Effect of transition metal (M: Fe, Co or Mn) for the oxygen reduction reaction with non-precious metal catalysts in acid medium

The effect of the metal for the oxygen reduction reaction (ORR) in acid medium with non-precious metal catalysts has been investigated. A series of non-precious metal catalysts with typical formulation M/N/C with M being Mn, Co or Fe have been prepared by incorporating N onto an active carbon matrix by means of thermal treatments under inert atmospheres. The N-containing active carbons were further treated with the M-containing precursors based upon Mn, Co or Fe phthalocyanines and thermally treated under inert atmosphere. The performance for the ORR in acid medium of all of the catalysts has been evaluated by means of electrochemical techniques. The activity, both in terms of onset potential for the ORR and maximum current density at representative potentials between 900 and 700 mV follows the trend Fe > Co > Mn. In addition, the performance of the Fe-based catalysts obtained during the different stages of the catalyst preparation has been also evaluated. The catalysts obtained after the pyrolysis step are the only ones showing measurable rates for the ORR. Although the amount of N and Fe incorporated onto the carbon matrix decreases the pyrolysis treatment, this treatment leads to the formation of the real active sites for the ORR irrespectively of the nature of the transition metal.