Electrochemical Performance of Fe₂O₃@PPy Nanocomposite as an Effective Electrode Material for Supercapacitor

6.7 ml pyrrole (monomer) added dropwise slowly with 100 ml distilled water on the magnetic stirrer with continuous rotation for one hour. After that, the 9.8 ml H₂SO₄ was added dropwise slowly with a continuous rotating magnetic stirrer for one hour. Then 20 ml distilled water with added slowly to 2.8 g of K₂Cr₂O₇ with a continuous rotation using with magnetic stirrer up to reach the saturated state. Hence K₂Cr₂O₇ solution with slowly added to the pyrrole solution. The stirring process was held continuously overnight (24 h). These whole processes are carried out within the ice path and its atmosphere. Finally, the resultant polypyrrole centrifuged methods after 24 h on the magnetic stirrer and Oven at 80 °C for 12 h.

The FeCl₃.6H₂O is diluted in 200 ml distilled water and stirred for 30 min at 80 °C. After that, NH₃OH was added dropwise slowly reaching the pH value up to 11 and thus the solution was stirred for 2 h at 80 °C. These whole processes are carried out within the ice path and its atmosphere. The resultant dried for 12 h at 80 °C in a hot air oven after the centrifuge method to obtain the Fe₂O₃.

One gram of Fe₂O₃ is added to the mixture of 100 ml distilled water and add dropwise 9.8 ml H₂SO₄ within the solution. Hence continuous stirrer 10 min after that addition of 6.7 ml (monomer) of pyrrole adds to the solution and continuous stirrer in one hour. K₂Cr₂O₇ (2.8 g) in 20 ml distilled water dilute and stirrer under ice path set up for one hour leads to Fe₂O₃+PPy to get 3.2 g of yield. (Fig. 1 shows the Overall Schematic Diagram for PPy/Fe₂O₃ nanocomposites Sample Preparation). Table I. Shows the chemical concentration of Ppy, Fe₂O₃ and Ppy@Fe₂O₃ nanocomposites.

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The crystal nature, morphological and elemental properties of the as-prepared samples are explored through X-ray diffraction (XRD, Rigaku D/Max Ultima III), Fourier transform Infra-red spectroscopy (FTIR, SHIMADZU, IRTRACER 100), Field emission-scanning electron microscopy (JSM-7500F), Transmission electron microscopy (TEM, Hitachi H-600) and Energy dispersive X-ray spectroscopy analysis (JSM-7500F). Further, the chemical composition of the as-prepared sample was assessed through X-ray Photoelectron spectroscopy (XPS, ESCA 3400 spectrometer). All electrochemical characterization like CV, GCD and EIS has been carried out using with SP-150, Biologic Science Instruments, and France). The working electrode slurry paste was prepared through 80:10:10 of active material: carbon black: PVDP are mixed with NMP solution. The as-prepared slurry was pasted on Ni-Foam (1*3 cm²) and an dried in electric oven. Herein, 3 M KOH electrolyte was used to analyze the electrochemical performance of the as-prepared working electrode. The measurements were recorded for their respective conditions. The specific capacitance was evaluated from CV and GCD curves for several current densities according to the following Eqs. 1 and 2,

$$\begin{eqnarray}&&{{\boldsymbol{C}}}_{{\boldsymbol{s}}}=\displaystyle \frac{\displaystyle \int i{dv}}{2\,* \,s\,* \,m}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{{\boldsymbol{C}}}_{{\boldsymbol{s}}}=\displaystyle \frac{{\boldsymbol{i}}{\rm{\Delta }}{\boldsymbol{t}}}{m{\rm{\Delta }}V}\end{eqnarray} \tag{ 2 }$$

Herein, $\displaystyle \int i\,{\boldsymbol{dv}}$ is the integral area of the CV curve, s is the scan rate (V s⁻¹), i is the current (A), Δt is the discharging time (sec), m is the loading mass (g) on the electrode surface, and ΔV is the applied potential window (V).

Figure 2 depicts the elemental vibrational modes of PPy, Fe₂O₃, and PPy/Fe₂O₃ nanocomposites studied using FTIR spectroscopy. The peaks of the monomer component of PPy were detected at 792 cm⁻¹ and 926 cm⁻¹, and these peaks are associated with the C–H wagging. The distinctive function peak at 1049 cm⁻¹ is due to C–H in plane deformation vibration, whereas the usual peak seen at 1114 cm⁻¹ is related to C–H out-of-plane deformations. N–C and C–H stretching vibrations occurred at 1305 and 1204 cm⁻¹, respectively.

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The stretching vibration of the pyrrole ring and the monomer (pyrrole) ring are sensed at 1474 cm⁻¹ and 1559 cm⁻¹, respectively. The N–H stretching and C–N stretching vibrations are responsible for the peaks at 3117 cm⁻¹ and 1684 cm⁻¹. The observation of all distinctive peaks confirms the synthesis of PPy by its components. The Fe–O vibration modes are responsible for the peaks in the Fe₂O₃ spectra at 1023 cm⁻¹ and 679 cm⁻¹. Peaks in O–H bending vibrations were found at 1416 cm-1 and 567 cm⁻¹. In the spectrum of Fe₂O₃@PPy (Fig. 2), the peak at 3436 cm⁻¹ is due to the −OH group, while the three characteristic peaks of PPy shifted near higher wave numbers specify the interactions between PPy and Fe₂O₃. Peaks at 554 and 680 cm⁻¹ in the PPy/Fe₂O₃ nanocomposite spectrum are attributable to Fe–O bond stretching, confirming the existence of Fe₂O₃ in the nanocomposite. The exclusion of peaks 620 cm⁻¹, 792 cm⁻¹, 1049 cm⁻¹, and 1305 cm⁻¹ for polypyrrol and 790 cm⁻¹, and 1689 cm⁻¹ for Fe₂O₃ in the Fe₂O₃@PPy spectrum, as well as the presence of some new peaks such as 930 cm⁻¹, 1116 cm⁻¹, 1563 cm⁻¹, and 2925 cm⁻¹, confirms the formation of chemical composition to establish a Fe₂O₃@PPy nanocomposites.

X-ray diffraction study majorly used for analyzing the amorphous ⁵¹ and crystalline structure of the prepared samples to confirm its diffraction performance for the pure PPy, Fe₂O₃ and Fe₂O₃@PPy composites towards the complex formation confirmation. Figure 3 shows the XRD profile for prepared pure PPy, Fe₂O₃, and Fe₂O₃@PPy nanocomposites. The diffraction (2θ) peak broadly identified between 10°–30° has previously been reported as nearly 2θ between 20°–28° by A. Kassim et al. and resembles the characteristic peak of pure PPy. ⁵² Which is clearly confirms the pure polypyrrol (Fig. 3a) in the nature of amorphous without any further crystalline peaks. Figure 3b shows the some crystalline peaks for pure Fe₂O_3, As per the observation of the JCPDS data Card no. 13–534, the diffraction peaks are shifted from the original value of Fe₂O₃ such as (102), (110), (113), (024) and (116) planes of Fe₂O₃ at 2θ = 27.98°, 38.54°, 41.40°, 47.72°, and 57.81° respectively.

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The appearance of the broad peak occurred in Fig. 3c for the diffraction of PPy/Fe₂O₃ nanocomposites. Thus similar to pure PPy without any Fe₂O₃ crystalline peaks, which means the Fe₂O₃ completely mixed with polymer and that the absences of crystalline peaks in Fe₂O₃@PPy nanocomposites confirm the complex formation between the PPy and Fe₂O₃.

Figures 4 and 5 show scanning electron micrographs and EDAX images of PPy, Fe₂O₃, and Fe₂O₃@PPy nanocomposite films at different magnification. Figure 4 depicts the structure of a polypyrrole thin film (a). The PPy film has a high porosity and a homogenous granular spherical shape. Figure 4 depicts a well-defined linked hexagonal nanocrystalline morphology of Fe₂O₃ (b). Every Fe₂O₃ nanoparticle is totally covered by PPy in the SEM micrograph of the Fe₂O₃@PPy hybrid nanocomposite [Fig. 4c]. Because of the availability of diffusion channels within the polymeric network, the introduction of Fe₂O₃ nanoparticles into the polymeric network provides some porosity and is favorable for electrochemical applications. This is because conduction happens more easily in porous structures, increasing the charge-discharge between the anode and the cathode.

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The uniform surface morphology of Fe₂O₃@PPy through the scanning electron microscope with an energy dispersive X-ray (EDAX) profile given in Fig. 4 for prepared nanocomposites (Fe₂O₃@PPy). The traced elements are dispersed as 58% (C K), 28% (O K), 7% (N K) and 7% (Fe K) in prepared Fe₂O₃@PPy composites sample. Significant volume of carbon was perceived for PPy/Fe₂O₃ as homogenously distributed throughout the entire surface of Fe₂O₃@PPy. ⁵³ The detector region of interest (ROI) observed as follows 11 (Carbon), 10 (Oxygen), and 6 (Nitrogen and Iron), which is confirm the presence of PPy and Fe₂O₃ in a uniform manner on the surface to create a porous nature of structural morphology to enhance the electrical properties.

Figures 5a–5d shows the trace of the Transmission Electron Microscope image for PPy/Fe₂O₃ nanocomposites. It is perceived that the black shade of Fe₂O₃ nanocomposites is captured into the light shade space of PPy surfaces with a regular diameter of 100–200 nm. It is confirmation of the nanocomposites in the range of nanoscale besides confirm the binding energies for the formation of Fe₂O₃@PPy nanocomposites. The different magnification of TEM micrographs for Fe₂O₃@PPy nanocomposites shows with a the clear shape and little aggregation. Simultaneously, TEM also exhibited that Fe₂O₃@PPy occurred in the range of nanometer scale with the sample was in the nature of crystalline. The presence of Fe₂O₃ trapped particles observed with the black shades and the TEM images indicate the presence of polymer chain with the light shade.

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The XPS is used to determine the valence states of Fe₂O₃@PPy, the evident of XPS spectrum given in Fig. 6. The high resolution Fe 2p spectrum shows two core peaks at 700 eV (Fe 2p) and 720 eV (Fe 2p) with a separation of 20 eV in addition with two peaks, which is significantly approximating that of earlier report of Fe₂O₃. ^54,55 The higher resolution peak of C 1s observed at 290 eV. The peak for Fe de-convoluted into four subpeaks at 695, 705, and 720 eV, consistent to sp2 graphitic lattice (C–C and C=C), in addition with the functional group of O–C=O, C=O, and C–O. ⁵⁶ The functional group of Fe–O–H, C=O, O–Fe and H–O–H high resolution for O 1s spectrum observed as shifting from 530.4 eV to 540 eV. ^57,58

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XPS is used to analyse the surface chemical nanocomposites as well as to reveal the presence of Fe₂O₃@PPy nanocomposite. The full range spectrum of Fe₂O₃@PPy, as shown in Fig. 6, reveals the presence of five major elements: C, O, N, S, and Fe, with atomic masses of 61.7%, 23.5%, 11.7%, 2.8%, and 0.3% respectively. The 1.5 c/s of resolution is N 1 s scan indicates the presence of nitrogen namely pyrrolic N (400.1 eV, 11.7%) from PPy. ⁵⁹ Herein, this nitrogen can act as a catalytic active cite in the oxygen reduction reaction. ⁶⁰ Obviously, the chemical bonds of nitrogen in PPy have been reformed into other types of nitrogen subsequently high temperature treatment and hydrothermal reaction. The Fe 2p spectrum in Fig. 7 shows three peaks in 695 eV, 705 eV and 720 eV, corresponding to Fe 2p^3/2, Fe 2p^1/2, and Fe 2p which can be used for qualitatively signifying the ionic states of iron. ⁶¹ The peak at around 720 eV is consistent with the characteristic peak of Fe³⁺, proving the presence of Fe³⁺. ⁶² The primary oxygen containing clusters are O²⁻, implying that the presence of ferric is Fe₂O₃ rather than FeOH. The presence of the OH⁻ class on the surface of Fe₂O₃@PPy is determined by the dominant peak of hydroxyls at 540 eV, which is consistent with the results of FTIR analysis. ⁶³

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Brunauer–Emmett–Teller's (BET) theory aims to explain the physical adsorption of gas molecules on a solid and serves as the basis for an important analysis technique for the measurement of the specific area of materials. The surface area is one of the most important quantities for characterizing novel porous materials. The BET analysis is the standard method for determining surface areas from nitrogen adsorption isotherms and was originally derived for multilayer gas adsorption onto a flat surface. Table II. shows the surface area and pore size of the Fe₂O₃ and Fe₂O₃@Ppy nanocomposites.

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The surface area and pore diameter of the nanomaterial were characterized by nitrogen (N₂) adsorption and desorption isotherms at 77.3 K using a Quantachrome NOVA Station A. The nitrogen adsorption/desorption isotherms measurements were performed to identify the surface area and pore diameter distribution of the prepared materials. The BET surface of Fe₂O₃, relative pressure increases up to 0.8 (p/p_o) at volume remains constant then the relative pressure increases and also volume adsorbed increases. In the BET surface composite of Fe₂O₃/PPY, relative pressure increases up to 1.0 (p/p_o) and volume adsorbed increases. The BET pore diameter of Fe₂O_3, volume decreases at the same time pore diameter is constant the pore diameter increases volume remains constant. The BET pore diameter composite of Fe₂O₃/PPY volume decreases particular value then again increases, then volume decreases pore diameter increase as shown in Fig. 7.

The electrochemical impedance spectroscopy (EIS) study has been acquainted as one of the major methods for analytical the electrical and ionic conductivity of the vital recital of supercapacitors. ^64,65 The EIS spectra are closely parallel in technique with by a point at lower frequency and an arc at higher frequency range, accompanying represent of the extensive time of an electrochemical stability of the composites material. At actual higher frequencies, the interrupt at real axis (Z') and imaginary axis (Z'') are signifies a collective resistance of solution resistance of electrolyte medium, characteristic resistance of substrate further more contact resistance at the electroactive material/current collector interface which mean electrode resistance (R_e). ⁶⁶ Figure 9 shows the Nyquist plot for (a) pure PPy, (b) Fe₂O₃ and (c) Fe₂O₃@PPy nanocomposite were swiped at 1 Hz to 1 MHz and subsequent derivation of real and imaginary part of resistance recorded from 0 ohm to 940 ohm for pure PPy, 0 ohm to 8 ohm for pure Fe₂O₃ and 0 ohm 6 ohm for Fe₂O₃@PPy. The evident of the lower resistance of 2 ohm for PPy, 1 ohm for Fe₂O₃ and, 0.5 ohm for Fe₂O₃@PPy, which is mentioned as the Fe₂O₃@PPy based nano composites support to the electrode performance in the course of the of electrochemical supercapacitor function and that the consistent report will be deliberated in Figs. 8a3–8c3. Inset, the figure shows the equivalent circuit model of the Fe₂O₃/PPy nanocomposite. From this circuit model, the Fe₂O₃@PPy nanocomposites show a lesser solution (0.5 Ω) and charge-transfer (0.31 Ω) resistance.

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The Cyclic Voltammetry curves of PPy, Fe₂O₃ and Fe₂O₃@PPy nanocomposite carried out at various scan rates in 3 M aqueous KOH electrolyte. In Fig. 7, the CV curves at minor scan rate from 5, 10, 20, 30, and 40 mV s⁻¹ exhibit a pair of redox peak profiles specifying the pseudocapacitive behavior and high reversibility. CV curves are leads to be quasi rectangular curve even at highest scan rate at 40 mV s⁻¹ is indicating an effective redox charge storage behaviour of Fe₂O₃@PPy nanocomposites. The pure PPy current density is starting as 25, 50, 100, 112.5, and 125 mA for the scan rate of potential range 5, 10, 20, 30, and 40 mV s⁻¹ respectively. The pure Fe₂O₃ current density is starting from 2.5, 10, 20, 26 and 32 mA for the scan rate of potential 5, 10, 20, 30, and 40 mV s⁻¹ respectively. The nanocomposite Fe₂O₃@PPy current density is occurring as follow 29, 40, 48, 50, and 56 mA, respectively. Depictive of the shape of oxidation and reduction curves of CVs at higher scan rate 40 mV s⁻¹ is remaining due to the most possible contact between the electrolyte and electrode is greatly condensed with the cumulative scan rate. The as-prepared Fe₂O₃@PPy nanocomposites shows the highest capacity of 395.45 C g⁻¹ at 5 mV s⁻¹. This remarkable electrochemical results of Fe₂O₃@PPy due to provide the huge electroactive sites in the electrolyte medium. Table III. Shows the comparison results of the previously published research articles.

Figure 7 shows the galvanostatic charge–discharge curvatures at 0–0.4 V sweep rate, in addition that the galvanostatic charge discharge curves of current density of obtained 1, 3, 5, 7, 9, 11, 13 and 15 A g⁻¹ for pure PPy (Fig. 7a), pure Fe₂O₃ (Fig. 7b) and Fe₂O₃@PPy (Fig. 7c) nanocomposite, respectively. The as-prepared all nanomaterials are shown the non-linear charging and discharging profile, which is confirm the higher significance of pseudocapacitance behavior. However, the Fe₂O₃@PPy nanocomposite displays an ideal quadrilateral form, the pure PPy has an ovate form, which can be credited to the circumstance that PPy has higher conductive and internal resistance, and the Fe₂O₃ in the nanocomposite of PPy/Fe₂O₃ enable the interfacial for electron transport. ⁶⁷

An extensive cycling life is an essential prerequisite for electrochemical supercapacitors. The cycling life investigation over 10,000 cycles for the Fe₂O₃@PPy nanostructure electrode was characterized using GCD test at higher current density of 10 A g⁻¹ for 10,000 cycles as shown in Fig. 9. Hence, the Fe₂O₃@PPy nanostructure electrode is retaining of 98% after 10,000 cycles for long-term stability analysis.

The results achieved in this present effort direct for simplistic synthesis and optimization of Fe₂O₃@PPy nanocomposite constituents for the purpose of supercapacitor electrode material for energy storage applications.