Dependence of the Activity of Sputtered Co–C–N Oxygen Reduction Electrocatalysts on Heat-Treatment Temperature

Abstract

The variations of the structure and the activity of a sputtered Co–C–N sample for the oxygen reduction reaction (ORR) with heat-treatment temperature are studied. The sample begins to show activity for ORR in acidic electrolyte at a critical heat-treatment temperature of , at which both substantial nitrogen release from the originally amorphous films and the transformation to a heterogeneous structure (-containing graphitic carbon) occurs. The sample shows activity in alkaline electrolyte when heated above , the activity increases substantially near and the highest activity occurs at . This suggests that active sites, apparently created below , which are active below , are blocked in acidic electrolyte. When N loss, graphitization and formation of occurs at , sites apparently become available in acidic electrolyte. The variation of the transformation temperature, , with and in sputtered combinatorial libraries was mapped using electron microprobe and X-ray diffraction studies. decreases strongly as the Co content increases since Co is a graphitization catalyst and increases weakly as the N content increases.