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Three-dimensional NiCo2O4@MnMoO4 core-shell nanoarrays for high-performance asymmetric supercapacitors Yuliang Yuan, Weicheng Wang, Jie Yang, Haichao Tang, Zhizhen Ye, Yujia Zeng, and Jianguo Lu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01966 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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Three-dimensional NiCo2O4@MnMoO4 core-shell nanoarrays for high-performance asymmetric supercapacitors

Yuliang Yuan,a Weicheng Wang,a Jie Yang,a Haichao Tang,a Zhizhen Ye,a Yujia Zengb and Jianguo Lu*a

a

State Key Laboratory of Silicon Materials, School of Materials Science and

Engineering, Zhejiang University, Hangzhou 310027, P.R. China b

Shenzhen Key Laboratory of Laser Engineering, College of Optoelectronic

Engineering, Shenzhen University, Shenzhen, 518060, P.R. China

*Corresponding author (Jianguo Lu). Fax: +86-571-87952124 Tel.: +86-571-87952187 E-mail address: [email protected]

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Abstract Design of new materials with sophisticated nanostructure has been proven to be an efficient strategy to improve their properties in many applications. Herein, we demonstrate the successful combination of high electron conductive materials of NiCo2O4 with high capacitance materials of MnMoO4 by forming a core-shell nanostructure. The NiCo2O4@MnMoO4 core-shell nanoarrays (CSNAs) electrode possesses high capacitance of 1169 F g–1 (4.24 F cm−2) at a current density of 2.5 mA cm−2, obviously larger than the pristine NiCo2O4 electrode. The asymmetric supercapacitors (ASCs), assembled with NiCo2O4@MnMoO4 CSNAs as binder-free cathode and active carbon (AC) as anode, exhibit high energy density of 15 Wh kg−1 and high power density of 6734 W kg−1. Cycle performance of NiCo2O4@MnMoO4 CSNAs//AC ASCs, conducted at current density of 20 mA cm−2, remain 96.45% of the initial capacitance after 10000 cycles, demonstrating its excellent long term cycle stability. Kinetically decoupled analysis reveals that the capacitive capacitance is dominant in the total capacitance of NiCo2O4@MnMoO4 CSNAs electrode, which may be the reason for ultra long cycle stability of ASCs. Our assembled button ASC can easily light up a red LED for 30 min and a green LED for 10 min after being charged for 30 seconds. The remarkable electrochemical performance of NiCo2O4@MnMoO4 CSNAs//AC ASCs is attributed to its enhanced surface area, abundant

electroactive

sites,

facile

electrolyte

infiltration

into

NiCo2O4@MnMnO4 nanoarrays and fast electron and ion transport path.


the

3D

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Introduction With growing increase in the demands of portable energy, one of the most urgent challenge is to explore efficient electrochemical energy storage devices.1,2 Among all the probable candidates, supercapacitors may be the most ideal because of their high power density, rate capability, safe operation and long cycling stability,3,4 which especially make them the best ideal choice for a hybrid electric vehicles and other short-term power-type devices.5−8 Up to now, the investigation of supercapacitors mainly focused on carbon materials, transition metal oxides/hydroxides and some conducting polymers. Generally, carbon materials work as a supercapacitors electrode based on the mechanism of electrical double-layer capacitance, which is poor in energy density as the fact that charge can be only stored at the electrode surface, whereas transition metal oxides/hydroxides and conducting polymers store energy based on the mechanism similar to the behavior of battery, where a faradic reaction occurred at the electrode surface, giving them the ability to store charge far more than an electrical double-layer capacitor. For the past years, binary transition metal oxides such as NiCo2O4,9−12 ZnCo2O4,13,14 ZnWO4,15,16 NiMoO4,17−21 CoMoO4,17,22−26 MnMoO425,27−29 and ZnSnO430 have been reported for a better electrochemical performance than the corresponding single component oxides, probably due to their richer oxidation states and higher electrical conductivities. Recently, metal sulfides like MnCo2S4,31 NiGa2S4,32 and NiO/Ni3S233 have also been reported with more than one metal or with two chemical compound in combination. It seems that multi-element oxides will be promising in practical use because they may provide considerable capacitance as well as high power density. As can be summarized from above, a perfect supercapacitor


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should possess the virtue of large specific capacitance, fast charge-discharge ability and long lifespan. Unfortunately, it is difficult for electrodes with single component metal oxides or one dimensional structure to satisfy all these demands.34−36 Luckily, core-shell nanostructure could be an option. In the designing of core-shell nanostructure for supercapacitors electrode, one major factor to be considered is the conductivity of active materials, especially when thick electrode with high mass loading is used. Due to the intrinsic low electric conductivity of these transition metal oxides with band gaps ranging from 3 to 4 eV, one can not ensure their outstanding electron transport performance, especially at high rate. Among the various kinds of transition metal oxides, NiCo2O4 has proved to possess a quite good conductivity. Hu et al. reported the electrical transport properties of NiCo2O4 nanoplate with conductivity up to 62 S cm‒1.37 Chen et al. use linear sweep voltammetry revealed the conductivity of NiCo2O4 is at least two order of magnitude higher than that of NiO and Co3O4 samples.38 This outstanding conductivity makes NiCo2O4 really suitable for electron transport when serve as a core material. Whereas to MnMoO4, D. C. electric conductivity on its pressed ploycrystall pellet exhibit a conductivity of 10‒8 S cm‒1,39 which is a rather low conductivity, but this do not affect its role as the shell material with low mass loading. So a combination of NiCo2O4 and MnMoO4 could be a rational design of supercapacitor’s electrode with predictable excellent performance. This idea of introducing better conductivity oxide into electrode design has also been reported in many other literature like the combination of NiO-Co3O440 and CuO-Co3O4.41 Herein, we designed a 3D core-shell nanostructure of NiCo2O4@MnMoO4. The nanostructure composed of NiCo2O4 nanowire as the core and MnMoO4 nanosheets as the shell. NiCo2O4 has been reported to be a high electrical conductivity material


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among transition metal oxides42 and MnMoO4 has been proved to be a high specific capacitance material.20 A facile two-step hydrothermal process was adopted for synthesis and a piece of nickel foam was used as the substrate. With the conductive substrate, NiCo2O4@MnMoO4 CSNAs can be applied as the supercapacitors electrode

directly

without

any

addition

of

polymer

binders.

The

3D

NiCo2O4@MnMoO4 CSNAs owns the merits of high surface area, fast electron transport path, abundant free space for electrolyte infiltration and volume expansion accommodating during charge and release. It is of great importance to combine the characteristic of high conductivity and high specific capacitance to achieve high energy density and power density for supercapacitors.

Experimental section A piece of commercial nickel foam was pre-weighed before put into the autoclave. 2.5 mmol NiCl2 6H2O, 5 mmol CoCl2 6H2O, 9 mmol CO(NH2)2 and 2 mmol CTAB were dissolved in 50 ml deionized water. The solution was magnetically stirred for 10 min at room temperature and then transferred to 50 ml autoclave with PTFE liner. The pre-weighed nickel foam (20×40×1.8 mm3) was then immersed into the solution. The autoclave was sealed and maintained at 100 °C for 6 h in an electric oven. After being cooled to room temperature naturally, the precursor products on the nickel foam were carefully washed with deionized water and dried at 60 °C overnight. Afterwards, the sample was annealed at 350 °C for 3 h at a ramping rate of 5 °C min−1 to transform into NiCo2O4 nanowires. The mass loading of the mesoporous NiCo2O4 nanowires on nickel foam was calculated to be around 3 mg cm−2. For the second step, 2 mmol of MnCl2 4H2O and 2 mmol of Na2MoO4 2H2O were dissolved in 30 ml of deionized water under magnetic stirring. The obtained


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white emulsion was transferred to a 50 ml stainless steel autoclave with teflon liner. The nickel foam with NiCo2O4 nanowires (Ni/NiCo2O4) prepared in the first step was rinsed in the emulsion and maintained at 160 °C for 10 min. After cooled down to room temperature naturally, the sample was washed with distilled water several times and annealed at 350 °C for 3 h. The mass loading of MnMoO4 grown on NiCo2O4 nanowire was calculated to be around 0.5 mg cm−2. For preparing MnMoO4 on nickel foam, the same synthesizing method was adopted using nickel foam instead of Ni/NiCo2O4 as the substrate. The ASCs were assembled with NiCo2O4@MnMoO4 CSNAs as binder free cathode and AC as anode. The AC anode was prepared by mixing AC, acetylene black and polyvinylidene fluoride (PVDF) in N-Methyl pyrrolidone (NMP) with mass ratio of 8:1:1. The uniform mixed paste was finally coated on nickel foam. Mass ratio of cathode and anode active materials are calculated based on capacity matching principle: m+ C− × ∆V− = m− C+ × ∆V+

(1)

where m+ and m- denote the mass of active materials of cathode (NiCo2O4@MnMoO4) and anode (AC). C+ and C- denote the specific gravimetric capacitance of cathode and anode materials, and ∆V+ and ∆V- are the voltage changes of cathode and anode materials during discharge in the three electrode measurement, respectively. The obtained products were characterized by X-ray diffraction (PANalytical B.V. Empyrean 200895) with a CuKα radiation, field-effect scanning electron microscope (FESEM, Hitachi S-4800), and transmission electron microscope (TEM, FEI F20). The chemical compositions of nanocomposites were analyzed by X-ray photoelectron


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spectroscopy (XPS, Thermo ESCALAB 250) with a monochromatic AlKα (1486.6 eV) radiation source. Electrochemical measurements (CHI 760E Electrochemical Workstation) were carried out in a three-electrode arrangement using 3 M KOH aqueous solution as the electrolyte. A platinum plate and a saturated calomel (Hg/Hg2Cl2) electrode were used as counter and reference electrode respectively. All potentials were referred to the SCE reference electrode. Nickel foam with active materials (~2 cm2 with mass loading of ~7 mg) was used as working electrode directly. As for cyclic voltammetry (CV), the capacitance is estimated by integrating the area under current-potential curve and divided by scan rate, mass or area of the electrode and potential window:

C=

1 mν 2∆V

∫

2 ∆V /ν

0

i (V ) dV

(2)

Herein, C represents area capacitance, m represents mass of the electrode (or A, area of the electrode to obtain areal capacitance), i represents current density, V and v represent potential change and scan rate. As for galvanostatic charge-discharge (GCD), the capacitance is described as follow:

C=

I dis ∆t m∆V

(3)

where Idis is the discharge current and ∆t is the discharging time corresponding to the specified potential change of ∆V. The energy density (E) and power density (P) of ASCs can be described as: 1 E = CV 2 2

P=

E ∆t


(4)

(5)

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Results and discussions Morphology and structure of the as prepared NiCo2O4 and NiCo2O4@MnMoO4 CSNAs were studied by SEM, as shown in Figure 1a and Figure 1c. The NiCo2O4 nanowires were densely covered on nickel foam with abundant space between each other. Each NiCo2O4 nanowire is long and thin with over 1 µm in length and 100 nm in diameter (Figure 1b). This kind of structure is favorable for the immersion of electrolyte due to the abundant space between nanowires when applied as an electrode. After a second hydrothermal process, the nanowires are covered by a thick layer of MnMoO4 nanosheets as can be seen in Figure 1d. As a result of MnMoO4 nanosheets grown on NiCo2O4 nanowire, the diameter of NiCo2O4@MnMoO4 core-shell nanocolumn is much larger than NiCo2O4 nanowire, but still enough interval space retained between each nanocolumn, facilitating the ions transporting and accommodating the volume expansion. In addition, MnMoO4 was grown on NiCo2O4 nanowire sheet by sheet, making each NiCo2O4@MnMoO4 core-shell nanocolumn a highly porous structure. This kind of morphology is of great benefit for sufficient reaction of active materials and electrolyte. A low magnification comparison between Figure 1a and Figure 1c illustrate that after a second growth process, the original NiCo2O4 nanowire array can be well maintained, making NiCo2O4@MnMoO4 CSNA a highly porous structure without dead volume. Therefore, electrolyte can percolate into the electrode through the free space between each NiCo2O4@MnMoO4 column and percolate deeper though the space between each MnMoO4 nanosheet, leading to a complete contact between active materials and electrolyte. XRD pattern of NiCo2O4@MnMoO4 CSNAs is shown in Figure 2, in which the diffraction peaks are ascribed to NiCo2O4 (PDF 02-1074) and Ni (PDF 04-0850). The


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three highest peaks at 44.5°, 51.8° and 76.4° belong to (111), (200) and (220) of metallic nickel, which come from the nickel substrate. The peaks positioned at 31.1°, 36.6°, 59.0°, 64.7° and 76.6° belong to (220), (311), (511), (440) and (533) of face cubic NiCo2O4, respectively. All peaks match well with the stand diffraction spectrum given by the International Centre for Diffraction Data (ICDD). Note that no peaks of MnMoO4 are observed on the pattern, which means that the MnMoO4 may be amorphous. The detailed structure of NiCo2O4 nanowires and NiCo2O4@MnMoO4 CSNAs was studied by TEM and high-resolution TEM (HRTEM). Figure 3a shows the TEM image of NiCo2O4 nanowire. The NiCo2O4 nanowire is composed of numerous interconnected nanoparticles with grain diameter of ~50 nm. The rough surface derived from interconnected nanoparticles give rise to enhancive surface area which offer plenty of electro-active surface sites. The HRTEM of NiCo2O4 nanowire confirmed the (111) and (222) face of cubic NiCo2O4 phase, with a lattice spacing of 0.47 nm and 0.23 nm (Figure 3b). Selected area electron diffraction (SAED) patterns of NiCo2O4 in the inset of Figure 3b display disordered diffraction spots signify multi-oriented crystals. Combining the results of low magnification morphology, diffraction fringes and diffraction spots, it can be deduced that it’s the crystallized grain which form the NiCo2O4 nanowire. Figure 3c and Figure 3d show the TEM images of NiCo2O4@MnMoO4 nanocolumn. It is observed that NiCo2O4 nanowire is covered by large amount of nanosheets, which would be MnMoO4. SAED characterization on the center of NiCo2O4@MnMoO4 nanoarray shows both diffraction spots and dispersed concentric circles, whereas the SAED characterization on the edge shows only dispersed concentric circles. Combining the results of NiCo2O4 in Figure 3a and Figure 3b, it can be speculated that the diffraction spots


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come from crystallized particles of NiCo2O4 and the dispersed concentric circles come from the amorphous MnMoO4. This speculation is also consistent with the result of XRD, where only peaks of NiCo2O4 appear in XRD, but no peaks of MnMoO4 can be observed. The XRD and TEM results reveal the fact that NiCo2O4@MnMoO4 nanocolumn is combined with polycrystal NiCo2O4 nanowire and amorphous MnMoO4 nanosheets. The surface chemical composition and oxidation states of NiCo2O4@MnMoO4 CSNAs were further analyzed by XPS, as shown in Figure 4. The Ni 2p core level spectrum is deconvoluted into four peaks by Gaussion curve fitting (Figure 4a). The binding energy at 855.85 eV and its satellite peak at 861.48 eV correspond to Ni 2p3/2 level, whereas the binding energy peak at 873.65 eV and its satellite peak at 879.5 eV correspond to the Ni 2p1/2 level. The results are in good agreement with characteristics of Ni2+ and Ni3+ in NiCo2O4.43 Figure 4b displays the Co 2p spectrum, where two major peaks at binding energies of 779.56 eV and 794.85 eV are ascribed to 2p3/2 and 2p1/2 of Co ions, correspond to characteristic of Co2+ and Co3+ in NiCo2O4.44 The above results confirm the typical characteristics of Ni 2p and Co 2p spectra of NiCo2O4. Figure 4c exhibits the Mn 2p spectrum, which is resolved into a pair of doublets: 641.46 eV and 653.69 eV corresponding to 2p3/2 and 2p1/2 of Mn ions, respectively. According to the previously reports, binding energy in the intervals of 639.9−640.8 eV, 640.5−642.3 eV, and 642.5−643.2 eV can be attributed to Mn2+, Mn3+, and Mn4+,45−46 respectively. It is reasonable to conclude that Mn cations exhibit an oxidation state of Mn3+ in our case. The XPS spectrum of Mo 3d (Figure 4d) displays two distinct peaks located at 231.59 eV and 234.63 eV with 3d5/2-3d3/2 spin-orbit split of 3.04 eV. This characteristic is attributed to Mo cations at lower oxidation state of Mo5+, which is in close agreement with previous studies.47−48 These

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results are the typical characteristics of Mn 2p and Mo 3d spectra of MnMoO4. Thus, the NiCo2O4@MnMoO4 CSNAs were confirmed in our case. To verify the applicability of NiCo2O4@MnMoO4 CSNAs as supercapacitor electrode. The electrochemical properties were investigated in a three electrode cell in 3 M KOH electrolyte. CV and GCD measurements were performed and shown in Figure 5. Figure 5a, b and c depict the CV curves of MnMoO4, NiCo2O4 and NiCo2O4@MnMoO4 electrode at scan rate of 2 mV s–1. All of the three electrode exhibit obvious oxidation and reduction peak between 0-0.5 V (vs SCE). Each of the curve shows a pair of oxidation and reduction peak with similar shape and height, illustrating good reversibility of the three electrode. Figure 5d, e and f depict the GCD curves of MnMoO4, NiCo2O4 and NiCo2O4@MnMoO4 electrode at series of current densities. Each of the curve shows typical faradic charge storage behavior with both inclined plateau in charge and discharge curve, which is in accordance with CV results. The capacitance as a function of current density calculated from GCD curves were displayed in Figure 6. Figure 6a, b and c describe the capacitance of MnMoO4, NiCo2O4 and NiCo2O4@MnMoO4 electrode, respectively. The capacitance is 918, 778, 676, 617, 558, 485, 411 and 441 F g–1 for MnMoO4 electrode, 803, 763, 699, 656, 629, 553 484, and 429 F g–1 for NiCo2O4 electrode and 1169, 1023, 838, 728, 666, 557, 471 and 385 F g–1 for NiCo2O4@MnMoO4 electrode. The results show that the NiCo2O4@MnMoO4 CSNAs electrode exhibits both improved specific gravimetric capacitance and specific area capacitance in the current range of 2.5-30 mA cm–2, illustrating the virtue of NiCo2O4@MnMoO4 CSNAs electrode. Figure 6d, e and f depict the cycling performance of MnMoO4, NiCo2O4 and NiCo2O4@MnMoO4 electrode at a current density of 30 mA cm–2 for 5000 cycles. As to the electrode of MnMoO4, the capacitance increased in the first 300 cycles, then decreased slowly


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with the cycle number increase, with capacitance retention of 94.07 % at last. For electrode of NiCo2O4, the capacitance decreased during the whole cycling process, and end with 85.52 % of capacitance retention. Electrode of NiCo2O4@MnMoO4 showed a middle cycling performance compared with MnMoO4 and NiCo2O4, with 92.58 % capacitance retention at last. Therefore, an improved cycling performance was achieved by grown MnMoO4 on NiCo2O4 nanowire. We assume the second growth of MnMoO4 nanosheet provide a protective cover for NiCo2O4 nanowire that lead to better cycling performance. Electrochemical impedance spectroscopy (EIS) measurements were carried out in three electrode configuration to gain more insights into the charge transfer behavior. Figure 7 shows the impedance spectra of MnMoO4 (Figure 7a), NiCo2O4 (Figure 7b) and NiCo2O4@MnMoO4 (Figure 7c) recorded at an open circuit potential of 0.2 V (vs SCE) between 0.1 Hz to 100 KHz. According to Dunn and Tolbert,49 the circuit model in Figure S5 can be used to describe the electrochemical impedance behavior of our electrode. Where Rs represents resistance of electrolyte solution, Rct represents faradic charge transfer resistance, CPEdl represents double-layer capacitance of electrode/electrolyte interface and CPEpse represents pseudocapacitance of electrode. In Figure 7a, b and c, the black dots represent raw data and hollow circles represent fitted data. It can be observed that the fitted data conform well with the raw data, confirm the rationality of fitting by the equivalent circuit. Charge transfer resistances of MnMoO4, NiCo2O4 and NiCo2O4@MnMoO4 electrode are 1.46 Ω, 0.23 Ω and 0.12 Ω with same electrode area of 1 cm2. It seems NiCo2O4 is more electroactive than MnMoO4, while NiCo2O4@MnMoO4 electrode, with larger surface area and more reaction sites, became the most electrochemical active electrode among the three electrode. Figure 7d gives the Nyquist plot of NiCo2O4@MnMoO4 electrode with the


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various of cycling in a three electrode cell. It can be observed that charge transfer resistance increased with the increase of cycling number, with 0.12 Ω at 50 cycles, 0.17 Ω at 2500 cycles and 0.56 Ω at 5000 cycles. There is a conductance deterioration during cycling, but even at 5000 cycles, the charge transfer resistance is still favorable. The total capacitance of NiCo2O4@MnMoO4 CSNAs electrode is kinetically separated into two components: double-layer capacitance which is governed by surface-confined interaction and intercalation capacitance which is governed by diffusion-controlled effects. To quantitatively separate these two charge storage contributions, CV results at different scan rates were analyzed by using Dunn’s method. Specifically, the current response at fixed potential can be described as the combination of surface capacitive effects and diffusion-controlled intercalation. i (V ) = k1v + k2 v 0.5

(6)

For analytical purpose, the formula can be rearranged to:

i (V ) / v 0.5 = k1v 0.5 + k2

(7)

where k1v and k2v0.5 correspond to the charge contributions from surface charge accumulation

and

diffusion-controlled

intercalation,

respectively.

Thus,

by

determining k1 and k2, each of these two contributions can be confirmed. Figure 8 describes the total areal capacitance, double-layer capacitance and intercalation capacitance as function of scan rate, which enables us to determine the fraction of double-layer capacitance from the total capacitance. It can be observed that the total capacitance decreased with the increase of scan rate. This phenomenon is attributed to the fact that intercalation process can’t keep up with the potential change


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at high scan rate. Therefore, only the intercalation part decreases with the scan rate, whereas the capacitive capacitance keeps almost constant. The specific data shows that the capacitive capacitance keeps constant at 0.65 F cm−2 with the increase of scan rate. The capacitive capacitance proportions of NiCo2O4@MnMoO4 CSNAs are 78.9%, 89.3%, 93.7%, and 97.6% at the scan rate of 2, 5, 10, and 20 mV s−1, respectively. Clearly, capacitive capacitance becomes more pronounced when the charging rate increases. It can be speculated that the high capacitive capacitance share a major part of charge storage from intercalation, which could help to keep the nanostructure of electrode integrity and therefore in favor of long cycle stability. A novel asymmetric supercapacitor (ASC) device was assembled by using NiCo2O4@MnMoO4 CSNAs as cathode and AC as anode (Figure 9a). Nyquist plot for the assembled ASC was recorded in a frequency range from 0.01 Hz to 100 KHz, as shown in Figure 9b. Two arcs were observed on the Nyquist plot, which we infer, belong to anode at middle frequency (Figure S6b) and cathode at high frequency (Figure S6c). A sloping line at low frequency with gradient larger than 45o suggest the pseudocapacitive diffusion behavior other than semi-infinite diffusion (Warburg resistance) of our device. Based on this assumption, the model in Figure S6f was adopted as the equivalent circuit. Of which Rs represents electrolyte resistance, CPEdl represents double-layer capacitance at electrode/electrolyte interface, CPEpse represents pseudocapacitance of cathode, Rc and Ra represent charge transfer resistance of cathode and anode. Fitting procedure was processed by Zsimpwin. It can be observed that the fitted data comform well with the raw data both in Nyquist plot (Figure S6d) and Bode plot (Figure S6e), confirm the rationality of our fitting procedure. Charge transfer rensistance of cathode (Rc) exhibit as small as 0.21 Ω with internal resistance (Rs) of 0.7 Ω. These values illustrate the high conductivity and high


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electroactive properties of our assembled NiCo2O4@MnMoO4//AC ASC. The characteristic frequency f0 for a phase of 45o is 56 Hz for the assembled ASC. This frequency signify the point at which the resistive and capacitive impedance are equal. The corresponding time constant τ (1/f0) was obtained to be 30 ms for our ASC. The operating voltage of the ASCs is set at 1.6 V, as can be observed in the GCD test in Figure 9d. According to charge matching principle, the mass loading of NiCo2O4@MnMoO4 CSNAs//AC ASCs were calculated to be 3.08 mg and 8.8 mg for cathode and anode, respectively. The CV curves of NiCo2O4@MnMoO4 CSNAs//AC ASCs were recorded at scan rates from 5 to 100 mV s−1 at the operating voltage range between 0 and 1.6 V (Figure 9c). The CV curves with distorted semi-rectangular shape with a pair of redox peaks and GCD curves with inclined platform demonstrate a combination of double-layer capacitance and faradic capacitance behavior of our ASCs. Ragone plot (Figure 9e) of NiCo2O4@MnMoO4 CSNAs//AC ASCs is derived from GCD, which shows the relationship between energy density and power density. The plot reveals our ASCs device presents a high energy density of 15 W h kg−1 at power density of 336 W kg−1 and a high power density of 6734 W kg−1 at energy density of 5.05 W h kg−1. Device performance of ASCs reported in recent years have been concluded as tables (Table S1 and S2), our device shows pretty good performance compared with the devices reported in the tables. Long cycle performance of NiCo2O4@MnMoO4 CSNAs//AC ASCs was conducted for 10000 cycles at current density of 20 mA cm−2 (Figure 9f). The ASCs device shows excellent cycle stability with 96% retention after 10000 cycles. With the excellent electrochemical performance of our ASCs, we can readily light either a red light-emitting diode (LED) or a green LED with two cells in series, as displayed in Figure 10a. Figure 10b exhibits the brightness variation of a red LED


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after being lighted up. All the pictures of LED were captured by a digital camera. When the button ASCs were charged for 30 seconds at current density of 10 mA cm−2, it can drive a red LED lighting for at least 30 min. It is worth noting that no apparent brightness changed in the first 10 min. Using the above ASCs to drive a green LED, the light emission can last for at least 10 min. These results stress the fact that our NiCo2O4@MnMoO4 CSNAs//AC ASCs possess outstanding energy storage capability.

Conclusions Novel NiCo2O4@MnMoO4 CSNAs has been successfully synthesized through a facile two-step hydrothermal process free from template on nickel foam and directly applied as supercapacitors electrode. The NiCo2O4@MnMoO4 CSNAs electrode exhibits excellent electrochemical performance with an extremely large capacitance of 1169 F g‒1 (4.24 F cm−2) at current density of 2.5 mA cm−2. The assembled NiCo2O4@MnMoO4 CSNAs//AC ASC exhibits high energy density of 15 W h kg−1 and high power density of 6734 W kg−1 with excellent cycle stability for 10000 cycles. Kinetically analysis reveals that the capacitive capacitance is dominant in the total capacitance of NiCo2O4@MnMoO4 CSNAs electrode, significantly increasing as the scan rate increases, which may be the reason for the ultralong cycle stability of ASCs. The assembled button ASC can easily light up a red LED for 30 min and a green LED for 10 min after being charged for 30 seconds. This outstanding electrochemical performance is attributed to the sophisticated nanostructure NiCo2O4@MnMoO4 with high electron conductive material of NiCo2O4 as core and large capacitance material of MnMoO4 as shell. Other merits like enhanced surface area, facile electrolyte


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infiltration into the 3D NiCo2O4@MnMnO4 also contribute a lot to the whole capacitance.

Acknowledgements This work is supported by National Natural Science Foundation of China under Grant No. 51372002 and the Shenzhen Science and Technology Project under Grant No. JCYJ20150324141711644.


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Figure 1. (a and b) SEM images of NiCo2O4 nanowires on nickel foam at different magnifications. (c and d) SEM images of NiCo2O4@MnMoO4 nanocolumns on nickel foam at different magnifications.


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Figure 2. XRD pattern of NiCo2O4@MnMoO4 on nickel foam (black line). Stand diffraction pattern of metallic nickel (PDF 04-0850) (red line) and face centered cubic NiCo2O4 (PDF 02-1074) (blue line).


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Figure 3. (a) TEM image of NiCo2O4 single nanowire. (b) Lattice diffraction fringes of NiCo2O4 recorded at high-resolution TEM (HRTEM), the inset image shows the selected area electron diffraction (SAED) of NiCo2O4. (c) TEM image of NiCo2O4@MnMoO4, the inset image shows the SAED of NiCo2O4@MnMoO4 corresponding to the area where both core and shell are characterized as the red circle assign. (d) TEM image of NiCo2O4@MnMoO4, the inset image shows the SAED of NiCo2O4@MnMoO4 corresponding to the area where only shell is characterized as the red circle assign.


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Figure 4. (a-d) Deconvolution of Ni 2p, Co 2p, Mn 2p and Mo 3d core-level XPS spectra of NiCo2O4@MnMoO4 CSNAs, respectively.


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Figure 5. CV curve of (a) MnMoO4, (b) NiCo2O4, (c) NiCo2O4@MnMoO4 at scan rate of 2 mV s–1. GCD curves of (d) MnMoO4, (e) NiCo2O4, (f) NiCo2O4@MnMoO4 at series of current densities.


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Fig 6. Areal capacitance and specific capacitance of (a) MnMoO4, (b) NiCo2O4, (c) NiCo2O4@MnMoO4 at series of current densites. Cycling performance of (d) MnMoO4, (e) NiCo2O4, (f) NiCo2O4@MnMoO4 at current density of 30 mA cm–2 for 5000 cycles.


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Fig 7. EIS of (a) MnMoO4, (b) NiCo2O4, (c) NiCo2O4@MnMoO4 in three electrode configuration and (d) EIS of NiCo2O4@MnMoO4 at different cycles in three electrode configuration.


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Figure 8. Comparison of charge storage for NiCo2O4@MnMoO4 nanocolumn arrays at scan rates of 2, 5, 10, 20 mV s−1. The total capacitance is decoupled into double layer capacitance and faradic capacitance.


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Figure 9. (a) Schematic diagram of an assembled ASC. (b) Nyquist plot of NiCo2O4@MnMoO4 CSNAs//AC ASCs. (c) CV curves of NiCo2O4@MnMoO4 CSNAs//AC ASCs at series of scan rates. (d) GCD curves of an ASC at series of current densities. (e) Ragone plot of the ASCs device. (f) Cycling performance of the ASCs device for 10000 cycles at current density of 20 mA cm−2.


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Figure 10. (a) Application of the NiCo2O4@MnMoO4 CSNAs//AC ASCs to drive a red and green LED with two cells in series. (b) Brightness variation of a red LED with time after being lighted up.


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Figure 1. (a and b) SEM images of NiCo2O4 nanowires on nickel foam at different magnifications. (c and d) SEM images of NiCo2O4@MnMoO4 nanowires on nickel foam at different magnifications. 218x152mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2 XRD pattern of NiCo2O4@MnMoO4 on nickel foam (black line), stand diffraction pattern of metallic nickel (PDF 04-0850) (red line) and face centered cubic NiCo2O4 (PDF 02-1074) (blue line). 297x230mm (150 x 150 DPI)


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Figure 3 (a) TEM image of NiCo2O4 single nanowire. (b) Lattice diffraction fringes of NiCo2O4 recorded at high-resolution TEM (HRTEM), the inset image shows the selected area electron diffraction (SAED) of NiCo2O4. (c) TEM image of NiCo2O4@MnMoO4, where the inset image shows the SAED of NiCo2O4@MnMoO4 corresponding to the area where both core and shell are characterized as the red circle assign. (d) TEM image of NiCo2O4@MnMoO4, where the inset image shows the SAED of NiCo2O4@MnMoO4 corresponding to the area where only shell is characterized as the red circle assign. 133x131mm (256 x 256 DPI)


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Figure 4 (a-d) Deconvolution of Ni 2p, Co 2p, Mn 2p and Mo 3d core-level XPS spectra of NiCo2O4@MnMoO4 CSNAs, respectively. 297x249mm (150 x 150 DPI)


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Figure 5. CV curve of (a) MnMoO4, (b) NiCo2O4, (c) NiCo2O4@MnMoO4 at scan rate of 2 mV s–1. GCD curves of (d) MnMoO4, (e) NiCo2O4, (f) NiCo2O4@MnMoO4 at series of current densities. 297x161mm (150 x 150 DPI)


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Fig 6. Areal capacitance and specific capacitance of (a) MnMoO4, (b) NiCo2O4, (c) NiCo2O4@MnMoO4 at series of current densites. Cycling performance of (d) MnMoO4, (e) NiCo2O4, (f) NiCo2O4@MnMoO4 at current density of 30 mA cm–2 for 5000 cycles. 297x169mm (150 x 150 DPI)


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Fig 7. EIS of (a) MnMoO4, (b) NiCo2O4, (c) NiCo2O4@MnMoO4 in three electrode configuration and (d) EIS of NiCo2O4@MnMoO4 at different cycles in three electrode configuration. 297x252mm (150 x 150 DPI)


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Figure 8. Comparison of charge storage for NiCo2O4@MnMoO4 nanocolumn arrays at scan rates of 2, 5, 10, 20 mV s−1. The total capacitance is decoupled into double layer capacitance and faradic capacitance. 297x229mm (150 x 150 DPI)


Langmuir

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Figure 9. (a) Schematic diagram of an assembled ASC. (b) Nyquist plot of NiCo2O4@MnMoO4 CSNAs//AC ASCs. (c) CV curves of NiCo2O4@MnMoO4 CSNAs//AC ASCs at series of scan rates. (d) GCD curves of an ASC at series of current densities. (e) Ragone plot of the ASCs device. (f) Cycling performance of the ASCs for 10000 cycles at current density of 20 mA cm−2. 297x158mm (150 x 150 DPI)


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Figure 10. (a) Application of the NiCo2O4@MnMoO4 CSNAs//AC ASCs to drive a red and green LED with two cells in series. (b) Brightness variation of a red LED with time after being lighted up. 148x58mm (300 x 300 DPI)


Langmuir

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Table of contents graphic 240x190mm (96 x 96 DPI)


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Three-Dimensional NiCo2O4 ... - ACS Publications

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