Density Functional Theory Study of the Effect of 3d and 4d Transition Metals On Mg-Based Complex Metal Hydrides for Hydrogen Storage Applications

     Light chemical elements are essential to establish a reversible hydrogen storage system [1]. Typical elements considered among the light elements are: Li, Be, B, C, N, O, F, Na, Mg, and Al. Complex metal hydrides (CMHs) have been identified as promising candidates for on board hydrogen storage applications [2]. Ti-doped CMHs have shown to improve the kinetics of complex metal hydride decomposition and the thermodynamic reactions relative to the pristine material. Thus, the role of Ti and other 3d/4d transition elements in different types of CMHs need further studying in order to achieve the goals established by the DoE [3].Main emphasis of this research has been to study NaMgH₃ as a promising material for on-board hydrogen storage applications.

    In the present work, catalytic additives with improved performance over Ti were identified in order to generalize the work on the mechanism for 3d/4d doping in NaMgH₃ to systems with higher gravimetric densities and more favorable thermodynamics.For this purpose, bulk and surface models of NaMgH₃ with impurities from the 3d/4d element block were investigated using plane wave density functional theory (DFT) calculations. In this respect, cohesive energies of pure and doped bulk/surface CMHs as well as the adsorption/substitution energies were calculated from first principles. Furthermore, the effect of substitutionally placed Ti on various sites on surface models was studied via DFT coupled Molecular Dynamics (MD) at elevated temperatures, and revealed remarkable trends.

 

References:

      1)            Choudhury, P.; Bhetanabotla, V.R.; Stefanakos, E.; “Manganese Borohydride as a Hydrogen –Storage Candidate: First-Principles Crystal Structure and

                     Thermodynamic Properties.” J. Phys. Chem. C. 2009. 113. 13416-13424.

      2)            Alapati, S. V; Johnson, K. J.; Scholl, D. S. “Identification of Destabilized Metal Hydrides for Hydrogen Storage Using First Principles Calculations.” J. Phys. Chem.

                     B. 2006. 110. 8769-8776.

      3)            Lovvik, O.M; Opalka, S. M.; “Density Functional calculations of Ti-enhanced NaAlH4.” Physical Review B. 2005. 71. 1-9.