Structure and Stability of Hydrogen Clathrates of Ammonia Borane
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1216-W10-02
Structure and stability of hydrogen clathrates of ammonia borane Alexander Abramov and Maciej Gutowski Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK ABSTRACT Structural rules have been formulated for the molecular crystal of ammonia borane (AB, NH3BH3) and clathrates thereof. These rules are similar to the “ice rules” in water. Periodic structures of possible clathrates of AB have been predicted. A rigorous analysis of the uniform space filling tessellations of 3D space has been performed resulting in the identification of two promising structures for AB clathrates. Additional five structures have been identified exploiting features and properties of AB molecules. The screening, or evaluation, of proposed periodic structures has been performed on the basis of their stability determined at the density functional level of theory. Hydrogen capacity of the most stable periodic structure (cantitruncated cubic honeycomb) is estimated to be 21 wt%, 19% chemically bound in AB and 2 wt% of H2 physisorbed in the cages of AB. INTRODUCTION On-board hydrogen storage remains one of the most challenging obstacles to overcome on a way to hydrogen economy. The transportation sector requires accessible hydrogen in a vast amount stored on-board of a car or truck. Unfortunately, none of the known thus far materials stores reversibly a sufficient amount of hydrogen per unit of mass or volume, that would be also kinetically easily accessible [1, 2]. Metal hydrides might have high hydrogen content but fast extraction of H2 requires elevated temperatures, not attainable in fuel cells used for mobile applications. Storage of hydrogen based on its physisorption avoids the kinetic limitations, but the density of stored hydrogen is unacceptably low. We have recently suggested that a possible solution to the problem of hydrogen storage could be a hybrid material, in which a fraction of hydrogen is chemically bound and the remaining part is physically bound [3]. Such a material would be an implementation of the concept of “hierarchical hydrogen storage” [3]. Each level of hydrogen storage would have different characteristics that become advantageous in different circumstances. In our previous work we suggested that hydrogen clathrates of a high hydrogen content material, like ammonia borane, could serve as models of hierarchical hydrogen storage [3]. The physical binding provides hydrogen that is kinetically easily accessible, whereas the chemical binding assures a high overall hydrogen density. AB was selected due to its high hydrogen content, ability to form extended dihydrogen bonded networks, and due to relatively strong hosthost and host-guest interactions. We have developed structural rules (construction principles) for the crystal and clathrates of ammonia borane, which are similar to the “ice rules” governing orientation of water molecules in ice and hydrates. The first construction principle requires that each hydrogen atom of AB is engaged in two dihydr
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