Role of oxygen vacancy in dissociation of oxygen molecule on SOFC cathode: Ab initio molecular dynamics simulation

The oxygen reduction reaction (ORR) at the cathode of the solid oxide fuel cell (SOFC) is investigated by the ab initio MD simulation, focusing on the role of the oxygen vacancy on the oxygen dissociation and reduction reaction. The oxygen molecule easily adsorbs on the nickel atom at the NiO surface as the monodentate superoxide ion (O2−(ad)), which turns quickly to the bidentate peroxide ion (O22 −(ad)) regardless of the existence of vacancies at surface. The bidentate O22 −(ad) in the vicinity of the oxygen vacancy dissociates easily, while that on the surface without vacancy is kept undissociated. Nickel atoms next to the oxygen vacancy have more electrons compared with those in the bulk NiO, and therefore, are able to donate more electrons to the adsorbed oxygen molecules, which results in the dissociation of the oxygen molecule. Detailed Mulliken charge analyses in term of bond overlap population on the dissociation process have revealed that dissociation undergoes with two series steps of O22 −(ad) via O− state either in the form of V••O or O−(ad) into O2 − state in the form of V×O or O2 −(ad). Moreover, the NEB analyses reveal that the energy decreases monotonically along the reaction path of oxygen dissociation on the NiO surface with vacancies, while there exists the activation energy of about 0.6 eV in the case without vacancies. Since the dissociation of O22 −(ad) takes place quickly upon encounter with oxygen vacancy at the surface, it is expected that the probability of meeting O22 − and V••O per time and area determines the reaction rate of ORR.