Controlled Surface Reconstruction on Ferromagnetic Oxides: Spin Pinning Effect to the Oxyhydroxide Layer and Its Enhanced Oxygen Evolution Activity

Abstract

The production of hydrogen by water electrolysis suffers from the kinetic barriers in the oxygen evolution reaction (OER) that limits the overall efficiency. As spin-dependent kinetics exist in OER, the spin alignment in active OER catalysts is critical for reducing the kinetic barriers in OER. It is effective to facilitate the spin polarization in ferromagnetic catalysts by applying external magnetic field, which increases the OER efficiency. However, more active OER catalysts tend to have dynamic open-shell orbital configurations with disordered magnetic moments, without showing an apparent long-range interatomic ferromagnetism; thus controlling the spin alignment of these active catalysts is challenging. In this work, we report a strategy with spin pinning effect to make the spins in active oxyhydroxides more aligned for higher intrinsic OER activity. Such strategy bases on a controllable reconstruction: ferromagnetic oxides with controlled sulfurization can evolve into stable oxide<sub>FM</sub>/oxyhydroxide configurations with a thin oxyhydroxide layer under operando condition. The spin pinning effect is found at the interface of oxide<sub>FM</sub>/oxyhydroxide. The spin pinning effect can promote spin selective electron transfer on OER intermediates to generate oxygens with parallel spin alignment, which facilitates the production of triplet oxygen and increases the intrinsic activity of oxyhydroxide by ~ 1 order of magnitude. Under spin pinning, the spins in oxyhydroxide can become more aligned after magnetization as long-range ferromagnetic ordering is established on the magnetic domains in oxide<sub>FM</sub>. The OER kinetics are facilitated accordingly after magnetization, implying that the spin pinning effect is involved in the rate-determining step and this step is spin dependent. The spin polarization process in OER under spin pinning is also believed to be sensitive to the existence of active oxygen ligand (O(-)) in oxyhydroxide. When the O(-) is created in 1<sup>st</sup> deprotonation step under high pH, the spin polarization of ligand oxygens will be facilitated, which reduces the barrier for subsequent O-O coupling and promotes the O<sub>2</sub> turnover.

Low-temperature water electrolysis: fundamentals, progress, and new strategies

“…Gracia et al have systematically investigated and found that the spin-polarized electrons in electrocatalysts can promote the formation of parallel spin aligned oxygen by quantum spin-exchange interactions (QSEIs) to enhance the OER kinetics. 255,260,261 Various strategies have been developed to control spin selectivity and facilitate spin polarization of OER electrocatalysts, including functionalizing metal oxides with extrinsic chiral molecules as a spin polarizer with a chiralinduced spin selectivity (CISS) effect, 253,[262][263][264] engineering magnetic structures and domains of materials, 265,266 and promoting electron spin polarization and ferromagnetic ordering with external magnetic fields. 251,267,268 Recently, Garce ´s-Pineda et al reported direct magnetic enhancement of the OER on magnetic oxides in 1 M KOH electrolyte using a moderate magnet (r0.45 T) and suggested a hypothesis of magnetic field-induced spin selectivity effect accounting for the enhanced OER activity (Fig.…”

“…To address this challenge, a new strategy of constructing ferromagnetic-antiferromagnetic (FM-AFM) or ferromagnetic-paramagnetic (FM-PM) core-shell composite particles has been developed to employ the interfacial spin coupling with the spin pinning effect to control the desired spin polarization state of the surface metal oxyhydroxide layer and adsorbed oxygen intermediate species. 266 For example, composite particles consisting of FM Co 3Àx Fe x O 4 cores and PM Co(Fe)O x H y shells have been developed for electrocatalytic OER in 1 M KOH. The FM Co 3Àx Fe x O 4 core was first premagnetized to align the magnetic domains and establish long-range ordering.…”

Nickel-based Single-Molecule Catalysts with Synergistic Geometric Transition and Magnetic Field-assisted Spin Selection Outperforms RuO2 for Oxygen Evolution

“…Interestingly, NiGA under magnetic field outperformed most of the state-of-the-art OER electrocatalysts, including RuO2. 60 To identify the spin selectivity effect behind the observed magnetic field-induced enhancement and eliminate any possible contribution from other indirect effects, including magnetohydrodynamic (MHD) [95] and magnetic hypothermia effects, we conducted a series of control measurements. Magnetic hypothermia is only relevant in an alternating magnetic field and is therefore not applicable in this study, where we use a permanent magnet.…”

“…However, after Garcia's [53,54] prediction about the spin-dependence of electrocatalytic water splitting, significant research has explored magnetic catalyst-induced spin effect experimentally. Most spin-based enhancement has been reported in ferromagnetic bulk catalysts, [59,60] with only a few examples of atomic or molecular systems. [61,62] Coordination complexes are ideal for studying the spin effects in molecular catalysts because they possess a well-defined crystal field, and the orbital occupancy and binding energy can be tuned by selecting the ideal ligands and crystal structure.…”