First principle study of hydrogenation of MgB ₂: An important step toward reversible hydrogen storage in the coupled LiBH ₄/MgH ₂ system

Recent experiments [F. E. Pinkerton, M. S. Meyer, G. P. Meisner, M. P. Balogh, and J. J. Vajo, J. Phys. Chem. C111, 12881 (2007) and J. J. Vajo and G. L Olson, Scripta Mater. 56, 829 (2007)] demonstrated that the recycling of hydrogen in the coupled LiBH ₄/MgH ₄ system is fully reversible. The rehydrogenation of MgB ₂ is an important step toward the reversibility. By using ab initio density functional theory calculations, we found that the activation barrier for the dissociation of H ₄ are 0.49 and 0.58 eV for the B and Mg-terminated MgB ₄(0001) surface, respectively. This implies that the dissociation kinetics of H ₄ on a MgB ₄(0001) surface should be greatly improved compared to that in pure Mg materials. Additionally, the diffusion of dissociated H atom on the Mg-terminated MgB ₄(0001) surface is almost barrier-less. Our results shed light on the experimentally-observed reversibility and improved kinetics for the coupled LiBH ₄/MgH ₄ system.