Engineering doping level for enhanced thermoelectric performance of carbon nanotubes/polyaniline composites
Graphical abstract
Introduction
With increasing amount of exhausted heat dissipated to the environment as waste energy, efficient energy harvesting technologies are urgently demanded for sustainable and renewable energy development. Thermoelectric (TE) materials which can harvest the waste heat from surrounding environment and directly transform the heat energy into electricity, have attracted extensive interest as a promising strategy for green power generation [[1], [2], [3], [4]]. Typically, the energy conversion efficiency of TE materials is evaluated by a dimensionless figure of merit: ZT = S2σT/κ, where S represents Seebeck coefficient, σ represents electrical conductivity, T is absolute temperature, and κ is thermal conductivity [5,6]. In addition, with low κ for organic TE materials, the TE performance can also be assessed by power factor (PF = S2σ). Therefore, both high σ and S are required to pursuit outstanding TE properties. However, the inverse proportion of σ and S severely restricts the optimization of TE performance, and thereby great efforts are required to synergically improve S and σ [7,8].
In the last decades, a variety of TE materials have been investigated, and organic conducting polymers have drawn particular interest because of the versatile merits of lightweight, low cost, intrinsic low thermal conductivity, and mechanical flexibility, which enable potential applications especially for wearable and portable electronics on complex geometry heat surfaces, such as human skin [[9], [10], [11], [12], [13]]. Among them, with the advantages of facile synthesis and environmental stability, polyaniline (PANI) has attracted widespread attention and tremendous efforts have been made to improve its TE efficiency [[14], [15], [16]]. To date, secondary doping engineering via the camphorsulfonic acid (CSA)/m-cresol solution processing, which promotes ordered stacking of PANI moleculars, has been exploited as one of the most efficient routes to improve TE performance of PANI films [[17], [18], [19], [20]]. With increasing of doping level, σ is greatly improved accompanied by decreased S ascribing to the increasing of carrier concentration. Consequently, a maximum PF of 11 μW m−1 K−2 could be achieved with typical optimal doping level of PANI/CSA = 2:1 (mole ratio) [19,21].
In addition, incorporation of carbon nanomaterials, especially carbon nanotubes (CNTs) which possess unique electronic and mechanical properties, to construct CNTs/conducting polymer hybrid composites, has become another promising effective strategy to achieve superior TE efficiency by integrating the merits of the individual constitute and introducing synergistic effects [[22], [23], [24], [25], [26], [27], [28]]. Wang et al. demonstrated enhanced TE performance of single-walled CNTs (SWNTs)/PANI composite films with high power factor of 217 μW m−1 K−2, higher than individual PANI and SWNTs components [29]. Although both S and σ were improved synergistically ascribing to the strong π-π interactions and energy filtering effect on the interface of CNTs and PANI, most of the reported CNTs/PANI composite films were fabricated with fixed amount of CSA (mole ratio of PANI/CSA is 2:1), and there is few reports to emphasize the importance of doping level on the TE efficiency of CNTs/PANI composites [[29], [30], [31]]. Considering that the intrinsic high conductivity of CNTs network can enable good charge transport, the carrier conduction of CNTs/PANI composites would not solely depend on the conductivity of polymer matrices that are strongly correlated with their doping level, and thereby the TE properties of CNTs/PANI composites would be distinctive compared with typical pure PANI films upon doping process. Recently, our group fabricated amine-functionalized carbon nanotubes (A-CNT)/PANI composite films, which exhibited optimal TE performance at low doping degree of PANI [32]. Despite this result demonstrated the crucial role of doping level in enhancing TE performance of CNTs/PANI composites, the amine group on the surface of CNTs would be covalently linked with aniline, which cause complicated carrier conduction [33]. Furthermore, few work about morphology and structure changes of the composites upon decreasing doping level was investigated to elucidate the enhanced TE efficiency.
Herein, typical unmodified single-walled CNTs were utilized to synthesize a series of CNTs/PANI composite with varied CNTs contents, and then their TE performance was investigated upon doping degrees modulation. For comparison, pure PANI films were also studied, which exhibited apparent structure transition and inferior TE efficiency with decreasing doping level. Moreover, the microstructure of the composite films was characterized by TEM, Raman, FTIR, and UV–Vis analysis. Upon decreasing doping degree of PANI, S was enhanced accompanied by decreased σ. Nevertheless, it is worth noting that rather less decrease of σ was observed with higher amount of CNTs in the composites, suggesting the contribution of CNTs network for the efficient carrier transport. With properly modulating the doping level of PANI, optimal TE properties of CNTs/PANI composite films were achieved with low doping degree, and the optimum doping level was greatly dependent on the CNTs content in the composites.
Section snippets
Materials
Single-walled carbon nanotubes (CNTs) (purity: >95%, outer diameter around 1–2 nm, length about 5–30 μm) were received from the Timesnano company, Chengdu, China. Aniline, ammonium peroxidisulfate (APS), ammonium hydroxide, hydrochloric acid (HCl), and camphorsulfonic acid (CSA) were obtained from Sinopharm Chemical Reagent Co. Ltd.
Sample preparation
The CNTs/PANI composites were obtained via in-situ polymerization of aniline in the presence of CNTs. First, CNTs was dispersed in 1 M HCl with sonication. A
Microstructure of CNTs/PANI composites
The morphology of the CNTs/PANI composites with varied CNTs contents were investigated by TEM measurement and shown in Fig. 1. Ascribing to strong π-π conjugation interactions, CNTs bundles serve as template and induce the PANI chains stacking along the surface of CNTs, and thereby forming a typical core-shell structure with PANI layer wrapped around CNTs [35,36]. Furthermore, the thickness of PANI layer depends on the constituent proportion. With an increase amount of CNTs, the thickness of
Conclusions
In this work, a series of CNTs/PANI composites with varied CNTs loadings were fabricated and doped with various amount of CSA to modulate doping level. With decreasing doping level, structure change of PANI chains from benzenoid units to quinoid, as well as state change of polarons from delocalization in expanded coil structure to localization in compact coil were observed from FTIR, Raman, and UV–Vis analysis, resulting in increased Seebeck coefficient accompanied by deteriorate electrical
CRediT authorship contribution statement
Hui Li: Conceptualization, Formal analysis, Investigation, Writing – original draft. Yuan Liang: Investigation, Formal analysis. Yalong Liu: Investigation. Siqi Liu: Formal analysis. Pengcheng Li: Conceptualization, Formal analysis, Data curation, Writing – review & editing. Chaobin He: Supervision, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 51803156), Hubei Provincial Natural Science Foundation of China (2019CFB190), Youths Science Foundation of Wuhan Institute of Technology (No. K201803), Scientific Research Program of Hubei Provincial Department of Education (No. B2020055), and the Postgraduate education innovation fund of Wuhan Institute of Technology (No.CX2019077).
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Hui Li and Yuan Liang contribute equal to this work.