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

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.

Spontaneous Demetallation of Mn(II)Nx Catalytic Sites in Proton Exchange Membrane Fuel Cell Conditions

“…As far as increasing the number of carbons around the active site is concerned, a typical example of increasing the number of carbons around FeN (4+2) /C (an Fe-hexa-aza type site) is given in our previous thermodynamic work published in ref 4. There, as shown in Figures 1, 2, and 3, 4 we first analyzed the equilibrium in acid conditions of an Fe ion in a hexa-aza site, starting with two 2,2′ bipyridine ligands, to obtain By increasing the number of benzene rings, the equilibrium constant, K c , at a given temperature, for each of the three hexaaza sites in the same acid medium decreased from 1.95 × 10 −10 to 1.79 × 10 −10 (by addition of two benzene rings) to 7.71 × 10 −12 (by the addition of 14 more benzene rings). The stability of the same site increases therefore by adding more benzene rings around it.…”

“…It was emphasized in the literature that theory and modeling became very important instruments to determine the parameters that affect the activity and stability of electrocatalysts used in PEM fuel cells. Currently, in most theoretical works, the Kohn–Sham density functional theory (DFT) is applied. − Nevertheless, the semiempirical and empirical methods, such as the use of thermodynamics for modeling the chemical stability of electrocatalysts used in proton exchange membrane (PEM) fuel cells, ,− have not lost their significance and continue to be used due to their simplicity and sufficient accuracy.…”

“…This work is about the comparison of the thermodynamic stability toward demetallation in the acidic medium of PEM fuel cells of (i) the porphyrin-like MnN 4 /C (Figure a), (ii) MnN (2+2) /C, the site illustrated in Figure a, and (iii) MnN (4+2) /C, the potential site illustrated in Figure a. The choice to compare these three sites was inspired by the proposition by Li et al of the corresponding FeN (2+2) /C as a durable site for the Fe/N/C electrocatalysts, as well as the proposition by other authors ,, of the corresponding hexa-aza site FeN (4+2) /C that was also presented as a durable site and as an alternative to FeN (2+2) /C.…”

“…This was achieved by vaporizing a very thin, porous layer of nitrogen-doped carbon onto the surface of the unstable catalyst at high temperature. This had the effect of transforming certain unstable porphyrinic FeN 4 catalyst sites (known as S1-type sites) into FeN 4 sites (known as S2-type sites), , which were more stable in terms of their demetallation in fuel cell. While the stability of the latter sites considerably improved the stability of the catalyst in the cell, the stabilization of the improved catalyst was not complete.…”

“…It is generally agreed that most of these sites are retained by 4N in direct coordination with the metal ion. Other nitrogen atoms may, however, complete this coordination 4 or simply dope the carbon support.…”