First-principles Calculations Of Defect Formation,Diffusion And H₂Dissociation In Chemical Hydrides

Chemical hydrides have received considerable attention because of their high volumetric as well as gravimetric densities. However, hydrogen release temperature is relatively high and the hydrogenation and/or rehydrogenation kinetics is slow making it not suitable for practical application. Understanding the breaking of hydrogen bond and diffusion of constituent atoms, which are the basic and essential steps in desorption process, from atomic scale can help to address which factors are the rate-limiting for hydrogen release and find scenario to adjust its kinetic rate.This thesis focuses on the (de)sorption-related processes in chemical hydrides, such as defect formation, defect transportation, hydrogen molecule dissociation. All results are obtained by a combination of first-principles calculations based on density functional theory and nedged elastic band method. Our results can address some basic scientific issues in chemical hydrides:(i) which kind of defects is formed favorably,(ii) what is the migration mechanism for dominant defects,(iii) what is the catalytic mechanism, etc. It is expected that the present results can guide the design of chemical hydrides for on-board applications. The major results are as follows.1. The effect of Ti atom on hydrogenation of Al(111) surfaceFirst-principles calculations are presented to explore the dissociation of H2and subsequent diffusion of atomic hydrogen over both clean and Ti-doped Al(111) surfaces. We find that Ti atom prefers to substitute one of the Al atoms from the subsurface layer. The activation barrier for the dissociation of H2over clean Al(111) surface is as high as1.28eV, while the energy barrier is reduced by about0.60eV when Ti atom substitutes for one of Al atoms from the surface or subsurface. However, H atoms are restricted by Ti strongly when Ti is in the surface layer. In the case of Ti locating in the subsurface layer, the diffusion barrier for H atom away from Ti atom is modearte. The present results can explain the experimentally observed improvement in absorption kinetics of H2through ball mill when Ti-based additives were introduced into NaAlH4.2. Hydrogen vacancy formation and diffusion in We have studied the formation and diffusion events of H vacancies with neutral and charged states in Na3AlH6on the basis of first-principles calculations. We find that hydrogen vacancy in Na3AlH6is in charged states. The barrier of local diffusion of H vacancy is lower than that of corresponding non-local diffusion, and H diffusion in Na3AlH6is dominated by mobility of positive H vacancies. Our results also verify that the observed highly mobile species in Na3AlH6in the anelastic spectroscopy experiments is probably positive H vacancies in the form of local diffusion.3. Intrinsic defect formation and migration in LiNH2Intrinsic defects in LiNH2are comprehensively studied using density functional theory in this thesis. We consider H-, Li-and N-related vacancies and interstitials in all possible charged states. Defect induced local structures and defect formation energies are examined. The energetically favorable H-and Li-related defects are in positively or negatively charged states, i.e., VH, Hi+, VLi-, and ILi+. The Li-related defects are predicted to diffusion readily. The predicted activation energies for self-diffusion show that the formation of H interstitial is the bottleneck for H transport and Li-related defects are the predominant diffusion species in LiNH2.4. The effect of F and Cl on desorption of MgH2Experimental evidence shows that TiF3or TiCl3is a better catalyst than TiH2and Ti in view of enhancing both the absorption and desorption kinetics in MgH2. This hints that halogen may have impact on desorption kinetics of MgH2. This work considers H-, Cl-and F-related defects in rutile MgH2. It is found that energetically favorable H-related defects in MgH2are positively and negatively charged vacancy; and the extrinsic defects F and Cl are prone to substitute for H atom in the neutral form. The energies needed to move all charged state of H atom near F or Cl are calculated. We find that F and Cl cannot lead to the decrease of removal energies for neutral and negative H atoms, but both of them can decrease obviously removal energy for positive H atom.