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Review

PANI-Based Thermoelectric Materials

College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China
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Authors to whom correspondence should be addressed.
Organics 2025, 6(3), 33; https://doi.org/10.3390/org6030033
Submission received: 27 April 2025 / Revised: 5 June 2025 / Accepted: 26 June 2025 / Published: 22 July 2025

Abstract

Polyaniline (PANI) based thermoelectric materials have attracted much attention in flexible energy harvesting devices due to their unique molecular structure, excellent chemical stability, and low cost. However, the intrinsic thermoelectric performance of intrinsic PANI makes it difficult to meet the needs of practical applications due to its low electronic transport properties. This review focuses on the preparation methods and key strategies for developing high-performance PANI-based thermoelectric materials. It aims to comprehensively update knowledge regarding synthesis methods, microstructures, thermoelectric properties, and underlying mechanisms. The overall goal is to provide timely insights to promote the development of high-performance PANI-based thermoelectric materials.

Graphical Abstract

1. Introduction

In this generation, the depletion of fossil fuels has plunged the world into a dual energy and environmental crisis. The overexploitation of these resources has led to drastically diminishing reserves, economic constraints, and ecological degradation, while emissions from combustion are contributing to climate change and pollution. Against this background, thermoelectrics have emerged as a promising solution, showing broad application prospects in temperature gradient power generation and refrigeration technologies [1,2,3,4,5]. The heat to electricity conversion mechanism in thermoelectric materials is primarily driven by the Seebeck effect [6,7]. This phenomenon can be described as follows: under a temperature gradient, charge carriers (electrons or holes) in the semiconductors migrate from the hot end to the cold end. Their accumulation at the cold end generates an internal potential difference upon balance, enabling the direct conversion of thermal energy into electrical energy. The Seebeck coefficient (S) is defined as the ratio of the induced thermoelectric voltage (ΔV) to the applied temperature gradient (ΔT), S = ΔV/ΔT. The energy conversion efficiency is determined by the dimensionless thermoelectric figure of merit zT, expressed as follows:
z T = S 2   σ T κ
where σ is the electrical conductivity, κ is the total thermal conductivity, and T is the absolute temperature. S, σ and κ are the three significant parameters that determine the zT value, which influence and constrain each other. The σ can be expressed as follows:
σ = n q μ
where n is the carrier concentration, q is the elementary charge, and µ is the mobility. Notably, the product of S2σ is defined as the power factor (PF), which is a comprehensive metric for evaluating the electrical performance of thermoelectric materials.
Based on the type of charge carriers, thermoelectric materials can be classified into p-type (dominated by holes) and n-type (dominated by electrons) [8,9,10]. To date, high-performance thermoelectric materials mainly include bismuth telluride (Bi2Te3), lead telluride (PbTe), silicon germanium (SiGe) alloys, and others [11,12]. Although these materials exhibit high zT values (zT ~1), they face multiple challenges, including reliance on scarce elements such as tellurium, elevated raw material costs, and the inclusion of toxic substances, like lead [13]. Additionally, their synthesis requires energy-intensive high-temperature sintering processes with significant material consumption. Their inherently poor mechanical properties hinder their application in emerging trends that require shaped, flexible, or miniaturized device configurations [14,15]. Conductive polymers, usually composed of abundant light elements such as C, H, O, combine resource sustainability with environmental compatibility. They effectively circumvent the dependence of traditional thermoelectric materials on scarce elements and have become an emerging focus in this field [16]. Most conductive polymers exhibit p-type thermoelectric behavior. Typical representatives include poly(3,4-ethylene dioxy-thiophene) (PEDOT), polyacetylene (PA), polyaniline (PANI), polypyrrole (PPy), and polythiophene (PT), as shown in Figure 1 [17,18,19]. Among these, PANI has garnered significant research interest, due to its distinctive combination of tunable doping level, conductivity, and chemical stability [20].
In contrast to inorganic semiconductor materials, conductive polymers have a quasi-one-dimensional molecular structure, with the main chain consisting of carbon atoms hybridized by sp2 to form alternating single and double bonds (Figure 2a) [21]. The sp2 hybridized orbital consists of four bonds. Three of these bonds overlap in a head-to-head manner to form a stable domain-specific electronic structure with high bond energies and low domain separation [22]. The fourth bond is the π-electron, a non-bonding electron in the sp2 orbital in the PZ direction. These π electrons overlap in a side-by-side fashion along the orbitals of the carbon atoms that are arranged consecutively in the main chain of the conductive polymer, which in turn form π bonds. With the electrons on the π-bonds being unpaired electrons, their bond energies are low and easy to leave the domain, allowing for free transport between atoms [17,22]. The large delocalized π-bonds provide a large carrier transport channel for carrier transport. Currently, most of the conductive polymers are p-type semiconducting materials, and their electrical conducting mechanism depends on the removal of electrons from the highest occupied molecular orbital (HOMO) energy level by oxidizing agents, which generates hole carriers [17,21]. Migration of these carriers is realized by delocalized or interchain jumps in the conjugated π-system, and the reduction of the HOMO-lowest unoccupied molecular orbital (LUMO) energy gap further enhances the σ. Therefore, electrical conductance can be conferred by chemical or electrochemical doping. The conducting mechanism of PANI is different from that of other conductive polymers in that it is achieved by protonate doping rather than the conventional redox mechanism [23,24,25].
At present, among the various structural models of PANI, the benzoquinone structure proposed by MacDiarmid is the most widely accepted model [26]. Specifically, PANI consists of both oxidizing and reducing units. The oxidizing and reducing units in the PANI molecule can be converted into each other by redox reactions. The degree of oxidation of PANI can be expressed by the magnitude of the n value. When the n value fluctuates between 0 and 1, the corresponding apparent color and thermoelectric properties of PANI change (Figure 2b–e). When n = 1 and m = 0, PANI is in a fully reduced state, and the molecular chain is an all-benzene ring structure (Figure 2b); when n = 0 and m = 1, PANI is in a fully oxidized state, and the molecular chain is a repeating structure with one benzene ring and one quinone ring arranged alternately (Figure 2c); n = m = 0.5 for the intermediate oxidized state consisting of three benzene rings and one quinone ring (Figure 2d). This state, which is often referred to as the emeraldine base (EB), is neutral. When it is doped (protonated), i.e., When it is doped (protonated), i.e., when the acid protonates the imine nitrogen, it is called emeraldine salt (ES) (Figure 2e). At room temperature, the ES formed by acid-doped PANI has high stability and excellent σ, which is considered to be the most valuable form of PANI for applications.
PANI typically exhibits a random heterogeneous microstructure consisting of a combination of crystalline and amorphous domains. This structural feature is significantly different from the homogeneous and highly crystalline crystal structure commonly found in inorganic thermoelectric materials. In conductive polymers, crystalline domains consist of ordered stacks of rigid conjugated chains with strong π–π interactions, which facilitate efficient electron migration along these ordered paths. In contrast, amorphous domains have limited electron transport due to the disordered arrangement of molecular chains (Figure 2f) [22,27]. Therefore, PANI can be regarded as multiphase systems consisting of highly conductive crystalline domains dispersed and embedded in an amorphous matrix with low conductivity. The classical conduction models of PANI include hopping conduction and percolation theory: (1) for the former, at low doping levels or disordered states, charge carriers hopping between localized states, is often described by variable range hopping (VRH) or Arrhenius-type thermal activation; (2) for the latter, when conducting domains in PANI for a network, the connectivity beyond a percolation threshold enables macroscopic conduction.
Quantum conduction models of PANI mainly include the polaron and bi-polaron models and the Su–Schrieffer–Heeger (SSH) models:
(1) Polaron and bi-polaron model: The undoped state of EB is an insulator or semiconductor with low σ. The doping process involves a protonation of the polymer backbone by a protonic acid, thereby introducing more carriers and significantly improving the σ to obtain ES. This process not only increases the n but also triggers a local transformation of the polymer backbone from a benzene ring (benzenoid) to a quinone ring (quinoid) conformation, accompanied by localized structural distortion [27,28,29]. Polarons are induced by proton doping and are localized radical cation states with a certain delocalization ability along the main chain, which helps to improve the µ of carriers. As the doping level increases, polarons can further couple to form bi-polarons, whose energy levels are closer to the Fermi level, which reconstructs the electronic structure while enhancing the delocalization and density of carriers, thereby significantly improving the conductive properties of polyaniline [27,28,29].
(2) SSH model: This model is a seminal quantum mechanical model originally developed to describe the electronic properties of polyacetylene, a linear conjugated polymer [30]. While PANI is chemically more complex, key concepts from the SSH model—especially those related to electron–phonon coupling, bond alternation, and soliton/polaron formation—have been adapted to explain charge transport in PANI. In PANI, polarons and bi-polarons act as the primary charge carriers upon doping. These quasi-particles cause local lattice distortions, which modify the electronic structure and create in-gap states, enabling band-like or hopping transport depending on disorder and doping level.
Density functional theory (DFT) calculations provide an in-depth understanding of the changes in the electronic structure of PANI at different doping levels [31,32,33,34,35]. According to calculations performed by Adrián et al. [35] using the PBEh hybrid functional, EB exhibits two distinct optical transition energies of approximately 2.2 eV and 4.0 eV, respectively. This is consistent with the absorption peaks measured in the experiment (~2.0 eV and 3.8 eV). Among them, the low-energy transition corresponds to the intrachain charge transfer transition between the reduced state (benzene ring) and the oxidized state (p-benzoquinone structural unit) in the aniline chain, while the high-energy transition originates from the typical π–π*. After the ES is formed by proton doping, new polaron and bi-polaron states are introduced into the system within the original energy gap. These intermediate states allow the emergence of more electronic transition paths, resulting in multiple absorption peaks at approximately 1.6 eV, 2.2 eV, and 4.1 eV, and 4.1 eV. This is in good agreement with the measured 1.5, 2.8, and 4.1 eV by experiment. This formation not only enriches the electronic state density but also increases the n and μ , which helps to improve the thermoelectric properties of doped PANI.
However, as a thermoelectric material, although PANI has the advantages of low cost, low thermal conductivity, and easy processing compared with inorganic materials, it still has the significant drawbacks of low σ and low S, which leads to poor PF, restricting its application in the thermoelectric field [36,37]. To overcome the above bottlenecks, the research focus of PANI-based thermoelectrics in recent years has been mainly concentrated on two aspects: one is to regulate its electronic structure and carrier behavior through chemical doping strategies to improve its intrinsic thermoelectric performance; the other is to introduce inorganic functional phases to construct composite materials in order to achieve synergistic optimization of PF and heat transport behavior [38,39]. This study focuses on reviewing recent progress in the development of PANI and its composites, and analyzes the preparation process, as well as the structure-property correlation and the underlying mechanism. At the same time, the challenges for future research in this field are indicated.

2. Synthesis of PANI-Based Thermoelectric Materials

The thermoelectric properties of intrinsic PANI make it difficult to meet the demands of practical applications. Most current research focuses on the synthesis of an organic–inorganic composite system, as shown in Table 1 [40]. The core strategy is to incorporate inorganic nanofillers (such as carbon nanotubes (CNTs), graphene, etc.) into the PANI matrix to form composites with heterogeneous interfaces, and optimize the electrical properties and enhance phonon scattering [19,41,42]. The main preparation methods include mechanical mixing, solution-mediated mixing, in-situ polymerization, and layer-by-layer self-assembly. Recently, interfacial polymerization was also developed to enhance the thermoelectric performance of PANI without the incorporation of inorganic phases.

2.1. Mechanical Mixing

The mechanical mixing method is a basic process for the preparation of organic–inorganic composite thermoelectric materials. This method uses PANI powder and inorganic nanofillers (Bi2Te3, CNTs, etc.) as the raw materials, which are mechanically mixed by ball milling. The mixture is then consolidated by pressing or sintering. For example, Zheng et al. [43] mixed BiCuSeO with PANI powder by ball milling, and hot pressed this into the BiCuSeO/PANI composite. Although the mechanical mixing method has the advantage of easy operation, the inorganic nanoparticles are prone to agglomeration during the mixing process, making it difficult to achieve uniform dispersion within the PANI matrix.

2.2. Solution-Mediated Mixing

The solution-mediated mixing method is employed to disperse the PANI and inorganic nanoparticles in a solution medium, achieve uniform dispersion through stirring or ultrasound, and obtain the composite material by evaporation of the solvent, filtration, or freeze-drying and other methods.
Prabu et al. [44] dispersed PANI and AgBiSe2 in N,N-dimethylformamide (DMF) under stirring and ultrasonication. The mixture was then poured into a surface dish, evaporated and dried to obtain PANI/AgBiSe2 composite films. Ugraskan et al. [45] dispersed graphitic carbon nitride (g-C3N4) into a solution of PANI-camphor-sulfonic acid (CSA) in m-cresol, followed by drop-coating and drying to obtain a composite film. However, the dispersion of PANI by this method is poor, resulting in poor homogeneity. To improve the dispersion of PANI, Singh et al. [46] milled PANI with CSA, and dispersed this into m-cresol under stirring, then introduced g-C3N4 into the solution under ultrasonication; the mixture was coated on the substrate dropwise and dried into a uniform film (Figure 3a). Wang et al. [47] dissolved CSA-doped PANI and 3,6-dioctyldecyloxy-1,4-benzenedicarboxylic acid (SAS) in m-cresol under stirring and obtained a series of PANI-SAS thin films with different SAS contents by drop-drying. The van der Waals force interactions between the long alkyl chains of SAS and the PANI molecular chains induced a more extended conformation and a more ordered arrangement along the PANI molecular chains. However, an excess of SAS produced agglomeration, which was not conducive to optimizing electrical properties. Ozlek et al. [66] utilized the solution-mediated dispersion method to make slurries of graphene with different contents of graphite and 12 wt% of PANI–CSA. Graphite/graphene/polyaniline (GGP) composites were obtained by scratch-coating and drying. When the content of graphene exceeds 14 wt%, it is easy to agglomerate and deteriorate the thermoelectric properties, and this can be partially mitigated by applying pressure to destroy the aggregation zone as shown in Figure 3b,c. Yao et al. [48] dispersed CSA doped PANI (as the matrix material) into m-cresol (as the solvent) and incorporated single-walled CNTs (SWCNTs) by magnetically stirring. Finally, the PANI/SWCNT composite thin films with a gradient distribution of SWCNT content were prepared by drop-coating (Figure 3d–g). It was found that the SWCNTs could be uniformly dispersed in the polymer matrix to form a continuous phase at low SWCNT loading; however, with the elevation of the SWCNT content, the nanotubes agglomerated due to the π–π interactions and van der Waals forces, forming a locally enriched region. Therefore, how to effectively inhibit the agglomeration of the dispersed phase in the solution-mediated mixing method is still a core problem.

2.3. Layer-by-Layer Self-Assembly

Layer-by-layer (LbL) self-assembly is a method that utilizes LbL alternating deposition to form molecular aggregates with an ordered structure through spontaneous binding of the layers with the help of strong interactions (e.g., covalent bonding) or weak interactions (e.g., electrostatic attraction, hydrogen bonding, coordination bonding, etc.) between molecules in each layer. It has the advantages of adjustable composition, thickness, and structure, wide material compatibility, and mild preparation conditions. The organic–inorganic composites prepared by this technique can achieve a homogeneous composite of two phases at the molecular scale. PANI/GP/PANI/DWNT multilayer composite films were prepared by LbL self-assembly using protonate-doped PANI, ionic surfactant-treated negatively charged graphene (GP), and negatively charged double-walled CNTs (DWNT) by Cho et al. [49,50]. Subsequently, the above ionic surfactants were replaced with a high-conductivity conducting polymer, PEDOT:PSS, which promotes the dispersion of graphene and CNTs in solvents. Meanwhile, the surface of graphene and CNTs was modified in the self-assembled material to generate new LbL self-assembled PANI/GP-PEDOT:PSS/PANI/DWNT-PEDOT:PSS multilayer composite thermoelectric films, which is conducive to the enhancement of the thermoelectric properties of the composite film (Figure 4a). Cho et al. [51] assembled PANI/DWNT-GO films by alternately layering PANI and DWNT in water via an assembly method and suspending the CNTs in water using GO as an effective dispersant. The composite formed a three-dimensional network of nanotubes with polymer-like entanglements as GO stripped DWNT and dispersed uniformly in the film.

2.4. In-Situ Polymerization

In-situ polymerization is the reaction of unsaturated bonds or functional groups in monomer molecules under specific reaction conditions (e.g., initiators, catalysts, or other specific reaction conditions), which causes the monomer molecules to link together to form long chains or networks of polymer molecules [23,52]. This reaction allows for the quantitative synthesis of polymers with a specific degree of oxidation and in a highly conductive state. Typically, this method triggers the oxidative polymerization of monomers with the aid of a water-soluble initiator in an acidic medium. The oxidation process of An is closely related to the type of oxidizing agent, while it is significantly dependent on several reaction conditions, such as the concentration of An, the pH of the reaction system, the ratio of oxidizing agent to An, the selected reaction solvent, and the reaction temperature [67]. Various combinations of oxidant types and reaction conditions lead to differences in the rate and extent of An oxidation, as well as in the structure and properties of the products.
For instance, Ji et al. [53] synthesized a bilayer structure by performing in situ polymerization of HCl doped PANI directly on a highly conductive SWCNT film (Figure 4b). The interfacial properties between the SWCNT film and PANI can be modulated by tuning the An polymerization rate. At high An concentrations, rapid polymerization leads to PANI chain aggregation, forming a loosely adhered layer with limited π–π interactions and weak interfacial coupling (Figure 4c). Conversely, at low An concentrations, the reduced polymerization rate enables the SWCNT film to template PANI growth directionally, resulting in a compact and well-defined layer (Figure 4d). Huang et al. [54] used CNTs as templates to grow PANI on CNTs by in-situ polymerization. The SEM photographs clearly show that the relatively smooth CNTs bundles are entangled and stacked with each other to form a continuous and dense two-dimensional network structure (Figure 4e), whereas, in the composite material, the surface of the CNTs bundles shows a snowflake-like morphology (Figure 4f), which is mainly attributed to the polymerization of PANI on the CNTs.
Li et al. [68] dispersed CNTs in an An solution using in-situ polymerization to obtain PANI/SWCNTs composites, followed by sequential de-doping and doping processes to design the interface. Due to the strong π–π interactions, the ordered PANI chains with protonated conformation were arranged along the CNT surface, forming a highly ordered PANI interfacial layer with excellent σ. Wang et al. [55] prepared PANI/MWCNTs by electrochemical polymerization on SWCNTs, with the SWCNT as the working electrode, the Ag/AgCl electrode as the reference electrode, and the platinum mesh as the counter electrode. The synthesized emerald salts at high deposition voltage (~1 V) showed a spherical shape. In the PANI/Bi2Si2Te6 nanosheet composites prepared by in situ oxidative polymerization, the thickness of the PANI layer covering the surface of Bi2Si2Te6 nanosheets increased from 3 nm to 18 nm, when PANI was increased from 5 to 19 wt%. At high PANI content, the interlayer π–π stacking density increased, and the PANI skeleton was ordered along the (010) crystallographic direction to construct a three-dimensional continuous conductive pathway [56].

2.5. Interfacial Polymerization

Interfacial polymerization is primarily conducted at the interface between two phases (e.g., an aqueous phase and an organic phase). Typically, an aqueous phase containing An is mixed with an oxidizing agent solution and then brought into contact with another immiscible organic phase (e.g., toluene or chloroform) to form a liquid–liquid interface. The oxidizing agent triggers the oxidative dehydrogenation of An at the interface, leading to the growth of polyaniline chains [69]. Ultimately, the polymer is deposited at the interface in the form of thin films or particles. The morphology and thickness of the polyaniline can be precisely controlled by regulating the reaction time, temperature, oxidizing agent concentration, and interface properties. For example, Lee et al. [52] suggested that, when the synthesis of PANI was carried out in the traditional aqueous system, the oxidative polymerization of An monomers in acidic medium was prone to trigger side reactions such as neighbor coupling and Michael addition due to the concentration of reaction sites on the surface of the particles and in the inner part of the particles, leading to the branching of the molecular chain or cross-linking and the formation of a disordered structure. For inhibition of the side reaction, chloroform was introduced as the organic phase to construct a heterogeneous reaction system, and PANI was enriched at the interface to form a stable colloid at low temperatures due to its amphiphilic property, i.e., the hydrophilic ammonium group and hydrophobic benzene ring; the oxidizing agent, ammonium persulfate, preferentially triggered the interfacial polymerization of the monomers in the aqueous phase by releasing the radicals, and with the increase in molecular weight, the polyaniline became insoluble in the aqueous phase and the organic phase. The biphasic insolubility drives polymer chains toward the interface, generating a growth-constrained microenvironment. Within this region, interfacial tension induces dominant dyadic coupling, enabling directional alignment of molecular chains. This process triggers radial polymer growth to self-assemble nanofibers, which subsequently undergo phase-directed aggregation via the dispersed phase, ultimately forming a three-dimensional honeycomb architecture (Figure 5a). Compared with the traditional disordered products, this interfacial polymerization strategy significantly enhanced the structural regularity of polyaniline, suppressed the side reactions, and provided a morphologically controllable synthetic route for a high-performance thermoelectric conductive polymer. Lee et al. [23] went on to obtain PANI with a low density of structural defects by preparing it in a biphasic solution, which was then filtered, washed and dried, and deprotonated to obtain PANI EB, which exhibits typical metallic behavior in the range of 5 K to 300 K after doping with camphor sulfonic acid. The material exhibits a continuous decrease in resistivity with lowering temperature, while its infrared spectra align perfectly with the Drude model, confirming a metal-like diffusion mechanism of free carriers. XRD analysis reveals that the tightly π–π stacked intermolecular chains (face-to-face spacing ~3.5 Å) possess an ultralow defect density, a structural feature that significantly enhances carrier mobility. This work pioneers the demonstration of metallic transport properties in conductive polymers, matching the behavior of conventional metals.
Zhang et al. [57] also used a bidirectional interfacial approach to add an anionic surfactant to the surface of an aqueous hydrochloric acid solution, which could assist the An monomers to preorganize and polymerize the monomers into ordered polymeric chains under the anionic head groups of the surfactant monolayer, while forming a confined environment between the surfactant monolayer and the water surface to provide space for the formation of the 2D PANI lattice (Figure 5b). The PANI film size is 130–160 µm2 and thickness varies from tens to hundreds of nanometers (Figure 5c). Wang et al. [58] demonstrated that PANI morphology can be precisely modulated by tuning the molar ratio of ammonium persulfate (APS) to an An monomer ([APS]/[An]). At low APS ratios (0.1–0.3), An undergoes slow oxidation to generate short-chain oligomers, which are doped with sulfosalicylic acid (SA) to form phenazine-structured nanosheets (PANI0.1) and comb-like lamellae (PANI0.3) (Figure 5d,e). Increasing the APS ratio to 0.5 accelerates polymerization, yielding long-chain conjugated linear PANI nanowires with delocalized electrons that enhance σ (Figure 5f). At a ratio of 0.7, protonation induces chain segment curling, leading to nanotube formation (PANI0.7) (Figure 5g). When the ratio reaches 0.9, SA/An micelles self-assemble into hollow microspheres via hydrogen bonding, while exothermic polymerization drives inner monomer expansion to rupture the shell, creating perforated hollow structures (PANI0.9) (Figure 5h). Excess APS (ratio 1.2) triggers simultaneous oxidant penetration inside and outside the micelles, resulting in particles (PANI1.2) (Figure 5i).
Two-dimensional polyaniline nanosheets (~30 nm) were synthesized at low temperatures using the ice template method by Choi et al. [59]. The system guides the orderly assembly of monomers through ice hydrogen bonding orientation, and synergistically low temperature inhibits the three-dimensional disordered growth and substrate protonation erosion to ensure interfacial stability (Figure 5j). Structural characterization shows that the PANI films synthesized by this method have a long-range orthorhombic crystal system characteristic system (P222, a = 17.356, b = 7.012, c = 8.579 Å), in which the b-axis π–π stacking (3.51 Å) and c-axis backbone arrangement (4.29 Å) construct anisotropic conductive channels (Figure 5l). X-ray diffraction analyses show that the strong diffraction signals of (020) crystalline surface (Figure 5k) originate from the lattice expansion mode dominated by π–π stacking at the edge of the conjugated benzene ring, confirming the ice template-induced growth mechanism of the molecular planar optimum orientation. This work provides a new idea for the interface-limited synthesis of 2D conjugated polymers through the synergistic regulation of hydrogen bonding orientation and dynamics.

3. Mechanisms for Enhancing Thermoelectric Properties of PANI-Based Materials

PANI exhibits significant promise for thermoelectric applications owing to its tunable doping level and robust environmental stability. However, its practical application faces substantial limitations stemming from intrinsically low PF. For example, acid-doped PANI pressed samples exhibit pronounced molecular chain disorder, which critically impedes both σ and the S, leading to a low PF of 0.28 μWm−1K−2 [60]. Research indicates that forming composites of PANI with highly conductive inorganic carbon materials (e.g., graphene, carbon nanotubes, etc.) offers a viable route to markedly enhance thermoelectric performance. The record-high PF (2710 μWm−1K−2) of PANI-based materials was achieved in the PANI/Graphene–PEDOT:PSS/PANI/DWNT–PEDOT:PSS multilayer composite by Cho et al. (Table 1) [52]. This substantial performance enhancement is attributed to synergistic interfacial mechanisms. Firstly, strong π–π stacking interactions among PANI and PEDOT:PSS and carbon nanomaterials promote planarization of the PANI molecular chain. This enhances the degree of π-conjugation and facilitates carrier mobility through interfacial conjugation coupling. Secondly, the inorganic phase co-assembles with PANI via interfacial interactions, including hydrogen bonds, to establish a three-dimensional conductive network. This extended network elongates charge transport path and bolsters structural stability. Consequently, effectively tuning the interfacial structure and electronic coupling between PANI and inorganic nanofillers has emerged as a critical research focus for optimizing their comprehensive thermoelectric performance. Current investigations primarily target four strategic avenues: (1) modulation of doping levels; (2) ordering of molecular chains; (3) organic–inorganic interface effect; (4) bridging effects. These approaches provide a foundational framework for developing high-performance PANI-based thermoelectric composites.

3.1. Modulation of Doping Level

The intrinsic state of PANI typically exhibits extremely low σ (~10−7 S cm−1), which can be enhanced by 6–8 orders of magnitude to ~101–103 S cm−1 through the introduction of delocalized carriers (e.g., polarons/bi-polarons) via protonic acid doping, as shown in Table 1. Common doping methods include chemical doping, electrochemical doping, and protonic acid doping [70]. Typical dopants encompass inorganic acids (e.g., HCl, H2SO4) and organic acids such as CSA and dodecyl-benzenesulfonic acid (DBSA). By precisely controlling the doping level, the carrier concentration can be modulated, thereby optimizing the thermoelectric performance through balancing σ and the S [71]. Li et al. [61] optimized the thermoelectric properties of CNTs/PANI composites by modulating the CSA to PANI molar ratio. It was revealed that increasing the CSA doping level induces a conformational transition in PANI chains from compact coils to an extended, ordered chain-like structure, establishing predominantly polaronic charge transport. This structural reorganization substantially enhances interchain electronic coupling and charge transport. In contrast, low doping levels promote charge localization within PANI chains, increase quinone unit density, and reduce the σ (Figure 6a). Horta-Romarís et al. examined PANI doped with dodecyl benzene sulfonic acid (DBSA) across molar concentrations. They observed a near-constant S (~1 μV K−1) but progressively increasing σ with doping level, reaching a maximum of ~30 S cm−1 at 1.25 M DBSA. This behavior arises from steric hindrance between the hydrophobic alkyl chains of DBSA and the PANI backbone, which restricts conformational freedom and enhances chain rigidity. The resultant structural constraint promotes a transition from disordered coiled conformation to planarized chain arrangements (Figure 6c,d).
In a study of HCl-doped PANI, Noby et al. [62] demonstrated a strong correlation between doping level and σ. As the HCl concentration increased from 0.1 M to 5 M, the σ of PANI rose from 1.5 S cm−1 to 3.7 S cm−1. This enhancement was attributed to two synergistic effects at elevated acid concentrations: (1) optimized π–π stacking and reduced defect density, and (2) improved molecular ordering that facilitates continuous conductive pathway formation. They collectively lowered the charge-transport barriers and increased the μ, resulting in an improvement in the overall σ.

3.2. Ordering of Molecular Chains

In PANI-based thermoelectric materials, the orientational ordering of molecular chains serves as a critical strategy for modulating thermoelectric performance [63,64]. Ideally, highly ordered PANI chains aligned along the π–π stacking direction can establish low-energy-barrier delocalized charge transport channels, enabling concurrent enhancement of σ and S. Researchers have engineered molecular chain alignment to synergistically enhance both σ and S, proposing a novel strategy to achieve high thermoelectric performance in PANI. Zhang et al. [57] were the first to build a restricted space at the interface of the hydrochloric acid solution by an anionic surfactant, which induced the oriented alignment and ordered polymerization of An monomers to form a 2D PANI with a long-range ordered structure. The material exhibits a rhombohedral lattice (a = b = 20.8 Å, γ = 115°) and Cl ions bridged densely stacked multilayers (layer spacing of 3.59 Å), with highly ordered molecular chains forming large-size crystal domains in the range of 130–160 µm2. The enhanced molecular chain ordering significantly strengthens the electron delocalization effect through improved π–π stacking, thereby achieving an out-of-plane σ of 15 S cm−1.
In addition to the directional polymerization of molecular chains through the optimization of reaction conditions, the construction of organic–inorganic composite systems can further improve molecular ordering through multiscale synergism [65,72]. Specifically, the inorganic component can act as a structure-directing agent to provide a template for the molecular chain arrangement, while the interfacial charge transfer can enhance the consistency of the π–π stacking, thus jointly optimizing the thermoelectric properties [73,74,75]. Ji et al. [53] reported the preparation of composite films of a PANI/SWCNT bilayer structure with HCl by in situ surface polymerization. When the concentration of An was gradually increased, the S showed a rising trend and peaked at about 22 μV K−1. Meanwhile, the σ decreased from about 1600 S cm−1 to about 350 S cm−1 with the increase of An concentration. This phenomenon originates from the amorphous and curly PANI chains produced by the intensive and rapid polymerization process in high concentration An solution, which leads to a decrease in σ due to the formation of discontinuous charge transport paths. With the decrease in An concentration, the reaction rate slows down and π–π interactions play a dominant role in the An polymerization on the surface of SWCNTs. The PANI chain arrangement under the low-concentration condition exhibits higher ordering and densification compared with that of the PANI layer generated in the high-concentration system, as shown in Figure 6e, thus leading to the enhancement of σ. In addition, SWCNTs exhibit a synergistic effect in the in situ polymerization of PANI, inducing the formation of a phenazine structure with intrinsically high S in the PANI molecular chains, which effectively enhances the S of the composites.
Li et al. [76] prepared CNT/PANI nanofibers by wet spinning (Figure 6f). The σ of the CNT/PANI fibers gradually increased with the increase in CNT content. When the mass fraction of CNTs in CNTs/PANI fibers reaches 84 wt%, the σ of the fibers can reach up to 1856 S cm−1. The increase in σ is mainly attributed to the introduction of a network of intrinsically highly conductive CNTs and the π–π conjugation between PANI and SWCNTs, which induces an ordered arrangement of PANI on SWCNTs, facilitating carrier transport within the fibers and thus optimizes the thermoelectric properties (Figure 6g,h).
The structural components of graphene are similar to those of CNTs, both of which are derived from sp2 hybridized carbon-based material systems. Among them, the two-dimensional planar ordered structure of graphene forms a strong π–π conjugated coupling effect with PANI. Based on the above structural characteristics, the PANI/graphene composite system has also become an important thermoelectric system. In the graphite/graphene/polyaniline (GGP) composites prepared by Ozlek et al. [66], the π–π interaction between graphene and PANI-CSA promotes the orderly arrangement of PANI chains on the graphene surface, forming a stable conductive network and enhancing the transport capacity of electrons between graphite layers. When the graphene content exceeds 14 wt%, the agglomeration phenomenon is significant, resulting in a decrease in thermoelectric performance. For the composite strategy of improving the orderliness of PANI molecular chains, in addition to carbon-based materials, structural optimization can also be achieved through synergy with other functional materials. For example, inorganic nanomaterials (such as metal oxides and metal nanoparticles): induce directional chain arrangement through template effects and interfacial charge transfer; ionic liquids: guide chain orientation assembly through ordered ion layers; biopolymers (cellulose nanocrystals): achieve regular chain coating through hydrogen bonds or electrostatic effects; conductive polymer: blends synergistically reduce chain entanglement through π–π stacking, etc. [77,78,79,80,81,82].

3.3. Organic–Inorganic Interface Effect

Another mechanistic explanation for the enhanced thermoelectric performance of PANI/inorganic composites involves the synergistic effect of the organic/inorganic interface, specifically through the energy filtering effect induced by the interfacial energy barrier and optimized carrier transport. The former arises from the formation of an energy barrier at the organic/inorganic interface, which selectively transports high-energy carriers through an energy filtering effect, thus increasing the S. The latter originates from the highly conductive layer or chemical bonding formed at the interface, which provides a low resistance transmission path for carriers, effectively reducing contact resistance and improving σ. These combined mechanisms overcome the limitations of traditional S and σ values, enabling decoupling optimization of the parameters of thermoelectric performance.
Wang et al. [55] reported that PANI/SWCNT bilayer films prepared by electrochemical polymerization exhibited excellent thermoelectric properties at room temperature with a maximum PF of 155.3 ± 7.2 μW m−1 K−2, corresponding to a σ of 654.9 ± 41.8 S cm−1 and a S of 48.7 ± 0.5 μV K−1. This performance enhancement originates from the π–π interaction between the PANI and SWCNT interfaces, which can optimize the carrier transport paths, while the interfacial energy filtration effect significantly improves the S of the composite films.
Wen et al. [83] developed PEDOT PSS/SWCNT@PANI composite fibers with a dual-interfacial structure, showing a σ of 2472 ± 23.3 S cm−1, a S of 43.5 ± 0.7 μV K−1, and an optimal PF of 467.8 ± 10.5 μW m−1 K−2. As shown in Figure 7a, the energy barrier between PEDOT: PSS and SWCNTs in this composite is 0.12 eV, which is theoretically conducive to the filtration of low-energy carriers. However, in practice, the poor dispersion of SWCNTs in the PEDOT:PSS matrix results in a limited number of interfaces between the two, which significantly limits the full utilization of the interfacial effect. To overcome the defects of the binary composites, a PANI layer was introduced to promote the uniform dispersion of SWCNTs in the PEDOT:PSS matrix, forming a double interface and generating a double energy barrier. The energy barriers between SWCNT/PANI and PANI/PEDOT:PSS were 0.08 eV and 0.04 eV, respectively. The uniformly distributed interfacial structure significantly improves the S by selectively screening out low-energy carriers [84]. Meanwhile, the PEDOT:PSS/SWCNT@PANI composite fiber further achieves overall improvement in the thermoelectric properties due to the highly oriented and aligned structure (Figure 7b).
In the PANI/SWCNT/Te ternary composites prepared by Wang et al. [85], there is a significant difference in the work function: 4.8 eV for PANI/SWCNT and 4.95 eV for PANI/Te. (Figure 7c). The interfacial barrier formed can effectively inhibit the transport of low-energy carriers, thus significantly improving the S of the system. In addition, the one-dimensional nanostructures possessed by SWCNT and Te help to improve the interfacial contact and enhance the carrier transport. Based on this, the synergistic interfacial effects enable the composite system to achieve a high PF of 101 μW m−1 K−2 (Figure 7d).
Wang et al. [77] effectively controlled the interface electronic structure of PANI/CNT composites using HCl doping. Before doping, the PANI/CNT composite exhibited a σ of ~0.43 S cm−1 and a S of 131 μV K−1. Following doping with HCl gas, the σ increased significantly to around 29.95 S cm−1, while the S decreased slightly to 124 μV K−1. The HOMO and LUMO of PANI were determined to be 5.03 eV and 3.78 eV, respectively. CNTs possess metallic characteristics and a band-gap-free feature. A notable energy barrier of ~0.6 eV between PANI and CNT, selectively allows high-energy carriers to pass through, thereby enhancing the S. Upon HCl doping, the σ of PANI is significantly augmented. Nevertheless, due to π–π interactions affecting PANI nestled between CNTs, the doping level remains insufficient, maintaining the interface barrier. Consequently, a synergistic optimization of thermoelectric performance is achieved (Figure 7e–g). This suggests that adjusting the interfacial band structure, particularly the energy barrier height, proves instrumental in enhancing the thermoelectric properties of polymer based composites.

3.4. Bridging Effects

Another theoretical explanation for the significant and unusual increase in the σ of organic/inorganic nanocomposites is that this phenomenon may be closely related to the role of inorganic nanostructures, especially inorganic nanowires, as “charge migration bridges” within the composite system. Specifically, nanowires can form continuous or quasi-continuous conductive channels between organic substrates, thus providing efficient migration pathways for charge carriers (e.g., electrons or holes). This bridging effect significantly reduces electron scattering and interfacial barriers, thereby improving overall electron transport efficiency [86]. In addition, the anisotropic structure and high aspect ratio of the nanowires contribute to the formation of an efficient network of electron channels, thereby enhancing the σ. Li et al. [86] reported that, in synthesized PANI/CNT composites, the CNT/PANI film exhibited excellent thermoelectric performance at a CNT content of 94 wt%, achieving a maximum σ of approximately 2012 S cm−1 and a S of around 37.0 μV K−1. At room temperature, the film attained a power factor of 273 μW m−1 K−2, which is more than 5 and 43 times those of the pure CNT and pure PANI films, respectively (Figure 7h). This remarkable enhancement can be attributed to the unique interconnected structure of the composite. Specifically, pure CNT exhibits a loosely bundled morphology, with these bundles randomly arranged, making it difficult to form a uniform network film (Figure 7i). In contrast, in the 94 wt% CNT/PANI composite, PANI effectively promotes the orderly stacking of CNT bundles (Figure 7j). Simultaneously, the interconnected PANI acts as a conductive bridge between adjacent CNT bundles. The low concentration of PANI ensures strong interfacial interactions with the CNTs, significantly reducing the charge hopping barrier between neighboring CNTs and thereby greatly enhancing charge transport across the entire network.

4. Conclusions and Outlook

Compared with inorganic thermoelectric materials, PANI-based materials have significant advantages, such as low cost, easy processing, and environmental friendliness. However, their thermoelectric properties are still limited by low intrinsic electrical transport properties, and the development process has long lagged behind that of inorganic materials. Recent research has focused on enhancing the thermoelectric performance of PANI-based materials through various mechanisms. This review provides a comprehensive overview of the synthesis methods, microstructures, thermoelectric properties, and underlying mechanisms. It can be concluded that multiscale interfacial synergies between nanofillers and polymer matrices enhance the performance of PANI-based thermoelectric materials. Specifically, inorganic nanophases (e.g., CNTs, and Bi2Te3) can significantly modulate the alignment and stacking of polymer molecular chains. Functional groups (e.g., carboxyl, hydroxyl) or delocalized π-electronic systems (e.g., the sp2-hybridized structure of CNTs) on the nanofiller surfaces can induce oriented alignment of PANI chains along the filler surface via hydrogen bonding, π–π interactions, or electrostatic forces. This orientational ordering can effectively reduce the disordered entanglement defects between the polymer chains and form a long-range continuous conjugated structure network, thus reducing the activation energy barrier for carrier-leap transport between the chain segments. Meanwhile, the ordered stacking of molecular chains can enhance the π-electron cloud overlap and broaden the migration channel of delocalized polaritons, further improving intrinsic mobility. On the other hand, the energy barrier formed by the energy level mismatch at the organic/inorganic heterogeneous interface can produce an energy filtering effect, which allows high-energy carriers to pass through the interfacial barrier while low-energy carriers are selectively scattered, leading to enhanced S. The above mechanisms can effectively improve the performance of PANI-based thermoelectric materials through a synergistic effect.
Future research could develop novel interfacial modification/polymerization methods to enhance the thermoelectric properties of PANI-based materials: (1) modification strategies could include introducing special functional groups onto the aniline backbone to tailor the microstructure and physical properties of PANI; (2) employing various methods such as emulsion polymerization and enzyme catalysis, it is possible to effectively construct ordered interfaces and enhance carrier transport behavior, thereby providing new research insights for the design and construction of high-performance PANI thermoelectric materials. Furthermore, most of the current understanding of the thermoelectric properties of PANI is derived from experimental observations. However, these explanations are largely based on speculative interpretations of empirical results, and no rational model has yet been established. In particular, there is no systematic theoretical framework to explain and regulate the thermoelectric properties in PANI-based composites. While extensive studies have focused on p-type PANI thermoelectrics, research on n-type PANI remains critically underdeveloped, particularly regarding stable doping strategies and carrier transport mechanisms. Therefore, establishing a unified theoretical framework—integrating carrier transport mechanisms, microstructure-property relationships, and doping strategies—is essential to rationally design high-performance PANI based thermoelectrics beyond current empirical approaches.

Author Contributions

Writing—original draft preparation and data curation, M.C.; validation, D.X.; resources, H.Z.; writing—review, editing and funding acquisition, P.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by NSAF with grant number U2230131. And the APC was funded by MDPI.

Acknowledgments

The authors acknowledge Priority Academic Program Development of Jiangsu Higher Education Institutions and Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University.

Conflicts of Interest

The authors declare no competing financial interests.

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Figure 1. Common conductive polymer structures: poly (3,4-ethylene dioxy-thiophene) (PEDOT), poly-pyrrole (PPy), polyacetylene (PA), polythiophene (PT) and polyaniline (PANI).
Figure 1. Common conductive polymer structures: poly (3,4-ethylene dioxy-thiophene) (PEDOT), poly-pyrrole (PPy), polyacetylene (PA), polythiophene (PT) and polyaniline (PANI).
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Figure 2. (a) Schematic illustration of sp2 hybridization and conjugated π bonds in conductive polymers. Aniline (An) in different redox states: (b) Completely reduced state (c) Completely oxidized state (d) Intermediate state (e) Doped state. (f) The microstructure of PANI randomly composed of crystalline and amorphous domains.
Figure 2. (a) Schematic illustration of sp2 hybridization and conjugated π bonds in conductive polymers. Aniline (An) in different redox states: (b) Completely reduced state (c) Completely oxidized state (d) Intermediate state (e) Doped state. (f) The microstructure of PANI randomly composed of crystalline and amorphous domains.
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Figure 3. (a) Schematic flow diagram of the preparation of PANNII/g-C3N4. Reproduced from [46], with permission. (b) Interfacial SEM images of GGP films containing 14 wt% graphene before and after (c) hot pressing. Reproduced from [66], with permission. Scanning electron microscopy (SEM) images of SWNT/PANI composite films with varying SWNT content: (d) 0 wt%, (e) 30 wt%, (f) 64 wt%, and (g) 80 wt%. At lower SWCNT loading (f), CNTs demonstrate homogeneous dispersion, while higher SWNT concentrations (g) result in significant CNT aggregation. Reproduced from [48], with permission.
Figure 3. (a) Schematic flow diagram of the preparation of PANNII/g-C3N4. Reproduced from [46], with permission. (b) Interfacial SEM images of GGP films containing 14 wt% graphene before and after (c) hot pressing. Reproduced from [66], with permission. Scanning electron microscopy (SEM) images of SWNT/PANI composite films with varying SWNT content: (d) 0 wt%, (e) 30 wt%, (f) 64 wt%, and (g) 80 wt%. At lower SWCNT loading (f), CNTs demonstrate homogeneous dispersion, while higher SWNT concentrations (g) result in significant CNT aggregation. Reproduced from [48], with permission.
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Figure 4. (a) Illustration of the LbL deposition process. Reproduced from [50], with permission. (b) Schematic representation of the PANI/SWCNT bilayer composite manufacturing process. Reproduced from [53], with permission. (c) SEM images of pure SWCNT film. (d) PANI surface grown on SWCNT film. Reproduced from [53], with permission. (e) CNT and (f) CNTF/PANI composites; the inset is a magnified view. Reproduced from [54], with permission.
Figure 4. (a) Illustration of the LbL deposition process. Reproduced from [50], with permission. (b) Schematic representation of the PANI/SWCNT bilayer composite manufacturing process. Reproduced from [53], with permission. (c) SEM images of pure SWCNT film. (d) PANI surface grown on SWCNT film. Reproduced from [53], with permission. (e) CNT and (f) CNTF/PANI composites; the inset is a magnified view. Reproduced from [54], with permission.
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Figure 5. (a) Three-dimensional honeycomb structure of EB. Reproduced from [52], with permission. (b) Mechanism of PANI surface polymerization (for simplicity, Cl counter ions are not shown) and molecular structure of 2D PANI. APS: ammonium persulfate; SMAIS: surfactant monolayer-assisted interfacial synthesis. Reproduced from [57], with permission. (c) AC-HRTEM image of PANI. Reproduced from [57], with permission. (di) Scanning electron microscope images of PANI0.1, PANI0.3, PANI0.5, PANI0.7, PANI0.9 and PANI1.2. Inset: TEM images of different morphologies of PANI. Reproduced from [58], with permission. (j) Schematic illustration of the synthesis procedure of 2D PANI-ICE on ice surfaces. Inset: TEM photomicrographs confirm the formation of 2D nanosheets with almost no defects. Reproduced from [59], with permission. Structural characterization of PANI-ICE by selected area electron diffraction (l) and X-ray powder diffraction (k). Reproduced from [59], with permission.
Figure 5. (a) Three-dimensional honeycomb structure of EB. Reproduced from [52], with permission. (b) Mechanism of PANI surface polymerization (for simplicity, Cl counter ions are not shown) and molecular structure of 2D PANI. APS: ammonium persulfate; SMAIS: surfactant monolayer-assisted interfacial synthesis. Reproduced from [57], with permission. (c) AC-HRTEM image of PANI. Reproduced from [57], with permission. (di) Scanning electron microscope images of PANI0.1, PANI0.3, PANI0.5, PANI0.7, PANI0.9 and PANI1.2. Inset: TEM images of different morphologies of PANI. Reproduced from [58], with permission. (j) Schematic illustration of the synthesis procedure of 2D PANI-ICE on ice surfaces. Inset: TEM photomicrographs confirm the formation of 2D nanosheets with almost no defects. Reproduced from [59], with permission. Structural characterization of PANI-ICE by selected area electron diffraction (l) and X-ray powder diffraction (k). Reproduced from [59], with permission.
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Figure 6. Modulation of PANI doping level and molecular chain ordering to enhance its thermoelectric properties: (a) Schematic structure of CNTs/PANI composites at different CSA doping levels (Reproduced from [61], with permission.) and its (b) PF variation. With the increase of CSA doping level, the PANI chains gradually changed from compact coil conformation to an extended and ordered chain structure. (c) Schematic diagram of the mechanism by which the spatial site-barrier interaction between the hydrophobic alkyl chain of DBSA and the PANI backbone promotes the transformation of the PANI chain from a disordered coiled to a planarized structure and (d) its thermoelectric properties. (e) SWCNT as a template to induce the formation of PANI/SWCNTs from different concentrations of An. Reproduced from [53], with permission. (f) Schematic mechanism of CNTs/PANI fiber preparation by wet spinning and its (g) σ, S, (h) PF.
Figure 6. Modulation of PANI doping level and molecular chain ordering to enhance its thermoelectric properties: (a) Schematic structure of CNTs/PANI composites at different CSA doping levels (Reproduced from [61], with permission.) and its (b) PF variation. With the increase of CSA doping level, the PANI chains gradually changed from compact coil conformation to an extended and ordered chain structure. (c) Schematic diagram of the mechanism by which the spatial site-barrier interaction between the hydrophobic alkyl chain of DBSA and the PANI backbone promotes the transformation of the PANI chain from a disordered coiled to a planarized structure and (d) its thermoelectric properties. (e) SWCNT as a template to induce the formation of PANI/SWCNTs from different concentrations of An. Reproduced from [53], with permission. (f) Schematic mechanism of CNTs/PANI fiber preparation by wet spinning and its (g) σ, S, (h) PF.
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Figure 7. Interfacial modulation of PANI–inorganic composites to optimize their thermoelectric properties: (a) SWCNT/PANI/PEDOT:PSS band diagram of the double interface energy barrier. Reproduced from [83], with permission. (b) PEDOT: schematic diagram of carrier transport in PSS/SWCNT@PANI composite fiber. The introduction of the PANI layer promotes the homogeneous dispersion of SWCNTs in the PEDOT:PSS matrix and forms a dual interface, creating a double energy barrier. Reproduced from [83], with permission. (c) Interfacial band diagrams of PANI/SWCNTs and PANI/Te. (d) PF of PANI films, Te nanorod films, SWCNT films and ternary PANI/SWCNT/Te nanocomposite films with 10 wt% Te content. Reproduced from [85], with permission. (e) Undoped high-resistance PANI, high potential barrier hinders carriers from passing. Reproduced from [77], with permission. (f) HCl-doped PANI increases carrier concentration, but PANI at the interface is almost unaffected, and the potential barrier remains unchanged, limiting carrier transport. Reproduced from [77], with permission. (g) Schematic diagram of band bending at the interface between CNT and PANI and the corresponding band structure. (h) Corresponding PF of CNT/PANI composites with different CNT contents at room temperature. Reproduced from [86], with permission. SEM images of (i) CNT and (j) 94 wt% CNT/PANI films. Reproduced from [86], with permission.
Figure 7. Interfacial modulation of PANI–inorganic composites to optimize their thermoelectric properties: (a) SWCNT/PANI/PEDOT:PSS band diagram of the double interface energy barrier. Reproduced from [83], with permission. (b) PEDOT: schematic diagram of carrier transport in PSS/SWCNT@PANI composite fiber. The introduction of the PANI layer promotes the homogeneous dispersion of SWCNTs in the PEDOT:PSS matrix and forms a dual interface, creating a double energy barrier. Reproduced from [83], with permission. (c) Interfacial band diagrams of PANI/SWCNTs and PANI/Te. (d) PF of PANI films, Te nanorod films, SWCNT films and ternary PANI/SWCNT/Te nanocomposite films with 10 wt% Te content. Reproduced from [85], with permission. (e) Undoped high-resistance PANI, high potential barrier hinders carriers from passing. Reproduced from [77], with permission. (f) HCl-doped PANI increases carrier concentration, but PANI at the interface is almost unaffected, and the potential barrier remains unchanged, limiting carrier transport. Reproduced from [77], with permission. (g) Schematic diagram of band bending at the interface between CNT and PANI and the corresponding band structure. (h) Corresponding PF of CNT/PANI composites with different CNT contents at room temperature. Reproduced from [86], with permission. SEM images of (i) CNT and (j) 94 wt% CNT/PANI films. Reproduced from [86], with permission.
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Table 1. Room temperature TE properties of PANI and its nanocomposites.
Table 1. Room temperature TE properties of PANI and its nanocomposites.
Materialσ
(S cm−1)
|S|
μV K−1
PF
μW m−1 K−2
Synthesis
Methods
MechanismsRef.
PANI~1000IPOMC[24]
PANI/BiCuSeO9.255~877MMOIE[43]
PANI/AgBiSe2740SMMOIE[44]
PANI/g-C3N4~5.8~35070.75SMMOIE[45]
PANI/g-C3N4~12~48~2.8SMMOIE[46]
PANI/SAS~400~2831SMMOMC[47]
PANI/SWCNT76965176SMMOIE[48]
PANI/Graphene/DWNT10801301825LbLOIE[49]
PANI/Graphene-PEDOT:PSS/PANI/DWNT-PEDOT:PSS18851202710LbLOIE[50]
PANI/DWCNT/GO9601151260LbLOIE[51]
PANI (CSA doping)800IPOMC[52]
PANI/SWCNT1320~2690LbLOIE[53]
PANI/SWCNT223842.7407ISPMBL[54]
PANI/SWCNT654.948.7155.3ISPOIE[55]
PANI/TiO2/CNT218322.9114.5ISPOIE[56]
2DPANI~16IPOMC[57]
PANI/PEDOT:PSS260012.1240.83ISPMDL[58]
PANI35IPOMC[59]
PANI/DBSA~27~10.20.28ISPMDL[60]
PANI/SWCNT185630.8176SMMOMC[61]
PANI/HCl3.7ISPMDL[62]
PANI/g-C3N4/rGO 110.397.1118.3SMMOIE[63]
EDOT:PSS/SWCNT@PANI247243.5467.8ISPOIE[64]
PANI/CNT~2012 ~37273 ISPOIE[65]
NOTE: Mechanical Mixing (MM), Solution-Mediated Mixing (SMM), Layer-by-layer self-assembly (LbL), In-situ polymerization (ISP), Interfacial polymerization (IP), Modulation of Doping level (MDL), Ordering of molecular chains (OMC), Organic–inorganic interface effect (OIE), Bridging effects (BE).
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Chen, M.; Xie, D.; Zhou, H.; Zong, P. PANI-Based Thermoelectric Materials. Organics 2025, 6, 33. https://doi.org/10.3390/org6030033

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Chen M, Xie D, Zhou H, Zong P. PANI-Based Thermoelectric Materials. Organics. 2025; 6(3):33. https://doi.org/10.3390/org6030033

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Chen, Mengran, Dongmei Xie, Hongqing Zhou, and Pengan Zong. 2025. "PANI-Based Thermoelectric Materials" Organics 6, no. 3: 33. https://doi.org/10.3390/org6030033

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Chen, M., Xie, D., Zhou, H., & Zong, P. (2025). PANI-Based Thermoelectric Materials. Organics, 6(3), 33. https://doi.org/10.3390/org6030033

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