Why Cobalt macrocyclic complexes are not efficient catalysts for the oxygen reduction reaction, under acidic conditions

Abstract

The activation of the oxygen reduction reaction (ORR) is of a primary importance for the development of fuel cells. Cobalt macrocyclic complexes, e.g. those of porphyrins, phthalocyanines, tetraazaannulenes (TAA), cyclam, have been tested in order to replace platinum, the best catalyst but a rare and expensive metal. However it has been noted in some studies that these complexes suffer from degradation in an acidic environment. Suspicions have already been expressed that such degradation are caused by hydrogen peroxide or demetallation, but the reason for the loss of activity of these catalysts has never been experimentally established. In the present work, it has been clearly demonstrated that cobalt complexes are stable in the presence of acids when the metal centre is at the Co(III) oxidation state but are demetallated when it is reduced to the Co(II) oxidation state, which occurs during the oxygen reduction reaction. Co2+ is a borderline Lewis acid in Pearson's classification and thus can be substituted by H+ in an acidic environment.