Chemical Properties of AB5 Hydrides

La2Ni4 octahedra, La2Ni2 tetrahedra, and Ni4 tetrahedra. The unit cell is doubled along the c axis because of the formation of a super-lattice, which is a consequence of long range correlations between occupied and unoccupied Ni4 tetrahedra. 

With reference to rule 3 (Section 9.2.3) regarding metal atom mobility, we note that AGf values at 298 K for LaNis and LaH2 are about —67 and — 171kJ respectively. Thus the following disproportionation reaction is highly favored [29], 

However, disproportionation on the surface of polycrystalline LaNi occurs readily at room temperature and constitutes the alloy activation process described 

All AB5 alloys are very brittle and are pulverized to fine particles in the hydriding-dehydriding process (see Section 9.3.1, Alloy Activation). Thus, electrodes must be designed to accommodate fine powders as the active material. There are several methods of electrode fabrication Sakai et al. [35] pulverize the alloy by subjecting it to several hydrogen absorption-desorption cycles, then coat the resulting particles with Ni by chemical plating. The powder is then mixed with a Teflon dispersion to get a paste, which is finally roller pressed to a sheet and then hot pressed to an expanded nickel mesh. The fabrication of a simple paste electrode suitable for laboratory studies is reported by Petrov et al. [37]. 

In order to fully understand the electrochemical behaviour of AB, hydrides, a knowledge of their chemical properties is required. Van Vucht et al. [25] were the first to prepare LaNij hydride and it is arguably the most thoroughly investigated H—storage compound. It reacts rapidly with hydrogen at room temperature at a pressure of several atmospheres above the equilibrium plateau pressure. PC isotherms for this system are shown in Fig. 3. 

LaNij has the CaCu, structure, space group P6lmmm [26] the hexagonal metal lattice is shown in Fig. 4. The crystal structure of LaNi Dy has been determined [27, 28] and is illustrated in Fig. 5. There are three types of interstitial D sites LUyNi tetrahedra, and M4 tetrahedra. The unit cell is doubled along the c-axis because of the formation of a superlattice which is a consequence of long-range correlations between occupied and unoccupied 

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