The Codopant Effect On Hydrogen Storage Properties Of Naalh₄

NaAlH4has received a significant attention and has been extensively researched due to its high theoretical hydrogen storage capacity and relatively low dehydrogen temperature. However, its commercial utilization was hampered by the poor reversibility and hydrogen/dehydrogen kinetics. Recent studies indicated that the hydrogen storage capacity of NaAlH4can be greatly improved by dopants. Thus, in this paper, the effects of complex dopants on NaAlH4were investigated using XRD and gaseous hydrogen storage properties test methods. The mechanisms of different kinds of dopants were explained by First-Principles calculation. The main results are obtained as follows.The doped-NaAlH4was prepared by ball-milling NaH/Al with TiF3/ZrCl4under hydrogen atmosphere. The hydrogen storage properties of NaAlH4doping by TiF3, ZrCl4or by a combination of both have been investigated. The results show that co-doping exhibited synergy effect. Moreover, the co-doping sample shows lower dehydrogen temperature and better kinetic properties than that of the single doping. The hydrogen storage properties of NaAlH4doping by different ratios of the two dopants were tested to further understand of the synergy effect. The results show that the sample doped with4mol%TiF3and1mol%ZrCl4has the better kinetic performance. At the temperature of180℃, the sample desorbed more than62%of the total hydrogen capacity in five minutes. The activiation energy is83.27kJ/mol. Furthermore, the main reasons for low the hydrogen capacity are the incomplete hydrogen reaction and the formation of dead materials halide sodium according to the analysis of sample composition.The TiN and ZrN are good dopants as they can not reaction with NaH or Al. Based on the before achievements, the dehydrogen performance and microstructure of the co-dopants system TiN and ZrN were investigated. The results show that the sample doped with4mol%TiN and1mo1ZrN has the better performance than the single doped ones. Moreover, the TiN and ZrN are stable dopants which can stably exist in hydrogen/dehydrogen circulation.The effects of Ti, Zr single and co-doped of NaAlH4have been studied by means of First-Principles theory calculation. By the calculation of defect formation energy, the defect TiNa was found to be easier to formation than TiAl. Thus, Ti is prefer to substitution Na rather than Al when it enter the body of NaAlH4. The complex defect TiNa+ZrNa is easy to formation for the Ti, Zr co-doped. By the calculation of doped structures and analysis of the changes between density of state, charge density and bond order, the doped atoms were indicated to have interplay with the AlH4. The interaction between H and doped atom is stronger than the interaction between H and Al. For the three complex co-defects TiNa+TiNa, ZrNa+ZrNa and TiNa+ZrNa, the level of weaken for Al-H in TiNa+ZrNa is the biggest. So, the two elements complex defect have the bigger influence on NaAlH4than the single ones, and it may beneficial for their decomposes.