DFT study on hydrogen desorption from NaMgH ₃ (001) surface

NaMgH ₃ is the only magnesium based ternary hydride with Na and it has attracted the attention as a possible candidate for hydrogen storage material because it has high gravimetric hydrogen density (6%) and high volumetric hydrogen density (88 kg/cu m). Hydrogen disassociation in this material occurs at 1 atm and almost 670 K, which is well above the operable range for on board hydrogen storage applications and a major impediment for practical applications. To enhance the dehydrogenation process on transition metal, modified-NaMgH ₃(001) surface was studied by using density functional theory calculations. For the surface calculations, three types of models can be identified, i.e., hydrogen desorption from the surface of the NaMgH ₃ within the NaMgH ₃, and at the bulk/surface interface. Hydrogen desorption process is facilitated in NaMgH ₃ by doping the material with Ti at the surface site and Zn as a co-dopant at the bulk site. The dehydrogenation energy of the co-doped surface structure is low, in comparison to the pristine model. When doped with transition metals from the 3d and 4d block elements, the dissociation barrier can also be lowered, in some cases with better results than Ti and Zn. Carbon doping was the most promising based on the considerations of the activation barrier and dehydrogenation energy cost. The results predict that these types of model structures are potential useful materials for hydrogen storage application. This is an abstract of a paper presented at the 2012 AIChE Spring Meeting and 8th Global Congress on Process Safety (Houston, TX 4/1-5/2012).