Free-standing highly conducting PEDOT films for flexible thermoelectric generator

Energy 170 (2019) 53e61

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Free-standing highly conducting PEDOT films for flexible thermoelectric generator Dan Ni, Haijun Song, Yuanxun Chen, Kefeng Cai* Key Laboratory of Advanced Civil Engineering Materials (Tongji University), Ministry of Education, School of Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 August 2018 Received in revised form 26 November 2018 Accepted 18 December 2018

Recently, organic thermoelectric (TE) materials especially conducting polymers have attracted increasing attention. In this work, we successfully synthesized ultrafine poly (3,4-ethylenedioxythiophene) (PEDOT) nanowires (NWs) (~10 nm) by a simple self-assembled micellar soft-template method and then obtain highly flexible free-standing PEDOT NW films by vacuum-assisted filtration. The films are with very high electrical conductivity (~1340 S cm
1). After being treated with 6 M H2SO4 and then with 1 M NaOH at room temperature, the film shows an enhanced power factor of 46.51 mW m
1K
2 (Seebeck coefficient of 25.5 mV K
1, electrical conductivity of 715.3 S cm
1), which increases by 54% compared with that of the pristine film. To the best of our knowledge, it outperforms the TE performance of all reported one dimensional conducting polymer-based films. In addition, the TE performance of the film almost remains unchanged even after being bent for 200 times, indicating excellent flexibility. A flexible TE prototype composed of six strips (7 mm  30 mm) of the as-prepared PEDOT NW films connected in series shows an output power of 157.2 nW at a temperature difference of 51.6 K. The free-standing PEDOT NW films show promise to a new generation of wearable TE devices. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Flexible PEDOT Free-standing Highly conductive Thermoelectric

1. Introduction Thermoelectric (TE) generator can convert waste heat to electrical energy without any noise, vibration or gas emission [1,2]. TE material is the most important component for TE generator. A high performance TE material possesses high electrical conductivity and Seebeck coefficient while poor thermal conductivity, since the TE performance is evaluated by a dimensionless figure-of-merit (ZT), ZT ¼ S2sT/k, where S is the Seebeck coefficient, s is the electrical conductivity, k is the thermal conductivity, and T is the absolutely temperature. The ZT value is very difficult to increase due to the three parameters being strongly interdependent (increasing s is usually accompanied by an increased k and decreased S). It has been reported that the ZT value can be enhanced through various strategies, such as nano engineering [3,4], band engineering [5,6], multiple localization transport behavior [7], broadfrequency phonon scattering [8] and adding magnetic or supermagnetic nanoparticles [9,10]. The ZT values above 2 have been reported for several inorganic TE material systems. However, compared with organic materials, inorganic TE materials have

* Corresponding author. E-mail address: [email protected] (K. Cai). https://doi.org/10.1016/j.energy.2018.12.124 0360-5442/© 2018 Elsevier Ltd. All rights reserved.

significant drawbacks that include the high cost of raw materials, toxicity, shortage of natural resources, and potential for heavy metal pollution [11]. Moreover, they usually require a complicated preparation process, which increases cost. They are also difficult to be incorporated into TE devices and difficult to recycle. In addition, they are usually brittle, which is not suitable for present explosive growth of flexible and wearable devices [12]. Therefore, looking for low-cost, eco-friendly, flexible and high performance TE materials is very urgent. Conjugated polymers (CPs), including poly (3,4ethylenedioxythiophene) (PEDOT) [13,14], polyaniline (PANI) [15,16], and polypyrrole (PPy) [17e20], possess many advantages such as light weight, nontoxic, abundant, easy to process, easy to be incorporated into devices, environmentally-benign, low intrinsic thermal conductivity (0.1e0.5 W m
1 K
1) [21], regulable electrical conductivity, etc. [22e24]. Therefore, the TE properties of CPs and CP-based materials recently have attracted increasing attention [21,25e27]. PEDOT is a very promising CP. However, because of insolubility and infusibility, until now, the majority of studies on TE properties are based on poly (styrenesulfonate) (PSS) doped PEDOT (PEDOT:PSS) due to its water-solubility. Various methods have been employed to improve the TE properties of PEDOT:PSS films and

54

D. Ni et al. / Energy 170 (2019) 53e61

much progress has been made [14,28e32]. However, the commercial PEDOT:PSS product (PH1000, H. C. Starck) is very expensive (about 9000 RMB per liter and only containing ~1.3 wt% PEDOT:PSS), so it is very desirable to find an alternative method, which can fully take the advantage of PEDOT and at the same time reduce the cost. In fact, a lot of efforts have been put to the synthesis and TE properties of PEDOT. Hu et al. [33] prepared PEDOT with different morphologies by a chemical oxidation polymerization in reverse microemulsions and investigated their TE performance. They found that the electrical conductivity, Seebeck coefficient and also power factor (PF ¼ S2s) of the PEDOT follow the same sequence of bulk < globular nanoparticle < nanorod or ellipsoidal nanoparticle < nanotube < nanofiber (The PEDOT nanofibers are with a PF of 16.4 mWm
1K
2). Hence, one-dimensional (1D) PEDOT nanostructures are the most desired. Until now, besides the chemical oxidation polymerization in reverse microemulsions, several other methods have been reported for synthesis of 1D PEDOT nanostructures. One is interfacial polymerization without using any template [14,34] and the 1D PEDOT synthesized is with very low electrical conductivity. One is electrodeposition within an anodic aluminium oxide template [35,36] or a porous polycarbonate membrane [37] or a template prepared by a lithographic technique [38]. One is vapor phase polymerization within nanoscale channels of a mold [39]. Although the above latter two methods can produce well-defined 1D PEDOT, for example, the PEDOT NWs show an extremely high electrical conductivity (7619 ± 771.6 S cm
1) [39], it is difficult to fabricate cost-effective 1D PEDOT in large volume. Another one is self-assembled micellar soft-template method [40,41]. Zhang et al. [42] recently used this method to synthesize PEDOT NWs with diameter of 6e20 nm and the PEDOT NW film shows a power factor (PF]S2s) of 10.35 mW m
1K
2 (S of 15 mV K
1 and s of 460 S cm
1). And they further treated the film with hydrazine and obtained improved properties (S~50.55 mV K
1, s~140 S cm
1 and PF~35.8 mW m
1K
2) [41]. Thus, this soft-template method followed with post-treatment is very promising. In this work, we successfully prepared a highly flexible freestanding PEDOT NW film with very high electrical conductivity (1340 S cm
1) by a modified self-assembled micellar soft-template method and then vacuum-assisted filtration. In order to further optimize the TE properties, the film was treated with acid (H2SO4) and/or base (NaOH), which is first time reported. The optimal PEDOT NW film shows a PF of 46.51 mW m
1K
2 (S of 25.5 mV K
1 and s of 715.3 S cm
1) at room temperature. It is the state-of-theart performance for CP NW film. Finally, a flexible TE power generator consisting of six optimal PEDOT NW film-Ag thermocouples on a polymide was assembled to demonstrate TE power generation. 2. Experimental 2.1. Raw materials The anionic surfactant sodium dodecyl sulfate (SDS) and the oxidant iron (III) chloride anhydrous (FeCl3) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. 3,4ethylenedioxythiophene (EDOT, 99.9%) monomer was purchased from Bohong Electronic Chemicals Co., Ltd., China. All the reagents were used directly without further purification. 2.2. Preparation of PEDOT NWs PEDOT NWs were prepared by a modified self-assembled micellar soft-template approach. The typical synthesis procedure

of PEDOT NWs is described as follows. 30 mmol SDS was dissolved in 100 ml deionized (DI) water, then 15 mmol FeCl3 was added into the transparent solution and stirred for 1e2 h in an oil bath at 50  C. When the color of the solution turned into deep yellow, 7 mmol EDOT monomer was slowly injected into the solution and the polymerization was carried out at 50  C. The solution was slowly cooled down in the oil bath for 10 h. Finally, the product was washed with DI water and methanol for several times. 2.3. Preparation of flexible and free-standing PEDOT NW film The as-prepared product was ultrasonically dispersed in methanol. A film (with thickness about 5 mm) was prepared by vacuumassisted filtration of a certain amount of the dispersion onto a porous PVDF membrane. Finally, the as-prepared film was dried at 60  C in vacuum for 12 h. 2.4. Post-treatment of the flexible and free-standing PEDOT NW film First, a free-standing PEDOT NW film was cut into small strips (1.5 cm  1 cm). Second, the strips were treated with different concentration H2SO4 to study the effect of H2SO4 concentration on TE performance of the PEDOT NW film at 25  C. Third, the strip treated with optimal concentration H2SO4 (6 M H2SO4) was further treated with different concentration NaOH to study the effect of NaOH concentration on TE performance of the PEDOT NW film at 25  C. The most optimal NaOH concentration was 1 M. In addition, the 6 M H2SO4 treated strips were further treated with 1 M NaOH at different temperatures. Finally, the best TE performance was obtained from the sample treated with 6 M H2SO4 and then treated with 1 M NaOH at 25  C. Note that the treating time was 30 min both for acid or base treatment. 2.5. Preparation of flexible TE generator A most properly post-treated PEDOT films were cut into strips (30 mm  7 mm), and then these strips were pasted on a polyimide substrate. After that, each strip was connected in series by Ag paste as conductive connection to obtain a prototype power generator. 3. Measurement and characterization Electrical conductivity was measured using a steady-state fourprobe 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 thermal electromotive force and temperature difference between two ends on one side of each film. The temperature dependence of the Seebeck coefficient and electrical conductivity was measured by MRS-3L thin-film TE test instrument system (Wuhan Giant Instrument Technology Co., Ltd, Wuhan, China). A homemade output power collector was used to examine the performance of the device. The temperature difference was established by a heating module. A voltmeter (Agilent 34970) was used to collect the output voltage of the generator, and a microammeter was used to collect the output current of the circuit. Through adjusting the value of load resistance, different output powers were obtained. The details for the measurement is shown in Figure S1. The crystallinity of the PEDOT NW film was examined by X-ray diffraction (XRD, Bruker D8 Advance) using Cu Ka (l ¼ 1.5406 Å) radiation. The doping level and oxidation level of the PEDOT NW film before and after post-treatments were analyzed by X-ray photoelectron spectroscopy (XPS) with Al Ka radiation (1486.6 eV) using an ESCALAB 250Xi system X-ray photoelectron spectrometer,

D. Ni et al. / Energy 170 (2019) 53e61

Raman spectroscopy with a Jobin Yvon HR800 Raman spectrometer using a laser diode at an excitation wavelength of 514.5 nm, Fourier transform infrared (FT-IR) spectroscopy on an EQUINOX 55 FTIR spectrometer, and UVeviseNIR absorption spectroscopy with a Shimadzu UV-2501PC UVeviseNIR spectrometer. The surface morphology of the films was observed by field-emission scanning electron microscopy (FESEM) (Nova NanoSEM 450) and that of the PEDOT NWs were observed by transmission electron microscopy (TEM) (JEM-2100F). The sample for TEM observation was prepared by dispersing a small amount of the PEDOT NWs in ethanol and ultrasonicated for 10 min before being dropped on a copper grid. 4. Results and discussion The as-prepared PEDOT NW film can be easily peeled off from the PVDF membrane (See Figure S2 in supplementary information). The film can be bent at any angle from 0 to 360 , indicating good flexibility (Fig. 1a), which is significant for flexible and wearable devices [23]. Fig. 1b and c shows the TEM images of the PEDOT NWs. The diameter of a single PEDOT nanowire is ~10 nm and the NWs tend to entangle with each other into bundles because of the strong p-p stacking interactions between PEDOT chains and the high surface energy of ultrafine PEDOT NWs [41]. The electrical conductivity and Seebeck coefficient of the asprepared films are 1340 S cm
1 and ~15 mV K
1 (see Fig. 2a). The electrical conductivity is about 2.5 times as high as that of the PEDOT NW film reported in Ref. [42] (see Table 1), and the PF is ~30.15 mW m
1K
2, which increases by 30% compared with that of the PEDOT NW film before treatment [41], indicating our modification of the self-assembled micellar soft-template method is very successful. This should be because our PEDOT NWs were undergone a longer time growth. Proper post-treatment has recently shown as an efficient means to improve the TE properties of PEDOT-based films, such as PEDOT:tosylate [13,46] and PEDOT:PSS [30,47e49]. For example, Fan et al. [50] significantly improved the TE performance of PEDOT:PSS through the post-treatment with H2SO4 and NaOH, and the PF was 334 mW m
1K
2. Inspired by the above works, we also

Fig. 1. (a) Digital photos of the film bent with different angles. TEM images of the PEDOT NWs at (b) low and (c) high magnifications.

55

treated the PEDOT NW films with H2SO4 and/or NaOH. Fig. 2a shows the effect of different-concentration H2SO4 treatment on the electrical conductivity of the PEDOT NW film. It is seen from Fig. 2a that the electrical conductivity of the PEDOT NW film increases with the increase of the H2SO4 concentration first and reaches a peak value of 1721 S cm
1 (when the H2SO4 concentration is 6 M), and then decreases with further increase of the H2SO4 concentration. A similar trend was also reported for PEDOT:PSS film [51]. However, the Seebeck coefficient of the film slightly decreases when the H2SO4 concentration increases from 1 M to 10 M (see Figure S3). A similar result was also obtained for the PEDOT:tosylate film treated with H2SO4 [46]. This slightly decrease in Seebeck coefficient is probably due to increase in carrier concentration (see Table S1). Obviously, the 6 M H2SO4 treated film (called as sample 6A hereinafter) shows the highest PF. In order to further improve the TE properties of the PEDOT NW films, the sample 6A was further treated with differentconcentration NaOH for 30 min at 25  C. The result is depicted in Fig. 2b. The electrical conductivity rapidly decreases with the NaOH concentration increase from 0 to 0.5 M and then decreases slowly with the increase of the NaOH concentration, while the Seebeck coefficient increases rapidly at the beginning and then increases slowly with the NaOH concentration. This is because a higher electrical conductivity usually accompanies a lower Seebeck coefficient [52]. It is seen from Fig. 2b that when the film is treated with 1 M NaOH, it shows a highest PF, ~46.51 mW m
1K
2 (with the s of 715.3 S cm
1 and the S of 25.5 mV K
1), which increases by 54% compared with the pristine film and by 30% compared with the film treated with hydrazine (PF~35.8 mW m
1K
2) [41]. For simplicity, the film treated with 6 M H2SO4 and then treated with 1 M NaOH is called as sample 6A1B, and the film only treated with 1 M NaOH is called as sample 1B hereinafter. Fig. 2c shows the TE properties of the sample 6A as a function of treating temperature treated with 1 M NaOH. The electrical conductivity decreases with the treating temperature, while the Seebeck coefficient slightly increases with the treating temperature, and obviously the PF of the film decreases with the treating temperature. In addition, the effect of bending (the bending angle from 0 to 360 , see Fig. 1) times on the TE performance is shown in Fig. 2d. The almost changeless TE performance indicates excellent flexibility of the films. It is widely accepted that the morphology tuning and evolution can significantly influence the TE properties of CPs [33,53]. In order to know the relation between the morphology evolution and the TE performance, the films before and after treatment were observed by FESEM (see supplementary information Figure S4a-c). The surface of the pristine film is rough with some microcracks (Figure S4a), while the surface of the sample 6A is smooth and compact, in which wire-like structures can be clearly observed under FESEM (Figure S4b). For sample 6A1B, the surface becomes rougher and some entangled PEDOT NWs protruding from the surface with some microcracks (Figure S4c). The surface morphology variation before and after the treatment agrees well with the change of electrical conductivity of the films (see Figure S4a-c). In order to shed light on the ordered structure of a macromolecular arrangement, the films were characterized by XRD, UVeViseNIR, and FTIR. Fig. 3a illustrates the XRD patterns of the pristine and post-treated PEDOT films. The pristine film exhibits two distinct peaks: one very strong at 2q ¼ 6.4 and one weaker at 2q ¼ 12.8 . The sharp and strong peak at 2q ¼ 6.4 , indicating good crystallinity of the NWs, corresponds to the interchain ring-packing along an orthorhombic a-axis [54]. The weaker peak at 2q ¼ 12.8 is the (200) plane reflections of the (100) plane (2q ¼ 6.4 ). For sample 6A, the intensity of the peaks become stronger. However, the peaks become weak after being treated with NaOH (for both the

56

D. Ni et al. / Energy 170 (2019) 53e61

Fig. 2. TE performance of the PEDOT NW films as a function of (a) H2SO4 concentration for treatment, (b) NaOH concentration for treatment after being treated with 6 M H2SO4, (c) treating temperature with 1 M NaOH after being treated with 6 M H2SO4, (d) TE performance of the sample 6A1B as a function of bending times.

Table 1 Comparison of the characteristics of the PEDOT NWs prepared by different methods. Methods

S ( mV K
1)

s(S cm
1)

electrochemical polymerization method with porous AAO membranes Lithographically pattered nanowire electrodeposition method on Si3N4-coated silica substrates electrochemical polymerization method with porous AAO membranes

/
122

1.34  10
3 / 7e40 ~9.2

/

100

/

liquid-bridge-mediated nanotransfer printing with vapor-phase polymerization

/

7619

/

~200 30 e170 50 e250 95

vapor-phase polymerization on hard carbon fiber paper

/

130

/

250

chemical oxidation polymerization in reverse micro mulsions self-assembled micellar soft-template approach self-assembled micellar soft-template approach

48.0 ~20.78 15

71.4 ~541 1340

~16.4 ~23.36 ~30.15

200 12 10

sample 1B and 6A1B). This means that the degree of the ordered alignment of the film increases after being treated with H2SO4 and decreases after being treated with NaOH, respectively. The higher degree of ordered alignment, the higher the carrier mobility, and hence the higher electrical conductivity of the films. Thus, the sequence of the electrical conductivity of the films is:6A > pristine PEDOT NW film> 6A1B. UVeViseNIR spectra have been used to analyze the concentration change of PEDOT (neutral), PEDOT2þ (bipolaron) and PEDOTþ (polaron) in the PEDOT films [54]. It is well-known that PEDOT has three main absorbance bands: the neutral chain shows the absorption around 600 nm, the chain in polaron state shows the absorption around 900 nm, and the chain in bipolaron state shows the broad absorption in the IR region (1250 nm) [46,55,56]. Fig. 3b shows that the pristine PEDOT NW film possesses only one absorption band covering the beginning of the IR region, indicating that the charge carriers are only bipolarons. For the sample 6A, the absorption in the IR region is enhanced, suggesting the increased concentration of bipolarons, which accounts for the enhancement

PF (mWm
1K
2)

D (nm) Film forming Cost/yield  √ (substrate) 

Ref.

high low high low

[43] [38]

high low

[44]

high medium √(substrate) medium low  low ✓ low large ✓ low large

[39]



[45] [33] [41] this work

of the electrical conductivity. Nevertheless, after being treated with 1 M NaOH (for the sample 6A1B), the bands at around 600 nm and 900 nm are all strengthened with drastically decreased in the broad IR absorption. The change in three bands revels that the NaOH treatment can lead to carrier type changing from bipolaron to polaron and then to neutral state [13], which results in a lower electrical conductivity. It is obviously seen from Fig. 3b that the bipolaron concentration of the sample 1B is lower than that of the sample 6A and 6A1B, indicating that the TE performance can be tuned by the acid and base co-treatment. Fig. 3c shows the FTIR spectra of the pristine PEDOT NW film, 6A, 1B and 6A1B samples, respectively. The characteristic peaks of the pristine PEDOT film at 1533, 1507 and 1370 cm
1 are attributed to the stretching modes of C]C and CeC in the thiophene ring; the peaks at 1160, 1114 and 1065 cm
1 are due to CeOeC bond stretching in the ethylenedioxy ring deformation mode; and the peaks at 993, 921, 860 and 709 cm
1 are attributed to the deformation modes of CeSeC in the thiophene ring, respectively [57e59]. Fig. 3d shows the enlarged spectra ranging from 1600 to

D. Ni et al. / Energy 170 (2019) 53e61

57

Fig. 3. XRD patterns (a), UVeViseNIR spectra (b), and (c) and (d) FTIR spectra of the pristine PEDOT NW film, 6A, 1B, and 6A1B samples.

1200 cm
1. The peak at 1533 cm
1 marked with dot line has an obvious shift. The peaks at 1533 cm
1 shifts to 1538 cm
1 after being treated with 6 M H2SO4; conversely, after the film being treated with NaOH, the peak shifts to a lower wavenumber (1527 cm
1 for the sample 1B and 1526 cm
1 for the sample 6A1B). The shift of the peak at 1533 cm
1 is strongly dependent on the doping level of the conducting polymer, in which the P-doped state is known to be highly stabilized by electron-donating ethylenedioxy group [60]. After being treated with 6 M H2SO4, the peak intensity of the film increases and a redshift occurs, which is ascribed to the increasing doping level [61]. Thus, the electrical conductivity of the film is enhanced. Raman spectroscopy is an effective characterization method to study the doping behavior of CP. Raman spectra show that after being treated with H2SO4, its Raman band between 1400 and

1500 cm
1 originating from Ca ¼ Cb stretching vibration of PEDOT [62] shifts to a higher wavenumber, and that the shift for the sample 6A is greater than that for the 3 M H2SO4 treated film (Figure S5), suggesting the increasing doping level of the PEDOT NW film after being treated with 6 M H2SO4, and hence the higher electrical conductivity of the film. Fig. 4a illustrates the Raman spectra of the pristine PEDOT NW film, 6A, and 6A1B samples. It is seen from Fig. 4a that the Raman band between 1400 and 1500 cm
1 increases in the spectra for the samples 6A and 6A1B. And it is clearly seen from Fig. 4b that the band shifts back from 1425 to 1423 cm
1 for the 6A1B sample, revealing that the base treatment is a dedoping process. This is the reason for the decreased electrical conductivity for the sample 6A1B. As described above, we have qualitatively analyzed the reasons for the TE performance improvement after acid and base treatment

Fig. 4. (a) Raman spectra of the pristine film, 6A and 6A1B samples. (b) The zoom-in spectra of (a) ranging from 1470 to 1380 cm
1.

58

D. Ni et al. / Energy 170 (2019) 53e61

one after another. Due to the importance of doping and oxidation to the electrical conductivity and Seebeck coefficient of CPs, the doping and oxidation levels are quantitatively measured by XPS in order to gain a deep insight into the mechanism for the TE performance enhancement of the PEDOT films. XPS survey spectra of the samples are shown in Fig. 5a. Fig. 5bed depict the XPS S2p core-level spectra of the pristine PEDOT NW film, the 6A and 6A1B samples, respectively. As shown in Fig. 5bed, the S2p signals include three components, and each one can be fitted with a spin-split doublet, S2p1/2 and S2p3/2, with the energy splitting of 1.2 eV, an area ratio of 1:2 and the same full width half maximum [41]. For the sulfur atom from PEDOT chains, two doublets respectively at 163.6 eV (S1_2p3/2) and 164.8 eV (S1_2p1/2), and 164.6 eV (S2_2p3/2) and 165.8 eV (S2_2p1/2) are corresponding to the neutral (S0) and partially oxidized state (Sdþ) in the polymer chains [63]. The last component located at 167.8 eV (S3_2p3/2) and 169.0 eV (S3_2p1/2) are ascribed to the sulfur atom of the dodecyl sulfate anions (DS
) or DS
and SO2
[41,54], indicating the 4 incorporation of DS
anions in the films and the formation of highly doped state after 6 M H2SO4 treated. The doping and oxidation levels of the samples can be estimated from their corresponding S2p signals. The doping level is calculated using the equation: doping level ¼ AS DS
=ðAS0 þ ASdþ Þ for the pristine PEDOT NW film and doping level ¼ ðAS DS
þ AS SO4 2
Þ=ðAS0 þ ASdþ Þ for the treated samples, and the oxidation level is calculated from the equation: oxidation level ¼ ASdþ =ðAS0 þ ASdþ Þ, where A0S , AdSþ, A
S_DS, and AS
SO2
4

stand for the peak area of the neutral PEDOT, oxidized PEDOT, sulfur atom from DS
and SO2
4 , respectively. The doping levels are calculated to be 39.0%, 42.1%, and 25.7% for the pristine PEDOT, 6A and 6A1B samples, respectively. Meanwhile, the oxidation levels of these samples are calculated to be 49.2%, 47.4% and 49%, respectively. Note that the doping level (39.0%) of the pristine PEDOT is

higher than that (32%) of the PEDOT film reported in Ref. [41], indicating that more DS
anions are doped into the present pristine PEDOT NW film, which is the reason for the higher conductivity of our PEDOT NW film. In addition, it should be noted that the oxidation level of all the samples are very close. Similar phenomenon was also reported by Johansson et al. [64], in which they proposed that once the dedoped PEDOT is exposed to an atmospheric environment, it immediately transformed into doped state. Due to the very similar oxidation level, the difference in electrical conductivity of the PEDOT films is ascribed to the different doping levels. As discussed above, the electrical conductivity is dependent on the doping level, thus, the higher doping level, the higher electrical conductivity. For the sample 6A, a higher electrical conductivity is obtained due to SO2
4 ions are doped into the polymer chains, which agrees with the results of FTIR spectra, Raman spectra and UVeViseNIR spectra. XPS O1s core-level further testifies that SO2
4 are doped into the 6A sample (Figure S6, the doping levels calculated from the ratio of AO1_1s/AO2_1s are 14.5%, 18.2% and 10.6% for the pristine PEDOT, 6A and 6A1B samples, respectively). For the NaOH treated PEDOT (Figure S6c), the doping level (10.6%) is lowered, indicating that the counter-anion exchange between the DS
and OH
[44,65], which results in a lower electrical conductivity. The XPS S2p core-level spectra (see Figure S7) of the 6A1B sample at different temperatures also show that the film with a high electrical conductivity is often with a high doping level. On the basis of the above discussion, the enhancement mechanism of the present film after being treated with H2SO4 is different from that of the H2SO4 treated PEDOT:PSS (one of the main reasons for the enhancement of electrical conductivity is due to the removal of PSS [66]).

Fig. 5. (a) XPS survey spectra of the pristine PEDOT film, 6A and 6A1B samples. XPS S2p core-level spectra of (b) pristine PEDOT film, (c) 6A and (d) 6A1B samples.

D. Ni et al. / Energy 170 (2019) 53e61

59

Fig. 6. (a) A schematic plot of the prototype power generator based on the 6A1B film. (b) TE voltage generated by the prepared generator versus DT, (c) temperature dependency of Seebeck coefficient of the 6A1B sample, (d) the output voltage and output power curves according to the different load resistance at various DT.

4.1. Flexible thermoelectric generator A prototype power generator based on the 6A1B sample has been fabricated shown as in Fig. 6a. Notably, the generator is very flexible (See supporting information-video 1), which means that it is possible to implement such a generator in wearable electronic devices. Fig. 6b shows the TE voltage (V) generated by the prepared power generator versus DT (temperature difference). Obviously, V is proportional to the DT, which is due to the Seebeck effect [67]. When 51.6 K temperature difference is applied, the V reaches 7.1 mV, and the calculated output voltage per 1 K is about 23 mV according to the equation: V ¼ DT  SN, where S is the Seebeck coefficient and N is the number of the PEDOT strips, which is a somewhat lower than the value, ~25.45 mV, measured at room temperature. This may be because of the heat loss caused by the Ohmic contact between the wires and the PEDOT strips, as well as between the Ag paste and the PEDOT strips. In theory, the slopes of the lines in Fig. 6b should be the same, however, in fact they slightly decrease with decreasing DT. This is because the Seebeck coefficient of the film gradually increases with temperature (Fig. 6c). Fig. 6d shows that the output voltage and power of the TE generator as a function of load resistance at various temperature difference. The temperature difference was established by a heating module, and the output voltage and power of the device were measured with a voltmeter by controlling the temperature gradient. As the DT increases, the output voltage proportionally increases, which leads to a higher output power. A maximum output power is obtained when the load resistance is about 60 U. The maximum power is 157.2 nW when DT ¼ 51.6 K, and by dividing the cross-sectional area and the number of legs, the normalized maximum power density is 75 mW cm
2, which is much higher than the values reported in Refs. [68,69]. Although this is just a demonstrative work, the flexible, eco-friendly and lowcost generator is very promising for practical applications. In

addition, the output power can be further increased through modifying the generator structure, increasing the number of TE legs, combining high TE performance N-type flexible materials to assemble PeN type TE generators. 5. Conclusions In summary, we successfully synthesized ultrafine PEDOT NWs (~10 nm) by a modified self-assembled micellar soft-template approach and obtained flexible free-standing PEDOT NW film with high electrical conductivity (~1340 S cm
1) by vacuum-assisted filtration. The doping level of the flexible free-standing PEDOT NW film was improved by treated with 6 M H2SO4, contributing to the highest electrical conductivity (~1721 S cm
1). After further optimizing the doping level by common acid and base, the best performance of the film exhibits a S value of 25.5 mV K
1, a s value of 715.3 S cm
1, and a corresponding PF of 46.51 mW m
1K
2. The TE performance of the film almost does not change with bending times, indicating excellent flexibility. A prototype power generator consisting of 6 legs of the optimum film has been fabricated. An output voltage of 7.1 mV and a power of 157.2 nW are generated at a temperature difference of 51.6 K. This work shows the highly conductive, flexible, low-cost and free-standing PEDOT NW films are very promising for application in wearable TE generators. Acknowledgements This work was supported by the Key Program of National Natural Science Foundation of China (5163210), National Basic Research Program of China (973 Program) under Grant No.2013CB632500, and the Foundation of the State Key Lab of Advanced Technology for Material Synthesis and Processing (Wuhan University of Technology). Finally, the authors thank Dr. Yong Du for the measurement of temperature dependency of Seebeck coefficient of the samples.

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