Degradation of hydrogen absorbing capacity in cyclically hydrogenated TiMn2

Abstract Degradation of the hydrogen absorbing capacity in cyclically hydrogenated TiMn2 (Ti–60 at.% Mn) with Laves structure was studied by measurement of pressure–composition–isotherm (PCT) curves, X-ray diffraction and microstructure observation. Characteristic changes with cyclic hydrogenation are observed as follows. The hydrogen absorbing capacity remarkably decreases, the lattice constant increases, the amount of hydrogen retained in TiMn2 increases and it approaches almost zero by dehydrogenation at 673 K in vacuum. Nano-sized regions with Moire patterns are produced in TiMn2 with hydrogenation and Debye rings corresponding to titanium hydride δ-TiH appear in the diffraction pattern. Based on these observations it is concluded that the degradation of hydrogen absorbing capacity after cyclic hydrogenation is attributable to the introduction of retained hydrogen, heterogeneous strain and/or nano-sized regions.