nickel-iron hexacyanoferrate coordination polymer nanocomposite for electrochemical energy storage

Accepted Manuscript Title: Preparation of graphene/nickel-iron hexacyanoferrate coordination polymer nanocomposite for electrochemical energy storage Author: Shahram Ghasemi Sayed Reza Hosseini Parvin Asen PII: DOI: Reference:

S0013-4686(15)00277-7 http://dx.doi.org/doi:10.1016/j.electacta.2015.02.002 EA 24270

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

1-11-2014 1-2-2015 1-2-2015

Please cite this article as: Shahram Ghasemi, Sayed Reza Hosseini, Parvin Asen, Preparation of graphene/nickel-iron hexacyanoferrate coordination polymer nanocomposite for electrochemical energy storage, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.02.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation of graphene /nickel-iron hexacyanoferrate coordination polymer nanocomposite for electrochemical energy storage Shahram Ghasemia,b, Sayed Reza Hosseinib, Parvin Asenb a b

Faculty of chemistry, University of Mazandaran, Babolsar, Iran

Nanochemistry Research Laboratory, Faculty of Chemistry, University of Mazandaran, 47416-95447 Babolsar, Iran

*Corresponding author. Tel: +981135302397 Fax: +98 1135302350. E-mail address: [email protected], [email protected] (S. Ghasemi)

Graphical abstract

Abstract

A new graphene/nickel-iron-hexacyanoferrate (graphene/Ni-Fe-HCF) nanocomposite was constructed and its electrochemical behavior was investigated. First, graphene oxide (GO) was deposited by electrophoretic deposition (EPD) technique onto stainless steel (SS). Then, it was electrochemically reduced to graphene (ERGO/SS) by applying constant potential at 1.1 in NaNO3. Ni-Fe-HCF hybrid were formed onto ERGO/SS from solution containing NiCl2, FeCl3 and K3 Fe(CN)6 by chronoamperometry. The surface morphology of constructed electrode was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM indicates the formation of nanoparticles in the range of 20-60 nm. Also, crystal structure of nanocomposite was characterized by using X-ray diffraction. The performance of prepared electrode was investigated by various electrochemical methods

using cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS). Results show that Ni-Fe-HCF hybrid has characteristics of battery-type material. Ni-Fe-HCF/ERGO nanocomposite has higher capacity (67.77 mAh g-1) than ERGO (32.5 mAh g-1) and Ni-Fe-HCF (20.97 mAh g-1) at 0.5 Ag-1. Also, it has more capacity than Ni-HCF/ERGO (44.58 mAh g-1) or Fe-HCF/ERGO (44.72 mAh g-1) at same current density. In addition, EIS results show Ni-Fe-HCF/ERGO has the lowest charge transfer resistance than ERGO and Ni-Fe-HCF. Cycle life studies resolve that Ni-Fe-HCF/ERGO shows good stability in 0.5 M KNO3 at pH=5. Key words: Nickel-iron hxacyanoferrate, Graphene, Nanocomposite, Electrophoretic deposition, Battery-type material.

1. Introduction: The formation of thin film of Fe 4  Fe CN

6  3

(PB) on platinum foil was first reported by

Neff [1]. It is recognized as the first synthetic coordination polymer compound which has general formula

M

A k

M 

B

A B CN 6  where M and M are transition metals with different

formal oxidation numbers [2]. Coordination polymers are a family of materials composed of 1D chain, 2D sheet and 3D network, containing organic ligands and inorganic metal ions connected to each other via strong bonds. Metal hexacyanoferrates (MHCFs, M: Ni, Co, Cu and Zn) known as PB analogues are coordination polymers because of the ability of polymerization and formation 3D structure. PB analogues have various applications in different research area such as chemical sensor and biosensor, electrocatalyst and charge storage [3-8]. There are three categories of samples which can be used in the energy storage systems: capacitor-type materials (EDLC), battery-type materials (such as Ni(OH)2 and NiHCF), and pseudocapacitive materials (such as MnO2 and RuO2). MHCFs are battery type materials that can be used as active material in batteries and hybrid supercapacitor [9-11]. PB analogues are desirable for use as battery electrodes because of tunable open channels that allow insertion of both molecular and ionic species. In battery-type materials, the energy density is generally twice that stored in

capacitor-type materials [10]. Cyclic voltammograms of MHCFs show characteristic peaks due to their redox reactions. The main drawback of PB analoguesis gradual dissolution during potential cycling [8]. In order to overcome this problem, several strategies such as application of conducting polymer

[8, 12] and preparation of hybrid MHCFs [8] have been introduced. Kulesza et al. [13, 14] reported the synthesis of hybrid Ni-Co-HCF and Ni-Pd-HCF with good stability. MHCFs are attractive candidate to construct various composites, which can be used in charge storage systems. Safavi et al. prepared stable hybrid Ni-Co-HCF on stainless steel (SS) and investigated its application in supercapacitor [8]. Poly(3,4-ethylenedioxythiophene)/MHCF (Ni-Co-Fe) [15], FeOOH@CoHCF [16], MnHCF/MnO2 [17], CuHCF [18], MnO2/NiHCF [10] and hybrid Ni-CoHCF [8] are some reported HCF-based capacitors. Carbon-based materials such as activated carbon, carbon nanotube, carbon aerogel and graphene employed as the most promising materials for supercapacitor. Among them, graphene is attractive candidate due to its large surface area (2675 m2 g-1) and high electrical conductivity [19, 20]. However, the conductivity of graphene film is mainly limited by agglomeration of graphene nanosheets because of the Van der Waals attraction between neighboring sheets [21]. It is believed that the agglomeration reduces the effective surface area of the graphene and thus can’t reflect the capacitance of an individual graphene sheet [20-23]. However, various parameters can influence on the capacitance of graphene and improve it such as preparation method and degree of reduction. The electrophoretic deposition (EPD) is an economical method in the preparation of thin film of graphene [24] which is done in two steps. In the first step, when electric field is applied, charged particles in suspension move toward the electrode with opposite charge and in the second step, they deposit on the electrode surface and form a coherent film.Various oxygen functional groups such as hydroxyl and carboxyl are formed on graphene oxide (GO) nanosheets during the chemical exfoliation and allow the formation of stable aqueous suspension used in EPD [25-27]. By applying voltage between electrodes, GO nanosheets with negative charge, immigrate toward the positive electrode and form a thin film on it.The deposition rate, thickness and uniformity of GO film can be controlled byvaryingthe time of deposition and passed current during EPD method [24]. To date, different substrate such as nickel foam, carbon fiber, carbon paper, carbon cloth and etc. have been used to deposit graphene. Moreover, SS has been considered in the electrode fabrication due to its availability and corrosion resistance. It was reported that GO film presents lower capacitance than reduced graphene oxide (RGO). Elimination of oxygen groups can increase the conductivity of GO film; consequently, capacitive behavior increases [28]. Electrochemical reduction of graphene oxide (ERGO) is one of the proposed methods to remove the functional groups. In comparison to chemical

treatment method, which use hazard material such as N2H4 and KOH, electrochemical reduction is simple, cost effective and eco-friendly technique to obtain ERGO film [29]. Composites of RGO with various materials including conducting polymer [30], metal oxide such as Co3O4 and SnO2 [31, 32] have been prepared to improve the electrochemical performance of energy storage device. Composites display better energy and power density in comparision with carbon materials. Scholz and Reddy [33] prepared hybrid of nickel-iron-hexacyanoferrate (Ni-Fe-HCF) by chemical precipitation in 1996. Also, Kumar et al. [34] synthesized electrochemically hybrid thin film of Nix– Fe(1−x)Fe(CN)6 on glassy carbon electrode and used it as electrocatalyst for H2O2. To the best our knowledge, there is no report on the use of hybrid Ni-Fe-HCF and NiFe-HCF/graphene (Ni-Fe-HCF/ERGO) nanocomposite. Herein, Ni-Fe-HCF/ERGO was constructed and its electrochemical behavior was investigated by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS).

2. Experimental 2.1. Chemicals and apparatus Graphite powder, NaNO3, H2SO4, H2O2, KMnO4, HCl, NiCl2.6H2O were purchased from Merck. KNO3 and K3 Fe(CN)6 were purchased from Fluka. FeCl3.6H2O was purchased from BDH. The electrochemical experiments were carried out using AUTOLAB 302N (Netherland) electrochemical analyzer with three-electrode cell. The employed electrodes were MHCF/ERGO/SS (M=Ni, Fe), platinum foil and Ag|AgCl (3M KCl, Azar electrode, Iran) as working, auxiliary and reference electrode, respectively. The characterizations of electrodes were carried out withfield-emission scanning electron microscopy (KYKY-EM 3200), X-ray diffraction (XRD) by X-ray diffractometer (GBC MMA) using Cu Kα irradiation. AFM was carried out in ambient conditions using Ara-research (0101/A, Iran), operating in non-contact mode. Fourier transform infrared (FT-IR) spectra were recorded with KBr pellet on a VECTOR- 22 (Bruker) spectrometer. UV-V is absorption spectrum was performed using UV-V is spectrometer PG Instruments Ltd.

2.2. Synthesis of graphite oxide Graphite oxide was synthesized by modified Hummer ҆s method [35]. Typically, 1 g graphite and 1 g NaNO3 were mixed with 23 mL concentrated sulfuric acid in an ice bath. 4 g KMnO4 was added gradually to yield a green-purple mixture. Then, the mixture was maintained at 0 °C for 1 h under stirring. Afterward, 180 mL double-distilled water was added slowly during

1 h. Temperature of mixture increased to 98 ᵒC. After 15 min, 10 mL of H2O2 was transferred to the suspension in order to terminate the reaction. The color of suspension changed from brown to yellow. The resulting product was filtered and rinsed with 5% HCl solution and then washed several times with distilled water to adjust the pH~6. The obtained product was dried at room temperature.

2.3. Preparation of GO/SS In order to obtain SS with mirror like-surface, SS substrate was polished galvanostatically by applying 5 A cm-2 for 2 min in a bath containing 50 vol. % phosphoric acid, 25 vol. % sulfuric acid, and balanced distillated water. After electropolishing, SS substrate was rinsed with water and then the area of 1 cm2 was assigned by covering with polytetrafluoroethylene tape. The electrophoretic cell consisted of two electrodes; SS (grade 304, 1 cm2) and platinum (Pt) foil as positive and negative electrode, respectively. These electrodes were placed vertically and immersed in suspension containing 1.5 mg mL-1 graphene oxide while the distance between them was kept 1 cm. GO nanosheets were deposited onto SS by applying a constant voltage of 5 V for 10 min. Afterward, GO/SS was dried at room temperature for 1 h.

2.4. Electrochemical reduction of GO/SS Three-electrode cell was used in order to reduce GO to ERGO. ERGO/SS was prepared by applying potential of -1.1 V to GO/SS in 0.5 M NaNO3 solution for 1000 s [29]. After reduction, ERGO/SS was washed with distilled water and then dried in the oven at 70 ᵒC for 1 h.

2.5. Preparation of Ni-Fe-HCF/ERGO/SS Electrochemical formation of Ni-Fe-HCF onto ERGO/SS and SS was carried out in cell containing 0.5 mM NiCl2.6H2O, 0.5 mM FeCl3.6H2O, 0.5 mM K3 Fe(CN)6, 0.5 mM HCl and 0.1 M KNO3 under constant potential of 0.35 V versus Ag|AgCl during 300 s. After electrodeposition, it was rinsed with distillated water and dried in oven at 70 ᵒC for 1 h. In order to compare the electrochemical behavior of Ni-Fe-HCF/ERGO nanocomposite with single-metal HCF/ERGO (Ni or Fe), various electrodes were prepared at the same condition and voltage from solution containing NiCl2 or FeCl3, respectively.

2.6. Atomic absorption analysis

The sample solutions were prepared by dissolving the film of Ni-Fe-HCF in 2 cm3 concentrated sulfuric acid and then the samples were diluted with water to 200 cm3. Atomic absorption measurements were carried out by nova (400 P, Germany). These processes were repeated for three times.

3. Result and discussion GO film on SS was deposited by electrophoretic deposition and electrochemically reduced to ERGO film by chronoamperometric method at -1.1 V vs. Ag|AgCl in 0.5 M NaNO3 solution. Schematic illustration of deposition and electrochemical reduction of GO film is presented in Fig. 1A. Also, I-t curve obtained from electrochemical reduction of GO film is shown in Fig. 1 B. It can be seen that during the reduction process, the current increases over time, which is due to the removal of functional groups and the formation of π-conjugation structure [29]. Consequently, the conductivity and capacity of the film improve during such process. Cyclic voltammograms of ERGO and GO are shown in Fig. 1C. ERGO has broader shape than GO, indicating the increase in capacity of the GO film through reduction process which is due to the elimination of functional groups [29]. The suspension of GO is characterized by UV-Vis. From Fig. 2A, maximum absorption at 229 nm is attribute to π-π* of aromatic C-C bonds. Also, a shoulder at 300 nm is due to n-π* transitions of C-O bonds [36] . Fig. 2B shows FT-IR spectra of GO and ERGO. The spectrum of GO displays O-H of water at 3440 cm-1, C-O of alkoxy and epoxy at 1059 and 1200 cm-1. Also, C=O of carbonyl and carboxyl are observed at 1740 and 1620 cm-1, respectively. When GO film is electrochemically reduced to ERGO, the adsorption bonds of oxygen functionalities decrease substantially [37, 38] Ni-Fe-HCF was depositedon ERGO/SS by applying constant voltage and the crystal structure was determined by XRD (Fig. 3). The diffraction peaks at 2θ=65, 75 and 82 º indicate the presence of (002), (220) and (112) planes of PB in cubic system. These peaks are appeared at highangles which may be due to the preparation method and particle size of PB. The peak positions shift toward the larger 2θ values with decreasing the size of particles [39, 40]. The broad peak is observed at 2θ=25 º which is corresponded to (002) plane of ERGO film. The sharp peak around 2θ=45º arises from SS substrate. Fig. 4A and B displays the morphology of the prepared Ni-Fe-HCF film on SS. It is covered with agglomerated nanoparticles. The appearance of cracks on Ni-Fe-HCF film is due to oxygen evolution process occurred during the electrochemical oxidation of water. Fig. 4C

and D shows the surface morphology of ERGO film. In ERGO film, aggregation occurs and appears as wrinkles on the surface. The local swelling of nanosheets may be due to the evolution of gas bubbles from electrolysis of water underneath the deposited GO film on SS substrate during EPD [24, 41, 42]. Fig. 4E and F show SEM images of Ni-Fe-HCF nanoparticles formed uniformly on ERGO film. The average sizes of spherical Ni-Fe-HCF nanoparticles are in the range of 20-60 nm. It seems that the potentiostatic method provides a suitable way to form Ni-Fe-HCF nanoparticles on graphene film. AFM images provide morphological information about ERGO and Ni-Fe-HCF/ERGO. Fig. 5 shows topology of the surface of ERGO/SS and Ni-Fe-HCF/ERGO/SS as 2D (A) and 3D (B) images recorded over area of 10 μm ×10 μm. The graphene wrinkles can be observed on the surface of prepared electrode. Compared with image of ERGO, the surface of the nanocomposite is rougher than ERGO, which can be attributed to the growth of Ni-Fe-HCF nanoparticles on ERGO film. Fig. 6A shows cyclic voltammograms of ERGO and Ni-HCF, Fe-HCF and Ni-Fe-HCF on ERGO substrate in 0.5 M KNO3 (pH=5) at scan rate of 0.02 V s1

. Cyclic voltammogram of ERGO exhibits rectangular shape, indicating good capacitive

behavior. The specific capacitance primarily originates from the double-layer capacitance. After deposition of Ni-HCF, Fe-HCF and Ni-Fe-HCF on ERGO/SS, the shape of the curves reveal that the electrochemical behaviors are different from electric double-layer capacitance of graphene which indicate the capacity originated mainly from the battery-type behavior of MHCFs. In Fe-HCF/ERGO, one couple redox peaks appears at half wave potential of 0.2 V, which may be related to the redox transition of Fe (II/III) couple coordinated via nitrogen atom of CN group (high spin iron ion). Also, low spin iron ions coordinated with carbon atom of the ligand is active at higher potential and appears as a redox couple at half wave potential of 0.88 V [43-45]. In peaks appeared at half wave potential of 0.2 V, cation exchange contribute in electron transfer reaction:

KFe III  Fe II CN

6   e   K   K 2 Fe II  Fe II CN 6 

PB

(1)

Everittʼs salt (ES) or Prussian white

In the second couple of redox peaks appeared at half wave potential of 0.88 V, the following mechanism occurs:

Fe III  Fe III  CN 6   e   K   KFe III  Fe II  CN 6  BG (Berlin green)

(2)

PB

In cyclic voltammogram of Ni-HCF/ERGO, two redox peaks are observed. One couple of redox peaks appear at half wave potential of 0.48 V [43]. The reaction is:

KNi II  Fe III CN

6   e   K   K 2 Ni II Fe II CN 6 

(3)

Another redox peaks appeared at half wave potential of 0.61 V is due to following reaction:

II II  Fe III  CN 6   e   K   KNi1.5  Fe II  CN 6  Ni1.5

(4)

A comparison between Ni-Fe-HCF/ERGO and single-metal HCF/ERGO suggest that the peak positions and intensity are different. Therefore, the composition of Ni-Fe-HCF isn’t a simple mixture of two single-metal HCF. From area under cyclic voltammogram, which is in relation with accumulated charge, it can be observed that the charge reserved on the surface of Ni-Fe-HCF/ERGO nanocomposite is more than single-metal HCF/ERGO. When Ni-FeHCF nanoparticles are formed on ERGO surface, two couple of redox peaks appears due to the faradic reaction of Ni-Fe-HCF on ERGO surface. The presence of the two well-separated redox peaks is the characteristic of a battery-type material. It seems that the redox peaks of Ni-Fe-HCF hybrid superimpose on ERGO cyclic voltammogram. In other words, charge accumulation on ERGO is increased and charge can restore on both ERGO and Ni-Fe-HCF. A comparison between Ni-Fe-HCF hybrids on ERGO film and SS is shown in Fig. 6B. The cyclic voltammogram of Ni-Fe-HCF/ERGO/SS is wider than Ni-Fe-HCF/SS which arises from the double layer capacitance of the graphene and a large quantity of charge accumulate on the surface of Ni-Fe-HCF/ERGO. A decrease in Ni-Fe-HCF particle size to the nanoscale regime can provide both higher amounts of total stored charge and faster charge/discharge rates [46]. Moreover, ERGO improves the conduction of electrons throughout the film and also provides excellent interfacial contact between Ni-Fe-HCF and ERGO [47]. It seems that Ni-Fe-HCF/ERGO

has better performance than other electrodes for energy storage. It is necessary to mention that SS has a small area surrounded by its cyclic voltammogram curve. SS do not show capacitive behavior and therefore low charges can accumulate on it (Fig. S1).

The stoichiometry of Ni-Fe-HCF film was determined by atomic absorption method.The results show that the hybrid film has stoichiometry of K1.80NiII0.70FeII0.50FeII(CN)6. Moreover, atomic absorption shows that hybrid of Ni-Fe-HCF isn’t a simple mixture. Fig.7 A shows the electrochemical behavior of Ni-Fe-HCF/ERGO nanocomposite at various scan rates. As the scan rate increases, the potential differences between oxidation and reduction peaks shift somewhat which arises from the internal resistance of the electrode. Fig.7 B and C shows the influence of scan rate increment on anodic and cathodic peak currents at Ni-Fe-HCF/ERGO. Peak currents appeared at potential of around 0.2 and 0.8 V are linearly proportional to the scan rate in the range of 0.005-0.05 V s-1, which indicates the presence of a surface-confined electrochemical behavior. It can be attributed to dynamic of charge propagation in the layer of Ni-Fe-HCF/ERGO. The capacity of Ni-Fe-HCF/ERGO is negatively correlated with the scan rate. The capacity of Ni-Fe-HCF/ERGO decreases with increasing the scan rate which can be attributed to the diffusion of ions from the electrolyte into the active materials. At high scan rate, the diffusion effect limits the migration of the electrolytic ions to the accessible outer surface and effective interaction between ions and electrode material reduces. Higher scan rates is accompanied with the insufficient time available for ion diffusion and adsorption of ions inside the active materials. In pseudocapacitive materials, difference between the peak potentials remains small with increasing the scan rate while in battery-type materials it increases [48]. In pseudocapacitive materials, redox peaks overlap with each other when the sweep rate increases but in batterytype materials, redox peaks remains separate even at high scan rates. These features distinguish battery-type and pseudocapacitive materials. In order to obtain information about the ability of Ni-Fe-HCF/ERGO, Ni-HCF/ERGO, FeHCF/ERGO and Ni-Fe-HCF as electrode material, constant current charge-discharge measurement was carried out in 0.5 M KNO3 (pH=5) between 0 and 1 V (vs. Ag|AgCl) (Fig. 8). The curves of Ni-Fe-HCF/ERGO, Ni-HCF/ERGO, Fe-HCF/ERGO and Ni-Fe-HCF/SS reveal a slope variation of the time dependence on potential indicating battery-type behavior of MHCFs [49]. This arises from the electrochemical redox reactions at the interface between electrode material and electrolyte. In carbon materials such as ERGO, a quasi-linear voltage-time response is due to the typical capacitor behavior. In charge-discharge curves of Ni-Fe-

HCF/ERGO/SS, plateaus can be observed which arise from the faradic reaction of Ni-FeHCF on the surface of ERGO. The shape of curves are the characteristic of Ni-Fe-HCF and ERGO which are in agreement with the results of the cyclic votammograms. The observed behavior shows that Ni-Fe-HCF has characteristics of a battery-type material.

Electrochemical redox reactions included in charging and discharging process of Ni-Fe-HCF may be ascribed to the following reaction: KNi II Fe II  Fe III  CN 6   K   e  K 2 Ni II Fe II  Fe II  CN 6 

(5)

Capacities of constructed electrodes were obtained from charge-discharge curves. The capacity of Ni-Fe-HCF/SS, ERGO/SS, Ni-HCF/ERGO, Fe-HCF/ERGO and Ni-FeHCF/ERGO at current density of 0.5 Ag-1 are 20.97, 32.5, 44.58, 44.72 and 67.77 mAh g-1, respectively. The capacity of Ni-Fe-HCF/ERGO is much higher than Ni-HCF/ERGO and FeHCF/ERGO which suggest Ni-Fe-HCF hybrid has better performance than single-metal HCF (Ni or Fe) on ERGO film during charge-discharge. The constant current charge-discharge curves of Ni-Fe-HCF/ERGO/SS at different current densities are shown in Fig. 9. With increasing current density, capacity decreases. The capacities are 67.77, 63.5, 58.72, 55.33, 52.25 and 45.83 mAh g

-1

at current densities of 0.5,

-1

0.6, 0.7, 0.8, 0.9 and 1 A g , respectively. The columbic efficiency (  ) is evaluated from galvanostatic charge-discharge curves using Eq. (6):



td 100% tc

(6)

Where td and tc are the time of discharge and charge, respectively. The values of  are in the range of 95 % for Ni-Fe-HCF/ERGO/SS electrode. It is seen that the color of Ni-Fe-HCF on SS substrate switches reversibly between blue and colorless during the charge-discharge process. On Ni-Fe-HCF/ERGO film, the color change occurs but due to presence of graphene (a black brown film) this process is less obvious. According to the equation (5), the color change is due to the redox process of Ni-Fe-HCF hybrid during charge-dischargeprocess. The stability of Ni-Fe-HCF films depend on pH of solution. Decomposition of Fe-HCF can take place in neutral or alkaline solutions [50] but degradation may occur slowly in acidic solution.

Fe 4  Fe CN

6   3H 2O  Fe OH 3  3Fe Fe CN 6   3H  

4

Fe  Fe  CN 6   3OH   Fe  OH 3  Fe  CN  6

(7) (8)

It seems that the instability of Ni-Fe-HCF may be related to the damage in Fe-HCF. Ni-FeHCF can maintain its stable response in weakly acidic solution. Ni-Fe-HCF/ERGO nanocomposite shows high stability in 0.5 M KNO3 with pH=5. Consequently, pH=5 was chosen as a suitable pH in order to investigate the electrochemical behavior. Also, ERGO film is instable in strong acidic and alkaline solutions and may detach from SS but its stability and electrochemical behavior is good in the range of 4-5. Fig. 10 A shows cyclic stability of single-metal HCF/ERGO (Ni or Fe) and Ni-Fe-HCF on ERGO and SS at pH=5. Cyclic voltammograms of Ni-Fe-HCF/ERGO/SS for 1st and 500th cycles at scan rate of 0.02 V s-1 are shown in Fig. 10 B. The shape, peak position and width of cyclic voltammograms change somewhat at different cycle numbers. The reversibility of redox reactions on Ni-Fe-HCF/ERGO is nearly good during potential cycling but the area of cyclic voltammograms reduces slightly which indicates the slight decline of capacity. The presence of graphene decreases the decomposition of Ni-Fe-HCF so the capacity of Ni-FeHCF/ERGO remained up to 95 % of initial value after 500 cycles. In the absence of graphene, the loss of stability of Ni-Fe-HCF on SS occurred quickly with repeated cycles so the capacity decreased to 80 % of initial value. Also, a comparison between the stability of Ni-Fe-HCF/ERGO with single-metal HCF/ERGO (Ni or Fe) revealed that Ni-Fe-HCF/ERGO has higher stability than Ni-HCF/ERGO and Fe-HCF/ERGO. Power and energy density are two important parameters. They can be calculated according to following equations (9, 10) from charge-discharge curves at various current densities:

E  0.5. P

Cs .  V 3.6

E t disch arg e



.3600

2

(9) (10)

E and P are energy (Wh kg-1) and power density (W kg-1), respectively. Ragone plot (power density vs. energy density) of Ni-Fe-HCF/ERGO/SS, Fe-HCF/ERGO/SS, Ni-HCF/ERGO/SS ERGO/SS and Ni-Fe-HCF/SS are shown in Fig. 11. The energy density of Ni-FeHCF/ERGO increases from 22.91 to 33.88 Wh kg-1 while the specific power decreases from 500 to 250 W kg-1 as the current density decreases from 1 to 0.5 A g-1. Ni-Fe-HCF/ERGO/SS is a ʺhigh powerʺ battery-type electrode (with supercapacitive-like behavior). EIS analysis is a powerful technique to evaluate the fundamental behavior of electrode materials. Fig. 12 shows the Nyquist plots for ERGO/SS and hybrid Ni-Fe-HCF on both ERGO and SS substrate in 0.5 M KNO3 (pH=5). Measurements were recorded at the open

circuit potentials in the frequency range of 20 KHz to 10 mHz with ac amplitude of 10 mV and equilibrium time of 1 s. The Nyquist plots for ERGO/SS and Ni-Fe-HCF/ERGO/SS and Ni-Fe-HCF/SS consist of a capacitive semicircle at high frequencies. The semicircle is related to the faradaic process which arises from the presence of electron transfer limiting step. Also, its diameter is equal to the faradaic charge transfer resistance (Rct). The diameter of semicircles for Ni-Fe-HCF/ERGO and Ni-Fe-HCF/SS are small, indicating the fast redox reactions of Fe(II)/Fe(III) at the electrode surface. Also, in intermediate frequencies, an approximately 45ᵒ lines can be observed for Ni-Fe-HCF/ERGO and Ni-Fe-HCF/SS. These straight lines arise from Warburg resistance (W) caused by frequency dependence of ion diffusion-transport from electrolyte to the electrode surface. Moreover, the vertical lines are observed for Ni-Fe-HCF/ERGO and ERGO/SS in low frequencies due to capacitive behavior. It is a result of the fast diffusion and quick adsorption of ions from the solution onto the electrode surface. The equivalent circuit accordance with the Nyquist plots is shown in Fig. 12. In this model, Rs represents the solution resistance caused by ionic resistance of the electrolyte. Rct and W are joined in series which can be related to the faradic impedance.In order to model the double layer capacitor (Cdl), constant phase element (CPE) is used rather than capacitor because of the surface roughness and distribution of the reactions on the surface [51]. CPE and Rct are in parallel in equivalent circuit which indicates the charge transfer limiting processes. Rct can be determined directly by Nyquist plots as semicircle arc diameter. The calculated Rct values for ERGO, Ni-Fe-HCF/SS and Ni-Fe-HCF/ERGO are 46.25, 41.35 and 26.78 Ω, respectively. The Ni-Fe-HCF/ERGO has the lowest Rct value compared with other active materials, which indicates the fast electron transfer rate between electrode material and electrolyte.

4. Conclusions Ni-Fe-HCF/ERGO nanocomposite was constructed by electrophoretic deposition of GO onto SS, electrochemical reduction of GO (ERGO/SS) and then electrodeposition of Ni-Fe-HCF onto ERGO/SS. SEM images of Ni-Fe-HCF/ERGO reveal the presence of Ni-Fe-HCF nanoparticles in the range of 20-60 nm on ERGO surface. Electrochemical performance of Ni-Fe-HCF/ERGO nanocomposite was examined by cyclic voltammetry, galvanostatic charge-discharge and EIS. Nonlinear charge-discharge curves as well as the presence of redox peaks in cyclic voltammograms are evidences of battery-type materials. The results show that Ni-Fe-HCF/ERGO nanocomposite exhibit higher capacity than single-metal

HCF/ERGO (Ni or Fe) and Ni-Fe-HCF/SS. The improvement in the capacity of Ni-FeHCF/ERGO can be attributed to the battery-type behavior of Ni-Fe-HCF and double layer capacitance of ERGO. Stability of the nanocomposite was examined at pH=5 in 0.5 M KNO3. Ni-Fe-HCF/ERGO retains its capacity up to 95 % of initial value after 500 cycles. Therefore, Ni-Fe-HCF/ERGO nanocomposite is a promising active material for energy storage in the future.

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Fig. 1: (A) Schematic illustration of deposition and electrochemical reduction of GO film, (B)

I-t curve of electrochemical reduction of GO and (C) cyclic voltammograms of GO and ERGO.

Fig. 2: (A) UV-Vis spectrum of GO and (B) FT-IR spectra of GO and ERGO. Fig. 3: XRD pattern of Ni-Fe-HCF/ERGO/SS. Fig. 4: SEM images of (A and B) Ni-Fe-HCF/SS, (C and D) ERGO/SS and (E and F) Ni-FeHCF/ERGO in two magnification. Fig. 5: (A) 2D and (B) 3D AFM images of ERGO/SS and Ni-Fe-HCF/ERGO/SS. Fig. 6: (A) Cyclic voltammograms of ERGO, Ni-HCF/ERGO, Fe-HCF/ERGO and Ni-FeHCF/ERGO and (B) Cyclic voltammograms of ERGO, Ni-Fe-HCF/SS and Ni-FeHCF/ERGO in 0.5 M KNO3 (pH=5) at scan rate of 0.02 V s-1. Fig. 7: (A) Cyclic voltammograms of Ni-Fe-HCF/ERGO in 0.5 M KNO3 (pH=5) at various scan rates: a) 0.005, b) 0.01, c) 0.02, d) 0.030,e) 0.040 and f) 0.050 V s-1. (B) and (C) cathodic and anodic peak currents vs. scan rate of the first and second set of the redox peaks, respectively Fig. 8: Galvanostatic charge-discharge curves of Ni-Fe-HCF/SS, ERGO/SS, NiHCF/ERGO/SS, Fe-HCF/ERFO/SS and Ni-Fe-HCF/ERGO/SS at current density of 0.5 A g-1 in 0.5 M KNO3 (pH=5). Fig. 9: Galvanostatic charge-discharge curves of Ni-Fe-HCF/ERGO/SS at different current densities.

Fig. 10: (A) Capacity retention of Ni-Fe-HCF/SS, Ni-HCF/ERGO, Fe-HCF/ERGO, Ni-FeHCF/ERGO calculated from cyclic voltammetry at scan rate of 0.02 V s-1. (B) Cyclic voltammograms of Ni-Fe-HCF/ERGO at different cycle number in 0.5 M KNO3 (pH=5) Fig. 11: Ragone plots of Ni-Fe-HCF/SS, ERGO/SS, Ni-HCF/ERGO, Fe-HCF/ERGO and NiFe-HCF/ERGO. Fig.12: Nyquist plots of Ni-Fe-HCF/SS, ERGO/SS and Ni-Fe-HCF/ERGO at open circuit potentials.

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