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Abstract:
Co-oxide based materials are promising electrocatalysts for oxygen evolution reaction (OER), crucial reaction for water electrolyzers. In this work, the mechanism undergone by water molecules (or hydroxide ions in alkaline media) on the surface of the material is analyzed. Correlations between the rate determining steps of the mechanism, overpotential, temperature, oxidation state and quantification of dissolved oxygen are stablished. In the first part the cobalt oxide based amorphous catalyst (CoCat) in potassium phosphate solution (neutral media) is used as a case study. The objective is to determine experimentally fundamental parameters (undefined) about the transition state (and state) determining the overall OER kinetics: enthalpic, entropic and overpotential contributions to the energetic barrier. Multitemperature Tafel analysis combined with electrochemical impedance spectroscopy is used to fit modified Eyring-Polanyi equations to determine the parameters. Results are consistent with both theoretical and (other) experimental works. These results reinforce the hypothesis that the adsorbate evolution mechanism in its acid-base variant is the path undergone for OER at low overpotentials. The second part the study uses amorphous cobalt oxides electrodeposited with the aid of tartrate (CoOx) in NaOH 0.1 M electrolyte. The study included operando XAS experiments in an electrochemical flow cell designed for this purpose. With the aid of a mass balance model and a phase-fluorometry based oxygen detector, the device was designed and optimized to perform operando XAS experiments for several hours in the station KMC-3 of BESSY II synchrotron quantifying the oxygen evolved. With this device, potential steps were exerted on the CoOx sample. As a result, a correlation between the amount of oxygen in the cell and the presence of Co(IV) in the material was determined. The changes in the electrocatalyst are potential-dependent rather than time-dependent. Similarities can be stablished between the OER mechanism on CoCat and CoOx.