An Analysis of Thermodynamic Characteristics of Metal-Hydride Systems for Hydrogen Storage, Using a Modified Scheme of the Perturbation Theory

Abstract

The use of hydrides of intermetallic compounds (IMC) to implement the working processes of thermo sorption compressors, heat pumps, storage systems, cleaning, and programmable supply of hydrogen is due to a number of unique properties of these hydrogen sorbents. These include, first of all, the fact that the saturation of IMCs with a large sorption capacity occurs under relatively "mild" thermodynamic conditions, as well as the selectivity of sorption processes and the presence of the effect of thermodesorptional activation of atoms and molecules of hydrogen isotopes. The work is devoted to the description of phase equilibria in IMC hydrides. The proposed approach to the problem of calculating phase equilibria in metal hydrides consists in determining the properties of the lattice gas of H atoms and the H₂ molecular phase (which is in equilibrium with it) within the framework of a single method, the modified perturbation theory. The thermodynamic description of the hydrogen subsystem in the region of disordered α-, β-phases is carried out on the basis of the model of the non-ideal (interacting) lattice gas of hydrogen atoms. In this case, both the direct interaction between hydrogen atoms and the indirect "deformation" contributions to the potential energy due to the expansion of the lattice during hydrogen dissolution are taken into account. Simulation of phase transitions in IMC-hydrogen systems based on the modified perturbation theory scheme gives a correct description of the main features of phase diagrams in a wide range of hydrogen pressures. From the condition of the equality of the chemical potentials of the H-subsystem of the hydride and H₂-phase (calculated on the H atom), equations are obtained, relating the pressure of the gaseous H₂ phase to the hydride parameters c and T (phase diagrams). The proposed computational procedure does not use adjustable parameters or empirical correlations. It relies on the atomic characteristics of the hydrogen subsystem and metal matrix, which have an unambiguous physical meaning. The object of this research is the LaNi₅ intermetallic hydride. Of particular interest is the position of the critical point of the β→α-transition in the LaNi₅-hydrogen system, for which there are no experimentally obtained parameter values. The paper presents the calculated values of the β→α-transition critical parameters T_c=445 K, pc=87 atm. The calculated data on the thermodynamic parameters of the α→β-transition (enthalpy, entropy, and pressure on the isotherm plateau) make it possible to describe the solubility of hydrogen in LaNi₅ at pressures up to 500 atm and are in good agreement with the available experimental data in the literature.