# Compositing Nanostructured Polyaniline with Single-Walled Carbon Nanotubes for High Thermoelectric Performance

Authors: Qunyi Luo; Xin Wu; Erqiang Wang; Cun-Yue Guo
Journal: International Journal of Energy Research
 (2023)

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## Abstract

In the face of the growing energy crisis and demand on environmental protection, organic thermoelectric (TE) materials have been coming under the spotlight from global researchers as a potential countermeasure. However, there have been few studies on the relationship between the morphology of conductive polymers (CPs) and their TE performance. In this work, nanostructured polyaniline (PANI) was synthesized by interfacial polymerization with the addition of organic solvents of different polarities, including cyclohexane (CYH), carbon tetrachloride (CCl4), toluene (MB), dichloromethane (DCM), and dimethyl sulfoxide (DMSO). Physical mixing PANI with single-walled carbon nanotubes (SWCNTs) affords PANI/SWCNT composites. The composite of PANI nanobelts and 10 wt% SWCNTs achieves a power factor (PF) of 5.13 ± 0.56 μW m-1 K-2 at room temperature (RT). Spherical PANI doped with 50 wt% SWCNTs followed by pressing affords maximal room temperature PF value of 178.10±3.77 μW m-1 K-2. A prototype TE generator (TEG) constructed with seven pairs of legs of as-prepared PANI/SWCNT composites and their n-type counterparts generates voltage of 10.27±0.15 mV and output power of 36.65±1.18 nW at temperature difference of about 50°C. This work illustrates that well-regulated PANI nanostructure could significantly affect the TE properties of PANI/SWCNT composites, which are promising in future preparation of high-performance polymer-based composites regarding the fabrication of TE devices suitable for harnessing low-grade waste heat.


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## Body

## 1. Introduction

Thermoelectric (TE) material is a new functional material that uses solid internal carrier migration to achieve mutual conversion of thermal and electrical energy, with the advantages of small size, lightweight, no noise, no pollution, and long service life, effectively improving energy utilization efficiency [1, 2]. As a promising green material, TE materials have potential applications, not only in temperature control but also uses in power generation from temperature difference, automobile exhaust, and industrial waste heat in addition to TE cooling [3]. TE materials can be broadly classified into conventional inorganic TE materials and organic TE materials. Inorganic TE material, represented by Bi2Te3, PbTe, and SnSe, which have been extensively applied in numerous scenarios [2]. Recently, Suriakarthick et al. used the solvothermal method to synthesize polycrystalline Cu2SnSe3 (CTSe), and this nanocrystal is an excellent candidate for the development of low-cost thermoelectric devices [4]. However, what cannot be ignored is the limitations of the application of inorganic TE materials; in most cases, they exhibit excellent TE performance only at high temperature. In addition, their currently complex and expensive process properties make them difficult in large scale energy utilization [5, 6]. As promising new materials, organic TE materials, such as poly(3,4-ethylenedioxythiophene) (PEDOT), polyaniline (PANI), and polypyrrole (PPy), have been considered as other feasible solutions in the utilization low-grade heat [7].As one of the classic conducting polymers, PANI has emerged as an organic TE material with excellent stability, chemical redox reversibility, and a unique doping/dedoping process [8–11]. In the 1980s-1990s, researches on TE properties based on PANI were widely carried out, but the performance is at a low level at that time [12–14]. Due to the nonnegligible TE performance defects compared to traditional TE materials, hybrid organic-inorganic materials have become an emerging research trend [15]. It is worth noting that a novel class of nanomaterials based on carbon nanotubes (CNTs) has rapidly developed since 1991 due to its unique mechanical, electrical, and excellent electron transport capabilities [16]. Among these, PANI/single-walled carbon nanotubes (SWCNTs) composites can be widely used in devices in the energy field. In 2006, Gupta and Miura used electrochemical polymerization to prepare PANI onto SWCNTs; the electrode of PANI/SWCNT composites showed much higher specific capacitance than pure PANI and SWCNTs, attaining 485 F/g in the situation of 73 wt% PANI deposited onto SWCNTs [17]. Subsequently, Liu et al. fabricated flexible PANI/SWCNT composite films with enhanced TE properties by employing in situ electrochemical polymerization process, enabling their use in various flexible energy devices in the future [18]. In addition, Feng et al. applied N,N-dimethylformamide (DMF) to make PANI uniformly wrapped in SWCNTs to improve their interfacial morphology, followed by pressing treatment to enhance the interfacial adhesion between two substances. At room temperature (RT), the power factor (PF) could reach 114.4 μW m-1 K-2; meanwhile, the maximum PF value could be 402 μW m-1 K-2 at 410 K [9]. Furthermore, several researchers noticed the effect of PANI nanostructure on TE properties. Sun et al. synthesized PANI nanotubes (PANI NT) doped by a β-naphthalene sulfonic acid (NSA), setting a sample without a specific nanostructure as a reference. The Seebeck coefficient (S) for the PANI NT produced with the 4 : 1 ANI/NSA ratio was 212.4 μV K-1 at 300 K, seven times more than that of the reference sample. Meanwhile, electrical conductivity increased from 0.0045 to 0.0077 S cm-1, nearly doubling [19]. Wu et al. also prepared PANI nanostructures by doping different acids; the findings demonstrated that doping level, doping acid types, and as-prepared nanostructure morphologies significantly impacted TE characteristics [20].In this work, PANI salts with significant morphological differences were mainly prepared by interfacial polymerization. Compared to traditional polymerization, interfacial polymerization, where the chemical reaction occurs at the liquid-liquid or liquid-air interface, shows fascinating prospects in the controllable fabrication of films, capsules, and fibers [21]. In 2003, Huang et al. synthesized PANI nanofibers by aqueous/organic interfacial polymerization. Nevertheless, they judged that the fibrous shape of PANI produced by interfacial polymerization was not significantly affected by the polarity of the organic phase [22]. After that, due to interfacial nucleation and aqueous growth being presented in the process of ANI polymerization [23], He and Lu adjusted the relative rate in the above polymerization methods to obtain PANI nanostructures with transitional morphologies from three key variables, the polarity of the organic phase, the amount of ammonium persulfate (APS), and the concentration of acetic acid [24]. However, the mechanism details have not been addressed due to the lack of comparison components and characterization analysis within each critical factor. Furthermore, there have been few studies on the connection between the different morphology of conductive polymers and their TE performance. Building on the work of previous researchers, we prepared PANI with varying morphologies by using five organic solvents, including cyclohexane (CYH), carbon tetrachloride (CCl4), toluene (MB), dichloromethane (DCM), and dimethyl sulfoxide (DMSO); with significantly polarity differences (Table S1), the polarity of organic layer affected the distribution of ANI concentration in aqueous and organic phases, which in turn affected the rate of polymerization occurring at the two-phase interface. Among the five solvents, DMSO has excellent solubility in the aqueous phase and is used as the comparison component. In addition, we set the control component with the only aqueous solution to verify the effect of the organic phase on the nanostructure. Flexible PANI/SWCNT hybrid films were prepared by ultrasonic dispersion of PANI and SWCNTs, and the interfacial interaction (π-π conjugated interaction) between these two substances not only contributes to the reconfiguration of composite materials but also improves TE properties by compensating the deficiency of PANI in electrical conductivity [9, 25]. With 90 wt% PANI nanobelts, the composite achieves a PF of 5.13±0.56 μW m-1 K-2 at room temperature. The formation of nanobelts helps to provide a smoother “path” for carrier transport, while the mobility is also significantly enhanced [19]. By varying the content of SWCNTs, the highest PF reaches 70.08±3.12 μW m-1 K-2 at RT attributed to spherical PANI doped with 50 wt% SWCNTs. The PF can be further elevated to 178.10±3.77 μW m-1 K-2 after pressing at 20 MPa for 3 min due to lower film thickness and enhanced interfacial adhesion. After that, n-type PANI/SWCNT composite films were synthesized by adding sodium dodecyl sulfate (SDS) and polyethyleneimine (PEI) to fabricate a TEG with seven p-n junctions, and the voltage is 3.6±0.2 mV, output power is 4.17±0.32 nW at ΔT≈15°C (the temperature difference between human skin and ambience) [26, 27].


## 2. Materials and Methods

### 2.1. Chemicals and Materials

Aniline (ANI, AR, ≥99.5%), cyclohexane (CYH, AR, 99.5%), dimethyl sulfoxide (DMSO, AR), acetate (AcOH, AR, 99.5%),N,N-dimethylformamide (DMF, AR, 99.5%), carbon tetrachloride (CCl4, for HPLC, ≥99.0%), sodium dodecyl sulfate (SDS, AR, 92.5-100.5%), and ammonium persulfate (APS, AR,98.5%) were purchased from Shanghai Macklin Biochemical Co., Ltd. Toluene (MB, AR) and dichloromethane (DCM, AR) were provided by Sinopharm Chemical Reagent Co., Ltd. Ethylene imine polymer (PEI) was bought from Shanghai Aladdin Biochemical Technology Co., Ltd. Single-walled carbon nanotubes (SWCNTs, diameter: 1-2 nm, purity: >90%) were obtained from Shenzhen Nanotech Port Co., Ltd. Load resistances (R=1 kΩ) were bought from Shenzhen Risym Co., Ltd.


### 2.2. Preparation of PANI with Different Nanostructures

The synthesis process of PANI powders is shown in Scheme1. Firstly, 3 mmol of APS was added into 25 mL of AcOH solution (1 M) to prepare the aqueous phase with continuous stirring. After 10 min, 5 mmol of ANI well dispersed in 15 mL CYH was dropped (2-3 s per drop) into the aqueous phase in the stationary state. The interfacial polymerization proceeded at RT for 15 h. Next, the PANI particles were filtered (pore diameter: 0.22 mm) and washed several times with deionized water and ethanol, followed by vacuum drying at 60°C. In the end, products were finely ground to prepare the PANI powder. As for the synthesis of PANI with different nanostructures, the steps were the same as above, except for replacing CYH with CCl4, MB, DCM, or DMSO.Scheme 1
The preparation route of PANI/SWCNT composite films combined by pressure method (take interfacial polymerization as an example).


### 2.3. Preparation of PANI/SWCNT Composite Films

At first, a certain amount of SWCNTs was added into 15 mL ethanol (EtOH), performing ultrasonic dispersion (about 90 W) for 40 min (the mode of working for 5 s, then resting for 5 s) at RT. Then, PANI powders were mixed with dispersed SWCNTs in different weight fractions (10, 30, 50, 70, and 90 wt%), spreading (about 15 W) for 20 min. The following procedures for vacuum filtering, lavation, and drying were similar to the previous steps of preparing PANI powders.


### 2.4. Follow-Up Treatment on PANI/SWCNT Composite Films

Before executing the “cold-pressing” method [9], we investigated the critical factor of pressure and pressing time by control variate method. As a result, the optimum pressing treatment conditions are 20 MPa, 180 s. Then, to prepare the n-type composite films, p-type films (synthesized in section 2.3), 0.5 g of SDS, and 0.5 g of PEI were added to 15 mL of EtOH with a similar operation of ultrasonic dispersion. Subsequently, the products were filtered by nylon membranes, washed, and dried in a vacuum atmosphere of 60°C. At last, these two composites were assembled successively on polyimide (PI) substrate and connected each junction with copper foil and conductive silver glue. The final TEG is shown in Figure S5.


### 2.5. Characterization

The morphologies were observed by scanning electron microscope (SEM, Hitachi SU8220) and transmission electron microscope (TEM, Hitachi HT7700Ex). And the energy dispersive spectroscopy (EDS) mapping picture was obtained with SEM. Furthermore, chemical structure and covalent states were gained by Raman spectra (Renishaw 532 nm-in Via) and Fourier transform infrared (FT-IR) spectrometer (Bruker VERTEX 70). Elemental composition and chemical state analyses were received from X-ray photoelectron spectra (XPS, Kratos, Analytical Axis Supra). X-ray diffraction (XRD, SmartLab) was employed in exploring the degree of order and crystal structure. Thermal stability was observed by thermogravimetric analysis (TGA, TA SDTQ 600). Finally, the thin film TE test system (MRS-3RT, Wuhan Joule Yacht Science and Technology Co., Ltd.) measured the TE parameters (electrical conductivities and the Seebeck coefficient). The output voltage was gained by a multifunction digital multimeter (ZTY0103A, Zhejiang Chint Electrics Co., Ltd.).


## 2.1. Chemicals and Materials

Aniline (ANI, AR, ≥99.5%), cyclohexane (CYH, AR, 99.5%), dimethyl sulfoxide (DMSO, AR), acetate (AcOH, AR, 99.5%),N,N-dimethylformamide (DMF, AR, 99.5%), carbon tetrachloride (CCl4, for HPLC, ≥99.0%), sodium dodecyl sulfate (SDS, AR, 92.5-100.5%), and ammonium persulfate (APS, AR,98.5%) were purchased from Shanghai Macklin Biochemical Co., Ltd. Toluene (MB, AR) and dichloromethane (DCM, AR) were provided by Sinopharm Chemical Reagent Co., Ltd. Ethylene imine polymer (PEI) was bought from Shanghai Aladdin Biochemical Technology Co., Ltd. Single-walled carbon nanotubes (SWCNTs, diameter: 1-2 nm, purity: >90%) were obtained from Shenzhen Nanotech Port Co., Ltd. Load resistances (R=1 kΩ) were bought from Shenzhen Risym Co., Ltd.


## 2.2. Preparation of PANI with Different Nanostructures

The synthesis process of PANI powders is shown in Scheme1. Firstly, 3 mmol of APS was added into 25 mL of AcOH solution (1 M) to prepare the aqueous phase with continuous stirring. After 10 min, 5 mmol of ANI well dispersed in 15 mL CYH was dropped (2-3 s per drop) into the aqueous phase in the stationary state. The interfacial polymerization proceeded at RT for 15 h. Next, the PANI particles were filtered (pore diameter: 0.22 mm) and washed several times with deionized water and ethanol, followed by vacuum drying at 60°C. In the end, products were finely ground to prepare the PANI powder. As for the synthesis of PANI with different nanostructures, the steps were the same as above, except for replacing CYH with CCl4, MB, DCM, or DMSO.Scheme 1
The preparation route of PANI/SWCNT composite films combined by pressure method (take interfacial polymerization as an example).


## 2.3. Preparation of PANI/SWCNT Composite Films

At first, a certain amount of SWCNTs was added into 15 mL ethanol (EtOH), performing ultrasonic dispersion (about 90 W) for 40 min (the mode of working for 5 s, then resting for 5 s) at RT. Then, PANI powders were mixed with dispersed SWCNTs in different weight fractions (10, 30, 50, 70, and 90 wt%), spreading (about 15 W) for 20 min. The following procedures for vacuum filtering, lavation, and drying were similar to the previous steps of preparing PANI powders.


## 2.4. Follow-Up Treatment on PANI/SWCNT Composite Films

Before executing the “cold-pressing” method [9], we investigated the critical factor of pressure and pressing time by control variate method. As a result, the optimum pressing treatment conditions are 20 MPa, 180 s. Then, to prepare the n-type composite films, p-type films (synthesized in section 2.3), 0.5 g of SDS, and 0.5 g of PEI were added to 15 mL of EtOH with a similar operation of ultrasonic dispersion. Subsequently, the products were filtered by nylon membranes, washed, and dried in a vacuum atmosphere of 60°C. At last, these two composites were assembled successively on polyimide (PI) substrate and connected each junction with copper foil and conductive silver glue. The final TEG is shown in Figure S5.


## 2.5. Characterization

The morphologies were observed by scanning electron microscope (SEM, Hitachi SU8220) and transmission electron microscope (TEM, Hitachi HT7700Ex). And the energy dispersive spectroscopy (EDS) mapping picture was obtained with SEM. Furthermore, chemical structure and covalent states were gained by Raman spectra (Renishaw 532 nm-in Via) and Fourier transform infrared (FT-IR) spectrometer (Bruker VERTEX 70). Elemental composition and chemical state analyses were received from X-ray photoelectron spectra (XPS, Kratos, Analytical Axis Supra). X-ray diffraction (XRD, SmartLab) was employed in exploring the degree of order and crystal structure. Thermal stability was observed by thermogravimetric analysis (TGA, TA SDTQ 600). Finally, the thin film TE test system (MRS-3RT, Wuhan Joule Yacht Science and Technology Co., Ltd.) measured the TE parameters (electrical conductivities and the Seebeck coefficient). The output voltage was gained by a multifunction digital multimeter (ZTY0103A, Zhejiang Chint Electrics Co., Ltd.).


## 3. Results and Discussion

### 3.1. Morphology and Structure of PANI

To distinguish between the PANI powers prepared from different organic solvents, we named these products according to the abbreviations of the organic phases, which were called PANI-CK (blank control component), PANI-CYH, PANI-CCl4, PANI-MB, PANI-DCM, and PANI-DMSO. As demonstrated in Figure 1, the morphological differences between PANI powders are apparent. In the blank control group (Figures 1(a) and 1(a’)), PANI-CK is composed of nanoparticles (NPs) and can be considered a “barbed” structure. With CYH being added to act as an organic layer, PANI-CYH is formed into nanotubes intertwined with each other (Figures 1(b) and 1(b’)). The outer diameter of PANI-CYH nanotubes is about 200 nm, and the inner diameter takes up about one-third of the width. According to Figures 1(c) and 1(c’), when CYH is replaced with CCl4, PANI-CCl4 turns to be nanobelts, accompanied by a small number of doped nanotubes. Subsequently, using MB as an organic phase, the micromorphologies transform from nanobelts to nanowires (Figures 1(d) and 1(d’)), roughly the same width as nanotubes of PANI-CYH. With the enhancement of solvent polarity, the NPs of PANI-DCM (Figures 1(e) and 1(e’)) are tightly bound together to form nanoclusters. Compared with PANI-CK, the nucleation effect of PANI-DCM NPs is more significant, verifying the remarkable effect of polar solvents on nanostructures. As one of the most polar solvents, DMSO, which promotes orderly alignment of polymer chains and highly extended polymer conformation [25], was easily mixed with AcOH aqueous solution. As shown in Figures 1(f) and 1(f’), PANI-DMSO presents a closed nanosphere-like structure, and the diameter of the nanospheres is about 10 μm.Figure 1
SEM images in low magnification mode of (a) PANI-CK, (b) PANI-CYH, (c) PANI-CCl4, (d) PANI-MB, (e) PANI-DCM, and (f) PANI-DMSO. SEM images in high magnification mode of (a’) PANI-CK, (b’) PANI-CYH, (c’) PANI-CCl4, (d’) PANI-MB, (e’) PANI-DCM, and (f’) PANI-DMSO.
(a)(b)(c)(d)(e)(f)To observe the compositions of PANI under a more microscopic perspective, PANI powders were dissolved in DMF using ultrasonic dispersion for 30 min, and the final images taken by TEM are displayed in FigureS1. Each type of PANI is constructed by NPs with a diameter of approximately 20 nm as shown in Figure S1a. Obviously, by adding solvent to the organic phase and changing the polarity, the polymerization process and state of NPs can be efficiently modified, thus dramatically influencing the nanostructure of PANI. Meanwhile, the theories of interfacial nucleation and aqueous growth can be confirmed from the noteworthy dissimilitude in SEM and TEM images. The formation of PANI NPs takes place in interfacial nucleation on account of the minimum size; then, PANI NPs act as nucleation centers for further growth of polymerization products. As the polarity of organic solvents increases (the polarity of the selected organic solvents is displayed in Table S1), the concentration distribution of primary products in aqueous and organic phases will be profoundly affected, impacting the further growth of PANI NPs generated in the aqueous phase. In the blank control group, the assemblage of NPs is highly dispersed, and a part of NPs can be detected (Figure 1(a’)), which is attributed to the restricted growth in the absence of organic solvents. Besides, the relative rates of polymerization reactions are recorded in Figure S2. Using single-phase polymerization, PANI-CK and PANI-DMSO were rapidly synthesized by detecting the remarkable change in liquid color. And then, in the interfacial polymerization, the order of the relative synthesis rates is as follows: PANI-DCM > PANI-CYH > PANI-CCl4 > PANI-MB, in which the nucleation growth theory can be verified. Indeed, the follow-up growth of PANI NPs leads to multiple shapes of PANI on the nanoscale; subsequently, along with the enhancement of polarity degree, the nanostructure of PANI converts to nanotubes, nanobelts, nanowires, nanoclusters, and nanospheres, respectively. In the group added with DMSO, PANI NPs are wrapped in layers and eventually form nanospheres; the size is much larger than other products, exhibiting significant development in AcOH-DMSO hybrid solutions. From the above SEM images, the degree of aggregation in PANI NPs is generally positively correlated with the polarity of organic phases, which results in the morphological variations of PANI products.


### 3.2. Structure Characterization of PANI

XRD is used to characterize the crystal structures of PANI-CK, PANI-CYH, PANI-CCl4, PANI-MB, PANI-DCM, and PANI-DMSO. The pristine PANI powders have three characteristic peaks that appear at about2θ=14.6° (the repeat unit of the PANI chain), 20.4° (the periodicity perpendicular to the polymer backbone chains), and 25.5° (the periodicity parallel with the polymer backbone chains) [28, 29]. The three characteristic peaks mentioned above correspond to (011), (020), and (200) crystal planes of PANI in sequence [30]. As demonstrated in Figure 2(a), the characteristic peaks at 20.4° and 25.5° are well matched to the pristine PANI powders; however, the peak generally at 14.6° shifts obviously to approximately 17.4°, which can be considered the addition of doped oxygen ions by AcOH replacing the larger ionic radius of nitrogen and carbon ions in the crystal structure. Meanwhile, due to the crystal transition caused by doped oxygen ions, the peak at 17.4° is split into 2-3 subpeaks in the range of 17.4°-18.1°. In PANI-CK, the sharper peak at 25.5° indicates the ordered molecular arrangement where it can be deduced from the scattered PANI NPs in SEM images. When it comes to PANI-CYH, PANI-CCl4, and PANI-MB, the intensity and location of the characteristic peaks among these three PANI salts are approximately the same; the disappearance of the peak at 25.5° illustrates the long-range disordered structure compared to the control group; besides, the more notable peaks in the range of 17.4°-18.1° displays the short-range ordered system due to the synthesis of the identical nanostructured particles in each group. Compared with the above PANI powders, PANI-DCM is a particular salt with a small number of similar units in chains, and the remarkable peak at 20.4° reflects the disordered arrangement, agreeing with the results of SEM observations. As for PANI-DMSO, the dramatic board peak at 19.6° is composed of several sub-peaks at 17.4°-18.1° and the characteristic band at 20.4°, revealing the superior repetition of nanosphere for PANI-DMSO samples. At the same time, the existence of the peak at 25.5° exhibits excellent monodistribution of the periodicity of PANI chains, and the addition of DMSO affords more ordered molecular arrangement of PANI chains [31, 32]. Compared with the PANI powders prepared by interface aggregation, in the pristine PANI powders, the intensity of the characteristic peak at 26° is significantly higher than that of others, revealing the excellent periodicity parallel with the polymer backbone chains. On the other hand, it demonstrates that different polymerization methods can significantly affect the crystal structure of PANI.Figure 2
Structure characterization and thermal analysis of PANI-CK, PANI-CYH, PANI-CCl4, PANI-MB, PANI-DCM, and PANI-DMSO, including (a) XRD patterns, (b) Raman spectra, (c) TGA traces, and (d) DGA curves.
(a)(b)(c)(d)Raman spectroscopy can elaborate chemical structure and covalent bond information. As shown in Figure2(b), Raman spectra of PANI doped with organic solvents are similar to that of PANI-CK, revealing the same type of polymer contained in these samples. The characteristic bands of pure PANI, such as the 1165 cm-1 band attributed to the C-H bending of quinoid/benzenoid units; the peak at 1252 cm-1 is ascribed to C-N stretching vibration; the 1312 cm-1 peak due to C–N+ stretching in bipolaron form (cationic imine units of the quinoid ring); the characteristic band at 1332 cm-1 assigned to C-N·+ stretching in polaron form (cationic amine units of the benzenoid ring); the peak at 1455 cm-1 is ascribed to the C=N stretching of the quinoid di-imine units; 1536 cm-1 is related to C=C stretching of the quinoid ring; then 1593 cm-1 band is defined as C-C stretching of the benzenoid ring. These bands not only demonstrate the coexistence of quinoid and benzenoid structures in PANI but also illustrate the conjugated structure caused by the re-arrangement of electron clouds [33, 34]. Meanwhile, the peak at 1408 cm-1 cannot be neglected, which usually appears in PANI powders with lower electrical conductivity [35]. As a result, this characteristic peak can be used as an indicator for determining the electrical conductivity of samples; due to the lack of corresponding peaks, PANI-CCl4, PANI-MB, and PANI-DSMO are regarded as the products with higher conductivity, which is consistent with results shown in TE test. The relative ratio of the quinoid and benzenoid rings in the PANI chain can be determined qualitatively from the intensity of absorption peaks corresponding to the skeletal vibration of both rings. When DMSO is used as the organic solvent, the increment at 1560 cm-1 illustrates the rise in quinoid ring proportion and reduction of benzene ring proportion, revealing the higher degree of oxidation in PANI-DMSO. The spectrum of PANI-MB displays a similar ratio of both rings. As for other samples, the intensity of C=C stretching in the quinoid ring is slightly smaller than that in the benzenoid ring, suggesting the lower oxidation level of these PANI salts compared with the emeraldine state.Figure2(c) depicts the TGA traces of PANI with different nanostructures, and the temperature ranges in primary stages of weight loss are observed from DGA curves (Figure 2(d)). There are five steps in the decomposition process. The first stage of weight loss below 100°C is attributed to evaporation of water and remaining organic solvents in the samples; the second step in the range of 100-240°C corresponds to the decomposition of AcOH and unreacted ANI monomers; moreover, the sharp downtrend curve displayed in the third stage (240-320°C) should be regarded as the rupture of partial chains and slight oligomers in organic polymers. Subsequently, as the temperature rises to the fourth section (320-515°C), the dramatically declining curve is ascribed to the degradation of macromolecular chains in the majority. At last, the fifth section above 515°C shows further thermal fracture of broken carbon chains in PANI samples [36, 37]. As a result, the thermal stability of PANI powders is negatively correlated with solvent polarity. In the first stage, the variation in weight loss can be inferred as a distinction in moisture content since only a tiny amount of organic solvent remains after lavation. In the case of identical drying time, other operations may affect the moisture content; a possible explanation for this might be the variation of growth in the aqueous phase illustrated in the previous analysis. With the enhancement of solvent polarity, PANI NPs preferentially develop in the AcOH solution, which can be explained by the well-known counterion-induced PANI solubility in the double-phase method [22]. In a word, the moisture content of PANI powders perhaps becomes a reduced reference to verify the nucleation growth process, but the exact mechanism remains to be investigated. As for PANI-DMSO, the single-phase method results in unexpected moisture content, whereas the change in thermal stability remains in line with the original trend.As a complementary characterization to the Raman spectrum, FT-IR spectroscopy has been employed to describe the structure of the main chains in several PANI samples. As displayed in Figure3(a), the absorption bands at 2980 cm-1, 1570 cm-1, 1490 cm-1, 1300 cm-1, 1147 cm-1, 812 cm-1, and 690 cm-1, which are related to the C-H stretching vibrations, C=C stretching of quinoid rings, C-C stretching mode of benzenoid rings, C-N stretching of secondary aromatic amine, in-plane bending of C-H bond, C-H out-of-plane bending of benzenoid rings, and benzenoid unit of doped PANI, respectively [19, 30]. With the increase of the polarity, the intensity of C-H stretching bonds is strengthened dramatically, reflecting the highly aggregated structure doped by strong polar solvents. Moreover, these PANI samples doped by AcOH exhibit an absorption peak at 1065 cm-1 attributed to the C-O stretching of the AcO- group, confirming that AcOH has been doped successfully in PANI chains [20]. As the defining peak of PANI conductivity, the band at 1147 cm-1 is regarded as the electronic-like band and serves as a measurement of the degree of electron delocalization on PANI [38]. Based on this theory, PANI-CCl4 and PANI-MB have extraordinary electrical conductivity owing to the remarkable peak at 1147 cm-1, reflecting higher carrier mobility than other PANI samples.Figure 3
Spectral analysis of PANI-CK, PANI-CYH, PANI-CCl4, PANI-MB, PANI-DCM, and PANI-DMSO, including (a) FT-IR spectra, XPS spectra of (b) full-range, and the deconvolution of (c) C1s and N1s (d) for the PANI with different morphologies.
(a)(b)(c)(d)XPS is a powerful tool for exploring the doping degree of conducting polymers. From the peak position and the shape of the spectrum, the chemical composition and molecular structure on the surface of samples can be obtained. In addition, the peak intensity is related to the content of oxidation states in the samples. The XPS full-range spectra for several PANI powders are gained in Figure3(b), revealing the presence of oxygen (531.8 eV), nitrogen (399.8 eV), and carbon (284.8 eV). Compared to spectra of pure PANI, the new peak at 531.8 eV is defined as the O1s peak coming from the dopant AcO- used to dope PANI, indicating the successful synthesis of PANI in the AcOH solution. Figure 3(c) represents the deconvoluted C1s XPS spectra of PANI powders prepared in the previous study, which is divided into four peaks corresponding to four different carbon species. The first one at 284.8 eV corresponds essentially to C-C or C-H bonds. The second peak at 286 eV is attributed to C-N or C=N bonds, and the third one at 288 eV is generally assigned to C-O bonds caused by the counterion and oxygen contamination. The final peak at around 289.5 eV reflects C=O bonds owing to surface oxidation [39, 40]. It is worth noting that the rising XPS peak signal area of C=O bonds in PANI-DMSO shows a higher oxidation level than the other PANI powders, verifying the findings in Raman analysis. In addition, the chemical environment of C atoms remained essentially unaltered after the variation of solvent polarity, which suggests that the protonation did not occur directly at C atoms, according to the comparison of C1s devolution data between the PANI-CK and PANI samples prepared by interfacial polymerization. Moreover, N1s spectra of PANI products were collected in Figure 3(d), which can be resolved into four peaks centered at 398.6 eV, 399.8 eV, 400.7 eV, and 403.5 eV, respectively. These peaks are defined as the quinoid imine (=N-), benzenoid amine (-NH-), positively charged imine (=NH+-) in the bipolaronic form, and protonated amine (-NH2+-) in the polaron state [33]. After the vibration of polarity, some structural changes were observed in XPS spectra. Based on the peak area ratio for various N1s species (Table S2), in contrast to PANI-CK, the overall proportion of oxidized segments (=N-, =NH+-) in PANI-DMSO increases significantly from 30.77% to 63.10%, while that of reduced components (-NH-, -NH2+-) shows the downward trend from 69.23% to 36.90%, revealing the highest degree of oxidation among all PANI salts. As for PANI-MB, the area ratio of imine and amine sections is about 1 : 1, indicating the approximately emeraldine base structure, which is consistent with the above findings. Meanwhile, the higher protonation of the N+ species (=NH+-, -NH2+-) in PANI-CCl4, PANI-MB, and PANI-DMSO confirms effective protonation of products formed by the addition of protons to the neutral polymer chain. Additionally, quinoid imine units (=N-) are preferentially protonated from the comparison of area proportion in both N+ species. As a result, considering these facts, it can be found that the polarity of solvents effectively protonates the polymer chains by regulating the amount of protonic acid involved in the reaction, making a profound impact on the oxidation degree of PANI salts.


### 3.3. TE Performance and Analysis of the PANI/SWCNT Composites

Faced with difficulty in measuring the TE properties of pure PANI in powder form on the instrument, a feasible approach is to prepare PANI/SWCNT hybrid material to reflect the differentiation of TE performance for PANI with different morphologies. The electrical conductivity (σ), Seebeck coefficient, and power factor (PF=S2σ) of PANI/SWCNT composite films with varying contents of SWCNTs at RT are illustrated in Figures 4(a)–4(c), respectively, which reflect the effect of SWCNT content on the TE performance. When the mass ratio of PANI and SWCNT is 9 : 1, it is clear to obtain the higher electrical conductivity value of PANI-CCl4/SWCNT (60.96±4.72 S cm-1) and PANI-DCM/SWCNT (54.71±3.44 S cm-1) than the other, mainly owing to the excellent electron transfer capability confirmed together by conductive characteristic peaks shown in the above spectral analysis. Meanwhile, as a slight sacrifice of the Seebeck coefficient in the high carrier concentration, the optimal PF values are 5.13±0.56 μW m-1 K-2 and 4.78±0.15 μW m-1 K-2, corresponding to PANI-CCl4/SWCNT and PANI-MB/SWCNT, respectively. Accompanied by an upward and downward trend in electrical conductivity and Seebeck coefficient, the maximum power factor is 70.08±3.12 μW m-1 K-2 at 50 wt% SWCNTs, attributing to PANI-DMSO/SWCNT composite films. In addition, PANI-MB/SWCNT hybrid material also maintains a high PF value of 66.55±2.14 μW m-1 K-2 when the mass ratio of PANI and SWCNTs is 1 : 1. As a one-dimensional nanomaterial with a perfectly connected hexagonal structure, SWCNTs exhibit excellent electrical properties. As Figure 4 demonstrates, 50 wt% of SWCNTs is the critical value of high thermoelectric properties. A greater interface effect between SWCNTs and PANI leads to further optimization of carrier transportation when the mass fraction of SWCNTs is less than 50%. As a consequence, SWCNTs are evenly distributed in the composite films and form an interwoven network (Figure S3). The formation of aggregations and bigger bundles, however, suggests poor dispersion of SWCNTs overloading above 50 wt%, which results in low conductivity and Seebeck coefficient. This is in line with the outcomes shown in Figure 4 as well [29]. Moreover, the electrical conductivity and Seebeck coefficient are interdependent; for example, the conductivity increased, and the Seebeck coefficient decreased through the enhancement of doping level [41]. Notably, when the masses of PANI and SWCNTs are the same, PANI-DMSO/SWCNT has the highest electrical conductivity, whereas the Seebeck coefficient is at a low level (42.31±1.01 μV K-1). In contrast, the Seebeck coefficient of PANI-CCl4/SWCNT is the highest among all the hybrid material, but the PF value is only about 37.05±1.98 μW m-1 K-2 due to poor electrical conductivity. From the TE performance of the above hybrid material, it is worth noting that electrical conductivity is the dominant factor affecting the value of PF, while the Seebeck coefficient is highest and hardly changed at 50 wt% SWCNTs.Figure 4
Effect of the SWCNT content on the (a) electrical conductivity, (b) Seebeck coefficient, and (c) power factor of PANI-X/SWCNT composite films (X=CK,CYH,CCl4,MB,DCM,DMSO).
(a)(b)(c)The variation of TE properties of different composites is explained as follows. Firstly, because of the almost insolubility of PANI powders in EtOH and ultrasonication at low power, the nanostructures of PANI salts remain the same as verified in FigureS3. With the doping of SWCNTs, the aggregation of PANI salts is significantly reduced, while the introduction of the nanotubular system can provide a pathway for carrier transportation. From the EDX mapping images, obviously, the oxygen is accumulated in PANI-DMSO particles compared with the control group, consisting of the results in XPS spectra. Next, the spectral analysis of several PANI/SWCNT composites is illustrated in Figure S4, revealing the same compositions from similar peak positions; whereafter, the apparent discrepancy in peak intensity reflects the differentiation in the interior structure of composites. For instance, from the prominent peaks at 17.4° and 25.5° in PANI-DMSO compared to the other (Figure S4a), the microarrangement of PANI-DMSO/SWCNT composite is higher than that doped by other organic solvents, supporting its excellent TE properties (Figure 5(a)). As two composites with better performance, a reasonable explanation can be deduced by comparing TE properties between PANI-MB/SWCNT and PANI-DMSO/SWCNT hybrid material at different SWCNT contents. Considering the theoretical basis, the electrical conductivity of a semiconductor can be given by [19, 20]
(1)σ=neμ.where n is carrier concentration (oxidation level), e is electric charge, and μ is carrier mobility. At first, the electric charges in PANI/SWCNT hybrid material are the same, and more H+ protons are bonded with quinoid imine units (=N-) in the main chain of PANI-DMSO salts to enhance the carrier concentration, according to the area ratio displayed in Table S2. Nevertheless, compared with PANI-MB in nanowires, the lack of connections between nanospheres causes poor carrier mobility in PANI-DMSO. When the content of SWCNTs is raised to 50 wt%, it is easy to understand that the combination of nanotubes and nanospheres makes it easier to transfer charger carriers inside tubes by giving them a “smoother” path [19, 20]. Combined with the excellent doping level, it is not difficult to comprehend the maximum PF value corresponding to PANI-DMSO/SWCNT composite films at 50 wt% SWCNTs. As for PANI-DCM, the lowest electrical conductivity and Seebeck coefficient are caused by the poor molecular arrangement of PANI chains reflected in Figures 1(e) and 2(a), affecting the carrier transportation between PANI particles and SWCNTs. In view of the optimal TE performance of the composite films at 50 wt% SWCNT, the mass ratio of PANI and SWCNTs adopted in the subsequent film samples is constantly 1 : 1.Figure 5
Comparison of the TE properties for PANI-X/SWCNT composite films (a) before and (b) after the pressing treatment at RT (X=CK,CYH,CCl4,MB,DCM,DMSO).
(a)(b)Pressing can significantly improve the TE performance of the composites (Figure5). It is worth noting that the electrical conductivity increases a lot while the Seebeck coefficient maintains slight growth. For instance, the conductivity value of PANI-DMSO/SWCNT after pressing (917.88±33.12 S cm-1) is dramatically higher than that of pristine PANI-DMSO/SWCNT (391.37±8.43 S cm-1), while the Seebeck coefficient achieves a slight rise from 42.31±1.01 μV K-1 to 44.06±0.73 μV K-1. As a result, the power factor of the pressed composites (178.10±3.77 μW m-1 K-2) (Figure 5(b)) is greatly promoted compared with that of the sample before pressing (70.08±3.12 μW m-1 K-2) (Figure 5(a)). The positive effect of pressing process on TE properties is further demonstrated by other hybrid materials, such as the nearly double growth of PF value in PANI-MB/SWCNT composites. The pressing-induced enhancement in TE performance might be mainly caused by the declined thickness of the composite films macroscopically. Since the subsequent preparation process is the same, there is little variation in the thickness of the composite films made in different polar environments. The differences in film thickness are mainly due to the pressing process. Before pressing, the thickness is within the range of 22.3±6.7 μm, after pressing, the thickness turns to be 11.2±3.8 μm. The significant change in film thickness caused the remarkable improvement in the thermoelectric properties of pressed complexes. From the microscopic perspective, PANI particles and SWCNTs become more compact because of the elimination of most voids due to the pressing treatment, thus shortening the transport distance of charge carriers in the networks formed by PANI and SWCNTs.In recent years, the development of PANI-based composite systems has gathered more on the composites of PANI and CNTs. In this work, PANI/SWCNT composites exhibited thermoelectric properties which are several times larger than either of their individual components (for pure SWCNTs, the electrical conductivity is248.75±10.16 S cm-1, the Seebeck coefficient is 34.98±0.32 μV K-1, the power factor is 30.45±1.50 μW m-1 K-2). Meng et al. reported the preparation of CNTs-based MWNT/PANI sheet composite and found that the improvement of thermoelectric properties for the composite should be attributed to the size-dependent energy-filtering effect caused by the nanostructured PANI coating layer enwrapped around the CNTs [42, 43]. Moreover, the orderly arrangement of PANI molecules on the outer wall of CNTs through π-π interactions (as shown in Figure 6) during the mixing process is also considered to be a key factor in the thermoelectric performance enhancement [44]. Besides the comparison between TE properties, Figure 6 explains the effect of SWCNTs and pressing method by using PANI-DMSO as an example. In Figure 6(a), the typical peak at 2θ=21° represents pure SWCNTs. With the introduction of SWCNTs, the characteristic peak of PANI can also be detected in the PANI-DMSO/SWCNT composite films, but the broad peak at 19.6° has been converted into conventional characteristic peaks at 17.4° and 20.4°, respectively. Meanwhile, the weakening for the peak at 20.4° and sharpening for the peak at 25.5° indicates the ordered arrangement of PANI chains and the enhancement of interface interaction between PANI particles and SWCNTs by the presence of SWCNTs [29]. In addition, the rising peak at 22.8° representing a periodicity caused by π-π stacking of PANI/SWCNT-linked structures further confirms the existence of stronger interface interaction, hence improving TE performance effectively by facilitating carrier transport. After pressing, the disappearance of the peak at 20.4° indicates the more organized distribution in main chains of PANI than that before pressing. As shown in Figure 6(b), the thermal stability of PANI-DMSO/SWCNT composites is extensively promoted by adding SWCNTs, which agrees well with the XRD results. By taking pressing treatment, however, the remaining mass ratio of the film drops to 80% after burning at 800°C, which might be attributed to partial fracture in macromolecular chains. Furthermore, FT-IR and Raman spectra can profoundly expound the chemical state and interaction between PANI and SWCNTs. In FT-IR spectra (Figure 6(c)), the peak at 1640 cm-1 is the typical band for SWCNTs, corresponding to stretching modes of the sp2 hybridized C=C bonds that form SWCNTs backbone [45]. Consequently, the coexistence of the typical peaks at 1640 cm-1 and 1490 cm-1 confirms the synthesis of PANI/SWCNT composites. From the Raman spectrum (Figure 6(d)), SWCNTs exhibit distinct bands at 1500 cm-1 (G- mode) and 1590 cm-1 (G+ mode), which are the criterion for determining the orderliness of SWCNTs. Besides, the D band at 1350 cm-1 and the 2D band at 2650 cm-1 illustrate the degree of disorder and stacking, respectively. Notably, the Raman peaks of SWCNTs dominate that of the PANI/SWCNT composite, from which it can be inferred that the fundamental backbone of the composite films is composed of SWCNTs. And after pressing, the decline in the intensity of the typical peaks at 1640 cm-1 and 1490 cm-1 (Figure 6(c)), the blue shift of G- band, and the increment of the G/D ratio (Figure 6(d)), suggest stronger π-π-conjugated interaction between PANI and SWCNTs [29, 46].Figure 6
Spectral and thermal analysis for PANI-DMSO, SWCNTs, PANI-DMSO/SWCNT, and PANI-DMSO/SWCNT-pressed, including (a) XRD patterns, (b) TGA traces, (c) FT-IR spectra, and (d) Raman spectra.
(a)(b)(c)(d)Furthermore, the temperature-dependent variation in the TE properties of PANI-DMSO/SWCNT composite films is investigated in the range of 25-145°C (Figure7(a)). With the rising temperature, the electrical conductivity drops steadily, reflecting a classic metallic conducting behavior of the films. In addition, the Seebeck coefficient first increases and then decreases with the rising temperature, obtaining the highest value of 49.04±0.15 μV K-1 at 125°C. Therefore, the power factor reaches a maximum value of 188.68±4.50 μW m-1 K-2 at 105°C, showing a broad prospect for this composite in utilizing low-grade waste heat.Figure 7
TE properties of PANI-DMSO/SWCNT composite films at (a) variable temperatures, (b) different bending cycles, (c) specific parameters (open circuit voltage, current and output power) of the TE module, and (d) thermal stability after different heating cycles.
(a)(b)(c)(d)Moreover, the flexibility of PANI-DMSO/SWCNT composite films at RT is examined by multiple bending cycles. After 1000 bending cycles, the PF can maintain about 75% of its original value (Figure7(b)), which might be ascribed to the instability of SWCNTs, manifesting that there is room for improvement in mechanical flexibility. At last, a prototype TEG with seven pairs of p-n junctions connected by copper wire and conductive silver adhesive was prepared to preliminarily examine the output behavior of TE device fabricated with such hybrid films (Figure S5a). In the fabrication of the TEG, p-type material is directly derived from PANI-DMSO/SWCNT composites after being pressed, and n-type material is converted from the p-type material doped by SDS and PEI, in which the rest of the operations are the same. As a result, the variation of each parameter (including open circuit voltage, current, and output power) of TEG with temperature difference is presented in Figure 7(c). In order to calculate the output power, we added the load resistance (R) of 1 kΩ; then, open circuit voltage (E) and output voltage (U) were recorded to analyze the internal resistance (r) of TEG. Under simulated skin contact conditions (ΔT=15°C), E=3.6±0.12 mV, U=2.1±0.10 mV, and R=1 kΩ, and r of TEG was estimated using
(2)r=E−UI=E−URU.AtΔT=15°C, r=714 Ω, this is considered to be a constant value in the variable temperature environment due to the stability of the overall structure in TEG.Moreover, the output power (P) is calculated using
(3)P=EI–I2r.Approximately,E and I grow in the same trend, rising linearly with the increasing temperature difference which indicates that r is a constant value. When ΔT≈50°C, E=10.27±0.15 mV, I=6.57±0.21 μA, P=36.65±1.18 nW, and powerdensity=11.54±0.37 nW/g with only seven couples of p- and n-legs, exhibiting the potential value in the energy reuse.To simulate the application of this TEG in harvesting body heat, the hot-end temperature was set at 37°C, while the cold end was in the ambience (around 22°C). As shown in FigureS5b, TEG generates the voltage of 3.6±0.20 mV, and output power is 4.17±0.32 nW, manifesting potential to harness low-grade waste heat in portable and wearable electronics. In addition, to test the thermal stability of TEG, the same PANI-DMSO/SWCNT composite was divided into different numbers of heating cycles, from 0 cycle to 100 cycles; in each heating cycle, the film was set into the vacuum environment at 105°C (optimal temperature) for 2 min. As shown in Figure 7(d), TE properties of the composites remained basically stable in the increment of heating cycles, and the PF was still maintained above 210 μW m-1 K-2 after 100 heating cycles, showing excellent thermal stability.


## 3.1. Morphology and Structure of PANI

To distinguish between the PANI powers prepared from different organic solvents, we named these products according to the abbreviations of the organic phases, which were called PANI-CK (blank control component), PANI-CYH, PANI-CCl4, PANI-MB, PANI-DCM, and PANI-DMSO. As demonstrated in Figure 1, the morphological differences between PANI powders are apparent. In the blank control group (Figures 1(a) and 1(a’)), PANI-CK is composed of nanoparticles (NPs) and can be considered a “barbed” structure. With CYH being added to act as an organic layer, PANI-CYH is formed into nanotubes intertwined with each other (Figures 1(b) and 1(b’)). The outer diameter of PANI-CYH nanotubes is about 200 nm, and the inner diameter takes up about one-third of the width. According to Figures 1(c) and 1(c’), when CYH is replaced with CCl4, PANI-CCl4 turns to be nanobelts, accompanied by a small number of doped nanotubes. Subsequently, using MB as an organic phase, the micromorphologies transform from nanobelts to nanowires (Figures 1(d) and 1(d’)), roughly the same width as nanotubes of PANI-CYH. With the enhancement of solvent polarity, the NPs of PANI-DCM (Figures 1(e) and 1(e’)) are tightly bound together to form nanoclusters. Compared with PANI-CK, the nucleation effect of PANI-DCM NPs is more significant, verifying the remarkable effect of polar solvents on nanostructures. As one of the most polar solvents, DMSO, which promotes orderly alignment of polymer chains and highly extended polymer conformation [25], was easily mixed with AcOH aqueous solution. As shown in Figures 1(f) and 1(f’), PANI-DMSO presents a closed nanosphere-like structure, and the diameter of the nanospheres is about 10 μm.Figure 1
SEM images in low magnification mode of (a) PANI-CK, (b) PANI-CYH, (c) PANI-CCl4, (d) PANI-MB, (e) PANI-DCM, and (f) PANI-DMSO. SEM images in high magnification mode of (a’) PANI-CK, (b’) PANI-CYH, (c’) PANI-CCl4, (d’) PANI-MB, (e’) PANI-DCM, and (f’) PANI-DMSO.
(a)(b)(c)(d)(e)(f)To observe the compositions of PANI under a more microscopic perspective, PANI powders were dissolved in DMF using ultrasonic dispersion for 30 min, and the final images taken by TEM are displayed in FigureS1. Each type of PANI is constructed by NPs with a diameter of approximately 20 nm as shown in Figure S1a. Obviously, by adding solvent to the organic phase and changing the polarity, the polymerization process and state of NPs can be efficiently modified, thus dramatically influencing the nanostructure of PANI. Meanwhile, the theories of interfacial nucleation and aqueous growth can be confirmed from the noteworthy dissimilitude in SEM and TEM images. The formation of PANI NPs takes place in interfacial nucleation on account of the minimum size; then, PANI NPs act as nucleation centers for further growth of polymerization products. As the polarity of organic solvents increases (the polarity of the selected organic solvents is displayed in Table S1), the concentration distribution of primary products in aqueous and organic phases will be profoundly affected, impacting the further growth of PANI NPs generated in the aqueous phase. In the blank control group, the assemblage of NPs is highly dispersed, and a part of NPs can be detected (Figure 1(a’)), which is attributed to the restricted growth in the absence of organic solvents. Besides, the relative rates of polymerization reactions are recorded in Figure S2. Using single-phase polymerization, PANI-CK and PANI-DMSO were rapidly synthesized by detecting the remarkable change in liquid color. And then, in the interfacial polymerization, the order of the relative synthesis rates is as follows: PANI-DCM > PANI-CYH > PANI-CCl4 > PANI-MB, in which the nucleation growth theory can be verified. Indeed, the follow-up growth of PANI NPs leads to multiple shapes of PANI on the nanoscale; subsequently, along with the enhancement of polarity degree, the nanostructure of PANI converts to nanotubes, nanobelts, nanowires, nanoclusters, and nanospheres, respectively. In the group added with DMSO, PANI NPs are wrapped in layers and eventually form nanospheres; the size is much larger than other products, exhibiting significant development in AcOH-DMSO hybrid solutions. From the above SEM images, the degree of aggregation in PANI NPs is generally positively correlated with the polarity of organic phases, which results in the morphological variations of PANI products.


## 3.2. Structure Characterization of PANI

XRD is used to characterize the crystal structures of PANI-CK, PANI-CYH, PANI-CCl4, PANI-MB, PANI-DCM, and PANI-DMSO. The pristine PANI powders have three characteristic peaks that appear at about2θ=14.6° (the repeat unit of the PANI chain), 20.4° (the periodicity perpendicular to the polymer backbone chains), and 25.5° (the periodicity parallel with the polymer backbone chains) [28, 29]. The three characteristic peaks mentioned above correspond to (011), (020), and (200) crystal planes of PANI in sequence [30]. As demonstrated in Figure 2(a), the characteristic peaks at 20.4° and 25.5° are well matched to the pristine PANI powders; however, the peak generally at 14.6° shifts obviously to approximately 17.4°, which can be considered the addition of doped oxygen ions by AcOH replacing the larger ionic radius of nitrogen and carbon ions in the crystal structure. Meanwhile, due to the crystal transition caused by doped oxygen ions, the peak at 17.4° is split into 2-3 subpeaks in the range of 17.4°-18.1°. In PANI-CK, the sharper peak at 25.5° indicates the ordered molecular arrangement where it can be deduced from the scattered PANI NPs in SEM images. When it comes to PANI-CYH, PANI-CCl4, and PANI-MB, the intensity and location of the characteristic peaks among these three PANI salts are approximately the same; the disappearance of the peak at 25.5° illustrates the long-range disordered structure compared to the control group; besides, the more notable peaks in the range of 17.4°-18.1° displays the short-range ordered system due to the synthesis of the identical nanostructured particles in each group. Compared with the above PANI powders, PANI-DCM is a particular salt with a small number of similar units in chains, and the remarkable peak at 20.4° reflects the disordered arrangement, agreeing with the results of SEM observations. As for PANI-DMSO, the dramatic board peak at 19.6° is composed of several sub-peaks at 17.4°-18.1° and the characteristic band at 20.4°, revealing the superior repetition of nanosphere for PANI-DMSO samples. At the same time, the existence of the peak at 25.5° exhibits excellent monodistribution of the periodicity of PANI chains, and the addition of DMSO affords more ordered molecular arrangement of PANI chains [31, 32]. Compared with the PANI powders prepared by interface aggregation, in the pristine PANI powders, the intensity of the characteristic peak at 26° is significantly higher than that of others, revealing the excellent periodicity parallel with the polymer backbone chains. On the other hand, it demonstrates that different polymerization methods can significantly affect the crystal structure of PANI.Figure 2
Structure characterization and thermal analysis of PANI-CK, PANI-CYH, PANI-CCl4, PANI-MB, PANI-DCM, and PANI-DMSO, including (a) XRD patterns, (b) Raman spectra, (c) TGA traces, and (d) DGA curves.
(a)(b)(c)(d)Raman spectroscopy can elaborate chemical structure and covalent bond information. As shown in Figure2(b), Raman spectra of PANI doped with organic solvents are similar to that of PANI-CK, revealing the same type of polymer contained in these samples. The characteristic bands of pure PANI, such as the 1165 cm-1 band attributed to the C-H bending of quinoid/benzenoid units; the peak at 1252 cm-1 is ascribed to C-N stretching vibration; the 1312 cm-1 peak due to C–N+ stretching in bipolaron form (cationic imine units of the quinoid ring); the characteristic band at 1332 cm-1 assigned to C-N·+ stretching in polaron form (cationic amine units of the benzenoid ring); the peak at 1455 cm-1 is ascribed to the C=N stretching of the quinoid di-imine units; 1536 cm-1 is related to C=C stretching of the quinoid ring; then 1593 cm-1 band is defined as C-C stretching of the benzenoid ring. These bands not only demonstrate the coexistence of quinoid and benzenoid structures in PANI but also illustrate the conjugated structure caused by the re-arrangement of electron clouds [33, 34]. Meanwhile, the peak at 1408 cm-1 cannot be neglected, which usually appears in PANI powders with lower electrical conductivity [35]. As a result, this characteristic peak can be used as an indicator for determining the electrical conductivity of samples; due to the lack of corresponding peaks, PANI-CCl4, PANI-MB, and PANI-DSMO are regarded as the products with higher conductivity, which is consistent with results shown in TE test. The relative ratio of the quinoid and benzenoid rings in the PANI chain can be determined qualitatively from the intensity of absorption peaks corresponding to the skeletal vibration of both rings. When DMSO is used as the organic solvent, the increment at 1560 cm-1 illustrates the rise in quinoid ring proportion and reduction of benzene ring proportion, revealing the higher degree of oxidation in PANI-DMSO. The spectrum of PANI-MB displays a similar ratio of both rings. As for other samples, the intensity of C=C stretching in the quinoid ring is slightly smaller than that in the benzenoid ring, suggesting the lower oxidation level of these PANI salts compared with the emeraldine state.Figure2(c) depicts the TGA traces of PANI with different nanostructures, and the temperature ranges in primary stages of weight loss are observed from DGA curves (Figure 2(d)). There are five steps in the decomposition process. The first stage of weight loss below 100°C is attributed to evaporation of water and remaining organic solvents in the samples; the second step in the range of 100-240°C corresponds to the decomposition of AcOH and unreacted ANI monomers; moreover, the sharp downtrend curve displayed in the third stage (240-320°C) should be regarded as the rupture of partial chains and slight oligomers in organic polymers. Subsequently, as the temperature rises to the fourth section (320-515°C), the dramatically declining curve is ascribed to the degradation of macromolecular chains in the majority. At last, the fifth section above 515°C shows further thermal fracture of broken carbon chains in PANI samples [36, 37]. As a result, the thermal stability of PANI powders is negatively correlated with solvent polarity. In the first stage, the variation in weight loss can be inferred as a distinction in moisture content since only a tiny amount of organic solvent remains after lavation. In the case of identical drying time, other operations may affect the moisture content; a possible explanation for this might be the variation of growth in the aqueous phase illustrated in the previous analysis. With the enhancement of solvent polarity, PANI NPs preferentially develop in the AcOH solution, which can be explained by the well-known counterion-induced PANI solubility in the double-phase method [22]. In a word, the moisture content of PANI powders perhaps becomes a reduced reference to verify the nucleation growth process, but the exact mechanism remains to be investigated. As for PANI-DMSO, the single-phase method results in unexpected moisture content, whereas the change in thermal stability remains in line with the original trend.As a complementary characterization to the Raman spectrum, FT-IR spectroscopy has been employed to describe the structure of the main chains in several PANI samples. As displayed in Figure3(a), the absorption bands at 2980 cm-1, 1570 cm-1, 1490 cm-1, 1300 cm-1, 1147 cm-1, 812 cm-1, and 690 cm-1, which are related to the C-H stretching vibrations, C=C stretching of quinoid rings, C-C stretching mode of benzenoid rings, C-N stretching of secondary aromatic amine, in-plane bending of C-H bond, C-H out-of-plane bending of benzenoid rings, and benzenoid unit of doped PANI, respectively [19, 30]. With the increase of the polarity, the intensity of C-H stretching bonds is strengthened dramatically, reflecting the highly aggregated structure doped by strong polar solvents. Moreover, these PANI samples doped by AcOH exhibit an absorption peak at 1065 cm-1 attributed to the C-O stretching of the AcO- group, confirming that AcOH has been doped successfully in PANI chains [20]. As the defining peak of PANI conductivity, the band at 1147 cm-1 is regarded as the electronic-like band and serves as a measurement of the degree of electron delocalization on PANI [38]. Based on this theory, PANI-CCl4 and PANI-MB have extraordinary electrical conductivity owing to the remarkable peak at 1147 cm-1, reflecting higher carrier mobility than other PANI samples.Figure 3
Spectral analysis of PANI-CK, PANI-CYH, PANI-CCl4, PANI-MB, PANI-DCM, and PANI-DMSO, including (a) FT-IR spectra, XPS spectra of (b) full-range, and the deconvolution of (c) C1s and N1s (d) for the PANI with different morphologies.
(a)(b)(c)(d)XPS is a powerful tool for exploring the doping degree of conducting polymers. From the peak position and the shape of the spectrum, the chemical composition and molecular structure on the surface of samples can be obtained. In addition, the peak intensity is related to the content of oxidation states in the samples. The XPS full-range spectra for several PANI powders are gained in Figure3(b), revealing the presence of oxygen (531.8 eV), nitrogen (399.8 eV), and carbon (284.8 eV). Compared to spectra of pure PANI, the new peak at 531.8 eV is defined as the O1s peak coming from the dopant AcO- used to dope PANI, indicating the successful synthesis of PANI in the AcOH solution. Figure 3(c) represents the deconvoluted C1s XPS spectra of PANI powders prepared in the previous study, which is divided into four peaks corresponding to four different carbon species. The first one at 284.8 eV corresponds essentially to C-C or C-H bonds. The second peak at 286 eV is attributed to C-N or C=N bonds, and the third one at 288 eV is generally assigned to C-O bonds caused by the counterion and oxygen contamination. The final peak at around 289.5 eV reflects C=O bonds owing to surface oxidation [39, 40]. It is worth noting that the rising XPS peak signal area of C=O bonds in PANI-DMSO shows a higher oxidation level than the other PANI powders, verifying the findings in Raman analysis. In addition, the chemical environment of C atoms remained essentially unaltered after the variation of solvent polarity, which suggests that the protonation did not occur directly at C atoms, according to the comparison of C1s devolution data between the PANI-CK and PANI samples prepared by interfacial polymerization. Moreover, N1s spectra of PANI products were collected in Figure 3(d), which can be resolved into four peaks centered at 398.6 eV, 399.8 eV, 400.7 eV, and 403.5 eV, respectively. These peaks are defined as the quinoid imine (=N-), benzenoid amine (-NH-), positively charged imine (=NH+-) in the bipolaronic form, and protonated amine (-NH2+-) in the polaron state [33]. After the vibration of polarity, some structural changes were observed in XPS spectra. Based on the peak area ratio for various N1s species (Table S2), in contrast to PANI-CK, the overall proportion of oxidized segments (=N-, =NH+-) in PANI-DMSO increases significantly from 30.77% to 63.10%, while that of reduced components (-NH-, -NH2+-) shows the downward trend from 69.23% to 36.90%, revealing the highest degree of oxidation among all PANI salts. As for PANI-MB, the area ratio of imine and amine sections is about 1 : 1, indicating the approximately emeraldine base structure, which is consistent with the above findings. Meanwhile, the higher protonation of the N+ species (=NH+-, -NH2+-) in PANI-CCl4, PANI-MB, and PANI-DMSO confirms effective protonation of products formed by the addition of protons to the neutral polymer chain. Additionally, quinoid imine units (=N-) are preferentially protonated from the comparison of area proportion in both N+ species. As a result, considering these facts, it can be found that the polarity of solvents effectively protonates the polymer chains by regulating the amount of protonic acid involved in the reaction, making a profound impact on the oxidation degree of PANI salts.


## 3.3. TE Performance and Analysis of the PANI/SWCNT Composites

Faced with difficulty in measuring the TE properties of pure PANI in powder form on the instrument, a feasible approach is to prepare PANI/SWCNT hybrid material to reflect the differentiation of TE performance for PANI with different morphologies. The electrical conductivity (σ), Seebeck coefficient, and power factor (PF=S2σ) of PANI/SWCNT composite films with varying contents of SWCNTs at RT are illustrated in Figures 4(a)–4(c), respectively, which reflect the effect of SWCNT content on the TE performance. When the mass ratio of PANI and SWCNT is 9 : 1, it is clear to obtain the higher electrical conductivity value of PANI-CCl4/SWCNT (60.96±4.72 S cm-1) and PANI-DCM/SWCNT (54.71±3.44 S cm-1) than the other, mainly owing to the excellent electron transfer capability confirmed together by conductive characteristic peaks shown in the above spectral analysis. Meanwhile, as a slight sacrifice of the Seebeck coefficient in the high carrier concentration, the optimal PF values are 5.13±0.56 μW m-1 K-2 and 4.78±0.15 μW m-1 K-2, corresponding to PANI-CCl4/SWCNT and PANI-MB/SWCNT, respectively. Accompanied by an upward and downward trend in electrical conductivity and Seebeck coefficient, the maximum power factor is 70.08±3.12 μW m-1 K-2 at 50 wt% SWCNTs, attributing to PANI-DMSO/SWCNT composite films. In addition, PANI-MB/SWCNT hybrid material also maintains a high PF value of 66.55±2.14 μW m-1 K-2 when the mass ratio of PANI and SWCNTs is 1 : 1. As a one-dimensional nanomaterial with a perfectly connected hexagonal structure, SWCNTs exhibit excellent electrical properties. As Figure 4 demonstrates, 50 wt% of SWCNTs is the critical value of high thermoelectric properties. A greater interface effect between SWCNTs and PANI leads to further optimization of carrier transportation when the mass fraction of SWCNTs is less than 50%. As a consequence, SWCNTs are evenly distributed in the composite films and form an interwoven network (Figure S3). The formation of aggregations and bigger bundles, however, suggests poor dispersion of SWCNTs overloading above 50 wt%, which results in low conductivity and Seebeck coefficient. This is in line with the outcomes shown in Figure 4 as well [29]. Moreover, the electrical conductivity and Seebeck coefficient are interdependent; for example, the conductivity increased, and the Seebeck coefficient decreased through the enhancement of doping level [41]. Notably, when the masses of PANI and SWCNTs are the same, PANI-DMSO/SWCNT has the highest electrical conductivity, whereas the Seebeck coefficient is at a low level (42.31±1.01 μV K-1). In contrast, the Seebeck coefficient of PANI-CCl4/SWCNT is the highest among all the hybrid material, but the PF value is only about 37.05±1.98 μW m-1 K-2 due to poor electrical conductivity. From the TE performance of the above hybrid material, it is worth noting that electrical conductivity is the dominant factor affecting the value of PF, while the Seebeck coefficient is highest and hardly changed at 50 wt% SWCNTs.Figure 4
Effect of the SWCNT content on the (a) electrical conductivity, (b) Seebeck coefficient, and (c) power factor of PANI-X/SWCNT composite films (X=CK,CYH,CCl4,MB,DCM,DMSO).
(a)(b)(c)The variation of TE properties of different composites is explained as follows. Firstly, because of the almost insolubility of PANI powders in EtOH and ultrasonication at low power, the nanostructures of PANI salts remain the same as verified in FigureS3. With the doping of SWCNTs, the aggregation of PANI salts is significantly reduced, while the introduction of the nanotubular system can provide a pathway for carrier transportation. From the EDX mapping images, obviously, the oxygen is accumulated in PANI-DMSO particles compared with the control group, consisting of the results in XPS spectra. Next, the spectral analysis of several PANI/SWCNT composites is illustrated in Figure S4, revealing the same compositions from similar peak positions; whereafter, the apparent discrepancy in peak intensity reflects the differentiation in the interior structure of composites. For instance, from the prominent peaks at 17.4° and 25.5° in PANI-DMSO compared to the other (Figure S4a), the microarrangement of PANI-DMSO/SWCNT composite is higher than that doped by other organic solvents, supporting its excellent TE properties (Figure 5(a)). As two composites with better performance, a reasonable explanation can be deduced by comparing TE properties between PANI-MB/SWCNT and PANI-DMSO/SWCNT hybrid material at different SWCNT contents. Considering the theoretical basis, the electrical conductivity of a semiconductor can be given by [19, 20]
(1)σ=neμ.where n is carrier concentration (oxidation level), e is electric charge, and μ is carrier mobility. At first, the electric charges in PANI/SWCNT hybrid material are the same, and more H+ protons are bonded with quinoid imine units (=N-) in the main chain of PANI-DMSO salts to enhance the carrier concentration, according to the area ratio displayed in Table S2. Nevertheless, compared with PANI-MB in nanowires, the lack of connections between nanospheres causes poor carrier mobility in PANI-DMSO. When the content of SWCNTs is raised to 50 wt%, it is easy to understand that the combination of nanotubes and nanospheres makes it easier to transfer charger carriers inside tubes by giving them a “smoother” path [19, 20]. Combined with the excellent doping level, it is not difficult to comprehend the maximum PF value corresponding to PANI-DMSO/SWCNT composite films at 50 wt% SWCNTs. As for PANI-DCM, the lowest electrical conductivity and Seebeck coefficient are caused by the poor molecular arrangement of PANI chains reflected in Figures 1(e) and 2(a), affecting the carrier transportation between PANI particles and SWCNTs. In view of the optimal TE performance of the composite films at 50 wt% SWCNT, the mass ratio of PANI and SWCNTs adopted in the subsequent film samples is constantly 1 : 1.Figure 5
Comparison of the TE properties for PANI-X/SWCNT composite films (a) before and (b) after the pressing treatment at RT (X=CK,CYH,CCl4,MB,DCM,DMSO).
(a)(b)Pressing can significantly improve the TE performance of the composites (Figure5). It is worth noting that the electrical conductivity increases a lot while the Seebeck coefficient maintains slight growth. For instance, the conductivity value of PANI-DMSO/SWCNT after pressing (917.88±33.12 S cm-1) is dramatically higher than that of pristine PANI-DMSO/SWCNT (391.37±8.43 S cm-1), while the Seebeck coefficient achieves a slight rise from 42.31±1.01 μV K-1 to 44.06±0.73 μV K-1. As a result, the power factor of the pressed composites (178.10±3.77 μW m-1 K-2) (Figure 5(b)) is greatly promoted compared with that of the sample before pressing (70.08±3.12 μW m-1 K-2) (Figure 5(a)). The positive effect of pressing process on TE properties is further demonstrated by other hybrid materials, such as the nearly double growth of PF value in PANI-MB/SWCNT composites. The pressing-induced enhancement in TE performance might be mainly caused by the declined thickness of the composite films macroscopically. Since the subsequent preparation process is the same, there is little variation in the thickness of the composite films made in different polar environments. The differences in film thickness are mainly due to the pressing process. Before pressing, the thickness is within the range of 22.3±6.7 μm, after pressing, the thickness turns to be 11.2±3.8 μm. The significant change in film thickness caused the remarkable improvement in the thermoelectric properties of pressed complexes. From the microscopic perspective, PANI particles and SWCNTs become more compact because of the elimination of most voids due to the pressing treatment, thus shortening the transport distance of charge carriers in the networks formed by PANI and SWCNTs.In recent years, the development of PANI-based composite systems has gathered more on the composites of PANI and CNTs. In this work, PANI/SWCNT composites exhibited thermoelectric properties which are several times larger than either of their individual components (for pure SWCNTs, the electrical conductivity is248.75±10.16 S cm-1, the Seebeck coefficient is 34.98±0.32 μV K-1, the power factor is 30.45±1.50 μW m-1 K-2). Meng et al. reported the preparation of CNTs-based MWNT/PANI sheet composite and found that the improvement of thermoelectric properties for the composite should be attributed to the size-dependent energy-filtering effect caused by the nanostructured PANI coating layer enwrapped around the CNTs [42, 43]. Moreover, the orderly arrangement of PANI molecules on the outer wall of CNTs through π-π interactions (as shown in Figure 6) during the mixing process is also considered to be a key factor in the thermoelectric performance enhancement [44]. Besides the comparison between TE properties, Figure 6 explains the effect of SWCNTs and pressing method by using PANI-DMSO as an example. In Figure 6(a), the typical peak at 2θ=21° represents pure SWCNTs. With the introduction of SWCNTs, the characteristic peak of PANI can also be detected in the PANI-DMSO/SWCNT composite films, but the broad peak at 19.6° has been converted into conventional characteristic peaks at 17.4° and 20.4°, respectively. Meanwhile, the weakening for the peak at 20.4° and sharpening for the peak at 25.5° indicates the ordered arrangement of PANI chains and the enhancement of interface interaction between PANI particles and SWCNTs by the presence of SWCNTs [29]. In addition, the rising peak at 22.8° representing a periodicity caused by π-π stacking of PANI/SWCNT-linked structures further confirms the existence of stronger interface interaction, hence improving TE performance effectively by facilitating carrier transport. After pressing, the disappearance of the peak at 20.4° indicates the more organized distribution in main chains of PANI than that before pressing. As shown in Figure 6(b), the thermal stability of PANI-DMSO/SWCNT composites is extensively promoted by adding SWCNTs, which agrees well with the XRD results. By taking pressing treatment, however, the remaining mass ratio of the film drops to 80% after burning at 800°C, which might be attributed to partial fracture in macromolecular chains. Furthermore, FT-IR and Raman spectra can profoundly expound the chemical state and interaction between PANI and SWCNTs. In FT-IR spectra (Figure 6(c)), the peak at 1640 cm-1 is the typical band for SWCNTs, corresponding to stretching modes of the sp2 hybridized C=C bonds that form SWCNTs backbone [45]. Consequently, the coexistence of the typical peaks at 1640 cm-1 and 1490 cm-1 confirms the synthesis of PANI/SWCNT composites. From the Raman spectrum (Figure 6(d)), SWCNTs exhibit distinct bands at 1500 cm-1 (G- mode) and 1590 cm-1 (G+ mode), which are the criterion for determining the orderliness of SWCNTs. Besides, the D band at 1350 cm-1 and the 2D band at 2650 cm-1 illustrate the degree of disorder and stacking, respectively. Notably, the Raman peaks of SWCNTs dominate that of the PANI/SWCNT composite, from which it can be inferred that the fundamental backbone of the composite films is composed of SWCNTs. And after pressing, the decline in the intensity of the typical peaks at 1640 cm-1 and 1490 cm-1 (Figure 6(c)), the blue shift of G- band, and the increment of the G/D ratio (Figure 6(d)), suggest stronger π-π-conjugated interaction between PANI and SWCNTs [29, 46].Figure 6
Spectral and thermal analysis for PANI-DMSO, SWCNTs, PANI-DMSO/SWCNT, and PANI-DMSO/SWCNT-pressed, including (a) XRD patterns, (b) TGA traces, (c) FT-IR spectra, and (d) Raman spectra.
(a)(b)(c)(d)Furthermore, the temperature-dependent variation in the TE properties of PANI-DMSO/SWCNT composite films is investigated in the range of 25-145°C (Figure7(a)). With the rising temperature, the electrical conductivity drops steadily, reflecting a classic metallic conducting behavior of the films. In addition, the Seebeck coefficient first increases and then decreases with the rising temperature, obtaining the highest value of 49.04±0.15 μV K-1 at 125°C. Therefore, the power factor reaches a maximum value of 188.68±4.50 μW m-1 K-2 at 105°C, showing a broad prospect for this composite in utilizing low-grade waste heat.Figure 7
TE properties of PANI-DMSO/SWCNT composite films at (a) variable temperatures, (b) different bending cycles, (c) specific parameters (open circuit voltage, current and output power) of the TE module, and (d) thermal stability after different heating cycles.
(a)(b)(c)(d)Moreover, the flexibility of PANI-DMSO/SWCNT composite films at RT is examined by multiple bending cycles. After 1000 bending cycles, the PF can maintain about 75% of its original value (Figure7(b)), which might be ascribed to the instability of SWCNTs, manifesting that there is room for improvement in mechanical flexibility. At last, a prototype TEG with seven pairs of p-n junctions connected by copper wire and conductive silver adhesive was prepared to preliminarily examine the output behavior of TE device fabricated with such hybrid films (Figure S5a). In the fabrication of the TEG, p-type material is directly derived from PANI-DMSO/SWCNT composites after being pressed, and n-type material is converted from the p-type material doped by SDS and PEI, in which the rest of the operations are the same. As a result, the variation of each parameter (including open circuit voltage, current, and output power) of TEG with temperature difference is presented in Figure 7(c). In order to calculate the output power, we added the load resistance (R) of 1 kΩ; then, open circuit voltage (E) and output voltage (U) were recorded to analyze the internal resistance (r) of TEG. Under simulated skin contact conditions (ΔT=15°C), E=3.6±0.12 mV, U=2.1±0.10 mV, and R=1 kΩ, and r of TEG was estimated using
(2)r=E−UI=E−URU.AtΔT=15°C, r=714 Ω, this is considered to be a constant value in the variable temperature environment due to the stability of the overall structure in TEG.Moreover, the output power (P) is calculated using
(3)P=EI–I2r.Approximately,E and I grow in the same trend, rising linearly with the increasing temperature difference which indicates that r is a constant value. When ΔT≈50°C, E=10.27±0.15 mV, I=6.57±0.21 μA, P=36.65±1.18 nW, and powerdensity=11.54±0.37 nW/g with only seven couples of p- and n-legs, exhibiting the potential value in the energy reuse.To simulate the application of this TEG in harvesting body heat, the hot-end temperature was set at 37°C, while the cold end was in the ambience (around 22°C). As shown in FigureS5b, TEG generates the voltage of 3.6±0.20 mV, and output power is 4.17±0.32 nW, manifesting potential to harness low-grade waste heat in portable and wearable electronics. In addition, to test the thermal stability of TEG, the same PANI-DMSO/SWCNT composite was divided into different numbers of heating cycles, from 0 cycle to 100 cycles; in each heating cycle, the film was set into the vacuum environment at 105°C (optimal temperature) for 2 min. As shown in Figure 7(d), TE properties of the composites remained basically stable in the increment of heating cycles, and the PF was still maintained above 210 μW m-1 K-2 after 100 heating cycles, showing excellent thermal stability.


## 4. Conclusions

In summary, a series of PANI salts with different nanostructures were successfully prepared by adjusting the polarity of organic solvents. Compared with the control group in which PANI is composed of scattered nanoparticles and can be considered a “barbed” structure, the promotion of polarity from varying organic solvents (CYH, CCl4, MB, DCM, and DMSO) affords hollow nanotubes, nanobelts, nanowires, nanoclusters, and nanosphere due to the disparate growth of PANI NPs generated in the aqueous phase. Spectral analyses reflect quite distinct oxidation level of these PANI samples: PANI-DMSO has the highest degree of oxidization, while PANI-MB is in the emeraldine state, and the polymer chains of other PANI products are mainly composed of benzene-type structures. Combining with the effect of internal structure on carrier mobility, PANI-CCl4 (shaped like nanobelts, with 10 wt% SWCNTs) achieves a remarkable power factor of 5.13±0.56 μW m-1 K-2 at room temperature. High oxidation level together with excellent π-π-conjugated interaction with SWCNTs, at 50 wt% SWCNTs, PANI-DMSO/SWCNT composite reaches the maximum PF value of 178.10±3.77 μW m-1 K-2 after pressing. Moreover, a prototype TE generator with seven pairs of p-n junctions generates high voltage of 10.27±0.15 mV and output power of 36.65±1.18 nW when ΔT≈50°C. This work takes advantage of nanostructured PANI which is closely related to the TE properties of PANI/SWCNT hybrid materials. The high TE performance demonstrated in these composites is expected to be exemplary and beneficial for future research on polymer-based TE composites.



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*Source: 6989497-2023-02-07.xml*