J. Phys. Chem. C 2007, 111, 19141-19147
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Hydrothermal Synthesis and Pseudocapacitance Properties of r-MnO2 Hollow Spheres and Hollow Urchins Maowen Xu,† Lingbin Kong,§ Wenjia Zhou,† and Hulin Li*,†,‡ College of Chemistry and Chemical Engineering of Lanzhou UniVersity, Lanzhou 730000, People’s Republic of China, College of Material Science and Engineering of Nanjing UniVersity of Aeronautics and Astronautics, Nanjing 210016, People’s Republic of China, and State Key Laboratory of Gansu AdVanced Non-ferrous Metal Materials, Lanzhou UniVersity of Technology, Lanzhou 730050, People’s Republic of China ReceiVed: August 22, 2007; In Final Form: October 11, 2007
In this work, R-MnO2 hollow spheres and hollow urchins are synthesized via a simple hydrothermal process without using any template or organic surfactant. The effect of the reaction time on the microstructure and morphology of samples is observed systemically. Meanwhile, the forming mechanism of hollow-structured R-MnO2 is carefully investigated by using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The results of nitrogen adsorption-desorption experiments and electrochemical measurements show that the product obtained by hydrothermal reaction for 6 h has large specific surface area, uniform pore-size distribution, and excellent capacitance performance, which make it have a potential application as a supercapacitor electrode material.
Introduction In recent years, due to environmental issues and depleting fossil fuels, interest in the development of alternative energy storage/conversion devices with high power and energy densities catering to the present day demands has increased to a greater extent.1 Electrochemical capacitors (ECs) or supercapacitors (SCs) have gained enormous attention owing to their higher power density and longer cycle life compared to secondary batteries and higher energy density than conventional electrical double-layer capacitors.2-4 In particular, electrochemical capacitors based on hydrous ruthenium oxides exhibit much higher specific capacitance than conventional carbon materials and better electrochemical stability than electronically conducting polymer materials.3,5 However, the high cost of this noble metal material limits it from commercialization. Hence, much effort has been aimed at searching for alternative inexpensive electrode materials with good capacitive characteristics, such as NiO, CoOx, MnO2, etc. Manganese oxides are an important and well-studied class of materials in catalysts, ion-exchangers, and batteries6 and have more recently been investigated as ECs with the anticipation that MnO2 will serve as a low-cost replacement for hydrous RuO2, the state-of-the-art ECs metal oxide.7,8 The cumulative evidence published thus far establishes that the electrochemical performance of MnO2 critically depends not only on their crystal structure and surface properties but also greatly on their textural properties including morphology, surface area, pore volume, and pore dimension.3,7,9 It is well-known that MnO2’s with hollow interiors have attracted significant interest owing to their unique properties, such as high specific surface area, low density, and good permeation.10-13 Recently, Wang et al. synthesized hollow * Corresponding author. E-mail:
[email protected]. Tel.: +86-931-8912517. Fax: +86-931-891-2582. † Lanzhou University. ‡ Nanjing University of Aeronautics and Astronautics. § Lanzhou University of Technology.
Figure 1. (a) XRD patterns for the standard values of JCPDS 440141 and the samples obtained at 110 °C for different reaction times; (b) the schematic illustration of R-MnO2.
MnO2 nanoshells using a polymer bead as template;14 Xie and co-workers prepared MnO2 hollow urchins using a mild reduction route.15,16 However, to the best of our knowledge, there have been no reports on MnO2 with hollow structures obtained by a hydrothermal method until recently. In the present study, we synthesized different hollowstructured R-MnO2 through a simple hydrothermal synthesis method. The effect of the reaction time on the microstructure and morphology of samples was also observed systemically. At the same time, the forming mechanism of hollow-structured
10.1021/jp076730b CCC: $37.00 © 2007 American Chemical Society Published on Web 11/30/2007
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Figure 2. (a) Low-magnification and (b) high-magnification FESEM images of the products obtained by hydrothermal reaction for 3 h.
R-MnO2, the textile properties, and the electrochemical performances of the synthesized products are presented and discussed. Experimental Section Synthesis. All chemical reagents were analar (AR) grade and used as received. The samples were synthesized via a simple hydrothermal process as follows. An amount of 1.262 g of KMnO4 was dissolved in 85 mL of deionized water, and then 2 mL of sulfuric acid (98 wt %) was slowly dropped into the above solution. After stirring for 15 min, 0.768 g of Cu scraps was added into the solution. The resultant system was transferred into a stainless steel autoclave and held at 110 °C for different times. When it was cooled down to room temperature naturally, the as-prepared products were filtered, washed with distilled water, and dried at 80 °C for 12 h. Characterization. The crystal structure of the product was characterized by X-ray diffraction (XRD, D/MAX 2400 diffractometer). The morphology and microstructure of the synthesized materials were investigated by field emission scanning electron microscopy (FESEM, JSM-6700F, Japan), high-resolution transmission electron microscopy (HRTEM, JEM-2010, Japan), and transmission electron microscopy (TEM, Hitachi 600 Japan). Nitrogen adsorption and desorption experiments were carried out at 77.3 K by means of an Autosorb-1 (Quantachrome Instruments) analyzer. The surface area was calculated using the Brunauer-Emmett-Teller (BET) equation. Pore-size distributions were calculated by the Barrett-JoynerHalenda (BJH) method. Electrochemical Measurement. The working electrodes of electrochemical capacitors were formed by mixing the prepared powder with 15 wt % acetylene black and 5 wt % poly(tetrafluoroethylene) (PTFE) binder of the total electrode mass. A small amount of distilled water was then added to those mixtures to make them more homogeneous. The mixtures were pressed onto nickel foam current collectors (1.0 × 1.0 cm2) to fabricate electrodes. All electrochemical measurements were done in a three-electrode experimental setup. Platinum foil with the same area as the working electrode and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. All the electrochemical measurements were carried out in 1 M Na2SO4 aqueous electrolyte by using a CHI 760B electrochemical workstation (CHI Instruments). Results and Discussion The phase and purity of the as-synthesized samples were determined by powder XRD. It can be seen from Figure 1a, after 3 h of hydrothermal reaction, only four very weak peaks are observed, and the main peaks can be indexed to the R-MnO2 phase (JCPDS, card no: 44-0141), suggesting the R-MnO2
forms in the process of hydrothermal treatment for 3 h. With increasing of hydrothermal reaction time, all of these peak intensities increase significantly and match very well with the standard XRD pattern (Figure 1a, bottom). No characteristic peaks are observed for the impurities, indicating high purity and crystallinity of the final sample. Figure 1b illustrates the schematic structure of R-MnO2 which is constructed from double chains of octahedral [MnO6] forming 2 × 2 tunnels. The tunnel structure makes it attractive as an electrode material in batteries and supercapacitors. The morphologies of the products were observed by FESEM. The panoramic morphology of R-MnO2 obtained by hydrothermal reaction for 3 h is shown in Figure 2a, which reveals that the sample has a spherical morphology with 1 to ∼3 µm diameter. The high-magnification SEM image (Figure 2b) demonstrates that the products are built up of many interleaving thin plates. In order to understand the effect of reaction time on the microstructure and morphology of samples, timedependent experiments were carried out and the resultant products were analyzed by FESEM. Interesting morphologies and microstructures of R-MnO2 prepared by hydrothermal reaction for 6 h are observed in Figure 3. As can be seen from its overall view (Figure 3a), with increasing reaction time, the aggregate continuously grows in size to form some urchin-like R-MnO2. Its high-magnification images (Figure 3, parts b and c) reveal that the surface of the urchin-like sphere consists of numerous compactly growing nanowires, which indicates that the thin plates gradually changed into nanowires after hydrothermal reaction for 6 h. During the process of analyzing FESEM, a few spheres with an interior cavity (Figure 3, parts b and c) are also observed, suggesting that the as-synthesized R-MnO2 spheres might have a hollow structure. Figure 4 displays SEM images of the products obtained by hydrothermal reaction for 12 h. As can be seen from its overview, the samples consist predominantly of hollow urchins with visible cavities. The high-magnification FESEM images of the sample (Figure 4, parts c and d) exhibit that the shell of these hollow urchins is formed by densely aligned nanorods with uniform diameters and lengths. The above results indicate that the reaction time seems important for the fabrication of the hollow-structured R-MnO2. Hence, the experiment is also carried out through hydrothermal reaction for 24 h, and the FESEM results display that the urchin structure disappeared completely after reaction for 24 h (Figure 5). The products obtained after 24 h are only nanorods. The above analysis results confirmed that the reaction time plays an important role in synthesizing hollow-structured R-MnO2.
R-MnO2 Hollow Spheres and Hollow Urchins
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Figure 3. Morphologies of the products obtained by hydrothermal reaction for 6 h: (a) low-magnification, (b and c) SEM images of broken R-MnO2 microspheres.
Figure 4. Morphologies of the products obtained by hydrothermal reaction for 12 h: (a and b) low magnification; (c and d) profile images of R-MnO2 hollow urchins.
Figure 5. Morphologies of the products obtained by hydrothermal reaction for 24 h.
The interesting morphology and microstructures of R-MnO2 obtained for hydrothermal reaction of 12 h were further studied using TEM. The TEM images of Figure 6a further confirmed that the R-MnO2 obtained for hydrothermal reaction of 12 h was composed of intercrossed nanorods. Figure 6b displays the HRTEM image of R-MnO2 shown in Figure 6a. It shows clear lattice fringes, which confirms the single-crystallinity of the R-MnO2 obtained for hydrothermal reaction of 12 h. The lattice
spacing of 0.69 nm between adjacent lattice planes in the image corresponds to the distance between two (110) crystal planes. On the basis of the above evolution of the time-dependent crystallinity and morphology, an “Ostwald ripening process” could be used to explain the formation of the different structured R-MnO2.16-18 During the reaction procedure, a large number of nuclei are formed in a short time through a well-known “Ostwald ripening process” at first, and then a slow crystal
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Figure 6. TEM and HRTEM images of R-MnO2 obtained for hydrothermal reaction of 12 h.
Figure 7. TEM of R-MnO2 obtained by different reaction times: (a) 3, (b) 6, (c) 12, and (d) 24 h.
growth follows. With the reaction going on, the aggregate continuously grows in size and density to form a sphere with a solid core. This stage might last for several hours, and then an interior cavity is gradually formed via a core evacuation process due to higher surface energies. A subsequent increase in the hydrothermal dwell time not only leads to the complete damage of the urchin structure but also increases the size of the individual nanorods. The crystallinity of the as-prepared R-MnO2 increases with the reaction time (see the XRD analysis), which confirms that Ostwald ripening (crystallites grow at the expense of the smaller ones) is the underlying mechanism in this hollowing process. Such a core evacuation process is also observed in the synthesis of titania and NiO hollow spheres.19,20 Investigations also reveal that 12 h would be sufficient for the R-MnO2 hollow urchins to reach their equilibrium size.
In order to further understand the forming mechanism of the different structured R-MnO2, the samples obtained from different reaction times were analyzed carefully by TEM investigations. As shown in Figure 7, several obvious evolution stages could be clearly observed. In the initial stage (shorter reaction time, 3 h), only a close-grained sphere is observed; after hydrothermal reaction for 6 h, the surface of the R-MnO2 sphere has changed to flower-like nanostructure which consists of nanoflakes and nanowires. When the reaction time was prolonged to 12 h, an interior cavity sphere is easily observed; after reaction for 24 h, the sphere structures disappear completely and only nanorods can be observed. The results of TEM investigation further confirm the above Ostwald ripening forming mechanism of the different structured R-MnO2.
R-MnO2 Hollow Spheres and Hollow Urchins The structure of R-MnO2 obtained by different hydrothermal reaction times has been further studied by the nitrogen adsorption-desorption experiments. The BET surface area was calculated to be 86.7 m2‚g-1 for the 3 h hydrothermally treated sample. Interestingly, with an increase in the hydrothermal reaction time to 6 h, the sample shows a higher specific surface area, 108.6 m2‚g-1. Although XRD shows a more amorphous materials at shorter reaction times than longer, the specific surface area is more dependent on the pore structure where the N2 adsorption-desorption takes place. As explained previously in the evolvement of R-MnO2, the materials prepared in a shorter reaction time exhibit more agglomerated nanoplates, and many of them are solid spheres, while after 6 h of hydrothermal treatment, the surface of the spheres becomes much slacker and many of the spheres have a cavity structure. When part b of Figure 8 is compared with part c, the sample obtained by hydrothermal reaction for 12 h shows a little lower specific surface area, 92.8 m2‚g-1, which is because many of the hollow spheres are broken and the slack thin-plates structure has changed to densely aligned nanorods. As for the sample obtained by hydrothermal reaction for 24 h, its specific surface area only reached 52 m2‚g-1. It also can be seen from Figure 8, that the samples obtained by hydrothermal reaction for 3 and 6 h exhibit a similar hysteresis loop in the low relative pressure (P/P0) range of 0.45 to ∼0.95, whereas the sample prepared by hydrothermal reaction for 12 h displays a different hysteresis loop in the high relative pressure (P/P0) range of 0.9 to ∼1. The hysteresis loop in the low relative pressure (P/P0) range of 0.45 to ∼0.95 might be ascribed to the presence of a mesoporous structure in the interleaving nanoplates or cavities of the hollow spheres, and the hysteresis loop at P/P0 ) 0.9 to ∼1 might result from the internanorods space.21,22 The special high BET surface area and mesoporous structure of the hollow spheres and urchins provide the possibility of efficient transport of electrons and ions, which leads to the high electrochemical capacity of these materials. As an excellent supercapacitor electrode material, it should have not only high capacity but also low electronic resistance. In order to evaluate the microstructure of as-synthesized materials and their electrochemical property, these samples are fabricated to electrodes of supercapacitors and characterized with electrochemical impedance spectroscopy (EIS) measurements first. Figure 9 shows the complex plane plots of the impedance of the R-MnO2 electrodes prepared at different hydrothermal reaction times (at 0.25 V, near OCP). The EIS of the R-MnO2 electrode is composed of a partially overlapped semicircle and a straight slopping line. Such a pattern of the EIS can be fitted by an equivalent circuit shown in the inset of Figure 9a. For all of the materials, at very high frequencies, the intercept at real part Z′ (which is equal to Rb) is almost the same, which means that the electrodes obtained by different materials have the same combination resistance of ionic resistance of electrolyte, intrinsic resistance of active materials, and contact resistance at the active material/current collector interface.7 The semicircle in the highfrequency range associated with the surface properties of the porous electrode corresponds to the faradic charge-transfer resistance (Rct).7,23 The charge-transfer resistance of the R-MnO2 materials is in the order of 6 h < 12 h < 3 h < 24 h prepared materials. At the lower frequencies, a straight sloping line represents the diffusive resistance (Warburg impendence, W) of the electrolyte in electrode pores and the proton diffusion in host materials.24 The diffusing lines for the materials with a pore network, especially for the R-MnO2 obtained by 6 h, come closer to an ideal straight line along the imaginary axis (Z′′),
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Figure 8. Nitrogen adsorption-desorption isotherms of R-MnO2 obtained by different reaction times. The inset shows BJH pore-size distributions: (a) R-MnO2 obtained by reaction for 3 h; (b) R-MnO2 obtained by reaction for 6 h; (c) R-MnO2 obtained by reaction for 12 h.
which indicates that the materials have low diffusion resistance. The sample obtained by 6 h shows the minimum Rct and diffusive resistance, indicating that it has an excellent porous structure. For more informative representation, experimental impedance data, Z(f), were transformed to capacitance, C(f), according to the equation Z′′ ) (2πfC)-1.25 The frequency dependence of capacitance (normalized to electrode geometric area) for different electrodes is shown in Figure 9b. For frequencies from 0.01 to ∼2 Hz, the electrode fabricated by R-MnO2 obtained through hydrothermal reaction for 6 h has the maximum capacitance, indicating that ions could more easily penetrate into the pores of the material formed by reaction for 6 h and are electrochemically more accessible over a reasonable frequency range for the electrode.
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Figure 11. Charge/discharge curves of R-MnO2 formed by hydrothermal reaction for 6 h in the potential range from -0.2 to 0.8 V at different current densities.
Figure 9. (a) Complex plane impedance plots of the R-MnO2 electrode prepared at different hydrothermal reaction times at an ac voltage amplitude of 5 mV. The inset of (a) is the equivalent circuit. (b) Frequency dependence of capacitance for different electrodes. Figure 12. Cycle life of the R-MnO2 electrode at 5 mA in 1 M Na2SO4 electrolyte; the inset shows charge/discharge curves of the R-MnO2 electrode in the potential range from -0.2 to 0.8 V at 5 mA.
Figure 10. CV curves of R-MnO2 prepared by hydrothermal reaction for 6 h at different scan rates (5, 10, 20 mV‚s-1, from inside to outside).
According to the above analysis, it is conclusive that the R-MnO2 obtained by hydrothermal reaction for 6 h has an optimum surface area and high-quality pore network and hence showed the best electrochemical capacitive properties. Thus, further experiments were performed on the sample which formed by reaction for 6 h. The typical rectangle-like shape of all cyclic voltammogram (CV) curves (Figure 10) measured at various scan rates in 1 M Na2SO4 solution for R-MnO2 formed by reaction for 6 h reveals the perfect electrochemical capacitive behavior of the synthesized sample. 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,1,7 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, and therefore, it shows almost ideal capacitive behavior. However, with the scan rate increasing, the effective interaction between the ions and the electrode is greatly reduced; the deviation from rectangularity of the CV becomes obvious. In order to get more information about the ability of the synthesized R-MnO2 as an electrode material in a supercapacitor, constant current charge/discharge measurement was carried out in 1 M Na2SO4. Figure 11 shows the constant current charge/ discharge curves of R-MnO2 at different current densities (2.5, 5, and 10 mA). 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:3
C)
I [(dV/dt)w]
where I (mA) and dV/dt (mV‚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 2.5, 5, and 10 mA are 167, 147, and 124 F‚g-1, respectively. The results indicate that the material has good rate capacitance. It is wellknown that a high rate capacity is one of the most important electrochemical performances in the applications of electrodes and batteries.26 Therefore, the excellent rate capability of this sample makes it attractive particularly for practical applications.
R-MnO2 Hollow Spheres and Hollow Urchins The long-term cycling stability of a composite electrode made of the powders obtained by hydrothermal reaction for 6 h is investigated, and the variation of specific capacitance over 350 cycles is depicted in Figure 12. A little increase of capacitance is observed during the first 20 cycles, and thereafter the capacitance begins to decrease. After 350 cycles, the capacitance is still about 89% of the first cycle. This demonstrates that, within the voltage window -0.2 to ∼0.8 V, the charge and discharge processes do not seem to induce significant structural or microstructural changes of the R-MnO2 electrode material as expected for pseudocapacitance reactions. The long-term stability implies that the R-MnO2 is a good candidate as a material for supercapacitor electrodes. Conclusion In summary, various hollow sphere and urchin structured R-MnO2’s have been synthesized by a simple and facile hydrothermal method. The different structured materials are formed by a ripening-splitting crystal growth process. The novel hollow sphere or urchin structured R-MnO2 materials possess a high loosely mesoporous cluster structure consisting of thin plates or nanowires and exhibit enhanced rate capacity and cycleability. The good cycleability and high rate capability coupled with the low cost and environmentally benign nature of manganese may make this material attractive for large applications. Furthermore, the hydrothermal method has excellent reliability, selectivity, and efficiency for synthesizing inorganic materials with uniform and distinct morphologies. References and Notes (1) Subramanian, V.; Zhu, H.; Vajtai, R.; Ajayan, P. M.; Wei, B. J. Phys. Chem. B 2005, 109, 20207.
J. Phys. Chem. C, Vol. 111, No. 51, 2007 19147 (2) Sarangapani, S.; Tilak, B. V.; Chen, C. P. J. Electrochem. Soc. 1996, 143, 3791. (3) Reddy, R. N.; Reddy, R. G. J. Power Sources 2004, 132, 315. (4) Conway, B. E. Electrochemical Supercapacitors; Kluwer Academic/ Plenum Publishers: New York, 1999. (5) Hu, C. C.; Tsou, T. W. Electrochem. Commun. 2002, 4, 105. (6) Thackeray, M. Prog. Solid State Chem. 1997, 25, 1. (7) Xu, M. W.; Zhao, D. D.; Bao, S. J.; Li, H. L. J. Solid State Electrochem. 2007, 11, 1101. (8) Fischer, A. E.; Pettigrew, K. A.; Rolison, D. R.; Stroud, R. M.; Long, J. W. Nano Lett. 2007, 7, 281. (9) Xing, W.; Li, F., Yan, Z.; Lu, G. Q. J. Power Sources 2004, 134, 324. (10) Wang, D.; Song, C.; Hu, Z.; Fu, X. J. Phys. Chem. B 2005, 109, 1125. (11) Ye, L.; Wu, C.; Guo, W.; Xie, Y. Chem. Commun. 2006, 4738. (12) Wang, Y.; Su, F.; Lee, J. Y.; Zhao, X. S. Chem. Mater. 2006, 18, 1347. (13) Duan, G.; Cai, W.; Luo, Y.; Sun, F. AdV. Funct. Mater. 2007, 17, 644. (14) Wang, L.; Ebina, Y.; Takada, K.; Sasaki, T. Chem. Commun. 2004,1074. (15) Li, Z.; Ding, Y.; Xiong, Y.; Yang, Q.; Xie, Y. Chem. Commun. 2005, 918. (16) Li, B.; Rong, G.; Xie, Y.; Huang, L.; Feng, C. Inorg. Chem. 2006, 45, 6404. (17) Ostwald, W. Z. Phys. Chem. 1900, 34, 495. (18) Bao, S. J.; Bao, Q. L.; Li, C. M.; Chen, T. P.; Sun, C. Q.; Dong, Z. L.; Gan, Y.; Zhang, J. Small 2007, 3, 1174. (19) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 3492. (20) Wang, Y.; Zhu, Q.; Zhang, H. Chem. Commun. 2005, 5231. (21) Tao, Y.; Kanoh, H.; Abrams, L.; Kaneko, K. Chem. ReV. 2006, 106, 896. (22) Meng, Y.; Chen, D.; Jiao, X. J. Phys. Chem. B 2006, 110, 15212. (23) Zhang, S. S.; Xu, K.; Jow, T. R. Electrochim. Acta 2004, 49, 1057. (24) Lin, C.; Ritter, J. A.; Popov, B. N. J. Electrochem. Soc. 1998, 145, 4097. (25) Jang, J. H.; Kato, A.; Machida, K.; Naoi, K. J. Electrochem. Soc. 2006, 153, A321. (26) Bao, S. J.; Li, C. M.; Li, H. L.; Luong, J. H. T. J. Power Sources 2007, 164, 885.
