Density functional theory study of the effect of 3d and 4d transition metals on NaMgH₃ complex metal hydride for hydrogen storage applications

In order to achieve the Hydrogen Economy, it is necessary to use hydrogen as an energy carrier. Hydrogen as an energy carrier is a possible solution to neutralize the climate changes occurred due to carbon emission. It is also highly attractive to ensure a smooth and efficient provision of energy without depending on other nations. Vehicles powered by fuel cells are one of the applications in which hydrogen can be used as an energy carrier. However, the absence of a commercially viable hydrogen storage technology is delaying the large-scale use of fuel cell powered vehicles. In order to promote research in this area, the U.S. Department of Energy (DoE) has set targets for the year 2010 and 2015 for a system of on board hydrogen storage. Previous results suggest that the addition of transition metals enhances the desorption kinetics of complex metal hydride and lowers the hydrogen dissociation temperature and activation energies. In the present work, study of catalytic additives with improved performance over Ti allows us to generalize the work on the procedure for 3d/4d doping in NaMgH3 and use it in systems with higher gravimetric densities and more favorable thermodynamics. For this purpose, plane wave density functional theory (PW-DFT) calculations helps us examine bulk and surface models of NaMgH3, Mg2FeH6 and KMgH3 complex metal hydride (CMHs) with impurities from the 3d element block. In this respect, first-principles calculations determined the cohesive energies of pure and doped bulk/surface CMHs as well as the adsorption/substitution energies. Furthermore, DFT coupled Molecular Dynamics (DFT-MD) calculations at elevated temperatures elucidate the effect of Ti placed on different sites on surface models. The results indicate that 5 out of 7 additives at the bulk model are thermodynamically stable relative to the pure model; therefore, possible. The most favorable elements in this regard came out to be; Ti, V, and Co in that order. Afterwards, from dynamics calculations performed at surface slab models, we conclude that Ti additives replacing sodium lattice sites and Ti additives placed at a four-fold vacant position are the stable arrangement in this system with potential formation of TiMgxHx complexes. Meanwhile, alloyed systems with 4d transition metals may be extremely stable for on-board hydrogen storage applications. Also, dynamics calculations performed at the surface slab models suggests that Ti additives replacing sodium lattice sites and Ti additives placed at a four-fold hollow site are the preferred, stable arrangements in NaMgH3 with possible formation of TiMgxHx complexes. This conclusion may serve as a guideline for the future design of nano-structured complex hydride for on-board hydrogen storage utilization.