Activating Octahedral Center in Co‐Doped NiFe₂O₄ via Bridging Amorphous MoS_x for Electrocatalytic Water Oxidation: A Case for e_g Orbital Regulation in Spinel Oxide

Abstract

Moderate e g filling for octahedral metal cations (M Oh ) is strongly correlated with the electrocatalytic water oxidation performance in the oxides system. Here, the e g fillings of Ni Oh and Fe Oh in NiFe 2 O 4 -based spinel are controllably regulated by introducing an external radical of catalytically inactive MoS x as an electron acceptor via a novel ultrasonic anchored pyrolysis strategy. The electron occupied in e g orbit of M Oh emigrates with the amount of Mo-S hanging on the apical of octahedral sites, and results in a salutary transition from high to medium e g occupancy state, as confirmed by the X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. In addition, benefiting from the abundant unsaturated S atoms in amorphous MoS x , the M Oh at the surface furthest activates and consequently shows a superior water oxidation performance. Density functional theory also reveals that the e g fillings of Ni and Fe decrease to 1.4 and 1.2 after MoS x modification, which can effectively reduce the free energy of the OOH* intermediates in the oxygen evolution reaction process. This work opens an avenue for further releasing the electrocatalytic activity of octahedral sites through bridging external phases with rational electron-capturing/donating capability.

Synthesis and advantages of spinel-type composites

“…155 Further, amorphous MoS x was also successfully grown in situ on the surface of Co-doped NiFe 2 O 4 (NFCO) via the same method, which proved the effectiveness and wide applicability of the ultrasonic-assisted reduction method. 156…”

“…Therefore, to take full advantage of each component, constructing spinel-based composites is regarded as an effective strategy for improved performance. 22,23…”

Enhancing Co–O Covalency by Low Electronegativity Anion-Induced Charge Rebalance in a (Co,Fe)S2/Co3O4 Composite for Efficient Oxygen Evolution Reaction

“…Inspired by our previous works, the electronic structure of TMOs can be efficiently optimized by anchoring a second phase, and the corresponding OER enhancement is usually explained by the beneficial electron transfer in a composite. − Nevertheless, whether the M–O covalency can be strengthened by bridging a suitable ligand is an uncertain issue to be answered. According to Tarascon et al’s idea, the less electronegative anion replacement in oxides can reinforce the overlap by shifting the energy level of the p band toward the metal d band .…”