Site Substitution of Ti in NaAlH4 and Na3AlH6

摘要

The hydrogen release and uptake kinetics of sodium alanate, NaAlH4, are greatly improved after doping with small amounts of transition metals, such as Ti. However, after 15 years of active research, an atomic-level understanding of the effect of Ti on the reaction rate is still lacking. One theory suggests that Ti atoms may be incorporated in bulk crystals of NaAlH4 where they exist as charged impurities, increasing the Fermi level, decreasing the formation energies of native charged defects, and therefore aiding in mass transport via bulk diffusion. Using first-principles density-functional theory (DFT) calculations, we examine the local structure, charge states, and energetics of substitutional Ti impurities in both NaAlH4 and its decomposition product, Na3AlH6. In contrast to previous studies which considered only simple metal substitutions, we study a much broader class of possible substitutions by varying the number of hydrogen ions bound to Ti and charge states of the resulting aggregate defects. The minimum energy configurations have been determined using ab initio molecular dynamics simulated annealing runs followed by conjugate gradientenergy minimization. Temperature-dependent free energies, which have been shown to be a necessary component when analyzing the equilibrium concentrations of native defects in sodium alanates, are included in this study. We find that the lowest Ti defect in NaAlH4 is a substitution on the Al site with two additional H atoms, in an overall q = - 1 charge state; it is compensated by the formation of positively charged A1H4 vacancies. As a result, the concentration of A1H4 vacancies is greatly increased at low temperatures, but this effect becomes less significant at typical temperatures of hydrogen release where the intrinsic concentration of AlH4 vacancies reaches that of substitutional Ti (e.g., around 350 K for 0.001% Ti). The predicted change in the Fermi level in NaAlH4 is not large enough to account for the experimentally observed lowering of the activation energy, ruling out bulk substitution of Ti as the main catalytic mechanism in Ti-doped sodium alanates. For Na3AlH6, we find a competition between two defect structures, one of them being a Ti substitution on the Na site in a q = +1 charge state and the other being a neutral Ti substitution on the Al site with an additional bound H. Ti substitution in Na3AlH6 is found to have insignificant effects on the Fermi level (less than 20 meV at T = 400 K) and concentrations of native defects.