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Construction of hierarchical CNT/rGO supported MnMoO nanosheets on Ni foam for high performance aqueous hybrid supercapacitors Xuemei Mu, Jingwei Du, Yaxiong Zhang, Zhilin Liang, Huan Wang, Baoyu Huang, Jin Yuan Zhou, Xiaojun Pan, Zhenxing Zhang, and Erqing Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09005 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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ACS Applied Materials & Interfaces

Construction of hierarchical CNT/rGO supported MnMoO4 nanosheets on Ni foam for high performance aqueous hybrid supercapacitors Xuemei Mu,†,‡ Jingwei Du,†,‡ Yaxiong Zhang,† Zhilin Liang,† Huan Wang,† Baoyu Huang,† Jinyuan Zhou,† Xiaojun Pan,† Zhenxing Zhang,∗,† and Erqing Xie∗,† School of Physical Science and Technology, Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China. E-mail: [email protected]; [email protected] Phone: +86 931-8912753. Fax: +86 931-8913554

Abstract Rationally designed conductive hierarchical nanostructures are highly desirable to support pseudocapacitive materials in achieving high-performance electrodes for supercapacitors. Herein, manganese molybdate nanosheets were hydrothermally grown with graphene oxide (GO) on three-dimensional nickel foam-supported carbon nanotube structures. Under the optimal graphene oxide concentration, the obtained carbon nanotubes/reduced graphene oxide/MnMoO4 composites (CNT/rGO/MnMoO4 ) ∗

To whom correspondence should be addressed School of Physical Science and Technology, Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China. ‡ These authors contributed equally. †

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as binder-free supercapacitor cathode perform a high specific capacitance of 2374.9 F g−1 at the scan rate of 2 mV s−1 and good long-term stability (97.1% of the initial specific capacitance can be maintained after 3000 charge/discharge cycles).The asymmetric device with CNT/rGO/MnMoO4 as the cathode electrode and the carbon nanotubes/activated carbon on nickel foam (CNT-AC) as the anode electrode can deliver an energy density of 59.4 Wh kg−1 at the power density of 1367.9 W kg−1 . These superior performances can be attributed to the synergistic effects from each component of the composite electrodes: highly pseudocapacitive MnMoO4 nanosheets and three-dimensional conductive Ni foam/CNTs/rGO networks. These results suggest the fabricated asymmetric supercapacitor can be a promising candidate for energy storage devices.

Keywords Supercapacitors, Manganese Molybdate, Carbon Nanotubes, Graphene Oxide, Nickel foam

1

Introduction

In the past decades, energy storage devices have attracted great attention for energy storage applications. 1,2 Supercapacitor also known as electrochemical capacitor has been extensively investigated because of its high energy, high power density and excellent cycle stability properties. 3,4 However, the electrochemical reactions usually involve a complicated multi-ion and multi-electron transfer processes which are largely limited to the specific surface area and conductivity of the current collector. 5 Due to its inherent three-dimensional (3D) cross-linked macroporous structures and prominent electrical conductivity, commercial nickel foam (NF) is mostly used current collector to load active material as electrode for supercapacitor. 6–9 However, its limited specific surface area restricts the mass loading of active materials and hinders the electrochemical performance of electrodes.


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Combining with carbon nanostructures (like graphene, reduced graphene oxide, carbon nanotubes etc.) has been demonstrated to be an effective approach to achieve Ni foam with high specific surface area and abundant conductive channels. 10–14 Among them, carbon nanotubes (CNTs) have been often grown on the Ni foam because of their high specific surface area as well as favorable mechanical properties, high conductivity, and the mature preparation process. 15–18 However, the rare defects of CNTs prepared by chemical vapor deposition (CVD) hamper their combination with active materials and then decrease the performance especially the stability of the electrode. 19 So many treatments have been applied to modify the surface of CNTs. Zhao et al. reported the HNO3 -treated CNTs can well support NiMn layered double hydroxide which manifested an energy density up to 88.3 Wh kg−1 as well as long-term stability. 20 Li et al. reported PPy-coated CNTs can fully utilize the electrical conductivity of CNT networks and perfectly combine the pseudocapacitive material MnO2 . 21 But these treatments are usually cumbersome associated with strong acids or conductive polymers which will destroy the backbone of Ni foam and reduce the stability. Graphene oxide (GO), which typically consists of two randomly distributed regions: hydrophobic polyaromatic areas of unoxidized benzene rings and hydrophilic regions containing carboxylic acid, epoxy, and hydroxyl groups, is a perfect amphiphilic surfactant and structure-directing agent for pseudocapacitive material growth. 22–25 By reduction of GO during the hydrothermal growth with CNTs on Ni foam, the obtained rGO will have good electrical contacts to CNTs due to their similar atomic structure through π-π attractions, leading to threedimensional hierarchical conductive networks and thus enhanced capacitance through additional faradaic reactions. 26,27 Therefore, it is suggested that pseudocapacitive materials combined with hierarchical conductive CNT/rGO networks on Ni foam can possess large specific surface area, fast ion and electron transfer channels, good rate performance, high energy density and power density, and long-term stability, rendering themselves promising candidates for future energy storage devices. Ternary transition-metal oxides/hydroxides have been explored as pseudocapacitive ma-



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terials since they can offer high energy densities resulting from their stable redox reactions. 28,29 For example, molybdates like NiMoO4 , 30,31 CoMoO4 , 32 MnMoO4 33 and their composites all can offer excellent electrochemical performance. Among these materials, MnMoO4 and its hydrates have attracted unique attention due to their high electrical conductivity, environment benignity, inherent large working voltage window, and extraordinary stability. 34,35 Cao et al. reported MnMoO4 ·4H2 O nanoplates grown directly on Ni foam as a binder free electrode presented a great specific capacitance of 2300 F g−1 at 4 mA cm−2 and excellent cycling stability. 36 As in our earlier work, MnMoO4 ·nH2 O nanosheets directly grown on Ni foam performed good electrochemical performance especially the asymmetric MnMoO4 ·nH2 O//AC (activated carbon) device delivered a high energy density of 31.6 Wh kg−1 at the power density of 935 W kg−1 . 37 However, the mass loading of active material was severely limited to the surface area of Ni foam current collector. Besides, the specific capacitance of active material was also unsatisfactory for large-scale production. In this work, MnMoO4 nanosheets were hydrothermally grown with GO on three-dimensional nickel foam-supported carbon nanotube structures (NF-CNT). The NF-CNT were prepared by CVD growing CNTs on Ni foam. The optimized CNT-rGO networks provide suitable sites for MnMoO4 nanosheets growth, resulting in CNT/rGO/MnMoO4 composite electrodes with superior specific capacitance, better cycling stability and higher mass loading than CNT/MnMoO4 or MnMoO4 electrodes on the same Ni foams. In view of the high specific surface area and good conductivity of NF-CNT, the negative electrode was prepared by dip coating of AC on it (CNT-AC). The assembled asymmetric device CNT/rGO/MnMoO4 //CNTAC performed excellent electrochemical performance.


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2 2.1

Experimental section Growth of CNTs on Ni foam

CNTs were grown by CVD on Ni foam. Firstly, the Ni foam was ultrasonically cleaned by acetone to remove the organic pollutant, successionally by 1 M HCl solution to remove NiO on the surface and then was stored in ethanol. Secondly, Ni foam was coated with 0.05 M Al(NO3 )3 in ethanol and then annealed at 600 ◦ C for 30 min to form a thin Al2 O3 layer as a barrier layer. Thirdly, 10 mM Ni(NO3 )2 in ethanol as a catalyst was dip-coated onto Ni foam and put in 1-inch quartz-tube furnace. A mixed gas flow of C2 H2 , H2 , and Ar (flow rates were 10, 20, and 200 standard cubic centimeter per minute (SCCM), respectively) was used for CNTs growth. The growth temperature and time were 700 ◦ C and 10 min, respectively. The obtained CNT on Ni foam was labeled as NF-CNT.

2.2

Fabrication of CNT/rGO/MnMoO4 cathode electrode

MnMoO4 and rGO/MnMoO4 hybrid nanosheets were separately grown on the NF-CNT substrates by hydrothermal process. Initially, the prepared NF-CNT was treated by ultrasonic for 30 s and then soaked for 10 min in 3 M HCl aqueous solution to remove the unstable CNTs on Ni foam and modify the CNT surface hydrophilic. The precursor solution for hydrothermal growth was 40 ml mixture of 0.05 M Na2 MoO4 and 0.05 M MnCl2 aqueous solutions with different GO concentrations (0, 0.1, 0.2, and 0.4 mg ml−1 ). Then the treated NF-CNT substrate and the precursor solution were transferred into a 50 ml PPL (para polyphenylene) lined stainless steel autoclave liner and maintained at the optimal growth temperature of 150 ◦ C for 6 h with a ramp rate of 2 ◦ C min−1 . Finally, the CNT/MnMoO4 or CNT/rGO/MnMoO4 composites were washed by deionized water several times and dried at 60 ◦ C for over 8 h. The obtained composites were separately labeled as 0 GO, 0.1 GO, 0.2 GO, and 0.4 GO according to the used GO concentrations.



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2.3

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Fabrication of CNT-AC anode electrode

AC, acetylene black, and PVDF (polyvinylidene fluoride) (5 wt.% dissolved in NMP (1Methyl-2-pyrrolidinone)) were mixed in a weight ratio of 80:10:10. Then, the mixture was uniformly dip-coated on the NF-CNT substrate and dried at 60 ◦ C for over 8 h. The obtained anode was labeled as CNT-AC. For control experiments, the anode with only AC on Ni foam was prepared, which was labeled as NF-AC.

2.4

Calculation

The specific capacitance from CV curves can be calculated by the equation (1) 15 R

I(v)dv m ∗ ∆V ∗ S

Cs =

(1)

Where Cs is the specific capacitance (F g−1 ) , I is the response current (A), m is the mass of activate material (mg), ∆V is the voltage window (V), and S is the scan rate (mV s−1 ). The specific capacitance from GCD curves can be calculated by the equation (2) 38

Cs =

I ∗ ∆t m ∗ ∆V

(2)

Where Cs is the specific capacitance (F g−1 ), I is the discharge current (A), m is the mass of active material (mg), ∆t is the discharge time (s), and ∆V is the voltage window (V). The specific capacitance of both half and full cells can be calculated by the aforementioned two equations. The energy density of the device can be calculated by the equation

E=

1 ∗C ∗V2 2

(3)

E t

(4)

and the power density is given by P =


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Where E is the energy density (Wh kg−1 ), C is the specific capacitance (F g−1 ) calculated by GCD curves of device, V is the voltage window (V), P is the power density (W kg−1 ), and t is the discharge time (s).

2.5

Material characterization

The morphologies were characterized by scanning electron microscope (SEM, MIRA3 TESCAN) with an accelerating voltage of 5 kV. The fine structures were characterized by transmission electron microscopy (TEM, FEI Tecnai F30, operated at 300 kV). The crystal structures were tested by X-ray diffraction (XRD, Philips, X’pert Pro, Cu Kα, 0.154056 nm) and micro-Raman spectroscopy (JY-HR800, 532-nm wavelength YAG laser). The electrochemical tests were carried out by electrochemical work station (CorrTest, CS 310) at room temperature. EIS measurements were obtained in frequencies from 100 kHz to 0.1 Hz at open circuit voltage with an AC voltage perturbation amplitude of 5 mV. The mass of the active material (including both MnMoO4 nanosheets and rGO) was weighed by a microbalance (Mettler, XS105DU) with a tolerance of less than 0.01 mg.

3 3.1

Results and discussion Fabrication and Characterization

Typically, CNTs were grown on Ni foam by CVD at atmospheric pressure, and then MnMoO4 nanosheets were hydrothermally grown with or without GO on the NF-CNT substrates to obtain the hierarchical nanostructured composites. Fig. 1 schematically illustrates the fabrication process of CNT/MnMoO4 and CNT/rGO/MnMoO4 composites. Compared with our previous work of MnMoO4 nanosheets on Ni foam, 37 CNTs here can not only increase the specific surface area of Ni foam but also facilitate the electron and ion transfer during the electrochemical reaction, leading to the increased areal mass loading and enhanced performance. However, for the hydrothermal growth without GO, MnMoO4 nanosheets will grow 7 ACS Paragon Plus Environment


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at the position of defects. The binding between nanosheets and CNTs is mainly physical contact due to rare defects of CVD-grown CNTs. This binding is not favorable for fast electrochemical process and long-term stability of the electrode. Therefore, GO was introduced into the hydrothermal growth of MnMoO4 on NF-CNT. During the growth, the GO sheets will combine with CNT by π-π attractions and MnMoO4 nanosheets will grow at both the positions of CNT defects and GO sheets. The hydrothermally reduced GO and CNTs will form three-dimensional porous conductive CNT/rGO networks as well as structure-directing agents for MnMoO4 growth. These networks can offer extra electron transfer channels not only along CNT growth direction but also between CNTs which can further increase the specific surface area to support high mass loading of active material, ensure excellent electrical conductivity and long-term stability of the electrode. Meanwhile, the GO concentration in precursor will affect the mass loading, morphologies and thus electrochemical performance of the electrode.

3.2

Morphologies and characterization

Fig. 2 (a) and (b) show the SEM images of CVD-grown carbon nanotubes on Ni foam. The Ni foam is covered uniformly and fully by CNTs. The diameter of prepared CNTs is quite homogeneous (about 10-20 nm) which can provide very large surface area for subsequent material growth ( Fig. S1). There are also some metallic nickel nanoparticles at the end of nanotubes as a catalyst in CVD process. Fig. 2 (c)-(f) show the morphologies of CNT/rGO/MnMoO4 samples with different GO concentrations (0, 0.1, 0.2, and 0.4 mg ml−1 , respectively). From which that the morphologies change under different GO concentrations. When there is no GO in the precursor solution (Fig. 2 (c)), the nanosheets grow along CNTs. These CNT/nanosheets networks present dense structure and cover the Ni foam uniformly. When the GO concentration is 0.1 mg ml−1 (Fig. 2 (d)), the nanosheets are assembled to mushroom-like clusters because of the intimate connections between rGO and MnMoO4 nanosheets. When further increasing the GO concentration to 0.2 mg ml−1 8 ACS Paragon Plus Environment

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(Fig. 2 (e)), the clusters link together. The cross-linked rGO nanosheets as ideal conductive networks can greatly shorten the distance of charge transfer and increase the surface area. When the GO concentration continually increases to 0.4 mg ml−1 (Fig. 2 (f)), no clusters can be seen and the morphologies are very likely to the sample without GO, indicating over GO will aggregate and hinder the growth of active material. Therefore, a point of interest is to study the sample with 0.2 mg ml−1 GO concentration. Fig. 3 (a) shows the typical TEM images of CNT/rGO/MnMoO4 (0.2 GO) from which MnMoO4 nanosheets and rGO nanosheets cover over CNTs. Compared with the TEM image of CNT/MnMoO4 ( Fig. S2), rGO nanosheets could provide more growing points than CNTs. The high resolution TEM image (HRTEM) of crystalline MnMoO4 nanosheets (Fig. 3 (b)) clearly reveals an interplanar spacing of 0.24 nm, corresponding to the (021) crystal face of MnMoO4 . In order to identify the distribution of rGO nanosheets and MnMoO4 nanosheets, the corresponding elemental mapping image is shown in Fig. 3 (c). The C, Mn, and Mo elemental segregations are ubiquitous indicating the MnMoO4 nanosheets were grown well on the CNT/rGO conductive networks. The XRD pattern of CNT/rGO/MnMoO4 (0.2 GO) can be well indexed to the triclinic structure of MnMoO4 ·nH2 O (PDF#78-0220) demonstrating the results of HRTEM (Fig. 3 (d)). The peaks marked with a rhombus can be attributed to the metallic nickel. Raman spectrum also confirms the triclinic structure of MnMoO4 as shown in Fig. 3 (e). The peaks between 200 and 1000 cm−1 are ascribed to stretching vibration modes of MnMoO4 39 and the peaks at 1348 and 1580 cm−1 are ascribed to the D band and G band of CNT/rGO, respectively. Compared with the Raman spectrum of NF-CNT (Fig. S3), the ID /IG becomes larger from 0.80 to 1.01, indicating more defects are introduced by rGO which is beneficial for MnMoO4 growth. 40

3.3

Half cell tests

The electrochemical behavior of CNT/rGO/MnMoO4 composite electrodes was studied using a three-electrode system with 2 M NaOH aqueous solution as the electrolyte. The CV 9 ACS Paragon Plus Environment


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curve of NF-CNT is rather rectangular and its integrated area is much smaller than that of the CNT/rGO/MnMoO4 samples (as shown in Fig. S4), indicating the capacitance is significantly increased after growth of rGO/MnMoO4 . In order to further evaluate the kinetic process of electrode, the slope of the linear plot (log(I ) vs log(v ),I is the peak current and v is the scan rate) (Fig. S5) is calculated to be 0.6754, 0.6267, 0.6696, and 0.6798 for 0, 0.1, 0.2, and 0.4 GO, respectively. The slope is greater than 0.5 and less than 1 which indicates that the pseudocapacitance contributes mainly from MnMoO4 rather than the double-layer capacitance from carbon materials (CNT and rGO) to the total capacitance. 41 In order to obtain optimal GO concentration in the precursor solution, CV curves of CNT/MnMoO4 and CNT/rGO/MnMoO4 were recorded at the same scan rate of 50 mV s−1 in a potential window of -0.1∼0.7 V. It can be seen from Fig. 4 (a) that all the CNT/rGO/MnMoO4 samples have similar redox peaks due to the same active material MnMoO4 in electrochemical process. The 0.2 GO sample has the smallest voltage distance between the oxidation and reduction peaks which indicates its outstanding conductivity and excellent reversibility. 42 Meanwhile, the GO concentration can affect the mass loading of MnMoO4 material. The electrode mass loadings (including both MnMoO4 nanosheets and rGO) are 0.68, 0.77, 0.67, and 0.53 mg cm−2 for the 0, 0.1, 0.2, and 0.4 GO samples, respectively. The rGO accounts for about 8.4% of the active material (only include MnMoO4 and rGO) for the 0.2 GO sample which can be calculated by TGA (Thermo Gravimetric Analysis) (Supporting Information Fig. S3 (b)). The variation of mass loading can be explained as follows: The defects of GO can provide more anchor points for MnMoO4 growth so the sample with 0.1 GO has bigger mass loading than the sample without GO; However, further increasing GO concentration will lead to stacking of GO sheets and hinder MnMoO4 growth, resulting in decreased mass loading. The specific capacitances corresponding to the samples with 0, 0.1, 0.2, and 0.4 GO are 1004.4, 1216.9, 1585.1, and 1405.3 F g−1 at the scan rate of 5 mV s−1 , respectively (Fig. 4 (b)). It is worthy of note that the 0.2 GO sample always performs the largest specific capacitance at all scan rates.


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The EIS (shown in Fig. 4 (c)) also demonstrates the difference induced by different GO concentrations and the insert shows the equivalent circuit for fitting. 43 The EIS spectra of the 0 GO, 0.1 GO, and 0.2 GO samples at high frequency can be well evaluated by this simple circuit model. But the 0.4 GO sample has very different chemical reaction mechanism in the frequencies from 100 kHz to 0.1 Hz. The fitting results are shown in Fig. S6 and the selected parameters are listed in Table S1. Rs is the equivalent series resistance related to solution resistance and block resistance (internal resistance of the electrode). 44 The 0.2 GO sample has the minimum Rs implying its best conductivity and outstanding charge/discharge rate capability. The Rct (charge-transfer resistance) is reduced by the introduction of GO suggesting fast ion transport within the electrode. 45 The cross binding of CNT/rGO networks make the high-speed electron and ion transfer channels to reduced Rs and Rct . The almost invariable Cdl (double-layer capacitance) and decreased Cf (Faradic capacitance) demonstrate the relative decreased pseudo capacitance after the introduction of GO. Fig. 4 (d) shows the cycling stability of all four samples. After 3000 cycles the samples with 0 GO, 0.1 GO, 0.2 GO, and 0.4 GO retain 67.0, 78.7, 97.1, and 66.6% of the initial specific capacitance, respectively. The enhanced cycling stability of the samples by GO can be ascribed to the increase of 3D electron and ion transfer channels, abundant growth points, dense and strong hierarchical structures. However, over GO like 0.4 GO will aggregate together and lead to inferior stability. In view of the half cell results, the 0.2 GO sample has the best electrochemical performance. The best performance of the 0.2 GO sample can be ascribed to its proper conductive CNT/rGO networks and appropriate growth points. The detailed CV and GCD curves of the 0.2 GO sample at different scan rates are shown in Fig. S7. With the increase of scan rate, the shapes of the CV curves do not change obviously even at the high scan rate of 100 mV s−1 . The calculated specific capacitances are 2374.9, 1585.1, 1233.9, 1011.6, 817.3, 703.9 F g−1 at the scan rates of 2, 5, 10, 20, 50, 100 mV s−1 , respectively. Moreover, the specific



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capacitances calculated from GCD are 2264.7, 1468.9, 1161.2, 920.3, 697.7 F g−1 at the current densities of 15, 20, 30, 50, 100 mA cm−2 , respectively. The specific capacitance of the 0.2 GO sample is much higher than those of molybdates, such as PPy@MnMoO4 composite (462.9 F g−1 at 5 mV s−1 ), 46 MnMoO4 ·4H2 O nanoplates (2300 F g−1 at 4 mA cm−2 ), 36 Co3 O4 @CoMoO4 core-shell architectures (1902 F g−1 at 1 A g−1 ), 47 NiMoO4 nanosheets (1888 F g−1 at 1 A g−1 ). 48 For the real applications of supercapacitor, the charge of the anode needs to match the charge of its counterpart cathode. However, the anode material AC has limited specific capacitance and low energy density, making it unfavorable for device fabrication. Here the NF-CNT is used as the current collector instead of the generally used Ni foam to load AC (CNT-AC). The structure of CNT-AC can effectively prevent the stacking of AC compared with the NF-AC (AC was dip-coated on Ni foam) and take more full use of the surface area of AC with comparative mass loading. The electrochemical performance of the CNT-AC and the NF-AC samples is shown in Fig. 5 from which the CNT-AC is better than the NF-AC in view of both specific capacitance and rate capability at the same mass loading (3.7 mg cm−2 ). The CNT-AC can reach a specific capacitance of 234 F g−1 at the current density of 5 mA cm−2 which is much larger than the NF-AC. The CV and GCD curves of CNT-AC are shown in Fig. S8 and the rectangular shapes of CV and triangular shapes of GCD curves at low and high scan rates/current densities indicate the double-layer capacitance of AC.

3.4

Full cell tests

Based on the half cell results of the samples, the CNT/rGO/MnMoO4 with 0.2 GO was chosen to assemble a full cell. The schematic illustration of aqueous asymmetric CNT/rGO/ MnMoO4 //CNT-AC supercapacitor is shown in Fig. 6 (a): the CNT/rGO/MnMoO4 as cathode and CNT-AC as anode were separated by a separator and soaked in 2 M NaOH electrolyte. The two of this kind supercapacitor device in series can light up 13 red and 6 orange 5-mm paralleled light emitting diode (LED) indicators implying large output power 12 ACS Paragon Plus Environment

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of the device (Fig. 6 (b)). Meanwhile, two devices in series also can light up a blue 5mm LED indicator implying high output potential of the device. Charge balance between the CNT/rGO/MnMoO4 cathode and CNT-AC anode was achieved by adjusting the mass loading of the active materials and verified by the areal ratio of CV measurements at the same scan rate of 20 mV s−1 (Fig. 6 (c)). A series of CV measurements at 50 mV s−1 have been initially checked at different potentials from 1.0 V to 1.8 V in order to find out the optimal operating potential window. As can be seen from Fig. 6 (d), the device is stable up to an operational voltage of 1.6 V, namely its optimal voltage window. At this optimal voltage, the measured CV curves are shown in Fig. 7 (a). The specific capacitances of the device can reach 161.4, 134.1, 122.8, 110.3, 95.0, and 84.0 F g−1 at the scan rates of 2, 5, 10, 20, 50, and 100 mV s−1 , respectively. Fig. 7 (b) shows the GCD curves at different current densities and the triangular shapes further indicate the superior electrochemical capacitive performance. The calculated specific capacitances can reach 148.4, 111.9, 94.5, 86.9, and 77.5 F g−1 at the current densities of 5, 10, 20, 30, and 50 mA cm−2 , respectively . The corresponding energy densities of the device can reach 59.4, 44.8, 37.8, 34.7, and 31.0 Wh kg−1 at the power densities of 1367.9, 2637.7, 5273.3, 7865.7, and 12263.7 W kg−1 , respectively. As shown in the Ragone plot of Fig. 7 (c), compared with other molybdate based asymmetric supercapacitor devices like AC//Co3 O4 @NiMoO4 , 49 NiMoO4 NSs//AC, 50 CoMoO4 /MnO2 NFs//AC ASC 51 and their composites (NiMoO4 -CoMoO4 //AC 52,53 ), our devices perform much higher energy density and power density. In addition, cycling stability is one of the most pivotal elements of supercapacitor in real applications. As shown in Fig. 7 (d), the cycling stability of the CNT/rGO/MnMoO4 //CNT-AC supercapacitor still has 87.5% of initial specific capacitance after 1000 charge/discharge cycles at the current density of 15 mA cm−2 . The excellent performance of the device could be attributed to the perfect 3D conductive structure of Ni foam/CNT/rGO and excellent electrochemical properties of MnMoO4 .



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4

Conclusions

In summary, the hierarchical nanostructured CNT/rGO/MnMoO4 composite electrodes were constructed by a facile two-step method: the Ni foam-CNTs substrates were prepared by CVD growth of CNTs on Ni foam and subsequently the MnMoO4 nanosheets were hydrothermally grown with GO on the NF-CNT. Under the optimal GO concentration, the binder-free CNT/rGO/MnMoO4 electrode exhibits a high specific capacitance of 2374.9 F g−1 at the scan rate of 2 mV s−1 and good cycling stability (97.1% of the initial specific capacitance can be maintained after 3000 charge/discharge cycles). These unexpected performance can be attributed to the synergistic effects from each component of composite electrodes: highly pseudocapacitive MnMoO4 nanosheets and three-dimensional conductive CNT/rGO networks on Ni foam that boosts the electrical conductivity and increases the mass loading thus resulting in enhanced redox-based pseudocapacitance. In addition, the asymmetric device with CNT/rGO/MnMoO4 as the cathode and CNT-AC as the anode can perform an energy density of 59.4 Wh kg−1 at the power density of 1367.9 W kg−1 . This value has surpassed those of previously reported asymmetric supercapacitors consisting of nickel or cobalt molybdates, suggesting the fabricated asymmetric supercapacitor can be a promising candidate for energy storage devices. Meanwhile, the novel hierarchical structures pave a new way to improve the energy density and cycling stability of supercapacitor.

Associated Content Supporting Information More structural characterizations and electrochemical measurements. The Supporting Information is available free of charge on the ACS Publication website at DOI:


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Acknowledgement This research was supported by the National Natural Science Foundation of China (Nos: 51302122, 51572118), the Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University (No: LZUMMM2016010), the Fundamental Research Funds for the Central Universities (No: lzujbky-860268), and the National Science Foundation for Fostering Talents in Basic Research of the National Natural Science Foundation of China in Lanzhou University.

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Figures

Figure 1: Schematic illustration of CNT/MnMoO4 and CNT/rGO /MnMoO4 fabrication process on Ni foam.



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Figure 2: (a) Low and (b) high magnification SEM images of NF-CNT. SEM images of CNT/rGO/MnMoO4 sample with different GO concentrations: (c) 0 mg ml−1 GO, (d) 0.1 mg ml−1 GO, (e) 0.2 mg ml−1 GO and (f) 0.4 mg ml−1 GO.


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Figure 3: The CNT/rGO/MnMoO4 sample (0.2 GO): (a) Typical TEM image, (b) High resolution image of nanosheets, (c) ) Typical STEM image of the CNT/rGO/MnMoO4 and the corresponding elemental mapping images for C, Mo and Mn, (d) XRD pattern, (e) Raman spectrum.



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Figure 4: (a) CV curves of the CNT/rGO/MnMoO4 samples with different GO concentrations at the same scan rate of 50 mV s−1 . (b) Specific capacitance of samples with different GO concentrations as a function of scan rate. (c) EIS of the CNT/rGO/MnMoO4 samples. (d) Cycling performance of the CNT/rGO/MnMoO4 samples.

Figure 5: Specific capacitance of NF-AC and CNT-AC as a function of (a) scan rate and (b) current density.


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Figure 6: (a) Schematic illustration of device structure. (b) Digital photographs of LED indicators lighted up by two devices in series. (c) CV curves of CNT-AC and CNT/rGO/MnMoO4 electrodes at 20 mV s−1 in their respective working voltage window. (d) CV curves of the device at different voltage windows at the same scan rate of 50 mV s−1 .



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Figure 7: (a) CV curves of the device at different scan rates. (b) GCD curves of the device at different current densities. (c) Ragone plot of the CNT/rGO/MnMoO4 //CNT-AC device. (d) Cycling stability of the device at a current density of 15 mA cm−2 .


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TOC


rGO-Supported ... - ACS Publications

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