| Hydrogen is one of the most promising energy source because it is sustainable,green and high-efficient.The magnesium,boron,and aluminum hydrides with high hydrogen content are potential hydrogen storage materials.Developments of catalysts to improve the dehydrogenation properties of typical high-content hydrides such as MgH2 and LiBH4 have attracted many attentions.Novel complex hydrides with great prospects,such as LiCa(AlH4)3,are also synthesized.From calculations based on density functional theory(DFT),the effects of Cu-dopant on MgH2 are studied,and the thermodynamic destabilization and the improvement of kinetic behavior is discussed.Moreover,the coexistence of Ti and VLi in LiBH4 is investigated,and the improvement on the dehydrogenation properties from complex Ti/VLi defect is also shown.Also,the crystal structure of LiCa(AlH4)3,especially the unknown position of H atoms is predicted,and the bonding character is revealed by calculation of electronic structures.The main contents of this thesis are:1.The dehydrogenation behavior of Cu-doped MgH2 system is investigated.The results show that the Cu dopant prefers interstitial sites in bulk and surfaces rather than substituting Mg.Moreover,Cu dopant forms CuH4 cluster with four adjacent H ions,which lengthens Mg-H bonds.The electronic structure further shows that Mg-H bond is ionic and Cu-H is covalent.Bader charge analysis show that after Cu-doing more electrons locate on Mg while fewer are on H.So the electron transfer from Mg to H is reduced by Cu,causing the weakening of the ionic Mg-H bond.The density of states(DOS)shows that in bulk MgH2 interstitial Cu causes stronger weakening of Mg-H bond than that at substitution site.However,in the DOS of the former case,the energy gap is large,which is unfavorable to the excitation of electrons from Mg-H bonding to anti-bonding states.Thus,Cu at substitution site is more effective to improve the dissociation of Mg-H bond.When doping Cu on surface,Cu 3d states hybridize with both Mg and H states,which weakens the Mg-H bonds.Apparently,Cu doping lowers the thermodynamic stability of MgH2.On the kinetic aspect,in bulk MgH2 Cu-doping reduces the formation energy of H vacancy(VH).Furthermore,Cu at substitution site lowers the diffusion barrier of H atoms,although such barrier is higher for H atoms close to interstitial Cu.Thus,Cu-doping,especially Cu at substitution site,could improve bulk H diffusion.Moreover,the activation energy of surface desorption(Eact)calculated from one-step approximation shows that Cu-doping dramatically reduces Eact,from 2.90 eV to 1.83 eV and from 2.25 eV to 1.48 eV on(001)and(110)surfaces,respectively.Therefore,the catalytic effect of Cu-doping on dehydrogenation of MgH2 is expectable.2.The dehydrogenation behavior of Ti and Li vacancy(VLi)co-existed LiBH4 system is also investigated.The calculated occupation energy shows that Ti prefers substituting Li in bulk while favors the interstitial site on(010)surface.Moreover,the occupation energy of Ti in bulk in much higher than on surface,indicating that Ti hardly enters into bulk LiBH4 lattice.The formation energy of Li vacancy(VLi)shows that it is very unlikely to form VLi in pure LiBH4,but there is significant mutual stabilization between Ti and VLi,which dramatically lowered the formation energy of VLi in Ti-doped system.On surface,such mutual stabilization is stronger,causing lower formation energy of composite Ti/VLi defect than individual Ti and VLi,indicating common existence of composite Ti/VLi defect.The calculated electronic structure shows that both Ti 3d and Li 1s states shift to lower energy range after the formation of composite defect which actually results in the mutual stabilization.Furthermore,the peaks of B-H hybridization are lowered by Ti,VLi,and composite defect,which leads to weakened B-H bonds and decreased thermodynamic stability of LiBH4.The composite Ti/VLi defect could also significantly lower the formation energy of VH in bulk LiBH4,implying that Ti/VLi could improve the dissociation of B-H bonds.The calculated H2 desorption energy(Ed)show that different choices of desorbing H atoms lead to completely different results even on pure surface,indicating possible competition between different desorption paths.Moreover,Ed on Ti/VLi decorated surface is much lower than that on pure surface,showing that the composite defect reduces the thermodynamic destability of LiBH4 surface.From kinetic point of view,the desorption barrier for H2 on Ti/VLi decorated surface is close to experimental results and lower than that on pure one.Therefore,Ti/VLi composite defect should be the critical factor affects the dehydrogenation properties of LiBH4.3.The crystal structure of LiCa(AlH4)3 is predicted based on the experimentally determined space group P63/m(No.176).The Wycoff position of H atoms are predicted at 12i and 6h.Also,several possible crystal structure from structural analogues in the inorganic crystal structure database(ICSD)are constructed and checked.The fi(?)l predicted crystal structure of LiCa(AlH4)3 is analogous to CdTh(Mo04)3,and the calculated crystal parameters agree well with experimental results.The crystal structure of LiCa(AlH4)3 can also be seen as hexagonal packing of AIH4 layers filled by Li and Ca within the triangle sites,similar to structure of Ca(AlH4)2 crystal.In comparison with LiAlH4,the Li-AlH4 interaction in LiCa(AlH4)3 is more covalent,while the Ca-AlH4 interaction is more ionic than in Ca(AlH4)2.The increasing of covalence for Li-AlH4 interaction is larger,causing weakened Al-H bonds comparing with LiAlH4 and Ca(AlH4)2.Moreover,hybridizations between Li and H are more localized in LiCa(AlH4)3,causing the weakening of Al-H bonds.Therefore,this novel mixed alanate is promising hydrogen storage material.Additionally,the dehydrogenation reaction pathway of LiCa(AlH4)3 is predicted by calculation of reaction enthalpy,providing theoretical guidance on further development of catalyst for LiCa(AlH4)3. |
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