The formation of metal hydride and its decomposition attract great interests due to its relevance for the hydrogen storage, which is an essential step for hydrogen economy. Magnesium is one of the promising candidates to meet DOEs goal for automotive application. [1]The challengenes of its slow kinetics and high thermodynamic stability are however remained. To shed light on the origin of the kinetics hindrance of the hydrogen uptake, Mg-H system are studied using the first principle thermodynamics.[2][3]
First, the hydrogen adsorption within submonlayer regime were studied. For the hydrogen adsortion on Mg(0001) surface, it is found that fcc hollow sites are energetical most favorable and adsorbed hydrogen atoms tend to form island. For hydrogen adsorption on vicinal surfaces Mg(102) and Mg(101), similar energetics has been found. The dissociation barrier for H2 on Mg(102) surface is calculated to be as large as 1.3 eV, which is roughly same with the dissociation barrier on (0001) surfaces. From these results, it is concluded that the hydrogen uptake on magesium surfaces is structure insensitive. With increases coverage further (> 1ML) on Mg(0001), it is found that the hydrogen will penentrate into the subsurface region spontaneously and form a H-Mg-H trilayer. Further hydrogen uptake will lead to the stacking of H-Mg-H trilayers, a possible prcursor for the transition to the metal hydride (Rutile).
Based on these energetics, the thermodynamics phase diagram are built up using so-called ab initio thermodynamics, where the effect of temperature and pressure are included. It is found that when hydrogen chemical potential is lower than -0. 32 eV, where the formation of bulk metal hydride is thermodynamic unstable, none of structures considered in above are stable, and formation of metal hydride is therefore kinetically hindered.
Reference: [1] T. Vegge, Phys. Rev. B. 2004, 70, 035412.[2] K. Reuter and M. Scheffler, Phys. Rev. B. 2001, 65,035406.[3] Hammer, B.; Hansen, L. B.; Nørskov, J. K. Phys. Rev. B 1999, 59, 7413.
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