Oxygen evolution reaction (OER) is a crucial half reaction of electrochemical water splitting and metal-air batteries. But its sluggish four-electron reaction leads to a high overpotential. Current commercial OER catalysts are mainly noble metal-based materials, but their high cost restricts their broad application. Therefore, extensive efforts have been devoted to exploring low-cost and efficient OER catalysts. Nonprecious metal-based materials have been regarded as promising OER catalyst candidates, due to their abundancy on the earth, controllable morphologies and tunable chemical states. Among various nonprecious metal-based materials, metal-organic frameworks (MOFs) have attracted much attention, because of their large specific surface area and rich metal centers. However, their poor electrochemical activities, stabilities and conductivities severely affect their application in OER catalysis. To improve the activities of MOFs, several methods have been adopted, such as synthesizing ultrathin nanosheets, growing MOFs on nickel foam or carbon cloth, doping heteroatoms, and introducing synergistic interactions between two materials. In 1970, Wagner proposed a space-charge theory, which indicates that the carrier property can be tuned through adjusting interface. Inspired by this theory, constructing metal oxide-catalyst interface seems to be a promising strategy to improve activities of catalysts. CeO₂ is a well-known cocatalyst due to its reversible Ce³⁺/Ce⁴⁺ redox. Previous works have demonstrated that OER performance can be effectively improved through introducing CeO₂ since it can speed up the electron mobility and induce strong interaction between CeO₂ and metal sites. In this work, an efficient OER catalyst was achieved through introducing CeO₂ into FeNi MOF catalyst. FeNi MOF nanosheet arrays grown on nickel foam was firstly prepared via a solvothermal process. Then CeO₂ nanoclusters (5 nm) were coated onto FeNi MOF surface by electrodeposition. A series of characterizations were employed to study the morphology, structure and surface electronic state information of the as-obtained CeO₂/FeNi MOF. From X-ray photoelectron spectroscopic analysis, the doping of CeO₂ clusters and the strong electronic interaction between CeO₂ clusters and FeNi MOF induce the formation of Fe/Ni-O-Ce bonds and optimize the electronic structures of Fe/Ni sites, which will enhance OER activities. The OER performance tests confirm that CeO₂/FeNi MOF indeed exhibits a superior OER activity than FeNi MOF alone. The hybrid catalyst delivers a higher mass activity (235.4 A·g^-1) and a faster turnover frequency (0.065 s^-1) than those of FeNi MOF (43.8 A·g^-1, 0.018 s^-1). Compared with FeNi MOF, CeO₂/FeNi MOF also shows better OER kinetics, as evidenced by a decreased Tafel slope, a reduced charge transfer resistance. Besides, CeO₂/FeNi MOF presents an outstanding stability (50 h, 50 mA·cm^-2). All these features make our CeO₂/FeNi MOF a potential catalyst in the future application. The interfacial strategy through introducing CeO₂ to modulate Fe and Ni active sites may open a door for developing high-performance OER catalysts in future.

Key words: MOF, CeO₂, interfacial effect, catalyst, oxygen evolution reaction