Carbon Aerogel Composites as Promising

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Mesoporous MnO2/Carbon Aerogel Composites as Promising Electrode Materials for High-Performance Supercapacitors Gao-Ren Li,* Zhan-Ping Feng, Yan-Nan Ou, Dingcai Wu, Ruowen Fu, and Ye-Xiang Tong* MOE Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Institute of Optoelectronic and Functional Composite Materials, Sun Yat-Sen University, Guangzhou 510275, China Received October 18, 2009. Revised Manuscript Received December 18, 2009 MnO2 as one of the most promising candidates for electrochemical supercapacitors has attracted much attention because of its superior electrochemical performance, low cost, and environmentally benign nature. In this Letter, we explored a novel route to prepare mesoporous MnO2/carbon aerogel composites by electrochemical deposition assisted by gas bubbles. The products were characterized by energy-dispersive spectrometry (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The MnO2 deposits are found to have high purity and have a mesoporous structure that will optimize the electronic and ionic conductivity to minimize the total resistance of the system and thereby maximize the performance characteristics of this material for use in supercapacitor electrodes. The results of nitrogen adsorption-desorption experiments and electrochemical measurements showed that these obtained mesoporous MnO2/carbon aerogel composites had a large specific surface area (120 m2/g), uniform pore-size distribution (around 5 nm), high specific capacitance (515.5 F/g), and good stability over 1000 cycles, which give these composites potential application as high-performance supercapacitor electrode materials.

1. Introduction In recent years, electrochemical supercapacitors as a kind of attractive energy-storage/conversion device are attracting wide interest owing to their higher power density and longer life cycle compared with batteries and higher energy density than conventional dielectric capacitors, which show potential applications in electric vehicles, power sources, portable electronics, and other devices.1 Various materials were investigated for electrochemical supercapacitors including (i) carboneous materials, (ii) conducting polymers, and (iii) transition-metal oxides.2 Among transition-metal oxides, amorphous hydrated ruthenium oxide exhibits remarkably high specific capacitance and excellent reversibility because of the ideal solid-state pseudofaradaic reaction.3 However, the high cost, low porosity, and toxic nature of RuO2 limit its practical application. Therefore, some cheap and environmentally friendly metal oxides have received more and more attention. MnO2 as a promising material for electrochemical supercapacitors has attracted much attention because of its high specific capacitance, ability to charge-discharge rapidly, good cycle stability, low cost, and environmentally benign nature.4 Furthermore, MnO2 can be used in neutral aqueous electrolytes, unlike *To whom correspondence should be addressed. (1) Conway, B. E. Electrochemical Capacitors: Scientific Fundamentals and Technological Applications; Kluwer Academic/Plenum: New York, 1999. (2) Sarangapani, S.; Tilak, B. V.; Chen, C. P. J. Electrochem. Soc. 1996, 143, 3791. (3) (a) Hu, C.-C.; Chang, K.-H.; Lin, M.-C.; Wu, Y.-T. Nano Lett. 2006, 6, 2690–2695. (b) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 2001, 142, 2699. (c) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699– 2703. (4) (a) Long, J. W.; Sassin, M. B.; Fischer, A. E.; Rolison, D. R.; Mansour, A. N.; Johnson, V. S.; Stallworth, P. E.; Greenbaum, S. G. J. Phys. Chem. C 2009, 113, 17595–17598. (b) Bai, Y.-H.; Zhang, H.; Xu, J.-J.; Chen, H.-Y. J. Phys. Chem. C 2008, 112, 18984–18990. (c) Shanmugam, S.; Gedanken, A. J. Phys. Chem. B 2006, 110, 24486–24491. (d) Benedetti, T. M.; Bazito, F. F. C.; Ponzio, E. A.; Torresi, R. M. Langmuir 2008, 24, 3602–3610. (e) Nakayama, M.; Tagashira, H. Langmuir 2006, 22, 3864–3869. (f) Jun, Y.-S.; Martin, S. T. Langmuir 2006, 22, 2235–2240. (g) Nakayama, M.; Konishi, S.; Tagashira, H.; Ogura, K. Langmuir 2005, 21, 354–359. (h) Wang, Y.; Zhitomirsky, I. Langmuir 2009, 25, 9684–9689. (i) Lv, G.; Wu, D.; Fu, R. J. Non-Cryst. Solids 2009, 355, 2461–2465.

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RuO2 3 xH2O and NiOOH, which can only be used in strong acidic or alkaline electrolytes, thus causing environmental problems. It has been especially emphasized that the electrochemical characteristics of MnO2 materials strongly depend on their structural parameters such as polymorphs, morphology, particle size, and bulk density.5 Up to now, various nanostructures of MnO2, such as nanoparticles,6 nanorods,7 nanowires,8 nanofibers,9 nanobelts,10 nanotubes,11 nanosheets,12 branched (5) (a) Athouel, L.; Moser, F.; Dugas, R.; Crosnier, O.; Belanger, D.; Brousse, T. J. Phys. Chem. C 2008, 112, 7270–7277. (b) Devaraj, S.; Munichandraiah, N. J. Phys. Chem. C 2008, 112, 4406–4417. (c) Xu, M.; Kong, L.; Zhou, W.; Li, H. J. Phys. Chem. C 2007, 111, 19141–19147. (6) (a) Xie, S.; Zhou, X.; Han, X.; Kuang, Q.; Jin, M.; Jiang, Y.; Xie, Z.; Zheng, L. J. Phys. Chem. C 2009, 113, 19107–19111. (b) Sayle, T. X. T.; Catlow, C. R. A.; Maphanga, R. R.; Ngoepe, P. E.; Sayle, D. C. J. Am. Chem. Soc. 2005, 127, 12828. (c) Espinal, L.; Suib, S. L.; Rusling, J. F. J. Am. Chem. Soc. 2004, 126, 7676. (d) Wei, W.; Cui, X.; Chen, W.; Ivey, D. G. J. Phys. Chem. C 2008, 112, 15075–15083. (7) (a) Liang, S.; Teng, F.; Bulgan, G.; Zong, R.; Zhu, Y. J. Phys. Chem. C 2008, 112, 5307–5315. (b) Wang, X.; Li, Y. D. Chem. Commun. 2002, 764. (c) Wang, L.-C.; Liu, Y.-M.; Chen, M.; Cao, Y.; He, H.-Y.; Fan, K.-N. J. Phys. Chem. C 2008, 112, 6981–6987. (d) Qu, Q.; Zhang, P.; Wang, B.; Chen, Y.; Tian, S.; Wu, Y.; Holze, R. J. Phys. Chem. C 2009, 113, 14020–14027. (e) Sui, N.; Duan, Y.; Jiao, X.; Chen, D. J. Phys. Chem. C 2009, 113, 8560–8565. (8) (a) Zhou, F.; Zheng, H. G.; Zhao, X. M.; Guo, Q. X.; Ni, X. M.; Shen, T.; Tang, C. M. Nanotechnology 2005, 16, 2072. (b) Li, Q.; Olson, J. B.; Penner, R. M. Chem. Mater. 2004, 16, 3402. (c) Xiong, J.; Xie, Y.; Li, Z. Q.; Wu, C. Z. Chem.;Eur. J. 2003, 9, 1645. (d) Wang, X.; Li, Y. D. Chem.;Eur. J. 2003, 9, 300. (e) Wang, X.; Li, Y. D. J. Am. Chem. Soc. 2002, 124, 2880. (9) Zheng, Y. H.; Cheng, Y.; Bao, F.; Wang, Y. S.; Qin, Y. J. Cryst. Growth 2006, 286, 156. (10) Li, G. C.; Jiang, L.; Pang, H. T.; Peng, H. R. Mater. Lett. 2007, 61, 3319– 3322. (11) (a) Luo, J.; Zhu, H. T.; Fan, H. M.; Liang, J. K.; Shi, H. L.; Rao, G. H.; Li, J. B.; Du, Z. M.; Shen, Z. X. J. Phys. Chem. C 2008, 112, 12594–12598. (b) Zheng, D. S.; Sun, S. X.; Fan, W. L.; Yu, H. Y.; Fan, C. H.; Cao, G. X.; Yin, Z. L.; Song, X. Y. J. Phys. Chem. B 2005, 109, 16439. (c) Umek, P.; Gloter, A.; Pregelj, M.; Dominko, R.; Jagodic, M.; Jaglicic, Z.; Zimina, A.; Brzhezinskaya, M.; Potocnik, A.; Filipic, C.; Levstik, A.; Arcon, D. J. Phys. Chem. C 2009, 113, 14798–14803. (12) (a) Zhu, H. T.; Luo, J.; Yang, H. X.; Liang, J. K.; Rao, G. H.; Li, J. B.; Du, Z. M. J. Phys. Chem. C 2008, 112, 17089–17094. (b) Omomo, Y.; Sasaki, T.; Wang, L. Z.; Watanabe, M. J. Am. Chem. Soc. 2003, 125, 3568. (c) Wang, N.; Cao, X.; He, L.; Zhang, W.; Guo, L.; Chen, C.; Wang, R.; Yang, S. J. Phys. Chem. C 2008, 112, 365– 369. (d) Wang, L. Z.; Takada, K.; Kajiyama, A.; Onoda, M.; Michiue, Y.; Zhang, L. Q.; Watanabe, M.; Sasaki, T. Chem. Mater. 2003, 15, 4508. (e) Kadoma, Y.; Uchimoto, Y.; Wakihara, M. J. Phys. Chem. B 2006, 110, 174–177.

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nanostructures,13 and other nanostructures,14 have been synthesized by different methods. Compared with hydrous RuO2 with high specific capacitance (SC) values ranging from 720 to 1000 F/g, MnO2 exhibits a lower electrochemical SC values that are usually about 150-300 F/g and are far from its theoretical value of ca. 1400 F/g.5,15-18 Therefore, it is significant and necessary to further improve the capacitive performance of MnO2. Recently, some efforts focused on mesoporous MnO2 with pore sizes of 2-50 nm because confining d-electrons to the thin walls between pores can endow such materials with unusual electrical, magnetic, and optical properties.19 With the characteristics of mesoporous materials, it will also be possible to optimize the electronic and ionic conductivity of MnO2 to minimize the total resistance of the system and thereby maximize the performance characteristics of this material for use in supercapacitor applications.20 The present work focuses on the facile creation of open and mesoporous morphologies of MnO2/carbon aerogel composites for use as high-performance supercapacitor electrode materials. At present, the preparation of mesoporous silicas, alumino-silicates, alumino-phosphates, and related materials has been already well established. However, it has proved difficult to synthesize transition metal oxides including MnO2 in the form of mesoporous materials, although the challenge has seen important advances in recent years.21 The synthesis of mesoporous materials usually involves the use of a hard template (e.g., mesoporous silica) within the pores of which the mesoporous solid is formed, followed by template dissolution, or a soft template (e.g., alkyl amine) around which the mesoporous solid is assembled. However, sometimes the above routes may have some shortcomings. For example, if the temperature range within which the target phase forms does not coincide with the stability range of the template, the desired phase may not be obtained. Here, we investigated a novel and facile route for the preparation of mesoporous MnO2 on carbon aerogels via electrochemical deposition assisted by gas bubbles. It is well-known that the (13) (a) Cheng, F. Y.; Zhao, J. Z.; Song, W. N.; Li, C. S.; Chen, J.; Shen, P. W. Inorg. Chem. 2006, 45, 2038. (b) Zheng, D. S.; Yin, Z. L.; Zhang, W. M.; Tan, X. J.; Sun, S. X. Cryst. Growth Des. 2006, 6, 1733. (14) (a) Li, B. X.; Rong, G. X.; Xie, Y.; Huang, L. F.; Feng, C. Q. Inorg. Chem. 2006, 45, 640. (b) Ding, Y. S.; Shen, X. F.; Gomez, S.; Luo, H.; Aindow, M.; Suib, S. L. Adv. Funct. Mater. 2006, 16, 549. (c) Li, Z. Q.; Ding, Y.; Xiong, Y. J.; Yang, Q.; Xie, Y. Chem. Commun. 2005, 918. (d) Jana, S.; Pande, S.; Sinha, A. K.; Sarkar, S.; Pradhan, M.; Basu, M.; Saha, S.; Pal, T. J. Phys. Chem. C 2009, 113, 1386–1392. (e) Wu, C.; Xie, Y.; Wang, D.; Yang, J.; Li, T. J. Phys. Chem. B 2003, 107, 13583–13587. (15) Liu, R.; Lee, S. B. J. Am. Chem. Soc. 2008, 130, 2942–2943. (16) Fischer, A. E.; Pettigrew, K. A.; Rolison, D. R.; Stroud, R. M.; Long, J. W. Nano Lett. 2007, 7, 281–286. (17) Dong, X.; Shen, W.; Gu, J.; Xiong, L.; Zhu, Y.; Li, H.; Shi, J. J. Phys. Chem. B 2006, 110, 6015–6019. (18) Subramanian, V.; Zhu, H.; Vajtai, R.; Ajayan, P. M.; Wei, B. J. Phys. Chem. B 2005, 109, 20207–20214. (19) (a) Dong, B.; Xue, T.; Xu, C.-L.; Li, H.-L. Microporous Mesoporous Mater. 2008, 112, 627–631. (b) Jiao, F.; Bruce, P. G. Adv. Mater. 2007, 19, 657–660. (c) Xue, T.; Xu, C.-L.; Zhao, D.-D.; Li, X.-H.; Li, H.-L. J. Power Sources 2007, 164, 953–958. (d) Zhu, S.; Zhou, Z.; Zhang, D.; Wang, H. Microporous Mesoporous Mater. 2006, 95, 257–264. (e) Rhodes, C. P.; Long, J. W.; Doescher, M. S.; Dening, B. M.; Rolison, D. R. J. Non-Cryst. Solids 2004, 350, 73–79. (20) Yuan, Q.; Yin, A.-X.; Luo, C.; Sun, L.-D.; Zhang, Y.-W.; Duan, W.-T.; Liu, H.-C.; Yan, C.-H. J. Am. Chem. Soc. 2008, 130, 3465–3472. (21) (a) Jiao, F.; Hill, A. H.; Harrison, A.; Berko, A.; Chadwick, A. V.; Bruce, P. G. J. Am. Chem. Soc. 2008, 130, 5262–5266. (b) Demir-Cakan, R.; Hu, Y.-S.; Antonietti, M.; Maier, J.; Titirici, M.-M. Chem. Mater. 2008, 20, 1227–1229. (c) Yuan, Q.; Yin, A.-X.; Luo, C.; Sun, L.-D.; Zhang, Y.-W.; Duan, W.-T.; Liu, H.-C.; Yan, C.-H. J. Am. Chem. Soc. 2008, 130, 3465–3472. (d) Bhattacharyya, S.; Gedanken, A. J. Phys. Chem. C 2008, 112, 659–665. (e) Yuan, Q.; Liu, Q.; Song, W.-G.; Feng, W.; Pu, W.-L.; Sun, L.-D.; Zhang, Y.-W.; Yan, C.-H. J. Am. Chem. Soc. 2007, 129, 6698–6699. (f) Jiao, F.; Jumas, J.-C.; Womes, M.; Chadwick, A. V.; Harrison, A.; Bruce, P. G. J. Am. Chem. Soc. 2006, 128, 12905–12909. (g) Jiao, F.; Harrison, A.; Jumas, J.-C.; Chadwick, A. V.; Kockelmann, W.; Bruce, P. G. J. Am. Chem. Soc. 2006, 128, 5468–5474. (h) Shibata, H.; Ogura, T.; Mukai, T.; Ohkubo, T.; Sakai, H.; Abe, M. J. Am. Chem. Soc. 2005, 127, 16396–16397.

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mesoporous carbon materials including carbon aerogels, wormholelike mesoporous carbons, and ordered mesoporous carbons used as substrates can obviously increase the active sites, enhance the electric conductivity, improve the homogeneity of electrochemical reaction, and reduce the ionic resistance of the metal oxide and, consequently, further increase both the power and energy densities of the electrode.22 In this study, mesoporous MnO2 with a high density of pores has been successfully synthesized on the substrate of carbon aerogels accompanying hydrogen evolution, which can be deliberately suppressed in typical electrodeposition processes to produce dense crystalline walls of pores. The growth rates of deposits can easily be well controlled by deposition potentials, current densities, or salt concentrations. The results of nitrogen adsorption-desorption experiments and electrochemical measurements showed that the obtained products had a large specific surface area, uniform pore-size distribution, and good capacitance performance, which give the composites potential application as high-performance supercapacitor electrode materials.

2. Experimental Section The detailed preparation procedure of carbon aerogels can be obtained in ref 23. According to the predetermined formulations, all reactants, including formaldehyde, resorcinol, deionized water, and cetyltrimethylammonium bromide, were mixed with a magnetic stirrer and then were transferred into a glass vial (20 mL). The vial was sealed and then was put into a water bath (85 °C) to cure for 5 days. After curing, at first the gels were directly dried in air at room temperature for 2 days, then further dried under an infrared lamp with an irradiation temperature of about 50 °C for 1 day, and finally dried in an oven at 110 °C at ambient pressure for 3 h. Subsequently, the resultant resorcinolformaldehyde aerogels were pyrolyzed at 300-900 °C for 3 h in flowing N2. The electrodeposition of mesoporous MnO2 was carried out in a simple three-electrode electrochemical cell with the carbon aerogels/Cu as the working electrode (WE, 1.0 cm2), a graphite electrode as the counter electrode (CE, 2.5 cm2), and a saturated calomel electrode (SCE) as the reference electrode (RE) that was connected to the cell with a double salt bridge system (ShangHai Yueci). The electrolyte was an aqueous solution of 0.1 M Mn(NO3)2. The electrodeposition was performed by potentiostatic electrolysis with a potential of -1.30 V versus SCE at room temperature for 120 min. The electrolysis plot of the current versus time is shown in Supporting Information Figure S1. The surface morphologies of the obtained mesoporous MnO2 deposits were observed by field emission scanning electron microscopy (FE-SEM, JSM-6330F) and transmission electron microscopy (TEM, JEM-2010HR). The as-deposited products were also characterized by X-ray diffraction (XRD, PIGAKU, D/MAX 2200 VPC) to determine the deposit structures. Chemicalstate analysis of deposits was carried out by X-ray photoelectron spectroscopy (XPS) using an ESCALAB 250 X-ray photoelectron spectrometer. All XPS spectra were corrected using the C 1s line at 284.6 eV. Curve fitting and background subtraction were accomplished. The samples were also characterized by Brunauer, Emmett, and Teller (BET) nitrogen sorption surface area measurements (Micromeritics ASAP 2010). Specific surface areas of the prepared deposits were calculated by the BET method, and pore sizes were calculated using the Barrett, Joyner, and Halenda (BJH) method (for large pores) or density functional theory (22) Chang, J.-K.; Lin, C.-T.; Tsai, W.-T. Electrochem. Commun. 2004, 6, 666– 671. (23) (a) Wu, D. C.; Fu, R. W.; Dresselhaus, M. S.; Dresselhaus, G. Carbon 2006, 44, 675–681. (b) Fu, R. W.; Wu, D. C. Preparation of carbon aerogels. China Patent, Appl. No. 200410027355.6, 2004. (c) Wang, J.; Yang, X.; Wu, D.; Fu, R.; Dresselhaus, M. S.; Dresselhaus, G. J. Power Sources 2008, 185, 589–594.

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Figure 1. TEM images of mesoporous MnO2 deposited on carbon aerogels with different magnetification. Scheme 1. General Schematic Representation of the Electrochemical Synthesis of Porous MnO2 Nanostructures Assisted by Gas Bubbles

(DFT) method (for small pores) on the basis of the adsorption branch of the nitrogen sorption isotherms. A Chi750B electrochemical workstation was used for the electrochemical measurements. The mesoporous MnO2/carbon aerogel composites as electrodes were studied for redox supercapacitor applications in 1.0 M Na2SO4 electrolyte. The graphite sheet was used as a counter electrode, and the SCE was used as the reference electrode. The cyclic voltammetry experiments were performed between 0 and 0.9 V versus SCE at a scan rate of 2-40 mV/s.

3. Results and Discussion Under electrodeposition with a constant potential of -1.30 V (vs SCE), Mn2þ (0.1 M Mn(NO3)2) can be successfully converted to MnO2 via eqs 1 and 2. Simultaneously, the water in the deposition solution could be also electroreduced with H2 gas release according to eq 3. Very interestingly, this electrodeposition gives rise to the formation of mesoporous MnO2. This can be attributed to no deposit growth toward the gas bubbles because of no metal ions being available there, which finally leads to the electrodeposition only happening between gas bubbles and, accordingly, the formation of mesoporous structures. The growth of mesoporous MnO2 is illustrated in Scheme 1. Supporting Information Figure S2 shows the SEM image of mesoporous MnO2 deposited in a solution of 0.1 M Mn(NO3)2 at -1.30 V. The porosity corresponds to the interstitial pores formed between nanoparticles. Figure 1 shows the TEM images of mesoporous MnO2 with different magnification. From Figure 1b, the sizes of pores can be estimated as about 10-15 nm. NO3 - þ H2 O þ 2e f NO2 - þ 2OH -

ð1Þ

2Mn2þ þ 4OH - þ O2 f 2MnO2 þ 2H2 O

ð2Þ

2H2 O þ 2e f H2 v þ 2OH -

ð3Þ

A typical energy-dispersive spectrometry (EDS) spectrum of MnO2 products is shown in Supporting Information Figure S3, Langmuir 2010, 26(4), 2209–2213

which shows that the obtained deposits are pure MnO2. Supporting Information Figure S4 shows the XRD pattern of the synthesized MnO2 products. All the diffraction peaks can be readily indexed to a pure tetragonal symmetry of R-MnO2 with a space group of I4/m (87) (JCPDS 44-0141). No characteristic peaks for other manganese oxides were detected, indicating the high purity of the as-prepared products. In addition, the powder pattern shows a large background that starts at 20°, and it arises from the carbon aerogel. The XPS spectra of the R-MnO2 deposits are shown in Supporting Information Figure S5. The detected peak at the bonding energy of 642 eV, which corresponds to Mn 2p3/2, indicates that the element Mn of the as-prepared samples is present in the chemical state of Mn4þ. The peak appearing at the binding energy of 654 eV, which can be assigned to Mn 2p1/2, further confirms the sole existence of Mn4þ. Therefore, on the basis of the results of EDS, XRD, and XPS, no unoxidized Mn, suboxide MnO, Mn2O3, and Mn3O4 phases were found in MnO2 products, and the high purity MnO2 was successfully obtained. In addition, quantitative XPS analysis demonstrates that the atomic ratio of Mn to O is approximately 1:2, which is in good agreement with the XRD results. The atomic concentration of O in oxide is determined on the basis of deconvoluted peak areas in order to separate it from the contributions of hydroxide and H2O. As the obtained MnO2 deposits have a mesoporous structure, a high surface area is expected. In order to examine the surface properties of the synthesized mesoporous structures, the porosity was characterized by nitrogen sorption analysis using standard BET techniques. The adsorption-desorption isotherm is shown in Figure 2a, indicating a hysteresis loop characteristic to mesoporous materials. The hysteresis loop in the low relative pressure (P/P0) range of 0.45-0.90 might be ascribed to the presence of a mesoporous structure, and the hysteresis loop at P/P0 = 0.901.0 might result from the interplates space.4c According to the BET method, the resulting MnO2 mesoporous structures have a specific surface area of about 120 m2/g. The pore-size distribution DOI: 10.1021/la903947c

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Figure 3. CVs of (a) mesoporous MnO2/carbon aerogel composites and (b) bare carbon aerogels in 1 mol/L Na2SO4 aqueous solution at 20 mV/s.

Figure 2. (a) Adsorption-desorption isotherms and (b) pore-size distributions of mesoporous MnO2 deposited on carbon aerogels in solution of 0.1 M Mn(NO3)2 at -1.30 V.

of deposits is shown in Figure 2b, which shows a pore-size distribution of around 5 nm calculated from the adsorption branch of the isotherm using the BJH model. The high BET surface area and mesoporous structure of the MnO2 deposit provide the possibility of efficient transport of electrons and ions, which can lead to the high electrochemical capacity of these materials. The electrochemical performances of the prepared mesoporous MnO2/carbon aerogel composites were investigated by cyclic voltammetry experiments. The potential window was chosen in the range of 0-0.9 V versus SCE. The representative cyclic voltammograms (CVs) of mesoporous MnO2/carbon aerogel composites and bare carbon aerogel electrodes in 1.0 M Na2SO4 at 20 mV/s are presented in Figure 3a and b, respectively. The nearly symmetrical rectangular shapes are clearly exhibited in the CVs, which show the symmetric current-potential characteristics of electrochemical double layer capacitor of mesoporous MnO2. This indicates a good capacitor with relatively low solution resistance. CVs of mesoporous MnO2/carbon aerogel composites in 1.0 mol/L Na2SO4 aqueous solution at various scan rates are shown in Supporting Information Figure S6. The CV curve at the faster scan rate has a bigger area than the lower scan rate one, but it should be noted that this does not indicate it has more charge at the higher scan rate. It is well-known that the specific capacitance is an important parameter to evaluate the performance of electrochemical supercapacitors. The mean specific capacitance of the mesoporous MnO2 electrode can be estimated by the equation C = Q/ΔVw, where Q is the charge amount corresponding to the area within the rectangular voltammogram curve, ΔV is the value of potential window, and w is the mass of the material. The mean specific capacitances of mesoporous MnO2 electrodes in 1.0 mol/L Na2SO4 were calculated as 515.5, 456.0, 447.6, 426.2, and 332.4 F/g at sweep rates of 2, 5, 10, 20, and 40 mV/s, respectively. Compared with the specific capacitance of bare carbon aerogel (182 F/g) and bulk MnO2/carbon aerogel (327 F/g) at 2 mV/s, the specific capacitance enhancement for the mesoporous MnO2/carbon aerogel composite electrodes may 2212 DOI: 10.1021/la903947c

be attributed to electrochemical activity of the mesoporous MnO2 phase. The crystalline phase in mesoporous MnO2 is a critically favorable parameter for electron/ion insertion reactions involving large hydrated cations. The whole charging/discharging process may mainly involve (i) cation transport in the electrolyte; (ii) adsorption/desorption of cations at the surface sites of electrodes, which may be dependent on the ion size and the dehydration/hydration rate; and (iii) cation extraction/insertion into solid MnO2 matrix. Therefore, the relative high capacity of MnO2 may be attributed to the mesoporous structure with high specific surface area. The decrease of capacitance with the sweep rate increase can be explained as follows. For crystalline samples of MnO2, the following mechanism was usually proposed for charge storage in MnO2. This mechanism involves intercalation/extraction of protons (H3Oþ) or alkali cations such as Liþ, Naþ, Kþ, and so forth into the bulk of oxide particles with concomitant reduction/ oxidation of the Mn ion.24 MnO2 þ Mþ þ e - T MnOOM ðMþ ¼ Liþ , Naþ , Kþ , or H3 Oþ Þ

ð4Þ

When the scan rate is slower, the diffusion of ions from the electrolyte can gain access to almost all available pores of the electrode, which will lead to a complete insertion/extraction reaction and accordingly enhance the reduction/oxidation process. However, the effective interaction between the ions and the electrode will be greatly reduced when the scan rate is increased, and accordingly the voltammetric charge will be reduced, which can be attributed to some hindered exchange of charged components between the solution and electroactive sites in less accessible parts of the film surface.25 In this study, the diffusion of Naþ ions into the pores of mesoporous MnO2/carbon aerogel composite electrodes will evidently decrease with increasing scan rate, and will finally only be limited to the outer surface of the MnO2 electrode when the scan rate is higher. The effective utilization for the redox reaction will decrease with increasing scan rate. Therefore, the main reason for such a behavior is that the higher scan rate prevents the accessibility of Naþ ions to all the pores of the electrode. The electrochemical stability of mesoporous MnO2 was examined by subjecting a mesoporous MnO2/carbon aerogel composite (24) (a) Pang, S. C.; Anderson, M. A.; Chapman, T. W. J. Electrochem. Soc. 2000, 147, 444. (b) Kuo, S. L.; Wu, N. L. J. Electrochem. Soc. 2006, 153, A1317. (25) (a) Ardizzone, S.; Fregonara, G.; Trasatti, S. Electrochim. Acta 1990, 35, 263–267. (b) Depauli, C. P.; Trasatti, S. J. Electroanal. Chem. 1995, 396, 161–168.

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electrode for CVs for a long number of cycles. The cycling process was performed at a scan rate of 20 mV/s for 1000 cycles. Supporting Information Figure S7 shows the variation of specific capacitance as a function of cycle number. As revealed from this data, a little decrease of specific capacitance is observed during 1000 cycles, and it is still about 97% of the first cycle after 1000 cycles. The system can withstand over 1000 cycles without any significant decrease in the specific capacitance. Therefore, this demonstrates that, within the voltage window 0-0.9 V, the charge and discharge processes do not seem to induce significant structural or microstructural changes in the mesoporous MnO2 as expected for pseudocapacitance reactions. The long-term stability implies that the mesoporous MnO2/carbon aerogel composites are good candidates as a material for supercapacitor electrodes. This kind of material may have future application for supercapacitors.

4. Conclusions In summary, herein we developed a simple and facile electrochemical deposition method for the preparation of mesoporous structures with a high density of pores. Novel mesoporous MnO2/ carbon aerogel composites have been successfully synthesized in aqueous electrolytes by electrochemical deposition accompanying hydrogen evolution, which is crucial for the growth of mesoporous structures. The high BET surface area and mesoporous structure of the products provide the possibility of efficient transport of electrons and ions. The prepared mesoporous MnO2/ carbon aerogel composites have been successfully employed as

Langmuir 2010, 26(4), 2209–2213

Letter

supercapacitor electrodes and give a highest specific capacitance (515.5 F/g). The high specific capacitance and good cycle ability coupled with the low cost and environmentally benign nature of the MnO2/carbon aerogel composite may make this material attractive for large applications. Furthermore, the unique synthetic mechanism of mesoporous structures is expected to be applicable for other metal oxides of technological importance. Acknowledgment. This work was supported by NSFC (20603048, 20873184, 90923008, and J0730420), Guangdong Province (2008B010600040 and 9251027501000002), and Sun Yat-Sen University (09lgpy17). Supporting Information Available: Electrolysis plot of current versus time; SEM image of mesoporous MnO2 deposited on carbon aerogels; EDS pattern of mesoporous MnO2 deposited on carbon aerogels; XRD pattern of mesoporous MnO2 deposited on carbon aerogels; XPS spectra of mesoporous MnO2 deposited on carbon aerogels; CVs of mesoporous MnO2/carbon aerogel composites; plot of specific capacitance versus cycle number of mesoporous MnO2/ carbon aerogel electrode; TEM images of mesoporous MnO2 on Ti sheet; SEM images of bare carbon aerogel and MnO2 deposited on carbon aerogel; adsorptiondesorption isotherms and pore-size distributions of aerogel before deposition of MnO2. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la903947c

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