Single-site and nanosized Fe-Co electrocatalysts for oxygen reduction: Synthesis, characterization and catalytic performance

The impregnation of Ketjen Black (C) with iron and cobalt phthalocyanines (MPc) taken one by one or as a 1:1 stoichiometric mixture, followed by heat treatment at 600 degrees C under inert atmosphere, gave materials containing arrays of single metal ions coordinated by four nitrogen atoms (M-N-4 units). Increasing the pyrolysis temperature to 800 degrees resulted in the formation of carbon-supported, nanosized metal particles. A key role of the carbon support in determining the material structure at either temperature investigated was demonstrated by TPD, EXAFS, XANES and XRPD studies. These also showed that a Fe-Co alloy is obtained at 800 degrees C when the impregnation of Ketjen Black involves a mixture of FePc and CoPc. Electrodes coated with the different Fe, Co and Fe-Co materials, containing ca. 3 wt% metal loadings, were scrutinized for the oxygen reduction reaction (ORR) in alkaline media by linear sweep voltammetry. For comparative purposes, two Pt electrocatalysts containing 3 and 20 wt% metal were investigated. The electrochemical activity of all materials was analyzed by Tafel and Koutecky-Levich plots as well as chronopotentiometry. The Fe-containing electrocatalysts have been found to be highly active for the ORR in alkaline media with convective limiting currents as high as 600 A g Fe-1 at room temperature and onset potentials as high as 1.02 V vs. RHE. It has been found that (i) the ORR mass activity of the Pc-derived electrocatalysts is superior to that of the Pt catalysts investigated; (ii) the activity of FePc and FePc-CoPc/C, heat treated at either 600 or 800 degrees C, is superior to that of the corresponding Co materials; (iii) the electrocatalysts obtained at 600 degrees C are fairly more active than those obtained at 800 degrees C. (C) 2010 Elsevier B.V. All rights reserved.