Oxygen electroreduction catalysis: effect of sulfur addition on cobalt tetraazaannulene precursors

The introduction of sulfur under thiourea form during the elaboration of cobalt tetraazaannulene based electrocatalysts, has provided under rotating disk electrode conditions in H2SO4 solutions, an enhancement in activity and stability of the previously synthesized materials. In HClO4 solutions, whereas stability was noticed, the effect of sulfur addition was revealed to be detrimental. On the basis of X-ray photoelectron spectroscopy experiments, this behavior difference emphasizes a model of an active site in which two cobalt ions are linked through oxygen ions to carbon sulfur (C-S) bonds. For all the active investigated samples the ratio Co/C-S bonds was found to be approximately 2 and the active site density turns out to be proportional to the amount of Co. The main difference between the most active samples in H2SO4 and HClO4 lies electrochemically in the predominance of 4 electron reduction in H2SO4 solution associated with a higher density of negative charges surrounding oxygen atoms and with the presence of imide groups in the most active samples to be very likely. (C) 1999 Elsevier Science Ltd. All rights reserved.