First-principles Study On Hydrogen Storage Properties Of Al Coordination Hydrides And MgH₂ By Metal Doping

Exploring high-efficiency,safe and reversible hydrogen storage materials is an important part of realizing the"Hydrogen Economy".The aluminum-based coordination hydrides and magnesium-based metal hydrides have the advantages of high hydrogen storage density and low production price,which has been extensively studied as hydrogen carriers.However,aluminum-based coordination hydrides have high thermal stability,while magnesium hydride has high thermodynamic stability and relatively slow hydrogen evolution reaction kinetics,which lead to a severe restrictions of practical application.Therefore,exploring active substances and their catalytic mechanisms is the key to improving the hydrogen storage performance of hydrides.In this paper,the de-hybridization effect of transition metal doping on AlH₄groups in Al-based coordination hydrides and the weakening effect of Al-H bonds are explored.At the same time,the effect of metal doping on the thermodynamic stability and bonding of MgH₂hydrogen storage materials is also explored.The main research contents are summarized as follows:1.By analyzing the surface energy and work function of transition metals（TMs=Sc,Ti,V,Cr,Mn,Fe,Ni and Cu）,the adsorption model of"cluster+substrate"was established,and the adsorption and dissociation process of AlH₄groups on the transition metal surface was explored.The study of the model shows that the change of bond length in the adsorption model of"cluster+substrate"is closely related to the work function,atomic charge and adsorption energy.The analysis of the electronic structure properties of the adsorption model shows that the doping of transition metal can dehybridize the AlH₄group,which reveals the weakening mechanism of AlH₄group.2.The thermodynamic stability of the doping of metals（TMs=Al,Ti,V,Fe,Co,Ni and Cu）with MgH₂hydrogen storage materials was investigated.The metal doping system was established by constructing MgH₂supercell and replacing Mg atom with metal atom.Comparing the changes of Mg-H bond before and after doping,it can be found that doping of metal atoms is beneficial to weaken Mg-H bond.The analysis of enthalpy of formation of doping system shows that the doping of metal atoms is beneficial to reduce the enthalpy of formation of MgH₂hydrogen storage material,which improves the hydrogen storage performance of MgH₂hydrogen storage material.3.The MgH₂ hydrogen storage materials were double-doped with Fe,Co and Ni metals and extended to ternary magnesium hydride in an attempt to elucidate the synergistic effect in multi-component systems.In particular,the bonding properties between alloying elements and hydrogen atoms and the electronic structure of doping systems are investigated.By comparing the enthalpies of formation of these systems,it can be concluded that the doping of Fe,Co and Ni are more favorable for dehydrogenation than pure Mg₁₈H₃₆.Finally,the bonding strength of Mg-H and the thermodynamic properties of the doping systems were studied by Bader charge analysis and phonon calculation,so as to discuss the bonding use and thermodynamic stability of the doping model.