Polypyrrole Hybrid Nanocomposite Electrode Materials with Outstanding Specific Capacitance

Abstract

This paper discusses the results of our investigation of the effect of processing parameters on the electrochemical properties of poly(vinylidene fluoride) single-walled carbon nanotube sheets and PVDF-CNTs modified by solution cast polyimide coating, followed by electrodeposition of polypyrrole. The polyimide-coated single-wall carbon nanotube sheet–PI/SWCNTs composite consists of SWCNT and PVDF (9:1) wt.% and 0.1–1 wt.% polyimide. The processing temperature varied from 90 to 250 °C. SEM images validated the nanostructure, while EDX confirmed the material composition. EIS analysis showed that the composite electrode material processed at 90 °C and followed by electrodeposition of polypyrrole has the lowest bulk resistance (65.27 Ω), higher porosity (4.59%), and the highest specific capacitance (209.16 F/g), indicating superior ion transport and charge storage. Cyclic voltammetry and cyclic charge–discharge analyses revealed that the hybrid composite electrode processed at 90 °C achieved a specific capacitance of 655.34 F/g at a scan rate of 5 mV/s, demonstrating excellent cycling stability over 10 cycles at a current density of 0.5 A/g. In contrast, composite electrodes processed at 180 °C and 250 °C showed decreased performance due in part to structural densification and low porosity. These findings underscore the critical role of processing temperatures in optimizing the electrochemical properties of PI/SWCNT composites, advancing their potential for next-generation energy storage systems.

1. Introduction

The fast-paced evolution of technology and the growing energy demand have driven the need for sustainable, efficient, and high-performance energy storage systems [1,2,3]. Traditional energy storage technologies, such as lithium-ion batteries and supercapacitors, have made significant strides; however, there remains a critical need for materials that can deliver higher energy densities, longer life cycles, and improved safety [4]. In this context, hybrid nanocomposites, particularly those incorporating carbon-based materials, have emerged as promising candidates due to their unique structural, electrical, and mechanical properties [5,6,7]. Single-walled carbon nanotube sheets (SWCNTs) and graphene are among the most studied carbon-based materials for energy storage applications. SWCNTs are renowned for their high electrical conductivity, mechanical strength, and large surface area, which make them ideal for use in electrode materials [8,9]. Similarly, graphene, with its two-dimensional structure, exceptional electrical conductivity, and high specific surface area, has been extensively researched for its potential to enhance energy storage performance [10,11,12,13]. When combined with other nanomaterials, these carbon-based materials can synergistically enhance the overall properties of the resulting composites, making them suitable for advanced energy storage devices [14,15]. Polyvinylidene fluoride (PVDF) and polyimide (PI) are polymers that have garnered attention in the field of energy storage due to their excellent chemical stability, mechanical properties, and processability [16,17,18]. PVDF in particular is known for its high dielectric constant and electroactive properties, which are beneficial for capacitor applications [19,20,21,22]. PI, on the other hand, offers thermal stability and good mechanical strength, which are critical for the longevity and durability of energy storage devices [23,24,25,26,27,28]. Integrating SWCNTs and PVDF-PI into hybrid nanocomposites combines the advantageous properties of each component, potentially leading to materials with superior performance characteristics [29].

Recent studies have focused on optimizing the composition, processing conditions, and structural configuration of PI/SWCNTs-PPy hybrid nanocomposites to achieve enhanced energy storage efficiency [30]. For instance, the dispersion of SWCNTs within the polymer matrix is a key factor that influences the composite’s electrical conductivity and mechanical integrity [31,32]. Techniques such as in situ polymerization, solution casting, and electrospinning have achieved uniform dispersion and strong interfacial interactions between SWCNTs and the polymer matrix [18,33,34]. Additionally, the introduction of dopants and the use of surface modification techniques can further enhance the electrochemical performance of these nanocomposites [35,36].

This study aims to examine PI/SWCNT hybrid nanocomposites, with a particular emphasis on the influence of processing conditions, especially temperature, on their energy storage efficiency. By exploring the impact of different synthesis methods, processing parameters, and structural modifications, this research aims to deepen the understanding of the relationship between these factors and the electrochemical performance of the materials. The ultimate aim of this study is to contribute to the development of next-generation energy storage systems with improved efficiency, durability, and sustainability.

2. Materials and Methods

2.1. Materials

The reagents used in this investigation, including 4,4-oxydianiline (ODA) (97% purity), pyromellitic dianhydride (PMDA) (99% purity), N-methyl pyrrolidone (NMP) (99% purity), and reagent-grade pyrrole monomer (98% purity), were all purchased from Sigma-Aldrich. The dopant, p-Toluene sulfonic acid, was purchased from Sigma-Aldrich, St. Louis, MO, USA. Single-walled carbon nanotube sheets (SWCNTs) were obtained from TuballTM, Leudelange, Luxembourg. All chemicals were utilized without any further purification.

The material selection and composition in this study were determined based on key considerations aimed at enhancing the performance of SWCNT–polyimide nanocomposites for high-temperature and safety-critical applications in sustainable energy storage systems. One crucial factor considered was the processing temperature, which was adjusted to achieve structures ranging from dense to porous. Additionally, variations in the CV scan rate were employed to influence the time scale of electrochemical processes. The SWCNT sheets utilized in this research comprised 90% carbon nanotubes (CNTs) and 10% polyvinylidene fluoride (PVDF), as specified by the manufacturer. Furthermore, a 10 wt.% polyimide (PI) resin was applied as a thin coating on the CNTs to enhance compatibility, thermal stability, and mechanical durability.

2.2. Synthesis and Processing of Hybrid Nanocomposites

2.2.1. Fabrication of Polyimide/Single-Walled Carbon Nanotube (PI/SWCNT) Hybrid Nanocomposites

The single-walled carbon nanotube (SWCNT) sheet containing up to 90 wt.% SWCNTs dispersed in a poly(vinylidene fluoride) (PVDF) matrix was used to fabricate the PI/SWCNT hybrid nanocomposite. A 10 wt.% solid-content PAA solution, synthesized by reacting equimolar amounts of PMDA and ODA in N-methyl-2-pyrrolidone (NMP) (Scheme 1), was uniformly applied to the SWCNT sheet via the solution casting method. The coated CNT sheet was first cured in a vacuum oven at 70 °C for six hours to remove the solvent. This was followed by a stepwise thermal treatment, gradually increasing the temperature to 90 °C for an additional six hours under a vacuum of 28 Hg. This process facilitated the conversion of PAA into polyimide, yielding a stable and mechanically robust composite material (Figure 1a). After cooling, the composite sheet was carefully detached from the substrate, resulting in a PI-SWCNT/PVDF sheet uniformly coated with polyimide. This composite sheet was then utilized as the working electrode for the electrochemical deposition of polypyrrole (PPy). The rigorous fabrication process ensured the film’s structural integrity and stability, minimizing shrinkage and defects. The same procedure was repeated for samples processed at 180 °C and 250 °C, enabling a comparative analysis of how different processing temperatures influence the material’s properties and electrochemical performance.

The choice of three specific processing temperatures (90 °C, 180 °C, and 250 °C) was intentional, guided by the thermal transitions and structural evolution of polyimide (PI) and carbon nanotube (CNT) composites. These temperatures represent critical points for evaluating the effects of low, medium, and high thermal treatments on the composites’ structural integrity and electrochemical properties. At 90 °C, the focus was on achieving uniform polyimide formation while preserving porosity and ensuring CNT dispersion. At 180 °C and 250 °C, higher temperatures were used to explore the impacts of structural densification and thermal degradation, respectively. The intervals were selected to span a wide range of thermal conditions, allowing a comprehensive analysis of the relationship between temperature, microstructure, and electrochemical performance. While more granular temperature points could offer additional insights, the chosen set provides a practical and efficient framework for identifying significant trends and optimizing the processing conditions.

2.2.2. Electrodeposition and Doping of Polypyrrole (PPy)

A 0.5 M pyrrole (Py) solution was prepared by dissolving pyrrole in 100 mL of water, with the addition of 0.0225 M p-toluene sulfonic acid. The solution was stirred thoroughly until fully dissolved, resulting in a clear mixture. The SWCNT-PI film served as the working electrode. Each electrode was immersed in the 0.5 M pyrrole solution within a three-electrode cell setup, connected to a Gamry 3000 Potentiostat (Gamry Instruments, Warminster, PA, USA) (Figure 1b). A glassy carbon counter electrode and an Ag/AgCl reference electrode were used to facilitate the potentiostatic electrochemical deposition of polypyrrole at 2 V for 700 s. Upon completing the polypyrrole deposition, the electrodes were rinsed with ethanol, dried in a vacuum oven at 100 °C to eliminate moisture, and subsequently weighed.

3. Characterization

3.1. Morphological Characterization: Using Scanning Electron Microscopy

The surface morphology and composition were analyzed using a Thermo Fisher SCIOS DualBeam Scanning Electron Microscope from Thermo Fisher Scientific, Waltham, MA, USA. Surface and cross-sectional imaging were conducted to assess the amount of electrodeposited polypyrrole (PPy) and the porosity induced by the deposition process. Additionally, the influence of PPy on the morphology of the PI/CNTs-PPy composite electrode was examined. Since the samples were inherently electrically conductive, Ag sputtering was not required.

3.2. Cyclic Voltammetry

Electrochemical characterization was performed using a Gamry 3000 Potentiostat in a three-electrode cell setup with an Ag/AgCl reference electrode. Cyclic voltammetry was performed within a voltage range of 0 to 1 V, using scan rates of 5, 10, and 25 mV/s, respectively, over 1 to 10 cycles. This analysis aimed to determine the nanocomposite electrode’s peak current, total charge stored, and specific capacitance. Equation (1a) was used to calculate the specific capacitance, Cp in F/g, where I (A) is the response current obtained during the voltage sweep ∆V (V) at given scan rates v (mV/s) of the specific active material m (g). ∫IdV is the integrated area under the cyclic voltammetry curve.

$$C_p={\displaystyle \frac{\int IdV}{2m\times \upsilon \times ∆V}}$$

(1a)

The specific capacity of the material (Csp) in mAh/g was calculated using Equation (1b).

$$C_{sp}={\displaystyle \frac{\int IdV}{2m\times \upsilon \times 3.6}}$$

(1b)

3.3. Gravimetric Cyclic Charge/Discharge

Samples were cycled under galvanostatic conditions at a current density of 0.5 A/g, from 0 V to 0.8 V. The specific capacitance (C_p) and specific capacity, C_sp from charge–discharge curves were calculated by using Equations (2) and (3), respectively. Where I_m is the discharge current density (Ag⁻¹), ∆t is the discharge time (s), and ∆V is the drop in voltage (V) during that discharge period [37].

$$C_p={\displaystyle \frac{I_m∆t}{∆V}}$$

(2)

$$C_{sp}={\displaystyle \frac{I_m∆t}{∆V\times 3.6}}$$

(3)

3.4. Electrochemical Impedance Spectroscopy

EIS measurements were conducted over a frequency range from 1 MHz to 0.01 Hz, and the results were modeled using a Randle’s cell equivalent electrical circuit model that included a solution resistance, pore resistance, double-layer capacitor/coating capacitor (Cc). EIS models for calculating specific capacitance (Cp) are based on the Bode and Nyquist plots using Equation (4), where f is the corresponding Bode frequency (s⁻¹) at the apex of the Nyquist plot, Z’max is the max imaginary impedance (Ω), and m is the mass of active material (g). Equation (5) is the Randle circuit model, often used to describe electrochemical impedance in systems like batteries, fuel cells, and supercapacitors. The basic Randles model includes a resistor (Rs), a capacitor (C), a charge transfer resistance (Rct), and a Warburg impedance element (W), which accounts for diffusion effects. Rs represents the solution resistance, Rct denotes the charge transfer resistance, C is the double-layer capacitance, ZW(ω) corresponds to the impedance of the Warburg element, and ω is the angular frequency of the AC signal (ω = 2πf, where f is the frequency).

$$C_p={\displaystyle \frac{1}{2\pi f{Z’}_{max}\times m}}$$

(4)

$$Z\left(\omega \right)=R_o+{\displaystyle \frac{1}{\frac{1}{R_{ct}}+j\omega C+\frac{1}{Z_W\left(\omega \right)}}}$$

(5)

4. Results and Discussions

4.1. Scanning Electron Microscopy (SEM)

Figure 2 is the SEM images of neat CNTs, PI/CNTs-90 °C, PI/CNTs-180 °C and PI/CNTs-250 °C. The images reveal distinct morphological changes in the PI/CNT composites processed at different temperatures. Pure CNTs display a loosely entangled network with high porosity and visible voids, maximizing electrolyte accessibility. At 90 °C, the composite shows an optimal balance of porosity and CNT dispersion within the polyimide matrix, forming an interconnected network that facilitates efficient ion transport and enhances electrochemical performance. At 180 °C, the structure becomes denser, with reduced porosity and partially encapsulated CNTs, limiting ion mobility and active surface area. At 250 °C, the surface appears highly compact with minimal porosity and fully embedded CNTs, indicating thermal degradation and structural densification that hinder charge storage. The 90 °C-treated sample achieves the best trade-off between porosity and CNT distribution, contributing to its superior electrochemical properties.

Figure 3 shows the SEM images of the PI/CNTs-PPy composites processed at 90, 180, and 250 °C and demonstrates the significant morphological differences that impacted their electrochemical performance. At 90 °C, the PPy is uniformly distributed across the CNT/polyimide matrix, with visible porosity that promotes efficient ion transport and charge storage, resulting in optimal electrochemical performance. In contrast, the 180 °C sample shows less uniform PPy deposition, with larger aggregates and reduced porosity, which hinder ion mobility and decrease the active surface area. The 250 °C processed composite is highly compact with minimal porosity, and PPy is poorly distributed, indicating thermal degradation and structural densification. These morphological changes at higher temperatures negatively impact ion accessibility and charge storage capacity, underscoring 90 °C as the ideal processing condition for achieving superior performance.

The EDS spectra shown in Figure 4 compare the elemental compositions of pure CNTs and PI/CNTs-PPy composites. The spectrum for pure CNTs shows 100% carbon by weight and atomic percentage, confirming its high purity. In contrast, the spectrum for PI/CNTs-PPy composites reveals 85.09% carbon and 14.91% iron by weight, with an atomic composition of 96.37% carbon and 3.63% iron. The detected iron likely originates from residual catalysts used during synthesis or processing. This reduction in carbon percentage and the presence of iron in the composite indicate the successful integration of additional components, with iron potentially affecting the composite’s electrochemical and catalytic properties.

SEM images (Figure 2) reveal the morphological structure of the composites, confirming the uniform distribution of CNTs within the polyimide matrix. The EDX spectra (Figure 4) validate the presence of carbon and minor amounts of iron, indicating the successful integration of CNTs and the interaction with the polyimide matrix.

4.2. Cyclic Voltammetry

Figure 5 illustrates the transient current–time curve obtained during electrochemical deposition of polypyrrole onto PI/SWCNT composite working electrodes processed at 90 °C, 180 °C, and 250 °C over a deposition period of 700 s. The composite processed at 90 °C exhibits the highest steady-state current, reaching approximately 0.08 A, indicating that it forms the most efficient PPy layer for ion transport and charge transfer. In contrast, the steady-state current for the 180 °C processed composite stabilizes at a lower current of 0.04 A, suggesting higher resistance and a denser structure with reduced ion diffusion. The steady-state current for the 250 °C processed composite stabilizes at around 0.06 A, performing better than 180 °C but still lower than 90 °C. This suggests that while 250 °C creates a stable PPy layer, it does not achieve the same level of ion mobility and electrochemical efficiency as 90 °C. Overall, processing at 90 °C proves to be the optimal temperature for polymerization, delivering the best electrochemical performance for energy storage applications.

Figure 6 presents the cyclic voltammograms (CVs) of PI/CNTs hybrid nanocomposites processed at 90 °C, followed by the electrodeposition of polypyrrole (PPy). The measurements were conducted using an Ag/AgCl reference electrode and a graphite rod counter electrode, with CV tests run at scan rates of 5 mV/s, 10 mV/s, and 25 mV/s. Figure 6a shows the results after one cycle; the peak current increases with the increase in the scan rate from 5 mV/s (black) to 25 mV/s (blue), indicating typical capacitive behavior where faster scan rates allow less time for ion diffusion, leading to higher current peaks. Figure 6b, displaying the CVs after five cycles, follows a similar trend, with increased peak currents at higher scan rates. Figure 6c shows the CV results after 10 cycles, continuing this trend, with further increased peak currents as the scan rate increases. Compared to the data obtained after one and five cycles, the enhanced current response indicates that the electrode’s performance improves with continued cycling due to formation of a more stable and efficient electrochemical interface.

Figure 7 presents the cyclic voltammograms (CVs) of PI/CNTs hybrid nanocomposites processed at 180 °C, followed by the electrodeposition of polypyrrole (PPy). The CV measurements were conducted using an Ag/AgCl reference electrode and a graphite rod counter electrode at scan rates of 5 mV/s, 10 mV/s, and 25 mV/s. Figure 7a shows the results after one cycle; the peak current increases as the scan rate increases from 5 mV/s to 25 mV/s, indicating typical capacitive behavior where faster scan rates lead to higher current peaks due to reduced time for ion diffusion. Figure 7b, displaying the CVs after five cycles, shows a consistent trend of increasing peak current with higher scan rates, with an overall higher current response compared to the one-cycle data. Figure 7c illustrates the CV results after 10 cycles, where the peak currents continue to increase with higher scan rates. The current response is further enhanced compared to the five-cycle data, indicating that the electrode’s performance continues to improve with repeated cycling, potentially due to the formation of a more stable and efficient electrochemical interface. This suggests that repeated cycling improves the electrode’s electrochemical performance by enhancing the utilization of active material and stabilizing the electrode surface.

Figure 8 presents the cyclic voltammograms (CVs) of PI/CNTs hybrid nanocomposites processed at 250 °C, followed by the electrodeposition of polypyrrole (PPy). The CV measurements were conducted using an Ag/AgCl reference electrode and a graphite rod counter electrode at a scan rate of 5 mV/s. In Figure 8a, after one cycle, the peak current increases with the scan rate, indicating typical capacitive behavior. However, the current response is lower compared to composites processed at lower temperatures, suggesting a reduction in electrochemical performance possibly due to the high processing temperature affecting the material’s structure. Figure 8b shows the CVs after five cycles, where the peak currents are slightly higher than in the one-cycle data, indicating that repeated cycling improves the electrode’s performance. Despite this, the overall current response remains lower than that of composites processed at lower temperatures, implying that the high processing temperature might have negatively impacted the material’s active surface area or conductivity. Figure 8c illustrates the CV results after 10 cycles, showing a continued increase in peak currents with the scan rate, though the overall enhancement in performance with cycling is modest. This suggests that the high processing temperature may limit the material’s ability to achieve optimal electrochemical performance. These results highlight that while the PI/CNTs hybrid nanocomposites processed at 250 °C exhibit capacitive behavior and some improvement with cycling, the high processing temperature significantly affected the material’s structure, leading to lower overall electrochemical performance compared to composites processed at lower temperatures. This emphasizes the importance of processing conditions for the best possible performance in energy storage applications.

Figure 9 displays the cyclic voltammograms (CVs) of PI/CNTs hybrid nanocomposites processed at three different temperatures—90, 180, and 250 °C—followed by the electrodeposition of polypyrrole (PPy). Cyclic voltammetry (CV) measurements were carried out using an Ag/AgCl reference electrode and a graphite rod counter electrode for one cycle at scan rates of 5 mV/s, 10 mV/s, and 25 mV/s. In Figure 9a, the CV results obtained at a scan rate of 5 mV/s are shown; the composites processed at 90 °C show the highest peak current, indicating improved capacitive behavior and excellent electrochemical performance. The peak currents decrease for the composites processed at 180 °C and further decrease for those processed at 250 °C, suggesting that higher processing temperatures may negatively impact the electrochemical properties, possibly due to structural changes in the material that reduce its conductivity or active surface area. Figure 9b, which displays the CVs at a scan rate of 10 mV/s, shows a similar trend wherein the composites processed at 90 °C continue to exhibit the highest peak current, followed by those processed at 180 °C and 250 °C. Although the increased scan rate leads to higher current peaks, the relative performance trend remains consistent, with lower processing temperatures yielding better electrochemical results. Figure 9c, illustrating the CVs at a scan rate of 25 mV/s, further reinforces this pattern. The composites processed at 90 °C show the best electrochemical performance, with the highest peak currents, while the performance decreases with increasing processing temperature, with the composites processed at 250 °C showing the lowest peak currents. This consistent pattern across different scan rates underscores the significant impact of processing temperature on the electrochemical performance of the composites, highlighting that the 90 °C processing condition consistently produces the best results, making it the best temperature for preparing these composites for high-performance energy storage applications. Conversely, higher temperatures may lead to detrimental structural changes that reduce the effectiveness of the material.

Figure 10 shows the cyclic voltammograms (CVs) of PI/CNTs hybrid nanocomposites processed at 90, 180, and 250 °C, followed by the electrodeposition of polypyrrole (PPy). The CV measurements were conducted using an Ag/AgCl reference electrode and a graphite rod counter electrode after five cycles, with scan rates of 5 mV/s, 10 mV/s, and 25 mV/s. Figure 10a shows the CVs at 5 mV/s; the composite processed at 90 °C demonstrates the highest peak current, indicating superior electrochemical performance. As the processing temperature increases to 180 °C and 250 °C, the peak current decreases, suggesting a reduction in electrochemical activity, likely due to structural changes that negatively impact the material’s conductivity or surface area. Figure 10b shows the CVs at 10 mV/s; there is a consistent trend wherein the 90 °C processed composite again exhibits the highest peak current, followed by the 180 °C and 250 °C samples. The higher scan rate leads to increased currents overall, but the decline in performance with rising temperature remains evident. Figure 10c shows the CVs at 25 mV/s; the composite processed at 90 °C continues to exhibit the best electrochemical performance with the highest peak currents, while the composites processed at 180 °C and 250 °C show progressively lower currents. This consistent pattern across different scan rates emphasizes the significant impact of processing temperature on the electrochemical behavior of the composites, highlighting that the 90 °C processing condition provides the best performance, whereas higher temperatures seem to adversely affect the material’s structure and efficiency.

Figure 11 presents the cyclic voltammograms (CVs) of PI/CNTs hybrid nanocomposites processed at 90, 180, and 250 °C, followed by the electrodeposition of polypyrrole (PPy). The CV measurements were conducted using an Ag/AgCl reference electrode and a graphite rod counter electrode after 10 cycles, with scan rates of 5 mV/s, 10 mV/s, and 25 mV/s. Figure 11a shows the CVs at 5 mV/s; the composite treated at 90 °C demonstrates the highest peak current, indicating superior electrochemical performance. As the processing temperature increases to 180 °C and 250 °C, the peak currents decrease, suggesting a reduction in electrochemical activity, likely due to structural changes that negatively impact the material’s conductivity and active surface area. Figure 11b, which presents the CVs at 10 mV/s, continues this trend, with the 90 –processed composite showing the highest peak current, followed by the 180 °C and 250 °C samples. Although the increase in scan rate results in higher current peaks overall, the decline in performance with increasing temperature remains consistent. Figure 11c, displaying the CVs at 25 mV/s, further reinforces this pattern, with the composite processed at 90 °C demonstrating the best electrochemical performance and the highest peak currents. The performance decreases with an increasing processing temperature, with the 250 °C sample showing the lowest peak currents. This consistent pattern across different scan rates and cycles highlights that the PI/CNTs hybrid nanocomposites processed at 90 °C consistently exhibit the best electrochemical performance, making it the optimal temperature for preparing these composites for high-performance energy storage applications. Conversely, higher processing temperatures seem to negatively impact the material’s structure negatively, leading to lower overall electrochemical performance.

Cyclic voltammetry (CV) was performed at scan rates of 5 mV/s, 10 mV/s, and 25 mV/s to analyze capacitive behavior. The results demonstrated a consistent increase in peak current with higher scan rates, reflecting typical capacitive behavior. The composite processed at 90 °C exhibited the highest specific capacitance across all scan rates, indicating its superior performance. The results revealed that composites processed at 90 °C exhibited superior performance due to optimal porosity development, facilitating efficient ion transport. At higher temperatures, structural degradation caused a collapse of the porous network, thereby reducing the active surface area and increasing bulk resistance. This degradation limited ion mobility and adversely affected electrochemical efficiency. Life cycle tests showed consistent performance across 10 cycles at a current density of 0.5 A/g. The 90 °C composite retained its electrochemical properties well, maintaining stability across all tested scan rates (5 mV/s to 25 mV/s).

Figure 12 and Figure 13 and

Table 1. Calculated specific capacitance from cyclic voltammetry.

SWCNTs/PI-PPy Electrode	Specific Capacitance (F/g)
	5 mV/s	10 mV/s	25 mV/s
90 °C	655.34	446.48	277.69
180 °C	370.99	272.95	162.68
250 °C	234.42	162.78	69.02

and

Table 2. Calculated specific capacities from cyclic voltammetry.

SWCNTs/PI-PPy Electrode	Specific Capacities (mAh/g)
	5 mV/s	10 mV/s	25 mV/s
90 °C	182.04	124.02	77.14
180 °C	103.05	75.82	45.19
250 °C	65.12	45.22	19.17

illustrate the variation of specific capacitance and specific capacity with processing temperature. The specific capacitance and specific capacity decreased with increasing processing temperature. The specific capacitance was calculated from the cyclic voltammograms (CVs) at different scan rates (5 mV/s, 10 mV/s, and 25 mV/s) for PI/CNTs hybrid nanocomposites processed at various temperatures (90 °C, 180 °C, and 250 °C) and tested after 1 cycle, 5 cycles, and 10 cycles. Across all conditions, the specific capacitance decreases as the scan rate increases, which is typical for capacitive materials. This trend occurs because slower scan rates provide more time for ion diffusion and better interaction with the electrode surface, leading to higher capacitance values. In the one-cycle tests, the specific capacitance at 90 °C starts around 651.68 F/g at 5 mV/s, decreases to approximately 236.25 F/g at 10 mV/s, and then slightly increases to about 257.61 F/g at 25 mV/s. This pattern suggests that ion movement becomes more restricted at higher scan rates, particularly during the initial cycle. In the five-cycle tests, the specific capacitance shows improvement, with values around 626.80 F/g at 5 mV/s, 438.22 F/g at 10 mV/s, and 256.43 F/g at 25 mV/s, indicating enhanced charge storage capability as the electrode material stabilizes with repeated cycling. The ten-cycle tests show further gains, reaching approximately 655.34 F/g at 5 mV/s, 446.48 F/g at 10 mV/s, and 277.69 F/g at 25 mV/s, demonstrating even better charge storage and material stability with more cycles. With the impact of processing temperature, it is clear that the composites processed at 90 °C consistently exhibit the highest specific capacitance across all scan rates for 1 cycle, 5 cycles, and 10 cycles, followed by those processed at 180 °C and 250 °C. This pattern suggests that lower processing temperatures are more favorable for maximizing specific capacitance, likely due to better preservation of the material’s structure and electrochemical properties. As the processing temperature increases, the material likely undergoes structural degradation, which diminishes its electrochemical performance. Despite the decline in specific capacitance at higher scan rates, the electrodes maintain strong performance even after 5 and 10 cycles, indicating their good potential for high-rate applications. The PI/CNTs hybrid nanocomposites demonstrate excellent specific capacitance, particularly at lower scan rates and with increased cycling, emphasizing their potential for high-performance energy storage applications, especially when processed at the best processing temperatures. Cyclic voltammetry (CV) was performed at various scan rates (5 mV/s to 50 mV/s) to analyze capacitive behavior. Galvanostatic charge–discharge (GCD) tests were conducted at 0.5 A/g, revealing a specific capacitance of 655.34 F/g at 90 °C. A life cycle test demonstrated stability over 10 cycles, indicating good electrochemical durability.

Figure 14 demonstrates that the processing temperature significantly influences the capacitance retention and overall electrochemical stability of SWCNTs/PI–PPy composite electrodes over 10 cycles at a current density of 0.5 A/g. The electrode processed at 90 °C shows the best performance, maintaining capacitance retention around 100% throughout the 10 cycles with minimal degradation, indicating excellent stability and suggesting that 90 °C is an optimal processing temperature. In contrast, the electrode processed at 180 °C exhibits a noticeable decline in capacitance retention after the fourth cycle, dropping to around 80% by the tenth cycle. This suggests that this temperature may lead to material degradation or poor component interaction, affecting long-term performance. The 250 °C processed electrode, while more stable than the 180 °C sample, does not surpass the performance of the 90 °C electrode, indicating that although higher temperatures may enhance certain material properties, they do not necessarily improve long-term electrochemical stability. Overall, the 90 °C processing temperature is the most effective for achieving high capacitance retention and stability, making it the preferred choice for optimizing the performance of these composite electrodes in energy storage applications.

4.3. Galvanostatic Charge/Discharge

Figure 15 presents charge–discharge curves, showing the electrochemical properties of SWCNT//PI–PPy composites processed at 90 °C, 180 °C, and 250 °C at an applied energy density of 0.5 A/g. The composite processed at 90 °C shows the longest discharge time, indicating the highest energy storage capacity, with a smooth and symmetric curve suggesting good capacitive behavior and efficient charge storage and release. The 180 °C composite exhibits a shorter discharge time and slight asymmetry, reflecting intermediate performance and some polarization effects. The 250 °C composite has the shortest discharge time and the most asymmetric curve, indicating the lowest energy storage capacity and higher resistance, likely due to material degradation or structural changes at the higher temperature. Overall, the 90 °C processed composite demonstrates the best electrochemical performance, emphasizing the importance of optimizing processing temperatures for supercapacitor applications. Higher processing temperatures negatively impact energy storage capacity and charge/discharge efficiency, as shown by the poorer performance of the 180 °C and 250 °C samples.

Figure 16 and Figure 17 and

Table 3. Summary of specific capacitance and capacities obtained from charge–discharge cycles for 700-s deposition at 0.5 A/g.

PI/CNTs–PPy Electrode Material	Specific Capacitance (F/g) 0.5 A/g	Specific Capacity (mAh/g)
90 °C	265.98	73.88
180 °C	134.58	37.38
250 °C	112.65	31.29

illustrate the impact of processing temperature on the electrochemical performance of SWCNTs/PI composite electrodes electrodeposited with polypyrrole (PPy), as measured by galvanostatic charge–discharge (GCD) at a current density of 0.5 A/g. In Figure 16, the specific capacitance is shown to decrease significantly as the processing temperature increases. The composite electrode processed at 90 °C exhibits the highest specific capacitance, reaching approximately 266 F/g. However, with an increase in processing temperature to 180 °C, the specific capacitance drops sharply to around 135 F/g and further decreases to 113 F/g at 250 °C. This decline suggests that higher processing temperatures may negatively affect the composite’s ability to store charge, likely due to thermal degradation or alterations in the microstructure, such as reduced porosity or increased internal resistance, which hinder effective ion transport. Similarly, Figure 17 shows a corresponding decrease in specific capacity with increasing processing temperatures. The specific capacity of the composite electrode at 90 °C is the highest, around 74 mAh/g. As the temperature rises to 180 °C, the specific capacity drops to about 37 mAh/g and further decreases to approximately 30 mAh/g at 250 °C. This trend parallels the decline observed in specific capacitance, reinforcing the idea that higher processing temperatures degrade the material’s electrochemical properties, potentially due to reduced active surface area, decreased porosity, or changes in the internal resistance of the material. When considering both figures together, it becomes evident that processing temperature plays a critical role in determining the electrochemical performance of PI/SWCNTs hybrid nanocomposite electrodes. Lower processing temperatures, particularly around 90 °C, are more favorable for achieving higher specific capacitance and specific capacity. This is likely because lower temperatures help preserve the material’s microstructure, ensuring higher porosity, better ion mobility, and lower internal resistance, which contribute to more efficient charge storage. On the contrary, higher processing temperatures, such as 180 °C and 250 °C, appear to impair these properties, leading to diminished electrochemical performance. This degradation may be attributed to factors such as thermal degradation, which can alter the material’s structure, reduce active sites, and increase the resistance to ion movement, ultimately lowering both specific capacitance and specific capacity. Additionally, it is important to note the discrepancy between the specific capacitance values obtained from GCD measurements in Figure 15 and Figure 16 and those derived from cyclic voltammetry (CV) curves. GCD measurements, which involve a constant current charge–discharge process, tend to produce lower apparent capacitance due to higher polarization and greater voltage drops. In contrast, CV is more sensitive to surface redox reactions and captures more transient processes, often leading to higher capacitance values.

Figure 18 compares the energy densities and power densities obtained in this study as a function of PI/SWCNT processing temperature and PPy electrodeposition with those reported previously by Zhang et al. [38], Yang et al. [39] and Ajuria et al. [40]. The results obtained in this study show that the composite electrode material processed at 90 °C followed by electrodeposition of PPy have outstanding electrochemical properties.

Galvanostatic charge–discharge (GCD) tests were conducted at a current density of 0.5 A/g. Specific capacitance values for composites processed at 90 °C were observed to be 655.34 F/g at 5 mV/s, 446.48 F/g at 10 mV/s, and 277.69 F/g at 25 mV/s after 10 cycles. These results highlight the superior electrochemical stability of the 90 °C processed composite. A life cycle test demonstrated stability over 10 cycles, indicating good electrochemical durability.

4.4. Electrochemical Impedance Spectroscopy (EIS) Analysis

Figure 19 shows electrochemical impedance spectroscopy (EIS) Bode plots for PI/SWCNT–PPy composite samples processed at different temperatures (90 °C, 180 °C, and 250 °C) after a 700-s polypyrrole (PPy) deposition. An impedance magnitude plot (Log(Z) vs. Log(Fr)) is shown in Figure 19. The sample processed at 90 °C exhibits the lowest impedance across the frequency range, indicating superior conductivity and minimal resistance. This is further supported by the analysis of solution resistance, coating resistance, and pore resistance, where the 90 °C sample demonstrated the lowest values for all three parameters, particularly a solution resistance of 2.307 ohms, a coating resistance of 94 ohms, and a pore resistance of 301.8 ohms. These factors collectively suggest that the 90 °C sample has well-formed conductive and porous structures, facilitating efficient ion transport and charge transfer. The 180 °C sample shows higher impedance, reflecting increased resistance and reduced conductivity. The analysis reveals that the solution resistance is 87.67 ohms, significantly higher than the 90 °C sample, while the coating resistance is extremely high at 8700 ohms, indicating the poor conductivity of the coating material. The pore resistance is also the highest among the samples at 13,930 ohms, suggesting substantial limitations in ion transport within the porous structure. This combination of high coating and pore resistance highlights the compromised electrochemical performance of the 180 °C sample, despite two notable time-constant phase changes in Figure 20 (one at 30 Hz and another at 500 Hz). The phase changes reflect a balance between capacitive and resistive properties, though the resistance remains significantly higher than the 90 °C sample. In contrast, the 250 °C sample exhibits the highest impedance overall, signifying the poorest conductivity and the most resistive behavior. The solution resistance is relatively low at 51.8 ohms, but the coating resistance, at 400 ohms, is still considerably higher than the 90 °C sample, indicating suboptimal conductivity. The pore resistance is measured at 2991.9 ohms, lower than the 180 °C sample but still much higher than the 90 °C sample, reflecting limited ion transport. The phase angle plot in Figure 20 for the 250 °C sample demonstrates the least capacitive behavior, with a time-constant phase change occurring at a much lower frequency of 7 Hz, underscoring a significant resistive nature and poor charge transfer capabilities. The large shift in phase angle towards resistive behavior at low frequencies indicates substantial impedance and limited capacitive performance.

The EIS Bode plots (Figure 19) reveal the impedance characteristics of composites processed at different temperatures. The 90 °C sample exhibits the lowest impedance across the frequency range, indicating superior conductivity and minimal resistance. In contrast, the composites processed at 180 °C and 250 °C show significantly higher impedance, reflecting reduced conductivity and increased resistance. These trends are attributed to structural densification and reduced porosity at higher temperatures. Detailed impedance values are summarized in

Table 4. Summary of theoretical porosity, resistance, and specific capacitance obtained using EIS.

Material	Bulk Resistance (Ω) (EIS)	Weigh Gain (%)	Specific Capacitance (F/g) (EIS)	Porosity (EIS)
PI/SWCNTs (90 °C)	65.27	1.669 (0.003287 g)	209.16 (23.15 Ω)	4.59
PI/SWCNTs (180 °C)	2366.4	1.313 (0.002284 g)	5.40 (1236 Ω)	0.35
PI/SWCNTs (250 °C)	143.62	0.567 (0.004875 g)	123.90 (26.35 Ω)	1.52

and

Table 5. Summary of resistance values obtained using (EIS).

Material	Solution Resistance (Ω)	Charge Transfer Resistance (Ω)	Pore Resistance (Ω)	Coating Resistance (Ω)
PI/SWCNTs (90 °C)	2.307	67.58	301.8	94
PI/SWCNTs (180 °C)	87.67	2465	13,930	8700
PI/SWCNTs (250 °C)	51.8	194.79	2991.9	400

.

The Nyquist plots in Figure 21 reveal the impact of processing conditions on the electrochemical performance of PI/CNTs–PPy hybrid nanocomposites, highlighting the consequences of failed coatings. The 90 °C sample demonstrates superior performance with a bulk resistance of 65.27 Ω, specific capacitance of 209.16 F/g (23.15 Ω impedance), and a weight gain of 1.669% (0.003287 g), supported by a small semicircle in the Nyquist plot, indicating uniform coating, low surface roughness, and effective ion diffusion, with a porosity of 4.59 (Table 4). In contrast, the 180 °C sample shows characteristics of a failed coating, with a bulk resistance of 2366.4 Ω, drastically reduced specific capacitance of 5.4 F/g (1236 Ω impedance), and a weight gain of only 1.313% (0.002284 g) (Table 4). Its large semicircle reflects severe surface roughness, poor uniformity, and high charge transfer resistance, compounded by Warburg impedance effects and a low porosity of 0.35, indicating limited ion transport and inhomogeneity. The 250 °C sample presents mixed results, with a bulk resistance of 143.62 Ω, a specific capacitance of 123.90 F/g (26.35 Ω impedance), and the lowest weight gain at 0.567% (0.004875 g) (Table 4). The large semicircle in its Nyquist plot indicates poor coating uniformity, sluggish charge transfer, and thermal degradation, as evidenced by a phase shift at 7 Hz, despite a slight improvement in porosity to 1.52 compared to the 180 °C sample. These results demonstrate that failed coatings lead to increased impedance, reduced capacitance, and poor ion transport, with the 90 °C sample emerging as the most efficient due to its balanced electrochemical properties, emphasizing the importance of optimizing processing temperatures for effective and uniform coatings.

4.5. Impact of Processing Temperature on Polymerization and Performance

The superior performance of the composite processed at 90 °C can be attributed to the optimized polymerization process and structural characteristics achieved at this temperature. At 90 °C, the poly(amic acid) (PAA) precursor undergoes controlled imidization, forming a uniform polyimide (PI) layer. This temperature ensures sufficient porosity and uniform dispersion of carbon nanotubes (SWCNTs) within the matrix, which enhance ion transport pathways and minimize bulk resistance. Moderate thermal treatment also prevents excessive densification or degradation of the material, preserving the active surface area required for efficient charge storage.

In contrast, higher processing temperatures (180 °C and 250 °C) lead to structural densification and thermal degradation. At 180 °C, the partial collapse of the porous network reduces ion mobility. It increases internal resistance, while at 250 °C, significant thermal degradation results in a compact and less porous structure, further hindering electrochemical performance. Therefore, the processing temperature of 90 °C represents an optimal balance, where polymerization and structural integrity are maintained, enabling superior electrochemical behavior, as evidenced by the highest specific capacitance and stable cycling performance.

5. Conclusions

This study demonstrates the significant influence of processing temperature on the structural and electrochemical properties of PI/SWCNTs-PPy hybrid nanocomposites for energy storage applications. Processing at 90 °C emerged as the optimal condition, resulting in the highest specific capacitance of 655.34 F/g and excellent cycling stability over 10 cycles. These superior results are attributed to the development of an optimal porous structure that facilitates efficient ion transport, minimizes internal resistance, and ensures uniform PPy deposition.

In contrast, composites processed at 180 °C and 250 °C showed diminished performance, with reduced specific capacitance and cycling stability. These declines were primarily due to structural degradation at higher temperatures, such as the collapse of the porous network and increased bulk resistance, which hinder ion mobility and adversely impact electrochemical efficiency. While cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) techniques provide valuable insights into the electrochemical performance of the composites, they do not fully capture the microstructural features or fine mechanisms of charge carrier transport and capture. Future studies will integrate advanced characterization methods, such as X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations, to provide a comprehensive understanding of these mechanisms.

The findings herein underscore the critical role of temperature control in achieving a balance between structural integrity and enhanced electrochemical properties. Future research should focus on scaling this optimized process and exploring alternative polymer matrices or advanced dopants to further enhance energy storage capabilities. These insights contribute to the advancement of next-generation energy storage systems, where processing precision is paramount for performance optimization. Future research could explore the scalability of this process and investigate alternative polymer matrices to enhance performance further.