTs content at room temperature along the in-plane direction. (b) The calculated thermoelectric power factor. 309x120mm (150 x 150 DPI)


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Figure 3. (a), (b) SEM images and (c), (d) TEM images of p-type and n-type CNTs; (e) FTIR spectra; (f) TGA curves; (g) XRD; and (h) Raman spectra of p-type and n-type CNTs. 435x259mm (300 x 300 DPI)



Figure 4. (a) Diagram of forming procedure. (b) Photo of folded BHF stripe (80 mm × 10 mm); (c) The mini LED in the T layer circuit of the P-type cannot be lit. However, when changed to (d) B layer circuit, the mini LED was lit. (e) Top SEM image; (T) SEM layer; (f) Fractured cross sections; (g) Bottom (B) layer of p-type BHF; (h-l) n-type BHF with reference to corresponding relationship. 218x299mm (300 x 300 DPI)


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Figure 5. (a) ATR-FTIR spectra; (b) XRD of PEBA-Li+ film and T/B layers of p- and n-type BHFs; (c) Raman spectra for T/B layers of p- and n-type BHFs (d) Fluorescence spectrum of PEBA, insets are chromaticity coordinates (0.17, 0.18) and electronic photos in UV light state; (e) TGA curves of p- and n-type BHFs; (f) Typical stress-strain curves of PEBA-Li+ film and p- and n-type BHFs. 260x294mm (300 x 300 DPI)



Figure 6. (a) Illustration of proposed formation of BHFs, where green part and black lines denote PEBA matrix and CNTs, respectively; (b) Schematic diagram of interaction force (gray arrows) and its diffusion path (blue arrows); (c) Magnified of CNT ball in the B layer; (d) SEM image of BHF cut off of the T layer along the dotted line (inset); (e) EDS scanning trace profile curves of BHF. 278x287mm (300 x 300 DPI)


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Figure 7. (a) Completed TE device consists of p-type and n-type (80 mm long ×10 mm wide) BHFs; (b) Infrared thermal image of the TE device and the distribution of temperature at the area on the right side of the solid line box; (c) TE output voltage as a function of temperature gradient (∆T) for TE modules consisting of 1 to 3 couples of p- and n-type BHFs. 287x153mm (300 x 300 DPI)



50x38mm (300 x 300 DPI)


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Flexible Thermoelectric Device Based on Poly(ether-b-amide12) and

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