Construction of Hierarchical Î±-MnO2 Nanowires@Ultrathin Î´-MnO2

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Construction of hierarchical #-MnO2 nanowires@ ultrathin #MnO2 nanosheets core-shell nanostructure with excellent cycling stability for high-power asymmetric supercapacitor electrodes Zhipeng Ma, Guangjie Shao, Yuqian Fan, Guiling Wang, Jianjun Song, and Dejiu Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11300 • Publication Date (Web): 24 Mar 2016 Downloaded from http://pubs.acs.org on March 24, 2016

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ACS Applied Materials & Interfaces

Construction of hierarchical α-MnO2 nanowires@ ultrathin δ-MnO2 nanosheets core-shell nanostructure with excellent cycling stability for high-power asymmetric supercapacitor electrodes Zhipeng Ma, †,‡ Guangjie Shao,*, †,‡ Yuqian Fan, † Guiling Wang, † Jianjun Song † and Dejiu Shen* ,‡ †

College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao, 066004, China

‡

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, 066004, China

ABSTRACT: The poor electrical conductivity and the mechanical instability are two major obstacles for realizing high performance of MnO2 as pseudocapacitors materials. The construction of unique hierarchical core-shell nanostructures therefore plays an important role in the efficient enhancement of the rate capacity and the stability of this material. We herein report the fabrication of a hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure by adopting a facile and practical solution-phase technique. The novel hierarchical nanostructures are consisted of ultrathin δ-MnO2 nanosheets with a few atomic layers well growing on the surface of the ultralong α-MnO2 nanowires. The first specific capacitance of

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hierarchical core-shell nanostructure reached 153.8 F g-1 at the discharge current density of as high as 20 A g-1, and the cycling stability is retained 98.1% after 10000 charge-discharge cycles, higher than those in the literatures. The excellent rate capacity and stability of the hierarchical core-shell nanostructures can be attributed to the structural features of the two MnO2 crystals, in which 1D α-MnO2 nanowire core provides a stable structural backbone and the ultrathin 2D δMnO2 nanosheet shell creates more reactive active sites, besides, the synergistic effects of different dimensions also contributes to the superior rate capability.

KEYWORDS: Manganese dioxide, Nanowires, Nanosheets, Core-shell Nanostructures, Cycling stability, Supercapacitors 1. INTRODUCTION Recently, electrochemical supercapacitors have been attracting tremendous interest of researchers due to their long cycling life, high power density, and the ability to bridge the energy-power gap between conventional Li-ion batteries and capacitors.1-4 With the development of nanotechnology, nanostructured electrode materials with different dimensions have exhibited outstanding electrochemical performance in producing supercapacitors.5-7 Nanowires are currently regarded as a new class of one-dimensional material which facilitates the electrical transport along the axial direction. Since enormous progress has been made in carbon nanotube area, a wide variety of nanowires, including transition metal oxides(e.g.Co3O4,8,

9

Fe3O4,10,11

NiCo2O4,12 MnO2,13-15 etc.) have been fabricated into one dimensional nanowires to acquire satisfactory electrochemical properties as electrode materials. Besides, two-dimensional nanosheets with high external surface area and good electronic properties have also been widely applied in the fabrication of high power electrochemical supercapacitors.7, 16, 17 In particular,


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single-layer graphene flakes that exhibited excellent electronic properties were firstly obtained by Geim et al using mechanical stripping method.18 The discovery triggered the study in the ultrathin inorganic graphene-like 2D materials as electrode materials.19-28 There are many layered inorganic materials with weak or strong chemical bonds coupling between layers, for instance NiO,17 MnO2,22, 23,27,28 MoO2,25 Co3O4,26 etc. Thus, it can be imagined that when the thickness of layered materials is reduced to single or a few layers, some extraordinary variations in their physical and chemical properties would probably occur Among various materials for supercapacitor, MnO2 has been extensively studied as one of the most promising active materials for supercapacitors,, given their ease of large-scale production, high theoretical capacity (1370 F g−1 as the oxidation state of Mn ion changes from 4+ to 3+ over a potential window of 0.8 V), economic and environmental advantages and natural abundance.29 However, due to some intrinsic drawbacks including poor electrical conductivity and sluggish ionic transport rate, supercapacitors solely using single phase MnO2 as electrodes often suffer from low rate capacity and poor cycling stability.30 To overcome these defects and enhance the overall electrochemical performance of MnO2, one promising solution is using another type of transition metal oxide material to design and fabricate hierarchical MnO2@metal oxides core-shell nanostructures, for example, Co3O4 nanowires@ MnO2 nanosheets,31 ZnCo2O4@MnO2

core-shell

nanostructures,32

ultralong

MnO2

nanowires@NixMn1-xOy

nanoflakes,33 etc. These hierarchical nano materials have several apparent merits, including good electrical conductivity and short ion diffusion path for 1D nanowire core within these architectures, large external surface area with great ion adsorption for 2D nanosheets in the architectures, and the potential synergistic effect between the two different dimensional nanomaterials, which can enhance the electrochemical performance of supercapacitors. In Fact,


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with a fast electronic and ionic transport rate and high external surface area, hierarchical coreshell nanostructures consisted of two types of metal oxides with different dimensions have been well-studied as materials for superior performance pseudocapacitors.31-36 Despite these advantages, it is still highly desirable but challenging to develop a method which is simple and easy for mass production of transition-metal oxide core-shell nanostructures, especially combined 1D and ultrathin 2D nanomaterials to make hierarchical nanostructures with ultrahigh active sites and excellent electrochemical stability. Herein, we present a facile and effective approach for mass production of hierarchical αMnO2 nanowires@ultrathin δ-MnO2 nanoflakes core-shell nanostructure. The hierarchical nanostructures are composed of α-MnO2 nanowires as the backbone coated with a few atomic layers of δ-MnO2 nanoflakes. Firstly, uniform sized ultralong α-MnO2 1D nanowires with good electron transport property are synthesized using a hydrothermal method. Ultrathin δ-MnO2 2D nanosheets are then grown on the surface of α-MnO2 nanowires to produce hierarchical coreshell nanostructure using chemical bath deposition method. Ultrathin δ-MnO2 nanosheets with only few atomic layers in this architecture are expected to provide high external surface area and large reactive active sites for fast charge transfer, which is favorable for the electrochemical reaction at electrode-electrolyte interface. In addition, it is well known that the crystal structure of MnO2 with different tunnel size can strongly influence the insertion/extraction of protons or cations during charge/discharge process. Alpha-type MnO2 with 2×2 tunnels (~4.6 Å) is generally considered to be a stable structured supercapacitor material. However, smaller tunnels are not beneficial for the fast insertion/extraction of cations.37 Compared with α-MnO2, layer structured δ-MnO2 has wider ionic diffusion tunnels (~6.9 Å). However, its crystal structure is very unstable during electrochemical reaction.22, 37 Fortunately, our designed hierarchical core-


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shell nanostructures successfully offset the structural limitation of α- and δ-MnO2 by the potential synergistic effect of different dimensions, which can stabilize crystal structure during charge-discharge process and contributes to the enhancement in high rate capability and cycling stability for pseudocapacitors. 2. EXPERIMENT SECTION 2.1. Synthesis of hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets coreshell nanostructur. All the chemical reagents were of analytically pure grade and used as received. A hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure was synthesized using a two-step solution phase method. In the first step, ultralong α-MnO2 nanowires were prepared by a hydrothermal treatment of KMnO4 aqueous solution. KMnO4 of 0.1 g and NH4F of 0.9 g were added to 40 mL deionized water under magnetic stirring and then transferred into a Teflon-lined autoclave (inner volume: 100 mL). The precursor solution was heated to 200 oC at a heating rate of 2 oC min-1 and maintained at 200 oC for 24 hours in a furnace, and then cooled to room temperature. The prepared brown precipitate was filtered and rinsed several times with distilled water and ethanol and dried at 80 °C for 12 h. In the second step, chemical bath deposition was adopted to grow ultrathin δ-MnO2 nanosheets on thesurface of α-MnO2 nanowires. Typically, the as-synthesized ultralong α-MnO2 nanowires were dispersed in 50 mL deionized water and sonicated for 30 min. Then, manganous acetate acid of 0.49 g, EDTA (ethylenediaminetetraacetic acid disodium salt) of 1.49 g and proper amount of SDS (sodium dodecyl sulfonate) were dissolved into the above α-MnO2 nanowire solution under vigorous stirring at 30 °C. Subsequently, 50 mL of aqueous solution containing NaOH of 0.5 g was added dropwise to the above solution. Then, K2S2O8 (1.6 g) aqueous solution of 50 mL was added dropwise to initiate the chemical deposition reaction. The solution was kept


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in a water bath at 30 ºC for 12 h to obtain the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructures. Afterwards, the products were filtered and washed with ethanol solution, and dried at 80ºC. The schematic illustration of the typical experiment process is shown in Figure 1. For comparison, ultralong α-MnO2 nanowires and ultrathin δ-MnO2 nanosheets were also synthesized by a hydrothermal method and chemical deposition reaction, respectively. 2.2. Materials characterization. The XRD patterns were performed on a RigakuSmart Lab X-ray diffractometer operated at 40 kV using a Cu Kα radiation at a scanning rate of 5 o min−1. The morphology and microstructure of the powders were observed by field-emission scanning electron microscopy (FE-SEM, Carl Zeiss Super55 operated at 5 kV) and field-emission transmission electron microscope (TEM, Hitachi HT7700 operated at 120 kV), respectively. The surface area was measured on a V-Sorb 2800P analyzer by nitrogen adsorption-desorption isotherms at 77 K. Atomic force microscopy (AFM) was characterized with a commercial instrument (Bruker Multimode 8) to observe surface morphologies at room temperature in nitrogen. 2.3. Evaluations of electrochemical properties. All the electrochemical performances were measured in a three-electrode configuration cell consisting of a working electrode, a counter electrode, and a saturated Hg/HgO electrode as the reference electrode in 6 M KOH aqueous electrolyte. The working electrodes were composed of 75 wt % as-synthesized active material, 15 wt % acetylene black as conductive additive, and 10 wt % PTFE (1 wt % suspension in water) as binder. The mixed slurry was coated on nickel foam with an area of 1 cm × 1 cm and dried at 80 °C for 12 h, and then pressed under a pressure of 10 MPa for 10 min. The loading mass of the active material is about 1-2 mg cm-2. The preparation of the counter electrode was


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same to that of the working electrode. The homogenous slurry obtained by mixing activated carbon, acetylene black, and PTFE in a weight ratio of 75:15:10 was smeared onto nickel foam of 2.5 cm × 2.5 cm. The cells were assembled and tested by galvanostatic charge−discharge testing system (NEWARE, Shenzhen, China) at the different current density within voltage window of −0.1 to 0.55 V vs Hg/HgO. The cyclic voltammetry (CV) measurements were carried out from − 0.1 to 0.55 V at the different scanning rates. The electrochemical impedance spectroscopy (EIS) was measured within a frequency range of 0.01 to 100000 Hz. Both CV and EIS were performed on a CHI 660E electrochemical workstation. The specific capacitance of electrode the materials was estimated from the charge-discharge test according to the following equation: Csp=I×∆t/(∆V×m ), where Csp is the specific capacitance (Fg−1 ), I is the chargedischarge current (A), ∆t is the discharge time (s), ∆V is the charge-discharge potential window (V), and m is the mass of the active material (g). 3. RESULTS AND DISCUSSION The crystal structure of the as-prepared products was investigated by the X-ray diffraction (XRD). Figure 2a shows the XRD patterns of the ultralong α-MnO2 nanowires, α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanowires and δ-MnO2 nanosheets. It can be seen from Figure 2a-1 and a-3 that the MnO2 nanowires and nanoflakes are well indexed as pure alpha and delta-phase MnO2 (JCPDS card no.44-0141 and 43-1456) phases, respectively. In addition, both α and δ-MnO2 peaks can be clearly identified in Figure 2a-2 without any impurity peaks. The morphologies of the ultralong α-MnO2 nanowires, ultrathin δ-MnO2 nanosheets and αMnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanowires are shown in Figure 2b-d. It is indicated in Figure 2b that the as-synthesized α-MnO2 nanowires with smooth surface (inset


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of Figure 2b) have a uniform diameter of ca. 50 nm and their length extends to several tens of micrometers, resulting in exceptionally large aspect ratios. Figure 2c shows a typical SEM image of the pristine δ-MnO2 nanosheets prepared by liquid phase precipitation with a magnified SEM image presented in the inset of Figure 2c. It is suggested that the nanosheets have ultrathin and lamellar structure with ample graphene-like wrinkles and folds. However, these ultrathin δMnO2 nanosheets with lateral size of several micrometers are interwoven with each other. When the ultrathin δ-MnO2 nanosheets was grown on the surface of the ultralongα-MnO2 nanowires by chemical bath deposition, the prepared products show a unique hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure, as illustrated in Figure 2d. Obviously, scarcely δ-MnO2 is packed in the space between the nanowires, which indicates that ultrathin δ-MnO2 nanosheets are prefer to grow on the surface of α-MnO2 nanowires. A magnified SEM image shown in the inset of Figure 2d reveals that the ultrathin δ-MnO2 nanosheets are uniformly coated on the α-MnO2 nanowire surface, forming the perfect hierarchical core@shell nanostructures with a high dispersity. Moreover, the diameter of each individual hierarchical nanostructure is ca.300-400 nm, which is larger than that of α-MnO2 nanowires. For further analyzing the elemental compositions of the ultralong α-MnO2 nanowires, ultrathin δ-MnO2 nanosheets and α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanowires, energy-dispersive X-ray spectrometry (EDS) maps were measured, as illustrated in Figure S1 (See Supporting Information). Figure S1a-c shows the typical images of the ultralong α-MnO2 nanowires, ultrathin δ-MnO2 nanosheets and α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanowires, respectively, as well as the corresponding recorded mappings of the elemental signals. It is clearly found that the K−K signal is also evenly distributed besides


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Mn-K and O-K signals in the three products. For ultralong α-MnO2 nanowires, K element may arise from the trace adsorption for K+ ion at the surface of nanowires in the reaction solution. However, K+ ion in the ultrathin δ-MnO2 nanosheets exists in the layered crystal structure, which stabilizes to the crystal structure of δ-MnO2 during charge-discharge. In addition, the nitrogen adsorption/desorption curves for the investigation of the BET surface areas of the three products measured are shown in Figure S2 (See Supporting Information). It can be seen from Figure S2 that the isotherms of the three as-synthesized products are ascribed to Type II, indicating that these products belong to typical mesoporous material. The surface area of the hierarchical αMnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructures is calculated to be 91.5 m2g-1, which is higher than that of ultralong α-MnO2 nanowires (30.3 m2g-1) and ultrathin δMnO2 nanosheets (58.4 m2g-1). The hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructures with high surface area can provide large active sites and fast charge transfer at electrode-electrolyte interface, thus enhancing the high rate capability of surpercapacitors. Figure 3a shows a typical transmission electron microscope (TEM) image of the ultralong α-MnO2 nanowires. It is clearly found that the as-synthesized products by a hydrothermal method have a uniform nanowire structure with the diameter of ca. 50 nm. Moreover, the selected area electron diffraction (SAED) pattern illustrated in the inset of Figure 3a displays a orderly spot array, which are indexed to the (310) and (200) planes of α-MnO2 crystal structure with 2×2 tunnels (Figure 3c), which is also well demonstrated in the XRD analysis. To further investigate the microstructure of the ultralongα-MnO2 nanowires, high resolution TEM (HRTEM) image is shown in Figure 3b. It reveals a crystal orientation of ultralong α-MnO2 nanowires along [010] direction. Additionally, according to the fast Fourier transform (FFT)


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pattern in the inset of Figure 3b, the interplanar spacings between the adjacent lattice planes are approximately 0.31 and 0.49 nm, corresponding to the (310) and (200) crystal plane of α-MnO2, respectively. For ultrathin δ-MnO2 nanosheets, the graphene-like lamellar structure with a lateral size above several micrometers is observed, and the nanosheets are nearly transparent to the electron beam, as shown in Figure 3d. Meanwhile, the SAED pattern (inset of Figure 3d) displays two diffraction rings with polycrystal characteristic, which is consisting with the (200) and (110) reflections of layered δ-MnO2 structure (Figure 3g). In addition, the ultrathin δ-MnO2 nanosheets tend to get wrinkled and fold, offering us the chance to directly measure the thickness of the as-obtained nanosheets from the vertical edges in Figure 3e. It indicates that the lattice fringes along this direction with the spacing of ca. 0.72 nm, corresponding to the (001) crystal planes of δ-MnO2, is larger than the theoretical thickness (0.69 nm) of the single-layered δ-MnO2 nanosheet. The difference could result from the hydrated cation between the inserted layers of the ultrathin δ-MnO2 nanosheets.22, 23 Notably, the thickness of the ultrathin nanosheets is only 23 atomic layers with ca. 2.2 nm. The a few atomic layers of the ultrathin δ-MnO2 nanosheets are expected to possess some extraordinary physical and chemical characteristics. The atomic force microscopy (AFM) image measurement for further analysis is shown in Figure 3f, which declares that the height of the ultrathin δ-MnO2 nanosheets along the black line is approximately 2.0 nm, matching well with the measured thickness of 2.2 nm for ultrathin δ-MnO2 nanosheets along the direction of the vertical edges in Figure 3e. Figure 3h shows the typical TEM image of hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructures in which the ultrathin δ-MnO2 nanosheets uniformly grow on the surface of the ultralong α-MnO2 nanowires. It can be seen that the thickness of the shell, corresponding to the size of the ultrathin δ-MnO2 nanosheets, is about 150 nm. Moreover, the size of the ultrathin δ-MnO2 nanosheets


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within core-shell nanostructures is smaller than that of the nanosheets synthesized solely by chemical bath deposition, due to the introduction of the α-MnO2 nanowires which created more nucleation sites during the formation of hierarchical core-shell nanostructures. In addition, the SAED image shown in the inset of Figure 3h reveals the diffraction patterns of two types of MnO2 crystal structures. The diffraction spot arrays and rings are attributed to the reflections of α-MnO2 and δ-MnO2, respectively, corresponding to the structures of the inset of Figure 3a and d. The HRTEM images of the interfaces of the core and shell in the nanostructure (Fig. 3h) shown in Figure 3i and j display the interplanar spacing of 0.49 nm for the (200) plane of αMnO2 core nanowires and 0.72 nm and 0.25 nm for the (001) and (110) planes of δ-MnO2 shell nanosheets, respectively. Besides, there are no distinct boundaries between the contacted surfaces of the core-shell nanostructure. Thus, we speculate that the ultrathin δ-MnO2 nanosheets are grown on the surface of α-MnO2 nanowires through crystal epitaxial growth to form hierarchical core-shell nanostructures. As we know, the capacitance of MnO2 with different crystal structures originates from the adsorption and the intercalation/extraction of cations at the surface of the material. The charge-discharge reaction for MnO2 is usually described as: MnO2+M++e-↔ (MnOOM)surface.38 The ultrathin δ-MnO2 with the thickness of 2-3 atomic layers provides large surface area and active sites, which is beneficial for cations adsorption and redox reactions at the interface of the electrode and electrolyte, as shown in Figure 3k. The electrochemical properties of ultralong α-MnO2 nanowires, ultrathinδ-MnO2 nanosheets and hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructures were studied using galvanostatic charging-discharging, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. Figure 4a displays the typical charge-discharge curves of the three products at a current density of 500 mA g-1. Apparently, the


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hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure shows a specific capacitance of 310.2 F g-1, which is much higher than the α-MnO2 nanowires (207.5 F g1

) or δ-MnO2 nanosheets (277.7 F g-1). The excellent electrochemical behavior of hierarchical α-

MnO2 nanowire@ ultrathin δ-MnO2 nanosheet core-shell nanostructure was further investigated by galvanostatic charging-discharging test at the current densities of 1-20 A g−1, as shown in Figure 4b. The large distortion in the charge-discharge curves is ascribed to the Faradic pseudocapacitance contribution of the hierarchical core-shell nanostructure. Figure 4c presents the calculated specific capacitance of the hierarchical α-MnO2 nanowires@ ultrathin δ-MnO2 nanosheets core-shell nanostructure as a function of current density. It can be seen that the sample exhibits a high specific capacitance of 230.7 F g-1 at the moderate current density of 1 A g-1. The specific capacitance can stabilize at 153.8 F g-1 as the current density increased to 20 A g-1, indicating excellent rate property. Moreover, the cycling stability test of the hierarchical αMnO2 nanowires@ ultrathin δ-MnO2 nanosheets core-shell nanostructure at the high current density of 20 A g-1 is also shown in Figure 4d. The capacitance retention is as high as 98.1 % after 10000 cycles, suggesting that the hierarchical core-shell nanostructure reveals superior cycling stability and potential application for high performance pseudocapacitors. In addition, the SEM test was conducted to observe the possible transformation of morphology and nanostructure for the hierarchical core-shell nanowires after the long-term cycling, as illustrated in Figure S3 (See Supporting Information). Although the ultrathin δ-MnO2 nanosheets on the surface of α-MnO2 nanowires become slightly thicker after 10000 cycles, the core-shell nanowires still retain their original structural characteristics, which an important proof for the nature of the hierarchical core-shell nanostructure. Table 1 shows the comparison between the as-prepared

hierarchical

α-MnO2

nanowires@ultrathin

δ-MnO2


nanosheets

core-shell

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nanostructures as a supercapacitor material and previously reported results. It can be seen that the capacitance of the as-obtained hierarchical core-shell nanowires was not as high as some of the reported core-shell nanostructures, for instance Co3O4@MnO2 nanoarchitectures,31 ZnCo2O4/MnO2 nanocone forests,32 and MnCo2O4.5@δ-MnO2 hierarchical nanostructures,35 which is attributed to the good electrical conductivity and high theoretical capacity of the core or shell nanomaterials for the core-shell nanostructures. Although our capacitance is not the highest, the cycling stability is distinctly superior to that in the literatures and highly competitive to supercapacitors. According to the above analysis, the superior high rate capability and excellent cycling stability of the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure is attributed to the synergistic effects of MnO2 with two different dimensions and crystal structures. Within the hierarchical core-shell nanostructure, the 1D α-MnO2 nanowire core serves as a stable structural backbone during charge-discharge, while the 2D δ-MnO2 nanosheet shell provides a high external surface area and a large number of reactive active sites for the electrochemical reaction at electrode-electrolyte interface. Figure 5a shows the cyclic voltammetry (CV) curves of ultralong α-MnO2 nanowires, ultrathin δ-MnO2 nanosheets and the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructures at a low scan rate of 2 mV s-1. Compared with a perfect rectangular shape indicating pure EDLC behavior at the electrode/electrolyte interface and no redox reaction, the CV curves for three samples were distorted. The large distortion is ascribed to the Faradic pseudocapacitance formation at the interface of MnO2 electrode/electrolyte. Furthermore, it can be clearly seen that the area of CV curves for the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure is larger compared with αMnO2 nanowires and δ-MnO2 nanosheets, indicating excellent pseudocapacitive behavior.


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Notably, a pair of reversible redox peaks appears at around 0.3/0.5 V vs Hg/HgO in 6 M KOH, which corresponds to K+ ions insertion/extraction at the electrode/electrolyte interface of the hierarchical MnO2 core-shell nanostructure. Moreover, a nearly symmetrical CV shape for the hierarchical core-shell nanostructure still maintains as the scan rate increases to 500 mV s1

(Figure 5b), demonstrating superior reversibility and excellent rate capability. Additionally, the

electrochemical impedance spectroscopy (EIS) measurements are preformed, and the impedance spectra of the three products in Figure 5c display similar shape with a straight line at low frequency and a single semicircle at high frequency. The diameter of the semicircle at the real axis represents the charge transfer resistance, which usually represents the resistance of the electrochemical reaction at the electrode/electrolyte interface.

43

It can be seen from the inset of

Figure 5c that the charge transfer resistance of the hierarchical α-MnO2 nanowires@ultrathin δMnO2 nanosheets core-shell nanostructure and the ultrathin δ-MnO2 nanosheets are comparative and are smaller than that of the ultralong α-MnO2 nanowires, owing to the high surface area and large reactive active sites of the ultrathin δ-MnO2 nanosheets. In addition, the Bode plots of three products are given in Figure 5d. It is found that the phase angle of the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure is closer to ideal value of -90° compared to α-MnO2 nanowires and δ-MnO2 nanosheets, indicating the excellent capacitance characteristic with fast electrolyte ion response. In

order

to

evaluate

the

potential

application

of

the

hierarchical

α-MnO2

nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure, a two-electrode asymmetric supercapacitor was fabricated using the hierarchical core-shell nanostructure as the positive electrode ,the nitrogen-doped carbon (NC) cellular foam synthesized by the method described in our previous works as the negative electrode, and 6 M KOH aqueous as the electrolyte.44 In the


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asymmetric supercapacitor, the mass ratio between the positive and negative materials is 1:1.2. The energy density was calculated using the following equation: E =1/2CV2 Where C is the total cell discharge specific capacitance and V is the cell voltage window. The power density was calculated using the following equation: P=E/∆t Where E is the energy density and ∆t is the discharge time. Additionally, to estimate the stable potential window of the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure and NC, CV measurements on the three electrode cell in 6 M KOH aqueous solution before evaluating the asymmetric cell were performed (Figure 6a). The hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets coreshell nanostructure electrode was measured within a potential window of −0.1 to 0.55 V (vs. Hg/HgO), while NC was measured with a potential window of −1.0 to 0 V (vs. Hg/HgO) at a scan rate of 10 mV s−1. Therefore, the stable potential window for the fabricated asymmetric supercapacitor can be extended to 1.55 V in 6 M KOH aqueous. Figure 6b displays the galvanostatic charge-discharge curves of the asymmetric supercapacitor at different current densities. It can be seen that the asymmetric supercapacitor exhibits a high specific capacitance of 276.9 F g-1 at the current density of 1 A g-1. The specific capacitance can stabilize at 73.8 F g-1 as the current density increased to 10 A g-1, suggesting excellent rate performance. Moreover, the cycling stability test of the asymmetric supercapacitor at 1 A g-1 is also shown in Figure S4. The capacitance retention is as high as 98.8 % after 2000 cycles, indicating that the asymmetric supercapacitor reveals superior cycling stability. In addition, the excellent electrochemical behavior of the asymmetric supercapacitor was further investigated by cyclic voltammetry (CV)


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measurements, as shown in Figure 6c. The typical CV curves of the asymmetric supercapacitor with the potential window ranging from 0 to 1.55 V deliver good symmetry at various scan rates, suggesting a desirable charge-discharge behavior. Figure 6d shows the Ragone plot of the asymmetric the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure//NC supercapacitor. A maximum energy density of 78 Wh kg-1 and a maximum power density of 21.7 kW kg-1 are obtained based on the total mass of the MnO2 and and NC. Compared to similar asymmetric systems, such as MnO2//AC, MnO2//graphene hydrogel, MnO2//graphene, and hollow Cs-MnO2 nanofibers//hollow Cs supercapacitors reported previously,45-49 the asymmetric supercapacitor in this study displays a superior energy density, which proves that the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure is a very promising electrode material for high power asymmetric supercapacitors. 4. CONCLUSIONS In summary, a unique α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell hierarchical nanostructure has been fabricated through a facile and effective two-steps solution-phase approach which is easily scaled up for mass production. Compared with the individual ultralong α-MnO2 nanowires and ultrathin δ-MnO2 nanosheets, the constructed hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure as the electrode for pseudocapacitors exhibits desirable higher rate capacity and more excellent cycling stability. The outstanding electrochemical properties of the hierarchical core-shell nanostructure are attributed to the structural features and the synergistic effects of MnO2 with two different dimensions: a 1D α-MnO2 nanowire core as a stable structural backbone and a 2D δ-MnO2 nanosheet shell offering large surface and more reactive active sites. Thus, the fabricated electrode materials with a novel hierarchical core-shell nanostructure are promising for high performance energy storage device.


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ASSOCIATED CONTENT Supporting Information Additional EDS, BET, SEM data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Prof. Guangjie Shao. Tel.: 0086-335-8061569; Fax: 0086-335-8059878. E-mail address: [email protected] Prof. Dejiu Shen. Tel.: 0086-0335-8055799; Fax: 0086-0335-8055799. E-mail address: [email protected] ACKNOWLEDGMENT We are grateful for the financial support from the Natural Science Foundation of Hebei Province (B2012203069, B2012203070) and support from education department of Hebei province on natural science research key projects for institution of higher learning (ZH2011228). REFERENCES (1) Wang, G.; Zhang, L.; Zhang, J. A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797-828. (2) Lang, X.; Hirata, A.; Fujita, T.; Chen, M. Nanoporous Metal/Oxide Hybrid Electrodes for Electrochemical Supercapacitors. Nat. Nanotechnol. 2011, 7, 232-236. (3) Faraji, S.; Ani, F. N. Microwave-Assisted Synthesis of Metal Oxide/Hydroxide Composite Electrodes for High Power Supercapacitors - A Review. J. Power Sources 2014, 263, 338-360.


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Figure 1 Fabrication process of the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure.


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Figure 2 XRD patterns (a) and SEM images of the ultralong α-MnO2 nanowires (b), δ-MnO2 nanosheets (c) andα-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanowires (d).


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Figure 3 (a) TEM, (b) HR-TEM and (c) crystal structure images of the ultralong α-MnO2 nanowires, the inset of b corresponding to the fast Fourier transform pattern. (d) TEM, (e) HRTEM, (f) AFM and (g) crystal structure images of the ultrathin δ-MnO2 nanosheets, the inset of d corresponding to the SAED pattern. (h) TEM, (i, j) HR-TEM and (k) schematic images of the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure. The inset of a, d and h are the corresponding the SAED pattern.


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Figure 4 (a) Galvanostatic charge-discharge curves of the ultralong α-MnO2 nanowires, the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanowires and δ-MnO2 nanosheets. (b) Galvanostatic charge-discharge curves and (c) specific capacitance of the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure at different current densities. (d) Long-term cycle performance of the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure at 20 A g-1.


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Figure 5 (a) CV curves, (c) Nyquist plots and (d) Bode plots of the ultralong α-MnO2 nanowires, the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanowires and δMnO2 nanosheets. (b) CV curves of the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure at various scan rates


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Figure 6 (a) Comparative CV curves of the nitrogen-doped carbon (NC) cellular foam and the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanowires electrodes performed in a three electrode cell in 6 M KOH aqueous solution at a scan rate of 10 mV s−1. (b) Galvanostatic charge-discharge curves of the asymmetric supercapacitor at different current densities. (c) CV curves of the asymmetric supercapacitor at various scan rates. (d) Ragone plot of the asymmetric supercapacitor. The inset of b demonstrates the schematic illustration of the asymmetric supercapacitor configuration and digital image of a green-light-emitting diode (LED) lighted by the asymmetric supercapacitor device.


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Table 1 Comparison of the electrochemical properties between the as synthesized hierarchical αMnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure and the recent reported data in the literatures Electrode materials

Electrolyte

Specific Capacitance

Capacitance retention

reference

Single-layer MnO2 nanosheets

1 M Na2SO4

171 F g-1 at 3 A g-1

91% after 10000 cycles

[23]

Co3O4@MnO2 nanoarchitectures

1 M LiOH

960 F g-1 at 0.1 A cm-2

93.8% after 1000 cycles

[31]

ZnCo2O4/MnO2 nanocone forests

1M KOH

1526 F g-1 at 10 A g1


[32]

α-MnO2 [email protected] Mn0.75Oy Nanoflakes

0.5 M Na2SO4

Around 400 F g-1 at 2 A g-1


[33]

Co3O4 nanowires@MnO2 nanoflakes

2M KCl

181.3 F g-1 at 10 A g-1


[34]

1 M Na2SO4

211.2 F g-1 at 10 A g-1


[35]

CoMoO4@MnO2 coree-shell nanosheet

1M KOH

1037 F g-1 at 0.04 A cm-2


[39]

NiCo2O4@MnO2 core–Shell nanosheet

1M NaOH

906.7 F g-1 at 0.04 A cm-2


[40]

MnO2@MnO2 core-shell nanostructures

0.5 M Na2SO4

108 F g-1 at 10 A g-1


[41]

MnO2 nanotubes

1 M Na2SO4

365 F g-1 at 0.25 A g-1


[42]


This work

MnCo2O4.5@δ-MnO2 hierarchical nanostructures

α-MnO2 nanowires@ultrathin δMnO2 nanosheets

6 M KOH

153.8 F g-1 at 20 A g-1


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TOC Figure

The excellent cycling stability of the hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure are attribted to the structural features and the synergistic effects of MnO2 with two different dimensions: a 1D α-MnO2 nanowire core as a stable structural backbone facilitating the electrical transport along the axial direction and a 2D δ-MnO2 nanosheet shell offering large surface and more reactive active sites.


31

Construction of Hierarchical Î±-MnO2 Nanowires@Ultrathin Î´-MnO2

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