The origin of magnetization-caused increment in water oxidation

In recent years, using an external magnetic field to promote the oxygen evolution reaction (OER) on magnetic catalysts has attracted great attention. It is an appealing topic, but with a puzzle where the increment comes from. The early theoretical work has predicted that the production of triplet dioxygen in OER can be facilitated by the spin-polarization effect in a single domain of magnetic catalysts. Recent progress has revealed that the M-O radical species is the key for promoting the triplet dioxygen formation, which is evident by the pH-dependent enhancement by magnetization. However, magnetization of a ferromagnetic material only changes its magnetic domain structure in the case of bulk ferromagnetic catalytic structures. It does not directly change the spin orientation of unpaired electrons in the material. Thus, where would be the OER enhancement coming from? The confusion further lies in that each magnetic domain is a small magnet and theoretically the spin-polarization promoted OER already occurs on these magnetic domains, and thus the enhancement should have been achieved without magnetization. Here, we demonstrate that the enhancement comes from the disappeared domain wall between magnetic domains. The proportion of domain wall occupied surface before magnetization determines the degree of enhanced OER by magnetization. Due to the spin-facilitated promotion effect, the OER on surface occupied by magnetic domains are more favourable than that on surface occupied by domain walls. Magnetization leads to the evolution of the magnetic domain structure, from a multi-domain one to a single domain one, in which the domain wall disappears. Consequently, the occupied by the domain wall is reformatted into one by a single domain, on which the OER follows the spin-facilitated pathways and thus the overall increment on the electrode occurs. This study fills the missing gap for understanding the magnetic field enhanced OER and it further explains the type of ferromagnetic catalysts which can give increment by magnetization.