Elsevier

Synthetic Metals

Volume 224, February 2017, Pages 27-32
Synthetic Metals

Thermoelectric properties of the PEDOT/SWCNT composite films prepared by a vapor phase polymerization

https://doi.org/10.1016/j.synthmet.2016.11.031Get rights and content

Highlights

  • PEDOT/SWCNT composite films were prepared via a vapor phase polymerization method.

  • The thermoelectric properties of the composite films were measured.

  • The thermoelectric properties of the composite films are enhanced compared with pristine PEDOT.

  • The enhancement mechanism of the PEDOT/SWCNT films was proposed.

Abstract

The poly(3,4-ethylenedioxythiophene)/single walled carbon nanotube (PEDOT/SWCNT) composite films with various SWCNT contents (0–50 wt.%) have been prepared by a vapor phase polymerization technique for the first time. The Seebeck coefficients of the composite films increase with the SWCNT contents. The maximum power factor is 37.8 μW mK−2 for a composite film with 35 wt% SWCNT, which is 1.7 times as high as that of the film without SWCNTs. The composite film with 35 wt% SWCNT has then been treated with a 0.04 wt% NaBH4/DMSO solution and the power factor increases to 44.1 μW mK−2 after the treatment. The PEDOT chains are more orderly arranged and with decreased doping level by forming composites with SWCNT and may be de-doped further after further treated with NaBH4/DMSO. Therefore, the Seebeck coefficient of the composite film has been further enhanced.

Introduction

Thermoelectric (TE) materials have attracted more and more attentions recently due to their unique ability to directly convert heat to electricity and vice versa. The performance of TE materials is evaluated by a figure of merit, ZT = αT/κ, where α is Seebeck coefficient; σ is electrical conductivity; and κ is thermal conductivity, which is the sum of both electronic (κe) and lattice (κl) component. A high ZT value requires a high α, a high σ and a low κ. However, it is very difficult to modulate the three parameters to get a high ZT value because each of the parameters is correlated to the carrier concentration of a material: as the carrier concentration increases, both κe and σ increase, while α decreases.

Inorganic TE materials are metal alloys, including Bi2Te3 based alloys and PbTe based alloys. Even though they have shown relatively high ZT values, they exhibit some significant shortcomings: There are limited resources and the main elements are heavy or rare metals, which are hazardous and very expensive; they usually require a complicated preparation process; and they are also very difficult to incorporate into TE devices and difficult to be recycled. In contrast, conducting polymers, such as polyaniline (PANI), poly(3,4-ethylenedioxythiophene) (PEDOT), and polypyrrole (PPy), possess many advantages, such as lightweight and nontoxic, intrinsic low thermal conductivity, abundance resource, flexible, and, especially, easy mass-production for large-area applications. Hence, conducting polymers have attracted increasing attention in the last years although their ZT values are still relatively low compared with traditional inorganic TE materials. Moreover, TE performance of conducting polymers can be improved by various methods [1], [2], [3], [4], [5].

Generally, two strategies have been used to improve the TE properties of conjugated polymers. One is to tune the doping levels of polymers. For example, Bubnova et al. [6] reported a tosylate (Tos) optimum doped PEDOT film with a ZT value of 0.25; Kim et al. [7] reported a PEDOT/polystyrenesulfonate (PSS) film with a ZT = 0.42 via adjusting the doping level by treating the film with ethylene glycol and dimethyl sulfoxide (DMSO). Although Weathers et al. [8] recently revealed that the ZT value in ref. [7] did somewhat overstate due to the samples measured for electrical properties being different from that for thermal conductivity, many research groups have confirmed that a post treatment can improve the TE properties of PEDOT based films[8], [9]. Another is to form a composite with inorganic TE nanostructures, as the composite may inherit the merit of both polymer and inorganic TE nanostructures, even having a synergetic effect [10]. Carbon nanomaterials including carbon nanotubes (CNTs) and graphene (GN) have been found to enhance the TE properties of conjugated polymers as effective fillers [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Our group first reported the phenomenon of that electrical conductivity and Seebeck coefficient simultaneously increased with GN contents in PANI/GN composites [13]. We also found such a phenomenon in PEDOT:PSS/reduced graphene oxide (rGO) composites [14], PPy/CNT composites [15], [16], PEDOT/CNT composites [23], and even PEDOT:PSS/Bi2Te3 composites [24]. The carrier mobility in the composites mentioned above has all been significantly increased. Many other works have also reported on enhanced TE properties by forming composites with carbon nanomaterials. For example, Yu et al. [20] reported that PEDOT:PSS/CNT composites, prepared by a physical mixing, has an enhanced TE performance, and the power factor reached 160 μW/mK2. Yao et al. [17] reported PANI/SWCNT hybrid films with a maximum TE power factor of 176 μW m−1K−2 at room temperature (RT). Most recently, Wang et al. [25] reported a record ZT value of 0.5 for PEDOT/CNT composite films treated by tetrakis(dimethylamino)ethylene. These studies suggest that CNTs are a promising candidate for further development of lightweight and low-cost polymeric composites for TE applications.

PEDOT is one of the most extensively explored conducting polymers. Usually PEDOT films are prepared by either a chemical oxidative polymerization or an electrochemical polymerization from its monomer in a liquid phase [26], [27]. Recently, Fabretto et al. [28] prepared a PEDOT film with an electrical conductivity of ∼3400 Scm−1 via a vacuum vapor-phase polymerization (VPP) method. Compared with the other methods, VPP is simpler, faster and more convenient to manufacture highly conductive polymer films. Therefore, it is of interest to investigate the TE properties of the PEDOT film prepared by the VPP method. Our group [29], [30] recently has successfully prepared PEDOT films by the VPP method in air and then treated the films with H2SO4 and a mixture of NaBH4 and DMSO, respectively. The TE properties of the films have been significantly improved by the treatments. The enhancement mechanism for the two treatments is different: the former (treated with H2SO4) is doping and the latter is dedoping. A peak power factor of 98.1 μW/mK has been achieved at RT for a sample treated with a 0.04 wt% NaBH4/DMSO solution. Xu et al. [31] reported the TE properties of PEDOT/rGO composite films prepared by the VPP method. The composite films display an enhanced TE performance compared to that of neat PEDOT. However, the TE properties of PEDOT/CNT composite films prepared by the VPP method have not been reported so far. Moreover, the TE properties of PEDOT/CNT composite films treated with NaBH4/DMSO solution have rarely been reported in the literature, either.

In this work, PEDOT/SWCNT composite films are fabricated by the VPP method. The PEDOT/SWCNT composite films have then been treated with a 0.04% NaBH4/DMSO solution and enhanced TE properties of the composite films are obtained. To understand the enhancement mechanism, the films have been characterized by TEM, X-ray photoelectron spectroscopy (XPS), UV–vis-NIR absorption spectra and Raman spectroscopy, respectively.

Section snippets

Experimental

Iron p-toluenesulfonate heptanedionate (Fe(Tos)3·6H2O; 99%) and polyethylene glycol–polypropylene glycol–polyethylene glycol (PPP, Mw = 5800 Da) were obtained from Sigma-Aldrich. 3,4-ethylenedioxythiophene (EDOT; 99.9%) monomer was purchased from Bohong Electronic Chemicals Co., Ltd, Yancheng, China. SWCNT (diameter of 1–2 nm) dispersant was purchased from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences (Chengdu, China). Sodium borohydride (NaBH4; 96%), pyridine (Py) and DMSO (99%)

Characterization

Seebeck coefficient was obtained from the slope of the linear relationship between thermal electromotive force and temperature difference (∼10 K) of two points on each film. Electrical conductivity was measured using a steady-state four-probe technique with a square wave current (∼10 mA in amplitude). Thermal conductivity was measured by TC3010 using a transient hot wire method (Xia Xi Electronic Technology Co., Ltd, Xi’an, China). Samples for thermal conductivity measurements were prepared by

Results and discussion

Fig. 1 shows the electrical conductivity, Seebeck coefficient and power factor of the PEDOT-Tos-PPP/SWCNT composite films as a function of SWCNT content at RT. The electrical conductivity increases with the SWCNT contents until the SWCNT content is ∼5 wt% and then decreases with the SWCNT contents. The reason for the increase of electrical conductivity of the composite film with a low SWCNT content (≤ 5 wt.%) may be as follows. The SWCNTs added acted as templates resulting in highly ordered

Conclusions

PEDOT/SWCNT composite films have been prepared by a vapor phase polymerization method and followed by 0.04 wt% NaBH4/DMSO treatment. Quasi-one-dimensional SWCNT/PEDOT core/shell nanostructures have been synthesized using SWCNT as a hard template and PPP as a soft template in the polymerization process. The results indicate the Seebeck coefficient of the film has increased from 15.5 μV K−1 to 24.1 μV K−1 with the SWCNT contents increasing from 0 to 50 wt%. The maximum power factor of

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program) under Grant no. 2013CB632500, National Natural Science Foundation of China (51271133), Doctoral Program of Henan Insititute of Engineering (D2016016), and the foundation of the State Key Lab of Advanced Technology for Material Synthesis and Processing (Wuhan University of Technology).

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