O2 Reduction Mechanism on Non-Noble Metal Catalysts for PEM Fuel Cells. Part I: Experimental Rates of O2 Eectroreduction, H2O2 Electroreduction, and H2O2 Disproportionation

One Fe/N/C cathode electrocatalyst for PEM fuel cells with a nominal loading of 0.2 wt % Fe was prepared by heat-treating at 950 degrees C in NH3 a nonmicroporous furnace carbon black impregnated with iron acetate. 02 reduction reaction (ORR) occurs on this catalyst with an apparent 4e transfer, while H2O2 electroreduction is sluggish, thereby minimizing the importance of the peroxide electroreduction pathway. It is also shown that H2O2 disproportionates only slowly into H2O and (1)/O-2(2) on this Fe/N/C catalyst. Therefore, the electrochemical-chemical pathway of O-2 to H2O2 followed by H2O2 disproportionation is minor during ORR. It is concluded that the present Fe/N/C catalyst reduces O-2 to water following mainly a direct 4e pathway. It is also noticed that the initial porosity of the carbon support may drastically influence the ORR mechanism. The same investigation was extended to other M/N/C electrocatalysts, where M = Cr, Mn, Co, Ni, and Cu, prepared according to the same procedure as used for Fe/N/C. A correlation was found between the ORR activity and the activity for H2O2 disproportionation. This is explained by the occurrence of a common rate-limiting intermediate for the two reactions: the oxo-ferryl-like cation radical.