Defect-Induced Atomic Arrangement in CoFe Bimetallic Heterostructures with Boosted Oxygen Evolution Activity.

Three CoFe-bimetallic oxides with different compositions (termed as CoFeO x -A/N/H) are prepared by thermally treating metal-organic-framework (MOF) precursors under different atmospheres (air, N 2, and NaBH 4 /N 2 ), respectively. With the aid of vast oxygen vacancies (O v ), cobalt at tetrahedral sites (Co 2+ (Th)) in spinel Co 3 O 4 is diffused into interstitial octahedral sites (Oh) to form rocksalt CoO and ternary oxide CoFe 2 O 4 has been induced to give the unique defective CoO/CoFe 2 O 4 heterostructure. The resultant CoFeO x -H exhibits superb electrocatalytic activity toward water oxidation: overpotential at 10 mA cm -2 is 192 mV, which is 122 mV smaller than that of CoFeO x -A. The smaller Tafel slope (42.53 mV dec -1 ) and higher turnover frequency (785.5 h -1 ) suggest fast reaction kinetics. X-ray absorption spectroscopy, ex situ characterizations, and theoretical calculations reveal that defect engineering effectively tunes the electronic configuration to a more active state, resulting in the greatly decreased binding energy of oxo intermediates, and consequently much lower catalytic overpotential. Moreover, the construction of hetero-interface in CoFeO x -H can provide rich active sites and promote efficient electron transfer. This work may shed light on a comprehensive understanding of the modulation of electron configuration of bimetallic oxides and inspire the smart design of high-performance electrocatalysts.