Computational study of hydrogen adsorption on lanthanum ferrite (lafeo3) (0 1 0) surface

Thumbnail Image
April, 2016
Journal Title
Journal ISSN
Volume Title
The electrochemical hydrogen storage mechanism of LaFeO3, a functional material widely studied as potential negative electrode material in nickel-metal hydride batteries, still remains unsettled, and requires more simulation studies to provide molecular-level insight into how the hydrogen adsorbs on the LaFeO3 perovskite oxide surface. Spin-polarized density functional theory calculations with the PBE exchange-correlation functional within the GGA and GGA+U using the plane-wave pseudopotential PWscf code in the Quantum-ESPRESSO package have been used to investigate the bulk structures and electronic properties, as well as the adsorption of H2 on lanthanum ferrite (LaFeO3). Adsorption energy calculations predict that for the use of LaFeO3 as anodic material in Ni-MH batteries, the most preferred adsorption site for H2 is the Fe-O bridge site where the O coordinates to another Fe atom and a La atom, and the Fe in the bridge coordinates to two other O atoms and two La atoms. When one molecule of the H2 is adsorbed at H-H molecular, dihydrogen and dihydride bond distances on this Fe-O bridge site of the LaFeO3 (0 1 0) surface, -Fe(µOH)La and -FeH are formed with adsorption energy in the range -0.455 to -0.456 eV. Similar adsorption sites do not necessarily give the same products on addition of H2. Even though some Fe-O bridge sites give -Fe(µOH)La and -FeH others give LaOH and FeH. At the O top end-on sites, while some give LaOH and FeH, others give H2O or FeOH depending on the H-H distances. All the O-top sites with the exception of that which coordinates to only La at the terminal position form H2O in at least one of the forms of H2 in an end-on attachment to O atom, leaving the surface to create an oxygen vacancy. Nonetheless, it is only at the O site which connects just two La atoms and in the dihydride H-H bond distance end-on configuration that involves dislodgement of H2O from the surface and vacancy hopping oxygen transport. Molecular hydrogen on the LaFeO3 (0 1 0) surface can only form H2O at the O top end-on site where the O coordinates to two Fe atoms and a La atom. Fe and La dihydrogen and dihydride bonds do not form on LaFeO3 (0 1 0) surface and also no hydrogen is adsorbed on the La atom. O atom facilitates the addition of hydrogen on the surface. Adsorbing another H2 molecule on LaFeO3 (0 1 0)/H2 system for a more realistic adsorption reveals that the adsorption at Fe-O bridge* site where the second H2 is at a dihydride bond distance leads to the formation of H2O through an unstable and unfavourable adsorption process. Fe changes oxidation state from +3 to +2 when one and two molecules of H2 are adsorbed. These results improve our understanding of the electrochemical hydrogen storage mechanism of LaFeO3.  
A thesis submitted to the Department of Chemistry, College of Science, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirements for the award of the degree Master of Philosophy in Physical Chemistry