(112d) Effect of Dopants in the Hydrogen Kinetics and Storage Capacities of Metal Hydrides

Among the major problems in making the hydrogen-powered fuel cell vehicles feasible in practice is the on-board storage system for hydrogen. The United States Department of Energy has set target system energy densities at value of 6.5 wt% (usable H2 weight > 6.5% of the storage system weight) and 62-kg H2/m3 by 2010. Ideally, storage and release of hydrogen should take place at temperatures between 0°C and 100°C and pressures of 1?10 bar and on time scales suitable for transportation applications. Although each current storage method possesses desirable attributes, no approach satisfies all of the efficiency, size, weight, cost and safety requirements for personal transportation vehicles. Research needs for improved compressed gas storage include the development of novel materials that are strong, reliable, and low in cost. Solid-state hydrogen storage (storage in metal hydrides, chemicals, and nanostructured materials) offers perhaps the best opportunities for meeting the requirements for onboard storage. While many metal hydrides are reported to be capable of meeting the gravimetric and the volumetric storage densities, favorable kinetics and thermodynamics (dehydrogenation of these metal hydrides at room temperature) is one factor that needs to be resolved in order to make these materials efficiently available for hydrogen storage. There is evidence of the benefits of adding dopants, such as carbon and titanium, to metal hydrides to improve their hydrogen kinetics and storage capacities. For instance, a remarkable improvement in hydrogen adsorption/desorption kinetics by MgH2 metal hydride is observed when it is ball milled with graphite/carbon [1]. Carbon was also shown to improve the hydrogen storage kinetics of Lithium Beryllium Hydrides [2], and Ti-doped sodium aluminum hydride when graphite was used as a co-dopant. In the later case, the graphite addition helped decrease the dehydrogenation temperature by as much as 15° C [3]. Hence, the goal of this work is to understand the effect of dopants, such as carbon and titanium, in the hydrogen storage kinetics of hydrides such as alanates and borates at an atomic level using theoretical/computational methods. First Principles Density Functional Theory and molecular simulations are used to characterize the role of dopants in the desorption kinetics of these hydrides. The effect of dopants on the metal-hydrogen bonding nature is studied using the total and the partial electronic density of states and the electronic charge distributions before and after doping.

1- S. Dal Toè , S. Lo Russo , A. Maddalena , G. Principi, A. Saber, S. Sartori, T. Spataru ?Hydrogen desorption from magnesium hydride?graphite nanocomposites produced by ball milling? Materials Science and Engineering B 108 (2004) 24?27. 2- Ghouri M.M. and Mainardi D.S., "The Role of Carbon in the Hydrogen Storage Kinetics of Lithium Metal Hydrides" In: Proceedings AIChE 2006 Annual Meeting, San Francisco, CA, Nov 12-17, 2006. 3- J Wang, A. D. Ebner, T. Prozorov, R. Zidan, J. A. Ritter ?Effect of graphite as a co-dopant on the dehydrogenation and hydrogenation kinetics of Ti-doped sodium aluminum hydride? Journal of Alloys and Compounds 307 (2000) 157?166.