Electrocatalytic activity and stability of substituted iron phthalocyanines towards oxygen reduction evaluated at different temperatures

A significant amount of work has been done in the field of the transition metal (TM) macrocyclic-catalyzed oxygen reduction reaction (ORR), but not as a function of temperature and substitution in an acidic electrolyte to simulate the environment of an operating PEM fuel cell. The electrocatalytic activity of several iron-based phthalocyanine macrocycles for the ORR were evaluated using cyclic voltammetric (CV) and rotating disc electrode (RDE) techniques. The activities of these substituted iron phthalocyanines were compared to those of the unsubstituted species in the temperature range of 20-80 °C. The activity was also evaluated in the presence of hydrogen peroxide, both a possible by-product of the ORR and potential de-activator of iron phthalocyanines. The kinetics and corresponding parameters such as overall ORR electron transfer numbers, reaction rate constants, Tafel slopes, electron transfer numbers in the rate-determining step, and electron transfer coefficients were all measured in the temperature range of 20-80 °C. A mechanism for the different FePc-catalyzed ORR's was suggested based on the experimental results. The effect of substitution and temperature on ORR kinetics was also studied in this paper.