Facile synthesis of hierarchical hollow ε-MnO2 spheres and their application in supercapacitor electrodes

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Facile synthesis of hierarchical hollow ε-MnO2 Spheres and their application in supercapacitor electrodes Dandan Han, Xiaoyan Jing, Pengcheng Xu, Yuansheng Ding, Jingyuan Liu

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S0022-4596(14)00284-9 http://dx.doi.org/10.1016/j.jssc.2014.06.041 YJSSC18533

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Journal of Solid State Chemistry

Received date: 1 April 2014 Revised date: 13 June 2014 Accepted date: 24 June 2014 Cite this article as: Dandan Han, Xiaoyan Jing, Pengcheng Xu, Yuansheng Ding, Jingyuan Liu, Facile synthesis of hierarchical hollow ε-MnO2 Spheres and their application in supercapacitor electrodes, Journal of Solid State Chemistry, http: //dx.doi.org/10.1016/j.jssc.2014.06.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile Synthesis of Hierarchical Hollow ε-MnO2 Spheres and Their Application in Supercapacitor Electrodes Dandan Han a, Xiaoyan Jing b, Pengcheng Xu a*, Yuansheng Ding a and Jingyuan Liub

a

College of Chemistry and Pharmaceutical Engineering, Jilin Institute of Chemical Technology, Jilin 132022, PR China b Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, PR China

∗ Corresponding author. Tel.: +86 0432 6331 5369; fax: +86 0432 6331 5369. E-mail address: [email protected] Address: College of Chemistry and Pharmaceutical Engineering, Jilin Institute of Chemical Technology, Jilin132022, Jilin province, P. R. China

E-mail address: Dandan Han, E-mail: [email protected] Pengcheng Xu, E-mail: [email protected]

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Abstract: The hierarchical hollow microspheres of ε-MnO2 have been synthesized through a facile chemical method at room temperature followed by selective removal of manganese carbonate structures with HCl. The microstructure and morphologies of the resulting materials are investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy. The results indicate that the product obtained by simple reaction for 3 min has porous shell with excellent permeability, uniform pore-size distribution. Electrochemical properties were characterized by cyclic voltammetry, galvanostatic charge/discharge and impedance spectra. As a result, the hierarchical hollow ε-MnO2 showed the specific capacitance of 115 F·g-1 at 0.5 A·g-1. These results demonstrate that the ε-MnO2 as electrode materials have potential application for high-performance supercapacitors.

Keywords: Hierarchical hollow manganese dioxide, Nanosheet, Electrochemical performance

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1. Introduction Over the past decade, porous inorganic materials have attracted great interest for their wide applications such as in the fields of in ion exchange, catalysis processes, and supercapacitors [1-3], because of their peculiar and fascinating properties that cannot be obtained in their bulk counterparts [4]. Thus, the change of material structure from solid to porous provides a potential way to improve their properties and application. Among the important functional metal oxides, different crystallographic forms of MnO2 are identified as better materials for supercapactor applications due to their earth abundance, low cost, environmental friendliness and the ability to charge and discharge rapidly in a wide potential window. Recently, various morphologies of MnO2 have been prepared in different types, such as wires, rods, belts, and tubes [5-8]. Chen and co-workers reported the synthesis of nanoporous hollow MnO2 spheres by a hydrothermal condition [9]. Ma’ group prepared sea urchin shaped MnO2 and 3D hierarchical clew like ε-MnO2 nanostructures through a facile hydrothermal method under different reaction conditions [10], Golikand et al synthesized α- and γ-MnO2 nanowires with diameter about 30-70 nm though a cathodic electro-synthesis route [11]. As one of the most attractive hierarchical architectures, on the other hand, porous hollow manganese oxide nanostructures have displayed the enhanced interfacial area for electron/ion adsorption and unhindered mass transport during electrochemical process. However, most of the reported synthesis methods are either hydrothermal mediated [12-15] or tedious and expensive combinations of procedures such as use of noble metals [16, 17]

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and electrochemical deposition [18-20], which are employed to generate bulk nanostructures and films of MnO2 for supercapacitor applications. In this paper, we report a facile method to synthesize hierarchical hollow ε-MnO2 architectures via the direct reaction between MnCO3 and KMnO4 within 3 min at room temperature. This method is quite facile, secure, controllable and energy-saving. The electrochemical performances of the as-prepared ε-MnO2 as the electrodes of supercapacitors have also been investigated. It is found that the as-prepared ε-MnO2 architectures provide more possibility to serve as an ideal host material for the adsorption of the H+ and Na+ ions due to the nanoporous structure.

2. Experimental Section 2.1. Synthesis procedure All the chemicals are of analytical grade and were used without further purification. The synthesis of MnCO3 microspheres was carried out on the basis of the previous work of Qian’s group [21]. The well-mixed aqueous solutions of MnSO4, and NaHCO3 were maintained at room temperature for 3 h. The as-prepared products were filtered, washed with distilled water, and dried in air at 60°C for 12 h. The as-prepared MnCO3 spheres were dispersed in 20 mL of distilled water, and then 0.03M KMnO4 solution was added under stirring. After 2 min, 2.5M HCl was added into the above solutions, and the mixtures were maintained at room temperature with stirring for 1 min. Finally, the hollow ε-MnO2 was separated, washed with double distilled water several times, and dried in a vacuum at 60°C for 12 h. 2.2. Characterization

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X-ray diffraction (XRD) measurement was examined on a Rigaku D/max TTR-III diffractometer using Cu Kα radiation (λ = 0.15405 nm). The morphologies of the samples were inspected on a scanning electron microscope (SEM, JSM-6480A, Japan Electronics). Transmission electron microscopy (TEM) micrographs were performed on a FEI Tecnai G2 20 S-TWIN transmission electron microscope with a field emission gun operating at 200 kV. 2.3. Preparation of electrodes and electrochemical characterization To evaluate the electrochemical properties of the ε-MnO2 microstructures, working electrodes were prepared as follows. The as-prepared materials, graphite, acetylene black, and poly (tetrafluoroethylene) (PTFE) were mixed in a mass ratio of 75:10:10:5 and dispersed in ethanol to produce a homogeneous paste. Graphite, acetylene black and PTFE were used as the conductive agent and binder, respectively. Then the mixture was coated onto the nickel foam substrate (1 cm2) with a spatula. Finally, the fabricated electrodes were dried at 60 ºC for 1 day in a vacuum oven. The loading level of the active material on the current collector is typically 0.02 g·cm-2. Cyclic voltammetry (CV), chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS) measurements were carried out using conventional three-electrode configuration on a CHI 660 E electrochemical workstation. The Ni foam coated with MnO2 was used as the working electrode, and platinum foil (1 cm2) and a saturated calomel electrode (SCE) were used as the counter and reference electrodes. All the experiments were carried out using freshly prepared 1.0 M aqueous NaSO4 electrolyte in double distilled water. The EIS measurements were carried out in the frequency

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range from 100 kHz to 0.05 Hz at open circuit potential with an ac perturbation of 5 mV.

3. Results and discussion 3.1. Material characterization X-ray diffraction (XRD) was carried out to determine the phase and purity of the prepared samples, and the results of (a) MnCO3 sphere precursor and (b) hierarchical hollow ε-MnO2 are given in Fig. 1. All of the reflections of the XRD pattern in Fig. 1a can be readily indexed to pure hexagonal phase of MnCO3, according to reported data (JCPDS Card No. 86-0173). The strong peaks in the XRD pattern are mainly attributed to the high crystallinity. As shown in Fig.1b, the patterns of the as-prepared sample match well with the standard patterns of the hexagonal ε-MnO2 (JCPDS 30-0820, a =2.80, c =4.45 Å). No peaks for other types of MnO2 were observed, indicating that the ε-MnO2 have been successfully prepared from the pure phase hexagonal MnCO3. The morphology and structure of the precursor MnCO3 sphere are examined by SEM and TEM. Fig. 2a and c present the low-magnification SEM and TEM images of MnCO3 sphere, it is obvious to see that the sample mainly contains uniform microspheres with diameters of 1-1.3µm. The texture of the nanospheres is further identified by the high magnification (Fig. 2b and d). As shown, the surface of MnCO3 sphere is not smooth. Moreover, a large yield tetrahedron or pyramid structures are observed on the surface of spheres, which may be formed by the self arrangement of MnCO3 nanoparticles. It is worth noting that those unique morphologies of MnCO3

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microstructures are different from reported other MnCO3 samples [9, 21, 22, 23]. The hollow MnO2 with hierarchical structure was obtained by the oxidation of the spherical precursor crystals in KMnO4 solution at room temperature followed by removal of MnCO3 with HCl. Fig. 3a and b show the SEM images of MnO2 hollow spheres, indicating that the as obtained MnO2 maintains the frame structure of the precursor. The cavity and rough surface can also be seen from the picture. As shown in Fig. 3e, the spectra of energy dispersive spectroscopy (EDS) demonstrates that the MnO2 spheres all consisted of carbon, manganese and oxygen, which is in agreement with those observed in the XRD pattern. TEM has been employed to further investigate the structure. Fig. 3c and d display the TEM image of a representative MnO2 microsphere with a diameter of about 1.5 µm, which is slightly larger than the precursor spheres. The different brightnesses in the TEM image can also illuminate the hollow structure of the sample. It is worth noting that the microspherical cages have a shell consisting of interconnected, irregularly shaped, and primary sheets with a thickness of 50 nm, which results in an average shell thickness of about 100 nm. The hollow structure with porous shell should be favorable to improve the capacitance behavior of an electroactive material due to unhindered diffusion and accession of electrolyte ions into the inner space/matrix [24]. 3.2. Growth mechanism of hierarchical hollow MnO2 spheres It is assumed that the formation of micrometersized spheres around spherical precursor crystals is analogous to that reported previously [25]. It can be explained as a fundamental solid-state phenomenon, the so-called Kirkendall effect [22, 26] which

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deals with the movement of the interface between diffusion couples. The formation process of the MnCO3 microsphere and hierarchical hollow MnO2 spheres can be proposed as Fig. 4. When the reactants are mixed, the generated MnCO3 small particles are formed. Further increasing the hydrothermal reaction time, the dispersed MnCO3 small particles spontaneously aggregate to microspheres, driven by the minimization of interfacial energy. Consequently, irregular growth of MnCO3 particles leads to a rough surface with specific geometric configuration. Correspondingly, the MnO2 shell with MnCO3 intermediate core is prepared at room temperature using a freshly prepared solution of dilute KMnO4 to oxidize the surface of the MnCO3 intermediate. KMnO4 and MnCO3 can react with each other readily and form a diffusion pair. The coupled reaction/diffusion at the crystal/solution interface might lead to the quick formation of a MnO2 shell around the outside surfaces of the MnCO3 crystals. After removal of the MnCO3 intermediate core with HCl, well crystallized MnO2 hierarchical hollow structure was obtained. There has always been a strong correlation between the crystallinity structure and the surface area of the system on the electrochemical properties as discussed later. 3.3. Electrochemical characterizations To explore the potential applications of the as-synthesized hierarchical hollow MnO2 micropheres, the sample was fabricated as supercapacitor electrodes and characterized by CV, EIS, and galvanostatic charge/discharge measurements in electrochemical energy storage. The CV responses of the MnO2 sample carried out at different scan rates (10-50 mV·s-1) in a fixed potential range of -0.2 to 0.8 V in

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aqueous 1 M Na2SO4 electrolyte are shown in Fig. 5a. The individual MnO2 sample electrode was subjected to one time stabilization/activation by repeated CV cycles (25 numbers) at sweep rates of 50 mV·s-1 in the similar potential range, before performing the CV experiments for accessing the electrochemical performance. The curves at different scan rates show no peaks, indicating that the electrode is charged and discharged at a pseudoconstant rate over the complete voltammetric cycle. Since the redox reactions depend on the insertion-deinsertion of protons from the electrolyte [27-29], In neutral electrolyte as studied here, there are generally two parallel mechanisms proposed, based on the adsorption and intercalation involving surface and bulk phenomena during the charge storage in porous MnO2-based electrodes [30, 31]. The first possible mechanism is ascribed to the rapid intercalation of alkali metal cations such as Na+ in the electrode during reduction and deintercalation upon oxidation. MnO 2 + C + + e - ←⎯ → MnOO - C + The other possible mechanism is based on the adsorption of the H+ and Na+ ions on the surface, rather than in the bulk of the sample [30, 31]. MnO 2 ( surface ) + C + + e - ←⎯ → ( MnO-2 C + ) ( surface) where C can be either H+ or Na+ ion. From the CV graphs it is realized that there is an increase in the anodic (positive) and cathodic (negative) current density with increase in the scan rate, and at low scan rates (5 mV·s-1), the diffusion of ions from the electrolyte can gain access to almost all available pores of the electrode, leading to a complete insertion reaction, therefore, it shows almost ideal capacitive behavior.

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However, with the scan rate increasing, the effective interaction between the ions and the electrode decreases, the deviation from rectangularity of the CV becomes obvious. From the CV measurements at different scan rates ( v , mV·s-1), the specific capacitance (Cs, F·g-1) values are estimated using the equation [31-34]: Cs =

Vc 1 i × VdV ∫ V vw(Va − Vc ) a

(1)

where w(g) is active weight of the electrode material. The specific capacitance values of the samples at different scan rates were drawn from the integration of potential (V versus SCE) versus specific capacitance (Cs, F·g-1) graphs, as shown in Fig. 5b. The calculated specific capacitance values for the electrode at scan rates of 10, 20, 30, 40 and 50 mV·s-1 are 189, 158, 139, 126 and 112 F·g-1, respectively. As the scan rate increased, the specific capacitance values decrease, which due to the limitation of the ion diffusion rate to satisfy electronic neutralization during the redox reaction. In order to get more information about the ability of the synthesized MnO2 as an electrode material in a supercapacitor, constant current charge/discharge measurement was carried out in 1M Na2SO4. The rate dependent discharge profile taken from the first cycles of corresponding voltage-time profile of the sample is shown in Fig. 5c. During the charging and discharging steps, the charge curves are very symmetric to their corresponding discharge counterparts in the whole potential region and the slope of every curve is potential-independent and maintains a constant value at a specified current. The specific capacitance of the electrode is obtained from the following equation [32-34]:

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C=

I [( dV / dt ) w]

(2)

where I (A) and dV/dt (V·s-1), respectively, denote the applied galvanostatic current and the slope of chronopentionmetric curve, w(g) represents the mass of electroactive material. Therefore, the specific capacitances of the electrode at 0.5, 1, 1.5 and 2 A·g-1 are 115, 100, 77 and 60 F·g-1(Fig. 5d), respectively. The capacitance measured at 0.5 A·g-1 which is higher than the reported [35]. The decrease of the capacitance with the increase of the discharge current density is likely caused by the increase of potential drop due to the resistance of the hollow spheres and the relatively insufficient reduction and deintercalation of the active material under higher discharge current densities. The specific capacitance obtained at the slowest scan rate is more close to that of full utilization of the active material of the electrode. The EIS has been employed to study the kinetic features of MnO2 sample electrode during their charge storage process. The impedance of hierarchical hollow ε-MnO2 sample before and after 500th cycles was measured in the frequency range of 100-0.05 kHz at open circuit potential with an ac perturbation of 5 mV (Fig. 6). The measured impedance spectra was analyzed using the CNLS fitting method based on the equivalent circuit [36], which is given in the inset of Fig. 6. It is found that two major characteristic features in the high and low frequency regions are attributed to various resistance phenomena during different interfacial processes in Faradaic reactions. Rs is contact resistance at the active material/current collector interface. Cf and Cs are film capacitor in parallel and pseudocapacitance, respectively. Rect and Rict are electron-transfer and charge-transfer resistor, respectively. The slope of the 45° 11

portion of the curve is called the Warburg resistance (W), which is a result of the frequency dependence of ion diffusion/transport in the electrolyte. At very high frequencies, the intercept at real part (Z′) is a combinational resistance of ionic resistance of electrolyte, intrinsic resistance of substrate, and contact resistance at the active material/current collector interface (Rs) [37]. The semicircle in the high-frequency range signifies charge transfer process in the electrode-electrolyte interfaces. The charge transfer resistance at the electrode electrolyte interface arises due to the discontinuity in the charge transfer process because of conductivity difference between the solid oxide (electronic conductivity) and liquid electrolyte phase (ionic conductivity) [38, 39]. At the high frequency region, the intercept of the semicircles at real part (Z′=1.6Ω) signifies a combination of ionic resistance of electrolyte, intrinsic resistance of substrate, and contact resistance at the active material/current collector interface. From the Nyquist plots in Fig. 6, the cumulative resistances at higher frequency region are same for the MnO2 samples before and after charge-discharge cycles, suggesting combined contributions from ionic resistance of electrolyte, intrinsic resistance of substrate, and contact resistance at the active material/current collector interface. The distinctive semicircle of the MnO2 sample after charge-discharge cycles in the medium frequency region is small as compared to that before the cycles sample suggesting lower diffusion resistance during insertion/de-insertion of H+ and Na+ ions inside the matrix of porous MnO2 sample. A major difference is the slope of the 45° portion of the curve called the Warburg resistance (ZW), which is a result of the frequency dependence of ion

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diffusion/transport in the electrolyte to the electrode surface [40]. Moreover, the increased Warburg resistance after 500 cycles is attributed to the increased diffusion and migration pathways of electrolyte ions during the charge/discharge processes. Long cyclic stability is one of the most important requirements for practical competence of supercapacitor devices. The cyclic specific capacitance performance corresponding capacitance retention plots of hollow MnO2 sample for 500 cycles at current density of 2.0 A·g-1 is presented in Fig. 7. A slight decrease in capacitance is observed up to about 300 cycles, and thereafter, the capacitance begins to stabilize gradually to a value of 50 F·g-1 over 320 cycles. This demonstrates that, within the voltage window of -0.2 to 0.8 V, the charge and discharge processes do not seem to induce significant structural or microstructural changes of the hollow MnO2 electrode material, as expected for pseudocapacitance reactions. Any structural changes in hollow MnO2 affect the stability of the material and decrease the measured specific capacitance values. Therefore, the hollow MnO2 with long-term stability and reversibility is a suitable material for pseudocapacitor electrodes.

4. Conclusions In summary, MnCO3 spheres with unique surface morphology and uniform feature sizes were successfully prepared at room temperature. Hierarchical hollow structured ε-MnO2 spheres were obtained by shape-preserving KMnO4 oxidation of micrometer-scale manganese carbonate structures, followed by the selective removal of MnCO3 with HCl. The electrochemical measurement of supercapacitor electrodes

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indicated that the exceptional cyclic, structural and electrochemical stabilities, and very low ESR value from impedance measurements promised good utility value of hierarchical hollow MnO2 material in fabricating a wide range of high performance electrochemical supercapacitors.

Acknowledgements This work was supported by Doctoral Scientific Research Foundation, Youth Foundation of Jilin Provincial Science & Technology Department (20140520097JH). Project in the Science & Technology Program of Educational Commission during the Twelfth Five-year Plan Period (2012290). “ChunMiao” Talents Special Project in Science and Technology of Educational Commission of Jilin Province.

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Captions Fig. 1 XRD patterns of the as-prepared (a) MnCO3 sphere precursor (b) hierarchical hollow ε-MnO2.

Fig. 2 FESEM images (a, b) and TEM images (c, d) of MnCO3 sphere precursor at different magnifications.

Fig. 3 FESEM images (a, b) and TEM images (c, d) of MnO2 sphere at different magnifications and EDS spectra of the hollow MnO2 spheres (d). Insert is the edge of a MnO2 shell.

Fig. 4 Schematic illustration of facile synthesis of hierarchical hollow MnO2 spheres. Fig. 5 (a) Cyclic voltammetry curves of hierarchical hollow MnO2 at scan rate of 10, 20, 30 , 40 and 50mV·s-1; (b) specific capacitance as a function of scan rate of MnO2 spheres; (c) the charge/discharge curves of hierarchical hollow MnO2 between voltage limits of -0.2 to 0.8 V at rates varied from 0.5 A·g-1 to 2 A·g-1 and (d) Current density dependent specific capacitance and capacitance retention plots of the samples.

Fig. 6 Nyquist plots of MnO2 electrode before and after 500 cycles. Inset shows the electrical equivalent circuit used for fitting impedance spectra.

Fig. 7 The specific capacitance and capacitance retention as a function of cycle number at a current density of 2A·g-1 in 1.0 M Na2SO4 solution.

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20 30 40 50

Fig.1

19

60

2Theta (degree)

10 2

11 0

10 1

10 0

12 2 21 4 30 0

02 4

Intensity (a.u.)

a

b

JCPDS-30-0820

70 80 90

11 0

11 3 20 2 11 6

10 4

01 2

Fig.2

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Spectrum2

Mn O

(e)

C

Mn

Mn

0 0.5 1 1.5 Full Scale 290 cts Cursor: 0.000

2

2.5

3

3.5

4

4.5

5

5.5

Fig.3

21

6

6.5

7

7.5

8

8.5

9

9.5

10

10.5

11 keV

Fig.4

22

50mV/s

6

Specific Current (A/g)

40mV/s 30mV/s

4

20mV/s 10mV/s

2 0 -2

b

250

Sp. capacitance(Cs,F/g)

a

200 150 100 50

-4

0 -0.2

0.0

0.2

0.4

0.6

0.8

10

Potential(V vs.SCE)

0.8

0.5A/g 1 A/g 1.5A/g 2 A/g

0.6

30

40

50

Scan rate (mV/s)

d Sp. capacitance(Cs,F/g)

Potential(V vs.SCE)

c

20

0.4 0.2 0.0

125

100

75

50

-0.2 0

100

200

300

400

0.5

500

Time(s)

Fig.5

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1.0 1.5 Current density (A/g)

2.0

30

Before charge-discharge cycles After 500 charge-discharge cycles

-Z''( Ω )

25

Cf

Cd Rs

20

W

15

Rict

Cs

Rect

W

10 5 0 0

5

10

15

Z'( Ω )

Fig.6

24

20

25

30

80

70 80 60 60 50

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40

20

30

Capacitance retention (%)

Specific capacitance(F/g)

100

0 0

100

200

300

400

500

Cycle number

Fig.7

Graphic abstract legend (TOC Figure) The MnO2 shell with MnCO3 intermediate core is prepared at room temperature, the hollow structure with excellent permeated shell enhanced the discharge capacity and electrochemical stability.

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Graphic abstract Facile Synthesis of Hierarchical Hollow ε-MnO2 Spheres and Their Application in Supercapacitor Electrodes Dandan Han a, Xiaoyan Jing b, Pengcheng Xu a*,Yuansheng Ding a and Jingyuan Liu b a

College of Chemistry and Pharmaceutical Engineering, Jilin Institute of Chemical

Technology, Jilin 132022, PR China b

Key Laboratory of Superlight Materials and Surface Technology, Ministry of

Education, Harbin Engineering University, Harbin 150001, PR China