Theoretical Study On The Effect Of O Vacancy On The Hydrogen Storage And Diffusion Properties Of LaFeO₃

ABO₃ perovskite oxide can be used as a new cathode material for NiMH batteries because it has good reaction activity,high discharge capacity,good hydrogen storage.It has strong application value,which has aroused widespread attention in related research fields.Since the ABO₃ type oxide has a special crystal structure,oxygen vacancies are easily generated,so that the material exhibits a variety of physical and chemical properties.However,the mechanism of the effect of O vacancies on the hydrogen storage properties of the La FeO₃ system remains unclear.In this paper,based on the transition state theory,studies on the effects of an O vacancy on hydrogen storage in LaFeO₃ are of considerable interest,the adsorption of H atoms in the La Fe O₃?010?surface system with an O vacancy is investigated,and the diffusion process of H atoms from the surface to the interior of the bulk is explored.By calculating the vacancy formation energy of the unit cell and the LaFe O₃?010?surface system,it was found that the O vacancy is more easily formed on the surface layer of the surface system.H₂ molecules have four kinds of adsorption modes on the LaFeO₃?010?surface with an O vacancy.The first adsorption mode is that the H₂molecules are completely dissociated adsorbed onto two different O atoms to form the two-OH groups.Second,the two H atoms are adsorbed onto an O atom,forming H₂O.The H₂O molecule is adsorbed on the surface of the defect by means of physical adsorption,so it is easier to separate from the surface of the defect-containing an O-vacancy,thereby forming an O-vacancy again,resulting in a secondary defect of the crystal surface.Third,the H₂ molecule is dissociated and an H atom is adsorbed onto an O atom to form-OH,where H-O is a covalent bond;another H atom is adsorbed onto a Fe atom to form a Fe-H metal bond.The fourth adsorption mode is a mechanism between physical adsorption and chemical adsorption and finally forms a FeH₂ structure.The transition state search calculations for the first three chemical adsorption processes were found the activation energy barrier exists in the three kinds of adsorption and dissociation processes,which means that the reaction experiences some difficulty in spontaneous occurrence,and external conditions are needed to stimulate these reactions.Compare the activation energy barrier values found the activation energy barrier of-OH and Fe-H is the lowest,which indicates that the reaction can easily occur.Therefore,the main reaction channel for dissociation adsorption is the third adsorption method that forms the-OH and Fe-H bonds.With the presence of O vacancies,the hydrogen absorption capacity of Fe atoms in LaFeO₃is improved,and the interaction between Fe-H is enhanced.It was found that the preferentially selected adsorption site of H atom is on the surface O atom of the LaFeO₃?010?plane containing an O vacancy,but the Fe atom can also be adsorbed.The adsorption capacity of H atoms in the surface layer is the largest,and the adsorption capacity of H can be reduced with the deep diffusion to the bulk phase.The adsorption of H atoms in the system is linked to the nearest neighbor atom with a weak covalent bond or a metal band.The O,Fe atoms in the surface all can adsorb H atoms,H atoms are mainly stored near the O atom in the bulk phase.The diffusion of H atoms from the surface layer to the bulk phase is carried out around the O atom,and the rotation jumps,continuously progressing deeper into the body phase.The energy barrier required for the diffusion of H atoms from the surface to the bulk phase is calculated.It can be seen that the step-by-step diffusion path and the direct diffusion path both have positive activation energy barrier in the H atom from the surface to the sub-surface diffusion process.And the step diffusion is smaller than the direct diffusion activation energy barrier,and the reaction is more likely to occur.If the H atom spans the barrier from the surface to the sub-surface,it will be easier to spread inward.The presence of O vacancy can reduce the activation energy barrier of diffusion of H atoms from the surface to the subsurface and improve the diffusion properties of the LaFeO₃?010?surface system.