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 MIL-101(Cr) with various surface Co-III and Co-II 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 N-2 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 Co-III content and displays higher OER activities, whereas Co/MIL-101(Cr)-R has a higher surface Co-II content and promotes higher ORR responses. However, Co/MIL-101(Cr)-O has a higher Co-III/Co-II 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. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.