| Magnesium hydride, with light weight and high hydrogen storage capacity, is one of the most potential hydrogen storage materials. However, the slow hydrogen storage kinetics and high hydrogen desorption temperature limit its practical application. The hydrogen storage properties of Mg can be facilitated by alloying with transition metals. In this work, the stability, interatomic interaction, and hydrogen storage properties of layered Mg-Ti alloy are studied via first-principles. Also, the possible factors, which might influence the hydrogenation properties of the layered Mg-Ti alloy, are analyzed. The ways improving hydrogen storage properties of Mg alloys are investigated.Firstly, the stability of Mg(0001)/Ti(0001) interface is investigated. The interface is constructed by 7 Mg atomic layers and 8 Ti atomic layers with an 18 ? vacuum above the atomic layers. The stable Mg(0001)/Ti(0001) interface is obtained through optimizing the parameters of the separation and the lattice misfit(the scaling factor)with a formation energy of-0.216 J/m2. The atomic arrangement of the Mg and Ti layers is in an anti-symmetric manner(AA model), and the separation and the scaling factor are 2.40 ? and 0.911, respectively. Hydrogen adsorption in the interface zone is further studied. Adsorption of hydrogen in the area that faces the position of Mg atoms in the front layer of Mg side(the top site) owns the lowest adsorption energy of-0.991 e V, which is lower than the adsorption of H on the Mg(0001) surface. In the near interface zones the attraction of H by Ti is stronger than that by Mg. However,the adsorption energy of H in the Mg side is in the range from-0.13 to-0.41 e V,which is favoring the reversible(de)hydrogenations. The maximum hydrogen storage capacity and the reversible hydrogen mass fraction of this system are 5.15 wt% and2.06 wt%, respectively.Secondly, Mg/n Ti/Mg multi-interface system is studied where n(= 1 to 8) is the number of the inserted Ti atomic layers. We also calculate the hydrogen adsorption energies of the interface system with distinct n. The AA model of the Mg/3Ti/Mg system is also the most stable structure with separation between Mg and Ti atomic layers of 2.50 ?, the scaling factor of 1.02, and the formation energy of-0.558 J/m2.The stability of the hydrogenated Mg/3Ti/Mg system is weaker than the Mg/Ti single interface system, but stronger than the adsorption of H on the Mg(0001) surface. The maximum hydrogen storage capacity and the reversible hydrogen mass fraction of thissystem are 6.61 wt% and 5.45 wt%, respectively. Both are higher than that of the Mg/Ti interface system.Thirdly, the electronic mechanisms of the stability and the hydrogenation of Mg-Ti interface systems are clarified. In the Mg-Ti interface systems, both the Mg and Ti atomic layers lost their electrons resulting an accumulation of electrons in the interface zone. There is a connection between the hydrogenation stability and the electronic accumulation, the higher the electronic accumulation the stable of the hydrogen adsorption is. The reactivity of the metal atoms at the interface is stronger than that in the inner of the system. In the system before hydrogenation there is interaction between Mg and Ti atoms in some degree, but after the adsorption of hydrogen the Ti-H interaction is stronger than the Mg-H interaction. Thus the insert of Ti atomic layers could improve the hydrogenation properties of Mg by altering the charge distribution of Mg atomic layer and interacting with H atoms.Fourthly, relationship between the hydrogenation properties and the distance of atomic layers is studied. It was found that the average adsorption energy of hydrogen atom is sensitive to the distance between atomic layers. Therefore for the layered hydrogen storage materials adjusting of the distance of the atomic layers is an effective approach to control the(de)hydrogenation properties.In conclusion, the charge distribution and the separation between atomic layers is greatly influence the hydrogen storage properties of Mg-Ti interface system. Insert of Ti atomic layers could alter the charge distribution of Mg atomic layers and promote its hydrogenation. The Mg-Ti interface zone also accelerates the hydrogen adsorption.However, by strong interaction between Ti and H atoms, H attracted by Ti atom is difficult to be released, and therefore the overall reversible hydrogen storage capacity of this system is lowered. Thus in the Mg-Ti layered alloys, thickness of the insert Ti atomic layers should not too large, instead inducing more interface zones is an alternative way to achieve the high hydrogen storage capacity and the good reversibility. |
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