Electroreduction of oxygen on Co-based catalysts:: determination of the parameters affecting the two-electron transfer reaction in an acid medium

Co-based catalysts for the oxygen reduction reaction (ORR) in an acid medium have been prepared from cobalt acetate (CoAc) adsorbed on nine different carbons (previously enriched in surface nitrogen or not). The catalysts were obtained by heat-treating these materials at 900 degreesC in a reducing environment rich in NH3. In this work, the emphasis was mainly placed on the electrochemical production of H2O2 as measured by the rotating ring-disk electrode (RRDE) technique. It is shown that all Co-based catalysts are active for ORR. The activity and specificity of the catalysts for peroxide production depend essentially on three factors: (i) the potential applied to the disk, (ii) the type of carbon support; and (iii) the concentration of the cobalt precursor. At identical Co loadings (2000 ppm), the percentage of peroxide produced at the disk (%H2O2) reaches a maximum in the 0.3-0.1 V versus SCE potential range and decreases for more negative potentials. When the potential is set at a constant value (100 mV versus SCE for instance), a strong effect of the carbon support on %H2O2 and on the ring current I-R is noticed, with lower values of %H202 and IR corresponding to higher nitrogen content at the surface of the catalysts, while higher values of disk current ID are obtained under the same conditions. A figure of merit for the electroreduction of oxygen to hydrogen peroxide was obtained for each catalyst by multiplying ID (representing their activity for ORR) by %H2O2 (representing their specificity for H2O2 production). According to this figure of merit, the best catalysts for peroxide production are made with Ketjenblack, Black Pearls, Vulcan, and Norit carbon supports. For Co loadings higher than 2000 ppm, it is shown that increasing the loading by more than one order of magnitude (from 2000 to 50,000 ppm) has practically no effect on %H2O2 and I-R, while I-D decreases. (C) 2004 Elsevier Ltd. All rights reserved.