Electrochemical Properties of Rare Earth Hydrogen Storage Alloy Electrodes and Structural Analysis by Means of the Rietveld Method

Abstract

Hydrogen storage capacity and electrochemical capacity were investigated on four types of rare earth based metal hydrides. The hydrogen capacities of metal hydrides obtained by PCT (Pressure-Composition-Temperature) measurements showed a good agreement with the electrochemical ones determined using an electrochemical cell. In the alloys studied, MmNi_3.8. Al_0.3Mn_0.4Co_0.5sh owed the greatest capacity while MmNi_3.5Al_0.8Co_0.7the smallest. Structural analysis using the Rietveld method was carried out for MmNi_3.5Al_0.8Co_0.7a nd MmNi_3.8Al_0.3Mn_0.4Co_{0, 5} The basic structure of both alloys was found to agree with that of LaNi₅ (space groups P 6Immm). It was also found that aluminum atoms coordinate to 3 g site. X-Ray diffraction and the Rietveld analysis data indicated that the a axis of MmNi_3.5Al_0.3Mn_0.4Co_0.5is longer than that of MmNi_3.5Al_0.8Co_0.7, whereas the c axis of the former is shorter than that of latter. This suggests that the length of a axis is one of the essential factors that determine the electrochemical capacity.