Vapor-Phase Deposition of Electroactive Polymers onto Electrospun Commodity Polyacrylonitrile Nanofibers

Abstract

Electroactive polymers (EAPs) offer unique electrical and mechanical properties suitable for energy, environmental, and biomedical applications. Integrating EAPs with nanofibrous substrates can further enhance performance through the high surface area, interconnected porosity, mechanical flexibility, and tunable morphology of electrospun nanofibers. However, achieving uniform integration of EAPs on high-aspect-ratio nanofibers remains challenging due to poor coating homogeneity, limited penetration, and the reliance on toxic solvents in conventional deposition techniques. Here, a solvent-free vapor-phase deposition (VPD) process was developed to produce environmentally friendly and tunable EAP coatings on electrospun polyacrylonitrile (PAN) nanofibers. Electrospinning conditions were optimized to produce uniform PAN nanofibers with an average diameter of 350 nm. Poly­(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), poly­(3-hexylthiophene) (P3HT), and poly­(2,2′-bithiophene) (PBTH) were deposited via oxidative vapor-phase polymerization using iron­(III) chloride (FeCl₃) as the oxidant. Increasing FeCl₃ concentration enhanced polymer deposition, resulting in increased nanofiber diameter and weight. Energy dispersive spectroscopy (EDS) confirmed increased sulfur-to-nitrogen ratios for PEDOT, PBTH, and P3HT coatings prepared with higher oxidant concentrations. Fourier transform infrared spectroscopy (FTIR) verified complete EAP coverage by the appearance of characteristic EAPs CC and thiophene vibrations and the disappearance of PAN nitrile band. Four-point probe measurements showed that nanofiber conductivity was maximized at intermediate oxidant concentrations (2–3 M FeCl₃), with the highest values of 0.41 S cm^–1 for PPy-coated PAN nanofibers and 0.11 S cm^–1 for PEDOT-coated nanofibers. Cyclic voltammetry (CV) demonstrated reversible redox behavior and quasi rectangular voltammograms for PEDOT-coated nanofibers at scan rates of 1–8 mV s^–1, highlighting their potential as conductive platforms for charge storage and charge-transport applications.

Introduction

EAPs have garnered significant attention in materials science due to their unique ability to switch reversibly between insulating and conducting in response to an electric field. , Prominent EAPs with exceptional electrical and optical qualities, redox stability, and biocompatibility include PEDOT, polyaniline (PANI), PPy, polythiophene (PTh), as well as its derivatives such as P3HT, and PBTH. These polymers, characterized by conjugated molecular backbones, can undergo reversible redox reactions, allowing them to switch between conductive and insulating states. − This property makes EAPs highly valuable in applications such as organic light-emitting diodes, organic thin-film transistors, organic photovoltaic cells, field-effect transistors, antistatic, anticorrosion coatings, electrochromic devices, chemical sensors, batteries and biosensors.

However, a key challenge in utilizing EAPs is their poor processability, nonscalability (i.e., difficulty in translating lab-scale synthesis or thin-film fabrication into large-area, high-throughput, and reproducible manufacturing) and lack of structural integrity when used alone. To overcome this, researchers have explored coating EAPs onto alternative polymeric scaffolds, particularly electrospun nanofibers, which offer a high surface area, interconnected porosity, and tunable mechanical properties.

Electrospinning is a technique that allows the fabrication of nanoscale fibers from commodity polymers, making it suitable for a wide range of applications requiring high surface area, tunable porosity, and controlled morphology. These attributes are critical not only for cell adhesion and proliferation and controlled molecular transport with respect to biomedical applications, but also for applications in energy storage and conversion, conductive and electroactive systems, filtration and environmental remediation, sensing platforms, and lightweight structural and functional coatings. An electrospinning setup consists of a high voltage power source, a syringe/extruder, a pump, and a drum or collection plate. Electrospun scaffolds serve as excellent substrates for EAP coatings, enhancing their functionality while maintaining structural stability. Although porous or fibrous structures of EAPs have been successfully prepared using techniques such as electrospinning, phase separation, self-assembly, and template-assisted synthesis, these methods often face challenges, including limited structural control, scalability issues, and poor mechanical stability. , One effective approach to circumvent these issues involves utilizing a commodity polymer in its nanofibrous form via electrospinning as a base material for subsequent EAP coating. Commodity or carrier polymers play a crucial role in electrospinning by providing a stable matrix for fiber formation and influencing nanofiber morphology. The viscosity, molecular weight, conductivity, and surface tension of these polymers determine fiber uniformity, diameter, and defect formation. Polymers such as PAN, poly­(vinyl chloride), polycaprolactone, polyethylene, polypropylene, poly­(lactic-co-glycolic acid), and poly­(ethylene oxide) have been extensively applied for electrospinning due to their tunable properties.

EAPs offer promises for various applications, but conventional coating or deposition methods with commodity polymers like dip-coating and electrochemical polymerization face scalability and adhesion challenges. Solvent-based techniques, such as dip-coating, involve dissolving the EAPs in organic solvents before applying them onto nanofiber scaffolds. While this method is straightforward, it presents issues such as poor adhesion, inconsistent polymer distribution, and potential degradation of the nanofiber structure due to solvent interactions. Furthermore, while EAP-containing materials with various structural architectures, including self-assembled, template synthesized, or blended electrospun nanofibers frequently achieve high electrical conductivities by utilizing large specific surface areas composed primarily of pure conducting polymer domains, , balancing these electrical metrics with mechanical performance and macroscopic-scale fabrication remains a critical challenge. For instance, evaluating a PVA nanofibrillar structural matrix coated with PEDOT has demonstrated excellent structural and mechanical properties despite displaying limited electrical conductivity. Additionally, the use of toxic organic solvents in solution-phase processes raises environmental and health concerns, making these conventional solution-based methods less desirable for large scale applications. , Electrochemical polymerization provides a more controlled approach to EAP deposition. This method allows for direct polymerization onto conductive substrates under an applied voltage. However, it is largely restricted to conductive surfaces, limiting its applicability for insulating nanofiber scaffolds. Furthermore, achieving uniform polymerization on complex three-dimensional nanofiber structures remains challenging due to variations in current distribution across the scaffold.

To effectively deposit EAPs onto nanofibers, VPD has emerged as a promising alternative to traditional solvent-based methods such as dip coating and electrochemical polymerization. VPD enables the deposition of EAPs onto various substrates without the use of solvents, thereby eliminating issues related to solvent toxicity and substrate swelling or degradation. Studies have demonstrated that VPD can produce uniform and conformal coatings of PPy, PEDOT, and other EAPs, significantly enhancing the structural integrity and electrical properties of the substrate. Despite these advantages, VPD still faces several challenges. One critical issue is the control of oxidant concentration, which directly influences polymer growth, uniformity, and conductivity. Excessive oxidant loading can lead to polymer aggregation and increased brittleness, whereas insufficient oxidant concentrations may result in incomplete EAP polymerization. Additionally, certain oxidants used in VPD, such as FeCl₃, can be highly reactive, potentially affecting the stability of the underlying nanofibers. Furthermore, the scalability of VPD for large-area deposition remains an ongoing challenge. While laboratory-scale studies have demonstrated successful polymer coatings on small substrates, translating these findings to industrial-scale production requires further optimization of reaction parameters, such as monomer diffusion rates, reaction time, and temperature control.

Using VPD, EAPs such as PEDOT and PPy can be uniformly coated onto electrospun nanofibers to create high-performance electrodes for energy-storage devices. − VPD using FeCl₃ or Fe (III) tosylate produces conformal, ultrathin coatings typically 5–12 nm thick while preserving the nanofiber porosity essential for ion transport. PEDOT-coated PAN nanofibers exhibit high conductivity, mechanical strengthening, and do not require metal current collectors due to intimate polymer fiber interfaces. Electrospun PEDOT:PSS systems, further enhanced by depositing an additional conductive polymer layer through vapor-phase growth rather than relying solely on solution processing, exhibit over 90% capacitance retention after 1000 cycles in all-solid-state supercapacitors. Additionally, VPD-grown PPy and PEDOT coatings significantly improve charge transport and mechanical flexibility, making them well suited for wearable applications.

This paper aims to advance the understanding and control of EAP deposition on electrospun PAN nanofibers (Figure ). To overcome the challenges of an efficient VPD system, this work focuses on optimizing the electrospinning of PAN coupons, developing a controlled VPD process for EAP coating, and examining FeCl₃-treated mats under varying oxidant concentrations. By systematically analyzing the deposition behavior and process efficiency, the study seeks to establish how oxidant levels influence coating uniformity and polymer loading, while ultimately aiming to enable the fabrication of conductive PAN-based nanofiber composites suitable for potential applications in energy storage.

1.

VPD of EAPs onto electrospun nanofiber mats. (A) Electrospinning of PAN nanofiber mats using a rotating drum collector followed by cutting into coupons. (B) Polymerization of EAPs onto oxidant-loaded PAN nanofiber coupons via VPD. Image partially created with BioRender.com.

Experimental Section

Materials

PAN (Ambeed, Lake Forest, CA, USA) with an average molecular weight of 150,000 g·mol^–1 was used to prepare polymer solutions for nanofiber production. N,N-Dimethylformamide (DMF, EMD Millipore, Burlington, MA, USA) served as the solvent for dissolving PAN. Iron­(III) chloride (FeCl₃, Thermo Fisher Scientific, Waltham, MA, USA) was used as the oxidant for VPD of EAPs. The monomers 3,4-ethylenedioxythiophene (EDOT) (TCI Chemicals, Tokyo, Japan), pyrrole (Py) (Sigma-Aldrich, St. Louis, MO, USA), 3-hexylthiophene (3-HT) for P3HT (Ambeed, Lake Forest, CA, USA), and 2,2-bithiophene (BTH) for PBTH (Ambeed, Lake Forest, CA, USA) were used as EAPs precursors for deposition onto oxidant containing PAN nanofibers. Methanol (MeOH, Fisher Chemical, Waltham, MA, USA) was used to remove residual oxidants and unreacted monomers after polymerization, and acetone (Thermo Fisher Scientific, Waltham, MA, USA) was used for cleaning materials associated with the deposition process.

Electrospinning of PAN Nanofibers

Figure A summarizes the workflow for electrospinning PAN nanofiber mats. In this study, PAN solutions at a concentration of 10% w/v are prepared by dissolving PAN powder in DMF and stirring until a homogeneous and clear mixture was obtained. For the electrospinning setup, a 20-gauge flat-tip needle is used as the spinneret, and the prepared polymer solution is loaded into a syringe connected to a syringe pump via PTFE tubing. A high-voltage power supply is then connected to the spinneret, and a grounded rotating drum collector is covered with wax paper to aid with nanofiber mat retrieval. During electrospinning execution with electrospinner (SprayBase Dublin, Ireland), the polymer solution flow rate is set to 0.04 mL/min, and 18.9 kV is applied between the needle tip and the collector. Collector rotation speed is a critical parameter influencing fiber alignment and mat uniformity, as has been similarly observed in previous electrospinning studies. , For the present system, the collector rotation speed was optimized experimentally and adjusted to 1,200 rpm to ensure uniform fiber deposition, and the process was continued for 4.5 h to allow the formation of a consistent nanofiber mat. After electrospinning, the fiber mat was carefully removed from the collector and placed in a vacuum desiccator at room temperature for 72 h to eliminate residual solvent.

VPD of EAPs on PAN Nanofibers

For this research work, the VPD deposition process was developed on the principle of chemical VPD (CVPD) to coat PAN nanofiber coupons with EAPs (PEDOT, PPy, P3HT, and PBTH). The process involves the oxidative polymerization of monomer vapors onto FeCl₃-soaked PAN fibers under controlled vacuum conditions (Figure B).

First, electrospun PAN mats are cut into 2 cm × 2 cm coupons after being dried in a vacuum desiccator for 72 h to remove residual solvent. The dried coupons are then soaked in FeCl₃ solutions (1 to 5 M) for 30 min, followed by a two-stage drying process, initially in open air for 24 h and then under vacuum for up to 72 h to optimize oxidant retention. For EAP deposition, FeCl₃-coated PAN coupons are placed inside a VPD chamber, along with 4 g of monomer. The chamber is evacuated to 97,600 Pa, and the temperature is maintained at optimal temperature, shown in Table , to facilitate monomer vaporization.

1. Parameters for EAP Deposition on PAN Coupons.

Monomer	Temperature (°C)	Pressure (mm Hg)	Time (min)	Monomer Boiling Point (°C) at Normal Atmosphere
EDOT	30	732	150	193
Py	30	732	150	129
BTH	120	732	150	260
3HT	120	732	150	226

The polymerization reaction proceeds for 2 h, during which EAPs vapors diffuse and react with FeCl₃ on the fiber surface, forming a uniform EAP coating. After deposition, the coupons are washed with MeOH to remove unreacted FeCl₃ and monomer residues, ensuring the stability of the PEDOT layer. The washed coupons are then dried under vacuum and characterized. The schematic diagram shown in Figure summarizes the overall process followed for this study.

Material Characterization

The morphology of the nanofibers and EAP-coated samples was examined by scanning electron microscopy (SEM). SEM imaging was performed using AXIA ChemiSEM system (Thermo Fisher Scientific) with Everhart–Thornley Detector (ETD) Prior to imaging, samples were sputter coated with approximately 2 nm of Au using a Quorum Technologies EMS150T ES sputter coater. SEM images were acquired at a spot size of 2.5 mm and a consistent magnification of 5000× to visualize fiber morphology and surface features. EDS was conducted using a 20 kV accelerating voltage to obtain enhanced signal for elemental composition characterization of the coated nanofibers.

The coated nature and chemical interactions of the nanofibers were examined using attenuated total reflectance (ATR) FTIR, performed on a Bruker Tensor II spectrometer equipped with a Harrick SplitPea ATR attachment to verify the presence of characteristic functional groups associated with the deposited EAP layers.

Electrical conductivity was measured by a linear four-point probe technique using a Keithley 6514 Electrometer and Keithley 2450 SMU (Keithley Instruments, USA) coupled with a Jandel Universal Probe (Jandel Engineering Ltd., UK) equipped with four collinear tungsten carbide tips (1.0 mm spacing, ∼100 μm tip radius) and micrometer-controlled X–Y positioning. All measurements were controlled via a user-friendly graphical interface using Keithley KickStart software. Measurements were performed on three independent replicate coupons (n = 3). Data are reported as mean ± standard deviation, with error bars representing the standard deviation.

CV analysis for the nanocomposites was performed using a Squidstat Plus 2550 potentiostat (PL-24X-M2) controlled via Squidstat User Interface (v2.03.24.2025). The CV measurements were conducted within a potential window of −0.400 to 0.430 V versus open circuit, starting and ending at 0 V, to capture both oxidative and reductive peaks. Multiple scan rates of 1 −8 mV s^–1 were applied with a 0.5 mV sampling interval and an α-factor of 42%. For electrochemical testing, 2032-type coin cells were assembled by stacking a zinc foil anode, a 0.5-in. Whatman GF/D separator presoaked in 2 M ZnSO₄, and a PAN + PPy or PAN + PEDOT cathode within the casing, with additional electrolyte added as needed. To ensure rigorous electrode normalization and data reproducibility, all subsequent current responses obtained via CV were analyzed with respect to the geometric surface area of the cathode at an average mass loading of 4 mg cm^–2. The assembled batteries were crimped at 1000 psi for 5 s using an MSK-110 hydraulic crimping machine, rinsed with deionized water and ethanol to remove excess electrolyte, and labeled for identification prior to testing.

Results and Discussion

EAP Deposition on PAN Coupons with Varying FeCl₃ Concentration

Figure illustrates the effect of varying FeCl₃ concentrations on the VPD of PEDOT, P3HT, PBTH, and PPy onto PAN nanofiber mats, highlighting trends in increased fiber mat weight, fiber diameter, and elemental ratios, specifically S/N for PEDOT, P3HT, PBTH, and C/N for PPy in the resulting nanocomposites. The presented data was generated using five independent batches of samples, with each batch containing three replicate coupons to ensure reproducibility (n = 15 total samples). Quantitative variability is expressed via error bars representing the standard deviation. Across all systems, increasing FeCl₃ concentration results in a steady rise in fiber mat weight, reflecting enhanced polymer loading. For example, the PAN + PEDOT system shows a weight gain rising from 73% at 1 M FeCl₃ to 267% at 5 M, while PAN + P3HT increases from 64% to 231% over the same oxidant concentration range. This polymer loading is directly reflected in the average fiber diameter, which increases consistently with higher FeCl₃ concentrations. SEM images further support these findings, showing more densely packed and partially fused fiber networks at higher FeCl₃ levels.

2.

Deposition of EAPs on PAN nanofiber coupons at varying FeCl₃ oxidant concentrations, illustrating the effect of oxidant loading on coating formation. Measurements were obtained from three coupons of five independent electrospun mats (n = 15 total samples). Data presented are the average between replicates. Error bars represent the standard deviation between all measurements.

Interfiber bridging becomes especially apparent at 4 and 5 M FeCl₃, indicating that excessive polymer deposition leads to physical connections between adjacent fibers. EDS analysis complements morphological data by demonstrating an increase in the sulfur to nitrogen (S/N) ratio for sulfur-based EAPs like PEDOT, P3HT, and PBTH, signaling greater sulfur incorporation from the polymer structure. In the case of PAN + PPy, an elevated carbon to nitrogen (C/N) ratio confirms successful deposition of PPy, as PAN is rich in nitrogen compared to PPy. It should be noted that EDS measurements performed in an SEM primarily probe the near-surface elemental composition of the sample (on the order of a few micrometers), and therefore reflect surface or coating chemistry rather than the bulk composition of the nanofiber mat. It is to be mentioned that, in the present study, the thickness of the vapor-phase deposited coating and the overall nanofiber mat were not quantitatively measured. However, due to the conformal nature of vapor-phase polymer growth and the porous three-dimensional architecture of electrospun mats, the EDS signal is expected to predominantly reflect the surface-modified fiber network rather than the internal bulk of the scaffold. Precise determination of coating thickness is challenging because of fiber curvature, surface roughness, and the absence of well-defined planar cross sections.

These results collectively demonstrate that vapor-phase polymerization provides a versatile and broadly applicable route for coating PAN nanofibers with chemically diverse EAPs including PEDOT, P3HT, PBTH, and PPy using a unified processing framework. Polymer growth occurs without the use of solvents, preserving nanofiber integrity while enabling direct surface deposition. Importantly, modulation of a single parameter, the FeCl₃ oxidant concentration, is sufficient to systematically control polymer loading, fiber diameter evolution, and inter fiber connectivity across all EAPs.

FTIR Analysis of Coated Nanofibers

The FTIR spectra shown in Figure validate the successful surface deposition of four distinct EAPs (PEDOT, P3HT, PBTH, and PPy) on PAN nanofibers with increasing FeCl₃ concentrations from 1 to 5 M. PEDOT-coated samples show a C–O–C stretching peak at 1,130 cm^–1 and broad bands in the 1187–1048 cm^–1 and 972–674 cm^–1 regions, associated with C–O–C and C–S–C vibrations, respectively. P3HT-coated fibers exhibit intensified peaks at 1,462 cm^–1 (CC stretch), within 1000–1250 cm^–1 (C–H deformation), and prominently at 825 cm^–1 (C–H bending), indicating thiophene ring substitution. PBTH coatings are marked by the increasing CC stretch near 1,600 cm^–1, characteristic of the bithiophene unit.

3.

FTIR spectra of coated PAN nanofibers, confirming successful polymer deposition and characteristic functional groups.

Similarly, PPy-coated fibers display a strengthening CC stretch at 1540 cm^–1, along with features from C–N stretching and out-of-plane C–H vibrations. In all cases, a concurrent reduction in the PAN CN peak at ∼2243 cm^–1 indicates increasing surface coverage and masking by the polymer layers. Collectively, these spectral changes confirm FeCl₃-facilitated, surface-localized polymerization and increasing deposition of each polymer onto the PAN nanofibers.

It should be noted that FTIR performed in attenuated total reflectance mode probes only the near surface region of the sample, with an effective penetration depth on the order of a few micrometers and therefore reflects surface chemistry rather than the bulk composition of the nanofiber mat. Within this surface sensitive context, the FTIR- ATR spectra shown in Figure validate the successful surface deposition of four distinct EAPs including PEDOT, P3HT, PBTH, and PPy on PAN nanofibers with increasing FeCl₃ concentrations from 1 to 5 M.

Analysis of Conductivity

Conductivity measurements obtained via four-point probe analysis are shown in Figure . The data indicates that the highest conductivity for most EAP-coated PAN samples was achieved with coatings prepared using 2 M FeCl₃. The optimal oxidant concentration for PBTH was 3 M FeCl₃. It is critical to clarify that the primary objective of this work was to demonstrate the processing versatility and broad applicability of this solvent-free VPD method for the preparation of various EAP coated samples, rather than to present a standardized comparison across normalized doping levels. Because charge carriers were generated inherently during the in situ polymerization stage, the carrier generation efficiency and final carrier density varied naturally depending on the specific chemical compatibility between each unique polymer backbone and the FeCl₃ oxidant interface. Consequently, the observed differences in performance among these polymers do not utilize a standard doping control framework.

4.

Electrical conductivity of EAP-coated PAN nanofiber composites as a function of oxidant concentration. Data presented are the average between independent replicates. Error bars represent the standard deviation between measurements (n = 3).

At higher FeCl₃ concentrations (∼3–5 M), a decrease in conductivity was observed. This trend could be caused by at least three main phenomena: (i) localized overoxidation of the conducting polymers, a well-documented phenomenon in which excessively strong oxidizing environments lead to irreversible chemical changes in the polymer backbone breaking conjugation, disrupting π-electron delocalization, and introducing structural defects that raise resistance and diminish electronic conductivity; , (ii) an accelerated proliferation of polymerization initiation sites driven by the elevated oxidant concentration, which minimizes the effective molecular weight and yields localized oligomeric boundaries that hinder macroscale charge transpor; , (iii) morphological disruption caused by the rapid aggregation of dense oxidant salts on the high aspect ratio fiber templates. In the absence of direct spectroscopic analysis, it is probable that a synergy of chain truncation, localized overoxidation, and morphological inhomogeneity drives the heightened resistance recorded at these upper oxidant thresholds. Consequently, although higher oxidant concentrations promote greater polymer deposition, the adverse effects of overoxidation and morphological defects outweigh the benefits, leading to reduced electrical conductivity.

Among the tested polymers, PAN + PPy and PAN + PEDOT exhibited significantly higher electrical conductivity, reaching up to 0.41 S cm^–1 and 0.11 S cm^–1 at 2 M FeCl₃, respectively. In contrast, PAN + P3HT and PAN + PBTH remained in the ∼10^–4 S cm^–1 range or lower. This limited conductivity arises not only from intrinsically lower charge transport in these polythiophene derivatives but also from their susceptibility to dedoping and structural disruption in protic environments. Oxidation of the thiophene backbone (p-doping) generates polarons and bipolarons that enable high electronic conductivity, whereas reduction, overoxidation, or chemical degradation removes conjugation and charge carriers, driving the polymer toward an insulating state. − Exposure to water or alcohol can exacerbate this effect: small, hydrated anions suppress efficient p-doping, water uptake swells amorphous regions and disrupts π–π stacking, and solvent-induced phase separation or roughening interrupts percolation pathways. − More polar derivatives, such as PBTH, are particularly prone to these effects due to enhanced hydrogen-bonding interactions and backbone disorder, which shorten effective conjugation length and reduce mobility even when the formal redox level is nominally maintained. Consequently, these protic-solvent interactions, combined with structural and redox sensitivity, readily convert P3HT and PBTH from moderately conductive to practically insulating materials under ambient or aqueous processing conditions.

The obtained conductivity from the system is in the 10^–4–10⁴ S cm^–1 range, enabling applications in bioelectronics, wearable sensors, transparent electrodes, and energy storage devices. − It has been observed that once the conductivity of EAP nanofiber mats reaches the 10°–10² S cm^–1 range, several applications become feasible. In bioelectronics, conductive PPy and PEDOT scaffolds enable electrical stimulation and recording for cardiac and neural tissue engineering while maintaining high porosity and compliance, and they have been used for stem-cell differentiation, nerve guidance, and myocardial patches requiring low-impedance cell coupling. , Highly conductive nanofiber membranes are also used in wearable strain, pressure, and biochemical sensors, where high surface area and low intrinsic resistance enable stable resistance changes, improved electromechanical sensitivity, and low-voltage operation. Continuous PEDOT:PSS nanofiber mats can function as transparent, conformal electrodes on nonplanar substrates, offering an alternative to brittle ITO in flexible displays, solar cells, and heaters. , In energy storage and conversion, CVD-grown or electropolymerized PPy, PEDOT, and PANi on electrospun fibers enhance charge transport in supercapacitor and battery electrodes, where conductivities in the tens to hundreds of S cm^–1 support fast charge–discharge rates and mechanical robustness. − Accordingly, although the conductivities obtained here are below the tens to hundreds of S cm^–1 typically required for high-rate energy storage, they confirm that EAP-coated PAN nanofiber mats constitute a promising and tunable platform for further development toward electrochemical and flexible energy device applications.

CV Analysis

CV analysis was conducted using an aqueous ZnSO₄-based zinc-ion coin-cell architecture for comparative CV measurements for two main reasons. First, the CR2032 coin-cell structure introduces realistic geometric constraints, controlled stack pressure, and limited electrolyte volume configurations that directly mimic practical energy storage device conditions. Coin-cells are traditionally prepared for low mass requirement and proof-of-concept battery research. This ensures that the mass transport behaviors observed, such as the diffusion-limited constraints of PAN+PPy at higher scan rates, reflects real-world device physics. Second, the mildly acidic, aqueous 2 M ZnSO₄ electrolyte provides an exceptionally stable screening medium for conductive polymers and aqueous zinc-ion battery fabrication due to the voltage profile that can probe the highly reversible redox chemistry of PEDOT and PPy coatings without inducing electrolyte degradation, electrode deterioration, or structural dissolution of the underlying commodity PAN nanofiber core.

CV analysis shown in Figure of PAN nanofibers coated with PEDOT and PPy, prepared using 2 M FeCl₃ as the oxidant. This concentration was selected as it yielded the highest conductivity among the tested range of 1–5 M.

5.

CV analysis of PAN nanofibers coated with PEDOT and PPy, demonstrating electrochemical behavior.

The CV profiles of PAN + PEDOT and PAN + PPy composites recorded at scan rates ranging from 1 to 8 mV s^–1 reveal distinct electrochemical behaviors associated with each polymer coating. In both cases, the current response increases proportionally with the scan rate, consistent with capacitive or pseudocapacitive charge storage characteristics. In the PEDOT curve, the current response increases progressively with scan rate, accompanied by a quasi-rectangular shape, characteristic of dominant capacitive and reversible redox processes. The anodic and cathodic currents are well-defined and symmetric, indicating stable electron transfer kinetics and uniform charge distribution along the PEDOT layer.

In contrast, the PPy curve exhibits a comparatively narrower potential window and lower current density, implying limited charge storage capability and higher internal resistance. The deviation from ideal rectangularity in PPy at higher scan rates suggests a diffusion-controlled process, where ion transport through the PPy matrix becomes restricted. The difference in peak broadness and slope further indicates that the PEDOT layer enables more efficient electron mobility and polarization response than PPY under equivalent electrochemical conditions.

PAN nanofibers coated with PEDOT and PPy using 2 M FeCl₃ exhibit behavior that has been similarly observed on FeCl₃-oxidized PEDOT and PPy films and PAN-based electrodes. PAN + PEDOT shows quasi-rectangular CVs with well-defined anodic and cathodic currents that increase proportionally with scan rate, indicating capacitive or pseudocapacitive charge storage and efficient electron transfer, with high conductivity. PAN + PPy displays narrower potential windows, lower current densities, and deviations from ideal rectangularity at higher scan rates, reflecting diffusion-limited behavior, partly diffusion-controlled pseudo capacitance, and reduced nondiffusive charge storage, as similarly reported for PPy coatings on carbon or PAN substrates. Unmodified PAN exhibits low currents and nonrectangular CVs, highlighting that PEDOT or PPy coatings enhance current response, with PEDOT providing the larger, more rectangular, and capacitive electrochemical behavior, consistent with previous observations.

Conclusion

Electrospun PAN nanofibers coated with EAPs were fabricated and investigated through SEM, EDS, FTIR, four-point probe conductivity measurements, and CV. Uniform PAN nanofibers were produced by optimized electrospinning conditions and served as stable substrates for FeCl₃-assisted VPD of PEDOT, PPy, P3HT, and PBTH. Increasing oxidant concentration from 1–5 M resulted in progressive polymer loading, reflected by higher mat weight and fiber diameter, with SEM confirming denser and partially fused fiber networks at higher concentrations. EDS analysis showed increased S/N ratios for sulfur-containing polymers and increased C/N ratios for PPy, verifying successful deposition.

FTIR measurements showed the growth of characteristic thiophene- and pyrrole-based vibrational peaks and a simultaneous suppression of the PAN nitrile peak, indicating complete surface masking by the EAP layers. Conductivity testing revealed that coatings produced with 2 M FeCl₃ yielded the highest performance for PEDOT, PPy, and P3HT, while PBTH reached maximum conductivity at 3 M. PPy and PEDOT exhibited the greatest charge transport, achieving 0.41 S/cm and 0.11 S/cm, respectively, whereas P3HT and PBTH remained in the lower conductivity range due to their greater susceptibility to protic-solvent–induced disorder, reduced effective conjugation length, and redox-driven degradation of their conductive pathways under ambient and aqueous conditions.

CV of PEDOT- and PPy-coated nanofibers showed reversible redox activity, with PAN + PEDOT demonstrating a more rectangular profile and higher current response, indicating improved charge mobility and lower internal resistance. In contrast, PAN + PPy exhibited restricted ion transport at higher scan rates.

The developed EAP-coated PAN nanofibers demonstrate that controlled oxidant concentration plays a critical role in governing polymer growth, coating uniformity, and electrical properties. These results confirm that VPD provides an effective route to producing conductive, structurally stable nanofiber composites suitable for use in energy-storage and electrochemical applications.

Glossary

Abbreviations

EAPs

electroactive polymers

VPD

vapor-phase deposition

PAN

polyacrylonitrile

SEM

scanning electron microscopy

EDS

energy-dispersive X-ray spectroscopy

FTIR

fourier transform infrared spectroscopy

ATR

attenuated total reflectance

DMF

N,N-dimethylformamide

EDOT

3,4-ethylenedioxythiophene

PPy

polypyrrole

PEDOT

poly­(3,4-ethylenedioxythiophene)

P3HT

poly­(3-hexylthiophene)

PBTH

poly­(2,2′-bithiophene)

FeCl₃

iron­(III) chloride

MeOH

methanol

CV

cyclic voltammetry

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

This work was supported by the National Science Foundation Partnership for Research and Education in Materials (DMR # 2122041).

The authors declare no competing financial interest.