Mathieu De Koninck

Cu x Co3 − x O4 Used as Bifunctional Electrocatalyst Physicochemical properties and electrochemical characterization for the oxygen evolution reaction

Cu x Co 3-x O 4 (x = 0 and 1) powders were prepared by a sol-gel method which favors high oxide specific surface areas with a larger value for Co 3 O 4 , ascribed to a larger powder mesopore volume. X-ray diffraction measurements reveal that CuCo 2 O 4 particles are less crystalline than Co 3 O 4 with crystallite size 10 times smaller. The sol-gel method allows formation of spinel oxide particles that do not contain any resistive CuO phase. X-ray photoelectron spectroscopy analyses have shown that Co 3 O 4 contains Co 2+ and Co 3+ species at the surface, tetrahedral Co 2+ cations being predominant. In the case of CuCo 2 O 4 , Cu + , Cu 2+ , and possibly Cu 3+ cations are also detected, octahedral Cu 2+ showing the highest concentration among the copper species. Composite film electrodes, based on mechanically mixed Co 3 O 4 or CuCo 2 O 4 particles, carbon black Vulcan XC-72R, and poly(vinylidene fluoride-co-hexafluoropropylene), were formed on a glassy carbon disk surface. The highest intrinsic electrocatalytic activity for the oxygen evolution reaction is obtained for the CuCo 2 O 4 composite electrode containing the larger amount of oxide particles. Cyclic voltammetry experiments suggest that the surface Co 2+ /Co 3+ ratio is decreased when the electrodes are immersed into the KOH electrolyte, which may be associated to the formation of a superficial CoOOH layer.

Cu x Co3 − x O4 Used as Bifunctional Electrocatalyst II. Electrochemical Characterization for the Oxygen Reduction Reaction

Composite film electrodes containing mechanically mixed Co 3 O 4 or CuCo 2 O 4 particles, carbon-black Vulcan XC-72R, and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) were formed on the glassy carbon disk surface of a rotating ring-disk electrode (RRDE) and studied for the oxygen reduction reaction (ORR) in O 2 -satwated 1 M KOH solution. The highest current densities were observed with CuCo 2 O 4 and they increased with the oxide content in the film, hence clearly demonstrating the excellent intrinsic electrocatalytic activity of CuCo 2 O 4 for this reaction. The results also showed that CuCo 2 O 4 is a better electrocatalyst than Co 3 O 4 with higher current densities and a greater number of electrons exchanged per O 2 molecule. It was found that the copper-cobalt spinel oxide component favors a total of 4e - in the oxygen reduction process. At the CuCo 2 O 4 -based composite electrode, direct reduction of O 2 into OH - ions (rate constant k 1 ) and the peroxide pathway (formation of HO 2 - ions and their reduction into OH - ions, rate constants k 2 and k 3 ) are proceeding in parallel, with a k 1 /k 2 ratio that increases with the overpotential when the oxide content is greater than 23.5%. At the Co 3 O 4 -based composite electrode, k 1 is very weak with a k 1 /k 2 ratio that decreases rapidly with the overpotential.

The electrochemical generation of ferrate at pressed iron powder electrode: comparison with a foil electrode

Abstract In this work, Fe(VI) (or ferrate) was generated by electrochemical oxidation of iron electrodes, made by pressing iron (99.5%) powder, in 14 M NaOH at room temperature. For comparison purpose, an iron foil electrode was also used to evaluate the effect of porosity on the yield for Fe(VI) generation. The cyclic voltammograms (CVs) of pure iron pellet revealed a broad anodic wave between −1 and 0.5 V versus Hg/HgO corresponding to iron dissolution and passivation of the electrode. These processes are followed by an irreversible oxidation wave attributed to the oxygen evolution reaction (OER) and the generation of ferrate. On the return scan, the cathodic wave at about 0 V versus Hg/HgO is associated with the reduction of electrochemically generated ferrate. The electrochemical generation of ferrate occurred with higher concentration and yield at an iron pellet electrode than at a foil electrode due to the porous structure of the pellet electrode, which favors iron dissolution. However, the electrochemically active thickness of the pellet electrode was estimated to be only about 1% of the actual pellet thickness.