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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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,† and Renzhi Ma*,‡ †

State Key Laboratory of Powder Metallurgy, School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, China ‡ International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan S Supporting Information *

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 complement the shortcomings of α-Ni(OH)2 nanoplates to compose a novel composite. The αNi(OH)2 nanoplates/graphene composite presents a high specific capacitance of 1954 F g−1 at 5 A g−1. The reason for the improving performance is attributed to graphene, which provides an improved conductivity and increased 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 2 V LED sustainable for about 7 min, which may bring great prospects for further fundamental research and potential applications in energy storage devices. KEYWORDS: α-Ni(OH)2 nanoplates, graphene, composite, electrodes, supercapacitor

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great prospects for applications in supercapacitors.14−16 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 multicrystal phases), but also due to its excellent chemical stability, low cost, and potential applications in batteries.17−20 Even so, there are also 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 as an ideal supporting matrix for the active material with large specific surface area, fast electron transport, favorable electrochemical stability, and good physical and mechanical properties.26−28 Yang 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 a scan rate of 5 mV s−1. Zhang et al.29 synthesized an αNi(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

INTRODUCTION In contemporary society, the gradual depletion of fossil fuel resources and the pollution environment problem have become more and more prominent. There is an urgent need for a kind of cost-effective and green renewable energy to solve the existing pollution problems of nonrenewable energy.1,2 Supercapacitors, compared with the traditional battery, appear to be a novel energy storage device with a remarkable storage capacity, high reversibility, and excellent cycle life, and can be a promising energy storage device with great applications in many fields because of the characteristics of safety and environmental friendliness.3−7 Therefore, the study of supercapacitors is extensively carried out in academia and industry throughout the world.8 Because the electrode materials directly affect the energy storage principle and electrochemical properties of supercapacitors, the primary focus is on the study of electrode active materials which could have good electrochemical stability, large specific area, and fast diffusion rate for ions and electrons.9 In recent years, the supercapacitor electrode materials have been explored for energy storage applications including these categories: transition metal oxides/hydroxides and conductive polymers.10−13 Compared with the polymers, transition metal oxides/hydroxides have been intensively researched and show © XXXX American Chemical Society

Received: December 18, 2017 Accepted: March 14, 2018 Published: March 14, 2018 A

DOI: 10.1021/acsaem.7b00309 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

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.

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 for the demerits. Since the performance of electrode materials is significantly affected 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 supercapacitor system.33−36 In this study, an economical and environmentally friendly hydrothermal process is employed to synthesize α-Ni(OH)2 nanoplates with novel morphology. The α-Ni(OH)2 nanoplate/graphene composite was synthesized through a one-step hydrothermal process. The

on graphenes (GS) performed a specific capacitance of 1335 F g−1 at current density of 2.8 A g−1. Singh et al.31 synthesized a porous plate-like Ni(OH)2-reduced graphene oxide (Ni(OH)2r-GO) with the capacitance of 1795 F g−1 at current density of 1 A g−1. Bag and Retna32 obtained stacking of layer-like αNi(OH)2 in the reduced graphene oxide (rGO) with capacitance of 1671.67 F g−1 at 1 A g−1. As described, the previous studies have made a great effort regarding how to enhance the electrochemical properties of Ni(OH)2 by combination with other conductive materials or preparation of 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, we were inspired to B

DOI: 10.1021/acsaem.7b00309 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Fabrication Asymmetric Supercapacitor Cells (ASCs). The electrochemical performance of a fabricated 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, and 1 M KOH aqueous solution as the electrolyte solution, and the electrodes were separated by the diaphragm. The type of active carbon is YEC-8A, and it is produced by the Fuzhou Yuanyi-Carbon Company. The surface area of AC is larger than 2100 m2 g−1, and the 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 q+ = q−, and the mass of the materials of the positive electrode and negative electrode should be calculated by the following equations:38

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 the significant advantage of excellent electrical conductivity, it is beneficial to the electronic transport, offering more sufficient reactions and 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 performances exhibit superior pseudocapacitor applications with high capacitance capabilities and a utility application in the fabrication asymmetric supercapacitor cells.

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q = C ΔVm

EXPERIMENTAL SECTION

m+ /m− = (C −ΔV −)/(C +ΔV +)

All chemicals of analytical grade in this work were purchased from the China National Pharmaceutical Group. They were used without further purification. Deionized water was used during the whole experiment. Materials Synthesis. α-Ni(OH)2 nanoplates were synthesized by a typical hydrothermal process. A 1 mmol portion of nickel acetate, 1 mmol of hexadecyl trimethylammonium 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 with ethanol and deionized water several times, and it 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 30 mL of deionized water was alternated with 15 mL of water and 15 mL of graphene; the concentration of graphene is 0.5 g L−1 (the detailed synthesis of graphene was shown in 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 Kα radiation (λ = 1.54184 Å). In order to study the microstructure and morphology of the sample, a scanning electron microscope (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 the product was analyzed by a Quadrasorb SI-3MP instrument. Three-Electrode Electrochemical Measurement. The working electrodes of 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 mixing them together. Subsequently, the mixture was 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 were performed in a three-electrode cell with a 1 M KOH aqueous electrolyte solution. Meanwhile, a Hg/HgO electrode, a platinum plate, and the as-prepared material 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 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 the basis of the following equation:37

Here, the following abbreviations apply: 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 ΔV is the potential window (V). According to the equation q+ = q−, the loading amounts of positive material and negative material in ACSs are 260 and 880 μg, respectively.

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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 for 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 spacings of the previous two characteristic peaks are 0.71 and 0.36 nm. 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 of ultrahigh purity. The morphology and size of the materials were identified by SEM. Figure 1b,c presents a representative SEM image of the 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 αNi(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 maintain 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), and (220) crystallographic planes of α-Ni(OH)2. Figure 1f depicts the HRTEM image, which can further analyze the 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, matched with the XRD results. Importantly, due to the hierarchical structure of ultrathin nanosheets, as-prepared αNi(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. In order to further improve electrochemical performances, the graphene modified α-Ni(OH)2 nanoplate composite was

C = (I Δt )/(mΔV ) Here, the following abbreviations apply: C represents the specific capacitance (F g−1), I refers to the discharging current (A), Δt represents the discharging time (s), m indicates the mass load of active materials (g), and ΔV refers to the discharging potential window (V). C

DOI: 10.1021/acsaem.7b00309 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 2. Microscopic 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 part c shows the corresponding SAED pattern.

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) corresponding specific capacitance, and (d) GCD cycling performance at the current density of 4 A g−1. The inset is the relevant Coulombic efficiency.

which is similar to Figure 1a. The main peaks can be wellidentified as (003), (006), (101), and (110), respectively. Meanwhile, the characteristic peak of graphene could not be

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, D

DOI: 10.1021/acsaem.7b00309 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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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) corresponding specific capacitance at various current densities, and (d) cycling performance at the current density of 5 A g−1. The inset is the relevant Coulombic efficiency.

the reason for a decrease in the size of the α-Ni(OH)2/graphene composite of about 500 nm. 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), and (202) crystallographic planes of α-Ni(OH)2, which is consistent with the XRD result. The corresponding HRTEM image in Figure 2d indicates that the interplanar spacing is about 0.26 nm, corresponding to the (101) planes of αNi(OH)2. It is noteworthy that the α-Ni(OH)2 nanoplates/ graphene composite possesses a higher specific surface area of 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 are shown 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 know, CV curves represent the capacitive behavior of the corresponding electrode materials, which consist of a pair of prominent redox peaks, and the behavior is derived from the quasireversible redox process

observed. To further prove the existence of graphene, the TG technique was conducted and shown in Figure S2 in Supporting Information, which indicates that the mass fraction of graphene in the 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, indicating that the obtained composite material is successfully incorporated with graphene. In addition, X-ray photoelectron spectroscopy (XPS) has been evaluated to further analyze the composition and chemical bonding states of each element in the α-Ni(OH)2 nanoplate/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 thickness of 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 forming a composite 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 group 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 the asprepared 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

α ‐Ni(OH)2 + OH− = γ ‐NiOOH + H 2O + e−

This redox process happens in the interface between the αNi(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. The cathodic peak that occurred at 0.36 V represents the reduction process under the negative current density. The E

DOI: 10.1021/acsaem.7b00309 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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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, and (d) cycling performance at 5 A g−1 under the measurements of the ASCs. The inset is the relevant Coulombic efficiency.

area of the α-Ni(OH)2 nanoplates/graphene composite is apparently bigger than the pure Ni foam in the same current density, which proved 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 values of the αNi(OH)2 nanoplates/graphene composite than those of pure αNi(OH)2 (Supporting Information, Figure S6). Indeed, the higher specific surface area is conducive to more intimate contact with the electrolyte and a more sufficient faradaic reaction occurring during the charging/discharging process.6 As a result, the α-Ni(OH)2 nanoplates/graphene composite has the potential to exhibit superior electrochemical properties compared to those of the pure α-Ni(OH)2. Moreover, the electrochemical stability of the composite as supercapacitor 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 that of pure α-Ni(OH)2, and the specific capacitance remains about 1602 F g−1 after 1000 cycles. This convincingly shows that the material has a good, stable cycle performance and outstanding charge storage ability. The reason for its favorable electrochemical performance may be as follows. On the basis of the consequence of TEM, the morphology of nanowires is generated by forming a composite with graphene. The above-mentioned slender nanowires might act as the conducting wire, which may result from the intrinsic high electron conductivity of graphene. On 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 pene-

relative GCD curves were observed in Figure 3b at different current densities (with a potential window of 0−0.55 V), revealing a great reversibility of the redox reactions. Figure 3c represents the specific capacitance of α-Ni(OH)2 nanoplates calculated on the basis of the discharge curves, which are 1284, 1130, 1081, 876, and 654 F g−1 at the current densities of about 2, 4, 5, 10, and 20 A g−1, respectively. Figure 3d illustrates 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 αNi(OH)2 nanoplates retained 76% after 1000 cycles. Fabrication of α-Ni(OH)2 nanoplates combined with graphene may be helpful for enhancement of electrochemical performance. According to Figure 4a, the CV curves of the αNi(OH)2 nanoplates/graphene composite clearly show the capacitive behavior in the scan range 2−50 mV s−1. On one hand, the pair of redox peaks are well-maintained when the CV curves expand with the increasing of scan rates. This illustrates the good electrochemical stability of the 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 0−0.55 V. It can 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. These 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 F

DOI: 10.1021/acsaem.7b00309 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 6. LED lit by fabricated ASCs. (a) Structure of the ASCs. Photograph of an LED lit by two serried cells at (b) the initial time, (c) 1 min, (d) 2 min, (e) 3 min, (f) 4 min, (g) 5 min, (h) 6 min, and (i) 7 min.

ratio of two electrodes about 0.29 (it calculated by the equation q+ = q−), and the faradaic 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 ASC studies. Figure S8c shows GCD curves with different potential windows at the current density of 5 A g−1. On the basis of the results of GCD, on 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 coulombic 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 increase of scan rates, the peak shifting 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 were 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 shows 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 of 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.45,46 The cycling performances of the ASCs were presented in Figure 5d. The specific capacitance 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 approximately 100% after cycling 1000 cycles in a wide potential window 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. 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 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 plays the part of the anode with AC

tration 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 indicates great conductivity and low internal resistance of electrode.39−41 The increased electrical conductivity enables better conduction of electrons, and a full reaction of active materials. This 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 nanowire 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 in contact 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. In addition, outstanding electrical conductivity in combination with graphene benefits the shortened ion pathway, and accelerated ion diffusion. To estimate the possibility of the α-Ni(OH)2 nanoplates/ graphene composite for practical application, asymmetric supercapacitors (ASCs) were assembled by using an α-Ni(OH)2 nanoplates/graphene composite as an anode and activated carbon (AC) as a cathode. Meanwhile, 1 M KOH alkaline solution was used as the electrolyte for the two-electrode cells, with the diaphragm as separator. The electrochemical properties of AC are 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 (1.55 V) was determined by the CV curves of anode active material (from 0 to 0.55 V) and cathode active material (from −1 to 0 V) at the scan rate of 5 mV s−1 (Supporting Information, Figure S8a). Meanwhile, Figure S8b of the Supporting Information clearly indicates the different CV curves at different potential windows for the ASCs with the mass G

DOI: 10.1021/acsaem.7b00309 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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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 2 V LED sustained for 7 min as an example. We assumed that the outstanding performances of the α-Ni(OH)2 nanoplates/ graphene composite are attributed to the unique nanoplate morphology by efficiently ensuring the synergetic effect of two materials (α-Ni(OH)2 and graphene), which offered a larger specific surface area. Also, this 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 combination with graphene, indicating that it could be a promising active electrode materials for high performance supercapacitors and other electrochemical energy storage devices.

as the cathode. A 1 M KOH solution acts as an electrolyte dispersed throughout the ASCs. Most interestingly, Figure 6b−i shows the pictures of an LED lit by ASCs at different time points, and the inset is the detailed view of the LED. In the initial time shown in Figure 6b, it is seen that the LED lights are very bright. They still retain a certain brightness after being lit for 7 min, as shown in Figure 6i. This fully proved that this composite material has a promising application in practical battery devices. The data from the Ragone plot (the energy density E (Wh kg−1) and the power density P (W kg−1)) were calculated by the next equation: E = (CΔV2)/7.2 and P = E3600/Δt, where C (F g−1) is specific capacitance for the ASCs, ΔV (V) is the potential window, and Δt (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 were calculated under different current densities as shown in Figure 7. It shows

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00309. Experimental and characterization details, including XRD, TGA, EDS, XPS, SEM, and CV (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaohe Liu: 0000-0003-1297-9597 Ning Zhang: 0000-0002-3033-0276 Renzhi Ma: 0000-0001-7126-2006 Notes

The authors declare no competing financial interest.

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Figure 7. Ragone plot of α-Ni(OH)2 nanoplates/graphene composite// AC ASC tested at the different current density.

ACKNOWLEDGMENTS This work was supported by the 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).

that the energy density can reach 103 Wh kg−1 with a high power density of 3871 W kg−1 under a high current density of about 5 A g−1. The most important thing is that 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 the α-Ni(OH)2 nanoplates/graphene composite. These results are more remarkable than those from some previously reported nickel hydroxide materials. The detailed data are shown in Table S2 of Supporting Information.47−49

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CONCLUSIONS In summary, we developed a facile synthetic strategy 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 on the basis of the above synthetic strategy. Compared with pure α-Ni(OH)2, the composite exhibited favorable 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 retained an excellent cycling stability after 1000 cycles. H

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DOI: 10.1021/acsaem.7b00309 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX