Energetics and thermodynamic stability of potential Fe (II) -hexa-aza-active sites for O_(2) reduction in PEM fuel cells

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(Fe^(II)N_((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(K c)for the demetallation reaction indicate that the iron(II)-hexa-aza-macrocyclic complexes and Fe^(II)N_((4+2))/C are chemically stable in an acid medium at 298 and 353 K.Compared with two other potential model sites(Fe^(II)N_((4+2))/C and Fe II N(2+2)/C)that were thought to be present in the same Fe-based catalysts,K c of Fe^(II)N_((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 K c for Fe^(II)N_((4+2))/C or Fe II N(2+2)/C,revealing the great chemical stability of Fe^(II)N_((4+2))/C.In this work,we discuss about a novel proposition that the two catalytic sites active in these Fe-based catalysts are Fe II N_(4)/C and Fe^(II)N_((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.