A Co/metal–organic-framework bifunctional electrocatalyst: The effect of the surface cobalt oxidation state on oxygen evolution/reduction reactions in an alkaline electrolyte

•A bifunctional electrocatalyst based on Co and MIL-101(Cr) was prepared.•The high surface CoIII content is beneficial for the OER activity.•The more surface CoII promotes the higher ORR activity.•The surface CoIII/CoII ratio may dominate the overall bifunctional activity.•The interaction between MIL-101(Cr) and Co species enhances ORR durability.

A bifunctional electrocatalyst based on MIL-101(Cr) with various surface CoIII and CoII contents was prepared via an oxidation or reduction treatment during an impregnation process. The electrocatalysts were characterized by X-ray powder diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and N2 adsorption–desorption. The catalytic activities of the electrocatalysts toward the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in an alkaline electrolyte were tested using a rotating disk electrode technique. The results exhibit Co/MIL-101(Cr)–O has a higher surface CoIII content and displays higher OER activities, whereas Co/MIL-101(Cr)–R has a higher surface CoII content and promotes higher ORR responses. However, Co/MIL-101(Cr)–O has a higher CoIII/CoII ratio of ∼0.89 and displays the highest overall bifunctional activities, combining both the OER and ORR processes together. Co/MIL-101(Cr)–R and Co/MIL-101(Cr)–O also exhibit a higher durability than Co/MIL-101(Cr) does, most likely because of the enhanced interaction between the MIL-101(Cr) support and the active surface Co species that stem from the reduction or oxidation treatment.