Thermodynamical Modeling of P-C Isotherms for Metal Hydride Materials

Abstract

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.