The reduction reaction of diatomic oxygen molecules (O₂(g)) to split into monatomic oxide ions (O^2-) is attracting attention for the oxide cathode reactivity to reduce cathodic over-potential in solid oxide fuel cells and other electrochemical applications. Various experimental and theoretical approaches have been reported to understand the rate determining step (RDS) of the oxygen reduction reaction (ORR) on the surface of oxide cathodes, mainly composed of 3d transition metal like (La, Sr)CoO₃. Although it is generally accepted that RDS of ORR on oxide cathodes is the process for a surface adsorbate of to meet with surface oxygen vacancy (Vö(s)) [i], an atomistic analysis on the adsorption of oxygen molecule and the splitting reaction is still under debated; how the electrons are transferred and how the splitting of oxygen molecule proceeds. On the other hand, the most stable form of O₂(g) is so-called “triplet” oxygen, which has a pair of up-spin electrons in two orbitals and the highest molecular orbital of is completely unoccupied. This electron configuration indicates that, the oxygen molecule, during ionization process, initially accept only two down-spin electrons to form bonded to surface Ni ions, and subsequently a pair of up- and down-spin electrons to dissociated into two O^2-on surface. One may, then, come up with a simple question whether or not oxygen molecules have such spin preference during ORR. If it is true, ORR on oxide is, to a certain extent, influenced by the spin polarization of donated electrons, and therefore, the spin configuration of the oxide surface.

We employed ab initio molecular dynamics (ai-MD) simulation and a nudged elastic band (NEB) density function theory simulation to make precise analysis on the electron transfer process. The material we chose was nickelous oxide (NiO), which is well known to exhibit an anti-ferromagnetic (af) order below Néel temperature around 523 K and becomes the disordered paramagnetic (pm) state above the temperature. The ai-MD simulation was carried out using Ni₂₄O₂₄ and Ni₂₃O₂₃ supercells with and without Vö(s), respectively, connected to a vacuum cavity placed with 8 O₂(g) molecules above NiO slab. In the latter case, Vö(s) was introduced as a Schottky pair to maintain charge neutrality. The ai-MD simulation was made with and without spin polarization corresponding to af- and pm-NiO.

The simulation results show that, when a O₂(g) molecule approaches to the NiO surface, O₂(g) is immediately charged with electrons, primarily owing to the increase of the coordination number of Ni^2+/3+ ions, from monodentate to bidentate regardless of the presence of Vö(s) and spin polarization [ii]. No splitting reaction was observed on surface without Vö. The adsorbates further dissociated on the surface of pm-NiO with Vö, but no splitting reaction proceeds on the surface of af-NiO even with the presence of Vö. The NEB simulation suggests that almost monotonous decrease of total energy with the increase of coordination number for pm-NiO, but a thermally activated process with its activation energy of 0.7 eV is necessary for the af-NiO case, because of additional spin dipole moment for the latter. Also observed was spin reversal during the electron transfer. The results indicate significant influence of spin polarization on ORR. A strong effect of af-order on the oxygen chemisorption on undoped and Li-doped NiO was discovered in late 60’s, but no reasonable explanation has been given [iii]. A similar strong influence of spin order of oxide cathode is anticipated in other 3d transition metal oxide systems with ferromagnetic and antiferromagnetic configuration, and the extent of “spin buffering capacity” seems a quite important factor that govern the reactivity of ORR on oxide surface.