High-Performance Asymmetric Supercapacitor Based on Hierarchical

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High-performance Asymmetric Supercapacitor Based on Hierarchical NiMn2O4@CoS Core-shell Microspheres and Stereotaxically Constricted Graphene Ning Hu, Lei Huang, Wenhao Gong, and Pei Kang Shen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04265 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

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ACS Sustainable Chemistry & Engineering

High-performance Asymmetric Supercapacitor Based on Hierarchical NiMn2O4@CoS Core-shell Microspheres and Stereotaxically Constricted Graphene Ning Hu, Lei Huang, Wenhao Gong, Pei Kang Shen* Collaborative Innovation Center of Sustainable Energy Materials, Guangxi University; Guangxi Key Laboratory of Electrochemical Energy Materials, Guangxi University; State Key Laboratory of Processing for Non-ferrous Metal and Featured Materials, Guangxi University, Nanning, 100 Daxue Road, 530004, PR China. *E-mail： [email protected]

Abstract Three

dimensional

self-assembled

hierarchical

NiMn2O4@CoS

core-shell

microspheres are synthesized via a facile hydrothermal and post-electrodeposition methods on Ni substrate. The microspheres are irregularly consisted of many nanoflakes, whose diameter is approximately 1.8μm. The NiMn2O4@CoS composites were used as electrode materials exhibiting a ultrahigh specific capacitance and excellent cycle performance for pseudocapacitors. The specific capacitance of composites electrode can reach to 1751 F/g at a current density of 1 A/g and 1270 F/g at a higher current density of 30 A/g. Most importantly, the specific capacitance still maintains 95% after 5,000 cycles 1

ACS Paragon Plus Environment

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at 10 A/g. A NiMn2O4@CoS// stereotaxically constricted graphene asymmetric supercapacitor device also shows a high energy density of 44.56 Wh kg-1 at the power density of 700.51 Wk g-1, a enormous power density of 20.99 kW kg-1 at 29.1 Wh kg-1. Moreover, the capacitance still remains 94% even after 5,000 cycles at 10 A/g. This outstanding electrochemical performance make NiMn2O4@CoS possibly become the next candidate electrode material for supercapacitors’ application.

Keywords:

NiMn2O4@CoS; Hierarchical core-shell microsphere; Stereotaxically Constricted Graphene; Pseudocapacitors

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Introduction With the rapid development of the global economy, the two themes of energy and environment gradually attract researchers’ attention. Researchers devote themselves to develop renewable energy for replacing traditional fossil energy to ease energy shortages and reduce environmental pollution1-2. Various electrochemical energy storage and conversion devices have been explored for practical application, such as recharge batteries (LIBs, SIBs and metal-air batteries), supercapacitors (SCs) and fuel cells (FCs) etc. To meet the property of high power density, high energy density and long lifetime for practical demands, supercapacitors have attracted researchers’ considerable interests3-4. Supercapacitors (SCs) are also called as electrochemical capacitors which it can be classified into two types based on charge-discharge mechanism, electrochemical double layer capacitors (EDLCs) governed by charge diffusion and adsorption at the electrode surface and pseudocapacitors controlled by Faradaic reaction at the electrode materials3, 5-6.

Comparing with other energy devices, SCs have many excellent advantages including

high power density, long cycle performance and faster charge and discharge time etc7 which make SCs become a candidate of energy sources for high power energy application such as electric vehicles and power girds6. But the practical application of SCs are huge restricted. For example, the specific capacitance of carbon-based electrode materials are lower and the cycle performance of

transition metal oxides are not

satisfied owing to the huge volume change in charge/discharge process8 9. In recent years, 3



various micro-nanostructure of

transition metal oxides (TMOs) or mixed transition

metal oxides (MTMOs) were synthesized for SCs’ electrode materials. They are used as pseudocapacitors electrode materials owing to TMOs and MTMOs exhibit high theoretical specific capacity. For example, the simple metal oxide RuO210, Co3O411-12, NiO13-15, Fe2O316-17 and MnO218-19 as SCs’ electrode materials show excellent electrochemical property20. However, their practical application is largely restricted. Comparing with simple TMOs, the MTMOs usually show higher electrical conductivity due to lower activation energy for electron transfer. More significantly, the multifarious valence states and bimetallic ion synergies effects are conducive to chemical reactions in charge-discharge process21-22. Taking into account the above factors, many researchers turn their attention to MTMOs for high performance electrode materials21. According to previous reports, MTMOs with different crystal structures show excellent electrochemical properties, because ternary oxides can offer more redox reactions than TMOs, such as nanowires NiCo2O423, mesoporous MnCo2O424, microspheres CoMn2O425, submicron-tube FeCo2O4 etc26. However, a few reports have investigated NiMn2O4 as electrode materials for electrochemical capacitiors27-28. Moreover there is no report about NiMn2O4 and CoS composites as electrode materials for SCs. In this work, to obtain excellent high performance electrode material, hierarchical NiMn2O4 (NMO) microspheres grown on Ni substrate were fabricated by a simple hydrothermal synthesis. In order to reduce the volume change of NMO microspheres and enhance SCs’ electrochemical property in charge and discharge process, NMO 4


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microspheres were coated by CoS (CS) thin layer on Ni substrate by a Cyclic Voltammetric (CV) method. The thickness of CoS layer will be enhanced with CV’s scan rate decreasing and cyclic numbers and Co2+, CS(NH2)2 concentration increasing. NMO@CS electrode displays ultrahigh specific capacitance and excellent cycling performance than pure NMO microsphere electrode in the 1M KOH three-electrode testing. The main reason is that the CoS thin can provide some capacitance because CoS thin can participate in electrochemical reaction with electrolyte and the surface of the NiMn2O4 microspheres were tightly coated by a CoS thin, so the NiMn2O4 microspheres can be fixed in a narrow space, which makes NiMn2O4 microspheres keep structural integrity and stability in charge discharge process, and the synergistic interaction between Co, Ni and Mn ions contributes to enhance electrochemical reaction and maintain structural stability. Moreover a NMO@CS// stereotaxically constricted graphene (SCG) asymmetric supercapatitors device also shows superior energy density, power density and outstanding cycling stability.

Experimental section Preparation of hierarchical NiMn2O4 microsphere on Ni substrate All chemical reagents were analytical grade and used without further purification. To remove nickel foam surface oxides, nickel foam was ultrasonicated in 3M HCl, ethyl alcohol and DI water for each 30 minutes. 1 mmol Ni(NO3)2▪6H2O, 2 mmol 5



Mn(NO3)2▪4H2O and 6 mmol hexamethylenetetramine (HTM) were dissolved in 40 ml DI water and 20 ml ethyl alcohol and the mixed solution were stirred and transferred to glass bottle, and then the washed nickel foam was immersed in bottle bottom. The bottle was sealed and maintained at 95 oC for 12 h. After cooling down naturally room temperature, the nickel foam was obtained by washing the surface impurity in water and ethyl alcohol and dried at 80 oC for 12h under vacuum. Then the as-prepared product was heated at 400 oC

in air for 3h to obtain the NMO microsphere. The weight of the

as-prepared NMO microsphere was measured before hydrothermal synthesis and after annealing and the mass on nickel foam was approximately 1 mg cm-2.

Electro-deposition of CoS on hierarchical NiMn2O4 microsphere 3D hierarchical NiMn2O4 (NMO) microsphere was used as working electrode for depositing CoS (CS) layer. Saturated calomel electrode (SCE) and Pt wire as reference electrode and counter electrode. 10 mM CoCl2▪6H2O and 1M CS(NH2)2 were dissolved in DI water as deposition bath. The electrochemical deposition was carried out by using CV technology under the potential range -1.2 V to 0 V at 5 mV s-1 for 5 cycles. The as-obtained NMO@CS electrode was dried at 80 oC for 8 h on vacuum. The weight of CoS was measured before and after electro-deposition, and the mass was about 0.3 mg cm-2.

Assembling of NiMn2O4@CoS// SCG asymmetric supercapacitor

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The asymmetric supercapacitor device (ASC) was fabricated by NMO@CS core-shell microsphere as positive, stereotaxically constricted graphene (SCG) as negative, glass fiber and 1M KOH solution as separator and electrolyte. A typical negative electrode preparation was mixed by SCG and PVDF in a mass ratio of 90 : 10. Some NMP solution were added to the mixed material forming uniform slurry which was coated on nickel foam. After drying at 80 oC for 8 h on vacuum, the coin configuration battery was assembled by as-prepared electrodes for two-electrode testing in a voltage window of 0 V to 1.4 V.

Electrochemical measurements The electrochemical property was investigated in a three-electrode device including 1M KOH aqueous solution as electrolyte, as-obtained NMO or NMO@CS as working electrode, saturated calomel electrode (SCE) and Pt wire as reference electrode and counter electrode. Cyclic voltammetry (CV) and galvanostatic charge and discharge (GCD) were conducted from the potential 0.05 V to 0.65 V in three-electrode. Electrochemical impedance spectroscopy (EIS) measurements with the amplitude of

5

mV from the frequency range of 100 kHz to 10 m Hz. All electrochemical testing were performed at electrochemical workstation (IM6, Zahner-Elektrik, Germany).

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Physical characterization The crystallographic information and phase structure of the as-prepared samples was tested by a D/Max-Ⅲ X-ray diffractometer (Rigaku Co., Japan) with Cu Kα radiation, a voltage of 30 kV, and a current of 30 mA. The scan range was 2θ=10° to 90°, and the scan rate was 10° 2θ min-1. A field-emission scanning electron microscope (FESEM; SU8820, Hitachi Co., Japan) and transmission electron microscope (TEM; Titan ETEM G2 80-300, FEI Co., USA) were employed to examine the morphological and analysis of the element products.

Results and discussion Fig.1 graphically shows the synthetic process of 3D hierarchical NiMn2O4@CoS core-shell microsphere composites. Firstly, 3D hierarchical NMO microspheres were grown on Ni substrate by the hydrothermal and subsequently heating treatment. Secondly, the NMO microspheres were uniformly coated by CoS (CS) film layer via a facile electrodeposition method to obtain NMO@CS core-shell composites. The phase and composition of hierarchical NMO@CS core-shell microsphere were investigated by XRD. As illustrated in Fig.S1, the as-obtained NMO@CS shows diffraction peaks at 29.8°, 36.6°, 42.6°, 57.7° and 62.6°, corresponding to the

(220),

(311), (400), (511) and (440) lattice planes, which accurately match with the standard values (PDF#84-0542). At the XRD pattern, CoS layer diffraction peaks were not clearly 8


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observed, and it is conjectured that CoS is amorphous or has a lower crystallinity owing to only a little mass loading. However the CoS layer was detected by XPS and EDX pattern analysis29. From the data of Table.S1, the element percentage of NiMn2O4@CoS can be seen. The elementary composition and valence state change of NMO@CS core-shell microspheres were investigated by XPS. As shown in Fig.2a, Mn 2p, Ni 2p, Co 2p, S 2p and O 1s were detected on the Ni substrate surface. Fig.2b shows Mn 2p2/3 peak at binding energies of 642.8 eV and Mn 2p1/2 peak at binding energies of 653.6 eV. Using a Gaussian fitting method, the Mn were fitting to three peaks, and the Mn2+ peaks located at 638.2 eV and 653.6 eV and another one peak located at 642.9 eV which was assigned to Mn3+.30 Similarly, the Ni 2p spectrum (Fig.2c) consists of Ni 2p1/2 and Ni 2p2/3 peaks at and two shake-up satellites peak. The fitting peaks at 863.3 eV and 881.3 eV were assigned to Ni2+, the peaks at 855.6 eV, 861.5 eV, 873.3 eV, and 879.1 eV corresponded to Ni3+. Fig.2d shows two strong peaks of Co 2p at 781 eV and 797 eV, and two shake-up satellites peaks. The fitting peaks at 780.9 eV and 796.9 eV were characteristic of Co3+, corresponding to the peaks at 785.4 eV and 802.8 eV were assigned to Co2+. Moreover, the peak of S2p2/3 located at 161.8 eV (as shown in Fig.2e) owing to the deposited Co-S film layer31. In addition, as displayed in Fig.2f, the O 1s fitting peaks located at 529.7 eV, 530.4 eV and 531.5 eV which were corresponding with metal-O bond, metal-O-H bond and H-O-H bond, respectively32-33. To explore the microstructure and morphology of hierarchical NMO and NMO@CS 9



core-shell microspheres, SEM and TEM means were used as illustrated Fig.3. As shown in the low–magnification SEM images (Fig.3a and 3d), the hierarchical NMO and NMO@CS core-shell microspheres homogeneously grown on nickel foam surface. Under higher-magnification image (Fig.3b), it can be clearly seen that 3D hierarchical NMO microspheres were orderly assembled by many nanoflakes (~ a few nanometer thickness). A high-resolution image (Fig.3c), the average diameter of 3D hierarchical NMO microspheres can be measured about 1.8 μm. Fig.3e and 3f show the hierarchical NMO@CS core-shell microspheres after electrodeposition. The SEM images can be clearly observed that NMO microspheres surface were coated by CoS layer, and the original structure of microspheres still keep integrated. Moreover, the NMO microspheres were connected by the interlaced CoS layer which contributed to improve stability of microspheres structure. As can be shown from the TEM images of Fig.3g and 3h, the NMO@CS microspheres demonstrate folding nanosheets morphology with transparent distinction which exhibit ultrathin nature. The diameter of microsphere was about 1.8 μm corresponding with SEM images. With a higher magnification image of TEM (Fig.3i), a thin layer of CoS can be detected in surface of NMO microsphere. The lattice spacing is confirmed to be 0.21 nm in the HRTEM image (Fig.3j) corresponding to the (400) plane of NMO microsphere. The corresponding FFT image of NiMn2O4 microsphere shows that the diffraction spots are assigned to (311) (200) and (400) planes which are identify with XRD result. Fig3.l and 3m show the HRTEM images of NMO and CoS and correspond to the FFT image of CoS. The EDX spectra (Fig.3n) demonstrates the 10


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mapping analysis of manganese, nickel, cobalt, sulfur and oxygen elements which suggests that the CoS thin layer covers on the surface of NMO microspheres. The as-obtained NMO@CS core-shell microspheres were further investigated as an electrode materials for SCs. Fig.4a shows the CV curves of NMO microspheres and NMO@CS core-shell microspheres electrode at a scan of 2 mV/s in a voltage window of 0.05 V - 0.65 V. The redox peaks of NMO@CS electrode from ~0.45V and ~0.55V move to ~0.25V and ~0.3V to more negative position after the CoS was coated in NMO surface, and the NMO@CS core-shell electrode show higher peak current and the CV’s area are twice as large as pure NMO indicating that the NMO@CS core-shell have higher capacitance at the same scan rate, and the redox peaks shifting to the left comparing with the pure NMO microspheres. Fig.4b displays the CV profile of the NMO@CS core-shell electrode at scan of 2, 5, 8, 10, 15, 20, 30, 40 and 50 mV/s in the voltage of 0.05 V - 0.65 V, and the redox peaks current density become stronger and the peaks move to left comparing pure NMO microspheres (as shown in Fig.S2). The couple redox peaks were attributed to ~0.25V and ~0.3V which can be attributed to the reversible redox reactions of Ni(II)⇄Ni(III) , Co(III)⇄Co(IV) and Mn(III)⇄Mn(IV). Herein, the charge-discharge mechanism of NMO@CS electrode materials can be explained as follows (1-4)34-35. NiMn2O4 + OH- + H2O ⇄ NiOOH + 2MnOOH + e-

(1)

MnOOH + OH- ⇄ MnO2 + H2O + e-

(2)

CoS + OH- ⇄

(3)

CoSOH + OH-

CoSOH + e⇄

CoSO + H2O + e-

(4) 11



From the CV curves, the two redox peaks can be obviously seen indicating that the capacitances are mainly from pseudocapacitance characteristics owing to the Faradaic redox reactions concerning about M-O/M-O-OH, where the M represents Mn, Ni and Co ions

36,

which identified with pseudocapacitor property of as-obtained NMO@CS

core-shell electrode. Moreover, the oxidation and reduction peaks divert to higher and lower voltage, respectively. Fig.4c shows the galvanostatic charge-discharge (GCD) curves of NMO@CS core-shell and NMO microspheres at the same current density of 1 A/g in the potential window of 0.05-0.65 V, and the discharge time of NMO and NMO@CS electrodes can reach ~996s and ~1582s indicating that the NMO@CS electrode has much higher specific capacitance. At different current density of 1, 2, 3, 5, 8, 10, 20 and 30 A/g , the discharge time of NMO@CS electrodes can reach 1582, 635, 402, 229, 111, 81, 47 and 27s, respectively which was longer at the same current density than NMO electrode (Fig.S3), moreover the platform of GCD according with redox reaction process. The corresponding discharging specific capacitance at various current density illustrate in Fig.4e. The specific capacitance of NMO@CS electrode can reach to 1727 F/g and 1270 F/g at 1 A/g and 30 A/g which are much higher than pure NMO microspheres at the same current density. Furthermore, the specific capacitance of as-prepared NiMn2O4@CoS composites electrode at 1 A/g is superior to other NiMn-based, NiCo-based and MnCo-based composites electrode comparing with previous literatures as shown in Table.S2. Herein, the NMO@CS electrode exhibits 12


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excellent electrochemical performance for pseudocapacitance. A fraction of the capacity is possibly provided by nickel foam substrate, but it is negligible in previous report. Cycling performance is an important factor for evaluating electrode materials properties. The long cycle performance of as-prepared electrode was investigated by GCD method in three electrode system. The cycling performance curves of NMO and NMO@CS electrodes at 10 A/g are shown in Fig.4f. The NMO@CS electrode exhibits that the specific capacitance still maintains 1406.7 F/g and only 5% capacitance loss after 5000 cycles, but the NMO electrode remains 975 F/g and 18% capacitance loss which indicates that NMO@CS core-shell exhibits more excellent cycling stability even after long-term cycles. In addition, to further investigate the ion diffusion and charge transfer in core-shell microspheres structure, the EIS measurement was tested. The Nyquist plots of NMO microspheres and NMO@CS core-shell microspheres electrode show in Fig.S4, and the inset displays semicircle image at high frequency region. It can be seen that the values of the resistances are ~2.0Ω and ~0.5Ω from the zoom image, respectively. The linear part of NMO@CS electrode are more vertical in low frequency and smaller diameter of semicircle in high frequency indicating that the NMO@CS core-shell structure has lower solid liquid interfacial impedance and faster ion diffusion rate than pure NMO electrode37. To further investigate NMO@CS core-shell microspheres electrode for practical application, Fig.6a displays that an asymmetric supercapacitor (ASC) device was assembled by NMO@CS core-shell microspheres as positive electrode and 13



stereotaxically constricted graphene (SCG) on nickel foam as negative electrode with glass fiber as separator and 1M KOH electrolyte. The SEM image and XRD pattern of SCG can be seen in Fig.S5 and S6. To detect the electrochemical property of ASC device, the device was measured by two electrode in 1M KOH electrolyte. The CV curves of two electrode at scan of 10 mV/s are shown in Fig.S7. It can be clearly observed that NMO@CS and SCG exhibit excellent electrochemical at the voltage window of -0.8 V – 0 V and 0.05 V - 0.65 V. Herein, the voltage window of ASC can be selected at 0 - 1.4 V. Fig.5a shows the CV profiles of ASC at a scan of 5, 10, 20, 30, 40, 50, 100 and 200 mV/s between 0 to 1.4 V. The CV curves of this device show a rectangular-like similar to double-layer capacity which shows that the ASC device exhibits a capacitive nature. With the scan rate increasing, the profile almost keep same, which manifests excellent electrochemical reversibility in charge and discharge process. To further investigate the electrochemical performance of ASC device, the GCD was test at current density of 1, 2, 3, 5,10, 20, and 30 A/g as shown in Fig.5b. The curves of ASC device charge and discharge exhibit higher symmetry and capacitive property according with CV curves. Fig.5c displays the specific capacitance of ASC device which can reach 163.7 F/g at current density of 1 A/g . Moreover, the capacitance still retains 107.1 F/g at a higher current density of 30 A/g. More importantly, the ASC capacitance remains 65.4% from 1 A/g to 30 A/g which exhibits superior rate performance. As illustrated in Fig.5d, the specific capacitance of ASC device maintains at 113 F/g and still 94% capacitance retention at 10 A/g after 5000 14


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cycles, and the coulombic efficiency is approximately approach 100%, indicating that ASC device exhibits outstanding cycling performance and excellent symmetry of charge and discharge process. Fig.S8 shows that the impedance value of NMO@CS //SCG ASC device in zoom image and

the values of the resistances are ~0.5Ω and ~1.5Ω before and

after 5000 cycles, which are higher after 5000 cycles which is possibly owing to the smash and loss of the electrode material and the corrosion of the Ni current collector in aqueous solution38-39. Energy density and power density were two most important characteristics to further evaluate the electrochemical property for SCs’ practical application. The Rogone plot demonstrates the relationship between power density and energy density as shown in Fig.6b. In this work, the fabricated ASC device delivers maximum energy density of 44.56 Wh kg-1 at a power density of 700.51W kg-1, and 29.1 Wh kg-1 at 20.99 kW kg-1. The value of energy density and power density is larger than previous reports, such as Fe2O3/GH//GH ( 25.6 Wh kg-1 at 347 W kg-1 )40, NiCo2O4@MnO2//AC ( 35 Wh kg-1 at 163 W kg-1 )41, NiCo2O4 //AC ( 15.4Wh kg-1 at 375W kg-1 )42 and NiO//Carbon ( 10 Wh kg-1 at 1000 W kg-1 )43.

Conclusions In summary, 3D hierarchical NiMn2O4@CoS core-shell microspheres were synthesized via a simple hydrothermal process and post-electrodeposition method for 15



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SCs’ application. The NMO microspheres were closely coating by a thin CoS layer, thereby it can make the microsphere structure more fasten on Ni substrate. Moreover, the unique microsphere structure consisting of many nanoflakes and synergistic effects of NMO and CoS accelerate the movement of ions and electrons. As a result, the hierarchical NMO@CS core-shell microspheres exhibit outstanding electrochemical performance for pseudocapacitors. In addition, the NiMn2O4@CoS//SCG-ACS also shows higher specific capacitance and superior cycle performance which delivers a maximum energy density of 44.56 Wh kg-1 at 700.51 W kg-1 and 29.1 Wh kg-1 at the power density of 20.99 kW kg-1. Therefore, the NiMn2O4@CoS compositions have the potential to next generation binder-free and no-conductive electrode material for SCs’ application.

Acknowledgements This work was supported by the National Basic Research Program of China (2015CB932304), (2015A030312007),

the

Natural

Guangxi

Science

Science

Foundation

and

of

Technology

Guangdong Project

Province

(AA17204083,

AB16380030) and the Danish project of Initiative toward Non-precious Metal Polymer Fuel Cells (4106-000012B).

16


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Associated content Supporting Information The Supporting Information is available free of charge on the ACS Publications website. SEM images, TEM images, Electrochemical test figures, and Table.

Author information Corresponding Author *E-mail: [email protected].

ORCID Pei Kang Shen: 0000-0001-6244-5978

Notes The authors declare no competing financial interest.

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Captions: Fig.1 Schematic illustration of fabrication process for hierarchical NiMn2O4@CoS core-shell microspheres on Ni substrate Fig.2 (a) XPS survey of NiMn2O4@CoS core-shell microspheres. (b-f) High resolution Mn2p, Ni2p, Co2p, S2p and O1s spectra for NiMn2O4@CoS core-shell microspheres

Fig.3 SEM images of the NiMn2O4 microspheres (a-c) and NiMn2O4@CoS core-shell microspheres (d-f) at different magnifications. (g-i) TEM images of NiMn2O4@CoS core-shell microspheres. (j-k) HRTEM image of NiMn2O4@CoS and the corresponding FFT pattern of NiMn2O4. (i-m) HRTEM image and the corresponding FFT pattern of CoS. (n) The EDX mapping of the hierarchical NiMn2O4@CoS composite. Fig.4

(a) CV curves of NiMn2O4 and NiMn2O4@CoS in the range of 0.05V-0.65V at 2mV/s.

(b) The CV curves of NiMn2O4@CoS electrode in the range of 0.05V-0.65V at different scan rates. (c) GCD curves of based on the NiMn2O4 and NiMn2O4@CoS at 1A/g. (d) The GCD curves of the NiMn2O4@CoS at various current densities. (e) The specific capacitance of the NiMn2O4 and NiMn2O4@CoS at different current density. (f) The cyclic performances based on NiMn2O4 and NiMn2O4@CoS at a current density of 10 A/g. Fig.5 (a) The CV curves of NiMn2O4@CoS//SCG asymmetric supercapacitor at different scan rates. (b) The GCD curves of NiMn2O4@CoS//SCG asymmetric supercapacitor at different current density. (d) Cycling performance of NiMn2O4@CoS//SCG asymmetric supercapacitor at a current density of 10A/g. Fig.6 (a) Schematic illustration of NiMn2O4@CoS//SCG asymmetric supercapacitor device based on NiMn2O4@CoS electrode ( positive ) and SCG electrode ( negative ) . (b) The Ragone plots relating the power density and energy density of the NiMn2O4@CoS//SCG asymmetric supercapacitor device with some advanced devices.

25



Fig.1

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Fig.2

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Fig.3

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Fig.4

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Fig.5

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Fig.6

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TOC: For Table of Contents Use Only

Synopsis: A hierarchical NiMn2O4@CoS core-shell microspheres are synthesized via a facile hydrothermal methods which exhibited ultrahigh capacitance and excellent cyclic performance.

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TOC


High-Performance Asymmetric Supercapacitor Based on Hierarchical

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