Ab Initio Alloying of Mg for Hydrogen Storage
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1098-HH03-05
Ab Initio Alloying of Mg for Hydrogen Storage Deniz Kecik, and Mehmet Kadri Aydinol Metallurgical and Materials Engineering, METU, Middle East Technical University Department of Metallurgical and Materials Engineering, Ankara, 06531, Turkey ABSTRACT A candidate hydrogen storing material should have high storage capacity and fast dehydrogenation kinetics. On this basis, magnesium hydride (MgH2) is an outstanding compound with 7.6 wt % storage capacity, despite its slow dehydriding kinetics and high desorption temperature. Therefore in this study, formation energies of alloyed bulk MgH2, adsorption energies on alloyed magnesium (Mg) and MgH2 surface structures were calculated by total energy pseudopotential methods. Also, the effect of substitutionally placed dopants to the dissociation of hydrogen molecule (H2) at the surface of Mg was investigated via Molecular Dynamics (MD). The results show that 31 out of 32 selected dopants decreased the formation energy of bulk MgH2, within a range of ~ 37 kJ/mol-H2 where only Sr did not display any such effect. The most favorable elements in this respect are; P, K, Tl, Si, Sn, Ag and Pb. Moreover, surface adsorption energy values display that all elements are adsorbed substitutionally on the clean (0001) surface of Mg where adsorption on MgH2 (001) surface is possible only for alloying elements other than Zn, Au, In, Ag, Li, Tl, Cd, Na and K. Finally, results of MD simulations point out that the elements giving rise to the dissociation of hydrogen molecule came out to be Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Ru, Rh ve Hf. INTRODUCTION Hydrogen, being a promising energy carrier, has been drawing attention recently considering especially the decline in the fossil fuel reserves and environmental concerns related to the fossil fuel based economy. There exist several methods for hydrogen storage, as mentioned by Zuttel [1], such as storing in gaseous as well as in liquid form, physisorption in carbon nanotubes and storing in metal and complex hydrides. Many metals are available for reacting with hydrogen and forming a stable metal hydride. Besides, the advantage of metal hydrides is that they are ready to store hydrogen in large amounts on a reversible basis [2]. Low cost, high storage capacity and low desorption temperature are generally searched as criteria for metal hydrides. Magnesium hydride, in this respect, is a promising material being economic, light weight and storing high amounts of hydrogen (7.6 wt %) despite its slow dehydriding kinetics. Therefore, in general terms, what is desired to be accomplished is to decrease the desorption temperature and improve dehydriding kinetics through adding alloying elements. As also previously studied by several authors [3-4], effects of alloying on both bulk and surface structures of MgH2 constitutes the main outline of this study. Therefore, it is mainly aimed to propose new magnesium alloys with improved hydrogen desorption characteristics where the context of the study basically comprises: calculation of formation ene
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