A new lattice gas model has been developed, describing the hydrogen storage in hydride-forming materials. This model is based on the mean-field theory and Bragg-Williams approximation. To describe first-order phase transitions and two-phase coexistence regions, a binary alloy approach has been adopted. A complete set of equations describing pressure-composition isotherms and equilibrium electrode potential curves of hydride forming materials in both solid-solution and two-phase coexistence regions has been set up. The proposed model defines both the equilibrium pressure and equilibrium potential as explicit functions of the normalized hydrogen concentration, using eight physically well-defined parameters. Gibbs free energies, entropies, and phase diagrams of both model ͑LaNi y Cu 1.0 ͒ and commercial, MischMetal-based, AB 5 -type materials at different compositions and temperatures have been simulated. Good agreement between experimental and theoretical results for the pressure-composition isotherms obtained in the gas phase and the equilibrium potential curves measured in electrochemical environment has been found in all cases.
A kinetic model has been developed, describing the kinetics of the hydrogen storage reactions in hydrideforming materials under equilibrium conditions. Based on first principles chemical reaction kinetics and statistical thermodynamics, the model is able to describe the complex processes occurring in hydrogen storage systems, including phase transitions. A complete set of equations, governing pressure-composition isotherms in both solid-solution and two-phase coexistence regions has been obtained. General expressions for rate constant dependencies have been proposed, using well-defined phase-dependent Hamiltonians for the hydrogen energy state at the surface and in the bulk of hydride-forming materials. The characteristics of both model ͑LaNi y Cu 1.0 ͒ and commercial, misch-metal-based AB 5 -type materials at different compositions and temperatures have been simulated. Good agreement between experimental and theoretical results for the pressurecomposition isotherms has been found in all cases.
Recently, a lattice gas model was presented and successfully applied to simulate the absorption/desorption isotherms of various hydride-forming materials. The simulation results are expressed by parameters corresponding to several energy contributions, e.g., interaction energies. However, the use of a model system is indispensable in order to show the strength of the simulations. The palladium-hydrogen system is one of the most thoroughly described metal hydrides found in the literature and is therefore ideal for this purpose. The effects of decreasing the thickness of Pd thin films on the isotherms have been monitored experimentally and subsequently simulated. An excellent fit of the lattice gas model to the experimental data is found, and the corresponding parameters are used to describe several thermodynamic properties. It is analyzed that the contribution of H-H interaction energies to the total energy and the influence of the host lattice energy are significantly and systematically changing as a function of Pd thickness. Conclusively, it has been verified that the lattice gas model is a useful tool to analyze thermodynamic properties of hydrogen storage materials.
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