(42c) Theoretical Thermodynamic and Kinetic Studies of Hydrogen Desorption in Ti-Doped Sodium Alanates

Pristine and titanium-doped sodium alanates are one of the pioneering and widely researched materials for hydrogen storage. Limitations related to the high temperatures for release and the slow release rate of hydrogen that are inherent in pristine sodium alanates can be overcome with the use of titanium. This study aims at theoretically understanding the thermodynamic and kinetic profiles of Ti-doped sodium alanates.

The formation of TiAl3 species is evident and stoichiometrically favorable from the atomic compositions of titanium and aluminum in Ti-doped NaAlH4 models. However, the pathways for TiAl3 formation or the intermediate phases that result and take part in dehydrogenation reactions are not completely understood. Six thermodynamic reaction pathways in Ti-doped sodium alanates that lead to the formation of TiAl3 and hydrogen release are studied in this work to propose the most favorable one. Of all the possible reaction pathways, the three-step reaction that proceeds with aluminum hydrides as intermediates in the first, TiAl in the second, and the final product TiAl3 in the third step of the reaction is the most favorable one. This reaction pathway leads to the formation of TiAl3 and aluminum in its +3 oxidation state present in aluminum hydride species responsible for the formation of Ti-Al alloys.

In this work, the free energy barriers associated with the reactions involved in the evolution of hydrogen via the first step of decomposition of pristine sodium alanates (NaAlH4→1/3Na3AlH6+2/3Al+H2) are also studied. Results from our calculations suggest a four step reaction that includes the transition from AlH4- to AlH63- anions, Al clustering and H2 evolution. The free energy barrier associated with one molecule of H2 release lies in the range of 80-124 kJ/mol H2 and the enthalpy of activation lies between 82-120 kJ/mol H2. The rate determining step for this mechanism is found to be the hydrogen evolution from associated AlH3 species. The role of Ti dopants in improved kinetics of Ti-doped sodium alanates is elucidated from our DFT/DFT-MD calculations. Ti dopants stay on the surface; serve as catalytic species in splitting hydrogen from AlH4/AlH3 groups as well as initiators for Al nucleation sites in Ti-doped NaAlH4.