Synthesis and enhanced electrochemical ...

Journal of Alloys and Compounds 757 (2018) 466e475

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Synthesis and enhanced electrochemical performance of PANI/Fe3O4 nanocomposite as supercapacitor electrode Thibeorchews Prasankumar a, Biny R. Wiston a, C.R. Gautam b, Rajangam Ilangovan c, Sujin P. Jose a, * a b c

School of Physics, Madurai Kamaraj University, Madurai 625021, India Advanced Glass and Glass Ceramics Research Laboratory, Department of Physics, University of Lucknow, Lucknow 226007, India National Centre for Nanoscience and Nanotechnology, University of Madras, Madras 600025, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 February 2018 Received in revised form 3 May 2018 Accepted 8 May 2018 Available online 10 May 2018

Conducting polymer nanocomposites associated with metal oxides are emerging class of pseudocapacitive materials that exhibit enhanced electrochemical performance in energy storage applications. In this work, we fabricated polyaniline (PANI)/Fe3O4 nanocomposite (PFNC) through the in situ polymerization of aniline in presence of microwave synthesised Fe3O4 nanoparticles. The prepared PFNC was characterised by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The surface morphology was investigated by scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analysis. TEM studies revealed the presence of Fe3O4 nanoparticles wrapped by PANI with size in the range of 40e60 nm. The porous structure is validated by the associated morphology, where the voids are created by the formation of microspheres out of the nanospheres. These spaces boost the electrochemical activity of the PFNC by facilitating more sites for the insertion of electrolytic ions during the electrochemical reaction process. The fabricated PFNC on carbon felt offers the enhanced electrochemical properties and exhibits high specific capacitance (572 F g1 at 0.5 A g1), pronounced cycling stability (>5000 cycles at 1 A g1) with good capacitance retention (82%). An excellent rate performance of 71.9% is also exhibited by the PFNC electrode with ten times the original current density (from 0.5 A g1 to 5 A g1). These out-standing characteristics prove that PFNC has a great potential to be exploited as highly efficient electrode materials for supercapacitors. © 2018 Elsevier B.V. All rights reserved.

1. Introduction The rapidly changing lifestyle, escalating fuel scarcity and rising environmental concern have made sustainable, reliable and clean energy source as the greatest quest of the century [1,2]. Rising demand for portable consumer electronics, quickly growing digital communication media and the need for storing the produced renewable energy insist the need to pinpoint an appropriate energy storage solution. Supercapacitors have gained considerable attention owing to their superior power capability, good safety record, better reversibility and longer cycling life [3e5]. Carbon and its derivatives like graphene [6], carbon nanotubes [7], activated carbon [8], carbon nanofibers [9] etc. store charge through nonFaradaic electrostatic charge accumulation and are called Electric

* Corresponding author. E-mail address: [email protected] (S.P. Jose). https://doi.org/10.1016/j.jallcom.2018.05.108 0925-8388/© 2018 Elsevier B.V. All rights reserved.

Double Layer Capacitors (EDLC). The electro-active transition metal oxides [10], sulphides [11], carbides, nitrides [12], conducting polymers [13] and their composites store charge through fast, reversible Faradaic redox reaction and are known as pseudocapacitors. Pseudocapacitors stretch the possibility of having high energy density and high power density in the same materials [14]. Due to this reason, they became the most studied supercapacitor materials. Transition metal oxides ranging from MnO2 [15], RuO2 [16], Co3O4 [17], Fe3O4 [18] and V2O5 [19] have been reported to have fairly good electrochemical activity. Through smart design and configuration, supercapacitors are expected to produce revolutionary breakthroughs in the field of energy storage leading to a wider range of technological applications. RuO2 was the first investigated pseudocapacitive electrode owing to its high theoretical capacitance. It was found to have very good electrochemical activity [16]. The commercialisation of RuO2 is limited by its high cost, toxicity and sparse supply [20]. High active surface area for

T. Prasankumar et al. / Journal of Alloys and Compounds 757 (2018) 466e475

electrode-electrolyte interaction [21], porosity and good conductivity [22] play major roles in the enhancement of electrochemical performance of pseudocapacitors. In order to obtain materials with high capacitance performance and good cycle life, researchers are striving to have a sensible combination of two or more materials that produce synergistic effect. It is possible to design the architectures that enable to tap the advantages of the synergistic effects of individual components and maximize the electrochemical performance of the electrodes. Among the various transition metal-oxides, magnetite (Fe3O4) has been reported to have high conductivity and commendable electroactivity. It is an attractive material due to its natural abundance, non-toxicity and low environmental impact [23,24]. Chung et al. reported nano-sized cellular Fe3O4 thin film electrodes for their applications in supercapacitors [25]. Wang et al. investigated the electrochemical performance of high surface area (165.05 m2 g1) Fe3O4 nanoparticles, prepared by hydrothermal method [26]. Reyes et al. obtained Fe3O4 nanoparticles with a specific capacitance of 36 F g1 [27]. The specific capacitance of Fe3O4 reported so far, are comparatively lower than other transition metal oxides. Electrochemical activity of Fe3O4 based materials like Fe3O4/Fe-CNT [28], G/Fe3O4@Fe/ZnO [29], Fe3O4@FLG [30], and Fe3O4-C hybrid nanocomposites [31] has been investigated by various researchers. Conducting polymers like polyaniline (PANI), polypyrrole (PPy), polythiophene (PT), poly (3,4-ethylenedioxy thiophene) (PEDOT), etc. were reported to have good flexibility, controllable thickness, and high electrical conductivity (up to 103 S cm1) but with less stability [32]. Polyaniline (PANI) has the highest capacitance among the various conducting polymers. Moreover, it is environmentally benign and has controlled molecular structures like fully reduced form, half-oxidized form and fully oxidized form [33]. The morphology of the PANI based nanostructures can easily be controlled to obtain different nanostructures like nanospheres, nanotubes, nanofibers, nanoflowers, nano-tentacles and nanotowers [34,35]. Metal oxide-polymer composite is expected to give enhanced electrochemical performance due to the synergistic effect between the metal oxide and the polymer [13]. PANI based combinations like RuO2/PANI [36], NiO/PANI [37], Fe2O3/PANI [38], NiCo2O4@PANI [39], MnOx/PANI [40], MoS2/PANI [41] and SnO2/ PANI [42] have been reported to exhibit higher electrochemical performance than their individual counterparts. Similarly, the specific capacitance of Fe3O4 is expected to improve by enveloping it with a suitable conducting polymer like PANI [43]. Fe3O4-polymer composites like Fe3O4@PANI and Fe3O4@PPy microspheres have been reported for bio-medical applications [44] whereas PANI/Fe3O4 nanocomposite has been used for chromium removal from polluted ground water [45]. A few reports are available for the preparation of ternary composites involving PANI and Fe3O4 [46e48]. The demerits involved in them are to maintain the proper chemical stoichiometry, uniform crystallinity and large area of the synthesized materials etc. This complicates the whole synthesis procedures. Microwave method is the best suit for metal oxide synthesis since they provide greater chances for the formation of metastable phases of metal oxides, homogeneity and the formation of unique microstructures with controlled morphology [49]. Here in this report, PANI/Fe3O4 nanocomposite (PFNC) was fabricated by the microwave synthesis of Fe3O4 nanoparticles and followed by an additional procedure of in situ polymerization of aniline monomer. This synthetic route serves as a novel one to fabricate the binary PFNC. The inexpensive aniline monomer is considered as an ideal matrix for Fe3O4 NPs to be existent and the developed materials exhibit unique morphology and homogeneity than previously reported PANI/Fe3O4 and showed enhanced electrochemical performance [50]. To the best of our knowledge,

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fabrication of PFNC through in-situ polymerization of aniline in the presence of microwave synthesized Fe3O4 nanoparticle has not been reported as electrode material for supercapacitors. 2. Experimental 2.1. Materials Iron (II) Sulfate Heptahydrate (FeSO4$7H2O, 99.5%), Aniline monomer (C6H7N, 99%), Ammonium persulfate (APS) ((NH4)2S2O8, 98%), acetylene black, polyvinylidene difluoride (PVDF) and N-methylpyrrolidone (NMP, 99%) were purchased from Alfa Aesar Chemical Reagent Corporation, United Kingdom. Hydrochloric acid (HCl) and Ammonia solution were purchased from Merck Chemical Reagent Corporation, Germany. All the chemicals were used as received without any further treatment. Double Distilled water was used throughout the experiment. 2.2. Material characterization The crystal structure and average particle size of Fe3O4, PANI and PFNC were analysed from the Powder X-Ray Diffraction (XRD) pattern recorded between 2q ¼ 10 and 80 , on Bruker Advanced D8 Powder Diffractometer equipped with Xcelerator detector (Cu Ka radiation, l ¼ 1.5418 Å). The characteristic vibrations of the samples were examined using the infra-red absorption spectra obtained on Perkin-Elmer Fourier Transform Infra-Red spectrometer (attenuated total reflection mode) in the wave number range of 4000 cm1 to 500 cm1, with a resolution of 4 cm1 averaging over 15 scans. The surface morphology of the samples was studied by field emission scanning electron microscope (FE-SEM: Hitachi 3000-N). TEM images were studied using a JEOL 2100 field emission gun transmission electron microscope. For preparing the TEM samples, the powder samples were taken and bath sonicated in isopropyl alcohol for 30 min. A few drops of the samples were then cast onto the holey-carbon grid and it was then allowed to dry in vacuum overnight. X-ray Photoelectron Microscopy (XPS) measurements were carried out with PHI 5000 Versa Probe ULVAC instrument. A BrunauereEmmetteTeller (BET) analyser (Model Autosorb iQ, Quantachrome Instruments) was used to study the surface area, pore size by nitrogen gas absorptionedesorption. 2.3. Synthesis of Fe3O4 nanoparticles At first, 0.1 M solution of FeSO4$7H2O (Ferrous sulfate) was dissolved in distilled water and stirred. Then, NH3$H2O (ammonia solution) was added drop wise (10 drops) to the above solution. The mixture was continuously stirred for 30 min and during the stirring, the colour changes from light green to dark brown and then to black. The homogeneous solution was then irradiated in a microwave oven (Samsung - CE1041DFB) for 6 min at a power of 600 W. The mixture was then cooled to room temperature. An external magnetic field was applied to the homogeneous solution to separate Fe3O4 magnetic particles from non-magnetic impurities. The mixture was then washed several times using distilled water. The precipitate was filtered and dried in a vacuum oven at a temperature of 60 C for 12 h. The as-synthesized product obtained through this method was Fe3O4 nanoparticles. The formation of Fe3O4 nanoparticle is supported by the following equation 3 FeSO4$7H2O þ H2O þ 6 NH3$H2O/Fe3O4þ 3(NH4)2SO4 þ 24H2O þ H2[

(1)

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2.4. Synthesis of PANI Polyaniline (PANI) nanoparticles were prepared by chemical oxidative polymerisation method. In this method, APS acts as an oxidizing agent. In a typical synthesis, Aniline monomer (3.5 mL) mixed with 30 mL distilled water was taken in a beaker and placed in an ice-bath. A mixture containing 0.3 M of APS and 1 M hydrochloric acid (HCl) was taken in a burette. The mixture was added drop wise to the aniline solution and stirred constantly for an hour in an ice-water bath. Polymerisation occurs slowly to form PANI. After polymerisation, the material was filtered with ethanol. Then it was dried in vacuum oven for 8 h at 60 C to obtain PANI nanoparticles.

measurements such as Cyclic Voltammetry (CV), Galvanostatic charge - discharge (GCD) studies and Cycling stability were carried out in Biologic SP-50 using three-electrode system configuration. Electrochemical impedance spectroscopy (EIS) analysis was also carried out for the samples in a typical three electrode cell using autolab potentiostat galvanostat (Autolab PGSTAT 302N). 1 M H2SO4 solution was optimized to be the suitable electrolyte for the proposed composite. A platinum electrode was used as the counter electrode and Ag/AgCl electrode was used as the reference electrode during the electrochemical measurements. 3. Results and discussion 3.1. Structure and morphology

2.5. Synthesis of PANI/Fe3O4 nanocomposite (PFNC) The polyaniline/Fe3O4 nanocomposite (PANI/Fe3O4) was prepared via in-situ polymerization of aniline in the presence of microwave synthesized Fe3O4 nanoparticles. 0.2 g of as-synthesized Fe3O4 nanoparticle was dispersed into 30 ml of distilled water. 3.5 ml of aniline monomer was mixed with the above solution. The mixture was placed in an ice bath over a magnetic stirrer. 0.3 M of APS (3.42 g) is mixed with 1 M solution of HCl in a separate beaker. This solution was then transferred into a burette. The solution containing APS and HCl was added drop wise to the above mixture for about 30 min. After the polymerization, the material was filtered using ethanol to remove the impurities. The final product was dried in vacuum oven for 12 h at 60 C to obtain the PANI/Fe3O4 nanocomposite (PFNC). The synthesis and electrochemical performance of PFNC electrode are depicted in Scheme 1. 2.6. Electrochemical measurements For the preparation of working electrode, 75 wt% of active material (PFNC), 20 wt% of acetylene black and 5 wt% of polyvinylidene difluoride (PVDF) were mixed with the addition of 1-methyl-2pyrrolidone (NMP) to form a homogeneous slurry. The slurry was coated onto a piece of carbon felt, having an area of 1 cm2. The electrode was dried in vacuum oven for 12 h. The loaded mass of the working electrode was 2 mg cm2. The electrochemical

Structural analysis of Fe3O4, PANI and PFNC was performed via X-ray diffraction (Fig. 1). XRD pattern of the synthesized Fe3O4 nanoparticles is shown in Fig. 1a. The prominent peaks were observed at 2q ¼ 30.18 , 35.58 , 43.29 , 57.02 and 62.8 corresponding to (220), (311), (400), (511) and (440) planes respectively. These data match very well with the JCPDS File No. 19-0629 of Fe3O4 nanoparticles. It is revealed that the synthesized Fe3O4 nanoparticles have cubic spinel structure [51]. The average crystallite size of the prepared magnetite nanoparticles was calculated using the Scherrer's formula [22].

D¼

Kl bcosq

and was found to be 49.6 nm. The XRD pattern of the synthesized PANI and PFNC are shown in Fig. 1b and c respectively. PANI shows broad peaks centred at 2q ¼ 20.3 and 30.8 corresponding to (020) and (022) reflection planes of PANI in its emeraldine salt form [52]. PANI exhibits low crystallinity due to the presence of repeated benzenoid and quinoid rings. The peak centered at 20.3 corresponds to periodicity parallel to the polymer chains [53]. PFNC has the characteristic reflection peaks of both Fe3O4 and PANI in it. The intensity of Fe3O4 reflection peaks is suppressed in PFNC and this is due to the encapsulation of Fe3O4 by the amorphous polymer, PANI [54]. These results confirmed the formation of PFNC and the average particle size is calculated to be 71 nm.

Scheme 1. Schematic illustration of synthesis and electrochemical performance of PFNC.


Fig. 1. XRD patterns of (a) Fe3O4, (b) PANI and (c) PFNC.

The chemical bonding was also investigated in the prepared samples and the composite by Fourier transform infrared spectroscopy. FTIR spectra of Fe3O4, PANI and PFNC are shown in Fig. 2.

Fig. 2. FTIR spectra of (a) Fe3O4, (b) PANI and (c) PFNC.

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In the Fe3O4 spectrum, three main absorption peaks were observed at 552 cm1, 1641 cm1 and 3426 cm1 corresponding to Fe-O bond vibration, O-H bending and O-H stretching [55] respectively. The peak obtained between 700 cm1 and 500 cm1 denotes C-Cl stretching thereby validating Cl doping of PANI [53]. The band at 3475 cm1 is assigned to N-H stretching mode. The peaks at 1138 cm1 and 827 cm1 are attributed to the in-plane and out-ofplane C-H bending of 1, 4- substituted phenyl ring respectively. C ¼ N stretching mode along with N-H deformation in quinonoid ring is observed at 1584 cm1. The peaks at 1240 cm1 and 1294 cm1 are associated with C-N stretching of benzenoid ring. The peaks observed at 1481 cm1 and 1574 cm1 corresponding to C]C vibration of quinoid ring and benzene ring [45] respectively. The FTIR of PFNC contains the typical characteristic peaks of both Fe3O4 and PANI. The small shifts in the peaks are due to the interaction between Fe3O4 and the nitrogen atom of aniline ring [56] which very well endorse the composite formation in the PFNC sample. Thermal stability is an important property for hybrid polymer materials or composites [57]. TGA curve of PFNC is given in Fig. S1 and it indicates fairly good stability of the composite up to 410 C. The morphology of the individual components and the fabricated nanostructures of PFNC were investigated by the microscopic images, SEM and TEM. The SEM image of synthesized Fe3O4 nanoparticle is shown in Fig. 3a. It shows the spherical morphology of the prepared Fe3O4 nanoparticles. Fig. S2 shows the SEM images of the prepared PANI. The SEM image of PFNC (Fig. 3b) depicts the agglomerated spherical morphology. Because of the magneto-dipole interactions between the particles, the PFNC nanospheres agglomerate to form microspheres creating irregular voids. These spaces boost the electrochemical activity of the PFNC by facilitating more sites for the insertion of electrolytic ions during the electrochemical reaction process. The TEM image of the Fe3O4 nanoparticle is depicted in Fig. 3c. It confirms the monodispersed spherical morphology of Fe3O4 nanoparticles. The TEM image of PFNC is shown in Fig. 3d. It reveals the irregular wrapping of PANI over Fe3O4 nanoparticles, but the spherical morphology and monodispersity of Fe3O4 are pretty preserved in the composite. It also confirms the heterogeneous nucleation during the composite formation, such that the PANI is coated over the Fe3O4 nanoparticles and form monodispersed PFNC. The PFNC has highly porous spherical granular morphology and shows the microspheres formed of the nanospheres and building voids during this formation process. This porous structure of the composite will facilitate the ion transfer during the electrochemical reaction. Normally, XPS provides further information about the chemical state of the constituent atoms. The survey spectrum of PFNC is shown in Fig. S3 and clearly indicates the presence of carbon, nitrogen, oxygen and iron in the corresponding oxidation states in the sample. The obtained results are in good agreement with the literature. C 1s of carbon spectrum (Fig. 4a) resolved into three components 285.1, 286.1 and 287.6 eV and are assigned to C-C, C-O and C]O, respectively [58]. The core level of oxygen spectrum O 1s (Fig. 4c) has four peaks located at 530.8, 531.7, 532.5 and 534.6 eV. The peak at 530.8 eV denotes O2 state, the formation of oxide with Fe. The presence of OH, CeO, OeC]O, and H2O in the sample may be due to air ambience [59]. The de-convolution peaks of the N 1s (Fig. 4b) indicate two distinct peaks, related to the different forms of nitrogen. The peak at 399.2 eV corresponds to pyridine-N (¼Ne) and the peak at 400.1 eV may be assigned to pyrrolic-N (eNHe). The core level spectrum of Fe 2p (Fig. 4d) is deconvoluted into five peaks. The peaks at 710.8 and 724.5 eV indicate Fe2þ oxidation state and the peaks located at 714.2 and 727.1 eV represent Fe3þ oxidation state. The satellite peak is detected at 719.2 eV which represents ℽ-Fe2O3 [60].

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Fig. 3. SEM images (a, b) and TEM images (c, d) of Fe3O4 and PFNC.

Fig. 4. XPS spectra of PFNC (a) C 1s (b) N 1s (c) O 1s and (d) Fe 2P

The specific surface area and pore size distribution of the electrode material are obtained through a standard nitrogen adsorption in BrunauereEmmetteTeller (BET) analysis. The nitrogen adsorption-desorption isotherms of Fe3O4, PANI and PFNC are shown in Fig. 5aec. Surface area and pore volume distribution of the electrode materials are listed in Table 1. The specific area of Fe3O4, PANI and PFNC was found to be 39.32 m2 g1, 53.44 m2 g1

and 64.71 m2 g1 respectively. From the table, it is clear that PFNC shows more pore volume than Fe3O4 and PANI electrodes. Fig. 5d denotes the comparison of the surface area for the individual electrode materials. The high surface area and high porous nature of PFNC may result in better ionic transportation through the electrode material, leading to enhanced supercapacitor performance.


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Fig. 5. Nitrogen adsorption e desorption curves for (a) Fe3O4, (b) PANI and (c) PFNC. (d) Bar diagram of the surface area for the electrode materials.

Table 1 Surface area and pore volume of the electrode materials. Electrode

Surface area (m2 g̶1)

Pore volume (cm3 g̶1)

Pore size (nm)

Fe3O4 PANI PFNC

39.32 53.44 64.71

0.251 0.367 0.462

7.62 5.46 4.73

coupling of these Faradaic reactions occurs at the electrode surface enhance the supercapacitor performance. The reaction is given below [50]. Charging: PANI þ H2SO4 / [PANI$Hþ∙]HSO4 and Fe2O3$FeO / Fe2O3$FeOþ (HSO4)

3.2. Electrochemical properties The cyclic voltammograms of the Fe3O4, PANI, and PFNC are shown in Fig. 6aec respectively. The Cyclic Voltammetry (CV) was performed in 1 M H2SO4 solution at different scan rates ranging from 10 mV s1 to 100 mV s1 in a fixed potential range of 0.6e1.0 V (vs. Ag/AgCl) to evaluate the rate-dependent performance of the samples. The peak current increases with increase in scan rate demonstrating the high power capability and reversibility of the material. Fe3O4 shows prominent anodic peak at 0.65 V and cathodic peak at 0.30 V indicating the quasi-reversible reaction, Fe(II) 4 Fe(III) [61]. The redox peaks of PANI are due to the transformation of PANI from the half doped emeraldine form to fully doped pernigraniline form during the Faradaic process [62]. PFNC shows higher current response than pristine Fe3O4 and PANI. The electrochemical mechanism of PFNC is explained as follows. During charging, PANI experiences oxidative doping and during discharging it experiences dedoping. It is supposed that there is a preferential oxidation of Fe(II) center in Fe2O3$FeO to Fe(III) during the intake of HSO4 ions into the composite and these are expelled in the discharge step due to the reduction of Fe back to Fe(II). The

Discharging: [PANI$Hþ$]HSO4 / PANI þ Hþ þ HSO4 and Fe2O3$FeOþ (HSO4) / Fe2O3$FeO þ HSO4 The enhanced current response and the increased area of CV curves of PFNC are compared with that of the individual counterparts, Fe3O4 and PANI. The comparison of CV curves of Fe3O4, PANI and PFNC at a scan rate of 10 mV s1 is shown in Fig. 7a. From these curves, it is evident that the synergistic effect of binary is responsible for the enhanced capacitance performance associated with the individual components. The galvanostatic charge-discharge (GCD) studies of Fe3O4, PANI and PFNC were performed for various current densities between 0.25 A g1 and 5 A g1. The GCD graphs of Fe3O4, PANI and PFNC are shown in Fig. 6def. The deviation in the linearity accounts to the fast Faradaic redox reaction at the electrode-electrolyte interface. This also supports the fact that the charge storage in Fe3O4, PANI and PFNC is due to the fast Faradaic reactions as predicted by the CV

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Fig. 6. CV curves (aec) at various scan rates and GCD curves (def) at various current densities of Fe3O4, PANI and PFNC.

results. The charge-discharge profiles of Fe3O4, PANI and PFNC at a current density of 1 A g1 are depicted in Fig. 7b. The specific capacitance of PFNC was calculated to be 572 F g1 whereas the

individual Fe3O4 and PANI show only 14 F g1 and 491 F g1 respectively at a current density of 0.5 A g1. The increased specific capacitance of PFNC may be due to synergistic effect between Fe3O4

Fig. 7. (a) Comparison of CV curves at a scan rate of 10 mV s1 and (b) comparison of GCD curves at a current density of 1 A g1.


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Table 2 Comparative specific capacitance performance of PANI and Fe3O4 based supercapacitor electrodes. S. No

Electrode material

Substrate

Specific capacitance at applied load

Reference

1 2 3 4 5 6 7

PANI/Fe3O4 nanocomposite Fe3O4/CNTs/PANI ternary composite CNT film-Fe3O4-PANI composite rGO/Fe3O4/PANI composite PAni-DBSA-Fe3O4 nanocomposite Pyrrole treated Fe3O4 nanowires PAni-DBSA-Fe3O4 nanocomposite

Carbon felt Ni mesh Carbon film Flexible film Stainless Steel Stainless Steel Stainless Steel

572 F g1 at 260 F g1 at 201 F g1 at 283 F g1 at 213 F g1 at 190 F g1 at 228 F g1 at

Our work [46] [47] [48] [50] [66] [67]

and PANI and the correlated morphology of the prepared PFNC. Specific Capacitance (Cs) of the electrode can be calculated from the chargeedischarge measurements using the relation [63].

Z I V dt Cs ¼ 2 m Vf Vi

(2)

R where, I is the current intensity, V dt is the area under the experimental discharge curve, m is the mass deposited on carbon felt and (Vf -Vi) is the active potential window during discharge. At low current densities, the fully accumulated charges in the inner active sites of the PFNC electrode result in high specific capacitance value of 572 F g1. Furthermore, it is found that the specific capacitance of PFNC electrode material is considerably higher than that of the previous reports [46e48,50,66,67], even surpassed the performance of the ternaries and tabulated in Table 2. A graph (Fig. 8a) is plotted between specific capacitance and corresponding

(0.5 A g1) (0.5 A g1) (20 mV s1) (1.0 A g1) (1 mA cm2) (0.5 A g1) (1 mA cm2)

current density and it is found that the specific capacitance decreases with increase in current density. It is because of the fast ionic motion into the pores of the PFNC electrode during the charge-discharge process [64]. Fig. 8b denotes the bar diagram of the specific capacitance values of Fe3O4, PANI and PFNC electrodes at different current densities. From this diagram, it is confirmed that at the lowest current density, the highest specific capacitance of the material is observed and vice versa. In addition, we measured the electrochemical property of the bare carbon felt (Fig. S4) in order to find out the contribution of electrochemical performance by the current collector employed in the experiment. Electrochemical Impedance Spectroscopy (EIS) stands out as a powerful tool for the analysis of interfacial process and thereby, the evaluation of rate constant, ionic and electronic conductivity, double layer capacitance etc. Impedance measurements were performed within the frequency range 100 kHz to 1 Hz. In the Nyquist plots (Fig. 8c), the depressed semi-circle at high frequency region is due to the possibility of the electrode blocking the ionic exchange of the Faradaic process, which is generated at the electrode and

Fig. 8. (a) Specific capacitance values of the electrodes at various current densities, (b) Bar graph for the specific capacitance values of the electrodes at various current densities, (c) Nyquist plots and (d) Cycling stability and coulombic efficiency of PFNC at a constant current density as a function of cycle number.

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electrolyte interface [65]. The straight line in the low frequency region is due to the Warburg diffusion impedance. PANI has clear semicircle with linear curve exhibits excellent electrochemical interactions between the electrode and electrolyte during electrochemical reactions. Fe3O4 has higher intrinsic resistance due to its semiconducting properties which are obvious from EIS spectra. Overall, the PFNC has the lowest resistivity and highest diffusion of ions at the electrode - electrolyte interface during the electrochemical reaction which is decipherable in EIS spectra with small semicircle and with the linear part in mid frequency region compared to other two electrodes (PANI and Fe3O4). The electron transfer resistance Rct of PFNC is only 1.80 U, which is fairly less than that of PANI (2.06 U), suggests better conductivity and thereby effortless ion transportation benefited from the interconnected conductive pathways [21]. Hence PFNC electrode demonstrates lower resistance and good capacitive behaviour appreciable for the supercapacitor electrode materials. Long-term stability is an important aspect for evaluating the performance of supercapacitor electrodes for practical applications. The cycling stability of PFNC electrode was determined by charge/ discharge cycling at a current density of 1 A g1. The specific capacitance as a function of cycle number is shown in Fig. 8d, along with the columbic efficiency of the tested electrode for 5000 charge-discharge cycles. There is a decrement in the obtained specific capacitance values for the initial cycles and then it reaches the stable capacitance. Increase in the capacitance values after the initial cycles may be due to the complete wetting of the prepared electrode by the electrolyte. PFNC electrode exhibits an excellent electrochemical stability of 82% of its initial capacitance due to its large interaction area between the electrode and the electrolyte. The synergistic effects of Fe3O4 and PANI in PFNC provide the efficient electrochemical performance and cycling stability. PANI is coated on the surface of the Fe3O4 nanoparticle and this coated PANI serves as conductive networks. Also, Fe3O4 bridges the PANI network and makes sure the electrical conductance between PANI and Fe3O4 [46]. This also offers channels for the charge transfer in the PFNC electrode. The porous structure of the PFNC will facilitate the ion transfer by the abundant interstitial spaces of the interfacial region between the electrode and the electrolyte, and the voids in the microspheres preventing volume expansion due to the insertion and extraction of ions during the electrochemical reaction. This is validated by the associated morphology obtained by SEM analysis where the voids are created by the formation of microspheres. These voids support the insertion and extraction of electrolytic ions, in turn, leading to a remarkable capability for charge storage, facile mass transfer and structural integrity. Hence, the reported PFNC remains as potential electrode material for long-life supercapacitors. 4. Conclusions We fabricated a highly efficient PFNC by in-situ polymerisation of aniline in the presence of microwave synthesized Fe3O4 nanoparticles. This fabricated material was used for the preparation of supercapacitor electrodes. The XRD and FTIR confirm the formation of composite whereas the SEM and TEM studies depict the correlated morphology for the improved electrochemical performance of the PFNC. The composite exhibits pseudocapacitance with an enhanced specific capacitance of 572 F g1 at a current density of 0.5 A g1. PFNC demonstrated remarkable cycle life of 5000 cycles with note-worthy capacitance retention of 82% at a current density of 1 A g1. These results showcase that PFNC has improved electrochemical performance owing to the synergistic effect of the individual components, the high stability of Fe3O4 nanoparticles and high pseudocapacitance of PANI. The simplicity in the synthesis of

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