Kinetics in the hydriding and dehydriding reaction and the thermal cycling effect of the $Mg_2Ni-H$system on the damage of hydrogenation properties were studied. And thermal analysis method was used to study the hydrogen occupation site in the Mg and $Mg_2Ni$.
In order to remove heat effect and to obtain intrinsic kinetic data of hydriding and dehydriding reaction, three types of reactor were used in this study. The heat effect was almost removed by applying Ni thermal ballast and using a highly heat conductive Cu tube reactor. Comparing reaction rate data with theoretical rate equations, based on the scale-like sphere model and nucleation and growth model, the rate controlling step of reaction of $Mg_2Ni$ was determined.
At above the allotropic transformation temperature of $Mg_2NiH_4$, at the initial stage the hydriding reaction rate was controlled by forced flow of hydrogen but at the later stage the surface reaction was the rate controlling one. The apparent activation energy at the later stage is about 4.7kcal/mol. The rate controlling step of dehydriding reaction was diffusion of hydrogen in the α phase. The apparent activation energy of dehydriding reaction of $Mg_2Ni$ is found 14.8kcal/mol.
At below the allrotropic temperature of $Mg_2NiH_4$, the hydriding reaction rate was controlled by the chemisorption of hydrogen at the surface of the particle at higher temperature range. But as the reaction temperature was decreased, the reaction rate was controlled by a bulk reaction (the interfacial reaction at the α/β interface or the diffusion of hydrogen through β phase). The apparent activation energy for the hydriding reaction is 38.0kcal/mol.
Hydrogen storage capacity and plateau presure were not changed by the absorption-desorption cycles. Only the reaction rate was decreased. The structure reordering model can not be adapted for this system. It was suggested that the decrease of hydrogenation rate was due to the change of surface state caused by the coalescence of the particle and surface diffusion.
The number of the thermal desorption peak were 1 and 2 for Mg hydride and the $Mg_2Ni$ hydride respectively. For $Mg_2Ni$ system, considering that the number of thermal desorption peaks corresponding to the occupation site, the two site occupation model by Darriet et al was proved. And for the thermal desorption of hydride, the theoretical rate equations, based on the scale-like sphere model, are presented.
