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 Ni2+/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.