Advanced Supercapacitors Based on Î±-Ni(OH)2 Nanoplates

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Advanced Supercapacitors Based on #-Ni(OH)2 Nanoplates/ graphene Composite Electrodes with High Energy and Power Density Fenglin Wang, Xiaohe Liu, Fashen Chen, Hao Wan, Yifan Lin, Ning Zhang, and Renzhi Ma ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00309 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 14, 2018

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ACS Applied Energy Materials

Advanced Supercapacitors Based on α-Ni(OH)2 Nanoplates/graphene

Composite

Electrodes

with

High Energy and Power Density Fenglin Wang,† Xiaohe Liu,*,† Fashen Chen,† Hao Wan,† Yifan Lin,† Ning Zhang,† Renzhi Ma*,‡ †

State Key Laboratory of Powder Metallurgy, School of Materials Science and

Engineering, Central South University, Changsha, Hunan 410083, China Email: [email protected] ‡

International Center for Materials Nanoarchitectonics (WPI-MANA), National

Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Email: [email protected] ABSTRACT: In order to solve the lack of energy sources, researchers devote themselves to the study of green renewable and economical supercapacitors. We demonstrate herein that the α-Ni(OH)2 nanoplates/graphene composites are fabricated as active electrodes in supercapacitors with excellent cycling stability, high energy density and power density. The advantages of graphene can complementary the shortcomings of α-Ni(OH)2 nanoplates to compose a novel composite. The α-Ni(OH)2 nanoplates/graphene composite present a high specific capacitance of 1954 F g–1 at 5 A g–1. The reason for improving performance is attributed to graphene, which provides an improved conductivity and increased the specific surface area by interweaving with α-Ni(OH)2 nanoplates. It is particularly worth mentioning that the assembled asymmetric supercapacitor cells yield a high specific capacitance of 309 F g–1 at 5 A g–1 and light a 2V LED sustainable for about 7 minutes, which may bring great prospects for further fundamental research and potential applications in energy storage devices. KEYWORDS:

α-Ni(OH)2

Nanoplates,

Graphene,

Supercapacitor

ACS Paragon Plus Environment

Composite,

Electrodes,

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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INTRODUCTION In contemporary society, the gradual depletion of fossil fuel resources and the pollution environment problem become more and more prominent. It urgently needs a kind of cost-effective and green renewable energy to solve the existing pollution problems of one-time energy.1,2 Supercapacitors, compared with the traditional battery, appear as a novel energy storage device with a remarkable storage capacity, highly reversible, excellent cycle life, which can be a promising energy storage device and has great application in many fields for the characteristics of safety and environmental friendly.3-7 Therefore, the study of supercapacitors is extensively carried out in the academia and industry from all the world.8 Because the electrode materials directly affects the energy storage principle and electrochemical properties of supercapacitors, it primarily focus on the study of electrode active materials which could have good electrochemical stability, large specific area and fast diffusion rate of ions and electrons.9 In recent years, the supercapacitors electrode materials have been explored in energy storage files including three categories: transition metal oxides/hydroxides and conductive

polymers.10-13

Compared

with

the

polymers,

transition

metal

oxides/hydroxides have been intensively researched and shown great prospect for applications in supercapacitors.14,15,16 For instance, Ni(OH)2 electrode material attracts a particular interest not only due to its advantages in terms of microstructure (such as considerable theoretical capacity, various morphologies and multi-crystal phases), but also due to its excellent chemical stability, low cost, and potential applications in batteries.17-20 Even so, it also exists some shortcomings such as low electron transport and poor cycles, which result in poor rate capability and chemical performance as the electrode.21-25 Recently, the graphene has attracted attention to be an ideal supporting matrix for the active material with large specific surface area, fast electron transport, favorable electrochemical stability, good physical and mechanical properties.26-28 et al.23 reported α-Ni(OH)2 grown on graphene, with a hierarchical network-like structure, showing the high specific capacitance of 1760.72 F g–1 at scan rate of 5 mV ACS Paragon Plus Environment

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s–1,. Zhang et al.29 synthesized α-Ni(OH)2/graphite composite, which had 3D hierarchical porous morphology and the capacitance reached 1956 F g–1 at 1 A g–1. Wang et al.30 reported the hexagonal Ni(OH)2 nanoplates grown on graphenes (GS) performed a specific capacitance of 1335 F g–1 at current density of 2.8 A g–1. Upendra singh et al.31 synthesized a porous plate like Ni(OH)2-reduced graphene oxide (Ni(OH)2-r-GO) with the capacitance of 1795 F g–1 at current density of 1 A g–1. Sourav Bag and C, Retna Raj32 obtained a stacking of layer-like α-Ni(OH)2 in the reduced graphene oxide (rGO) with the capacitance of 1671.67 F g–1 at 1 A g–1. As described, the previous studies have made a great effort on how to enhance the electrochemical properties of Ni(OH)2 by combining with other conductive materials or preparing the different morphologies. However, it is still difficult to get a high specific capacitance at a large current density in the charge/discharge process. On the basis of previous studies, it inspires us to synthesize a novel composite material with a unique morphology for enhancing capacitive performance by developing graphene composite with Ni(OH)2, which can make full use of the merits and compensate the demerits. Since the performance of electrode materials is significant effected by its morphologies and the specific surface area, a material with a high specific surface area, a uniform morphology and a special porous structure, is necessary to exhibit better properties in a supercapacitors system.33-36 In this study, an economical and environmentally friendly hydrothermal process is employed to synthesis α-Ni(OH)2 nanoplates with novel morphology. The α-Ni(OH)2 nanoplates/graphene composite was synthesized through a one-step hydrothermal process. The composite shows a unique morphology and a porous structure, which supply a large specific surface area as well as short electron and ion diffusion paths during the electrochemical reactions. As graphene has a significant advantage of excellent electrical conductivity, it is beneficial to the electronic transport, offering more sufficient reactions and an outstanding stability as a current collector. The prepared composite, with a unique morphology and a porous structure, presents a higher specific capacitance than α-Ni(OH)2 nanoplates at higher current density. Meanwhile, its capacitor ACS Paragon Plus Environment


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performances exhibit superior pseudocapacitor applications with high capacitance capabilities and a utility application in the fabrication asymmetric supercapacitor cells. EXPERIMENTAL SECTION All chemicals of analytical grade in this work were purchased from China National Pharmaceutical Group. They were used without further purification. Deionized water was used during the whole experiments. Materials Synthesis: α-Ni(OH)2 nanoplates were synthesized by a typical hydrothermal process. 1 mmol of nickel acetate, 1 mmol of hexadecyl trimethyl ammonium bromide (CTAB), and 5 mmol of urea were mixed in a beaker with 30 ml of deionized water. The mixture was magnetic stirred at room temperature for 0.5 h and placed in a 50 mL Teflon-lined autoclave, which was heated to 120 °C and maintained for 12 h in a convection oven. The obtained powder was collected and washed by ethanol and deionized water several times, which was dried under vacuum at 60 °C for 12 h. Meanwhile, the prepared α-Ni(OH)2 nanoplates/graphene composite was obtained by the above process, except the 30 ml of deionized water was alternated to 15 ml water and 15 ml graphene, the concentration of graphene is 0.5 g L-1 (The detail synthesis of graphene was shown at the supporting information). Materials

Characterization:

Typically,

the

crystallographic

structures

and

lattice parameter of materials were tested by a RIGAKU Rint-2000 X-ray diffractometer equipped with Cu- Ka radiation (λ= 1.54184 Å). In order to study the microstructure and morphology of sample, a scanning electron microscopy (FEI, Helios Nanolab 600i) was adopted. The transmission electron microscopy (TEM), selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) can further study the crystallinity, morphology, size and thickness of the material, which are tested by a Tecnai G2 F20 field emission transmission electron microscope operated at 200 kV. The specific surface area of product was analyzed by a Quadrasorb SI-3MP instrument. Three-Electrode

Electrochemical

Measurement:

The


working

electrodes

of

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supercapacitors were obtained by adding the active materials, acetylene black, and polyvinylidene fluoride (PVDF) (in a mass ratio of 8:1:1) into a mortar and adequately mixed together, subsequently dropped with N-methy1-2-pyrrolidone (NMP) to obtain a homogeneous slurry. Then the slurry was pressed onto the surface of a cleaned nickel foam (1 cm × 1 cm) and dried under vacuum at 60 °C for 12 h. The electrochemical properties of supercapacitors was performed in a three-electrode cell with 1 M KOH aqueous electrolyte solution. Meanwhile, an Hg/HgO electrode, a platinum plate and the as-prepared were served as the reference electrode, the counter electrode and the working electrode, respectively. The loading mass of α-Ni(OH)2 and α-Ni(OH)2/graphene are 280 μg and 220 μg , respectively. To analyze the electrochemical performance of active materials, the cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) were tested with a Gamry Interface 1000 electrochemical workstation. The specific capacitance was calculated by the discharge time of galvanostatic discharge curves on basis of the following equation37: C = I ∗△ t /m ∗△ V

Where C represents the specific capacitance (F g−1), I refers to the discharging current (A), △ t represents the discharging time (s), m means the mass load of active materials (g), △ V refers to the discharging potential window (V). Fabrication Asymmetric Supercapacitor Cells (ASCs): The electrochemical performance of fabrication asymmetric supercapacitor was implemented in a two-electrode system. It was assembled with the α-Ni(OH)2 nanoplates/graphene as positive electrode, active carbon(AC) as negative electrode, 1M KOH aqueous as electrolyte solution, and the electrodes were separated by the diaphragm. The type of active carbon is YEC-8A, and it is produced by Fuzhou Yuanyi-carbon Company. Surface area of AC is larger than 2100 m2 g-1, and particle size of AC is about 10 μm. In order to get the best performance, the conservation of charge between the two electrodes should follow = and the mass of the materials of the positive electrode and negative electrode should be calculated by the next equations 38: q = C ∗△ V ∗ m ACS Paragon Plus Environment


⁄ = ∗△ ⁄ ∗△

Where C refers to the specific capacitance of positive and negative electrodes (F g−1), m represents the mass of active materials on positive and negative electrodes (g), and △ V is the potential window (V). According to the equation = , the loading amount of positive material and negative material in ACSs are 260 μg and 880 μg, respectively. RESULTS AND DISCUSSION The crystal structures and lattice parameters of the materials were identified by XRD. Figure 1a depicts a typical XRD pattern of the material synthesized without graphene at 120 °C for 12 h. All the reflections match with the standard card of α-Ni(OH)2 (JCPDS 38-0715). The peaks of the XRD pattern represent the (003), (006), (101) and (110) diffraction planes at 2θ values of 12.3°, 24.9°, 33.3°, and 59.4° , respectively. The basal spacing of the previous two characteristic peaks are 0.71nm and 0.36nm. The XRD pattern indicates that α-Ni(OH)2 has a high crystallinity. Furthermore, no other peaks such as β-Ni(OH)2 or any impurity exist, showing that the synthetic materials are in ultrahigh purity. The morphology and size of materials was identified by SEM. Figure 1b and Figure 1c present a representative SEM image of obtained α-Ni(OH)2, indicating that hexagonal α-Ni(OH)2 nanoplates with a mean width of about 5 µm were achieved under current conditions. With careful observation, the α-Ni(OH)2 hexagonal nanoplates are composed of abundant ultrathin nanosheets around the edges. It is clearly seen that many ultrathin nanosheets adhere to the edge of the nanoplates and keep a well-regulated morphology. Figure 1d is a TEM image of α-Ni(OH)2. Herein, a large number of ultrathin nanosheets aggregated on the edge of the nanoplates can be clearly observed. Figure 1e displays a typical selected area electron diffraction pattern, revealing the polycrystalline nature of the α-Ni(OH)2 hexagonal nanoplates as a result of the hierarchical structure of ultrathin nanosheets. The diffraction rings of material can be indexed to the (101), (110), (202), (120), (303), (220) crystallographic planes of α-Ni(OH)2. Figure 1f depicts the HRTEM image, which can further analyze the


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structure of obtained α-Ni(OH)2. The lattice fringes were measured to be about 0.26 nm, corresponding to distance spacing of the (101) crystallographic plane of α-Ni(OH)2, it is matched with the XRD results. Importantly, due to the hierarchical structure of ultrathin nanosheets, as-prepared α-Ni(OH)2 has a large specific surface area of about 132.54 m2 g−1, indicating abundant active sites, which may be beneficial to the electrochemical performances with favorable rate capability and great cycling stability.



Figure 1. Hydrothermal synthesis of α-Ni(OH)2 nanoplates obtained at 120 °C for 12 h. (a) XRD pattern, (b),(c) SEM images, (d) TEM image, (e) SAED pattern, and (f) HRTEM image.

Figure 2. Mircosocpic characterizations of the α-Ni(OH)2 nanoplates/graphene composite. (a) AFM image of the as-prepared graphene with the height profile, (b) SEM image, (c) TEM image and (d) HRTEM image. Inset in c) shows the corresponding SAED pattern. In order to further improve electrochemical performances, graphene modified α-Ni(OH)2 nanoplates composite was manufactured through hydrothermal reaction with graphene at 120 °C for 12 h. Figure S1 of supporting information shows the XRD pattern of α-Ni(OH)2 nanoplates/graphene composite, which is similar to Figure 1a. The main peaks can be well identified as (003), (006), (101) and (110), respectively. Meanwhile, the characteristic peak of graphene could not be observed. To further prove the existence of graphene, the TG technique was conducted and shown in Figure S2 at supporting information, which indicates that the mass fraction of graphene in the


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composite is 17.88 %. The EDS mapping (Figure S3) clearly shows that the strongest signals for Ni, O and C were found in the nanoplate, indicates that the obtained composite material is successful incorporated with graphene. In addition, the X-ray photoelectron spectroscopy (XPS) have been evaluated to further analyze the composition and chemical bonding states of each element in the α-Ni(OH)2 nanoplates/graphene composites, as presented in Figure S4. Figure 2a presents the AFM image of the graphene with the height profile, it is clear that the graphene is an ultrathin sheet structure with the thickness about 1 nm, which can provide a large specific surface area. The SEM image (Figure 2b) of as-prepared product suggests that the well-distributed α-Ni(OH)2 nanoplates/graphene could be synthesized after compositing with graphene. Figure 2c depicts a representative TEM image of α-Ni(OH)2/graphene composite. Herein, α-Ni(OH)2 nanoplates and graphene folded together can be clearly detected. By incorporating graphene, the hexagonal α-Ni(OH)2 nanoplates are decorated homogeneously by graphene. Through careful observation and analysis, it can be found that a large of slender nanowires attached on the base nanoplates, which might act as the conducting wires. Moreover, the nanowires are interwoven with each other, forming a similar three-dimensional conductive network and a self-supported conductive matrix, which can effectively make improvements to the conductivity, the specific surface area and the pore volume/structure. More importantly, compared with pure α-Ni(OH)2, the sizes of as-prepared composite were decreased to about 500 nm. The graphene may supply a number of active sites for adsorbing metal ions and enable Ni(OH)2 to have good distribution, it could be the reason for decreasing to the size of α-Ni(OH)2/graphene composite about 500nm. The small-sized or even nanosized materials are more favorable to electrochemical reaction at high current density. The diffraction rings in the typical SAED pattern of α-Ni(OH)2 nanoplates/graphene present a polycrystalline nature owing to the microstructure of assembled nanoplates. The SAED pattern (the inset of Figure 2c ) can be indexed to the (101), (110), (202) crystallographic planes of α-Ni(OH)2, which are consistent with the XRD result. The corresponding HRTEM image in Figure 2d indicates that the interplanar spacing is about 0.26 nm, ACS Paragon Plus Environment


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corresponding to the (101) planes of α-Ni(OH)2. It is noteworthy that α-Ni(OH)2 nanoplates/graphene composite possesses a higher specific surface area about 164.00 m2

g−1,

which

may

provide

more

accessible

sites

for

electrochemical

interactions/reactions. The electrochemical characterizations of as-prepared products were analyzed by cyclic voltammetry (CV), galvanic charge/discharge (GCD) and cycling performance, which were demonstrated in Figure 3. All electrochemical characterizations were carried out in three-electrode system in 1 M KOH aqueous electrolyte. Figure 3a presents a typical CV curve of α-Ni(OH)2 nanoplates under different scan rates. As we known, CV curves represent the capacitive behavior of the corresponding electrode materials, which consist of a pair of prominent redox peaks, and it is derived from the quasi-reversible redox process:

- + = - + + ! This redox process happen in the interface between the α-Ni(OH)2 and the KOH aqueous electrolyte during the potential sweep. The anodic peak was detected at 0.47 V (vs. Hg/HgO) under the positive current density, indicating an oxidation process. Cathodic peak occurred at 0.36 V represents to the reduction process under the negative current density. The relatively GCD curves were observed in Figure 3b at different current densities (with a potential window of 0 - 0.55 V), it reveals a great reversibility of the redox reactions. Figure 3c represents the specific capacitance of α-Ni(OH)2 nanoplates calculated based on the discharge curves, which are 1284, 1130, 1081, 876, and 654 F g−1 at the current densities about 2, 4, 5, 10, 20 A g−1, respectively. Figure 3d illustrates that the electrochemical stability of α-Ni(OH)2 nanoplates, which was tested under a current density of 4 A g−1 with a potential window of 0 - 0.55 V for 1000 cycles. The specific capacitance of α-Ni(OH)2 nanoplates retained 76% after 1000 cycles.


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Figure 3. Electrochemical characterizations of α-Ni(OH)2 nanoplates. (a) CV curves at various scan rates, (b) Galvanostatic charge/discharge (GCD) curves at various current densities, (c) The corresponding specific capacitance, (d) The GCD cycling performance at the current density of 4 A g−1, the inset is the

relevant coulombic

efficiency. By fabricating α-Ni(OH)2 nanoplates combined with graphene, it may be help for enhancement of electrochemical performance. According to Figure 4a, the CV curves of α-Ni(OH)2 nanoplates/graphene composite clearly shows the capacitive behavior at the scan range with 2 - 50 mV s−1. On the one hand, the couple of redox peaks are well maintained when the CV curves expand with the increasing of scan rates. It illustrates the good electrochemical stability of composite under the high scan rates. On the other hand, the peak current increases significantly with increasing scan rates, which shows that the kinetics of interfacial faradic redox reactions are fast enough to facilitate the transmission of electron and ion. The GCD curves shown in Figure 4b are recorded at the different current densities with a potential window ranging with 0 - 0.55 V. It can



Figure 4. Electrochemical characterizations of α-Ni(OH)2 nanoplates/graphene composite. (a) CV curves at various scan rates, (b) Galvanostatic charge/discharge curves at various current densities, (c) The corresponding specific capacitance at various current densities, (d) The cycling performance at the current density of 5 A g−1, the inset is the relevant coulombic efficiency. be found that the discharging time of composite is much longer than that of pure α-Ni(OH)2 nanoplates in the same current density, indicating that it has a higher specific capacitance. The corresponding specific capacitance values of the composite are presented in Figure 4c. There were 1954, 1610, 1460, 1340, 1250, 1175, 1096 at 5, 7 10, 15, 20, 25, 30 A g−1, respectively. According to Figure S5, it can be seen that the enclosed CV curve area of the α-Ni(OH)2 nanoplates/graphene composite are apparently bigger than the pure Ni foam in the same current density, which improved that the contribution of nickel foam is negligible. Compared with pure α-Ni(OH)2, the composite exhibits an enhanced specific capacitance in Figure S5b of Supporting Information. The typical electrochemical impedance spectroscopy shows the smaller Rs and Rct value of α-Ni(OH)2 nanoplates/graphene composite than those of pure


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α-Ni(OH)2

(Supporting Information, Figure S6). Indeed, the higher specific surface

area is conducive to more intimate contact with the electrolyte and more sufficient faradaic reaction occurring during the charging/discharging process.6 As a result, the α-Ni(OH)2 nanoplates/graphene composite is hopeful to exhibit a superior electrochemical properties than the pure α-Ni(OH)2. Moreover, the electrochemical stability of the composite as supercapacitors materials was tested at a current density of 5 A g−1 with a potential window of 0.55 V, as given in Figure 4d, which is much better than pure α-Ni(OH)2, and the specific capacitances remains about 1602 F g-1 after 1000 cycles. It convincingly shows that the material has good stable cycle performance and outstanding charge storage ability. The reason for its favorable electrochemical performance may be as follows. Based on the consequence of TEM, the morphology of nanowires is generated by compositing with graphene. The above-mentioned slender nanowires might act as conducting wire, which may result from the intrinsic high electron conductivity of graphene. On the one hand, the nanowires can penetrate the base nanoplates to effectively enhance their conductivity contacts.27, On the other hand, the nanowires are interwoven with each other, forming a similar three-dimensional conductive network, to facilitate the penetration of electrolyte into the active electrode and further improve the conductivity of electrode. The result of EIS is further confirmed that α-Ni(OH)2 nanoplates/graphene composites have a smaller Rct than pure α-Ni(OH)2, which indicating great conductivity and low internal resistance of electrode. 39-41 The increased electrical conductivity enables a better conducting of electrons, and a full reaction of active materials react. It also facilitates the conduction of electrons under a large current density during the charge /discharge process, so a higher specific capacitance of 1954 F g−1 can be maintained under a large current density at 5 A g−1. In addition, the three-dimensional nanowires network structure can effectively improve the specific surface area and the abundant pore structure according to the consequence of BET, and it is favorable for the active material to be sufficiently contacted with the electrode and electrolyte.42-44 Furthermore, the self-supported conductive matrix



obtained by interweaved nanowires, presents good mechanical properties. It can effectively maintain the structural stability of the materials during charge and discharge, improving the cycle stability of supercapacitors. In summary, it reveals a greatly improved electrochemical performance for the α-Ni(OH)2 nanoplates/graphene composite because of its special morphology and favorable structure. Besides, the outstanding electrical conductivity by combining with graphene benefits to shortened ion pathway, and accelerated ion diffusion.

Figure 5. Electrochemical characterizations of asymmetric capacitors. (a) CV curves at different scan rates, (b) Galvanostatic charge/discharge curves at different current densities, (c) calculated specific capacitance, (d) cycling performance at 5 A g−1 under the measurements of the ASCs, the inset is the relevant coulombic efficiency. To estimate the possibility of α-Ni(OH)2 nanoplates/graphene composite for practical application, asymmetric supercapacitors (ASCs) were assembled by using α-Ni(OH)2 nanoplates/graphene composite as a anode and activated carbon (AC) as a cathode. Meanwhile, 1M KOH alkaline solution was used as the electrolyte for the


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two-electrode cells, and the diaphragm as separator. The electrochemical properties of AC were shown in Figure S7 of the Supporting Information. The potential windows of ASCs can be extended by taking full advantage of the positive and negative electrodes to achieve a larger operating voltage for the two-electrode cells. The total voltage for ASCs (1.55V) was determined by the CV curves of anode active material (from 0 to 0.55V) and cathode active material (from -1 to 0 V) at the scan rate of 5 mV s–1 (Supporting Information, Figure S8a). Meanwhile, the Figure S8b of the Supporting Information clearly indicates the different CV curves at different potential windows for the ASCs with the mass ratio of two electrodes about 0.29 (it calculated by the equation = ), and the faradic reactions occurred with the voltage increased to 1.55 V. However the oxygen evolution reaction also can be found when the voltage increased to 1.65 V. Thus, we choose 1.55 V as the profit voltage for the ASCs studies. Figure S8c shows GCD curves with different potential windows at the current density of 5 A g-1. Based on the results of GCD, on the one hand, the specific capacitance has increased with the gradually increased voltage windows. On the other hand, the charge time is getting longer with the gradually increased voltage windows, which means that the columbic efficiency is getting lower than the voltage window of 1.55 V. According to Figure 5a, the CV curves for ASCs were preserved well at a variety of scan rates from 2 to 50 mV s−1. With the increasing of scan rates, the peaks shift to the more positive or negative position is not obvious, suggesting the superior reversibility of the reaction under big current density. The GCD curves (Figure 5b) of the ASCs was tested at the current density from 5 to 20 A g−1. Figure 5c illustrates the corresponding specific capacitance values calculated by the discharge time, which exhibits that the specific capacitance is about 309, 198, 161, 123, 106 F g−1 under the current densities 5, 7, 10, 15, 20 A g−1, respectively. In particular, the ASC shows a considerable specific capacitance of 309 F g−1 under a big current density about 5 A g−1, which is bigger than the values obtained from previous reports for other Ni(OH)2-based ASCs, as shown in Table S1 of supporting information. The cycling performances of the ASCs were presented in Figure 5d. The specific capacitance ACS Paragon Plus Environment


increased gradually during the first 150 cycles, and retained a steady level in the remainder of the cycling. It is apparent that the specific capacitance values remain an approximately 100% after cycling 1000 cycles in a wide potential window of 0−1.55 V, which reveals that the fabricated ASCs with considerable specific capacitance and good cyclic stability can act as low-cost energy storage devices.

Figure 6. LED lighted by fabricated ASCs. (a) Structure of The ASCs, (b) The photograph of a LED lighted by two serried cells for the initial time, (c) 1 min, (d) 2 min, (e) 3 min, (f) 4 min, (g) 5 min. (h) 6 min, (i) 7 min. The asymmetric supercapacitors were further assembled in series to verify their practical application. Figure 6 shows that the two series of assembled ASCs device (charging to the 3.1 V) can efficiently light up a 2 V LED lights. Figure 6a shows that a structure of ASCs is assembled by the sequence anode/separator/cathode. In this experiment, the α-Ni(OH)2 nanoplates/graphene composite play the part of the anode and AC as the cathode. 1 M KOH solution acts as an electrolyte dispersed throughout the ASCs. Most interestingly, Figure 6b to 6i are the pictures about a LED lighted by ASCs at different time points, and the inset is the detail view for the LED. In the initial time shown in Figure 6b, it is seen that the LED lights are very bright. It still remains certain brightness after lighted 7 minutes, as shown in Figure 6i. It fully proved that this composite material has a promising application in practical battery devices.


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Figure 7. Ragone plot of α-Ni(OH)2 nanoplates/graphene composite//AC ASC tested at the different current density. The data of Ragone plot (the energy density E (Wh kg−1) and the power density P (W kg−1)) were calculated by the next equation: E = ∗△ ⁄7.2 and P = & ∗ 3600⁄△ * , where (F g−1) is specific capacitance for the ASCs, △ (V) is potential window and △ * (s) is discharge time of GCD curves for the ASCs. According to the equation, the

results of Ragone

plot for

α-Ni(OH)2

nanoplates/graphene//AC ASC was calculated under different current density as shown in Figure 7. It exhibits the energy density reach up to 103 Wh kg−1 with a high power density of 3871 W kg−1 under high current density about 5 A g−1. The most important thing is the maximum power density of 15.857 kW kg−1 can be achieved in optimized ASCs. Meanwhile, the energy density still maintains 35.37 Wh kg−1, which has exhibited a practical application in electrochemical energy storage devices, and the excellent electrochemical properties of the asymmetric supercapacitor as discussed above can be due to the high energy and power of α-Ni(OH)2 nanoplates/graphene



composite. This results are remarkable than some previously reported nickel hydroxide materials. The detail data was shown in Table S2 of supporting information. CONCLUSIONS In summary, a facile synthetic strategy we developed for the fabrication of α-Ni(OH)2 nanoplates using CTAB as the surfactant agent. Moreover, the α-Ni(OH)2 nanoplates could be readily modified with graphene based on the above synthetic strategy. Compared with pure α-Ni(OH)2, the composite exhibited favourable electrochemical properties with higher specific capacitance and cycling stability. The electrochemical characterizations suggested that the composite revealed a superior specific capacitance of 1954 F g−1 at 5 A g−1 and remained an excellent cycling stability after 1000 cycles. Meanwhile, the assembled ASCs provided a high specific capacitance of 309 F g−1 at 5 A g−1. We further demonstrated the practical application of this material, lighting a 2V LED sustained for 7 minutes as an example. We assumed that the outstanding performances of the α-Ni(OH)2 nanoplates/graphene composite are attributed to the unique nanoplates morphology by efficiently ensuring the synergetic effect of two materials (α-Ni(OH)2 and graphene ), which offered a larger specific surface area. And it improves the electrical conductivity and promotes the faster diffusion and migration of ions between the interfacial electrolyte and electrodes during the charge/discharge process by combining with graphene, indicating that it could be a promising active electrode materials for high performance supercapacitors and other electrochemical energy-storage devices. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Experimental details of graphene, XRD, the TG data, the EDS mapping, the N2 adsorption–desorption isotherm, XPS patterns of the ɑ-Ni(OH)2/graphene, CV curves


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of the pure Ni foam and α-Ni(OH)2 nanoplates/graphene composite, the comparing GCD cycling performance and EIS between the pure α-Ni(OH)2 and α-Ni(OH)2 nanoplates/graphene composite, CV curves and GCD curves of AC, the comparing CV curve of the positive electrode and the negative electrode, CV curves and GCD curves of α-Ni(OH)2 /graphene composite//AC ASCs with different potential window, XRD,SEM images for different α-Ni(OH)2/graphene composites, the data of the specific capacitance about some previous reported nickel hydroxide based composite for ASCs, the data of comparison of the maximum energy density and power density of some previous reported nickel hydroxide based composite as the electrode materials. AUTHOR INFORMATION Corresponding Authors * Email: [email protected] * Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (51372279), Hunan Provincial Natural Science Foundation of China (13JJ1005). X. L. acknowledges support from Shenghua Scholar Program of Central South University. R. M. acknowledges support from JSPS KAKENNHI (15H03534, 15K13296).



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Graphic Abstract


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Advanced Supercapacitors Based on Î±-Ni(OH)2 Nanoplates

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