1-s2.0-3D core_shell hierarchies of MnOOH ultrathin-main

journal homepage: /locate/nanoenergyAvailable online at RAPID COMMUNICATION3D core/shell hierarchies of MnOOH ultrathin nanosheets grown on NiO nanosheet arraysfor high-performance supercapacitorsJianqing Sun a,c,1,Wenyao Li a,b,1,Bingjie Zhang a,Gao Li a,Lin Jiang a,Zhigang Chen a,Rujia Zou a,d,n,Junqing Hu a,na State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,College of MaterialsScience and Engineering,Donghua University,Shanghai201620,PR Chinab School of material engineering,Shanghai university of engineering science,Shanghai201620,Chinac Key Laboratory of Organofluorine Chemistry,Shanghai Institute of Organic Chemistry,Chinese Academy ofSciences,345Ling-Ling Road,Shanghai200032,Chinad Center of Super-Diamond and Advanced Films(COSDAF),Department of Physics and Materials Science,City University of Hong Kong,Hong Kong,ChinaReceived30September2013;received in revised form3December2013;accepted7December2013Available online30December2013KEYWORDSNiO;MnOOH;Core/shell hierar-chies;Cycling stability;Energy/powerdensity;SupercapacitorAbstractAs the most promising electrode material for supercapacitors,core/shell hybrid material willenhance the electrochemical performance comparing with single component constituent,thus hasrecently drawn our research.Herein,we have designed and synthesized3D hierarchical hetero-structures of dense MnOOH ultrathin nanosheets grown on porous hierarchical NiO nanosheet arraysby facial and rational process.In this configuration,porous hierarchical NiO nanosheet arrays serveas fast ion and electron transport model and dense MnOOH ultrathin nanosheets enhance thecontact surface area and assist ions penetrate into the core region to realize the release of potentialelectrochemical properties of NiO nanosheet arrays,and thus these heterostructures provide intenseneeded critical function for efficient use of metal oxide and hydroxide in energy storage.As anelectrode,the as-fabricated3D NiO@MnOOH core/shell nanosheet hierarchies exhibited favorableelectrochemical performances,i.e.,high specific capacitance of1625.3F/g at a current density of4A/g with a remarkable rate capability and excellent energy density(80.0Wh/kg),as well as goodcycling stability(105.7%of the initial capacitance after5000cycles).It suggests that they shouldhave a promising potential for the next generation energy conversion–storage devices.&2014Elsevier Ltd.All rights reserved.2211-2855/$-see front matter&2014Elsevier Ltd.All rights reserved./10.1016/j.nanoen.2013.12.006n Corresponding author.E-mail addresses:rjzou@(R.Zou),hu.junqing@(J.Hu).1These authors contributed equally to the work.Nano Energy(2014)4,56–64IntroductionAs by utilizing and optimizing the advantages of different components,core/shell configuration heterostructured nanomaterials can offer remarkable synergetic properties and multifunctionalities[1–3],they have been of great scientific and technologic interests,according to their high versatility and applicability in nanotechnology including electronics[4],energy conversion–storage devices[5], biomedical science[6],catalysis[7]etc.For supercapaci-tors,the most promising candidates for high-capacitance electrochemical energy storage devices,such an expected core/shell hybrid configuration will enhance the electro-chemical performance comparing with a single component constituent,and thus has recently drawn our research. Considerable efforts have been triggered to the synthesis and supercapacitor properties of the core/shell nanomater-ials from some transition metal oxide based systems,while a well-defined micro-/nano-structure for a higher capaci-tance required by practical energy storage applications is still a challenge.In addition,even a few of successful strategies such as oxidation[8],chemical templates method [9]and pulsed laser deposition[10]have been presented for the synthesis of transition metal oxide or hydroxide core/ shell architectures,a facile and high-efficiency synthetic method is still greatly needed for the supercapacitor applications.Transition metal(Ru,Ni,Co,Mn,Fe,etc.)oxides and hydroxides are excellent candidates for design of high cycling stability and high energy/power density supercapa-citor electrodes[11–13],due to multi-oxidation states for charge transfer and reversible adsorption.RuO2once was anticipated as an ideal supercapacitor electrode material with a capacitance as high as1300F/g[14],however,the expensive nature and toxicity limit its wide application. So,other low-cost and nontoxic transition metal oxides/ hydroxides such as NiO[15],Co3O4[16],MnO2[17],MnOOH [18]and Fe2O3[19]have been developed as alternatives. Even though improved electrochemical performances have been revealed,the involved redox reactions are typically at a low charge/discharge rate,and only part of active super-capacitive materials contributed to better rate and cycling performance,leading to a weak energy/power density.To meet the requirement of high capacitance,cycling stability and energy/power density,one promising route is to choose hybrid supercapacitive materials and rationally design inte-grated smart configuration.Within the configuration,struc-tural features and electroactivities of individual component are fully manifested,the interface/chemical distribut-ions are homogeneous at a nanoscale and a fast ion and electron transfer is guaranteed,that is,a synergistic effect is realized.For a supercapacitor,core/shell hybrid hetero-structured nanomaterials have been confirmed to be unique as facilitating the electroactivities of each component,ion diffusion and electron transfer,therefore electrochemical performance is dramatically promoted[20].Particularly, ordered nanostructures grown directly on current collectors are very helpful to electron transport and microstrain[21], when combined with highly porous or hollow morphologies [22,23],the3D core/shell heterostructures represent an attractive configuration for fast reaction kinetics,due to the significantly enhancing of surface area.Herein,we present a3D core/shell configuration of denseMnOOH ultrathin nanosheets grown on hierarchical NiOnanosheet arrays as an electrode material for supercapaci-tors.Such electrode material demonstrates a high specificcapacitance of1625.3F/g at a current density of4A/g,which is among the highest value reported for transitionmetal oxides or hydroxides grown on Ni foam substrate[9,26,29,30].Furthermore,it also exhibits a remarkablerate capability and a good cycling stability(105.7%of theinitial capacitance after5000cycles);more importantly,ahigh energy density of80.0Wh/kg also has been achieved.To our best knowledge,this electrochemical performancesof as-fabricated NiO@MnOOH core/shell nanosheet hierar-chies are the best among the reported hybrid supercapa-citor systems,indicating a great potential for the nextgeneration energy conversion–storage devices[2,20,26]. ExperimentalAll reagents were of analytical grade and used withoutfurther purification.Preparation of NiO hierarchiesTypically,a piece of Ni foam was carefully cleaned with HClaqueous,acetone and deionized water.A solution forchemical bath deposition(CBD)was prepared by mixingnickel sulfate(NiSO4)aqueous with NH3ÁH2O,then the as-cleaned Ni foam was vertically immersed into a50mL(voluminal)beaker which contained the mixed solution at651C for1.5h,resulting a light-blue material formed on theNi foam.After being rinsed and dried in a vacuum oven at601C for2h,the material was annealed at4501C for1hunder N2flow,resulting a black material which was char-acterized to be NiO hierarchies.Preparation of NiO@MnOOH core/shell hierarchiesMnOOH nanosheets were grown onto the surface of porous NiO hierarchies by a facial potentiostatic electrochemical deposition process.In detail,MnOOH nanosheets were deposited at0.9V(vs.SCE)in a solution contain0.02M manganese ammonium(MnAc2)and0.02M ammonium acetate (NH4Ac)with10%DMSO for10min at7251C,changing the deposition time can get different mass contents of MnOOH. After deposition,the as-prepared NiO@MnOOH electrode material was rinsed with deionized water and absolute ethanol several times and then placed in a vacuum oven at601C for2h.Structure characterizationAs-prepared products were characterized with a D/Max-2550PC X-ray diffractometer(XRD;Rigaku,Cu-Kαradiation),a micro-Raman spectroscopy(Raman;Renishaw,inVia-Reflex),a X-ray photoelectron spectroscopy(XPS,ESCALab250),a scanning electron microscopy(SEM;HIT ACHI,S-4800)and a transmission electron microscopy(TEM;JEOL,JEM-2100F)equipped with an energy-dispersive X-ray spectrometer(EDX).573D core/shell hierarchies of MnOOH ultrathin nanosheets grown on NiO nanosheet arraysElectrochemical characterizationElectrochemical measurements were performed on an Auto-lab Electrochemical Workstation(PGSTAT302N)using a three electrode electrochemical cell and1M LiOH as the electro-lyte at room temperature.The Ni-foam-supported hierarch-ical heterostructures($1cm2area)acted directly as the working electrode.A Pt plate and Ag/AgCl were used as the counter electrode and the reference electrode,respec-tively.All potentials were referred to the reference electrode. The specific capacitance(F/g)and current density(A/g) were calculated based on the mass of two active materials, i.e.,NiO and MnOOH.The mass of NiO was weighed on an XS analytical balance(Mettler Toledo;δ=0.01mg),and the mass of MnOOH was calculated by the Faraday0s law. Results and discussionThe SEM image in Figure1a shows the aligned and cross-linked hierarchical NiO nanosheet arrays.In fact,a hier-archy is self-assembled by many aligned thin NiO nanosheets on a major NiO nanoflake,forming a multi-hierarchy with many individual irregular interspaces,as shown in high-magnification SEM imaging,Figure1b.The composed major nanoflake has a thickness of50–100nm,while the aligned thin nanosheets are as thin as10–15nm.Amazingly,these thin NiO nanosheets are full of numerous pores with a size of 10–20nm,as demonstrated in Figure1b and c(marked with blue arrows).So,it is believed that these porous NiO hierarchies will have a high surface area and considerably large interspaces,which is beneficial for electrolyte pene-trating and electron transferring.High-resolution TEM (HRTEM)image in Figure1d shows the NiO nanosheet is a single crystal in nature;in this image,the lattice fringes give an interplanar spacing of0.24nm and0.15nm,corre-sponding to{À111}and{02À2}lattice planes of the NiO crystal,and the corresponding fast Fourier transform(FFT) diffraction pattern can be indexed to[211]zone axis of the NiO crystal.As is well known,NiO is an excellent supercapacitor electrode material with remarkably high capacitance[26], however,the poor cycling stability and feeble energy/power density block its further application.To overcome these weakness and far more improve the electrochemical perfor-mance,we now consider designing a core/shell configuration by hybridizing NiO with an additional pseudocapacitor electrode material,e.g.,metal hydroxide.Here,as-grown aligned and cross-linked hierarchical NiO nanosheet arrays can serve as a robust scaffold for the loading of this additional supercapacitive material.To achieve synergy, the newly involved material would be required to fulfill the following requirements:to contribute effective specific capacitance,to enlarge the surface area but not to prevent contacting between the hierarchical NiO nanosheet arrays and ions in the electrolyte,as well as to maintain the structural integrity[2,21].For these aims,in our study,the electrodeposition is demonstrated to be a convenient and controllable method to grow another electrode material onto the surface of NiO nanosheets(Figure S1,see ESI†).In a typical process,the hybrid hierarchical configuration was obtained by electrodeposit assist growth of the ultrathin MnOOH nanosheets on the hierarchical NiO nanosheet arrays,receiving a mass ratio of15.4%(MnOOH/NiO). Obviously,the mass ratio of as-electrodeposited MnOOH nanosheets to the NiO hierarchies is low,however,the MnOOH nanosheets densely cover the surface of NiO hier-archies,the volume ratio can reach$100%(MnOOH/NiO) Figure1(a and b)SEM images of as-grown hierarchical NiO nanosheet arrays.(c and d)TEM and HRTEM images of a NiO nanosheet, a lower-right inset in(d)is the corresponding FFT diffraction pattern.J.Sun et al. 58according to Figure 2a,hence the hybrid hierarchies looking like a spongy morphology.As seen from Figure 2b,as-grown MnOOH nanosheets are very thin,showing a thickness of 3–5nm,so the NiO hierarchies are effectively and completely covered by these fine MnOOH nanosheets,leading to a large volume of interspaces inside these structures.It is believed that such spongy morphology can facilitate ions penetrating into the core region through the fine MnOOH nanosheets to realize the release of potential electrochemical properties of porous hierarchical NiO nanosheet arrays.Figure 2c shows the MnOOH nanosheets stripped from as-fabricated NiO@MnOOH core/shell nanosheet hierarchies,also demon-strating they are very thin;the first five diffraction rings (from the one with the smallest diameter)in the electron pattern are interpreted as (002),(311),(312),(402)and (006)re flections,respectively,of the MnOOH crystals.Shown in Figure 2d is a HRTEM image of a MnOOH nanosheet,in which an interplanar spacing from the lattice fringes is measured to be 0.46nm and 0.46nm,correspond-ing to {020}and {200}lattice planes of the MnOOH crystal,and a lower-right inset of the corresponding FFT pattern can be indexed to [100]zone axis of the MnOOH crystal.X-ray diffraction (XRD)patterns of as-fabricated hier-archical NiO nanosheet arrays and NiO@MnOOH core/shell nanosheet hierarchies on the Ni foam substrate are shown in Figure 3a,respectively.It shows that the hybrid products are composed of three crystalline phases,i.e.,cubic Ni (JCPDS:65-2865;a =3.524Å),cubic NiO (JCPDS:47-1049;a =4.177Å)and tetragonal MnOOH (JCPDS:18-0804;a =b =8.6Åand c =9.3Å).No characteristic peaks peculiar to impurities of other crystalline phases,such as Ni(OH)2and MnO 2are observed,which indicates the level of impurities in the sample is lower than the resolution limit of XRD ($5at%).X-ray photoelectron spectroscopy (XPS)analysis was taken to get more composition information of the hybrid hierarchies.A survey scan spectrum shows the presence of Ni,Mn and O of as-prepared NiO@MnOOH core/shell nanosheet hierarchies,Figure S2(see ESI †).Figure 3b of Ni2p region shows two major peaks with binding energies at 855.5eV and 873.3eV ,corresponding to Ni2p 3/2and Ni2p 1/2spin-orbit peaks,respectively,of NiO phase [24].Figure 3c of Mn2p region demonstrates two peaks at the binding energies of 642.7eV and 654.2eV originated from Mn2p 3/2and Mn2p 1/2spin-orbit peaks,respectively,indicat-ing that element Mn is in the chemical state of Mn (III)[25].Figure 3d is the O1s region,the black curve is well fit to the spectrum which is corresponding to the oxide,hydroxide and water peaks (red,green and blue peaks),respectively ,it strongly demonstrates the existence of MnOOH by excluding the contribution of NiO material [25].The local energy dispersive X-ray spectroscopy (EDX)and Raman spectrum analysis are also conducted (Figures S3and S4,see ESI †),which provide further evidence for the con figuration of the NiO@MnOOH core/shell nanosheet hierarchies.As the fabricated 3D NiO@MnOOH core/shell nanosheet heterostructures are composed of porous hierarchical NiO nanosheet arrays and spongy MnOOH ultrathin nanosheets,the transportation of electrolytes through their nanopores as well as the interspaces is possibly more feasible for ef ficient redox reactions and more quickly for charge transport during Faradaic charge-storage process.However ,the charge-storage ef ficiency of the materials varies due to the differences in morphologies and microstructures.In order to identify whether such architectures are favorable for high-rate capacitive energy storage,the speci fic capacitance values of the obtained electrode material should be determined by cyclic voltammetry (CV)and galvanostatic charge –discharge (CD)measurements,which are useful for examining electrochemicalperformance.Figure 2(a and b)SEM images of the NiO@MnOOH core/shell nanosheet hierarchies.(c and d)TEM and HRTEM images of the MnOOH nanosheets,two lower-right insets in (c and d)are the corresponding SAED and FFT patterns,respectively.593D core/shell hierarchies of MnOOH ultrathin nanosheets grown on NiO nanosheet arraysElectrochemical measurements were performed on an Autolab (PGSTAT302N)potentiostat with a three-electrode con figuration system in a 1M LiOH at room temperature.Considering the Ni foam could in fluence the final electro-chemical performance with a small capacitance contribu-tion,we compared it with NiO@MnOOH core/shell nanosheet hierarchies (Figure S5,see ESI †)and found that the speci fic capacitance of Ni foam was very small (1.5F/g).In order to reasonably illustrate the contribution of the active material,the mass of Ni foam was subtracted from NiO@MnOOH electrode neglecting the small contribution of the Ni foam.The variation of the capacitance with the electrodeposition time is examined and plotted in Figure S6(See ESI †),(their corresponding SEM imaging characteriza-tion is shown in Figure S7,see ESI †),and it is found that the highest capacitance corresponds to 2min deposition of the MnOOH nanosheets.This illustrates the convenience of the electrodeposition in tailoring electrode surface and obtain-ing high capacitance.For comparison,electrochemical properties of as-grown pristine hierarchical NiO nanosheet arrays also have been studied (Figure S6a ,ESI †).Figure 4a shows the cyclic voltammetry (CV)curves of the NiO@M-nOOH core/shell nanosheet hierarchies on Ni foam at different scan rates,i.e.,2,5,10,20,50and 80mV/s.It can be seen that the redox peaks of the NiO@MnOOH core/shell nanosheet hierarchies have a decided change as the increase of the scan rate.Figure 4b is the CV curves of pristine hierarchical NiO nanosheet arrays and NiO@MnOOH core/shell nanosheet hierarchies at 50mV/s.For the hier-archical NiO nanosheet arrays,the CV curve exhibits a relatively symmetrical shape corresponding to the faradic oxidation/reduction reactions of NiO +OH -2NiOOH +e -[11,26].However ,the CV curve demonstrates a signi ficant difference with NiO@MnOOH core/shell nanosheet hierar-chies which increases the surface roughness of the compo-site material as well as offering another transition metal oxide after adding MnOOH [47].Compared with the pristine hierarchical NiO nanosheet arrays,the obvious enlarged enclosed area of CV curves of NiO@MnOOH core/shell nanosheet hierarchies indicates the increase for new redox waves of MnOOH +OH À2MnO 2+H 2O +e Àand MnO 2+Li ++e À2MnOOLi [27,28,48].That is to say,the transition metal oxide (NiO)and hydroxide (MnOOH)are hybridized together harmoniously,and the electroactivities of two individual components are suf ficiently manifested.The electrochemical performances of the NiO@MnOOH core/shell hierarchies are further demonstrated by galvano-static charge –discharge (CD)testing.And the potential used to calculate to the speci fic capacitance of the electrode according to the charge –discharge curves is the maximum potential.CD curves of the NiO@MnOOH core/shell nanosheet hierarchies were recorded at various current densities,as shown in Figure S8.At the current density of 4A/g,a comparison of the CD curves from the NiO@MnOOH core/shell nanosheet hierarchies and pristine hierarchical NiO nanosheet arrays is given in Figure 4c.It is interesting to find that the charge time of the hybrid core/shell hierar-chies is almost equal to that of the pristine,corresponding to a relatively similar charge transfer resistance,(EIS spectra in Figure S9,see ESI †).And because of the participation of MnOOH material,the core/shell hybrid con figuration exhibiting a longer discharge time reveals much better electrochemical performance when compared with the pristine hierarchical NiO nanosheet arrays.AsFigure 3(a)XRD patterns of hierarchical NiO nanosheet arrays and NiO@MnOOH core/shell nanosheet hierarchies on Ni foam substrate.(b –d)XPS spectra of NiO@MnOOH core/shell nanosheet hierarchies:Ni2p,Mn2p and O1s spectrum,respectively.J.Sun et al.60shown in Figure 4d,the NiO@MnOOH core/shell nanosheet hierarchies have a speci fic capacitance of 1890.5,1625.3,1568.0,1312.0and 1223.3F/g at the current density of 1.7,4,8,16,and 20A/g,respectively,much higher than the pristine hierarchical NiO nanosheet arrays (1881.3,1390.0,965.3,472.0and 313.4F/g at 2,4,8,16,and 20A/g,respectively)(their area capacitances shown in Figure S10,see ESI †),in particular ,it also among the highest value of the reported single or hybrid transition metal oxides and hydroxides electrode materials [9,26,29,30,40].Moreover ,such high capacitance of the present core/shell nanosheet hierarchies (1890.5F/g)is maintained remarkably up to 64.7%of that measured at 20A/g,still much better than the pristine NiO hierarchies (16.6%)and also better than the other reported values [41,42,43].We further studied the electrochemical performances of NiO@MnO 2nanosheet hierarchies by similar fabrication method for a comparison and poor electrochemical performance was detected (Figure S6a and Figure S11,See ESI †).These results imply that the additional component of MnOOH nanosheets within these hybrid heterostructures not only didn ’t prevent the capacitance releasing of NiO material,but also provided more electrochemical redox reactions and enlarged the surface area to boost the energy conversion –storage and retention capability.This advanced con figuration for the NiO@MnOOH core/shell nanosheet hierarchies is schematically illustrated in Figure 5a,which is quite different from the capacitance enhancement by normal core/shell nanostructure arrays in previous reports,such as Ni/NiO inverse opal nanostructure [31],and Ni@MnO 2opal nanostructure [44]in which only the shell material was high-active material while the core material mainly served as the current collector.The long-term cycling stability of the as-fabricated NiO@MnOOH core/shell nanosheet hierarchies and pristine hierarchical NiO nanosheet arrays were examined by their CV cycles at a scan rate of 50mV/s,as shown in Figure 5b.The capaci-tance of the core/shell nanosheet hierarchies gradually increases to 105.7%with a slight fluctuation from 0to 3500cycles and keeps almost constant after that,which reveals the NiO@MnOOH core/shell nanosheet hierarchies for electrode have a good long-term cyclic performance.While the hierarchical NiO nanosheet arrays lost 22.4%of the initial capacitance over 5000cycles with obvious fluctuations in 1500–3500cycles.The fluctuations may ascribe to the in fluence of the electrolyte temperature and the destruction/reconstruction of the NiO nanostruc-tural [45–46].CV curves of the as-fabricated NiO@MnOOH core/shell nanosheet hierarchies and NiO nanosheet hier-archies after 5000cycles are presented in Figure S12(see ESI †).It shows only oxidation peak of NiO hierarchies has a clear change,further proves the good cycling stability of NiO@MnOOH core/shell nanosheet hierarchies compares with the pristine hierarchical NiO nanosheet arrays.Power and energy densities are two important para-meters for evaluating the electrochemical performance of the supercapacitors.The Ragone plots of the NiO@MnOOH core/shell nanosheet hierarchies and hierarchical NiO nanosheet arrays are exhibited in Figure 5c,respectively.Here,a high energy density (80.0Wh/kg)was obtained of the NiO@MnOOH core/shell nanosheet hierarchies,which reveals a much better performance than mostEDLCsFigure 4(a)CV curves of NiO@MnOOH core/shell nanosheet hierarchies at different scan rates.(b)CV curves of NiO@MnOOH core/shell nanosheet hierarchies and pristine hierarchical NiO nanosheet arrays at 50mV/s.(c)Galvanostatic charge –discharge (CD)curves of NiO@MnOOH core/shell nanosheet hierarchies and pristine hierarchical NiO nanosheet arrays at 4A/g.(d)A comparison of capacitances for NiO@MnOOH core/shell nanosheet hierarchies and pristine hierarchical NiO nanosheet arrays as a function of the current density.613D core/shell hierarchies of MnOOH ultrathin nanosheets grown on NiO nanosheet arrays[32,34,35]and pseudocapacitors [36,37].Furthermore,the power density of the NiO@MnOOH core/shell nanosheet hierarchies reaches 17.1kW/kg at an energy density of 24.1Wh/kg,which is among the best reported literatures so far [33–38].More importantly,such high power density value can meet the power demands of PNGV (Partnership for a New Generation of Vehicles,15kW/kg),thus making them great potential in hybrid vehicle systems as power-supply components [10,39].Normally,the energy and power limitations observed at high rates are believed due to complexly distributed resistance and the tortuous diffusion pathways within the porous textures [33].In this study,it is noteworthy that the NiO@MnOOH core/shell nanosheet hierarchies on Ni foam current collector substrate that we fabricated have a porous core and a fluffy shell,which enhances ion diffusion in outer and inner supercapacitive materials and increases the electrolyte-material contact area,as unambiguously proofed in our research above.In summary,this study demonstrated a facile,high-ef ficiency,and potentially scalable technique for fabricating 3D NiO@MnOOH core/shell hierarchies on Ni foam substrate for pseudocapacitor electrodes.As-fabricated NiO@MnOOH core/shell nanosheet hierarchies exhibited outstanding electrochemical performances such as a high capacitance (1625.3F/g at 4A/g)with a remarkable rate capability and a good cycling stability (105.7%after 5000cycles);more importantly,a high energy density 80.0Wh/kg and a high power density 17.1kW/kg of this material were also achieved.It suggests the NiO@MnOOH core/shell nanosheet hierarchical nanostructures have a promising potential for the next generation energy conversion –storage devices.AcknowledgmentsThis work was financially supported by the National Natural Science Foundation of China (Grant nos.21171035and 51302035),the Key Grant Project of Chinese Ministry of Education (Grant no.313015),the PhD Programs Foundation of the Ministry of Education of China (Grant nos.20110075110008and 20130075120001),the National 863Program of China (Grant no.2013AA031903),the Science and Technology Commission of Shanghai Municipality (Grant no.13ZR1451200),the Fundamental Research Funds for the Central Universities,the Hong Kong Scholars Program,the Program Innovative Research Team in University (No.IRT1221),the Shanghai Leading Academic Discipline Project (Grant no.B603),and the Program of Introducing Talents of Discipline to Universities (No.111-2-04).Appendix A.Supplementary materialSupplementary data associated with this article can be found in the online version at /10.1016/j.nanoen.2013.12.006.References[1]S.Xuan,Y .X.J.Wang,J.C.Yu,K.C.Leung,Langmuir 25(2009)11835–11843.[2]J.P .Liu,J.Jiang,C.W.Cheng,H.X.Li,J.X.Zhang,H.Gong,H.J.Fan,Adv .Mater .18(2011)2076–2081.Figure 5(a)Schematic of the charge-storage advantage of the NiO@MnOOH core/shell nanosheet hierarchies,in which both the NiO core and MnOOH shell materials contribute to the charge-storage.(b)Cycle performance of the NiO@MnOOH core/shell nanosheet hierarchies and pristine hierarchical NiO nanosheet arrays during 5000cycles at 50mV/s.(c)The Ragone plots of the electrodes made of the NiO@MnOOH core/shell nanosheet hierarchies and pristine hierarchical NiO nanosheet arrays.J.Sun et al.62。