Electrocatalytic reduction of dioxygen at macrocycle conducting polymer electrodes in acid media

Functionalized transition metal macrocyclic monomers were synthesized in order to prepare electron-conducting polymer-based electrodes. Cobalt tetra(o-aminophenyl)porphyrin and cobalt 4,4',3'',4'''-tetraaminophthalocyanine macrocycles were electropolymerized at a vitreous carbon electrode to give, respectively, poly(CoTAPP) and poly(CoTAPc). The electrocatalytic behaviour of such electrodes, particularly the influence of the macrocyclic skeleton towards dioxygen reduction, was examined. Both electrodes are active in oxygen reduction. However, the modified electrode containing cobalt phthalocyanine seems to be the most active. The Tafel slope determined at 20 degrees C at the onset of the reduction wave for both modified electrodes was 58 +/- 5 mV/decade, while in a higher overpotential region it was close to 116 +/- 20 mV/decade. The number of electrons involved during the reduction of dioxygen was determined using the Koutecky-Levich equation. The main reaction product at a poly(CoTAPP) electrode is hydrogen peroxide, whereas at a poly(CoTAPc) electrode the product is mainly water, since the last electrode allows a four-electron reduction process to occur at higher overpotentials. (C) 1997 Elsevier Science S.A.