Conductive Polymer-Based Thermoelectric Composites: Preparation, Properties, and Applications

: Thermoelectric (TE) materials are capable of realizing the direct conversion between heat and electricity, holding a giant prospect in the sustainable development of modern society. Conductive polymers (CPs) are suitable for the preparation of TE materials given their low-cost, lightweight, flexible, and easy processing properties. With the accelerating pace of flexible composite development, there is intensive interest in their emerging applications in various aspects such as wearable electronics and thermoelectric sensors. In order to further improve the thermoelectric properties, a series of new methods have been proposed to prepare conductive polymer-based thermoelectric composites and improve their thermoelectric properties. In this review, we discuss the compositing methods, properties, and applications of conductive polymer-based TE composites. The challenges and future development directions in the design and application of conductive polymer matrix composites are also pointed out.


Introduction
Nowadays, environmental issues and energy depletion are the global challenges we are facing, thus green, low-carbon, and sustainable development have become a worldwide consensus.Exploring a novel type of steady, efficient, and environmentally friendly energyconversion path to harvest secondary energy including waste heat, solar energy, and greenhouse gases is the key to facing these challenges [1].
Thermoelectric (TE) materials are a type of green and eco-friendly material, having the function of direct conversion between thermal energy and electrical energy depending on the mobility of solid internal carriers, even under very low temperature gradients relative to the environmental temperature.TE generators (TEGs) have the merits of a compact structure, no moving parts, zero emissions, and long lifetime.They can effectively improve the efficiency of waste heat utilization and greatly reduce carbon dioxide emissions [2,3].At present, TE materials have been applied in the high-tech field and military field to a certain extent such as in temperature control, power, waste heat harvesting, infrared detection, aerospace, and computer chip cooling [4].Generally speaking, TE materials mainly consist of traditional inorganic TE materials and organic TE materials.The former mainly depends on inorganic semi-conductors (such as PbTe, Cu 2 Se, SnSe, etc.) whose synthesis technology and performance research are relatively mature, but their flaws (like the raw material cost, mechanical properties, suitable condition, and difficulty of processing) restrict their wide application in industry [5][6][7][8][9].In contrast, the latter, represented by conductive polymers (CPs) including poly (3,4-ethylenedioxythiophene) (PEDOT) [10][11][12], polyaniline (PANI) [13,14], polypyrrole (PPy) [15][16][17], polycarbazole (PCz), and their derivatives have shown immense application potential in the TE field due to advantageous traits including abundant resource, facile synthesis, cost effectiveness, and low thermal conductivity (0.1-1 W m −1 K −1 ).However, the TE performance of organic TE (OTE) materials still lags far behind that of inorganic TE (ITE) materials.Therefore, improving the TE performance of OTE materials is crucial.In recent years, a large number of research results have shown that the formation of composite or hybrid TE materials is an effective means to improve the TE performance because composites can combine the advantages of constituents, enrich the processing means, reduce costs, and optimize the TE performance.In this review article, composites of CPs and carbon material and the composites of CPs with other inorganic components reported in recent years are mainly introduced [18][19][20].
TE effects generally refer to the Seebeck effect, the Peltier effect, and the Thomson effect.The Seebeck effect, also named as the first thermoelectric effect, originates from the asymmetry in the electron transport distribution arising from a temperature gradient, causing an electromotive force (Figure 1a).The Seebeck coefficient and the voltage are denoted as S and ε AB , respectively, and ε AB is defined as Equation (1).
where S AB is the relative Seebeck coefficient between material A and material B, which is decided by their own absolute Seebeck coefficient (S AB = S A − S B ), the unit is µV K −1 .
causing an electromotive force (Figure 1a).The Seebeck coefficient and the voltage are denoted as S and εAB, respectively, and εAB is defined as Equation (1).
where SAB is the relative Seebeck coefficient between material A and material B, which is decided by their own absolute Seebeck coefficient (SAB = SA − SB), the unit is μV K −1 .The Peltier effect, also named as the second thermoelectric effect, originates from the heat absorption and heat release phenomena at the two junctions of conductor A and conductor B, respectively, leading to the temperature gradient (as shown in Figure 1b).The heat QP absorbed or released due to the Peltier effect is named as Peltier heat, which is defined as Equation (2).
where ПAB is the relative Peltier coefficient between conductor A and B, the unit is V; I is the current in the circuit, the unit is A; t is the duration time, the unit is s.
The Thompson effect refers to the fact that when a current flows through a conductor with a certain temperature gradient, the conductor needs to absorb or release a certain amount of heat, except to generate irreversible Joule heat (as shown in Figure 2).The heat (QT) absorbed or released due to the Thompson effect is named as Thompson heat, which is defined as Equation (3).
where μ is the Thompson coefficient, V K −1 ; I is the current, A; t is the time, s; ΔT is the temperature gradient at the ends of the conductor.The Peltier effect, also named as the second thermoelectric effect, originates from the heat absorption and heat release phenomena at the two junctions of conductor A and conductor B, respectively, leading to the temperature gradient (as shown in Figure 1b).The heat Q P absorbed or released due to the Peltier effect is named as Peltier heat, which is defined as Equation (2).
where II AB is the relative Peltier coefficient between conductor A and B, the unit is V; I is the current in the circuit, the unit is A; t is the duration time, the unit is s.The Thompson effect refers to the fact that when a current flows through a conductor with a certain temperature gradient, the conductor needs to absorb or release a certain amount of heat, except to generate irreversible Joule heat (as shown in Figure 2).The heat (Q T ) absorbed or released due to the Thompson effect is named as Thompson heat, which is defined as Equation (3).
where µ is the Thompson coefficient, V K −1 ; I is the current, A; t is the time, s; ∆T is the temperature gradient at the ends of the conductor.The efficiency of a TE material is often expressed in a dimensionless thermoelectric figure of merit (ZT) [22][23][24], which can be written as where σ, κ, and T are the electrical conductivity (S cm −1 ), the thermal-conductivity (W m −1 K −1 ), and temperature (K), respectively.TE efficiency is denoted by the ZT value, which increases with elevated S, σ, and T, but decreases with elevated κ (Equation ( 4)).The electric performance part (S 2 σ) is the thermoelectric power factor (PF). Equation (4) shows that TE materials with high thermoelectric conversion efficiency should possess the properties of high S, high σ, and low κ.
Conductive polymer-based composites have drawn great attention, and many groups have been conducting research in this field.Although some researchers have previously reviewed the application of conductive polymers, there have been few comprehensive reviews on recent advances in the preparation and novel applications of conductive polymer-based thermoelectric composites.In this article, we review in more detail the various composite preparation methods, TE property tuning, and application states regarding conductive polymer-based TE composites including conductive polymer-carbon material composites, and conductive polymer-inorganic component composites.Furthermore, concluding remarks and suggestions as well as a brief outlook on future development in flexible conductive polymer-based composites are provided.

Preparation Method of Conductive Polymer-Based Composite Materials
At present, conductive polymers and inorganic materials such as Bi₂Te3, CuSe [25,26] thermoelectric composites have achieved excellent thermoelectric properties.At the same time, the composites formed by CPs and carbon nanotubes (CNTs), graphene, and other materials have also been widely studied.In such composites, the π-π interaction between molecules can make different components pile up more closely.Moreover, the high electrical conductivity of carbon materials can make up for the weak conductivity of CPs to a certain extent, and through the π-π interaction between the two components, molecular arrangement can be improved, which is conducive to carrier transport [27][28][29].
For composite materials containing CPs, the common preparation methods mainly include physical mixing, in situ polymerization/reaction, electrochemical polymerization/deposition, and layer-by-layer assembly.In the following section, these methods will be briefly introduced, and then the application of CP-based TE composites prepared by The efficiency of a TE material is often expressed in a dimensionless thermoelectric figure of merit (ZT) [22][23][24], which can be written as where σ, κ, and T are the electrical conductivity (S cm −1 ), the thermal-conductivity (W m −1 K −1 ), and temperature (K), respectively.TE efficiency is denoted by the ZT value, which increases with elevated S, σ, and T, but decreases with elevated κ (Equation ( 4)).The electric performance part (S 2 σ) is the thermoelectric power factor (PF). Equation (4) shows that TE materials with high thermoelectric conversion efficiency should possess the properties of high S, high σ, and low κ.
Conductive polymer-based composites have drawn great attention, and many groups have been conducting research in this field.Although some researchers have previously reviewed the application of conductive polymers, there have been few comprehensive reviews on recent advances in the preparation and novel applications of conductive polymer-based thermoelectric composites.In this article, we review in more detail the various composite preparation methods, TE property tuning, and application states regarding conductive polymer-based TE composites including conductive polymer-carbon material composites, and conductive polymer-inorganic component composites.Furthermore, concluding remarks and suggestions as well as a brief outlook on future development in flexible conductive polymer-based composites are provided.

Preparation Method of Conductive Polymer-Based Composite Materials
At present, conductive polymers and inorganic materials such as Bi 2 Te 3 , CuSe [25,26] thermoelectric composites have achieved excellent thermoelectric properties.At the same time, the composites formed by CPs and carbon nanotubes (CNTs), graphene, and other materials have also been widely studied.In such composites, the π-π interaction between molecules can make different components pile up more closely.Moreover, the high electrical conductivity of carbon materials can make up for the weak conductivity of CPs to a certain extent, and through the π-π interaction between the two components, molecular arrangement can be improved, which is conducive to carrier transport [27][28][29].
For composite materials containing CPs, the common preparation methods mainly include physical mixing, in situ polymerization/reaction, electrochemical polymerization/deposition, and layer-by-layer assembly.In the following section, these methods will be briefly introduced, and then the application of CP-based TE composites prepared by each method is illustrated.The TE properties of some typical conductive polymer-based composite materials are shown in Table 1.

Physical Mixing
The composites can be prepared by direct mixing of the two components.CPs and carbon materials are usually mixed in the liquid phase [30][31][32], and the two components in the composite are uniformly distributed.The π-π interaction between the polymer and the conjugated system of carbon materials will make the contact between the two components closer.
Liang et al. [33] used a physical mixing method to mix PPy nanowire powder with SWCNTs to successfully prepare PPy nanowire/SWCNT nanocomposites with unique morphology and high thermoelectric properties, as shown in Figure 3.The PF of the composite film could reach 21.7 ± 0.8 µW m −1 K −2 when the mass ratio of SWCNT to PPy nanowire was 60 wt%, around 987 times higher than that of the pure PPy nanowires.Polyaniline is a typical conductive material with the merits of facile synthesis and environmental stability.Toshima et al. [34] prepared composite materials by combining polyaniline with Bi 2 Te 3 through the physical mixing method.The thermoelectric properties were much higher than that of the composite prepared by solution mixing due to the dispersion of Bi 2 Te 3 nanoparticles in polyaniline.This shows that the preparation method of the composite film has a great influence on its thermoelectric properties.In recent years, Wang et al. [35] prepared PANI/Te nanorod composites with high thermoelectric properties by mixing PANI and Te nanorods in the liquid phase by the physical mixing method.By adjusting the content of the Te nanorods, the room temperature PF could be optimized to 105 µW m −1 K −2 .Subsequently, they used Bi 2 Te 3 nanorods and PANI to prepare n-type PANI-based composites [36].Flexible power generators were prepared using p-type PANI/Te nanorod composite materials and n-type PANI/Bi 2 Te 3 composite materials, which exhibited good air stability.Liu et al. [37] prepared PANI/SWCNT composite thin film integrated electrodes by simple physical mixing and solution deposition.Due to the strong π-π interaction between the conductive polymer and the carbon material, the composite obtained high capacitance and high stability.
each method is illustrated.The TE properties of some typical conductive polymer-based composite materials are shown in Table 1.

Physical Mixing
The composites can be prepared by direct mixing of the two components.CPs and carbon materials are usually mixed in the liquid phase [30][31][32], and the two components in the composite are uniformly distributed.The π-π interaction between the polymer and the conjugated system of carbon materials will make the contact between the two components closer.
Liang et al. [33] used a physical mixing method to mix PPy nanowire powder with SWCNTs to successfully prepare PPy nanowire/SWCNT nanocomposites with unique morphology and high thermoelectric properties, as shown in Figure 3.The PF of the composite film could reach 21.7 ± 0.8 µW m −1 K −2 when the mass ratio of SWCNT to PPy nanowire was 60 wt%, around 987 times higher than that of the pure PPy nanowires.Polyaniline is a typical conductive material with the merits of facile synthesis and environmental stability.Toshima et al. [34] prepared composite materials by combining polyaniline with Bi2Te3 through the physical mixing method.The thermoelectric properties were much higher than that of the composite prepared by solution mixing due to the dispersion of Bi2Te3 nanoparticles in polyaniline.This shows that the preparation method of the composite film has a great influence on its thermoelectric properties.In recent years, Wang et al. [35] prepared PANI/Te nanorod composites with high thermoelectric properties by mixing PANI and Te nanorods in the liquid phase by the physical mixing method.By adjusting the content of the Te nanorods, the room temperature PF could be optimized to 105 µW m −1 K −2 .Subsequently, they used Bi2Te3 nanorods and PANI to prepare n-type PANI-based composites [36].Flexible power generators were prepared using p-type PANI/Te nanorod composite materials and n-type PANI/ Bi2Te3 composite materials, which exhibited good air stability.Liu et al. [37] prepared PANI/SWCNT composite thin film integrated electrodes by simple physical mixing and solution deposition.Due to the strong π-π interaction between the conductive polymer and the carbon material, the composite obtained high capacitance and high stability.[38], based on the PEDOT: PSS/Bi0.5Sb1.5Te3system, compared the effects of film forming methods on the thermoelectric properties of composites in the physical mixing method.They found that the TE properties of the composite films  [38], based on the PEDOT: PSS/Bi 0.5 Sb 1.5 Te 3 system, compared the effects of film forming methods on the thermoelectric properties of composites in the physical mixing method.They found that the TE properties of the composite films obtained by dropping film were better than those obtained by spinning film, and this difference may be due to the morphological differences caused by different film forming methods.Guo's group [39] prepared PE/PEDOT/SWCNT ternary composite materials by the physical mixing method.The composite films were prepared by the vacuum filtration method and double solvent method, respectively.The composite films prepared by the double solvent method had higher TE properties, and the maximum PF value was 158.81 ± 8.83 µWm −1 K −2 .The introduction of polyethylene (PE) not only further improved the TE properties, but also gave the composite material high flexibility and self-healing properties.These results indicate that better TE properties may be obtained by choosing suitable composites of multiple conductive polymers.obtained by dropping film were better than those obtained by spinning film, and this difference may be due to the morphological differences caused by different film forming methods.Guo's group [39] prepared PE/PEDOT/SWCNT ternary composite materials by the physical mixing method.The composite films were prepared by the vacuum filtration method and double solvent method, respectively.The composite films prepared by the double solvent method had higher TE properties, and the maximum PF value was 158.81 ± 8.83 µWm −1 K −2 .The introduction of polyethylene (PE) not only further improved the TE properties, but also gave the composite material high flexibility and self-healing properties.These results indicate that better TE properties may be obtained by choosing suitable composites of multiple conductive polymers.

In Situ Polymerization/Reaction
It is easy to obtain better TE properties of CP-based composites by in situ polymerization than by physical mixing.In this process, monomers are first mixed with another component of the composite, and then the polymer is formed by the polymerization reaction.This method can make the polymerization products arrange in a tightly and orderly manner on the surface of another component to obtain the desired composite topography [28,40].
Wang et al. [41,42] fabricated multi-walled carbon nanotube/polypyrrole composites (MWCNT/PPy) with an in situ polymerization method using p-toluenesulfonic acid (TSA) as the dopant and FeCl3 as the oxidant, where the MWCNT content varied from 0 to 20 wt%.The PF value increased with the MWCNT content and the maximum value was 2.079 µW m −1 K −2 in this range, which was enhanced 25 times compared with the pure PPy.To investigate the effects of the preparation methods on the TE properties, this team also fabricated MWCNT/PPy using the in situ polymerization and interfacial polymerization methods, respectively, in which the MWCNT content varied from 0 to 68 wt%.Results showed that the composites prepared with the in situ polymerization method had higher electrical conductivity, and the PF reached 2.20 µW m −1 K −2 when the MWCNT content was 68 wt%, 27.5 times higher as that of the pure PPy.Baghdadi et al. [43] fabricated onedimensional nanocomposites based on polypyrrole-carbon nanotubes with the in situ

In Situ Polymerization/Reaction
It is easy to obtain better TE properties of CP-based composites by in situ polymerization than by physical mixing.In this process, monomers are first mixed with another component of the composite, and then the polymer is formed by the polymerization reaction.This method can make the polymerization products arrange in a tightly and orderly manner on the surface of another component to obtain the desired composite topography [28,40].
Wang et al. [41,42] fabricated multi-walled carbon nanotube/polypyrrole composites (MWCNT/PPy) with an in situ polymerization method using p-toluenesulfonic acid (TSA) as the dopant and FeCl 3 as the oxidant, where the MWCNT content varied from 0 to 20 wt%.The PF value increased with the MWCNT content and the maximum value was 2.079 µW m −1 K −2 in this range, which was enhanced 25 times compared with the pure PPy.To investigate the effects of the preparation methods on the TE properties, this team also fabricated MWCNT/PPy using the in situ polymerization and interfacial polymerization methods, respectively, in which the MWCNT content varied from 0 to 68 wt%.Results showed that the composites prepared with the in situ polymerization method had higher electrical conductivity, and the PF reached 2.20 µW m −1 K −2 when the MWCNT content was 68 wt%, 27.5 times higher as that of the pure PPy.Baghdadi et al. [43] fabricated one-dimensional nanocomposites based on polypyrrole-carbon nanotubes with the in situ polymerization method.The fabricated composites had a TE performance with 0.77 µV m −1 K −2 of PF and 1 × 10 −3 of ZT at 25 • C, because of the π-π stacking.
Liang et al. [19] prepared PPy/SWCNT composite materials by in situ chemical oxidation polymerization.First, SWCNTs were mixed with the pyrrole monomer by ultrasonication, and then ferric sulfate was used to initiate the polymerization reaction.After vacuum filtration, large-area, stretchable, ultra-flexible, and mechanically stable TE films were obtained.The preparation diagram is shown in Figure 5. Yao et al. [44] prepared nanocomposites based on CNTs and ordered PANI with an in situ polymerization method, in which single-walled nanotubes (SWCNTs) and aniline were used as the template and reactant, respectively.An ordered chain structure was formed in the process of the SWCNTdirected preparation.Furthermore, the values of the related TE parameters including electrical conductivity, S, and PF were far higher (2 orders of magnitude) than that of pure PANI.The PF value and ZT value could approach 2 × 10 −5 W m −1 K −2 and 0.004 at 25 • C, respectively, indicating the proposed way effectively contributed to enhanced TE properties.To investigate the effects of graphene content on the performance of P3HT, Saini et al. [45] prepared P3HT-graphene nanocomposites by the in situ polymerization of a 3-hexylthiophene monomer with graphene.The photoelectric and thermal properties of the P3HT composites were related to the graphene content, and such nanocomposites can be further used in organic device applications.To enhance the optical, electrical, and electrochemical properties of polycarbazole (PCz) nanocomposites, Das et al. [46] first fabricated a SnO 2 -PCz nanocomposite using one-pot synthesis coupled with the in situoxidative polymerization method.Results showed that the fabricated composite had better thermal stability than the pure PCz, and its TE performance was related to the SnO 2 content, according to which we could modulate its thermal and electrical properties effectively.Liang et al. [19] prepared PPy/SWCNT composite materials by in situ chemical oxidation polymerization.First, SWCNTs were mixed with the pyrrole monomer by ultrasonication, and then ferric sulfate was used to initiate the polymerization reaction.After vacuum filtration, large-area, stretchable, ultra-flexible, and mechanically stable TE films were obtained.The preparation diagram is shown in Figure 5. Yao et al. [44] prepared nanocomposites based on CNTs and ordered PANI with an in situ polymerization method, in which single-walled nanotubes (SWCNTs) and aniline were used as the template and reactant, respectively.An ordered chain structure was formed in the process of the SWCNT-directed preparation.Furthermore, the values of the related TE parameters including electrical conductivity, S, and PF were far higher (2 orders of magnitude) than that of pure PANI.The PF value and ZT value could approach 2 × 10 −5 W m −1 K −2 and 0.004 at 25 °C, respectively, indicating the proposed way effectively contributed to enhanced TE properties.To investigate the effects of graphene content on the performance of P3HT, Saini et al. [45] prepared P3HT-graphene nanocomposites by the in situ polymerization of a 3-hexylthiophene monomer with graphene.The photoelectric and thermal properties of the P3HT composites were related to the graphene content, and such nanocomposites can be further used in organic device applications.To enhance the optical, electrical, and electrochemical properties of polycarbazole (PCz) nanocomposites, Das et al. [46] first fabricated a SnO2-PCz nanocomposite using one-pot synthesis coupled with the in situ-oxidative polymerization method.Results showed that the fabricated composite had better thermal stability than the pure PCz, and its TE performance was related to the SnO2 content, according to which we could modulate its thermal and electrical properties effectively.

Electrochemical Polymerization/Deposition
Electrochemical polymerization is a simple and effective method for polymer synthesis.Due to the independence of the active sites and functional units of monomers, the molecular design is more flexible.Composite materials can be prepared by electrochemical polymerization/deposition in which the formation of TE composites can be realized in one step or two steps.
Canobre et al. [47] prepared PPy/CNT composite materials by the chemical synthesis method and electrochemical method, respectively.The results showed that due to the synergistic effect between the component materials, the PPy/CNT composite electrode prepared by the electrochemical method had a higher current density and coulomb efficiency than the electrode prepared by the chemical method.Wei et al. [48] synthesized PPy/SWCNT composites by electrochemical polymerization and studied the synergistic effect of the composites on oxygen reduction.The composites prepared by the electrochemical method showed a better synergistic effect than that through chemical polymerization, and afforded the best performance when the mass ratio of PPy to SWCNTs was 1:2.

Electrochemical Polymerization/Deposition
Electrochemical polymerization is a simple and effective method for polymer synthesis.Due to the independence of the active sites and functional units of monomers, the molecular design is more flexible.Composite materials can be prepared by electrochemical polymerization/deposition in which the formation of TE composites can be realized in one step or two steps.
Canobre et al. [47] prepared PPy/CNT composite materials by the chemical synthesis method and electrochemical method, respectively.The results showed that due to the synergistic effect between the component materials, the PPy/CNT composite electrode prepared by the electrochemical method had a higher current density and coulomb efficiency than the electrode prepared by the chemical method.Wei et al. [48] synthesized PPy/SWCNT composites by electrochemical polymerization and studied the synergistic effect of the composites on oxygen reduction.The composites prepared by the electrochemical method showed a better synergistic effect than that through chemical polymerization, and afforded the best performance when the mass ratio of PPy to SWCNTs was 1:2.
Guo et al. [49] prepared highly flexible poly(ANi-co-Py)/SWCNT TE material by the electrochemical polymerization method in which conductive glass was used as the working electrode.Results showed that the values of S and PF could approach 33.0 ± 3.2 µV K −1 and 83.2 ± 3.3 µW m −1 K −2 , respectively.The σ could approach 1035.3 ± 9.0 S cm −1 when the SWCNT content was 60 wt%.To further improve the TE performance of the poly(ANi-co-Py)/SWCNT composite, they adapted a similar method with the working electrode of a SWCNT strip and an auxiliary dopant of sodium dodecyl sulfate [50], and the prepared composite obtained 111.4 ± 3.2 µW m −1 K −2 at room temperature.Additionally, Guo et al. [51] developed PANI/SWCNT thermoelectric composites using a dynamic threephase interfacial electro-polymerization method by employing dimethyl sulfoxide as an additive.When the content of the SWCNTs reached 50 wt%, the PF value of the composite reached 236.4 ± 5.9 µW m −1 K −2 at room temperature.
Jiang et al.
[52] prepared PANI/SWCNT composite films by in situ electropolymerization.The composite of the two components was confirmed by polarization Raman spectroscopy and showed obvious anisotropy.The electrical conductivity of the composite film was obviously better than that of the conductive polymer, and the electrical conductivity value changed with the change in the measurement direction.Therefore, the composite material is an excellent material for the preparation of an efficient ammonia sensor.Chen and Guo [12] prepared PEDOT by the dynamic three-phase interface electropolymerization method, and then synthesized flexible composites with SWCNTs by the physical mixing method, as shown in Figure 6.The composites exhibited good TE properties with a maximum PF of 253.7 ± 10.4 µW m −1 K −2 .Furthermore, to further improve the TE performance, SWCNTs were acidified before the PEDOT/SWCNT composites were fabricated, of which the maximum PF could reach as high as 350.0 ± 47.6 µW m −1 K −2 at 298 K and 510.2 µW m −1 K −2 at 412 K.These studies present a new strategy for the preparation of high-performance composite thermoelectric materials.
Guo et al. [49] prepared highly flexible poly(ANi-co-Py)/SWCNT TE material by the electrochemical polymerization method in which conductive glass was used as the working electrode.Results showed that the values of S and PF could approach 33.0 ± 3.2 µV K −1 and 83.2 ± 3.3 µW m −1 K −2 , respectively.The σ could approach 1035.3 ± 9.0 S cm −1 when the SWCNT content was 60 wt%.To further improve the TE performance of the poly(ANi-co-Py)/SWCNT composite, they adapted a similar method with the working electrode of a SWCNT strip and an auxiliary dopant of sodium dodecyl sulfate [50], and the prepared composite obtained 111.4 ± 3.2 µW m −1 K −2 at room temperature.Additionally, Guo et al. [51] developed PANI/SWCNT thermoelectric composites using a dynamic three-phase interfacial electro-polymerization method by employing dimethyl sulfoxide as an additive.When the content of the SWCNTs reached 50 wt%, the PF value of the composite reached 236.4 ± 5.9 µW m −1 K −2 at room temperature.
Jiang et al.
[52] prepared PANI/SWCNT composite films by in situ electropolymerization.The composite of the two components was confirmed by polarization Raman spectroscopy and showed obvious anisotropy.The electrical conductivity of the composite film was obviously better than that of the conductive polymer, and the electrical conductivity value changed with the change in the measurement direction.Therefore, the composite material is an excellent material for the preparation of an efficient ammonia sensor.Chen and Guo [12] prepared PEDOT by the dynamic three-phase interface electropolymerization method, and then synthesized flexible composites with SWCNTs by the physical mixing method, as shown in Figure 6.The composites exhibited good TE properties with a maximum PF of 253.7 ± 10.4 µW m −1 K −2 .Furthermore, to further improve the TE performance, SWCNTs were acidified before the PEDOT/SWCNT composites were fabricated, of which the maximum PF could reach as high as 350.0 ± 47.6 µW m −1 K −2 at 298 K and 510.2 µW m −1 K −2 at 412 K.These studies present a new strategy for the preparation of high-performance composite thermoelectric materials.

Microwave Plasma
Ibrahim et al. [53] prepared PANI/CNT nanocomposite films by the microwave plasma polymerization method, followed by ultrasonically mixing the aniline and carbon nanotubes and injecting them into the plasma reactor.After that, the plasma functionalized the carbon tube surface and polymerized the aniline simultaneously.The polymerization reaction begins on the walls of the carbon tubes.In this process, the thickness of the composite film can be controlled by the load of carbon nanotubes and the time of the spray pulse.Through a series of characterization, it was confirmed that the CNTs were well-

Microwave Plasma
Ibrahim et al. [53] prepared PANI/CNT nanocomposite films by the microwave plasma polymerization method, followed by ultrasonically mixing the aniline and carbon nanotubes and injecting them into the plasma reactor.After that, the plasma functionalized the carbon tube surface and polymerized the aniline simultaneously.The polymerization reaction begins on the walls of the carbon tubes.In this process, the thickness of the composite film can be controlled by the load of carbon nanotubes and the time of the spray pulse.Through a series of characterization, it was confirmed that the CNTs were well-dispersed in the composite film, the combination of the CNTs and PANI was confirmed, and the composite film possessed high electrical conductivity.

Sequential Infiltration Synthesis
The sequential infiltration synthesis (SIS) method is a kind of atomic layer deposition that can control the proportion and material distribution of the two components by changing the SIS conditions (temperature, time, etc.).Ham et al. [54] prepared a PANI-InO x composite film with high electrical conductivity using the sequential permeation method.The composite film showed a higher pseudo-capacitance than the single-phase component, possibly because the oxygen vacancy at the interface of the two components could enhance the charge transfer.

Layer-by-Layer Assembly
Layer-by-layer assembly can separate and alternate different components into films, thus forming a composite material.The π-π interaction between different layers in the assembly process can produce a more ordered molecular arrangement and ideal morphology.Yu et al. [55] developed composite materials assembled by layers, and at the same time introduced electrochemical polymerization to construct multi-layer structural composites.They first dispersed MWCNTs in solution with polydiallyl dimethyl ammonium chloride (PDDA) and sodium deoxycholate (DOC), respectively, and then assembled the MWCNTs in layers by alternating between the two solutions.The MWCNT/PEDOT composites were then synthesized by the electrochemical polymerization of MWCNT multilayer films.The highest power factor obtained was 155 µW m −1 K −2 .
Table 1.Preparation method of typical CP-based composites and TE properties at room temperature.

Solvent Evaporation
Manna's group [56] prepared Cu 2 Te/P3HT composite materials by solvent evaporation and self-assembled Cu 2 Te into a nanodisk network in an rr-P3HT thin film by solvent evaporation.Cu 2 Te and P3HT were uniformly combined to form a mutually transmitted nanostructure.This special composite structure showed a strong photocurrent and improved the overall photoelectric response.Therefore, the solvent evaporation method has great application prospects in the future.

Applications
CPs with economic significance, good environmental stability, optical, and electronic properties have drawn increasing attention in the formation of high-performance TE composites.Applications of CPs and CP-based TE composites include sensors, supercapacitors, electromagnetic shielding electromagnetic interference (EMI), catalytic applications, corrosion suppression, and so on [57].Applications in sensors include biosensors, wearable sensors, and so on.A detailed description of these applications is as follows.

Sensors
CP-based composite materials have the potential for the fabrication of miniaturized sensors that enable small sample volumes, portability, and high-density arrays.

Wearable Sensor
Sadi and Kumpikait ė [58] developed a high-performance wearable sensor based on a PPy/SWCNT/polydopamine (PDA) composite material, in which PPy could significantly improve the electrical conductivity, PDA improved the absorption of SWCNT molecules, and in practical applications, the material showed high electrical conductivity and durability.Gao et al. [59] combined PEDOT: PSS with SWCNTs to develop a wearable self-powered dual-mode sensor with a high S that could independently detect changes in pressure and temperature.This has good application prospects in the field of electronic skin and personal health detection.Based on PEDOT: PSS and reduced graphene oxide (rGO) composite materials, Du et al. [60] developed a self-powered thermoelectric wearable sensor with simple structure that could accurately identify various hand movements, with great application prospects in motion detection.Based on the PEDOT: PSS and CNT composite, He et al. [61] prepared a new self-supporting and self-powered temperature-strain sensor that could effectively detect strain deformation and temperature change under self-powered conditions; this sensor also had a high mechanical stress and stable thermoelectric performance.In order to prepare wearable thermoelectric materials suitable for waste heat recovery, Li et al. [62] prepared CNTs/PANI composite fibers with different CNT contents by the wet spinning method.These composites had efficient carrier transport and superior orientation.When the content of carbon nanotubes was 84%, the S, σ, and PF could reach 30.8 µV K −1 , 1856 S cm −1 , and 176 µW m −1 K −2 , respectively, which has great application potential in the field of wearable thermoelectrics.

Fire Warning Sensor
Chen et al. [63] developed a sensing system for fire warning based on the P3HT/SWCNT composite material, which not only used the thermoelectric effect, but also increased the photoelectric effect.The synergistic effect of light and heat enabled the device to have a strong output voltage and response time.The high thermoelectric properties of the P3HT/SWCNT composite materials could accurately identify and in a timely manner warn of fire, which has strong application potential in daily life and safety protection.Du et al. [64] prepared a wearable fire alarm material by the in situ polymerization of PPy with montmorillonite and ammonium polyphosphate.This material has good flame retardancy and accurate sensing ability, and has a good application prospect in the field of fire warning and electronic textiles in the future.

Other Types of Sensors
In order to improve the performance of glutamate biosensors, Maity and Kumar [65] prepared glutamate biosensors by modifying MWCNT/PPy composites with platinum nanoparticles.A series of characterization and tests showed that the material had obvious advantages of high sensitivity and short response time.Jiang et al. [66] fabricated a coaxial Ag/polymer nanowire array based on super wettability-based nanofabrication and polymeric swelling-induced resistance change, which was used as an organic gas sensor.This sensor exhibited outstanding performance, even the toluene gas content was very low (0.5 ppm).Xu et al. [67] prepared a CP hydrogel for strain sensors based on hydroxypropyl methyl cellulose, PEDOT: PSS, and polyacrylamide.The sensors had a wide detection range and ultra-high sensitivity, of which the values were 2-1600% and GF of 2.97-17.58with short response time and good stability.
3.1.4.Some Special Applications of CPs-Based Composite Sensors Roy et al. [68] prepared taste sensors by means of SWCNTs and electropolymerized pyrrole, which could distinguish various tea leaves by the impedance measurement technology of the sensing electrode.In addition, Wu et al. [69] prepared a sensor by wrapping a PPy/SWCNT composite material on a Au electrode, which could quickly detect the local anesthetic levobupivacaine.Min et al. [70] prepared a PPy/SWCNT composite material with a large specific surface area and high electron transfer rate.The composite material of tyrosinase-PPy-SWCNT was prepared by combining it with the biosensor tyrosinase.The amperometric detection of dopamine obtained a high sensitivity and low detection limit, which has great potential in the application of neurological diseases.

Supercapacitors
Supercapacitors are highly expedient energy storage units that are widely used in portable electronic devices, powering vehicles, and so on [71].Meng et al. [72] fabricated highly flexible self-supporting porous graphene/PANI films with high-performance supercapacitor applications using a simple template method.The supercapacitor rate performance could remain at 94% when the current density changed from 0.5 to 10 A g −1 , while that value was maintained at 89% for the 3D-rGO film under the same conditions.To improve the flexibility and stability of PEDOT:PSS used for supercapacitors, Du et al. [73] fabricated PEDOT:PSS/CNP, where CNFs was used as the building blocks.The optimized PEDOT:PSS/CNP was suitable for the electrodes of supercapacitors, and the supercapacitor assembled based on the fabricated composite material had excellent electrochemical performance with only 4.2% loss in capacitance after 10,000 charge/discharge cycles.
[74] described a three-component composite based on PPy and MWC-NTs doped with tetra(n-octyl) ammonium bromide.The electrochemical and mechanical stability were enhanced due to the presence of tetra(n-octyl) ammonium bromide.The specific capacitance was 600 F g −1 , about four times as high as that for the polymer without tetra(n-octyl) ammonium bromide.Liu et al. [37] prepared PANI/SWCNT composite thin film integrated electrodes.Due to the strong π-π interaction between the conductive polymer and the carbon material, the composite obtained a high capacitance and high stability.Results showed that the capacitance could achieve 446 F g −1 , accompanied by good stability, losing less than 2% capacitance compared with the fresh film after 13,000 charging/discharging cycles.Banu et al. [75] used PCz/magnesium oxide cobalt (MgCo 2 O 4 )/graphene oxide ternary nanocomposites as active electrodes and applied them to supercapacitors.Through an electrochemical study of the material, it was found that it had high power and energy storage capacity, and that the composite material had high stability compared with the material applied alone.

Corrosion Inhibition
Corrosion usually occurs when metals are exposed to wet or non-neutral environments.Murat et al. [76] fabricated PCz, PCz/nanoclay, and PCz/Zn films using chemical and electrochemical methods.When a 3.5 wt% NaCl solution was used to test the anticorrosion efficiency, the composite material prepared by the chemical method had a stronger anti-corrosion ability.Zhu et al. [77] reported a novel composite coating based on PPy and functionalized graphene (ZGP) composites.The hydrophilic groups and PPy films improved the dispersion of the coating, and the ZGP raw material significantly improved the corrosion resistance of the composite coating.The performance tested in 3.5 wt% NaCl solution showed that the composite coatings based on PPy and ZGP-2 possessed excellent anti-corrosion properties.This opens up a new way for the future application of PPy-based composites in corrosion inhibition.

EMI Shielding
To deal with the issue of electromagnetic pollution, also named EMI, arising from electronic devices and wireless communication, EMI shielding materials are necessary.The prepared ferrite nanoparticles were functionalized to PANI via in situ polymerization [78].To enhance the EMI shielding, based on the PANI obtained from the chemical oxidative method and NiFe nanoparticles, nanocomposite films that served as an EMI shielding material were fabricated [79].The maximum shielding effectiveness was achieved (52 dB) when the thickness of the composite film was 0.5 mm.

Catalytic Applications
The PANI composite can be used in coupled reactions [80].PANI/Pd nanotubes synthesized by the templating method were also used as chemical catalysts [81].Huang et al. fabricated and characterized a PANI/Au composite with the hydrothermal method, where the reactive template was FeCl 3 and methyl orange [82].In addition, amperometry and CV were used to trail the electrocatalytic response.The electrode modified with the PANI/Au composite had higher catalytic activity compared with that modified with the pure PANI.
In past decades, CP-based composites have made great progress and have made significant achievements in wearable devices and biointegration applications.Among them, PEDOT-based composites are the most widely studied TE materials with the best thermoelectric properties, and have great application potential.Although the thermoelectric performance of the PPy-based composites is not very high, it also has great research significance because of its low cost.TE materials have great potential for future practical applications, for example, thermoelectric devices can be used to convert the heat of automobile exhaust into electricity to power the automobile.Installing thermoelectric equipment in a data center room can convert the huge amount of heat in the room into electricity, which then powers the computers, and so on.Although we are still a long way from functional application needs for CP-based composite TE materials, the practical and efficient application of thermoelectric materials will be realized in the future through further innovation in the synthesis and selection of composite materials.

Conclusions and Outlook
Conductive polymer-based TE composites have witnessed continuing advances in recent years.This review presents the preparation, properties, and applications of conductive polymer-based TE composites in which typical conductive polymers such as PPy, PANI, P3HT, PEDOT, etc. are included.As elaborated in this review article, the addition of high-electrical conductivity carbon materials or other inorganic materials into conductive polymers can effectively improve the electrical conductivity of conductive polymers.At the same time, the composites, along with the generation of interfaces among components, usually affect the material morphology, carrier transport mechanism, etc., so the thermoelectric performance can be further improved.If there is π-π interaction between the two components like in binary composites, the molecular arrangement will be modified, which is conducive to efficient carrier transport.In addition, conductive polymer-based TE composites have a wide range of application prospects, and research in wearable devices and implantable electronic products is also increasingly expected.Although conductive polymer-based composites are currently one of the hot research directions and have achieved high thermoelectric properties, the underlying mechanism in the composite formation process is not quite clear, more preparation methods have yet to be developed, and the efficiency of some devices in application is far from ideal.Therefore, it is necessary to further improve the energy collection efficiency and expand the application scenarios while maintaining their flexibility and mechanical properties.
To enhance the TE performance of conductive polymer-based TE composites and explore the in-depth mechanism in forming composites, researchers should focus on designing new conductive polymers, optimizing the conjugate main and side chains, adjusting the positions of substituents, and devising new doping strategies in the future.In addition, new materials such as MXenes, quantum dots, MOFs, etc. can also be tried to compound with conductive polymers.Rational design of the structure and distribution of inorganic fillers, optimization of interface transport, improvement in the compositing method, etc. will undoubtedly help to establish a comparatively solid theoretical basis and further improve the thermoelectric properties of composite materials.In terms of application, the stretchability of the wearable device prepared by the conductive polymerbased TE composite material is a great advantage, and the subsequent improvement should lie in the enhancement of the conversion efficiency of the device under the premise of ensuring the stretchability.Thermoelectrics should advance hand in hand with other energy sources like light, sound, magnetoelectricity, etc. to develop integrated thermoelectric, photothermoelectric, acoustic thermoelectric, and other element conversion equipment.Therefore, it is of great significance to develop much wider application scenarios such as fire warnings, medical assistance, etc., and explore multi-functional flexible wearable devices.

Figure 2 .
Figure 2. Schematic illustration of the Thomson effect.

Figure 2 .
Figure 2. Schematic illustration of the Thomson effect.

Figure 3 .
Figure 3. Preparation procedure of the nanocomposite free-standing films.Reproduced with permission [34].Copyright 2016, Elsevier Ltd.Given its good TE performance and environmental stability, PEDOT-based TE composites have attracted great attention from researchers.Kim et al. 31] reported the preparation of a PEDOT: PSS/CNT thermoelectric composite using the physical mixing method, where PEDOT: PSS, doped with DMSO, was used to disperse CNTs in water.The schematics of CNTs dispersed by GA and PEDOT: PSS and the formation of a segregated network are shown in Figure 4. Results show that the composite with the best TE properties in the present work contained 35 wt% SWCNT and 35 wt% PEDOT: PSS, achieving a ZT value of 0.02.Du et al.[38], based on the PEDOT: PSS/Bi0.5Sb1.5Te3system, compared the effects of film forming methods on the thermoelectric properties of composites in the physical mixing method.They found that the TE properties of the composite films

Figure 3 .
Figure 3. Preparation procedure of the nanocomposite free-standing films.Reproduced with permission [34].Copyright 2016, Elsevier Ltd.Given its good TE performance and environmental stability, PEDOT-based TE composites have attracted great attention from researchers.Kim et al. 31]  reported the preparation of a PEDOT: PSS/CNT thermoelectric composite using the physical mixing method, where PEDOT: PSS, doped with DMSO, was used to disperse CNTs in water.The schematics of CNTs dispersed by GA and PEDOT: PSS and the formation of a segregated network are shown in Figure4.Results show that the composite with the best TE properties in the present work contained 35 wt% SWCNT and 35 wt% PEDOT: PSS, achieving a ZT value of 0.02.Du et al.[38], based on the PEDOT: PSS/Bi 0.5 Sb 1.5 Te 3 system, compared the effects of film forming methods on the thermoelectric properties of composites in the physical mixing method.They found that the TE properties of the composite films obtained by dropping film were better than those obtained by spinning film, and this difference may be due to

Figure 4 .
Figure 4. Schematics of CNTs dispersed by GA and PEDOT: PSS (a,b) and the formation of a segregated network during the drying of water-based polymer emulsions that occurred after the addition of stabilized CNTs (c,d) [31].Copyright © 2010, American Chemical Society.

Figure 4 .
Figure 4. Schematics of CNTs dispersed by GA and PEDOT: PSS (a,b) and the formation of a segregated network during the drying of water-based polymer emulsions that occurred after the addition of stabilized CNTs (c,d) [31].Copyright © 2010, American Chemical Society.

Figure 5 .
Figure 5. (a) Fabrication procedure for the TE film.(b) A photograph of the fabricated nanocomposite TE film [19].Royal Copyright 2016, Society of Chemistry.

Figure 5 .
Figure 5. (a) Fabrication procedure for the TE film.(b) A photograph of the fabricated nanocomposite TE film [19].Royal Copyright 2016, Society of Chemistry.