Enhanced supercapacitor performance of reduced graphene oxide/polypyrrole composite for energy storage application

Abstract

The combined properties of reduced graphene oxide/polypyrrole (rGO/Ppy) have drawn a lot of interest in composite electrodes for supercapacitors. In this work, chemical bath deposition (CBD) method was used to develop rGO/Ppy composite thin films. The CBD method enables the synthesis of self-supported electrode materials, optimizing the physicochemical characteristics and promoting efficient charge transfer of electrodes for energy storage systems. The characterization of rGO/Ppy thin films by X-ray diffraction, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy revealed the presence of amorphous Ppy with a spherical morphology on rGO layers. The rGO/Ppy thin films exhibited remarkable electrochemical properties, obtaining 987 F g⁻¹ specific capacitance at scan rate of 5 mV s⁻¹ and retaining 87% electrochemical stability after 3,000 cycles. Moreover, an aqueous asymmetric supercapacitor device (rGO/Ppy//H₂SO₄//WO₃) demonstrated a 61 F g⁻¹ specific capacitance and 11 Wh kg⁻¹ specific energy at 0.474 kW kg⁻¹. This study reveals an easy method for enhancing Ppy charge storage capacity and cyclic stability through composition of rGO for supercapacitor applications.

1 Introduction

Supercapacitors represent an advanced energy storage technology, distinguished by their higher power density, quick charge and discharge, and outstanding cyclic stability (Mansuer et al., 2021; Miao et al., 2021; Mane et al., 2020a). Supercapacitors, are commonly used in several fields such as wearable and lightweight electronic devices (Wen et al., 2021), power systems (Shayeghi et al., 2021), electric and hybrid vehicles (Muzaffar et al., 2019), and others (Plebankiewicz et al., 2020; Liang et al., 2019). Pseudocapacitive materials, including transition metal compounds and conducting polymers (CPs), operate via a faradaic charge storage mechanism, which involves electron transfer during a chemical state change. These materials exhibit significantly higher specific capacitance compared to materials like carbon nanotubes (CNTs) and graphene, which rely on an electrical double-layer capacitance (EDLC) mechanism for charge storage. The EDLC mechanism involves ion adsorption and desorption in response to an applied bias potential. The CPs are regarded one of the best choices for supercapacitor electrodes due to their exceptional advantages, including excellent electrical conductivity, rapid redox processes, and mechanical flexibility. Researchers have focused extensively on several noteworthy CPs, like polyaniline, polypyrrole (Ppy), polythiophene, and their derivatives (Pawar et al., 2024a). Among these, Ppy stands out as a particularly intriguing polymer, offering a significant theoretical specific capacitance (>600 F g⁻¹) and superior electric conductivity (Ding et al., 2021). However, Ppy exhibits lower electrochemical stability due to ion doping and dedoping mechanisms, which causes volumetric changes during the charge storage process, thereby compromising structural integrity and reducing cycle stability. Additionally, the dense structure of Ppy limits ion transport within the active material, leading to lower energy density. Consequently, there has been a lot of interest in modifying Ppy structure to create a porous architecture that can mitigate volumetric expansion and enhance performance However, it has poor cycle and rate capability due to insufficient structural stability in the charging and discharging activities. Combining Ppy material with EDLC materials is an effective way of improving Ppy electrochemical cycle stability (Huang et al., 2016).

Carbon-derived materials like graphene oxide (GO), CNTs, and reduced graphene oxide (rGO) are preferred for synthesizing porous materials due to their ability to form composites with polymers effectively. These materials offer several advantages, including a greater electrical conductivity, higher specific surface area, and enhanced stability (Kulandaivalu and Sulaiman, 2019). Additionally, their solution processability is a significant benefit, facilitating the integration of these materials into pseudocapacitive materials. For rGO, the edges are covered with numerous functional groups that include oxygen, for example, carboxyl (-COOH), aldehyde (-CHO), and hydroxyl (-OH) groups. These functional groups offer as active sites for chemical bond formation, enhancing the reactivity of rGO sheets with Ppy and facilitating the formation of rGO-based Ppy composites. The formation process is driven by strong van der Waals interactions between the highly π-conjugated Ppy chains and the aromatic graphene basal planes. Additionally, the large surface area of rGO sheets provides robust interfacial EDLC and offers a stable platform for Ppy deposition, which helps to buffer the volumetric changes of the polymer throughout charging and discharging (Rashti et al., 2020; Barakzehi et al., 2020; Wang et al., 2022).

Recent literature highlights the effective synthesis of Ppy/graphene composites through polymerization of Ppy and rGO, facilitated by the good dispersion and accessibility of rGO and pyrrole monomers in aqueous solutions. For instance, (Ye and Feng, 2014) prepared one-dimensional hollow Ppy nanotubes combined with GO using easy one-step reduction self-assembly method and enhanced the specific capacitance (C_s) and surface area compared to Ppy and rGO prepared in the study. (Zhu and Xu, 2018) developed a Ppy/rGO composites series via chemical polymerization, which achieved C_s of 290 F g⁻¹ with significant electrochemical stability of 97.5% over 20,000 cycles. (Sarmah and Kumar, 2019) developed ternary molybdenum disulfide (MoS₂)-rGO/Ppy nanocomposites via hydrothermal method and exhibited a C_s of 1875 F g⁻¹ and 91% cyclic stability over 10,000 cycles. (Ramesh et al., 2017) studied the supercapacitive properties of a Co₃O₄@Ppy/multi-walled carbon nanotube (MWCNT) nanocomposite and achieved 609 F g⁻¹C_s in a 6 M KOH electrolyte. Eeu and coworkers (Eeu et al., 2013) produced a ternary Ppy/rGO/Fe₂O₃ film via a single-pot chronoamperometry method, which obtained a 126 F g⁻¹C_s in a 1 M KCl electrolyte. (Li et al., 2018) synthesized a ternary nanocomposite of Ppy/Ni(OH)₂/sulfonated graphene oxide (SGO) using hydrothermal method to anchor Ni(OH)₂ nanoflakes on SGO, followed by chemical oxidative polymerization of Ppy to coat the surface of materials and achieved C_s of 1632.5 F g⁻¹ with an 86% retention of capacitance over 1,000 cycles. (Jose et al., 2021) developed nanocomposite of rGO draped palladium oxide/Ppy by electrodeposition method, achieved 595 F g⁻¹C_s in a 1 M H₂SO₄ electrolyte with an 88% cyclic stability over 5,000 cycles. Zhang and coworkers (Zhang et al., 2020) produced Ppy/graphene oxide@graphene aerogel (PGO@GA) by pulse electropolymerization and obtained 625 F g⁻¹C_s with excellent cycling life retaining 85.7% over 5,000 cycles. The majority of research detailed the production of rGO and Ppy composite as powder and the use of binder-assisted coating techniques, such as polyvinylidenefluoride (PVDF), to prepare electrodes for supercapacitors. This study focuses on rGO/Ppy composite thin film that is directly produced over a stainless steel substrate as a more efficient method of improving interfacial contact than binder-assisted coating techniques. This type of binder-free electrode is advantageous because it offers rapid electron transport, straightforward electrolyte diffusion, and improved access to electro-active regions by lowering material resistance. Consequently, it is possible to immediately achieve synergistic properties to improve the C_s, rate performance, and stability of electrode components (Thorat et al., 2023). Notably, there is a lack of research on the development of rGO/Ppy composites via chemical bath deposition (CBD) method directly on conductive support and their application as electroactive materials in supercapacitors.

The present study uses CBD method for synthesis of Ppy and rGO/Ppy composite thin films on stainless steel sheet. This type of binder-free electrode is more efficient, offering rapid electron transit, straightforward electrolyzed diffusion, and improved access to electro-active regions by lowering material resistance. Consequently, it is possible to quickly achieve synergistic characteristics in order to improve the efficiency, stability, and C_s of electrode materials (Patil et al., 2017). The main advantages of CBD method include its ability to produce adherent and homogeneous thin films on a large scale with high repeatability (Mane et al., 2020b). The rGO was introduced in Ppy to improve porosity, surface area, and specific energy. However, research on the supercapacitive characteristics of rGO/Ppy composites developed with CBD method is still lacking. The rGO, Ppy, and rGO/Ppy thin films are characterized using X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), contact angle, Brunauer, Emmett and Teller (BET), and X-ray photoelectron spectroscopy (XPS). Additionally, in a three-electrode setup, the electrochemical properties of rGO, Ppy, and rGO/Ppy composite thin films were studied using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques. Furthermore, an aqueous asymmetric supercapacitor (ASC) device was developed employing rGO/Ppy electrode as the cathode, tungstate oxide (WO₃) as the anode, and 1 M sulfuric acid (H₂SO₄) as the aqueous electrolyte.

2 Experimental

2.1 Chemicals and materials

To synthesise rGO/Ppy thin films, pyrrole monomers, ammonium persulfate (APS), and H₂SO₄ were purchased from Thomas Baker Ltd. And utilised exactly as received. Double distilled water (DDW) was utilized as a solvent throughout the experiments. Film deposition was carried out on stainless steel (SS) substrates (304 grade). Before film deposition, the SS substrates were polished with fine sandpaper and then ultrasonically cleaned for 15 min using DDW. After that, the SS substrate was left to dry at ambient temperature.

2.2 Deposition of thin films

2.2.1 Deposition of rGO/Ppy electrode

The GO was produced via modified Hummers’ method and afterwards chemically reduced. The process of rGO preparation is provided in the Supplementary Material. The dilution method was used for producing the 1 mg mL⁻¹ density rGO suspension (Malavekar et al., 2020). To evenly distribute the rGO sheets, the resulting solution was sonicated for 5 h. Firstly, 0.03 M pyrrole and 0.05 M APS were prepared in 50 mL of 1 M H₂SO₄ solution. At the ambient temperature (300 K), each solution was gradually added to rGO suspension. The rGO/Ppy composite thin film synthesis using CBD is schematically represented in Figure 1. A SS substrate was placed in the resulting solution, which was vigorously stirred (at 500 rpm speed) and left to polymerize. After polymerization, well adherent, smooth, black-coloured rGO/Ppy thin film was formed on the SS substrate. After thin film formation, samples washed repeatedly with DDW to remove any residual reactants and air-dried at room temperature (300 K). The structural interaction and charge transfer process of rGO sheets and Ppy are influenced by several factors, such as the rate of polymerization and the different van der Waals interactions that take place. Hence, to examine the influence of rate of polymerization reaction, rGO/Ppy thin films were deposited at varied time period such as 1, 2, and 3 h are denoted as RP1, RP2, and RP3, respectively. Through non-covalent functionalization, rGO and Ppy interact to create a nanocomposite material with rGO and Ppy acting as electron acceptor and donor, respectively. The pyrrole monomer is chemically oxidized with H₂SO₄, which serves as a dopant, and produces dimer, semi-dine, and trimer of pyrrole. During rGO/Ppy composites chemical polymerization mechanism, the interactions between Ppy surface and backbone caused positively charged monomers, or pyrrole cations, to adsorb on the negatively charged rGO surface due to some interfacial forces like π-π stacking, hydrogen bonding, and electrostatic forces (Moyseowicz and Gryglewicz, 2020). Consequently, increasing numbers of nucleation sites undergo crucial polymerization, assisted by a large specific surface area of rGO network, resulting in the consistently deposition of Ppy with an excellent coverage on the surface. The interaction and production of Ppy on rGO surface is seen in Figure 2. By forming hydrogen bonds among Ppy radical and rGO hydroxyl group, the spherical particles of Ppy and rGO nanosheet are bonded together (Ding et al., 2021). Protonated N, formed from H₂SO₄ and single pairs on -OH saturated rings, produces stronger hydrogen bonds due to electrostatic force of attraction. This promotes an appropriate interaction between the two molecules. The stability of the composite is enhanced by the presence of π-π stacking across Ppy quinoid rings and π bonds in rGO basal planes. Supplementary Figure S1 illustrates the mass loading of Ppy and rGO/Ppy thin films, ranging from 0.21 to 0.4 mg cm⁻². The mass loading of the thin film appears to increase with the polymerization time. The mass loading of the films in CBD is influenced by the formation and growth process of nucleus. Maximal thickness results from improving coagulation during growth via boosting the number of nucleation sites (Patil et al., 2014).

FIGURE 1

FIGURE 2

2.2.2 Deposition of WO₃ electrode

Particularly in the supercapacitor field, WO₃ is a promising material for negative window electrodes. The WO₃ superior supercapacitive characteristics can be attributed to two aspects. From an electrochemical perspective, WO₃ exhibits superior supercapacitive characteristics such as a large theoretical capacitance (1112 F g⁻¹), in aqueous electrolytes, a broad negative operating potential window (∼0.8 V), excellent electrochemical supercapacitive performance, resulting in less chemical corrosion of current collectors. The WO₃ thin film was deposited onto SS sheet using the CBD method. After producing a 0.15 M sodium tungstate (Na₂WO₄) solution in 80 mL of DDW, diluted H₂SO₄ was gradually added while being constantly stirred to get the pH down to 2 ± 0.1. The cleaned SS sheet substrate was then placed vertically in a reaction bath and kept at 353 K for 8 h. After the deposition period, WO₃ coated SS sheet was removed from the reaction bath and carefully cleaned with DDW.

2.3 Electrochemical measurements

Based on a three-electrode setup, the experiment was performed in 1 M H₂SO₄ solution at ambient temperature. The platinum plate served as the counter electrode, saturated calomel electrode (SCE) as the reference electrode, and Ppy, rGO/Ppy, and WO₃ electrodes as a working electrode. The CV, GCD, and EIS were performed by an Zive MP1 electrochemical workstation. The EIS was tested using an AC signal delivered at amplitude of 10 mV over a frequency range of 0.1 Hz–100 KHz.

The following Equations 1–4 were used to compute a specific capacitance (C_s), specific energy (S_E), and specific power (S_P),where, ‘m’ is mass of materials deposited on 1 × 1 cm² area (g), ‘ν’ is scan rate measured (V s⁻¹), ‘ΔV’=(V₁-V₀) is operating potential window (V), ‘I’ is current density (A), and ‘t’ is the discharge time (s).

3 Results and discussion

3.1 Structural and elemental characterizations of ppy and rGO/Ppy composite thin films

The XRD method was used to examine the structural properties of synthesized Ppy and RP series electrodes (Figure 3a). The XRD patterns of Ppy and RP series electrodes reveal comparable diffraction spectra, with no notable material-related peaks. The diffraction peaks indicated by ‘Δ’ in Figure 3a originate from the SS substrate. The absence of any notable material peak in the obtained spectra within the 10°–80° range indicates the formation of amorphous Ppy and RP series material over the SS substrates. Due to the deposition performed at room temperature, the energy required for particle arrangements in a certain direction is only accessible. This lowers particle mobility and causes a disordered particle arrangement that produces amorphous thin films (Patil et al., 2023). The absence of a rGO peak in XRD of rGO/Ppy composite electrodes is probably because of the low rGO content. The XRD pattern of rGO powder is displayed in Supplementary Figure S2 in order to ensure rGO formation. The (002) plane of graphite is shown by the diffraction peak at 24°, matching PDF No. 41-1487 (Mehta et al., 2021).

FIGURE 3

Raman spectra were recorded in order to obtain more understanding of bonding between molecular in Ppy and RP series electrodes (Figure 3b). In Ppy spectrum, the peaks at 1591 cm⁻¹ are caused by the π conjugated structure of the polymer backbone. The peak associated with the polymers ring stretching mode is located at 1345 cm⁻¹ (Biswas and Drzal, 2010). At 1053 cm⁻¹, the peak displays C-H in-plane Ppy bending modes. The polaronic and bipolaronic quinoid structures exhibits ring deformation, as displayed by the two mild peaks placed at 992 and 931 cm⁻¹, respectively (Kulandaivalu et al., 2019). In rGO Raman spectrum (Supplementary Figure S3), the sp² hybridised carbon atom of graphene and the peak connected to the C-C ring is located at 1591 cm⁻¹ (G band). The D band at 1345 cm⁻¹ represents the carbon atoms sp³-hybridization. Increased D band intensity is indicative of rather substantial defects in the rGO sheet that cause the sp² hybridization to be distorted to sp³. The quantitative study of the rGO sheet is supported by the G peak. The spectra of RP series composites exhibit the two rGO peaks (D and G bands), as well as the previously described distinctive peaks of bare Ppy. The I_D/I_G intensity ratio is used to quantify the degree of disorder and oxidation in the hexagonal C-C ring (Pawar et al., 2024a). A larger ratio of I_D/I_G reveals more degree of disorder. The I_D/I_G intensity ratio for rGO is 0.82. For RP series composites, a 0.85 I_D/I_G ratio suggests significant disorder and the development of rGO stacking in Ppy material. The large D peak at about 1345 cm⁻¹ suggests that Ppy particles are separating multilayered rGO sheets. Layered rGO sheets with sp² hybridization provide a simple lateral channel for electron transfer, potentially lowering the electrode’s series resistance (Pawar et al., 2023).

FE-SEM was used to examine the morphology of Ppy and RP composite samples at two magnifications (×5,000 and 20,000x). The Ppy surface texture is depicted in Figures 4a,b, revealing the dense, spherical, nanoparticle-like morphology. Furthermore, the randomly interconnected particles with diameter of 100 nm generate a lot of empty space and gaps. Due to the porous surface rapid ion migration with a shorter diffusion length is possible, advantageous for charge storage (Patil et al., 2022). After Ppy composited with rGO, rGO layers were observed in Figures 4c,d, but Ppy particles with large quantity were not observed. This might be because less polymerization time and lower amount of Ppy particles were embedded in rGO layers. With increasing polymerization time, rGO layer receives additional Ppy particles over rGO sheets. The rGO surface was covered with Ppy spherical particles, as illustrated in Figures 4e,f, here Ppy particles might function as spacers that prohibit rGO sheets from restacking and develop a porous (with pores of 1 μm diameter in size) electrode with evenly spaced material. Variations in the time of polymerization impact on growth rate, which subsequently influences the deposition of Ppy particle on rGO sheets. Conversely, a higher polymerization time (Figures 4g,h) may lead graphene sheets to collapse or restack, resulting in a more condensed structure. Combining an additional rGO surface with a Ppy particles structure boosts interfacial area and the number of active sites, enhancing the electrochemical reaction (Moyseowicz et al., 2020).

FIGURE 4

Supplementary Figure S4 displays the cross-section SEM image of RP2 electrode. The measured film thickness as for RP2 electrode is about 0.4 μm. Ppy nanoparticles are arranged and attached to rGO sheets in the HR-TEM image of RP2 composite (Figures 4i,j), which is consistent with the FE-SEM results. The HR-TEM images with a lack of a fringe pattern indicates the materials amorphous nature. This work demonstrates that the surface texture of RP series electrodes is influenced by the polymerization time. Variations in polymerization time affect the growth rate, which effects the deposition of Ppy particles over rGO sheets.

The contact angle studies were carried out to assess the interfacial characteristics of the electrode and electrolyte. The contact angle confirms whether the material is hydrophilic or hydrophobic. The surface exhibits hydrophilicity if the contact angle (θ) between electrolyte and material is below than 90°. On the other hand, the surface is hydrophobic if θ is higher than 90° (Bagde et al., 2024). The contact angle value is significantly associated with the size and roughness of the surface pores. (Krainer and Hirn, 2021). For Ppy, RP1, RP2, and RP3 electrodes, the contact angle values are 55°, 41°, 29°, and 35°, respectively, confirming the hydrophilic behaviour of the electrodes (Figures 5a,d). The morphology of the electrode surface varies with polymerization time, which impacts wettability. The RP2 film low contact angle may be attributed to its porous structure (Pawar et al., 2024a). The electrodes porous structure increases its capacity for interacting with the electrolyte and offers a lot of active redox sites. This indicates that liquid enters a porous structure of scattered Ppy particles in rGO sheets, lowering the angle of contact resulting in a hydrophilic surface (Pawar et al., 2023).

FIGURE 5

The XPS study revealed the precise chemical oxidation states of every element included in RP2 sample. The survey spectrum of RP2 contains peaks of C1s, N1s, and O1s, showing the presence of the C, N, and O elements with atomic percentages of 71.72, 13.39, and 14.89%, respectively (Figure 6a). In C1s spectrum of RP2, five peaks were detected (Figure 6b). These peaks showed binding energies of 288.6 eV (O-C=O functional group), 287.6 eV (C=O carbonyl functional group), 286.0 eV (C-O functional group), 285.0 eV (C-N functional group), and 284.1 eV (C=C group) (Chen et al., 2020). The N1s spectrum (Figure 6c) reveals three peaks that represent nitrogen bound to various hydrogen atoms at binding energies of 400.3 eV (-NH⁺- group), 399.5 eV (-NH- amine nitrogen group), and 397.8 eV (=N- imine nitrogen group) (Moyseowicz and Gryglewicz, 2019). Additionally, O1s spectrum (Figure 6d) shows three peaks that correspond to the usual oxygen bonds of the -OH, -COOH-, and O-C groups, with corresponding binding energy values of 532.7, 531.8, and 530 eV, respectively (Han et al., 2017).

FIGURE 6

The N₂ adsorption-desorption isotherm studies are used for examining the material specific surface area. A graph of the N₂ adsorption-desorption isotherms for Pare and RP2 sample is displayed in Supplementary Figure S5 (at 77 K temperature of liquid nitrogen). According to the International Union of Pure and Applied Chemistry (IUPAC) standards, it shows the type-IV isotherms, indicating the presence of aggregated mesopores particles (Kulandaivalu and Sulaiman, 2019). The Ppy and RP2 exhibit specific surface areas of 26 and 54 m² g⁻¹, respectively. Consequently, the observed variation in surface area corresponds with some change in RP2 sample surface morphology. RP2 has a higher surface area, indicating that using rGO in composite materials improves their overall surface area. The huge loading surface of rGO nanosheets allows for the formation of Ppy particles, which improve the surface area and C_s of composite electrodes. The way a supercapacitor electrode stores charge depends on its surface morphology. The P3 electrode large surface area and porous structure enhance its capacity to interact with the electrolyte and produce a large number of active redox sites. Due to their larger surface area compared to other electrodes, P3 electrodes exhibit better C_s (Mane et al., 2021).

3.2 Electrochemical studies of ppy, and rGO/Ppy composite thin films

A three-electrode setup was used to investigate the way properly tailored morphologies affect the electrochemical properties of Ppy and RP series electrodes. The electrochemical behaviour of produced Ppy and RP series electrodes was assessed by CV analysis in a 1 M H₂SO₄ electrolyte and these results are displayed in Figure 7. The CV technique is highly effective in comprehending the electrochemical reactions involved in supercapacitive at the electrode and electrolyte interface. It is commonly used to assess reversible faradic reaction and ascertain the electrode or device operating potential window. The comparative CV curves of Ppy and RP series electrodes at a constant 50 mV s⁻¹ scan rate are displayed in Figure 7a, within the potential region of −0.4 to +0.6 V/SCE. A pseudocapacitive charge storage mechanism is shown by the non-rectangular appearance of the CV curves. Compared to other electrodes, RP2 electrode enclose increased area and more current density in the CV curve, showing RP2 electrode may facilitate rapid charge transfer and boost the electrode current response capabilities (Xie et al., 2020). As RP2 offers the highest wettability and the maximum specific surface area, it also has the highest C_s. Figures 7b–e depicts the CV curves for Ppy, RP1, RP2, and RP3 electrodes from 5 to 100 mV s⁻¹ scan rates. For Ppy, RP1, RP2, and RP3 electrodes, the corresponding C_s values at 5 mV s⁻¹ scan rate are 356 ± 7.1, 564 ± 11.4, 987 ± 15.3, and 723 ± 10.8 F g⁻¹, respectively.

FIGURE 7

Further examination of charge storage processes, such as diffusion-controlled and capacitive mechanisms, the power law was applied for CV measurements, as seen below (Equation 5) (Pawar et al., 2024a),here, ‘i_peak’ represents maximum current determined at the oxidation potential, whereas ‘a’ and ‘b’ are variables. The ‘b’ value is determined by the main charge storage mechanism, which can range between 0.5 and 1. Charge storage is mostly focused on the inner side of the electrode if b is near to 0.5, driven by diffusion of electrolytic ions and redox reactions. If b = 1, total charge storage is dominated by surface adsorption (Nwaji et al., 2023). For Ppy, RP1, RP2, and RP3, the values of ‘b’ computed from log i vs. log ν graph (Figure 7f) are 0.53, 0.62, 0.84, and 0.66, respectively indicate that the diffusion process is the most effective way to store the largest amount of charge. The RP2 electrode b values, which range from 0.5 to 1 i. e 0.84, show that RP2 electrode overall capacity to store charge was influenced by both capacitive and diffusive processes (battery type). Compared to 0.5, the value 0.84 is substantially closer to 1.0. This demonstrates that the majority of the reaction rate is controlled by surface and near-surface mechanisms. The residual gap below 1.0 demonstrates that bulk diffusion continues to have a negligible, quantifiable impact on the total electrochemical processes. Link the 0.84 value to the physical traits of RP2, such as open channels, porous structure, and maximum mass loading that ease ion movement. The rGO served as a spacer and provided pathways for charge transfer, which ultimately led to the development of smaller Ppy particles. These tiny Ppy particles placed between rGO sheets quickly undergo redox processes (Patil et al., 2017). A modified power law Equation 6 is applied to calculate the approximate contribution from both of these charge storage mechanisms in the total current (Malavekar et al., 2021),where k₁ν and k₂ν^0.5 indicate the represent the current contributions from surface and bulk redox processes on the material inner side, respectively. Using Equation 6, the determined currents for capacitive and diffusion-controlled mechanism for Ppy and RP series electrodes are depicted in Figures 8a–d along with their respective contributions. The C_s value rises as drops the scan rate due to a rise in the diffusion-controlled mechanism current. At a scan rate of 100 mV s⁻¹, RP2 electrode stores around 20% of charge by diffusion-controlled processes. However, at a 5 mV s⁻¹ scan rate, this increases to 53%. The porous structure of RP2 electrode promotes faster electrolyte ion penetration, leading to diffusion-controlled process contribution at low scan rate (Chodankar et al., 2016). At higher scan rate charge storage contribution done by capacitive behavior kinetics.

FIGURE 8

The comparative GCD graphs of Ppy and RP series electrodes at a of 1 A g⁻¹ current density are depicted in Figure 9a. The GCD technique is often utilized to evaluate C_s of electrodes and devices as well as the stability of electrochemical cycles. The RP2 electrode has the longest discharge time duration in comparison to the other electrodes, showing its better electrochemical characteristics. The GCD plots with 1–3 A g⁻¹ current densities for Ppy, RP1, RP2, and RP3 electrodes are depicted in Figures 9b–e. At a current density of 1 A g⁻¹, the C_s values of Ppy, RP1, RP2, and RP3 were determined from GCD are 287 ± 10.5, 531 ± 11.8, 888 ± 14.4, and 706 ± 11.2, respectively. Figure 9f shows that C_s decreases with scan rate for all electrodes due to the shorter duration of reversible redox reactions at increased scan rates. The interaction between Ppy and rGO, and the significant electroactive surface area supplied by the spherical particle-like morphology over rGO sheets, are responsible for the enhanced electrochemical characteristics of RP2 electrode.

FIGURE 9

The EIS technique, also referred to as AC impedance spectroscopy, is one of the most effective ways to comprehend the impedance behaviour of supercapacitor at the electrode and electrolyte interface. For EIS, a little AC signal with frequencies that vary between several mHz to kHz is used to charge the supercapacitor cell. Responding to applied AC signals, the output signals represent the current. The Nyquist plots are analyzed with an amplitude of 10 mV at frequencies ranging from 0.1 Hz to 100 kHz, to ascertain the ion diffusion and the electrode material electrical conductivity. The Nyquist graphs display half semicircles in the higher frequency region and excellent capacitive behaviour with straight lines in the lower frequency region, as illustrated in Figure 10a. The electrolyte resistance (R_s), charge transfer resistance (R_ct), constant phase element (Q), Warburg impedance (W), and goodness of the fit (χ²) constitute the equivalent circuit used to match the EIS data, as shown in the inset of Figure 10a. The R_s values are 0.56, 0.42, 0.15, and 0.37 Ω cm⁻², while those of R_ct are 120, 82, 50, and 64 Ω cm⁻² for Ppy, RP1, RP2, and RP3 electrodes, respectively. The computed R_s, R_ct, W, Q, and χ² values are summarized in Table 1. A comparatively small value of R_s and R_ct indicates that RP2 has superior charge transfer rates because of its porous electrode structure, which increases electro-active surface for electrochemical processes. To assess the electrochemical stability of Ppy and RP2 electrode, 3,000 GCD cycles were performed with a current density of 3 A g⁻¹. After 3,000 cycles, Ppy, RP1, RP2, and RP3 electrodes demonstrated an excellent 76, 81, 87, and 84% capacitive retention, respectively (Figure 10b). This is ascribed to the optimised composition of rGO and Ppy in the self-supported electrode. For supercapacitor applications, Table 2 compared the C_s and electrochemical stability of RP2 electrode with previously reported Ppy and rGO composite electrodes (Hao et al., 2021; Shibing and Jiachun, 2014; Xu et al., 2015; Shu et al., 2016; Zhang et al., 2017; Wang et al., 2022; Alves et al., 2017). Optimization of polymerization time in rGO/Ppy composite material led to high electrochemical properties and optimal stability for RP2, showing that the impact of the synthesis method for producing composite materials.

FIGURE 10

3.3 Electrochemical studies of rGO/Ppy//H₂SO₄//WO₃ device

The ASC device was built to test the practical level feasibility of prepared electrodes for charge storage. The ASC device contains RP2 and WO₃ electrodes as cathodes and anodes in aqueous 1 M H₂SO₄ electrolyte, respectively. Figure 11a shows a schematic representation of rGO/Ppy//H₂SO₄//WO₃ device. Supplementary Material (Sections 3 and 4), as well as Supplementary Figures S6, S7, reveal a structural and electrochemical study of WO₃ negative electrode. The ASCs are a novel type of device that consists of two distinct electrode materials with different mechanisms for storing charge or electroactive materials with variable ratios of redox active sites. The operating voltages of each electrode determine the final voltage of ASCs. The balance of the charges stored (q) on the two component electrodes is required to reach the maximum operating potential and S_E i.e., q- = q+. To establish a proper charge balance, the mass ratios of the two electrode materials were determined using the following Equation 7 (Pawar et al., 2024b),here, ‘ΔV’ is the operating potential window (V), ‘m’ is deposited mass (g), and ‘C_s’ is the specific capacitance (F g⁻¹) of the particular electrode. The values of cathode and anode are shown by the ‘+’ and ‘–’ sign symbols, respectively. Based on this formula the m⁺/m ^̶ value is 0.36.

FIGURE 11

The CV curves with different potentials (between +0.8 and +1.3 V) are measured at a constant 50 mV s⁻¹ scan rate in order to optimize the electrochemical potential window of the ASC (Figure 11b). The device displays a larger area enclosed by the CV curve at an operating voltage of +1.3 V, which probably results in an increase in S_E of ASC device. The device charge storage properties were analysed by recording CV curves from 5 to 100 mV s⁻¹ scan rates and GCD plots from 1.5 to 3.5 A g⁻¹ current densities (Figures 11c,d). Through CV analysis, an optimal C_s of 61 F g⁻¹ is showed at a scan rate of 5 mV s⁻¹. Additionally, the C_s value derived from GCD is 50 F g⁻¹ at a current density of 1.5 A g⁻¹. As scan rates and current densities increased, C_s values declined due to restricted charge storage time. The expected ASC device electrochemical characteristics reveals the enhancing actions of the faradic rGO and pseudocapacitive Ppy electrode, which maintain perform even at more current densities.

The rGO/Ppy//H₂SO₄//WO₃ ASC device charge transfer mechanism is assessed using the EIS technique at frequencies between 0.1 Hz to 100 kHz and the fitted equivalent circuit is shown in the inset (Figure 12a). The initial X-axis intercept indicates a R_s value of 0.3 Ω cm⁻², while the diameter of the preceding semicircle indicates a R_ct value of 7 Ω cm⁻². A straight line represents the W (0.010 Ω cm⁻²). This low impedance improves electrochemical processes at the electrode-electrolyte contact without causing a considerable ohmic loss (Sadavar et al., 2022).

FIGURE 12

Longer supercapacitor lifespan reduces maintenance costs for portable electronic equipment, leading to additional practical applications. The rGO/Ppy//H₂SO₄//WO₃ ASC device stability curve, measured at a 3.5 A g⁻¹ current density after 3,000 cycles is depicted in Figure 12b. The ASC exhibits 74% cyclic stability over 3,000 GCD cycles. The relatively stable cycling behavior can be attributed to the rGO/Ppy composite architecture, in which the rGO framework provides an electrically conductive and mechanically supportive network for Ppy. In addition, the porous structure facilitates electrolyte-ion transport and provides accessible active sites, which can help reduce the adverse effects associated with repeated Ppy redox reactions. The observed capacitance loss can be mainly associated with the repeated volume expansion and contraction of Ppy during the reversible doping/de-doping processes occurring during charge–discharge cycling. During these processes, electrolyte ions repeatedly enter and leave the Ppy matrix, resulting in structural strain. Continuous cycling can consequently induce expansion/shrinkage, loss of electrical contact, and gradual deterioration of the electrochemically active Ppy sites. These effects can reduce the effective electrode/electrolyte interface and hinder ion and electron transport, ultimately leading to the observed decrease in capacitance. In addition, the rGO/Ppy composite architecture can partially alleviate, but cannot completely eliminate, these degradation processes. This composite structure helps maintain the electrode integrity during repeated cycling; however, prolonged repeated redox-induced structural changes in Ppy can still result in irreversible morphological and interfacial changes. Supplementary Figure S8 displays XRD and SEM analyses of RP2 electrode both before and after cycling stability. XRD analysis revealed no appreciable changes in the XRD pattern during charging-discharging cycles (Supplementary Figure S8a). The RP1.0 electrode XRD pattern exhibits less intense peaks upon cyclic stability than before to cycling. The results demonstrate that in an H₂SO₄ electrolyte solution, graphite crystallinity reduces during potential cycling. (Supplementary Figures S8b,c) show FE-SEM images of RP2 electrode surfaces before and after cyclic stability at ×5,000 magnification. After 3,000 cycling, morphologies are slightly changed. The charging and discharging mechanism randomly distribute interconnected Ppy spherical particles across rGO sheets, resulting in a rough surface. The moisture content causes randomly distributed aggregates to form a rough surface on the sample.

Additionally, Figure 12c displays the computed S_E and S_P values for the ASC device. The ASC device achieved a maximum S_E of 11 Wh kg⁻¹ at 0.474 kW kg⁻¹ of S_P. Compared to earlier reports for rGO/Ppy electrode-based energy storage devices, the S_E and S_P for the ASC device are better. (Lim et al., 2014) created a device based on C-Ppy/graphene and achieved S_E of 8 Wh kg⁻¹ at 115 W kg⁻¹. Using a Ppy/rGO electrode, Zhu and coworkers (Zhu et al., 2015) developed a device and reported a S_E of 7 Wh kg⁻¹ at 89 W kg⁻¹. (Majumder et al., 2017) developed a device based on Ppy/CuO/Eu₂O₃–2 and demonstrated S_E of 4 Wh kg⁻¹ at 249 W kg⁻¹.

4 Conclusions

In conclusion, the binder-free CBD method was used to develop amorphous, Ppy spherical nanoparticles over rGO sheets. Changing the deposition time, which modifies the growth kinetics of RP series electrodes and affecting its physico-chemical characteristics. As a result, an optimized deposition time (RP2 electrode) produces an optimum growth rate, which produce the porous morphology, resulting in an increase in wettability, and improves electrochemical activity. The rGO and Ppy inherent redox properties, result in increased electrochemical performance, with optimal C_s of 987 F g⁻¹ at a scan rate of 5 mV s⁻¹. The enhanced electrochemical properties of RP2 composite was ascribed to large number of redox active electrochemical sites, quick charge-transporting channels, high surface area due to random stacking of rGO sheets and Ppy particles. Also, in the ASC device fabrication, the optimized RP2 electrode serves as the cathode and WO₃ serves as the anode. The ASC device showed a significant C_s of 61 F g⁻¹, along with a S_E of 11 Wh kg⁻¹ and S_P of 0.474 kW kg⁻¹. This work demonstrates the effectiveness of conducting rGO as a composite material can boost the electrode properties of Ppy.

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