PPy core-shell heterostructured nanobelts for supercapacitor

Accepted Manuscript Self-assembling hierarchical NiCo2O4/MnO2 nanosheets and MoO3/PPy coreshell heterostructured nanobelts for supercapacitor Si-Wen Zhang, Bo-Si Yin, Chang Liu, Zhen-Bo Wang, Da-Ming Gu PII: DOI: Reference:

S1385-8947(16)31732-6 http://dx.doi.org/10.1016/j.cej.2016.11.144 CEJ 16145

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

12 October 2016 11 November 2016 25 November 2016

Please cite this article as: S-W. Zhang, B-S. Yin, C. Liu, Z-B. Wang, D-M. Gu, Self-assembling hierarchical NiCo2O4/MnO2 nanosheets and MoO3/PPy core-shell heterostructured nanobelts for supercapacitor, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.11.144

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Self-assembling hierarchical NiCo2O4/MnO2 nanosheets and MoO3/PPy core-shell heterostructured nanobelts for supercapacitor Si-Wen Zhang 1, Bo-Si Yin 1, Chang Liu 1, Zhen-Bo Wang 1,*, Da-Ming Gu 1 1

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and

Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No.92 West-Da Zhi Street, Harbin, 150001 China. E-mail: [email protected]; Tel.: +86-451-86417853; Fax: +86-451-86418616 ABSTRACT A high energy/power density aqueous asymmetric supercapacitor device is assembled by self-assembled NiCo2O4/MnO2 composite as a positive electrode and MoO3@PPy composite as a negative electrode in Na2SO4 electrolyte. Due to the synergistic effect of electronic conductivity of PPy and high-rate metal oxides, the heterostructure electrodes reveal better charge transport and cycling stability. The overall areal capacitance retentions for the NiCo 2O4/MnO2 and MoO3@PPy electrode materials are 97.5% and 86.2% after 6000 cycles, respectively. Such unique nanoarchitecture in the hybrid device further presents remarkable electrochemical performance with high capacitance and ideal cycle life at high rates. The novel device with an expanded operating voltage window of 1.6 V, presents a high energy density of 60.4 Wh kg-1 and a maximum power density of 2400 W kg-1. The device demonstrates a good cycle life with 88.2% capacitance retention after 10000 cycles. This strategy for the choice provides a promising route for the next-generation device of energy storage and conversion with high energy, high power density, and long life. Keywords: Aqueous asymmetric device, NiCo2O4/MnO2 nanosheet, MoO3@PPy nanobelt,

1

Low-cost, Long cycle life

1. Introduction With the increasing interests in electric energy storage for portable systems and mobile electronics, many efforts have been dedicated to the new generation energy devices. Among miscellaneous conversion devices and electrochemical energy storage, supercapacitors (SCs) is hopeful as energy-type and long life candidates[1-8]. Transition metal oxides as positive electrodes of SCs have been diffusely explored with high energy densities and areal capacitance deriving from faradic redox reactions[9-15]. Among the transition metal oxides, spinel nickel cobaltite (NiCo 2O4) has been received vast interest recently in supercapacitor applications because of its low-cost, high availability and environmental friendliness [3]. Particularly, NiCo 2O4 has much higher redox activity and better electrical conductivity compared to cobalt oxides and nickel oxides[16-21]. At the same time, MnO2 [22-25] also has been widely investigated as pseudocapacitive material due to its natural abundance, low toxicity and high theoretical capacitance. Therefore, NiCo2O4 with higher conductivity can support the active electrode material (MnO2) forming 3D hierarchical hybrid nanostructure for high-performance supercapacitor device. For the negative electrode, Khomenko’s group reported an asymmetric capacitor with manganese oxide/activated carbon[26]. Cheng and his coworkers investigated MnO2 nanowire/graphene//graphene asymmetric electrochemical capacitors[27].

Liu

et

al

studied

CoO@Polypyrrole

nanowire

array

used

in

supercapacitor[28]. As is known, metal oxides (MOs) in general exhibit higher electrochemical energy storage ability arising from faradic redox reactions than carbon-based materials. Among

2

various transition metal oxides, MoO3 with a typical two-dimensional layered structure is one of the most actual interest because of its unique structure is beneficial for small ions to insert/remove such as H+, Na+ and K+ [29] and multiple oxidation states that enable rich redox reactions to happen for pseudocapacitive[30], which contributes to high specific capacitance. Nevertheless, its poor inherent electrical conductivity usually affects its high performance in device and reduces faradic redox kinetics. To address this problem, an effective way to improve electrochemical performance of MoO3 is to coat MoO3 with a conductive polymer. Polypyrrole (PPy) is an interesting conductive polymer due to its high electrical conductivity, low cost and the performance of coating forbidding the structural collapsing happened to transition metal oxide after charge-discharge process[31-33]. Herein, we report a novel low-cost high-performance aqueous asymmetric device with self-assembled NiCo2O4/MnO2 composite as a positive electrode and MoO3@PPy composite as a negative electrode. And for all we know, this is the first study to report the new design. The device achieves a specific capacitance of 170.1 F g-1 with a relatively wide operational voltage of 1.6 V and a high energy density of 60.4 Wh kg-1 at a power density of 950.1W kg-1. For comparison, we also constructed a button cell with 1 mol L-1 Na2SO4 aqueous electrolyte and PVA-Na2SO4 gel as soild-state electrolyte, respectively. The results show that the performance of the button cell was much higher in aqueous electrolyte.

2 Experimental 2.1 Synthesis of NiCo2O4/MnO2 nanosheet arrays Firstly, NiCo 2O4 nanosheet arrays were synthesized on Ni foam (NF) by a facile hydrothermal reaction followed with a simple post-annealing process. In a typical synthesis, 0.2 g

3

Ni(NO3)2·6H2O, 0.4 g Co(NO3)2·6H2O, 1.2 g NH4F and 3 g urea were dissolved in 70 mL of deionized water under magnetically stirring for 1 h in air. The as-obtained solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and then a piece of clean NF was immersed into the reaction solution. After that, the autoclave was heated to 120 oC inside a conventional oven for 3 h, and then cooled to room temperature. The product on the NF was washed with distilled water repeatedly, and then dried at 60 oC for 12 h. Afterward, the sample was calcined at 350 oC for 2 h at a ramping rate of 5 oC min-1 to transform into NiCo2O4 nanosheets. Next, the NF was put again into a solution containing 0.1 mol L-1 KMnO4 in an autoclave, and the autoclave was subsequently maintained at 180 oC for 30 min. Finally, the sample was washed with distilled water, and annealed at 200 oC in air for 1 h to obtain NiCo 2O4/MnO2 nanosheet arrays. The mass loading of the NiCo 2O4 on the NF was calculated to be about 2.1 mg. The mass loading of the NiCo2O4/MnO2 nanosheets on the NF was about 3 mg. 2.2 Preparation of MoO3@PPy nanobelts The MoO3 nanobelts were prepared by a hydrothermal method. In a typical synthesis, 40 mL of H2O2 (30%) was added dropwise into 4 g of molybdenum powders in the ice-water bath under magnetic stirring for 4 h to remove the redundant H2O2, forming a clear orange solution. The orange solution was transferred and sealed in a 100 mL Teflon-lined stainless autoclave. The autoclave was heated to 180 oC for 24 h, then cooled to room temperature. The MoO3@PPy nanocomposite was prepared by in situ oxidative polymerization. In a typical synthesis, 0.1 g MoO3 nanobelts were fully dispersed in 20 mL deionized water at ambient temperature and then transferred into an ice bath. 0.2 mL of pyrrole was added into the above

4

solution and stirred for 30 min. Then, 0.4 g (NH4)2S2O8 (dissolved in 10 mL of deionized water) was slowly dropped into the above solution. The mixture was kept at ice bath for 4 h under stirring. Finally, the obtained composite was washed with water and ethanol, respectively, and then dried under vacuum at 60 oC for 12 h. The mass loadings of the MoO3 and MoO3@PPy on the NF were calculated to be about 1.5 mg and 2 mg, respectively. 2.3 Fabrication of Solid-state Asymmetric Device (AAD) The AAD were assembled by using MoO3@PPy nanobelts as the negative electrode and NiCo 2O4/MnO2 nanosheet arrays as the positive electrode with a separator (NKK TF40, 40 µm thickness, low ESR type, purchased from SCM industrial Chemical CO.,LTD) and PVA/Na2SO4 gel as a solid electrolyte. PVA/Na2SO4 gel was prepared as follows: in a typical process, 6 g PVA was dissolved in 60 mL deionized water with stirring at 85 oC for 1 h. Then, 1 mol L-1 Na2SO4 was slowly dropped into the above solution at 80 oC under stirring until the solution became clear. The NiCo 2O4/MnO2 electrode, the MoO3@PPy electrode and the separator were soaked in the gel for about 3 min, and then were assembled together. The device was kept at 60 oC for 12 h to remove excess water in the electrolyte. 2.4 Characterization The obtained product was characterized by scanning electron microscopy (SEM, Hitachi-4800), and X-ray powder diffraction (XRD, Rigaku Dmax-rB, Cu Kα radiation, λ = 0.1542 nm, 40 kV, 100 mA). X-ray photoelectron spectroscopy (XPS) was carried out by using the Physical Electronics PHI model 5700 instrument and a field emission transmission electron microscopy (FETEM) (JEM-2100). Electrochemical characteristics of the as-obtained products were studied on an CHI660 electrochemical work station (Chenhua,

5

Shanghai) using cyclic voltammetry, chronopotentiometry and electrochemical impedance test by configuring the sample into a three-electrode cell, where the sample was used as the working electrode, Pt foil as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. 2.5 Calculation Methods 2.5.1 Single Electrode The areal capacitance (Ca) of the electrode could be calculated from its charge/ discharge curve by the following equation [5]: Ca = I ∆t / S ∆V

(1)

where C (mF cm−2) is areal capacitance, I (mA) represents the discharge current, and S, ∆ t (s) are designated as the effective electrode area and total discharge time, respectively. 2.5.2 AAD Energy density (E) and average power density (P) could be calculated as [34]: E=1/2CsV2

(2)

P=E/t

(3)

where Cs is specific capacitance calculated before, V is the cell voltage and t is the discharge time.

3 Results and Discussion 3.1 Positive electrode material The structure and morphology of the NiCo2O4/Ni foam (NF) nanosheets are confirmed by scanning electron microscopy (SEM) together with energy-dispersive X-ray spectroscopy (EDS). Fig. S1a-b shows large quantity of sheet-like structure on the substrate. EDS analysis

6

in Fig. S1c-g confirms the existence of Co, Ni and O in the NiCo 2O4 nanosheets. The representative synthesis process of the NiCo2O4/MnO2 nanosheets electrode is shown in Fig. 1a. Fig. 1b and 1c display a low-magnification SEM image and a high-magnification SEM image of heterostructure NiCo2O4/MnO2 nanosheet arrays grown on NF. The nanosheets are consistently grown on NF to form a coating on the surface and the uniform coverage of MnO2 nanoflakes on each NiCo2O4 nanosheet surface can be seen clearly. Fig. 1d shows X-ray diffraction (XRD) patterns of the NiCo2O4 and NiCo2O4/MnO2 nanosheet arrays. Apart from a strong peak from the NF substrate, all the other diffraction peaks can be well indexed to spinel NiCo 2O4 phase (JCPDS 20-0781) and the tetragonal MnO2 phase (JCPDS 44-0141). The detailed elemental composition of the as-prepared NiCo2O4/MnO2 nanosheets is further analyzed by EDS as shown in Fig. 1e-h. It can be seen that the NiCo 2O4/MnO2 nanosheets are composed of Mn, O, Ni and Co elements. The more detailed elemental composition of the as-prepared material is further characterized by X-ray photoelectron spectroscopy (XPS) in Fig. 1i. The survey spectra indicate the presence of Co, Ni, Mn and O, and the C signal may be attributed to adventitious carbon. The Mn 2p region exhibits two main peaks at 644.4 and 657.7 eV, showing that the element Mn is in the formation of MnO2 [35,36] It can confirm that the NiCo2O4/MnO2 nanosheets are successfully prepared.

Fig. 1

The TEM images of the NiCo2O4/MnO2 nanosheets are shown in Fig. 2a and b. The dark shadow in Fig. 2a is due to the wrinkles of the nanosheets. The high-magnification TEM of

7

Fig. 2b clearly shows that the mesopores are uniformly distributed on the whole surface of the nanosheets. Their size is about 2-5 nm. It is generally known that the mesoporous structure in nanosheets may accelerate the mass transport in electrode/electrolyte interface and enhance the electrochemical performance of electrode [37,38]. The result of high resolution TEM (HRTEM) shows that the interplanar spacing of lattice fringes is 0.23 nm, corresponding to that of the (222) lattice planes of the spinel NiCo2O4 (Fig. 2c). The selected-area electron diffraction (SAED) pattern (Fig. 2d) presents well-defined diffraction rings, suggesting its polycrystalline characteristics.

Fig. 2

To get the optimized electrochemical performance of the nanosheets electrode, time controlled experiments are conducted. As shown in Fig. S2a and b, MnO 2 nanoflakes prepared after 15 min of reaction time are thin and sparsely distributed on the surface of NiCo 2O4 nanosheets. With the increase of reaction time to 30 min, NiCo2O 4 nanosheets are gradually covered by MnO 2 nanoflakes as presented in Fig. S2c and d. When the reaction time reaches 45 min, NiCo2O 4 nanosheets are covered by thick and dense MnO2 nanoflakes in Fig. S2e-f. The effect of different reaction time on electrochemical impedance spectroscopy (EIS) of as-prepared materials is also presented in Fig.S3a-d. All the three samples exhibit the similar shape at lower frequency and higher frequency region. At the high frequency, the intersection of the curve at the real part displays the resistance of the electrochemical system (Rs ) and the

8

semicircle diameter reflects the charge-transfer resistance (Rct)[39]. Rs of the sample which reaction time is 45 min (0.78 Ω) is greater than the others (about 0.25 Ω and 0.66 Ω corresponding to 30 min and 15min, respectively). Rct (0.05 Ω) of the sample prepared for 30 min reaction time is also the smallest, indicating the best electroactivity and electrical conductivity. It can confirm that the electrical conductivity of the sample depends on the MnO2 layer thickness. When the reaction time reaches 45 min, NiCo2O 4 nanosheets are covered by thick and dense MnO 2 nanoflakes. The worse electrical conductivity of NiCo 2O4/MnO 2 nanocomposite should be resulted from the difficulty of electronic diffusion from MnO2 nanoflakes to NiCo2O 4 nanosheets.

Fig. 3

The electrochemical performance of the NiCo2O4/MnO2 nanosheets is tested in a three-electrode cell with 3 mol L-1 KOH as the electrolyte. Fig. 3a shows the cyclic voltammetry (CV) curves of the NiCo 2O4/MnO2 nanosheets and pure NF at 100 mV s-1. The results reveal that the pure NF has little contribution to the total capacitance of the integrated NiCo 2O4/MnO2 nanosheets electrode. The electrochemical performance of the NiCo2O4 nanosheets electrode is also evaluated by CV and galvanostatic charge-discharge (GCD) measurements as shown in Fig. S4a-b. It is worth noting that the CV curves consist of a pair of redox peaks corresponding to the voltage plateaus revealed in GCD curves. The electrode exhibits a typical faradic behavior due to the faradic redox reactions between M-O and

9

M-O-OH, where M refers to Ni and Co[40]. The representative CV curves of the NiCo 2O4/MnO2 nanosheets are presented in Fig. 3b. The rectangular shape of CV curves reveals an ideal pseudocapacitance performance of the sample. Fig. S5a shows the CVs of the NiCo 2O4 and NiCo2O4/MnO2 core-shell nanoflake arrays self-supported on NF at a scanning rate of 100 mV s-1. It is worth noting that the CV integral area of the NiCo2O4/MnO2 electrode material is apparently larger than that of the pure NiCo2O4, indicating that the NiCo2O4/MnO2 material has a larger areal capacitance than the NiCo2O4. It should be attributed to the additional pseudocapacitance contributed by the MnO2 shell, which can adsorb K+ cation on the electrode surface and/or possibly intercalate and deintercalate K+ ion [41]. GCD measurements are further performed in the voltage range of 0 and 0.45 V. A symmetric near-linear triangular shape of NiCo2O4/MnO2 electrode is observed in Fig. 3c, further manifesting its pseudocapacitance behavior and excellent reversibility. Fig. S5b shows the comparison of the NiCo 2O4 and NiCo2O4/MnO2 electrodes for GCD curves. It is clear to see that the NiCo2O4/MnO2 nanoflakes array electrode has higher discharging time than NiCo2O4 nanosheets array electrode. Its areal capacitances are 13.9, 13.8, 12.7, 11.1, 9.8 F cm-2 at current densities of 4, 6, 8, 10, 15 mA cm-2, respectively (Fig. 3d). According to the charge-discharge curves, the areal capacitances comparison of the NiCo 2O4 and NiCo 2O4/MnO2 electrodes are also calculated based on (Fig. S5d). It is clear that the areal capacitance is largely enhanced for the NiCo2O4/MnO2 nanoflakes array electrode. Thus, the unique array electrode provided a short ion diffusion path way and effective electron transfer which is further proved by the EIS measurements (Fig. 3e and Fig. S5c). The Rs of the NiCo 2O4/MnO2 nanoflakes array is about 0.25 Ω, demonstrating the lower internal resistance

10

than the NiCo 2O4 nanosheets electrode (0.83 Ω). The overall areal capacitance retention for the NiCo2O4/MnO2 electrode is 97.5% after 6000 cycles in Fig. 3f. In addition, the mesoporous structure of the nanosheets can accelerate the diffusion of electrolyte within electrode, which may result in the enhanced performance of the NiCo 2O4/MnO2 electrode. 3.2 Negative electrode material The growth schematic of MoO3@PPy nanobelts electrode is illustrated in Fig. 4a. First, the MoO3 nanobelts are prepared by a simple hydrothermal process. Using ammonium persulphate (APS) as an oxidant, PPy is further immobilized onto the nanobelts by a simple chemical polymerization. The morphological images of MoO3 and MoO3@PPy composite are shown in Fig. 4b-c and Fig. S6a-b. The prepared MoO3 presents 1D nanobelts with 1 µm length and 100 nm width (Fig. S6a-b). In the PPy@MoO3 nanocomposite (Fig. 4b-c), the MoO3 nanobelts are homogeneously coated by PPy with the thickness of about 30-50 nm. The EDX spectrum (Fig. S6c-f) demonstrates that the MoO3 nanobelts are mainly composed of Mo and O. Except for Mo and O elements, it can be seen from Fig. 4d-g that the as-prepared nanobelts also contain C and N elements. The result shows the PPy is successfully coated on the surface of MoO3 nanobelts. The XRD patterns of the MoO3 nanobelts and MoO3@PPy nanocomposite are shown in Fig. 4h. All the diffraction peaks can be well indexed to (JCPDS card no. 05-0508) which is orthorhombic structure. However, the weaker peak intensity of the MoO3@PPy nanocomposite is caused by the coating of semi-crystalline PPy [42,43].

Fig. 4

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Fig. 5a-b is the TEM micrographs of the as-prepared MoO3@PPy nanocomposite. The 1D MoO3@PPy nanobelts exhibits the width of 130-150 nm and length of 1.2 µm. According to the low-resolution TEM image in Fig. 5c, the PPy shell is homogeneous with thickness of 30-50 nm. The results agree with those of SEM. The MoO3@PPy nanobelts also present a single-crystal structure as shown in Fig. 5d.

Fig. 5

The electrochemical study for the MoO3@PPy is conducted in a three-electrode system with 1 mol L-1 Na2SO4 electrolyte. For comparison, both the MoO3@PPy and pure NF are investigated. The CV curves of the MoO3@PPy and NF are compared in Fig. 6a, which reveals that the pure NF almost has no contribution to the total capacitance. The electrochemical performance of the MoO3 nanobelts is also investigated by CV and GCD measurements in Fig. S7a-b. The CV curves contain a pair of redox peaks at -0.4V and -0.8V corresponding to the voltage plateaus revealed in GCD curves, which presents the pseudocapacitance

behavior.

These

peaks

present

the

reversible

and

fast

intercalation/deintercalation process of sodium ions in the MoO3 phase[44]. Additionally, CV curves of the MoO3@PPy electrode collected at various scan rates of 10 to 100 mV s-1 are shown in Fig. 6b. All of the CV curves of MoO3@PPy nanocomposite electrode present a rectangular shape without obvious redox peaks, which revealed an ideal capacitive behavior[45]. Fig. 6c shows the galvanostatic charge-discharge curves of the MoO3@PPy electrode at different current densities. The comparison of CV and GCD curves for the MoO3

12

nanobelts and MoO3@PPy nanocomposite is presented in Fig. S8a-b. It is interesting that the operating voltage window of MoO3@PPy nanobelts electrode is higher than pure MoO3 nanobelts electrode, which may be attributed to the lower internal resistance and the better conductivity, suggesting that the MoO3@PPy nanobelts are good electrode material in storage device. Its areal capacitances are 2.8, 2.2, 1.2, 0.9, 0.8 F cm-2 at current densities of 4, 6, 8, 10, 15 mA cm-2, respectively (Fig. 6d). Its maximal areal capacitance of 2.9 F cm-2 observed is about 6-fold higher than that of MoO3 electrode (0.5 F cm-2) in Fig. S8d. The electrical conductivity of the electrode decides the capacitive performance. The capacitance of the bare MoO3 nanobelts is low due to the poor internal conductivity. Fig. 6e and Fig. S8c show the Nyquist plots of the MoO3@PPy and MoO3 nanobelts electrodes in 1 mol L-1 Na2SO4 aqueous solution. The impedance of MoO3@PPy nanocomposite is lower (1 Ω) than MoO3 nanobelt (1.36 Ω), indicating the good electrical conductivity of heterostructure. The cycling stability is significant to estimate the application of electrode materials. The overall areal capacitance retention of the MoO3@PPy electrode is 86.2% after 6000 cycles (Fig. 6f). Therefore, the MoO3@PPy core/shell nanobelts electrode shows a good electrochemical stability for long-term cycles and achieves a better electrochemical behavior.

Fig. 6 3.3 Aqueous asymmetric supercapacitor device Using PVA-Na2SO4 gel as the electrolyte, a solid-state hybrid device is constructed and investigated here. For comparison, the performance of the hybrid device using 1 mol L-1 aqueous Na2SO4 as electrolyte is also presented. The design of the device is schematically

13

displayed in Fig. S9a. Fig. S9b-c show the CV and GCD curves comparison of the aqueous electrolyte and gel electrolyte-based devices, respectively. As can be seen, the electrochemical performance of aqueous electrolyte-based device is higher than that of PVA-Na2SO4 gel electrolyte-based device. This phenomenon is explained by the EIS testing shown in Fig. S9d. Obviously, the Rs value for the solid-state device (7.62 Ω) is larger than that of the aqueous device (4.61 Ω). It means that the ion diffusion and charge transfer in the solid-state device which is sluggish are not good as that of the aqueous one. Based on the above reasons, it can be believed that electrochemical performance of the device with aqueous electrolyte is more excellent.

Fig. 7

To further evaluate the two electrodes for real applications, an aqueous asymmetric supercapacitor device (the total mass of the active materials is 5 mg) was fabricated using NiCo 2O4/MnO2 nanoflakes and MoO3@PPy nanobelts as the positive and negative electrodes, respectively (Fig. 7a). The CV curves of the optimized hybrid device at various scan rates is shown in Fig. 7b. As can be seen, even at a scanning rate of 100 mV s-1, the shape of CV curve remains undistorted, indicating low contact resistance in the device. GCD test is also performed with different current densities in the voltage window of 0-1.6 V as shown in Fig. 7c. The symmetrical charge/discharge characteristic and the quick I-V response represent good capacitance property of our device. According to the CV results for single electrode, the CV curves of the device are obtained at 30 mV s-1 with the operating cell voltage windows

14

from 0-1.4 to 0-2.4 V as shown in Fig. 7d. The device shows a typical capacitance behavior with nearly rectangular CV curves, demonstrating good stability under different voltage windows. The specific capacitance of the device is calculated from the GCD curves as presented in Fig. 7e. Its specific capacitances are 156.5, 170.1, 112.2, 82.4 and 67.5 F g-1 at 4, 6, 8, 10 and 15 mA cm-2, respectively. The long-term cycle property of the device is further proved in Fig. 7f. Obviously, its capacitance tends to slowly decrease and maintains about 88.2% of the original capacitance after 10000 cycles, exhibiting a good cyclic stability. Three devices connected in series could easily light up a blue LED that shows the use of devices in real application (Fig. 7f inset).

Fig. 8

The Ragone plots of NiCo2O4/MnO2//MoO3@PPy derived from the GCD curves are shown in Fig. 8. The as-assembled button cell delivers a maximum energy density of 60.4 Wh kg-1 at 960 W kg-1 and 24 Wh kg-1 at 2400 W kg-1 (The calculated based on the total mass of the NiCo2O4/MnO2 and MoO3@PPy electrodes). It is also much superior than many recent energy storage devices (The negative electrodes of all the devices above consist of carbon materials), such as CoO@PPy//AC (43.5 Wh kg-1 at 87.5 W kg-1)[28], NiCo2O4@MnO2 //AC(35 Wh kg-1 at 163 W kg-1)[46], Ni(OH)2/graphene // RGO (31 Wh kg-1 at 420 W kg-1)[47],V2O5/ECF // ECF (22.3 Wh kg-1 at 1500W kg-1)[48], and Ni(OH)2-CNTs //AC (32.5 Wh kg-1 at 1800 W kg-1)[49].

4 Conclusions 15

In summary, an aqueous asymmetric supercapacitor device based on low-cost NiCo 2O4/MnO2 nanosheets array and MoO3@PPy nanobelts is assembled by a simple way. Compared with the previous reports, our newly designed device demonstrates excellent stability in a large potential of 1.6 V and exhibits the excellent energy density of 60.4 Wh kg-1 and the high power density of 2400 W kg-1. The 11.8% decay after 10000 cycles of the device manifests the outstanding cycling stability. Our work not only provides the possibility of high power/energy density devices with long life, but also presents a novel design for energy storage and conversion devices.

Acknowledgments This research is financially supported by the National Natural Science Foundation of China (Grant No. 21273058 and 21673064), China postdoctoral science foundation (Grant No. 2012M520731), Heilongjiang postdoctoral financial assistance (LBH-Z12089) and the support from King Saud University of Saudi Arabiavisiting professor program.

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1272-1278.

23

List of figure captions Fig. 1 Structural characterization of NiCo 2O4/MnO2 nanoflakes composite.(a) Preparation process for the NiCo 2O4/MnO2 nanocomposite. (b-c) SEM images of NiCo2O4/MnO2 nanoflakes grown radially on the NF. (d) Typical XRD patterns of NiCo2O4 and NiCo 2O4/MnO2 nanoflakes. (e-h) EDX mapping of a typical nanosheets. XPS spectrum of NiCo 2O4/MnO2 nanoflakes (i) The survey spectrum NiCo 2O4/MnO2 nanoflakes. Inset shows a Mn 2p spectrum. Fig. 2 (a) Low-magnification TEM image, (b) Enlarged TEM image and (c) HRTEM image of NiCo 2O4/MnO2 nanoflakes. (d) The corresponding SAED pattern in (c). Fig. 3 Electrochemical characterizations of NiCo2O4/MnO2 nanoflakes in three electrodes system. (a) The CV curves of NiCo 2O4/MnO2 nanoflakes and the pure NF at a scanning rate of 100 mV s-1, (b) The CV curves at scan rates between 10 and 100 mV s-1, (c) Charge/discharge curves at current densities ranged from 4 to 15 mA cm-2. (d) The areal capacitance as a function of the current densities of the NiCo2O4/MnO2 nanoflakes. (e) EIS spectra of the NiCo2O4/MnO2 nanoflakes electrode. (f) Cycling performance at current density of 4 mA cm-2 Fig. 4 Structural characterization of MoO3@PPy nanobelts composite.(a)Schematic diagram illustrating the synthesis procedure of MoO3@PPy nanobelts. (b-c) SEM images of MoO3@PPy nanobelts. (d-g) EDX mapping of a typical nanobelt. (h) Typical XRD patterns of MoO3 and MoO3@PPy nanobelts. Fig. 5 (a) Low-magnification TEM image. (b) Enlarged TEM image. (c) HRTEM image of MoO3@PPy nanobelts. (d) The corresponding SAED pattern in (c)

24

Fig. 6 Electrochemical characterizations of MoO3@PPy nanobelts in three-electrode system. (a) The CV curves of MoO3@PPy nanobelts and the pure NF at a scanning rate of 100 mV s-1, (b) The CV curves at scan rates between 10 and 100 mV s-1, (c) Charge/discharge curves at current densities ranged from 4 to 15 mA cm-2. (d) The areal capacitance as a function of the current densities of the MoO3@PPy nanobelts. (e) EIS spectra of the MoO3@PPy nanobelts electrode. (f) Cycling performance at current density of 4 mA cm-2 Fig. 7 The NiCo2O4/MnO2//MoO3@PPy device. (a) Comparative CV curves of NiCo 2O4/MnO2 nanoflakes and MoO3@PPy nanobelts electrodes performed in a three-electrode cell. (b) CV curves at different scanning rates. (c) Charge-discharge curves at different current densities. (d) CV curves of the assembled device collected in different scan voltage windows. (e) Specific capacitance calculated from the charge/discharge curves as a function of current density. (f) Cycling performance at current density of 4 mA cm-2. Inset shows a blue LED powered by the devices. Fig. 8 Ragone plots of the NiCo2O4/MnO2//MoO3@PPy hybrid device and others from the literatures.

25

(a)

NiCo 2O4 nanosheets

NiCo 2O4/MnO2 nanosheets

(b)

(c)

(c) (d)

NiCo2O4

Intensity(a.u.)

NiCo2O4/MnO2

2µm

MnO2

200nm 20

40

60

(f)

Mn 2p3/2

Mn 2p3/2

I n t e n s i t y ( a .u .)

In ten sity (a .u .)

630

640 Energy650(eV) Binding

500nm

500nm

1000

800

600

400

Binding Energy (eV) Fig. 1

26

660

670

Binding Energy (eV)

C 1s

O 1s

670 665 660 655 650 645 640 635 630

Ni 3s M n 3s M n 3p

M n LLM Ni 2p3

M n 2p1 M n 2p3

(h)

Mn 2p1/2 Mn 2p 1/2

Co 2p3

(g)

500nm

Intensity(a.u.)

500nm 500nm

80

2θ(degree)

(i)

O KLL

(d) (e)

200

0

(a)

(b)

pore

wrinkles

20 nm

50 nm (c)

(d) 0.23 nm

5 nm

5 1/nm

Fig. 2

27

(a)

(b)

(b)-1

10 mV s-1

10 mV s

-1

30 mV s

-1

50 mV s

-1

30 mV s

0.15 0.15

-1

50 mV s

0.10 0.10

-2) Current(Acm cm-2 Current(A )

NiCo2O4/MnO2

-1

-1

80 mV s

80 mV s

-1

-1

0.05 0.05

100 mV s

100 mV s

0.00 0.00 -0.05 -0.05 -0.10 -0.10 -0.15 -0.15 -0.20 -0.20

0.0 0.0

0.1 0.1

0.5

0.2 0.3 0.2 0.3 Potential(V) Potential(V)

0.4 0.4

0.5 0.5

0.0 0.0

0.1 0.1

0.2 0.2

0.3 0.3

0.4 0.4

0.5 0.5

Potential(V) Potential(V)

(c) 0.4

4 mA cm-2

18

6 mA cm-2

16

8 mA cm-2 10 mA cm-2

Potential(V)

0.20 0.20

(a)

Ni NiCo2 O4/MnO2

Ni

0.3

15 mA cm-2

0.2 0.1

Areal Capacitance(F cm -2)

Current(A cm-2 ) Current(A cm-2)

0.20 0.20 0.15 0.15 0.10 0.10 0.05 0.05 0.00 0.00 -0.05 -0.05 -0.10 -0.10 -0.15 -0.15 -0.20 -0.20

0.0

(d)

14 12 10 8 6 4 2 0

0

500

1000

1500

2000

2500

3000

3500

4

6

8

10

12

14

16

-2

Time(s)

Current Density(mA cm ) 120

10

(e)

(f)

Capacitance retention(%)

-Z''(Ω)

100

5

80 60 40 20 0

0 0

1

2

3

4

5

6

7

8

9

1000

10

2000

3000

4000

Cycle number

Z'(Ω)

Fig. 3

28

5000

6000

PPy

(a) Hydrothermal

MoO3

(NH4)2S2O8

Pyrrole

MoO3 nanobelts

MoO3/PPy nanobelts

(b)

(c)

1µm

100nm

(e)

3µm

3µm (f)

(h)

(g)

3µm

3µm

MoO3 MoO3/PPy

Intensity(a.u.)

(d)

20

40

60

2θ(degree)

Fig. 4

29

80

(a)

(b)

1µm

100nm (d)

(c)

30-50nm 5 1/nm

20nm

Fig. 5

30

(a)

0.15

0.15

0.10

10 mV s-1 30 mV s-1

Ni

Current(A cm-2)

Current(A cm-2)

(b)

MoO3/PPy

0.05 0.00 -0.05 -0.10

0.10

50 mV s-1

0.05

100 mV s-1

80 mV s-1

0.00 -0.05 -0.10

-0.15

-0.15 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

-1.0

-0.8

-0.6

Potential(V)

(c)

0.2

4 mA cm

8 mA cm

Potential(V)

-2

10 mA cm 15 mA cm

-2 -2

-0.4 -0.6 -0.8 -1.0 0

200

400

600

800

0.2

(d)

2

1

0

1000 1200 1400 1600

4

6

Time(s)

8

10

12

14

16

Current Density(mA cm-2)

120

(e)

30

0.0

-2

Areal Capacitance(F cm )

6 mA cm 2

-0.2

-0.2

Potential(V) 3

-2

0.0

-0.4

(f) Capacitance retention(%)

100

25

-Z''(Ω)

20 15 10 5

80 60 40 20 0

0 0

10

20

30

40

0

50

1000

2000

3000

4000

Cycle number

Z'(Ω)

Fig. 6

31

5000

6000

(a)

0.15

NiCo2O4/MnO2

0.10

Current(A cm -2 )

C u r ren t(A cm -2 )

0.10

MoO3/PPy

0.05 0.00 -0.05 -0.10

1.8

10 mV s-1

(b)

-0.20 -1.0 -0.8 -0.6 -0.4 -0.2

0.0

0.2

4 mA cm-2 6 mA cm-2

30 mV s-1

1.6

50 mV s-1

1.4

0.04 0.02

80 mV s-1

1.2

10 mA cm-2

1.0

15 mA cm-2

100 mV s

-1

0.00 -0.02 -0.04 -0.06

0.8 0.6

0.2 0.0

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

0.4

8 mA cm-2

0.4

-0.08 -0.10

-0.15

(c)

0.08 0.06

Potential(V)

0.20

0

100

Potential(V)

Potential(V)

200

300

400

500

600

700

Time(s) B

(d)

0-1.4V 0-1.6V 0-1.8V 0-2.0V 0-2.2V 0-2.4V

C u rren t(A cm -2 )

0.04 0.02 0.00 -0.02 -0.04 -0.06 -0.08

160

120

(e)

(f) 100 C apacitance retention(% )

0.06

180

Specific capacitance(F g -1)

0.08

140 120 100 80 60 40 20

0.0

0.5

1.0

1.5

Potential(V)

2.0

2.5

60 40 20 0

0

-0.10

80

4

6

8

10

12

14 -2

Current density(mA cm )

Fig. 7

32

16

0

5000 Cycle number

10000

Energy density(Wh kg-1)

102 Ref.45 Ref.28

Ref.47

Ref.44 Ref.46

101

This work NiCo2O4/MnO2//AC CoO/PPy//AC Ni(OH)2/graphene// RGO Ni(OH)2/CNT//AC V2O5/ECF//ECF

100 101

102

103

Power density(W kg-1)

Fig. 8

33

104

Self-assembling hierarchical NiCo2O4/MnO2 nanosheets and MoO3/PPy core-shell heterostructured nanobelts for supercapacitor Si-Wen Zhang 1, Bo-Si Yin 1, Chang Liu 1, Zhen-Bo Wang 1,*, Da-Ming Gu 1

120 10

2

10

1

Capacitance retention(%)

100 80 60 40 100 101

20

102

103

104

Power density(W kg-1)

0 0

5000 Cycle number

34

10000

Highlights ·

A novel low-cost high-performance aqueous asymmetric device was designed

·

The device presents a maximum energy/power densities of 60.4 Wh kg-1 and a

kg-1.

·

The capacitance of the device can still maintains 88.2% after 10000 cycles.

35

2400 W