reduced graphene oxide composites for high performance electrochemical supercapacitor

reduced graphene oxide composites for high performance electrochemical supercapacitor

Accepted Manuscript Title: Facile hydrothermal synthesis of mesoporous nickel oxide/reduced graphene oxide composites for high performance electrochem...

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Accepted Manuscript Title: Facile hydrothermal synthesis of mesoporous nickel oxide/reduced graphene oxide composites for high performance electrochemical supercapacitor “Your article is registered as a regular item and is being processed for inclusion in a regular issue of the journal. If this is NOT correct and your article belongs to a Special Issue/Collection please contact [email protected] immediately prior to returning your corrections.”–> Author: Peiqi Cao Lincai Wang Yanjie Xu Yanbao Fu Xiaohua Ma PII: DOI: Reference:

S0013-4686(14)02555-9 http://dx.doi.org/doi:10.1016/j.electacta.2014.12.107 EA 23983

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

8-10-2014 24-11-2014 17-12-2014

Please cite this article as: Peiqi Cao, Lincai Wang, Yanjie Xu, Yanbao Fu, Xiaohua Ma, Facile hydrothermal synthesis of mesoporous nickel oxide/reduced graphene oxide composites for high performance electrochemical supercapacitor, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.12.107 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.

Facile hydrothermal synthesis of mesoporous nickel oxide/reduced graphene oxide composites for high performance electrochemical supercapacitor

Peiqi Caoa, Lincai Wanga, Yanjie Xua, Yanbao Fub, Xiaohua Maa*

Department of Materials Science, Fudan University, Shanghai, 200433, China

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Environmental and Energy Technologies Division, Lawrence Berkeley National

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Laboratory, University of California, Berkeley, CA 94720, USA

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Corresponding author: Xiaohua Ma

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E-mail address: [email protected]

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Postal address: 220 Handan road, Shanghai, China, 200433

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Phone number: +86-021-55664024

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Highlights

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1.

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Mesoporous NiO/RGO composites were fabricated trough a facile hydrothermal

route with the help of SDS template. The NiO/RGO composites exhibit high specific capacitance and good cycling stability.

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The unique graphene network structure and mesoporous structure of NiO particles improve the capacitive performance of the composites effectively.

Abstract: : Mesoporous NiO/reduced graphene oxide composites are successfully synthesized by a facile hydrothermal route. The XRD, FT-IR, Raman, SEM, TEM, and BET analysis are performed for characterizing the microstructure of the as-prepared composites. It can be found that the NiO particles with mesoporous structure are randomly anchored onto the surface of graphene sheets. The electrochemical

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performance is evaluated by CV, EIS and galvanostatic charge-discharge tests.

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Experimental data show that the NiO/RGO composites exhibit very high specific

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capacitance (1016.6 F g-1) and excellent cycling stability (94.9% capacitance retention

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after 5000 cycles), which are due to the 3D graphene conductive network and the

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meosporous structure promoting efficient charge transport and electrolyte diffusion.

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The results suggest that the NiO/RGO composites are a promising supercapacitor

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electrode material.

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storage

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Key words: Nickel oxide, Reduced graphene oxide, Mesoporous structure, Energy

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1. Introduction Today, with the increasing demand for energy and growing concerns about environmental pollution, development of clean and high efficiency energy storage systems, such as lithium ion batteries, electrochemical supercapacitors and Ni-H

batteries, have been put forward. Among all these candidates, supercapacitors have received a considerable amount of attention due to their higher power density, faster charge-discharge property, longer cycle life and simpler principles compared with traditional batteries[1-4]. In general, there are two main categories of electrode materials for supercapacitors based on their different working principles: (1) carbon based materials with high surface area, charging and discharging by an electric double

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layer mechanism; (2) redox active materials such as transition metal oxides and

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conductive polymers, which contribute pseudocapacitance through fast, reversible

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redox reactions[5]. Usually, electrical double layer capacitors (EDLC) exhibit high

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power density, long cycle life, but very low specific capacitance; while

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pseudocapacitors can theoretically provide 10-100 times higher capacitance but

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actually suffer from low utilization rate of active materials and poor conductivity[6-8].

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To solve these problems, efforts have been made to prepare hybrid materials, which

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combine large surface area pseudocapacitance materials with carbon materials to

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improve the electrochemical performance[9-12].

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Recently, much research on transition metal oxides being used as the electrode

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materials has been done. Among these oxides, ruthenium oxide has attracted great interest due to its high specific capacitance, wide potential windows and high

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electrochemical stability[13-15]. Unfortunately, the high cost of raw materials greatly restricts its commercialization. Hence, more efforts have been made to seek for alternative inexpensive electrode materials such as nickel oxide, cobalt oxide, and manganese oxide[16-24]. Among these candidates, nickel oxide (NiO) appears to be a

promising alternative because of its high theoretical specific capacitance (2573 F g-1), low fabrication cost and environmental impact[25]. However, its relatively poor electrical conductivity and low accessible surface areas usually lead to limited capacitance and poor reversibility during the charge-discharge process in experiments. One effective solution to this problem is to transform the materials from bulk forms into porous or hollow structures to enlarge their active surface areas. In the recent

including

nanoparticles,

nanoflowers,

nanoplates

and

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morphologies

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years, researchers have prepared NiO with different structures and surface

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mesoporous[26-29].

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Meanwhile, coupling NiO particles with some conductive additive materials such

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as carbon nanotubes and graphene could also help to improve their electrochemical

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performance by accelerating the electron transformation among active materials.

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Recently, graphene has attracted a lot research interest in energy-storage technologies

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due to its unique properties like ultra large surface area (2630 m2 g-1), and extremely

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high electrical conductivity[30, 31]. Application of graphene or graphene based

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materials in supercapacitors has been attempted by some researchers. Reports based

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on metal oxides/graphene composites, such as MnO2/graphene, Co3O4/graphene, TiO2/graphene and SnO2/graphene, have shown that the addition of graphene could

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help to improve the electrochemical properties of metal oxides drastically, and the specific capacitance of some composites can even be improved more than 50%[32-35]. Inspired by the previous works, herein, we report a simple and environmentally

friendly method to prepare NiO/reduced graphene oxide composites with mesoporous structure by a hydrothermal route. As electrochemical capacitor electrode materials, the physicochemical characteristics and electrochemical properties of the synthesized products were studied systematically. The experimental data demonstrate that the NiO/RGO composites with specialized microstructure can deliver attractive specific capacitance and have very good electrochemical stability, which suggests their

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potential application in supercapacitors.

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2. Experimental

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2.1 Synthesis of reduced graphene oxide

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Graphene oxide was prepared from natural graphite powders by a modified

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Hummers method, in which the pre-oxidation of graphite was followed by oxidation

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with Hummers method[36]. Graphite powder (3.0 g) was first added with stirring into

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concentrated H2SO4 (24 mL), in which K2S2O4 (2.5 g) and P2O5 (2.5 g) were

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dissolved at 80 ℃ in advance. The mixture was stirred for 4.5 h at 80 ℃ in oil bath.

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After cooling down to room temperature, the solution was diluted with distilled water

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and settled overnight. The as-prepared product was collected by filtration and washed by distilled water, and then dried at 80 ℃ for 10 h. Then the pre-oxidized graphite

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(2.5 g) and NaNO3 (1.25 g) were mixed and put into concentrated H2SO4 (115 ml) in a flask with ice bath. KMnO4 (7.5 g) was gradually added into the mixture with vigorous stirring to keep the temperature below 20 ℃. After removing the ice bath, the mixture was stirred at 35 ℃ for 0.5 h followed by addition of H2O (300 ml). The

color of the solution turned to yellow after additional stirring at 98 ℃ for 1 h. Then the solution was further diluted with distilled water and H2O2 (30%, 30 mL) was added into it. After being settled overnight, the remaining product was centrifuged and washed with distilled water until the pH became neutral. Then dry in vacuum to get the brown flake graphene oxide (GO). RGO can be obtained simply by a reduction

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reaction with the help of hydrazine hydrate as reductant.

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2.2 Synthesis of NiO/RGO composites

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The NiO/RGO composite (NGC) was prepared by a facile one pot hydrothermal

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route. Firstly, sodium dodecyl sulfate (10 g) and 5 ml ethanol were added to 100 mL

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of 1.5 mg ml-1 RGO suspension, and then stirred for 30 min to form homogenous

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solution. Next, NiCl2·6H2O (2.5 g) and urea (20 g) were dissolved in the solution with

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ultrasonication for 1 h to make Ni2+ absorbed on the surface of RGO sheets. Then the

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mixture was put into a Teflon lined autoclave and heated at 100 ℃ for 4 h. After

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cooling to room temperature, the gray precipitate was filtered and repeatedly washed

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by deionized water and dried in a vacuum oven at 60 ℃ overnight. Finally, the

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precursor was calcined at 260 ℃ for 5 h in air to obtain the mesoporous NiO/RGO composite. Different NiO/RGO composites can be obtained by changing the feeding

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weight ratio of NiCl2·6H2O to RGO. In addition, pure RGO and NiO were also synthesized using the same procedure for comparison.

2.3 Characterization

The surface morphology of the products was characterized by field emission scanning electron microscopy (FESEM, Ultra 55, Germany). Transmission electron microscopy (TEM, JEM 2011, Japan) was also utilized to characterize the microstructure of the products. The crystal structure was analyzed by X-ray diffraction (XRD, D8 Advanced, Germany) using Cu Kα radiation. IR spectra were obtained on a Fourier transform infrared (FT-IR,Nicolet Nexus 470, America)

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spectrometer in the range from 4000 to 400 cm-1. The samples and KBr crystal were

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ground together, and then the mixture was pressed into a flake for IR spectroscopy.

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Raman spectra were recorded on a Dilor LABRAM-1B multi-channel confocal

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microspectrometer with 514 nm laser excitation. Brunauer-Emmett-Teller (BET) and

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area and mesopore volume of products.

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the Barrett-Joyner-Halenda (BJH) equations were used to evaluate the specific surface

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2.4 Preparation of electrodes and electrochemical measurement

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The working electrodes were fabricated as follows. First, the as-prepared products,

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acetylene black and Polyvinylidene fluoride (PVDF) were mixed in a mass ratio of

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80:10:10 and dispersed in N-methylketopyrrolidine (NMP). Then the mixture was coated on a nickel foam current collector and dried at 120 ℃ for 10 h under vacuum.

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The loading mass of each electrode was about 5 mg. The electrochemical measurements were performed on a CHI600E electrochemical workstation (Shanghai Chenhua Instrument Co., China) in a three-electrode system. Cyclic voltammetry (CV), galvanostatic discharge-charge and electrochemical impedance spectroscopy

(EIS) were measured in 6 M KOH aqueous solution at room temperature using a Pt wire as the counter electrode and Ag//AgCl (in sat. KCl) as reference electrode. The specific capacitance of the electrode material was calculated from the CV curves according to the following equation[37]:

(F/g) is the specific capacitance,

current,

(A) is the oxidation or reduction

(g) represents the loading mass of electrode material,

(mV s-1)

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where

(V) is the voltage range of one sweep

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indicates the potential scan rate, and

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segment.

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3.1 Physicochemical characterizations

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3. Results and discussion

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X-ray diffraction measurements were employed to investigate the phase and

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structure of the synthesized products. Fig. 1(a) shows the XRD patterns of reduced

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graphene oxide (RGO), a broad peak (002) centered at around 2θ=24º was observed,

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which is the typical peak for graphene. Fig. 1(b) and Fig. 1(c) show the XRD patterns

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of pure NiO and mesoporous NiO-RGO composites (NGC). Compared to pure NiO, an additional small and broad diffraction peak appears at 2θ=24º, which can be

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indexed into the disorderedly stacked RGO sheets. However, the peak is much weaker than that of the as-prepared RGO, which is perhaps due to a more disordered stacking and quite uniform dispersion of the graphene sheets in the composites[38]. Both XRD patterns of pure NiO and NiO/RGO composites exhibit the characteristic peaks of

NiO (space group Rm) rock salt, such as 2θ=37.2º(111), 43.2º(200), 62.9º(220), 75.1 º(222) and 79.2º(311), which are in good agreement with the standard powder diffraction patterns of NiO rock salt structure (JCPDS card no. 040835)[39]. (Fig. 1.) The FTIR spectra of the products are shown in Fig. 2. In the spectrum of graphene (Fig. 2a), peaks at 1070, 1365, 1625 and 3450 cm-1 can be assigned to C-O stretching

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vibrations, C=O functional groups, C=C functional groups, and stretching mode of

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hydoxyl groups, respectively[40-42]. The presence of these peaks suggests that there

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are some oxygen-containing groups like hydroxyl group, carboxy group and carbanyl

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group remained on the RGO sheets. Fig. 2(c) shows the FT-IR spectrum of NiO/RGO

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composites, the peaks at about 1100, 1625 and 3450 cm-1 are still attributed to the

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skeletal vibration of oxygen-containing functional groups of the graphene sheets,

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while the sharp peak at around 500 cm-1 in the low frequency region can be ascribed

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to the Ni-O and Ni-O-Ni vibrations[43], same as the characteristic peak in the

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spectrum of pure NiO showed in Fig. 2(b), suggesting the existance of graphene and

(Fig. 2.)

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NiO in the composite.

Fig. 3. shows the Raman spectrum of graphene, pure NiO and NiO/RGO

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composites. The D band around 1351 cm-1 and the G band around 1580 cm-1 can be clearly observed in the spectra of the composites(Fig. 3c), corresponding to the A1g mode breathing vibration of six member sp2 carbon rings and the E2g vibrational mode of sp2 bonded carbon atoms[44]. And a relatively weak peak located at about 500 cm-1

is in accordance with pure NiO. Such a result further confirms that the crystalline structure of NiO/RGO composites has been obtained. (Fig. 3.) The morphology of the as-prepared products was examined by scanning electron microscope and transmission electron microscope. Fig. 4(a) shows the SEM image of mesoporous NiO particles. It can be seen that the NiO spheres have good dispersion

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and uniform sizes (average diameter of about 300 nm), and a unique tremella-like

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structure of the spheres can be observed. The high-magnification TEM image (Fig. 4b)

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also shows that the NiO particles possess abundant porous structure, not only on the

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surface but also in the interior of the spheres. This special morphology is due to the

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sodium dodecyl sulfate used in the synthesis process as a template[45]. After being

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calcined, sodium dodecyl sulfate decomposed and escaped from the precursor,

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resulting in the abundant pores all over the particles and higher surface area, which

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facilitate the transport of electrolyte ions during rapid charge-discharge process. Fig.

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4(c) and (d) show the SEM images of NiO/RGO composites collected under different

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magnifications. It is evident that the NiO particles kept the same porous tremella-like

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spherical structure and particle size compared with the pure NiO. Moreover, the RGO sheets are uniformly coated with the NiO particles, and overlap each other to form a

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3D network structure; at the same time, the anchoring NiO particles could act as a spacer to prevent the restacking of individual graphene sheets. This kind of structure increases the effective liquid-solid interfacial area, provides a fast path for the transport of electrolyte ions, and consequently facilitates the Faraday reaction.

(Fig. 4.) Generally, the specific surface area, pore diameter and pore volume all play an important part in determining the electrochemical properties of electrode material. To characterize the specific area and porosity, nitrogen sorption measurements of the products were carried out. The nitrogen adsorption/desorption isotherms and the corresponding Barrett-Joyner-Halenda (BJH) pore size distribution plot of the

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as-prepared NiO/RGO composite are shown in Fig. 5. According to International

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Union of Pure and Applied Chemistry (IUPAC) classification, the isotherm of NGC

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displays typical type Ⅳ with H2 type hysteresis loop[46], in the medium relative

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pressure region, the adsorbed N2 increases steadily with the increase of relative

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pressure due to capillary condensation and multilayer adsorption in the mesopores,

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and the climb in the high pressure region can be attributed to adsorption in the voids

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among the nanoflakes. This kind of curve reveals the existence of imperfect and

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complex cylindrical channels of uniform size in the material. The BET surface area of

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NGC is 274.3 m2 g-1, which is pronouncedly larger than previously reported value[46],

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as a result of the abundant pore structure in the composites. Besides, the pore diameter

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of the sample is mainly distributed at around 5-7 nm, and the average pore size is 6.7 nm, which further confirms that the NiO/RGO composites maintains the mesoporous

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structure.

(Fig. 5.)

3.2 Electrochemical studies

The mesoporous NiO and NiO/RGO composites were used as supercapacitor electrode materials and their electrochemical properties were studied by using cyclic voltammetry (CV), galvanostatic discharge-charge and electrochemical impedance spectroscopy (EIS) measurements. Cyclic voltammetry tests were performed in a potential range of 0-0.5 V at different scan rates using 6 M KOH as electrolyte under a three electrode system, and with an Ag/AgCl electrode and platinum wire as the

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reference electrode and counter electrode, respectively. Fig. 6. shows the CV curves

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of graphene, pure NiO and NiO/RGO composites at the voltage scan rate of 10 mV s-1.

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It can be seen that all the curves for NiO systems exhibit a pair of electrochemical

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redox peaks in the potential range of 0.23-0.45 V. The two strong current peaks

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indicate that the capacitance of NiO is mainly resulted from pseudocapacitive

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capacitance based on a fast redox process according to the following equation:

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The anodic peak (positive current) at around 0.4V is related to the oxidation of NiO to

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NiOOH, and the cathodic peak (negative current) at about 0.25 V is for the reverse

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process, which is the signature of pseudocapacitance[49, 50]. And the two peaks

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present very good symmetry, suggesting that the redox reactions were highly reversible and may have good potential for repeated cycling. Mennwhile, it is easily

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observed that the potential of both anodic and cathodic peaks of NGC shift negatively compared to those of pure NiO, and the current densities of the redox couple peaks are much higher than those of bare NiO, these may indicate a lower energy barrier for the

reaction and the improvement in the electrochemical reaction

kinetics of NiO[51]. (Fig. 6.) It is generally accepted that the area covered by the CV curves can be used to estimate the capacitance of a material. Evidently, the area under the CV curve for NiO/RGO composite is much larger than that of pure NiO and RGO, demonstrating that NGC presents higher capacitance. To evaluate the electrochemical performance in

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detail, the CV curves of NGC were measured at different scan rates (Fig. 7(a)). The

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specific capacitance of the composites was determined from the CV curves using the

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following equation[52]:

(F/g) is the specific capacitance,

(A) is the oxidation or reduction

(g) represents the loading mass of electrode material,

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current,

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where

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indicates the potential scan rate, and

(mV s-1)

(V) is the voltage range of one sweep

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segment. Fig. 7(b) summarizes the relationship between scan rate and the specific

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capacitance of the NiO-RGO composites. Correspondingly, the specific capacitance

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of NGC were 1016.6, 800.4, 633.4, 520.8 and 402.6 F g-1 at 1, 2, 5, 10, 20 mV s-1,

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which are much higher than those achieved from pure NiO (502.2, 451.1, 385.5, 329.5 and 267.2 F g-1), and also superior to most of the similar electrode material ever

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reported[53, 54]. (Fig. 7.) For comparison, the galvanostatic charge and discharge tests were also conducted and the results are shown in Fig. 8(a). The charge-discharge curves exhibit

a plateau at 0.2-0.35 V, which is characteristic of pseudocapacitance. Besides, the very long discharging time of the curves suggest the outstanding electrochemical performance of NGC, hence verifying the CV results. The significantly enhanced capacitance of the NiO after combination with graphene sheets can be explained as follows. On one hand, the combination of NiO particles with the highly conductive graphene sheet leads to a great increase in the overall electronic conductivity,

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allowing for rapid and efficient charge transport. On the other hand, the anchoring

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NiO particles could act as a spacer to prevent the restacking of individual graphene

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sheets, thus form a 3D network structure with large surface area, and consequently a

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higher electrochemical active surface for giving play to the full advantages of NiO

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pseudocapacitance and double-layer capacitance of graphene. Last but not least, the

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structure of mesoporous NiO particles has unique advantages like large specific

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surface and appropriate aperture size for ion transport itself[55]. All these possible

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reasons lead to the outstanding electrochemical performance of NGC.

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In order to compare the capacitive properties of NiO/RGO composites with

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different component ratio, NGC samples were obtained by changing the feeding

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weight ratio of NiCl2•6H2O to RGO during the synthesis process. The estimations based on the TG curves (shown in Fig. S2) indicate that mass percentage of RGO in

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these composites are 4.56%, 9.38%(NGC sample used for all kinds of characterization), 14.35%, 18.07%. Fig. 8(b) shows the specific capacitance of these composites at a scan rate of 1 mV s-1, it can be seen that the capacitance remarkably increases with the increase of RGO content in the mass ratio range of 0% to 10%.

When the content of RGO is up to 9.38%, the composite achieves the highest specific capacitance, and then the capacitance decreases with further increasing the RGO content. The decrease of capacitance in the composites with high RGO content can be caused by the serious agglomeration of RGO sheets, which will result in a decrease of the specific surface area. Besides, the relatively low specific capacitance of RGO itself will lower the capacitance of the whole composite if the content of RGO is too

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much.

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(Fig. 8.)

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Since the electrochemical impedance spectroscopy (EIS) analysis has been

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recognized as one of the principal methods for examining the fundamental behavior of

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electrode materials, the RGO, pure NiO and NGC electrodes were also measured from

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100 KHz to 0.1 Hz with alternate current amplitude of 5 mV in 6 mol·L-1 KOH. The

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corresponding Nyquist plots are displayed in Fig. 9. All the EIS curves were

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composed of an arc in high frequency region and a line in low frequency region. It is

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generally believed that the intercept with the real axis at high frequency reflects the

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electrochemical reaction impedance of the electrode (Faraday resistance), the

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semicircle in the high frequency region is related to charge transfer resistance, and the straight line indicates the diffusion resistance (Warburg impendence) of electrolyte in

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electrode pores and the proton diffusion in host marerials[56]. When comparing the EIS curves of pure NiO and NGC, the semicircle line profile in the high frequency region are very similar, but difference of the sloping lines are easily observed: the sloping line of NGC is more straight than that of NiO, which indicates that the

diffusion resistance of NGC electrode is lower than that of pure NiO electrode. This may be caused by the 3D network structure formed with graphene sheets and NiO particles, which provides large surface area and abundant pore structure for the diffusion of electrolyte ions. The fitting results based on the equivalent circuit given as inset in Fig. 10 further confirm such a conclusion. Here, Re, Rct, Zw, Cdl and Clc represent the electrode resistance, the charge transfer resistance, Warburg impedance,

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double-layer capacitance and limit capacitance[57]. As the parameters shown in Tabel

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(Fig. 9.) &(Table 1)

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1, the Rct and Zw for NGC electrode are smaller than those of pure NiO electrode.

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As the cycle stability is also a very important index for evaluating the

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supercapacitive performance of the material, the cycling test was conducted between

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the potential range of 0-0.5 V for over 5000 cycles. As shown in Fig. 10., the specific

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capacitance of NGC remains over 94.9% of the initial capacitance even after 5000

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cycles, indicating that the composite exhibits long cycling stability. It is also noticed

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that the specific capacitance of NGC has an increase in the first 1000 cycles and

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reaches a maximum value at about 1200 cycles, which is probably caused by the

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penetration of electrolyte ions and the gradual activation of the active materials[58]. In strong contrast to NGC, pure NiO shows poor cycle stability with only 73.9%

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retention of the initial capacitance. The long-term stability implies that the NiO/RGO composite is a promising electrode material for supercapacitors. (Fig. 10.) 4. Conclusion

In summary, we have successfully synthesized mesoporous NiO/RGO composites through a facile hydrothermal method. The as-prepared composites have a unique 3D network structure formed with graphene sheets and anchored NiO particles, which exhibit large specific surface area and abundant pore structure. Electrochemical tests demonstrate that the NiO/RGO composites have good electrochemical capacitive performance and provide a very high specific capacitance of 1016.6 F g-1 with

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excellent cycling stability. The enhanced electrochemical performance is mainly due

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to the good electrical conductivity and larger surface area and higher porosity, which

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can increase the contact between electrolytes with active materials, and shorten ions

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low-cost, high-performance supercapacitors.

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diffusion path. All these characteristics demonstrate its great potential applications in

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Acknowledgments

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This work was supported by Shanghai Leading Academic Discipline Project,

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Project Number: B113.

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Figures

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(a) )

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1μ μm

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M Fig. 3.

(d) )

1μ μm

1μ μm

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Fig. 4.

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(c) )

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(b)

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(a)

Fig. 5.

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M Fig. 6.

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(b)

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(a)

Fig. 7.

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(b)

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(a)

Fig. 8.

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Fig. 10.

Table 1 Parameters fitted from EIS spectra.

Rs (Ω)

Rct (Ω)

Zw

Cdl (F)

Cl (F)

0.2938

0.0203

0.1504

0.001468

0.0306

NiO

0.2842

0.4657

0.1447

0.001385

0.1879

NGC

0.2801

0.2813

0.0810

0.001575

0.2021

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Sample

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RGO

Figure captions: Fig. 1. XRD patterns of Graphene (a), NiO (b), and NGC (c).

Fig. 2. FT-IR spectra of Graphene (a), NiO (b), and NGC (c).

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Fig. 3. Raman spectra of Graphene(a), NiO (b), and NGC (c).

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Fig. 4. SEM images of pure NiO (a) and NGC (c,d). TEM image of NiO (b).

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Fig. 5. The N2 adsorption/desorption isotherms (a) and pore size distribution (b) of

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NGC.

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Fig. 6. CV curves of NGC(a), pure NiO(b) and RGC(c) at a scan rate of 10mVs-1 in

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6M KOH.

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Fig. 7. CV curves of NGC at scan rates of 20 mVs-1, 10 mVs-1, 5 mVs-1, 2 mVs-1 and

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1 mVs-1 in 6M KOH (a). Specific capacitance calculated from the corresponding CV

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curves for each scan rate (b).

Fig. 8. Galvanostatic charge/discharge curves of NGC at current densities of 2 A g-1, 1 A g-1, 0.5 A g-1 and 0.2 A g-1 in 6M KOH (a). The specific capacitance of the NGC composites of different RGO contents (b).

Fig. 9. Nyquist plots of RGO, NiO and NGC electrodes. Voltage perturbation amplitude: 5 mV, frequency range: 100 kHz to 100 mHz. The insert are the partial enlarged view of Nyquist plots and the equivalent electrical circuit mode plot.

Fig. 10. Cycling performance of NGC electrode and pure NiO electrode in the voltage

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range of 0-0.5 V at scan rate of 10 mVs-1 in 6M KOH.