A novel composite electrode material derived from bisferrocenyl-functionalized GO and PANI for high performance supercapacitor

Article abstract of DOI:10.1016/j.electacta.2020.136712

A novel bisferrocenyl based epoxy compound was synthesized to be used for further surface modification of graphene oxide (GO). GO surface was firstly modified with ethylenediamine (EDA) as an amine linker and then bisferrocenyl based epoxy compound was covalently grafted to the GO surface through the epoxide ring opening reaction. Afterwards, polyaniline (PANI) nanofibers were physically adsorbed onto the surface of the modified GO to obtain a potentially high-performance nanocomposite for battery-type supercapacitor applications. The surface modification, crystalline structure, morphology, composition, and microstructure of the synthesized nanocomposites were further investigated using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) and N₂ adsorption/desorption analyses, respectively. Also, the electrochemical behavior of the synthesized nanocomposites was investigated by electrochemical methods of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), galvanostatic charge-discharge measurement (GCD). The final nanocomposite electrode exhibited a charge storage capacity of 272 mAh g^?1, and capacity retention of 89 % over 3000 CV cycles. A high energy density of 69.3 Wh kg^?1 and high power density of 6171 W kg^?1 were achieved in a symmetrical two-electrode configuration.

Full text of DOI:10.1016/j.electacta.2020.136712

Journal Pre-proof
A novel composite electrode material derived from
bisferrocenyl-functionalized GO and PANI for high performance
supercapacitor
Elmira Payami , Reza Teimuri-Mofrad , Iraj Ahadzadeh ,
Reza Mohmmadi
PII:
S0013-4686(20)31105-1
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https://doi.org/10.1016/j.electacta.2020.136712
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29 June 2020
1 July 2020
Please cite this article as:
Elmira Payami ,
Reza Teimuri-Mofrad ,
Iraj Ahadzadeh ,
Reza Mohmmadi , A novel composite electrode material derived from bisferrocenyl-functionalized
GO and PANI for high performance supercapacitor, Electrochimica Acta (2020), doi:
https://doi.org/10.1016/j.electacta.2020.136712
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A novel composite electrode material derived from bisferrocenyl-functionalized GO and
PANI for high performance supercapacitor
Elmira Payamia,b, Reza Teimuri-Mofrada,*, Iraj Ahadzadehb, Reza Mohmmadia
a Department of Organic and Biochemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
b Department of Physical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
Corresponding author: Tel.: +98 413 339 3105; Fax: +98 413 334 0191;
E-mail address: teymouri@tabrizu.ac.ir (Reza Teimuri-Mofrad)
Credit author statement
Elmira Payami: Investigation, Writing-Original draft preparation, Conceptualization,
Methodology, Software, Reza Teimuri-Mofrad: Supervision, Iraj Ahadzadeh: Validation,
Reviewing and Editing, Reza Mohammadi: Visualization.

Abstract
A novel bisferrocenyl based epoxy compound was synthesized to be used for further surface
modification of graphene oxide (GO). GO surface was firstly modified with ethylenediamine
(EDA) as an amine linker and then bisferrocenyl based epoxy compound was covalently grafted
to the GO surface through the epoxide ring opening reaction. Afterwards, polyaniline (PANI)
nanofibers were physically adsorbed onto the surface of the modified GO to obtain a potentially
high-performance nanocomposite for battery-type supercapacitor applications. The surface
modification, crystalline structure, morphology, composition, and microstructure of the
synthesized nanocomposites were further investigated using Fourier transform infrared
spectroscopy (FT-IR), X-ray diffraction (XRD), field emission scanning electron microscopy
(FESEM), energy dispersive X-ray (EDX) and N2 adsorption/desorption analyses, respectively.
Also, the electrochemical behavior of the synthesized nanocomposites was investigated by
electrochemical methods of cyclic voltammetry (CV), electrochemical impedance spectroscopy
(EIS), galvanostatic charge-discharge measurement (GCD). The final nanocomposite electrode
exhibited a charge storage capacity of 272 mAh g-1, and capacity retention of 89 % over 3000
CV cycles. A high energy density of 69.3 Wh kg-1 and high power density of 6171 W kg-1 were
achieved in a symmetrical two-electrode configuration.
Keywords: Ferrocenyl based compound, Polyaniline, Battery-type supercapacitor,
Nanocomposite.

1. Introduction
Supercapacitors are among the most important energy storage and conversion systems that have
attracted the attention of related scientific society in recent decades. The important features and
advantages of supercapacitors over other energy storage systems are as follows: high power
density, superior durability, low maintenance cost, safety, and fast charge/discharge rate [1-3].
Depending on the charge storage mechanism, supercapacitors are divided into two different
classes: an electrostatic attraction as in electrical double layer capacitors (EDLCs) and battery-
type faradic redox reaction in pseudocapacitors [4, 5]. EDLCs store charge using electrostatic
attraction of electrolyte ions between the electrode/electrolyte interface (Helmholtz layer),
whereas pseudocapacitors store energy via fast faradaic redox reaction of the electroactive
compounds such as ferrocene (Fc), transition metal oxide, and conductive polymers on the active
electrochemical sites with electrolyte [6-10]. Graphene, activated carbon, carbon nanotubes are
the materials used in EDLCs. Among these carbon allotropes, graphene is one of the most
promising candidates for use in the electrode materials because of its prominent properties
including large surface area, high conductivity, excellent mechanical and chemical stabilities,
and low cost [11-15]. Despite these outstanding features, graphene-based electrode shows low
capacitance due to the limited exploitation of large surface area; therefore, its use in pure state is
not suitable for supercapacitor applications [16]. Unlike carbon based materials, the
pseudocapacitive compounds such as conductive organic polymers including PANI [17, 18],
polypyrrole (PPY) [19-22], and polythiophene (PTh) [23, 24], and some common transition
metal oxides (mostly V2O5, MnO2, RuO2, and metal sulfides) exhibit superior capacitive
behavior compared to EDLCs due to their rather fast reversible faradaic redox reactions [25, 26].
Among the various conductive polymers, PANI as one of the typical conductive organic

polymers is the best candidate for supercapacitor applications because of its low cost, high
pseudocapacitance, good environmental stability [27-29]. PANI despite its unique features often
suffers from poor cycling stability and rate capability. Many attempts have been taken to modify
the performance of the PANI. One of these approaches is the synthesis of PANI with various
structures such as nanowires, nanorods, nanotubes or nanofibers, which improves the surface
area and electrolyte ion diffusion and consequently improves the capacitance and rate capability
compared to the intact PANI. Synthesis of PANI nanocomposites with various carbon based
materials such as graphene oxide, carbon nanotube, and amorphous carbon is another way to
improve the properties of PANI. The synergistic effects between PANI and carbon based
materials lead to larger capacitance, good rate capability, and cycle durability [30-33].
Organometallic compounds exhibit good optoelectronic properties due to their electron
delocalization and therefore received much attention of scientists. Among the organometallic
materials ferrocene (Fc) due to its unique features such as superior redox reversible properties,
high stability, and non-toxicity can be applied in various fields such as electrocatalysis,
electroanalysis, and biosensors [34, 35]. Modification of GO nanosheets with ferrocenyl based
compounds can be improved the electrochemical properties of the GO [36-40].
In general, hybridizing the battery-type and EDLCs-type electrode materials provide many
significant advantages and improvements in the performance of the electrochemical properties
compared to their individual components. Modified GO was prepared via two different methods
including non-covalent and covalent functionalization approaches. In the case of non-covalent
interaction, modified GO nanosheets were prepared via a strong π-π electronic interaction
between GO layers and other appropriate materials. In the former functionalization method, the

active oxygenic groups on the GO surface play an important role in the covalently grafting of
electroactive materials onto GO surfaces [41].
In this work, a novel bisferrocenyl based epoxy compound was synthesized, investigated via
some analysis, and then this synthesized electroactive compound was covalently grafted onto GO
surfaces through ring-opening reaction. The PANI nanofibers was then treated with modified GO
to obtain a novel Fc-based nanocomposite. Furthermore, various analyses methods were
employed to investigate the structural and electrochemical properties of the synthesized
nanocomposite. The final nanocomposite exhibits good electrochemical performance. The
enhancement in electrochemical performance of the final nanocomposite can be attributed to the
synergistic effects between both battery-type components including Fc and PANI nanofiber and
EDLCs-type component including GO nanosheets.
2. Experimental section
2.1. Materials
All of materials used during synthesis processes are given below:
α-acetyl-γ-butyrolactone, hydrochloric acid (HCl), sulfuric acid 98% (H2SO4), sodium acetate,
sodium iodide (NaI), potassium carbonate (K2CO3), epichlorohydrin (ECH), sodium hydroxide
pellets (NaOH), tetrabutylammonium iodide (Bu4NI), dimethylformamide (DMF), methanol
(MeOH), graphite flake, potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrogen
peroxide (H2O2), EDA, potassium hydroxide pellets (KOH), p-toluenesulfonic acid (PTSA),
toluene, aniline (double-distilled before use), ammonium persulfate (APS). All other materials
were of analytical grade and were used without any further purification.
2.2. Synthesis procedures

2.2.1. Preparation of ethylferrocene (EtFc)
EtFc was prepared according to our previously reported procedure [42].
2.2.2. Preparation of 5-chloro-2-pentanone
A mixture of 12 ml conc. HCl, 14 ml H2O, and 10 g (78.13 mmol) α-acetyl-γ-butyrolactone
were placed in a two necked distilling flask equipped with a condenser and the receiver was
immersed in an ice-bath. The entire reaction vessel was quickly immersed into the oil bath
preheated at 100 ℃. After about 10 min the color was changed from yellow to orange and then to
black. After 20 ml of distillate have been collected, 7 ml H2O was added to the distilling flask
and another 5 ml of distillate was collected. Afterwards, the yellow organic layer was separated
and the aqueous layer was extracted with dichloromethane and dried over Na2SO4, and the
solvent was evaporated. An amount of 6 g 5-chloro-2-pentanone was obtained as yellow, viscous
oil in 64 % yield [43].
2.2.3. Synthesis of 5-chloro-2,2-bis(ethylferrocenyl)pentane (BisFc-Cl)
A 50 ml, two necked flask equipped with a dropping funnel and gas inlet tube was charged with
a 5 ml H2SO4 and 5 ml dry MeOH, this mixture was stirred for 24 h at room temperature. After
this time, EtFc (26 mmol, 5.58 g) and 5-chloro-2-pentanone (13 mmol, 1.5 ml) were slowly
added into the reaction flask and were stirred at 65 ℃ for 30 h. After completion of the reaction
(controlled by TLC), the reaction mixture was cooled slowly to room temperature and was
neutralized with aqueous sodium carbonate, then extracted with dichloromethane (3×30 ml) and
dried over anhydrous Na2SO4. After the solvent removal, the crude product was purified by
column chromatography (n-hexane). BisFc-Cl was obtained as brown-orange viscous oil in 60%
yield [44].

1
FT-IR (KBr): 3090 (Ar-H), 2964, 2871 (C-H), 1640, 1454 (C=C), 483 (C-Fe); H NMR (400
MHz, CDCl3): δ 1.09-1.19 (6H, m, CH2-CH3), 1.55-1.67 (5H, m, -CH3, C-CH2), 1.92-2.01 (2H,
m, C-CH2-CH2), 2.24-2.37 (4H, m, Cp-CH2), 3.43-3.47 (2H, m, CH2-Cl), 3.84-4.10 (16H, m,
Cp) ppm.
2.2.4. Synthesis of 5-acetoxy-2,2-bis(ethylferrocenyl)pentane (BisFc-OAc)
To a solution of BisFc-Cl (0.1 g, 0.19 mmol) in 2 ml DMF were added sodium acetate (0.05 g,
0.57 mmol) and NaI (0.3 g, 0.19 mmol) and this solution was heated to 120 ℃ overnight. After
this time period, the reaction mixture was cooled down to room temperature then partitioned
between water and dichloromethane. The organic layer was washed several times with water,
dried over anhydrous Na2SO4, and the solvent was evaporated. The crude product was purified
by column chromatography 9:1 (n-hexane-ethyl acetate) to afford BisFc-OAc as brown-orange
viscous oil in 94% yield [45].
FT-IR (KBr): 3090 (Ar-H), 2964 (C-H), 1737 (C=O), 1646, 1456 (C=C), 1239 (C-O), 484 (C-
1
Fe); H NMR (400 MHz, CDCl3): δ 1.09-1.19 (6H, m, CH2-CH3), 1.49-1.62 (5H, m, -CH3 and
C-CH2), 1.86-1.91 (2H, m, C-CH2-CH2), 2.04 (3H, s, (C=O) CH3), 2.25-2.35 (4H, m, Cp-CH2),
3.92-4.10 (18H, m, O-CH2 and Cp) ppm.
2.2.5. Synthesis of 4,4-bis(ethylferrocenyl)pentanol (BisFc-OH)
To a solution of BisFc-OAc (0.53 g, 0.96 mmol) in MeOH (40 ml) was added K2CO3 (1.41 g,
10.27 mmol). This suspension was stirred at room temperature for 2 h. Afterwards the reaction
mixture was diluted with distilled water and extracted with dichloromethane (3×10 ml). The
combined organic layers were dried over anhydrous Na2SO4 and the solvent was concentrated to
afford BisFc-OH as brown-orange viscous oil in 100% yield [46].

1
FT-IR (KBr): 3382 (OH), 3090 (Ar-H), 2963, 2872 (C-H), 1641, 1455 (C=C), 483 (C-Fe); H
NMR (400 MHz, CDCl3): δ 1.08-1.18 (6H, m, CH2-CH3), 1.47-1.48 (2H, m, C-CH2),1.57-1.63
(4H, m, -CH3 and OH), 1.81-1.90 (2H, m, C-CH2-CH2), 2.24-2.37 (4H, m, Cp-CH2), 3.54-3.56
(2H, m, CH2-OH), 3.90-4.07 (16H, m, Cp) ppm.
2.2.6. Synthesis of 2-[(4,4-bis(ethylferrocenyl)pentoxy)methyl]oxirane (BisFc-Epo)
A 25 ml, two necked round bottom flask equipped with dropping funnel and condenser was
charged with BisFc-OH (0.72 g, 1.4 mmol), NaOH (0.08 g, 2.05 mmol), and a catalytic amount
of Bu4NI (as a phase transfer catalyst). ECH (0.2 g, 2.1 mmol) was added dropwise to the
mixture and stirred at 30 ℃for 48 h. After two days, the reaction mixture was diluted with water
and extracted with dichloromethane (3×10 ml). The combined organic layers were dried over
anhydrous Na2SO4 and after the solvent was evaporated, the crude product was purified with
column chromatography 9:1 (n-hexane-ethylacetate) to afford BisFc-Epo as brown-orange
viscous oil in 75% yield [39].
1
FT-IR (KBr): 3089 (Ar-H), 2955 (C-H), 1644, 1458 (C=C), 482 (C-Fe); H NMR (400 MHz,
CDCl3): δ 1.08-1.18 (6H, m, CH2-CH3), 1.39-1.51 (2H, m, C-CH2)1.57-1.62 (3H, m, -CH3),
1.78-1.90 (2H, m, C-CH2-CH2), 2.23-2.36 (4H, m, Cp-CH2), 2.58-2.61 and 2.76-2.79 (2H, m,
CH2 epoxide), 3.10-3.13 (1H, m, -CH- epoxide), 3.32-3.43 (3H, m, -O-CH2-CH-CH2 and -CH2-
CH2-O-), 3.62-3.67 (1H, m, -CH2-CH2-O-), 3.90-4.05 (16H, m, Cp) ppm.
2.2.7. Preparation of PANI
PANI was prepared chemically by an interfacial polymerization method according to the
literature [47].
2.2.8. Preparation of GO

The GO product was prepared from graphite using Hummers method [48]. In brief, pristine
graphite (5 g), NaNO3 (2.5 g), and 130 ml conc. (98%) H2SO4 was placed into a 2-liter round
bottomed flask. This mixture was vigorously stirred for 1 h at 0 ℃. After that, 15 g KMnO4 was
slowly added to the above mixture and stirred for 72 h at room temperature. As the reaction
progressed, the dark color of the mixture was changed to light brown. Then, 350 ml distilled
water was slowly added to the mixture and after the mixture was cooled to 25 ℃, 60 ml H2O2
(30%) and 200 ml HCl (10%) solution was added into the flask, respectively. Finally, the GO
product was obtained after centrifuging/washing process (with HCl (5%) and water). After
freeze-drying, the GO nanosheets were obtained as a brownish powder.
2.2.9. Modification of GO with ethylenediamine (GO-EDA)
An amount of 200 mg of GO was added in 50 ml deionized water (DI) (4 mg/ml) and this
mixture was exfoliated by ultrasonication till homogeneous dispersion was formed. Afterwards,
1 ml EDA was slowly added into this suspension under vigorous stirring. The suspension was
stirred for 24 h at room temperature. After this time, the modified GO powder was centrifuged
and washed several times with ethanol and DI water, respectively. The collected powder was
dried in a vacuum oven at 60 ℃[49].
2.2.10. Synthesis of chemical-fonctionalized of GO-EDA with Fc moiety (GO-EDA-BisFc)
An amount of 100 mg of GO-EDA was dispersed in 50 ml dry toluene for 30 min. A catalytic
amount of PTSA and BisFc-Epo (300 mg) was added to this suspension and this mixture was
refluxed for 24 h at 110 ℃. At the end of the reaction, the solid powder was centrifuged at 1200
rpm and the obtained powder was thoroughly washed with DI water and dichloromethane,

respectively, until the filtrate became colorless followed by drying in a vacuum oven at 60 ℃ to
obtain the GO-EDA-BisFc as a black powder [50].
2.2.11. Synthesis of GO-EDA-BisFc/PANI nanocomposite
An aqueous suspension of PANI (2 mg ml-1) was added into the modified GO suspension in DI
water (2 mg ml-1) and the obtained mixture was stirred vigorously for 3 days at 25 ℃. At the end,
after centrifuging, the remaining precipitate was dried in a vacuum oven to gain the GO-EDA-
BisFc/PANI nanocomposite.
2.3. Characterization techniques
1
The H NMR data were collected on Bruker FT-400 spectrometers using TMS as an internal
1
reference and CDCl3 as a solution, respectively (The H NMR spectra are given in the
Supporting Information). FT-IR spectra were recorded using KBr disk on the Bruker-Tensor 270
spectrometer. The surface morphology, elementary composition, phase analysis, surface area and
pore size were analyzed using FESEM (MIRA3 TESCAN), EDX spectroscopy (MIRA3
TESCAN) and XRD analysis (PANalytica X pertPRO (Germany)) with a Cu-Kα (λ = 1.5406 Å)
radiation source, Brunauere-Teller (BET) (Belsorpmini II aaparatus at 196 ℃), and Barrete-
Jovnere-Halenda (BJH) (Belsorpmini II aaparatus at 196 ℃).
2.4. Electrochemical experiments
All electrochemical measurements were carried out in a conventional standard three electrode
cell configuration: 2 µl of a mixture of 80 wt. % synthesized materials (GO, GO-EDA, PANI,
GO-EDA-BisFc, and GO-EDA-BisFc/PANI nanocomposites), 10 wt. % carbon black and, 10 wt.
% polyvinylideneflouride in NMP was coated on a surface of carbon paper as the working
electrodes, a platinum wire as an auxiliary electrode and an Ag/AgCl as the reference electrodes.

All of electrochemical measurements were performed in 1 M H2SO4 aqueous solution as a
background electrolyte at room temperature. The electrochemical properties of GO, GO-EDA,
PANI, GO-EDA-BiFc, and GO-EDA-BisFc/PANI nanocomposites were measured by CV
(potential range of voltammograms between -0.2 to 0.8 V vs. Ag/Ag Cl), GCD and EIS
spectroscopy (EG&G PARSTAT 2263).
3. Results and discussion
3.1. Synthesis and characterization of Fc-based epoxy compound (BisFc-Epo)
The synthesis procedures of the novel bisferrocenyl based epoxy compound are shown in scheme
1.
A coupling reaction between EtFc and 5-chloro-2-pentanone as a coupling agent in dry
MeOH/H2SO4 leads to the formation of BisFc-Cl. Subsequently, BisFc-Cl was converted to the
acetate form by treatment with sodium acetate in DMF at 120 ℃, followed by the further
hydrolysis of the acetate groups with methanolic K2CO3 in 2 h to afford BisFc-OH. Finally,
BisFc-OH was reacted with epichlorohydrin in the presence of Bu4NI as a phase transfer catalyst
at 30 ℃for 48 h and BisFc-Epo was obtained as a novel ferrocenyl based epoxy compound.
1
The H NMR spectra of BisFc-Cl, BisFc-OAc, BisFc-OH, and BisFc-Epo in CDCl3 are
presented in Fig. S1. Also, the FT-IR spectra of the synthesized compounds were collected
between 400 and 4000 cm-1 and deposited as electronic supplementary information (see Fig. S2).
Electrochemical behaviors of the BisFc-Epo were investigated by the CV method (Fig. 1) that
performed in a potential range of 0.0 to 0.8 V and 0.1 M LiClO4 as a background supporting
electrolyte. Two pairs of chemically reversible redox peaks were emerged on the CV curve in
Fig. 3 that well separated together. These peaks were associated to the Faradic redox reaction of
Fc/Fc+ redox couples.

3.2. Synthesis of functionalized GO with BisFc-Epo (GO-EDA-BisFc)
The preparation process of the functionalized GO is illustrated in scheme 2. As can be seen from
the scheme 2, in the first step, the GO nanosheets were reacted with EDA as a linker in DI water.
EDA as a nucleophile was attacked the epoxy rings on the GO nanosheets and opened them by
ring opening reaction and the modified GO with EDA (GO-EDA) was formed. In the second
step, the active NH2 functional groups on the surface of the GO-EDA were reacted with the
BisFc-Epo in the dry toluene. Then, the epoxy ring on the BisFc-Epo was opened and covalent
bond was formed between the GO-EDA and BisFc-Epo.
3.3. Synthesis of GO-EDA-BisFc/PANI nanocomposite
After the functionalization of the GO nanosheets, both of the modified GO and neat PANI are
physically combined to form the final nanocomposite (Scheme 3). There is the synergetic effect
between modified GO nanosheets and neat PANI including hydrogen bonding, π-π stacking, and
electrostatic interactions (Fig. 2) [51, 52].
The FT-IR spectra for the BisFc-Epo, GO, PANI, GO-EDA, GO-EDA-BisFc, GO-EDA-
BisFc/PANI are shown in Fig. 3. As shown in the Fig. 3, in the FT-IR spectrum of BisFc-Epo the
peak located at about 500 cm-1 can be attributed to Fe-C band in the Fc rings. The peaks at
around 2900 and 3000 cm-1 can be related to the aliphatic and aromatic C-H stretching vibration,
respectively. The peak at around 1640 cm-1 can be assigned to the C=C vibration of Fc rings and
the peak at 1109 cm-1 can be attributed to the (C-O-C) in the epoxy ring. In the FT-IR of neat
PANI, the well defined characteristic peaks corresponding to the quinoid and benzenoid rings
were appeared at 1576 cm-1 and 1490 cm-1, respectively [53, 54]. The absorption peak at about

1300 cm-1 can be assigned to the C-N stretching vibration band. The observed peaks at around
1100 cm-1 and 800 cm-1 can be attributed to the C-H in-plane bending vibration and the out-of-
plane vibration, respectively [55]. The stretching vibration of N-H band was appeared at about
3400 cm-1. These data confirmed the formation of neat PANI. For FT-IR spectrum of GO, the
characteristic peaks were observed at about 3300 cm-1, 1700 cm-1, 1578 cm-1, 1200 cm-1, and
1050 cm-1 corresponding to the stretching vibration of O-H, C=O, C=C, C-O, and C-O-C bands,
respectively [56]. In the FT-IR spectrum of the GO-EDA, the appearance of the new peaks at
1360 cm-1, 2924 cm-1, and 3400 cm-1, which were respectively assigned to the stretching
vibration of C-N, C-H, and N-H bands, indicating the successful modification of GO with EDA
[57]. In the FT-IR spectrum of the GO-EDA-BisFc, a new peak at about 480 cm-1 was attributed
to the Fe-C, which reflects the covalenty-grafted of GO with BisFc-Epo compound. All
characteristic peaks of the neat PANI and the GO-EDA-BisFc were appeared in the FT-IR
spectrum of the final nanocomposite. The intensity some of oxygenated functional groups in the
spectra of the GO-EDA-BisFc/PANI were reduced, which may be because of the reduction of
these functional groups during the preparation of the final nanocomposite [58]. Finally, these
findings proved that the final nanocomposite was successfully synthesized.
The surface morphology of the all of the synthesized compounds was analyzed using FESEM
analysis. Fig. 4 illustrates the FESEM images of (a) PANI, (b) GO, (c) GO-EDA, (d) GO-EDA-
BisFc, and (e & f) GO-EDA-BisFc/PANI nanocomposites. As demonstrated in Fig. 4a, the neat
PANI was exhibited nanofibers type structure, which aggregated together. The FESEM image of
GO nanosheets (Fig. 4b) showed the layered structure with lots of wrinkles and crumpled
structure which can be related to the presence of various functional groups such as epoxy rings,

carboxylic acid, carbonyl and OH groups on the surface of GO nanosheets. The FESEM
micrographs of the GO-EDA (Fig. 4c) showed the similar layered structure of GO, However,
wrinkles on the surface have increased due to the modification of GO surface with EDA. As can
be seen from the micrographs of GO-EDA-BisFc (Fig. 4d), after surface modification of GO
with Fc groups the FESEM image of GO-EDA-BisFc showed the rough surface structure. The
presence of PANI nanofibers in the final nanocomposite gives a microporous structure
nanocomposite (Fig. 4e & 4f). Short diffusion length and high surface area between active
materials and electrolyte ions as a result of such microporous structure suggested that the final
nanocomposite can be applied in electrode materials for supercapacitor applications.
EDX analysis is an important method in order to investigate the elemental composition of the
synthesized materials. The EDX curves of the (a) PANI, (b) GO, (c) GO-EDA, (d) GO-EDA-
BisFc, and (e) GO-EDA-BisFc/PANI nanocomposite were illustrated in Fig. 5. As can be seen in
Fig. 5c, the EDX pattern of GO-EDA reveals the presence of the N content in the modified GO
nanosheets with EDA. The EDX spectrum of the GO-EDA-BisFc (Fig. 5d) confirmed the
successful modification of GO surface with Fc moiety. The increasing of N and C contents in the
EDX profile of the final nanocomposite (Fig. 5e) proved the coexistence of the PANI nanofibers
and modified GO in the GO-EDA-BisFc/ PANI nanocomposite.
The phase composition and crystalline structure of the PANI, GO, GO-EDA, GO-EDA-BisFc,
GO-EDA-BisFc/PANI were determined by XRD analysis and the related patterns were shown in
Fig. 6. The XRD pattern of PANI exhibits rather low crystallinity attributable to the repetition of
benzenoid and quinoid ring in the PANI chains [59, 60]. The sharp diffraction peak appeared at

2θ= 11.5o can be assigned to the (001) crystal plane of GO. The value of the interlayer spacing of
GO sheets depended on the oxygen functionality on the GO surface and the amount of adsorbed
water layer [61]. Compared with GO, the (001) reflection peak of GO-EDA has been shifted to
the lower angle (2θ= 10.5o) with an interlayer spacing of about 0.85 nm. These results essentially
prove that a covalent bond is formed between GO sheets and EDA. As shown from the XRD
pattern of GO-EDA-BisFc, modification of GO surface with BisFc-Epo compound has disrupted
the layer structure of GO sheets and XRD pattern of GO-EDA-BisFc shows low crystallinity.
The GO-EDA-BisFc/PANI exhibits all relevant related peaks of the composite components
confirming the successful synthesis of the final nanocomposite.
Pore structure analysis of GO-EDA-BisFc and GO-EDA-BisFc/PANI nanocomposites were
characterized by nitrogen adsorption-desorption BET isotherms at 77 K and the related diagrams
were shown in Fig. 7. In both cases, diagrams showed a typical IV shape isotherm with a minor
hysteresis loop at high pressures. These experimental results indicated that the GO-EDA-BisFc
and GO-EDA-BisFc/PANI nanocomposites have almost mesoporous structure. According to
data obtained from BET analysis, the GO-EDA-BisFc/PANI nanocomposite has a higher specific
surface area (16.82 m2 g-1) and average pore diameter (28.3 nm) than GO-EDA-BisFc
nanocomposite (9.31 m2 g-1), (22.0 nm), respectively. It can be concluded that incorporation of
porous structure and large specific surface area increases the rate of diffusion of the electrolyte
ions by reducing the diffusion path length from the bulk electrolyte to the active materials which
further reflected as increased the electrochemical performance of as-prepared electrodes [62-64].
3.4. Electrochemical evaluations
Fig. 8a demonstrates the CV voltammograms of all of the as-prepared electrodes obtained at a
scan rate of 50 mV/s in the potential range of -0.2 to 0.8 V. On the basis of the CV results,

voltammograms of the GO nanosheets and GO-EDA displays a similar rather rectangular shape
due to the predominance of EDLC behavior in capacitance by the suitable surface charge
storage. In the case of PANI, the presence of two redox peaks can be assigned to the redox
process of PANI between the well-defined oxidation states: the leucoemeraldine form
(semiconducting state) and polaronic emeraldine form (conducting state) [65]. Therefore, the
PANI has a battery-type energy storage behavior. The CV curve of GO-EDA-BisFc showed a
non-rectangular shape with a significant broad anodic and cathodic peaks, which can be assigned
to the existence of the Fc core in this compound. The GO-EDA-BisFc provides high charge
storage capacity due to the possession of both of EDLC behavior and the faradaic battery-type
characteristics of GO nanosheets and Fc (as a high electroactive material), respectively. It is
evident from the Fig. 8a, the CV curve area of the GO-EDA-BisFc/PANI are larger than the
other samples, which may be resulting from the presence of the PANI and Fc as faradaic redox
active materials in the final nanocomposite. Shorter diffusion length compared to the other
samples, facilitating the diffusion and migration of the electrolyte ions and increasing charge
storage capacity in the final nanocomposite. Fig. 8b illustrates the CV curves at various scan
rates (10 to 200 mV/s). It can be noted that with increasing the scan rate, the peak current
increases. These observations mean that the final nanocomposite has good ratability. The specific
capacity, Cs (mAh g-1), of the final nanocomposite at various scan rates was calculated from CV
curves based on the following equation:
Vc
1
(1)
Cs 
I(V)dV

3.6mvVa
where I, m, and v are the current, mass of the active material, and the scan rate, respectively. The
GO-EDA-BisFc/PANI nanocomposite shows specific capacities of 243.4, 239.3, 214.6, 196.7,
182.5, and 156.3 mAh g-1 at a scan rate of 10, 25, 50, 75, 100, and 200 mV s-1, respectively. Fig.

8c exhibits the specific capacities of the final nanocomposite at various scan rates. As shown in
Fig 8c, as the scan rate increases, the specific capacity decreases. The high capacity performance
at low scan rates can be attributed to the this fact that the ions at low scan rates have enough time
to access and penetrate into the electroactive material [66].
Additionally, the kinetic of the reaction of the final nanocomposite was analyzed via CV method
at different scan rates (10 to 100 mV s-1) (Fig. 9a). Generally, the current response at a fixed
potential consists of two separate charge storage mechanisms including capacitive and diffusion-
controlled process. The current response (i) can obey a power-law relationship with the scan rate
(v) as follows (Equation 2) [67, 68]:
i  avb
(2)
where a and b are the adjustable parameters. The b-value is determined from the slope of log i vs.
log v as indicated in Fig. 9b. In general, slope b=1 assigned to the capacitive effect, while the
slope b=0.5 corresponds to the ion diffusion-controlled process. According to this plot, the
calculated b-values for all currents are close to 0.5, indicating that the charge storage mechanism
of final nanocomposite is mainly controlled via ion-diffusion rather than capacitive process.
Also, the contribution ratio of these two charge storage mechanisms can be calculated using the
following equation:
i  k1vk2 v
(3)
where k1 and k2 are the adjustable parameters. The first and the second parts of the Equation (3)
refers to the capacitive and intercalation of ions, respectively, which can be obtained by
modifying the above equation.

i
 k1 v k2
v
(4)
Once we know the values of k1 and k2, the contribution ratio of faradaic and non-faradaic
mechanisms can be easily calculated. Fig. 9c exhibits the contribution ratio of two charge storage
mechanisms of the final nanocomposite vs. scan rate. As a result, faradaic contribution decreased
by increasing the scan rates while non-faradaic contribution increased by increasing the scan
rates. This observation might be due to electrolyte diffusion limitation at high scan rates [67, 68].
The charge storage retentions of the PANI, GO-EDA-BisFc, and GO-EDA-BisFc/PANI
nanocomposite after 3000 CV cycles at a scan rate of 75 mV s-1 are represented in Fig.10. As
shown in Fig. 10, the pure PANI and GO-EDA-BisFc show the charge storage retention of 74%
(poor cycling stability) and 95% (excellent cycling stability), respectively. The swelling and
shrinking of the PANI backbone during charging/discharging process can be considered as the
main reason of the poor cycling stability of PANI [69]. The GO-EDA-BisFc/PANI
nanocomposite shows good cycling performance (89%) compared to the pure PANI. The
improving cycle life suggests a synergistic effect between modified GO nanosheets and neat
PANI.
Fig. 11a represents the GCD analysis of the all of the as-prepared electrodes. These experiments
were performed in H2SO4 1 M at a current density of 2.5 A g-1. As can be seen in Fig. 11a, the
nearly triangular shapes of the GCD curve of GO-EDA suggest the EDLC and an ideal
capacitive behavior of GO nanosheests. The GCD curves of PANI, GO-EDA-BisFc, and GO-
EDA-BisFc /PANI show deviation from the regular triangular shape corresponding to the typical

redox reaction of the Fc moiety and PANI nanofibers and their faradaic battery-type effects. An
important point that can be deduced from the GCD curves is the longer discharge time of the
final nanocomposite compared to other materials, which may be due to the presence of Fc and
PANI in the final nanocomposite structure. The presence of Fc as an electroactive material and
PANI as a conducting polymer improves specific charge storage capability by increasing the rate
of charge transfer and shorten ion diffusion distance for electrolyte accessibility. Fig. 11b shows
the GCD curves of GO-EDA-BisFc/PANI at various current densities. An apparent increase in
charge storage capacity was observed with decreasing current density. This observation reflects
the fact that at high current densities the diffusion of the electrolyte ions is slow. Low diffusion
of the ions leads to an incomplete insertion reaction and consequently the low specific charge
storage capability.
The specific charge storage capacity Cs (in mAh g-1) of the all of the as-prepared electrodes were
calculated from the corresponding GCD curves via using the Equation (5):
i.t
m
(mAhg1)
(5)
Cs 
where i is the discharging current (mA), ∆t shows the discharge time (h), m denotes the mass of
the active material on the working electrode (g). At a current density of 2.5 A g-1, the calculated
values of Cs for the PANI, GO-EDA, GO-EDA-BisFc, and GO-EDA-BisFc/PANI
nanocomposites are 168, 72, 201 and 272 mAh g-1, respectively. These findings, again reveals a
synergistic effect between GO nanosheets, Fc, and PANI in the final nanocomposite which may
lead to the observed enhanced charge storage capability. Table 1 shows a comparison between
the present work and previously reported GO modified with Fc moiety electrodes. As can be
seen from the table 1, the GO-EDA-BisFc/PANI nanocomposite appears to be a good candidate
material for supercapacitor applications. The specific capacities of the PANI, GO-EDA, GO-

EDA-BisFc, and GO-EDA-BisFc/PANI at different current densities were depicted in Fig. 11c.
Fig. 11c shows that with increasing current densities, due to the limitation of ions diffusion
between electrolyte and surface of active materials, the capacity of the nanocomposites
decreases. According to the Fig. 11c, the final nanocomposite possesses higher capacity
compared to the other materials. Fig. 11d exhibits the cycling stability of the GO-EDA-
BisFc/PANI nanocomposite after 3000 GCD cycles at 10 A g-1. As indicated in Fig. 11d, the
final nanocomposite possesses good cycling stability (90%). Additionally, GO-EDA-
BisFc/PANI nanocomposite shows high Coulombic efficiency (CE) of ⁓100 after 3000 GCD
cycles. CE values can be calculated easily using Equation (6):
td
tc
(6)
 
where tc and td correspond to the charging and discharging time, respectively. The high CE and
good cycle performance are attributed to the unique structure of the final nanocomposite.
Table 1. Comparison between this work and other reported works.
Samples
Electrolyte
Potential window (V)
Specific capacity
Ref.
(assumed here as battery-type
pseudocapacitor) (mAh/g)
1M HClO4
1M HClO4
1 M H2SO4
-0.2 to 0.8 (Ag/AgCl)
-0.2 to 0.8 (Ag/AgCl)
-0.8 to 1.4 (Ag/AgCl)
200 at (0.5 A/g)
120 at (0.5 A/g)
280 at (1 A/g)
[70]
[71]
[72]
P(FcA-co-ANI)
P(FcA-co-ANI)
CNT@TFcP/Cu

1 M H2SO4
1 M H2SO4
1 M H2SO4
1 M H2SO4
1 M H2SO4
-0.2 to 0.8 (Ag/AgCl)
-1 to 1.5 (Ag/AgCl)
-0.8 to 1.2 (Ag/AgCl)
-0.8 to 0.9 (Ag/AgCl)
-0.8 to 1.2 (Ag/AgCl)
233 at (2.5 A/g)
139 at (2 A/g)
89 at (1 A/g)
[73]
[39]
[37]
[48]
[80]
GO-Fc/Mn3O4/PANI
GO-PolyFc
bA-Fc/rGO
114 at (1 A/g)
95 at (14 A/g)
CNTs/Cs-Fc
AzFc/rGO/PANI
GO-B-(EtFc)-
Pr3/PANI0.5
1 M H2SO4
1 M H2SO4
1 M H2SO4
-0.2 to 0.8 (Ag/AgCl)
-0.2 to 0.8 (Ag/AgCl)
-0.2 to 0.8 (Ag/AgCl)
429 at (2.5 A/g)
208 at (2.5 A/g)
272 at (2.5 A/g)
[81]
[40]
GO-Fc/PANI
GO-EDA-
this
BisFc/PANI
work
The Nyquist plots of the GO-EDA-BisFc and GO-EDA-BisFc/PANI nanocomposite are shown
in Fig. 12. The electrical equivalent circuit (EEC) used to fit the experimental data of impedance
spectra and the fitting parameter are summarized in table 2. As shown in table 2, from a
comparison of CPEdl-T values of GO-EDA-BisFc and GO-EDA-BisFc/PANI, it was found that
the GO-EDA-BisFc/PANI nanocomposite exhibits a higher interfacial capacitance than GO-
EDA-BisFc nanocomposite. These values are in agreement with results of GCD and CV
analyses.

Table 2. Fitting parameters of EIS data on equivalent circuit (EEC).
Rs (Ω)
0.51
Rct (Ω.cm2)
1.27E-4
Element
CPEdl-T (S.s-n) CPEdl-P
W1-R
16.63
0.37
W1-T
5.96E-4
3.96E-5
W1-P
0.22
3.05E-6
5.12E-6
0.98
0.91
GO-EDA-BisFc
0.30
2.45 E-1
0.26
GO-EDA-BisFc/PANI
Additionally, a symmetric supercapacitor (SSC) based on GO-EDA-BisFc/PANI nanocomposite
was fabricated to evaluate final electrochemical performance of the synthesized nanocomposite.
The CV curves of symmetric device at various scan rates in a maximum working potential of 1.8
V are shown in Fig. 13a. As the scan rate increasing from 10 to 50 mV s-1, the area under CV
curves increased and also the curve’s shape remained unchanged. This result clearly indicates
that our fabricated device exhibits good cycle life and rate capability. The GCD curves collected
at different current densities (Fig. 13b). All GCD curves exhibit small ohmic drop, indicating its
good rate capability. The corresponding specific capacity values calculated by Eq. 5 and the
resulted plot are shown in the inset of Fig. 13b. The specific capacities were measured to 77,
73.3, 72, 66.7, and 46.7 mAh g-1 at 1, 2, 3, 4, and 5 A g-1, respectively. The electrochemical
performance of our fabricated device and other previously reported PANI nanocomposites are
shown in Ragone plot (energy density vs. power density) (Fig. 13c). The maximum energy
density of 69.3 Wh kg-1 and maximum power density of 6171 W kg-1 for fabricated device were
calculated using E=1/2 CV (Eq. 7) and P=E/∆t (Eq. 8) [74], respectively. According to the
Ragone plot, our fabricated symmetric cell has the highest energy density and power density
values among previously reported literature results. The Nyquist plot of symmetric
supercapacitor (Fig. 13d) exhibits a smaller semicircle diameter, indicating a lower charge

transfer resistance. As shown in Fig. 13 e, after 3000 CV cycles, our fabricated device retained
86 % of initial capacitance that indicates a good cycling stability. According to the
electrochemical results, our device can be regarded as potential active materials for
supercapacitor applications.
4. Conclusion
In summary, we have successfully synthesized functionalized GO with bisferrocenyl based
epoxy compound by ring opening reaction. Then, the PANI nanofibers adsorbed onto the
modified GO surface via physical treatment to obtain the final high performance nanocomposite.
The final nanocomposite was evaluated by different analyses method such as FT-IR, CV, GCD,
EIS, BET, FESEM, EDX and XRD. The results showed that the presence of Fc and PANI as
electroactive materials in the GO-EDA-BisFc/PANI nanocomposite control the charge storage
mechanism. Therefore, the major part of the charge storage mechanism is pseudocapacitance.
The GO-EDA-BisFc/PANI nanocomposite shows high discharge capacity (272 mAh g-1 at 2.5 A
g-1) and excellent capacity stability (89 %). Also, a high energy density of 69.3 Wh kg-1 and high
power density of 6171 W kg-1 were achieved in a symmetrical two-electrode configuration.
Based on the obtained data, it can be concluded that the GO-EDA-BisFc/PANI nanocomposite
can be used as a suitable candidate in the battery-type supercapacitor applications.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.

Acknowledgements
This project is supported by a research grant of the University of Tabriz (Number: SAD/796-
970305).
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