Journal ofMaterials Chemistry A

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State Key Laboratory for Modication of C

College of Materials Science and Engin

201620, China. E-mail: rjzou@dhu.edu.cn;

† Electronic supplementary informationand calculations. See DOI: 10.1039/c3ta14

Cite this: J. Mater. Chem. A, 2014, 2,4795

Received 12th November 2013Accepted 20th January 2014

DOI: 10.1039/c3ta14647b

www.rsc.org/MaterialsA

This journal is © The Royal Society of C

Hierarchical mesoporous NiCo2O4@MnO2 core–shell nanowire arrays on nickel foam for aqueousasymmetric supercapacitors†

Kaibing Xu, Wenyao Li, Qian Liu, Bo Li, Xijian Liu, Lei An, Zhigang Chen, Rujia Zou*and Junqing Hu*

We demonstrate the design and fabrication of hierarchical mesoporous NiCo2O4@MnO2 core–shell

nanowire arrays on nickel foam via a facile hydrothermal and electrodeposition process for

supercapacitor applications. In order to increase the energy density and voltage window, a high-voltage

asymmetric supercapacitor based on hierarchical mesoporous NiCo2O4@MnO2 core–shell nanowire

arrays on nickel foam as the positive electrode and activated carbon (AC) as the negative electrode was

successfully fabricated. The as-fabricated asymmetric supercapacitor device achieved a specific

capacitance of 112 F g�1 at a current density of 1 mA cm�2 with a stable operational voltage of 1.5 V and

a maximum energy density of 35 W h kg�1. The present NiCo2O4@MnO2 core–shell nanowire arrays

with remarkable electrochemical properties could be considered as potential electrode materials for next

generation supercapacitors in high energy density storage systems.

1. Introduction

Supercapacitors, a class of efficient energy storage devices, havedrawn signicant research attention in recent years because oftheir high power density, near-innite long cycling life, safeoperation, and environmental friendliness.1 Therefore, theycould have potential applications in portable electronics, hybridelectric vehicles and a number of microdevices.2 However, tomeet the energy demands for next-generation supercapacitorpractical applications, the energy density of supercapacitorsshould be improved without sacricing the power density andcycle life.3 According to the equation of energy density (E), E ¼0.5CDV2.4 An effective method can be used to improve theenergy density of supercapacitors by maximizing the speciccapacitance (C) and/or the cell voltage (DV). Though organicelectrolytes can be employed to improve the voltage window (uptoz3 V) according to the reports,5 their poor ionic conductivity,high cost and toxic nature limit large-scale commercial appli-cations.5b More recently, an effectively promising strategy toincrease the energy density and voltage window is to developasymmetric supercapacitors (ASCs) with a battery-type Faradaicelectrode (as the energy source) and a capacitor-type electrode(as the power source), which were based on higher ionicconductivities and more environmentally friendly aqueous

hemical Fibers and Polymer Materials,

eering, Donghua University, Shanghai

hu.junqing@dhu.edu.cn

(ESI) available: Supplementary gures647b

hemistry 2014

electrolytes than organic electrolytes.3a,6 ASCs can take advan-tage of different potential windows of the two electrodes toincrease the device maximum operating voltage in aqueouselectrolytes (up to 2.0 V), and thus signicantly improve theenergy density of the supercapacitors.4a,6,7 Recently, muchattention has been focused on improving the energy density byexploring various materials in ASCs, such as Ni(OH)2/gra-phene//porous graphene,3a Co9S8 nanorod//Co3O4@RuO2,5b

Ni(OH)2/CNT//AC,7b VOx//VN,8 RGO/MnO2//RGO paper9 andCoO@polypyrrole//AC.10

Among various electrode materials, spinel nickel cobaltite(NiCo2O4) has received tremendous interest recently in super-capacitor applications due to its low-cost, high availability andenvironmental friendliness.11 Most importantly, ternaryNiCo2O4 materials have higher electronic conductivity andelectrochemical activity than their binary nickel oxides or cobaltoxides.12 So far, there have been many reports on the synthesisand electrochemical properties of one dimensional (1D), twodimensional (2D) and three dimensional (3D) nano/micro-structures from NiCo2O4 materials.13 Jiang et al. reportedNiCo2O4 nanowires for supercapacitors with a specic capaci-tance of 760 F g�1 at 1 A g�1 with a potential window of 0–0.5 V.11a Lou et al. reported the ultrathin mesoporous NiCo2O4

nanosheets on nickel foam showing a specic capacitance of1450 F g�1 at 20 A g�1 with a small potential window (�0.1 to0.3 V).13a Most importantly, a higher conductivity NiCo2O4 canbe used as a backbone to support and provide effective electricalconnection to active electrode materials forming 3D hierar-chical hybrid nanostructures for high-performance super-capacitors, which can provide the synergistic effect of all

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individual constituents.14 For instance, Lou et al. reportedhierarchical NiCo2O4@MnO2 core–shell heterostructurednanowire arrays on nickel foam showing an areal speciccapacitance of 3.31 F cm�2 at 2 mA cm�2 with a potentialwindow of 0–0.6 V.14a Liu and coworkers synthesized hierar-chical NiCo2O4@NiCo2O4 core–shell nanoake arrays exhibit-ing high areal specic capacitances of 1.55 F cm�2 at 2 mA cm�2

in the voltage range of 0–0.55 V.14b More recently, Liu et al.synthesized 3D CoxNi1�xDHs/NiCo2O4/CFP hybrid compositeelectrodes and obtained a high areal capacitance of 2.17 F cm�2

at 10 mA cm�2 with a potential window of �0.1 to 0.45 V for x ¼0.5.14c Although excellent electrochemical properties have beenachieved, to the best of our knowledge, the small operatingvoltage window of the reported NiCo2O4-based electrode mate-rials is less than 0.6 V, which limits the energy density forsupercapacitor applications. Therefore, it is thus of greatinterest to the rational design and fabrication of NiCo2O4-basedelectrode materials with a large operating voltage window forsupercapacitor applications.

In this work, we demonstrated the design and synthesis ofhierarchical mesoporous NiCo2O4@MnO2 core–shell nanowirearrays on nickel foam via a facile hydrothermal route and elec-trodeposition process for supercapacitor applications. Asan electrode material for pseudocapacitors, the hybridNiCo2O4@MnO2 core–shell nanowire arrays showed a higherareal specic capacitance of 2.24 F cm�2 and a very long-termcycling stability (�113.6% capacitance retention aer 8000cycles). Moreover, an asymmetric supercapacitor based on hier-archical mesoporous NiCo2O4@MnO2 core–shell nanowire arrayson nickel foam as the positive electrode and activated carbon(AC) as the negative electrode was successfully fabricated. The as-fabricated ASC achieved a specic capacitance of 112 F g�1 at acurrent density of 1 mA cm�2 with a stable operational voltage of1.5 V, and a maximum energy density of 35 W h kg�1.

2. Experimental sectionSynthesis of mesoporous NiCo2O4 nanowires

All the reagents used were of analytical grade (purchased fromSinopharm) and used without further purication. MesoporousNiCo2O4 nanowires were synthesized through combining ahydrothermal reaction and a thermal annealing process. In atypical synthesis procedure, a piece of nickel foam was carefullycleaned with 3 M HCl solution in an ultrasound bath for 30 minto remove the NiO layer on the surface, and then washed withdeionized water and absolute ethanol several times.Ni(NO3)2$6H2O (0.1 mmol), Co(NO3)2$6H2O (0.2 mmol) andurea (0.9 g) were dissolved in deionized water (50 mL) undervigorous magnetic stirring. Aer stirring for 1 h, the as-obtainedsolution was transferred into a 60 mL polytetrauoroethylene(PTFE) (Teon)-lined autoclave, and then a piece of clean nickelfoam was immersed into the reaction solution. The autoclavewas sealed and maintained at 120 �C for 6 h in an electric oven.Aer being cooled to room temperature naturally, the productson the nickel foam were carefully washed with deionized waterand absolute ethanol with the assistance of ultrasonication, andthen dried at 60 �C overnight. Aerward, the samples were

4796 | J. Mater. Chem. A, 2014, 2, 4795–4802

calcined at 300 �C for 2 h at a ramping rate of 1 �C min�1 totransform into mesoporous NiCo2O4 nanowires. The massloading of the mesoporous NiCo2O4 nanowires on the nickelfoam was calculated to be around 1.2 mg cm�2 (for details, seethe ESI†).

Electrochemical deposition of MnO2 nanosheets

The nickel foam with mesoporous NiCo2O4 nanowires was usedas a working electrode for electrodeposition of MnO2 nano-sheets, which were deposited at 0.5 mA cm�2 (vs. SCE) in asolution containing 0.1 M manganese acetate (MnAc2) and0.02 M ammonium acetate (NH4Ac) for different times from 4 to20 min at room temperature. Aer depositing, the as-preparedNiCo2O4@MnO2 electrode materials were washed with deion-ized water and absolute ethanol and then dried at 60 �C over-night. Finally, the as-prepared NiCo2O4@MnO2 electrodematerials were annealed at 200 �C for 2 h in air.

Materials characterization

The as-synthesized products were characterized with a D/max-2550 PC X-ray diffractometer (XRD; Rigaku, Cu-Ka radiation), ascanning electron microscope (SEM; S-4800), a transmissionelectron microscope (TEM; JEM-2100F) equipped with anenergy dispersive X-ray spectrometer (EDX) and an X-rayphotoelectron spectrometer (XPS, PHI5000VersaProbe). Themass of electrode materials was weighed on a XS analyticalbalance (Mettler Toledo; d ¼ 0.01 mg).

Electrochemical measurement

Electrochemical performances were measured on an Autolab(PGSTAT302N potentiostat) using a three-electrode mode in a1 M NaOH solution. The reference electrode and counter elec-trode were SCE and platinum, respectively. The nickel foam-supported hybrid nanostructures of NiCo2O4@MnO2 electrodematerials or pristine NiCo2O4 electrode materials acted directlyas the working electrode.

Fabrication of an aqueous asymmetric supercapacitor

The NiCo2O4@MnO2 electrode (positive), the AC electrode(negative) and the lter paper separator were previouslysoaked in 1 M NaOH solution for about 24 h. Then theaqueous asymmetric supercapacitor was fabricated by a piece ofNiCo2O4@MnO2 electrode (positive) and AC electrode (negative)with a separator sandwiched in between, and 1 M NaOH solu-tion was used as the electrolyte. Aer that, the aqueous asym-metric supercapacitor was encapsulated by paralm to avoidleakage of the electrolyte. The specic capacitance, energy andpower densities of the as-prepared asymmetric supercapacitorwere all calculated based on the total mass of both the negativeand positive electrodes excluding the current collector.

3. Results and discussion

In our study, the fabrication process for hierarchical meso-porous NiCo2O4@MnO2 core–shell nanowire arrays on nickel

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foam as supercapacitor electrode materials is presented inFig. 1. Firstly, the commercial nickel foam was treated with HClsolution to remove the possible NiO layer on the surface. Aertreatment, no notable NiO was detected in the XRD pattern(Fig. S1†), and the nickel foam still retained its 3D porousstructure. Mesoporous NiCo2O4 nanowire arrays were grownvertically on the nickel foam through a simple hydrothermaland post annealing process. The 3D nickel foam was employedas the current collector due to its large and uniformmacroporesstructure, large supporting area, and high electrical conduc-tivity.10,15 Most importantly, the nickel foam with 3D structurecan provide efficient and rapid pathways for ion and electrontransport for electrode materials. The second is the controllableelectrodeposition of ultrathin MnO2 nanosheet coating on theNiCo2O4 nanowire arrays, because electrodeposition has beendemonstrated as an efficient and controllable method tosynthesize a range of nanomaterials including metals, metaloxides, and conducting polymers.2c,16

Scanning electron microscopy (SEM) and transmissionelectron microscopy (TEM) were employed to investigate themorphology and structure of the as-synthesized materials. Asseen in a low-magnication SEM image in Fig. 2a, the NiCo2O4

nanowires with a needle-like shape are uniformly grown on thenickel foam to form a new 3D nanostructure. The alignedNiCo2O4 nanowire has lengths of about 2–5 mm and diameters

Fig. 1 Schematic of the fabrication process for hierarchical meso-porous NiCo2O4@MnO2 core–shell nanowire arrays on nickel foam.

Fig. 2 (a and b) Low- and high-magnification SEM images of NiCo2O4

nanowire arrays. (c and d) Low- and high-magnification SEM images ofhierarchical NiCo2O4@MnO2 core–shell nanowire arrays.

This journal is © The Royal Society of Chemistry 2014

of�20–80 nm. A high-magnication SEM image, Fig. 2b, clearlyshows that individual NiCo2O4 nanowires consist of numeroussmall nanoparticles and a large quantity of mesoporous struc-tures, which is ascribed to the fact that the H2O and gases arereleased and lost during the intermediates' decomposition/oxidation through thermal annealing.17 An X-ray diffraction(XRD) pattern, Fig. S2,† reveals the overall crystal structure andphase purity of the products. All diffraction peaks can beindexed to the cubic phase NiCo2O4, in agreement with thereported values from the JCPDS card (no. 20-0781). Fig. 2cdisplays a low-magnication SEM image of hierarchicalNiCo2O4@MnO2 core–shell nanowires aer electrodeposition.It shows that the integration of MnO2 nanomaterial into themesoporous NiCo2O4 nanowire arrays does not deteriorate theordered structure. The high-magnication SEM image, Fig. 2d,shows that ultrathin MnO2 nanosheets cover uniformly thesurface of the mesoporous NiCo2O4 nanowires, and the MnO2

nanosheets are interconnected with each other, forming ahighly porous surface morphology. Moreover, within suchunique hierarchical core–shell nanowires, the open and freeinterspaces among these NiCo2O4 nanowire arrays can be effi-ciently utilized, which will make the electrolyte ions access thesurface of active materials more easily and improve the utili-zation rate of electrode materials.

The nanostructures of mesoporous NiCo2O4 nanowires andultrathin MnO2 nanosheets were further investigated by TEM.A low-magnication TEM image, Fig. 3a, shows that each as-grown NiCo2O4 nanowire is straight with a diameter of about20–80 nm, these nanowires are full of a lot of pores and thepore sizes are about several nanometers. Fig. 3b showsthe typical TEM image of two hierarchical mesoporousNiCo2O4@MnO2 core–shell nanowires. Each NiCo2O4 nanowireis uniformly covered by ultrathin MnO2 nanosheets, and thethickness of the outer MnO2 shell layer is about 20–40 nm. Ahigh-resolution TEM image (HRTEM), Fig. 3c, reveals that thed-spacing of 0.28 and 0.47 nm corresponds to the distance ofthe {220} and {111} planes, respectively, of the NiCo2O4 crystal.The corresponding fast Fourier transformation (FFT) pattern,Fig. 3d, can be indexed to the (220), (1�11) and (311) diffrac-tion planes of the cubic NiCo2O4 crystal. Moreover, energydispersive X-ray (EDX) spectrometry mapping, Fig. 3e, unam-biguously conrms the NiCo2O4@MnO2 core–shell hierar-chical structure. The EDX spectrum, Fig. S3,† furtherdemonstrated that the NiCo2O4@MnO2 core–shell nanowiresare mainly composed of Ni, Co, Mn and O. X-ray photoelectronspectroscopy (XPS) studies also determine the chemicalcomposition of the composites as shown in Fig. 3f. A surveyscan shows the presence of four elements (Ni, Co, Mn and O)within as-prepared hierarchical mesoporous NiCo2O4@MnO2

core–shell nanowires. Clearly, the C signal may be attributedto adventitious carbon. The Mn 2p3/2 peak is centered at642.1 eV and the Mn 2p1/2 peak is centered at 654.0 eV, with abinding energy separation of 11.9 eV, which is in good agree-ment with the previously reported peak binding energy sepa-ration observed in MnO2.18 Taken the above together, theNiCo2O4@MnO2 core–shell nanowires have been successfullysynthesized for their use as electrode materials.

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Fig. 4 (a) CV curves of the hierarchical mesoporous NiCo2O4@MnO2

core–shell nanowires. (b) A comparison of CV curves of the hierar-chical mesoporous NiCo2O4@MnO2 core–shell nanowires andNiCo2O4 nanowires at a scan rate of 20mV s�1. (c) A comparison of theCD curves of as-synthesized electrodematerials at a current density of2 mA cm�2. (d) Specific capacitances of as-synthesized electrodematerials at different current densities.

Fig. 3 (a and b) TEM images of mesoporous NiCo2O4 nanowires andhierarchical mesoporous NiCo2O4@MnO2 core–shell nanowires,respectively. (c and d) HRTEM images and the corresponding FFTpattern of NiCo2O4 nanowires, respectively. (e) EDX mapping of anindividual hierarchical mesoporous NiCo2O4@MnO2 core–shellnanowire. (f) XPS spectra of NiCo2O4@MnO2 core–shell nanowires,the inset is the Mn 2p region.

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The cyclic voltammogram (CV) tests of the as-synthesizedhierarchical mesoporous NiCo2O4@MnO2 core–shell nanowiresand NiCo2O4 nanowires were rstly carried out in a three-elec-trode conguration with a Pt plate counter electrode and a SCEreference electrode in 1 M NaOH aqueous electrolyte. The CVcurves, Fig. 4a, are recorded from the NiCo2O4@MnO2 core–shell nanowires at a scan rate of 5, 10, 20, and 50 mV s�1,respectively. A couple of redox peaks were observed within thepotential windows ranging from 0 to 0.6 V, and these peaksmainly resulted from redox reactions related to M–O/M–O–ON,where M represents Ni, Co or Mn ions and N represents H or Naions,11a,19 which indicates the pseudocapacitive nature of the as-prepared electrode materials. Interestingly, with increasingscan rate, the redox current increased. And also, the oxidationand reduction peaks shied toward higher and lower potential,respectively, with a large potential separation. It is consideredthat the specic capacitance of the electrode material can beestimated from the average area of the CV curve. Fig. 4b showsthe CV curves of NiCo2O4 and NiCo2O4@MnO2 electrodes at ascan rate of 20 mV s�1. Particular noteworthy is that the CVintegrated area of the NiCo2O4@MnO2 electrode materials isapparently larger than that of the pure NiCo2O4 electrodematerials, indicating that the NiCo2O4@MnO2 electrode mate-rials have a signicantly larger specic capacitance than unitary

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NiCo2O4 electrode materials. Such an improved electrochemicalperformance of the NiCo2O4@MnO2 electrode materials wasfurther conrmed by galvanostatic charge–discharge (CD)measurements performed at different current densities. Fig. 4cshows the comparison of CD curves for the NiCo2O4 andNiCo2O4@MnO2 electrodes at the same current density of 2 mAcm�1. As expected, the NiCo2O4@MnO2 electrode demonstratesmuch longer discharging time than the NiCo2O4 electrode. Itmeans that the NiCo2O4@MnO2 electrode material exhibitshigher specic capacitance values than the NiCo2O4 electrodematerial. It is worth noting that the nickel foam has almost nocapacitance contribution to the whole areal capacitance of twosuch electrode materials, which is discussed in the ESI(Fig. S4†). The areal capacitances of the NiCo2O4 andNiCo2O4@MnO2 electrode materials were calculated based ontheir corresponding CD curves (Fig. S5†), and typical results areshown in Fig. 4d. Correspondingly, the areal capacitance of theNiCo2O4@MnO2 electrode material with different electrodepo-sition times is examined and plotted in Fig. S6.† TheNiCo2O4@MnO2 core–shell nanowires aer 15 min electro-chemical deposition possess the highest areal capacitances upto 2.244, 1.974, 1.907, 1.83, 1.674, 1.556, 1.41, and 1.23 F cm�2

at 2, 5, 8, 10, 15, 20, 30 and 50 mA cm�2, respectively, while theNiCo2O4 nanowires only show an areal capacitance of 1.428 and0.35 F cm�2 at 2 and 50mA cm�2, respectively. To the best of ourknowledge, such good electrochemical performances of theNiCo2O4@MnO2 electrode material are much better thanpreviously reported values of hybrid nanostructures, forinstance, TiO2/NiO nanotube arrays (2.9 mF cm�2 at 0.4 mAcm�2),2b Co3O4@MnO2 core–shell nanowire arrays (0.56 F cm�2

at 11.25 mA cm�2),20a Ni–NiO core–shell inverse opal (7.8–9 mFcm�2),20b Co3O4@RuO2 nanosheet arrays (0.67 F cm�2 at 10 mAcm�2),5b WO3�x–MoO3�x core–shell nanowires (�216 mF cm�2

at 2 mA cm�2),20c and CNT/Ni hybrid network nanostructuredarrays (�0.901 F cm�2 at 0.69 mA cm�2).20d

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Cycling performance is a key factor in determining thesupercapacitors for many practical applications. In this study, along-term cycle stability of the as-synthesized products as anelectrode material was evaluated by repeating the CV test at ascan rate of 50 mV s�1 for 8000 cycles, Fig. 5. It can be clearlyseen that the specic capacitance retention of such two elec-trode materials gradually increases rst and then slightlydecreases. This indicates that there is an activation process ofthe electrodes at the beginning period of the CV cycling test.12a

During this process, the electrode will be completely activatedthrough the intercalation and de-intercalation of ions throughsome circulations, resulting in the increase of active pointsinside the electrode materials, hence enhancing the speciccapacitance. The overall specic capacitance retention for theNiCo2O4@MnO2 electrode material is �113.6% aer 8000cycles, while that of the NiCo2O4 electrode material is �99.1%aer 8000 cycles. The enhanced pseudocapacitive performancesof the NiCo2O4@MnO2 electrode material are mainly attributedto their uniquemesoporous core–shell structure which providesmore active sites for efficient electrolyte ion transportationnot only at the active materials surface but also throughoutthe bulk. The dramatic performance improvement of theNiCo2O4@MnO2 core–shell nanowires is also demonstrated byelectrochemical impedance spectroscopy (EIS) (Fig. S7†). Theequivalent series resistance (ESR) of the NiCo2O4@MnO2 core–shell nanowires (0.9 U) is much smaller than that of the bareNiCo2O4 nanowires electrode (1.5 U), indicating a lower diffu-sion resistance.17b

These excellent performances are contributed by the uniquedesign of the hierarchical mesoporous NiCo2O4@MnO2 core–shell nanowire arrays on nickel foam which includes thefollowing merits: (i) mesoporous NiCo2O4 nanowires with goodelectrical conductivity directly grown on nickel foam willprovide electron “superhighways” for charge storage anddelivery, which could overcome the limited conductivity ofMnO2 nanosheets themselves. (ii) The mesoporous structurescan act as an “ion reservoir” that can shorten the diffusiondistance from the external electrolyte to the interior surfaces

Fig. 5 Cycling performance of the as-synthesized electrode materialsfor 8000 cycles at a scan rate of 50 mV s�1.

This journal is © The Royal Society of Chemistry 2014

and thus minimize ion transport resistance, which will signi-cantly enhance the intercalation/deintercalation of ions andimprove the utilization rate of electrode materials even at highrates.4b The Brunauer–Emmett–Teller (BET) surface areas of theNiCo2O4 and NiCo2O4@MnO2 are 90.482 and 103.491 m2 g�1,respectively (Fig. S8†). (iii) Both NiCo2O4 and MnO2 are goodpseudocapacitive metal oxide materials, of which a material-combination will provide synergistic and multifunctionaleffects (as schematically illustrated in Fig. S9†). (iv) The hier-archical mesoporous NiCo2O4@MnO2 core–shell nanowirearrays directly grown on nickel foam could avoid “dead” volumecaused by the tedious process of mixing active materials withpolymer binder/conductive additives.

To further evaluate the NiCo2O4@MnO2 electrode for prac-tical applications, an asymmetric supercapacitor device wasfabricated by using the NiCo2O4@MnO2 electrode as the posi-tive and the AC on nickel foam as the negative with one piece oflter paper as the separator (denoted as NiCo2O4@MnO2//AC-ASC) (Fig. 6). In order to obtain the maximum performance ofthe NiCo2O4@MnO2//AC-ASC device, the charge of the positiveand negative electrode should be optimized based on testingresults of NiCo2O4@MnO2 and AC electrodes (for details, seethe ESI†). ASCs can take advantage of different potentialwindows of the two electrodes to increase the maximum oper-ating voltage of the device in the aqueous electrolyte (up to2.0 V). Symmetric supercapacitor applied voltage can splitequally between the two electrodes because of using the samematerial and having the same mass in each electrode. However,in ASCs, the voltage split depends on the capacitance of eachelectrode active material and ASCs can avoid the aqueouselectrolyte decomposition at 1 V which occurs in the symmetricsupercapacitors.4c Therefore, according to the comparative CVcurves of the NiCo2O4@MnO2 and AC electrodes at a scan rate

Fig. 6 Schematic illustration of the as-fabricated NiCo2O4@MnO2//AC-ASC device based on the hierarchical mesoporousNiCo2O4@MnO2 electrode (positive) and the AC electrode (negative)in 1 M NaOH electrolyte.

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of 20 mV s�1 (Fig. S10†), the AC electrode was measured withina stable potential window of �1.2 to 0.0 V, while that of theNiCo2O4@MnO2 electrode was measured from 0.0 to 0.6 V (bothvs. SEC), which indicates that such two electrode combinationsas asymmetric supercapacitors can afford a device with 1.5 Voperation voltage. The cell voltage is as large as 1.5 V, which ishigher than that of conventional AC-based symmetric super-capacitors in aqueous electrolytes (0.8–1.0 V). Accordingly,the potential window of 0–1.5 V was chosen for the overallelectrochemical performances of the NiCo2O4@MnO2//AC-ASCdevice.

Fig. 7a shows the CV curves of the NiCo2O4@MnO2//AC-ASCdevice at different voltage windows in 1 MNaOH electrolyte at ascan rate of 10 mV s�1. As expected, the stable electrochemicalwindows of the asymmetric supercapacitor device can beextended to 1.5 V. Both CV and CD tests were performedto evaluate the electrochemical performance of theNiCo2O4@MnO2//AC-ASC device at various scan rates andcurrent densities with a voltage window of 0–1.5 V (resultsshown in Fig. 7b and c). Fig. 7d shows calculations of thespecic capacitances of the asymmetric supercapacitor basedon the corresponding CD curves. The mass and areal speciccapacitance of the NiCo2O4@MnO2//AC-ASC device can achievethe maximum specic capacitance of 112 F g�1 (0.52 F cm�2) at1 mA cm�2, which is substantially higher than the values

Fig. 7 (a and b) CV curves of the NiCo2O4@MnO2//AC-ASC device atdifferent scan voltage windows and different scan rates, respectively.(c and d) CD curves and specific capacitances of the NiCo2O4@MnO2//AC-ASC device at different current densities, the inset is the digitalphotograph of the as-fabricated NiCo2O4@MnO2//AC-ASC device. (e)Cycle performance of the NiCo2O4@MnO2//AC-ASC device at a currentdensity of 18 mA cm�2, the inset shows the CD curves of the first10 cycles for the asymmetric supercapacitor. (f) EIS spectra of theNiCo2O4@MnO2//AC-ASC device before and after cycle tests.

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reported previously, such as hierarchical porous NiO//carbon(38 F g�1),21a NiO//activated carbon (73.4 F g�1)21b and NiO//rGO(50 F g�1).21c The inset in Fig. 7d is the digital photograph of theas-fabricated NiCo2O4@MnO2//AC-ASC device. A long-termcycle stability of the as-fabricated asymmetric supercapacitorwas evaluated by repeating the CD tests at a large currentdensity of 18 mA cm�2 for 5000 cycles. As shown in Fig. 7e, theoverall specic capacitance can still be retained at about �71%aer 5000 cycles, which is comparable to that of the porousNiO//carbon device (50% aer 1000 cycles),21a NiO//Ru0.35V0.65O2 (83.5% aer 1500 cycles),22a Ni(OH)2–MnO2//RGO(76% aer 3000 cycles)19b and AC//NiMoO4$xH2O (80.6% aer1000 cycles).22b The inset in Fig. 7e shows the CD curves of theas-fabricated NiCo2O4@MnO2//AC-ASC device for the rst 10cycles. It clearly demonstrates that the as-fabricated devicemaintains a good electrochemical reversibility with �99%Coulombic efficiency. Even aer a long period of the charge–discharge tests (>135 000 s), Fig. S11,† the CD curves remainundistorted and essentially symmetric. EIS spectra wererecorded in the frequency range of 100 kHz to 0.01 Hz forfurther understanding of the electrochemical behavior of theas-fabricated device. Fig. 7f shows the Nyquist impedancespectra of the asymmetric supercapacitors before and aer5000 cycles. Noticeably, the EIS spectra are similar in terms ofthe curve shape except that a small change of equivalent seriesresistance (ESR, the intercept on the real axis) in the highfrequency range. The magnitudes of ESR obtained for theasymmetric supercapacitors before and aer 5000 cycles are1.34 and 2.29 U, respectively, indicating that the as-fabricatedasymmetric supercapacitor devices have a relatively goodstability.

Power density and energy density are the two key parametersto characterize the performance of the electrochemical super-capacitors. Although it is a challenge to compare the perfor-mance of all types of supercapacitors due to differentmeasurement conditions, like potential window, charge–discharge rates, material mass loadings, a rough comparisonbetween the developed electrode and documented work canstill be made.5b Fig. 8 shows the Ragone plots of theNiCo2O4@MnO2//AC-ASC device based on CD tests in a voltagewindow of 0–1.5 V. Data of a few typical examples reported inthe literature to date were also provided for a rough compar-ison. It is worth noting that the maximum energy densityobtained for our asymmetric supercapacitor devices is 35 W hkg�1 at a power density of 163 W kg�1. Signicantly, theobtained energy density is higher than that of the symmetricalsupercapacitors such as CNTs//CNTs supercapacitors (<10 W hkg�1),23 graphene/MnO2//graphene/MnO2 supercapacitors (8.1W h kg�1),3a AC//AC supercapacitors (<10 W h kg�1),24 rGO//rGOsupercapacitors (<4 W h kg�1),25 and 3D graphene/MnO2//3Dgraphene/MnO2 (6.8 W h kg�1).26 Furthermore, we alsocompared the specic energy and power density of theNiCo2O4@MnO2//AC-ASC device to the previously reporteddevices with aqueous electrolytes, such as hierarchical porousNiO//carbon (�10 W h kg�1),21a graphene//MGC (30.4 W hkg�1),6 GHCS//GHCS–MnO2 (22.1 W h kg�1),27 GNCC//AC(19.5 W h kg�1)28 and NiCo2O4–rGO//AC (23.3 W h kg�1).29

This journal is © The Royal Society of Chemistry 2014

Fig. 8 Ragone plot of the NiCo2O4@MnO2//AC-ASC device. Thevalues reported for others devices are given here for a comparison.

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4. Conclusions

In summary, hybrid NiCo2O4@MnO2 core–shell nanowirearrays directly grown on nickel foam have been successfullydeveloped by a facile hydrothermal and electrodeposition route.An asymmetric supercapacitor device based on hierarchicalmesoporous NiCo2O4@MnO2 core–shell nanowire arrays as thepositive electrode and activated carbon (AC) as the negativeelectrode can achieve a specic capacitance of 112 F g�1 at acurrent density of 1 mA cm�2 with a stable operational voltageof 1.5 V and a maximum energy density of 35 W h kg�1. It isbelieved that the as-fabricated asymmetric supercapacitordevice has great potential applications in high energy densitystorage systems.

Acknowledgements

This work was nancially 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 Foundation ofthe Ministry of Education of China (Grant Nos. 20110075110008and 20130075120001), the National 863 Program of China(Grant No. 2013AA031903), the Science and TechnologyCommission of Shanghai Municipality (Grant No.13ZR1451200), the Fundamental Research Funds for theCentral Universities, Program for Changjiang Scholars andInnovative Research Team in University (Grant No. IRT1221),the Shanghai Leading Academic Discipline Project (Grant No.B603), and the Program of Introducing Talents of Discipline toUniversities (No. 111-2-04).

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