Morphology-Controlled Polyaniline Nanofibers via Rapid Polymerization for Enhanced Supercapacitor Performance

Abstract

Polyaniline (PANI) nanofibers (NFs) were synthesized via two chemical oxidative polymerization approaches: a rapid mixing process and a conventional stirred tank method. PANI is a promising electrode material for supercapacitors due to its conductivity, stability, and pseudocapacitive redox behavior. The rapid mixing route proved especially effective, as fast polymerization promoted homogeneous nucleation and yielded thin, uniform, and interconnected NFs, whereas conventional stirring produced thicker, irregular fibers through heterogeneous nucleation. Structural characterization (FTIR, UV-Vis, XRD, XPS, TGA) confirmed that both samples retained the typical emeraldine form of PANI, but morphological analyses (SEM, BET) revealed that only the rapid process preserved nanofiber uniformity and porosity. This morphological control proved decisive for electrochemical behavior: symmetric supercapacitor devices fabricated from rapidly synthesized NFs delivered higher specific capacitances (378.8 F g⁻¹ at 1 A g⁻¹), improved rate capability, and superior cycling stability (90.33% retention after 3000 cycles) compared to devices based on conventionally prepared NFs. These findings demonstrate that rapid polymerization offers a simple and scalable route to morphology-engineered PANI electrodes with enhanced performance.

1. Introduction

Polyaniline (PANI) is regarded as one of the most promising candidates among intrinsically conducting polymers (ICPs) owing to its facile synthesis, remarkable thermal stability, excellent optical and electrochemical properties, and distinct redox behavior [1,2,3]. The nanostructured form of PANI imparts enhanced characteristics, making it particularly attractive for high-performance applications. Consequently, nanostructured PANI has garnered significant research interest in recent years [4,5,6]. It has been effectively utilized in a variety of applications including sensors [7,8,9], electrodes [10], electrochromic smart windows [11,12] and electrochemical energy storage systems [13,14,15]. Nevertheless, the widespread use of PANI nanostructures remains limited, primarily because scalable and reproducible synthesis routes that can deliver uniform morphology, controlled nanostructure, and consistent electrochemical performance are still lacking. Variations in polymerization conditions, choice of oxidants, dopants, and synthesis techniques often result in significant differences in conductivity, stability, and structural integrity, which hinder the reliable production of high-quality nanomaterials for practical applications.

PANI is commonly synthesized through both chemical and electrochemical oxidative polymerization methods [16,17]. Among these, traditional chemical oxidative polymerization remains the preferred technique for bulk synthesis [1,2,3]. This approach involves the gradual addition of aniline monomer to an oxidizing agent in an acidic medium under vigorous mechanical stirring [18]. This method offers a simple and cost-effective route for producing NFs, making it attractive for large-scale preparation. However, NFs obtained in this way often suffer from severe aggregation and rapid sedimentation, which significantly limit their stability and practical applicability. This phenomenon arises because, during the initial phase of polymerization, homogeneous nucleation leads to the formation of well-dispersed NFs. As the reaction proceeds, continuous stirring induces heterogeneous nucleation, resulting in thicker, shorter, and irregularly shaped NFs that readily agglomerate. These findings indicate that while NFs are initially formed, prolonged polymerization facilitates secondary growth on existing fibers, promoting surface aggregation. Therefore, to achieve high-quality PANI NFs, it is crucial to terminate the polymerization at an early stage to prevent undesirable secondary growth [19,20,21].

A key innovation of this study is demonstrating that a rapid polymerization process facilitates homogeneous nucleation in bulk solution, which in turn yields PANI NFs with a well-defined morphology [22]. When PANI NFs are formed quickly, they behave as embryonic nuclei, which prefer homogeneous growth pathways and are less likely to migrate to heterogeneous nucleation sites. Experimentally, rapid mixing of reactants at ambient temperature has proven effective for promoting the swift formation of PANI NFs. Another critical factor influencing the nucleation behavior is mechanical stirring [22]. It has been consistently observed that mechanical agitation enhances heterogeneous nucleation, often resulting in larger and irregular particles compared to reactions conducted without stirring [23]. In contrast, under static conditions, PANI NFs tend to form discretely and with improved morphology [23]. Therefore, conducting the polymerization at room temperature without vigorous mechanical stirring presents a straightforward and efficient approach to synthesize high-quality PANI NFs in good yield [22,23].

In this study, PANI NFs were synthesized via chemical oxidative polymerization using two different approaches: rapid mixing and stirred tank reactor methods. The resulting nanostructures were evaluated for their morphological and electrochemical properties. PANI NFs synthesized through rapid mixing, without mechanical stirring, exhibited a well-defined, uniform morphology due to consistent homogeneous nucleation. In contrast, NFs obtained via the stirred tank reactor method were shorter, thicker, and showed irregular agglomeration, attributed to heterogeneous nucleation. Both types of NFs were utilized as electrode materials in symmetric SCs, and their performance was comparatively assessed. The PANI prepared under non-stirred conditions demonstrated superior C_sp, and lower R_s and R_ct, indicating enhanced electrochemical performance. These findings suggest that rapid mixing offers a simple, scalable, and cost-effective route for fabricating high-performance PANI NF-based electrodes for energy storage and conversion applications.

2. Experiment

2.1. Materials

Aniline monomers (C₆H₅NH₂), ammonium persulphate (NH₄)₂S₂O₈, sodium phytate (C₆H₁₇NaO₂₄P₆), dimethylformamide (C₃H₇NO), acetone (C₃H₆O) and sulfuric acid (H₂SO₄) were purchased from Sigma Aldrich (St. Louis, MI, USA). Commercially received aniline was further distilled for the removal of trace impurities and stored in the refrigerator as stock. The rest of the chemicals were used as received, because they were of research grade. Solutions were prepared in deionized water.

2.2. Preparation of PANI NFs

PANI NFs were synthesized via two distinct chemical polymerization techniques: rapid mixing in the absence of mechanical stirring (following our previously reported procedure [24]) and a conventional stirred tank reactor method. In brief, a 4.5% (w/v) sodium phytate solution was prepared in deionized water. Subsequently, 5 mL of this solution was combined with 1 mM aniline in 10 mL of deionized water. Separately, a 1.0 mM ammonium persulfate (APS) solution was prepared. Both solutions were pre-cooled in a refrigerator for 15 min prior to reaction. For the rapid mixing synthesis, 3 mL of the sodium phytate-aniline solution was mixed with 1.5 mL of APS solution in a 5 mL Eppendorf tube. The reaction was initiated by vortexing the mixture for 5–10 min, during which the solution turned green, indicating the formation of PANI. The product was collected by filtration, washed with acetone, and vacuum dried at 60 °C for 12 h, yielding a dark green powder. This sample was designated as PANI-A. For the stirred tank synthesis, the same solutions were reacted under mechanical stirring for 1 h. The resulting green dispersion was filtered, washed, and dried following the same procedure as above. This product was labeled PANI-B. Both samples were stored for further characterization and used in electrode fabrication.

2.3. Preparation of Electrodes

Initially, a gold sheet (Wuhan Corrtest Instruments (Wuhan, China), 1–6 mm, 99.99%)) was employed as the current collector. The surface was polished using a slurry of silica gel powder in deionized water and subsequently rinsed three times with deionized water to remove any residues. For electrode fabrication, 20 mg of the synthesized PANI sample was dispersed in 10 mL of dimethylformamide (DMF) and subjected to ultrasonic agitation for 30 min at room temperature. The resulting homogeneous PANI-DMF suspension was drop-cast onto the cleaned gold substrate (1 × 1 cm²). The coated electrode was then dried at ambient conditions, forming a uniform thin film of PANI on the current collector. The active material loading was maintained at 2 mg cm⁻² for all electrochemical measurements.

2.4. Fabrication of Symmetric SCs

To evaluate the practical electrochemical performance of both PANI-A and PANI-B NFs as electrode materials for SC applications, symmetric SC cells were assembled using two identical electrodes. The configurations were designated as PANI-A/separator/PANI-A and PANI-B/separator/PANI-B, respectively. In each assembly, Whatman filter paper (pore size: 20 µm) served as the separator, positioned such that one end extended into an external electrolyte reservoir to ensure continuous supply of the electrolyte via capillary action. The complete cell assemblies were sealed with epoxy resin and secured using plastic clips. Electrode preparation for both symmetric cells followed the procedure previously described.

2.5. Structural and Morphological Characterizations

The surface morphology of the synthesized PANI samples was examined using scanning electron microscopy (SEM, JSM-6701F, JEOL Ltd, Tokyo, Japan). The specific surface area (SSA) was determined by means of the Brunauer–Emmett–Teller (BET) method, employing N₂ adsorption–desorption isotherms at 77 K using a Micromeritics ASAP2020 instrument (Norcross, GA, USA). Functional groups were identified via Fourier-transform infrared spectroscopy (FTIR) using a Shimadzu Affinity-1S spectrometer (Shimadzu Corporation, Kyoto, Japan). Optical properties were characterized by UV-Vis spectroscopy, conducted on a Perkin Elmer spectrophotometer (Buckinghamshire, UK). Elemental composition was analyzed using X-ray photoelectron spectroscopy (XPS; KRATOS Axis Ultra DLD, Nanuet, NY, USA) with Mg Kα radiation (hυ = 1283.3 eV) as the excitation source. Crystalline structure was assessed by X-ray diffraction (XRD) using a JEOL diffractometer (Tokyo, Japan) with Cu Kα radiation (λ = 1.5405 Å). Thermal stability was evaluated via thermogravimetric analysis (TGA, Waltham, MA, USA) under N₂ atmosphere, with a heating rate of 10 °C min⁻¹.

2.6. Electrochemical Characterizations

The electrocatalytic performance of PANI-A and PANI-B NF electrodes was investigated using CV on a Gamry 3000 electrochemical workstation (Gamry Instruments, Warminster, PA, USA) in a three-electrode configuration. An Ag/AgCl (3 M KCl, saturated AgCl, BASi, West Lafayette, IN, USA) and a gold coil served as the reference and counter electrodes, respectively, while 1 M H₂SO₄ was used as the electrolyte. All electrochemical measurements were carried out at room temperature (~25 °C) under ambient laboratory atmosphere. The applied potential range extended from −0.2 V to +0.8 V. The C_sp was calculated from the CV curves using Equation (1) [25]:

$$C_{sp}={\displaystyle \frac{{\int }_{V1}^{V2}idV}{m∆Vv}}$$

(1)

where C_sp is the specific capacitance (F g⁻¹), I is the current (A), v is the scan rate (V s⁻¹), ΔV is the voltage window (V), and m is the mass of the active material (g).

To evaluate practical capacitive behavior, GCD tests were conducted on both symmetric SC devices under identical conditions across varying current densities. The C_sp values were extracted from discharge profiles using Equation (2) [26]:

$$C_{sp}=2{\displaystyle \frac{I\times ∆t}{∆v\times m}}$$

(2)

Here, the factor of 2 accounts for the symmetric cell configuration. Δt is the discharge time (s), ΔV is the voltage window (V), and m is the total mass of the active material in both electrodes.

Electrochemical impedance spectroscopy (EIS) was performed to analyze the R_s and R_ct of the SC cells. EIS measurements were conducted in the frequency range of 0.01 Hz to 1 MHz at a fixed potential of 0.8 V. The specific energy (E_cell, Wh kg⁻¹) and specific power (P_cell, W kg⁻¹) of cell were methodically evaluated using Equations (3) and (4) [27,28].

$$E_{cell }={\displaystyle \frac{1}{2}} C_{sp }{∆V}^2$$

(3)

$$P_{cell}={\displaystyle \frac{E_{cell}}{∆t}}$$

(4)

3. Results and Discussion

3.1. Structural and Morphological Study

SEM analysis (Figure 1) reveals that PANI-A, synthesized without mechanical stirring, exhibits uniform NFs (27–70 nm), while PANI-B, synthesized with agitation, forms thicker, rod-like structures (90–130 nm). The uniform morphology of PANI-A is attributed to homogeneous nucleation during rapid polymerization in a quiescent environment, which inhibits secondary growth. This nanofibrous structure enhances electrolyte penetration and improves electrocatalytic performance. In contrast, mechanical stirring in PANI-B promotes heterogeneous nucleation, leading to agglomerated, irregular fibers that impede ion/electron transport and reduce catalytic activity [22,23]. Therefore, optimal electrochemical morphology can be achieved through rapid mixing without mechanical agitation.

Since SSA is intrinsically linked to material morphology, the BET analysis was conducted to assess the SSA and pore volume of the synthesized samples. As shown in Figure 2a–d, the N₂ adsorption–desorption isotherms and pore size distribution curves confirm type-IV behavior, indicative of porous structures. PANI-A demonstrates a higher SSA (44.315 m² g⁻¹) and pore volume (0.042 cm³ g⁻¹) compared to PANI-B (34.228 m² g⁻¹, 0.036 cm³ g⁻¹). The increased SSA and porosity of PANI-A provide more active sites for ion diffusion, enhancing charge storage capacity. Therefore, PANI-A is more suitable for SC applications.

The structural features of PANI-A and PANI-B were examined using FTIR and UV-Vis spectroscopy, as presented in Figure 3a,b. Both samples exhibit nearly identical FTIR spectra (Figure 3a). Characteristic peaks at 1685 and 1558 cm⁻¹ correspond to C=C stretching vibrations of quinoid and benzenoid rings, respectively. The band at 1487 cm⁻¹ is attributed to C–N stretching within the quinoid structure. Additional peaks at 1284, 1104, and 820 cm⁻¹ represent C–N stretching, N=Q=N (Q = quinoid) vibrations, and C–H bending in benzene rings. The signals at 820 and 554 cm⁻¹ are assigned to P–O–C and O–Na stretching of sodium phytate phosphonates [29,30,31].

The UV-Vis spectra in Figure 3b indicate that PANI-A and PANI-B exhibit similar absorption features. Peaks at approximately 345 and 424 nm are attributed to the π-π* transition of the benzenoid ring and the polaron-π* transition, respectively. A broad absorption band centered at 787 nm corresponds to π-polaron transitions, resulting from interband charge transfer between benzenoid and quinoid units. This peak confirms the formation of the emeraldine salt form of PANI, which is known to be the only electrically conductive state of the polymer [32,33,34].

XPS analysis was conducted to further elucidate the elemental composition of the synthesized PANI samples, as shown in Figure 4. The survey spectra (Figure 4a) confirm the presence of phosphorus (P), carbon (C), nitrogen (N), and oxygen (O) in both PANI-A and PANI-B. Detection of P confirms successful doping of the PANI matrix. Protonation of imine N results in varied oxidation states, which were characterized through N 1s core-level spectra (Figure 4b,c). The deconvolution reveals four components at 399.425, 401.398, 401.902, and 402.958 eV for PANI-A, and at 399.450, 401.421, 402.093, and 402.984 eV for PANI-B, corresponding to imine N (–N=), amine N (–NH–), protonated amine (–NH⁺–), and protonated imine (–NH⁺=), respectively [35]. Additionally, the P 2p region shows well-defined 2p_3/2 and 2p_1/2 doublets at 133.489/134.199 eV (PANI-A) and 133.861/134.474 eV (PANI-B) (Figure 4d,e), confirming the successful incorporation of phytate dopant [36,37].

The XRD spectra of PANI-A and PANI-B NFs are presented in Figure 5a. Characteristic diffraction peaks at 15.21°, 20.14°, and 25.47° confirm the emeraldine form of PANI. Peaks at 15.21° and 20.14° correspond to the parallel and vertical periodic arrangements of the PANI chains, while the 25.47° peak reflects interactions between monomer units and the dopant. Notably, PANI-A exhibits more intense diffraction peaks compared to PANI-B. This enhanced nature facilitates improved ionic transport along the polymer backbone, contributing to superior electrocatalytic performance [38,39].

The thermal stability of PANI-A and PANI-B was assessed via TGA analysis (Figure 5b). Both samples exhibited three-step weight loss patterns: initial weight loss around 125 °C due to moisture evaporation, followed by de-doping of the dopant, and finally, thermal degradation of the PANI backbone [40]. While both samples showed similar moisture loss, PANI-B exhibited stability in the 370–620 °C range, whereas PANI-A remained stable up to 670 °C. This enhanced stability in PANI-A is attributed to its uniform, interconnected NF morphology, which promotes stronger polymer-dopant interactions. The degradation of phytate dopant groups begins near 550 °C, with final backbone decomposition occurring between 615 and 670 °C [41].

3.2. Electrochemical Study

The electrocatalytic performance of PANI-A and PANI-B electrodes was evaluated using CV, as shown in Figure 6a,b. Both materials exhibit nearly rectangular CV profiles with two distinct redox peaks, corresponding to transitions between leucomeraldine, emeraldine, and pernigraniline forms of PANI [42]. These profiles are characteristic of PANI and represent both electric double-layer and pseudocapacitive behavior, indicated by the presence of two redox couples [43]. Notably, the CV curves of PANI-A show a larger enclosed area and higher peak currents than those of PANI-B, suggesting superior charge storage capacity. The C_sp values were calculated using Equation (1) [25] and are summarized in

Table 1. C_sp of PANI-A and PANI-B at different scan rates.

Scan Rate (mV s−1)	Csp (F g−1) of PANI-A	Csp (F g−1) of PANI-B
20 50 100	600.30 580.00 551.70	462.50 420.00 375.00
150	481.60	343.33

.

As shown in Table 1, the C_sp values indicate that PANI-A NF electrodes outperform PANI-B in electrochemical performance. A general decline in C_sp with increasing scan rate is observed for both materials. This behavior is attributed to enhanced electrolyte penetration and efficient charge storage at lower scan rates. Conversely, at higher scan rates, limited ion diffusion into the electrode’s interior results in reduced electrolyte interaction, thereby lowering the effective capacitance [42].

Given that practical SC applications commonly employ a two-electrode configuration without a reference electrode, both PANI-A and PANI-B NFs were evaluated in such a setup. Symmetric devices were fabricated using either PANI-A (PANI-A//PANI-A) or PANI-B (PANI-B//PANI-B) electrodes and subjected to GCD and EIS. As shown in Figure 7a,b, both devices exhibited nearly symmetrical GCD curves at current densities of 1, 3, 5, and 10 A g⁻¹, indicative of good electrochemical reversibility [44]. The C_sp values, calculated from the GCD data using Equation (2) [26], are listed in

Table 2. C_sp of PANI-A and PANI-B symmetric SC devices at different current densities.

Current Density (A g−1)	Csp (F g−1) of PANI-A	Csp (F g−1) of PANI-B
1 3 5	378.80 361.10 350.55	342.90 324.50 302.75
10	344.00	271.95

.

The results indicate that the symmetric SC device based on PANI-A NFs consistently exhibits higher C_sp than the PANI-B-based device across all current densities, highlighting the superior energy storage capabilities of PANI-A. The C_sp values derived from the GCD discharge curves align closely with those obtained from CV analysis, further validating the enhanced electrochemical performance of PANI-A electrodes. As current density increases from 1 to 10 A g⁻¹, a decline in C_sp is observed for both devices (Table 2, Figure 8a), with PANI-B showing a more pronounced decrease. Notably, the PANI-A cell retains 90.81% of its initial C_sp (378.80 F g⁻¹ at 1 A g⁻¹), maintaining 344 F g⁻¹ at 10 A g⁻¹. In contrast, the PANI-B cell retains only 79.31% of its initial C_sp (342.90 F g⁻¹ at 1 A g⁻¹), dropping to 271.95 F g⁻¹ at 10 A g⁻¹. These findings confirm the superior rate capability and C_sp retention of PANI-A NFs. This enhanced performance is attributed to their uniform, interconnected morphology, which facilitates efficient ion diffusion and electron transfer due to improved conductivity and reduced R_ct. Consequently, the PANI-A electrodes offer greater active surface area for redox reactions, both at the surface and within the bulk of the material [45,46].

Cyclic stability is a critical parameter for evaluating the long-term performance of electrode materials in energy storage systems. To assess this, the fabricated symmetric SC devices were subjected to continuous charge–discharge testing via galvanostatic cycling (GCD) for 3000 cycles at a current density of 10 A g⁻¹, as shown in Figure 8b. The PANI-A based device demonstrated superior cycling stability, retaining 90.33% of its initial capacitance, compared to 78.18% retention exhibited by the PANI-B based device. This decline in capacitance over repeated cycles is primarily attributed to the intrinsic volumetric changes, swelling and contraction of the conducting polymer matrix during the charge–discharge process [47].

Figure 9 presents the Nyquist plots of symmetric SC devices fabricated with PANI-A and PANI-B electrodes, as obtained from EIS. Each spectrum can be interpreted in three distinct regions. The initial intercept on the X-axis reflects the R_s, which arises primarily from the gold substrate used as the current collector and the ionic resistance at the electrode-electrolyte interface. The second region, represented by a semicircle in the high-frequency range, corresponds to the R_ct and the formation of the electrical double layer within the active electrode material [28,48,49]. The third region appears as an inclined line at low frequencies, indicative of Warburg impedance, which is associated with ion diffusion limitations and sluggish electrolyte transport into the interior of the porous electrode structure [24].

The experimental impedance data for both SC devices were modeled using the equivalent circuit shown in the inset of Figure 9, comprising R_s, R_ct, Warburg impedance (W) for ion diffusion, and two capacitive elements: Q₁ and Q₂. Here, Q₁ represents the electrical double layer capacitance at the electrode-electrolyte interface, while Q₂ corresponds to the pseudocapacitance within the bulk of the electrode material. Both PANI-A and PANI-B devices demonstrated low R_s values of 0.380 Ω and 0.459 Ω, respectively. Notably, the PANI-A symmetric device exhibited a significantly lower R_ct (7.245 Ω) compared to PANI-B (11.551 Ω), which is attributed to its uniform and interconnected NF morphology. This structure facilitates shorter diffusion paths for electrolyte ions, enhancing their accessibility to inner active sites and promoting efficient faradaic reactions. These findings corroborate the superior electrochemical characteristics of PANI-A, as indicated by its improved ionic conductivity and reduced interfacial resistance. The Ragone plot comparing two cells indicates that the PANI-A shows the highest energy density of 121.2 Wh kg⁻¹ at a power density of 502.2 W kg⁻¹ (Figure 10). Further, it confirms that the PANI-A showed superior performance compared to PANI-B.

4. Conclusions

PANI NFs were synthesized via chemical polymerization using two different approaches: rapid mixing and a stirred tank reactor. The rapid mixing method yielded PANI-A NFs characterized by thin, uniform, and interconnected structures, attributed to homogeneous nucleation. In contrast, PANI-B NFs, produced in a stirred tank reactor with mechanical agitation, exhibited thicker, irregular fibers with structural discontinuities, indicative of heterogeneous nucleation. Symmetric SC devices fabricated from PANI-A demonstrated superior electrochemical performance, with C_sp of 378.8 F g⁻¹ at 1 A g⁻¹ and 344.0 F g⁻¹ at 10 A g⁻¹, retaining 90.81% of their initial capacitance. Comparatively, the PANI-B device showed lower capacitances (342.90 F g⁻¹ at 1 A g⁻¹ and 271.95 F g⁻¹ at 10 A g⁻¹) and a reduced retention of 79.31%. After 3000 charge–discharge cycles at 10 A g⁻¹, the PANI-A device maintained 90.33% of its capacitance, surpassing the 78.18% retention of the PANI-B device. Furthermore, EIS revealed lower R_s (0.38 Ω) and R_ct (7.245 Ω) values for PANI-A, signifying improved ionic and charge transfer dynamics compared to PANI-B. These findings underscore the fact that rapid mixing without mechanical stirring offers a facile, scalable, and reproducible route to produce highly interconnected and structurally robust PANI NFs with enhanced electrochemical characteristics suitable for SC applications.