Cobalt/Cobalt Oxide Surface for Water Oxidation

Water splitting to produce molecular hydrogen is an essential method to store sustainable energies. One of the bottlenecks for water splitting is the availability of an efficient and stable water-oxidizing catalyst. Herein, metallic cobalt foil, after the treatment under high potential (10-60.0 V), was used for water oxidation. The cobalt/cobalt oxide surface was characterized by various spectroscopic, microscopy, X-ray diffraction, and electrochemical methods. Diffuse reflectance infrared Fourier transform spectroscopy showed peaks for Co oxide at 489 and 595 cm^-1 attributed to the stretching of Co-O and bending of O-Co-O bonds in the CoO₆ octahedra. Small aggregated particles (ca. 50-100 nm) with a spherical morphology were detected by scanning electron microscopy, and high-resolution transmission electron microscopy from the mechanically separated particles indicated spacings of 2.5-2.6 Å corresponding to the interplanar spacings of the (011) plane for Co₃O₄. Selected area (electron) diffraction showed concentric rings centered on a bright central spot, indicating a polycrystalline material. Each ring is related to planes of different orientation and different interplanar spacing, attributed to metallic cobalt and cobalt oxides. X-ray diffraction resulted in patterns corresponding to Co₃O₄ and CoO(OH), and X-ray photoelectron spectroscopy could confirm the formation of metallic cobalt, Co(II) and Co(III) oxides on the surface of the electrode. These results suggest that the oxide on the surface of foil could be a mixture of different phases of Co oxide containing Co₃O₄ and CoO(OH). Under overpotentials of 460 and 780 mV, current densities of 0.144 and 0.5 A/cm² were observed at pH-14 without dropping of IR. To the best of our knowledge, the three-dimensional electrode obtained here is among the most efficient cobalt-based water-oxidizing electrodes under alkaline conditions reported so far and may be applied in large scale water-splitting systems.