Consequently, the absolute values of enthalpy (ΔH) and entropy (ΔS) become higher as the x ? 0.15. It also can be clear seen from Fig.?9 that the absolute values of both enthalpy and entropy increase gradually with the increasing of hydrogen concentration, which indicates the metal hydrides have lower reaction enthalpies at the initial stage of hydrogenation and the stability of hydrides increases with the increase of reaction time. According to the study of Li et?al.