Energetics and structure of single Ti defects and their influence on the decomposition of NaAlH4

Abstract

The energetics and structure of various types of single extrinsic Ti defects in NaAlH₄ bulk and (001) slab at the hydriding/dehydriding critical point environment were studied systematically. It is found that the most favorable situation is Ti substituting Al at the subsurface (Ti_Al(2nd)), which has the highest coordination number for extrinsic Ti ions. The most stable Ti defect in the 1st layer is located at the Al rich interstitial site, namely Ti_i(1st), accompanied with remarkable strength of Ti–H/Al bond and local geometry deformation at the 1st layer around Ti. Deeper insight of the formation mechanism of Ti defects is obtained by dividing the formation enthalpy of Ti defects into three terms, which are contributed from the cost of removing a substituted host atom if necessary, the cost of structure deformation, and the gain of bonding between Ti and its surrounding ions in the formation of the defects. This associates the formation energy directly with the local structure of Ti defects. For the first time, we adopt H_f(H), H_f(H–H), H_f(AlH₃) and H_f(Na) to discuss the hydrogen release ability of the Ti doped NaAlH₄. We find that TiAl₄H₂₀ and TiAl₃H₁₂ complexes are formed around Ti_Al(2nd) and Ti_i(1st) respectively, which significantly promotes the dehydriding ability of NaAlH₄. What is more, the catalyst mechanism of Ti on the decomposition of NaAlH₄ is linked to the AlH₃ mechanism according to our calculations.