MELT-SPINNING OF AB5.5-TYPE HYDRIDE-FORMING COMPOUNDS AND THE INFLUENCE OF ANNEALING ON ELECTROCHEMICAL AND CRYSTALLOGRAPHIC PROPERTIES

Non-stoichiometric almost single-phase AB5.5 materials were prepared using the melt-spinning technique. Characteristic of these materials is the anisotropically deformed crystal lattices (decreased a-axis and increased c-axis), resulting from the excess of B-elements incorporated in the hP6-CaCu5 structure. These materials show (i) an excellent long-term stability during electrochemical cycling in alkaline solution, indicating that the oxidation resistivity strongly depends on the extent of the crystal lattice deformation and (ii) a relatively poor electrocatalytic activity which can be attributed to the absence of highly catalytic second-phase alloys. Annealing of the as-prepared samples induces the deformed crystal lattice to relax by secreting the excess of B-elements. As a consequence, double-phase materials are formed, consisting of hydride-forming compounds with compositions close to the AB5 StoiChiometry and second-phase alloys. These double-phase materials combine a relatively poor long-term cycling stability with a high electrocatalytic activity.