Study On The Diffusion Mechanism Of H Atoms In LaFeO₃

As a new cathode material for Ni/H batteries, ABO₃ type oxide not only has good reaction activity, high discharge capacity and universal hydrogen storage phenomenon, but also is low cost, easily activated, and of good chemical stability, for which it has great potential application value. For ABO₃ type oxide, fundamental problems like H atom occupation,diffusion mechanism etc. are not apparent. Therefore, this paper takes the first-principle method to study the H₂ molecules in the adsorption properties of LaFeO₃（010）surface and occupation and diffusion of H atom in bulk phase of La Fe O₃（010）.The most stable adsorption surface of LaFeO₃（010） can be determined by calculating the surface energy. The results indicate that, there are three kinds of chemical adsorption modes of H₂ molecules on the surface of LaFeO₃（010）: The best adsorption occurs at O-O bridge location, where H₂ molecules are completely dissociated to be adsorbed on the surfaces of the two O atoms forming two-OH groups. At this position, the typical covalent bonds between H atom and surface O atom is formed through the orbital hybridization of H 1s and O 2p; The energy barrier of H₂ molecules dissociation is about 1.542 e V, indicating that the dissociative adsorption can be occurred only under certain thermal conditions. The second adsorption occurs at Fe-O bridge location, H₂ molecules are completely dissociated, where an H atom adsorbed on the O atoms to form-OH group, while the other H atom is adsorbed to the Fe atoms forming a metal bond. The third adsorption occurs at the top location of O atom,where two H atoms are adsorbed to the same surface O atom to form H₂ O molecules. However the surface oxygen vacancies can be easily formed after the H₂ O molecules escaping from the surface because of weak reaction in-between. In addition, H₂ molecules also can be physically adsorbed on La Fe O₃（010） surface.This article both studies the surface adsorption and bulk phase occupation of H atoms to LaFeO₃（010）. The results indicate that the best adsorption position of H atom is the top position of O₁ atom at（010） surface, at the same time, the Fe position can also store hydrogen;H atoms adsorbed on the surface of Fe atoms can easily spread to the adjacent O atoms,similarly H atoms can diffuse from the O atoms to the more stable adjacent O atoms, but the diffusion process can hardly occur between the Fe atoms. The best occupation position of H atoms in the subsurface layer is near the surroundings of O₃ atom, H diffusion from the surface to the subsurface, passing through the nearest neighboring position, which compared with the direct diffusion path is more favorable. The best diffusion path is: H atoms diffuse from the top location of the surface O₁ atom to the surroundings of O₁ atom of the subsurface,and then to the surroundings of O₃ atom of the subsurface. The best occupation position of H atoms in the third layer bulk phase possibly happen to be surroundings of O4 or O₅ atom.The diffusion of H atoms from the surroundings of O₃ atom to the O₅ atom can overcome thelower energy barrier, the surroundings of O₅ atom for the third bulk layer is the best occupation position, and H atoms diffusion from surroundings of the O₃ atom to the O₅ atom is the best diffusion path directed to the third layer bulk phase from the subsurface.Simultaneously, Fe atom in the third layer bulk phase can also store hydrogen, and H atom has tendency for O atom. Therefore, the best path for H atoms diffusion from the surface to the bulk phase is: H atom diffuses from the top position of the surface O₁ atom to the surroundings of O₁ atom in subsurface layer, then spread to the surroundings of O₃ atom in subsurface layer, and finally spread to the surroundings of O₅ atom in the third layer bulk phase. The total diffusion process is a gradual approach of H atom directed to the O atom inward, a transmission cycle of saltating and rotating.