Electrocatalysis of cathodic oxygen reduction by metal phthalocyanines: Part I. Introduction, cobalt phthalocyanine as electrocatalyst: Experimental part

Abstract

Macrocyclic organic N 4-complexes of certain metals, especially Fe and Co, are promising electrocatalysts for cathodic oxygen reduction, and consequently, they have been extensively investigated. However, no clear picture of the pertinent electrocatalytic mechanism has evolved as yet. Therefore, relatively thick layers of iron and cobalt phthalocyanine were selected as model systems to find, firstly, accurate kinetic descriptions of cathodic oxygen reduction on these electrocatalysts and, secondly, a general mechanism appropriate to the kinetic findings. Here, the kinetics of the oxygen reduction on cobalt phthalocyanine will be presented. It is found that, at low cathodic overpotential, hydrogen peroxide is the stable reaction product. Only at more negative potentials is it partially reduced further to water. No direct reduction of O 2 to H 2O occurs over the entire potential range investigated. Electrolysis of oxygen labelled with 18O shows that no O-O bond rupture occurs in the formation of hydrogen peroxide. The kinetic results are put in the context of the surface properties of the cobalt phthalocyanine film as determined by cyclic and ac voltammetry.