Kinetics of Oxygen Electroreduction on Me-N-C (Me = Fe, Co, Cu) Catalysts in Acidic Medium. Insights on the Effect of the Transition Metal

The influence of three different transition metals (Me = Fe, Co, Cu) on the oxygen reduction reaction (ORR) kinetics in acidic medium of Me-N-C catalysts synthesized using Me(II)-phthalocyanine as precursors is investigated in this work. Through a detailed electrochemical characterization using cyclic voltammetry and rotating ring-disk electrode, several kinetics parameters such as Tafel slope, reaction order for oxygen and proton, apparent activation energy, selectivity towards hydrogen peroxide production, and kinetics of reduction of adsorbed oxygen were determined. The behavior of these three catalysts is analyzed in detail. A comparison between each other of the catalysts, and with a Pt-based catalyst is done. The results obtained provide clear evidence of the important role played by each transition metal in the formation of more or less effective active sites. The ORR kinetics behavior can be well interpreted according to the occurrence of a redox-mediated coverage of the active sites at low overpotentials (close to the ORR onset), which has influence on the Tafel slope, as well as on the oxygen adsorption and activation energy of the process. The results clearly show that, among the other transition metals considered, Fe is the best performing one in carrying out the ORR.