Structural phase stability and thermodynamical properties of transition metal complex hydrides Na2MgTMH7 (TM=Sc−Cu) for hydrogen storage applications

Abstract

In order to predict ground state structure as well as thermodynamical properties of transition metal-based complex hydrides (TMCHs) and identify potential hydrogen storage materials for energy storage applications, we have performed first-principles density functional spin polarized total energy calculations for Na2MgTMH7 where TM ​= ​Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. To investigate the thermodynamical properties of transition metal-based complex hydrides (TMCHs) and to find potential hydrogen storage materials for energy storage applications. Our spin-polarized calculations show that all these materials are having a finite magnetic moment at the transition metal site with ferromagnetic or anti−ferromagnetic ordering as ground state configuration except the cases where TM ​= ​Sc, Co, Ni, and Cu. Our calculated enthalpy of formation and hydrogen site energy of TMCHs suggest that these systems are stable and the hydrogen is expected to release at a relatively low temperature compared to that of pure binary hydrides. We have performed the chemical bonding analyses between the constituents of TMCHs through density of states, charge density, electron localization function, Born effective charge, Bader effective charge, Mulliken charge, and COHP and these analyses suggest that there is an ionic bonding between hydrogen with Na, Mg, and transition metal with noticeable covalent bonding between hydrogen and Mg/TM.