Enhancement of the thermoelectric property of nanostructured polyaniline/carbon nanotube composites by introducing pyrrole unit onto polyaniline backbone via a sustainable method
Graphical abstract
Introduction
A thermoelectric (TE) device can directly convert ubiquitous heat with low quality to versatile and high-demand electricity, thus garnering extensive interest due to its potential application in industrial waste heat recovery and natural heat utilization [1], [2], [3], [4], [5]. Moreover, when a current is run across the TE device, it can be used as a solid-state heat pump for distributed spot-size refrigeration [1], [2]. Recently, many organic/inorganic composites were applied to fabricate the TE devices. The TE generator consisting with PEDOT:PSS/Te-NWs aerogel films and n-type carbon nanotube fibers has been fabricated by Wang et al with an output power of 1.28 μW at 60 K temperature gradient [6]. Wu et al fabricated the high-performance n-type thermoelectric composites, the corresponding TE device made up of five p−n junctions reaches a large output power of 3.3 μW under a 50 °C temperature gradient [7]. Photoinduced p- to n-type switching in thermoelectric solymer-carbon nanotube composites have also been prepared, when one side of this module comprising 15 double legs is attached to a glass fi lled with ice water, leaving the other side at room temperature, it generates a voltage of 5 mV [8].
The conversion efficiency of a thermoelectric material is determined by the dimensionless figure of merit (ZT) defined as S2σT/к, where S, σ, T, and к represent the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity, respectively [9], [10], [11], [12]. Therefore, materials with a high Seebeck coefficient and electrical conductivity and a low thermal conductivity are required to achieve high TE conversion efficiency. In addition, the TE performance of organic materials can also be evaluated by the power factor (PF = S2σ) due to their low intrinsic thermal conductivity [13], [14].
Owing to their nontoxicity, earth abundance, light weight and mechanical flexibility, organic conjugated polymers that can offer the possibility of acquiring desirable TE materials have been extensively investigated in recent years [15], [16], including a series of polymers and organic/inorganic composites developed by our group [16], [17], [18], [19], [20]. Based on the designable structure of organic TE materials, the alignment of polymer chains is considered an important factor enhancing the TE property of conducting polymers by increasing the electrical conductivity [21]. For instance, uniaxially aligned ribbon phase PBTTT [22] and aligned PCDTPT [23] with proper molecular weights exhibited a high carrier mobility along the polymer chain direction. Similarly, Yao et al demonstrated a method to tune the molecular chain alignment of a traditional polyaniline (PANi) by mixing it with camphorsulfonic acid (CSA) in m-cresol solution [24], [25]. Even though the electrical conductivity increased from 4.7 S cm−1 to 220 S cm−1 when the PANi was doped with CSA in m-cresol solution, the high boiling point of m-cresol and the complex preparation procedure make it difficult to achieve high mass production in an industrial process. Hence, another traditional conducting polymer, polypyrrole (PPy) which is a well-known aligned polymer was considered a candidate to improve the alignment of PANi molecular chain. In addition, the oxidative polymerization of aniline and pyrrole can share the same catalysis system. Furthermore, nanostructured conducting polymeric materials are of exceptional interest in the field of nanoscience and nanotechnology due to their low-dimensional structure, metal-like conductivity and other novel properties [26]. Low-dimensional PANi nanostructures have also received great attention in recent years [27]. However, the shape and dispersion characteristics of nanostructured PANi are unstable and vary with the different synthesis bath. A method developed by Li et al. figured out that the reproducible problem was caused by mechanical agitation, which triggered aggregation of PANi during the course of polymerization [28].
Herein, the nanostructured PANi, PPy and poly(aniline-co-pyrrole) random copolymer were prepared by a traditional oxidative polymerization without mechanical agitation. No environmentally polluting or harmful solvent was used throughout the experimental section. In terms of chemical engineering, the experimental procedure is attractive as a sustainable process for materials preparation. The combination of Fourier Transform Infrared (FTIR) spectra, Raman spectra and X-ray photoelectron spectroscopy (XPS) spectra revealed that the poly(aniline-co-pyrrole) random copolymer was successfully synthesized, and a strong interaction was also detected between the pyrrole and the aniline units on the random copolymer backbone. In addition, single wall carbon nanotubes (SWCNTs) with high surface area were applied to improve the membrane forming performance of insoluble conjugated polymers and the polymer/SWCNT composites were prepared. The nanostructured morphology of the synthesized polymer was observed by scanning electron microscopy (SEM); poly(aniline-co-pyrrole) random copolymers were well dispersed into the SWCNT bundles and formed a self-assembled layered morphology. The highest PF of 98.5 μW·m−1·k−2 was achieved. In addition, the relationship between the TE properties and the structure was investigated (Scheme 1).
Section snippets
Chemicals
All chemicals used in this work were analytical grade. Aniline, pyrrole and ammonium peroxydisulfate (NH4S2O8, APS) were purchased from Energy Chemical, Shanghai, China. Hydrochloric acid (HCl, 35–38%) was supplied by Sun Chemical Technology Co., Ltd., Shanghai, China. SWNTs (diameter: <3 nm, purity: >95.0 wt%) were received from Nanjing XFNANO Materials Tech Co., Ltd., Nanjing, China. All the reagents were used as received without further purification, and deionized water and anhydrous ethanol
Chemical structure of the synthesized polymers
Partial chain segment conformation, FTIR spectra, Raman spectra and XPS survey spectra were applied in this work to investigate the chemical structures of PANi, PPy and the random copolymer PANiPy.
Recently, with the development of organic semiconductors, understanding the mechanism of charge transport in conjugated polymers has been an important but challenging issue. As reported in the literature [29], [30], [31], the electrical conductivity of conducting polymers depends strongly on the
Conclusions
In conclusion, the PANiPy random copolymer was successfully synthesized via a green method, and no environmentally polluting or harmful solvent was used throughout the experimental section. In terms of chemical engineering, the experimental procedure is attractive as a sustainable process for materials preparation. The poly(aniline-co-pyrrole) random copolymer was successfully synthesized without mechanical agitation, and a strong interaction has formed between the pyrrole and the aniline units
Acknowledgments
We gratefully acknowledge financial support from the National Natural Science Foundation of China (Project No. 51773118), Shenzhen Sci & Tech Bureau (Project No. JCYJ20170818093417096), Science Foundation of Guangdong Province (No. 2017A030310397). Instrumental Analysis Center of Shenzhen University (Xili Campus).
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