Recent Advances in Polyaniline-Based Thermoelectric Composites

Open AccessCCS ChemistryMINI REVIEW1 Oct 2021Recent Advances in Polyaniline-Based Thermoelectric Composites Siqi Liu, Hui Li, Pengcheng Li, Yalong Liu and Chaobin He Siqi Liu Department of Materials Science and Engineering, National University of Singapore, Singapore 117574 , Hui Li *Corresponding authors: E-mail Address: email protected E-mail Address: email protected E-mail Address: email protected Hubei Key Laboratory of Plasma Chemistry and Advanced Materials, Hubei Engineering Technology Research Center of Optoelectronic and New Energy Materials, School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430205 , Pengcheng Li Hubei Key Laboratory of Plasma Chemistry and Advanced Materials, Hubei Engineering Technology Research Center of Optoelectronic and New Energy Materials, School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430205 , Yalong Liu Hubei Key Laboratory of Plasma Chemistry and Advanced Materials, Hubei Engineering Technology Research Center of Optoelectronic and New Energy Materials, School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430205 and Chaobin He *Corresponding authors: E-mail Address: email protected E-mail Address: email protected E-mail Address: email protected Department of Materials Science and Engineering, National University of Singapore, Singapore 117574 Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), Singapore 138634 https://doi.org/10.31635/ccschem.021.202101066 SectionsAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Polyaniline (PANI)-based thermoelectric (TE) materials have attracted considerable attention for flexible electronic energy harvesting devices because of properties such as solution processibility, excellent environmental stability, and low cost. However, the typical opposite dependence between electrical conductivity and Seebeck coefficient impedes their development, and tremendous efforts have been devoted to increasing their TE efficiency. This mini review focuses on key strategies in developing high-performance PANI-based TE materials, including doping engineering (fine-tuning of doping level), modulation of molecular orientation, inorganic nanoparticles incorporation to combine the merits of their components, interface engineering that introduces interfacial interactions and an energy-filtering effect, and other strategies. With properly optimized components, microstructure, and fabrication processing, which aid in the carrier transport properties, the overall performance of PANI-based TE composites has been greatly enhanced, promoting the improvement of PANI-based composites for sustainable energy utilization. Download figure Download PowerPoint Introduction Due to the depletion of fossil fuels and heightened environmental pollution, advanced green energy resources or technologies are urgently required to alleviate the current issues and fulfill the growing energy demand. Energy harvesting from environmental waste heat, such as solar energy and dissipated energy from power consuming systems, is considered as a promising strategy for sustainable energy utilization.1 Thermoelectric (TE) materials, which effectively utilize the surrounding waste thermal energy with direct conversion into useful electrical energy and enable reversible conversion as a cooler without any moving parts and noise pollution, have attracted significant attention and exhibited extensive applications in aerospace, cogeneration, and microsensors.2 Most commercial TE materials are inorganic TE materials, such as Bi2Te3 and PbTe. However, toxic components, manufacturing difficulties, and brittleness impede their widespread applicability, especially for wearable and portable electronic devices.3 In contrast, organic TE materials are now attracting ever-increasing interest, due to their versatile advantages, such as low thermal conductivity, light weight, solution processibility, and excellent flexibility.4,5 So far, numerous efforts have been devoted to pursuing better TE properties of organic TE materials, fulfilling the increasing demands of flexible energy harvesting devices. Generally, the performance of a TE material is quantified by the dimensionless figure of merit, ZT = S2σT/κ, where σ, S, T, and κ represent electrical conductivity, Seebeck coefficient, absolute temperature, and thermal conductivity, respectively. In addition, power factor (PF =S2σ) is also commonly adopted to evaluate the TE efficiency of organic materials.6 Therefore, from the definition of the ZT value, large S and σ, but low κ are favorable for high TE efficiency. However, these parameters are strongly interdependent, and the opposite dependence of S and σ greatly restricts the optimization of TE properties.7 The electrical conductivity (σ = nqμ, where n is the carrier concentration, q is the elementary charge, and μ is the charge carrier mobility) is proportional to n, which is closely related to doping level, and μ which is affected by chemical structure and morphology.8 In contrast, S is defined as: S = k B q ∫ E − E F k B T σ ( E ) σ d E where kB is the Boltzmann constant, E is the carrier-occupied energy level, and EF is Femi energy level, which represents the average transported entropy per charge carrier.9 When the Fermi level is close to the conduction band gap, n increases, which subsequently causes a reduction in transported energy of charge carriers, and thereby the reduction of S.10,11 This competing trend of S and σ complicates the improvement of TE materials and it is therefore crucial to optimize S and σ simultaneously, leading to enhanced TE efficiency. Polyaniline-Based Composites Among the abundant organic TE materials, conducting polymers are regarded as the most high potential candidates for TE applications,12 including polypyrrole (PPy), polyaniline (PANI), polythiophene (PTh), and poly(3,4-ethylenedioxythiophene) (PEDOT). Until now, poly(3-hexylthiophene) (P3HT) has been reported to reach a PF of 20 μW m−1 K−2 via immerse-doping with the ferric salt of triflimide Fe(TFSI)3,13 while the vapor phase-doped P3HT with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) approached a high PF of 27 μW m−1 K−2, accompanied with a larger S over 85 μV K−1.14 Meanwhile, PEDOT with various counter-ions were intensively studied, where PEDOT:poly(styrene sulfonate) (PSS) recorded a high PF over 334 μW m−1 K−2 via sequential treatment of common acid and base solution,15 and the tosyl anion-doped PEDOT (PEDOT:Tos) films demonstrated a maximum PF approaching 324 μW m−1 K−2.16 In comparison with these other conducting polymers, PANI has received significant attention because of the merits of low cost, easy synthesis, and excellent chemical stability. In the early years, PANI was primarily used after chemical oxidative polymerization with HCl doping, which possessed a rather low σ of several S cm−1.17,18 Even combined with highly conductive nanofillers, the σ values of the resulting composites were still undesirable and showed an inferior PF of less than 20 μW m−1 K−2, greatly impeding its application for TE devices.19 Aimed at improving σ and TE performance of PANI-based materials, secondary doping engineering was conducted to optimize the microstructural ordering in polymers, leading to greatly increased σ of 280 S cm−1 and PF of 11 μW m−1 K−2, respectively, for pure PANI film.20–24 After that, a variety of PANI-based composites with nanomaterials possessing high σ or S, such as inorganic TE materials, carbon nanomaterials mainly carbon nanotubes (CNTs) and graphene (GN), and other conducting polymers, have been extensively investigated, and great progress has been achieved using various techniques (Figure 1). By properly optimizing the components, microstructure, and fabrication process, the majority of the reported PANI-based TE composites has steadily improved to higher PF above 200 μW m−1 K−2 in recent years, and a more attractive PF up to 2710 μW m−1 K−2 has been approached with a PANI/GN-PEDOT:PSS/PANI/double-walled carbon nanotube (DWCNT)-PEDOT:PSS multilayer film at room temperature (RT), comparable with the TE performance of commercial inorganic materials.30 Figure 1 | TE PFs of PANI-based composites from 2010 to now. (The inset images have been reproduced with permission from ref 25. Copyright 2010 American Chemical Society; ref 26. Copyright 2015 American Chemical Society; ref 27. Copyright 2018 American Chemical Society; ref 28. Copyright 2019 American Chemical Society; ref 29. Copyright 2021 American Chemical Society.) Download figure Download PowerPoint To date, huge efforts have been devoted to pursuing high-performance PANI-based TE composites, and many effective processing methods have been applied to enhance TE efficiency including doping engineering, modulation of molecular orientation, inorganic nanoparticles incorporation, interface engineering, and other strategies (Figure 2). Specifically, upon properly tuning the doping parameters, such as dopant, solvent, and doping level, the consequently modulated morphology and energy level of PANI, which are strongly correlated to carrier concentration and carrier mobility, result in optimized σ and S, thus high TE efficiency.21,24 Moreover, the σ and PF of the PANI-based TE composites can also be significantly improved by constructing ordered polymer chain microstructure, which effectively decreases the interchain hopping activation energy and contributes to enhanced carrier mobility without sacrificing S.1,37 Besides the optimization of polymer microstructure, incorporating inorganic nanoparticles to fabricate hybrid composites is another straightforward route to pursuing high TE properties by combining the unique properties of inorganic TE nanoparticles that possess a high S and a highly conductive PANI polymer. Besides, strong interchain π–π interactions which lead to better charge transport properties and effective energy-filtering effects that restrict low-energy carriers are introduced by properly engineering PANI-filler interfaces, contributing to simultaneously enhanced σ and S.38 Furthermore, other novel techniques such as the modification of nanofillers and ionic TE effects have also been developed to fabricate flexible PANI-based composites with enhanced TE performance. In this mini review, we will give a brief summary of the primary reports of PANI composites with enhanced TE properties. In each section, the specific strategy and rational design in approaching high TE performance of PANI-based composite materials will be described. Finally, we will highlight the current challenges and perspectives for the improved TE performance and efficiency enhancement of PANI-based composite materials. Figure 2 | Schematic illustration of key strategies to enhance the performance of PANI-based TE composite materials. (The inset images have been reproduced with permission from ref 31. Copyright 2021 Elsevier; ref 32. Copyright 2019 Elsevier; ref 25. Copyright 2010 American Chemical Society; ref 33. Copyright 2021 American Chemical Society; ref 34. Copyright 2021 MDPI; ref 35. Copyright 2018 American Chemical Society; ref 29. Copyright 2021 American Chemical Society; ref 36. Copyright 2018 Royal Society of Chemistry.) Download figure Download PowerPoint Strategies to Optimize the Performance of PANI-Based TE Composites Doping engineering Since acid doping can greatly increase carriers through protonation of the imine nitrogen sites to increase σ several S cm−1, abundant dopants, such as boric acid,39 5-sulfosalicylic acid (SSA),40–42 PSS,28 methane sulfonic acid,43 HCl,44–49 and especially camphorsulfonic acid (CSA) with m-cresol as the solvent, have been utilized to develop various PANI-based TE materials. Although great progress has been achieved for PANI-based composites, their TE efficiency is still far less than commercial organic materials, which greatly impedes their widespread applications in energy harvesting.17 Since the microstructures and energy band of PANI are greatly dependent on doping level, the tuning of the doping degree is expected to be a quite effective strategy in optimizing TE efficiency of PANI composites. Doping level manipulation has been proven to greatly affect the PANI chain conformation and σ. Typically, with increasing doping level, polarons tend to be increased and transferred from localization to delocalization with expanded chain conformation, resulting in rapidly increased σ at the beginning of the doping process. Further increasing the dopant cannot effectively protonate PANI chains or even cause undesirable screening effect to contract the chain with more compact coil conformation, resulting in unchanged or decreased σ with higher doping.17,50–52 However, increasing carrier density leads to deteriorated S as doping level increases. Therefore, maximum σ and TE performance of PANI is mainly achieved with a protonation degree of 50% (mole ratio of PANI:CSA = 2:1), which has been widely applied for the fabrication of PANI-based materials.53 Contrasting with previous reports, Li et al.31,32,54,55 demonstrated the significantly enhanced TE performance of CNTs/PANI composites at low doping levels. As shown in Figures 3a–3d, they investigated the effect of doping level, which had been fine-tuned by decreasing the amount of CSA, on the TE properties of single-walled CNTs (SWCNTs)/PANI composite films.31,54 Ascribed to the strong π–π interactions promoting the formation of a unique PANI interface layer and the existence of a CNT-interconnected network which provides conductive pathways to enable efficient carrier transportation, σ was decreased less and accompanied with a greatly increased S as doping level decreased, leading to a high PF value of 321 μW m−1 K−2 at a PANI/CSA mole ratios of 3:1 with CNTs loading of 69 wt %. Furthermore, amine-functionalized CNT (A-CNTs)/PANI composites also exhibited greatly enhanced PF of 401 μW m−1 K−2 with a properly reduced doping level. Subsequently, they reported greatly enhanced TE efficiency of typically unmodified SWCNTs/PANI films by immersing in ammonium hydroxide and ethanol, subsequently, to partially remove CSA and control doping level.32,55 With the dedoping process, conductive, ordered, and protonated PANI chains were transferred to an insulating, disordered PANI emeraldine base (PANIeb) that reduced carrier density, while the interconnected SWCNTs networks would compensate the deteriorating effects of PANI chains on carrier transport, leading to greatly enhanced S accompanied with a slightly decreased σ. Based on these investigations, the TE efficiency of PANI-based composites is expected to be further enhanced by properly modulating the doping level, as most of them were prepared with a traditional doping level of 50%. Figure 3 | (a) Schematic images and (b) TE properties of SWCNTs/PANI composite films with varying molar ratios of PANI/CSA. Reproduced with permission from ref 31. Copyright 2021 Elsevier. (c) Schematic representation of carrier transport and (d) TE properties of SWCNTs/PANI composites after dedoping. Reproduced with permission from ref 32. Copyright 2019 Elsevier. Download figure Download PowerPoint Molecular self-assembly Along with doping engineering, molecular self-assembly is another important route to enhance TE efficiency of PANI composites, which efficiently improves the carrier mobility and σ without deteriorating S. So far, several strategies have been explored to enhance molecular orientation of PANI, such as the secondary doping of PANI, the introduction of strong interactions between PANI and fillers, electrospinning process, mechanical stretching, and so on. Many studies have focused on secondary doping engineering to promote electron delocalization along the chains and enable good dissolution and processing of PANI in a variety of common organic solvents, contributing to improved TE efficiency and enhanced processibility.56 Since Cao et al.20,57,58 reported that functionalized protonic acid, especially CSA, can effectively protonate PANIeb to give doped PANI solutions in weakly or nonpolar organic solvents, and induce significant enhancement in σ within the range of 100–400 S cm−1, secondary doping engineering has received great attention for the processing of PANI composites. MacDiarmid et al.22,50,59 studied the viscosity, electronic spectra, and σ of PANI doped with CSA in varied solutions of m-cresol and chloroform. It was shown that PANI chain conformation changed from compact to expanded coil with enhanced electron delocalization along the polymer chains and σ approaching 200 S cm−1 was achieved as the m-cresol proportion increased, ascribed to strong interactions between polymer chains and m-cresol and hydrogen bonding with the carbonyl group from CSA and hydroxyl group in m-cresol. Furthermore, vapor exposure of m-cresol also introduced an effective secondary doping effect on PANI.23 Recently, Yao et al.24,60 revealed increased carrier mobility and a decreased hopping barrier, accompanying the increased PANI-ordered regions as the m-cresol proportion increased. This led to remarkably improved σ from 4.7 to 220 S cm−1 with a slight increase of S when m-cresol increased from 0% to 100%, and thereby a high PF of 11 μW m−1 K−2 was achieved, 60 times higher than that from chloroform. In recent years, with the merits of solution processing and enhanced σ via secondary doping engineering, PANI hybrid composites have experienced rapid development and promising results have been reported.61–63 Yao et al.64 prepared SWCNTs/PANI hybrid films by solution mixing the two components. Chemical interactions between PANI and m-cresol induce PANI chain transition of compact-to-expand in the solution process, which was then further oriented to form a highly ordered PANI interface layer on the surface of SWCNTs ascribing to strong π–π interactions between PANI and CNTs. The enhanced and ordered polymer chains benefit increased carrier mobility, resulting in improved σ and S. Consequently, the σ of SWCNTs/PANI hybrid composites was gradually increased with the increment of SWCNTs content, reaching a maximum PF of 176 μW m−1 K−2 with 64 wt % SWCNTs. Similar behavior has also been observed for SWCNTs/PANI composites prepared by in situ polymerization which induced stronger interactions between the two constituents.63 In addition, Wang et al.26 reported a series of DWCNTs/PANI composite films by mixing the CNTs dispersion with PANIeb or PANI-CSA (Figures 4a and 4b). The conducting polymer can effectively bridge nanotubes and decrease electrical contact resistance between CNTs, enabling good charge transport at CNTs junctions and leading to superior σ. Moreover, with the addition of DWCNTs, largely increased carrier mobility and decreased carrier concentration was observed, contributing to the simultaneous improvement in σ (610 S cm−1) and S (61 μV K−1). Thus, PF reached 220 μW m−1 K−2 with CNTs amount of 30 wt %, one of the highest values among previously reported PANI composites. Figure 4 | (a) σ, S, and (b) PF of DWCNTs/PANI composite films with varied CNTs content, where the inset presents the crucial role of PANI-CSA to connect CNTs junctions. Reproduced with permission from ref 26. Copyright 2015 American Chemical Society. (c) TEM images of 25 wt % SWNT/PANI composites. (d) TE properties of SWNT/PANI composites with varying SWNT content. Reproduced with permission from ref 25. Copyright 2010 American Chemical Society. (e) Schematic illustration of the synthesis process of PPy/GNs/PANI composites. (f) PF of PPy/GNs/PANI composites with 32 wt % GNs loading at different temperatures. Reproduced with permission from ref 65. Copyright 2017 American Chemical Society. (g) Schematic illustration of DMSO-induced, greatly enhanced, and ordered PANI nanostructures along SWCNT interfaces by electrochemical polymerization methods. Reproduced with permission from ref 33. Copyright 2021 American Chemical Society. Download figure Download PowerPoint Yao et al.25 prepared HCl-doped SWCNTs/PANI via in situ polymerization with dispersed CNTs and aniline monomers, in which the PANI chains were grown along the outer surface of SWCNTs to form ordered chain packing because of π–π interactions between conjugated SWCNTs and PANI, leading to enhanced σ of 125 S cm−1, S of 40 μV K−1, and a resultant PF over 20 μW m−1 K−2 with SWCNTs content of 41.4 wt % (Figures 4c and 4d). Abad et al.66 prepared GN nanoplatelet (GNPs)/PANI composite pellets by mechanical blending and cold pressing techniques. Due to the addition of highly conductive GNPs and the strong interactions between PANI and GNPs, which promotes the increment of PANI quinoid rings structure and facilitates charge transport, σ was increased with the addition of GNPs, in accordance with the percolation law from 0.48 S cm−1 of raw PANI to 123 S cm−1 of 50 wt % GNPs/PANI composite. Combined with the increased S, a maximum PF reaching 14 μW m−1 K−2 was obtained. Recently, Wang et al.65 proposed an efficient strategy by solution mixing of PANI and PPy/GNs composite to fabricate a PPy/GNs/PANI ternary nanocomposite with improved TE efficiency (Figures 4e and 4f). With strong π–π interaction and hydrogen bonding between PANI and PPy, PANI molecules tend to be more expanded and ordered stacking, which can efficiently promote carrier transport and enhance carrier mobility, while the additional barriers may generate energy-filtering and quantum-confinement effects, which can reduce carrier concentration in the composites. Consequently, optimized σ, S, and PF values of 500 S cm−1, 32.4 μV K−1, and 52.5 μW m−1 K−2, respectively, were achieved with 32 wt % GNs at 90 °C, higher than GNs/PANI and GNs/PPy. Similar enhancement was also found in in situ-polymerized PANI/GN composite films, where the π–π interactions between in situ-synthesized PANI and GN led to the PANI conformation changing from coil to a more expanded structure. Meanwhile, the improved σ was also aided by the greatly improved dispersion homogeneity of GNPs inside the PANI matrix during the in situ polymerization process.67 Furthermore, with the assistance of the electrospinning process, Wang et al.68 reported electrospun CNT/PANI nanofibers with more oriented PANI chains along the CNTs axis. The ordered molecule alignment contributed to the 80% higher σ in the orientation to the S was Recently, et reported that with the of in the during the electrochemical polymerization of PANI, greatly enhanced ordered molecular of PANI were introduced (Figure Therefore, higher σ and a on S of composites were at the SWCNT with the composites, leading to an enhanced PF over μW m−1 Moreover, more ordered can be by mechanical stretching, which polymer chain alignment to the leading to enhanced As for PANI films, a high σ of S cm−1 be achieved by to at The more and ordered polymer chains by were also found by et in the multilayer composite results showed that as the the PANI and PEDOT chains tend to be by on a 20 PANI-CSA Thus, the interactions between the were which facilitates the carrier transport and leads to increased σ with in S. incorporation to the PF of PANI-based composites is by inorganic nanoparticles that possess high σ or S while the merits of organic PANI inorganic materials have been as into the PANI including and and so on. the one to an enhanced σ, et into the PANI With loading over %, the σ of the PANI-based composites was enhanced by several of over 1 S Although the S decreased as loading increases, the optimized PF of the composites increased by of from μW m−1 K−2 of PANI to 3 μW m−1 K−2 with % In contrast, the S of pure PANI was reported to increase with the incorporation of high S inorganic Wang et reported the of with PANI layer was observed to be to the surface by a of and electronic interactions between and PANI conjugated The improved carrier transport properties by the interface and high S of leads to an enhanced PF of μW m−1 K−2 at and μW m−1 K−2 at Moreover, ascribed to the efficient between interfaces, the composite exhibited a rather low κ of to a high ZT reaching at from nanofillers, the incorporation of other inorganic nanoparticles and with PANI have been reported by et PANI has also been on (Figure and the in S and σ can be by the (Figures and As a after the incorporation of high S the film exhibited a maximum PF of μW m−1 K−2 at Besides, et acid for flexible TE applications (Figures and where different of PANI on the surface of affected the TE properties of the composites. maximum PF was reported to μW m−1 K−2 with two of PANI (Figure as the increased content of PANI led to a higher σ the S. To good for flexible TE was into the matrix and the film exhibited a high PF of μW m−1 K−2 at Moreover, inorganic TE have also been used to with Wang et composite films TE The hybrid films showed a transition as the treatment temperature increased to ascribed to the rapid reduction in the concentration of carriers from PANI after treatment and the electronic carriers from gradually their The transition temperature is correlated to the and a higher concentration leads to a transition By varying the treatment temperature and content, a S of μV with a PF of μW m−1 K−2 was achieved at Figure | (a) electron of composites. (b) Seebeck coefficient and (c) electrical conductivity of composite films as a of content. Reproduced with permission from ref 34. Copyright 2021 (d) electron images of with (e) PF of composites at with increasing PANI (f) PF value of a film with other reported materials. Reproduced with permission from ref 35. Copyright 2018 American Chemical Society. Download figure Download PowerPoint engineering Aimed at high TE efficiency of PANI-based composite