Effect on specific capacity and de-hydrogenation efficiency in doped-MgH2

Abstract

Effect on specific capacity and de-hydrogenation efficiency of MgH2 are studied with sequential doping of La, Ce, Al, Li and Ni using Density Functional Theory. Doping of La and Ce enhance the unit cell volume and thus decrease the volumetric capacity, while, doping of Al, Li and Ni decrease the unit cell volume and subsequently increase the volumetric capacity. Among them, doping of Ni decreases the unit cell volume the most and hence provides highest volumetric capacity. Gravimetric capacities get increased with the following trends of doping; Ce < La < Ni < Al < Li in accordance to the mass of the dopant. It is proved from the analysis of density of states that the desorption of H2 will occur from the nearest neighbor H atoms of dopant La and Ce, while, for doping of Al, Li and Ni, desorption will take place from the whole crystal structure. H2 desorption barriers decrease according to the following trends of dopants; La > Ce > Al > LiNi. With co-doping of Ni, MgH2R-Ni is formed, and it exhibit decrement of unit cell volume in comparison to MgH2R. Lesser average bond length of MgH2RNi leads to localization of density of states across Fermi energy enabling easier electron transfer with lesser H2 de-sorption energy.