Stability of CaNi5Hx stored at temperatures between 20 and 150 degrees C

The stability of CaNi5Hx stored at different temperatures was studied as a function of time. In general AB(5) metal hydrides are known to be metastable with a tendency to disproportionate at elevated temperatures. In the present study samples of CaNi5 were stored in the hydrided state (as CaNi5Hsimilar to4.7) at temperatures between 20 and 150degreesC. After different periods of time, up to 120 days, the hydrogen absorption capacity was measured electrochemically. Significant capacity decays were observed at temperatures of 40degreesC and higher. The capacity decay with storage time tended to stop at a level of 147 mAh/g corresponding to 1.83 hydrogen atoms per calcium atom. X-ray diffraction revealed that the CaCu5 structure was preserved after the degradation. A mathematical model for the decay is proposed.