A Review on the Synthesis Methods, Properties, and Applications of Polyaniline-Based Electrochromic Materials

Abstract

Polyaniline (PANI), characterized by its proton-coupled redox mechanism and multicolor reversibility, is widely investigated for adaptive optical interfaces. Compared to inorganic oxides, PANI offers advantages in cost-effectiveness, mechanical flexibility, and molecular tunability; however, its practical implementation faces challenges related to kinetic limitations and environmental instability. This review presents a comprehensive analysis of PANI-based electrochromic materials, examining the intrinsic correlations among synthesis methodologies, microstructural characteristics, and optoelectronic performance. Synthesis strategies, including chemical oxidative polymerization, electrochemical deposition, and template-assisted techniques, are evaluated. Emphasis is placed on resolving the trade-off between optical contrast and switching kinetics by constructing high-surface-area porous nanostructures and inducing chain ordering via functional dopants to shorten ion diffusion paths and reduce charge transfer resistance. Fundamental electrochromic properties are subsequently discussed, with specific attention to degradation mechanisms triggered by environmental factors, such as pH drift, and stabilization strategies involving electrolyte engineering and composite design. Furthermore, the review addresses the evolution of applications from single-band monochromatic displays to dual-band smart windows for decoupled visible/near-infrared regulation and multifunctional integrated systems, including electrochromic supercapacitors and adaptive thermal management textiles. Finally, technical challenges regarding long-term durability, neutral color development, and large-area manufacturing are summarized to outline future research directions for PANI-based optical systems.

1. Introduction

Electrochromic materials are a class of functional materials whose optical properties, such as color or transmittance, can undergo reversible and stable changes under an applied electrical potential [1,2,3]. This phenomenon, known as electrochromism, originates from electrochemically induced redox reactions that alter the electronic structure of the material, leading to variations in light absorption in the visible or near-infrared regions [4,5,6]. A typical electrochromic process involves electron transfer accompanied by ion insertion and extraction, enabling controllable and energy-efficient modulation between different optical states, which is aligned with global efforts to mitigate industrial CO₂ emissions [7] and adapt to long-term climate dynamics [8]. According to their composition, electrochromic materials are generally classified into inorganic and organic systems. Inorganic electrochromic materials, represented by tungsten oxide (WO₃), nickel oxide (NiO), and molybdenum oxide (MoO₃), typically exhibit excellent cycling stability and high optical contrast, but suffer from limited color diversity, intrinsic brittleness, and relatively high fabrication costs [9,10,11,12]. In contrast, organic electrochromic materials, including conjugated polymers and small organic molecules, offer advantages such as structural tunability, low density, mechanical flexibility, and solution processability, although their long-term stability and environmental tolerance remain challenging [13,14,15,16]. The complementary characteristics of inorganic and organic electrochromic materials have stimulated extensive interest in polymer-based electrochromic systems as promising candidates for next-generation electrochromic devices. Despite the dominance of inorganic electrochromic materials—most notably tungsten oxide (WO₃)—which serve as the industry benchmark due to their exceptional cyclic stability and high optical contrast, they are often limited by intrinsic brittleness, high vacuum-processing costs, and a restricted color palette (typically transitioning only between transparent and blue). In contrast, PANI offers distinct competitive advantages: (1) Rich Multicolor Reversibility (yellow-green-blue-purple) enabling aesthetic versatility; (2) Mechanical Flexibility compatible with roll-to-roll processing for wearable electronics; and (3) Low-Cost Solution Processability. However, acknowledging that PANI still faces challenges regarding environmental stability compared to the robust lattice of inorganic oxides is crucial for targeting appropriate applications.

Polyaniline (PANI) is one of the earliest intrinsically conducting polymers to be systematically studied, and its electrochromic behavior was first reported in the late 1970s and early 1980s during investigations of conductive polymers [17,18,19]. Owing to its unique redox chemistry, polyaniline can reversibly interconvert among the leucoemeraldine, emeraldine, and pernigraniline oxidation states, each corresponding to distinct optical colors. Unlike many other conducting polymers, polyaniline can be proton-doped without altering its backbone structure, allowing its electrical conductivity and optical properties to be simultaneously regulated through simple acid–base chemistry [20,21]. This synergistic redox–proton doping mechanism endows polyaniline-based materials with rich color modulation, relatively low operating voltages, and fast electrochromic responses. In addition, polyaniline exhibits good chemical stability in acidic environments, low-cost and readily available monomer sources, and compatibility with various fabrication strategies, including chemical and electrochemical polymerization as well as composite and hybrid material design. These attributes make polyaniline an attractive platform for developing high-performance and multifunctional electrochromic materials. This growing significance is reflected in the academic literature. As illustrated in Figure 1, a bibliometric analysis based on Web of Science data reveals a substantial growth trajectory in publications related to “Polyaniline” and “Electrochromic” over the past two decades. Although the annual publication count peaked in 2019, the field has maintained a robust level of research activity in recent years (2020–2025), underscoring the enduring importance and established maturity of this field.

Against this background, this review aims to provide a comprehensive overview of recent advances in polyaniline-based electrochromic materials, with particular emphasis on the intrinsic relationships between synthesis methods, structural/morphological characteristics, key electrochromic properties, and practical applications. Common preparation strategies, including chemical oxidative polymerization, electrochemical deposition, template-assisted synthesis, and composite construction, are first summarized, with a focus on how processing parameters influence microstructure and electrochromic behavior. Subsequently, critical performance metrics such as optical contrast, coloration efficiency, switching speed, and cycling stability are systematically discussed, along with the major factors governing device durability and lifetime. Finally, representative applications in smart windows, display devices, sensors, and emerging multifunctional integrated systems are reviewed. Current challenges, including long-term stability, color neutralization, and scalable manufacturing, as well as future research directions, are also highlighted to guide the rational design and application of next-generation polyaniline-based electrochromic materials.

2. Synthesis Methods of Polyaniline-Based Electrochromic Materials

2.1. Chemical Oxidative Polymerization

Chemical oxidative polymerization is one of the most widely employed and scalable routes for the synthesis of polyaniline (PANI) electrochromic materials. In a typical procedure, aniline salts (e.g., aniline hydrochloride) are oxidized in acidic aqueous media using oxidants such as ammonium persulfate (APS), yielding the emeraldine salt (ES), which represents the most electrochemically active and electrochromically relevant oxidation state of PANI. According to the IUPAC technical report by Stejskal and Gilbert, this “standard” synthesis exhibits excellent reproducibility across laboratories, with polymer yields consistently exceeding 90% and an average room-temperature conductivity of 4.4 ± 1.7 S·cm⁻¹ for HCl-doped PANI (based on 59 samples) [22]. The reaction mechanism is commonly described via a radical cation pathway, in which APS initiates the oxidation of aniline to form reactive intermediates that undergo head-to-tail coupling and chain growth, while protonic acids simultaneously induce doping and stabilize the conjugated backbone, thereby enabling reversible electrochromic redox transitions.

As schematically illustrated in Figure 2, the electrochromic functionality of PANI arises from the reversible interconversion between its different oxidation and protonation states, namely leucoemeraldine, emeraldine base, and emeraldine salt. The fully reduced leucoemeraldine form can be transformed into the conductive emeraldine salt through oxidative doping, which involves electron transfer accompanied by counterion incorporation. Alternatively, the insulating emeraldine base can be converted into the electroactive emeraldine salt via acidic protonation without changing the overall oxidation state of the polymer backbone. These coupled redox and protonation–deprotonation processes generate polaronic and bipolaronic charge carriers along the conjugated chain, modulating both electrical conductivity and optical absorption, and thus constitute the fundamental mechanism underlying the electrochromic behavior of PANI [23].

From the perspective of electrochromic performance and durability, synthesis conditions strongly influence defect density, chain organization, and doping efficiency, which together govern charge transport and long-term cycling stability. The strength and dosage of the oxidant play a critical role: strong oxidants such as APS typically afford high conversion and conductivity but may induce over-oxidation and structural defects when used excessively, leading to degraded optical contrast and poor cycling endurance [24]. In addition, the electrochromic working potential window significantly affects stability. Wang and co-workers demonstrated that PANI films cycled within the 0.4–0.8 V (yellow ↔ green) range exhibited superior stability, with a coloration time of approximately 4.5 s and a high coloration efficiency of 159.48 cm²·C⁻¹, whereas operation involving deeper oxidation states (e.g., 0.4–1.2 V or 0.8–1.2 V, yellow/green ↔ blue) resulted in rapid performance degradation [25]. These results clearly indicate that avoiding over-oxidation is essential for achieving long electrochromic lifetimes. At the device level, Ding et al. further showed that appropriate anion doping and microstructural optimization could dramatically enhance cycling durability, with modified PANI retaining approximately 78% optical contrast after 6500 cycles, compared to only 6.7% for pristine PANI [26].

The nature and concentration of the acid dopant fundamentally determine the protonation degree, electronic structure, and, thus, the electrochromic behavior of PANI. Stejskal and co-workers systematically investigated chemical oxidative polymerization in various inorganic and organic acids, reporting that strongly acidic media typically produce protonated PANI with conductivities in the range of 1–10 S·cm⁻¹. In contrast, polymerization in carboxylic-acid-containing media often requires sulfuric acid generated in situ from APS decomposition to achieve effective protonation, resulting in lower conductivity. The authors also proposed partial sulfonation of the benzene rings as a possible origin of the exceptionally wide conductivity range observed for PANI bases (10⁻¹¹–10⁻⁷ S·cm⁻¹) [27]. Complementary to synthesis-stage control, Blinova et al. demonstrated that post-synthetic reprotonation of PANI bases provides a powerful and practical strategy for fine-tuning conductivity. By immersing PANI bases in aqueous solutions of different acids, the conductivity could be continuously adjusted over 10⁻⁹–10⁰ S·cm⁻¹, and in phosphoric acid systems, an empirical relationship was established: pH = 0.77 − 0.64 log(σ [S·cm⁻¹]). Notably, surface wettability could be modulated simultaneously, with water contact angles decreasing from 78° (PANI base) to 44° after reprotonation in 1 mol·L⁻¹ H₃PO₄, highlighting the coupling between doping level, interfacial properties, and ion transport relevant to electrochromic switching [28].

Extending this concept, Stejskal’s group conducted a comprehensive screening of 42 inorganic and organic acids for PANI reprotonation. The highest conductivity (1.22 S·cm⁻¹) was achieved using 50% tetrafluoroboric acid, while most strong inorganic acids yielded conductivities on the order of 10⁻¹ S·cm⁻¹, and sulfonic acids typically produced 10⁻²–10⁻¹ S·cm⁻¹. In addition to electrical properties, reprotonation markedly influenced density (1.19–2.06 g·cm⁻³), water contact angle (29–102°), and volume change (−14% to +33%), underscoring the versatility of acid-controlled property modulation for electrochromic film design [29].

To further enhance conductivity while maintaining favorable electrochromic kinetics, Li and Wan introduced a “doping–dedoping–redoping” strategy to fabricate porous PANI films. Using acids such as HCl, HClO₄, H₂SO₄, H₃PO₄, and p-toluenesulfonic acid (p-TSA) for redoping, they achieved exceptionally high room-temperature conductivities of 200–300 S·cm⁻¹, comparable to PANI–CSA systems processed in m-cresol. The high conductivity was attributed to the preservation of an expanded-chain conformation, which facilitates efficient charge transport and is particularly advantageous for fast electrochromic switching [30].

Synthesis temperature is another critical parameter affecting morphology–conductivity relationships. Paschoalin et al. investigated in situ polymerization of HCl-doped PANI on PET substrates at 0, 10, and 20 °C, accompanied by parallel studies on PANI powders. UV–Vis spectra consistently showed characteristic polaron absorption bands near 420 nm and 800 nm, confirming formation of the emeraldine salt. XRD analysis revealed that the PANI powders were polycrystalline with similar degrees of crystallinity (66.3%, 62.8%, and 63.0%, respectively), indicating that crystallinity was not the dominant factor governing conductivity. Instead, AFM and FE-SEM analyses demonstrated that increasing synthesis temperature led to rougher surfaces and less compact polymeric aggregates. Consequently, the conductivity of PANI/PET films decreased dramatically from 219.00 ± 0.02 S·m⁻¹ (0 °C) to 0.15 ± 0.01 S·m⁻¹ (20 °C), while PANI powder conductivity dropped from 969.82 ± 1.03 S·m⁻¹ to 253.91 ± 0.72 S·m⁻¹ [31]. These results clearly indicate that lower synthesis temperatures favor more ordered morphologies and continuous charge-transport pathways, which are beneficial for electrochromic efficiency and stability.

Beyond temperature and acid control, morphological engineering and functional additives offer additional routes to optimize electrochromic performance. Porous and one-dimensional nanostructures effectively shorten ion-diffusion pathways and reduce charge-transfer resistance. For example, Erro et al. reported that highly porous PANI nanofiber films achieved an ultrafast switching time of T₉₀ ≈ 20 ms at an optical contrast of ΔT ≈ 10%, directly demonstrating the kinetic advantages of porous architectures [32]. Yu and co-workers incorporated interfacially polymerized PANI nanofibers into PANI/PSS films, increasing electrochromic contrast from 0.49 to 0.67 and limiting contrast decay to <4% after 2000 cycles, accompanied by a reduction in charge-transfer resistance from 94 Ω to 76 Ω [33].

Functional dopants such as organic dyes can further modulate morphology and electrochemical behavior. Stejskal and colleagues examined the effects of 37 organic dyes during chemical oxidative polymerization, finding that PANI synthesized from aniline hydrochloride retained conductivities on the order of 1 S·cm⁻¹ with predominantly globular morphology, whereas dye effects were more pronounced for polypyrrole systems [34]. Building on this concept, Cao et al. developed a one-step ethyl orange (EO)-assisted synthesis that induced the formation of PANI nanorods with enhanced conductivity (13.5 S·cm⁻¹) and excellent dispersibility (ζ-potential 34 mV). When incorporated into waterborne polyurethane (WPU) coatings at 0.5 wt%, EO–PANI significantly improved electrochemical stability, exhibiting a low-frequency impedance magnitude of approximately 10¹⁰ Ω at 0.01 Hz in 3.5 wt% NaCl, nearly two orders of magnitude higher than that of conventional WPU coatings (~10⁸ Ω) [35]. Such results highlight how morphology- and dopant-engineering strategies developed within chemical oxidative polymerization frameworks can be directly translated into robust electrochromic and multifunctional device architectures.

2.2. Electrochemical Polymerization

Electrochemical polymerization (electropolymerization/electrodeposition) has been widely applied to the construction of polyaniline (PANI) electrochromic films and devices due to its ability to achieve in situ deposition of polymer films on conductive substrates and precisely regulate the film growth process by applying potential, current, and total charge [36,37]. Depending on the control parameters, electrochemical polymerization is generally categorized into potentiostatic mode (typically corresponding to chronoamperometry, CA) and galvanostatic mode (typically corresponding to chronopotentiometry, CP, or galvanostatic electropolymerization, GS), both of which possess distinct advantages in growth kinetics, structural controllability, and device integration [38]. In potentiostatic mode, the polymerization reaction is confined within a preset potential window, which helps inhibit the over-oxidation of PANI, thereby offering better control over its oxidation state and doping level. For instance, studies employing potentiostatic electrodeposition (CA) to deposit PANI on Fluorine-doped Tin Oxide (FTO) substrates, using an electrolyte of 0.3 M aniline and 1 M H₂SO₄, compared the deposition effects at +0.8, +0.9, and +1.0 V (vs. SCE), revealing a close relationship between the “deposition potential window—film quality—electrochemical reversibility [39].” Similarly, in Indium Tin Oxide (ITO) substrate and acidic medium systems, the analysis of PANI nucleation and growth mechanisms through potentiostatic electrodeposition discovered that nucleation kinetics have a decisive impact on film compactness and uniformity [40]; these mechanistic findings further support the advantages of the potentiostatic method in regulating film structure. Furthermore, in situ characterization techniques provide direct evidence for observing growth dynamics during potentiostatic electrodeposition; specifically, in situ spectroscopic ellipsometry has been used to monitor the thickness evolution of PANI films in real time, revealing the direct influence of electrochemical conditions on deposition kinetics and film growth behavior [41].

In contrast, the galvanostatic electrochemical polymerization method directly regulates deposition quality by controlling the amount of charge passed, making it particularly suitable for reproducible preparation and large-area film growth, and thus it is widely used in device research. Recent research explicitly adopted the galvanostatic method to prepare PANI electrochromic films, achieving an optical contrast of approximately 50% at 630 nm, with a response time as fast as 0.1 s. Additionally, this method demonstrated reversible switching between multiple color states such as transparent, yellow, green, blue, and purple, highlighting the potential of the galvanostatic method in optimizing electrochromic kinetics and multi-color regulation [42]. As illustrated in Figure 3a, the electrochemical process of PANI involves two coupled aspects: (i) the electrosynthesis step (alpha), where aniline is first oxidized by electron loss to reactive intermediates (radical-cation–type species) that subsequently couple to form oligomers and finally grow into the PANI backbone; and (ii) the reversible backbone redox steps (beta and gamma) after film formation, where the reduced segments are oxidized to a more conductive doped state through the beta transition (one-electron transfer), and are further driven toward a more oxidized state through the gamma transition accompanied by electron transfer and deprotonation (one electron coupled with two protons). Consistently, the cyclic voltammogram in Figure 3b shows negligible response for the blank electrode, while the PANI-coated electrode exhibits a set of characteristic redox peaks: an anodic beta1 peak at low potential (around 0.2 V vs. SCE) with its corresponding cathodic beta2 peak (near 0 V), an anodic gamma1 peak (around 0.45 V) paired with a cathodic gamma2 peak (around 0.4 V), and a pronounced high-potential alpha feature (around 0.75–0.85 V) associated with the oxidative electrosynthesis and growth process. These alpha, beta, and gamma processes collectively indicate that PANI electrochromism is fundamentally governed by proton-coupled backbone redox (beta and gamma) together with interfacial protonation and deprotonation and counter-ion charge compensation, enabling efficient ion–electron coupled transport and thus driving the macroscopic switching between bleached and colored states [42,43]. In the field of flexible electronic devices, combining galvanostatic electropolymerization with cyclic voltammetry, researchers have successfully prepared PANI films with tunable nanostructures on ITO/PET substrates, meeting multifunctional requirements for electrochromism and energy storage, further highlighting the advantages of the galvanostatic method in flexible electrode adaptability and large-scale preparation [44]. To further clarify the influence of electrochemical polymerization modes on film formation, cyclic voltammetry (CV) and chronoamperometry (CA) were applied to electropolymerize aniline on ITO-coated glass substrates. The CV-derived polyaniline (PANI) films exhibited nanofibrous morphologies with widths of 50–150 nm and a superior photoelectrochemical response, with photocurrent increasing from 0.1 to 0.4 mA·cm⁻², compared with ~0.2 mA·cm⁻² for CA-prepared films, highlighting their potential for optoelectronic applications [45].

In electrochemical polymerization systems, electrolytes and substrates are not merely passive background conditions; they directly participate in the type of doping ions, nucleation behavior, interface adhesion, and synergistic ion/electron transport of PANI, thus significantly influencing electrochromic response speed and cycling stability [38,46]. Regarding electrolytes, the ion species and solvent environment can significantly alter interface wettability, charge transfer kinetics, and ion diffusion resistance, thereby regulating electrochromic kinetics [47,48]. An optimized, simple, and low-cost method has been applied to the prototype preparation of electrochromic devices (ECDs), involving the incorporation of the ionic liquid (IL) 1-(4-vinylbenzyl)-3-methylimidazolium chloride ([VBMIM]Cl) [49] into the conductive polymer PANI and depositing it on FTO substrates via electrodeposition [50]. By adjusting the IL to its optimal concentration, the EC-2 electrode demonstrated efficient bleaching time (tb) and coloration time (tc) superior to other electrodes, and exhibited multiple reversible color changes from dark green to light green. The electrode maintained almost its original electrochemical activity after multiple cycles, and portable ECDs prepared based on this show promise as ideal candidates for smart displays and smart window applications [50]. Another typical example is the Zn²⁺ system; compared to aqueous Zn²⁺ electrolytes, PANI in organic Zn²⁺ electrolytes exhibited significantly enhanced electrochromic performance, with coloration/bleaching times of 2.0/2.4 s, an optical contrast reaching 72.2%, and a coloration efficiency as high as 205 cm²·C⁻¹, while maintaining 92.7% of its optical contrast after 10,000 cycles. This is attributed to the better interface wettability and faster charge transfer kinetics provided by the organic electrolyte. The choice of substrate is also crucial in PANI electrochemical polymerization [51]. Transparent conductive substrates like ITO and FTO not only provide electron transport channels but also influence nucleation density, film continuity, and adhesion through surface energy, roughness, and effective electrochemically active area. Research on FTO substrates revealed a coupling relationship between “substrate—potential—film quality,” emphasizing the need to re-optimize the deposition potential window (e.g., +0.8 to +1.0 V vs. SCE) according to different substrate conditions to avoid over-oxidation and uncontrolled roughness [39]. Similarly, nucleation mechanism studies on ITO substrates indicate that the nucleation-growth path, determined jointly by the substrate and electrolyte, ultimately affects film uniformity and long-term stability [40]. Additionally, pre-nucleation electrodeposition strategies have been proposed to prepare PANI films with better uniformity and adhesion, successfully constructing multi-patterned electrochromic devices [52].

Another core advantage of electrochemical polymerization lies in its ability to precisely regulate film thickness and morphology through parameters such as deposition time, charge amount, and current density, which are critical for balancing optical modulation amplitude, response speed, and cycle life. To improve kinetic performance while maintaining sufficient optical modulation, increasing research focuses on morphology engineering to build high-density ion channels and shorten diffusion paths. For example, Xing et al. further grew NiO nanorod arrays on electropolymerized PANI films, constructing a PANI/NiO composite electrochromic film. This one-dimensional nanostructure significantly increased the density of ion transport channels, achieving an optical modulation amplitude of approximately 72%, with fast coloration/bleaching response times of 0.61/0.73 s [53].

Beyond pure morphology control, composite structures and interfacial synergistic effects can also quantitatively enhance electrochromic performance. Addressing the limitation of PANI films in infrared (IR) electrochromic response due to their dense structure, researchers synthesized polyaniline/carbon quantum dots (PANI/CQDs) films (leveraging quantum dots’ giant performance enhancement capability [54]) using sulfuric acid and CQDs as dopants. The tetrahedral structure of sulfuric acid promoted the formation of highly ordered layered PANI, while the synergistic effect of CQDs facilitated a loose and porous surface morphology. For the PANI/CQDs porous film electropolymerized with a charge of 0.5 C (PANI/CQDs-5), it achieved 53% optical contrast at 850 nm, and emissivity changes (Δε) of 0.41, 0.46, and 0.42 in the 3–5 μm, 8–14 μm, and 2.5–25 μm ranges, respectively. The coloration and bleaching switching times were 2.5 s and 3.6 s, with a retention rate exceeding 95% after 100 cycles [55]. Furthermore, electropolymerized PANI/graphene oxide (GO) composite films achieved a coloration efficiency of 59.3 cm²·C⁻¹, significantly higher than the 50.0 cm²·C⁻¹ of pure PANI films, and maintained 53.1% capacitance retention after 1000 galvanostatic cycles, indicating that the interaction between GO and the PANI matrix effectively enhanced structural stability [56]. It is worth noting that morphology engineering has extended to infrared regulation and thermal management; for instance, a self-wrinkling PANI device structure achieved an emissivity modulation of 0.375 in the 8–14 μm band and a temperature regulation capability of approximately 8.9 °C [57]. To overcome the inherent instability of the PANI molecular structure, researchers also prepared triphenylamine cross-linked PANI derivative (CPANI) films via electrodeposition. This device achieved maximum emissivity modulations of 0.5 and 0.7 in the 3–5 μm and 8–14 μm bands, respectively, and maintained an average modulation capability of about 70% after 10,000 cycles [58].

In summary, electrochemical polymerization has established its core status in the preparation of high-performance polyaniline electrochromic materials due to its precise control over redox states, film thickness, and micromorphology. Whether through the potentiostatic mode to optimize nucleation kinetics and crystal structure, or utilizing the galvanostatic mode to achieve consistent large-area deposition, or further breaking through the performance bottlenecks of single materials via electrolyte engineering and nanocomposite strategies, this technology system demonstrates extremely high flexibility and adaptability. By precisely balancing the relationship between “nucleation—growth—structure” and combining novel functionalized substrates with electrolyte systems, electrochemical polymerization not only significantly enhances the optical contrast and response speed of devices but also provides solid technical support for the realization of long-life, multi-band (visible-infrared) synergistically regulated smart optoelectronic and thermal management devices.

2.3. Template-Assisted Synthesis

Based on the previously discussed chemical synthesis and electrochemical polymerization methods, template-assisted synthesis has been widely employed as an important structural regulation strategy to tailor the micro- and nanostructures of polyaniline (PANI), thereby alleviating the ion-diffusion limitations associated with the dense film structures that often form during conventional chemical or electrochemical polymerization processes [59,60]. By introducing predesigned spatial confinement or structure-directing agents during polymerization, template methods enable effective control over PANI nanostructures, producing high–specific surface area architectures such as nanofibers, nanotubes, and porous networks. These structures typically provide more electrolyte-accessible interfaces and more continuous ion-transport pathways, which is beneficial for improving doping/de-doping kinetics and cycling stability [61].

Hard-template approaches generally rely on rigid materials with regular pores or cavities. For example, anodic aluminum oxide (AAO) features tunable pore diameters, uniform channels, and excellent structural stability, enabling confinement of aniline polymerization within one-dimensional nanopores [62]. Classic studies have demonstrated highly ordered PANI nanotube arrays whose morphology and dimensions are directly constrained by the template pores, providing a clear route toward oriented one-dimensional electroactive structures with shortened diffusion paths [63]. Beyond AAO, macroporous hard templates such as inverse opals have also been used to construct hierarchically interconnected pore architectures. For instance, an inverse-opal WO₃ skeleton coated with a PANI layer to form a core–shell inverse-opal composite film enables reversible multicolor switching, improves coloration/bleaching times to 3.8/6.14 s, and increases coloration efficiency to 201.1 cm²·C⁻¹; these enhancements are attributed to rapid ion diffusion and enlarged effective reaction area enabled by the hierarchically porous core–shell inverse-opal structure [64]. In addition, using inorganic nanochannels/nanotube arrays as “hard scaffold templates” can yield quantifiable benefits: encapsulating and patterning electrodeposited PANI within TiO₂ nanotube arrays produces composite electrochromic films for information display and delivers higher optical contrast and faster switching kinetics in the near-infrared region (relative to single-component films) [65].

In contrast, soft-template methods mainly employ surfactants, micelles, or molecular self-assembly systems to guide PANI nanostructure formation under mild conditions. A representative recent example uses itaconic acid micelles as a soft template to produce PANI nanotubes; the mechanism has been interpreted as the cooperative directing effect of the aniline/itaconic acid micellar system and the electric double-layer structure on the polymerization process, enabling controllable formation of one-dimensional hollow nanostructures [66]. In addition, organic dyes can serve as structure-directing agents or soft templates for morphology control. As shown schematically in Figure 4, a one-step methyl orange (MO)-assisted synthesis enables the formation of PANI microtubes with aspect ratios up to ~20. MO molecules assemble into one-dimensional supramolecular templates in acidic solution and interact with aniline oligomers via hydrogen bonding and π–π stacking, thereby guiding anisotropic PANI growth during oxidative polymerization. As a result, the electrical conductivity is enhanced from 0.5 to 4.6 S·cm⁻¹ (≈8× that of spherical PANI), while the microtubes maintain stable conductivity for over 30 days and exhibit good dispersibility in various solvents, indicating favorable processability and potential as conductive fillers in composite systems [67]. Beyond conventional surfactant systems, bio-based self-assembled templates can couple structural control with optical functionality: chiral nematic phases formed by cellulose nanocrystals (CNCs) can template optically active PANI/CNC films, providing a materials basis for introducing designable microstructural order and optical responses in flexible and sustainable systems [68]. For studies closer to scale-up and practical electrochromic applications, soft-template-controlled electrodeposition has also enabled scalable, large-area broadband modulation. For example, PVSK was used as a template to regulate PANI electrodeposition, producing PVSK–PANI films with uniform spherical morphology and enhanced electrochemical activity without compromising broadband IR performance. The resulting devices exhibited maximum emissivity changes (Δε) of 0.47/0.51/0.49 (3–5/8–14/2.5–15 μm) for 2 × 2 cm² devices, and 0.55/0.46/0.46 for 20 × 20 cm² devices, together with an infrared temperature modulation up to 10.4 °C at 70 °C, demonstrating the effectiveness of soft-template morphology control for large-area electrochromic thermal management [69].

Furthermore, template concepts are often combined with layer-by-layer assembly or “in situ morphology templating” to simultaneously optimize electron/ion pathway organization and interfacial stability. For instance, a layered PANI/Ag nanowire composite film used in multicolor electrochromic smart windows with dual-band modulation accelerates electron transport and shortens switching times (coloration/bleaching of 6.7/18.0 s at 525 nm) while increasing coloration efficiency to 45.5 cm²·C⁻¹ [70]. Alternating assembly of PANI with MXenes (e.g., (Ti₃C₂T_x)) to form PANI/MXene films can strengthen interfacial interactions and, through the layered structure, expose more active sites for ion/electron insertion–extraction, thereby improving both electrochromic and energy-storage performance [71]. In addition, “self-wrinkling” introduces wrinkle microstructures as an in situ physical template into PANI functional layers, achieving higher infrared emissivity modulation (Δε = 0.375, 8–14 μm), faster response (1.8/2.6 s), and an ~8.9 °C temperature regulation capability, indicating that templated microstructures are also effective for broadband thermo-optical regulation device [57]. Relatedly, although pre-nucleation electrodeposition does not belong to traditional “externally added templates,” it produces loose porous structures via a “nucleation-first, growth-later” process design and shows a clear advantage in electrochemical activity (maximum active charge of 22.3 mC·cm⁻²). It can therefore be regarded as another form of “process templating” for structural regulation within electrodeposition systems [72].

2.4. Composite and Hybrid Material Synthesis

Despite Polyaniline (PANI) holding a prominent position in the field of electrochromism due to its rich multicolor transitions and high coloration efficiency, single-component materials remain significantly constrained by a “structure–transport–life” bottleneck. While early studies demonstrated functional switching, they exposed three intrinsic weaknesses: First, the significant volume expansion and contraction induced by repeated doping/dedoping processes lead to structural fatigue and delamination, severely limiting cycling life to typically less than a few thousand cycles. Second, the drastic drop in conductivity in the reduced state hinders charge transport, resulting in distinct kinetic hysteresis that cannot be resolved by film thickness optimization alone. Third, the dense film structure limits the effective infiltration depth of ions, making it intrinsically difficult to maintain high activity in thick films required for high optical contrast. To overcome this “structure–transport–life” bottleneck, constructing organic-inorganic hybrids or multi-component composites has become a mainstream strategy. The core lies in leveraging synergistic effects to engineer composite structures at the micro/nano-scale featuring “rigid supporting skeletons,” “continuous conductive networks,” and “efficient ion channels,” thereby synchronously enhancing optical modulation amplitude, response rate, and long-term stability [49,50,51,52].

Among the various composite strategies, the PANI–Metal Oxide composite system primarily embodies the “structural hybridization” design philosophy, where the inorganic phase typically functions as both a structural support and an ion transport carrier. This strategy aims to prevent polymer collapse via the inorganic skeleton and optimize electrolyte accessibility through pore engineering. For instance, a TiO₂ porous framework constructed using Electrostatic Spray Deposition (ESD), combined with in situ electropolymerized PANI, forms a nanocomposite film with both high specific surface area and excellent interfacial bonding. This system utilizes a “donor-acceptor coupling + porous channel” mechanism to significantly enhance electrochromic kinetics, achieving a high optical modulation amplitude of 76.9% at 600 nm, a switching time shortened to less than 3.6 s, and a coloration efficiency of 78 cm² C⁻¹ [73]. Beyond kinetics improvement, structural hybridization also enables electrochromism–energy-storage integration by distributing charge storage and buffering the volume variation of PANI through a robust inorganic framework. For example, WO2.86/PANI hybrid films combine the multicolor electrochromism of PANI in the visible region with the near-infrared modulation of tungsten oxide, allowing synergistic or decoupled regulation of the VIS–NIR dual bands. In a typical LiClO4–PC electrolyte, selective transport of Li+ ions and electrons enables three distinct optical states (bright: VIS/NIR transparent; cool: VIS transparent while NIR blocked; dark: both VIS and NIR blocked), demonstrating multi-mode switching for advanced smart-window energy management [74]. Meanwhile, MnO₂/PANI systems exploit the pseudocapacitive contribution of MnO₂ to share charge storage and mitigate structural fatigue, forming electrochromic supercapacitors in which the voltage evolution during galvanostatic charge–discharge is synchronized with the optical transmittance modulation; notably, the device can retain about 73.46% of its initial capacitance with nearly 100% Coulombic efficiency after 15,000 cycles at 0.4 mA cm⁻², highlighting the stabilizing role of the inorganic phase [75].

As further illustrated in Figure 5, a flexible dual-band electrochromic device integrating porous PANI with oxygen-deficient tungsten oxide nanowires (W₁₈O₄₉) exhibits typical galvanostatic charge–discharge behavior over a range of current densities (0.01–0.10 mA cm⁻²) (Figure 5a) and a corresponding current-density-dependent areal capacitance (Figure 5b). More importantly, the transmittance and voltage evolve synchronously during a full switching cycle (Figure 5c): a potentiostatic coloration step at −1.5 V for 60 s rapidly decreases the transmittance and consumes 28.6 mWh m⁻², followed by a galvanostatic discharge at 0.05 mA cm⁻² that releases 14.7 mWh m⁻² while the transmittance recovers, and a final potentiostatic bleaching step at 1.5 V for 30 s outputs an additional 10.6 mWh m⁻² to complete optical recovery. This quantified energy-flow/optical-response coupling directly visualizes real-time electrochromic–energy-storage dual-functionality and supports the energy-recycling concept reported for such bifunctional smart windows [76]. The device-level photographs (Figure 5d,e) further demonstrate practical feasibility by interconnecting prototype units to power an external LED [76].

In contrast, the PANI–Carbon composite system focuses more on “transport hybridization,” with the core objective of establishing interconnected electron pathways and optimizing ion transport channels. In this system, reduced graphene oxide (rGO) and carbon nanotubes (CNTs) act as “permanent current collectors”(analogous to the interface engineering seen in MOF-derived heteroatomic porous carbon [77]), effectively overcoming the kinetic hysteresis caused by the drastic drop in conductivity of polyaniline (PANI) in its low oxidation state (bleached state). Taking the ternary PB/rGO/PANIsystem as an example, electrodeposited films utilize rGO to construct a continuous conductive network, Prussian Blue (PB) to enhance spectral stability, and PANI to contribute fast chromic response. This synergistic effect of “layered structure—pore organization—interfacial charge transfer” significantly enhances the electrochromic kinetics of the device. Studies show that smart windows based on this composite electrode achieve an optical modulation amplitude (ΔT) of 41% at 625 nm, with coloring and bleaching times of 12.1 s and 12.6 s, respectively. More importantly, the device retains 92% of its initial performance (maintained at 38%) after 3000 cycles, fully demonstrating its potential in long-term electrochromic and thermal management systems [78].

Building on this, the advantages of carbon-based skeletons are particularly prominent in the field of flexible wearables. PANI@MWCNTs textile electrodes employ an integrated design of “fiber substrate + carbon nanotube conductive skeleton + PANI chromic layer.” The carbon network not only endows the device with mechanical durability under deformation but also supports rapid electrochemical response (coloring/bleaching times of ~2.15/1.89 s) and, combined with the infrared regulation capability of PANI, realizes efficient thermal management functions [79]. Similarly, the composite of 2D MXene materials (such as) with PANI leverages high conductivity and layered ion channels to further reduce interfacial impedance and improve the cycling stability of dual-function (chromic + energy storage) devices [80].

While the integration of inorganic oxides (e.g., TiO₂ or WO₃) with PANI prevents polymer collapse, a critical analysis reveals a recurring trade-off: simply physically mixing or electrodepositing PANI onto inorganic skeletons often introduces high interfacial resistance, which can dampen the coloration efficiency compared to pure polymer films. Consequently, recent research highlights that physical hybridization alone is insufficient, and strategies must shift towards “interfacial chemical engineering” to create strong chemical bonds. At the micro-interface level, La₂O₃/PANI composite films modified with silane coupling agents demonstrate the critical role of “chemical bonding”: strong interfacial interactions effectively suppress the volumetric collapse of the polymer skeleton. When the molar ratio of La₂O₃ to aniline is optimized to 1:3.5, the material not only significantly enhances cycling durability but also achieves a coloration efficiency of 22.81 cm² C⁻¹ and rapid response times (1.29 s/1.33 s) [81]. At the mesoscopic scale, constructing conjugated polymer dual networks (CPDN) offers a new pathway for flexible multifunctionalization; a PEDOT/PANI dual network integrated via a one-step electrochemical polymerization not only exhibits an extremely high coloration efficiency (322 cm² C⁻¹) and fast switching (~1s), but also realizes visual energy storage monitoring (yellow-purple transition) and excellent capacitance retention (86.4% after 1000 cycles) with a specific capacitance of 357 Fg⁻¹ in flexible electrochromic-supercapacitor devices [82]. Furthermore, synergistic design has extended to the device physics level. A novel dual-band smart window utilizes the “built-in electric field” at the Au/PANI Schottky junction interface to accelerate carrier transport. This field-assisted effect significantly promotes polaron/bipolaron transitions, not only improving response speeds to τc/τb = 0.9/1.3 s (at 633 nm) but also expanding operation modes from three to four, enabling independent and flexible modulation of visible and near-infrared light in large-scale (55 cm²) devices [83].

Collectively, composite synthesis follows the principle that “the skeleton defines transport limits, while the interface determines efficiency. Metal oxide hybrids primarily target stability and diffusion constraints through rigid pore designs, whereas carbon-based composites resolve kinetic and flexibility bottlenecks via efficient conductive networks. These multi-scale synergistic designs significantly reduce ion diffusion paths and charge transfer impedance without sacrificing PANI’s optical contrast. This paves the way for broad-spectrum, fast-response, and durable electrochromic applications, establishing a solid foundation for next-generation smart windows and flexible electronics.

2.5. Additive Manufacturing and Patterning Strategies

The electrochromic phenomenon in polyaniline (PANI) is fundamentally governed by the reversible redox transitions between its oxidation states (leucoemeraldine, emeraldine, and pernigraniline), accompanied by the insertion and extraction of ions (e.g., H⁺, Li⁺) and solvent molecules. While the synthesis methods discussed in previous sections (Section 2.1, Section 2.2, Section 2.3 and Section 2.4) focus on tailoring the intrinsic molecular and microstructural properties of PANI, additive manufacturing and patterning strategies represent the critical engineering bridge that translates these “processable precursors/inks” into device-level architectures with defined spatial and thickness distributions. This “bottom-up” fabrication approach directly influences key device metrics—including optical modulation amplitude (ΔT/ΔR), coloration efficiency (CE), switching kinetics, and cycling stability—while enabling the scalable, low-temperature integration required for smart windows, flexible displays, and wearable electronics.

The core technological chain for additive manufacturing typically follows a sequence of “ink formulation → patterned deposition → electrolyte integration.” The development of stable, printable dispersions containing PANI nanostructures (such as 2D nanosheets, nanofibers, or composites) is a prerequisite. For instance, Huang et al. demonstrated that inkjet-printed 2D lamellar PANI inks could overcome the limitations of bulk agglomeration. By creating a stable ink with optimized rheology, they achieved flexible, patterned electrochromic devices exhibiting a high optical contrast of approximately 76% at 750 nm, a high coloration efficiency of 259.1 cm²·C⁻¹, and rapid response times (1.8 s for coloration and 2.4 s for bleaching) [84]. These results align well with the “structure-property” correlations discussed earlier: the 2D porous morphology significantly shortens ion diffusion paths and maximizes the effective electrochemically active area, ensuring high performance even under printed conditions. Extending this to wearable applications, inkjet-printed layer-stacked PANI on textile substrates has achieved a reflectance modulation of 22.9%, maintaining reversible operation after 1000 bending cycles, which highlights the direct correlation between material microstructure and device-level mechanical reliability [85].

Patterning goes beyond merely printing static shapes; it involves the precise spatial control of ion migration to realize multi-zone or programmable dynamic displays. Recent work has proposed controlling the three-dimensional migration of ions within the device to achieve multi-pattern PANI electrochromism. Under galvanostatic driving, such devices can increase reflectance to ~34% within 15 s and maintain it at ~30% during continuous operation, demonstrating the feasibility of programmable pattern evolution [85]. While highly promising for anti-counterfeiting and information display, this approach imposes stringent requirements on the uniformity of the underlying PANI film. Consequently, studies focusing on the real-time regulation of electropolymerization kinetics are essential to ensure the film homogeneity and reproducibility required for high-resolution patterning.

Equally critical to the manufacturing strategy is the design of the electrolyte and encapsulation system, which dictates the scalability, stretchability, and environmental stability of the final device. Solid-state or gel electrolytes are preferred for large-area and flexible manufacturing. For example, modular devices based on electrochromic ion gels** have demonstrated a high ionic conductivity of ~3.84 × 10⁻³ S·cm⁻¹, achieving a transmittance modulation of 81.3% with response times of ~5.3 s/5.1 s, while remaining stable under 10% mechanical strain [86].

As devices scale from laboratory prototypes to large-area applications (e.g., smart windows), engineering challenges such as the potential drop (iR drop) across electrodes and edge effects become prominent. To address this, hierarchical current collector designs are often integrated with PANI deposition. Hierarchical metal mesh electrodes (e.g., pitch ~100 μm, line width ~2 μm) have been utilized to ensure uniform driving voltages across large areas, enabling efficient broad-spectrum modulation. Additionally, manufacturing processes can induce functional microstructures; for instance, self-wrinkling PANI surfaces formed through structural self-assembly have been shown to enhance infrared emissivity modulation (Δε ≈ 0.375) and thermal regulation (~8.9 °C) without the need for complex lithography [57].

In summary, additive manufacturing and patterning serve as a vital link in the PANI research framework, connecting “synthesis methods” to “practical applications.” It transforms material design ability into spatial programmability and scalable production. Future research in this domain requires a holistic optimization of “printable PANI inks,” “high-conductivity gel electrolytes,” and “low-impedance interfaces” to realize the full potential of PANI in next-generation optical and thermal management systems.

To provide a holistic view of the preparation strategies discussed in this section,

Table 1. Comparative Overview of PANI Synthesis Methods.

Synthesis Method	Mechanism	Key Advantages	Intrinsic Limitations	Typical Outcome	Ref.
Chemical Oxidative	Radical oxidation in acidic media (e.g., with APS)	Scalable and cost-effective High yield (>90%) Tunable conductivity	Risk of over-oxidation Hard to control film thickness Exothermic heat issues	Granular/Globular morphology Conductivity: ~4.4 S·cm−1	[22,24,30,35]
Electrochemical	Anodic oxidation (Potentiostatic/Galvanostatic)	Precise control (thickness/state) Binder-free in situ film Good substrate adhesion	Limited to conductive substrates Hard to scale up Electrolyte dependent	Nanofibrous/Porous films Fast switching (0.1–2.0 s) High contrast (~50%)	[36,39,42,52]
Template-Assisted	Spatial confinement (Hard AAO/Soft micelles)	High surface area (nanotubes) Short diffusion paths Ordered structures	Template removal is complex Multi-step processing High cost	Ordered Nanorods/Tubes Ultrafast kinetics (~20 ms) High dispersibility	[62,63,66,68]
Composite and Hybrid	Structural (Oxides) or Transport (Carbon) hybridization	Oxides: Structural support (Stability) Carbon: Conductive network Multifunctionality (Vis/NIR)	Interface engineering complexity Transparency vs. loading trade-off Volume mismatch risks	Rigid porous networks High stability (>15,000 cycles) Dual-band modulation	[73,74,75,76,79]

systematically compares the four primary synthesis methodologies: chemical oxidative polymerization, electrochemical polymerization, template-assisted synthesis, and composite/hybrid construction. It highlights the distinct mechanisms, key advantages, and intrinsic limitations of each approach, along with their typical morphological outcomes and performance metrics. This comparative summary offers a clear guideline for selecting appropriate synthesis routes to balance the critical trade-offs between scalability, structural precision, and functional stability in electrochromic device fabrication.

3. Electrochromic Properties of Polyaniline-Based Materials

3.1. Optical Properties

The optical behavior of PANI is governed by redox-induced band structure rearrangements, manifesting as a transition sequence from transparent leucoemeraldine to green emeraldine salt and purple pernigraniline. These state changes correspond to distinct UV-Vis absorption signatures, endowing the material with precise tunability. Consequently, Optical contrast (ΔT) and response kinetics are key indicators for measuring electrochromic performance. Electropolymerized PANI films usually show large ΔT in the visible range; for instance, electrodeposited films can reach approximately 50% optical contrast at 630 nm with a rapid response time of ~0.1 s, demonstrating effective utilization of the active area [42]. To further break through thermodynamic stability limits and enhance performance, composite structural engineering plays a significant role. Recent studies combined with Density Functional Theory (DFT) reveal that TiO₂ doping can significantly elevate the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) energy levels of oxidized PANI, facilitating electron excitation and polaron formation. As visualized in Figure 6, this energy level modulation enables distinct multicolor switching, where the film transitions from neutral yellow (−0.5 V) to conducting green (+0.3 V) and finally to insulating blue (+1.0 V) during the oxidation process. Experiments confirm that PANI-TiO₂ composite films not only possess expanded electrochemically active areas and optimized ion transport efficiency but also achieve an optical contrast of 73.8%, with coloration and bleaching times shortened to 0.46 s and 0.94 s, respectively, significantly outperforming pure PANI films. Moreover, this composite exhibits excellent infrared regulation capabilities under low voltage (<1 V), with an emissivity variation (Δε) as high as 0.61. This enhanced thermal regulation is corroborated by infrared thermal imaging (Figure 5, bottom), which shows that under identical stimulation, the PANI-TiO₂ composite achieves a higher surface temperature (62.2 °C) compared to pure PANI (57.3 °C), directly reflecting its superior emissivity modulation capability [87]. Similar micro/nano-structural optimizations have been validated in other systems; for example, PANI doped with p-TSA and featuring a long-range ordered/short-range disordered layered structure can further increase ΔT to 70%–80%, while maintaining fast coloring/bleaching responses (~2.8 s/5.2 s) and extremely high coloration efficiency (328.5 cm²⋅C⁻¹) [88].

Coloration efficiency (CE), used to quantify the change in optical density per unit charge density, is another core parameter for evaluating energy efficiency. Highly crystalline PANI films, after chain structure optimization, can reach a CE of 150 cm²⋅C⁻¹ [89].To further enhance energy efficiency and enrich color dimensions, complementary device designs based on PANI show immense potential. For instance, a study on a complementary system utilizing a PANI electrode and 1-methyl-4,4′-bipyridyl iodide (MBI) demonstrates that MBI, acting as a cathodic electrochromic layer with electrolyte functions, not only simplifies the device architecture but also achieves an exceptional CE of up to 140.63 cm²⋅C⁻¹. This system covers a full color gamut of red, yellow, green, blue, and purple over a wide voltage range, with each color state possessing a high color contrast of up to 73.8 and rapid response (1 s/1.9 s). Combined with screen-printed PVA barrier layer technology, this material can realize patterned anti-counterfeiting displays, proving the feasibility of high-CE materials in applications like smart packaging [90]. Furthermore, heterostructure designs greatly enrich spectral characteristics: PANI/WO_2.86 dual-band films exhibit four reversible color states (light blue, yellow-green, dark blue, and dark green), achieving independent regulation of visible and near-infrared regions [74]; while PANI-MnO₂ nanocomposite films display systematically tunable UV–Vis–NIR spectra, utilizing synergistic effects to further enhance ΔT and response range [75].

Beyond inorganic oxide composites, composites of PANI with carbon quantum dots (CQDs) and graphene derivatives also exhibit excellent tunability of color states and transmission characteristics, enabling regulation of color and transmittance across a broad spectrum (including the IR region), further expanding applications in multi-spectral electrochromic and thermal control systems [91]. In all these systems, morphology, film thickness, and crystallinity largely determine the final optical performance: thin, porous, or nanostructured PANI-based films typically exhibit higher ΔT and faster response speeds, mainly benefiting from shorter ion diffusion paths and larger electrochemically active interfacial areas. In summary, through the optimization of molecular ordering, nanostructures, and heterogeneous interface designs (such as energy level matching and chemical bonding), PANI-based materials still hold huge potential for enhancing electrochromic performance in the visible and near-infrared regions, providing a solid performance foundation for applications such as smart windows, display devices, and adaptive optical surfaces.

3.2. Electrochemical Properties

Since PANI operation relies on coupled ion-electron transport to maintain charge neutrality, its electrochemical performance is fundamentally governed by mass transport kinetics. In dense films, restricted ion diffusion across the interface often limits switching efficiency. Over the past years, a consistent message has emerged: once PANI films become relatively dense or when conductivity drops in low-oxidation (bleached) states, device performance is increasingly limited by coupled electron/ion transport, making it essential to reduce charge-transfer resistance and improve the utilization of redox-active sites via composite scaffolds, interfacial bonding, and electrolyte engineering. For example, introducing anthraquinone-1-sulfonate (AQS) as a redox-active dopant and incorporating AQS into gel electrolytes can strengthen reversible faradaic contributions while providing more continuous ion pathways, leading to improved electrochemical reversibility and cycling stability that are favorable for electrochromic operation [92].

From the standpoint of charge transport, another representative direction is to integrate “dynamic” conductive pathways or inorganic frameworks to mitigate the intrinsic conductivity/structural bottlenecks of PANI. Reversible metal electrodeposition (RME), where a metal is reversibly deposited/dissolved during operation, effectively introduces an adaptive electronic percolation and interfacial charge-transfer route for PANI across different oxidation states, thereby accelerating electrochemical kinetics and enabling multi-mode regulation in functional devices [93]. In parallel, coupling PANI with inorganic phases such as layered vanadium oxides (VOx) can stabilize electrochemical interfaces and facilitate ion-insertion/extraction processes; notably, in situ Raman evidence has been used to track intermediate formation and reversible evolution during Zn²⁺ insertion/extraction, supporting the role of “interfacial stabilization + transport-channel construction” in enhancing reversibility and durability [94]. Collectively, these advances indicate that improvements in the electrochemical performance of PANI systems usually arise from simultaneous optimization of electronic pathways and ion-accessible interfaces, rather than merely increasing intrinsic conductivity.

Cyclic voltammetry (CV) is the most direct tool for assessing PANI electrochemical behavior and for linking electrochemistry to electrochromic performance. In typical PANI-based electrodes, anodic/cathodic peaks (or peak sets) reflect reversible redox transitions among oxidation states accompanied by ion doping/de-doping; peak potential shifts and peak separations provide insight into polarization and charge-transfer kinetics, whereas the integrated CV area (charge) correlates with the amount of electrochemically addressable redox sites. Scan-rate dependence further helps distinguish surface-controlled pseudocapacitive responses from diffusion-limited processes; when nanostructuring, conductive hybrid frameworks, or redox-active dopants shorten diffusion paths and improve electrolyte accessibility, CV responses generally exhibit reduced polarization and more stable cycling. As a practical illustration, recent work presents CV curves together with a schematic of PANI oxidation states and corresponding doping/de-doping processes, which is particularly useful in reviews to close the “CV features → redox/ion compensation → transport kinetics” loop. Specifically, Figure 7 presents the characterization of PANI-AQS composites, rigorously following this analytical logic: The CV curves of electrodes with varying doping ratios (Figure 7a) first reveal the evolution of redox peaks. Under different scan rates (Figure 7b), the optimized electrode demonstrates stable capacitive behavior, indicating efficient kinetics. A direct comparison (Figure 7c) highlights that the PANI-AQS (3:1) electrode exhibits significantly larger integral areas and distinct redox pairs (a/a’, b/b’, c/c’) compared to pure PANI. The origin of these enhanced features is elucidated in the mechanism schematic (Figure 7d), which shows how AQS groups facilitate the protonation/deprotonation cycles between Emeraldine salt and Leucoemeraldine base states. This mechanism translates into superior energy storage metrics, as quantified by the Galvanostatic Charge–Discharge (GCD) curves at 1 A g⁻¹ (Figure 7e) and 1 mA cm⁻² (Figure 7f). Finally, the summary data (Figure 7g) confirms that the optimal aniline/AQS molar ratio of 3:1 maximizes both specific and areal capacitance (>400 F g⁻¹), validating the efficacy of the doping strategy [92].

3.3. Switching Performance

Switching performance of electrochromic materials is commonly evaluated by the coloration time (τ_c) and bleaching time (τ_b), often reported as the time required to reach 90% of the full optical change (t₉₀) [4]. For polyaniline (PANI)-based systems, switching is governed by coupled electron transport, ion migration, and interfacial charge-compensation processes. Because PANI exhibits pronounced conductivity differences among redox/protonation states, and because ion diffusion becomes increasingly rate-limiting in dense or thick films, switching is frequently asymmetric, with faster coloration and slower bleaching. This asymmetry becomes more prominent as film thickness increases or device dimensions scale up, where longer ionic diffusion distances and stronger interfacial polarization can dominate τ_b.

At the materials level, creating highly accessible ion-transport pathways is an effective route to accelerate switching. For example, highly porous PANI nanofiber films have demonstrated ultrafast t₉₀ (~20 ms) at a modest ΔT (~10%), illustrating the direct kinetic advantage of shortened diffusion lengths and enlarged electrochemically accessible surface area [32]. In composite architectures, introducing a nanofiber network can simultaneously improve kinetics and durability: incorporating interfacially polymerized PANI nanofibers into PANI/PSS films increased the electrochromic contrast (from 0.49 to 0.67), limited contrast decay to <4% after 2000 cycles, and reduced the charge-transfer resistance (from 94 Ω to 76 Ω), consistent with improved coupled ion/electron transport [33].

At the device level, a representative example of switching kinetics under realistic, step-potential operation is provided by the single-component PANI multicolor dual-band electrochromic smart window reported by Wang et al. The device achieved ΔT = 65% at 633 nm and ΔT = 59% at 1600 nm, and after 10,000 cycles, it exhibited only minor losses in modulation amplitude [95]. Importantly, the supporting data explicitly quantified switching times as τ_c/τ_b = 5.9/16.9 s at 633 nm and τ_c/τ_b = 11/32.5 s at 1600 nm, providing a clear, quantitative demonstration of the typical kinetic feature in dual-band PANI devices: coloration is relatively fast, whereas bleaching is more strongly constrained by ionic back-diffusion and interfacial polarization, especially at longer wavelengths and in device-scale configurations [95]. Expanding the scope from purely optical modulation to multifunctional electrochemical storage, recent work on electrochromic supercapacitors (ECSCs) provides a system-level verification of how structure regulation can overcome kinetic bottlenecks [96]. As illustrated in Figure 8, a symmetric ECSC based on PANI–p-TSA composite electrodes and a PVA–H₂SO₄ gel electrolyte was constructed (Figure 8a). In this system, p-toluenesulfonic acid (p-TSA) serves a dual function: acting as a dopant counter-ion for charge compensation and regulating polymer chain growth via multi-sulfonic groups to form a cross-linked morphology with continuous conductive networks and open ion channels, thereby effectively shortening the ion diffusion path. This structural optimization translates directly into superior rate capability: Cyclic voltammetry (CV) curves maintain excellent symmetry (Figure 8b), and Galvanostatic Charge–Discharge (GCD) profiles exhibit stable potential evolution across a wide current density range of 1–10 mA·cm⁻² (Figure 8c). This serves as robust evidence that interfacial ion supply remains efficient even under fast driving conditions, with no significant diffusion limitations. Consequently, the device retains an areal capacitance exceeding 120 mF·cm⁻² even at high current densities (Figure 8d), indicating that redox-active sites are fully utilized during rapid switching. Electrochemical Impedance Spectroscopy (EIS) further corroborates this kinetic advantage (Figure 8e), where the small charge-transfer resistance and typical low-frequency Warburg behavior quantify the contribution of “reduced interfacial impedance and diffusion resistance” to enhanced dynamic response. Regarding durability, the PANI–p-TSA system retains approximately 79% of its capacitance after 3000 cycles (Figure 8f), demonstrating that its kinetic pathways remain stable during long-term repetitive switching. Furthermore, the device delivers a competitive areal energy density of ~10 μWh·cm⁻² at relatively high power densities (Figure 8g); this high-power characteristic inherently reflects the system’s capability for rapid charge injection/extraction, which is the physical basis for fast electrochromic response. Finally, the stable electrochemical response under mechanical bending (Figure 8h) and the reversible photo-electric coupling verified by reflectance spectra (Figure 8i) collectively confirm the system’s ability to maintain efficient “electron/ion transport–optical modulation” synchronization under dynamic operating conditions.

Beyond electrode design, electrolyte engineering can critically influence switching kinetics and long-term stability by controlling ionic conductivity, interfacial impedance, and the uniformity of ion flux during repeated redox cycling. A recent Perspective summarizes design strategies for ionogels in electrochromic devices, which are directly relevant to PANI systems [104]. In brief, ionogels immobilize an ionic liquid (or ion-conducting medium) within a polymer network, thereby suppressing leakage/evaporation while maintaining continuous ion-conduction pathways. By tuning network polarity, crosslink density, and microphase morphology, ionogels can balance high ionic conductivity with mechanical integrity [105], enhance electrode/electrolyte wetting and adhesion, reduce interfacial resistance, and stabilize ion supply during cycling. Mechanistically, these effects help mitigate local ion depletion and polarization—factors that often manifest as increasing τ_b, switching hysteresis, and cycle-to-cycle drift in PANI-based electrochromic devices.

3.4. Environmental Stability

Environmental stability is one of the key issues that must be addressed before polyaniline (PANI) electrochromic materials can be deployed in practical engineering applications. Compared with inorganic transition-metal oxides, the electrochromism of PANI relies more strongly on the coupled processes of redox transition–protonation/deprotonation–ionic charge compensation. As a result, external pH, temperature, and humidity can alter the doping state of the polymer, ion-transport kinetics, and the interfacial chemical environment, thereby triggering reliability problems at the device level, including decreases in ΔT, increases in t₉₀, reductions in CE, and shortened cycle life. In addition, an electrochromic device may develop “local environmental drift” during operation; for example, proton generation/consumption associated with PANI electrochemical reactions can dynamically shift the interfacial pH near the electrode, coupling electrochemical switching–local chemical environment–aging into an intrinsic feedback loop [106].

From the perspective of pH, the conductivity and optical state of PANI are closely tied to its degree of protonation. Acidic conditions favor electroactive/protonated forms (e.g., emeraldine salt), whereas neutral/alkaline environments promote deprotonation (toward emeraldine base) and weaken effective counterion doping, typically resulting in lower carrier density, degraded spectral reversibility, and poorer coupled ion/electron transport. A direct visual manifestation is provided by Figure 9a: protonated PANI-based films immersed in buffer solutions show minimal appearance change at pH 3.0 over 1–30 min, while clear time-dependent darkening (and a more pronounced shift at higher pH) is observed at pH 7.4 and especially pH 10.0, consistent with pH-triggered deprotonation/dedoping and macroscopic optical-state drift [107]. Importantly, pH is not merely an external boundary condition in EC devices. Figure 9b (from the “electrochemical proton pump” concept) illustrates that under anodic polarization, PANI can release protons (PANI–H → PANI + H⁺), actively acidifying a confined thin-layer region; the reverse process can also be electrochemically driven [106]. This mechanistic picture is directly relevant to EC reliability: repeated switching may induce interfacial pH excursions even under nominally constant bulk conditions, shifting the local protonation equilibrium and thereby perturbing ΔT, t₉₀, and CE over time (e.g., by changing the fraction of electroactive/protonated segments and the efficiency of ionic compensation) [106]. However, it is important to note a significant methodological weakness in many reported stability studies. Most cycle life tests are conducted in ideal, large-volume, buffered electrolytes, which artificially mask the degradation caused by these local chemical environment changes. In contrast, practical devices operate with limited electrolyte volume where “local environmental drift” is pronounced. Therefore, previous studies that do not account for this self-induced local pH shift likely overestimate the practical lifespan of PANI-based devices, and future protocols must simulate these confined environments to provide realistic durability data.

From the perspective of temperature, the influence of temperature on environmental stability generally involves both kinetic and aging contributions. At low temperature, higher electrolyte viscosity and lower ionic conductivity slow down doping/dedoping, leading to increased t₉₀, a longer τ_b, and stronger kinetic hysteresis. At high temperature, dopant/ion migration and loss can accelerate, interfacial side reactions can be promoted, and the mechanical stress associated with repeated swelling/shrinkage during cycling can be magnified, increasing the likelihood of crack formation and interfacial delamination. In wide-temperature-window device engineering, it has been demonstrated that appropriately designed transparent ion-conducting layers/electrolytes can help flexible PANI-based devices retain multicolor electrochromic responses over a broad range (e.g., −10 to 60 °C), indicating that the “electrolyte structure–ion mobility–temperature robustness” relationship can be substantially improved through materials design [108]. In parallel, mechanistic studies of thermal/oxidative aging have shown that heat and oxygen exposure can drive irreversible chemical evolution in conductive PANI films (e.g., accumulation of oxidative products and composition changes), providing a basis for understanding continuous degradation of optical and electrochemical performance at elevated temperatures [109].

From the perspective of humidity, water can be both beneficial and detrimental to PANI systems. On the one hand, water molecules can participate in proton transport and swelling, which may temporarily increase ion accessibility and modify conductivity. On the other hand, long-term humid or hygrothermal exposure can cause doping-state drift, enhanced interfacial hydration, reduced mechanical integrity, and fluctuations in electrolyte concentration due to water uptake/loss, ultimately leading to unstable responses and accelerated cycling decay. Humidity-dependent behavior in PANI-based devices has been reported to significantly affect optical modulation and CE, emphasizing that moisture management is not a secondary packaging detail but a stability-determining factor [110]. Moreover, the intrinsic sensitivity of PANI’s electrical response to relative humidity (via proton conduction and hydration effects) suggests a realistic risk of RH-driven electrical drift under service conditions. Accordingly, stability-oriented device designs typically combine packaging strategies with electrolyte engineering [111]. Ionogels, which immobilize ionic liquids within a polymer network, are widely adopted to suppress leakage/evaporation while maintaining continuous ion-transport pathways; by tuning network architecture (e.g., crosslink density and phase morphology) and improving interfacial adhesion/wetting, ionogels can reduce interfacial impedance and stabilize ion supply under temperature–humidity fluctuations, thereby mitigating kinetic drift and performance fading [104].

In terms of degradation mechanisms, environmental failure in PANI electrochromic systems is rarely triggered by a single factor; instead, it is usually the consequence of coupled chemical–electrochemical–mechanical processes, including: (i) over-oxidation or accumulation of deeply oxidized states under unfavorable conditions, causing irreversible damage to the conjugated structure and drift of the optical state; (ii) loss of protons/dopant ions or deprotonation-induced drift in conductivity and spectral characteristics; (iii) microcracking, pore collapse, and interfacial delamination driven by repeated humidity/temperature-induced swelling–shrinkage; and (iv) increasing charge-transfer resistance due to interfacial side reactions and product accumulation, which manifests as a longer τ_b, increased t₉₀, and accelerated decay of ΔT and CE. Operando spectroscopic electrochemistry (e.g., Raman tracking under working potentials) has been used to capture signatures consistent with structural rearrangement and side reactions at higher anodic potentials, supporting the linkage between operating window, structural evolution, and performance deterioration [112]. Therefore, improving environmental stability requires coordinated strategies at both the material and device levels: suppressing deep oxidation and irreversible chemistry while enhancing resistance to dedoping at the material level-for instance, recent single-atom modification and synthesis strategies [113] have proven effective in stabilizing the local coordination environment of the chromophore [114]; and stabilizing the interfacial environment via structured electrolytes, buffering/water management, and robust encapsulation at the device level, together with evaluation protocols that better reflect real service conditions (combined temperature–humidity stress, pH variation, long resting self-bleaching, and alternating rest/cycling tests).

To summarize the property optimizations discussed in this section,

Table 2. Summary of Electrochromic Performance of PANI-Based Materials.

Material System	Configuration	Optical Modulation (ΔT)	Response Time (tc/tb)	Efficiency (CE) or Stability	Ref.
Pure PANI	Electropolymerized Film	≈50% (630 nm)	0.1 s (Fastest)	N/A	[42]
PANI Nanofibers	Porous Nanofiber Film	≈10%	t90 ≈ 0.02 s	N/A	[32]
PANI/PSS	Nanofiber Composite	67	N/A	Decay < 4% (2000 cycles)	[33]
PANI/TiO2	Nanocomposite Film	73.8%	0.46 s/0.94 s	Δϵ = 0.61 (IR Emissivity)	[87]
PANI-p-TSA	Layered Structure	70–80%	2.8 s/5.2 s	CE = 328.5 cm2⋅C−1.	[88]
PANI/MBI	Complementary Bilayer	73.8%	1.0 s/1.9 s	CE = 140.6 cm2⋅C−1.	[90]
PANI Smart Window	Dual-Band Device	65% (Vis)/59% (NIR)	5.9 s/16.9 s	High retention (10,000 cycles)	[95]
PANI-p-TSA	Sym. Supercapacitor	N/A	N/A	79% retention (3000 cycles)	[96]

presents a quantitative comparison of key performance metrics—including optical contrast, switching kinetics, and coloration efficiency—across the various PANI-based systems reviewed. These data highlight how specific strategies, such as composite engineering and functional doping, directly translate into enhanced optoelectronic and electrochemical behaviors.

4. Applications of Polyaniline-Based Electrochromic Materials

4.1. Smart Windows

Electrochromic smart windows regulate solar radiation transmittance dynamically by driving reversible redox reactions in the electrochromic layer via an applied potential, thereby switching between different optical states. Solar radiation can be approximated into three bands: ultraviolet (UV), visible light (VIS, 400–780 nm), and near-infrared (NIR, 780–2500 nm). Since NIR energy accounts for approximately 43% of total solar irradiance, the decoupled regulation of “visible daylighting (VIS)” and “solar heat gain (dominated by NIR)” is considered a critical pathway for enhancing building energy efficiency potential and visual/thermal comfort [5,115]. As a representative conductive polymer, Polyaniline (PANI) combines the advantages of multi-color states, low cost, and solution/low-temperature processability. In the realm of smart windows, PANI occupies a unique niche distinct from traditional inorganic counterparts. Unlike WO₃-based windows, which primarily modulate visible light through a single monochromatic transition (transparent to deep blue) via ion intercalation into a rigid lattice, PANI-based windows leverage the redox reconfiguration of the polymer backbone to achieve dynamic multicolor control, allowing for more versatile aesthetic integration. In smart windows, PANI typically serves as the anodic electrochromic layer or as a counter electrode/ion storage layer to participate in the device’s charge balance. Furthermore, the coupled “redox—protonation/deprotonation—ion compensation” process of PANI makes it highly sensitive to the electrolyte and device structure. Consequently, to address these sensitivities, recent research has increasingly focused on constructing PANI/WO₃ hybrid architectures [116,117]. These designs effectively combine the superior structural stability of inorganic oxides with the fast switching kinetics and spectral diversity of conducting polymers, realizing the synergistic optimization of “kinetics—reliability—spectral selectivity.” [95]. Furthermore, integrating advanced load forecasting models based on multimodal large models [118] could further optimize the dynamic control strategies of these smart windows for building energy efficiency.

From the energy-saving perspective, conventional single-band smart windows mainly modulate VIS transmittance, which often struggles to reduce indoor heat load under strong sunlight without significantly sacrificing daylighting. By contrast, dual-band/multiband smart windows tend to employ multiple operating modes (e.g., bright, cool, and dark) to realize “on-demand daylighting” and “on-demand thermal shielding.” In such devices, PANI typically contributes in two ways. First, as an anodic layer, it provides charge compensation and ion storage for an inorganic cathodic layer (e.g., oxygen-deficient WO_(3−x)), reducing polarization and improving reversibility. Second, the absorption differences of PANI across its oxidation/doping states offer an additional tunable degree of freedom in the visible region (color/transmittance), complementing NIR modulation from inorganic components (e.g., LSPR-related absorption or ion-insertion-induced NIR changes).

Recent representative studies have provided clear, quantifiable evidence for the logic chain of “PANI participation → dual-band modulation → energy saving verification.” Early explorations employed oxygen-deficient tungsten oxide nanowires as the cathode and porous PANI as the anode to assemble flexible dual-band devices, achieving high optical modulation (85.3% at 1200 nm) and significant indoor temperature reduction (8.8 °C) [76]. Building on this foundation, to introduce a semi-solid system and enhance visual monitoring capabilities, a novel study constructed a semi-solid, polychromatic dual-band smart window by organically assembling monoclinic tungsten oxide nanowires (WO_3−x NWs), an AlCl₃-polyvinyl alcohol (PVA) electrolyte, and PANI. This device not only achieved independent regulation of “bright,” “cool,” and “dark” modes but also exhibited excellent comprehensive performance, including large optical modulation (74.9% at 700 nm) and rapid switching (t_c/t_b of 7.6/2.7 s at 700 nm). Its NIR modulation primarily stemmed from the Localized Surface Plasmon Resonance (LSPR) absorption and phase transition of the WO_3−x NWs combined with the synergistic effect of PANI polarons/bipolarons, while visible light absorption relied on the bandgap transition of the nanowires and the electronic transition of the PANI quinone unit. Uniquely, the multi-colored PANI endowed the window with the ability to visually monitor modulation modes, with transparent, green, and blue states corresponding to the three modes, respectively; simulations indicated a maximum temperature decrease of 4.3 °C [119].

Furthermore, addressing the issues of monotonous color changes and limited reaction kinetics in PANI films, layer-stacked structural designs were introduced for optimization. Researchers developed a dual-band smart window based on layer-stacked PANI/Silver Nanowires (Ag NWs) composite films. The incorporation of Ag NWs significantly enhanced electron transport, shortening the coloring/bleaching times at 525 nm to 6.7 s/18.0 s. This composite film outperformed single-layer PANI in speed and exhibited rich aesthetic color conversions (light yellow, yellow-green, blue-black) alongside excellent stability after 2500 cycles [70].

Beyond liquid-based systems, the pursuit of safety has driven the development of all-solid-state smart windows, yet sluggish ion diffusion remains a persistent kinetic bottleneck. Addressing this, a high-performance solid-state ECD was engineered using a PANI interface as a rapid H⁺ reservoir to drive Prussian blue electrochromism. By facilitating efficient proton insertion via an ion-exchange mechanism with a solid polymer electrolyte, this architecture overcomes the kinetic limitations typically associated with solid-state Li⁺ diffusion. Consequently, the optimized device delivers exceptional metrics that rival liquid counterparts—fast switching (1.2 s coloration, 3.0 s bleaching), high contrast (72.5%), and robust stability (>5000 cycles)—validated on a large-scale (20 × 30 cm²) flexible platform suitable for architectural integration [120].

Beyond optical modulation alone, PANI’s redox capacity enables “electrochromic energy storage” (EES), transforming a window into a transparent battery or supercapacitor. High-performance EES devices have been realized using an electroactive polyamide (EPA) containing pentaaniline segments; when matched with a V₂O₅ ion-storage component, the resulting electrochromic capacitive window (ECW) exhibits dual-band modulation (optical contrast > 53% in both VIS and NIR) together with a wide 2 V operating window and a high areal capacitance of 47.26 mF·cm⁻². Energy stored in the colored state can effectively drive small electronic devices, demonstrating practical energy-recovery capability. Meanwhile, to mitigate the cost barrier associated with conventional ITO-based devices, innovative ITO-free flexible transparent electrodes have emerged as a robust alternative. For example, metal nanowire networks, such as silver nanowires (AgNWs) and alloyed copper nanowires (e.g., Ag-Au coated CuNWs or Cu-Au alloys), offer superior mechanical flexibility and high conductivity, serving as reliable current collectors for bifunctional electrochromic energy storage devices [121,122,123]. Additionally, a cost-effective scheme using poly(o-methoxyaniline) (PMOANI) and a hybrid transparent electrode (ITO 60 nm/Al mesh) was proposed. This device maintained an excellent optical contrast (ΔT = 57%) and fast switching (~5 s) while significantly reducing ITO thickness, and it retained the dual-functionality with an areal capacitance of ~8 mF·cm⁻², enabling the timer display to be powered for ~20 min in the dark state [124]. Furthermore, integrating these flexible electrodes with self-regulating mechanisms has led to the development of stable, ITO-free smart windows capable of autonomous energy management [125].

To further strengthen both “high-contrast electrochromism” and “substantial energy storage/charge buffering” within a single platform, recent studies have explored hybrid electrodes that combine one-dimensional inorganic nanostructures with PANI for integrated electrochromic–energy-storage smart windows/electrodes. In a representative W18O49 nanowire/PANI hybrid film system, clear potential-dependent visible color states were observed (supporting multi-state output for smart-window/display scenarios), while cyclic voltammetry (CV) curves measured at different scan rates demonstrated stable and reversible electrochemical behavior, directly linking charge-storage characteristics with optical output within a single characterization framework [126]. The reported key quantitative metrics include an optical modulation amplitude of ΔT = 70.2% and an areal capacitance of 79.6 mF·cm⁻², providing direct data support for the review discussion that “PANI–inorganic nanostructure synergy can enhance ΔT while pseudocapacitive/charge-buffering effects improve energy utilization and reduce polarization.”

Meanwhile, PANI functionality can be extended by coupling with other environmental stimuli, such as temperature. A thermo–electrochromic dual-responsive device (T-ECD) was designed by combining a thermochromic hydroxypropyl cellulose/potassium chloride (HPC/KCl) hydrogel electrolyte with optothermal PANI films. Under solar irradiation, the photothermal effect of PANI drives a rapid hydrogel phase transition (30 s), much faster than ~3 min for conventional thermochromic devices. This system achieves switchable color tones (light yellow to purple) and a high solar modulation efficiency (60.89%). A model-house field test further showed that, compared with conventional glass, the indoor temperature can be reduced by 13.3 °C, highlighting the synergistic effect between PANI electrochromism and photothermal behavior [127].

Overall, compared with typical inorganic oxide systems (e.g., WO₃, NiO), PANI-based smart windows exhibit distinctive advantages: (i) strong color and spectral designability enabled by multiple oxidation/doping states; (ii) compatibility with flexible substrates and low-temperature processing, facilitating plastics, textiles, and large-area coating; and (iii) dual functionality—serving simultaneously as an optical modulator and an energy-storage unit. However, the engineering deployment of PANI smart windows depends more strongly on system-level optimization across “formulation–structure–packaging,” including porous/laminated structures (e.g., Ag-NW composites) to promote coupled electron/ion transport, composite/interfacial reinforcement to mitigate volume effects, and gel/ionogel electrolytes to suppress water-content fluctuations. The development trend of PANI smart windows is therefore shifting from “single visible tinting” toward comprehensive competition in “multi-mode spectral selectivity + reliability + low energy consumption/energy recovery,” with PANI remaining a unique and valuable platform material for visible tunability, charge balancing, and photothermal coupling [76].

4.2. Displays and Optical Devices

Electrochromic displays and optical devices are essentially electrically controlled spectral modulators [128,129]. Under an applied potential, the electrochromic layer undergoes reversible redox reactions accompanied by ion insertion/extraction to maintain charge neutrality, thereby switching between colored and bleached states. This alters absorption, reflection, or transmission in the visible region and even the near-infrared, enabling text/pattern/color-block displays as well as tunable optical filters. Unlike emissive displays, electrochromic devices often exhibit a “quasi-bistable” character—energy is mainly consumed during switching, while the optical state can often be maintained with no power or only minimal holding power. This makes electrochromics intrinsically attractive for flexible/wearable indicators, e-paper-like reflective displays, and low-power optical switches and filters. Polyaniline (PANI), governed by a coupled redox–protonation/deprotonation–ion compensation mechanism, can deliver multicolor reversible changes at relatively low driving voltages and is compatible with solution/low-temperature processing. These advantages allow PANI to function as an active display layer, and also enable performance enhancement through complementary-electrode designs and electrolyte engineering to achieve higher contrast, faster response, and more reliable flexible integration.

In the crossover space between “displays” and “optical devices,” complementary bilayer architectures based on PANI and viologen provide direct device-level evidence for multiwavelength output and low-energy operation [130]. Figure 10 summarizes a representative all-organic flexible electrochromic device in which PANI serves as the anodic electrochromic layer and viologen acts as the cathodic electrochromic layer. By using complementary redox processes, this bilayer design reduces polarization and improves reversibility, enabling multiwavelength color/spectral switching suited for both display and filtering. Specifically, Figure 10a presents transmittance spectra over 500–900 nm with clearly labeled ON/OFF/Initial states, demonstrating stable optical differentiation across a broad spectral window and supporting the concept of “visible display + NIR modulation/filtering.” Figure 10b shows stepwise absorbance switching and extended cycling behavior, indicating repeatable optical responses under repeated operation. Figure 10c further provides an expanded kinetic profile at monitored 800 nm, explicitly reporting fast response times of τc = 0.5 s and τb = 0.8 s, evidencing sub-second switching in the NIR. Figure 10d records the applied voltage waveform together with the current response, offering a direct mapping between the electrochemical driving process and the optical switching output—useful for establishing the “charge passed–switching kinetics–energy consumption” discussion. Therefore, this PANI–viologen bilayer device can serve as a representative example supporting the logic that “complementary electrode design enables multiwavelength display/filtering and low-power switching through fast τc/τb [130].”

For flexible and wearable electrochromic displays, manufacturable patterning and mechanical deformation stability remain key constraints for practical deployment. Inkjet printing of two-dimensional lamellar PANI provides a “printable–addressable–customizable pattern” route: printed PANI electrodes have been reported to achieve a high optical contrast of ΔT = 76% at 750 nm, a high coloration efficiency of CE = 259.1 cm²·C⁻¹, and relatively fast switching kinetics (τc/τb = 1.8/2.4 s), together with pseudocapacitive characteristics and mechanical compliance that are beneficial for flexible patterned displays and “display + electrochemical function” integration [84]. Moving to textile platforms, lamellar-PANI textile electrochromic displays (reflective readout) have demonstrated a reflectance modulation amplitude of ΔR = 22.9% and retained electrochromic function after 1000 bending cycles, underscoring the feasibility of sustaining “deformation–ion/electron transport pathways–optical readout stability” through structural and interfacial design [85]. In addition, highly integrated all-in-one electrochromic textiles (where a PANI chromic layer and a metallic reflective layer form the optical modulation unit) have achieved an ultrathin thickness of ~80 μm while maintaining stable chromic output in textile form factors, indicating that device-level integration and thinning are also important engineering routes toward wearable displays [131].

Low power consumption is a defining advantage of electrochromic displays compared with emissive technologies. For PANI-based systems, achieving low-power operation in practice typically depends on coordinated advances along three axes: (i) reducing polarization and ohmic losses via complementary structures or conductive networks, thereby lowering the required charge per unit optical change (ΔT/ΔR/ΔOD) and shortening t90 and τc/τb; (ii) employing solid/gel electrolytes that retain sufficient ionic conductivity in thin form factors to reduce the driving voltage and switching energy; and (iii) minimizing leakage and crosstalk through device architecture and encapsulation to improve retention stability. In this context, the PANI–viologen complementary bilayer shown in Figure 10 provides direct kinetic evidence for “complementary electrodes → fast switching → low-energy operation,” particularly with the reported τc = 0.5 s and τb = 0.8 s (800 nm). Meanwhile, inkjet-printed lamellar PANI and textile reflective-display examples further demonstrate that low-power electrochromic displays can be compatible with flexible and wearable formats, allowing the intrinsic advantages of electrochromism to translate into realistic use scenarios [130].

4.3. Sensors and Indicators

Electrochromic sensors/indicators can be viewed as electrochemical–optical coupled systems that “translate chemical information into readable optical signals,” holding promise for diverse diagnostic applications such as monitoring disease-associated biomarkers [132,133]. Target molecules/ions modulate the redox equilibrium, doping/de-doping degree, and ion-compensation process of the electrochromic layer, thereby changing absorption/transmittance/reflectance and producing reversible (or quasi-reversible) color/spectral responses. Owing to its pronounced proton-coupled characteristics (strong coupling between redox transitions and protonation/deprotonation), polyaniline (PANI) is particularly sensitive to H⁺/pH, alkaline volatile amines, and various ionic environments. Such chemical perturbations directly map onto the doping state of PANI and the distribution of polarons/bipolarons, manifesting as changes in the intensity/position of visible absorption bands, attenuation of ΔT/ΔA, and prolonged t90 (or response time), thus providing a material basis for visual monitoring.

For gases and volatile biomarkers—especially volatile amines generated during food spoilage—PANI-based “proton-sensitive electrochromism” has been leveraged for indirect quantification. Zohrevand and co-workers constructed FTO/PANI film electrodes and systematically compared the electrochromic behavior in the presence/absence of amines, showing that amines can markedly interfere with the electrochromic process of PANI. By tracking the electrochromic response at 420, 620, and 750 nm, triethylamine could be indirectly detected with a linear range of 0.10–7.01 μmol·L⁻¹ and a detection limit of 0.06 μmol·L⁻¹ [134]. The strategy was further extended to gas-phase monitoring of beef and fish spoilage, demonstrating the feasibility of PANI along the pathway “gas-phase chemistry → doping-state drift → optical readout.”

For ionic and acid–base indicators, a key advantage of PANI is that it supports both electrochemical readout and optical (color/absorption) readout, and the two are often governed by the same coupled factors: protonation degree, morphology/interface characteristics, and ion accessibility. In a recent example, PANI films drop-cast on FTO were evaluated as pH sensors using both electrochemical and optical modes: the electrochemical platform delivered a maximum sensitivity of 127.3 ± 6.2 mV·pH⁻¹, while the optical platform achieved a maximum integrated-absorbance sensitivity of 0.45 ± 0.05 (pH)⁻¹, and the authors highlighted a morphology-dependent trade-off (rougher/thicker films favor electrochemical response, whereas smoother surfaces benefit optical readout) [135]. As shown in Figure 11, the study directly correlates the surface roughness of PANI films with the sensitivities obtained by the two readout modes: integrated absorbance is used to quantify optical pH sensitivity, while open-circuit potential/electrode potential response is used to quantify electrochemical pH sensitivity. The results indicate that roughness modulates both optical and electrochemical responses, yet their optimal ranges differ, reflecting that morphological tuning alters effective interfacial reaction area, ion accessibility, and optical scattering/baseline stability, thereby shaping the combined performance of “ion transport → signal output → readout stability.”

Beyond pH/H⁺, PANI is also widely used as a functional layer for ion recognition and electrochemical signal amplification, offering a pathway toward integrated and miniaturized monitoring for environmental and health applications. For instance, an ion-imprinted PANI (IIPANI) prepared by electropolymerization and combined with a bismuth-modified carbon paste electrode enabled highly selective Ni(II) detection, with a linear range of 0.01–1 μM and a detection limit of 0.00482 μM. Notably, good selectivity was retained even in the presence of 1000-fold excess interfering ions (Cd(II), Co(II), Cu(II), Zn(II)), and the approach was successfully applied to river-water samples. These results suggest that PANI can serve not only as a visually addressable electrochromic layer but also as a structurally programmable interface for selective ion recognition and long-term monitoring [136].

From a device-level viewpoint of indicators/visual monitoring, PANI can also actively regulate the local chemical environment electrochemically, converting “chemical information” into interpretable optical/electrochemical outputs. Wiorek and co-workers demonstrated PANI films as electrochemical proton pumps: under anodic polarization, H⁺ can be released into thin-layer samples for rapid, controllable acidification. According to the reported description, the sample pH can be decreased from approximately pH ≈ 8 to pH ≈ 2–3 within ~2 min or less, enabling reagent-free alkalinity determination in water samples [106]. This “electrochemistry → local pH → material state” active-coupling concept is instructive for electrochromic indicators that must suppress environmental drift and improve readout consistency in complex matrices: by embedding controllable ion/proton regulation units, unstable external perturbations can be converted into calibrated and repeatable response windows.

4.4. Emerging Applications

Beyond smart windows, displays, and sensors, polyaniline (PANI)–based electrochromic materials have increasingly been explored in recent years toward multifunctional integrated devices. Representative emerging directions include (i) energy storage–electrochromic integrated devices (electrochromic energy-storage devices, EESDs/electrochromic supercapacitors/electrochromic capacitive windows), (ii) electrochromic textiles for wearable visual indicators and color-changing labels, and (iii) adaptive camouflage and infrared emissivity modulation devices for low detectability and thermal management. A common feature of these applications is that device performance is no longer evaluated solely by visible color change; instead, the reversible redox chemistry of PANI is leveraged to couple optical/spectral modulation with pseudocapacitive energy storage, often in conjunction with textile platforms, reflective architectures, or infrared regulation requirements. Consequently, system-level design of charge transport, ion compensation, and interfacial stability becomes a critical determinant of achievable performance and durability.

4.4.1. Energy Storage–Electrochromic Integrated Devices

The electrochromic response of PANI and its electrochemical energy storage behavior originate from the same fundamental process involving electron transfer accompanied by ion compensation. Accordingly, integrated devices can, in principle, simultaneously realize: (i) electrically controlled optical modulation (ΔT/ΔR), (ii) electrochemical energy output (operating voltage window, areal/gravimetric capacitance, and cycling stability), and (iii) visual indication of the state of charge through color or transmittance changes. From a device engineering perspective, high-performance integration typically relies on three synergistic strategies: (1) introducing highly conductive frameworks (e.g., two-dimensional materials or porous conductive networks) to reduce ohmic polarization and ensure electronic continuity across different oxidation states; (2) matching reversible ion-storage counter electrodes to minimize charge imbalance and parasitic reactions; and (3) employing gel or solid-state electrolytes to improve form-factor adaptability and long-term environmental robustness.

A representative example is provided by layer-by-layer assembled PANI/MXene integrated devices, whose coupled optical, kinetic, energetic, and cycling performances are summarized in Figure 12 [71]. Figure 12a shows the reversible color evolution of the device under different applied potentials, reflecting the multistate optical output associated with redox transitions of PANI during doping and dedoping. The corresponding transmittance spectra (Figure 12b) demonstrate an optical modulation amplitude of ΔT = 51.9% at approximately 700 nm in the initial cycles, which remains at ≈47.4% after 1000 cycles. Figure 12c further tracks the evolution of ΔT as a function of cycle number, where the modulation remains around 51.9% in the early stage, ≈51.5% at intermediate cycles, and ≈47.4% at 1000 cycles, indicating sustained optical functionality during long-term operation. Switching kinetics extracted at 700 nm (Figure 12d) show coloration/bleaching times of τc/τb = 7.0/2.0 s before cycling and 5.7/2.5 s after 1000 cycles, suggesting that the device retains second-scale response without orders-of-magnitude degradation. In terms of energy efficiency, the optical density–charge density relationship (Figure 12e) yields coloration efficiencies of CE = 118.2 cm²·C⁻¹ in early cycles and 113.3 cm²·C⁻¹ after 1000 cycles, corresponding to only a modest decline. Collectively, these results indicate that the combination of a two-dimensional conductive scaffold with PANI can simultaneously support large optical modulation, stable switching kinetics, and preserved coloration efficiency, which can be attributed to reduced charge-transfer resistance, improved utilization of electroactive sites, and relatively continuous ion-transport pathways that mitigate polarization and irreversible degradation.

A representative example of an electrochromic energy-storage device (ECESD) employs a sandwich-type configuration consisting of a PANI electrode/hydrogel electrolyte/Zn electrode, in which a highly transparent PAAm–7.5 M ZnCl₂ gel electrolyte is introduced. Owing to the optimized polymer–salt network, the gel electrolyte exhibits an average transmittance of approximately 94.1% in the visible range (380–800 nm), providing a low optical-loss background for device operation [137]. Within the working potential window, the device displays well-defined, voltage-controlled multicolor states. Quantitative electrochromic kinetics and energy-efficiency parameters measured at 550 nm reveal coloration/bleaching response times of t90 (tc/tb) = 6.9/6.3 s, a large optical modulation amplitude of ΔT = 75.7% (Tmax = 91.4%, Tmin = 15.7%), and a coloration efficiency of CE = 112.8 cm²·C⁻¹. During prolonged cycling, the ΔT of the PANI film decreases from 70.9% to 59.2%, corresponding to a retention of approximately 83.5%, indicating practical device-level stability under repeated operation.

Beyond battery-type configurations employing gel electrolytes, redox-active dopants combined with functionalized gel electrolytes have emerged as another effective strategy to enhance the overall performance of EESDs. A representative case involves the incorporation of anthraquinone-1-sulfonate (AQS) into both the PANI electrochromic film and a PVA-based gel electrolyte [92]. In this design, AQS introduces additional reversible faradaic sites at both the electrode–electrolyte interface and within the bulk gel phase, while simultaneously improving the continuity of ion-transport pathways. As a result, the device achieves rapid color switching within <2 s (from light green to deep black) and delivers a high areal capacitance of 194.2 mF·cm⁻² at 1 mA·cm⁻². Notably, after 5000 charge–discharge cycles, the device retains 72.1% of its initial capacitance, representing a substantial improvement over undoped counterparts. These results demonstrate that electrolyte redox activity and structural design, together with optimized ion transport, constitute a critical logic chain for improving cycling durability and multifunctional performance in EESDs.

Beyond inorganic conductive scaffolds, conductive polymer network engineering has also proven effective for enhancing integrated performance. For example, flexible devices based on PEDOT/PANI conjugated dual networks have been reported to achieve CE = 322 cm²·C⁻¹ and ΔT = 41% (610 nm) under sub-second switching conditions, while simultaneously delivering a high specific capacitance of 357 F·g⁻¹ (at 1 A·g⁻¹). Such architectures benefit from continuous electronic pathways across oxidation states and reduced kinetic limitations associated with the low-conductivity states of PANI, thereby enabling concurrent fast electrochromic response and substantial energy storage [82].

In addition, incorporating inorganic pseudocapacitive phases provides an effective route to buffer volume changes and stabilize long-term operation. Electrochemically deposited PANI/MnO₂ nanocomposite electrodes, when employed in integrated devices, exhibit ΔT = 46.12% (680 nm), a wide operating voltage window of 2.0 V, and an areal energy density of 49.99 μWh·cm⁻². In this system, the pseudocapacitive contribution and structural support of MnO₂ help mitigate mechanical and electrochemical degradation associated with repeated PANI doping/dedoping, thus improving compatibility between optical modulation and energy output [75].

Notably, PANI-based integrated devices are also evolving toward processable polymer architectures suitable for large-area implementations. Electroactive polyamides containing pentaaniline segments (EPA) have been employed in electrochromic capacitive windows (ECWs), achieving dual-band modulation of ΔT = 56.57% at 650 nm and 53.29% at 1250 nm, rapid optical switching (<5 s), and an areal capacitance of 47.26 mF·cm⁻², while operating in non-acidic electrolytes [124]. Similarly, smart window supercapacitors based on hyperbranched electroactive polyamides have reported ΔT > 59.51%, a voltage window of 2.4 V, and an areal capacitance of 85.76 mF·cm⁻², offering alternative material platforms for integrating optical bistability with energy storage in large-area devices [138].

Overall, the primary constraint in energy storage–electrochromic integrated devices arises from the intrinsic coupling between optical performance (ΔT, CE, τc/τb) and energy-storage metrics (voltage window, capacitance/energy density, cycling life). Progress toward practical applications, therefore, relies on coordinated optimization of electrode frameworks, electrolyte systems, and interfacial chemistry to balance optical output, kinetic response, and long-term reliability.

4.4.2. Electrochromic Textiles and Adaptive Camouflage/Infrared Emissivity Modulation

For wearable applications and adaptive camouflage, the key advantages of PANI lie in its compatibility with low-temperature, solution-based processing on fibrous or textile substrates, enabling reversible visible color changes while maintaining mechanical flexibility. Moreover, the evolution of carrier absorption associated with PANI doping states facilitates coupling with reflective structures and infrared-regulating mechanisms, extending functionality from the visible region to near- and mid-infrared thermal radiation control. Representative textile-based systems include PANI@MWCNTs composite coatings deposited on cotton fabrics, where the carbon nanotube network enhances electronic conductivity and mechanical compliance, allowing electrochromic operation to be retained under bending and deformation, similar to highly stretchable composite nanofiber films [139], while offering pathways toward infrared shielding and thermal management [79].

For adaptive camouflage and thermal management, more stringent requirements are imposed on devices, particularly regarding large and reversible emissivity modulation (Δε) in the mid-infrared atmospheric window and long-term cycling durability. A recent study achieved this by constructing a coordination-stabilized PANI network through co-doping with protonic acids and Zn²⁺ ions, reporting an emissivity modulation of Δε = 0.65 in the 8–14 μm range and retention of 98% of the modulation amplitude after 1000 electrochemical cycles. These results highlight the role of coordination stabilization and enhanced carrier delocalization in improving the reversibility and durability of infrared modulation [140]. In parallel, fully integrated electrochromic textile systems combining PANI functional layers with reflective and structural layers in thin, flexible architectures have begun to emerge, enabling simultaneous visible camouflage and infrared regulation on a single platform. Such systems provide device-level pathways toward wearable thermal management, unmanned platform concealment, and adaptive signal display [131].

To provide a comprehensive overview of the diverse applications discussed in this chapter,

Table 3. Performance Metrics of PANI-Based Integrated Applications.

Application Type	Device/Material System	Optical Metrics (ΔT/Δε)	Functional Metrics (Energy/Sensing)	Stability/Response	Ref.
Smart Window	PANI/WO3-x NWs	ΔT = 74.9%	Temp reduction: 4.3 °C	Fast switching (7.6/2.7 s)	[119]
Smart Window	Solid-State PANI/PB	ΔT = 72.5%	High H+ reservoir capacity	>5000 cycles	[120]
Printed Display	Inkjet-Printed PANI	ΔT = 76%	CE = 259.1 cm2·C−1	1.8 s/2.4 s	[84]
Sensor	PANI Film (pH)	Absorbance change	Sens. = 127.3 mV·pH−1	Dual-mode readout	[135]
Energy Storage	PANI/MXene	ΔT = 51.9%	CE = 118.2 cm2·C−1	≈47.4% mod. (1000 cyc)	[71]
Supercapacitor	PANI/MnO2	ΔT = 46.1%	E-Dens. = 49.9 μWh·cm−2	73.5% cap. (15,000 cyc)	[75]
EESD	PANI/Zn (Hydrogel)	ΔT = 75.7%	CE = 112.8 cm2·C−1	83.5% mod. retention	[137]
Camouflage	PANI/Zn2+ Co-doped	Δε = 0.65 (8–14 μm)	IR Emissivity modulation	98% retention (1000 cyc)	[140]

summarizes the device architectures and key performance metrics across smart windows, displays, sensors, and multifunctional integrated systems. This comparison highlights how PANI-based materials are tailored to meet specific functional requirements, from broadband optical modulation to electrochemical energy storage and sensing sensitivity.

5. Challenges and Future Perspectives

Despite significant progress in nanostructural engineering, multi-band spectral regulation, and multifunctional integration of polyaniline (PANI) electrochromic materials, the transition from laboratory prototypes to large-scale engineering applications remains constrained by several critical bottlenecks. Future evolution requires addressing the following intertwined scientific and technical challenges.

First, long-term cyclic stability and environmental durability constitute the core bottlenecks limiting practical application. Unlike inorganic oxide systems, PANI faces more complex decay mechanisms. At the electrochemical level, operation at high anodic potentials induces over-oxidation, leading to the formation of pernigraniline degradation products (e.g., hydrolysis to quinone-like species) and irreversible scission of the conjugated backbone [141,142], a process involving complex simultaneous reduction and degradation pathways similar to those found in complex electrochemical catalytic systems [143,144]. At the mechanical level, the intrinsic volume expansion and contraction accompanying repeated doping/dedoping (ion insertion/extraction) generate significant stress at the electrode–substrate interface that necessitates advanced “soft/hard interface” design strategies and strain recovery models inspired by metallic glass systems to prevent film delamination [145,146] and microstructural toughening mechanisms in ultra-high strength alloys [147], contact failure, and structural pulverization over long-term cycling. At the environmental level, the “local environmental drift” effect—such as interfacial pH fluctuations induced by the proton-pump effect—establishes a feedback loop of “electrochemical operation—local chemical environment alteration—accelerated aging”, which conceptually parallels the short-term degradation and long-term recovery dynamics observed in stressed systems [148]. However, conventional testing in large-volume buffered electrolytes often masks these local pH excursions. Future protocols must therefore focus on evaluating durability under realistic “confined electrolyte” conditions and coupled thermal-humidity stress. Consequently, stabilization strategies must shift from simple physical encapsulation to intrinsic stability design, including developing self-buffering electrolytes to mitigate ionic channel clogging [149], constructing cross-linked or interpenetrating polymer networks capable of accommodating volume deformation, and implementing “protective” electrochemical protocols that strictly limit the operating potential window.

Second, the limitation of spectral modulation, particularly the absence of a high-quality “neutral state,” remains a critical barrier preventing PANI from competing with commercial inorganic electrochromics in architecture and aesthetics [150,151]. Although PANI naturally exhibits vibrant colors (yellow, green, blue, purple), it intrinsically lacks the pure black or neutral gray state urgently needed for architectural smart windows and high-fidelity displays. Currently, the bleached state typically retains a residual pale yellow hue, while the dark state biases towards deep blue rather than neutral black, making it difficult to balance high transmittance with neutral coloration. While attempts to blend PANI with complementary chromophores (e.g., WO₃ or viologens) have shown promise, a critical evaluation reveals that simple physical blending often leads to complex manufacturing and compromised transparency in the bleached state due to spectral mismatch. Consequently, future research should prioritize molecular-level strategies, particularly the design of donor–acceptor (D–A) copolymers that can intrinsically broaden absorption bands to achieve high-fidelity neutral black states, or achieve spectral balance through compositing with carbon-based nanomaterials.

Third, a significant processing gap exists between laboratory preparation and large-scale manufacturing. Although chemical oxidative polymerization offers scalability potential, controlling batch-to-batch consistency in molecular weight, conductivity, and dispersion stability at an industrial scale remains challenging. In large-area device construction (e.g., >1 m²), the limited conductivity of transparent conductive electrodes leads to significant voltage drop (iR drop), triggering the “iris effect”—where edge switching is significantly faster than the center—thereby affecting visual uniformity [152]. Furthermore, the high cost proportion of high-quality flexible electrodes (e.g., high-performance ITO/PET or silver nanowire networks) and high-barrier encapsulation materials limits economic viability. Adopting unified metric architectures [153] and cost-preserving models [154] similar to those in critical infrastructure is essential to evaluate scalability. To bridge the gap between lab-scale prototypes and industrial applications, future manufacturing must move beyond simple coating. Technologies need to combine low-cost solution processing techniques (e.g., slot-die coating) with hierarchical metal-mesh current collectors and advanced surface engineering [155] to minimize ohmic losses, thereby ensuring large-area switching uniformity and reducing system costs [156].

Finally, the research paradigm in this field is evolving from single-material optimization to system-level functional integration, potentially incorporating advanced multi-objective optimization strategies tailored for complex electromagnetic devices [157,158]. (i) Multi-physics coupling, developing composite material systems capable of decoupled control over visible (color), near-infrared (heat gain), and mid-infrared (radiative cooling) bands to adapt to next-generation thermal management needs; (ii) Integrated energy systems, utilizing the pseudocapacitive properties of PANI to construct “zero-energy” smart windows or visual batteries, enabling energy harvesting, storage, and self-powered coloration, thereby supporting grid resilience and cooling efficiency in infrastructure [159,160], and (iii) Solid-state ionics, developing high-performance ionogels to replace liquid electrolytes, satisfying the rigorous mechanical flexibility requirements of wearable and textile electronics while eliminating leakage risks; and (iv) Intelligent system integration, incorporating advanced control algorithms [161] and visual processing techniques—such as motion decoupling [162], deblurring [163], and weather-degraded image restoration [164]—to enhance the environmental adaptability and display fidelity of PANI-based optical devices. Despite existing challenges, integrating polymer chemistry, interface engineering, and device physics through interdisciplinary research positions polyaniline to overcome current limitations and become a cornerstone material in future smart optical and energy systems.

6. Conclusions

This review has comprehensively surveyed the landscape of polyaniline (PANI)-based electrochromic materials, elucidating the intrinsic correlations linking synthesis methodologies, structural modulation, and optoelectronic performance. The core functionality of PANI is underpinned by its unique coupled mechanism of “redox transitions—protonation/deprotonation—ionic charge compensation.” This mechanism not only distinguishes PANI from inorganic intercalation compounds but also provides a versatile handle for tuning optical properties through both electron flux and proton activity, positioning it as an ideal platform for dual-band spectral regulation and multifunctional integration.

In terms of material preparation, significant progress in synthesis strategies—ranging from scalable chemical oxidative polymerization to precise electrochemical deposition and template-assisted nanostructuring—has enabled the tailoring of molecular ordering and micro-morphology. These structural optimizations have been pivotal in overcoming the intrinsic kinetic limitations of PANI, effectively resolving the trade-off between high optical density (requiring thick films) and rapid switching response (requiring short diffusion paths). Notably, the design of composite architectures, incorporating rigid inorganic scaffolds or highly conductive carbon networks, has proven effective in mitigating volumetric fatigue during redox cycling and enhancing electrochemical reversibility, thereby significantly extending device cycle life.

Regarding applications, the role of PANI has undergone a fundamental shift from a simple monochromatic display material to a functional core of advanced photonic and energy systems. This evolution is characterized by three distinct trends: (1) the transition from single visible-light modulation to decoupled “visible–near-infrared” dual-band regulation, meeting the demands of next-generation smart windows for precise light–heat management; (2) the convergence from passive coloration to integrated “electrochromism–energy storage” systems, leveraging the pseudocapacitive properties of PANI to construct electrochromic supercapacitors and self-powered optical systems; and (3) the expansion from laboratory prototypes to environmentally adaptive systems, including PANI-based adaptive thermal management textiles and high-sensitivity potentiometric sensors.

Looking forward, the commercial viability of PANI electrochromics relies on bridging the gap between laboratory performance and practical service reliability. Core challenges remain in overcoming the environmental sensitivity inherent to the proton-coupled mechanism (e.g., pH drift), achieving high-fidelity neutral color states, and establishing cost-effective large-area manufacturing processes. Future breakthroughs will no longer depend on single-dimensional material modification but require a holistic engineering approach that synergizes polymer chemistry (intrinsic stability design), solid-state ionics (high-performance electrolytes), and interface physics (micro-stress management). Addressing these bottlenecks will position polyaniline as a cornerstone material for the next generation of sustainable, adaptive, and intelligent optical interfaces.