Theoretical Studies On The Effect Of Pd Substitution On The Structure And Property Of Mg₂Ni

The energy is the foundation and guarantee of the modern society development. With the rapid development of globe economy and the rising human demand, it is increasing of the energy consumption and waste emission . Ecological environment is suffering great challenge. Low-carbon and carbon-free energy sources become the new hotspot. Hydrogen energy meets the trend, which can achieve free emissions of carbon dioxide. It needs to find a good carrier to develop and utilize hydrogen energy . So countries around the world gathered the sight into the magnesium based hydrogen storage materials.In this paper, the research object is Mg-based hydrogen storage alloys. On the basis of the Mg₂Ni, the Mg or Ni atoms are replaced by Pd atoms respectively or synchronously,describing the improvement of the performance of hydrogen storage by the substitution of Pd. All calculations are based on ab initio density functional theory, using the pseudopotential and the basis set of plane wave in Vienna ab initio simulation package. The covalent interactions are dealed with PAW potential and PW91 generalized gradient function (GGA) is selected as exchange correlation function.The first chapter introduces the experimental and theoretical research status of magnesium based hydrogen storage alloys. The second chapter briefly introduces the theoretical base and basic knowledge involved in this article. Calculation is divided into three main parts (the third chapter to the fifth chapter):The first part, Put Pd atom whose radius is larger than Ni replace Ni atoms located in 3d position in Mg₂Ni. After hydrogen absorption, there is a greater cell deformation, and the volume of cell increases, which is effective for the diffusion of hydrogen in bulk. Calculated PDOS overlap areas integral below Ferm-level of orbit. The interaction of Ni-H is 0.66 in Mg₂Ni, while in alloy the Ni-H interaction reduces to 0.59, which weaks the bond between H and the major hydrogen atom Ni, and makes hydrogen emerge easily. It benefits to lower the temperature of dehydrogenation.The second part, Pd replace Ni atoms located in 3b. The colume of cell increases,so that the distance between Ni-H grows longer after the alloy hydrogen. According to analysis of electron density, the electron density at the saddle point close to the bond Ni– H is 0.70 in H₂-Mg₂Ni, while that is 0.68 in H₂-Mg₁₂Ni_6-x Pd_x(x=1,2,3).With Pd substitution, the attractiveness of original location Ni to H atom reduces, and improves the activation performance, which is conducive to the release hydrogen.The third part, calculate electron structures of the cell after substitution with Pd for Mg(6f/6i).And then add hydrogen atoms, compares changes in the cell after hydrogenation. Besides that,we attempted substitution both side. Pd increases negative power in side A and weaken the strong interaction between H and Ni, which is propitious to hydrogen activation. The 6i replacement is superior to that of 6f. This prediction is a guiding for experiments.The novel conclusions and ideas of this work are listed as follows:1. The crystal structures and electronic structures of Mg₂Ni and its hydrides were performed by the first-principles. We put Pd atoms replace each side of the Mg₂Ni, because of its greater electronegativity , larger radius than Mg or Ni, and the characteristics of strong catalytic activity. Calculate the changes of electronic structure before and after hydrogenation and substitution. It shows the feasibility of Pd substitution and makes has a good guide for experimental study.2. According to analysis of electron density, constructs electron density contour map by selecting three atoms in lower cell, compares the electron density close to the saddle point, and semi-quantitatively compares the Ni-H bond intensity. According to calculating the density of states, constructs TDOS and PDOS, compares the stability of the alloy before and after replacement through the bond peak position and peak height change.3. On the basis of the electronic density of states distribution curve, take integral calculations to overlap areas under the curve by the program compiled ourselves, which can quantitatively compare the bonding strength between atoms in the graph of DOS and the overlap of electron partial density of states in the graph of PDOS.