Structural And Bonding Feature Of Ti Substituted Mg₂Ni And Mg₂NiH₄:First Principle Calculations

With the depletion of traditional energy, it is the research focus to search for replaceable and renewable energy. Considering of economic, technical, mobility, environment friendly factors, hydrogen energy is likely to one of the most promising energy. Hydrogen energy will also promote the development of a variety of renewable energies, leading to a new energy revolution. In order to realize the massive application of hydrogen energy, an effective way of hydrogen storage is particularly critical and challengeable. Magnesium alloys are considered to be one of the most promising hydrogen materials because of their high! storage capacity, abundance and low cost. However, the high hydrogen desorption enthalpy (64 kJ/mol·H2) and stability severely limit its practical applications. To reduce the hydrogen desorption enthalpy of Mg2NiH4, a substantial amount of compositional modifications have been carried out, and shown that doping the foreign elements into Mg2NiH4 is an efficient way to decrease dehydrogenation temperature and expedite kinetics of Mg2NiH4. In the present work, the structural feature and electronic structure of Ti co-substitution Mg2Ni and Mg2NiH4 are carried out from first-principles calculations based on density functional theory, revealing the destabilizing mechanism. Hence, it will provide helpful theoretical basis to further study the doped Mg2NiH4 with the foreign elements. Such investigation will gain a comprehensive insight into the structures and properties of complex hydrides, and provide a valuable clue to design advanced hydrogen storage materials. Our research content is roughly as follows:Firstly, the structural and electronic property of complex ternary alloy Mg11TiMgNi5TiNi has been investigated based on density functional theory. Substitution preference of Ti for both Mg and Ni are studied from the minimum of electronic energy. The calculated formation enthalpies demonstrated that the structural stability of Mg11TiMgNi5TiNi is medium between Mg11TiMgNi6 and Mg12Ni5TiNi, showing that Ti co-substitution for both Mg and Ni in Mg2Ni is thermodynamically more favorable than Ti single-substitution for Ni. The calculated chemical affinity factor is less than single substitution cases, implying lower stability of hydrides of Mg11TiMgNi5TiNi, so Mg11TiMgNi5TiNi can be predicted as the more favorable alloy for hydrogen storage. Further calculated electronic structure shows that the Ni-Ni and Ni-Mg hybridizations are lowered due to the strong Ti-Ni covalent bonding.Secondly, a comparative study of single- and co- substitution of Ti on dehydrogenation of Mg2NiH4 has been carried out from first-principles calculations based on density functional theory. In comparison with Ti single-substitution, the formation enthalpy in co-substituted Mg2NiH4 is higher, showing the lower thermodynamic stability. In Ti co-doped Mg2NiH4, the average Ni-H bond length of entire unit cell is larger, implying weaker Ni-H bonds and lower stability of complexes NiH44-. Ti co-substitution further shows a more favorable dehydrogenation effect on Mg2NiH4 due to lower hydrogen dehydrogenation energy, and significantly reduces the dehydrogenation reaction enthalpy of Mg2NiH4 to 70% compared to the pure state. Further calculated electronic structure demonstrates that the co-substituted Mg2NiH4 displays a metallic behavior with the Fermi level locating at the doping band, and the underlying mechanism for improving dehydrogenation properties of co-substituted Mg2NiH4 can be attributed to the weakened Ni-H interactions together with the narrowed energy gap.