Effect of measurement parameters on thermodynamic properties of La-based metal hydrides

Pressure–concentration isotherms (PCIs) of LaNi_5−xAl_x (x = 0.3 and 0.4) hydrides were measured using a volumetric method. Two important thermodynamic properties, enthalpy of formation (ΔH) and entropy of formation (ΔS), were calculated using the van't Hoff equation. The effects of the Al content on the hydrogen storage capacity, plateau pressure and thermodynamic properties were studied. Additionally, the effects of the charging/discharging pressure difference (ΔP_s) during each step of the absorption/desorption PCI measurement on the hydrogen storage capacity (wt%), equilibrium pressure (P_e), plateau slope, reaction enthalpy (ΔH) and entropy (ΔS) were studied for LaNi_4.6Al_0.4 hydride. All of these properties (P_e, ΔH, ΔS, etc.) showed a significant variation with ΔP_s. The effect of the temperature range on the estimation of the enthalpy of formation was investigated. It was observed that ΔH depends on the experimental temperature range.