Nanofoaming to Boost the Electrochemical Performance ofNi@Ni(OH)2 Nanowires for Ultrahigh Volumetric SupercapacitorsShusheng Xu, Xiaolin Li, Zhi Yang,* Tao Wang, Wenkai Jiang, Chao Yang, Shuai Wang, Nantao Hu,*Hao Wei, and Yafei Zhang

Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of Micro/Nano Electronics, School ofElectronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China

*S Supporting Information

ABSTRACT: Three-dimensional free-standing film electrodes havearoused great interest for energy storage devices. However, smallvolumetric capacity and low operating voltage limit their practicalapplication for large energy storage applications. Herein, a facile andnovel nanofoaming process was demonstrated to boost thevolumetric electrochemical capacitance of the devices via activationof Ni nanowires to form ultrathin nanosheets and porousnanostructures. The as-designed free-standing Ni@Ni(OH)2 filmelectrodes display a significantly enhanced volumetric capacity (462C/cm3 at 0.5 A/cm3) and excellent cycle stability. Moreover, the as-developed hybrid supercapacitor employed Ni@Ni(OH)2 film aspositive electrode and graphene-carbon nanotube film as negativeelectrode exhibits a high volumetric capacitance of 95 F/cm3 (at 0.25 A/cm3) and excellent cycle performance (only 14%capacitance reduction for 4500 cycles). Furthermore, the volumetric energy density can reach 33.9 mWh/cm3, which is muchhigher than that of most thin film lithium batteries (1−10 mWh/cm3). This work gives an insight for designing high-volumethree-dimensional electrodes and paves a new way to construct binder-free film electrode for high-performance hybridsupercapacitor applications.

KEYWORDS: nanofoaming, nickel nanowire, supercapacitor, volumetric capacity, energy storage

1. INTRODUCTION

A lot of electrochemical energy storage devices have beenprepared to moderate the energy and environment challenges.Obviously, the two most important for electrochemical energystorage devices are the battery and the supercapacitor. With thegrowing demand of energy, a single battery or supercapacitorcannot satisfy the needs of high electricity consumption, so weurgently need an energy storage device have both high energydensity and power density.1−4 Most recently, more and moreattention is paid to combining the merits of these two devices,namely, hybrid supercapacitors (HSC),5 which representativelyare composed of a battery as energy source electrode and asupercapacitor as power source electrode, such as TiO2@Ni(OH)2//mesoporous carbon,6 LiMn2O4//active carbon,7

and Li4Ti5O12//CNTs.8 Compared with traditional electric

double-layer capacitors, HSC can ensure a wider workingpotential window and larger capacity, resulting in a higherenergy density. Compared with a battery, HSC can deliver animproved power density for fast charge−discharge and long-term cycling stability. In consequence, HSC has opened up anew field for electrochemical energy storage and reduced thegap between battery and supercapacitor.Obviously, the electrochemical energy storage performance

of HSC is directly affected by electrode materials, such asmaterial structure, type, and property. To enhance the

electrochemical performance of HSC, many efforts have beenmade to research electrode materials, especially transition metaloxides and metal hydroxides, such as NiO,9,10 Ni(OH)2,

11,12

and Co3O4,13−15 due to their larger capacities and higher

energy densities than those of traditional carbonaceousmaterials.16−20 Among them, Ni(OH)2 has aroused greatinterest owing to its high theoretical specific capacity, excellentchemical stability, and easy preparation. However, its poorelectric conductivity often results in low rate capability andshort cycle life. The reason is that most Faradic materials arediffusion-controlled is that poor conductivity often leads to lowCoulombic efficiency and sluggish reaction kinetics.21 Up tonow, one effective method to solve these problems is directsynthesis of high-performance electrode materials on desiredsubstrates with high electrical conductivity. For example, Liu etal. have proposed a facile method to synthesize free-standingand additive-free three-dimensional (3D) Ni@NiO filmelectrode with excellent rate capability.22 Peng et al. synthesizedNiMoO4 nanosheet and nanorod arrays and studied electro-chemical properties for hybrid asymmetric supercapacitorapplication.23 Go et al. directly synthesized ultrathin NiCo2O4

Received: August 25, 2016Accepted: September 28, 2016Published: September 28, 2016

Research Article

www.acsami.org

© 2016 American Chemical Society 27868 DOI: 10.1021/acsami.6b10700ACS Appl. Mater. Interfaces 2016, 8, 27868−27876

nanosheets on conductive Ni nanofoam (NF) for energystorage electrodes with excellent electrochemical perform-ance.24 Compared with traditional methods adding binderand conductive additives to fabricate electrodes, this additive-free fabricate method can avoid using binder and conductiveadditives. Thus, all materials on the surface of the film can fullaccess with the electrolyte solution and take part in redoxreaction process, which significantly improves the overallelectrochemical properties and exhibits promising possibilityto improve the volumetric capacity of HSC.3D nanostructures constructed from low-dimensional seg-

ments have attracted increasing interest for electrochemicalenergy storage. However, these 3D nanostructures are oftensubjected to low volumetric capacitance. Several methods, suchas vacuum-assisted self-assembly,16 mechanical compression,17

and liquid-electrolyte-mediated dense integration,20 have beenreported to increase bulk density for the purpose of improvingthe volumetric capacity, often at the expense of gravimetriccapacity and rate capability. Thanks to enhanced physicochem-ical activity, large specific surface area, and excellent electricalconductivity, metal nanofoams (especially transition metals)have attracted considerable attentions as an important class offunctional materials. In this work, we demonstrate an efficientNi@Ni(OH)2 film electrode by a simple nanofoaming processusing hydrothermal method. Unlike other reported methods,our nanofoaming process can in situ synthesize a layer ofultrathin Ni(OH)2 nanosheets with porous nanostructures onthe surface of conductive Ni nanowires (NWs) substrate. Thisunique structure of electrode material has several advantagesincluding a large energy storage capacity of Ni(OH)2 andexcellent conductivity of Ni, ease of preparation, and low costof Ni NWs. Furthermore, our film electrode with 3Dconductive network is similar to the commercial nickel foamexcept that its pore is macroporous (pore size of 450−3200μm), so we call this Ni@Ni(OH)2 film a nanofoam (NF). Afterthe nanofoaming process, the Ni@Ni(OH)2 NF electrodesdisplay a significantly enhanced volumetric capacity of 462 C/cm3 at 0.5 A/cm3, long-term cycle stability, and excellent ratecapability. Graphene-carbon nanotube (G-CNT) film is chosenfor negative electrode for HSC owing to its large specificsurface area, excellent conductivity of 3D CNT network, andlarge bulk density.25 Moreover, unlike the traditional slurry-processed method, both positive and negative electrodes arebinder-free, which not only ensures fast ion diffusion but alsomakes sufficient use of active materials. As a consequence, theas-fabricated HSC device possesses large volumetric capaci-

tance of 95 F/cm3 at 0.25 A/cm3. Furthermore, the devicedisplays an ultra-high-volume energy density of 33.9 mWh/cm3,which is even higher than that of lithium thin film batteries.Our work may pave a new route to fabricate high-performancenanofoam film electrodes for other metal nanomaterials withsimilar structure.

2. EXPERIMENTAL SECTION2.1. Synthesis of Ni@Ni(OH)2 NF Positive Electrode.

Analytical-grade (AR) chemicals were used as received in allexperiments. Ultralong Ni NWs were synthesized using a facile one-step process under a magnetic field as reported in our work.26 The Nifilm was acquired by dispersing Ni NWs in a 1 wt %polyvinylpyrrolidone (PVP) ethanol solution, vacuum-filtered, andthen pressed at 10 MPa for 5 min. The obtained Ni film was dried in60 °C vacuum oven for 12 h. Ni@Ni(OH)2 NF film was preparedusing a moderate hydrothermal reaction. Typically, 32 mg of Ni filmand 30 mL of a 15% H2O2 solution were put into a 50 mL Teflon-lined autoclave reactor and heated to 180 °C for 12 h. After thehydrothermal process, the obtained materials was cleaned usingdeionized water and dried at 60 °C for 6 h. For the followingelectrochemical characterizations, the film was cut to size (0.5 × 0.5cm2). To get optimal experiment conditions, the weight of Ni film (32and 64 mg), concentration of H2O2 solution (0, 15, and 30%),temperature (160, 180, and 200 °C), and reaction time (6, 12, and 24h) were adjusted. NF-2 mg-15% (as shown in Table S1) was chosen asthe experiment condition due to its optimum electrochemicalproperties. All characterizations in article refer to this sample (labeledas Ni@Ni(OH)2 NF) except for special statements.

2.2. Characterizations. The microstructures and morphologies ofthe as-prepared materials were obtained by a scanning electronmicroscope (SEM, Ultra-55, Carl Zeiss, Germany) and transmissionelectron microscopy (TEM, JEM-2010, JEOL, Japan). The phase ofthe products was obtained using a D8 advanced X-ray diffractometer(XRD) with Cu Kα source in a 2θ range of 10−90°. X-rayphotoelectron spectroscopy (XPS) was carried out on a Kratos AxisUltraDLD with an Al Kα source (1486.6 eV). The chemical elementsof the material was analyzed by using energy dispersive X-rayspectroscopy (EDS, Oxford Instruments INCA PentaFET × 3, Model:7426). The specific surface area of the sample was obtained from theBrunauer−Emmett−Teller (BET) plot of the N2 desorption isothermmeasured at 77 K with an ASAP 2020 (Micromeritics, USA). The poresize distribution curves were determined from desorption branchisotherms by a nonlocal density functional method.

2.3. Electrochemical Measurements. Electrochemical tests ofindividual electrodes (Ni@Ni(OH)2 and G-CNT), including cyclicvoltammetry (CV), galvanostatic charge/discharge measurements(GCD), and the electrochemical impedance spectroscopy (EIS),were carried out at room temperature using a CHI760E electro-chemical working station (Chenhua, Shanghai, China). Film electrodes

Scheme 1. Schematic Illustration of Nanofoaming Process for Ni@Ni(OH)2 Nanofoams Fabricationa

a(a) Formation of β-Ni(OH)2 after 6 h hydrothermal reaction. (b) Transformation of β-Ni(OH)2 to amorphous Ni(OH)2 with mesopores after 12h hydrothermal reaction.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b10700ACS Appl. Mater. Interfaces 2016, 8, 27868−27876

27869

were directly used as the working electrode. A Pt foil electrode and aHg/HgO electrode acted as counter electrode and reference electrode,respectively. A hybrid asymmetric supercapacitor was fabricated byusing Ni@Ni(OH)2 NF positive electrode and G-CNT negativeelectrode, separate by porous nonwoven fabric into two face-to-faceelectrodes. For both three- and two-electrode cells, 6 M KOH wasused as the electrolyte. The EIS measurements were performed atopen-circuit potential over a frequency ranging of 100 kHz to 0.01 Hz.

3. RESULTS AND DISCUSSIONNanofoaming techniques were exploited to fabricate Ni@Ni(OH)2 NF film with ultrathin nanosheets and porousnanostructure by a facile one-step hydrothermal process, asshown in Scheme 1. The formation of ultrathin porousnanosheets on the surface of Ni NWs may be due to stronghydroxidizability of nickel by H2O2 in the autoclave under highpressure and temperature conditions. The Ni NWs act as Nisources for growing of Ni(OH)2 ultrathin nanosheets, as well asthe skeleton and current collector to support active materials.The reaction equation can be summarized as

+ →Ni H O Ni(OH)2 2 2 (1)

We proposed a formation mechanism of ultrathin and porousNi(OH)2 nanosheets into two processes as follows: (i)Transformation Ni to β-Ni(OH)2 on the surface of NWs.This can be demonstrated by HRTEM and XRD character-izations in Figure S1. (ii) β-Ni(OH)2 transformation toamorphous Ni(OH)2 with mesopores, which can be provedby following TEM, XRD and XPS characterizations. Differentfrom the traditional method, our nanofoaming techniques cangenerate a layer of porous ultrathin nanosheets on the surfaceof conductive Ni NWs. This unique porous core−sheathstructure can ensure excellent contact between conductive Nicore and active Ni(OH)2 sheath and provide a quick electronictransfer channel, leading to the boosted energy storage abilityfor Ni@Ni(OH)2 NF film electrode.The morphologies and nanostructures of as-synthesized raw

materials Ni NWs and Ni@Ni(OH)2 NF were obtained bySEM and TEM, as exhibited in Figure 1. It can be easilyobserved from Figure 1a that ultralong Ni NWs have beenobtained by a simple liquid-phase method under a magneticfield. Moreover, the higher magnification SEM inset imageexhibits that Ni NWs have an acicular structure with an uneven

surface, which can efficiently improve the specific surface areaof Ni NWs. Figure 1b displays the TEM image of a Ni NW;many thorns can be observed and evenly distributed on thesurface of Ni NW. To study the crystal structure of Ni NWs,HRTEM image obtained from the edge of a thorn wasdisplayed in Figure 1c. Clear and ordered lattice fringes can bedetected with a crystal spacing of about 0.21 nm, in goodagreement with the {111} spacing planes of the Ni face-centered cubic ( fcc) structure.27 Compared with Ni NWs inFigure 1a, for Ni@Ni(OH)2 NF, the embossments on thesurface of Ni NWs disappeared, and the diameter of the NWsbecome much thinner after nanofoaming treatments, asexhibited in Figure 1d. Furthermore, the surface of thenanowire become uneven, which facilitates full contact betweenelectrode material and electrolyte. Figure 1e exhibits the TEMimage of Ni@Ni(OH)2 NF. It is clear that embossments on thesurface disappeared after nanofoaming treatments, indicatingthat Ni on the surface of NWs has reacted with H2O2. From theHRTEM image in Figure 1f, numerous nanosheets andconsiderable mesopores (marked with red circles) can beobserved, which may be ascribed to the release of gas duringhydrothermal reaction. The nanosheets show a transparentfeature, further demonstrating its ultrathin nature. Moreover,these nanosheets are aligned and uniformly wrapped on the NiNWs. This resultant porous nanostructure is believed tofacilitate the ion diffusion and charge transport without anyadditives and binders. The porous nanostructures can befurther demonstrated by pore size distribution curves shown inFigure S2. For Ni NWs without any treatments, the pore size ismainly distributed around 4 nm. After a facile one-stepnanofoaming treatment, the distribution of Ni@Ni(OH)2 NFpore sizes becomes much wider and can be increased to morethan 10 nm. For battery-type Ni(OH)2, a substantial fraction ofcharge storage arise from redox reaction. Hence, a larger poresize is more conducive to electrolyte diffusion and iontransport, which can further enhance the energy storage abilityfor Ni@Ni(OH)2 NF electrode. It is worth mentioning thatBET surface area decreased after nanofoaming treatment; this isdue to the smaller surface area change compared withmacroscopic foaming process. The HRTEM image of Ni@Ni(OH)2 NF in Figure 1f displays a portion of amorphousphase without any lattice structure on the surface of NWscorresponding to amorphous Ni(OH)2. A crystal plane spacingof 0.21 nm corresponding to {111} spacing of Ni fcc structure isalso obtained. Expressively, when applied to energy storageelectrode, ultrathin porous Ni(OH)2 nanosheets directly grownon the surface of Ni NWs was expected to obtain large capacityand excellent rate performance due to its mesoporousnanostructure and short pathways for ion diffusion and electrontransfer.The detailed experimental conditions of Ni@Ni(OH)2

samples to determine the optimum ratio of Ni NWs withH2O2 are shown in Table S1. SEM images of samples from NF-32 mg-0% to NF-32 mg-10% (32 mg Ni nanowires hydro-thermal reacted with 30 mL of 10 wt % H2O2) are displayed inFigure S3. Interestingly, when the relative content of H2O2 islow, as with NF-32 mg-0% and NF-64 mg-5%, some β-Ni(OH)2 hexagonal platelets can be found around the NWs,which can be demonstrated by XRD and XPS characterizationsshown in Figure S4. With increasing H2O2 content, no β-Ni(OH)2 hexagonal platelets can be found from NF-64 mg-10% to NF-32 mg-10% (Figure S3e−l). Figure S5 shows theEDS of NF-32 mg-0% to NF-32 mg-15%. Only Ni and O

Figure 1. (a) SEM images of Ni NWs. Inset is the magnified SEMimage. (b) TEM image and (c) HRTEM image of Ni NWs. Inset is themagnified HRTEM image. (d) SEM images of Ni@Ni(OH)2 NF.Inset is the magnified SEM image. (e) TEM and (f) HRTEM imagesof Ni@Ni(OH)2 NF. Inset is the magnified HRTEM image.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b10700ACS Appl. Mater. Interfaces 2016, 8, 27868−27876

27870

elements can be found in each sample. Moreover, except forNF-32 mg-0% and NF-64 mg-5%, the contents of O increasewith the increase of H2O2 content, indicating that more Ni hasbeen transformed into amorphous Ni(OH)2 phase rather thanβ-Ni(OH)2, so we can deduce that when the content of H2O2 islow Ni would prefer to be oxidized into β-Ni(OH)2 while Niwould prefer to transform into amorphous Ni(OH)2 withincreasing H2O2 content as summarized in Figure S5h. For NF-32 mg-0% and NF-64 mg-5%, the formation of hexagonal β-Ni(OH)2 destroys the Ni NWs structures and 3D conductivenetwork results in poor electrochemical performance, while forNF-64 mg-10% to NF-32 mg-15%, amorphous Ni(OH)2nanosheets in situ grow on the surface of Ni NWs, thusretaining porous nanostructures and 3D conductive networkand leading to enhanced electrochemical performance. Addi-tionally, amorphous phase Ni(OH)2 can possess enhancedelectrochemical efficiency compared with hexagonal β-Ni-(OH)2 due to disorder. Thus, the results mentioned abovesuggested that as-synthesized Ni@Ni(OH)2 NF can provide anew insight for high-capacity electrochemical energy storageelectrode.The crystallographic structure and chemical composition of

Ni@Ni(OH)2 NF were characterized by XRD and XPS; theresults are exhibited in Figure 2. The diffraction angles (2θ) in

Figure 2a located at 44.62, 51.94, and 76.58° can be assigned to(111), (200), and (220) reflections of Ni, respectively.28,29 Alldiffraction angles of Ni NWs and Ni@Ni(OH)2 NF areconsistent with fcc Ni phase (Powder Diffraction File No. 04−0850, Joint Committee on Powder Diffraction Standards,[2004]). No impurities or any Ni(OH)2 characteristic peakscan be found from XRD due to the fact that Ni(OH)2 isamorphous phase rather than crystalline. This can be furtherdemonstrated by XPS spectra as exhibited in Figure 2b−d.From the survey spectrum in Figure 2b, C, O, and Ni can bedetected from Ni@Ni(OH)2 NF, among which C should comefrom residues of ethylene glycol (EG). Two major peaksaround 855.9 and 873.5 eV can be found from the high-resolution Ni 2p spectra, along with two satellite peaks at 861.4and 879.6 eV, which is consistent with amorphous Ni(OH)2from previous reports.30 The peak at 531.5 eV in the O 1s high-

resolution spectra (Figure 2d) is assigned to Ni−OH. The peakaround 533.1 eV corresponds to C−OH from residues of EG.31

Hence, the formation of amorphous Ni(OH)2 on the surface ofNi NWs after nanofoaming process has been further confirmed.To get the optimal experiment conditions, the samples with

different proportions of Ni NWs to H2O2, different reactiontemperatures, and reaction times were tested by electro-chemical measurements; the results are shown from FiguresS6−S8. After comprehensive comparison, the optimumconditions were chosen as 32 mg of Ni NWs and 30 mL and15% H2O2 heating at 180 °C for 12 h (NF-32 mg-15% in TableS1). Then, we studied the influence of thickness onelectrochemical property of the electrode; the results areshown in Figure S9. It is clear that the film with thickness of 45μm possesses the largest capacity. When the film is too thin, theactive material is not enough for redox reaction, while alongwith the increasing thickness of film electrode, the electrolytecannot sufficiently diffuse into film to contact with activematerial for redox reaction. Therefore, the electrochemicalproperty of the electrode would be attenuated when the film istoo thick or too thin.To demonstrate the potential application of Ni@Ni(OH)2

NF film electrode for energy storage applications, electro-chemical performances electrode were measured using CV andGCD techniques in a three-electrode cell. Figure 3a presentsthe CV curves of Ni@Ni(OH)2 NF at scan rate ranging of 2−100 mV/s at the potential of 0−0.6 V vs Hg/HgO. A couple ofstrong reversible redox peaks can be obtained in each curve,unlike electric double-layer capacitor materials with nearlyrectangular CV curve. This is because Ni(OH)2 belongs to abattery-type electrode, whose capacity is mainly originated fromFaradaic redox reaction as below29,32

+ ↔ + +− −Ni(OH) OH NiO(OH) H O e2 2 (2)

The oxidation peak and reduction peak are nearly symmetricin each curve, demonstrating the excellent reversibility of as-prepared Ni@Ni(OH)2 NF. With the scan rate increasing, theoxidation peak moves positively, and the reduction peak movesnegatively, mainly owing to the existence of internal resistancefor electrode material. However, the shape of CV curves showsno significant change even at a high scan rate of 100 mV/s,suggesting the small equivalent series resistance of the as-prepared electrode based on the close contact between activeNi(OH)2 nanosheets and Ni substrates.The GCD tests were studied to further assess the

electrochemical property of Ni@Ni(OH)2 NF film electrode,as shown in Figure 3b. The potential of two voltage plateaus onGCD curves match well with peak positions of CV curves(Figure 3a), corresponding to the reversible redox reaction.Moreover, GCD curves are nearly symmetric at current densityranging of 0.5−5 A/cm3, implying the high Columbic efficiencyand low polarization of Ni@Ni(OH)2 NF electrode.The volumetric capacity at different current densities

calculated from GCD curves is exhibited in Figure S10a. TheNi@Ni(OH)2 NF film electrode shows a volumetric capacity ashigh as 462 C/cm3 at 0.5 A/cm3, suggesting that electrolyticions can efficiency diffuse and migrate into the Ni(OH)2nanosheets when the current density is small. It is clear thatthe volumetric capacity decreases with increasing currentdensity. This is because at high current density the migrationof the electrolytic ions is limited, which results in a portion ofNi(OH)2 nanosheets on the surface becoming inaccessible forelectrochemical charge storage. Remarkably, the volumetric

Figure 2. (a) XRD patterns of Ni NWs and Ni@Ni(OH)2 NF. XPSspectra of Ni@Ni(OH)2 NF. (b) Survey spectrum and spectra for (c)Ni 2p and (d) O 1s.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b10700ACS Appl. Mater. Interfaces 2016, 8, 27868−27876

27871

capacity decreases to 208 C/cm3 while the current densityincreases to 5 A/cm3, indicating that a capacity retention of45% was obtained compared with the volumetric capacity at 0.5A/cm3. These results indicate the excellent capacity and ratecapability behavior of Ni@Ni(OH)2 NF electrode. Cycleperformance is another crucial criterion to evaluate electro-chemical energy storage ability of electrode material. FigureS10b reveals the cycle stability obtained at a large currentdensity of 4 A/cm3 for 4500 cycles. The volumetric capacityincreases up to 122% of the original capacity in the first 500cycles. This can be attributed to the activation process of activematerial Ni(OH)2 inside the 3D structure film electrode. After4500 cycles, the capacity could remain at 83% of the initialcapacity, suggesting the remarkable long-term cycle perform-ance of the as-prepared Ni@Ni(OH)2 NF film electrode.Furthermore, the Coulombic efficiency calculated from GCDcurves increases from 86 to 98% after cycling. Hence, theabove-mentioned results reveal the ultra-high-volume capacity,remarkable rate capability, and excellent cycle performance ofNi@Ni(OH)2 NF electrode material. We also compared theelectrochemical properties of Ni@Ni(OH)2 NF with those ofother previously reported materials, as listed in Table S2. It isobvious that our electrode possesses a competitive energystorage ability among listed electrode materials.To explore the remarkable electrochemical property of as-

prepared Ni@Ni(OH)2 NF, nickel foam treated by hydro-thermal reaction (marked as Ni−F) and Ni@Ni(OH)2 NWfilm treated by electrochemical reaction were chosen forcomparison. Figure 3c displays the CV curves of differentsamples at scan rate of 10 mV/s. All curves have two strongsymmetric peaks, suggesting that the capacity of these threesamples are mainly derived from active material Ni(OH)2owing to Faradaic redox reaction. Furthermore, Ni@Ni(OH)2NF possesses the maximum CV area, indicating a largercapacity than those of Ni−F and Ni@Ni(OH)2 NW. The sameresults can be found from GCD curves at 1 A/cm3 exhibited inFigure 3d. Ni@Ni(OH)2 NF electrode shows the longestdischarge time, indicating a higher electrochemical capacity

than those of other samples. Volumetric capacity of Ni−F, Ni@Ni(OH)2 NW, and Ni@Ni(OH)2 NF with different currentdensities is shown in Figure 3e, which are calculated to be 8, 88,and 462 C/cm3 at 0.5 A/cm3 for Ni−F, Ni@Ni(OH)2 NW, andNi@Ni(OH)2 NF, respectively. It is clear that the volumetriccapacity of Ni@Ni(OH)2 NF is much higher than those of Ni−F and Ni@Ni(OH)2 NW, indicating that the nanofoamingprocess can boost the electrochemical capacity of Ni@Ni(OH)2NF film electrode. When current density increases to 5A/cm3,Ni@Ni(OH)2 NF possesses a volumetric capacity of 208 C/cm3, which is also much higher than those of Ni−F (1.5 C/cm3) and Ni@Ni(OH)2 NW (76 C/cm3). Detailed electro-chemical tests of Ni−F and Ni@Ni(OH)2 NW are displayed inFigure S11. Figure 3f exhibits the EIS curves of three sampleswhich reveal that the three samples have similar and very lowsolution resistance (Rs), indicating the excellent conductivity ofthe samples. The remarkable electrochemical property of Ni@Ni(OH)2 NF electrode can be due to the following reasons:First, unlike Ni−F, nickel foam with a micrometer scale (FigureS12a,b), Ni@Ni(OH)2 NF electrode material exists innanoscale with ultrathin and porous nanostructures, greatlyimproving the surface area of electrode, promoting theelectrolyte access, and exposing more active sites to electrolyte.Second, compared with Ni@Ni(OH)2 NW (Figure S12c,d),conductive Ni NWs are wrapped with microporous andamorphous Ni(OH)2 nanosheets on the surface, furtherimproving surface area and ensuring efficient charge transportand ion diffusion. Third, the unique mesoporous core−sheathstructure ensures full contact between Ni substrate and activeNi(OH)2 nanosheets, which ensures excellent electrical contactand short pathways for ion diffusion and electron transfer.To evaluate Ni@Ni(OH)2 NF film for supercapacitor

application, G-CNT was chosen as negative material for itslarge volumetric capacitance and large bulk density. Cross-sectional SEM image of G-CNT in Figure 4a displays a denselayered and wrinkled structure of as-prepared film. Further-more, CNTs are uniformly incorporated with graphene to avoidthe restacking of the sheets and also efficiently improve the

Figure 3. (a) CV curves of Ni@Ni(OH)2 NF at scan rate from 2 to 100 mV/s. (b) GCD curves of the sample ranging from 0.5 to 5 A/cm3. (c) CVcurves at scan rate of 10 mV/s, (d) GCD curves at a current density of 1 A/cm3. (e) Volumetric capacity as a function of current density, (f) Nyquistplots of Ni−F, Ni@Ni(OH)2 NW, and Ni@Ni(OH)2 NF.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b10700ACS Appl. Mater. Interfaces 2016, 8, 27868−27876

27872

conductivity and mechanical strength of the film. Theelectrochemical property of the G-CNT film electrode wastested at room temperature in 6 M KOH solution. CV curves(Figure 4b) exhibit a near rectangular shapes without significantdeformation with the scan rate increase, indicating excellentcapacitance behavior. The GCD curves of G-CNT are shown inFigure 4c. It is clear that each curve is nearly linear andsymmetrical as current density increase from 0.5 to 5 A/cm3,demonstrating the typical characteristic of capacitor behavior.The volumetric capacitance at different current densities isdisplayed in Figure 4d. The G-CNT shows a volumetriccapacitance of 164, 146, 125, 95, and 84 F/cm3 at 0.5, 1, 2, 4,and 5 A/cm3, respectively. EIS is also performed, and thehomologous Nyquist plots are exhibited in Figure S13a. The Rsvalue is about 1.8 Ω which reveals the excellent conductivity ofG-CNT. The total equivalent series resistance (ESR) isestimated as 1.1 Ω, indicating the excellent charge transferbetween electrode and electrolyte. Additionally, the G-CNTelectrode possesses outstanding long-term cycle stability withonly 6% capacitance decrease after 2250 cycles (Figure S13b).All these results demonstrate the excellent electrochemicalproperties of as-prepared G-CNT electrode for HSCapplications.To estimate the energy storage ability of as-prepared film

electrode in practical application, we have fabricated a hybridasymmetric supercapacitor with Ni@Ni(OH)2 NF film and G-CNT film as positive electrode and negative electrode,respectively. The active electrode materials were placed on Nifoams which served as current collector, and then sandwichedwith a separator using 6 M KOH as electrolyte, as illustrated inFigure 5a. The charge balance between two electrodes abidesby the relation: q+ = q−. Electrode charge storage is often basedon capacitance (C), the voltage window (ΔE), and the volumeof the electrode material (V) according to the equationbelow:33

= × Δ ×q C E V (3)

We maintain the same surface area of two electrodes and adjustthe thickness ratio between two electrodes as V+(Ni@Ni(OH)2)/V−(G‑CNT) = 0.53, depending on the charge balance between thetwo electrodes.To assess the electrochemical performance and obtain the

suitable potential windows of our HSC, we performed CV testsof the device under various voltage window at 20 mV/s beforecharacterizing the electrochemical performance of the full-cell,as shown in Figure S14a. It shows a capacitive properties withnearly rectangular-like CV curves even at a voltage window upto 1.6 V. With the increase of the operation voltage window,the existence of redox peaks indicates that more Faradaicreactions occurred on Ni@Ni(OH)2 NF electrode. When theoperation potential of the HSC is increased from 1.1 to 1.6 V,the volumetric capacitance obtained from CV curves can beincreased from 27.2 to 53.6 F/cm3 (Figure S14a inset), with anenhancement of 197%. Importantly, the energy density of theHSC is increased to 417% calculate by the equation E = 0.5CV2. However, when the voltage increases to much higher,there is a serious limitation related with H2 evolution at thenegative electrode. Thus, the operation window was set as 1.6 Vfor further evaluate the capacitive behavior of as-fabricated HSCin subsequent study.Figure 5b exhibits CV curves of the assembled HSC from 2

to 100 mV/s tested at the 1.6 V voltage window in 6 M KOH.It is clear that all CV curves have nearly rectangular-like shapes,demonstrating an excellent capacitive property and fast charge/discharge behaviors of the as-fabricated HSC. When the scanrate increases, the form of the CV curves still maintains

Figure 4. (a) Cross-sectional SEM image of G-CNT film. Inset is themagnified SEM image. (b) CV curves of G-CNT at different scan ratesin 6 M KOH. (c) GCD curves of G-CNT at different constant currentdensities. (d) Volumetric capacitance of G-CNT as a function ofcurrent densities calculated from GCD curves.

Figure 5. (a) Schematic of the assembled structure of HSC based onNi@Ni(OH)2 NF positive electrode and G-CNT negative electrode.(b) CV curves of HSC device at different scan rates. (c) GCD curvesof HSC device at different current densities. (d) Volumetriccapacitance and area of the HSC device as a function of currentdensities calculated from GCD curves. (e) Cycle performance of HSCdevice at 1 A/cm3. Inset is the GCD curves of cycles 1 and 4500. (f)Ragone plots of HSC device.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b10700ACS Appl. Mater. Interfaces 2016, 8, 27868−27876

27873

symmetry and displays no obvious change, suggesting theexcellent electrochemical reversibility and rate capability of thedevice. Additionally, the GCD curves exhibited in Figure 5c arean approximately triangular shape with linear behavior, furtherconfirming high electrochemical properties and capacitor-likebehaviors of Ni@Ni(OH)2//G-CNT HSC. The volumetriccapacitance of a supercapacitor can be obtained from the GCDcurve depending on the following equation:34

= ·Δ ·ΔC I t V U/ (4)

where C refers to volumetric capacitance (F/cm3), I refers tothe charge/discharge current (A), Δt refers to the dischargetime (s), V is the total volume of active material on bothelectrodes (cm3), and ΔU refers to the voltage window (V).The volumetric capacitance of the HSC at different currentdensities is exhibited in Figure 5d. The volumetric capacitanceof assembled HSC is calculated to be 95, 85, 79, 70, and 50 F/cm3 at current density of 0.25, 0.5, 0.75, 1, and 2 A/cm3,respectively. Moreover, an areal capacitance from 1.6 F/cm2 at4.2 mA/cm2 to 0.4 F/cm2 at 67.2 mA/cm2 was achieved. TheEIS spectrum in Figure S14b shows that the as-fabricated HSChas low ionic and electronic resistances in the high-frequencyarea. A nearly vertical line in the low-frequency area is arepresentative characteristic of capacitive behavior, suggestingthe fast charge/discharge behavior of assembled HSC.Long-term cycle stability of the supercapacitor is very

essential for practical application, so a cycle stability assessmentwas performed for Ni@Ni(OH)2//G-CNT HSC by repeatingGCD tests between 0 and 1.6 V with a current density of 1 A/cm3, as displayed in Figure 5e. The increase of capacitance inthe first 300 cycles can be related to the surface wettingenhancement of electrode material by electrolyte in cyclingprocess.35 The volumetric capacitance can reach up to 110% oforiginal capacitance. After a total number of 4500 galvanostaticcharge/discharge cycles, a capacitance loss of 14% was observedfor our HSC device. It suggests our Ni@Ni(OH)2//G-CNTHSC possesses excellent long-term electrochemical stability,which is superior to that of asymmetric supercapacitorsreported by previous works, such as Ni(OH)2//AC (82%retention after 1000 cycles),36 Co7Ni3//AC (75.4% retentionafter 3000 cycles),37 and Co3O4@Ni(OH)2//RGO (84%retention after 1000 cycles).38

The volumetric energy density (E, mWh/cm3) and powerdensity (P, Wh/cm3) of a supercapacitor are obtainedaccording to following relationship:

=E CV0.5 2 (5)

=P E t/ (6)

where C refers to the volumetric capacitance (F/cm3), V refersto the cell potential (V), and t is the discharge time (s).39

Figure 5f displays the Ragone plot of Ni@Ni(OH)2//G-CNTHSC obtained from GCD curves (Figure 5c) at differentcurrent densities. At a low current density of 0.25 A/cm3, theenergy density and power density are calculated to be 33.9mWh/cm3 and 200 mW/cm3, respectively. The improvedenergy density can be attributed to the synergistic effectsbetween Ni@Ni(OH)2 positive electrode and G-CNT negativeelectrodes. On one hand, the high volumetric energy density isobtained because of the ultrahigh volumetric capacity ofpositive electrode owing to the Faradaic reaction of activeNi(OH)2 nanosheets. On the other hand, the operationpotential window is extended to 1.6 V due to the negative

electrode G-CNT (1.0 V). Furthermore, energy density of as-prepared HSC is far more larger than VOx//VN ASC (0.61 and850 mWh/cm3),40 CNT//Ni(OH)2 ASC (0.3 mWh/cm3and14 Wh/cm3),41 Mn3O4//Ni(OH)2 APSC (0.35 mWh/cm3 and7 Wh/cm3),42 VN//CNT ASC (0.54 mWh/cm3 and 400 Wh/cm3),43 MnO2//CoSe2/carbon cloth ASC,44 Fe2O3//MnO2SCs (0.4 mWh/cm3 and 60 Wh/cm3),45 TiO2@MnO2//TiO2@C-based SCs(0.3 mWh/cm3 and 190 Wh/cm3),46 andCo9S8//Co3O4@Ru2O ASC (1.21 mWh/cm3 and 13290 Wh/cm3).47 It is even higher than that of thin film lithium batteries(1−10 mWh/cm3).48,49 Even when the current densityimproves to 4 A/cm3, the energy density still remains at 8.4mWh/cm3 with a power density of 3217 Wh/cm3. Theexcellent electrochemical property of Ni@Ni(OH)2//G-CNTHSC should be ascribed to its novel and unique electrodearchitecture. Particularly, our facile nanofoaming process couldin situ grow Ni(OH)2 nanosheets on the surface of highconductivity Ni NWs, which could substantially alleviate theconductivity problem of Ni(OH)2. Moreover, the porousNi(OH)2 nanosheets are beneficial to shorten the pathways forion diffusion and electron transfer during charge storageprocesses, thus significantly improving the electrochemicalperformance and rate capability. Hence, our Ni@Ni(OH)2//G-CNT HSC device demonstrates excellent potential for energystorage applications.

4. CONCLUSIONSWe have proposed an efficient nanofoaming technique to boostthe electrochemical energy storage property of Ni@Ni(OH)2NF film electrode for ultrahigh volumetric capacitancesupercapacitors. Ultrathin Ni(OH)2 nanosheets with porousnanostructures can be obtained on the surface of Ni NWs by afacile vacuum-assisted filtration method and subsequentnanofoaming treatments. The as-obtained Ni@Ni(OH)2 NFfilm electrodes display a significantly enhanced volumetriccapacity of 462 C/cm3 at 0.5 A/cm3 and excellent cycle stability(83% capacity retention after 4500 cycles). A hybrid super-capacitor with Ni@Ni(OH)2 NF as positive electrode and G-CNT as negative electrode was developed. The HSC possessesan ultrahigh volumetric capacitance of 95 F/cm3 at 0.25 A/cm3,a large energy density of 33.9 mWh/cm3, and a long-term cyclestability of 86% retention after 4500 cycles at a 1.6 V operationvoltage window. These remarkable properties demonstrate thatour facile nanofoaming method can open up new possibilitiesfor metal oxide or hydroxide materials for high-energysupercapacitor applications.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b10700.

Ni−F, Ni@Ni(OH)2 NWs, and G-CNT fabricationprocesses; SEM, TEM, XRD, XPS, BET, and EDScharacterizations of Ni@Ni(OH)2 NF; detailed CV,GCD, and EIS data for Ni−F, Ni@Ni(OH)2 NWs, andNi@Ni(OH)2 NF (PDF)

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: zhiyang@sjtu.edu.cn.*E-mail: hunantao@sjtu.edu.cn.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b10700ACS Appl. Mater. Interfaces 2016, 8, 27868−27876

27874

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The support of National Key Research and DevelopmentProgram of China (2016YFC0102700), the National NaturalScience Foundation of China (61671299 and 81670958),Shanghai Science and Technology Grant (16JC1402000),Program of Shanghai Academic/Technology Research Leader(15XD1525200), and the Program for Professor of SpecialAppointment (Eastern Scholar) at Shanghai Institutions ofHigher Learning are gratefully acknowledged. The InstrumentalAnalysis Center of Shanghai Jiao Tong University and theCenter for Advanced Electronic Materials and Devices ofShanghai Jiao Tong University are also acknowledged.

■ REFERENCES(1) Miller, J. R.; Simon, P. Electrochemical Capacitors for EnergyManagement. Science 2008, 321, 651−652.(2) Yan, J.; Khoo, E.; Sumboja, A.; Lee, P. S. Facile Coating ofManganese Oxide on Tin Oxide Nanowires with High-PerformanceCapacitive Behavior. ACS Nano 2010, 4, 4247−4255.(3) Zuo, W.; Wang, C.; Li, Y.; Liu, J. Directly Grown NanostructuredElectrodes for High Volumetric Energy Density Binder-Free HybridSupercapacitors: A Case Study of CNTs//Li4Ti5O12. Sci. Rep. 2015, 5,7780.(4) Lior, N. Mirrors in the Sky: Status, Sustainability, and SomeSupporting Materials Experiments. Renewable Sustainable Energy Rev.2013, 18, 401−415.(5) Li, B.; Dai, F.; Xiao, Q.; Yang, L.; Shen, J.; Zhang, C.; Cai, M.Nitrogen-Doped Activated Carbon for a High Energy HybridSupercapacitor. Energy Environ. Sci. 2016, 9, 102−106.(6) Ke, Q.; Zheng, M.; Liu, H.; Guan, C.; Mao, L.; Wang, J. 3DTiO2@Ni(OH)2 Core-Shell Arrays with Tunable Nanostructure forHybrid Supercapacitor Application. Sci. Rep. 2015, 5, 13940.(7) Hu, X.; Deng, Z.; Suo, J.; Pan, Z. A High Rate, High Capacity andLong Life (LiMn2O4 + AC)/Li4Ti5O12 Hybrid Battery−Super-capacitor. J. Power Sources 2009, 187, 635−639.(8) Wang, G.; Zhang, L.; Zhang, J. A Review of Electrode Materialsfor Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797−828.(9) Cai, G.; Wang, X.; Cui, M.; Darmawan, P.; Wang, J.; Eh, A. L. S.;Lee, P. S. Electrochromo-Supercapacitor Based on Direct Growth ofNiO Nanoparticles. Nano Energy 2015, 12, 258−267.(10) Dam, D. T.; Wang, X.; Lee, J. M. Graphene/NiO Nanowires:Controllable One-Pot Synthesis and Enhanced PseudocapacitiveBehavior. ACS Appl. Mater. Interfaces 2014, 6, 8246−8256.(11) Xu, S.; Li, X.; Yang, X.; Wang, T.; Xu, M.; Zhang, L.; Yang, C.;Hu, N.; He, D.; Zhang, Y. A novel Ni@Ni(OH)2 Coaxial Core-SheathNanowire Membrane for Supercapacitor Electrodes with HighVolumetric Capacitance and Excellent Rate Capability. Electrochim.Acta 2015, 182, 464−473.(12) Salunkhe, R. R.; Lin, J.; Malgras, V.; Dou, S. X.; Kim, J. H.;Yamauchi, Y. Large-Scale Synthesis of Coaxial Carbon Nanotube/Ni(OH)2 Composites for Asymmetric Supercapacitor Application.Nano Energy 2015, 11, 211−218.(13) Liu, J.; Jiang, J.; Cheng, C.; Li, H.; Zhang, J.; Gong, H.; Fan, H.Co3O4 Nanowire@ MnO2 Ultrathin Nanosheet Core/Shell Arrays: ANew Class of High-Performance Pseudocapacitive Materials. Adv.Mater. 2011, 23, 2076−2081.(14) Xia, X.; Tu, J.; Zhang, Y.; Wang, X.; Gu, C.; Zhao, X.; Fan, H.High-Quality Metal Oxide Core/Shell Nanowire Arrays on Con-ductive Substrates for Electrochemical Energy Storage. ACS Nano2012, 6, 5531−5538.(15) Zhang, X.; Zhao, Y.; Xu, C. Surfactant Dependent Self-Organization of Co3O4 Nanowires on Ni Foam for High Performance

Supercapacitors: from Nanowire Microspheres to Nanowire PaddyFields. Nanoscale 2014, 6, 3638−3646.(16) Yang, C.; Zhang, L.; Hu, N.; Yang, Z.; Wei, H.; Zhang, Y.Reduced Graphene Oxide/Polypyrrole Nanotube Papers for FlexibleAll-Solid-State Supercapacitors with Excellent Rate Capability andHigh Energy Density. J. Power Sources 2016, 302, 39−45.(17) Choi, B. G.; Chang, S.; Kang, H.; Park, C. P.; Kim, H. J.; Hong,W.; Lee, S.; Huh, Y. S. High Performance of A Solid-State FlexibleAsymmetric Supercapacitor Based on Graphene Films. Nanoscale2012, 4, 4983−4988.(18) Liu, R.; Wan, L.; Liu, S.; Pan, L.; Wu, D.; Zhao, D. An Interface-Induced Co-Assembly Approach Towards Ordered MesoporousCarbon/Graphene Aerogel for High-Performance Supercapacitors.Adv. Funct. Mater. 2015, 25, 526−533.(19) Li, Y.; Dong, J.; Zhang, J.; Zhao, X.; Yu, P.; Jin, J.; Zhang, Q.Nitrogen-Doped Carbon Membrane Derived from Polyimide as Free-Standing Electrodes for Flexible Supercapacitors. Small 2015, 11,3476−3484.(20) Yang, X.; Cheng, C.; Wang, Y.; Qiu, L.; Li, D. Liquid-MediatedDense Integration of Graphene Materials for Compact CapacitiveEnergy Storage. Science 2013, 341, 534−537.(21) Yan, J.; Fan, Z.; Sun, W.; Ning, G.; Wei, T.; Zhang, Q.; Zhang,R.; Zhi, L.; Wei, F. Advanced Asymmetric Supercapacitors Based onNi(OH)2/graphene and Porous Graphene Electrodes with HighEnergy Density. Adv. Funct. Mater. 2012, 22, 2632−2641.(22) Liu, N. S.; Li, J.; Ma, W. Z.; Liu, W. J.; Shi, Y. L.; Tao, J. Y.;Zhang, X. H.; Su, J.; Li, L. Y.; Gao, Y. H. A Facile Synthesis ofMesoporous Co3O4/CeO2 Hybrid Nanowire Arrays for HighPerformance Supercapacitors. ACS Appl. Mater. Interfaces 2014, 6,13627−13634.(23) Peng, S.; Li, L.; Wu, H.; Madhavi, S.; Lou, X. ControlledGrowth of NiMoO4 Nanosheet and Nanorod Arrays on VariousConductive Substrates as Advanced Electrodes for AsymmetricSupercapacitors. Adv. Energy Mater. 2015, 5, 1401172.(24) Gao, G.; Wu, H.; Ding, S.; Liu, L.; Lou, X. HierarchicalNiCo2O4 Nanosheets Grown on Ni Nanofoam as High-PerformanceElectrodes for Supercapacitors. Small 2015, 11, 804−808.(25) Jiang, L.; Sheng, L.; Long, C.; Fan, Z. Densely Packed GrapheneNanomesh-Carbon Nanotube Hybrid Film for Ultra-High VolumetricPerformance Supercapacitors. Nano Energy 2015, 11, 471−480.(26) Wang, J.; Wei, L.; Zhang, L.; Jiang, C.; Kong, E. S.-W.; Zhang, Y.Preparation of High Aspect Ratio Nickel Oxide Nanowires and TheirGas Sensing Devices with Fast Response and High Sensitivity. J. Mater.Chem. 2012, 22, 8327−8335.(27) Li, Z.; Su, Y.; Liu, Y.; Wang, J.; Geng, H.; Sharma, P.; Zhang, Y.Controlled One-Step Synthesis of Spiky Polycrystalline NickelNanowires with Enhanced Magnetic Properties. CrystEngComm2014, 16, 8442−8448.(28) Dai, X.; Chen, D.; Fan, H.; Zhong, Y.; Chang, L.; Shao, H.;Wang, J.; Zhang, J.; Cao, C. Ni(OH)2/NiO/Ni Composite NanotubeArrays for High-Performance Supercapacitors. Electrochim. Acta 2015,154, 128−135.(29) Cai, G.; Wang, X.; Cui, M.; Darmawan, P.; Wang, J.; Eh, A. L. S.;Lee, P. S. Electrochromo-Supercapacitor Based on Direct Growth ofNiO Nanoparticles. Nano Energy 2015, 12, 258−267.(30) Jiang, W.; Yu, D.; Zhang, Q.; Goh, K.; Wei, L.; Yong, Y.; Jiang,R.; Wei, J.; Chen, Y. Ternary Hybrids of Amorphous NickelHydroxide−Carbon Nanotube-Conducting Polymer for Supercapaci-tors with High Energy Density, Excellent Rate Capability, and LongCycle Life. Adv. Funct. Mater. 2015, 25, 1063−1073.(31) Byon, H. R.; Gallant, B. M.; Lee, S. W.; Shao-Horn, Y. Role ofOxygen Functional Groups in Carbon Nanotube/Graphene Free-standing Electrodes for High Performance Lithium Batteries. Adv.Funct. Mater. 2013, 23, 1037−1045.(32) Li, M.; Ma, K.; Cheng, J.; Lv, D.; Zhang, X. Nickel-CobaltHydroxide Nanoflakes Conformal Coating on Carbon Nanotubes as aSupercapacitive Material with High-Rate Capability. J. Power Sources2015, 286, 438−444.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b10700ACS Appl. Mater. Interfaces 2016, 8, 27868−27876

27875

(33) Sui, L.; Tang, S.; Chen, Y.; Dai, Z.; Huangfu, H. X.; Zhu, Z.;Qin, X.; Deng, Y.; Haarberg, G. M. An Asymmetric Supercapacitorwith Good Electrochemical Performances Based on Ni(OH)2/AC/CNT and AC. Electrochim. Acta 2015, 182, 1159−1165.(34) Lai, H.; Wu, Q.; Zhao, J.; Shang, L.; Li, H.; Che, R.; Lyu, Z.;Xiong, J.; Yang, L.; Wang, X.; Hu, Z. Mesostructured NiO/NiComposites for High-Performance Electrochemical Energy Storage.Energy Environ. Sci. 2016, 9, 2053−2060.(35) Ganesh, V.; Pitchumani, S.; Lakshminarayanan, V. NewSymmetric and Asymmetric Supercapacitors Based on High SurfaceArea Porous Nickel and Activated Carbon. J. Power Sources 2006, 158,1523−1532.(36) Lang, J.; Kong, L.; Liu, M.; Luo, Y.; Kang, L. AsymmetricSupercapacitors Based on Stabilized α-Ni(OH)2 and ActivatedCarbon. J. Solid State Electrochem. 2010, 14, 1533−1539.(37) Zhang, J.; Cheng, J.; Li, M.; Liu, L.; Liu, F.; Zhang, X. Flower-Like Nickel-Cobalt Binary Hydroxides with High Specific Capacitance:Tuning the composition and asymmetric capacitor application. J.Electroanal. Chem. 2015, 743, 38−45.(38) Tang, C.; Yin, X.; Gong, H. Superior Performance AsymmetricSupercapacitors Based on a Directly Grown Commercial Mass 3DCo3O4@Ni(OH)2 Core-Shell Electrode. ACS Appl. Mater. Interfaces2013, 5, 10574−10582.(39) Shen, L.; Wang, J.; Xu, G.; Li, H.; Dou, H.; Zhang, X. NiCo2S4Nanosheets Grown on Nitrogen-Doped Carbon Foams as anAdvanced Electrode for Supercapacitors. Adv. Energy. Mater. 2015, 5,1400977−1400983.(40) Lu, X.; Yu, M.; Zhai, T.; Wang, G.; Xie, S.; Liu, T.; Liang, C.;Tong, Y.; Li, Y. High Energy Density Asymmetric Quasi-Solid-StateSupercapacitor Based on Porous Vanadium Nitride Nanowire Anode.Nano Lett. 2013, 13, 2628−2633.(41) Tang, Z.; Tang, C.; Gong, H. A High Energy DensityAsymmetric Supercapacitor from Nano-architectured Ni(OH)2/Carbon Nanotube Electrodes. Adv. Funct. Mater. 2012, 22, 1272−1278.(42) Feng, J.; Ye, S.; Lu, X.; Tong, Y.; Li, G. Asymmetric PaperSupercapacitor Based on Amorphous Porous Mn3O4 NegativeElectrode and Ni(OH)2 Positive Electrode: A Novel and High-Performance Flexible Electrochemical Energy Storage Device. ACSAppl. Mater. Interfaces 2015, 7, 11444−11451.(43) Xiao, X.; Peng, X.; Jin, H.; Li, T.; Zhang, C.; Gao, B.; Hu, B.;Huo, K.; Zhou, J. Freestanding Mesoporous VN/CNT HybridElectrodes for Flexible All-Solid-State Supercapacitors. Adv. Mater.2013, 25, 5091−5097.(44) Yu, N.; Zhu, M.; Chen, D. Flexible All-Solid-State AsymmetricSupercapacitors with Three-Dimensional CoSe2/Carbon ClothElectrodes. J. Mater. Chem. A 2015, 3, 7910−7918.(45) Lu, X.; Zeng, Y.; Yu, M.; Zhai, T.; Liang, C.; Xie, S.; Balogun, M.S.; Tong, Y. Oxygen-Deficient Hematite Nanorods as High-Perform-ance and Novel Negative Electrodes for Flexible AsymmetricSupercapacitors. Adv. Mater. 2014, 26, 3148−3155.(46) Lu, X.; Yu, M.; Wang, G.; Zhai, T.; Xie, S.; Ling, Y.; Tong, Y.; Li,Y. H-TiO2@MnO2//H-TiO2@C Core-Shell Nanowires for HighPerformance and Flexible Asymmetric Supercapacitors. Adv. Mater.2013, 25, 267−272.(47) Xu, J.; Wang, Q.; Wang, X.; Xiang, Q.; Liang, B.; Chen, D.;Shen, G. Flexible Asymmetric Supercapacitors Based Upon Co9S8Nanorod//Co3O4@RuO2 Nanosheet Arrays on Carbon Cloth. ACSNano 2013, 7, 5453−5462.(48) El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B. LaserScribing of High-Performance and Flexible Graphene-Based Electro-chemical Capacitors. Science 2012, 335, 1326−1330.(49) Hu, N.; Zhang, L.; Yang, C.; Zhao, J.; Yang, Z.; Wei, H.; Liao,H.; Feng, Z.; Fisher, A.; Zhang, Y.; Xu, Z. C. J. Three-DimensionalSkeleton Networks of Graphene Wrapped Polyaniline Nanofibers: anExcellent Structure for High-Performance Flexible Solid-State Super-capacitors. Sci. Rep. 2016, 6, 19777.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b10700ACS Appl. Mater. Interfaces 2016, 8, 27868−27876

27876