Non-stoichiometric Hydride-Forming Compounds: An Excellent Combination of Long-term Stability and High Electrocatalytic Activity*

A new concept is shown to be very effective in improving the long-term cycling stability of AB5-type electrode materials in alkaline media. An excess amount of B content incorporated within the AB5 crystal lattice is found to be essential in this respect. Both binary, LaNi,+~, and ternary, La(NiCu)5+~, compounds with the desired non stoichiometric composition can be easily prepared from the melt. Annealing at the appropriate temperature within the homogeneity region of these materials turned out to be important. The,compounds are characterized by an anisotropically deformed crystal lattice with decrdNed a-axis and increased c-axis. Advantageously, all non-stoichiometric materials show an extremely high electrocatalytic activity for the electrochemical hydro gen reaction. This makes them very suitable as negative electrode material in rechargeable Ni/MH l~atteries with good long-term cycling stability and excellent charge/discharge capability, even under severe conditions