Stability of the hydrogen absorption and desorption plateaux in LaNi5-H -: Part 5:: H capacity

A systematic study was made by gravimetry, carefully preserving the system mass zero, of the positions of the alpha/alpha+beta and alpha+beta/beta phase boundaries and the plateau lengths in LaNi5-H, while absorption-desorption cycling under various conditions of temperature and pressure. Our findings include: (i) in agreement with previous studies, the initial capacity of the virgin intermetallic is in excess of H/M=1.1; (ii) however, the initial desorption plateau is shorter than the initial absorption plateau by nearly 0.1 in H/M, owing to the trapping of Ii, possibly in lattice defects; (iii) absorption-desorption cycling at any temperature in impure hydrogen shortens the plateaux (capacity degradation), greatly exacerbated by elevated temperature; (iv) however, the total ii capacity remains relatively constant, with an exchange of H between 'reversible' sites and trap sites, manifest as the drift in the alpha/alpha+beta phase boundary noted above. The reality of the Ii trapping owing to activation has been proved by independent techniques, such as deep-inelastic neutron scattering. It is unlikely that this H is initially trapped in the form of LaHx, although this and other La-Ni hydrides do form during extended cycling at elevated temperatures. (C) 1999 Published by Elsevier Science S.A. All rights reserved.