Evolution of waste iron rust into α-Fe2O3/CNF and α-Fe2O3/PANI composites as an efficient positive electrode for sustainable hybrid supercapacitor

A green and sustainable approach to recycle the waste iron rust into a valuable α modification of Fe2O3 via simple grinding and calcination for application in a hybrid supercapacitor is reported. The α-Fe2O3 was coupled with carbon nanofibers (CNFs) and conducting polymer, polyaniline (PANI), to form composite hybrid supercapacitor electrode materials. The conventional hydrothermal, electrospinning, and in-situ polymerization processes were used to prepare composites. Further, X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), scanning electron microscopy (SEM), and energy-dispersive x-ray (EDAX) spectroscopy were used to study the structural, morphological, and compositional properties of the as-synthesized α-Fe2O3 and its composites with CNF and PANI. The α-Fe2O3/CNF and α-Fe2O3/PANI composites, coated on carbon rod, were used as electrodes in a three-electrode system to study electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge–discharge (GCD) in 1 M H2SO4. The XRD studies revealed the formulation of iron rust into α-Fe2O3 modifications with an average of 28 nm crystallite size. Uniform dispersion of α-Fe2O3 over CNF of 400–500 nm diameter and excellent covering of PANI over α-Fe2O3 nanomaterials were the morphological features observed for α-Fe2O3 /CNF and α-Fe2O3 /PANI composites, respectively. The electrochemical studies on α-Fe2O3/PANI composites exhibit higher performance as against Fe2O3/CNF with respect to specific capacitance, 192 Fg−1 (88.88 Fg−1); energy density, 11.28 Whkg−1 (3.084 Whkg−1); power density, 162 Wkg−1 (69.39 Wkg−1); and capacitance retention of 80% (75%) after 5000 charge–discharge cycles. The heavy dispersion of α-Fe2O3 over long CNF and PANI fibers with intimate contact resulted in abundant active sites for electrochemical reactions leading to the obtained result. The rust-derived α-Fe2O3 with PANI offers excellent stability to act as a potential candidate for sustainable hybrid supercapacitor application.

abundant active sites for electrochemical reactions leading to the obtained result. The rust-derived a-Fe 2 O 3 with PANI offers excellent stability to act as a potential candidate for sustainable hybrid supercapacitor application.

Introduction
The past few decades have witnessed rapid development in industrial automation, transportation, and a huge upsurge of modern household appliances. Ever-growing industrialization and urbanization, however, demand a huge amount of electrical energy to work with [1]. Today, most of the energy demands of the world are fulfilled by using non-renewable energy resources. The huge utilization of these resources, however, leads to depletion and eventually finishes off the reservoirs, global warming, pollution, and climate change [2]. At the outset, the alternative renewable energy resources, although clean, eco-friendly and abundant, demand an efficient energy harvesting system and smart storage devices to store the energy efficiently for later use. Because of the global concern, the development of efficient, clean energy devices utilizing natural materials and/or materials with a greener synthetic approach is especially challenging [3,4]. In this regards, the rapidly developing field of nanotechnology provides not only the exciting opportunities to design materials having desired electronic, chemical, mechanical, and morphological characteristics but also the greener synthetic approach for applications in the fields of advanced energy storage (batteries and electrochemical capacitors) [5,6], environmental remediation [7], photocatalysis [8,9], hydrogen production [10,11], electronics and energy conversions [12], sensors, etc., [13] Recently, a lot of interest is developed to explore energy storage materials for use in batteries, supercapacitors, and fuel cells. The supercapacitor, among them, have emerged as an eco-friendly and safe means of energy storage device due to its characteristics, such as comparatively small size, high power density (5-10 kWkg -1 ), long cycle life ([ 500,000 cycles), long shelf life, high efficiency (95%), and wide operating temperature range (-40 to ? 70°C). It is being considered as a future energy storage system that probably complements or even replaces conventional batteries and fuel cells [14].
The basic energy storage mechanism of the supercapacitor involves either charge separation at the electrode-electrolyte interface (electric double-layer capacitors, EDLC) or the Faradic redox reactions between the electrolyte and the electrode material (pseudo-capacitors). Carbon-based materials, such as graphene, carbon nanofibers (CNFs), activated carbon (AC), and carbon nanotubes (CNTs), serve as the main electrode materials in EDLC [15,16] with onedimensional carbon mesoporous nanofibers being popular [17].
So far, many efforts are being made to develop carbon fibers using verities of methods involving more or less complexity in preparation [18]. Electrospinning, one of the methods, working under precisely controlled conditions, has emerged as a versatile, efficient, continuous and industrially viable method to evolve long nanofibers with different morphologies having extremely high aspect ratio and large surface area to volume ratio [19]. Such fibers are highly desirable for use in energy storage applications since they exhibit transportation directionally and short ionic transport lengths [20]. The hydrothermal method, on the other hand, is a facile, low-cost method that has advantages, such as high yield of the products, control over morphology, and reproducibility. So far, several metal oxides and hydroxides have been synthesized using a hydrothermal technique for use as electrode materials in supercapacitors [21].
The EDLC-based supercapacitors exhibit long cycle life but produce relatively low specific capacitance, whereas pseudo-capacitors have high specific capacitance but show slow charge kinetics [22]. The hybrid capacitor combines the characteristics of EDLC and the pseudo-capacitor can exhibit features, like high energy, power densities, and long cycle life [23][24][25]. The pseudo-capacitor electrode materials, the metal oxides, are often coupled with conducting polymers for better electronic communications to yield hybrid-type supercapacitor materials [26]. A huge research literature is available on nanofibers and or polymer-coupled metal oxides exhibiting better performance in supercapacitance. For instance, single-type oxide systems, such as an electrospun-TiO 2 /CNF exhibited a high specific capacitance of 280.3 Fg -1 at current density 1 Ag -1 [27], hydrothermally grown V 2 O 5 /CNF composite exhibited a high capacitance of 227 Fg -1 at current density 1 Ag -1 [28], and MnO 2 /CNF has achieved specific capacitance of 311 Fg -1 at a scan rate of 2 mV/s [29] and ZnO/CNF displayed specific capacitance of 178.2 Fg -1 [30]. Among single oxides, iron oxide (Fe 2 O 3 ) is considered as an important electrochemical electrode material due to (i) its existence in several morphologies and crystallographic modifications [31], (ii) ease in variations in oxidation state (Fe 2?-$ Fe 3? ) (iii) outstanding theoretical specific capacitance (3625 Fg -1 ), (iv) high thermal stability (v) high corrosion resistance, (vi) abundance in nature and environmental friendliness [32][33][34], and (vii) use as positive as well as a negative electrode. Like many other electrodes, Fe 2 O 3 also suffers from poor electrical conductivity leading to lower capability and cycling stability [35]. However, the electrical conductivity can be improved by introducing carbon/ conducting polymer materials into Fe 2 O 3 forming composites [36]. A good kind of literature is available wherein a thermodynamically stable form of iron oxide, a-Fe 2 O 3 (Hematite), coupled with carbon nanofiber via methods, like hydrothermal [27][28][29][30][31][32][33][34][35][36][37][38][39], electrospinning [40,41], vapor growth [42], gel templating [43], and mechanical press [44], has been utilized as the efficient negative electrode in Li-ion/ Na-ion batteries and/or as supercapacitor in -1.2 to 0 V potential range. There are some reports where a-Fe 2 O 3 -based, ternary-type hybrid materials have been utilized as the positive electrode in aqueous electrolytes [45,46]. In all the cases, electrode material fabrication methods have involved the use of toxic, expensive chemicals and complicated chemical and physical treatments that can lead to environmental implications. There are only two reports available in the literature on the greener synthesis of a-Fe 2 O 3 , wherein a-Fe 2 O 3 was obtained by chemical and physical treatments on up-cycled industrial mill scale waste [47] and in another case annealing of a mixture of iron hydroxide and dextran chemicals at various temperatures [48]. However, to the best of our knowledge, there are no reports on the use of rustderived nano a-Fe 2 O 3 for application as a hybrid supercapacitor. The rusting process called corrosion is a weathering phenomenon capable of giving aggregates of nano-scaled iron oxide hydroxide, a rusted product, thus can be a cheap means for getting iron oxide nanomaterial [49] In view of this, we propose a scalable, simple, smart strategy to evolve nano-scaled a-Fe 2 O 3 to recycle huge iron rust waste available. The method is truly greener, cost-effective and requires no chemicals and other expensive treatments. The synthesized a-Fe 2 O 3 was further coupled with CNF and PANI to get binary composite electrode material.

Experimental details 2.1 Materials
The materials used for the synthesis of PANI-CNT were polyacrylonitrile (PAN, Mw = 150,000, Sigma-Aldrich), ammonium persulfate (APS, AR grade, E Merck), hydrochloric acid (HCl, Specific gravity 1.08 g/cc, Sigma-Aldrich), and aniline (Synthesis grade, Sigma-Aldrich). All the chemicals were used without further purification (except aniline, which is doubly distilled and stored in an airtight container in dark) and the solutions were prepared in doubly distilled water or as otherwise stated.
For the preparation of electrodes, the carbon rods, obtained from the exhausted AA-sized, alkaline, battery cell (Eveready make) were used. The carbon black (AR grade, Sigma-Aldrich) and polyvinylidene fluoride (PVDF, AR grade, Sigma-Aldrich) were used as conductivity enhancer and binder, respectively, to prepare the electroactive material on the carbon rod. The N-methyl pyrrolidone (NMP, AR grade, Sigma-Aldrich) was used as a solvent to get a-Fe 2 O 3 /CNF and a-Fe 2 O 3 /PANI deposit on the carbon rod. These deposits were used for measuring electrochemical properties. The N-N-dimethylformamide (DMF, AR Grade, Sigma-Aldrich) was used as a solvent to make the solution for electrospinning. A 0.1 M H 2 SO 4 solution, prepared by appropriate dilution of H 2 SO 4 (AR Grade Sp. gravity 1.840 g/ml, Sigma-Aldrich), was used as an electrolyte for measuring CV, GCD, and EIS characteristics of the composites.
peeling. The rust was ground for 4 h in a laboratoryscale stainless steel ball mill (Sisco, India) at 60-80 rpm, then cooled to room temperature, and washed several times with deionized water. It was then dried naturally and subjected to annealing in a Muffle furnace at 550°C for 5 h. After the heat treatment, the powder was cooled to room temperature and subjected again to another short cycle of grinding (for an hour) followed by annealing at 550°C (for an hour). Finally, after natural cooling, the product was dried under a vacuum and stored in a desiccator.

Fabrication of carbon nanofibers (CNFs)
The CNF was fabricated using an Electrospinning Assembly (ESPIN-NANO, PECO-Chennai). An electrospinning solution was prepared by dissolving polyacrylonitrile (10 wt %) in N-N-dimethylformamide (DMF) with a constant magnetic stirring for 2 h at 70°C. The clear solution was loaded into a 10-ml electrospinning syringe. A constant high voltage of 20 kV was maintained between the tip of the syringe needle and the base collector plate. The base plate to needle tip distance was maintained at 15 cm. The electrospinning process was carried out in a closed chamber by injecting solution from a syringe at a 0.8 ml/h flow rate. After electrospinning, the asprepared electrospun PAN fibers were removed and subjected further to a horizontal quartz tube furnace for carbonization. In this process, the PAN fibers were heated for stabilization at 220°C for an hour in the air atmosphere then carbonized at 600°C in the nitrogen atmosphere for another hour and finally kept at 400°C in the nitrogen environment for the next hour for activation. After natural cooling of quartz tube furnace, the CNF was collected and stored in a desiccator.

Preparation of a-Fe 2 O 3 /CNF composite by hydrothermal method
The hydrothermal synthesis was carried out by using a Teflon-lined hydrothermal autoclave (110 ml capacity). A solution for the hydrothermal process was prepared by sonicating 50 mg CNF in 50 ml of deionized water for 3 h to get uniform dispersion. To this dispersion, 100 mg of a-Fe 2 O 3 was added and stirred magnetically for an hour. This mixture was then transferred into a Teflon-coated stainless steel autoclave and heated at 140°C for 8 h. Finally, after natural cooling of autoclave, the content was removed, washed centrifugally, and finally dried at 70°C overnight in a hot air oven to obtain a-Fe 2 O 3 / CNF. Solution B was then transferred to an ice bath of * 5°C temperature. Solution A was then added dropwise into solution B with continuous magnetic stirring at such a rate that does not allow the temperature to exceed 10°C, and the stirring was continued for the next 6 h to complete the process of polymerization. Finally, the product was removed and centrifugally washed several times with deionized water and dried at 80°C overnight in a vacuum oven to get a-Fe 2 O 3 /PANI composite.

Electrode preparation method
For electrochemical testing, the composite (a-Fe 2 O 3 / CNF and a-Fe 2 O 3 /PANI) electrode materials coated on carbon rods via the dip coat method were used as electrodes.
A carbon rod was carefully removed from exhausted AA-sized, alkaline type, and battery cell (Eveready make) and washed thoroughly by water several times and then sonicated for 30 min in distilled water. The dried rods were polished by sandpaper (zero size) followed by successive washings several times with water, alcohol, and acetone and finally subjected to sonication for 30 min in acetone solvent for 30 min. These rods were further dried at 110°C in a vacuum for 2 h, cooled naturally, and stored in a desiccator before electrode fabrication. A coating solution was prepared by thoroughly sonicating a mixture of synthesized composites (a-Fe 2 O 3 /CNF and a-Fe 2 O 3 /PANI), Acetylene black, and PVDF in 85:10:5 weight percent ratio in N-methyl pyrrolidone solvent to obtain a homogeneous dispersion. A clean rod was then dipped in this dispersion and drown off slowly to obtain a uniform coating. Finally, the coated carbon rod was dried at 80°C in an oven for 4 h and used as the electrode for electrochemical measurements. The PVDF was used as a binder, while carbon black to maintain electrical conductivity. The exact weight of the active material coated on the carbon rod was computed by the weight difference technique.

Materials characterization
X-ray diffraction (XRD) measurements on a-Fe 2 O 3 , CNF and a-Fe 2 O 3 /CNF, a-Fe 2 O 3 /PANI composites were performed on Rigaku D/MAX-RB X-ray diffractometer with Cu-Ka radiation at a scan rate of 1 min -1 . The surface morphology and compositional analysis of CNF, a-Fe 2 O 3 , a-Fe 2 O 3 /CNF, and a-Fe 2 O 3 /PANI composites were investigated using Scanning Electron Microscope (Carl Zeiss EVO-18) equipped with energy-dispersive X-ray spectroscopy (EDAX). The detailed morphology, microstructures, and crystalline nature of a-Fe 2 O 3 were further investigated using transmission electron microscopy (TEM, JEOL 3010), high-resolution transmission electron microscope (HRTEM, Tecnai G2, F30), and selected area electron diffraction (SAED). The electrochemical performances of composite electrodes were measured on AutoLab electrochemical workstation (PGSTAT204) using a three-electrode system at room temperature in 1 M H 2 SO 4 electrolyte. For electrochemical characterization, a-Fe 2 O 3 /CNF and a-Fe 2 O 3 /PANI composites coated on graphite rod were used as the working electrode, a platinum wire as the counter electrode, and Ag/AgCl (calomel, 3.5 M KCl) as the reference electrode. The composite electrodes were dipped in electrolytes at least for an hour before actual measurements to establish the equilibrium. The cyclic voltammetry was performed at scan rate 5-100 mV/s in a potential range of 0-1 V for a-Fe 2 O 3 /CNF and -0.5 to 0.8 V for a-Fe 2 O 3 / PANI. The capacitance of the electrode was galvanostatically measured in the potential range of 0-1 V for a-Fe 2 O 3 /CNF and -0.5 to 0.8 V for a-Fe 2 O 3 /PANI at current density ranging from 1 to 10 Ag -1 . Electrochemical impedance spectroscopy was performed under open-circuit potential with frequency ranging from 10 -2 to 10 5 Hz.
3 Results and discussion The understanding of the corrosion mechanism of iron structures is necessary to explore the feasibility of rust products to transform them into usable nanomaterial forms by applying simple grinding and annealing technics. Under ambient environmental conditions, the process of rusting begins on the surface of iron structures via the creation of local inhomogeneities in terms of the accumulated dust, moisture, and oxygen, leading to the formation of several local electrochemical/concentration cells. The electrochemical reactions occurring at the cathode and anode lead to the formation of Fe 2? and OHions, respectively. These ions further migrate toward each other and meet in between the cathodic and anodic areas to form a molecular, hydrous, Fe(OH) 2 , FeOOH species as initial corrosion products. These products further grow by accumulating more and more Fe 2? and OHions to form a cluster of loosely held (via weak Vander-Waals and Hydrogen bondings) nanosized particles [49,50]. The corrosion product being porous, permit the access of corrosive environment (water and oxygen) to the local site, thus continuing corrosion further but following the exponential decay law. The natural aging and drying could convert Fe(OH) 2 and FeOOH products to FeO . xH 2 O and Fe 2 O 3 clusters. Simple annealing at this stage was sufficient to evaporate residual coordinated water to form a porous, anhydrous Fe 2 O 3 cluster. The process of strong mechanical grinding of these porous clusters finally breaks into nanoscale range Fe 2 O 3 as observed. It is to be noted that the usual chemical way of nanomaterial synthesis involves nucleation, growth, and termination steps, that may result in hard grains and clusters with the fear of inclusion of impurities.

X-ray diffraction (XRD)
The XRD patterns of the as-obtained a-Fe 2 O 3 , CNF, a- where B is the broadening of diffraction line measured at half its maximum intensity (in radians) and t the diameter of crystal particle, h the diffraction angle, and k is the wavelength of X-ray used [51]. We observed no diffraction peaks corresponding to any impurities/contaminant indicating the high purity of the sample obtained. The XRD patterns of CNF ( Fig. 1, navy blue curve) display a characteristic, broad peak due to (111) plane of carbon material exhibiting interlayer packing of hexagonal carbon sheets. When a-Fe 2 O 3 was coupled with CNF, we observe no change in the crystalline phase of a-Fe 2 O 3 and the XRD peak pattern of a-Fe 2 O 3 /CNF composite (Fig. 1, red curve) keeps the characteristics peak patterns of both the material phases, viz, a-Fe 2 O 3 and CNF, except the diminishing in the intensity of (111) peak of CNF phase which can be attributed to binding of a-Fe 2 O 3 with CNF in the composite. A similar observation was found in the case of the XRD pattern of a-Fe 2 O 3 /PANI composite (Fig. 1, blue curve) which displays a prominent peak corresponding to polyaniline (Emeraldine hydrochloride, JCPDS No 49-2500) and those corresponding to a-Fe 2 O 3 .

Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX)
The SEM images of CNF, a-Fe 2 O 3 , a-Fe 2 O 3 /CNF, and a-Fe 2 O 3 /PANI are shown in Fig. 2a-d Figure 2a shows the morphology of carbon nanofibers with the presence of a continuous interconnected network of fibers with an average diameter of 400-500 nm. The CNF was able to maintain its length and continuity in structure even after carbonization, giving a large aspect ratio and moderate interconnectivity among the fibers. Such type of structure is highly desirable for use in supercapacitor applications as it provides transportation directionality with short ionic length during charge storage [30]. Figure 2b shows the SEM image of the as-prepared a-Fe 2 O 3 , displaying nanocluster, made up of a large number of interconnected a-Fe 2 O 3 nanoparticles having somewhat irregular shapes. The SEM image of a-Fe 2 O 3 /CNF composite in Fig. 2c shows the quite heavy decoration of a-Fe 2 O 3 particles over carbon fibers with some segregated a-Fe 2 O 3 nanoparticles forming tiny plate-like clusters. The fiber network as observed in Fig. 2b was also observed in Fig. 2c indicating that no new morphology is generated in the process. The SEM of a-Fe 2 O 3 /PANI (Fig. 2d) displays quite uniform, thick distribution of PANI as a homogeneous layer over a-Fe 2 O 3 particles. This is especially required as PANI/ CNF provides the conducting surface over the poorly conducting iron oxide. This intimate and heavy junction between a-Fe 2 O 3 and conducting CNF/ PANI is essential for a fast redox reaction occurring during the electrochemical process. The EDAX pattern of a-Fe 2 O 3 /CNF is displayed in Fig. 2f. The presence of carbon, iron, and oxygen peaks and the absence of any other elements/impurities confirm the purity of a-Fe 2 O 3 /CNF composites. Similarly, the EDAX pattern of a-Fe 2 O 3 /PANI (Fig. 2e) displays the presence of nitrogen in addition to carbon, iron, and oxygen without impurities.

TEM, HRTEM, and SAED of a-Fe 2 O 3
The detailed morphology and microstructure of the as-prepared iron rust have been further investigated by TEM and HRTEM. Figure 3a, b shows TEM images of the as-prepared iron rust at different resolutions. The images revealed that iron rust was made up of aggregations of nanoparticles with sizes ranging from 20 to 50 nm, the average matching roughly with those obtained using the Scherrer formula (28 nm). Further, the interplanar distance measured out in the HRTEM image of 0.25 nm (Fig. 3c) can be indexed to the lattice fringes of a-Fe 2 O 3 (110) planes, which is prominently consistent with the results of XRD. Figure 3d shows the SAED pattern of a-Fe 2 O 3 NP's with bright spots in the diffraction circles confirming the crystalline nature of a-Fe 2 O 3 nanoparticles.  Figure 4a, c displays CV profile before cycling, while Fig. 4b, d displays CV profile after 5000 cycles for a-Fe 2 O 3 /CNF and a-Fe 2 O 3 /PANI at 5,10,20,30,40,50,75, and 100 mV/s scan rates, respectively. The approximately symmetrical, rectangular appearance of the CV profile with distinct redox peaks demonstrates the presence of pseudo-capacitive behavior. As revealed from the SEM ( Fig. 3a and b), the excellent interconnectivity established between a-Fe 2 O 3 nanoparticles and CNF greatly improves the electrical conductivity, and provides large surface areas with good interfacial contact and hence better electronic communication resulting in the observed electrochemical performance (Fig. 4a). The a-Fe 2 O 3 / PANI exhibited a much larger capacitive current of 180 mA (Fig. 4c) than Fe 2 O 3 /CNF (3 mA) (Fig. 4a) as against bare a-Fe 2 O 3 , which did not show any comparable current. The highest electrochemical performance of a-Fe 2 O 3 /PANI has resulted from the excellent interconnectivity among PANI fibers and nano-scaled a-Fe 2 O 3 leading to the synergistic effect between the two components. CV curves for a-Fe 2 O 3 /PANI display well-defined pairs of redox peaks during the anodic and cathodic sweeps. These redox peaks confirm the active participation of both a-Fe 2 O 3 and PANI. The sustenance redox reaction executed by a-Fe 2 O 3 /PANI composite during cyclic voltammetry has been proposed based on the facile electron exchange between Fe 3? and Fe 2? (as it exhibits variable oxidations states easily, requires no mediators) [32] and various forms of PANI [52] as under as follows:

(via emeraldine base/salt (EB/ES) intermediate formation).
Reduction: Fe þ3 OFe þ4 O 2 þ e À ! Fe þ2 OFe þ4 O 2 (4) pernigraniline base (fully oxidised) þ e ! leucoemeraldine base (fully reducedÞ: ð5Þ A couple of well-defined redox peaks were observed within 0.2-0.8 V potential window due to reversible interconversions as proposed above. This coupled with the excellent homogeneity between PANI and nanosized a-Fe 2 O 3 particles resulted in a fast faradic process giving high pseudo-capacitance and hence much higher specific capacitance. It is evident that with the increase in the sweep rate, the peak current notably increases keeping the CV profiles intact. A slight potential shift of the redox peaks can be ascribed to the internal charge transfer resistance [53]. With the increase of scan rate, the specific capacitance was found to diminish gradually as usual and considered as normal behavior due to faster diffusion of ions at a lower scan rate than higher [54]. After 5000 cycles (Fig. 4b, d), the area under the curve has decreased for a-Fe 2 O 3 /CNF and a-Fe 2 O 3 /PANI electrodes, indicating a decrease in specific capacitance of both materials.
The fundamental behavior of the electrode material was further explored through electrochemical impedance spectroscopy (EIS). Figure 5a, b shows the Nyquist plot of a-Fe 2 O 3 /CNF and a-Fe 2 O 3 /PANI within the frequency range of 0.01-10 5 Hz at opencircuit potential before cycling (black curve) and after 5000 cycles (red curve), respectively. EIS spectra of a-Fe 2 O 3 /CNF exhibit one depressed semicircle (Fig. 5a) in the high-frequency range and a sloped straight line in the low-frequency region corresponding to characteristic charge transfer process (Rct) and Warburg resistance (W) due to diffused limited process, respectively. The absence of a linear portion in the low-frequency range before and after cycling indicates the presence of negligible surface Warburg resistance in the electrolyte. This behavior is associated with the poor conductivity of a-Fe 2 O 3 making the charge transfer resistance too high so that electrochemical reaction becomes slow, thus making the diffusion process not a rate control step, as a consequence, the Warburg impedance becomes insignificant leading to the absence of line portion at the lowfrequency region [1]. Besides, a slow rise in the Faradic interfacial charge transfer resistance with Fig. 4 Cyclic voltammograms at different scan rates for a-Fe 2 O 3 /CNF a before cycling and b after 5000 cyclings; and for a-Fe 2 O 3 /PANI c before cycling and d after 5000 cyclings cycling (red curve) has been observed. This is associated with the partial deterioration of nanostructure and the corrosion of the current collector during a redox reaction, leading to observed lower conductivity after cyclings [32]. A notable reduction in semicircle diameter for a-Fe 2 O 3 /PANI (Fig. 5b, black curve) than for a-Fe 2 O 3 /CNF displays outstanding electrical conductivity of a-Fe 2 O 3 /PANI, contributing to much superior electrochemical performance. A straight line versus a high angle to the Z' axis before and after cycling has been noticed, suggesting the coexistence of abundant ion diffusion pathways and their rapid movements in the active material leading to differential capacitance [34,54]. Figure 5c, d shows GCD curves of a-Fe 2 O 3 /CNF and a-Fe 2 O 3 /PANI recorded at different current densities, respectively. A perfect triangular-shaped charge/discharge curve in GCD implies the presence of a pure EDLC-type capacitance. However, the GCD curves of both the electrodes exhibit a plateau at about 0.3-0.4 V as well as triangular behavior, indicating the presence of a pseudo-capacitive faradic feature. The specific capacitance of both the composites has been calculated from GCD curves at different current densities using Eq. (6): where C s is the specific discharge capacitance, i is the current density, Dt is the discharge time, and DV is the potential during discharge. The a-Fe 2 O 3 /PANI composite possesses a high specific capacitance of 192.29 Fg -1 at low current density (1 Ag -1 ), while the specific capacitance for a-Fe 2 O 3 /CNF was drastically decreased from 88.88 to 5.43 Fg -1 at the current density of 1-5 Ag -1 . It is to be noted that Fe 2 O 3 decorated carbon nanotubes (CNTs) synthesized using the microwave-assisted technique reported the capacitance 204 Fg -1 at 0.5 Ag -1 current density exhibiting an energy density of 28.3 Whkg -1 at a power density of 1Wkg -1 [55]. The relationship between specific capacitance and current density of a-Fe 2 O 3 /PANI composite material is superior at lower current density. All the curves clearly show the same trend of the slowly decreasing nature of specific capacitance with the increments in the discharge current density. As displayed in Fig. 5c, d, a-Fe 2 O 3 / PANI exhibited a longer discharge time (520 s) than a-Fe 2 O 3 /CNF composites (330 s) at 1 Ag -1 . Figure 6a displays the CV comparison of a-Fe 2 O 3 / CNF and a-Fe 2 O 3 /PANI at a scan rate of 5 mV/s. It is observed that a-Fe 2 O 3 /PANI exhibits a much higher area enclosed by a curve than Fe 2 O 3 /CNF, indicating a high capacitive nature of a-Fe 2 O 3 /PANI than that of Fe 2 O 3 /CNF composites. The characteristic transition redox peaks of PANI are visible at a scan rate of 5 mV/s. Figure 6b shows the relationship between the specific capacitance at different current densities. The specific capacitance of a-Fe 2 O 3 /CNF and a- Fg -1 at 1 Ag -1 current density, respectively. The specific capacitance was drastically decreased to 5.43 Fg -1 for a-Fe 2 O 3 /CNF (at a current density of 5 Ag -1 ) and to 84.04 Fg -1 for a-Fe 2 O 3 /PANI composite (at a current density of 10 Ag -1 ), respectively. The decreasing trend in specific capacitance with the increments in the current density is observed for both composite materials. The active surface of the electrodes may be probably becoming inapproachable for charge stockpile during the charge-discharge process, resulting in the comparatively inadequate Faradic redox reaction at higher discharge current densities, which accounts for the abatement of specific capacitance [56]. It is to be noted that Fe 2 O 3 nano-spindles, synthesized chemically via hydrothermal route, as the positive electrode in 0.5 M K 2 SO 4 electrolyte showed a specific capacitance of 159 Fg -1 at 0.1 Ag -1 current density [46]. The results are much comparable with mill scale-derived Fe 2 O 3 sprayed on large area iron/aluminum current collector electrode (method reported as a greener synthetic route) which gave 92 Fg -1 capacitance at 5 mVs -1 scan rate with 20% loss after 5000 cycles [47]. A dextran-based template-free chemical method reported as a greener method could exhibit better capacitance of 295 Fg -1 with loss of 15% capacitance in 1500 GCD cycles [48]. A chemical-free, simple mechanically press method adopted for the synthesis of a-Fe 2 O 3 /Ni foil/C reported having 40.07 Fg -1 capacitance and 20% retention loss in just 500 cycles [57]. A facile, low-cost hydrothermal method has been used to synthesize binary nanocomposites gave 121.25 and 294 Fg -1 specific capacitance with 72.3% (after 2000 cycles) and 82% (after 1000 cycles) capacity retention for a -Fe 2 O 3 /C [58] and a-Fe 2 O 3 / ordered porous C nanocomposites [54], respectively. An expensive, self-assembly chemical method was used to derive a-Fe 2 O 3 /C reported to give a C sp value of 200 Fg -1 but exhibited poor retention capacity [59]. An attempt to make a thin film of PANI over a-Fe 2 O 3 nanoparticles, however, exhibited poor performance [60], whereas microwave irradiation-synthesized composites, although exhibited good specific capacitance of 204 Fg -1 , showed poor retention capacity of 67% in just 1000 cycles [61]. The electrodeposited binary core-shell NW a-Fe 2 O 3 / PANI also exhibited poor performance in terms of volume capacitance (2.02 mF/cm) [62]. The comparison of the literature reported data with our findings is gathered in Table 1. It displays the data for only binary oxides (a-Fe 2 O 3 ) coupled with either PANI or form of carbon (CNF, C etc.), obtained via various methods, including facile, less expensive, and waste-derived/greener methods. It is observed that although the wastederived or green method of synthesis exhibits absolute capacitance lower than more expensive synthetic methods, the cost per kilowatt performance and the life cyclability can be competitive for large grid-scale storage applications.
The performance of a supercapacitor is expressed in terms of the energy density and power density, which can be calculated from Eqs. 7 and 8 respectively: where C s is the Specific Capacitance, DV is the potential window, and Dt is the discharge time. The Ragone plot, the plot of specific energy (Wh/Kg) versus the specific power (W/Kg), is shown in Fig. 6c for a-Fe 2 O 3 /CNF and a-Fe 2 O 3 /PANI electrodes. For a-Fe 2 O 3 /CNF composite, the increase in specific power from 69.39 to 410.63 Wkg -1 tend to decrease the specific energy from 3.08 to 0.1 Whkg -1 , whereas a-Fe 2 O 3 /PANI showed specific energy of 11.28 Whkg -1 at a specific power of 162.44 Wkg -1 with a maximum specific power density of 1623.78 Wkg -1 at a specific energy of 4.93 Whkg -1 ; the results confer good power characteristics of a-Fe 2 O 3 /PANI than of a-Fe 2 O 3 /CNF. The higher power output makes the material promising for applications where high-power output, as well as high energy capacity, is required. The cycling stability is considered as an effective element of supercapacitor for realistic application purposes. Figure 6d demonstrates long-term cycling properties of the a-Fe 2 O 3 /CNF (black curve) and a-Fe 2 O 3 /PANI (red curve) composite tested by continuous GCD measurements at a current density of 1 Ag -1 for 5000 cycles. The specific discharge capacitance of a-Fe 2 O 3 / CNF and a-Fe 2 O 3 /PANI electrodes drops by 25% and 20% of the initial capacitance, respectively, after 5000 cycles, displaying an excellent cycling performance for both the electrodes. The loss after 5000 cycles is attributed to the stacking and agglomeration of active sites in the process.

Conclusions
The decorations of waste iron rust-derived a-Fe 2 O 3 on CNF and PANI were successfully done by using electrospinning, hydrothermal, and in-situ polymerization methods to evolve Fe 2 O 3 /CNF and Fe 2 O 3 / PANI composite materials. The electrochemical characterizations were done by using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). The a-Fe 2 O 3 /CNF and Fe 2 O 3 /PANI as positive electrodes showed a maximum specific capacitance of 88.88 and 192.29 Fg -1 at 1 Ag -1 scan rate and the energy density of 3.08 and 11.28 Whkg -1 with the power density of 69.39 and 162.44 Wkg -1 , respectively. A loss of 25% for a-Fe 2 O 3 /CNF and 20% for Fe 2 O 3 /PANI in specific capacitance has been registered after 5000 continuous charge/discharge cycles, indicating a highly stable cycling performance. The facile, low-cost evolution of waste iron rust into useful a-Fe 2 O 3 nano-material and its subsequent utilization in supercapacitors exhibited better Block copolymer/iron oxide nanocomposites were prepared by carbonization of poly(t-butyl acrylate)-block-polyacrylonitrile (PtBA-b-PAN), and nano 4-hydroxylbenzoic acid-activated Fe2O3 hydrophobic nanomaterial g Exhibited pure EDLC-type super capacitance h Calculated from the area enclosed by CV characteristics, thus making it a prospectively promising electrode material especially for large grid-scale energy storage in sustainable hybrid supercapacitor.