Physicochemical and thermodynamic investigation of hydrogen absorption and desorption in LaNi3.8Al1.0Mn0.2 using the statistical physics modeling

In the present work, experimental absorption and desorption isotherms of hydrogen in LaNi3.8Al1.0Mn0.2 metal at two temperatures (T= 433 K, 453 K) have been fitted using a monolayer model with two energies treated by statistical physics formalism by means of the grand canonical ensemble. Six parameters of the model are adjusted, namely the numbers of hydrogen atoms per site n(alpha) and n(beta), the receptor site densities N-m alpha and N-m beta, and the energetic parameters P-alpha and P-beta. The behaviors of these parameters are discussed in relationship with temperature of absorption/desorption process. Then, a dynamic investigation of the simultaneous evolution with pressure of the two alpha and beta phases in the absorption and desorption phenomena using the adjustment parameters. Thanks to the energetic parameters, we calculated the sorption energies which are typically ranged between 276.107 and 310.711 kJ/mol for absorption process and between 277.01 and 310.9 kJ/mol for desorption process comparable to usual chemical bond energies. The calculated thermodynamic parameters such as entropy, Gibbs free energy and internal energy from experimental data showed that the absorption/desorption of hydrogen in LaNi3.8Al1.0Mn0.2 alloy was feasible, spontaneous and exothermic in nature.