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 O2 to H2O 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 O2 to H2O than those for the 2e– reduction of O2 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.