graphene hydrogels and pure graphene hydrogels

graphene hydrogels and pure graphene hydrogels

Nano Energy (]]]]) ], ]]]–]]] 1 Available online at www.sciencedirect.com 3 5 journal homepage: www.elsevier.com/locate/nanoenergy 7 9 RAPID COMM...

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Nano Energy (]]]]) ], ]]]–]]]

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Available online at www.sciencedirect.com

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High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels

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Ronghua Wanga, Chaohe Xub, Jong-Min Leea,1

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a School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore b College of Aerospace Engineering, Chongqing University, No. 174 Shazhengjie Road, Chongqing 400044, PR China

Received 22 June 2015; received in revised form 29 September 2015; accepted 28 October 2015

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KEYWORDS

Abstract

Graphene; Hydrogels; Nickel oxyhydroxide; Binder-free; Asymmetric supercapacitors

NiOOH nanosheet/graphene hydrogels (H–NiOOH/GS), with mesoporous NiOOH nanosheets uniformly dispersed within the highly interconnected 3D graphene network, are constructed and studied for the first time by a mixed solvothermal and hydrothermal reaction. The effect of solvent composition on the morphology, phase, dispersibility of nanocrystal and hydrogel strength is systematically studied. As binder-free electrodes of supercapacitors, H–NiOOH/GS delivers high capacitance of 1162 F g  1 at 1 A g  1 with excellent rate capability (981 F g  1 at 20 A g  1). The charge-storage mechanisms of H–NiOOH/GS are in-depth investigated by quantifying the kinetics of charge storage, which reveals that NiOOH exhibits both capacitive effects and diffusion-controlled battery-type behavior during charge storage. Additionally, solvothermal-induced pure graphene hydrogels (H-GS) are also prepared and used as the negative electrode for the first time, which show an impressive specific capacitance of 425 and 368 F g  1 at 5 and 40 mV s  1, respectively. Benefitting from the synergistic contribution of both positive and negative electrodes, the assembled H–NiOOH/GS//H-GS asymmetric supercapacitors achieve a remarkable energy density of 66.8 W h kg  1 at a power density of 800 W kg  1, and excellent cycling stability with 85.3% capacitance retention after 8000 cycles, holding great promise for energy storage applications. & 2015 Published by Elsevier Ltd.

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Tel.: +65 65138129. E-mail address: [email protected] (J.-M. Lee).

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http://dx.doi.org/10.1016/j.nanoen.2015.10.030 2211-2855/& 2015 Published by Elsevier Ltd.

Please cite this article as: R. Wang, et al., High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.030

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Introduction

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High-performance energy-storage and -conversion devices have attracted tremendous attention because of the rapid growth in energy consumption in the world [1]. Electrochemical capacitors (ECs) are one of the most promising devices due to their superior power density, ultrafast charging–discharging rate, long-cycle-life, and safety in use. However, supercapacitors suffer from relatively poor energy density, at least one order of magnitude lower than those of traditional batteries. To boost the energy density, one way is to rational design and explore novel-structured electrode materials with high specific capacitance [2]. Another solution is to develop asymmetric supercapacitors (ASCs) consisting of a battery-like Faradic electrode (as energy source) and a capacitor-like electrode (as power source), which have different potential ranges [3]. Compared with conventional symmetric ECs, the asymmetric configuration can greatly widen the operation-voltage window, resulting in a notable improvement in energy density. As to the electrode material, a high-performance EC electrode generally requires high electrical conductivity, large ion-accessible surface area, fast ionic transport rate and good electrochemical stability [4]. In these regards, graphene-based composite materials, such as metal oxide/ grapheme [5], metal hydroxide/grapheme [6] and polymer/ grapheme [7], have been demonstrated to be very promising EC electrodes. However, driven by the strong π–π interaction, graphene sheets will readily parallel re-stack to form graphite-like powders during processing, leading to a severe reduction in specific surface area and becoming a serious obstacle to making full use of the unique properties of grapheme [8,9]. Moreover, polymer binder and/or conductive additives are usually required to be mixed with graphene-based active materials to make electrodes, which further reduce the specific capacitive performance. Recently, three-dimensional graphene-based frameworks, especially graphene-based hydrogels, have received particular attention for potential applications in supercapacitors [7,10,11], supports for catalysts [12] and so on. The unique graphene gels consist of interconnected 3D porous frameworks with large specific surface areas, allowing multidimensional electron transport and rapid electrolyte ions diffusion [13]. In addition, the hydrogels can be used as binder-free electrodes, thus can effectively reduce the “dead surface” in traditional slurry-derived electrodes and facilitate more efficient charge and mass transportation [1,14]. For example, Duan et al. have demonstrated that a hydrothermally produced graphene hydrogel can give a specific capacitance of 190 F g  1 at 1 A g  1 [11]. Later, the same group further prepared functionalized graphene hydrogels (FGHs) by one-step chemical reduction of graphene oxide (GO) using hydroquinones as the reducing and functionalizing molecules simultaneously [13]. The FGHs showed a good specific capacitance of 441 F g  1 at 1 A g  1 in 1 M H2SO4 aqueous electrolyte, demonstrating exciting potential of the graphene hydrogels for energy storage applications. Although construction of graphene sheets into hydrogels has greatly improved its electrochemical performance, the specific capacitance of a pure graphene hydrogel is fundamentally

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limited by its electrical double layer (EDL) capacitance mechanism. To further improve their specific capacitance, many efforts have been made to composite graphene hydrogels with other pseudocapacitive materials (metal oxide (MnO2 [5], Mn3O4 [15], V2O5 [16]), metal hydroxide (Ni(OH)2 [6,10]), polymers [7], and so on) that have a higher theoretical specific capacitance. For instance, Ni (OH)2 particles/GS hydrogels have been prepared by a hydrothermal method, which exhibited a specific capacitance of 1250 F g  1 at a scan rate of 10 mV s  1 [10]. Similarly, V2O5/GS hydrogels were also constructed by the hydrothermal reaction [16]. The composite hydrogel is capable of delivering a high specific capacitance of about 320 F g  1 at a current density of 1.0 A g  1. Alternatively, Yan et al. employed a reduction-induced in situ selfassembly and constructed MnO2/GS hydrogels, which delivered a capacitance of 242 F g  1 at 1 A g  1 and 92 F g  1 at 8 A g  1 [5]. Despite these encouraging results of graphenebased hybrid hydrogels, the reported methods are mostly based on chemical reduction or hydrothermal reaction, using H2O as the reaction solvent. However, the influence of reaction solvents on the morphology, phase, nanocrystal dispersibility, and hydrogel strength of products has rarely been investigated. Moreover, the study of graphene-based hybrid hydrogels for asymmetric supercapacitors is also very limited despite their significant potential. In this paper, we demonstrate that a mixed solvothermal and hydrothermal reaction (DMF and H2O), instead of the commonly used hydrothermal reaction, can produce a new material, NiOOH nanosheet/GS hydrogels. Although many nickel-based materials (NiO, Ni(OH)2) have been intensively investigated as the electrode materials for ECs, the research of NiOOH in this field is rather limited, and NiOOH/GS hydrogels have never been constructed and studied in previous work. Our study revealed that the composition of solvent played a key role in determining the final phase of the products. The effect of DMF/H2O ratio on the morphology, dispersibility of nanocrystal and hydrogel strength was also systematically studied. As a binderfree electrode for ECs, the optimized NiOOH/GS hydrogels exhibited a specific capacitance of 1162, 1051 and 981 F g  1 at 1, 10 and 20 A g  1, respectively, demonstrating excellent electrochemical performance. Furthermore, an ASC, based on as-prepared NiOOH/GS hydrogels as the positive electrode and solvothermal-induced pure graphene hydrogels as the negative electrode, has been successfully constructed. The ASC delivers a high energy density of 66.8 W h kg  1 at a power density of 800 W kg  1 and exhibits remarkable cycling stability with retention of 85.3% of specific capacitance after 8000 cycles at a large operating potential of 1.6 V. These findings highlight the importance of the solvent to final products, and open up the possibility of NiOOH/GS hydrogels for applications in high-voltage ASCs with high energy and power densities.

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Experimental procedures

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Material synthesis

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GO was prepared from graphite powder (Alfa-Aesar) by the modified Hummers method [17].

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Please cite this article as: R. Wang, et al., High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.030

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105 Figure 1 (a) XRD patterns of hybrid hydrogels obtained in various solvents with different DMF/H2O ratio; (b) A digital photograph of NiOOH/GS hydrogels in D6–H1 solvent; (c) The survey XPS spectra of NiOOH/GS; (d) Core level C 1s spectrum; (e) Ni 2p XPS spectrum; (f) TG curves of freeze-dried NiOOH/GS hydrogels.

The 3D macroscopic NiOOH/GS hydrogels were prepared by a mixed solvothermal and hydrothermal reaction. Typically, the GO was first ultrasonically dispersed in a mixed solvent of N,N-dimethylformamide (DMF) and H2O. Then, nickelous acetate (Ni(OAc)2  4H2O) was added to the above dispersion with stirring. The mixed dispersion was ultrasonicated for 30 min and then solvothermally treated at 180 1C for 6 h to produce a 3D macroscopic NiOOH/GS monolith (hydrogel). Finally, the monolith was dialysed repeatedly with distilled water to remove the organic solvent, remaining salts and impurities. Ni/GS hydrogels or Ni(OH)2/GS hydrogels were obtained if the solvent was only DMF or only H2O, respectively. As a control, pure NiOOH without GS was also prepared via a similar procedure as NiOOH/GS hydrogels except no GO was added.

Pure GS hydrogels were prepared as follows: GO was ultrasonicated in DMF to form a dispersion (2 mg ml  1), which was solvothermally reacted at 180 1C for 6 h to produce pure GS hydrogels.

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Electrode preparation 117 The hydrogels were directly used in the wet state as an electrode, without adding any polymer binder or carbon black: the hydrogels were immersed in a 2 M KOH aqueous solution overnight to exchange interior water, then cut into small pieces and pressed between two nickel foams (1  1 cm2) to form an electrode. The mass loading was about 1–1.5 mg.

Please cite this article as: R. Wang, et al., High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.030

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For the powder materials, a slurry-coating process was used to fabricate the electrodes: active materials, carbon black and a PVDF binder were mixed in a mass ratio of 8:1:1 and homogenized in N-methyl-2-pyrrolidone (NMP) to form slurries. The homogenous slurries were drop-casted on a nickel foam and vacuum dried at 110 1C for 12 h.

where I is the applied current, dt is the time differential, m is the mass of active materials (including GS for NiOOH/GS hydrogel), and ΔV indicates the voltage range of one sweep segment. Specific capacitance can also be calculated from galvanostatic charge–discharge curves according to the following equations:

Characterization

IΔt Cs ¼ ΔVm

Morphologies of the as-prepared samples were characterized using field emission scanning electron microscopy (JEOL-JSM6700F microscope) and transmission electron microscopy (TEM, JEOL, JEM-2010F, 200 kV). X-ray diffraction (XRD) was carried out on a Bruker D8 Advance X-Ray Diffractometer with Cu-Kα as the radiation source (λ=0.154 nm). X-Ray photoemission spectroscopy (XPS) was performed on a KRATOS AXIS DLD spectrometer. Thermal gravimetric analysis (TGA) was conducted in air at a heating rate of 10 1C min–1. The nitrogen adsorption and desorption isotherms were measured using a Quantachrome Instruments Autosorb AS6B. A methylene blue adsorption method was further used to test the specific surface area of the hydrogels. Methylene blue is a common dye probe used to determine the surface area of graphitic materials, with 1 mg of adsorbed methylene blue covering 2.54 m2 of surface area [4,5]. First, a known mass of hydrogel was added into a standard concentration of methylene blue in DI water for 24 h to reach maximum adsorption. Afterwards, the methylene blue concentration was tested by analyzing the supernatant through UV–vis spectroscopy at a wavelength of 665 nm. By comparing to the initial standard concentration, the amount of adsorbed methylene blue can be calculated, and thus, the specific surface area of the hydrogels can be determined.

where I, Δt, ΔV and m represent the discharge current, discharge time, voltage drop upon discharging (excluding the IR drop) and the mass of active materials.

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A single electrode was tested in a three-electrode electrochemical cell with a Pt counter electrode and Hg/HgO reference electrode in a 2 M KOH solution. The electrochemical measurements for pseudocapacitors were carried out using a two electrode configuration, where NiOOH/GS and pure GS hydrogels were used as the positive and negative electrodes, respectively. All the electrochemical measurements were carried out with an ACHI660D electrochemical workstation. All the operating current densities were calculated based on the total mass of the system (the whole electrode including GS for the 3-electrode system, the total weight of the positive and negative electrodes for the 2-electrode system). The Nyquist plots were recorded by applying an AC voltage of 10 mV amplitude in the frequency range of 0.01–100 kHz.

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Calculation For 3-electrode system (half-cell): 1

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The specific capacitance (Cs, F g ) was calculated from the CV curves using the equations: R I dt Cs ¼ ð1Þ mΔV

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For full-cell

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The specific capacitance of the whole ASC in a twoelectrode cell (Ccell, F g  1) can be calculated from both CV curves and galvanostatic charge–discharge curves, using the same formula as that of half-cell, where M is the total weight of the positive and negative electrodes (M= m + + m  ): R Idt ð3Þ Ccell ¼ MΔV

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The energy density (E, W h kg ) and powder density (P, W kg  1) of the ASC were estimated using the following equations [2,5,18]: E ¼ 1=2  Ccell  ðΔV Þ2

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Electrochemical measurements

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Results Preparation of NiOOH/GS hydrogels and structural characterization Different from the commonly used chemical or hydrothermal reaction using pure H2O as the solvent, the construction of NiOOH/GS hydrogels was achieved by mixed solvothermal and hydrothermal reactions. A mixed solution of DMF and H2O was used as the reaction solvent, and the ratio of DMF/ H2O (denoted as Dx–Hy, while D and H represent DMF and H2O, x and y means the relative volume of DMF and H2O, respectively) plays a key role in determining the final phase of the products. As shown in Figure 1a, when there is no water in the system (D1–H0), Ni/GS hydrogels are produced with JCPDF number of 04-0850 (the red line). This is understandable because DMF can be an active reducing agent under suitable conditions, which reduces Ni2 + into Ni [17]. However, when the solution is pure H2O (D0–H1), an Ni (OH)2/GS composite (the green line, JCPDF:14-0117) is obtained as the final product, according to the reaction (Ni2 þ þ 2OH  -NiðOHÞ2 ). Noteworthy, when a mixed solution of DMF and H2O was used as the solvent, NiOOH/GS hydrogels were obtained instead of Ni/GS or Ni(OH)2/GS. As shown by the blue line in Figure 1a, with three different

Please cite this article as: R. Wang, et al., High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.030

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DMF/H2O ratios (D6–H1, D2–H1, and D1–H2), all diffraction curves can be well indexed to γ-NiOOH (JCPDF: 06-0075). It is believed that the generation of NiOOH results from the combined reactions between Ni2 + , DMF and H2O. We suppose that the reaction takes place as follows: 2Ni2 þ þ 4H2 OþHCONðCH3 Þ2 -2NiOOH þ 4H þ þHOCH2 NðCH3 Þ2 . However, the detailed reaction mechanisms and reaction path awaits further investigations. The composition of solvent not only determines the final phase of the products, but also has significant effect on the dispersibility of nanocrystals and hydrogel strength. Figure 1b displays the digital photographs of NiOOH/GS hydrogels obtained in D6–H1 system. Clearly, well-defined 3D macroscopic monoliths with different sizes have formed, and no agglomeration of NiOOH is observed on the surface of the hydrogel. The hydrogels were mechanically strong enough to be moved by a tweezer. However, for products obtained in D2–H1, D1–H1 and D1–H2 solvents, many small pieces broke away from the hydrogels, indicating inferior robustness; and also, a layer of free NiOOH was visible on the top of the hydrogels, suggesting an inhomogeneous distribution of NiOOH with increasing ratio of water (Figure S1). This is because H2O facilitates a higher hydrolysis and condensation rate of Ni2 + . The nucleation process is hard to control, thus resulting in severe agglomeration [8]. However, in the case of D6–H1, the trace amount of water results in decreasing yet well-controllable hydrolysis and condensation rate, contributing to a homogeneous distribution of NiOOH. Since a higher DMF/H2O ratio can

5 contribute to a better homogeneous of NiOOH and higher strength of the hydrogels, the samples prepared in D6–H1 solvents were characterized. As a control, pure NiOOH without GS was also prepared in a D6–H1 solvent via a similar procedure as NiOOH/GS hydrogels except no GO was added (Figure S2). X-ray photoelectron spectroscopy (XPS) measurements were performed to further investigate the chemical compositions and the valence states of the NiOOH/GS samples. It was revealed that the sample mainly composed of C, O and Ni elements, without any other impurities (Figure 1c). The C1s peak consists mostly of C =C bond and the peak intensities of oxygen-containing functional groups are rather low, indicating the effective elimination of the oxygen-containing groups (Figure 1d). For the Ni 2p spectrum (Figure 1e), the Ni 2p3/2 and Ni 2p1/2 peaks are centered at the binding energies of 856.2 and 873.8 eV (with a spin energy separation of 17.6 eV), accompanied by two prominent shake-up satellite peaks (862.3 and 880.0 eV). These are the typical characteristic peaks of Ni3 + species, distinctly verifying the formation of NiOOH [19,20]. Figure 1f is the TG curves of the hybrid. According to the thermo-decomposition reaction [21] (2NiOOH = 2NiO + 1/2 O2 + H2O), the amount of NiOOH was calculated to be about 68.8% from TG analysis. The microstructure of the 3D macroscopic NiOOH/GS hydrogel was characterized by SEM and TEM. As shown in Figure 2a and b, the freeze-dried NiOOH/GS hydrogel possesses a well-defined and interconnected 3D porous

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Figure 2 (a–c) SEM images of NiOOH/GS hydrogel after vacuum freeze drying. (d) TEM image and (e) HRTEM image of NiOOH/GS hydrogel. (f–i) EDX mapping of C, O and Ni elements. Please cite this article as: R. Wang, et al., High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.030

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network, with continuous pores in the range of one submicrometer to several micrometers. Closer observation clearly shows the building blocks of the framework consist of ultra-thin NiOOH/GS sheets, with a thickness of dozens of nanometers. Such 3D porous structures can facilitate the effective flow of electrolyte ions into the whole electrode, which is favorable to enhance charge storage reactions [13]. Figure 2d is a TEM image of NiOOH/GS sheets. Clearly, NiOOH nanosheets are highly dispersed and firmly attached on the GS. While for Ni/GS and Ni(OH)2/GS, only undefined morphology can be obtained (Figure S3). As demonstrated in previous reports, the 2D nano-plate-shaped material is favorable for shortening ion diffusion paths because of their large aspect ratio and can improve the utilization of active materials at large current densities [22]. The homogeneous growth of NiOOH on the surface of GS was further recognized by the elemental mapping, in which Ni, O, C elements are uniformly distributed in the products (Figure 2f–i). For comparison, only irregular agglomerates were obtained in the absence of GS (Figure S4), demonstrating the critical role of GS as a substrate and providing abundant active sites for the heterogonous nucleation and uniform growth of NiOOH [9]. Figure 2e shows the HRTEM image of NiOOH/GS. As marked by arrows, many cavities are observed in the NiOOH nanosheets. This means in addition to the macroporous nature of the hydrogel, the ultrathin NiOOH nanosheets themselves are mesoporous in nature. As a result, the hybrid hydrogel is fully porous, which is highly desirable for supercapacitor applications. The inset in Figure 2e shows clear lattice fringes with an interplanar spacing of 0.24 nm, corresponding well to the distance between (101) planes of NiOOH. The porous nature of NiOOH/GS architecture was also characterized by BET measurement (Figure 3). The N2 adsorption–desorption isotherms exhibited a typical IV hysteresis loop at a relative pressure between 0.4 and 0.9, characteristic of pores with different pore sizes. Remarkably, the specific surface area of NiOOH/GS can reach 186 m2 g  1, which is markedly contrasted to that of pure NiOOH (34 m2 g  1). BJH calculations disclosed the pore volume was 0.17 cm3 g  1 with an average pore diameter of 3.9 nm. As demonstrated in the previous reports [5,13], the BET measurement of the freeze-dried hydrogels could substantially underestimate its specific surface area due to partial re-stacking of some graphene sheets and the fusing of mesopores during the freeze-drying process. Therefore,

we further adopted the methylene blue (MB) dye adsorption method [5,13] to determine the intrinsic surface area of the wet hydrogels. With this approach, the specific surface area of the NiOOH/GS hydrogel reached E 850 m2 g  1, 4.5 times higher than its freeze-dried counterpart.

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Electrochemical characterizations of NiOOH/GS hydrogels

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As discussed above, the as-prepared NiOOH/GS hydrogel simultaneously possesses ultrahigh specific surface area and hierarchical porous structure, thus is expected to show excellent electrochemical performance. Figure 4a shows the CV curves of NiOOH/GS electrode (denoted as H–NiOOH/ GS) at various scan rates from 0.5 to 50 mV s  1. A pair of symmetric cathode and anode peaks are clearly observed within a potential range of 0–0.7 V, which is related to the reversible reaction of NiðOHÞ2 þOH  2NiOOH þH2 Oþ e  . Capacitive effects were characterized by analyzing the voltammetric response at various scan rates according to [23,24]:

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where i is the current, ν is the scan rate. Both a and b are adjustable parameters, with b-values determined from the slope of the linear plot of log i versus log ν. In general, slope b =1 involves nondiffusion-controlled surface redox reactions (i = aν, capacitive effect), while the slope b= 1/2 indicates the ideal diffusion-controlled redox intercalation process (i= aν1/2, battery-type behavior). The calculated bvalues of H–NiOOH/GS during cathodic scan are shown in Figure 4b. With the exception of peak potentials, b-values are close to 1, indicating the current is predominantly capacitive in nature. At peak potentials between 0.4– 0.45 V, the b-values are in the range of 0.6–0.8, suggesting the current arises from capacitive effects mixed with contributions from diffusion-controlled battery-type behavior. As reported in literatures, for nanostructured layered metal hydroxide (Ni(OH)2, Co(OH)2, etc.), the interlayer spacing is several nm and allows for the reversible intercalation of ions, therefore, they often exhibit both capacitive and battery-type behavior [23]. As for γ-NiOOH in this study, it has a hexagonal crystal structure with D-spacing between (003) of 0.69 nm, thus offers the diffusion paths for OH  (diameter of 0.274 nm) to intercalate. Noteworthy,

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Nitrogen adsorption and desorption isotherms of freeze-dried NiOOH/GS (a) and pure NiOOH (b).

Please cite this article as: R. Wang, et al., High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.030

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Figure 4 (a) CV curves of H–NiOOH/GS; (b) b-values of H–NiOOH/GS plotted as a function of potential for cathodic sweeps. Inset: power law dependence of current on sweep rate shows good linearity; (c) The plots of v1/2 versus i(V)/v1/2 used for calculating k1 and k2 at different potentials of the cathodic scan. (d) Bar chart showing total capacitance together with percentage contribution from capacitive and intercalation at different scan rates; (e) Charge–discharge curves of H–NiOOH/GS; (f) The corresponding specific capacitance of H–NiOOH/GS, T–NiOOH/GS and T–NiOOH versus current density.

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iðV Þ ¼ k1 v þk2 v 1=2

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As shown in Figures 4c and S5, k1 and k2 can be determined from the slope and y-axis intercept point of the straight line: i(V)/v1/2 = k1v1/2 + k2. Thus we are able to quantify, at specific potentials, the fraction of current due to each of the contributions. Using this analysis, we further calculated the contribution fraction of capacitive effects at

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this is the first time that the phenomenon NiOOH exhibits battery-type behavior besides of capacitive behavior was revealed and reported. As discussed above, the current response at a fixed potential can be described as a sum of two mechanisms (surface capacitive effects and diffusion-controlled intercalation processes) according to [23,24]:

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different scan rates (Figures 4d and S5). It is found that an increase in scan rate has little effect on the capacitive contributions, whereas the intercalation contribution decreases with the increase of scan rate. Since the redox intercalation process is diffusion-controlled, the time for ion diffusion into the host lattices will decrease upon increasing the scan rate, hence the contribution will decrease accordingly [24]. Therefore, the redox pseudocapacitive behavior will dominate the charge storage at high rates. To quantify the capacitance of the electrode, a series of galvanostatic charge–discharge measurements were conducted at different current densities. As shown in Figure 4e, the charge/discharge voltage profiles of the electrode displays a well-defined potential plateau at

Please cite this article as: R. Wang, et al., High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.030

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87 Figure 5 (a, b) SEM images of pure GS hydrogels after vacuum freeze drying. (c) CV curves of pure GS hydrogels and (d) the corresponding specific capacitance versus scan rate.

current densities from 1 A g  1 to 20 A g  1, agreeing well with the CV results. And also, the charge/discharge curves are quite symmetric with high Coulombic efficiency (100%), which implies fully reversibility of the electrode. The calculated specific capacitance, including contributions from intercalation processes, as a function of the discharge current density is plotted in Figure 4f. Notably, the electrode delivered a capacitance of 1162, 1160, 1064 and 1051 F g  1 at a current density of 1, 2, 5, 10 A g  1, respectively, which are much higher than that of pure NiOOH. Even at an ultrahigh current density of 20 A g  1, the capacitance can still reach 981 F g  1, maintaining a capacitance retention as high as 84.4%. The high specific capacitance as well as outstanding rate capability can be attributed to the unique microstructure of the electrode: (i) NiOOH nanosheets with 2D structure can shorten the electron diffusion distance and facilitate the surfacedependent Faradic reactions even at high scan/current rates. (ii) The 3D porous structure of the electrode is beneficial for the penetration and diffusion of the electrolyte, leading to fast ion transport. (iii) The interconnecting graphene network can provide 3D electron conducting channels within the electrode. In this way, both a facile ion-diffusion path and a fast electronic transfer superhighway are provided. To highlight the superiority of the as-prepared wet hydrogels as electrodes, the conventional binder-enriched electrodes of NiOOH/GS aerogels were also prepared by slurry coating technology, which were denoted as T–NiOOH/ GS. As revealed in Figure S6 and 4f, T–NiOOH/GS shows a capacitance of 1077 and 1045 F g  1 at 1 and 2 A g  1, respectively, comparable to that of H–NiOOH/GS and much higher than that of pure NiOOH (T–NiOOH). This clearly

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demonstrates the positive role of the graphene conductive network in improving the electrochemical performance. However, upon increasing the current density, the capacitance dramatically decreased to 773 and 665 F g  1 at 10 and 20 A g  1, which is 72% and 62% of that at 1 A g  1. Obviously, H–NiOOH/GS exhibits much better rate capability compared with T–NiOOH/GS, especially at high current densities. This verifies that wet hydrogels indeed possess great advantages as electrode materials over binderenriched electrodes, which can be mainly contributed to the following reasons: on one hand, due to the hydrophobic nature of graphene, aerogels prepared by freeze-drying of hydrogels usually have a worse electrolyte wettability and diffusion than hydrogels [25,26]; Moreover, as demonstrated by the MB adsorption results, the specific surface area of aerogels will dramatically decrease due to the partial overlapping of the graphene sheets during the freezedrying [11]. However, the direct application of wet hydrogels as electrodes can maintain the ultrahigh specific surface area and allow all of the active materials wetted by the electrolyte, thus lead to the faster ions transportation at high current densities. On the other hand, the binder-free fabrication enables a low interfacial resistance, which also promotes the charge transfer rate [27].

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Negative electrode 119 To evaluate the capacitive performance of the H–NiOOH/GS electrode in a full-cell configuration, we attempted to fabricate an ASC device. Because the H–NiOOH/GS electrode exhibits the exceptionally high specific capacitance, it is extremely important to explore a suitable negative electrode

Please cite this article as: R. Wang, et al., High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.030

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with high capacitance, so as to obtain an ASC with a high energy density. It is well known that graphene can serve as an excellent negative electrode in an alkaline electrolyte due to its high conductivity and superior electrochemical stability [28]. In the current study, pure graphene hydrogels were prepared via a solvothermal-induced self-assembly approach and were used for the first time as the negative electrode. As shown in Figure 5a and b, the freeze-dried graphene hydrogel also shows a highly interconnected 3D porous network structure, and the pore walls are composed of ultrathin graphene sheets. Such microstructure is favorable for the electrolyte ions to have full access into the whole electrode. The porous structure of the graphene hydrogel was further confirmed by nitrogen adsorption and desorption measurements. It is revealed that the freeze-dried graphene hydrogel has a high specific surface area of  262 m2 g  1 with a pore volume of 0.17 cm3 g  1 (Figure S7), comparable to the previous reports [4,13,29]. The pore size distribution obtained from the Barrett–Joyner–Halenda (BJH) method ranges from 2 to 40 nm with a peak at  3.9 nm. Again, because the freeze drying used to prepare BET sample will induce partial restacking of some graphene sheets and reduce the apparent specific surface area, we also used MB adsorption method to more accurately determine the solvated surface area of the wet graphene hydrogel to be E1770 m2 g  1. Figure S8 displays the full XPS spectra of freeze-dried graphene hydrogel, which shows the signal of C, O and N elements at 284.9, 400.3 and 533.1 eV, respectively, without any impurities. Noteworthy, 2.5% nitrogen was detected, indicating nitrogen was doped into GS during the solvothermal reaction. The C/O atomic ratio was calculated to be 7.1, suggesting the successful reduction of GO to GS [9,17]. Figure 5c shows the CV curves of GS hydrogel (H-GS) at various scan rates between  0.9 V and 0.1 V, using threeelectrode cells with Pt counter electrode and Hg/HgO reference electrode in a 2 M KOH solution. The CV curves show slight cathodic and anodic peaks, which may be induced by the existing oxygen-containing functional groups and/or nitrogen heteroatoms (Figure S8b and S8c) [30]. Remarkably, the pure GS hydrogel shows an impressive specific capacitance of 425 F g  1 at 5 mV s  1. Upon increasing the scan rate up to 40 mV s  1, the specific capacitance remains at 368 F g  1, 86.6% of that at 5 mV s  1, highlighting the excellent rate capability of H-GS. The specific capacitance derived from CV is much superior to the commonly used activated carbon [1–3] (AC, typicallyo200 F g  1), Fe2O3-based negative electrode (347 F g  1 at 10 mV s  1) [31], and In2O3-based electrodes (201 F g  1 at 100 mV s  1) [32]. Furthermore, the performance of H-GS was tested in a two-electrode system. The specific capacitances derived from galvanostatic discharge curves was calculated based 2IΔt on the following formula [11,29,33]: Csingle ¼ ΔVm , where I, Δt, ΔV and m represent the discharge current, discharge time, voltage drop upon discharging (excluding the IR drop) and the mass of one electrode. As shown in Figure S9, the electrode delivers 193.8, 184.3, 180.6, 167, 162.4 and 160.6 F g  1 at 1, 2, 4, 6, 8 and 10 A g  1, respectively. The performances are superior or comparable to the values from previous results, such as hydrothermally-prepared hydrogels ( 190 F g  1 at 1 A g  1) [11], hydrazine-reduced hydrogels [29] (144 and 191 F g  1 at 1 A g  1 for 5 mg ml  1 and

9 10 mg ml  1, respectively), electrochemically-prepared hydrogels [33] ( 190, 180, 164 F g  1 at 2, 4 and 8 A g  1, respectively) and so on. These outstanding electrochemical properties indicate that H-GS may be ideal to act as a negative electrode in this study.

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Asymmetric supercapacitors

71 Asymmetric supercapacitors were constructed using H–NiOOH/ GS as a positive electrode, H-GS as a negative electrode, and 2 M KOH as an electrolyte. To balance the charge storage, the masses of the positive and negative electrodes were optimized according to the following equation: mþ Cs  ΔV  ¼ m Cs þ ΔV þ

ð9Þ

where m is the mass, Cs is the specific capacitance, and ΔV is the voltage range for positive (+) and negative (–) electrodes, respectively. Accordingly to the CV test at 20 mV s  1 (Figure S10), the specific capacitance of H–NiOOH/GS is 1023 F g  1 while that of H-GS is 386 F g  1. Therefore, the optimized mass ratio between H–NiOOH/GS and H-GS should be m + /m  = (386 F g  1 * 1.0 V)/(1023 F g  1 *0.7 V)=0.54. Since the operating potential windows for H–NiOOH/GS and H-GS are 0.9 V–0.1 V and 0–0.7 V, we assume that the asymmetric supercapacitor of H–NiOOH/GS//H-GS can be cycled reversibly with a cell voltage up to 1.6 V in the 2 M KOH electrolyte. As expected, The CV curves at scan rates from 5 to 50 mV s  1 demonstrate that the asfabricated H–NiOOH/GS//H-GS asymmetric supercapacitor shows an excellent capacitive behavior at 0–1.6 V (Figure 6a). Note that contributions from both electric double-layer capacitance and pseudocapacitance can be clearly observed at all scan rates [2] .The corresponding specific capacitance at different scan rates are displayed in Figure 6b. A high specific capacitance of 161 F g  1 was achieved at 5 mV s  1. When the scan rate increases to 10, 20, 30, 40 and 50 mV s  1, the specific capacitance still maintained at 148, 132, 123, 117 and 111 F g  1, respectively. To further illustrate the electrochemical properties, the galvanostatic charge/discharge plots at different current densities were also presented in Figure 6c. The slight nonlinearity of the discharge curves, especially at lower current densities, indicates the contributions from the redox reaction of NiOOH, which agrees well with the CV results. Additionally, the charge–discharge curves are highly symmetric with the Coulombic efficiency at nearly 100%, suggesting the good reversibility of the ASC devices. As calculated, the H–NiOOH/GS//H-GS ASC achieved a high specific capacitance of 188 F g  1 at 1 A g  1 (Figure 6d), which is higher than the values obtained from recent reports for other NiO and Ni(OH)2-based ASCs, such as Ni(OH)2/graphite foam//activated microwave exfoliated graphite oxide (119 F g  1 at 1 A g  1) [22], H– [email protected](OH)2 nanowire//[email protected] (114 F g  1 at 1.2 A g  1) [34], Ni(OH)2/CNT//AC (112.5 F g  1 at 0.1 A g  1) [3], Ni(OH)2/GS//RuO2/GS [35] (160 F g  1 at 1 A g  1), NiO/graphene foam//porous nitrogen-doped carbon nanotubes (116 F g  1 at 1 A g  1) [36], porous NiO nanoflakes// RGO (50 F g  1 at 0.2 A g  1) [37]. More importantly, it also exhibits good rate capability with 66.5% of capacitance

Please cite this article as: R. Wang, et al., High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.030

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103 Figure 6 (a) CV curves and (b) corresponding specific capacitance of H–NiOOH/GS//H-GS ASCs measured at different scan rates. (c) Charge/discharge curves and (d) corresponding specific capacitance of H–NiOOH/GS//H-GS ASCs at different current densities. (e) Nyquist plots of H–NiOOH/GS//H-GS ASCs, the inset is the enlarged Nyquist plot from the high-frequency region. (f) Ragone plot of H–NiOOH/GS//H-GS ASCs, as well as other similar ASCs reported in literatures: Ref. [1], NiCoAl LDH//AC; Ref. [2], NiMoO4//AC; Ref. [5], MnO2/GS hydrogels//graphene hydrogels; Ref. [14], CoNi3O4/C//AC; Ref. [28], graphene foams/CNTs/MnO2//graphene foams/CNTs/Ppy; Ref. [34], H–[email protected](OH)2 nanowire//[email protected]; Ref. [36], NiO/graphene foam//porous nitrogen-doped carbon nanotubes; Ref. [37], porous NiO nanoflakes//RGO; Ref. [38], Ni(OH)2/GS//porous GS; Ref. [39], [email protected]@Ni(OH)2// [email protected]; Ref. [40], Ni–Co–S//graphene.

retention even at a high current density of 10 A g  1. Figure 6e displays the Nyquist plots of the fabricated H– NiOOH/GS//H-GS ASCs, which shows a typical diffusioncontrolled Warburg capacitive behavior with a diagonal line in the low frequency region and small depressed semicircle in the high frequency region [28]. The equivalent series resistance (ESR) and charge transfer resistance (Rct) are estimated to be  2.37 and 0.24 Ω, indicating good conductivity and low internal resistance of the ASCs. The excellent electrochemical performances of both H– NiOOH/GS and H-GS electrodes, the good matching between the positive and negative electrodes together

with the small resistance of the ASCs may account for the outstanding performance of the assembled ASCs. Energy and power density are two important parameters for ASC devices. The Ragone plots of the ASC derived from the discharge curves are presented in Figure 6f. An energy density as high as 66.8 W h kg  1 was achieved at a power density of 800 W kg  1, and it still maintains 44.4 W h kg  1 even at a high power density of 8000 W kg  1, outperforming many previously reported NiO or Ni(OH)2 based systems [3,22,34,36–39], as given in the same plot for comparison. The excellent performance is also comparable or superior to other asymmetric cells, such as NiMoO4//AC (60.9 W h kg  1

Please cite this article as: R. Wang, et al., High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.030

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at 850 W kg  1) [2], NiCoAl LDH//AC (58.9 W h kg  1 at 400 W kg  1) [1], graphene foams/CNTs/MnO2//graphene foams/CNTs/Ppy (22.8 and 6.2 W h kg  1 at 860 and 2700 W kg  1, respectively) [28], MnO2/GS hydrogels//graphene hydrogels [5] (21.2 and 18.4 W h kg  1 at 800 and 6610 W kg  1, respectively), Ni–Co–S//grapheme [40] (60 W h kg  1 at 1800 W kg  1), and CoNi3O4/C//AC (29.1 W h kg  1 at 130.4 W kg  1) [14]. The cycle life of the ASC was tested through a charge– discharge process at 4 A g  1. As shown in Figure 7a, the specific capacitance of the first cycle was 136 F g  1, then slightly decreased during the first 2000 cycles, and relatively stable in the following cycles. After 8000 cycles, specific capacitance remains at 116 F g  1 with 85.3% of the initial capacitance, revealing its good long-term cyclic stability. This was further confirmed by the corresponding charge–discharges curves as well as CV between the first and 8000th cycles (Figure 7b and c). The symmetric charge– discharge curves after 8000 cycles demonstrate the ASC maintained high reversibility during cycling (Figure 7d). Such a cycling performance is highly competitive with those of some other asymmetric supercapacitors, such as Ni(OH)2/ GS//RuO2/GS (ca. 92% retention after 5000 cycles) [35], MnO2/GS hydrogels//pure GS hydrogels (89.6% after 1000 cycles) [5], Ni(OH)2/CNT//AC (83% after 3000 cycles) [3], and NiMoO4//AC (85.7% after 10,000 cycles) [2].

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Different from the commonly used chemical or hydrothermal reaction, we have explored a novel mixed solvothermal/

hydrothermal reaction (DMF and H2O) and successfully prepared a new electrode material: NiOOH/GS hydrogels, in which mesoporous NiOOH nanosheets were uniformly dispersed within a 3D porous graphene framework. It was found that the solvent composition played a key role in determining the final phase of the products: only Ni/GS or Ni(OH)2/GS hydrogels can be produced by single solvothermal or hydrothermal reactions, respectively. Additionally, a higher DMF/ H2O ratio can contribute to a better dispersibility of nanocrystals and higher strength of the hydrogels. The application of NiOOH/GS hydrogels as a new electrode material for ECs was systematically studied. Benefitting from the ultrahigh specific surface area (850 m2 g  1), Q4 hierarchical porous structure and conductive network, NiOOH/GS hydrogels achieved high pseudo-capacitances (1162 F g  1 at 1 A g  1) and excellent rate capability (980 F g  1 at 20 A g  1) as binder-free electrodes. Investigation of charge-storage mechanisms reveal that both capacitive effects and diffusion-controlled intercalation process contribute to the stored charge. Furthermore, an optimized asymmetric supercapacitor was developed based on NiOOH/ GS hydrogels and solvothermal-induced pure graphene hydrogels. The H–NiOOH/GS//H-GS ASCs showed a high energy density of 66.8 W h kg  1 and 44.4 W h kg  1 at a power density of 800 and 8000 W kg  1, respectively, which is among the highest reported for ASCs in an aqueous electrolyte. Our ASCs also exhibit remarkable cycling stability with retention of 85.3% specific capacitance after 8000 cycles. Thus, pairing up NiOOH/GS hydrogels and pure graphene hydrogels for ASCs represents a new approach to high performance energy storages.

Please cite this article as: R. Wang, et al., High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.030

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This work is supported by Academic Research Fund (RGT27/ 13) of Ministry of Education.

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Appendix A.

Supplementary material

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.10.030.

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Please cite this article as: R. Wang, et al., High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.030