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Nano Energy (2014) 4, 56–64

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nCorresponding auE-mail addresses

hu.junqing@dhu.edu1These authors co

RAPID COMMUNICATION

3D core/shell hierarchies of MnOOH ultrathinnanosheets grown on NiO nanosheet arraysfor high-performance supercapacitors

Jianqing Suna,c,1, Wenyao Lia,b,1, Bingjie Zhanga, Gao Lia,Lin Jianga, Zhigang Chena, Rujia Zoua,d,n, Junqing Hua,n

aState Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of MaterialsScience and Engineering, Donghua University, Shanghai 201620, PR ChinabSchool of material engineering, Shanghai university of engineering science, Shanghai 201620, ChinacKey Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy ofSciences, 345 Ling-Ling Road, Shanghai 200032, ChinadCenter of Super-Diamond and Advanced Films (COSDAF), Department of Physics and Materials Science,City University of Hong Kong, Hong Kong, China

Received 30 September 2013; received in revised form 3 December 2013; accepted 7 December 2013Available online 30 December 2013

KEYWORDSNiO;MnOOH;Core/shell hierar-chies;Cycling stability;Energy/powerdensity;Supercapacitor

ont matter & 20140.1016/j.nanoen.2

thor.: rjzou@dhu.edu.c.cn (J. Hu).ntributed equally

AbstractAs 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 synthesized 3D 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-fabricated 3D NiO@MnOOH core/shell nanosheet hierarchies exhibited favorableelectrochemical performances, i.e., high specific capacitance of 1625.3 F/g at a current density of4 A/g with a remarkable rate capability and excellent energy density (80.0 Wh/kg), as well as goodcycling stability (105.7% of the initial capacitance after 5000 cycles). It suggests that they shouldhave a promising potential for the next generation energy conversion–storage devices.& 2014 Published by Elsevier Ltd.

Published by Elsevier Ltd.013.12.006

n (R. Zou),

to the work.

573D core/shell hierarchies of MnOOH ultrathin nanosheets grown on NiO nanosheet arrays for high-performancesupercapacitors

Introduction

As by utilizing and optimizing the advantages of differentcomponents, core/shell configuration heterostructurednanomaterials can offer remarkable synergetic propertiesand multifunctionalities [1–3], they have been of greatscientific and technologic interests, according to their highversatility and applicability in nanotechnology includingelectronics [4], energy conversion–storage devices [5],biomedical science [6], catalysis [7] etc. For supercapaci-tors, the most promising candidates for high-capacitanceelectrochemical energy storage devices, such an expectedcore/shell hybrid configuration will enhance the electro-chemical performance comparing with a single componentconstituent, and thus has recently drawn our research.Considerable efforts have been triggered to the synthesisand supercapacitor properties of the core/shell nanomater-ials from some transition metal oxide based systems, whilea well-defined micro-/nano-structure for a higher capaci-tance required by practical energy storage applications isstill a challenge. In addition, even a few of successfulstrategies such as oxidation [8], chemical templates method[9] and pulsed laser deposition [10] have been presented forthe synthesis of transition metal oxide or hydroxide core/shell architectures, a facile and high-efficiency syntheticmethod is still greatly needed for the supercapacitorapplications.

Transition metal (Ru, Ni, Co, Mn, Fe, etc.) oxides andhydroxides are excellent candidates for design of highcycling stability and high energy/power density supercapa-citor electrodes [11–13], due to multi-oxidation states forcharge transfer and reversible adsorption. RuO2 once wasanticipated as an ideal supercapacitor electrode materialwith a capacitance as high as 1300 F/g [14], however, theexpensive 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 havebeen revealed, the involved redox reactions are typically ata low charge/discharge rate, and only part of active super-capacitive materials contributed to better rate and cyclingperformance, leading to a weak energy/power density. Tomeet the requirement of high capacitance, cycling stabilityand energy/power density, one promising route is to choosehybrid supercapacitive materials and rationally design inte-grated smart configuration. Within the configuration, struc-tural features and electroactivities of individual componentare fully manifested, the interface/chemical distribut-ions are homogeneous at a nanoscale and a fast ion andelectron transfer is guaranteed, that is, a synergistic effectis realized. For a supercapacitor, core/shell hybrid hetero-structured nanomaterials have been confirmed to be uniqueas facilitating the electroactivities of each component, iondiffusion and electron transfer, therefore electrochemicalperformance is dramatically promoted [20]. Particularly,ordered nanostructures grown directly on current collectorsare very helpful to electron transport and microstrain [21],when combined with highly porous or hollow morphologies[22,23], the 3D core/shell heterostructures represent anattractive configuration for fast reaction kinetics, due tothe significantly enhancing of surface area.

Herein, we present a 3D 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 of 1625.3 F/g at a current density of 4 A/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 after 5000 cycles); more importantly, ahigh energy density of 80.0 Wh/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].

Experimental

All reagents were of analytical grade and used withoutfurther purification.

Preparation of NiO hierarchies

Typically, 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 NH3 �H2O, then the as-cleaned Ni foam was vertically immersed into a 50 mL(voluminal) beaker which contained the mixed solution at65 1C for 1.5 h, resulting a light-blue material formed on theNi foam. After being rinsed and dried in a vacuum oven at60 1C for 2 h, the material was annealed at 450 1C for 1 hunder N2 flow, resulting a black material which was char-acterized to be NiO hierarchies.

Preparation of NiO@MnOOH core/shell hierarchies

MnOOH nanosheets were grown onto the surface of porousNiO hierarchies by a facial potentiostatic electrochemicaldeposition process. In detail, MnOOH nanosheets weredeposited at 0.9 V (vs. SCE) in a solution contain 0.02 Mmanganese ammonium (MnAc2) and 0.02 M ammonium acetate(NH4Ac) with 10% DMSO for 10 min at 725 1C, changing thedeposition time can get different mass contents of MnOOH.After deposition, the as-prepared NiO@MnOOH electrodematerial was rinsed with deionized water and absoluteethanol several times and then placed in a vacuum ovenat 60 1C for 2 h.

Structure characterization

As-prepared products were characterized with a D/Max-2550 PCX-ray diffractometer (XRD; Rigaku, Cu-Kα radiation), a micro-Raman spectroscopy (Raman; Renishaw, inVia-Reflex), a X-rayphotoelectron spectroscopy (XPS, ESCALab250), a scanningelectron microscopy (SEM; HITACHI, S-4800) and a transmissionelectron microscopy (TEM; JEOL, JEM-2100F) equipped with anenergy-dispersive X-ray spectrometer (EDX).

J. Sun et al.58

Electrochemical characterization

Electrochemical measurements were performed on an Auto-lab Electrochemical Workstation (PGSTAT302N) using a threeelectrode electrochemical cell and 1 M LiOH as the electro-lyte at room temperature. The Ni-foam-supported hierarch-ical heterostructures (�1 cm2 area) acted directly as theworking electrode. A Pt plate and Ag/AgCl were used asthe 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 XSanalytical balance (Mettler Toledo; δ=0.01 mg), and themass of MnOOH was calculated by the Faraday0s law.

Results and discussion

The SEM image in Figure 1a shows the aligned and cross-linked hierarchical NiO nanosheet arrays. In fact, a hier-archy is self-assembled by many aligned thin NiO nanosheetson a major NiO nanoflake, forming a multi-hierarchy withmany individual irregular interspaces, as shown in high-magnification SEM imaging, Figure 1b. The composed majornanoflake has a thickness of 50–100 nm, while the alignedthin nanosheets are as thin as 10–15 nm. Amazingly, thesethin NiO nanosheets are full of numerous pores with a size of10–20 nm, as demonstrated in Figure 1b and c (marked withblue arrows). So, it is believed that these porous NiOhierarchies will have a high surface area and considerablylarge interspaces, which is beneficial for electrolyte pene-trating and electron transferring. High-resolution TEM(HRTEM) image in Figure 1d shows the NiO nanosheet is a

Figure 1 (a and b) SEM images of as-grown hierarchical NiO nanosha lower-right inset in (d) is the corresponding FFT diffraction patte

single crystal in nature; in this image, the lattice fringesgive an interplanar spacing of 0.24 nm and 0.15 nm, corre-sponding to {�111} and {02�2} lattice planes of the NiOcrystal, and the corresponding fast Fourier transform (FFT)diffraction pattern can be indexed to [211] zone axis of theNiO crystal.

As is well known, NiO is an excellent supercapacitorelectrode material with remarkably high capacitance [26],however, the poor cycling stability and feeble energy/powerdensity block its further application. To overcome theseweakness and far more improve the electrochemical perfor-mance, we now consider designing a core/shell configurationby hybridizing NiO with an additional pseudocapacitorelectrode material, e.g., metal hydroxide. Here, as-grownaligned and cross-linked hierarchical NiO nanosheet arrayscan serve as a robust scaffold for the loading of thisadditional supercapacitive material. To achieve synergy,the newly involved material would be required to fulfillthe following requirements: to contribute effective specificcapacitance, to enlarge the surface area but not to preventcontacting between the hierarchical NiO nanosheet arraysand ions in the electrolyte, as well as to maintain thestructural integrity [2,21]. For these aims, in our study, theelectrodeposition is demonstrated to be a convenient andcontrollable method to grow another electrode materialonto the surface of NiO nanosheets (Figure S1, see ESI†). Ina typical process, the hybrid hierarchical configuration wasobtained by electrodeposit assist growth of the ultrathinMnOOH nanosheets on the hierarchical NiO nanosheetarrays, receiving a mass ratio of 15.4% (MnOOH/NiO).Obviously, the mass ratio of as-electrodeposited MnOOHnanosheets to the NiO hierarchies is low, however, theMnOOH nanosheets densely cover the surface of NiO hier-archies, the volume ratio can reach�100% (MnOOH/NiO)

eet arrays. (c and d) TEM and HRTEM images of a NiO nanosheet,rn.

Figure 2 (a and b) SEM images of the NiO@MnOOH core/shell nanosheet hierarchies. (c and d) TEM and HRTEM images of theMnOOH 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 arrays for high-performancesupercapacitors

according to Figure 2a, hence the hybrid hierarchies lookinglike a spongy morphology. As seen from Figure 2b, as-grownMnOOH nanosheets are very thin, showing a thickness of 3–5 nm, so the NiO hierarchies are effectively and completelycovered by these fine MnOOH nanosheets, leading to a largevolume of interspaces inside these structures. It is believedthat such spongy morphology can facilitate ions penetratinginto the core region through the fine MnOOH nanosheets torealize the release of potential electrochemical propertiesof porous hierarchical NiO nanosheet arrays. Figure 2cshows the MnOOH nanosheets stripped from as-fabricatedNiO@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 electronpattern are interpreted as (002), (311), (312), (402) and(006) reflections, respectively, of the MnOOH crystals.Shown in Figure 2d is a HRTEM image of a MnOOHnanosheet, in which an interplanar spacing from the latticefringes is measured to be 0.46 nm and 0.46 nm, correspond-ing to {020} and {200} lattice planes of the MnOOH crystal,and a lower-right inset of the corresponding FFT pattern canbe 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/shellnanosheet hierarchies on the Ni foam substrate are shown inFigure 3a, respectively. It shows that the hybrid productsare composed of three crystalline phases, i.e., cubic Ni(JCPDS: 65-2865; a=3.524 Å), cubic NiO (JCPDS: 47-1049;a=4.177 Å) and tetragonal MnOOH (JCPDS: 18-0804;a=b=8.6 Å and c=9.3 Å). No characteristic peaks peculiarto impurities of other crystalline phases, such as Ni(OH)2and MnO2 are observed, which indicates the level ofimpurities in the sample is lower than the resolution limitof XRD (�5 at%). X-ray photoelectron spectroscopy (XPS)

analysis was taken to get more composition information ofthe hybrid hierarchies. A survey scan spectrum shows thepresence of Ni, Mn and O of as-prepared NiO@MnOOH core/shell nanosheet hierarchies, Figure S2 (see ESI†). Figure 3bof Ni2p region shows two major peaks with binding energiesat 855.5 eV and 873.3 eV, corresponding to Ni2p3/2 andNi2p1/2 spin-orbit peaks, respectively, of NiO phase [24].Figure 3c of Mn2p region demonstrates two peaks at thebinding energies of 642.7 eV and 654.2 eV originated fromMn2p3/2 and Mn2p1/2 spin-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 thespectrum which is corresponding to the oxide, hydroxide andwater peaks (red, green and blue peaks), respectively, itstrongly demonstrates the existence of MnOOH by excludingthe contribution of NiO material [25]. The local energydispersive X-ray spectroscopy (EDX) and Raman spectrumanalysis are also conducted (Figures S3 and S4, see ESI†),which provide further evidence for the configuration of theNiO@MnOOH core/shell nanosheet hierarchies.

As the fabricated 3D NiO@MnOOH core/shell nanosheetheterostructures are composed of porous hierarchical NiOnanosheet arrays and spongy MnOOH ultrathin nanosheets,the transportation of electrolytes through their nanopores aswell as the interspaces is possibly more feasible for efficientredox reactions and more quickly for charge transport duringFaradaic charge-storage process. However, the charge-storageefficiency of the materials varies due to the differences inmorphologies and microstructures. In order to identify whethersuch architectures are favorable for high-rate capacitive energystorage, the specific capacitance values of the obtainedelectrode material should be determined by cyclic voltammetry(CV) and galvanostatic charge–discharge (CD) measurements,which are useful for examining electrochemical performance.

Figure 3 (a) XRD patterns of hierarchical NiO nanosheet arrays and NiO@MnOOH core/shell nanosheet hierarchies on Ni foamsubstrate. (b–d) XPS spectra of NiO@MnOOH core/shell nanosheet hierarchies: Ni2p, Mn2p and O1s spectrum, respectively.

J. Sun et al.60

Electrochemical measurements were performed on anAutolab (PGSTAT302N) potentiostat with a three-electrodeconfiguration system in a 1 M LiOH at room temperature.Considering the Ni foam could influence the final electro-chemical performance with a small capacitance contribu-tion, we compared it with NiO@MnOOH core/shellnanosheet hierarchies (Figure S5, see ESI†) and found thatthe specific capacitance of Ni foam was very small (1.5 F/g).In order to reasonably illustrate the contribution of theactive material, the mass of Ni foam was subtracted fromNiO@MnOOH electrode neglecting the small contribution ofthe Ni foam. The variation of the capacitance with theelectrodeposition 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 thatthe highest capacitance corresponds to 2 min deposition of theMnOOH nanosheets. This illustrates the convenience of theelectrodeposition in tailoring electrode surface and obtain-ing high capacitance. For comparison, electrochemicalproperties of as-grown pristine hierarchical NiO nanosheetarrays also have been studied (Figure S6a, ESI†). Figure 4ashows the cyclic voltammetry (CV) curves of the NiO@M-nOOH core/shell nanosheet hierarchies on Ni foam atdifferent scan rates, i.e., 2, 5, 10, 20, 50 and 80 mV/s.It can be seen that the redox peaks of the NiO@MnOOHcore/shell nanosheet hierarchies have a decided change asthe increase of the scan rate. Figure 4b is the CV curves ofpristine hierarchical NiO nanosheet arrays and NiO@MnOOHcore/shell nanosheet hierarchies at 50 mV/s. For the hier-archical NiO nanosheet arrays, the CV curve exhibits arelatively symmetrical shape corresponding to the faradicoxidation/reduction reactions of NiO+OH-2NiOOH+e-

[11,26]. However, the CV curve demonstrates a significantdifference with NiO@MnOOH core/shell nanosheet hierar-chies which increases the surface roughness of the compo-site material as well as offering another transition metaloxide after adding MnOOH [47]. Compared with the pristinehierarchical NiO nanosheet arrays, the obvious enlargedenclosed area of CV curves of NiO@MnOOH core/shellnanosheet hierarchies indicates the increase for new redoxwaves of MnOOH+OH�2MnO2+H2O+e� and MnO2+Li+ +e�2MnOOLi [27,28,48]. That is to say, the transitionmetal oxide (NiO) and hydroxide (MnOOH) are hybridizedtogether harmoniously, and the electroactivities of twoindividual components are sufficiently manifested.

The electrochemical performances of the NiO@MnOOHcore/shell hierarchies are further demonstrated by galvano-static charge–discharge (CD) testing. And the potential usedto calculate to the specific capacitance of the electrodeaccording to the charge–discharge curves is the maximumpotential. CD curves of the NiO@MnOOH core/shellnanosheet hierarchies were recorded at various currentdensities, as shown in Figure S8. At the current density of4 A/g, a comparison of the CD curves from the NiO@MnOOHcore/shell nanosheet hierarchies and pristine hierarchicalNiO nanosheet arrays is given in Figure 4c. It is interesting tofind that the charge time of the hybrid core/shell hierar-chies is almost equal to that of the pristine, correspondingto a relatively similar charge transfer resistance, (EISspectra in Figure S9, see ESI†). And because of theparticipation of MnOOH material, the core/shell hybridconfiguration exhibiting a longer discharge time revealsmuch better electrochemical performance when comparedwith the pristine hierarchical NiO nanosheet arrays. As

Figure 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 50 mV/s. (c) Galvanostatic charge–discharge (CD)curves of NiO@MnOOH core/shell nanosheet hierarchies and pristine hierarchical NiO nanosheet arrays at 4 A/g. (d) A comparison ofcapacitances for NiO@MnOOH core/shell nanosheet hierarchies and pristine hierarchical NiO nanosheet arrays as a function of thecurrent density.

613D core/shell hierarchies of MnOOH ultrathin nanosheets grown on NiO nanosheet arrays for high-performancesupercapacitors

shown in Figure 4d, the NiO@MnOOH core/shell nanosheethierarchies have a specific capacitance of 1890.5, 1625.3,1568.0, 1312.0 and 1223.3 F/g at the current density of 1.7,4, 8, 16, and 20 A/g, respectively, much higher than thepristine hierarchical NiO nanosheet arrays (1881.3, 1390.0,965.3, 472.0 and 313.4 F/g at 2, 4, 8, 16, and 20 A/g,respectively) (their area capacitances shown in Figure S10,see ESI†), in particular, it also among the highest value ofthe reported single or hybrid transition metal oxides andhydroxides electrode materials [9,26,29,30,40]. Moreover,such high capacitance of the present core/shell nanosheethierarchies (1890.5 F/g) is maintained remarkably up to64.7% of that measured at 20 A/g, still much better thanthe pristine NiO hierarchies (16.6%) and also better than theother reported values [41,42,43]. We further studied theelectrochemical performances of NiO@MnO2 nanosheethierarchies by similar fabrication method for a comparisonand poor electrochemical performance was detected(Figure S6a and Figure S11, See ESI†). These results implythat the additional component of MnOOH nanosheets withinthese hybrid heterostructures not only didn’t prevent thecapacitance releasing of NiO material, but also providedmore electrochemical redox reactions and enlarged thesurface area to boost the energy conversion–storage andretention capability.

This advanced configuration for the NiO@MnOOH core/shell nanosheet hierarchies is schematically illustrated inFigure 5a, which is quite different from the capacitanceenhancement by normal core/shell nanostructure arrays inprevious reports, such as Ni/NiO inverse opal nanostructure[31], and Ni@MnO2 opal nanostructure [44] in which only the

shell material was high-active material while the corematerial mainly served as the current collector. The long-term cycling stability of the as-fabricated NiO@MnOOHcore/shell nanosheet hierarchies and pristine hierarchicalNiO nanosheet arrays were examined by their CV cycles at ascan rate of 50 mV/s, as shown in Figure 5b. The capaci-tance of the core/shell nanosheet hierarchies graduallyincreases to 105.7% with a slight fluctuation from 0 to3500 cycles and keeps almost constant after that, whichreveals the NiO@MnOOH core/shell nanosheet hierarchiesfor electrode have a good long-term cyclic performance.While the hierarchical NiO nanosheet arrays lost 22.4% ofthe initial capacitance over 5000 cycles with obviousfluctuations in 1500–3500 cycles. The fluctuations mayascribe to the influence of the electrolyte temperatureand the destruction/reconstruction of the NiO nanostruc-tural [45–46]. CV curves of the as-fabricated NiO@MnOOHcore/shell nanosheet hierarchies and NiO nanosheet hier-archies after 5000 cycles are presented in Figure S12 (seeESI†). It shows only oxidation peak of NiO hierarchies has aclear change, further proves the good cycling stability ofNiO@MnOOH core/shell nanosheet hierarchies compareswith the pristine hierarchical NiO nanosheet arrays.

Power and energy densities are two important para-meters for evaluating the electrochemical performance ofthe supercapacitors. The Ragone plots of the NiO@MnOOHcore/shell nanosheet hierarchies and hierarchical NiOnanosheet arrays are exhibited in Figure 5c, respectively.Here, a high energy density (80.0 Wh/kg) was obtained ofthe NiO@MnOOH core/shell nanosheet hierarchies, whichreveals a much better performance than most EDLCs

Figure 5 (a) Schematic of the charge-storage advantage of the NiO@MnOOH core/shell nanosheet hierarchies, in which both theNiO core and MnOOH shell materials contribute to the charge-storage. (b) Cycle performance of the NiO@MnOOH core/shellnanosheet hierarchies and pristine hierarchical NiO nanosheet arrays during 5000 cycles at 50 mV/s. (c) The Ragone plots of theelectrodes made of the NiO@MnOOH core/shell nanosheet hierarchies and pristine hierarchical NiO nanosheet arrays.

J. Sun et al.62

[32,34,35] and pseudocapacitors [36,37]. Furthermore, thepower density of the NiO@MnOOH core/shell nanosheethierarchies reaches 17.1 kW/kg at an energy density of24.1 Wh/kg, which is among the best reported literaturesso far [33–38]. More importantly, such high power densityvalue can meet the power demands of PNGV (Partnershipfor a New Generation of Vehicles, 15 kW/kg), thus makingthem great potential in hybrid vehicle systems as power-supply components [10,39]. Normally, the energy and powerlimitations observed at high rates are believed due tocomplexly distributed resistance and the tortuous diffusionpathways within the porous textures [33]. In this study, it isnoteworthy that the NiO@MnOOH core/shell nanosheethierarchies on Ni foam current collector substrate that wefabricated have a porous core and a fluffy shell, whichenhances ion diffusion in outer and inner supercapacitivematerials and increases the electrolyte-material contactarea, as unambiguously proofed in our research above.

In summary, this study demonstrated a facile, high-efficiency, and potentially scalable technique for fabricating3D NiO@MnOOH core/shell hierarchies on Ni foam substratefor pseudocapacitor electrodes. As-fabricated NiO@MnOOHcore/shell nanosheet hierarchies exhibited outstandingelectrochemical performances such as a high capacitance(1625.3 F/g at 4 A/g) with a remarkable rate capability anda good cycling stability (105.7% after 5000 cycles); moreimportantly, a high energy density 80.0 Wh/kg and a highpower density 17.1 kW/kg of this material were alsoachieved. It suggests the NiO@MnOOH core/shell nanosheethierarchical nanostructures have a promising potential forthe next generation energy conversion–storage devices.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (Grant nos. 21171035 and51302035), the Key Grant Project of Chinese Ministry ofEducation (Grant no. 313015), the PhD Programs Foundationof the Ministry of Education of China (Grant nos.20110075110008 and 20130075120001), the National 863Program of China (Grant no. 2013AA031903), the Scienceand Technology Commission of Shanghai Municipality (Grantno. 13ZR1451200), the Fundamental Research Funds for theCentral Universities, the Hong Kong Scholars Program, theProgram Innovative Research Team in University (No.IRT1221), the Shanghai Leading Academic Discipline Project(Grant no. B603), and the Program of Introducing Talents ofDiscipline to Universities (No. 111-2-04).

Appendix A. Supplementary material

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.nanoen.2013.12.006.

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Jianqing Sun received his B.S. degree(2011) and M.S. degree (2013) in Chemistryfrom Donghua University, PR China. Hejoined the Key Laboratory of OrganofluorineChemistry, Shanghai Institute of OrganicChemistry of the Chinese Academy ofSciences. His current research interestsfocus on design and synthesis of electrodefor energy conversion and storage devices.

Wenyao Li received his B.S. degree inInorganic Non-metallic Materials Engineer-ing from Jingdezhen Ceramic Institute(2009) and now is a Ph.D. candidate inMaterial Physics and Chemistry from Don-ghua University. His current research inter-ests focus on the preparation and propertiesof manganese-based metallic oxides andtheir composites for supercapacitors.

Bingjie Zhang joined the College of Materi-als Science and Engineering, Donghua Uni-versity as an undergraduate student in2010. Her research interests are designand synthesis of electrode for energy con-version and storage devices.

Gao Li received his B.S. degree in 2011 fromDonghua University, PR China. He joined thedepartment of State Key Laboratory forModification of Chemical Fibers and PolymerMaterials, College of Materials Science andEngineering, Donghua University in 2011 as aM.S. candidate. His research is synthesis oftransition metal oxides for supercapacitors.

J. Sun et al.64

Lin Jiang received his M.S. degree in 2013from Donghua University, Shanghai, PRChina. He joined the Department of OpticalThin Film, Shanghai Institute of TechnicalPhysics of the Chinese Academy of Sciences.His research interests are designing opticalthin-film for optical device

Zhigang Chen received his B.S. degree(2002) and M.S. degree (2005) in Chemistryfrom Central China Normal University, andcompleted his Ph.D. degree (2008) at FudanUniversity. He joined the faculty of DonghuaUniversity in Shanghai in 2008. Subse-quently, he was supported by Alexandervon Humboldt Foundation and worked as apostdoctoral fellow at the Max Planck Insti-tute for Colloids and Interfaces, Germany,

in 2008–2009. Now, Dr. Chen is an Associate Professor at DonghuaUniversity. His research interests include the synthesis of nanoma-terials and hybrid materials, design and construction of nanode-vices, and their applications in solar energy conversion andbiomedical applications.

Rujia Zou received his M.S. degree inplasma physics from Donghua University(2009) and his PhD in Material Science fromDonghua University (2012). At present, he isworking in Donghua University. He hasauthored and co-authored more than 40refereed journal publications and held over10 patents. His current research interestsare focusing on the properties of 1D inor-ganic nanomaterials (including nanotubes,

nanowires, and nanoribbons), including optical, (in-situ in TEM)electrical, mechanical property and their potential applications.

Prof. Junqing Hu received his Ph.D. fromthe University of Science & Technology ofChina, in 2000. From 2000 to 2008, heworked at the City University of Hong Kong,and then the National Institute for MaterialsScience, Tsukuba, Japan. At present, he is aFull Professor of Donghua University, China.He has authored and co-authored more than150 refereed journal publications and heldover 20 patents. His current research inter-

ests focus on synthesis, property measurements, and applications of1D nanostructures.