Ni-doped CoFe2O4 Hollow Nanospheres as Efficient Bi-functional Catalysts

Ni-doped spinel oxides NixCo1-xFe2O4 (x = 0, 0.25, 0.5, 0.75) hollow nanospheres electrocatalysts are synthesized with a simple hydrothermal approach. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) results reveal that the morphology, hollow and spinel structures of the cobalt ferrites remain unchanged with doping. The electrocatalytic activity of the Ni-doped CoFe2O4 with different doping contents has been studied and compared with the pure CoFe2O4 hollow nanospheres in alkaline solution by using rotating ring-disk electrode (RRDE) technique. For ORR, the Ni0.5Co0.5Fe2O4 (x = 0.5) exhibits as the most active catalyst with the highest diffusion limited current density and more positive onset potential. Whereas, the Ni0.75Co0.25Fe2O4 (x = 0.75) shows the best catalytic activity for OER with more negative onset potential (0.27 V vs. Ag/AgCl) and maximum current density (36.0 mA/cm2 at 1.0 V). X-ray photoelectron spectra (XPS) measurements reveal that the oxygen vacancy on the oxide surfaces increases, while the cations occupied ratio on octahedral/tetrahedral sites in spinel structures decreases along with the increasing of the Ni doping content. Combining with the charge transfer resistance measured by electrochemical impedance spectroscopy (EIS), these three factors work synergistically on the catalytic activities of the Ni-doped CoFe2O4 hollow nanospheres.