Revealing Spin Magnetic Effect of Iron-Group Layered Double Hydroxides with Enhanced Oxygen Catalysis

Abstract

The oxygen evolution reaction (OER) is the bottleneck limiting the reaction process of water splitting. The OER process involves the recombination of oxygen from diamagnetic singlet state OH or H2O to paramagnetic triplet state O2. The spin conservation for oxygenated intermediates must play an important role in the OER. However, the dynamic mechanism of magnetic field-induced spin polarization is still in its infancy. Herein, based on the spin-coupling interaction of iron group elements, three typical iron group layered double hydroxides (LDHs) were constructed to study the relationship among magnetic field, spin polarization, and OER activity. Combining experimental and theoretical studies, we revealed the spin-magnetic effect of iron group LDHs for enhancing the OER process. There is a positive correlation between the saturation magnetization and OER performance of iron group LDHs under different magnetic fields. The NiCoFe-LDHs (NCFL) endows the strongest OER activity (η10 = 230 mV) and saturation magnetization (Ms = 44 emu mg–1) compared with that of CoFe-LDHs (CFL, η10 = 372 mV, Ms = 21 emu mg–1) and NiFe-LDHs (NFL, η10 = 246 mV, Ms = 29 emu mg–1). The density functional theory calculations show that the Fe sites of NCFL endow a stronger spin-coupling interaction with OH, and Raman spectroscopy further proves the promotion for the formation of the O–O bond of NCFL. Applying an external magnetic field, due to the spin magnetic effect of iron group LDHs, the enhancement amplitude of OER activity is also positively correlated with the magnetism of the catalyst. NCFL has the strongest spin magnetic effect about −34.8 mV T–1 compared with NFL (−27.0 mV T–1) and CFL (−16.7 mV T–1). The overpotential of NCFL is only 206 mV under the condition of 700 mT magnetic field. In conclusion, we demonstrate the mechanism of underlying influence of the spin magnetic effect on the OER performance and provide insights into the relationship between catalysts and oxygenated intermediates. These insights would help to understand and design catalysts at the spintronic level.