Influence of heat treatment on the activity and structure of CoTETA/C catalysts for oxygen reduction reaction

The influence of heat treatment on the improvement of the catalytic activity of CoTETA/C catalysts is investigated. These non-precious metal oxygen reduction catalysts are prepared from carbon-supported cobalt triethylenetetramine (CoTETA/C) and heat treated in the temperature interval from 500 to 1000 degrees C in Ar atmosphere. Electrochemical characteristics are demonstrated in oxygen-saturated acid electrolyte by rotating disk electrode, cyclic voltammetry, as well as single fuel cell tests. The results show that the effect of heat treatment is important on the catalytic activity of CoTETA/C catalysts for the ORR and a maximum catalytic activity is obtained after heat treatment at 800 degrees C. The ORR reaction mechanism on the catalysts heat treated at 700, 800 and 900 degrees C is mainly through a 4e reaction path, while a 2e reaction is dominant on the catalysts heat treated at 500, 600 and 1000 degrees C. Tafel slopes of the CoTETA/C catalysts are all around -200mV/dec. X-ray absorption measurements reveal that the CoN4 centers are no longer detected after heat treatment. XRD results clearly confirm the formation of nanometallic alpha-Co with different sizes aggregated. A possible interpretation of the catalytic active sites is also discussed. (C) 2009 Elsevier Ltd. All rights reserved