However, there is increasing experimental evidence that the real active elements undergo electronic structural evolution under OER conditions, including charge, spin, orbital, and structure changes.

(4) These parameters are usually identified on the basis of the electronic structure of 3d transition metals (TMs) in pristine materials. Based on the volcano-shaped experimental data, many descriptors affecting the four-electron charge transfer steps on surface-active sites have been proposed, including orbital occupancy close to unity, (1) O 2p-band center relative to the Fermi level, (2) charge-transfer energy, (3) and metal–ligand covalency.

Understanding the mechanism and identifying the active sites of the OER are highly important for the design and development of efficient and robust electrocatalysts. The sluggish four-electron kinetics of the oxygen evolution reaction (OER) becomes a bottleneck for electrochemical water splitting. Increasing energy demands and environmental issues have prompted intense research on electrochemical water splitting for energy storage and conversion technologies. This study opens new avenues for the realization of electronic structure manipulation via unique 5f involvement. Density functional theory calculations demonstrated that the OER activity of Co 3+/4+ was synergistically enhanced by the covalent bonding of U 6+-5f in the U 5f–O 2p–Co 3d network. X-ray absorption spectroscopy revealed that the Co 2+ ion in pristine UCoO 4 was converted to high-valence Co 3+/4+, while U 6+ remained unchanged during the OER, indicating that only Co was the active site. UCoO 4 exhibits a low overpotential of 250 mV at 10 mA cm –2, surpassing other unitary cobalt-based catalysts ever reported. Herein, we report a new class of catalyst, namely, UCoO 4 with alternative CoO 6 and 5f-related UO 6 octahedra, as a unique example of a 5f-covalent compound that exhibits enhanced electrocatalytic oxygen evolution reaction (OER) activity because of the presence of the U 5f–O 2p–Co 3d network. Most studies have focused on regulating 3d transition-metal active ions through other d-block metals, while few have utilized f-block metals. Electronic structure modulation among multiple metal sites is key to the design of efficient catalysts.

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