Metallic and Carbon Nanotube-Catalyzed Coupling of Hydrogenation in Magnesium Xiangdong Yao,* ,² Chengzhang Wu, ‡ Aijun Du, ²,§ Jin Zou, ⊥ Zhonghua Zhu, ² Ping Wang, ‡ Huiming Cheng, ‡ Sean Smith, ²,§ and Gaoqing Lu* ,² Contribution from the ARC Centre for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology (AIBN), The UniVersity of Queensland, QLD 4072, Brisbane, Australia, Shenyang National Laboratory for Materials Science, Institute of Metals Research, Shenyang 110015, China, § Centre for Computational Molecular Science, The UniVersity of Queensland, QLD 4072, Brisbane, Australia, and ⊥ Centre for Microscopy and Microanalysis, The UniVersity of Queensland, QLD 4072, Brisbane, Australia Received July 31, 2007; E-mail: x.yao@minmet.uq.edu.au; maxlu@uq.edu.au Abstract: Synergistic effect of metallic couple and carbon nanotubes on Mg results in an ultrafast kinetics of hydrogenation that overcome a critical barrier of practical use of Mg as hydrogen storage materials. The ultrafast kinetics is attributed to the metal-H atomic interaction at the Mg surface and in the bulk (energy for bonding and releasing) and atomic hydrogen diffusion along the grain boundaries (aggregation of carbon nanotubes) and inside the grains. Hence, a hydrogenation mechanism is presented. Introduction A major challenge in realizing the hydrogen economy is the development of efficient and safe storage materials for hydro- gen. 1 Magnesium (Mg) and Mg-based alloys have been con- sidered as promising materials that could achieve practical hydrogen storage because of their low cost, high capacity, and excellent reversibility. 2 However, there exists two critical barriers to the practical utilization of these materials: the operational temperature is too high, and the hydrogenation kinetics is slow. 3 To overcome these barriers, tremendous efforts have been devoted in the past decade to developing new strategies, such as nanostructuring, 4 alloying, 5 and the use of catalysts. 6 It has been proven that high-energy ball milling could increase hydrogenation kinetics by reducing the grain size, activating the surface and introducing defects. 7 Additionally, Mg can be alloyed with other metallic elements, such as Ni to enhance the absorption kinetics, albeit at the cost of a partial reduction of gravimetric capacity. Transitional metals such as Fe, Ti, and V can also catalyze the hydrogen dissociation process and thus enhance the hydrogenation kinetics signifi- cantly at high temperatures (>573 K). 2,8 Recently, carbon materials, in particular carbon nanotubes, have been demon- strated to have an excellent catalytic effect on hydrogen storage in Mg based alloys by enhancing the hydrogen diffusion in MgH 2 -C systems. 9 In our recent studies, 10 we have demonstrated that the catalytic effects of combined transition metals such as Fe and Ti with carbon nanotubes (CNTs) as mixed dopants lead to significant acceleration of hydrogen dissociation and diffusion in nano- structured magnesium, approaching the goal of rapid hydroge- nation kinetics at practically meaningful low temperatures. Notably, the effect is significantly enhanced in comparison with the hydrogenation kinetics promoted by elemental Fe 8a or Ti 8b alone. This indicates that the synergistic interaction among metals and carbon nanotubes may be an effective strategy to significantly lower the operating temperature and to increase hydrogenation kinetics. Despite this progress, the hydrogenation kinetics still falls short of the requirements for practical applications. 10a It is therefore desirable to further explore and develop the synergistic effects of CNTs with other metallic catalysts in order to achieve faster hydrogenation kinetics. It has been shown that V and Ti are both individually effective in promoting the hydrogenation kinetics of Mg based alloys and that V is in fact much more effective than either Fe or Ti ² ARC Centre for Functional Nanomaterials, The University of Queen- sland. ‡ Shenyang National Laboratory for Materials Science. § Centre for Computational Molecular Science, The University of Queensland. ⊥ Centre for Microscopy and Microanalysis, The University of Queen- sland. (1) Schlapbach, L.; Zuttel, A. Nature 2001, 414, 353. (2) Shang, C. X.; Guo, Z. X. J. Power Source 2004, 129, 73. (3) Huot, J.; Liang, G.; Schulz, R. Appl. Phys. A 2001, 72, 187. (4) Seayad, A. M.; Antonelli, D. M. AdV. Mater. 2004, 16, 765. (5) Hirscher, M.; Becher, M. J. Nanosci. Nanotechnol. 2003, 3, 3. (6) Oelerich, W.; Klassen, T.; Bormann, R. J. Alloys Compd. 2001, 315, 237. (7) Zaluski, L.; Zaluska, A.; Strom-Olsen, J. O. J. Alloys Compd. 1995, 217, 245. (8) (a) Zaluska, A.; Zaluski, L.; Strom-Olsen, J. O. J. Alloys Compd. 1999, 288, 217. (b) Liang, G.; Huot, J.; Boily, S.; Van Neste, A.; Schulz, R. J. Alloys Compd. 1999, 292, 247. (c) Bazzanella, N.; Checchetto, R.; Miotello, A.; Sada, C.; Mazzoldi P.; Mengucci, P. Appl. Phys. Lett. 2006, 89, 014101. (d) Song, Y.; Guo, Z. X.; Yang, R. Phys. ReV.B 2004, 69, 094205. (9) (a) Orimo, S.; Fuji, H. Appl. Phys. 2001, A72, 167. (b) Kiyobayashi, T.; Komiyama, K.; Takeichi, N.; Tanaka, H.; Senoh, H.; Takeshita, H. T.; Kuriyama, N. Mater. Sci. Eng. B 2004, 108, 134. (c) Wu, C. Z.; Wang, P.; Yao, X.; Liu, C.; Chen, D. M.; Lu, G. Q.; Cheng, H. M. J. Alloys Compd. 2006, 414, 259. (10) (a) Yao, X.; Wu, C. Z.; Du, A. J.; Lu, G. Q.; Cheng, H. M.; Smith, S. C.; Zou, J.; He, Y. J. Phys. Chem. B 2006, 110, 11697. (b) Yao, X.; Wu, C. Z.; Wang, H.; Cheng, H. M.; Lu, G. Q. J. Nanosci. Nanotechnol. 2006, 6, 494. Published on Web 11/23/2007 15650 9 J. AM. CHEM. SOC. 2007, 129, 15650-15654 10.1021/ja0751431 CCC: $37.00 © 2007 American Chemical Society