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

[摘要] 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 /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 /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.

[学科分类] 电力

[关键词] active site;fuel cell;oxygen reduction reaction;thermodynamic stability