Theoretical Studies On The Structures And Properties Of MgScH_n~Q(n=10-20,Q=0,-1) Clusters

In recent years,the extensive use of fossil fuels is the main cause of environmental pollution and global warming.With the increasing demand for energy,fossil fuels continue to decrease,and the fossil fuels stored on earth are limited.Finding reliable,clean,flexible and low-cost renewable energy has become the main research direction of each research group.Hydrogen energy is an efficient,reproducible and green,pollution-free energy source.Hydrogen forms metal hydrides with some metals and alloys for solid-state storage at moderate temperatures and pressures,which makes them faster and more efficient than gas and liquid storage methods.Therefore,metal hydride materials,especially magnesium-based hydrogen storage materials,have high weight hydrogen storage density and are a safe and efficient on-board storage method.After studying the Mg-Sc-H system,we find that it shows excellent stability and high hydrogen storage capacity and can be used as potential hydrogen storage materials,but the potential mechanism of its excellent hydrogen storage performance has not been fully explored and needs further study.In this work,we investigate the structural evolution,electronic properties,stability,and hydrogen storage properties of medium-sized neutral and anionic MgScHnQ(n=10-20,Q=0,-1)clusters.The main research contents are as follows:1.Through CALYPSO and the smaller PBEPBE/3-21G theoretical calculation methods,a comprehensive structure prediction search is carried out on structures of MgScHn0/-clusters of different sizes.Then through the larger B3PW91/6-311+G(d)theoretical calculation method,the selected initial structure is geometrically optimized,and we determined the ground state and two metastable isomers of each size MgScHn0/clusters,determined the energy level order,symmetry and electronic state of structures.It is found that the ground state structures of neutral and anionic clusters are three-dimensional structures.The ground state structure of the diatomic MgSc0/-cluster is also studied.2.By calculating the average binding energy Eb,the second-order difference energy ?2E and the HOMO-LUMO energy gap of the ground state structures of MgScHn0/-clusters,the relative stability of the system is studied.The results showed that the neutral MgScH15 cluster is more stable than other clusters and has the largest energy gap(5.43 eV).3.In order to study the electronic structures of anionic MgScHn-(n=10-20)clusters in more detail,we use the time-dependent density functional theory to theoretically simulate the photoelectron spectroscopy(PES)of anionic clusters and obtain their vertical detachment energy(VDE)and adiabatic detachment energy(ADE),which facilitates the comparison of later theoretical and experimental spectra.4.We systematically study the hydrogen storage properties of MgScHn0/-clusters.It is found that the neutral MgScHn clusters can adsorb up to 15 hydrogen atoms,and the anionic MgScHn-clusters can adsorb up to 13 hydrogen atoms.The interaction between MgSc0/-clusters and hydrogen atoms is chemical adsorption.The adsorption energy range of neutral MgScHn clusters is 2.45-2.62 eV,and the adsorption energy range of anionic MgScHn-clusters is 1.87-4.68 eV.It shows that the existence of excess electrons on the anionic MgScHn-clusters can enhance the hydrogen storage capacity.More importantly,it is found that the ground state structure of the MgScH15 cluster has Cs symmetry and the largest mass hydrogen storage capacity of 17.8 wt%,which is the most stable cluster.5.We use molecular orbital analysis and adaptive natural density partitioning(ADNDP)methods to further investigate the properties of the chemical bonds of the most stable MgScH15 cluster.The molecular orbital of the MgScH 15 cluster is mainly composed of the interaction among the 2p orbital of Mg atom and the 3d orbital of transition metal Sc atom and the is orbital of H atoms,which promotes the stability of the MgScH 15 cluster.In conclusion,the results show that the MgScH15 cluster is the most stable clusters with Cs symmetry and the largest hydrogen storage capacity by weight,which is 17.8 wt%.The current findings are expected to provide guidance for further exploration of novel hydrogen storage nanomaterials.