Sihem Belkhiria

In the present study, the hydrogen-absorption properties of the LaNi₅ and the La_0.7Ce_0.1Ga_0.3Ni₅ compounds were determined and compared. This work is therefore divided into two parts: an experimental part that presents and discusses the kinetics and isotherms of hydrogen absorption in the two compounds at two different temperatures (298 K and 318 K). In addition, the temperature variations inside the hydride bed were determined. In the second section, the experimental isotherms were compared to a numerical model processed using statistical physics. Following that, thanks to the perfect agreement between the experimental data and the proposed model, the stereographic and energetic parameters associated with the hydrogen absorption reaction, such as the number of hydrogen atoms per receptor site (n₁, n₂), the densities of the sites (N_m1, N_m2), the half-saturation pressures (P₁, P₂) and the absorption energies (ΔE₁, ΔE₂) for each receptor site, were calculated. All of these parameters are acquired by making numerical adjustments to the experimental data. Thermodynamic functions, such as internal energy and Gibbs energy, which regulate the absorption process, were then identified using these parameters. For both compounds, all of the aforementioned were compared and discussed in relation to initial temperature and pressure. The results demonstrated that the hydrogen-storage properties in LaNi₅ are enhanced by more than 30% of stored mass and kinetics when Ce and Ga are substituted at the La sites.

The results of an experimental and mathematical study into the MmNi_4.2Mn_0.8 compound’s hydrogen storage properties are presented in the present research. Plotting and discussion of the experimental isotherms (P-C-T) for different starting temperatures (288 K, 298 K, 308 K, and 318 K) were carried out first. Then, the enthalpy and entropy of formation (ΔH₀, ΔS₀) were deduced from the plot of van’t Hoff. Following that, the P-C-T were contrasted with a mathematical model developed via statistical physics modeling. The steric and energetic parameters, such as the number of the receiving sites (n₁, n₂), their densities (N_m1, N_m2), and the energy parameters (P₁, P₂) of the system, were calculated thanks to the excellent agreement between the numerical and experimental results. Therefore, plotting and discussing these parameters in relation to temperature preceded their application in determining the amount of hydrogen in each type of site per unit of metal ([H/M]₁, [H/M]₂) as well as for the entire system [H/M] versus temperature and pressure besides the absorption energies associated with each kind of site (ΔE¹, ΔE²) and the thermodynamic functions (free energy, Gibbs energy, and entropy) that control the absorption reaction.