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Te nanorod composite films for flexible thermoelectric power generator
Accepted Manuscript Preparation and properties of PEDOT:PSS/Te nanorod composite films for flexible thermoelectric power generator Haijun Song, Kefeng Cai PII:
S0360-5442(17)30039-7
DOI:
10.1016/j.energy.2017.01.037
Reference:
EGY 10172
To appear in:
Energy
Received Date: 27 April 2016 Revised Date:
29 December 2016
Accepted Date: 7 January 2017
Please cite this article as: Song H, Cai K, Preparation and properties of PEDOT:PSS/Te nanorod composite films for flexible thermoelectric power generator, Energy (2017), doi: 10.1016/ j.energy.2017.01.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Preparation and Properties of PEDOT:PSS/Te Nanorod Composite Films for Flexible Thermoelectric Power Generator Haijun Song, Kefeng Cai*
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Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education; School of Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China
*Corresponding author: [email protected]
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ABSTRACT Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) functionalized Te (PF-Te) nanorods were in situ synthesized. A simple and efficient
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vacuum-assisted filtration method was employed to integrate the PF-Te nanorods with PEDOT:PSS to form PEDOT:PSS/PF-Te composite films. By varying the content of PF-Te nanorods, the Seebeck coefficient of the composites increases from 15.6 to 51.6 µV/K, while the electrical conductivity decreases from 1262 to 122.4 S/cm. A
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maximum power factor of 51.4 µW/mK2 is obtained from a sample containing 70 wt% PF-Te nanorods at room temperature. An 8 single-leg flexible thermoelectric
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generator prototype was fabricated using the as-prepared PEDOT:PSS/PF-Te composite film containing 70 wt% PF-Te on a polyimide substrate with Ag electrodes.
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The prototype produced an output voltage of 2.5 mV at a 13.4 K temperature difference between human body and the environment. Keywords: PEDOT:PSS; Te nanorods; Composite; Thermoelectric properties; Wearable device.
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ACCEPTED MANUSCRIPT 1. Introduction Recently, the large proliferation of portable/wearable electronic devices stimulates research interests in lightweight and highly-flexible renewable and
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sustainable energy sources like solar cells, piezoelectrics, and thermoelectrics (TE) [1-4]. TE devices can directly convert heat to electricity or vice versa, offering a promising potential to convert body heat continuously into electrical power [5]. The
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current state-of-the-art TE materials are alloys such as bismuth telluride and lead
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telluride, which are expensive, brittle and fragile making them unsuitable for kinetic applications [6]. In contrast, conducting polymers, because of their intrinsically low thermal conductivities, material abundance, flexibility, facile processability, and environment friendly [7], have been studied for flexible TE applications [8-10].
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The TE property of a material is evaluated by a dimensionless figure of merit ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, T is the absolute temperature and S2σ is the power factor.
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Therefore, an ideal TE material should simultaneously possess a high electrical
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conductivity, a high Seebeck coefficient and a low thermal conductivity. The intrinsically low thermal conductivity of conducting polymer that typically ranging from 0.11 to 0.4 W/mK [11] is especially beneficial to a high ZT value. However, the low power factor of the conducting polymers, which is 2 or 3 orders of magnitude lower than that of state-of-the-art inorganic TE materials, excluded them as candidates for TE materials over the past years. Generally, two strategies have been proposed to increase the power factor of conducting polymers. One is to adjust the doping level of 2
ACCEPTED MANUSCRIPT the polymers since it affects the free-carrier concentration and carrier mobility [12-14]. For example, a ZT value up to 0.25 for a tosylate (Tos) optimum doped Poly(3,4-ethylenedioxythiophene)(PEDOT) film [12] and a ZT value about 0.42 for a
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poly(styrenesulphonate) (PSS) optimally doped PEDOT film at room temperature (RT) [14] have been reported (Note that this ZT value is somewhat overstated due to the samples measured for electrical properties being different from that for thermal Another
strategy
is
to
design
conducting
polymer
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conductivity).
based
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nanocomposites [15,16]. The nanocomposites may take the advantages of both the low thermal conductivity of the polymer and high power factor of inorganic TE fillers. Numerous polymer-inorganic TE materials have been reported to improve the TE properties [17-22] and much progress has been made. For example, Yao et al. have
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prepared single-walled carbon nanotube (SWCNT)/polyaniline (PANI) hybrid film by a simple solution process, and the film shows a maximum ZT value of 0.12 due to the highly ordered PANI interface layer on the SWCNT surface [20]. Most recently, an
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n-type PEDOT/CNT composite film treated by tetrakis(dimethylamino)ethylene with
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a ZT ~ 0.5 has been reported, which is the largest ever reported ZT value for polymer based composite materials [22]. Among numerous conducting polymers, PEDOT:PSS is one of the most
promising candidates for TE application because of its high electrical conductivity, excellent stability, flexibility, and commercial availability [23]. The electrical conductivity of PEDOT:PSS can be up to >103 S/cm through post treatments [24,25]; however, its Seebeck coefficient still remains very low (~ 15-20 µV/K), which 3
ACCEPTED MANUSCRIPT influences its overall TE properties. Tellurium (Te) nanorods possess a high Seebeck coefficient (~ 408 µV/K [26]) and can be easily synthesized in water solution in the presence of a structure directing surfactant [27]. Previously, See et al. [28]
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synthesized a PEDOT:PSS coated Te nanorod composite with a Seebeck coefficient of 163 µV/K, which is 9 times larger than that of pure PEDOT:PSS. However, the electrical conductivity of the composite film was very low (~ 19.3 S/cm) and the film
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was drop-cast on a quartz substrate, restricting its application as flexible and wearable
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TE device. PEDOT:PSS thin film, prepared by a vacuum-assisted filtration method with common organic solvent, such as methanol, ethanol, dimethyl formamide (DMF), or dimethylsulfoxide (DMSO), with high electrical conductivity indicates that the method is an efficient and robust through selectively removal of PSS during the
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filtration [29,30]. In this work, we have combined the high conductive PEDOT:PSS with high Seebeck coefficient PF-Te by the vacuum-assisted filtration method using a PVDF membrane. The PVDF membrane plays an important role during the process: 1)
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the membrane is porous, through which excessive PSS is removed by the filtration; 2)
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the membrane also acts as a substrate with high strength and flexibility. The as-prepared PEDOT:PSS film exhibits much higher power factor (51.4 µW/mK2) compared with the TE fabrics (PEDOT:PSS coated commercial fabric with a power factor of ~0.04 µW/mK2 [9]). Besides, our method is more convenient and simpler than the printing method [31], in which a followed acid treatment and then heat treatment are necessary. Finally, a flexible power generation device has been designed based on the flexible PEDOT:PSS/PF-Te composite films. 4
ACCEPTED MANUSCRIPT 2. Experimental 2.1. Materials PEDOT:PSS aqueous solution (CLEVIOS PH1000) was purchased from H.C.
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Starck. Ethanol was obtained from Sinopharm Chemical Reagent Co., Ltd. Ascorbic acid and sodium tellurite (Na2TeO3, 97%) were purchased from Aladdin Industrial Corporation. Porous PVDF membrane (0.22 µm) was obtained from Shanghai Xingya
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Group. All materials were used without further purification.
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2.2. Synthesis of PEDOT:PSS Functionalized Te Nanorods
The synthesis process of PEDOT:PSS functionalized Te nanorods was followed a method described in ref.[28] In a typical procedure, 56 mmol ascorbic acid was dissolved in 400 ml of distilled water with stirring for 30 min to form a clear solution,
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followed by addition of 5 ml PEDOT:PSS with another 30 min stirring. Then 2.5 mmol Na2TeO3 was added to the mixture with vigorously stirring. The obtained suspension was heated to 90 °C and maintained at the temperature for 20 h. Then the
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resulting product was cooled down to room temperature naturally, and the precipitate
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was centrifuged at 9000 rpm for 30 min. After pouring off the PEDOT:PSS-rich supernatant, the precipitate was redispersed in distilled water and centrifuged again. The final PF-Te product was dried at 70 °C under vacuum for 12 h. For comparison, the bare Te nanorods were synthesized by the same method as that for PF-Te nanorods but in the absence of PEDOT:PSS. 2.3. Preparation of Flexible PEDOT:PSS/PF-Te Composite Films Fig. 1 schematically depicts the fabrication process of PEDOT:PSS/SPF-Te 5
ACCEPTED MANUSCRIPT composite films via a vacuum-assisted filtration method. Different amount of the as-prepared PF-Te was dispersed in 6 mL ethanol by sonication for 0.5 h. Then 200 µL of PEDOT:PSS aqueous solution was added into the above PF-Te dispersion, and
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the resulting mixture was sonicated for another 0.5 h. Finally, the PEDOT:PSS/PF-Te composite films were prepared by the vacuum-assisted filtration of the mixture onto porous PVDF membrane (0.22 µm). The as-prepared PEDOT:PSS/PF-Te composite
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films were then dried at 70 oC under vacuum for 12 h. For comparison, PEDOT:PSS
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film prepared by drop-casting PEDOT:PSS solution on a glass substrate without further treatment was denoted as PEDOT:PSS; and the PEDOT:PSS film prepared by the vacuum-assisted filtration method was denoted as F-PEDOT:PSS. 2.4. Measurement and Characterization
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Electrical conductivity was measured using a steady-state four-probe technique with a square wave current (~10 mA in amplitude) using Ecopia HMS-3000. The Seebeck coefficient was determined by the slope of the linear relationship between the
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thermal electromotive force and temperature difference (~5 - 15 K) between two ends
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on one side of each film. Through-plane thermal conductivity was measured using TC3010 thermal conductivity measuring instrument (Xi’an Xiatech Electronics Co., Ltd., China) based on the transient hot-wire method. All of the measurements were taken three times at 293 K with 19 % relative humidity and the average of three values was used for analysis (see details in Supporting Information). The synthesized Te nanorods and the prepared composite films were analyzed by XRD using Cu Kα radiation (D/MAX 2550VB3+/PCⅡ). The morphology and thickness of the films 6
ACCEPTED MANUSCRIPT were observed by scanning electron microscope (FEI Nova NanoSEM 450, the thickness of samples was presented in Supporting Information Table S1). The surface composition of the samples was analyzed by X-ray photoelectron spectroscopy (XPS)
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with Al Kα radiation (1486.6 eV) using an ESCALAB 250Xi system X-ray photoelectron spectrometer. 3. Results and discussion
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The crystallinity of the Te nanorods, F-PEDOT:PSS, and PEDOT:PSS/PF-Te
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composite film is evaluated by XRD, as depicted in Fig. 2. In Fig. 2A, all the peaks agree with the standard data of Te (JCPDS card, No. 36-1452), demonstrating that the synthesized nanorods are pure hexagonal Te. And the chemical reactions involved in this process are proposed as follows:
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TeO32- + 2H+→TeO2 + H2O
C6H8O6 + TeO2 → C6H4O6 + Te + 2H2O
(1) (2).
Initially, H+ resulting from the ionization of ascorbic acid reacts with TeO32- to
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generate TeO2, which is not dissolved in water at RT. When the reaction mixture is
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heated to 90 oC, the initially generated TeO2 solids gradually dissolved in the solution. And due to the reducibility of ascorbic acid, the Te4+ was slowly reduced to Te to form Te seeds [27]. The Te seeds grew up into Te nanorods. The XRD pattern of F-PEDOT:PSS is shown in Fig. 2B. The diffraction peak at 2θ = 17.6° is attributed to the PSS groups [32] and the peak at 2θ = 25.8o corresponds to the (0 2 0) plane of PEDOT [33,34]. And the diffraction peaks at 2θ =20.5° and 22.7° are resulting from the PVDF membrane [35]. Both diffraction peaks of PEDOT:PSS and Te nanorods 7
ACCEPTED MANUSCRIPT can be found in the XRD pattern of the PEDOT:PSS/PF-Te composite film (Fig. 2C and 2D). When the Te content is low in the composite film, its diffraction peaks are difficult to detect (Fig. 2C, red arrows) due to the coated PEDOT:PSS layer (see
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HRTEM image hereinafter). With the increase of Te content, diffraction peaks of Te gradually appear and they are very strong for the sample with 90 wt% PF-Te (see Fig. 2D).
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Fig. 3a shows the FE-SEM image of the PF-Te nanorods, indicating that the
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PF-Te nanorods are uniform with an average diameter of ~30 ± 10 nm. HRTEM image (Fig. 3b) clearly shows a thin layer (~2-3 nm) on the surface of the Te nanorod. It is deduced that the thin layer is PEDOT:PSS. In the hexagonal crystal structure of Te, the interactions between the spiral chains of Te atoms are thought to be a mixture
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of electronic and van der Waals forces [36,37]. During the growing process of the Te nanorods in the presence of PEDOT:PSS, the interaction between the van der Waals force from the Te atom layer and π interaction from the PEDOT:PSS chains at the
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polymer/nanocrystal interface results in a good interfacial adhesion between the two
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phases. This polymer layer can also serve to passivate the surface of the nanocrystalline rods and prevent oxidation during processing. Furthermore, an energy filtering effect may be introduced by this polymer/nanocrystal interface, which is beneficial for the enhancement of Seebeck coefficient [38]. Typical SEM images of the F-PEDOT:PSS film and PEDOT:PSS/PF-Te composite films with various PF-Te contents are presented in Fig. 4. The F-PEDOT:PSS film shows a smooth and flat surface (Fig. 4a). For the PEDOT:PSS/PF-Te composite films, 8
ACCEPTED MANUSCRIPT when the PF-Te content is low (≤50 wt%), only a few Te nanorods dispersed in the PEDOT:PSS matrix can be observed; when the PF-Te content increases to 70 wt%, randomly distributed Te nanorods can be observed (Fig. 4c), indicating a good
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dispersion of PF-Te nanorods in the PEDOT:PSS matrix; when the PF-Te content increases to a high value, i.e., 90 wt%, the composite film becomes very rough, friable and easy to flake off from the PVDF membrane naturally due to the PEDOT:PSS
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being unable to fully coat all the PF-Te nanorods (Fig.4d).
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The as-prepared PEDOT:PSS/PF-Te composite film with 70 wt% PF-Te has a smooth surface and is flexible and cuttable, making them ideal for integration into any shape for future applications (Figure S1(a)). For comparison, we have also prepared PEDOT:PSS/Te composite film, in which the Te nanorods are without being coated
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with PEDOT:PSS, and the PF-Te composite film by the same method. The obtained PEDOT:PSS/Te composite film separated into two layers naturally after being dried under vacuum at 70 oC for 12 h (Figure S1(b)). This may be because the density of Te
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is much higher than that of PEDOT:PSS and the interaction between the ex-situ
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synthesized Te nanorods and PEDOT:PSS is weak, leading to the Te nanorods subsiding first and leaving the PEDOT:PSS at the top during the vacuum-assisted filtration process. The PF-Te composite film (Figure S1(C)) shows a smooth surface with a metallic luster due to containing high content of Te nanorods. Also the composite film is tightly bonded with the PVDF membrane and shows strong strength and high flexibility. XPS spectra are used to analyze the surface composition of the as-prepared films. 9
ACCEPTED MANUSCRIPT Fig. 5(a) presents the XPS survey spectrum of the samples. For the PEDOT:PSS film and F-PEDOT:PSS film, their spectra are almost the same. After forming composite with PF-Te, two strong peaks at binding energy of 587 and 576 eV appear,
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corresponding to the Te 3d3/2 and Te 3d5/2 peaks [39]; besides, other peaks at 44, 823 and 874 eV also appear, corresponding to the peaks of Te 4d, Te 3p3, and Te 3p1 of Te nanorods, respectively [40,41]. These results demonstrate the successful
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combination of PEDOT:PSS and PF-Te. The characteristic S(2p) peaks are also
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recorded and shown in Fig. 5(b): the S(2p) peak with a binding energy of 166-170 eV originates from the sulphur atoms in the PSS units, whereas the peak with a lower binding energy of 162-166 eV corresponds to the sulphur atoms in the PEDOT units [17,42]. By comparing with the spectra of the PEDOT:PSS film and F-PEDOT:PSS
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film, the influence of the filtration process is evidenced by the reduction of the intensity of S(2p) from PSS and enhancement of the intensity of S(2p) from PEDOT. The ratio of PSS to PEDOT is determined by calculating the ratio of the integral area
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of peaks assigned to PSS and PEDOT. The ratio of PSS to PEDOT of the
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PEDOT:PSS film is 2.41:1, while this value decreases to 1.99:1 for the F-PEDOT:PSS film, leading to 17.4% reduction of PSS from the film. This phenomenon has also been observed after the PEDOT:PSS film being treated with other organic solvents, such as diethylene glycol, dimethyl sulphoxide, ethylene glycol, sorbitol, N-Methyl pyrrolidone, etc [14,42,43]. The reason for the phenomenon is explained as follows. In the PEDOT:PSS solution, there exist two types of PSS: one is complexed with PEDOT chain, and the another is “free PSS” that acts as a surfactant. During the 10
ACCEPTED MANUSCRIPT filtration process, the free PSS was removed and the removal of insulating PSS promoted carrier transport and hopping, resulting a high electrical conductivity [44] (see herein after). Besides, the peak of PSS has shifted to a lower energy after the
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filtration process, indicating that the electrostatic interaction between the PEDOT and PSS becomes weaker due to the presence of ethanol between PEDOT and PSS [45]. Moreover, the addition of PF-Te almost does not introduce any change in the S2p
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spectra.
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The room temperature TE properties of PEDOT:PSS, Te nanorods, PF-Te and PEDOT:PSS/PF-Te composite films changing with PF-Te content are shown in Table 1 and Fig. 6a. The PEDOT:PSS film exhibits an electrical conductivity of 0.21 S/cm. After the vacuum-assisted filtration process, its electrical conductivity has been
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dramatically enhanced to 1262 S/cm, similar to the value reported in ref.[30], which mainly attributes to the removal of PSS and rearrangement of PEDOT chains [24,25]. However, its Seebeck coefficient almost keeps constant before (~ 15.4 µV/K) and
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after (~ 15.6 µV/K) the filtration process. The Te nanorods show a high Seebeck
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coefficient of 367 µV/K, while its electrical conductivity is only 0.046 S/cm. When Te nanorods are coated with PEDOT:PSS, the electrical conductivity increases to 11.5 S/cm. Usually, in a simple two-phase composite system, the electrical conductivity cannot exceed the largest value of either individual component. While the as-prepared PF-Te sample shows a much higher electrical conductivity than both the PEDOT:PSS and Te nanorods. This phenomenon can be attributed to a potential interfacial interaction in polymer/nanocrystal system which would allow the TE performance of 11
ACCEPTED MANUSCRIPT the composite to surpass the prediction of mean-field models [46]. In the PF-Te film, the template effect of Te nanorods may result in a conformational organization of PEDOT:PSS interfacial layer, making a higher degree of organization than the bulk
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PEDOT:PSS. Therefore, a higher conductivity in the polymer/nanocrystal composite has obtained. Simultaneously, its Seebeck coefficient decreases to 151 µV/K, which is consistent with the results reported by See et al. [28]. The electrical conductivity of
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the composite films monotonically decreases with the increasing of PF-Te content,
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and the Seebeck coefficient shows an opposite tendency. The enhanced Seebeck coefficient is mainly ascribed to the energy filtering effect [38] at the interface between PEDOT:PSS and Te nanorods and the inherent high Seebeck coefficient of Te nanorods. Because of the energy-filtering, low-energy carriers are scattered by the
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potential barrier at the interface between two phases, whereas high-energy carriers cross the barrier. High-energy carriers can transfer more heat than low-energy carriers, thereby leading to an increase in S [38]. The decrease of electrical conductivity is
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mainly because of relative lower electrical conductivity of the PF-Te compared with
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that of F-PEDOT:PSS. The electrical conductivity decreases to 122.4 S/cm for the composite film containing 92 wt% PF-Te, this is also because the obtained film becomes rough and friable, making the carriers transport more difficult (see Fig. 4d) besides the above-mentioned reason. The maximum power factor value of the composite films reached 51.4 µW/mK2 from the sample with 70 wt% PF-Te, which is about twice as large as that of the F-PEDOT:PSS film and two orders of magnitude higher than that of the Te nanorods. This value is higher than that of the reported 12
ACCEPTED MANUSCRIPT PEDOT/PbTe composite (1.45 µW/mK2 [47]); PEDOT:PSS/Bi2Te3 based alloy nanosheet composite films (32.26 µW/mK2 [19]); PEDOT:PSS/Au nanoparticles (11.7 µW/mK2 at 50 oC [48]) and PEDOT/Bi2S3 composite (2.3 µW/mK2 with
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36.1wt% Bi2S3 [21]). But it is still lower than those of Te-Cu1.75Te/PEDOT:PSS composite film (84 µW/mK2 [49]) and rGO/PEDOT:PSS/Te NW hybrid film (143 µW/mK2 [50]).
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The through-plane thermal conductivity (κ ) of the composite films with various
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contents of PF-Te is shown in Fig. 6b. The κ of the PEDOT:PSS/PF-Te composite films increases linearly from 0.141 to 0.156 W/mK as the PF-Te content increases from 0 to 82 wt %. The κ of F-PEDOT:PSS is 0.141 W/mK. Usually conducting polymer films or conducting polymer based composite films have anisotropic
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properties: the κ is much lower than the in-plan thermal conductivity (κ//), and presently, it is a great challenge to measure the κ// of films. According to ref. [14], the ratio of the κ// and κ , κ///κ ,‐is about 1.40 ± 0.22 for PEDOT:PSS. By using the ratio,
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the κ// of the composite films calculated is from 0.197 to 0.218 W/mK as the PF-Te
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content increases from 0 to 82 wt %. The reason for such low thermal conductivity of the composite films is explained as follows: (1) the PEDOT:PSS matrix dominates the thermal conductivity; (2) the diameter of the Te nanorods being of order of the intrinsic mean-free-path of heat carrying phonons and the acoustic mismatch of Te and the polymer structures results in an efficient scattering of phonons [28,46,51]. After consideration the anisotropic issue of the composite film, a largest ZT value, 0.076, is obtained for the sample containing 70 wt% PF-Te at room temperature. 13
ACCEPTED MANUSCRIPT Subsequently, a prototype power generator based on the as-prepared PEDOT:PSS/PF-Te composite material has been fabricated as shown in Fig. 7a. The device consists of eight single-leg of PEDOT:PSS/PF-Te (70 wt%) composite film
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(length 25 mm, width 5 mm) pasted on a polyimide substrate and Ag paste is used for electrode. Fig. 7b shows the output performance of this device due to the temperature difference between a forearm (305.4 ± 0.2 K) and the ambient (292 K). One side of
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the device is attached to a forearm skin and the other side is exposed to the air by
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using a bubble film as an insulator. The output voltage of the module is 2.5 mV at a temperature difference of approximately 13.4 K. The output voltage Vo can be defined as: Vo = NS∆T, where N is the number of TE elements, S is the Seebeck coefficient and ∆T is temperature difference. Base on the values of N = 8, S = 27.5 µV/K, ∆T =
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13.4 K, the calculated Vo is 2.95 mV, which is a little higher than the experimental value, probably due to the inevitable contact resistance in the device. The result demonstrates a wearable TE power generator based on PEDOT:PSS/PF-Te flexible
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films for harvesting energy from the waste heat generated from the human body.
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4. Conclusions In
conclusion,
we
have
shown
the
facile
fabrication
of
flexible
PEDOT:PSS/PF-Te composite films using a vacuum-assisted filtration process. The obtained films are flexible and are ideal for wearable electronic devices. PEDOT:PSS contribute the high electrical conductivity in the composite films due to removal of insulating PSS during the filtration process and Te nanorods contribute the enhanced Seebeck coefficient owing to the high Seebeck coefficient of the nanorods and an 14
ACCEPTED MANUSCRIPT energy-filtering effect at the interface between PEDOT:PSS and Te nanorods. An optimized power factor of 51.4 µW/mK2 has been obtained for the sample containing 70 wt% PF-Te. The flexible TE generator module consisting of 8 legs of the
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as-prepared composite film with Ag paste as electrode has produced an output voltage of 2.5 mV at a 13.4 K temperature difference. This study provides a new approach to facilitate practical application of flexible TE generator for low-grade energy
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harvesting.
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Acknowledgements
This work was supported by the National Basic Research Program of China (973 Program) under Grant No. 2013CB632500, Key Program of National Natural Science Foundation of China (5163210), National Natural Science Foundation of China
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(51271133), and the foundation of the State Key Lab of Advanced Technology for
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EP
Material Synthesis and Processing (Wuhan University of Technology).
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[22] Wang H, Hsu J-H, Yi S-I, Kim S, Choi K, Wang G, Yu C. Thermally Driven Large N-Type Voltage Responses from Hybrids of Carbon Nanotubes and Poly(3,4-ethylenedioxythiophene) with Tetrakis(dimethylamino) ethylene. Adv Mater 2015;27:6855-6861. [23] Wang YJ. J. Phys.: Conf. Ser. Research progress on a novel conductive 18
ACCEPTED MANUSCRIPT polymer–poly (3, 4-ethylenedioxythiophene)(PEDOT). 2009;152:012023. [24] Ouyang J. Solution-processed PEDOT: PSS films with conductivities as indium tin oxide through a treatment with mild and weak organic acids. ACS Appl Mater
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ACCEPTED MANUSCRIPT [31] Bae EJ, Kang YH, Jang K S, Jang S-K, Cho SY. Enhancement of Thermoelectric Properties of PEDOT: PSS and Tellurium-PEDOT: PSS Hybrid Composites by Simple Chemical Treatment. Sci Rep 2016;6:18805.
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ACCEPTED MANUSCRIPT 1990;53:87–95. [40] Park H, Son W, Lee SH, Kim H, Lee JJ, Cho W, Choi HH, Kim JH. Aqueous chemical synthesis of tellurium nanowires using a polymeric template for
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der Gon AW, Salaneck WR, Fahlman M. The effects of solvents on the morphology and sheet resistance in poly (3, 4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS) films. Synth Met 2003;139:1–10.
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ACCEPTED MANUSCRIPT Adv Mater 2013;25:1629–1633. [47] Wang YY, Cai KF, Yao X. Facile fabrication and thermoelectric properties of PbTe-modified poly (3, 4-ethylenedioxythiophene) nanotubes. ACS Appl Mater
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[49] Zaia EW, Sahu A, Zhou P, Gordon MP, Forster JD, Aloni S, Liu Y-S, Guo JH, Urban JJ. Carrier Scattering at Alloy Nanointerfaces Enhances Power Factor in PEDOT: PSS Hybrid Thermoelectrics. Nano Lett 2016;16:3352-3359. [50] Choi J, Lee JY, Lee SS, Park CR, Kim H. High
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[51] Dames C, Chen G. Thermoelectrics Handbook: Macro to Nano; Rowe, D. M.,
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ACCEPTED MANUSCRIPT Table caption Table 1 TE parameters of the PEDOT:PSS, Te nanorods and PF-Te films at room temperature.
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Figure captions Fig. 1 Schematic illustration of the fabrication of flexible PEDOT:PSS/PF-Te composite film on PVDF via a filtration process.
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films with 50 wt% PF-Te (C), 90 wt% PF-Te (D).
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Fig. 2 XRD patterns of Te nanorods (A), F-PEDOT:PSS (B), and PEDOT:PSS/PF-Te composite
Fig. 3 (a) SEM image and (b) HRTEM image of the PF-Te nanorods.
Fig. 4 Surface SEM images of the F-PEDOT:PSS film (a) and PEDOT:PSS/PF-Te composite films with 50 wt% (b), 70 wt% (c), and 90 wt% (d) PF-Te nanorods.
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Fig. 5 XPS spectra of the PEDOT:PSS film, F-PEDOT:PSS film and PEDOT:PSS/PF-Te composite film with 70 wt% PF-Te, (a) XPS survey spectrum; (b) sulphur S(2p) spectra. Fig. 6 (a) electrical conductivity, Seebeck coefficient, and power factor and (b) through-plane
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thermal conductivity of the as-prepared PEDOT:PSS/PF-Te composite films as a function of
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PF-Te content at room temperature.
Fig. 7 (a) photograph of a flexible device consisting of eight (PEDOT:PSS/PF-Te)-Ag thermocouples on a polyimide substrate and (b) output performance of the device due to the temperature difference between a forearm (~305.4 K) and the ambient (~292 K).
23
ACCEPTED MANUSCRIPT Table 1 TE parameters of the PEDOT:PSS, Te nanorods and PF-Te films at room temperature. Electrical conductivity (S/cm)
Seebeck coefficient (µV/K)
Power factor (µW/mK2)
PEDOT:PSS Te nanorods PF-Te
0.21 0.046 11.5
15.4 367 151
5 × 10-3 0.62 26.2
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Samples
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Fig. 1 Schematic illustration of the fabrication of flexible PEDOT:PSS/PF-Te composite film on
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PVDF via a filtration process.
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Fig. 2 XRD patterns of Te nanorods (A), F-PEDOT:PSS (B), and PEDOT:PSS/PF-Te composite
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films with 50 wt% PF-Te (C), 90 wt% PF-Te (D).
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Fig. 3 (a) SEM image and (b) HRTEM image of the PF-Te nanorods
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Fig. 4 Surface SEM images of the F-PEDOT:PSS film (a) and PEDOT:PSS/PF-Te composite films with 50 wt% (b), 70 wt% (c), and 90 wt% (d) PF-Te nanorods
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Fig. 5 XPS spectra of the PEDOT:PSS film, F-PEDOT:PSS film and PEDOT:PSS/PF-Te
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composite film with 70 wt% PF-Te, (a) XPS survey spectrum; (b) sulphur S(2p) spectra.
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Fig. 6 (a) electrical conductivity, Seebeck coefficient, and power factor and (b) through-plane
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thermal conductivity of the as-prepared PEDOT:PSS/PF-Te composite films as a function of
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PF-Te content at room temperature.
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Fig. 7 (a) photograph of a flexible device consisting of eight (PEDOT:PSS/PF-Te)-Ag thermocouples on a polyimide substrate and (b) output performance of the device due to the
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temperature difference between a forearm (~305.4 K) and the ambient (~292 K).
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ACCEPTED MANUSCRIPT Highlights: 1. PEDOT:PSS/Te nanorods composite films were prepared. 2. A vacuum-assisted filtration method was employed to prepare the composite films.
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3. A flexible thermoelectric generator composed of the composite film was developed.
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4. The generator outputs a voltage of 2.5 mV at a 13.4 K temperature difference.
S0360-5442(17)30039-7
DOI:
10.1016/j.energy.2017.01.037
Reference:
EGY 10172
To appear in:
Energy
Received Date: 27 April 2016 Revised Date:
29 December 2016
Accepted Date: 7 January 2017
Please cite this article as: Song H, Cai K, Preparation and properties of PEDOT:PSS/Te nanorod composite films for flexible thermoelectric power generator, Energy (2017), doi: 10.1016/ j.energy.2017.01.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Preparation and Properties of PEDOT:PSS/Te Nanorod Composite Films for Flexible Thermoelectric Power Generator Haijun Song, Kefeng Cai*
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Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education; School of Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China
*Corresponding author: [email protected]
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ABSTRACT Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) functionalized Te (PF-Te) nanorods were in situ synthesized. A simple and efficient
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vacuum-assisted filtration method was employed to integrate the PF-Te nanorods with PEDOT:PSS to form PEDOT:PSS/PF-Te composite films. By varying the content of PF-Te nanorods, the Seebeck coefficient of the composites increases from 15.6 to 51.6 µV/K, while the electrical conductivity decreases from 1262 to 122.4 S/cm. A
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maximum power factor of 51.4 µW/mK2 is obtained from a sample containing 70 wt% PF-Te nanorods at room temperature. An 8 single-leg flexible thermoelectric
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generator prototype was fabricated using the as-prepared PEDOT:PSS/PF-Te composite film containing 70 wt% PF-Te on a polyimide substrate with Ag electrodes.
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The prototype produced an output voltage of 2.5 mV at a 13.4 K temperature difference between human body and the environment. Keywords: PEDOT:PSS; Te nanorods; Composite; Thermoelectric properties; Wearable device.
1
ACCEPTED MANUSCRIPT 1. Introduction Recently, the large proliferation of portable/wearable electronic devices stimulates research interests in lightweight and highly-flexible renewable and
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sustainable energy sources like solar cells, piezoelectrics, and thermoelectrics (TE) [1-4]. TE devices can directly convert heat to electricity or vice versa, offering a promising potential to convert body heat continuously into electrical power [5]. The
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current state-of-the-art TE materials are alloys such as bismuth telluride and lead
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telluride, which are expensive, brittle and fragile making them unsuitable for kinetic applications [6]. In contrast, conducting polymers, because of their intrinsically low thermal conductivities, material abundance, flexibility, facile processability, and environment friendly [7], have been studied for flexible TE applications [8-10].
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The TE property of a material is evaluated by a dimensionless figure of merit ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, T is the absolute temperature and S2σ is the power factor.
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Therefore, an ideal TE material should simultaneously possess a high electrical
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conductivity, a high Seebeck coefficient and a low thermal conductivity. The intrinsically low thermal conductivity of conducting polymer that typically ranging from 0.11 to 0.4 W/mK [11] is especially beneficial to a high ZT value. However, the low power factor of the conducting polymers, which is 2 or 3 orders of magnitude lower than that of state-of-the-art inorganic TE materials, excluded them as candidates for TE materials over the past years. Generally, two strategies have been proposed to increase the power factor of conducting polymers. One is to adjust the doping level of 2
ACCEPTED MANUSCRIPT the polymers since it affects the free-carrier concentration and carrier mobility [12-14]. For example, a ZT value up to 0.25 for a tosylate (Tos) optimum doped Poly(3,4-ethylenedioxythiophene)(PEDOT) film [12] and a ZT value about 0.42 for a
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poly(styrenesulphonate) (PSS) optimally doped PEDOT film at room temperature (RT) [14] have been reported (Note that this ZT value is somewhat overstated due to the samples measured for electrical properties being different from that for thermal Another
strategy
is
to
design
conducting
polymer
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conductivity).
based
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nanocomposites [15,16]. The nanocomposites may take the advantages of both the low thermal conductivity of the polymer and high power factor of inorganic TE fillers. Numerous polymer-inorganic TE materials have been reported to improve the TE properties [17-22] and much progress has been made. For example, Yao et al. have
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prepared single-walled carbon nanotube (SWCNT)/polyaniline (PANI) hybrid film by a simple solution process, and the film shows a maximum ZT value of 0.12 due to the highly ordered PANI interface layer on the SWCNT surface [20]. Most recently, an
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n-type PEDOT/CNT composite film treated by tetrakis(dimethylamino)ethylene with
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a ZT ~ 0.5 has been reported, which is the largest ever reported ZT value for polymer based composite materials [22]. Among numerous conducting polymers, PEDOT:PSS is one of the most
promising candidates for TE application because of its high electrical conductivity, excellent stability, flexibility, and commercial availability [23]. The electrical conductivity of PEDOT:PSS can be up to >103 S/cm through post treatments [24,25]; however, its Seebeck coefficient still remains very low (~ 15-20 µV/K), which 3
ACCEPTED MANUSCRIPT influences its overall TE properties. Tellurium (Te) nanorods possess a high Seebeck coefficient (~ 408 µV/K [26]) and can be easily synthesized in water solution in the presence of a structure directing surfactant [27]. Previously, See et al. [28]
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synthesized a PEDOT:PSS coated Te nanorod composite with a Seebeck coefficient of 163 µV/K, which is 9 times larger than that of pure PEDOT:PSS. However, the electrical conductivity of the composite film was very low (~ 19.3 S/cm) and the film
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was drop-cast on a quartz substrate, restricting its application as flexible and wearable
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TE device. PEDOT:PSS thin film, prepared by a vacuum-assisted filtration method with common organic solvent, such as methanol, ethanol, dimethyl formamide (DMF), or dimethylsulfoxide (DMSO), with high electrical conductivity indicates that the method is an efficient and robust through selectively removal of PSS during the
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filtration [29,30]. In this work, we have combined the high conductive PEDOT:PSS with high Seebeck coefficient PF-Te by the vacuum-assisted filtration method using a PVDF membrane. The PVDF membrane plays an important role during the process: 1)
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the membrane is porous, through which excessive PSS is removed by the filtration; 2)
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the membrane also acts as a substrate with high strength and flexibility. The as-prepared PEDOT:PSS film exhibits much higher power factor (51.4 µW/mK2) compared with the TE fabrics (PEDOT:PSS coated commercial fabric with a power factor of ~0.04 µW/mK2 [9]). Besides, our method is more convenient and simpler than the printing method [31], in which a followed acid treatment and then heat treatment are necessary. Finally, a flexible power generation device has been designed based on the flexible PEDOT:PSS/PF-Te composite films. 4
ACCEPTED MANUSCRIPT 2. Experimental 2.1. Materials PEDOT:PSS aqueous solution (CLEVIOS PH1000) was purchased from H.C.
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Starck. Ethanol was obtained from Sinopharm Chemical Reagent Co., Ltd. Ascorbic acid and sodium tellurite (Na2TeO3, 97%) were purchased from Aladdin Industrial Corporation. Porous PVDF membrane (0.22 µm) was obtained from Shanghai Xingya
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Group. All materials were used without further purification.
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2.2. Synthesis of PEDOT:PSS Functionalized Te Nanorods
The synthesis process of PEDOT:PSS functionalized Te nanorods was followed a method described in ref.[28] In a typical procedure, 56 mmol ascorbic acid was dissolved in 400 ml of distilled water with stirring for 30 min to form a clear solution,
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followed by addition of 5 ml PEDOT:PSS with another 30 min stirring. Then 2.5 mmol Na2TeO3 was added to the mixture with vigorously stirring. The obtained suspension was heated to 90 °C and maintained at the temperature for 20 h. Then the
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resulting product was cooled down to room temperature naturally, and the precipitate
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was centrifuged at 9000 rpm for 30 min. After pouring off the PEDOT:PSS-rich supernatant, the precipitate was redispersed in distilled water and centrifuged again. The final PF-Te product was dried at 70 °C under vacuum for 12 h. For comparison, the bare Te nanorods were synthesized by the same method as that for PF-Te nanorods but in the absence of PEDOT:PSS. 2.3. Preparation of Flexible PEDOT:PSS/PF-Te Composite Films Fig. 1 schematically depicts the fabrication process of PEDOT:PSS/SPF-Te 5
ACCEPTED MANUSCRIPT composite films via a vacuum-assisted filtration method. Different amount of the as-prepared PF-Te was dispersed in 6 mL ethanol by sonication for 0.5 h. Then 200 µL of PEDOT:PSS aqueous solution was added into the above PF-Te dispersion, and
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the resulting mixture was sonicated for another 0.5 h. Finally, the PEDOT:PSS/PF-Te composite films were prepared by the vacuum-assisted filtration of the mixture onto porous PVDF membrane (0.22 µm). The as-prepared PEDOT:PSS/PF-Te composite
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films were then dried at 70 oC under vacuum for 12 h. For comparison, PEDOT:PSS
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film prepared by drop-casting PEDOT:PSS solution on a glass substrate without further treatment was denoted as PEDOT:PSS; and the PEDOT:PSS film prepared by the vacuum-assisted filtration method was denoted as F-PEDOT:PSS. 2.4. Measurement and Characterization
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Electrical conductivity was measured using a steady-state four-probe technique with a square wave current (~10 mA in amplitude) using Ecopia HMS-3000. The Seebeck coefficient was determined by the slope of the linear relationship between the
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thermal electromotive force and temperature difference (~5 - 15 K) between two ends
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on one side of each film. Through-plane thermal conductivity was measured using TC3010 thermal conductivity measuring instrument (Xi’an Xiatech Electronics Co., Ltd., China) based on the transient hot-wire method. All of the measurements were taken three times at 293 K with 19 % relative humidity and the average of three values was used for analysis (see details in Supporting Information). The synthesized Te nanorods and the prepared composite films were analyzed by XRD using Cu Kα radiation (D/MAX 2550VB3+/PCⅡ). The morphology and thickness of the films 6
ACCEPTED MANUSCRIPT were observed by scanning electron microscope (FEI Nova NanoSEM 450, the thickness of samples was presented in Supporting Information Table S1). The surface composition of the samples was analyzed by X-ray photoelectron spectroscopy (XPS)
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with Al Kα radiation (1486.6 eV) using an ESCALAB 250Xi system X-ray photoelectron spectrometer. 3. Results and discussion
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The crystallinity of the Te nanorods, F-PEDOT:PSS, and PEDOT:PSS/PF-Te
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composite film is evaluated by XRD, as depicted in Fig. 2. In Fig. 2A, all the peaks agree with the standard data of Te (JCPDS card, No. 36-1452), demonstrating that the synthesized nanorods are pure hexagonal Te. And the chemical reactions involved in this process are proposed as follows:
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TeO32- + 2H+→TeO2 + H2O
C6H8O6 + TeO2 → C6H4O6 + Te + 2H2O
(1) (2).
Initially, H+ resulting from the ionization of ascorbic acid reacts with TeO32- to
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generate TeO2, which is not dissolved in water at RT. When the reaction mixture is
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heated to 90 oC, the initially generated TeO2 solids gradually dissolved in the solution. And due to the reducibility of ascorbic acid, the Te4+ was slowly reduced to Te to form Te seeds [27]. The Te seeds grew up into Te nanorods. The XRD pattern of F-PEDOT:PSS is shown in Fig. 2B. The diffraction peak at 2θ = 17.6° is attributed to the PSS groups [32] and the peak at 2θ = 25.8o corresponds to the (0 2 0) plane of PEDOT [33,34]. And the diffraction peaks at 2θ =20.5° and 22.7° are resulting from the PVDF membrane [35]. Both diffraction peaks of PEDOT:PSS and Te nanorods 7
ACCEPTED MANUSCRIPT can be found in the XRD pattern of the PEDOT:PSS/PF-Te composite film (Fig. 2C and 2D). When the Te content is low in the composite film, its diffraction peaks are difficult to detect (Fig. 2C, red arrows) due to the coated PEDOT:PSS layer (see
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HRTEM image hereinafter). With the increase of Te content, diffraction peaks of Te gradually appear and they are very strong for the sample with 90 wt% PF-Te (see Fig. 2D).
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Fig. 3a shows the FE-SEM image of the PF-Te nanorods, indicating that the
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PF-Te nanorods are uniform with an average diameter of ~30 ± 10 nm. HRTEM image (Fig. 3b) clearly shows a thin layer (~2-3 nm) on the surface of the Te nanorod. It is deduced that the thin layer is PEDOT:PSS. In the hexagonal crystal structure of Te, the interactions between the spiral chains of Te atoms are thought to be a mixture
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of electronic and van der Waals forces [36,37]. During the growing process of the Te nanorods in the presence of PEDOT:PSS, the interaction between the van der Waals force from the Te atom layer and π interaction from the PEDOT:PSS chains at the
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polymer/nanocrystal interface results in a good interfacial adhesion between the two
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phases. This polymer layer can also serve to passivate the surface of the nanocrystalline rods and prevent oxidation during processing. Furthermore, an energy filtering effect may be introduced by this polymer/nanocrystal interface, which is beneficial for the enhancement of Seebeck coefficient [38]. Typical SEM images of the F-PEDOT:PSS film and PEDOT:PSS/PF-Te composite films with various PF-Te contents are presented in Fig. 4. The F-PEDOT:PSS film shows a smooth and flat surface (Fig. 4a). For the PEDOT:PSS/PF-Te composite films, 8
ACCEPTED MANUSCRIPT when the PF-Te content is low (≤50 wt%), only a few Te nanorods dispersed in the PEDOT:PSS matrix can be observed; when the PF-Te content increases to 70 wt%, randomly distributed Te nanorods can be observed (Fig. 4c), indicating a good
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dispersion of PF-Te nanorods in the PEDOT:PSS matrix; when the PF-Te content increases to a high value, i.e., 90 wt%, the composite film becomes very rough, friable and easy to flake off from the PVDF membrane naturally due to the PEDOT:PSS
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being unable to fully coat all the PF-Te nanorods (Fig.4d).
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The as-prepared PEDOT:PSS/PF-Te composite film with 70 wt% PF-Te has a smooth surface and is flexible and cuttable, making them ideal for integration into any shape for future applications (Figure S1(a)). For comparison, we have also prepared PEDOT:PSS/Te composite film, in which the Te nanorods are without being coated
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with PEDOT:PSS, and the PF-Te composite film by the same method. The obtained PEDOT:PSS/Te composite film separated into two layers naturally after being dried under vacuum at 70 oC for 12 h (Figure S1(b)). This may be because the density of Te
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is much higher than that of PEDOT:PSS and the interaction between the ex-situ
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synthesized Te nanorods and PEDOT:PSS is weak, leading to the Te nanorods subsiding first and leaving the PEDOT:PSS at the top during the vacuum-assisted filtration process. The PF-Te composite film (Figure S1(C)) shows a smooth surface with a metallic luster due to containing high content of Te nanorods. Also the composite film is tightly bonded with the PVDF membrane and shows strong strength and high flexibility. XPS spectra are used to analyze the surface composition of the as-prepared films. 9
ACCEPTED MANUSCRIPT Fig. 5(a) presents the XPS survey spectrum of the samples. For the PEDOT:PSS film and F-PEDOT:PSS film, their spectra are almost the same. After forming composite with PF-Te, two strong peaks at binding energy of 587 and 576 eV appear,
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corresponding to the Te 3d3/2 and Te 3d5/2 peaks [39]; besides, other peaks at 44, 823 and 874 eV also appear, corresponding to the peaks of Te 4d, Te 3p3, and Te 3p1 of Te nanorods, respectively [40,41]. These results demonstrate the successful
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combination of PEDOT:PSS and PF-Te. The characteristic S(2p) peaks are also
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recorded and shown in Fig. 5(b): the S(2p) peak with a binding energy of 166-170 eV originates from the sulphur atoms in the PSS units, whereas the peak with a lower binding energy of 162-166 eV corresponds to the sulphur atoms in the PEDOT units [17,42]. By comparing with the spectra of the PEDOT:PSS film and F-PEDOT:PSS
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film, the influence of the filtration process is evidenced by the reduction of the intensity of S(2p) from PSS and enhancement of the intensity of S(2p) from PEDOT. The ratio of PSS to PEDOT is determined by calculating the ratio of the integral area
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of peaks assigned to PSS and PEDOT. The ratio of PSS to PEDOT of the
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PEDOT:PSS film is 2.41:1, while this value decreases to 1.99:1 for the F-PEDOT:PSS film, leading to 17.4% reduction of PSS from the film. This phenomenon has also been observed after the PEDOT:PSS film being treated with other organic solvents, such as diethylene glycol, dimethyl sulphoxide, ethylene glycol, sorbitol, N-Methyl pyrrolidone, etc [14,42,43]. The reason for the phenomenon is explained as follows. In the PEDOT:PSS solution, there exist two types of PSS: one is complexed with PEDOT chain, and the another is “free PSS” that acts as a surfactant. During the 10
ACCEPTED MANUSCRIPT filtration process, the free PSS was removed and the removal of insulating PSS promoted carrier transport and hopping, resulting a high electrical conductivity [44] (see herein after). Besides, the peak of PSS has shifted to a lower energy after the
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filtration process, indicating that the electrostatic interaction between the PEDOT and PSS becomes weaker due to the presence of ethanol between PEDOT and PSS [45]. Moreover, the addition of PF-Te almost does not introduce any change in the S2p
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spectra.
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The room temperature TE properties of PEDOT:PSS, Te nanorods, PF-Te and PEDOT:PSS/PF-Te composite films changing with PF-Te content are shown in Table 1 and Fig. 6a. The PEDOT:PSS film exhibits an electrical conductivity of 0.21 S/cm. After the vacuum-assisted filtration process, its electrical conductivity has been
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dramatically enhanced to 1262 S/cm, similar to the value reported in ref.[30], which mainly attributes to the removal of PSS and rearrangement of PEDOT chains [24,25]. However, its Seebeck coefficient almost keeps constant before (~ 15.4 µV/K) and
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after (~ 15.6 µV/K) the filtration process. The Te nanorods show a high Seebeck
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coefficient of 367 µV/K, while its electrical conductivity is only 0.046 S/cm. When Te nanorods are coated with PEDOT:PSS, the electrical conductivity increases to 11.5 S/cm. Usually, in a simple two-phase composite system, the electrical conductivity cannot exceed the largest value of either individual component. While the as-prepared PF-Te sample shows a much higher electrical conductivity than both the PEDOT:PSS and Te nanorods. This phenomenon can be attributed to a potential interfacial interaction in polymer/nanocrystal system which would allow the TE performance of 11
ACCEPTED MANUSCRIPT the composite to surpass the prediction of mean-field models [46]. In the PF-Te film, the template effect of Te nanorods may result in a conformational organization of PEDOT:PSS interfacial layer, making a higher degree of organization than the bulk
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PEDOT:PSS. Therefore, a higher conductivity in the polymer/nanocrystal composite has obtained. Simultaneously, its Seebeck coefficient decreases to 151 µV/K, which is consistent with the results reported by See et al. [28]. The electrical conductivity of
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the composite films monotonically decreases with the increasing of PF-Te content,
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and the Seebeck coefficient shows an opposite tendency. The enhanced Seebeck coefficient is mainly ascribed to the energy filtering effect [38] at the interface between PEDOT:PSS and Te nanorods and the inherent high Seebeck coefficient of Te nanorods. Because of the energy-filtering, low-energy carriers are scattered by the
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potential barrier at the interface between two phases, whereas high-energy carriers cross the barrier. High-energy carriers can transfer more heat than low-energy carriers, thereby leading to an increase in S [38]. The decrease of electrical conductivity is
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mainly because of relative lower electrical conductivity of the PF-Te compared with
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that of F-PEDOT:PSS. The electrical conductivity decreases to 122.4 S/cm for the composite film containing 92 wt% PF-Te, this is also because the obtained film becomes rough and friable, making the carriers transport more difficult (see Fig. 4d) besides the above-mentioned reason. The maximum power factor value of the composite films reached 51.4 µW/mK2 from the sample with 70 wt% PF-Te, which is about twice as large as that of the F-PEDOT:PSS film and two orders of magnitude higher than that of the Te nanorods. This value is higher than that of the reported 12
ACCEPTED MANUSCRIPT PEDOT/PbTe composite (1.45 µW/mK2 [47]); PEDOT:PSS/Bi2Te3 based alloy nanosheet composite films (32.26 µW/mK2 [19]); PEDOT:PSS/Au nanoparticles (11.7 µW/mK2 at 50 oC [48]) and PEDOT/Bi2S3 composite (2.3 µW/mK2 with
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36.1wt% Bi2S3 [21]). But it is still lower than those of Te-Cu1.75Te/PEDOT:PSS composite film (84 µW/mK2 [49]) and rGO/PEDOT:PSS/Te NW hybrid film (143 µW/mK2 [50]).
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The through-plane thermal conductivity (κ ) of the composite films with various
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contents of PF-Te is shown in Fig. 6b. The κ of the PEDOT:PSS/PF-Te composite films increases linearly from 0.141 to 0.156 W/mK as the PF-Te content increases from 0 to 82 wt %. The κ of F-PEDOT:PSS is 0.141 W/mK. Usually conducting polymer films or conducting polymer based composite films have anisotropic
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properties: the κ is much lower than the in-plan thermal conductivity (κ//), and presently, it is a great challenge to measure the κ// of films. According to ref. [14], the ratio of the κ// and κ , κ///κ ,‐is about 1.40 ± 0.22 for PEDOT:PSS. By using the ratio,
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the κ// of the composite films calculated is from 0.197 to 0.218 W/mK as the PF-Te
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content increases from 0 to 82 wt %. The reason for such low thermal conductivity of the composite films is explained as follows: (1) the PEDOT:PSS matrix dominates the thermal conductivity; (2) the diameter of the Te nanorods being of order of the intrinsic mean-free-path of heat carrying phonons and the acoustic mismatch of Te and the polymer structures results in an efficient scattering of phonons [28,46,51]. After consideration the anisotropic issue of the composite film, a largest ZT value, 0.076, is obtained for the sample containing 70 wt% PF-Te at room temperature. 13
ACCEPTED MANUSCRIPT Subsequently, a prototype power generator based on the as-prepared PEDOT:PSS/PF-Te composite material has been fabricated as shown in Fig. 7a. The device consists of eight single-leg of PEDOT:PSS/PF-Te (70 wt%) composite film
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(length 25 mm, width 5 mm) pasted on a polyimide substrate and Ag paste is used for electrode. Fig. 7b shows the output performance of this device due to the temperature difference between a forearm (305.4 ± 0.2 K) and the ambient (292 K). One side of
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the device is attached to a forearm skin and the other side is exposed to the air by
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using a bubble film as an insulator. The output voltage of the module is 2.5 mV at a temperature difference of approximately 13.4 K. The output voltage Vo can be defined as: Vo = NS∆T, where N is the number of TE elements, S is the Seebeck coefficient and ∆T is temperature difference. Base on the values of N = 8, S = 27.5 µV/K, ∆T =
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13.4 K, the calculated Vo is 2.95 mV, which is a little higher than the experimental value, probably due to the inevitable contact resistance in the device. The result demonstrates a wearable TE power generator based on PEDOT:PSS/PF-Te flexible
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films for harvesting energy from the waste heat generated from the human body.
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4. Conclusions In
conclusion,
we
have
shown
the
facile
fabrication
of
flexible
PEDOT:PSS/PF-Te composite films using a vacuum-assisted filtration process. The obtained films are flexible and are ideal for wearable electronic devices. PEDOT:PSS contribute the high electrical conductivity in the composite films due to removal of insulating PSS during the filtration process and Te nanorods contribute the enhanced Seebeck coefficient owing to the high Seebeck coefficient of the nanorods and an 14
ACCEPTED MANUSCRIPT energy-filtering effect at the interface between PEDOT:PSS and Te nanorods. An optimized power factor of 51.4 µW/mK2 has been obtained for the sample containing 70 wt% PF-Te. The flexible TE generator module consisting of 8 legs of the
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as-prepared composite film with Ag paste as electrode has produced an output voltage of 2.5 mV at a 13.4 K temperature difference. This study provides a new approach to facilitate practical application of flexible TE generator for low-grade energy
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harvesting.
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Acknowledgements
This work was supported by the National Basic Research Program of China (973 Program) under Grant No. 2013CB632500, Key Program of National Natural Science Foundation of China (5163210), National Natural Science Foundation of China
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(51271133), and the foundation of the State Key Lab of Advanced Technology for
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Material Synthesis and Processing (Wuhan University of Technology).
15
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ACCEPTED MANUSCRIPT Table caption Table 1 TE parameters of the PEDOT:PSS, Te nanorods and PF-Te films at room temperature.
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Figure captions Fig. 1 Schematic illustration of the fabrication of flexible PEDOT:PSS/PF-Te composite film on PVDF via a filtration process.
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films with 50 wt% PF-Te (C), 90 wt% PF-Te (D).
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Fig. 2 XRD patterns of Te nanorods (A), F-PEDOT:PSS (B), and PEDOT:PSS/PF-Te composite
Fig. 3 (a) SEM image and (b) HRTEM image of the PF-Te nanorods.
Fig. 4 Surface SEM images of the F-PEDOT:PSS film (a) and PEDOT:PSS/PF-Te composite films with 50 wt% (b), 70 wt% (c), and 90 wt% (d) PF-Te nanorods.
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Fig. 5 XPS spectra of the PEDOT:PSS film, F-PEDOT:PSS film and PEDOT:PSS/PF-Te composite film with 70 wt% PF-Te, (a) XPS survey spectrum; (b) sulphur S(2p) spectra. Fig. 6 (a) electrical conductivity, Seebeck coefficient, and power factor and (b) through-plane
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thermal conductivity of the as-prepared PEDOT:PSS/PF-Te composite films as a function of
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PF-Te content at room temperature.
Fig. 7 (a) photograph of a flexible device consisting of eight (PEDOT:PSS/PF-Te)-Ag thermocouples on a polyimide substrate and (b) output performance of the device due to the temperature difference between a forearm (~305.4 K) and the ambient (~292 K).
23
ACCEPTED MANUSCRIPT Table 1 TE parameters of the PEDOT:PSS, Te nanorods and PF-Te films at room temperature. Electrical conductivity (S/cm)
Seebeck coefficient (µV/K)
Power factor (µW/mK2)
PEDOT:PSS Te nanorods PF-Te
0.21 0.046 11.5
15.4 367 151
5 × 10-3 0.62 26.2
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Samples
24
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Fig. 1 Schematic illustration of the fabrication of flexible PEDOT:PSS/PF-Te composite film on
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PVDF via a filtration process.
25
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Fig. 2 XRD patterns of Te nanorods (A), F-PEDOT:PSS (B), and PEDOT:PSS/PF-Te composite
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films with 50 wt% PF-Te (C), 90 wt% PF-Te (D).
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Fig. 3 (a) SEM image and (b) HRTEM image of the PF-Te nanorods
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Fig. 4 Surface SEM images of the F-PEDOT:PSS film (a) and PEDOT:PSS/PF-Te composite films with 50 wt% (b), 70 wt% (c), and 90 wt% (d) PF-Te nanorods
28
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Fig. 5 XPS spectra of the PEDOT:PSS film, F-PEDOT:PSS film and PEDOT:PSS/PF-Te
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composite film with 70 wt% PF-Te, (a) XPS survey spectrum; (b) sulphur S(2p) spectra.
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Fig. 6 (a) electrical conductivity, Seebeck coefficient, and power factor and (b) through-plane
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thermal conductivity of the as-prepared PEDOT:PSS/PF-Te composite films as a function of
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PF-Te content at room temperature.
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Fig. 7 (a) photograph of a flexible device consisting of eight (PEDOT:PSS/PF-Te)-Ag thermocouples on a polyimide substrate and (b) output performance of the device due to the
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temperature difference between a forearm (~305.4 K) and the ambient (~292 K).
31
ACCEPTED MANUSCRIPT Highlights: 1. PEDOT:PSS/Te nanorods composite films were prepared. 2. A vacuum-assisted filtration method was employed to prepare the composite films.
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3. A flexible thermoelectric generator composed of the composite film was developed.
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4. The generator outputs a voltage of 2.5 mV at a 13.4 K temperature difference.