Researches On Phase Relation And Defect Thermodynamics Of Ti Doped NaAlH₄ With First-principles Method

The hydriding/dehydriding performance of complex hydrides are often poor, which may be resulted from high kinetic barriers, or intrinsically resulted from high requirements in thermodynamic driving force. If the reaction process is hindered merely by the kinetic barrier, it can be improved by suitable catalysts. However, if it is intrinsically due to the latter reason, any further search for suitable catalysts may be futile and the material is impractical for reversible hydrogen storage.There are several hot issues on researches of high-capacity hydrogen storage materials of complex hydrides: hydriding/dehydriding thermodynamic properties (such as hydriding/dehydriding thermodynamic reversibility and destabilization); micro mechanism of hydriding/dehydriding reaction and catalyst mechanism.The stability and hydriding/dehydriding performance of hydrides are often influenced by their environment. Chemical potential can be utilized to describe the interaction of hydride and its environment. It can also connect equilibrium hydrogen pressure and thermodynamic characteristics of hydridind/dehydriding, thus, chemical potential can be a link between theoretical and experimental studies. It is as well as a link between the related macro and micro phenomena. Although its capacity does not meet the practical criterion, NaAlH₄ is undoubtedly a prototype for the study of other high-capacity hydrogen storage materials. In this dissertation, we have established and used equilibrium phase diagram characterized with chemical potentials to study the thermodynamic properties of hydrogen absorption and desorption with NaAlH₄ as researched object. We find that the chemical potential range for a stable hydride phase can be illustrated directily from the phase diagram. The hydridind/dehydriding critical point with the lowest chemical potential of H can be obtained, which is directly related with equilibrium hydrogen pressure. The hydridind/dehydriding equation can be expressed directily from the phase equilibrirum relation at critical point. The energy required for the exchange of a H2 molecular between hydride and environment can be read from the coordination of critical point, and the reaction enthalpy can be derived based on it. In addition, we can calculate formation energy of various defects in hydrides with consideration of the practical experiment environment.We propose an approch to estimate the equilibrium hydrogen pressure of a target hydride from a reference hydride, which takes advantage of both first-principles calculations and experiments. That is, by taking experimental Van't Hoff's line of a hydride as reference, we may obtain the equilibrium hydrogen pressure of the target hydride through comparing the critical points of the target and reference hydrides. As an example, we have obtained the equilibrium hydrogen pressure of LiAlH₄ at different temperature from the experimental Van't Hoff's line of NaAlH₄ and the theretical relation between the critical points of NaAlH₄ and LiAlH₄. The obtained result is more accurate than that directly calculated from first-principles method. With this new approach, the equilibrium hydrogen pressure of a new complex hydride can be obtained rather quickly to a more accurate degree. This provides an efficient way to screen new potential high-capacity hydrogen storage materials of complex hydrides with good hydriding/dehydriding thermodynamic reversibility.For the first time, we use equilibrium phase diagram characterized with chemical potentials to study the destabilization of transition metals (TMs) to NaAlH₄. It is found that the chemical potential of H at critical point increases as TM-Al alloys are formed with lower chemical potential of Al. This is a destabilization mechanism of TMs to NaAlH₄. In this way, the destabilizing ability of TM to NaAlH₄ can be easily distinguished. It can also be adopted to study the hydridind/dehydriding thermodynamic properties under different chemical potentials of TM, which is generally not available in conventional methods.It is known that the catalytic mechanism of Ti-contained additives to NaAlH₄ is related to the micro hydriding/dehydriding mechanism of NaAlH₄, yet none of the mechanisms are clear. We believe that the first step of NaAlH₄ decomposition is the formation of some H-contained vacancy, which can be studied through calculating the intrinsic defects of NaAlH₄ systematically. Both atomic and electronic chemical potentials are considered when intrinsic defects are calculated since NaAlH₄ is an ionic crystal with a wide band gap. It is found that among the defects which lead to decomposition of NaAlH₄, the formation possibility of AlH₃ complex vacancy, V(AlH₃), is the highest in both bulk and (001) surface of NaAlH₄. The structure of V(AlH₃) is"vacancy+AlH5"pair, and AlH5 is considered as a precursor of Na3AlH6. AlH₃ molecule is not stable under the chemical environment at critical point, thus it is easy to decompose after desorption from NaAlH₄. Overall, we believe that it is likely to have an"AlH₃ intermediate state mechanism"during decomposition.We also systematically calculate single Ti defects in bulk and (001) slab with different depth under the chemical environment of the critical point. Ti_Al(2^nd) and Ti_i(Al rich) are the two single Ti defects with high formation possibility accompanied by TiAl+3H(12) and TiAl₄H₂₀ complexes respectively. It is found that a deeper insight on the formation mechanism of single Ti defect can be obtained when the defect deformation enthalpies are divided into three terms which are directly related to the local structures of Ti defects. In addition, for the first time, we adopt the formation enthalpy of four vacancies [H_f(V_H), H_f(V_H-H), H_f(V_AlH3) and H_f(V_Na)] within and around the two Ti-Al-H complexs to investigate the impact of Ti defects on the decomposition of NaAlH₄. It is found consistently that the two single Ti defects can effectively reduce all the vacancy formation enthalpies of NaAlH₄, but mainly at the regions inside the TiAl4H20 complex or outside TiAl3H12 complex.