| First-principles calculation method based on density functional theory has been fully developed in the recent years. Various programs and software packages are wildly applied in the fields of nature science, including condensed-matter physics, materials science, geology, chemistry and biology, etc. The method provides not only theoretical understanding but also important evidence to the designs of industrial materials. Now days, first-principles electronic structure calculations are among the most popular methods available in science studies.Borohydrides have potential applications on fuel cells and heat storages as they have high hydrogen gravimetric density. In addition, the high storage capacity, low price and light mass make them the most promising hydrogen storage materials.Borohydrides are ionic compounds and presented by chemical formula M(BH4)n (M=Li, Na, K, Mg, Ca, Ti, Al). LiBH4 and NaBH4 are the most potential hydrogen storage materials due to their higher capacity. However, they are thermally too stable for reversible hydrogen storage applications. Magnesium borohydride Mg(BH4)2 and calcium borohydride Ca(BH4)2 are ideal as they have partial reaction reversibility and are thermally less stable than alkali borohydrides. Thermal stability is a major key to the properties of hydrogen storage materials. Therefore, modifying thermal stability of borohydrides has been the object of many investigations of hydrogen storage properties in recent years.To develop borohydrides into suitable hydrogen storage materials, the destabilizing of borohydrides thermodynamically as well as enhancing of hydrogen decomposition kinetically is essential. In this paper, we calculate the electronic structure, formation energy, electronegativity, Bader Analysis, distribution of electrons and doping effects of borohydrides by first principles based on density function theory. Our calculations are performed using the Vienna Ab initio Simulation Package (VASP) and CASTEP module of MaterialsStudio4.3. The main content and results are the following:In the chapter 4, the lattice parameters, electron band structure and density of states, e lectron Localized Functions, Bader analysis, and formation energy of Borohydrides are calculated. In chapter 5, Ca(BH4)2 is selected as the studied object. The electronic structures of Ca(BH4)2 and Ca(BH4)2-M (M=Ti or Nb) are studied by first principles density functional theory calculations, and the dehydrogenation mechanism of Ti, Nb doping is also intensively investigated. The formation energy of dopants suggests that both Ti, Nb tend to occupy the interstitial sites. The electronic structure analysis shows that the interactions of Nb (Ti) and atoms in [BH4] groups generate intermediate phases which are conducive to the decomposition of Ca(BH4)2. The electronegativity calculation and Bader analysis also show Nb and Ti have good catalytic behaviors. By analyzing electron localized functions, [BH4] groups restructure after Ti or Nb is doped, and the catalytic function of Ti is relatively weaker than that of Nb. The calculated results are consistent with the catalytic dehydrogenation experiments of Ca(BH4)2.The catalytic mechanism of a doped element (Nb or Ti) is to restructure the oringal chemical bonds of a hydrogen storage material. The strong interactions between Nb (or Ti) and B atoms in adjacent groups will benefit the formation of Nb (or Ti)-B compounds and the escape of H atoms from [BH4] groups, and finally improve the dehydrogenation properties of Ca(BH4)2. The comparison of theoretical and experimental results confirmed the reliability and accuracy of the forecasts from theoretical calculations. |
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