COMPARISON OF ELECTROCATALYTIC SITES DENSITY ON ELECTRODES WITH Co₃O₄ AND NiCo₂O₄ COATINGS

The paper compares catalytic properties of spinel mixed oxides of Co₃O₄ and NiCo₂O₄, that we synthesized, as possible catalysts for the oxygen reduction reaction in an alkaline medium. This reaction is one of the most important in the development of alternative energy sources with high specific characteristics – metal-air batteries and fuel cells. One of the reagents in such systems is oxygen; another necessary component can be a metal (Zn, Li, etc.) or hydrogen. We have employed the method of comparing catalytic activity for catalysts deposited on flat electrodes and have tried to achieve the sufficient adhesion in order to use the samples as electrodes. The Co3O4 catalyst was obtained on the substrate by hydrothermal method from a solution containing 0.1 M Co(NO₃)₂ and 0.4 M urea. The synthesis of NiCo₂O₄ was carried out electrochemically from a solution containing 0.01 M Ni(NO₃)₂ and 0.02 M Co(NO₃)₂. Sample characterization was carried out with X-ray analysis; electrochemical characteristics were obtained using cyclic voltammetry in 1 M NaOH solution. The paper finds out that catalytically active sites are formed on the electrode surface with a layer of the corresponding oxide during the cathodic polarization. Formation of one center during the electrode reaction corresponds to a twoelectron transfer. Total number of the active sites can be determined from the amount of electricity. Surface coverage was determined from the amount of electricity as a function of the potential calculated from the cathodic branch of the voltammogram curve. We carried out the calculation by the method of numerical integration using the trapezoidal rule in the Excel package. Based on the calculations performed, the form and parameters of the isotherm of the surface filling with active sites as a function of the electrode potential were established. The filling of the surface with active sites is shown for the first time to take place in accordance with the Frumkin–Temkin isotherm. We calculated the isotherm parameters, the density of the sites on the surface, and the effective distance between them. Most centers were obtained for an electrode with a layer of NiCo₂O₄ – 2.27∙10¹⁷ cm^-2 . The original technique is of interest for comparing the catalytic activity of electrodes from various materials.

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