Improved activity of cathode catalysts for fuel cells by optimizing the conditions for preparation of carbon particles modified with cobalt-polyrrole complex

Carbon nanoparticles modified with a cobalt-adsorbed polypyrrole film (Co/PPy/C) were found to be a good electrocatalyst for O₂ reduction. The Co/PPy/C catalyst was synthesized by a multiple modification method for the purpose of obtaining Co-N₄ structure in high density. The conventional Co/PPy/C catalyst was prepared by electropolymerization of pyrrole onto the surface of carbon particles dispersed in the electrolyte solution containing pyrrole, followed by complexation using cobalt acetate. The new Co/PPy/C catalyst was prepared by repeating the above operation. Compared with those of the conventional catalyst, the cobalt ions and nitrogen atoms constituting active centers increased remarkably. The carbon nanoparticles prepared by the multiple modification method was suspended in a Nafion solution, and pseudo MEA (Membrane Electrode Assembly) was made by casting the solution onto an edge-plane pyrolytic graphite electrode. It was revealed that the new catalyst reduced O₂ mainly with four electrons, the number of electrons transferred n being 3.3 at a positive potential (E_p = 0.31 V vs. SCE). In addition, the catalytic activity of the catalyst was improved by optimizing the heat-treatment conditions (n = 3.8 at E_p = 0.42 V vs. SCE). These results show the potential use of the present catalyst as a novel cathode catalyst in the fuel cell.