First-principles Study Of The Influence Mechanism Of Element Substitution On The Re/dehydrogenation Ability Of MgH₂ Hydrogen Storage Materials

The MgH₂,which has high hydrogen storage capacity and cheap price,has attracted much attention among the studied hydrogen storage materials.However,its dehydrogenation temperature is high,and its performance of hydrogen absorption is poor.Hence,how to improve the hydrogen storage kinetics and decrease the desorption temperature are two key issues to study.This thesis focus on the influences mechanisms of the metal elements substitution on the re/dehydrogenation of MgH₂.All calculations were performed by the CASTEP code in MS6.0 which is a first-principles plane wave pseudopotential method within the density functional theory.As to the dehydrogenation process,results show that the width of band gap and the impurity energy levels are two key factors to determine the hydrogen storage properties of MgH₂.The impurity energy levels introduced by Fe,V,Nb,Ti are in the middle of energy gap,and those introduced by La are in the bottom of conduction band,the energy gap becomes narrower.In two cases above,the weakest Mg-H bond breaks easier,which is helpful to the MgH₂ dehydrogenation.The covalent bonds formed between Nb,Fe,Ti,V,La and neighbor hydrogen lead to the metal hydrides formation,which plays a catalytic role to the dehydrogenation of MgH₂.The electrostatic forces between Mg1 and surround hydrogens are no longer symmetrical duo to the doping,the H with weaker force to Mg releases easier,which improves the dehydrogenation properties of MgH₂.As to the rehydrogenation process,the results show that H atom can be adsorbed on the Mg?0001?surface stably,and with the increase of the number of hydrogen adsorption,the stability of system decreases,but the stable adsorption also can forms.This is the prerequisite for the generation of MgH₂.After the substitution of transition metal Fe,V,Ti,Nb and rare earth element La to Mg,the more stable adsorption systems are formed,which catalyzes the rehydrogenation of MgH₂.The bond length between substitution atoms and H was significantly shorter after the substitution of transition metal Fe,V,Ti,Nb to Mg.The H atoms are adsorbed strongly around the substitution atom,which provide a source of hydrogen for the formation of MgH₂.There are less effect of the substitution of the rare earth element La on the adsorption hydrogen system,and this can be obtained by analyzing the overlap population and electron density difference.At the same time,as a catalyst,the hydride formed by doping element and hydrogen atom can participate in the rehydrogenation reaction of MgH₂.