Bipotential deposition of nickel–cobalt hexacyanoferrate nanostructure on graphene coated stainless steel for supercapacitors

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Bipotential deposition of nickelecobalthexacyanoferrate nanostructure on graphenecoated stainless steel for supercapacitors

Shahram Ghasemi*, Reza Ojani, Solmaz Ausi

Faculty of Chemistry, University of Mazandaran, 47416-95447 Babolsar, Iran

a r t i c l e i n f o

Article history:

Received 28 April 2014

Received in revised form

16 June 2014

Accepted 6 July 2014

Available online xxx

Keywords:

Graphene

Electrophoretic deposition

Bipotential method

Nickelecobalt hexacyanoferrate

Supercapacitor

* Corresponding author. Tel.: þ98 113530239E-mail addresses: [email protected], s

Please cite this article in press as: Ghasemgraphene coated stainless steel for superj.ijhydene.2014.07.026

http://dx.doi.org/10.1016/j.ijhydene.2014.07.00360-3199/Copyright © 2014, Hydrogen Ener

a b s t r a c t

Graphene oxide (GO) was deposited on inexpensive and mechanically stable stainless steel

(SS) electrode by electrophoretic deposition (EPD) technique. GO was reduced electro-

chemically in NaNO3 to obtain electrochemically reduced graphene oxide (ERGO). Next,

Hybrid nickelecobalt hexacyanofarrate (NiCoHCF) nanoparticles were deposited from so-

lution containing Niþ2 and Coþ2 with ratio of 1:1 on ERGO/SS by bipotential method.

Morphological investigation of prepared sample by scanning electron microscopy showed

the presence of nanoparticles with diameters in the range of 15e50 nm. Crystal structure of

nanocomposite was investigated by X-ray diffraction technique. Electrochemical behavior

of prepared film indicates that hybrid nanocomposite has higher specific capacitance

(411 F g�1) than ERGO (185.2 F g�1) in KNO3 solution at current density of 0.2 A g�1. In other

words, pseudocapacitor that is formed based on the faradaic behavior of NiCoHCF can

improve the capacitive performance of ERGO.

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

Electrochemical supercapacitors also known as super-

capacitors or electrical double layer capacitors (EDLCs) [1]

have attracted many research interests due to their variant

applications in portable electronics [2], electric tools and dig-

ital cameras [2,3]. Specific surface of electrode is one of the

most important items in supercapacitors to obtain high spe-

cific capacitance by increasing double layer capacitance and

embedded numerous superficial electroactive materials for

pseudocapacitance purpose [4]. Graphene as nanoscale de-

rivative of carbon material has extraordinary properties, such

as strong mechanical strength, high thermal/electrical

7; fax: þ98 [email protected]

i S, et al., Bipotential decapacitors, Internationa

26gy Publications, LLC. Publ

conductivity [5e7] and large surface area (theoretically

2630 m2/g for single layer graphene) [6], where it is larger than

carbon nanotube (1315 m2/g) and graphite (10 m2/g) [2]. Many

methods have been proposed for preparation of graphene

[8e11], but in many of cases, modified Hummer's method

followed by exfoliation is considered as a simple chemical and

low cost procedure, which produce GO in large scale [7,12].

However EDLCs use carbon material as electrode, they suffer

from low energy density. In order to overcome this problem,

researchers suggested pseudocapacitors which incorporate

electroactive materials with multiple oxidation states to car-

bon materials [4,13].

Reduction of graphene oxide and removal of oxygenated

functional groups on GO surface increases capacitance

(S. Ghasemi).

position of nickelecobalt hexacyanoferrate nanostructure onl Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/

ished by Elsevier Ltd. All rights reserved.

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e92

behavior of GO [5,14]. Composites of reduced graphene oxide

with various materials including polymers [15,16], metal hy-

droxides and oxides (such as Co(OH)2 [17], MnO2 [18], ZnO [19],

CuO [20] and NiS [21]) have been prepared to modify the

properties and performance of supercapacitors.

Electrophoretic deposition technique (EPD) is an inter-

esting method for preparation of coatings and thin films [22],

which deposits films on the substrates with applying constant

electric field between two electrodes. During the EPD, charged

colloidal particles migrate toward conductive electrode with

opposite charges and deposit coherently on it [23]. EPD of

graphene on various substrates, such as SS [22], indium tin

oxide (ITO) [23] and nickel foam [24] has been reported. The

advantage of this method is preparation of thin film of gra-

phenewith controllable thickness and uniformity [22]. SS is an

inexpensive substrate, mechanically stable, corrosion resis-

tant and it takes less deposition time to prepare same thick-

ness of deposited materials rather than other substrates,

which make SS be suitable electrode material in super-

capacitors [25].

Metal hexacyanoferrates (MHCFs, M: Zn, Co, Ni, Cu), also

known as Prussian blue (PB) analogs, form a class of zeolitic

inorganic compounds, which present interesting properties in

various research area, such as electrocatalysis [26e28].

Various combinations of these materials were synthesized by

chemical and electrochemical methods. Kulesza et al. pre-

pared NiCoHCF and NiPdHCF [29,30] as both thin films and

bulk precipitates. Reddy et al. prepared FeNiHCF by co-

precipitation of Fe3þ and Ni2þ in the presence of HCF [31].

Because of good electrochemical behavior of MHCFs, they can

be used in the charge storage systems. Safavi et al. prepared

hybrid NiCoHCF on stainless steel (SS) by electrochemical

deposition and investigated its application as electrode ma-

terial in supercapacitors [32]. NiCoHCF as analogs of the well-

known Prussian blue coordination compound can provide

good electrochemical performance in energy storage systems

[29,32]. NieCo alloys with high corrosion/heat resistance as

well as their magnetic and electrical properties attracted

much attention in technological applications [25]. So far,

many researchers investigated the modification of GO with

different types of organic and inorganic nanostructured ma-

terials to improve the capacitive performance of it [6], but the

preparation of NiCoHCF nanocomposite for supercapacitor

purpose has not been reported. In this work, first, GO was

deposited on SS by EPD and then it was reduced electro-

chemically (ERGO). Finally, ERGO was modified by hybrid

NiCoHCF with bipotential method [33] in order to obtain

NiCoHCF/ERGO nanocomposite. Capacitive performance of

fabricated electrode was evaluated by cyclic voltammetry

(CV), electrochemical impedance spectroscopy (EIS) and gal-

vanostatic charge and discharge techniques.

Experimental

Chemicals

Graphite powder, NaNO3, KNO3, H2SO4, H3PO4, Ni(NO3)2,

Co(NO3)2, K3[(Fe(CN)6)], H3PO4 were of analytical grade and

Please cite this article in press as: Ghasemi S, et al., Bipotential degraphene coated stainless steel for supercapacitors, Internationaj.ijhydene.2014.07.026

were purchased from Merck. GO was synthesized by modified

Hummers method [12].

Preparation of GO/SS and ERGO/SS

A power supply was used to apply direct current between

electrodes. Prior to each deposition, steel substrates were

polished galvanostatically to remove any surface impurity by

applying 5 A cm�2 for 5 min in a bath containing 50 vol.%

phosphoric acid, 25 vol.% sulfuric acid, and balanced deion-

ized water. SS with mirror-like surface was formed at the end

of polishing.

At first, GO was dispersed in double distilled water

(1.5 g L�1) and sonicated for 4 h. Next, it was deposited by EPD

on SS by applying direct current voltage of 5 V between SS

sheet (316, 5 cm � 1 cm � 1.5 mm) as positive electrode and

platinum foil (5 cm � 1 cm� 1 mm) as negative electrode with

distance of 1 cm. The deposition time was set on 7 min in

order to obtain the desired thickness of film. Finally, GO was

electrochemically reduced at �1.1 V (vs. Ag/AgCl/KCl (sat.)) in

0.5MNaNO3 for 2700 s in order to convert it to ERGO [14] ERGO/

SS was dried in oven at 70 �C for 2 h.

Preparation of NiCoHCF/ERGO nanocomposite

Electrochemical deposition and measurements were carried

out using AUTOLAB 302N (the Netherland) electrochemical

analyzer system with three electrode cell. Platinum foil and

Ag/AgCl/KCl (sat.) were used as counter and reference elec-

trode, respectively. Deposition of NiCoHCF was carried out in

cell containing 0.5 mM Co(NO3)2, 0.5 mM Ni(NO3)2, 0.5 mM

K3[Fe(CN)6] and 0.4 M KNO3. Electrodepositing of NiCoHCF film

was conducted on ERGO/SS as working electrode by bipoten-

tial method [33]. According to this method, the nucleation

potential (Vnuc) of 0.7 V was applied for 1 s and the deposition

potential (Vdep) of 0.2 V was applied for 350 s (Fig. 1(A)).

Moreover, like hybrid nanocomposite, NiHCF/ERGO and

CoHCF/ERGO were fabricated from electrodepositing solution

containing Ni(No3)2 or Co(NO3)2.

CharacterizationX-ray diffraction (XRD) patterns of ERGO/SS and hybrid NiC-

oHCF/ERGO/SS were recorded on X-ray diffractometer (GBC

MMA, Instrument) using Cu Ka radiation. The structure and

surfacemorphology of ERGO/SS and hybrid NiCoHCF/ERGO/SS

were studied by field-emission scanning electron microscopy

(KYKY-EM3200).

Results and discussion

During the EPD, GO containing functional groups with nega-

tive charges, such as eCOO� are drawn to the positive elec-

trode by applying voltage that results in the deposition of

yellow brown film on SS. A dark brown film (ERGO) was

formed by electrochemically reduction of GO film. During the

electrochemical reduction, some oxygenated functional

groups of GO, such as eC]O existed on the exfoliated GO

sheets are significantly removed. Iet curve (Fig. 1(B)), recorded

during the electrochemical reduction of GO film indicates two




Fig. 2 e FT-IR spectrum of prepared GO and ERGO (A); XRD pattern

SEM images of prepared (a) ERGO/SS and (b) NiCoHCF/ERGO/SS

Fig. 1 e Schematic illustration of applied bipotential

method for deposition of NiCoHCF, NiHCF and CoHCF on

ERGO/SS (A); Iet curve of electrochemical reduction of GO to

ERGO at ¡1.1 V (vs. Ag/AgCl) in 0.5 M NaNO3 (B).

i n t e rn a t i o n a l j o u rn a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e9 3


sharp peaks at 40 s and 180 s followed by a decrease in

reduction current density up to 800 s and afterwards, the

reduction current increases with time due to more conduc-

tivity of ERGO film. The reduction current density approxi-

mately levels off for a longer time and it reaches to constant

situation, which means that the conductivity of the prepared

film increases to nearly constant values [14].

FT-IR spectroscopy (Fig. 2(A)), revealed the presence of

functional groups on GO at ~1100 (CeO of alkoxy or epoxy),

~1400 (OeH of carboxyl), ~1670 (carboxyl C]O) and

~1750 cm�1 (C]O of carbonyl). When GO is reduced to ERGO,

the functional groups are removed from spectrum [34,35].

Hybrid NiCoHCF film or Single Ni or CoHCF was electro-

chemically deposited on ERGO/SS by applying bipotential

method from corresponding salt solution. By applying bipo-

tential method nucleation and growth of films can be

controlled to obtain the film containing MHCF nanoparticles.

During such electrochemical deposition method, the

arrangement of nanoparticles on ERGO is controlled and

binding of NiCoHCF is improved, so that the deposited film

could be stabilized without any weight loss for long time

(Fig. 1(A)).

Fig. 2(B) shows the XRD patterns of as prepared ERGO (a)

and NiCoHCF/ERGO (b) on SS. In both diffraction patterns, the

sharp peak at 2q ¼ 25� indicates the (002) plane of ERGO and

the peak at 2q ¼ 44� corresponding to SS peak due to the

presence of Ni and Co as fundamental materials in SS [34,36].

In NiCoHCF/ERGO/SS, two peaks at 2q ¼ 41 and 51� corre-

sponding to (002) plane of cubic Ni and Co in hexagonal sys-

temwere appearedwhich indicate the successfully deposition

of Ni and Co on ERGO/SS. The elemental analysis achieved by

atomic absorption method show that NiCoHCF deposited on

of a) ERGO/SS and b) NiCoHCF/ERGO/SS nanocomposite (B);

nanocomposite (C).





ERGOhas stoichiometry of K1.80Ni0.30Co0.80FeII(CN)6 which is in

good agreement with other previously reported work [29,32].

NiCoHCF film contain 5.74% and 13.59 (% weight) of Ni and Co.

Also, results show that NiHCF and CoHCF films contain 18.26

and 19.22 (weight %) of Ni and Co. As it can be seen, in NiHCF

and CoHCF, the metal loading is somewhat different probably

due to different kinetic of complex formation.

The morphology of the prepared electrode containing

NieCo hybrid is shown in Fig. 2(C). The sample consisted of

nanoparticles with average size in the range of 15e50 nm that

grow on the surface of ERGO. Such arrangement of nano-

particles provides suitable surface area for contribution in the

process of charge/discharge.

Fig. 3(A) shows the cyclic voltammograms of NiCoHCF/

ERGO and other electrodes in 1 M KNO3 at scan rate of

10 mV s�1. The main reaction during the electrochemical re-

action of compounds could be described by the following

expression:

Fig. 3 e Cyclic voltammograms of a) ERGO/SS, b) CoHCF/ERGO/SS

scan rate of 10 mV s¡1 (A); Cyclic voltammograms of NiCoHCF/E

and (f) 45 mV s¡1 (B).


MNiCoFe3þðCNÞ6 þMþ þ e-4M2NiCoFe2þðCNÞ6 (1)

where Mþ shows alkali metal ions, such as sodium or potas-

sium [37e39]. The cyclic voltammogram of NiCoHCF hybrid

that consists of two sets of redox peaks is shown by P1 and P2and they are ascribed to the redox process of Kþ-free NiCoHCF

complex and Kþ-rich one (Fig. 3(A)) [37]. A comparison be-

tween cyclic voltammograms of NiCoHCF/ERGO/SS film,

single-metal hexacyanoferrates/ERGO and ERGO/SS suggests

that CV of NiCoHCF/ERGO/SS would probably results from the

superposition of voltammetric responses of the two single-

metal hexacyanoferrates but some changes in peak intensity

and position as well as the overlap of peaks are evidence that

the composition of hybrid-metal hexacyanoferrate is different

from a simple combination of two single-metal hex-

acyanoferrates (NiHCF and CoHCF), which was reported in

previous work [29,32]. Also, from Fig. 3(A), it can be observed

that the area under voltammogram of hybrid NiCoHCF is

, c) NiHCF/ERGO/SS, and d) NiCoHCF/ERGO/SS in 1 M KNO3 at

RGO/SS at various scan rates (a) 5, (b) 10, (c) 15, (d) 25, (e) 35





larger than this area for single cobalt or nickel hex-

acyanoferrate andmore charge is accumulated on the surface

of NiCoHCF/ERGO,whichmakes it good candidate as electrode

materials in supercapacitors.

Fig. 3(B) shows the effect of scan rates on the cyclic vol-

tammograms of NiCoHCF/ERGO in the range of 5e45 mV s�1.

With increase in the scan rate, two different anodic peaks

overlap with each other; however, the cathodic peaks do not

overlap with each other in all scan rates. The observed change

in peak potential with increase in scan rate is probably due to

some difficulty in charge transfer kinetics.

The potential of the prepared NiCoHCF/ERGO/SS in super-

capacitorwas examined by constant current charge/discharge

technique in 1 M KNO3. The constant current charge/

discharge curves of the NieCo hybrid and other prepared

electrodes were obtained from applying specific current of

0.2 A g�1 that is shown in Fig. 4(A). A slope variation of the time

dependence on potential was observed for electrochemical

redox process occurring at the electrode and electrolyte

interface. This observation is evidence of a pseudocapacitance

Fig. 4 e Galvanostatic discharge curves at current density of 0.2

applied current densities of a) ERGO/SS, b) CoHCF/ERGO/SS, c) N


behavior of MHCFs [40]. Minor IR loss in discharge curve was

observed in NiCoHCF/ERGO rather than single Ni or CoHCF/

ERGO indicating the lower internal resistance for prepared

electrode. One of the main parameters in energy storage de-

vices is low internal resistance, which results in decrease in

energy waste and consequently there is less unwanted heat

during charge/discharge process [16]. This feature presents

another advantage of hybrid NiCoHCF/ERGO rather than

NiHCF/ERGO or CoHCF/ERGO. According to these results,

pseudocapacitor based on NiCoHCF hybrid as an inorganic

material and ERGO as a carbonaceous material shows good

performance during the charge/discharge cycles. Also, the

formation of NiCoHCF hybrid leads tomore effective influence

on capability of ERGO in comparison to single MHCF so that

more specific capacitance would be obtained.

The specific capacitance of different modified SS was

investigated as a function of current density (Fig. 4(B)). A

decrease in specific capacitance was observed by increasing

the discharge current density. One of the main features of

prepared film is its high stability in electrolyte, whereas no

A g¡1 (A); and variation of specific capacitance at different

iHCF/ERGO/SS and NiCoHCF/ERGO/SS in the 1 M KNO3 (B).




Table 1 e Electrical parameter values for NiCoHCF/ERGO electrode from galvanostatic discharge curves at various currentdensities.

Current density (A g�1) Discharge time (s) Specific capacitance (F g�1) Specific energy (W h kg�1) Specific power (W kg�1)

0.2 1644 411.0 36.5 79.9

0.3 856 321.0 28.5 119.9

0.5 489 305.6 27.1 199.5

0.7 337 294.9 26.2 279.9

1 225 281.3 25.0 400.0

2 103 257.5 22.9 800.4

3 63.5 238.1 21.2 1199.6

4 46 230.0 20.4 1596.5

5 35 218.8 19.4 2000.6

6 27 202.5 18.0 2400.0

7 23 201.3 17.9 2798.6

8 19 190.0 16.9 3198.3

9 16.2 182.3 16.2 3600.0


evidence of instability was observed during the experiments.

Also, the prepared hybrid films on ERGO film show lower

charge transfer resistances in comparison to ERGO, which

makes it good candidate for electrochemical studies.

The discharge-specific capacitance (Cs) of the NiCoHCF/

ERGO/SS electrode is calculated from the discharge curves by

means of the following formula [41]:

Cs ¼ itmv

(2)

where i shows the discharge current (A), t is the discharge time

(s), m is the mass (g) of the active materials on the SS surface

(calculated with the subtraction of the weight of steel elec-

trode from NiCoHCF/ERGO film) and V is potential window

(0.8 V) of discharge step. The weight of deposited film was

0.1 mg. The maximum specific energy and specific power are

measured with considering the following equations:

E ¼ 0:5CsV2

3:6W h kg�1

� �(3)

P ¼ iV2m

Wkg�1� �

(4)

where V is the operating potential window (0.8 V). The specific

capacitance and electrical parameters of hybrid NiCoHCF/

ERGO/SS at different specific current are summarized in

Table 1. Moreover, the specific capacitances of ERGO, single Ni

and CoHCF on ERGO are equal to 185.2, 245.6 and 245.5 F g�1 at

0.2 A g�1 (Table 2). The specific capacitances of Ni or CoHCF/

ERGO respectively are lower than NiCoHCF/ERGO (411 F g�1)

thatmeans the preference of hybridNiCoHCF for singleMHCF.

Table 2 e Specific capacitance values of prepared electrodes cacorresponding current densities.

Electrode

0.2 (A g�1) 0.3 (A g�1)

ERGO (F g�1) 185.2 144.3

NiHCF/ERGO (F g�1) 245.6 228.4

CoHCF/ERGO (F g�1) 245.5 196.5

Ni/CoHCF/ERGO (F g�1) 411.0 321.0


Fig. 5 shows Nyquist plots of ERGO and NiCoHCF/ERGO film

on SS electrode in 1 M KNO3. Measurements were recorded at

the corresponding open circuit potentials at the ranges of

100 kHz to 10mHzwith the ac voltage amplitude of 10mV and

the equilibrium time of 5 s. On ERGO/SS and at high-frequency

a linear response with a slope of z2.15 is observed which is

followed by a near vertical line at low frequencies region. A

coupled process of mass transport by diffusion and charge

accumulation in the film contribute in this process. The slope

of the linear tail at high frequencies is higher than a pure

Warburg line (unity). Also, at low frequencies region, slope

becomes lower than a pure capacitance (infinity). The

impedance response of ERGO is a typical capacitive behavior

which can be observed in the cyclic voltammetry of ERGO.

For NiCoHCF/ERGO/SS and NiCoHCF deposited on SS

(NiCoHCF/SS), a capacitive semicircle at high frequencies fol-

lowed by Warburg impedance at medium frequencies and a

capacitive-like behavior at low frequencies are observed. In

the intermediate frequencies an approximately 45� line can be

detectedwhich is the characteristic of ion diffusion toward the

electrode structure. Semicircle in high-frequency region is due

to the faradaic process which arises from electron-transfer

limiting step. Its effective diameter is equal to the faradaic

charge transfer resistance, which is responsible for the elec-

tron-transfer kinetics of redox reactions at the electrode-

electrolyte interface [42]. NiCoHCF/ERGO combines the pseu-

docapacitive behavior of NiCoHCF with capacitive behavior of

ERGO. In Nyquist diagrams of both NiCoHCF/ERGO/SS and

NiCoHCF/SS, the diameter of high-frequency semicircles are

relatively low,which indicates that the rate of redox transition

of Fe(II)/Fe(III) is very high. This fact confirmed by cyclic

lculated from charge/discharge curves measured at

Current density

0.5 (A g�1) 0.7 (A g�1) 1 (A g�1) 2 (A g�1)

130.4 124.4 118.8 115.3

205.0 168.0 151.3 142.5

166.3 145.7 129.2 115.0

305.6 294.4 281.3 257.5




Fig. 5 e Nyquist plots of ERGO/SS, NiCoHCF/ERGO/SS and NiCoHCF/SS at corresponding open circuit potentials in the

frequency range from 10 mHz to 100 kHz with a 10 mV ac amplitude in 1 M KNO3. Inset: equivalent electrical circuit

comparable with Nyquist diagram.


voltammetry is due to the high reactivity of prepared NiC-

oHCF. An electrical equivalent circuitmodel was employed for

the analysis of Nyquist diagram (Fig. 5, inset). In this circuit, Rs,

Rct andW show solution resistance, charge transfer resistance

and Warburg element. C1 and CPE present capacitor and

constant phase element corresponding to the double layer

capacitance. The charge transfer resistances of NiCoHCF and

NiCoHCF/ERGO are calculated to be approximately 7.5 and

2.6 U respectively. Although NiCoHCF has low charge transfer

resistance on two electrodes but some difficulty in charge

transfer resistance on NiCoHCF/ERGO/SS is observed with

respect to NiCoHCF/SS. It seems that the behavior of NiCoHCF

nanoparticles deposited on ERGO is somewhat different with

Table 3 e Examples of specific capacitance of graphene/metal composites for supercapacitors reported in otherwork.

Electrode Current density(A g�1)

Specificcapacitance (F g�1)

References

MoS2/

graphene

1 243 [43]

Fe2O3/

graphene

1 226 [44]

CoO/

graphene

1 139.47 [45]

MnO2/

graphene

0.5 276 [46]

MnC2O4/

graphene

0.5 122 [47]


homogeneous film of NiCoHCF on SS which provide good

connectivity and charge propagation thought it.

Table 3 shows specific capacitance of some graphene/

metal nanocomposite electrodes,which have been reported in

work of others, indicating well efficiency of prepared NiC-

oHCF/ERGO electrode.

The stability of the NieCo hybrid nanostructure was

investigated by repeated CV during 800 cycles at a scan rate of

25 mV s�1. A decrease in the value of specific capacitance was

observed during the first cycles of charge/discharge process.

From Fig. 6 approximately 83% of the initial capacitance was

retained after 800 cycles which makes it suitable for practical

applications of supercapacitors.

Fig. 6 e Capacitance retention of NiCoHCF/ERGO/SS

calculated from CV at a scan rate of 25 mV s¡1.





Conclusions

GO was deposited by EPD method on SS. The prepared film

binds strongly to the surface of SS. After electrochemical

reductionofGOtoERGO,NiCoHCFhybridfilmwasdepositedon

the surface of electrode by means of bipotential method. Dur-

ing it, a short pulse is applied to nucleate the nanoparticles of

NiCoHCFon the surfaceof ERGOfollowedby constantpotential

to grow nanoparticles during longer time. The electrochemical

performance of graphene is improved by incorporation of

NiCoHCF through capacitive behavior and faradaic redox re-

action, so that the specific capacitance increased from 185.2 to

411F g�1 at 0.2A g�1. In addition,minor IR losswasobserved for

NiCoHCF/ERGO in comparison to single MHCF and NiCoHCF/

SS. And this is the evidence of the performance of NiCoHCF/

ERGOnanocomposite as electrodematerial in supercapacitors.

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Bipotential deposition of nickel–cobalt hexacyanoferrate nanostructure on graphene coated stainless steel for supercapacitors