Theoretical Studies On The Effect Of Sc, Co Doped On The Structure And Property Of The Mg₂Ni Surface

Magnesium-based hydrogen storage alloys are considerd to be one of the most potentialhydrogen storage materials, due to their extremely high hydrogen storage capability, richnatural resources, low cost and light weight. However, the high desorption temperature andslow sorption kinetics limit their practical applications. Tremendous studies such as elementsubstitute and composite materials, have been done to improve its thermodynamics, kinetics,electrochemistry and circle life to some extent. So far, the interaction between metal andhydrogen and the mechanism of thermodynamics and kinetics from the microcosmic pointview were still not clear and a theoretical model is needed to provide its reaction mechanism.From the point of the mechanism of hydrogen storage, hydrogen absorption process is startedfrom the surface adsorption, which is the most important step. So the surface properties havea great impact on analyzing the hydrogen absorption properties of Mg₂Ni.In this thesis, A first-principle based on density functional theory and plane wavepseudo-potential （PWP） method were used to research microscopic structure of themagnesium-based hydrogen storage materials. The PW91function of the generalized gradientapproximation （GGA） was employed for the exchange correlation of electrons and the planewave energy-cutoff was set to380eV in the calculations. First of all, the crystal structure andelectronic properties of the Mg₂Ni phase and three low-index surface were calculated to find apreferred hydrogen absorption surface. Moreover, hydrogen adsorptions on clean and Mg₂Nilow-index surfaces were investigated，by the analysis of the adsorption sites, adsorptionenergies and electronic structures.Finally, the adsorption sites, adsorption energies andelectronic structures of the Sc and Co doped surfaces were studied，to understand theimprovement of hydrogen adsorption by doping.The following conclusions are reached:1. By the analysis of structure and property of the Mg₂Ni bulk and the （100） and （010）surface, we find the interaction of Ni1-Ni2is stronger than that of Mg1-Ni1, and the electrondensity concentrates around the Ni, which explain that Ni have a great impact on thehydrogen adsorption. Further more, Mg₂Ni（100） and （010） surface have higher surface energy, which is benefited for the surface activation and hydrogen adsorption. Atoms interaction of（110） surface is significantly less than that of （100） and （010） surface, and the distancebetween atoms is larger, which is effective for the diffusion of hydrogen in bulk. However,the Mg₂Ni（110） surface has lower surface energy, hydrogen adsorption is more difficult.Therefore, Mg₂Ni（110） surface is not appropriate for hydrogen adsorption surface;Mg₂Ni（100） and （010） surface can be used as the hydrogen adsorption surface.2. On the clean Mg₂Ni（100） surface, adsorption energy of the stable adsorption site isthat G> D>F, and the adsorption capacity of adsorption site G which is part of the chemicaladsorption is largest. The steric hindrance of adsorption site C is larger than that of D.Compared to the clean surface, the stable adsorption site of Co doped Mg₂Ni（100） surface didnot change but the adsorption energy increased, which strongth the hydrogen adsorptionability. For the Sc doped Mg₂Ni（100） surface, site c becomes a stable adsorption site, that is,H prefers to adsorb on the g, f, c sites.3. On the clean Mg₂Ni（010） surface, adsorption energy of the stable adsorption site isthat C> F>B. Adsorption site G contains two Ni atoms, and its adsorption capacity is largest.Compared to the clean surface, the stable adsorption site of Co doped Mg₂Ni（010） surface didnot change but the adsorption energy increased, which strongth the hydrogen adsorptionability. For the Sc doped Mg₂Ni（010） surface, site e becomes a stable adsorption site, that is,H prefers to adsorb on the c, f, e sites.4. Hydroption molecule is easy to dissociate on the clean Mg₂Ni（010） surface, and theinteraction between hydroption atom and surface is stronger. Hydroption atom prefers toadsorb on Ni-Ni bridge site. In the three initial adsorption, the adsorption energy of the Hor1is largest. For the Co doped Mg₂Ni（010） surface, Co atom plays a catalytic role of hydrogenadsorption, which strongths the hydrogen adsorption ability. Howere the Mg-Ni bridge sitehas little influence.The novel conclusions and ideas of this work are listed as follows:1. Our study shows that the Mg₂Ni（100） and （010） surface can be used as the hydrogenadsorption surface. According to investigation of hydrogen adsorption on the Mg₂Ni surfaces,the most stable adsorption sites all contain Ni atom.2. Compared to the clean surfaces, the stable adsorption site of Co doped Mg₂Ni surfaces did not change but the adsorption energy increased, which strongth the hydrogen adsorptionability.3. Doping Sc strongths the hydrogen adsorption ability of c site on the （100） surface ande site on the （010） surface, which make them become the stable sites.4. Comparing with the Co-doped surface, the hydrogen adsorption capacity of theSc-doped surface is larger.