Impact of Heterometallic Cooperativity of Iron and Copper Active Sites on Electrocatalytic Oxygen Reduction Kinetics

The oxygen reduction reaction (ORR) is a key reaction in polymer electrolyte fuel cells and metal-air batteries. In these electrochemical systems, platinum group metals (PGMs) have been widely used as ORR electrocatalysts. Because of material cost and scarcity of platinum group metals, non-PGM electrocatalysts are considered to be an ideal alternative for mass production with low material cost. Many non-PGM electrocatalysts have been intensively studied such as pyrolyzed Fe-, N-doped carbon (Fe-N-C) catalysts. However, many non-PGM electrocatalysts including Fe-N-C still suffer from product selectivity due to the production of H2O2 as the byproduct. In this work, we synthesized an ORR electrocatalyst of Cu-, Fe-, and N-doped carbon nanotubes, (Cu,Fe)-N-CNT. This heterobimetallic catalyst showed the selective 4e(-) reduction of O-2 to H2O2 with ca. 99%. Kinetic analysis of the electrocatalytic ORR and hydrogen peroxide reduction reaction (HPRR) in acidic media revealed that (Cu,Fe)-N-CNT showed two orders of magnitude higher rate constants for the direct 4e(-) reduction of O-2 to H2O2 than those for the 2e(-) reduction of O-2 to H2O2, whereas a monometallic Fe-N-CNT showed the same order of magnitude, indicating that the heterometallic cooperativity had a drastic impact on the ORR kinetics. Our findings would open up possibilities to develop non-PGM ORR electrocatalysts with heterobimetallic active sites for the selective ORR.