Graphene Supported Ni-based Nanocomposites as Electrode Materials with High Capacitance

Accepted Manuscript Title: Graphene Supported Ni-based Nanocomposites as Electrode Materials with High Capacitance “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: Tao Meng Qian-Qian Xu Yin-Tao Li Xiang-Ying Xing Chun-Sheng Li Tie-Zhen Ren PII: DOI: Reference:

S0013-4686(14)02561-4 http://dx.doi.org/doi:10.1016/j.electacta.2014.12.113 EA 23989

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Electrochimica Acta

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22-9-2014 15-12-2014 17-12-2014

Please cite this article as: Tao Meng, Qian-Qian Xu, Yin-Tao Li, XiangYing Xing, Chun-Sheng Li, Tie-Zhen Ren, Graphene Supported Ni-based Nanocomposites as Electrode Materials with High Capacitance, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.12.113 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.

Graphene Supported Ni-based Nanocomposites as Electrode Materials with High Capacitance Tao Menga, Qian-Qian Xua, Yin-Tao Lia, Xiang-Ying Xinga, Chun-Sheng Lib and Tie-Zhen Rena* a

School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, P.R. China

b

College of Chemical Engineering, Hebei United University, Tangshan, Hebei 063009, P. R. China

* Corresponding author. E-mail: [email protected], Tel: +86 2260204909

Graphical abstract

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Highlights ► Research

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Ni based/graphene materials are prepared with a facile hydrothermal method.

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15%Ni(OH)2/graphene shows the highest specific capacitance of 2077 F/g.

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15%Ni/graphene by calcined 15%Ni(OH)2/graphene has a low capacitance of 633 F/g.

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The capacitance of 15%Ni(OH)2/graphene is 965F/g after 500 cycles at 50 mV/s.

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Abstract ： The Ni-based/graphene materials were synthesized by a simple hydrothermal method, using the graphene oxide

as the support precursor. The

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textural and structural properties of the as-prepared Ni-based/graphene samples were

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fully characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermalgravimetric analysis and Fourier transform infrared

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spectroscopy, respectively. Electrochemical properties were examined by cyclic

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voltammetry , electrochemical impedance spectroscopy and cyclic charge-discharge tests. The results showed that the 15%-β-Ni(OH)2/graphene had the high specific

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capacity of 2077 F g-1 at 5 mV s-1 in the range of 0.0 - 0.7V (vs. Ag/AgCl) and had a good cycle life with 965 F g-1 after 500 cycles at 50 mV s-1.

Keywords: Ni-based/graphene; electrochemical properties; Ni(OH)2; hydrothermal method; supercapacitor. 1. Introduction With the consumption of fossil fuels and environment changes, it is urgent to

find

new

energy and

efficient

energy storage devices.

Electrochemical

supercapacitors (ESs) are regarded as promising candidates for energy storage due to fast charge time, long cycle life, and high power density.[1, 2] Based on the charge conversion/storage mechanism, ESs can be classified as electrical double-layer capacitors (EDLCs) and the redox electrochemical capacitors (pseudocapacitors). The electrical charge of EDLCs based on carbon materials is stored at the electrode/electrolyte interface, while the metal oxides/hydroxides and conducting polymers have a nature properties of the pseudocapacitors by utilizing the

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capacitance trimming from reversible faradaic reaction occurring at the electrode surface.[3, 4]

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Great efforts have been made to study the materials based on the carbons and

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metal oxides/hydroxides for ESs. Carbon materials such as activated carbon,[5] carbon aerogels,[6] porous carbons,[7, 8] and carbon nanotubes (CNTs) [9] are

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usually utilized as electrode materials in supercapacitors due to their excellent

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conductivity, high surface area and stable chemical property, but the small double

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layer capacitance limits its application. For pseudocapacitors, the transition metal

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oxides/hydroxides have relatively higher capacitance and faster redox kinetics than EDLCs, while a relative lower capacitance stability and a shorter cycle life are major

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limitations of ESs. To utilize the good electrochemical properties of both carbon

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materials and metal oxides/hydroxides, one possible route is to integrate these two

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kinds of materials into the electrodes for ESs. Recently, graphene, consisting of a single layer of carbon atoms in a

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two-dimensional (2D) lattice, has been emerging as a fascinating material with its

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large surface, high electron mobility, and chemical stability.[10] Therefore, the graphene materials used in various electrical devices, such as electromechanical resonators,[11] dye-sensitized solar cells,[12] and supercapacitors [13] are in prospect. Compared with costly and toxic Ru,[14] the inexpensive and eco-friendly nickel metal,[15] oxides [16] and hydroxides [17] as the pseudocapacitors electrode materials with good capacitive characteristic for ESs have been widely studied. However, the nickel oxides/hydroxides have poor electrical conductivity, which

results in a decreased capacitive performance after long cycle and slow electron transport at high electrical rates, limiting its practical applications.[18, 19] To resolve the issue, we can incorporate graphene into the nickel compounds to maximize the electrochemical properties of the pseudocapacitive material. In this work, we proposed an application of the graphene as support for Ni-based materials in supercapacitors. The graphene oxide (GO) was employed as a precursor for preparation of the electrode materials. The plenty of the oxygen-containing functional groups (carboxyl, hydroxyl, carbonyl and epoxy group)

species.[20]

The

textural

and

structural

properties

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on GO sheets can provide a large amount of uniform anchor sites for the nickel of

the

as-prepared

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Ni-based/graphene electrode materials were characterized. The effect of the

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graphene support, Ni loading, the cyclic voltammetric window and the Ni phase of the Ni-based/graphene for supercapacitors were investigated. As a result, the

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15%-β-Ni(OH)2/graphene showed a high specific capacity of 2077 F g-1 at 5 mV s-1

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in the range of 0.0 - 0.7 V(vs. Ag/AgCl) and had a good cycle life.

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2.1 Synthesis of materials 2.1.1 preparation of GO

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2. Experimental

All chemicals in our experimental were of analytical grade and used

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method.[21]

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GO was produced by natural graphite according to a solvothermal synthesis

without any further purification. Graphite powder (1 g) was stirred in 23 ml of H2SO4

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solution (98 wt%) at 0 °C for 1 h. Then, KMnO4 (5 g) was gradually added into the

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above-mentioned solution in two hours under vigorous magnetic agitation. Continuing vigorous stirring for 3 hours at room temperature, the final suspension was transferred

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to a 50 ml of Teflon-lined autoclave and heated in an oven at 70 °C for 15 h, and then cooled down to room temperature naturally. 20 ml of H2O2 (30 wt%) was added into the mixture to remove the residual KMnO4. After the H2O2 adding into the mixture, it released a large amount of bubbles and the color of the mixture changed into brilliant yellow. Then, the mixture was filtered and washed with 50 ml 5 wt% HCl aqueous solution to remove the metal ions and then deionized water was used to remove the

acid. Finally, the dark brown solution of GO with ca. 6.2 mg ml-1 was obtained. 2.1.2 synthesis of electrode materials 200 ml of GO was sonicated for 30 min to exfoliate the GO sheets. Then, a certain amount of Ni(NO3)2·6H2O solution (10 ml) was added gradually and was sonicated further for 30 min to produce uniform dispersion. NH3·H2O (25 wt%) was added drop by drop into the solution which resulted in a brownish-black suspension with a pH value of 10. 1 ml of hydrazine was added after stirred the suspension for 30 minutes. The mixture was then transferred to a Teflon-lined autoclave (50 ml of

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Volume) and heated for 12 h at 180 °C. Then the as-synthesized black product was isolated by filtration, washed for several times with distilled water and 95% of ethanol,

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and finally dried in a vacuum oven at 70 °C for 24 h to get the β-Ni(OH)2/graphene

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solid powder, named as x%-β-Ni(OH)2/graphene, where x is the mass rate of Ni metal. The obtained x%-β-Ni(OH)2/graphene materials were calcined in a tube resistance

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furnace at 300 °C and 500 °C for 2 h to get the x%-NiO/graphene and

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x%-Ni/graphene with a heating rate of 2 °C/min and with a nitrogen flow rate of 20

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mL min-1. As a comparison, the β-Ni(OH)2 was obtained in the same conditions

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without adding graphene and graphene materials were prepared in the absence of the nickel precursors.

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2.2 Characterization of materials

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Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer with Cu-Kα radiation (λ≈0.154nm) at 40 kV and 40 mA in a

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scanning range of 10–80° (2θ). The morphologies of as-obtained products were observed by scanning electron microscopy (SEM, JSM-6490LV) with the sample

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dispersed on the Al pan. The images of Transmission electron microscopy (TEM) was taken on a FEI Tecnai G20 microscope, operating at 200 kV. The thermogravimetric

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analysis (TGA) was performed on a TA SDT Q600 instrument from room temperature to 800 °C with a heating rate of 10 °C min-1 in air and with ɑ-Al2O3 as the reference. Fourier transform infrared spectroscopy (FT-IR) of electrode materials were recorded by Bruker Vector 22 infrared spectroscopy in the range of 400 to 4000 cm-1 by using the KBr pellet technique.

2.3 Electrochemical tests The working electrodes for electrochemical capacitors were prepared by the following processes. 0.25 g of as-prepared material was mixed with acetylene black and polytetrafluoroethylene (PTFE), and their mass ratio was 75:20:5. The above slurry was made using ethanol as a solvent and coated onto nickel foam. After dried at 70 °C for 12 h, the coated nickel foam was pressed under the pressure of 10 MPa for 10 min as the electrode material. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)

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of electrodes were performed on a IM6 & ZENNIUM electrochemical workstation in three-electrode system. The working electrode (2cm×2cm) with the active-material

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mass around 50 mg was tested in 6 M KOH solution using Ag/AgCl as a reference

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electrode and a platinum wire as counter electrode. The pure graphene electrode was prepared as the same method mentioned above with the active-material mass of ca. 64 The CV tests were at the scans rate value of 5, 10, 20, 50 and 100 mV s-1,

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mg.

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respectively. EIS measurements were recorded in a frequency range of 100k to 1m Hz

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with AC oscillation of 5 mV. The cycle charge-discharge was performed on the

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Land-CT2001A (100 mA) cell in a symmetric two-electrode system with the cell potential ranges from 0.0 to 1.0 V for 1000 cycle. The button shaped electrode has a

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diameter about 2 cm with the active-material mass around 18 mg.

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3. Results and Discussion 3.1 Characterization of materials The phase structure and purity of the as-prepared samples were examined by

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XRD as represented in Fig. 1. The discernible diffraction peaks at 2θ=19.2°, 33.2°, 38.5°, 52.2°, 59.3°, 62.8°, 69.6° and 72.9° are matched to the standard data card

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JCPDS 14-0117, which is β-Ni(OH)2 with a hexagonal crystalline structure. The typical peak of GO is disappear after the hydrothermal treatment, suggesting the GO are reduced to graphene.[22] With the loading of β-Ni(OH)2 increased, the intensity of the diffraction peaks becomes slightly strong. Calcinations at 300 °C lead to the phase transition of β-Ni(OH)2 to NiO. As observed, the peaks of calcined samples with low intensity can be indexed as NiO and agrees well with the standard data card JCPDS

65-2901. Further calcinations at 500 °C give three main peaks at 2θ=44.5°, 51.8°, 76.4°, which are the characteristic peaks of Ni phase, corresponding to the standard card JCPDS 04-0850 with a space group of Fm-3m. The new broad peak at around 26° observed in the Ni-based/graphene materials means that the amorphous graphene becomes stronger during the calcination in N2 atmosphere.[22] SEM measurements are performed to investigate the morphologies and dimension of the obtained Ni-based/graphene materials. Fig. 2a shows the hexagonal crystalline structure of pure β-Ni(OH)2 and the size of the particles is about 260 nm

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and those hexagonal plates combine each other to form a bulk shape. Fig. 2b represents the morphology of 15%-Ni(OH)2/grapene, we can see the main part are

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irregular layered sheets which incompact with the size around 250 nm. For the sample

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of 25%-Ni(OH)2/graphene, it is obvious the curled structure of graphene and the β-Ni(OH)2 particles can be seen clearly in the Fig. 2c. In some areas, the β-Ni(OH)2

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particles as a unit contact with each other to form a big aggregated particles. In the

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Fig. 2d, the layered structure of graphene is also found in the calcined sample of

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15%-NiO/graphene and the particles are not shaped well after calcinations. Fig. 2e

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shows the 15%-Ni/graphene with most ball-like particles with a size of 10 nm - 80 nm. Those particles are the Ni metal, reduced by the graphene during the process of

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calcinations.[23, 24] Fig. 2f reveals the TEM image of the 15%-Ni(OH)2/graphene,

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we can see that the particles with a shape of hexagon are around 220 nm, which is

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similar as observed in SEM images. Meanwhile, the curly structure particles with thin layer are distributed randomly, which is the typical morphology of graphene.

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TGA is an effective analytical technique to evaluate the load of graphene in the

β-Ni(OH)2/graphene materials. As shown in Fig.3, it can be found that the curve of all

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the as-deposited β-Ni(OH)2/graphene samples has three-steps, corresponding to the weight loss process of dehydration and decomposition. The first stage, with weight loss about 9 wt% under the temperature of 200 °C, is related to the evaporation of residual (or absorbed) water molecules associated with the materials and the decomposition of residual organic functional groups on graphene sheet.[25] The second stage, the temperature between the 300 and 330 °C, may be the process of

decomposing the Ni(OH)2 into NiO.[26] In this step, the 15%-, 20%-, and 25%-β-Ni(OH)2/graphene have a weight loss of 5.3 wt%, 7.1 wt%, and 7.5 wt%, which are in good agreement with the theoretical weight loss value 4.8 wt%, 6.9 wt% and 8.3 wt%, respectively, caused by the decomposition of β-Ni(OH)2. The graphene materials are completely burned out in air at the temperature ranging from 330 to 430 °C in the last step, reflecting a sudden drop. Further heating to 800 °C can not lead to the weight loss changed obviously and the black samples is changed into green, indicating that the graphene are completely decomposed. The weights of the final

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remainder (NiO) are about 19.9 wt%，28.7 wt% and 34.3 wt% for 15%-, 20%- and 25%-Ni(OH)2/graphene.

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FT-IR is a useful tool to investigate the functional groups of compounds. Fig.4

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shows the FT-IR spectrum of Ni-based/graphene materials. The peaks at 3640, 517 and 450 cm-1 are related to the O-H stretching vibration, which indicates OH groups in

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a free configuration of the lattice vibration of hydroxyl groups and Ni-O-H bending

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vibrations.[26, 27] With the loading of β-Ni(OH)2 , the peaks at 3640 cm-1 is slightly

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increased. This confirms the existence of β-Ni(OH)2. The peak at 418 cm-1 in the

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15%-NiO/graphene corresponds to the stretching vibration of Ni-O, which indicating that NiO nanoparticles are successfully formed after annealing process.[28] The peak

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of 3448 cm-1 may be attributed to the vibration of -OH group (residual) of the

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graphene sheets.[29] The peaks located at 1582 cm-1 of all the Ni/graphene materials

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are related to the aromatic skeletal of C=C stretching vibration belonging to the feature of graphene sheets.[22]

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3.2. Electrochemical tests To explore the potential application, the samples were fabricated into electrodes

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and the relevant electrochemical behavior was characterized by electrochemical workstation in three-electrodes system. The electrochemical properties and specific capacitances of the β-Ni(OH)2, Ni-based/graphene electrode materials are investigated by using cyclic voltammetry (CV). Fig. 5a-d shows the CV curves of β-Ni(OH)2 and β-Ni(OH)2/graphene electrodes in 6 mol/L of KOH electrolyte at different scan rates

in the potential window ranging from 0.0 to 0.7 V ( vs. Ag/AgCl). All the CV curves of β-Ni(OH)2 and β-Ni(OH)2/graphene electrodes at the different scan rates have a couple of redox peaks. The capacitance characteristics are quite different from that of double-layer capacitance in which the shape of CV curve is normally close to an ideal rectangle. It indicates that the capacitance of β-Ni(OH)2 and Ni(OH)2/graphene materials mainly results from pseudocapacitive capacitance based on a redox mechanism and it can be explained by the following schemes: [30, 31] β-Ni(OH)2 + OH－ ↔

β-NiOOH + H2O + e－

（1）

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The shapes of CV loops at the scan rate of 5 mV s-1 (Fig. 5a-d ) have anodic peaks at rough value of 0.45 V, which is related to the oxidation of β-Ni(OH)2 to

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β-NiOOH, and the cathodic peak at about 0.30 V is ascribed to its reverse process. It

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is similar to the reported irregular-shaped agglomerates Ni(OH)2, in which the potentials of anodic peak and cathodic peak were 0.47 V and 0.27 V. [32] As the scan

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rate increased, the potentials of the anodic and cathodic peaks shift in more positive

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and negative directions, due to the limitation of the ion diffusion rate to satisfy

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electronic neutralization during the redox reaction. [16, 33] The graphene electrode

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shows the totally different shape of CV curves (Fig.5e) due to the absence of the Ni compounds. The redox peaks can not be observed and the symmetric shape appears in

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the potential window. The polarization becomes obviously when the potential is above

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0.4 V, suggesting the faradic capacitance can not be expressed correctly.

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The specific capacitance can be calculated from CV measurements by using the collected

discharging t2

current

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c  Q m  V   i  dt m  V t1

and

time

with

the

following

equation:

(2)

Where C (F g-1) is the specific capacitance of the as-prepared material, Q (C) is

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the quantity of electric charge. i (A) is the charge-discharge current, t is the discharging time, m (g) is the amount of active material, ∆V(V) is the potential drop during discharge. The specific capacitance of β-Ni(OH)2 and β-Ni(OH)2/graphene are listed in Table 1 with 1145 F g-1 for pure Ni(OH)2, 2077, 1987, 1212 and 55 F g-1 for 15%-, 20%-, 25%-β-Ni(OH)2/graphene, and graphene at 5 mV s-1, respectively. It is

obvious that the specific capacitances of all the β-Ni(OH)2/graphene electrode materials at different scan rates are higher than the pure β-Ni(OH)2, and much higher than the graphene electrode, in the potential window ranging from 0.0 to 0.7 V (vs. Ag/AgCl). This reveals there must be a positive synergistic effort between the graphene support and the β-Ni(OH)2 nanoparticles. The GO is the precursor for preparing the β-Ni(OH)2/graphene. The oxygen-containing groups of GO, as not only the support precursor but also an efficient dispersing agent, can prevent from the aggregation of nanoparticles and the recombination of the graphene.[34, 35]

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Meanwhile, the good electron mobility of graphene makes it easier for the electron transportation to the internal of the β-Ni(OH)2/graphene electrodes materials to reduce

all

the

β-Ni(OH)2/graphene

electrodes

materials,

the

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Among

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the resistivity. Hence, the electrochemical properties could be enhanced.[36]

15%-β-Ni(OH)2/graphene shows the highest specific capacitance and can be achieved

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2077 F g-1 at the scan rate of 5 mV s-1. The specific capacitance of

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β-Ni(OH)2/graphene electrode materials decreases with the loading of β-Ni(OH)2.

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This may due that the overmuch β-Ni(OH)2 can aggregate a big particle and limit the

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efficiency of the active sites. Graphene gives a big contribution for the improved supercapacitance. A specific capacitance of β-Ni(OH)2/graphene around 1667 F g-1 at

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5 mV s-1, prepared by a chemical precipitation method, was obtained within the

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voltage ranging from 0.1 to 0.6 V(vs. SCE).[32] The synthesized Ni(OH)2/graphene

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and Ag@Ni(OH)2/graphene by dispersed the hierarchical stacked nanoplates of Ni(OH)2 and Ag deposited Ni(OH)2 on graphene nanosheets showed improved

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electrochemical properties.[37] The specific capacitance of Ni(OH)2/graphene and Ag@Ni(OH)2/graphene are 582 F g-1 and 621 F g-1 even at scan rate of 2 mV s-1 with

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the voltage ranging from 0.0 to 0.5 V. The β-Ni(OH)2 nanocrystals on reduced graphene oxide by a facile one-step approach in a mixed binary solvent has a specific capacitance of 1843 F g-1 at a scan rate of 5 mV s-1 with the voltage ranging from 0.0 to 0.6 V (vs. Ag/AgCl). Those capacitance are improved when the assistance of graphene. The specific capacitance of the β-Ni(OH)2/graphene in our works are not only

impacted by the graphene support and the β-Ni(OH)2 loading but also influenced by the potential window.

Fig. 6a-c shows the CV curves of β-Ni(OH)2/graphene

electrodes under the voltage ranging from -1.0 to 0.0 V in 6 M of KOH (vs. Ag/AgCl). All the CV curves are of a standard rectangular shape in a wide voltage reversal at the scan rate of 5 mV s-1, indicating a reversible reaction and good capacitive behavior. The shapes of CV loops at different scan rates in our experiment are quasi-rectangular along the current-potential axis without obvious redox peaks, indicating that all samples have a good capacitive behavior in double-layer capacitance. The perfect

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standard rectangular shape is represented in pure graphene electrode (Fig. 6d), showing the feature of carbon materials. [38] The specific capacitance of the

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β-Ni(OH)2/graphene calculated by the equation (2) is listed in table 1. The

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capacitance of all the samples is decreased with the loading of Ni(OH)2, which is similar as tested in the voltage ranging from 0.0 to 0.7 V(vs. Ag/AgCl). Amongst

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them, the 15%-β-Ni(OH)2/graphene also has the highest specific capacitance (203 F

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g-1 vs. Ag/AgCl), closing to the reported double-layer capacitance of graphene. [39,

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40] Though the capacitance of graphene electrode increases to 122 F g-1,

the series

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of Ni based electrodes show that the value of capacitance is far less than those in the voltage widow ranging from 0.0 to 0.7 V. This can be explained by that the β-Ni(OH)2

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have little response in the potential window ranging from -1.0 to 0.0 V, which was fit

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for the carbon materials.[7, 39] It is reported that the α-Ni(OH)2 with anodic peak at

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0.37 V and cathodic peak at 0.19 V are attributed most probably to α-Ni(OH)2/γ-NiOOH redox couple.[15] The positions of both anodic and cathodic

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peaks shift to a higher potential with the number of cycling, which may indicate a continuous minor conversion from α-Ni(OH)2 and γ-NiOOH into β-Ni(OH)2 and

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β-NiOOH. In other words, the anodic and catodic peaks of both α- and β-Ni(OH)2 are greater than zero. Hence, the potential window range from 0.0 to 0.7 V is benefit for the faradaic-pseudocapacitance of Ni(OH)2. The anodic and cathodic peaks of β-Ni(OH)2-based materials in our work are around 0.45 and 0.3 V. They are higher than α-Ni(OH)2/γ-NiOOH redox couple (0.37 and 0.19 V), which can support that the redox process of the β-Ni(OH)2-based materials are β-Ni(OH)2/β-NiOOH.

Fig. 7a shows the cyclic voltammetric (CVs) curves of 15%-NiO/graphene material. The CV curves reveal a pair of distinct redox peaks, indicating that the capacitance of NiO/graphene mainly results from pseudocapacitive capacitance. The redox process of NiO as supercapacitor is not a simple absorption/desorption OH−, but the surface Faradaic reaction of Ni2+/Ni3+ occurs at the surface of NiO in alkaline solution. It can be explained by the following schemes:[16] NiO + OH− ↔ NiOOH + e−

(3)

From the CV loops, we can know the anodic peak around 0.45 V is related to the

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oxidation of nano NiO to NiOOH, and the cathodic peak at about 0.3 V is ascribed to its reverse process. The specific capacitance of the 15%-NiO/graphene are calculated

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by the equation (2) and listed in table 1. The 15%-NiO/graphene has the highest

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specific capacitance of 1563 F g-1 at a scan rate of 5 mV s-1, which is smaller than the 15%-β-Ni(OH)2/graphene, but much larger than the reported NiO.[16, 41] As the scan

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rate increased, the specific capacitance is decreased and the potentials of the anodic

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and cathodic peaks in the CV loops shift in more positive and negative directions, due

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to the limitation of the ion diffusion rate to satisfy electronic neutralization during the

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redox reaction.[16] The cyclic voltammetric (CV) curves of 15%-Ni/graphene material are similar to 15%-β-Ni(OH)2/graphene, suggesting the 15%-Ni/graphene

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electrode material also has a property of pseudocapacitive capacitance (Fig. 7b). The

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anodic and cathodic peaks of the 15%-Ni/graphene are also at around the 0.45 and

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0.3V. The specific capacity is calculated by the equation (2) and also listed in table 1. According to the data, the 15%-Ni/graphene has a capacity of 633 F g-1 at 5 mV s-1 the

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and

value

is

much

lower

than

the

15%-NiO/graphene

and

the

15%-β-Ni(OH)2/graphene. It has been demonstrated that the surface Ni atoms on the

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Ni nanoparticles react easily with O2 and water to form Ni oxide/hydroxide, which provides the pseudocapacitive behavior of the nickel nanoparticles. And the nickel hydroxide has been approved to be the main composition on the surface of the Ni nanoparticles. [15, 42] However, the nickel oxide/hydroxide species are not compact and thick enough to form the effective layer and to contribute the pseudocapacitance of Ni nanoparticles. Thus, the behavior of pseudocapacitance of Ni nanoparticles is

similar to those observed nickel oxide and nickel hydroxide; on the other hand, the low quantity of Ni oxide/hydroxide on the surface of Ni nanoparticles limits the capacitance in a low value. The pseudocapacitive properties of Ni-based/graphene material mainly depend on the Ni-based species. The CV curves of all the Ni-based materials have a similar shape. The anodic and cathodic peaks are around the same potentials. This may indicate that the reaction mechanism of the Ni-based/graphene materials are the transformation of Ni2+↔Ni3+.[43-45] Comparing with the β-Ni(OH)2/graphene

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species, we believe that the capacitance is determined by the structure and the surface reactivity.[44, 46] The layer structure of Ni(OH)2 with a inter-layer distance of 0.46

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nm enables the OH− ions, which are from the electrolyte solutions, to connect easily

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with the Ni species during the charging process.[46] After calcinations, the Ni(OH)2 is transformed to NiO by removing the hydroxyl with the inter-layer distance decreased.

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This will hinder the transportation of OH− ions and reduce the kinetics rate. Therefore,

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lower capacity can be calculated by CV curves. While, the Ni/graphene has the lowest

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capacity due to the few effective nickel oxide/hydroxide species on the Ni metal

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surface.

The cycle-life tests of β-Ni(OH)2 and Ni-based/graphene materials were

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conducted by 500 cycles between 0.0 and 0.7V at a scan rate of 50 mV s-1. Observing

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Fig. 8, the specific capacitance of 15%-β-Ni(OH)2/graphene and 15%-Ni/graphene

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become larger with the increase of cycle number at the beginning 100 cycling tests. This may due that the electrode materials are activated after several redox processes.

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With the increase of the cycle number, the specific capacitance has a slightly decrease from 1004 to 965 F g-1 for 15%-β-Ni(OH)2/graphene and from 389 to 376 F g-1,

A

respectively. The curves of pure β-Ni(OH)2 and 15%-NiO/graphene experience a downward trend, from 407 to 206 F g-1 and 367 to 286 F g-1, respectively. Among all the tested, the specific capacitance of pure Ni(OH)2 drops much. While the samples with the graphene have a good stability due to the super mechanics and electricity properties of graphene.[47, 48] The EIS analysis is one of the principal methods to examine the fundament

behavior of electrode materials for supercapacitor. The impedance of β-Ni(OH)2 and Ni-based/graphene materials were measured in the frequency range of 1m-100k Hz at open circuit potential with an AC perturbation of 5 mV (vs. Ag/AgCl). Fig. 9 shows the Nyquist plots of all the as-prepared materials, consisting of a small semicircle at high frequency and a linear part at low frequency. The internal resistance (Ri), which is the total resistances of the electrode (Rs), the electrolyte resistance (Relectrode) and the resistance at electrolyte/electrode interface (Rinterface) can be obtained from the point intersecting with the real axis in the very high frequency.[49] The Ri of the

PT

β-Ni(OH)2 and β-Ni(OH)2/graphene materials with the data around 0.05-0.07 Ω are the lowest among all the materials. The Ri of 15%-Ni/graphene is lower and around

RI

0.1 Ω, while the 15%-NiO/graphene has the highest Ri with a value of 0.12 Ω. The

SC

slightly lower Ri of β-Ni(OH)2/graphene materials may depend on the hydroxide in the structure of the β-Ni(OH)2. These hydroxide can promote the access that the

U

adsorption/desorption of OH− to react with Ni species during the redox reaction. The

N

Ni/graphene also has a lower internal resistances than the NiO/graphene, and this can

A

be explained by the conductor nature of Ni metal. The semicircle (Fig.9(inset)) in the

M

range of high frequency represents the charge-transfer resistance (Rct), and it increases with the increase of the semicircle diameters.[1, 16] In the low frequency, the slope of

D

all the samples are close to 90° and the sharp impedance line indicates all the

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electrode materials follow an ideal capacitor as the capacitance results from

EP

pseudocapacitive redox reaction.[19, 50, 51] The charging and discharging curves of Ni-based/graphene samples were

CC

measured at the current density of 0.5 A g-1 and 1 A g-1 with a potential window ranging from 0.0 to 1.0 V in 6 M of KOH by a two electrodes system. From the Fig.

A

10a, we can see that all the charge-discharge curves of the Ni-based/graphene electrode materials display the mirror-like shape, which indicates the high electrochemical reversibility of the samples.[52] The potential drop at the very beginning of the discharge process was caused by the overall internal resistance of the electrode, including the electronic resistance of electrode, the diffusing resistance of ions through the separator and in the nanopores of electrode and the current

collector.[16, 53] The slope of the discharge curve represents the discharge time of the samples. We can know that the 15%-Ni(OH)2/graphene with a small slop has the longest discharge time, which is corresponding to the highest capacitance. Fig 9b shows the relationship of cycle number to discharge capacitance tested at current of 0.5 and 1.0 A g-1. The figure for capacitance at the current density of 0.5 A g-1 of 15%-β-Ni(OH)2/graphene,

15%-NiO/graphene

and

15%-Ni/graphene

can

be

calculated as 62 F g-1, 38 F g-1 and 36 F g-1 at the beginning of cycles, respectively. When the current density was increased to 1 A g-1, the capacitance of

PT

15%-β-Ni(OH)2/graphene, 15%-NiO/graphene and 15%-Ni/graphene were 42 F g-1, 28 F g-1 and 24 F g-1, respectively. The capacitance at 0.5 A g-1 of all the

RI

Ni-based/graphene was larger than the results at 1 A g-1. The decrease of the

SC

capacitance with the increase of the discharge current density may be caused by the increase of potential drop due to the resistance of the Ni-based/graphene materials and

U

the relatively insufficient faradic redox reaction under higher discharge current

N

densities.[29, 54] The capacitance of all the samples tested at different current has

A

nearly no loss after 1000 cycle and this indicates that the Ni-based/graphene materials

M

have a good stability. However, the capacitance tested in two electrode system is lower than that in three electrode system and still needs to be improved in the future

TE

D

for industrial application.

4. Conclusions

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Ni-based/graphene as electrode materials for supercapacitor were prepared by a

CC

simple hydrothermal synthesis method using the GO as the precursor. The effect of the graphene support, Ni loading, the testing potential window and the Ni species

A

phase of the Ni-based/graphene for supercapacitor were investigated in detail. As a result, nickel hydroxide materials are more active electrode under the assistance of graphene. The 15%-Ni(OH)2/graphene had a biggest specific capacity of 2077 F g-1 at 5 mV s-1 in the potential window ranging from 0.0 - 0.7 V (vs. Ag/AgCl) and had a stable cycle life.

Acknowledgement This work was supported by the National Natural Science Foundation of China (21076056), Key Project of Chinese Ministry of Education (210010), Ph.D. Programs Foundation of Ministry of Education of China (20091317120005), Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and Hebei Provincial Key Lab of Green Chemical Technology & High Efficient Energy Saving, School of Chemical Engineering & Technology, Hebei University of

PT

Technology.

RI

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Figure Captions

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D

M

Electrochimica Acta, 90 (2013) 673.

A

supercapacitor performance of uniform pompon-like β-Ni (OH)< sub> 2 hollow microspheres,

A

CC

Fig. 1 XRD patterns of all prepared materials. Fig. 2 SEM images of pure Ni(OH)2 (a), 15%-Ni(OH)2/graphene (b), 25%-Ni(OH)2/graphene (c), 15%-NiO/graphene (d), 15%-Ni/graphene (e) and TEM image of 15%-Ni(OH)2/graphene (f). Fig. 3 TGA curves of 15%-, 20%- and 25%-Ni(OH)2/graphene materials. Fig. 4 FT-IR spectra of the Ni-based/graphene materials. Fig. 5 CVs curves of (a) 25%-, (b) 20%-, (c) 15%-Ni(OH)2/graphene, (d) pure Ni(OH)2 and (e) graphene electrodes at potential window of 0.0-0.7 V. Fig. 6 CVs curves of (a) 25%-, (b) 20%- , (c) 15%-Ni(OH)2/graphene and (d) graphene electrodes at potential window of -1.0-0.0V. Fig. 7 CVs curves of (a) 15%-NiO/graphene and (b) 15%-Ni/graphene electrodes at

RI

PT

potential window of 0.0-0.7V. Fig. 8 The specific capacitance by cycle tests of pure Ni(OH)2, 15%-Ni(OH)2/graphene, 15%-NiO/graphene and 15%-Ni/graphene at a scan rate of 50 mV s-1. Fig. 9 Nyquist impedance plots of all the electrode materials and (inset) the enlarged part in high frequency. Fig. 10 (a) Galvanostatic charging and discharging of Ni-based/graphene material at current density of 0.5A g-1 and (b) the relationship of specific capacitance with cycle number of Ni-based/graphene materials at the current density of 0.5 A g-1 and 1 A g-1.

Capacitances (F g-1) 20mV s

-1

50mV s

-1

100mV s

114

64

185

175

147

133

112

15%-Ni(OH) 2/graphene

203

196

192

162

120

graphene

122

112

100

77

75

25%-Ni(OH) 2/graphene

1212

1042

804

440

232

20%-Ni(OH) 2/graphene

1987

1496

1040

525

326

15%-Ni(OH) 2/graphene

2077

1730

1343

777

375

Ni(OH) 2

1146

990

684

407

228

15%-NiO/graphene

1563

1113

720

367

241

15%-Ni/graphene

633

483

362

222

156

graphene

55

52

50

47

45

M

TE

EP CC A

-1

119

-1.0-0.0

0.0-0.7

10mV s 127

20%-Ni(OH) 2/graphene

141

A

25%-Ni(OH)2/graphene

-1

N

5mV s

U

Composition

D

Potential window (V)

SC

Table 1 The specific capacitance of all the samples.

-1

TE

EP

CC

A D

PT

RI

SC

U

N

A

M Fig. 1

TE

EP

CC

A

Fig. 2

D

PT

RI

SC

U

N

A

M

TE

EP

CC

A D

PT

RI

SC

U

N

A

M Fig. 3

TE

EP

CC

A D

PT

RI

SC

U

N

A

M

Fig. 4

TE

EP

CC

A D

PT

RI

SC

U

N

A

M Fig. 5

TE

EP

CC

A D

Fig. 7

PT

RI

SC

U

N

A

M

Fig. 6

TE

EP

CC

A D

PT

RI

SC

U

N

A

M

Fig. 8

TE

EP

CC

A D

PT

RI

SC

U

N

A

M Fig. 9

TE

EP

CC

A D

PT

RI

SC

U

N

A

M

Fig. 10