Preparation and Capacitive Behavior of Dandelion-Like γ-MnO 2 Nanofibre / Activated CarbonMicrobeads Composite for the Application of Supercapacitor

Dandelion-like γ-manganese dioxide (γ-MnO2) nanofibre/activated carbon microbeads (ACMBs) composite is prepared by an in situ coating technique. The structure and morphology of the composite are characterized by scanning electron microscopy and X-ray diffraction. The results show that γ-MnO2 nanofibre is uniformly encapsulated on the surface of ACMB, and the composite finally becomes a dandelion-like microbead. Cyclic voltammetry, galvanostatic current charge/discharge, and cycle life measurements are used to evaluate the electrochemical behaviors of the composite. Since the composite is able to undergo pseudofaradic charge transfer reactions and hereto contributes together with the double-layer effect to the total capacitance of the material, the specific capacitance of the composite is as high as 375.9 F g−1 at a scan rate of 1 mV s−1, which is significantly higher than the pure ACMB. Besides, the capacitance retention of the supercapacitor using the composite as electrode-active material keeps still 93% after 1000 cycles.


Introduction
Currently, electrochemical supercapacitors are extensively studied as auxiliary energy storage devices to be used with rechargeable batteries.Their applications include electric vehicles, renewable energy, mobile generator devices, directcurrent power systems, and uninterruptible power supplies [1].On the basis of the energy storage mechanism, supercapacitors can be classified into two categories [2], namely the electrical double-layer capacitor and the faradic pseudocapacitor.The capacitance of the former comes from the charge accumulation at the electrode/electrolyte interface, therefore, strongly depending on the surface area of the electrode accessible to the electrolyte.The capacitance of the latter is due to the reversible faradic redox reaction of electroactive species on the electrode, such as surface functional groups and transition metallic oxides.It is obvious that the electrode takes the important part in the development of supercapacitors.
Carbon microbeads have several advantages, such as high electrical conductivity, good fluidity, excellent sphericity, easy-to-control pore size distribution and a relatively low cost [3].Usually, the carbon materials only possess doublelayer capacitance, while metallic oxide possesses faradic pseudocapacitance, and the faradic pseudocapacitance is almost 10-100 times higher than double-layer capacitance [4].Transition metallic oxides have been widely studied as promising materials for electrochemical capacitors due to their redox chemistry, large pseudocapacitance, and high power and energy density.
An improvement in the capacitance of carbon material can be realized by preparing carbon/metallic oxide composites.Recently, many carbon/transition metallic oxides or hydroxide composites for the application of supercapacitor have been widely developed, which are based on a combination of double-layer capacitance and faradic pseudocapacitance [5][6][7][8].With regard to metallic oxides, amorphous hydrated ruthenium oxide has been found to be an excellent electrode [9,10] because of its ideal pseudocapacitance behavior (as high as 760 F g −1 specific capacitance) and good reversibility.However, the high cost of this precious metallic oxide hinders its practical application.Hence, seeking for inexpensive metallic oxides, for example, NiO, CoO x , SnO 2 , Mn 3 O 4 , and MnO 2 , as alternative electrode materials is of great interest [11][12][13][14][15][16].With its low cost and environmental friendliness, the manganese oxides with various crystal structures have been intensively investigated as electrode-active materials of supercapacitor [17,18].
It has been found that appropriate MnO 2 modification on the surface of carbon material can apparently improve its capacitive behavior.It has been reported that the specific capacitance of the carbon aerogel loaded with MnO 2 can increase from 133 F g −1 to 219 F g −1 [19].Our group prepared the MnO 2 /carbon aerogel composite by chemical co-precipitation and obtained a high specific capacitance of 226.3 F g −1 [5].
Although some works have been carried out on composites of activated carbon, CNTs, carbon aerogel, and carbon fiber coated with MnO 2 , there is less reported on γ-MnO 2 nanofibre/ACMB composite.In this paper, dandelion-like γ-MnO 2 nanofibre/ACMB composites were prepared by an in situ coating technique.The electrochemical performances of γ-MnO 2 nanofibre/ACMB composite were investigated in detail.

Experimental
2.1.Preparation of γ-MnO 2 Nanofibre/ACMB Composite Electrode.All chemical reagents were the analytical grade and directly used as received.ACMB was prepared by a typical hydrothermal technique as follow.The glucose (1.5 mol L −1 ) was put into a Teflon-lined stainless steel autoclave of 100 mL capacity.The autoclave was sealed and maintained at 160 • C for 18 h in an oven, then cooled to room temperature naturally.A brown precipitate was collected, washed with distilled water and absolute ethanol.Then the obtained sample was dried in a vacuum oven at 80 • C for 12 h.Finally, the sample was carbonized at desired temperature 750 • C for 1 h in a flow of Ar, followed by concentrated nitric acid activation at 70 • C stirred for 24 h to form ACMB.
The in situ preparation process of γ-MnO 2 nanofibre/ACMB composite was as follow: firstly, 1.0 g ACMB was dispersed in 150 mL of distilled water by ultrasonic vibration for 10 min.Secondly, 1.9441 g MnSO 4 •H 2 O was added into the above suspension, and stirred for 30 min at room temperature.Subsequently, 3.1106 g K 2 S 2 O 8 was added into the above solution according to chemical stoichiometric ratio, and then refluxed 6 h at 100 • C under an uninterrupted stir.The resulting γ-MnO 2 nanofibre/ACMB composite precipitate was filtered and washed with distilled water and ethanol several times until the filtrate was about neutrality.Then the product was dried at 80 • C for 8 h in an oven.Finally, γ-MnO 2 nanofibre/ACMB composite was obtained.The content of γ-MnO 2 nanofibre in the dandelion-like γ-MnO 2 nanofibre/ACMB composite was about 20 wt.%.

Measurement Techniques of Structural Characterization.. (i) Scanning electron microscopy (SEM) (JSM-6610,
JEOL) was used to study the morphology and surface structure of the samples.(ii) X-ray diffraction (XRD) of samples was performed on a diffractometer (D/MAX-3C) with Cu Ka radiation (λ = 1.54056Å) and a graphite monochromator at 50 kV, 100 mA.

Evaluation of Electrochemical
Properties.The mixture containing 80 wt.% γ-MnO 2 nanofibre/ACMB, 10 wt.% acetylene black and 10 wt.% polyvinylidene fluoride (PVDF) was well mixed in N-methyl-2-pyrrolidone (NMP) until to form the slurry with proper viscosity, and then the slurry was uniformly laid on a Ni foam that was used as a current collector (area was about 1.5 cm 2 ) and then dried at 80 • C for 12 h.The Ni foam coating γ-MnO 2 nanofibre/ACMB composite was pressed for 1 min under a pressure of 1.6×10 6 Pa.The electrochemical performances of the prepared electrodes were characterized by cyclic voltammetry (CV) and charge/discharge tests.The used electrolyte was 6 mol L −1 KOH solution.The experiments were carried out using a three-electrode cell, in which the Ni foam and the saturated calomel electrode were used as counter and reference electrodes, respectively.The cyclic voltammetry and the charge/discharge measurements at constant current were performed by means of electrochemical analyzer systems, CHI660 (CH Instruments, USA).The cycle life was carried out by potentiostat/galvanostat (BTS6.0,Neware, Guangdong, China) on button cell supercapacitors, and the symmetrical button cell supercapacitors were assembled according to the order of electrode-separator electrode

Results and Discussion
3.1.Material Characterization. Figure 1 presents the SEM images of the ACMB and γ-MnO 2 nanofibre/ACMB composite.As being seen from Figure 1(a), the carbon microbeads are good dispersity, smooth surface, perfect spherical morphology, and about 2 μm average size.The images in Figures 1(b) and 1(c) show that the γ-MnO 2 nanofibre/ACMB composite keeps the spherical morphology of ACMB, but the surface of the ACMB sphere is covered by a lot of γ-MnO 2 nanofibers and finally becomes a dandelionlike appearance.The average diameter of the dandelion-like γ-MnO 2 nanofibre/ACMB composite is about 3 to 4 μm.The dandelion-like appearance of the γ-MnO 2 nanofibre/ACMB composite can provide not only bigger specific surface area, but also higher electron and ion conductivity as electrodeactive material of supercapacitor.Besides, γ-MnO 2 nanofibre can also play an important role as a pathway of electron transfer continuously and electrolyte conveyance during charging/discharging process for supercapacitor.
Figure 2 shows XRD patterns of (a) ACMB and (b) γ-MnO 2 nanofibre/ACMB composite.In Figure 2  The broad diffraction peaks suggest a highly disordered and amorphous structure of the ACMB.Simultaneously, the clear appearance of two diffraction peaks indicates that the ACMB is a partial graphitization carbon.Hence, the ACMB has an improved electrical conductivity.Besides two diffraction peaks corresponding to graphite planes of ACMB Figure 2(a), some new diffraction peaks can be observed in Figure 2 respectively.All characteristic peaks can be indexed to the (110), ( 201), ( 211), (221), and (520) planes of orthorhombic γ-MnO 2 (JCPDS card no.82-2169).Therefore, it can actually be considered that the composite is consisted of ACMB encapsulated by γ-MnO 2 nanofibre.For CV measurement, the specific capacitances of electrode can be estimated based on the following equation [20]: where i is a sampled current, dt is a sampling time span, and ΔV is the total potential deviation of the voltage window.Figure 3(b) shows cyclic voltammograms of the γ-MnO 2 nanofibre/ACMB electrode at different scan rates.The specific capacitances of ACMB and γ-MnO 2 nanofibre/ACMB are tabulated in Table 1.It can be found from Table 1 that the specific capacitance of the γ-MnO 2 nanofibre/ACMB electrodes is obviously higher than that of ACMB at every given scan rate.The reason is probably that the capacitance of the γ-MnO 2 nanofibre/ACMB electrodes is combination of double-layer capacitance (ACMB) and faradic pseudocapacitance (γ-MnO 2 ).In addition, it can also be seen from Figure 3(b) that the redox current of γ-MnO 2 nanofibre/ACMB electrode increases clearly with the increasing of the scan rate, indicating its good rate ability.

Electrochemical Characterization of γ-MnO
In order to gain a further understanding on the electrochemical performance of γ-MnO 2 nanofibre/ACMB composite material, the comparison of galvanostatic charge/ discharge curves for the ACMB electrode and γ-MnO 2 nanofibre/ACMB composite electrode at 0.5 A g −1 is shown in Figure 4(a).The charge/discharge curve of ACMB is almost linear, which indicates that ACMB has good electrical double-layer properties.In comparison, the charge/discharge curve of γ-MnO 2 nanofibre/ACMB composite electrode deviates from ideal linear, and it can be attributed to different capacitive behavior of γ-MnO 2 nanofibre/ACMB electrode.Besides, the charge/discharge curve of γ-MnO 2 nanofibre/ACMB exhibits longer charge/discharge duration than one of the ACMB, indicating an enhancing charge storage capacity, which is mainly ascribed to faradic capacitance of γ-MnO 2 .
The charge/discharge curves of the γ-MnO 2 nanofibre/ACMB composite electrodes at different current densi-ties are shown in Figure 4(b).As being seen from Figure 4(b), the discharge time of γ-MnO 2 nanofibre/ACMB composite electrode quickly drops as the current density increases.It may be because at the low current density, the ions have enough time to diffuse into the micropore of γ-MnO 2 nanofibre/ACMB; while at the high current density, the ions can only partially penetrate into the micropore of active material.The specific capacitance of supercapacitor can be evaluated from the charge/discharge test according to the following equation [21]: where I, Δt, ΔV , and m were the current used for charge/ discharge, A, the time elapsed for the charge or discharge cycle, s, the voltage interval of the charge or discharge, V, and the mass of activated carbon on the electrodes, g, respectively.
In order to gain a further understanding on the electrochemical performance of γ-MnO 2 nanofibre/ACMB composite material, ACMB and γ-MnO 2 nanofibre/ACMB was used as electrode-active materials of symmetrical supercapacitor, respectively.The variation of specific capacitances of the ACMB and γ-MnO 2 nanofibre/ACMB supercapacitor during 1000 cycles is shown in Figure 5.A significant improvement of the specific capacitance occurs before 200 cycles for the ACMB supercapacitor, which is mainly attributed to a gradual activation process of the active material ACMB.After 200 consecutive cycles, the capacitance of the ACMB supercapacitor was fixed at a steady value.However, it has been seen that the supercapacitor with γ-MnO 2 nanofibre/ACMB possesses markedly much higher specific capacitance than with ACMB.Furthermore, the specific capacitance of the supercapacitor with γ-MnO 2  nanofibre/ACMB displays excellent cyclic stability and only 7% decay after 1000 consecutive cycles.Therefore, the γ-MnO 2 nanofibre/ACMB can be considered as a promising electrode-active material for long-term supercapacitor applications.

Conclusions
Dandelion-like γ-MnO 2 nanofibre/ACMB composite was successfully prepared by an in situ coating process on the surface of ACMB.Dandelion-like γ-MnO 2 nanofibre/ACMB composite for the application of supercapacitor showed excellent electrochemical performances.The specific capacitance of the supercapacitor using γ-MnO 2 nanofibre/ACMB as electrode-active material is apparently higher than one of the supercapacitor using ACMB as electrode-active material because the specific capacitance of the γ-MnO 2 nanofibre/ACMB is combination of double-layer capacitance (ACMB) and faradic pseudocapacitance (γ-MnO 2 ).The specific capacitance of the γ-MnO 2 nanofibre/ACMB composite electrode calculated from CV curve is as high as 375.9 F g −1 at 1 mV s −1 , which is significantly higher than that of ACMB electrode (266.3F g −1 ).Besides, the capacitance retention of the supercapacitor with γ-MnO 2 nanofibre/ACMB composite as electrode-active material is up to 93% after 1000 cycles.Therefore, it is obviously proved that the γ-MnO 2 nanofibre/ACMB composite exhibits extensive potential as highefficiency electrode material of supercapacitor.
(a), the XRD pattern of ACMB reveals the broad characteristic peaks at 2θ = 23.5 • and 43.2 • , which correspond to diffraction from (002) plane and (101) plane of graphite.

2
Nanofibre/ACMB Composite.In order to evaluate the electrochemical characteristics of γ-MnO 2 nanofibre/ACMB composite, cyclic voltammetry and galvanostatic charge/discharge were used to characterize the electrochemical capacitance behavior.Figure3(a) shows cyclic voltammograms for ACMB electrode and γ-MnO 2 nanofibre/ACMB composite electrode.It can be found that the capacitance characteristic of the ACMB is typically electric double-layer capacitance, which can generally produce a CV curve close to the ideal rectangular shape.By contrast, the CV curve for the γ-MnO 2 nanofibre/ACMB composite electrode showed the presence of significant redox peaks, indicating that faradic reactions took place during the charge/discharge processes.Besides, it can be seen from Figure3(a) that the area surrounded by CV curve for the γ-MnO 2 nanofibre/ACMB composite electrode is apparently more than one of the ACMB electrode, indicating that the γ-MnO 2 nanofibre/ACMB electrode has much more specific capacitance than ACMB.The difference of CV curves for ACMB and γ-MnO 2 nanofibre/ACMB composite is attributed to the different capacitance mechanisms.The pure ACMB showed an electrical double-layer capacitance, whereas the γ-MnO 2 nanofibre/ACMB composite possessed a combination of both electrical double-layer capacitance and pseudocapacitance.