Abstract
The oxygen evolution reaction (OER) is the bottleneck that limits the energy efficiency of water-splitting. The process involves four electrons’ transfer and the generation of triplet state O2 from singlet state species (OH- or H2O). Recently, explicit spin selection was described as a possible way to promote OER in alkaline conditions, but the specific spin-polarized kinetics remains unclear. Here, we report that by using ferromagnetic ordered catalysts as the spin polarizer for spin selection under a constant magnetic field, the OER can be enhanced. However, it does not applicable to non-ferromagnetic catalysts. We found that the spin polarization occurs at the first electron transfer step in OER, where coherent spin exchange happens between the ferromagnetic catalyst and the adsorbed oxygen species with fast kinetics, under the principle of spin angular momentum conservation. In the next three electron transfer steps, as the adsorbed O species adopt fixed spin direction, the OER electrons need to follow the Hund rule and Pauling exclusion principle, thus to carry out spin polarization spontaneously and finally lead to the generation of triplet state O2. Here, we showcase spin-polarized kinetics of oxygen evolution reaction, which gives references in the understanding and design of spin-dependent catalysts.
Highlights
The oxygen evolution reaction (OER) is the bottleneck that limits the energy efficiency of water-splitting
Mechanisms and guided the subsequent design of highly active catalysts.[8,9]. This principle was further supported by the findings that the OER activities of transition metal oxides correlate strongly with the eg occupancy, which is related to the binding strength between the metal and the oxygen species.[10,11,12]
We have found that the spinpolarized kinetics of OER starts at the first electron transfer step, where ferromagnetic exchange happens between the ferromagnetic catalysts and the adsorbed oxygen species under the principle of spin angular momentum conservation
Summary
We begin with the discussion of the magnetic properties of the employed catalysts, CoFe2O4, Co3O4, and IrO2. The study of magnetic property reveals the optimal strength of the applied magnetic field for the alignment of the magnetic moment in ferromagnetic CoFe2O4. The CoFe2O4 and Co3O4 were prepared by a modified solid-state chemistry method as previously reported.[11] X-ray powder diffraction characterization was performed to confirm their crystal structures. C, d, the OER performance of the ferromagnetic CoFe2O4 is promoted obviously under the magnetic field while the changes in non-ferromagnetic catalysts Co3O4 and IrO2 are negligible. When a strong enough magnetic field (higher than the coercivity) is applied to a ferromagnetic material, the magnetic moment will (macroscopically) align along with the direction of the external magnetic field.
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