The origin of magnetization-caused increment in water oxidation

Abstract

Magnetization promoted activity of magnetic catalysts towards the oxygen evolution reaction (OER) has attracted great attention, but remains a puzzle where the increment comes from. Magnetization of a ferromagnetic material only changes its magnetic domain structure. It does not directly change the spin orientation of unpaired electrons in the material. The confusion is 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 upon magnetization. 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. The surface 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 spin-polarized OER and it further explains the type of ferromagnetic catalysts which can give increment by magnetization.

Spin-Modulated Oxygen Electrocatalysis

“…To clarify this puzzle about why magnetization can enhance the OER activity of multidomain catalyst under an external magnetic field, Xu et al investigated how magnetization can boost the OER activity of multidomain catalysts. They discovered that the enhancement of OER under the magnetic field is closely related to the domain walls . The researchers prepared NiFe thin films with the thickness varying from 200 to 800 nm (Figure F) and discovered that the magnetic domains have a stripe shape and the neighboring domains have an antiparallel direction (Figure G).…”

“…(J) The magnetic moment direction in neighboring domains and the domain wall between them. (K) The areas of magnetic domains and domain walls in NiFe films with different film thicknesses (Reproduced from ref . Copyright 2023 Springer Nature).…”

“…They discovered that the enhancement of OER under the magnetic field is closely related to the domain walls. 118 The researchers prepared NiFe thin films with the thickness varying from 200 to 800 nm (Figure 10F) and discovered that the magnetic domains have a stripe shape and the neighboring domains have an antiparallel direction (Figure 9G). As the film thickness decreased, the width of the domain gets shrunk, and thus, the area of domain walls increases.…”

Flexomagnetic Effect Enhanced Ferromagnetism and Magnetoelectrochemistry in Freestanding High-Entropy Alloy Films

“…Magnetism is also particularly attractive for tuning spin-related electron transfer in electrochemistry, especially in magneto­electrochemistry that couples the external magnetic field with electrocatalysts to control and understand the electrochemical reactions. − Unfortunately, numerous 3 d -transition-metal materials are diamagnetic or paramagnetic at room temperature, which significantly impedes the application of the magnetic boosting strategy . Noteworthily, introducing external inhomogeneous strain gradients (flexomagnetic effect) in electrocatalytic film can lead to phase transition and spin-reorientation following the atomic displacements, − thus modifying the magnetic ground state, and a flexomagnetism-driven magnetization enhancement is expected to be observed. ,, Furthermore, when an external magnetic field is applied to this engineered magnetic film, the enhanced magnetization can contribute to a more substantial electron transfer by weakening the scattering of electrons, − resulting in magnetic responses and a better activity. However, an experimental design for combining flexomagnetism with electrocatalysts is still lacking.…”

Spin-Selective Oxygen Evolution Reaction in Chiral Iron Oxide Nanoparticles: Synergistic Impact of Inherent Magnetic Moment and Chirality

“…While f-Fe 3 O 4 can manifest intrinsic spin polarization without external magnetization, , the introduction of H ext triggers domain wall reconstruction, transitioning these into a single domain and consequently boosting the OER activity. Conversely, superparamagnetic systems, such as s-Fe 3 O 4 , require the presence of a H ext to manifest spin polarization, as thermal fluctuations intrinsically disrupt their single-domain magnetizations. , Consequently, an improvement in the OER activity observed in f-Fe 3 O 4 and s-Fe 3 O 4 is logical.…”