Generalized Kubas Complexes as a Novel Means for Room Temperature Molecular Hydrogen Storage
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Generalized Kubas Complexes as a Novel Means for Room Temperature Molecular Hydrogen Storage Yong-Hyun Kim, Yufeng Zhao, M. J. Heben, and S. B. Zhang National Renewable Energy Laboratory Golden, CO 80401, U.S.A. ABSTRACT We propose that generalized Kubas complexes of molecular hydrogen with light metal elements, such as B and Be embedded in carbon nanostructures, or related Be and B materials, could offer breakthrough performance in room temperature hydrogen storage. First-principles local-density functional calculations show that hydrogen bound to these materials are intact, in similarity to physisorbed H2, but with a greatly enhanced adsorption energy in the range of 0.20.7 eV. The metal-H2 binding is attributed to the Coulombic interaction between holes created at the metal sites and σ electrons of the H2. Management of the hole density and electron-hole orbital overlap thus enables us to control the binding strength of H2 for optimal storage properties. INTRODUCTION Molecular hydrogen (H2) is the ultimate clean-burning fuel that can replace fossil fuels. Its practical application, however, has been impeded by lack of a safe, economical, and efficient way to store hydrogen at room temperature. The difficulty lies in the fact that this lightest element typically interacts with a host material (X) either in the molecular form (H2) through the ‘too weak’ van der Waals interaction, or in the atomic hydride form (X-H) by forming chemical bonds that are ‘too strong’. The former is called physisorption and the latter chemisorption. Virtually all presently known hydrogen storage materials bind hydrogen in one of these two limits. For physisorption, because the binding is too weak, hydrogen can not be bound at room temperature without extremely high external pressures, whereas chemisorbed hydrogen typically requires catalysts to break and form the strong X-H bonds. To circumvent both shortcomings, we focus our study on the intermediate binding mechanisms [1,2] between physi- and chemisorption. In contrast to the work of Jensen et al. [2], who studied the possibility of using solution phase metallic complexes to store dihydrogen, we seek to coordinate molecular hydrogen to solid supports. A Kubas complex is a molecular hydrogen complex formed with a transition metal atom such as W [1] and Ir [2], where the H-H bondlength is elongated from that of an isolated molecule, ranging from 0.8-1.0 Å. Because the bondlengths of the physi- and chemisorbed hydrogen are 0.75 and >1.7 Å, respectively [3-6], the Kubas complex is much closer to physisorption in binding energy. The calculated binding energies of stable ‘generalized’ Kubas complexes found in this study are 0.2-0.7 eV per H2, which is between the case of physi- (0.04 eV per H2) and chemisorption (>4 eV per H) [3-6]. Therefore, while retaining the merits associated with physisorbed H2, the H2 in the ‘generalized’ Kubas complexes will be more amenable to room temperature operation of hydrogen storage systems due to the enhanced adsorption energy [7]. Here, using first
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