Electrocatalytic aspects of iron phthalocyanine and its μ-oxo derivatives dispersed on high surface area carbon

The cyclic voltammetry of iron phthalocyanine (FePc) and its two μ-oxo derivatives obtained by mixing the materials in dry form with high-area Vulcan XC-72 carbon yielded a common set of voltammetric peaks. This provides evidence for the presence of a single type of surface species for the macrocycle in its various forms. The electrochemical activity of these dispersed specimens for O₂ reduction in alkaline media using thin porous coating-rotating disk techniques was found to be essentially the same for both μ-oxo derivatives. Comparable activities were observed in the case of bulk monomeric FePc only after polarizing the electrodes at fairly negative potentials. Some differences in activity were observed, however, for the materials in gas-fed electrodes of the type used in fuel cells in 4 M NaOH at 60°C. Based on the results obtained in this work and quantum mechanical considerations, it has been concluded that (i) the increase in conductivity induced by the exposure of FePc to dioxygen is most likely due to the formation of μ-oxo-type derivatives, and (ii) it is not necessary to invoke a metal spin crossover mechanism as the key factor in explaining the high electrocatalytic activity of FePc for the reduction of O₂, as has been proposed earlier in the literature.