Thermodynamic Stability in Acid Media of FeN₄-Based Catalytic Sites Used for the Reaction of Oxygen Reduction in PEM Fuel Cells

Two types of Fe-based catalytic sites have been proposed in the literature to perform the oxygen reduction reaction in PEM fuel cells running with FeN 4 -based electrocatalysts: [FeN 4 /C] or [FeN 4 C 12 ] derived from the molecular structure of iron-porphyrin, and [FeN 2+2 /C], derived from the structure of the iron complex with two phenanthroline molecules. The energetics and chemical thermodynamic stability (equilibrium constants, K c , for iron acid leaching) of these two types of sites and some of their oxygenated forms have been determined at pH ∼ 0. This does not consider the direct or indirect electrochemical oxidation of these catalytic sites or the electro-corrosion of their support. All evaluated FeN 4 -based sites are chemically stable in acid at both 298 and 353 K. It is their high values of T Δ S contribution to the Gibbs free energy for acid leaching which are responsible for the stability of these catalytic sites. The dioxygen and hydroxyl complexes of [FeN 4 C 12 ] and [FeN 2+2 /C] electrocatalysts demonstrate an improved stability with increasing temperature, explained by their electron withdrawing properties and their effect on the number of electrons in the antibonding orbitals. Complexes with dioxygen are more resistant to the action of acid than the ones formed by chemisorption of OH-groups.