Characterization of New Systems for the Catalytic Electroreduction of Oxygen by Electrochemistry and X-Ray Absorption Spectroscopy

Abstract

Electrochemical and X-ray absorption techniques have been used to determine the influence of heat treatment in electrocatalytic activity for O₂ reduction for two Co catalysts. The catalysts are cobalt phthalocyanine (catalyst 1) and polyacrylonitrile + cobalt acetate (catalyst 2) adsorbed on carbon black and heat treated at several temperatures. A maximum for the catalytic activity was obtained for PcCo at 850 °C and for the PAN + Co catalyst at 950 °C with subsequent decrease. The results obtained by XANES and EXAFS data clearly show that metallic cobalt aggregates with different size are synthesized in the range of increased activity. In the region of highest activity were observed the smallest cobalt clusters (20 Å). For higher temperatures these cobalt aggregates became bigger (100–200 Å), which corresponds to the decrease in the catalytic activity. TEM was used as a complementary technique and it confirms the influence of the annealing temperature in the size of the cobalt aggregates obtained. XANES measurements at the Co and N K edges confirm that CoN₄ centers and nitrogen atoms are no longer detected after heat treatment in the region of increased activity.