Hydrogen Storage Properties Of Light Metal Magnesium Nanoclusters And Theoretical Study On Corrosion Resistance Of Bimetallic Alloys With Metal Aluminum

Due to the advantages of low density and abundant reserves,metal magnesium has been widely concerned and studied.Studies have shown that magnesium can react with hydrogen to produce magnesium hydride(MgH2)and achieve solid-state storage of hydrogen,which is considered to be one of the ideal hydrogen storage materials in the future because of the advantages of high gravimetric hydrogen capacity,acceptable cost,high safety and abundant reserve etc.The theoretical hydrogen storage capacity of MgH2 can reach 7.6 wt%,which fully meets the requirements of large-scale vehicle hydrogen storage.However,for bulk MgH2 materials,its slow adsorption/desorption kinetics and high operating temperature(nearly 573 K at 1 bar H2)are still a difficult problem at present,which seriously restricts its wide application.In this paper,the dynamic and thermodynamic properties of MgH2 materials are improved by studying the two aspects of nanoscale and transition metal doping.In addition,as one of the lightest structural metals,magnesium is widely used in automotive and aerospace industries.Generally,the high-temperature strength and corrosion resistance of monometallic magnesium are relatively poor,which cannot meet the needs of industrial application.At present,the application of metal magnesium is mainly focused on its alloys that possess better physical and chemical properties.Here,we focus on the study of nanoclusters of magnesium aluminum alloy,and analyze their structure and electronic properties from the nanoscale.Further,through analyzing the O2 adsorption on magnesium aluminum alloy nanoclusters,we explore their corrosion resistance to oxygen in essence.According to the density functional theory,the main contents and results of our study are as follows:1.By searching the structure of magnesium clusters in different sizes,we find that the Mg55 nanocluster shows a special icosahedral structure with high symmetry.The structural analysis shows that Mg55 has four various substitution sites for transition metal atom(Ti and Nb).Comparing their binding energies and formation energies,it is found that the average binding energy and formation energy of the clusters after doping are smaller,indicating that the addition of transition metals is conducive to improving the stability of the structure,especially doping into the second layers.In addition,compared with the transition metal Ti,doping metal Nb has a better thermodynamic advantage in the energy,and the structure is relatively more stable.2.We further study their electronic properties.According to the Bader charge analysis,it is found that the addition of transition metals Ti and Nb can significantly change their local charge distribution,and the surrounding Mg atoms transfer more electrons to Ti and Nb atoms.The charge distribution at different doping sites is analyzed.We find that the second layer and the outermost layer are more favorable to change the charge distribution on the surface of clusters and activate the surface of clusters,which helps to promote the adsorption and dissociation of H2 on clusters.3.The adsorption process of H2 on all clusters is studied.And we find out their corresponding most stable adsorption sites and further analyze the dissociation mechanism of H2 on clusters.When the Ti and Nb atoms dope in the center and the second layer,the molecular hydrogen is still retained with an equilibrium distance more than 3 A from the nearest atom of nanoclusters,even 4.125 A and 4.126 A for Mg54Ti1 and Mg54Nb2,respectively,indicating that the adsorption of H2 on these clusters is weak physical adsorption and requires more energy to complete the adsorption and dissociation of H2.But when Ti and Nb atoms substitute the surface Mg atoms(such as Mg54TM3 and Mg54TM4),we find out that the hydrogen molecule easily adsorbs around Ti and Nb atoms,and the H-H bonds are significantly elongated from 0.740 A in free H2 to 0.954,0.814,0.803 and 0.902 A for Mg54Ti3,Mg54Ti4,Mg54Nb3 and Mg54Nb4,respectively.And the results obtained from the adsorption energy analysis are in agreement with the above discussion.4.The dissociation process of H2 on all clusters is studied.We find that the H atoms prefer to occupy the bridge sites of nanoclusters rather than the top sites and the hollow sites,indicating that the bridge sites are the most favorable adsorption site for H atoms.Further comparing their dissociation energy barriers,it is found that the addition of transition metals is indeed beneficial to reducing the reaction energy barrier of H2 dissociation and improving the kinetic process of H2 storage.Especially for surface doping,the yielding energy barriers can be neglected with the value of only 0.083 eV for H2@Mg54Ti4 and even 0.0 eV for H2@Mg54Ti3,H2@Mg54Nb3 and H2@Mg54Nb4,respectively.Such a low reaction energy barrier suggests that H2 will spontaneously dissociate on the top of Ti and Nb atoms.5.The magnesium aluminum alloy nanoclusters were properly searched and optimized.Further,we analyze their structure and energy characteristics.It is found that Mg atoms prefer to occupy the surface of the cluster,especially the vertex sites(OV)of the surface.When the ratio of Mg to A1 atoms is close to 1:1,the clusters are seriously distorted and become irregular.But when the ratio is quite small,the clusters can maintain a certain degree of symmetry without great deformation.We further find that the Mg12A143 and Mg12Al43-alloy nanoclusters possess highly symmetrical,and 12 Mg atoms completely occupy 12 OV sites of the surface.Additionally,the excess energies of Mg12Al43 and Mg12A143-clusters are appreciably lower relative to other nanoclusters,especially for Mg12Al43-with the value of-6.101 eV,which indicates that the Mg12Al43 and Mg12Al43-alloy clusters possess high thermodynamic stability.6.The electronic properties of magnesium aluminum alloy clusters are further studied.It is found that when the content of Al is high in the alloy,the corresponding HOMO-LUMO energy gap is larger.In general,the larger the cluster's energy gap,the greater its resistance to oxygen corrosion.For the Mg12Al43 and Mg12Al43-clusters,their energy gap values are significantly larger than those of the other alloys with different proportions.Especially for Mg12A14l3 with the value of 0.816 eV,this suggests it possesses highly resistant to oxidation.7.The adsorption of O atom and O2 molecule on Al55,Mg55,Mg12Al43 and Mg12Al43 clusters is studied in detail.Interestingly,it is found that the most stable adsorption site of O atom is the H1 for all clusters.Other sites energetically show weaker adsorption effect relative to the H1 sites.Compared with Al55 and Mg55,the O adsorption strength on the Mg12A143 and Mg12Al43 clusters is relatively weak,and the corresponding energy values are-4.531 eV and-4.526 eV,respectively.Similarly,the adsorption of O2 on Mg12Al43 and Mg12Al43-clusters also shows a consistent trend.The corresponding adsorption energies were-1.959 eV and-1.809 eV,respectively,which were significantly lower than those of Al55 and Mg55 clusters.These results show that the magnesium aluminum clusters show a certain advantage on the resistance to oxygen corrosion after forming the alloy clusters.