Ultrathin MnO2 nanoflakes deposited on carbon nanotube networks

Ultrathin MnO2nanoflakes deposited on carbon nanotube networks for symmetrical supercapacitors with enhanced performancePeng Sun,Huan Yi,Tianquan Peng,Yuting Jing,Ruijing Wang,Huanwen Wang, Xuefeng Wang*Shanghai Key Lab of Chemical Assessment and Sustainability,School of Chemical Science and Engineering,Tongji University,Shanghai200092,China h i g h l i g h t s g r a p h i c a l a b s t r a c tMnO2@CNTs/Ni mesh electrode ex-hibits a high specific capacitance.A symmetric supercapacitor(SSC) was assembled with enhanced performance.The SSC delivers a wide working voltage and superior energy density and powerdensity.a r t i c l e i n f oArticle history:Received16October2016 Received in revised form15November2016Accepted29November2016 Available online4December2016Keywords:Carbon nanotubesMnO2nanoflakesNi meshCore-shell structureSymmetric supercapacitor a b s t r a c tManganese dioxide is a promising electrode material for electrochemical supercapacitors,but its poor electronic conductivity(10À5~10À6S cmÀ1)limits the fast charge/discharge rate for practical applications. In the present work,we use the chemical vapor deposition(CVD)method to grow highly conductive carbon nanotube(CNT)networks onflexible Ni mesh,on which MnO2nanoflake layers are deposited by a simple solution method,forming a hierarchical core-shell structure.Under the optimized mass loading, the as-fabricated MnO2nanoflake@CNTs/Ni mesh electrode exhibits a high specific capacitance of 1072F gÀ1at1A gÀ1in three-electrode configuration.Due to advantageous features of these core-shell electrodes(e.g.,high conductivity,direct current path,structure stability),the as-assembled symmetric supercapacitor(SSC)based on MnO2@CNTs/Ni mesh has a wide working voltage(2.0V)in1M Na2SO4 aqueous electrolyte.Finally an impressive energy density of94.4Wh kgÀ1at1000W kgÀ1and a high power density of30.2kW kgÀ1at33.6Wh kgÀ1have been achieved for the as-assembled SSC,which exhibits a great potential as a low-cost,high energy density and attractive wearable energy storage device.©2016Elsevier B.V.All rights reserved.1.IntroductionSupercapacitors with superior power density,fast charge-discharging rate and long cycle life for energy storage devices have attracted intense research attention in recent years[1,2].It is noted that electrode materials play a key role towards the devel-opment of high performance supercapacitors in terms of the morphology,porosity,size,and electronic conductivity.Pseudoca-pacitive RuO2$xH2O has been studied extensively as an attractive electrode material for supercapacitors owing to its high energy density and large charge transfer-reaction pseudocapacitance which is based on fast and reversible redox reactions at the elec-trode surface[3,4];however,the high cost prevents its large-scale*Corresponding author.E-mail address:xfwang@(X.Wang).Contents lists available at ScienceDirect Journal of Power Sourcesjournal h omepage:www.elsevier.co m/lo cate/jp owsour/10.1016/j.jpowsour.2016.11.1120378-7753/©2016Elsevier B.V.All rights reserved.Journal of Power Sources341(2017)27e35applications.Among the pseudocapacitive electrode materials,MnO2is one of the most attractive candidate because of low cost,high natural abundance,and excellent theoretical specific capacitance (1370F gÀ1)[5].However,it is difficult to achieve the theoretical value in most experiments due to its low electronic conductivity (10À6to10À5S cmÀ1)[6],low ion diffusion constant and poor stability[5].For MnO2-based supercapacitors,several effective strategies have been carried out to address these issues.One way is to create various nanostructures,such as nanorods[7],nanowires [8],nanospheres[9],nanoflowers[10],nanotubes[11]and nano-flakes[12],which exhibit higher specific capacitance and rate/ cycling performance than their bulk counterparts.Another way is combining MnO2with highly conductive materials(especially for various carbons)to form a hybrid nanostructure[13e15].For instance,nanoporous gold was used as substrates to deposit nanocrystalline MnO2,resulting in a specific capacitance of the constituent MnO2(~1145F gÀ1)[6].Unfortunately,the high cost of Au has restricted its potential applications.Recently,to solve low energy density problem,asymmetric supercapacitors(ASCs)have been explored by combining MnO2as energy source and porous carbon as power source to increase the operation voltage.For example,an asymmetric electrochemical capacitor based on gra-phene as negative electrode and a MnO2nanowire/graphene composite as positive electrode was developed in neutral aqueous Na2SO4electrolyte,which exhibited a superior energy density of 30.4Wh kgÀ1in the high-voltage region of0e2.0V[8].Similar energy density has been obtained based on activated carbon nanofibers as negative electrode and MnO2/carbon nanofiber composites as positive electrode[16].In spite of these progresses, the energy density of most MnO2-based ASCs cannot exceed that of lead acid batteries(20e40Wh kgÀ1)and their power densities inevitably decrease due to the discrepancy in kinetics and specific capacities between the two electrodes[9,17,18].Therefore,for MnO2-based supercapacitor device it remains a huge challenge to increase specific capacitance and to realize high energy density with high power density at the same time.In this work,we report ultrathin MnO2nanoflakes deposited on carbon nanotube networks/Ni mesh with a well-designed core-shell nanostructure(MnO2@CNTs/Ni),which gives a high specific capacitance of1072F gÀ1at1A gÀ1in three-electrode system.In addition,a symmetric supercapacitor(SSC)has been assembled from two pieces of MnO2@CNTs/Ni electrodes using1M Na2SO4 solution as electrolyte,which exhibits wide working voltage of 2.0V and high power density of30.2kW kgÀ1at33.6Wh kgÀ1. Meanwhile,such a supercapacitor device shows very good stability and long cycle life.2.Experimental2.1.Synthesis of the CNTs/NiMnO2@CNTs/Ni was synthesized by a facile two-step method. The shiny andflexible Ni mesh(mesh number:200,1cmÂ1cm) wasfirstly immersed into1M H2SO4solution with sonication for 15min to remove the NiO oxidation layer on the surface.In thefirst step,carbon nanotubes(CNTs)were grown on Ni mesh(CNTs/Ni) via a chemical vapor deposition(CVD).The as-pretreated Ni mesh was soaked in30mL ethanol solution with0.1M polyethylene glycol(PEG)(C.P.,Sinopharm Chemical Reagent Co.,Ltd.)and0.1M Ni(NO3)2(A.R.,Sinopharm Chemical Reagent Co.,Ltd.)for3h and then dried in air.After that,the Ni mesh with catalyst was placed into a Al2O3tubular furnace.C2H2gas as the carbon source was pumped into the tube,which was heated up to550 C at a heating rate of5 C minÀ1and held at550 C for1h,and then cooled down naturally to room temperature.Generally,~0.3mg CNTs(average)is loaded to Ni substrate(1cmÂ1cm)after CVD process.2.2.Synthesis of the MnO2@CNTs/NiIn the second step,the as-synthesized CNTs/Ni was added into 10mL of5%(w/w)polyethylene glycol(PEG)aqueous solution with magnetic stirring for1h(ensure the adequate adsorption of PEG). Then10mL of0.05M KMnO4(A.R.,Sinopharm Chemical Reagent Co.,Ltd.)aqueous solution was added and heated at75 C in an oil bath with continuous magnetic stirring for2h.The mesh was taken out,washed repeatedly with deionized water after the solution was cooled down to room temperature.After that,the product was dried in a vacuum at60 C for12h to obtain the MnO2@CNTs/Ni composite.The mass loading of MnO2was confirmed by a highly sensitive balance with a precision down to±0.01mg,which was directly obtained by subtracting the substrate(CNTs/Ni)weight from the total weight of the substrate and the MnO2onto its sur-face.Different mass loadings of MnO2can be obtained by changing the amount of KMnO4.2.3.Materials characterizationThe morphology and microstructure of the products were characterized byfield emission scanning electron microscopy (FESEM;Hitachi S-4800)and transmission electron microscopy (TEM;JEOL,JEM-2100),X-ray diffraction(XRD;Bruker Focus D8 with Cu K a radiation),Raman spectroscopy(Renishaw In via, 514nm laser under ambient conditions)and BrunnerÀEmmetÀ-Teller(BET,NOVA2200e,Quanta-chrome,America).2.4.Electrochemical measurementThe electrochemical measurements including cyclic voltam-metry(CV)and galvanostatic charge-discharge(GCD)were con-ducted in an electrochemical workstation(CHI660D,Chenhua, Shanghai,China)at room temperature.In the three-electrode sys-tem,the MnO2@CNTs/Ni was directly used as working electrode, while platinum wire and a saturated calomel electrode(SCE)were used as counter and reference electrodes,respectively,in1M Na2SO4electrolyte.For comparison,CNTs/Ni synthesized by CVD method was tested with CV and GCD in1M Na2SO4electrolyte without any treatment.Further to explore the advantages of this material for real ap-plications,a symmetric supercapacitor was assembled from two pieces of MnO2@CNTs/Ni using1M Na2SO4solution as electrolyte, which were separated by a commonfilter paper.The specific capacitances can be calculated from galvanostatic tests by the equation[19]:C¼I D t=m D V(1) where I is the discharging current,t is the discharge time,D V is the potential window,and m is the mass of active material in the working electrode for three-electrode system(For symmetric cell system,m is the total mass of the active electrode materials in two electrodes).Energy density(E)and power density(P)can be calculated from galvanostatic tests by the following equations[19]:E¼hCðD VÞ2i.2(2) P¼E=D t(3)P.Sun et al./Journal of Power Sources341(2017)27e35 28where E ,C ,D V ,P and D t are the speci fic energy,speci fic capacitance,working voltage,speci fic power and discharge time,respectively.3.Results and discussion3.1.Morphological and structural characterizationsThe schematic diagram (Fig.1)exhibits the preparation process of the MnO 2nano flake@CNTs/Ni mesh,which involves two major steps.In the first step,CNT networks are grown on highly conductive and flexible Ni mesh in a CVD tube furnace using Ni(NO 3)2and C 2H 2as catalyst and carbon source,respectively.In the second step,MnO 2nano flakes are coated on the network of CNTs based on the spontaneous redox reaction given in equation (4)[20],eventually to form a MnO 2nano flake@CNT core-shell structure on Ni mesh.4MnO 4Àþ3C þH 2O ¼4MnO 2þCO 32Àþ2HCO 3À(4)In such a hybrid structure,MnO 2nano flakes overspread on CNTsand connect with each other on Ni mesh,forming a highly conductive three-dimensional (3D)network.In the process,PEG is used as an amphiphile for promoting the reduction reaction of KMnO 4under a rather mild condition (75 C,1atm)in comparison with other reported methods [21,22].Fig.2shows low-and high magni fication FESEM and TEM im-ages of the as-synthesized CNTs/Ni mesh.It was observed that CNTs with high density are grown uniformly on Ni mesh,forming a 3D hierarchical structure (Fig.2A,B).The CNTs/Ni keeps the ordered two dimensional woven structure of the Ni mesh substrate.Meanwhile,CNTs/Ni can be readily rolled up,which is appropriate for flexible device applications (Fig.S1in Supporting Information).Higher-magni fication FESEM images (Fig.2B)provide clearer in-formation about the Ni mesh growing CNT nanotubes.It can be seen that every CNTs/Ni fiber has the uniform diameter of approximately 63m m,which is larger than that of pristine Ni fiber diameter (ca.45m m)(Fig.S2in Supporting Information).These nanotubes are interconnected to form a 3D networks andtheFig.1.The fabrication steps of the MnO 2@CNTs/Ni.Fig.2.(A,B,C)FESEM and (D)TEM images of the CNTs grown on Ni mesh.P.Sun et al./Journal of Power Sources 341(2017)27e 3529average diameter of CNTs is~20nm(Fig.2C).The tubular structure can be further confirmed from the TEM image(Fig.2D).The FESEM and TEM images of as-synthesized MnO2@CNTs on Ni mesh are shown in Fig.3.In the low magnification FESEM image (Fig.3A)the MnO2@CNTs/Ni keeps two dimensional structure uniformly on Ni mesh substrate.With high magnification(Fig.3B) the cross-linked MnO2nanoflakes and CNTs with a hierarchical core-shell structure was clearly observed on the surface of CNTs, which could provide abundant space for electrolyte ions.In such a special structure,MnO2nanoflakes on CNTs can increase the effective contact area and supply fast paths for the insertion and extraction of electrolyte ions,which are beneficial to the Faraday reaction between MnO2and electrolyte ions[9,10,32].Fig.3C and D are the TEM images of MnO2@CNTs peeled off from Ni mesh.After coated with MnO2nanoflakes,CNTs could not be distinguished under TEM observation.The thickness of the MnO2layer is approximately40e50nm calculated by the diameter difference between MnO2@CNTs/Ni and CNTs/Ni(Figs.3C,2D).These MnO2 nanoflakes are generally less than4nm in thickness(Fig.3D),and the ultrathin MnO2nanoflakes are advantageous to the insertion and extraction of electrolyte ions without any obstruction.Fig.3E shows the selected-area electron diffraction(SAED)pattern of MnO2@CNTs/Ni(0.5mg cmÀ2mass loading of MnO2),indicating that the coated MnO2is amorphous.The elemental spatial distri-butions of MnO2@CNTs/Ni(0.5mg cmÀ2mass loading)were characterized by energy-dispersive spectroscopy(EDS)of individ-ual elements Mn,O,C and Ni,as shown in Fig.3F.Raman spectrum and XRD pattern are shown in Fig.4.Two most intense peaks D(disordered)and G(graphite-like)are obtained at 1360cmÀ1and1600cmÀ1(Fig.4A),which can be attributed to the characteristic peaks of CNTs[23].The G mode is assigned to the E2g phonon of C sp2atoms,and the D modes are caused by the Raman double resonant scattering from nonzero-center phonon modes which are originated from amorphous disorder and defects within the carbon lattice[24,25].The intensity ratio of the D band to the G band(I D/I G)is0.68for CNTs/Ni and0.82for MnO2@CNTs/Ni,indi-cating the disorder degree of CNTs is increased in MnO2@CNTs/Ni because of the KMnO4oxidation during the preparation process.It is generally accepted that the I D/I G ratio defines the defect density of carbon materials and electrochemical reactions are most likely to occur at the sites of the defects[26].In addition,a pronounced peak centered at638cmÀ1in low frequency range arises from MnO2Fig.3.(A,B)FESEM and(C,D)TEM images of MnO2@CNTs/Ni(0.5mg cmÀ2mass loading of MnO2).(E)SAED pattern and(F)EDS analysis of MnO2@CNTs/Ni(0.5mg cmÀ2mass loading).P.Sun et al./Journal of Power Sources341(2017)27e3530species in the MnO2@CNTs/Ni.In the XRD pattern of MnO2@CNTs/ Ni(Fig.4B),the strong peaks at2q¼29 ,44 ,52 ,77 are attributed to Ni mesh.However,only the unobvious peak(121)at2q¼37 and the peak(211)at2q¼43 for MnO2(JCPDS File Card No.14e0644) are faintly spotted,suggesting small mass loading and the amor-phous phase of MnO2in the composite(corresponds to the result of SAED pattern)[9,18].BrunauerÀEmmettÀTeller(BET)analysis re-veals the specific surface areas of CNTs/Ni,MnO2@CNTs/Ni (0.5mg cmÀ2and5.0mg cmÀ2mass loading).The N2adsorp-tionÀdesorption curves of CNTs/Ni,MnO2@CNTs/Ni(0.5mg cmÀ2 and5.0mg cmÀ2)are shown in Support Information(Fig.S6).As shown in Table S1,the specific surface areas of CNTs/Ni, MnO2@CNTs/Ni(0.5mg cmÀ2and5.0mg cmÀ2)are28.319,45.836, 6.589m2gÀ1,respectively.3.2.Electrochemical characterizationThe CV comparison for the MnO2@CNTs/Ni(0.5mg cmÀ2mass loading of MnO2),CNTs/Ni and Ni mesh in1M Na2SO4electrolyte at 20mV sÀ1is shown in Fig.5A,with a potential window fromÀ0.2e0.8V.Generally,the integral area of CV curves is pro-portional to specific capacitance at the same scan rate.It can be observed that the CV loop of the MnO2@CNTs/Ni is much larger than that of CNTs/Ni and Ni.Therefore,the capacitance value of the composite is much higher than that of CNTs/Ni and Ni,which is attributed to the positive synergistic effect between conductive CNTs and pseudocapacitive MnO2.The charge storage mechanism is based on surface adsorption of electrolyte cations Naþas well as proton incorporation according to the reaction[27]:MnO2þxNaþþyHþþ(xþy)e e4MnOONa x H y(5) In order to evaluate the contribution of MnO2to the capacitance of the MnO2@CNTs/Ni electrodes,we investigated the effect of the mass loading on the capacitance value,which will be useful for practical application.Different mass loading of MnO2from0.5to 5.0mg cmÀ2have been loaded by changing the amount of KMnO4 during the preparation process.Fig.5B and C shows the CV curves of MnO2@CNTs/Ni with different mass loadings of MnO2at the scan rate of10mV sÀ1and50mV sÀ1,respectively.The CV curves of low mass loading of MnO2show much more rectangular and symmetric than that of high loaded sample.Meanwhile,with increase of scan rates from10to50mV sÀ1,the CV curves deviate from ideal capacitor due to the large resistance in high mass loading MnO2. Fig.5D shows the CV curves of the MnO2@CNTs/Ni(0.5mg cmÀ2 mass loading)at different scan rates.With increasing scan rate from 5to200mV sÀ1,the current density also increases without any obvious changes in the shape of the CV curves,indicating a very good rate performance.The rectangular and symmetric shape of the CV curves is observed at high scan rates,implying the low contact resistance of the electrodes.Fig.6A shows the galvanostatic charge-discharge(GCD)curves of MnO2@CNTs/Ni(0.5mg cmÀ2mass loading)and CNTs/Ni in the potential range fromÀ0.2e0.8V(vs SCE)at a current density of 1A gÀ1.The much longer discharging time of the MnO2@CNTs/Ni electrode than that of the CNTs/Ni electrode indicates the increase of capacitance mainly from the MnO2component.GCD curves of the MnO2@CNTs/Ni electrode at different mass loadings and cur-rent densities are shown in Fig.6B,C.The nearly linear and almost symmetrical curves of MnO2@CNTs/Ni indicate that the electrode exhibits a good capacitive behavior.According to equation(1),we can calculate the capacitance values at different current densities from the discharging time.Fig.6D shows the specific capacitance values at different current densities for MnO2@CNTs/Ni with different mass loadings of MnO2.It is noted that an ultrahigh specific capacitance of1072F gÀ1is obtained for the electrode with a mass loading of0.5mg cmÀ2at a current density of1A gÀ1.The capacitance retention is approximately60%from1to10A gÀ1.Such superior capacitive behavior may be attributed to:(1)the core-shell structure of MnO2@CNTs and ultrathin MnO2nanoflakes(less than 4nm in thickness),which are beneficial to the insertion and extraction of electrolyte ions;(2)the ordered two dimensional woven structure of the Ni mesh substrate and the3D network structure of interconnected CNTs,which results in lower contact resistance(illustrated in Fig.S3in Supporting Information).When the mass loading is0.85,2.22,3.50and5.00mg cmÀ2,the specific capacitance values are684.0,340.1,305.9and288.0F gÀ1at1A gÀ1, respectively.The decrease of the specific capacitance with increasing MnO2mass loading is due to:(1)the additional MnO2 generates a low proton diffusion;(2)the electrical conductivity of the MnO2@CNT composite is decreasing at high mass loading[28]. As shown in Table S2in support information,the specific capaci-tance values obtained in this work are much better than previous works[29e31].To evaluate the charge transfer and electrolyte diffusion in CNTs/ Ni and MnO2@CNTs/Ni(0.5and5.0mg cmÀ2mass loading)elec-trodes,electrochemical impedance tests were carried out at0.4V with a frequency range from0.1to105Hz.Fig.S4shows theNyquistFig.4.Raman spectra(A)and XRD patterns(B)of the samples.The mass loading of MnO2(MnO2@CNTs/Ni)is0.5mg cmÀ2.P.Sun et al./Journal of Power Sources341(2017)27e3531Fig.5.Electrochemical properties of the samples in three electrode system.(A)CV curves for MnO 2@CNTs/Ni,CNTs/Ni and bare Ni mesh at a scan rate of 20mV s À1.(B,C)CV curves of MnO 2@CNTs/Ni with different mass loadings for MnO 2.The scan rates are (B)10mV s À1and (C)50mV s À1.(D)CV curves of the MnO 2@CNTs/Ni at different scan rates.The mass loading of MnO 2is 0.5mg cm À2.Fig.6.Electrochemical properties of the samples in three electrode system.(A)Galvanostatic charge-discharge curves for MnO 2@CNTs/Ni (0.5mg cm À2mass loading of MnO 2)and CNTs/Ni at 1A g À1.(B)Galvanostatic charge-discharge curves of MnO 2@CNTs/Ni with different mass loadings of MnO 2at 1A g À1.(C)Galvanostatic charge-discharge curves of the MnO 2@CNTs/Ni at different current densities.The mass loading of MnO 2is 0.5mg cm À2.(D)Speci fic capacitance values versus current density for MnO 2@CNTs/Ni at different mass loadings of MnO 2.P.Sun et al./Journal of Power Sources 341(2017)27e 3532plots of CNTs/Ni and MnO 2@CNTs/Ni.For each electrode,a semi-circle and a straight line can be observed in high-frequency region and low-frequency region,pared with MnO 2@CNTs/Ni electrodes,the almost vertical slope of CNTs/Ni impedance plot re flects the ideal capacitive behavior.As shown in the enlarged view of the high-frequency region in the inset,theFig.7.(A)Schematic structure of the flexible MnO 2@CNTs/Ni based symmetric supercapacitor (SSC)in 1M Na 2SO 4aqueous solution.(B)CV curves at different voltage windows.(C)CV curves of the SSC device at different scan rates.(D)Speci fic capacitances of the SSC device at different scan rates.(E)Cycling performance of the SSC device at 100mV s À1for 1000cycles.(F)Galvanostatic charge-discharge curves of the SSC device at various current densities.(G)Speci fic capacitances of the SSC device at different current densities.P.Sun et al./Journal of Power Sources 341(2017)27e 3533diameter of the semicircle for each electrode corresponds to the charge-transfer resistance caused by the Faradaic reaction on the electrode surface.The charge-transfer resistance of the CNTs/Ni and MnO2@CNTs/Ni(0.5and5mg cmÀ2mass loading)electrodes are about0.18,0.20and0.43U,respectively.The electrode with low mass loading of MnO2has higher electronic conductivity,which is beneficial to the charge transfer during the Faradaic reaction.As shown in the enlarged view of the high-frequency region,the intercept along the x axis is the internal resistance,including the electrolyte resistance,the intrinsic resistance of the active material and the contact resistance at the active material/current collector interface.The CNTs/Ni electrode shows the smallest internal resistance of3.56U;however,MnO2@CNTs/Ni(5mg cmÀ2mass loading)electrode shows the largest resistance of3.66U.To further explore the advantages of this material for practical application,a symmetric supercapacitor(SSC)has been assembled from two pieces of MnO2@CNTs/Ni with the mass loading of 0.5mg cmÀ2using1M Na2SO4solution as electrolyte.Schematic structure of theflexible SSC is illustrated in Fig.7A.We perform CV measurements on the MnO2@CNTs/Ni based SSC using a two-electrode system at different working voltage at10mV sÀ1 (Fig.7B).It is noted that the working voltage can be extended to 2.0V without obvious polarizations,which is higher than that of other MnO2-based SSCs[7,18,32].It is well known that water decomposition is the key factor that determines the stable voltage (normally about1V)of a capacitive device in aqueous electrolyte system.For the our SSC high working voltage(2.0V)can be reached,indicating as-prepared MnO2@CNTs/Ni electrode has high oxygen and hydrogen evolution over-potentials in1M Na2SO4 electrolyte.The CV curves of our SSC(the working voltage is2.0V) at different scan rates are shown in Fig.7C.The rectangular shape and symmetry of the CV scans can be observed even at the high scan rate of500mV sÀ1,indicating its good electrochemical per-formance.Fig.7D shows specific capacitance of our SSC at different scan rates.The capacitance retention from10to500mV sÀ1is up to 43.1%(140.2F gÀ1at10mV sÀ1and60.4F gÀ1at500mV sÀ1).As presented in Fig.7E,the long-term stability of the SSC was exam-ined by CV cycling at a scan rate of100mV sÀ1and the SSC keeps the capacitance retention of83.4%after1000cycles.Fig.7F shows the GCD curves of the MnO2@CNTs/Ni based SSC at different current densities over the working voltage of0e2.0V.The symmetrical charge-discharge characteristics represents a good capacitive characteristic for our supercapacitor.The specific capacitance of the SSC at different current densities are shown in Fig.7G.Calculated from Fig.7F according equation(1),the specific capacitances are170.0F gÀ1at1A gÀ1and91.0F gÀ1at20A gÀ1. The capacitance retention from1to20A gÀ1is up to53.5%.Ac-cording to equation(2)and equation(3),we can calculate energy density and power density from specific capacitances in Fig.7C and F.As shown in Fig.8A,the highest energy density of78.0Wh kgÀ1 (at a power density of1402W kgÀ1)and the highest power density of30.2kW kgÀ1(at an energy density of33.6Wh kgÀ1)have been obtained from CV curves.According to GCD curves,the highest energy density of94.4Wh kgÀ1(at a power density of 1000W kgÀ1)and the highest power density of20.0kW kgÀ1(at an energy density of55.6Wh kgÀ1)are achieved at a working voltage of2.0V.These results are much higher than that of other reported MnO2-based SSCs or ASCs,such as CNPs-MnO2//CNPs-MnO2(CNPs: carbon nanoparticles)SSC(4.8Wh kgÀ1,14.0kW kgÀ1)[7],gra-phene-MnO2//graphene-MnO2SSC(6.8Wh kgÀ1,62.0W kgÀ1) [28],graphene-MnO2//ACN(ACN:activated carbon nanofiber)ASC (51.1Wh kgÀ1,102.2W kgÀ1)[20],AC//MnO2-CNTs ASC (13.3Wh kgÀ1,600.0W kgÀ1)[32],Ni(OH)2-MnO2-RGO//FRGO ASC (54.0Wh kgÀ1,392.0W kgÀ1)[33],AGMn//aMEGO ASC (20.8Wh kgÀ1,32.3kW kgÀ1)[34],UPMNFs//FMCNTs ASC (47.4Wh kgÀ1,200.0W kgÀ1)[35]and AC//K0.27MnO2$0.6H2O ASC (25.3Wh kgÀ1,140.0W kgÀ1)[36](see Table S3in Supporting In-formation).To further demonstrate the practical application,our SSC device can successfully power a green light-emitting diode (LED,working voltage:1.5V)for5min,after being charged at 10A gÀ1for24s as shown in Fig.8B.Impressively,two SSC devices in series can even power12LEDs for more than1min(Fig.8B). 4.ConclusionThe MnO2@CNTs/Ni composite is prepared by a facile two-step method on aflexible and metallic Ni mesh under a mild reaction condition,in which MnO2nanoflakes and CNTs forming a hierar-chical core-shell structure.In such a unique structure,the electrode exhibits a high specific capacitance of1072F gÀ1at1A gÀ1in1M Na2SO4electrolyte.The as-assembled MnO2@CNTs/Ni based SSC exhibits a wide working voltage(2.0V)in1M Na2SO4electrolyte,Fig.8.(A)Ragone plot for the MnO2@CNTs/Ni based SSC with other MnO2-based supercapacitors reported previously.(B)One device can power a green light-emitting(LED, working voltage:1.5V)and two SSC devices in series can power12LEDs.(For interpretation of the references to colour in thisfigure legend,the reader is referred to the web version of this article.)P.Sun et al./Journal of Power Sources341(2017)27e3534high power density(30.2kW kgÀ1at an energy density of 33.6Wh kgÀ1),high energy density(94.4Wh kgÀ1at a power density of1000W kgÀ1)and long-term cycle stability(keeps a retention of83.4%after1000cycles).For practical application,one SSC was assembled to power1LED for5min and two SSC devices in series can power12LEDs for more than1min.In conclusion,the MnO2@CNTs/Ni composite is a very promising electrode material for assembling capacitor with both high power density and high energy density.AcknowledgementsThe authors gratefully acknowledge thefinancial support offered by NSFC Grants(21373152).Appendix A.Supplementary 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