Hydrogen Storage in Ni Nanoparticle-Dispersed Multiwalled Carbon Nanotubes Hyun-Seok Kim, † Ho Lee, ‡ Kyu-Sung Han, † Jin-Ho Kim, § Min-Sang Song, † Min-Sik Park, † Jai-Young Lee, † and Jeung-Ku Kang* ,† Department of Materials Science and Engineering, Korea AdVanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon, South Korea, Samsung Electronics Co. Ltd., San 24, Giheung, Yongin, South Korea, and Samsung AdVanced Institute of Technology, San 24, Giheung, Yongin, South Korea ReceiVed: NoVember 18, 2004; In Final Form: January 28, 2005 Hydrogen storage properties of mutiwalled carbon nanotubes (MWCNTs) with Ni nanoparticles were investigated. The metal nanoparticles were dispersed on MWCNTs surfaces using an incipient wetness impregnation procedure. Ni catalysts have been known to effectively dissociate hydrogen molecules in gas phase, providing atomic hydrogen possible to form chemical bonding with the surfaces of MWCNTs. Hydrogen desorption spectra of MWCNTs with 6 wt % of Ni nanoparticles showed that ∼2.8 wt % hydrogen was released in the range of 340-520 K. In Kissinger’s plot to evaluate the nature of interaction between hydrogen and MWCNTs with Ni nanoparticles, the hydrogen desorption activation energy was measured to be as high as ∼31 kJ/mol‚H 2 , which is much higher than the estimates of pristine SWNTs. C-H n stretching vibrations after hydrogenation in FTIR further supported that hydrogen molecules were dissociated when bound to the surfaces of MWCNTs. During cyclic hydrogen absorption/desorption, there was observed no significant decay in hydrogen desorption amount. The hydrogen chemisorption process facilitated by Ni nanopaticles could be suggested as an effective reversible hydrogen storage method. Since discovery of carbon nanotubes (CNTs) in 1991 by Iijima, 1 hydrogen storage in CNTs has shown great promise as a high-energy density absorbent. 2,3 In many theoretical and empirical results, superior hydrogen gas absorbing property has been reported under a high pressure and often at an extremely low temperature. 4-7 The interaction mechanism between carbon and hydrogen has been attributed to physisorption of molecular hydrogen inside the tubes and interstitial sites in tube bundles. 4-7,16 Recently, studies on hydrogenation under a high pressure and at an elevated temperature have shown the possibility that every carbon atom on CNTs could be a potential site for chemisorption of one hydrogen atom. 8-10 Since it needs ∼440 kJ/mol‚H 2 to break H-H bond of H 2 molecules, the chemical adsorption is unlikely to occur in gas phase except for these special environ- ments. Experimentally, the doping of dissociative catalysts such as alkali metals has also activated the chemical adsorption process, resulting in much higher hydrogen storage capacity than that for the physisorption mechanism. 11,12 However, these alkaline dopants have higher hydrogen affinity and lower hydrogen molecules dissociative activity than transition metals such as Ni, Co, and Pt. Accordingly, it was difficult to conclude which was responsible for the improved storage capacity. For the better understanding of the role of dopants, it is necessary to investigate the relationship between hydrogen storage proper- ties and the hydrogen molecules dissociative catalysts. In this paper, we selected Ni as a dissociative catalyst for hydrogen and employed the incipient wetness impregnation method, which enabled nanosized catalysts to be dispersed on the carbon surface. 13,14 The hydrogen storage properties were determined by evolution temperature and quantity measured by thermal desorption spectrum (TDS) analysis. We also evaluated the possibility of chemical adsorption process assisted by Ni nanoparticles present on nanotube surface. The MWCNTs were synthesized with microwave plasma enhanced CVD as presented in our previous works. 15,16 First, a cobalt layer with a thickness of 50 nm was deposited on p-type Si-substrate by rf magnetron sputtering at 100 W rf power and the pressure was adjusted to 30 mTorr by feeding Ar gas. Prior to the growth of CNTs in microwave PECVD, hydrogen was introduced and plasma treatment was conducted at 1100 W microwave power for 90 s. The plasma-treated cobalt seeds were used as catalytic seeds for the growth of MWCNTs. A mixture of H 2 (89.9%, vol %), CH 4 (0.1%), and O 2 (10%) was used as the gas source. The microwave power and working pressure during the growth of CNTs were 700 W and 30 Torr, respectively. The growth temperature was maintained at 750 °C using halogen lamp heating. No further process was performed to purify the soot or to open the tube end with as- produced samples. 0.005 g of MWCNTs was impregnated with 5, 10, 21, and 73 mM Ni nitrate acetone solutions of 10 mL. The metal loading amount was determined by concentration of the solution as reported by Joo et al. 14 After immersion, the black sample was dried in 60 °C and subsequently heat-treated in H 2 gas flow. To observe the hydrogen storage properties, hydrogen (99.999%) was charged under 4 MPa at 300 K for 2 h in 0.002 g MWCNTs with nanoparticle dispersion. The sample was placed in a quartz reactor, which was surrounded by liquid nitrogen cooled cryostat and wound by heating element with a programmable power supply. The injection port of gas chro- matograph was connected directly to the reactor and high-purity Ar (99.999%) of 1 atm was used as carrier gas. Hydrogen evolved from MWCNTs was probed with gas chromatograph equipped with TCD (thermal conductivity detection method) and the selected capillary column (CARBOXEN 1006PLOT). The temperature scanning range and its rate were set to 280- * Author to whom correspondence should be addressed. E-mail: jeungku@ kaist.ac.kr. † Korea Advanced Institute of Science and Technology. ‡ Samsung Electronics Co. Ltd. § Samsung Advanced Institute of Technology. 8983 J. Phys. Chem. B 2005, 109, 8983-8986 10.1021/jp044727b CCC: $30.25 © 2005 American Chemical Society Published on Web 04/19/2005