Enhanced oxygen catalytic activity by Fe3+ ions replaced in octahedral sites of Co3O4 nanofibers for Zn-air batteries

Co3O4-based nanomaterials are considered as potential oxygen catalysts with unique electronic structure. Herein, Co(3-x)FexO(4) (x = 0, 1, 1.5, and 2.0) nanofibers, in which the Co ions of octahedral sites are substituted by iron (Fe) in Co3O4, were fabricated. X-ray photoelectron spectroscopy and Mossbauer measurements reveal that the introduced Fe3+ ions exist mainly in trivalent form and replace Co3+ ions situated in octahedral sites of the Co3O4 cell, thus the formula for Co2FeO4 is (Co0.74Fe0.26)T(Co1.26Fe0.74)OO4. Theoretical calculations show that the appropriate replacement of Fe ions reduce the band center difference of O 2p-CoT 3d and O 2p-CoO 3d, elucidating the inductive Fe3+ ions in octahedral sites identifies as active sites and optimizes the activity of oxygen catalyst. Consequently, the optimum Co2FeO4 nanofibers exhibit a low overpotential of 350 mV at 10 mA cm(-2) for oxygen evolution reaction (OER) and a high limiting current density of 5.61 mA cm(-2) for oxygen reduction reaction (ORR). Moreover, the assembled Co2FeO4-based Zn-air batteries (ZABs) show a high gravimetric specific energy of 934.6 W h kg(-1).