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

Abstract

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 °C in NH 3 a nonmicroporous furnace carbon black impregnated with iron acetate. O 2 reduction reaction (ORR) occurs on this catalyst with an apparent 4e transfer, while H 2 O 2 electroreduction is sluggish, thereby minimizing the importance of the peroxide electroreduction pathway. It is also shown that H 2 O 2 disproportionates only slowly into H 2 O and 1 / 2 O 2 on this Fe/N/C catalyst. Therefore, the electrochemical−chemical pathway of O 2 to H 2 O 2 followed by H 2 O 2 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 H 2 O 2 disproportionation. This is explained by the occurrence of a common rate-limiting intermediate for the two reactions: the oxo-ferryl-like cation radical.