Effects Of Substitution And Doping On The Properties Of Hydrogen Storage Materials （Mg₂NiH₄、NaAlH₄）:a First-Principles Study

On the basis of more promising hydrogen storage materials Mg2NiH4and NaAlH4, the first principles calculations have been performed to study the effects of substitution and doping on the properties of these hydrogen storage materials. In this paper, the main research results are as follows:1.The crystal structures, electronic and thermodynamic properties of Yb-, Ca-and Sr-substituted Mg2NiH4hydrides have been investigated by the first principles method. We have evaluated the relative stabilizing effect of rare-earth element Yb and alkaline-earth elements Ca and Sr by computing the hydrogen desorption energy. The results show that the stabilizing effect of Yb is smaller than that of Ca and Sr. Furthermore, to identify the decomposition reaction, two different decomposition reactions are discussed. The decomposition enthalpy and temperature calculations indicate that the reaction, MMgNiH4�'MH2+1/2MgNi2+1/2Mg+H2(M=Yb, Ca,Sr), is the feasible decomposition reaction under decomposition temperature. The analysis of hydrogen desorption energy, decomposition enthalpy and temperature suggests that rare-earth-based YbMgNiH4hydride shows better therm odyn am ic dehydrogenation properties than alkaline-earth-based CaMgNiH4and SrMgNiH4hydrides.2. The crystal structures, electronic and dehydrogenation properties of La-doped NaAlH4(001) surface have been studied by the first principles method. The LaAl4H18complex is found in the structure of La-doped NaAlH4(001) surface. The analysis of bond length and electronic properties shows that it is easy to form lanthanum aluminum compound mentioned by experimental report, and that the interactions of La-H and La-Al weaken the interaction of Al-H. The calculated hydrogen desorption energies in H1, H2, H3and H4positions of NaAlH4(001) surface are3.8754,3.8751,3.9430and3.9432eV, respectively. These values are all larger than the largest hydrogen desorption energy (the value of H11position is2.8476eV) of La-doped NaAlH4(001) surface. This further illustrates that La doping favors the dehydrogenation reaction for NaAlH4system.3. The crystal structures, electronic and dehydrogenation properties of TiB2cluster-doped NaAlH4(101) surface have been calculated by the first principles method. One complex TiB2-Al3H12-AlH4is found in the structure of TiB2cluster-doped NaAlH4(101) surface, and the AlH3and AlIH5unit in complex favors the first decomposition reaction of NaAlH4. Analysis of bond length shows that TiB2cluster is not easy to decompose into other compounds during the dehydrogenation reaction. Hydrogen desorption energies from many positions of the complex are reduced significantly from those of NaAlH4(101) surface. It is proved that the effect of TiB2cluster is extended to the AlH4unit which is next to the AIH4unit connected to Ti atom by calculating the hydrogen desorption energy. These results suggest that doping TiB2cluster is able to promote the release of hydrogen from the NaAlH4(101) surface.