MnO2 Sandwich-Structured

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Design and Synthesis of MnO2/Mn/MnO2 Sandwich-Structured Nanotube Arrays with High Supercapacitive Performance for Electrochemical Energy Storage Qi Li, Zi-Long Wang, Gao-Ren Li,* Rui Guo, Liang-Xin Ding, and Ye-Xiang Tong MOE Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: We demonstrate the design and fabrication of novel nanoarchitectures of MnO2/Mn/MnO2 sandwich-like nanotube arrays for supercapacitors. The crystalline metal Mn layers in the MnO2/Mn/MnO2 sandwichlike nanotubes uniquely serve as highly conductive cores to support the redox active two-double MnO2 shells with a highly electrolytic accessible surface area and provide reliable electrical connections to MnO2 shells. The maximum specific capacitances of 937 F/g at a scan rate of 5 mV/s by cyclic voltammetry (CV) and 955 F/g at a current density of 1.5 A/g by chronopotentiometry were achieved for the MnO2/Mn/MnO2 sandwich-like nanotube arrays in solution of 1.0 M Na2SO4. The hybrid MnO2/Mn/MnO2 sandwich-like nanotube arrays exhibited an excellent rate capability with a high specific energy of 45 Wh/kg and specific power of 23 kW/kg and excellent long-term cycling stability (less 5% loss of the maximum specific capacitance after 3000 cycles). The high specific capacitance and charge−discharge rates offered by such MnO2/Mn/MnO2 sandwich-like nanotube arrays make them promising candidates for supercapacitor electrodes, combining high-energy densities with high levels of power delivery. KEYWORDS: Supercapacitor, hybrid electrode, nanotube array, MnO2, electrodeposition

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on highly conductive materials such as metal,31 conducting polymer,32,33 carbon nanotube,34,35 or graphene36 for enhanced performance. The enhanced performance was also obtained by coating MnO2 onto SnO2 nanowires,37 ZnO nanorods,38 and Zn2SnO4 nanowires.39 In all of the above cases, MnO2 is of relatively low weight fraction and usually has excellent rate and cycling performance; however, the energy and power densities of electrodes are sacrificed. To realize the practical applications for high-performance ECs that needs large capacitance and high energy storage, here we design and synthesize novel MnO2/Mn/MnO2 sandwichstructured nanotube arrays (SNTAs) with high MnO2 weight fraction as shown in Scheme 1a. The aligned SNTAs represent a new prime example of materials with a well-defined pore structure. The aims of designing such MnO2/Mn/MnO2 SNTAs are listed as the following: (i) The middle Mn layer will provide electron “superhighways” for charge storage and delivery because of its high electrical conductivity, which will overcome the key weakness (the limited electric conductivity) of MnO2. (ii) The MnO2/Mn/MnO2 SNTAs would relax the transport of ions because of the hollow nanostructures. In addition, the double thin layers of MnO2 in SNTAs would enable fast, reversible Faradaic reactions and provide short ion

upercapacitors, also called ultracapacitors or electrochemical capacitors (ECs), have become some of the most promising candidates for next-generation power devices because of their high power density, fast charging−discharging rate, and excellent cycle stability.1−4 To date, various materials, including carboneous materials,5 transition-metal oxides,6−9 conducting polymers,10 and hybrid composites,11−14 have been widely studied as electrodes for ECs. Among various electrode materials, the hydrous RuO2 exhibits the most promising performance.15 However, the high cost, rareness, and toxic nature of RuO2 have limited its commercial attractiveness. Up to now, there has been extensive interest in developing the inexpensive transition-metal oxide electrodes, such as MnO2,16−19 Co3O4,20 NiO,21 VOx,22 and TiO2,23 for ECs. Among inexpensive metal oxides, MnO2 has been considered to be one of the most attractive electrode materials for ECs owing to its high theoretical specific capacitance, low cost, natural abundance, and environmental friendliness.18 However, the poor electrical conductivity of MnO2 (10−5∼10−6 S/cm) remains a major challenge and limits rate capability for high power performance, thus hindering its wide application in an energy storage system.24−26 To improve electrical conductivity and maximize the utilization rate of MnO2, providing reliable electrical connection while keeping its film morphology has become one of the essential criteria in designing highperformance electrodes for MnO2-based ECs.27−30 Recently, the considerable research interest has been focused on hybrid composite nanostructures where thin MnO2 layers were loaded © 2012 American Chemical Society

Received: May 8, 2012 Revised: June 20, 2012 Published: June 25, 2012 3803

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Scheme 1. (a) Nanotube Array Architecture, Triple-Layered Structure, and High Conductivity in Electrodes Provide Ion and Electron “Superhighways”. (b) Illustration of the Formation of MnO2/Mn/MnO2 SNTAs

Figure 1. (a) SEM image of Mn NTAs synthesized via a ZnO nanorod array template (b, inset: side view); (c) XRD pattern of Mn NTAs; (d) SEM image of the fabricated MnO2/Mn/MnO2 SNTAs; (e) XRD pattern of MnO2/Mn/MnO2 SNTAs.

sandwich-like nanostructure in the wall of the MnO2/Mn/ MnO2 nanotube. The outer and inner symmetric MnO2 shells in the inset in Figure 2a indicate the uniform wraps, which are about 15 nm in thickness. The MnO2 shell shows a polycrystalline structure as shown in Figure 2b. The middle layer in the sandwich-like structure is metal Mn with a thickness of 12 nm, which is not oxidized. The middle Mn layer shows a single-crystal structure and has a preferential growth in [100] direction as shown in Figure 2c. The compositions in sandwichlike walls of MnO2/Mn/MnO2 nanotube arrays are determined by an electron probe microanalyzer and energy dispersive X-ray spectrometer. The ratio of Mn to O in Area 2 (the outer and inner layers) in the inset in Figure 2a is determined to be 1/2 (Figure 2d), indicating that pure MnO2 exists in Area 2. The ratio of Mn to O in Area 1 (middle layer) in Figure 2a is determined to be about 55/1 (Figure 2e), indicating almost pure Mn exists in Area 1. Quantitative analysis results show the compositions of 70.2 at. % MnO2 and 29.8 at. % Mn in MnO2/ Mn/MnO2 SNTAs. Nitrogen adsorption−desorption isotherms of MnO2/Mn/MnO2 SNTAs are shown in Figure 2f, and a high Brunauer−Emmett−Teller (BET) surface area of 91.3 m2/ g is obtained. To explore the advantages of MnO2/Mn/MnO2 SNTAs as electrodes for ECs, we studied their performance by preforming electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements. Figures S3(1) and (2) of the SI show impedance curves of MnO2/Mn/MnO2 SNTAs and MnO2 NTAs (Figure S4, SI), respectively, measured in a solution of 1.0 M Na2SO4. For MnO2/Mn/MnO2 SNTAs, a much lower equivalent series resistance (ESR) and a more vertical line in the low-frequency region are seen, indicating that MnO2/Mn/MnO2 SNTAs as electrodes have a much higher conductivity. Figure 3a(1) shows the CV curve of MnO2/Mn/MnO2 SNTAs at a scan rate of 5 mV/s in 1.0 M Na2SO4 aqueous solution. The shape of this CV curve is quasirectangular, indicating the ideal electrical double-layer capacitance behavior and fast charging−discharging process characteristic. At a scan rate of 5 mV/s, the Csp of MnO2/Mn/ MnO2 SNTAs is 925 F/g, which is much larger than 311 F/g of MnO2 NTAs at the same scan rate (Figure 3a(2)). The Csp versus scan rate, as summarized in Figure 3b, demonstrates that

diffusion paths. (iii) The SNTAs with double MnO2 shells would obviously enhance the utilization rate of MnO2 material because of anisotropic morphology, large specific surface area, and hollow nanostructures. (iv) The SNTAs directly growing on conductive substrate have an excellent electrical contact with current collectors, and this would let each MnO2/Mn/MnO2 nanotube effectively participate in electrochemical reactions with almost no “dead” volume. Electrochemical measurements show that the synthesized MnO2/Mn/MnO2 SNTAs exhibit high specific capacitance, excellent rate capability, high energy and power densities, and excellent long-term cycle stability and they are promising materials for high-performance ECs. Here the MnO2/Mn/MnO2 SNTAs were facilely synthesized by inner and outer surfacely oxidizing Mn nanotubes as shown in Scheme 1b. First, Mn nanotube arrays (NTAs) were synthesized. The schematic illustration of the procedure of fabricating Mn NTAs is shown in Figure S2 of the Supporting Information (SI). The details of fabrication are described in experimental section in the SI. The scanning electron microscopy (SEM) image of the synthesized Mn NTAs is shown in Figure 1a, which shows that the diameters of nanotubes are about 300 nm and the wall thicknesses are about 40 nm. The side view of Mn NTAs is shown in Figure 1b (inset), which shows their lengths are about 1.6 μm. The X-ray diffraction (XRD) pattern of Mn NTAs is shown in Figure 1c. The Mn peaks in the XRD pattern can be indexed to (221), (310), (311), and (321) crystal faces of hexagonal-close-packed (hcp) Mn phase (JCPDS 65-3344). No Mn oxides, hydroxides, or other impurities are seen in XRD, indicating that the pure Mn NTAs were successfully synthesized. Then the thermal annealing process in air at 150 °C for 15 min was performed to create double-faced MnO2 shells on inner and outer surfaces of Mn nanotubes, and accordingly the MnO2/Mn/MnO2 SNTAs were fabricated. The SEM image of the annealed sample is shown in Figure 1d, which shows the NTAs structures are kept well. Figure 1e shows the XRD pattern of MnO2/Mn/MnO2 SNTAs. The Mn phase, such as Mn(221), Mn(310), and Mn(311), and the MnO2 phase, such as MnO2(100), MnO2(002), and MnO2(110) (JCPDS 50-0866), are observed, indicating the coexistence of Mn and MnO2 in the annealed sample. The TEM image shown in Figure 2a confirms the 3804

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Figure 2. (a) TEM image of MnO2/Mn/MnO2 SNTAs (inset: higher magnification); (b) high resolution transmission electron microscopy (HRTEM) image of Area 2; (c) HRTEM image of Area 1; (d) energy-dispersive spectrometry (EDS) pattern of Area 2; (e) EDS pattern of Area 1; (f) N2 adsorption−desorption isotherms of MnO2/Mn/MnO2 SNTAs.

Figure 3. (a) Comparison of CVs for (1) MnO2/Mn/MnO2 SNTAs and (2) MnO2 NTAs at a scan rate of 5 mV/s; (b) comparison of specific capacitances for (1) MnO2/Mn/MnO2 SNTAs and (2) MnO2 NTAs at different scan rates. (The specific mass loading is 0.32 mg/ cm2).

MnO2/Mn/MnO2 SNTAs yield greatly improved capacitance performance with a 2−3 times increase in Csp at all scan rates compared with that of MnO2 NTAs. Galvanostatic charge−discharge curves of MnO2/Mn/MnO2 SNTAs were further measured as shown in Figure 4a. A high symmetric nature is observed in charging−discharging curves, indicating a good electrochemical capacitive characteristic and superior reversible redox reaction. The charge storage capacity of MnO2/Mn/MnO2 SNTAs is significantly improved, with a ∼78% increase in discharge time over MnO2 NTAs at a current density of 1.5 A/g. The summary plot of Csp versus current density (Figure 4b) demonstrates that MnO2/Mn/MnO2 SNTAs exhibit significantly enhanced capacitance performance. When the current density is 1.5 A/g, the Csp of MnO2/Mn/ MnO2 SNTAs is ∼937 F/g, which is about three times that of MnO2 NTAs. When the current density is increased to 24 A/g, the Csp of MnO2/Mn/MnO2 SNTAs still remains 660 F/g, which is much larger than 246 F/g of MnO2 NTAs. With charging−discharging rate increasing from 1.5 to 24 A/g, MnO2/Mn/MnO2 SNTAs only show a ∼30% Csp loss, which is much smaller than the ∼53% Csp loss of MnO2 NTAs, indicating a better rate capability of MnO2/Mn/MnO2 SNTAs. Such a superior rate capability of MnO2/Mn/MnO2 SNTAs can be attributed to the highly accessible specific surface area,

Figure 4. (a) Galvanostatic charge−discharge curves: (1) MnO2/Mn/ MnO2 SNTAs; (2) MnO2 NTAs at a current density of 1.5 mA/g; (b) summary plots of specific capacitances of (1) MnO2/Mn/MnO2 SNTAs and (2) MnO2 NTAs at the different current densities; (c) Ragone plots (energy density vs power density): (1) MnO2/Mn/ MnO2 SNTAs and (2) MnO2 NTAs; (d) comparison of cycling performance of (1) MnO2/Mn/MnO2 SNTAs and (2) MnO2 NTAs for 3000 cycles at 1.5 A/g. (The specific mass loading is 0.32 mg/cm2).

reduced diffusion length of ions, and increased electrical conductivity. For high-performance ECs, the high energy and power densities are expected. Figure 4c(1) shows Ragone plot (power density vs energy density) at various charge−discharge rates for MnO2/Mn/MnO2 SNTAs. The MnO2/Mn/MnO2 SNTAs deliver a high energy density of ∼52 Wh/kg at a high power density of ∼15 kW/kg (Figure 4c), much superior to MnO2 NTAs. In addition, the highest specific power, 23.9 kW/kg, of the synthesized MnO2/Mn/MnO2 SNTAs can adequately meet the power demands of the PNGV (Partnership for a New Generation of Vehicles), 15 kW/kg, demonstrating the 3805

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Nano Letters excellent capability of MnO2/Mn/MnO2 SNTAs for ECs as power supply components in hybrid vehicle systems. Excellent cycling stability is crucial for real EC operations. Herein, the long-term cycle stability of MnO2/Mn/MnO2 SNTAs is evaluated by repeating the charging−discharging test at a current density of 1.5 A/g for 3000 cycles. The Csp as a function of cycle number is presented in Figure 4d(1). It can be seen that the Csp increases slightly at the beginning and then decreases slightly. Finally, the Csp almost keeps stability until 3000 cycles. After 3000 cycles, the Csp decrease is only about 1.5% of the maximal Csp. The above results demonstrate MnO2/Mn/MnO2 SNTAs are highly stable during a cycling test. Compared with MnO2 NTAs, the MnO2/Mn/MnO2 SNTAs show a much higher cycle stability and much higher Csp as shown in Figure 4d. After 3000 cycles, the SNTA morphology of MnO2/Mn/MnO2 composites is still kept well as shown in SEM in Figure S6 in SI. All of the above results confirm that such a design of MnO2/ Mn/MnO2 SNTAs allows maximizing the performance of MnO2, and this may be attributed to the following reasons: first, the SNTAs with an anisotropic morphology, large surface area, and hollow nanostructures can create efficient diffusion paths for electrolyte ions, which will significantly enhance the intercalation of ions and the utilization rate of electrode material. Second, the conductive network in electrode material is well-built since double MnO2 shells are attached tightly on the Mn layers that have a high electrical conductivity (∼0.69 × 108 S/cm) and the MnO2/Mn/MnO2 SNTAs directly grow on conductive substrate. The high conductibility in electrode will favor the rate capability for high power performance and the fast charge−discharge. The utilization rate of electrode material also can be largely enhanced by the high conductibility of the electrode because of the slight polarization. This phenomenon can be seen by reconstructing cyclic chronopotentiometric curves from electrode potential and normalized charge (stored/ released charge scaled to total capacity). The MnO2/Mn/ MnO2 SNTAs show a much higher utilization rate than MnO2 NTAs as shown in inset in Figure S3b of the SI. In summary, this study reports a simple, cost-effective, and potentially scalable technique for fabricating novel MnO2/Mn/ MnO2 SNTAs for high-performance ECs. The unique SNTA architecture of MnO2/Mn/MnO2 composites allow high efficient utilization of MnO2 for charge storage with facilitated transport of ions and electrons and accordingly make the composite electrodes with excellent performance, such as large specific capacitance, excellent rate capability, high power and energy density, and excellent long-term cycle stability. Such MnO2/Mn/MnO2 SNTAs would open up new opportunities for the next generation high-performance ECs.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by NSFC (51173212, 21073240, and 90923008), Fundamental Research Fund for the Central Universities (11lgzd14), Research Fund for New Star Scientist of Pearl River Science and Technology of Guangzhou (2011J2200057), Guangdong Province (9251027501000002, 2011A010802004), and Open-End Fund of State Key Lab of Physical Chemistry of Solid Surfaces of Xiamen University (201113).

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic and analytical methods, capacitive equations, SEM images, impedance comparison curves, and N2 adsorption− desorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 3806

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