12068
Langmuir 2008, 24, 12068-12072
Controllable Synthesis of Hierarchical Nanostructured Hollow Core/ Mesopore Shell Carbon for Electrochemical Hydrogen Storage Baizeng Fang, Minsik Kim, Jung Ho Kim, and Jong-Sung Yu* Department of AdVanced Materials Chemistry, Korea UniVersity, 208 Seochang, Jochiwon, ChungNam 339-700, Republic of Korea ReceiVed June 9, 2008. ReVised Manuscript ReceiVed July 31, 2008 Hierarchical nanostructured hollow core/mesopore shell carbon (HN-HCMSC) represents an innovative concept in electrochemical hydrogen storage. This work deals with physical characteristics and electrochemical hydrogen storage behavior of the HN-HCMSCs, produced by a replica technique using solid core/mesopore shell (SCMS) silica as template. HN-HCMSCs with various core sizes and/or shell thicknesses have been fabricated through the independent control of the core sizes and/or shell thicknesses of the SCMS silica templates. The superb structural characteristics of the HN-HCMSCs including large specific surface area and micropore volume, and particularly well-developed three-dimensionally interconnected hierarchical nanostructure (hollow macroporous core in combination with meso-/ microporous shell), provide them with great potential for electrochemical hydrogen storage. A discharge capacity up to 586 mAh/g, corresponding to 2.17 wt % hydrogen uptake, has been demonstrated in 6 M KOH for the HN-HCMSC with a core size of 180 nm and a shell thickness of 40 nm at a discharge rate of 25 mA/g. Furthermore, the HN-HCMSC also possesses excellent cycling capacity retainability and rate capability.
1. Introduction Hydrogen is the most abundant element in the universe, which can be produced easily from renewable energy sources. It is also nonpolluting, forming water as a harmless byproduct during use, and has great potential as an energy source. However, hydrogen storage with a safe, effective, and cheap system is crucial for hydrogen cells or hydrogen-driven combustion engines.1,2 Hydrogen storage is a topical goal in the development of a hydrogen economy. Most research into hydrogen storage is focused on storing hydrogen in a lightweight, compact manner for mobile applications. Presently, three generic mechanisms are known for storing hydrogen in materials: absorption, adsorption, and chemical reaction. Hydrogen can be stored in the form of metal hydrides. This method uses an alloy that can absorb and hold large amounts of hydrogen by bonding with hydrogen and forming hydrides. Traditional alloy families include AB5 (i.e., LaNi5),3-5 AB2 (i.e., ZrV2),6 AB (i.e., FeTi),7 and A2B (i.e., Mg2Ni).8,9 Although AB5-type alloys have been widely used in commercial batteries, their reversible gravimetric storage densities are low (ca. 300 mAh/g).3 Furthermore, high temperatures are often required to release their hydrogen content. Mg-based alloy has high capacity (i.e., more than 600 mAh/g); however, their poor cycling life restricts the practical applications. In addition to the abovementioned alloys, metal oxide (i.e., ZnO),10 inorganic nanotubes * To whom correspondence should be addressed. E-mail: jsyu212@ korea.ac.kr. (1) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 414, 353. (2) Han, S.-C.; Lee, P.-S.; Zu¨ttel, A. J. Alloys Compd. 2000, 306, 219. (3) Cohen, R. L.; Wernick, J. H. Science 1981, 214, 108. (4) Liao, B.; Lei, Y. Q.; Chen, L. X. J. Power Sources 2004, 129, 358. (5) Kohno, T.; Yoshida, H.; Kawashima, F. J. Alloys Compd. 2000, 311, 5. (6) Song, M. Y.; Ahn, D.; Kwon, I. H.; Chough, S. H. J. Electrochem. Soc. 2001, 148, A1041. (7) Yukawa, H.; Takahashi, Y.; Morinaga, M. Comput. Mater. Sci. 1999, 14, 291. (8) Goo, N. H.; Woo, J. H.; Lee, K. S. J. Alloys Compd. 1999, 288, 286. (9) Abe, T.; Tachikawa, T.; Hatano, Y.; Watanabe, K. J. Alloys Compd. 2002, 332, 792. (10) Wan, Q.; Lin, C. L.; Yu, X. B.; Wang, T. H. Appl. Phys. Lett. 2004, 84, 124.
(i.e., BN, TiS2, MoS2),11 and microporous metal-organic frameworks12,13 have also been investigated for possible hydrogen storage. In recent years, much attention has been paid to elemental carbon, especially carbon nanotubes (CNTs) because of their unique characteristics such as high surface reactivity and strong gas adsorption. CNT was claimed to have hydrogen storage of greater than 50 wt % and to be the exclusive candidate carrier of hydrogen for the hydrogen vehicle. However, further investigation demonstrates that the claims of high storage capacities of CNT related to their characteristic morphology are unjustified,14 and hydrogen storage capacity in CNT was reported to be less than 1 wt %.15 Other carbonaceous materials investigated for hydrogen storage include graphite nanofibers (GNFs)14,16-19 and activated carbon.20,21 Unfortunately, to date, none of the investigated materials meets the benchmark of 6.5 wt % hydrogen storage from the U.S. Department of Energy at ambient temperature and pressure. As compared to conventional low temperature-high pressure hydrogen storage technology, electrochemical hydrogen storage has been proved as elegant and more efficient at ambient pressure and temperature. AB5 (typically LaNi5) and Mg-based alloy have exhibited a hydrogen storage capacity of ca. 1.0 wt %.22-24 Recently, nanostructured porous materials have attracted (11) Seayad, A. M.; Antonelli, D. M. AdV. Mater. 2004, 16, 765. (12) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. Science 2003, 300, 1127. (13) Fe´rey, G.; Latroche, M.; Serre, C.; Millange, F.; Loiseau, T.; PercheronGue´gan, A. Chem. Commun. 2003, 2976. (14) Schimmel, H. G.; Kearley, G. J.; Nijkamp, M. G.; Visserl, C. T.; Jong de, K. P.; Mulder, F. M. Chem.-Eur. J. 2003, 9, 4764. (15) Kajiura, H.; Tsutsui, S.; Kadono, K.; Kakuta, M.; Ata, M.; Murakami, Y. Appl. Phys. Lett. 2003, 82, 1105. (16) Chen, P.; Wu, X.; Lin, J.; Tan, K. L. Science 1999, 285, 91. (17) Ruffieux, P.; Gro¨ning, O.; Bielmann, M.; Mauron, P.; Schlapbach, L.; Gro¨ning, P. Phys. ReV. B 2002, 66, 245416. (18) Ritschel, M.; Uhlemann, M.; Gutfleisch, O.; Leonhardt, A.; Graff, A.; Ta¨schner, Ch.; Fink, J. Appl. Phys. Lett. 2002, 80, 2985. (19) Pinkerton, F. E.; Wicke, B. G.; Olk, C. H.; Tibbett, G. G.; Meisner, G. P.; Meyer, M. S.; Herbst, J. F. J. Phys. Chem. B 2000, 104, 9460. (20) Stro¨bel, B.; Jo¨rissen, L.; Schliermann, T.; Trapp, V.; Schu¨tz, W.; Bohmhammel, K.; Wolf, G.; Garche, J. J. Power Sources 1999, 84, 221. (21) Kadono, K.; Kajiura, H.; Shiraishi, M. Appl. Phys. Lett. 2003, 83, 3392.
10.1021/la801796c CCC: $40.75 2008 American Chemical Society Published on Web 09/10/2008
Controllable Synthesis of HN-HCMSC
much attention for electrochemical hydrogen storage, such as MoS2 nanotube (ca. 0.96 wt % hydrogen uptake),25 Cu(OH)2 nanoribbon,26 activated carbon (ca. 1.8 wt % hydrogen uptake),27,28 SWNTs (ca. 1.84 wt % hydrogen uptake),29 and ordered mesoporous carbon (OMC) (ca. 1.95 wt % hydrogen uptake).30,31 Relatively high hydrogen storage capacities have been achieved by nanostructured porous carbon materials with high specific surface area and highly developed micro-/mesoporosity. In this study, hierarchical nanostructured hollow core/ mesoporous shell carbons (HN-HCMSCs) with various core sizes or shell thicknesses have been fabricated and explored for the first time for electrochemical hydrogen storage. The fantastic nanostructure of the HN-HCMSCs, that is, highly developed three-dimensionally interconnected porosity network of macropores in combination with meso-/micropores, favors fast mass transport and effective adsorption/desorption of hydrogen, resulting in a hydrogen uptake up to 2.17 wt % along with excellent cycling capability retainability and rate capability.
2. Experimental Section 2.1. Synthesis of HN-HCMSCs with Various Core Sizes or Shell Thicknesses. HN-HCMSCs with various core sizes and/or shell thicknesses were synthesized using the submicrometer-size solid core/mesoporous shell (SCMS) silica as template.32 Core sizes and shell thicknesses of HN-HCMSCs were controlled by the core sizes and shell thicknesses of the SCMS silica template, respectively, while the micro- and mesoporosity of the HN-HCMSCs were controlled to some extent by the molar ratio of n-octadecyltrimethoxysilane (C18-TMS) to tetraethyl orthosilicate (TEOS), and the source and amount of carbon precursor incorporated into the SCMS silica template, typically, pitch, furfurfyl alcohol, divinylbenzene, and phenol.33-37 A synthesis scheme for the SCMS silica and HCMSC is shown in Scheme 1, where a solid silica sphere is used as a starting material. The silica sphere can be produced in various sizes by controlling the amount of TEOS added into the aqueous ammonia, and the SCMS silica can be produced in various sizes and various shell thicknesses by using the solid silica spheres with various sizes and adjusting the molar ratio of C18-TMS to TEOS, respectively.35-37 Mesosize and microsize thickness silica walls can be formed in the shell of the SCMS silica from the interaction of C18-TMS and TEOS, which are the primary sources of meso- and micropores of the replicated HCMSC. In general, the higher ratio of C18-TMS to TEOS tends to increase the mesopore size and to decrease the silica wall thickness in the shell of the SCMS silica, thus to some extent increasing the micropore volume in the (22) Simicic, M. V.; Zdujic, M.; Jelovac, D. M.; Rakin, P. M. J. Power Sources 2001, 92, 250. (23) Jurczyk, M.; Nowak, M.; Jankowska, E.; Jakubowicz, J. J. Alloys Compd. 2002, 339, 339. (24) Xue, J.; Li, G.; Hu, Y.; Du, J.; Wang, C.; Hu, G. J. Alloys Compd. 2000, 307, 240. (25) Chen, J.; Kuriyama, N.; Yuan, H.-T.; Takeshita, H. T.; Sakai, T. J. Am. Chem. Soc. 2001, 123, 11813. (26) Gao, P.; Zhang, M.; Niu, Z.; Xiao, Q. Chem. Commun. 2007, 5197. (27) Jurewicz, K.; Frackowiak, E.; Be´guin, F. Fuel Process. Technol. 2002, 415, 77–78. (28) Jurewicz, K.; Frackowiak, E.; Be´guin, F. Appl. Phys., A 2004, 78, 981. (29) Dai, G.-P.; Liu, C.; Liu, M.; Wang, M.-Z.; Cheng, H.-M. Nano Lett. 2002, 5, 503. (30) Vix-Guterl, C.; Frackowiak, E.; Jurewicz, K.; Friebe, M.; Parmentier, J.; Be´guin, F. Carbon 2005, 43, 1293. (31) Fang, B.; Zhou, H.-S.; Honma, I. J. Phys. Chem. B 2006, 110, 4875. (32) Yoon, S. B.; Sohn, K.; Kim, J. Y.; Shin, C.-H.; Yu, J.-S.; Hyeon, T. AdV. Mater. 2002, 14, 19. (33) Zhang, F.; Meng, Y.; Gu, D.; Yan, Y.; Yu, C.; Tu, B.; Zhao, D. J. Am. Chem. Soc. 2005, 127, 13508. (34) Yoon, S. B.; Chai, G. S.; Kang, S.; Yu, J.-S.; Gierszal, K. P.; Jaroniec, M. J. Am. Chem. Soc. 2005, 127, 4188. (35) Kim, M.; Yoon, S. B.; Sohn, K.; Kim, J. Y.; Shin, C.-H.; Hyeon, T.; Yu, J.-S. Microporous Mesoporous Mater. 2003, 63, 1. (36) Kim, J. Y.; Yoon, S. B.; Yu, J.-S. Chem. Commun. 2003, 790. (37) Yu, J.-S.; Yoon, S. B.; Lee, Y. J.; Yoon, K. B. J. Phys. Chem. B 2005, 109, 7040.
Langmuir, Vol. 24, No. 20, 2008 12069 Scheme 1. Synthesis of SCMS Silica and HN-HCMSC
corresponding HCMSC.38 Carbon precursor can be incorporated into the mesoporous channels separated by silica walls in the shell of the SCMS silica, and after carbonization of polymerized carbon precursor and removal of SCMS silica, HN-HCMSCs with macroporous hollow core in combination with meso-/microporous shell can be produced with various surface characterization (surface area, pore volume, meso- and microporosity). A typical synthesis route for SCMS silica with a core size of 190 nm and shell thickness of 42 nm is as follows. Five milliliters of aqueous ammonia (32 wt %) was added into a solution containing 125 mL of ethanol and 10 mL of deionized water. After the mixture was stirred at room temperature for ca. 15 min, 4 mL of TEOS (98%, ACROS) was added to the above-prepared mixture and stirred for ca. 6 h to yield uniform silica spheres (Sto¨ber silica solution). A mixture solution containing 2.78 mL of TEOS and 1.13 mL of C18TMS (90% tech., Aldrich) (i.e., molar ratio of TEOS to C18-TMS ) 4.7) was added into the colloidal solution containing the silica spheres and further reacted for 1 h. The resulting octadecyl group incorporated silica shell/solid core nanocomposite was retrieved by centrifugation, dried at room temperature, and further calcined at 823 K for 6 h under an oxygen atmosphere to produce the final SCMS silica material (Kaiser approach). Aluminum was incorporated into the silicate framework through an impregnation method to produce acidic points on the surface of the SCMS silica, which will catalyze polymerization of phenol and paraformaldehyde.39 A total of 1.0 g of SCMS silica was added to an aqueous solution containing 0.27 g of AlCl3 · 6H2O in 0.3 mL of water, and the resulting slurry was stirred for 30 min. The powder was dried in air at 353 K. Finally, the Al-impregnated SCMS silica was calcined at 823 K for 5 h in air to yield SCMS aluminosilicate. A typical synthesis route for HN-HCMSC capsules is as follows. 0.374 g of carbon precursor (i.e., phenol) was incorporated into the mesopores of 1.0 g of the SCMS template by heating at 100 °C for 12 h under vacuum. The resulting phenol-incorporated SCMS template was reacted with paraformaldehyde (0.238 g) under vacuum at 130 °C for 24 h to yield a phenol-resin/SCMS aluminosilicate composite. The composite was heated to 160 °C at a rate of 1 K/min and held for 5 h under a nitrogen flow. The temperature was then ramped at 5 K/min to 950 °C and held for 7 h to carbonize the cross-linked phenol resin inside the mesopores of the SCMS structure. The dissolution of the SCMS template using 2.0 N NaOH and washing in EtOH-H2O solution (volume ratio of EtOH to H2O ) 1:1) produced HN-HCMSC. By varying the amount of TEOS or adjusting the molar ratio of C18-TMS to TEOS, HN-HCMSCs with various core size and/or shell thickness were fabricated and denoted as HNHCMSC (x/y, here, x and y stand for the core size and the shell thickness in nm, respectively). Slight shrinkage in core size and shell thickness of the HCMSC has been observed as compared to those of the parent SCMS silica. For example, HCMSC180/40 spheres were obtained from SCMS190/42 spheres. Meso- and micropores of the HN-HCMSC are directly generated from the silica walls in the mesoporous shell of the SCMS silica. Carbonization of polymerized carbon precursor also intrinsically produces some micropores in the carbonized body. (38) Yu, J.-S.; Hyeon, T. Korean Patent 10-0500975, 2005. (39) Kim, J. Y.; Lee, S. J.; Yu, J.-S. Bull. Korean Chem. Soc. 2000, 21, 544.
12070 Langmuir, Vol. 24, No. 20, 2008
Fang et al.
Figure 2. Nitrogen adsorption-desorption isotherms for the HNHCMSC180/40. Figure 1. Representative TEM images for the HN-HCMSCs: (a) C180/40, (b) C260/40, (c) C340/40, and (d) C340/70.
2.2. Surface Characterization of the HN-HCMSCs. N2 adsorption and desorption isotherms were measured at 77 K on a KICT SPA-3000 Gas Adsorption Analyzer after the carbon was degassed at 423 K to 20 µTorr for 4 h. The specific surface areas were determined from nitrogen adsorption using the Brunauer-EmmettTeller (BET) equation. Total pore volume was determined from the amount of gas adsorbed at the relative pressure of 0.99. Micropore (pore size
