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Toward high thermoelectric performance for polypyrrole composites by dynamic 3-phase interfacial electropolymerization and chemical doping of carbon nanotubes
Composites Science and Technology 183 (2019) 107794
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Toward high thermoelectric performance for polypyrrole composites by dynamic 3-phase interfacial electropolymerization and chemical doping of carbon nanotubes
T
Wusheng Fana,b,1, Yichuan Zhanga,1, Cun-Yue Guob,∗∗, Guangming Chena,∗ a b
College of Materials Science and Engineering, Shenzhen University, Shenzhen, 518055, PR China School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing, 100049, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermoelectric Composite Polypyrrole Dynamic 3-phase interfacial electropolymerization Carbon nanotube
Polymer thermoelectric composites have received significant attention in recent years. To achieve high thermoelectric performance is one main aim, especially when being compared with their inorganic counterparts. Here, we report high-performance thermoelectric composites based on single-walled carbon nanotube (SWCNT) and polypyrrole (PPy), which are prepared by dynamic 3-phase interfacial electropolymerization and subsequent physical mixing. We found that both SWCNT content and chemical doping of SWCNT dramatically affected the composite thermoelectric performance. The maximum power factor at room temperature of the PPy composite reaches as high as 240.3 ± 5.0 μW m−1 K−2, which may be the highest value for PPy and its composites reported so far. The present study demonstrates that the dynamic 3-phase interfacial electropolymerization combining SWCNT chemical doping is effective to fabricate high-performance polymer thermoelectric composites.
1. Introduction The considerable interests in organic/inorganic thermoelectric composites arise from their key role in developing sustainable energyefficient technologies, since they can be used for direct energy-conversion between heat and electricity [1–3]. Excellent thermoelectric performance is vital to realize high thermoelectric energy conversion efficiency. Poly(3,4-ethylenedioxythiophene) (PEDOT) [4–8], polyaniline and (PANI) [9–12] and polypyrrole (PPy) [13–15] are usually selected as polymer matrices. Among them, PEDOT and PANI have achieved remarkable progresses and are regarded as the most promising candidates with high thermoelectric performance [16,17]. However, compared with PEDOT and PANI, relatively less attention has been paid to PPy despite its possessing stability and easy-to-prepare preparation procedure. The main reason may lie in its relatively low electrical conductivity, which constrains its application as thermoelectric material. Typically, two main polymerization methods, i.e. chemical oxidation polymerization and electropolymerization, have been employed to synthesize PPy or its composites. In contrast with chemical oxidation
polymerization, electropolymerization has distinct advantages of controlling the oxidation level and thickness of the sample film, contributing to the improvement of thermoelectric performance [18–20]. However, the electropolymerization method always leads to a tight attachment between the resultant polymer film and the electrode, resulting in serious problem of peeling off. Recently, free-standing PEDOT film with high thermoelectric performance was obtained via dynamic 3phase (solid (electrode)/liquid (oil)/liquid (water)) interfacial electropolymerization method [21,22]. It implies that this method may also afford a new effective approach to fabricate PPy-based thermoelectric composites. On the other hand, carbon nanotubes (CNTs) often serve as the important inorganic constituents for polymer thermoelectric composites due to their unique one-dimensional structure and outstanding electrical properties [23–25]. Especially, single-walled CNTs (SWCNTs) have more important application in thermoelectric energy conversion [26–28]. However, the achieved bulk electrical conductivities of the raw SWCNT films are always far away from expectations possibly due to the high junction resistances and Schottky barriers among adjacent SWCNTs. Fortunately, the employment of chemical doping can
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (C.-Y. Guo), [email protected] (G. Chen). 1 These two authors (W. Fan and Y. Zhang) contributed equally to this work. ∗∗
https://doi.org/10.1016/j.compscitech.2019.107794 Received 13 July 2019; Received in revised form 21 August 2019; Accepted 27 August 2019 Available online 27 August 2019 0266-3538/ © 2019 Elsevier Ltd. All rights reserved.
Composites Science and Technology 183 (2019) 107794
W. Fan, et al.
and then dried under vacuum at 60 °C for 24 h (Fig. 1c). The resultant PPy film was crushed and put into an ethanol solution (Fig. 1d). On the other hand, 300 mg of the as-purchased SWCNT were refluxtreated at 90 °C for 4 h in a mixed-acid solution (40 mL of concentrated sulphuric acid plus 120 mL of concentrated nitric acid) (Fig. 1a–b). As a result, an acid-doped SWCNT (a-SWCNT) film was obtained by dilution of the above mixture and subsequent vacuum filtration using a nylon millipore membrane (0.22 μm). Then, the a-SWCNT film was dried at 60 °C for 24 h (Fig. 1c). Similarly, the resultant a-SWCNT film was crushed and put into an ethanol solution (Fig. 1d). After that, a certain proportion of SWCNTs/ethanol and PPy/ethanol was mixed and ultrasonicated at room temperature for extra 20 min (Fig. 1e). Finally, the PPy/a-SWCNTs composite film was obtained by vacuum filtration and vacuum dried at 60 °C for 12 h (Fig. 1f). For comparison, the PPy/pristine SWCNT (p-SWCNT) films in which p-SWCNT did not undergo the acid-doping treatment were also prepared. The procedures were similar to Fig. 1, excluding the acid-doping treatments of SWCNT.
significantly improve their electrical conductivities [29,30], and hence it is considered to be a promising option to prepare composites with high thermoelectric performance. In the present work, we report the fabrication of flexible PPy composite films with high power factor by combining the 3-phase interfacial electropolymerization and chemical doping of SWCNTs. PPy was first synthesized via a dynamic 3-phase interfacial electropolymerization and then mixed with acid-doped SWCNT (a-SWCNT). The pristine SWCNT (p-SWCNT) and a-SWCNTs were compared to highlight the role of chemical doping in the enhancement of the composite thermoelectric performance. The PPy/a-SWCNT composites show excellent thermoelectric performance with the maximum power factor at room temperature of 240.3 ± 5.0 μW m−1 K−2, which may be the highest values for PPy and its composites reoirted to date [22]. 2. Experimental 2.1. Chemicals and materials
2.3. Characterization
Pyrrole (CP, 99%) was obtained from Sinopharm Chemical Reagent Co., Ltd. Single-walled carbon nanotubes (SWCNTs) (diameter: 1–2 nm, purity: > 90.0 wt%) with a commercial trademark of NTP8012 were provided by Shenzhen Nanotech Port Co. Ltd, China. Nitric acid (HNO3, A.R., 65%) and sulphuric acid (H2SO4, A.R., 98%) were purchased from Beijing Chemical Works. Hexafluorophosphoric acid (HPF6, 60 wt% solution in H2O) was obtained from J&K Scientific Ltd. All of the other reagents, including dichloromethane (CH2Cl2, A.R.), anhydrous ethanol (A.R.) and deionized water, were used as received without any further purification or treatment.
The morphologies of the PPy and its composites were observed with a high-resolution scanning electron microscope (HRSEM, Apreo S). The Raman spectra of the samples were recorded by a Raman spectrometer (Renidhaw inVia Raman Microscope) with an excitation wavelength of 514.5 nm. 2.4. Thermoelectric performance measurements The Seebeck coefficients were measured by a commercial Thin-Film Thermoelectric Parameter Test System (MRS-3RT, Wuhan Joule Yacht Science & Technology Co., Ltd) with a quasi-steady-state mode. The electrical conductivities were measured by a four point configuration method. At least five samples were measured and an average value was used.
2.2. Fabrication of PPy/a-SWCNT and PPy/SWCNT films The preparation procedure of PPy/a-SWCNT film is illustrated schematically in Fig. 1. Organic solvent of dichloromethane (25 mL) was first poured into a reaction beaker, and then pure pyrrole monomer (0.1 M) was added. After that, 0.1 M HPF6 aqueous solution (25 mL) was titrated into the above solution, where a water/oil interface occurred in the middle of the beaker after several minutes (Fig. 1a). Three electrodes were then put into the reaction system (Fig. 1b). A platinum wire, vertically immersed across the water/oil interface, was served as the working electrode (W). The other platinum wire and the Ag/AgCl electrode were used as the counter electrode (C) and reference electrode (R), respectively. Note that unlike the working electrode, both the counter and the reference electrodes were immersed only in the upper aqueous solution, but close to the water/oil interface. Electropolymerization of pyrrole monomers was carried out for 4 h under potentiostatic (1.3 V) condition using a CHI660E electrochemical workstation. When the dynamic 3-phase interfacial electropolymerization was completed, a dark blue PPy film was formed at the water/oil interface. Finally, the film was rinsed with ethanol and deionized water,
3. Results and discussion Fig. 2 presents the optical photos and SEM images for the freestanding film of the pure PPy, prepared by the dynamic 3-phase interfacial electropolymerization procedure. As clearly shown in the optical photos, the film side contacting with water phase (Fig. 2a) is much smoother and brighter than the other side with dichloromethane solvent phase (Fig. 2b). The SEM images in Fig. 2c–d further demonstrate that the surface microstructures are significantly different. The surface toward the water-phase side (Fig. 2c) is relatively smooth and dense, whereas the oil-phase side (Fig. 2d) has a 3D honeycomb-like porous structure with the pore size of 5–10 μm. Similar morphological differences between both sides of the polymer films were also reported in previous works of interfacial electropolymerization [19,31].
Fig. 1. Schematic illustration of the preparation procedure of the PPy/a-SWCNT composites. 2
Composites Science and Technology 183 (2019) 107794
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Fig. 2. (a, b) Optical photos and (c, d) SEM images of (a, c) the surface of the water-phase side and (b,d) the oil-phase side for the free-standing films of the pure PPy, synthesized by the dynamic 3-phase interfacial electropolymerization procedure.
Fig. 3. (a, c) SEM images and (b, d) Optical photos of (a, b) PPy/p-SWCNT and (c, d) PPy/a-SWCNT composite films containing 80 wt% SWCNT.
Fig. 3 shows the SEM and optical images of the PPy composites (containing 80 wt% SWCNT) containing p-SWCNT or a-SWCNT. As shown in Fig. 3a and c, p-SWCNT and a-SWCNT are homogeneously dispersed in the PPy matrix, while the composite surface of the latter one seems smoother. In addition, all composites reveal high flexibility (Fig. 3b and d), which is suitable for preparation of flexible thermoelectric devices. Therefore, chemical doping of SWCNT has little impact on the SWCNT dispersion and film flexibility. The thermoelectric performances of the pure PPy, the pure SWCNTs and their composites at room temperature are compared in Fig. 4. The average thickness of the film samples for thermoelectric measurements is around 20 μm. The effects of the chemical doping and the SWCNT content on the composite thermoelectric performance are discussed. Fig. 4a shows the increasing trends of electrical conductivities of PPy/aSWCNT and PPy/p-SWCNT composites with increased a-SWCNT or pSWCNT contents. Overall, the PPy/a-SWCNT composites display higher electrical conductivities than the PPy/p-SWCNT composites at the same SWCNT loadings, which may be attributed to a higher electrical conductivity of a-SWCNT (5707.5 ± 496.0 S cm−1) than the pristine one (774.7 ± 48.0 S cm−1). Fig. 4b shows the SWCNT content dependence of Seebeck coefficients for PPy/a-SWCNT and PPy/p-SWCNT composites. The Seebeck coefficients can be improved significantly after the addition of SWCNTs, and then change little till the SWCNT content of
Fig. 4. Dependences of a) electrical conductivities (σ), b) Seebeck coefficients (S) and c) power factors (PF = S2σ) on SWCNT content for the PPy composites.
80 wt%. Besides, compared with the a-SWCNT (~15.22 μV K−1), the larger Seebeck coefficient of the p-SWCNT (~50.80 μV K−1) contributes to larger Seebeck coefficients for the PPy/p-SWCNT composites than PPy/a-SWCNT composites at the same SWCNT contents. In our previous work, the SWCNT chemical doping leads to a dramatic increase of electrical conductivity and an obvious reduction of Seebeck coefficient in PEDOT/SWCNTs system [22]. Interestingly, in the present work, both the electrical conductivity and the Seebeck coefficient of the PPy composites are significantly enhanced simultaneously when the SWCNT is acid doped (Fig. 4a and b). As a consequence, as shown in Fig. 4c, the power factor of the PPy/a-SWCNT composites are higher than the PPy/ p-SWCNT composites. And the power factors are improved with 3
Composites Science and Technology 183 (2019) 107794
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SWCNT content. In addition, the power factor at room temperature is improved with increasing SWCNT content at a maximum of 240.3 ± 5.0 μW m−1 K−2 for PPy/a-SWCNT composites, possibly due to the interfacial interactions (p–p conjugated interactions and van der Waals forces). To our knowledge, the value is probably the highest power factor at room temperature for PPy and its composites reported to date. The present work demonstrates that the dynamic 3-phase interfacial electropolymerization combining the chemical doping of SWCNTs is an effective strategy towards the fabrication of high-performance polymer thermoelectric composites. Acknowledgements The authors thank National Natural Science Foundation of China (No. 51573190) for financial support. References [1] L. Wang, Z. Zhang, Y. Liu, B. Wang, L. Fang, J. Qiu, K. Zhang, S. Wang, Exceptinal thermoelectric properties of flexible organic-inorganic hybrids with nanodispersed and periodic nanophase, Nat. Commun. 9 (2018) 3817. [2] D. Qu, X. Li, H. Wang, G. Chen, Assembly strategy and performance evaluation of flexible thermoelectric devices, Adv. Sci. 6 (2019) 1900584. [3] G. Wu, Z.-G. Zhang, Y. Li, C. Gao, X. Wang, G. Chen, Exploring high-performance ntype thermoelectric composites using amino-substituted rylene dimides and carbon nanotube, ACS Nano 11 (6) (2017) 5746–5752. [4] F. Jiang, J. Xiong, W. Zhou, C. Liu, Y. Wang, F. Zhao, H. Liu, J. Xu, Use of organic solvent-assisted exfoliation of MoS2 for optimizing the thermoelectric performance of flexible PEDOT:PSS thin films, J. Mater. Chem. 4 (2016) 5265–5273. [5] K. Xu, G. Chen, D. Qiu, Convenient construction of poly(3, 4-ethylenedioxythiophene)/graphene pie-like structure with enhanced thermoelectric performance, J. Mater. Chem. A 1 (2013) 12395–12399. [6] S. Zhang, Z. Fan, X. Wang, Z. Zhang, J. Ouyang, Enhancement of the thermoelectric properties of PEDOT: PSS via one-step treatment with cosolvents or their solutions of organic salts, J. Mater. Chem. A 6 (16) (2018) 7080–7087. [7] C.-J. Yao, H.-L. Zhang, Q. Zhang, Recent progress in thermoelectric materials based on conjugated polymers, Polymers 11 (1–19) (2019) 107. [8] J. Xie, C.-E. Zhao, Z.-Q. Lin, P.-Y. Gu, Q. Zhang, Nanostructured conjugated polymers for energy-related applications beyond solar cells, Chem. Asian J. 11 (2016) 1489–1511. [9] J. Wu, Y. Sun, W. Xu, Q. Zhang, Investigating thermoelectric properties of doped polyaniline nanowires, Synth. Met. 189 (2014) 177–182. [10] F. Wang, S. Liu, C. Gao, T. Wan, L. Wang, L. Wang, L. Wang, Enhancement of the thermoelectric property of nanostructured polyaniline/carbon nanotube nanocomposites by introducing pyrrole unit onto polyaniline backbone via a sustainable method, Chem. Eng. J. 370 (2019) 322–329. [11] C. Cho, K.L. Wallace, P. Tzeng, J.-H. Hsu, C. Yu, J.C. Grunlan, Outstanding low temperature thermoelectric power factor from completely organic thin films enabled by multidimensional conjugated nanomaterials, Adv. Energy Mater. 6 (2016) 1502168. [12] K. Xu, C. Gao, G. Chen, D. Qiu, Direct evidence for effect of molecular orientation on thermoelectric performance of organic polymer materials by infrared dichroism, Org. Electron. 31 (2016) 41–47. [13] L. Liang, G. Chen, C.-Y. Guo, Enhanced thermoelectric performance by self-assembled layered morphology of polypyrrole nanowire/single-walled carbon nanotube composites, Compos. Sci. Technol. 129 (2016) 130–136. [14] J. Wu, Y. Sun, W.-B. Pei, L. Huang, W. Xu, Q. Zhang, Polypyrrole nanotube film for flexible thermoelectric application, Synth. Met. 196 (2014) 173–177. [15] M. Culebras, B. Uriol, C.M. Gómez, A. Cantarero, Controlling the thermoelectric properties of polymers: application to PEDOT and polypyrrole, Phys. Chem. Chem. Phys. 17 (2015) 15140–15145. [16] C. Gao, G. Chen, Conducting polymer/carbon particle thermoelectric composites: emerging green energy materials, Compos. Sci. Technol. 124 (2016) 52–70. [17] Y. Xue, C. Gao, L. Liang, X. Wang, G. Chen, Nanostructure controlled construction of high-performance thermoelectric materials of polymers and their composites, J. Mater. Chem. A 6 (45) (2018) 22381–22390. [18] N. Trinh, M. Saulnier, D. Lepage, S. Schougaard, Conductive polymer film supporting LiFePO4 as composite cathode for lithium ion batteries, J. Power Sources 221 (2013) 284–289. [19] L. Gao, X. Mao, W. Xiao, F. Gan, D. Wang, Electropolymerization of PEDOT on CNTs conductive network assembled at water/oil interface, Electrochim. Acta 136 (2014) 97–104. [20] T. Deguchi, H. Tomeoku, M. Takashiri, Preparation and characterization of electropolymerized poly(3,4-ethylenedioxythiophene) thin films with different dopant anions, Japan, J. Appl. Phys. 55 (2016) 06GK03. [21] W. Fan, C.-Y. Guo, G. Chen, Flexible films of poly(3,4-ethylenedioxythiophene)/ carbon nanotube thermoelectric composites prepared by dynamic 3-phase interfacial electropolymerization and subsequent physical mixing, J. Mater. Chem. A 6 (26) (2018) 12275–12280. [22] W. Fan, L. Liang, B. Zhang, C. Guo, G. Chen, PEDOT thermoelectric composites with
Fig. 5. Raman spectra of the p-SWCNT, a-SWCNT, PPy and their composites containing 60 wt% SWCNT.
increasing SWCNT contents for both composites. Consequently, the highest power factors for PPy/a-SWCNT and PPy/p-SWCNT composites reach as high as 240.3 ± 5.0 and 104.1 ± 9.7 μW m−1 K−2, respectively. Indeed, the value of 240.3 ± 5.0 μW m−1 K−2 is more than 10 times higher than those of the PPy/CNT systems prepared by chemical oxidative polymerization [13], surface coating [32] and other methods [33,34]. To our knowledge, this may be the highest value for PPy and its composites reported so far, which is even comparable to the highperformanced PEDOT composites [22]. This demonstrates that the present method, i.e. the dynamic 3-phase interfacial electropolymerization combining the chemical doping of SWCNTs, is very effective to improve the thermoelectric performance of PPy composites. Fig. 5 shows the Raman spectra of p-SWCNT, a-SWCNT, PPy/pSWCNT and PPy/a-SWCNT. The strongest band for pure PPy at 1587 cm−1 is attributed to the C]C backbone stretching vibration related to an overlap of two oxidized structures [32]. In addition, the two bands at 935 and 970 cm−1 are attributed to the ring-deformation vibrations of neutral PPy and the ring-deformation vibrations in bipolaron units. The band at 1060, 1245, 1380 cm−1 are assinged to the C–H out-of-plane deformation, anti-symmetrical C–H in-plane bending and ring-stretching vibrations, respectively [35]. Concerning the pSWCNT and a-SWCNT, the typical bands at 1591 and 1589 cm−1 are assigned to the E2g mode related to the vibration of sp2-bonded carbon atoms in 2D hexagonal lattices (G-band). In the PPy/a-SWCNT composite, the phenomenon that band at 1589 cm−1 shifts to 1592 cm−1, suggesting the existence of interfacial interactions (p–p conjugated interactions and van der Waals forces) between PPy molecules and aSWCNTs, which may result in the enhanced thermoelectric performance. 4. Conclusion In summary, PPy/SWCNT and PPy/a-SWCNT composites have been successfully fabricated by a dynamic 3-phase interfacial electropolymerization procedure of PPy and a subsequent mixing. The freestanding film of the pure PPy exhibits different surface structures: the surface toward the water-phase side is relatively smooth and dense, whereas the oil-phase side has a 3D honeycomb-like porous structure. Chemical doping of SWCNT has little influence on the SWCNT dispersion and composite film flexibility. In contrast, chemical doping of SWCNT has a significant effect on PPy composites’ thermoelectric performance. Compared with PPy/SWCNT composites, PPy/a-SWCNT composites display a higher electrical conductivity, a slight larger Seebeck coefficient and a considerable higher power factor at the same 4
Composites Science and Technology 183 (2019) 107794
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[23]
[24]
[25] [26]
[27]
[28]
[29]
excellent power factors prepared by 3-phase interfacial electropolymerization and carbon nanotube chemical doping, J. Mater. Chem. A 7 (22) (2019) 13687–13694. A. Dey, O.P. Bajpai, A.K. Sikder, S. Chattopadhyay, M.A.S. Khan, Recent advances in CNT/graphene based thermoelectric polymer nanocomposite: a proficient move towards waste energy harvesting, Renew. Sustain. Energy Rev. 53 (2016) 653–671. Y.-C. Zhang, D. Zheng, H. Pang, J.-H. Tang, Z.-M. Li, The effect of molecular chain polarity on electric field-induced aligned conductive carbon nanotube network formation in polymer melt, Compos. Sci. Technol. 72 (15) (2012) 1875–1881. C. Gayner, K.K. Kar, Recent advances in thermoelectric materials, Prog. Mater. Sci. 83 (2016) 330–382. C. Gao, G. Chen, In situ oxidation synthesis of p‐type composite with narrow‐bandgap small organic molecule coating on single‐walled carbon nanotube: flexible film and thermoelectric performance, Small 14 (12) (2018) 1703453. G. Wu, Z.-G. Zhang, Y. Li, C. Gao, X. Wang, G. Chen, Exploring high-performance ntype thermoelectric composites using amino-substituted rylene dimides and carbon nanotubes, ACS Nano 11 (6) (2017) 5746–5752. G. Wu, C. Gao, G. Chen, X. Wang, H. Wang, High-performance organic thermoelectric modules based on flexible films of a novel n-type single-walled carbon nanotube, J. Mater. Chem. A 4 (37) (2016) 14187–14193. I. Puchades, C.C. Lawlor, C.M. Schauerman, A.R. Bucossi, J.E. Rossi, N.D. Cox, B.J. Landi, Mechanism of chemical doping in electronic-type-separated single wall
[30]
[31]
[32]
[33]
[34]
[35]
5
carbon nanotubes towards high electrical conductivity, J. Mater. Chem. C 3 (39) (2015) 10256–10266. E. Bekyarova, M.E. Itkis, N. Cabrera, B. Zhao, A. Yu, J. Gao, R.C. Haddon, Electronic properties of single-walled carbon nanotube networks, J. Am. Chem. Soc. 127 (16) (2005) 5990–5995. M. Li, H. Zhu, X. Mao, W. Xiao, D. Wang, Electropolymerization of polypyrrole at the three-phase interline: influence of polymerization conditions, Electrochim. Acta 92 (2013) 108–116. L. Liang, C. Gao, G. Chen, C.-Y. Guo, Large-area, stretchable, super flexible and mechanically stable thermoelectric films of polymer/carbon nanotube composites, J. Mater. Chem. C 4 (3) (2016) 526–532. J. Wang, K. Cai, S. Shen, J. Yin, Preparation and thermoelectric properties of multiwalled carbon nanotubes/polypyrrole composites, Synth. Met. 195 (2014) 132–136. H. Song, K. Cai, J. Wang, S. Shen, Influence of polymerization method on the thermoelectric properties of multi-walled carbon nanotubes/polypyrrole composites, Synth. Met. 211 (2016) 58–65. I.M. Minisy, P. Bober, U. Acharya, M. Trchová, J. Hromádková, J. Pfleger, J. Stejskal, Cationic dyes as morphology-guiding agents for one-dimensional polypyrrole with improved conductivity, Polymer 174 (2019) 11–17.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Toward high thermoelectric performance for polypyrrole composites by dynamic 3-phase interfacial electropolymerization and chemical doping of carbon nanotubes
T
Wusheng Fana,b,1, Yichuan Zhanga,1, Cun-Yue Guob,∗∗, Guangming Chena,∗ a b
College of Materials Science and Engineering, Shenzhen University, Shenzhen, 518055, PR China School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing, 100049, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermoelectric Composite Polypyrrole Dynamic 3-phase interfacial electropolymerization Carbon nanotube
Polymer thermoelectric composites have received significant attention in recent years. To achieve high thermoelectric performance is one main aim, especially when being compared with their inorganic counterparts. Here, we report high-performance thermoelectric composites based on single-walled carbon nanotube (SWCNT) and polypyrrole (PPy), which are prepared by dynamic 3-phase interfacial electropolymerization and subsequent physical mixing. We found that both SWCNT content and chemical doping of SWCNT dramatically affected the composite thermoelectric performance. The maximum power factor at room temperature of the PPy composite reaches as high as 240.3 ± 5.0 μW m−1 K−2, which may be the highest value for PPy and its composites reported so far. The present study demonstrates that the dynamic 3-phase interfacial electropolymerization combining SWCNT chemical doping is effective to fabricate high-performance polymer thermoelectric composites.
1. Introduction The considerable interests in organic/inorganic thermoelectric composites arise from their key role in developing sustainable energyefficient technologies, since they can be used for direct energy-conversion between heat and electricity [1–3]. Excellent thermoelectric performance is vital to realize high thermoelectric energy conversion efficiency. Poly(3,4-ethylenedioxythiophene) (PEDOT) [4–8], polyaniline and (PANI) [9–12] and polypyrrole (PPy) [13–15] are usually selected as polymer matrices. Among them, PEDOT and PANI have achieved remarkable progresses and are regarded as the most promising candidates with high thermoelectric performance [16,17]. However, compared with PEDOT and PANI, relatively less attention has been paid to PPy despite its possessing stability and easy-to-prepare preparation procedure. The main reason may lie in its relatively low electrical conductivity, which constrains its application as thermoelectric material. Typically, two main polymerization methods, i.e. chemical oxidation polymerization and electropolymerization, have been employed to synthesize PPy or its composites. In contrast with chemical oxidation
polymerization, electropolymerization has distinct advantages of controlling the oxidation level and thickness of the sample film, contributing to the improvement of thermoelectric performance [18–20]. However, the electropolymerization method always leads to a tight attachment between the resultant polymer film and the electrode, resulting in serious problem of peeling off. Recently, free-standing PEDOT film with high thermoelectric performance was obtained via dynamic 3phase (solid (electrode)/liquid (oil)/liquid (water)) interfacial electropolymerization method [21,22]. It implies that this method may also afford a new effective approach to fabricate PPy-based thermoelectric composites. On the other hand, carbon nanotubes (CNTs) often serve as the important inorganic constituents for polymer thermoelectric composites due to their unique one-dimensional structure and outstanding electrical properties [23–25]. Especially, single-walled CNTs (SWCNTs) have more important application in thermoelectric energy conversion [26–28]. However, the achieved bulk electrical conductivities of the raw SWCNT films are always far away from expectations possibly due to the high junction resistances and Schottky barriers among adjacent SWCNTs. Fortunately, the employment of chemical doping can
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (C.-Y. Guo), [email protected] (G. Chen). 1 These two authors (W. Fan and Y. Zhang) contributed equally to this work. ∗∗
https://doi.org/10.1016/j.compscitech.2019.107794 Received 13 July 2019; Received in revised form 21 August 2019; Accepted 27 August 2019 Available online 27 August 2019 0266-3538/ © 2019 Elsevier Ltd. All rights reserved.
Composites Science and Technology 183 (2019) 107794
W. Fan, et al.
and then dried under vacuum at 60 °C for 24 h (Fig. 1c). The resultant PPy film was crushed and put into an ethanol solution (Fig. 1d). On the other hand, 300 mg of the as-purchased SWCNT were refluxtreated at 90 °C for 4 h in a mixed-acid solution (40 mL of concentrated sulphuric acid plus 120 mL of concentrated nitric acid) (Fig. 1a–b). As a result, an acid-doped SWCNT (a-SWCNT) film was obtained by dilution of the above mixture and subsequent vacuum filtration using a nylon millipore membrane (0.22 μm). Then, the a-SWCNT film was dried at 60 °C for 24 h (Fig. 1c). Similarly, the resultant a-SWCNT film was crushed and put into an ethanol solution (Fig. 1d). After that, a certain proportion of SWCNTs/ethanol and PPy/ethanol was mixed and ultrasonicated at room temperature for extra 20 min (Fig. 1e). Finally, the PPy/a-SWCNTs composite film was obtained by vacuum filtration and vacuum dried at 60 °C for 12 h (Fig. 1f). For comparison, the PPy/pristine SWCNT (p-SWCNT) films in which p-SWCNT did not undergo the acid-doping treatment were also prepared. The procedures were similar to Fig. 1, excluding the acid-doping treatments of SWCNT.
significantly improve their electrical conductivities [29,30], and hence it is considered to be a promising option to prepare composites with high thermoelectric performance. In the present work, we report the fabrication of flexible PPy composite films with high power factor by combining the 3-phase interfacial electropolymerization and chemical doping of SWCNTs. PPy was first synthesized via a dynamic 3-phase interfacial electropolymerization and then mixed with acid-doped SWCNT (a-SWCNT). The pristine SWCNT (p-SWCNT) and a-SWCNTs were compared to highlight the role of chemical doping in the enhancement of the composite thermoelectric performance. The PPy/a-SWCNT composites show excellent thermoelectric performance with the maximum power factor at room temperature of 240.3 ± 5.0 μW m−1 K−2, which may be the highest values for PPy and its composites reoirted to date [22]. 2. Experimental 2.1. Chemicals and materials
2.3. Characterization
Pyrrole (CP, 99%) was obtained from Sinopharm Chemical Reagent Co., Ltd. Single-walled carbon nanotubes (SWCNTs) (diameter: 1–2 nm, purity: > 90.0 wt%) with a commercial trademark of NTP8012 were provided by Shenzhen Nanotech Port Co. Ltd, China. Nitric acid (HNO3, A.R., 65%) and sulphuric acid (H2SO4, A.R., 98%) were purchased from Beijing Chemical Works. Hexafluorophosphoric acid (HPF6, 60 wt% solution in H2O) was obtained from J&K Scientific Ltd. All of the other reagents, including dichloromethane (CH2Cl2, A.R.), anhydrous ethanol (A.R.) and deionized water, were used as received without any further purification or treatment.
The morphologies of the PPy and its composites were observed with a high-resolution scanning electron microscope (HRSEM, Apreo S). The Raman spectra of the samples were recorded by a Raman spectrometer (Renidhaw inVia Raman Microscope) with an excitation wavelength of 514.5 nm. 2.4. Thermoelectric performance measurements The Seebeck coefficients were measured by a commercial Thin-Film Thermoelectric Parameter Test System (MRS-3RT, Wuhan Joule Yacht Science & Technology Co., Ltd) with a quasi-steady-state mode. The electrical conductivities were measured by a four point configuration method. At least five samples were measured and an average value was used.
2.2. Fabrication of PPy/a-SWCNT and PPy/SWCNT films The preparation procedure of PPy/a-SWCNT film is illustrated schematically in Fig. 1. Organic solvent of dichloromethane (25 mL) was first poured into a reaction beaker, and then pure pyrrole monomer (0.1 M) was added. After that, 0.1 M HPF6 aqueous solution (25 mL) was titrated into the above solution, where a water/oil interface occurred in the middle of the beaker after several minutes (Fig. 1a). Three electrodes were then put into the reaction system (Fig. 1b). A platinum wire, vertically immersed across the water/oil interface, was served as the working electrode (W). The other platinum wire and the Ag/AgCl electrode were used as the counter electrode (C) and reference electrode (R), respectively. Note that unlike the working electrode, both the counter and the reference electrodes were immersed only in the upper aqueous solution, but close to the water/oil interface. Electropolymerization of pyrrole monomers was carried out for 4 h under potentiostatic (1.3 V) condition using a CHI660E electrochemical workstation. When the dynamic 3-phase interfacial electropolymerization was completed, a dark blue PPy film was formed at the water/oil interface. Finally, the film was rinsed with ethanol and deionized water,
3. Results and discussion Fig. 2 presents the optical photos and SEM images for the freestanding film of the pure PPy, prepared by the dynamic 3-phase interfacial electropolymerization procedure. As clearly shown in the optical photos, the film side contacting with water phase (Fig. 2a) is much smoother and brighter than the other side with dichloromethane solvent phase (Fig. 2b). The SEM images in Fig. 2c–d further demonstrate that the surface microstructures are significantly different. The surface toward the water-phase side (Fig. 2c) is relatively smooth and dense, whereas the oil-phase side (Fig. 2d) has a 3D honeycomb-like porous structure with the pore size of 5–10 μm. Similar morphological differences between both sides of the polymer films were also reported in previous works of interfacial electropolymerization [19,31].
Fig. 1. Schematic illustration of the preparation procedure of the PPy/a-SWCNT composites. 2
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Fig. 2. (a, b) Optical photos and (c, d) SEM images of (a, c) the surface of the water-phase side and (b,d) the oil-phase side for the free-standing films of the pure PPy, synthesized by the dynamic 3-phase interfacial electropolymerization procedure.
Fig. 3. (a, c) SEM images and (b, d) Optical photos of (a, b) PPy/p-SWCNT and (c, d) PPy/a-SWCNT composite films containing 80 wt% SWCNT.
Fig. 3 shows the SEM and optical images of the PPy composites (containing 80 wt% SWCNT) containing p-SWCNT or a-SWCNT. As shown in Fig. 3a and c, p-SWCNT and a-SWCNT are homogeneously dispersed in the PPy matrix, while the composite surface of the latter one seems smoother. In addition, all composites reveal high flexibility (Fig. 3b and d), which is suitable for preparation of flexible thermoelectric devices. Therefore, chemical doping of SWCNT has little impact on the SWCNT dispersion and film flexibility. The thermoelectric performances of the pure PPy, the pure SWCNTs and their composites at room temperature are compared in Fig. 4. The average thickness of the film samples for thermoelectric measurements is around 20 μm. The effects of the chemical doping and the SWCNT content on the composite thermoelectric performance are discussed. Fig. 4a shows the increasing trends of electrical conductivities of PPy/aSWCNT and PPy/p-SWCNT composites with increased a-SWCNT or pSWCNT contents. Overall, the PPy/a-SWCNT composites display higher electrical conductivities than the PPy/p-SWCNT composites at the same SWCNT loadings, which may be attributed to a higher electrical conductivity of a-SWCNT (5707.5 ± 496.0 S cm−1) than the pristine one (774.7 ± 48.0 S cm−1). Fig. 4b shows the SWCNT content dependence of Seebeck coefficients for PPy/a-SWCNT and PPy/p-SWCNT composites. The Seebeck coefficients can be improved significantly after the addition of SWCNTs, and then change little till the SWCNT content of
Fig. 4. Dependences of a) electrical conductivities (σ), b) Seebeck coefficients (S) and c) power factors (PF = S2σ) on SWCNT content for the PPy composites.
80 wt%. Besides, compared with the a-SWCNT (~15.22 μV K−1), the larger Seebeck coefficient of the p-SWCNT (~50.80 μV K−1) contributes to larger Seebeck coefficients for the PPy/p-SWCNT composites than PPy/a-SWCNT composites at the same SWCNT contents. In our previous work, the SWCNT chemical doping leads to a dramatic increase of electrical conductivity and an obvious reduction of Seebeck coefficient in PEDOT/SWCNTs system [22]. Interestingly, in the present work, both the electrical conductivity and the Seebeck coefficient of the PPy composites are significantly enhanced simultaneously when the SWCNT is acid doped (Fig. 4a and b). As a consequence, as shown in Fig. 4c, the power factor of the PPy/a-SWCNT composites are higher than the PPy/ p-SWCNT composites. And the power factors are improved with 3
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SWCNT content. In addition, the power factor at room temperature is improved with increasing SWCNT content at a maximum of 240.3 ± 5.0 μW m−1 K−2 for PPy/a-SWCNT composites, possibly due to the interfacial interactions (p–p conjugated interactions and van der Waals forces). To our knowledge, the value is probably the highest power factor at room temperature for PPy and its composites reported to date. The present work demonstrates that the dynamic 3-phase interfacial electropolymerization combining the chemical doping of SWCNTs is an effective strategy towards the fabrication of high-performance polymer thermoelectric composites. Acknowledgements The authors thank National Natural Science Foundation of China (No. 51573190) for financial support. References [1] L. Wang, Z. Zhang, Y. Liu, B. Wang, L. Fang, J. Qiu, K. Zhang, S. Wang, Exceptinal thermoelectric properties of flexible organic-inorganic hybrids with nanodispersed and periodic nanophase, Nat. Commun. 9 (2018) 3817. [2] D. Qu, X. Li, H. Wang, G. Chen, Assembly strategy and performance evaluation of flexible thermoelectric devices, Adv. Sci. 6 (2019) 1900584. [3] G. Wu, Z.-G. Zhang, Y. Li, C. Gao, X. Wang, G. Chen, Exploring high-performance ntype thermoelectric composites using amino-substituted rylene dimides and carbon nanotube, ACS Nano 11 (6) (2017) 5746–5752. [4] F. Jiang, J. Xiong, W. Zhou, C. Liu, Y. Wang, F. Zhao, H. Liu, J. Xu, Use of organic solvent-assisted exfoliation of MoS2 for optimizing the thermoelectric performance of flexible PEDOT:PSS thin films, J. Mater. Chem. 4 (2016) 5265–5273. [5] K. Xu, G. Chen, D. Qiu, Convenient construction of poly(3, 4-ethylenedioxythiophene)/graphene pie-like structure with enhanced thermoelectric performance, J. Mater. Chem. A 1 (2013) 12395–12399. [6] S. Zhang, Z. Fan, X. Wang, Z. Zhang, J. Ouyang, Enhancement of the thermoelectric properties of PEDOT: PSS via one-step treatment with cosolvents or their solutions of organic salts, J. Mater. Chem. A 6 (16) (2018) 7080–7087. [7] C.-J. Yao, H.-L. Zhang, Q. Zhang, Recent progress in thermoelectric materials based on conjugated polymers, Polymers 11 (1–19) (2019) 107. [8] J. Xie, C.-E. Zhao, Z.-Q. Lin, P.-Y. Gu, Q. Zhang, Nanostructured conjugated polymers for energy-related applications beyond solar cells, Chem. Asian J. 11 (2016) 1489–1511. [9] J. Wu, Y. Sun, W. Xu, Q. Zhang, Investigating thermoelectric properties of doped polyaniline nanowires, Synth. Met. 189 (2014) 177–182. [10] F. Wang, S. Liu, C. Gao, T. Wan, L. Wang, L. Wang, L. Wang, Enhancement of the thermoelectric property of nanostructured polyaniline/carbon nanotube nanocomposites by introducing pyrrole unit onto polyaniline backbone via a sustainable method, Chem. Eng. J. 370 (2019) 322–329. [11] C. Cho, K.L. Wallace, P. Tzeng, J.-H. Hsu, C. Yu, J.C. Grunlan, Outstanding low temperature thermoelectric power factor from completely organic thin films enabled by multidimensional conjugated nanomaterials, Adv. Energy Mater. 6 (2016) 1502168. [12] K. Xu, C. Gao, G. Chen, D. Qiu, Direct evidence for effect of molecular orientation on thermoelectric performance of organic polymer materials by infrared dichroism, Org. Electron. 31 (2016) 41–47. [13] L. Liang, G. Chen, C.-Y. Guo, Enhanced thermoelectric performance by self-assembled layered morphology of polypyrrole nanowire/single-walled carbon nanotube composites, Compos. Sci. Technol. 129 (2016) 130–136. [14] J. Wu, Y. Sun, W.-B. Pei, L. Huang, W. Xu, Q. Zhang, Polypyrrole nanotube film for flexible thermoelectric application, Synth. Met. 196 (2014) 173–177. [15] M. Culebras, B. Uriol, C.M. Gómez, A. Cantarero, Controlling the thermoelectric properties of polymers: application to PEDOT and polypyrrole, Phys. Chem. Chem. Phys. 17 (2015) 15140–15145. [16] C. Gao, G. Chen, Conducting polymer/carbon particle thermoelectric composites: emerging green energy materials, Compos. Sci. Technol. 124 (2016) 52–70. [17] Y. Xue, C. Gao, L. Liang, X. Wang, G. Chen, Nanostructure controlled construction of high-performance thermoelectric materials of polymers and their composites, J. Mater. Chem. A 6 (45) (2018) 22381–22390. [18] N. Trinh, M. Saulnier, D. Lepage, S. Schougaard, Conductive polymer film supporting LiFePO4 as composite cathode for lithium ion batteries, J. Power Sources 221 (2013) 284–289. [19] L. Gao, X. Mao, W. Xiao, F. Gan, D. Wang, Electropolymerization of PEDOT on CNTs conductive network assembled at water/oil interface, Electrochim. Acta 136 (2014) 97–104. [20] T. Deguchi, H. Tomeoku, M. Takashiri, Preparation and characterization of electropolymerized poly(3,4-ethylenedioxythiophene) thin films with different dopant anions, Japan, J. Appl. Phys. 55 (2016) 06GK03. [21] W. Fan, C.-Y. Guo, G. Chen, Flexible films of poly(3,4-ethylenedioxythiophene)/ carbon nanotube thermoelectric composites prepared by dynamic 3-phase interfacial electropolymerization and subsequent physical mixing, J. Mater. Chem. A 6 (26) (2018) 12275–12280. [22] W. Fan, L. Liang, B. Zhang, C. Guo, G. Chen, PEDOT thermoelectric composites with
Fig. 5. Raman spectra of the p-SWCNT, a-SWCNT, PPy and their composites containing 60 wt% SWCNT.
increasing SWCNT contents for both composites. Consequently, the highest power factors for PPy/a-SWCNT and PPy/p-SWCNT composites reach as high as 240.3 ± 5.0 and 104.1 ± 9.7 μW m−1 K−2, respectively. Indeed, the value of 240.3 ± 5.0 μW m−1 K−2 is more than 10 times higher than those of the PPy/CNT systems prepared by chemical oxidative polymerization [13], surface coating [32] and other methods [33,34]. To our knowledge, this may be the highest value for PPy and its composites reported so far, which is even comparable to the highperformanced PEDOT composites [22]. This demonstrates that the present method, i.e. the dynamic 3-phase interfacial electropolymerization combining the chemical doping of SWCNTs, is very effective to improve the thermoelectric performance of PPy composites. Fig. 5 shows the Raman spectra of p-SWCNT, a-SWCNT, PPy/pSWCNT and PPy/a-SWCNT. The strongest band for pure PPy at 1587 cm−1 is attributed to the C]C backbone stretching vibration related to an overlap of two oxidized structures [32]. In addition, the two bands at 935 and 970 cm−1 are attributed to the ring-deformation vibrations of neutral PPy and the ring-deformation vibrations in bipolaron units. The band at 1060, 1245, 1380 cm−1 are assinged to the C–H out-of-plane deformation, anti-symmetrical C–H in-plane bending and ring-stretching vibrations, respectively [35]. Concerning the pSWCNT and a-SWCNT, the typical bands at 1591 and 1589 cm−1 are assigned to the E2g mode related to the vibration of sp2-bonded carbon atoms in 2D hexagonal lattices (G-band). In the PPy/a-SWCNT composite, the phenomenon that band at 1589 cm−1 shifts to 1592 cm−1, suggesting the existence of interfacial interactions (p–p conjugated interactions and van der Waals forces) between PPy molecules and aSWCNTs, which may result in the enhanced thermoelectric performance. 4. Conclusion In summary, PPy/SWCNT and PPy/a-SWCNT composites have been successfully fabricated by a dynamic 3-phase interfacial electropolymerization procedure of PPy and a subsequent mixing. The freestanding film of the pure PPy exhibits different surface structures: the surface toward the water-phase side is relatively smooth and dense, whereas the oil-phase side has a 3D honeycomb-like porous structure. Chemical doping of SWCNT has little influence on the SWCNT dispersion and composite film flexibility. In contrast, chemical doping of SWCNT has a significant effect on PPy composites’ thermoelectric performance. Compared with PPy/SWCNT composites, PPy/a-SWCNT composites display a higher electrical conductivity, a slight larger Seebeck coefficient and a considerable higher power factor at the same 4
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[23]
[24]
[25] [26]
[27]
[28]
[29]
excellent power factors prepared by 3-phase interfacial electropolymerization and carbon nanotube chemical doping, J. Mater. Chem. A 7 (22) (2019) 13687–13694. A. Dey, O.P. Bajpai, A.K. Sikder, S. Chattopadhyay, M.A.S. Khan, Recent advances in CNT/graphene based thermoelectric polymer nanocomposite: a proficient move towards waste energy harvesting, Renew. Sustain. Energy Rev. 53 (2016) 653–671. Y.-C. Zhang, D. Zheng, H. Pang, J.-H. Tang, Z.-M. Li, The effect of molecular chain polarity on electric field-induced aligned conductive carbon nanotube network formation in polymer melt, Compos. Sci. Technol. 72 (15) (2012) 1875–1881. C. Gayner, K.K. Kar, Recent advances in thermoelectric materials, Prog. Mater. Sci. 83 (2016) 330–382. C. Gao, G. Chen, In situ oxidation synthesis of p‐type composite with narrow‐bandgap small organic molecule coating on single‐walled carbon nanotube: flexible film and thermoelectric performance, Small 14 (12) (2018) 1703453. G. Wu, Z.-G. Zhang, Y. Li, C. Gao, X. Wang, G. Chen, Exploring high-performance ntype thermoelectric composites using amino-substituted rylene dimides and carbon nanotubes, ACS Nano 11 (6) (2017) 5746–5752. G. Wu, C. Gao, G. Chen, X. Wang, H. Wang, High-performance organic thermoelectric modules based on flexible films of a novel n-type single-walled carbon nanotube, J. Mater. Chem. A 4 (37) (2016) 14187–14193. I. Puchades, C.C. Lawlor, C.M. Schauerman, A.R. Bucossi, J.E. Rossi, N.D. Cox, B.J. Landi, Mechanism of chemical doping in electronic-type-separated single wall
[30]
[31]
[32]
[33]
[34]
[35]
5
carbon nanotubes towards high electrical conductivity, J. Mater. Chem. C 3 (39) (2015) 10256–10266. E. Bekyarova, M.E. Itkis, N. Cabrera, B. Zhao, A. Yu, J. Gao, R.C. Haddon, Electronic properties of single-walled carbon nanotube networks, J. Am. Chem. Soc. 127 (16) (2005) 5990–5995. M. Li, H. Zhu, X. Mao, W. Xiao, D. Wang, Electropolymerization of polypyrrole at the three-phase interline: influence of polymerization conditions, Electrochim. Acta 92 (2013) 108–116. L. Liang, C. Gao, G. Chen, C.-Y. Guo, Large-area, stretchable, super flexible and mechanically stable thermoelectric films of polymer/carbon nanotube composites, J. Mater. Chem. C 4 (3) (2016) 526–532. J. Wang, K. Cai, S. Shen, J. Yin, Preparation and thermoelectric properties of multiwalled carbon nanotubes/polypyrrole composites, Synth. Met. 195 (2014) 132–136. H. Song, K. Cai, J. Wang, S. Shen, Influence of polymerization method on the thermoelectric properties of multi-walled carbon nanotubes/polypyrrole composites, Synth. Met. 211 (2016) 58–65. I.M. Minisy, P. Bober, U. Acharya, M. Trchová, J. Hromádková, J. Pfleger, J. Stejskal, Cationic dyes as morphology-guiding agents for one-dimensional polypyrrole with improved conductivity, Polymer 174 (2019) 11–17.