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Cobalt-Doped MnO2 Hierarchical Yolk−Shell Spheres with Improved Supercapacitive Performance Chuan-Lin Tang,†,‡ Xiao Wei,†,‡ Yan-Mei Jiang,† Xue-Yan Wu,§ Li−Na Han,† Kai-Xue Wang,*,† and Jie-Sheng Chen† †

School of Chemistry and Chemical Engineering and §School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China S Supporting Information *

ABSTRACT: Cobalt-doped MnO2 with a hierarchical yolk−shell spherical structure has been prepared by a light-assisted method. The hierarchical spheres obtained are characterized by X-ray diffraction, nitrogen adsorption−desorption, and electron microscopy analyses. The supercapacitive performance of the material is evaluated by cyclic voltammetry and galvanostatic charge/discharge measurements. A specific capacitance of 350 F g−1 is achieved at a current density of 0.1 A g−1 for the spheres with a Co/Mn ratio of 0.5%, much higher than that of the undoped MnO2 with a similar hierarchical spherical structure, which is approximately 168 F g−1. A specific capacitance of 196 F g−1 is still retained after being charged/discharged at a current density of 2.0 A g−1 for 1000 cycles, demonstrating good cycling stability. Over 90% of the initial specific capacitance was retained, exhibiting excellent capacitance retention ability.

1. INTRODUCTION Supercapacitors with merits including high power density, superior reversibility, long cycle life, and relatively high energy density have been regarded as promising energy storage devices for hybrid electric vehicles, pulse laser techniques, and energy management systems. The energy storage of electrochemical supercapacitors occurs at or near the electrode/electrolyte interfaces following two mechanisms. The first one is based on quick formation of a double layer of charges or opposite ions at the electrode/electrolyte interface, called electric double-layer capacitors (EDLCs). Porous active carbons1 and silicons2−4 with large surface areas are generally used electrode materials for EDLCs. The other is based on a charge-transfer Faradaic process, generating pseudocapacitance.5 The most widely used active electrode materials for pseudocapacitors are transition-metal oxides6 and conducting polymers.7 The electrode materials play a key role in determining the electrochemical performance of the supercapacitors. It also has been demonstrated that the pseudocapacitance of transitionmetal oxides is much higher than the electrochemical doublelayered capacitance of carbon materials.8 For example, ruthenium oxide, the most extensively studied electrode material for supercapacitor, has a theoretical capacitance of as high as 720 F g−1.9,10 However, ruthenium is a very expensive and toxic element, and strongly acidic electrolytes such as sulfuric acid are usually involved in supercapacitor fabrication, hindering its general commercial application. Manganese oxide (MnO2) with advantages such as low cost, low toxicity, natural abundance, and environmental friendliness is considered as one of the most promising electrode materials for pseudocapacitors.11−19 The charge storage mechanism of manganese oxide is based on © XXXX American Chemical Society

surface adsorption of electrolyte cations and proton incorporation, accompanied by oxidation/reduction of Mn ions.20−24 A high theoretical capacitance of 1233 F g−1 is expected for complete reduction of MnIV into MnIII over a potential window of 0.9 V. However, the experimental capacitances of MnO2 materials are usually less than 100 F g−1, much lower than the theoretical value.25,26 These low capacitances are attributed to the relatively low specific surface area and poor electronic and ionic conductivity of MnO2 materials.27,28 Only the surface layer with a thickness of tens nanometers contributes to the specific capacitance of these materials.29−31 There are several strategies to improve the pseudocapacitive performance of MnO2 materials. Ghodbane and co-workers found that the specific capacitances increase in the following order: pyrolusite (28 F g−1) < Ni−todorokite < ramsdellite < cryptomelane < OMS-5 < birnessite < spinel (241 F g−1).32 Thus, manipulation of the crystal structure of MnO2 is a fundamental strategy to improve the specific capacitance of MnO2 materials. Second, decreasing the particle size and controlling the morphology can increase the specific surface area and shorten the mass-transportation distance of the electrode. Although the charge-storage mechanism in MnO2 materials is mainly Faradaic, the particle size and morphology are also efficient in enhancing the electrode kinetics of these materials.33,34 In addition, incorporation of other metal elements into MnO2 lattices has also been proved recently to be feasible in further improving the electrical conductivity and pseudo-capacitive performance of Received: December 23, 2014 Revised: March 17, 2015

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The Journal of Physical Chemistry C MnO2 materials.35−38 Metal elements, such as Ni, Cu, Fe, V, Co, Mo, Ru, Au, Ag, and Cu, have been successfully doped into the structures of MnO2 materials.39,40 The doping by other metal elements may vary the electronic structure of MnO 2 , consequently leading to better electronic conductivity and higher electrochemical performance.41 Thus, it is highly desirable to prepare heteroatom-doped MnO2 materials with controlled crystal structure and morphology in one step. In this work, yolk−shell-structured spheres composed of Codoped ramsdellite-type MnO2 nanoflakes were successfully prepared in one step via a simple and efficient light-assisted preparation method. The effect of doping on the pseudocapacitive properties of MnO2 is systematically investigated. The ramsdellite structure ensures a relatively high specific capacitance. The yolk−shell morphology and the nanoflakes facilitate the mass transport, improving the electrode kinetics. Incorporation of Co into MnO2 compounds would improve the electrical conductivity and thus pseudocapacitive performance of the spheres. These yolk−shell-structured spheres are promising electrode materials for high-performance supercapacitors.

on a CHI-660D electrochemical workstation. The cyclic voltammograms (CV) were obtained on a CHI660D electrochemical workstation at different scanning rates in a potential range of 0−1.0 V (vs SCE). Electrochemical impedance spectra were measured using a Zahner Zennium electrochemical workstation with an ac voltage signal of 5 mV in the frequency range between 100 and 0.1 kHz. The average specific capacitance values were calculated from the area of the CV plot according to eq 143 C = i/mv

(1)

where m is the total mass of active materials in the electrode, v is the potential sweep rate, and i is the even current response, which is obtained through integrating the area of the curve.

3. RESULTS AND DISCUSSION XRD patterns of MnO2 and Co−MnO2 are shown in Figure 1. The diffraction peaks in the XRD patterns of the as-prepared

2. EXPERIMENTAL SECTION 2.1. Preparation of Co-Doped MnO2. Yolk−shellstructured Co-doped MnO 2 (Co−MnO2 ) spheres were fabricated via a light-driven preparation route.42 Typically, 2.80 g of manganese(II) sulfate monohydrate (MnSO4.H2O), 0.084 g of cobalt(II) sulfate heptahydrate (CoSO4·H2O), and 6.50 g of potassium persulfate (K2S2O8) were dissolved in 200 mL of deionized water containing 2.0 mL of conc. HNO3. The solution was then transferred to a water-jacketed quartz reactor and ultraviolet (UV) irradiated under a 400 W high-pressure mercury lamp for 20 min with stirring. The products were filtered, washed with copious deionized water, and dried at 60 °C overnight. The temperature of the reaction system was controlled by jacketing the quartz reactor with ice and circulating water, eliminating the thermal effect of the UV irradiation on the reaction. For comparison, yolk−shell-structured MnO2 spheres without Co doping were prepared in the absence of cobalt(II) sulfate heptahydrate via the above procedure. 2.2. Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku Dmax-2200 diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 1.5418 Å). The morphology of the sample was observed with a scanning electron microscope (SEM, JSM-7401F, JEOL, Japan). The microstructure of the sample was characterized using a transmission electron microscope (TEM, JEM-2100F, JEOL, Japan) and a high-resolution transmission electron microscope (HRTEM, JEM-2100F, JEOL, Japan), operating at 200 kV. Nitrogen adsorption/desorption analyses of the samples were conducted on a Micromeritics ASAP 2010 M+C nitrogen adsorption instrument (Micrometritices Inc., USA) at 77 K. 2.3. Electrochemical Measurements. The electrochemical properties of the Co−MnO2 spheres were evaluated using a three-electrode cell configuration. The working electrode was fabricated by the following steps: Co−MnO2 hollow microsphere (80 wt %), acetylene blacks (10 wt %), and polytetrafluoroethylene (PTFE, 10 wt %) were well mixed and grounded in an agate mortar and then pressed onto a titanium mesh, which served as a current collector. Saturated calomel electrode (SCE) was used as a reference electrode and platinum net as a counter electrode. The electrolyte was a 1.0 M Na2SO4 aqueous solution. Galvanostatic charge−discharge was performed in the voltage range of 0−1.0 V at room temperature

Figure 1. XRD patterns of MnO2 and Co−MnO2 formed by lightassisted method.

samples are in good agreement with the ramsdellite MnO2, crystallized in an othorhombic system with cell parameters of a = 9.3720 Å, b = 2.8508 Å, and c = 4.4706 Å (JCPDS No. 44-0142). The peaks located at 22.16°, 37.14°, 42.28°, 55.74°, and 66.74° are ascribed to the typical (101), (210), (211), (212), and (610) diffractions, respectively. The broad nature of the peak profiles suggests the formation of small crystallites with low crystallinity. No diffraction peaks originating from cobalt oxide are observed in the XRD pattern of Co−MnO2, indicating that no crystallized cobalt oxide is formed. A distinct shift of the diffraction peaks to small angle is observed after Co doping. Given the fact that the radius of Co2+ (0.72 Å) is larger than that of Mn4+ (0.67 Å) in six coordination, the doping of Co will inevitably widen the interplanar spacings of the crystal structure of MnO2. The shift in the diffraction peaks suggests that Co has been successfully doped into the crystal structure of MnO2. As revealed by the elemental analyses conducted on an inductive-coupled plasma emission spectrometer (ICP), the content of Co doped in MnO2 is approximately 0.5 wt %. The morphologies of the samples are revealed by SEM observation (Figure 2). A large amount of relatively uniform spheres with an average diameter of about 400 nm is observed. These spheres are composed of well-defined nanoflakes, generating a flower-like morphology. As observed in the SEM and HRTEM images (Supporting Information, Figure S1), the thickness of the nanoflakes is approximately 3 nm, which may shorten the diffusion distance for ions. The assembly of these nanoflakes forms a hierarchically porous structure, facilitating penetration of electrolyte. No obvious difference in morphology B

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Figure 2. SEM images of (a) MnO2 and (b) Co−MnO2 spheres.

Figure 3. TEM images of (a) MnO2 and (b) Co−MnO2 spheres showing the yolk−shell structure. (c) Element mapping images from the same selected area. (d) HRTEM images of MnO2 and Co−MnO2.

observation. The diameters of the spheres and yolks are approximately 400 and 250 nm, respectively. Void space among the yolk and shell can be observed distinctly in the spheres. The radial orientation of the nanoflakes generates a hierarchically porous structure, which may facilitate penetration of electrolyte and ensure the structural robustness upon cycling.

is observed between MnO2 and Co−MnO2, indicating that Co doping has a negligible impact on the morphology of the samples. The TEM image presented in Figure 3 reveals that both the MnO2 and the Co−MnO2 spheres have an identical yolk−shell structure. The yolks and shells of the spheres are composed of radially oriented ultrathin nanoflakes, consistent with SEM C

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structured MnO2 spheres. When scanned at a rate over 20 mV s−1, the CV curves become oval shaped, attributing to intensified polarization, fast charge transfer, and cation diffusion.45 Fast, reversible successive surface redox reactions define that the shape of the voltammogram is close to that of the EDLC.46 The variation of specific capacitances of yolk−shell-structured MnO2 and Co−MnO2 spheres with different scan rates in 1.0 M Na2SO4 solution are shown in Figure 6. At a scan rate of 1.0 mV

Elemental mapping analyses of elements Mn and Co of a select area of a single Co−MnO2 nanosphere are shown in Figure 3c. The Co element is observed to be well distributed over the observed region, suggesting that Co is uniformly doped in manganese oxide. Clear lattice fringes are observed in the HRTEM images of MnO2 and Co−MnO2 (Figure 3d). The interplanar spacing of approximately 0.24 nm is assigned to the (210) plane of ramsdellite. A slight increase to 0.26 nm in the d spacing of the (210) plane is observed after cobalt doping, consistent with XRD analysis. The textural characteristic of yolk−shell-structured Co− MnO2 spheres is investigated by the nitrogen adsorption/ desorption analyses performed at 77 K (Figure 4). The nitrogen

Figure 6. Specific capacitances of MnO2 and Co−MnO2 yolk−shellstructured spheres at different scan rates in a 1 M Na2SO4 solution.

s−1, MnO2 spheres give a specific capacitance of approximately 154 F g−1. After being doped with Co, a specific capacitance of approximately 287 F g−1 is achieved. Co doping significantly improved the supercapacitive performance of MnO2 nanomaterials, consistent with the results reported in the literature.37,47 Increasing the scan rate to 20 mV s−1, a specific capacity of approximately 146 F g−1 is retained for the Co−MnO2 spheres, more than 50% of the value at 1.0 mV s−1. In general, the redox reactions depend on the insertion/extraction rate of protons or alkali cations from the electrolyte.43 A good capacitive characteristic that can be achieved at lower scan rate is ascribed to more working ions reaching the active surface, promoting the effective interaction among the ions and the electrode. The galvanostatic charge−discharge curves (the fifth cycle) of MnO2 and Co−MnO2 spheres at a current density of 0.1 A g−1 are shown in Figure 7a. The nearly symmetric charge−discharge curve reveals a good capacitive behavior and a highly reversible Faradaic reaction between Na+ and MnO2 spheres.34,35 At a current density of 0.1 A g−1, a specific capacitance of as high as 350 F g−1 is achieved for the Co−MnO2 spheres, much higher than that of the undoped MnO2 spheres (168 F g−1) and those of MnO2 materials reported in the literature.48,49 The high specific capacitance is attributed to the crystal structure, yolk−shell morphology, and Co doping of the Co−MnO2 spheres. The Co−MnO2 spheres have a ramsdellite crystal structure,

Figure 4. N2 adsorption−desorption isotherms of Co−MnO2 yolk− shell-structured spheres. (Inset) Pore size distribution curve.

adsorption/desorption isotherm of the sample can be categorized as type IV with a small hysteresis loop observed at a relative pressure of 0.7−1.0, indicating the existence of a meso- and macroporous structure.44 The aggregation of the nanoflakes in the yolk−shell structure generates plenty of mesopores, while the void space between the core and the shell contributes to the macropores. These meso- and macroporous structures form a three-dimensional (3D) electrolyte permeation pathway, facilitating mass transport. The specific BET surface area and pore volume of the yolk−shell Co−MnO2 sphere are 135 m2 g−1 and 0.36 cm3 g−1, respectively. The Barrett−Joyner−Halenda (BJH) pore size distribution calculated based on the desorption branch is in the range of 8−10 nm. The high surface area can usually provide more active adsorption sites, contributing to the specific capacitance. Figure 5 shows CV curves of Co−MnO2 and MnO2 in 1.0 M Na2SO4 electrolyte at different scan rates between 1.0 and 20 mV s−1 in a potential range of 0−1.0 V (vs SCE). The CV curves of both Co−MnO2 and MnO2 are nearly rectangularly shaped for scan rates up to 10 mV s−1, indicating the low contact resistance and an ideal capacitive behavior of the as-prepared yolk−shell-

Figure 5. Comparison of CV curves for Co−MnO2 and MnO2 yolk−shell-structured spheres in the potential range from 0 to 1.0 V at different scan rates. D

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Figure 7. (a) Galvanostatic charge/discharge curves of MnO2 and Co−MnO2 spheres at a current density of 0.1 A g−1. (b) Cycling stabilities of MnO2 and Co−MnO2 at a current density of 2.0 A g−1.

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delivering a relatively higher capacitance compared to those of pyrolusite and Ni−todorokite. The yolk−shell morphology formed by radially oriented nanoflakes forms a 3D porous system, facilitating the transport of ions and electrolyte. The ultrathin nanoflakes provide plenty of active surface for the reversible Faradaic reaction between Na+ and MnO2. Co doping is expected to enhance the electrical conductivity and the chemical diffusion coefficient of sodium in the manganese dioxide.50 Electrochemical impedance spectroscopy (EIS) measurements suggest that the faradic charge−transfer resistance Rct of MnO2 decreases from 9.7 to 7.8 Ω after being doped with Co (Supporting Information, Figure S2). Co doping may significantly increase the concentration of Mn3+ ions, and consequently, the extra eg electron of Mn3+ increases the conductivity of MnO2.41 The galvanostatic intermittent titration technique (GITT) was employed to determine the chemical diffusion coefficient of sodium in the manganese dioxide (Supporting Information, Figures S3−S5). The sodium ion diffusion coefficient of MnO2 increases distinctly after Co doping, contributing to the rate performance and specific capacitance of the samples. The cycling stabilities of the yolk−shell-structured MnO2 and Co−MnO2 spheres at 2.0 A g−1 are shown in Figure 7b. Co− MnO2 spheres retain over 90% of the initial specific capacitance after being cycled for 1000 cycles, demonstrating the electrochemical stability of the Co−MnO2 spheres. Generally, the fading in the capacitance upon electrochemical cycling results from loss of active material through partial dissolution of MnO2 or conversion to electrochemically irreversible species such as Mn2O3 and Mn3O4.51

ASSOCIATED CONTENT

S Supporting Information *

HRTEM image of cobalt-doped MnO2 nanoflakes, electrochemical impedance spectra, and galvanostatic intermittent titration measurements of cobalt-doped and undoped MnO2 spheres. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-21-34201273. E-mail: [email protected]. Author Contributions ‡

Chuan-Lin Tang and Xiao Wei contribute to this work equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (2013CB934102 and 2014CB932102) and the National Natural Science Foundation of China (21271128, 21331004, 20130117).

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REFERENCES

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CONCLUSION Co-doped ramsdellite manganese dioxide spheres with a yolk− shell structure have been successfully synthesized via a lightassisted method. The diameters of these spheres range from 350 to 500 nm. These spheres are composed of well-defined nanoflakes with a thickness of approximately 3 nm. The specific surface area and the pore size of the nanospheres are 135 m2 g−1 and 9 nm, respectively. The obtained Co−MnO2 exhibit a high specific capacitance of 350 F g−1 at 0.1 A g−1 and an excellent cycling stability of over 90% of the initial specific capacitance is retained. This work demonstrates the specific capacitance of MnO2 materials can be significantly improved by carefully controlling their crystal structures, morphologies, and structural incorporation of heteroatoms. These yolk−shell Co−MnO2 spheres are promising electrode material for high-performance supercapacitors. E

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DOI: 10.1021/jp512795g J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/jp512795g J. Phys. Chem. C XXXX, XXX, XXX−XXX