Energetics and thermodynamic stability of potential Fe(II)‐hexa‐aza‐active sites for O2 reduction in PEM fuel cells

Abstract

Abstract We present here a thermodynamic assessment of the stability behavior in acid environment at 298 and 353 K (80°C) of two iron (II) hexa‐aza‐macrocyclic complexes and of an hexa‐aza‐iron‐based site (FeIIN(4+2)/C) that should potentially be active for the oxygen reduction reaction in proton exchange membrane (PEM) fuel cells. The calculations of the equilibrium constant (Kc) for the demetallation reaction indicate that the iron (II)‐hexa‐aza‐macrocyclic complexes and FeIIN(4+2)/C are chemically stable in an acid medium at 298 and 353 K. Compared with two other potential model sites (FeIIN4/C and FeIIN(2+2)/C) that were thought to be present in the same Fe‐based catalysts, Kc of FeIIN(4+2)/C is two to three orders of magnitude smaller at 353 K, and three to four orders of magnitude smaller at 298 K, than Kc for FeIIN4/C or FeIIN(2+2)/C, revealing the great chemical stability of FeIIN(4+2)/C. In this work, we discuss about a novel proposition that the two catalytic sites active in these Fe‐based catalysts are FeIIN4/C and FeIIN(4+2)/C. This proposition is in agreement with the durability behavior of these catalysts in PEM fuel cells and also with their known physico‐chemical characterizations. The origin of the fast and slow decay behaviors of the different sites, which are active at the Fe–N–C‐based cathode of PEM fuel cells, is also discussed.

Keywords

• active site
• fuel cell
• oxygen reduction reaction
• thermodynamic stability