The Nature of the Chemical Bond in Hydrogen Storage Compounds

Abstract

Abstract The electronic structures of LaNi 5 and Mg 2 Ni both containing a variety of alloying elements, M, are investigated by the DV-Xα cluster method in order to understand alloying effects on the hydrogen absorption and desorption characteristics of them. It is found that hydrogen atoms make a strong chemical bond with Ni atoms rather than La or Mg atoms in pure LaNi 5 or in pure Mg 2 Ni, despite the larger affinity of La or Mg atoms for hydrogen than Ni atoms in the binary metal-hydrogen system. It is also shown that the nature of the chemical bond between the constituent atoms determines the stability of LaNi 5 and Mg 2 Ni hydrides. For example, for the LaNi 5 system, the ratio of the La(M)-Ni bond order to the Ni(M)-Ni bond order correlates well with the experimental data of the equilibrium plateau pressure of hydrogen. Similar results are also obtained for other systems, CaNi 5 , TiFe and ZrMn 2 . On the other hand, for the Mg 2 Ni system, both the Ni(M)-Mg bond strength and the Ni(M)-H bond strength correlate well with the measured enthalpy of formation for the hydride. In addition, an empirical rule between the unit cell volume and the stability of the hydrides is explained consistently on the basis of the present calculation. The present electronic approach is useful even in the systems where an empirical rule is violated (e.g., CaNi 5 and Mg 2 Ni).