Role Of Metal Impurity On Hydrogen Diffusion Of Magnesium Based Hydrogen Storage System: A Theoretical Study

The hydrogen storage capacity of magnesium hydride?MgH₂?is relatively high with a gravimetric density of 7.6 wt%,a volumetric density of 110 g/L,and an energy density of 9MJ/kg.T here i s a w ide a bundance of l ow cost m agnesium i n t he na ture,l eading M g a promising candidate for the commercial hydrogen storage materials.However,there are still two s erious s hortcomings for M gH₂ in p ractical a plications.First,M gH₂ is v ery s table because the release o f h ydrogen occurs at temperature in ex cess of 300?.Second,slow kinetics of dehydrogenatin and hydrogenation.In the introduction,we review the general situation of hydrogen storage materials in detail a nd i ntroduce t he a dvantage of M g-based h ydrogen s torage m aterials an d t heir application obstacles in highlight.The review continues to focus on solutions for hydrogen storage improvements,and some other existing problems.In chapter two,we introduce the first-principles m ethod based on de nsity f unctional t heory,t he related c omputational software VASP,and the sransition state theory.Using the fi rst-principles calculation m ethod,w e s ystematically study the s tructural stability of four typical impurity metals?Ti,Nb,Al,and In?doped in different layers of Mg?0001?surface.By comparing the substitution energy of impurity metals doped in different layers and analyzing the electronic structure of the doping system,we determine the stable congfiguration of defects.It is found that indium can be dissolved in Mg to form Mg-In solid solution and the other impurity matals prefer to substitute Mg atoms in the inner layers.There is a charge transfer on doped Mg?0001?surface mainly from the nearest neighbor Mg atoms to the impurity metal atoms.Then different Mg?0001?surface structures with the impurity metal doped are selected to study the hydrogen adsorption properties and compare with the hydrogen adsorption properties of pure Mg?0001?surface.In impurity metal doped Mg s ystem,the i nteraction be tween h ydrogen a tom a nd Ti or N b atoms is s ignificantly enhanced,favorable for the adsorption of hydrogen atoms.As for Al or In,the interaction is repulsive,maybe benefical for the diffusion of hydrogen.Based on the discussion of the hydrogen stable adsorption sites on the doped Mg?0001?surface,we continue to study the hydrogen diffusion properties on i mpurity metal doped Mg?0001?surface and the effect of doping impurity metal on the hydrogen diffusion barrier.We find that the diffusion of hydrogen atoms is relatively easy in Ti or Nb doped system compared with pure Mg system and thus benefical to the formation of multilayer H-Mg-H structure,which is crucial for the transition to Mg hydride.As for Al or In doped system,the existence of t he A l or In i mpurities,no m atter i n t he f irst or s econd l ayer,a dsorbed hydrogen atom w ill ke ep a way from t he dop ed a tom,a nd t he di ffusion will no m uch different from the pure Mg system.So the doped Al or In provides no helps to the formation of mu ltilayer H-Mg-H structure a nd its tr ansition to M g h ydride,th outh it mig ht b e benefical to hydrogen releasing from MgH₂.