No abstract

RNi5-type metal hydride materials have been simulated by using a simple model. These materials have been thoroughly studied with respect to their physical and electrochemical performance. Examples of simulated P–C–T curves are drawn. The simulations demonstrate good agreement with the experimental data reported for various RNi5-type hydrogen storage materials. As expected, the plateau pressure increases with increasing temperatures. It also becomes more difficult to insert hydrogen atoms at higher temperatures, which is generally accepted to occur. Finally, limitations of the model are discussed and possible solutions to overcome these limitations are proposed.

It is generally accepted that under ordinary conditions adopted for the study of metal-hydrogen (M-H) compounds: a) hydrogen can be described reasonably well as an ideal gas, b) the distribution of hydrogen atoms over the different sites of the lattice can be expressed by the Fermi-Dirac statistics, c) the average hydrogen-hydrogen interaction energy which is a function of the number of H-atoms in the lattice includes all the contributions coming from the electrons and the lattice. By considering all the above approximations we have derived a relation between the pressure of gaseous hydrogen and the number of hydrogen atoms in the bulk. The PCIs were obtained by calculating the standard chemical potential for an ideal gas from statistical mechanics and optimizing the other free parameters from the experimental PCIs data at one temperature. We were able to closely predict the PCIs at other temperatures when they were compared with the experimentally obtained PCIs for various RNi 5 -type hydrogen storage materials.

The phonon distribution of hydrogen storage α-LaNi 5 H with 4h, 6m, 12n, and 12o interstitial hydrogen was calculated by using first-principles potential surfaces with a 2×2×2 supercell model in order to investigate structural and thermodynamic properties. Frequency shifts due to the phonon contribution from the internal energies of 12n < 6m < 12o < 4h appeared in specific modes originating from interstitial hydrogen and in the upper-edge modes with nickel-lattice motion. The thermodynamic stability of 12n interstitial hydrogen in α-LaNi 5 due to the wide XZ storage space can be explained by its phonon amplitudes and the charge density around nickelbonded hydrogen.