Ab Initio Molecular Dynamics Simulation of Oxygen Adsorption and Electron Transfer on SOFC Cathode

Oxygen reduction reaction (ORR) at the oxide cathode of the solid oxide fuel cell (SOFC) is an important factor affecting the reaction rate of the whole processes. Dense oxide cathodes with mixed ionic and electronic conductivity are reportedly believed that the surface reaction is the rate-determining step in the series of cathode reaction process [1]. Recent experimental ac impedance measurement shows the surface reaction rate constant for the slowest step with 1/2 dependency of the partial pressure of oxygen regardless of cathode oxide materials [2,3], which suggests that the reaction between the absorbed peroxide ion (O₂²⁻(ad)) and the oxygen vacancy (Vö) at the cathode surface is a rate-limiting reaction. However, it is not straightforward to  observe such elementary steps experimentally under the operation condition of the SOFC since these complicated processes happen within a short time scale. Therefore, the discussion based on the atomistic point of view is desired to confirm the knowledge from the experimental measurement.

Under such circumstances, we have investigated the proton dynamics on the perovskite oxide surface, the dissociation process of hydrocarbons and alcohols on metal surfaces and so on by the ab initio molecular dynamics (MD) simulation, which takes into account both the chemical reaction with the accuracy of the ab initiocalculation and the dynamics of atoms simultaneously. Nickel oxide (NiO) lattices with and without the Schottky pair are employed as model systems here to simplify the system without the effect of other factors such as dopant and so on.

At the initial stage, the oxygen molecule easily absorbs on the nickel atom on the NiO surface as the monodentate state and changes relatively quickly to the bidentate adsorption regardless of the existence of vacancies. Then, the bidentate oxygen molecule in the vicinity of the oxygen vacancy dissociates easily while the bidentate oxygen molecule on the NiO surface without vacancy does not dissociate. The Mulliken population analysis reveals that nickel atoms next to the oxygen vacancy can donate more electron to the adsorbed oxygen molecule than those in the bulk NiO, which results in the dissociation of the oxygen molecule near the oxygen vacancy.

Moreover, the NEB analysis reveals that the monotonous energy decreases along the reaction path of dissociation of the oxygen molecule on the NiO surface with vacancies, whereas the activation energy of about 0.6 eV is necessary for the dissociation of oxygen on the NiO surface without vacancy. Since the dissociation takes place quickly after monodentate and subsequent bidentate adsorption, associated with the experimental results of the ac impedance measurement for the various cathode materials [2,3], it is concluded that the probability of meeting of O₂^2-(ad) and Vö determines the reaction rate of the ORR.

References

[1] T. Kawada, K. Masuda, J. Suzuki, A. Kaimai, K. Kawamura, Y. Nigara, J. Mizusaki, H. Yugami, H. Arashi, N. Sakai, H. Yokokawa, Solid State Ionics 121 (1999) 271.

[2] A. Takeshita, S. Miyoshi, S. Yamaguchi, T. Kudo, Y. Sato, Solid State Ionics 262 (2014) 378.

[3] A. Takeshita, Master thesis, The University of Tokyo (2014).