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

Abstract

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 FeN4-based electrocatalysts: [FeN4/C] or [FeN4C12] derived from the molecular structure of iron-porphyrin, and [FeN2+2/C], derived from the structure of the iron complex with two phenanthroline molecules. The energetics and chemical thermodynamic stability (equilibrium constants, Kc, 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 FeN4-based sites are chemically stable in acid at both 298 and 353 K. It is their high values of TΔS 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 [FeN4C12] and [FeN2+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.

Strategies for Engineering High‐Performance PGM‐Free Catalysts toward Oxygen Reduction and Evolution Reactions

“…There are only a few theoretical investigations on the stability of FeN x moieties. [ 61 ] Recently, four representative types of active sites (based on the FeN 4 C 12 moiety as discussed in Figure 4 [ 48 ] and FeN 2+2 /C moiety proposed by the INRS team [ 62 ] ) were investigated. FeN 4 C 12 moiety that bridges the zigzag edges in micropores ( Figure A) or integrates into a graphene layer (Figure 8B), and FeN 2+2 /C moiety that locates in micropores bridging the armchair sides (Figure 8C) or integrates into a graphene layer (Figure 8D) were modeled.…”

“…FeN 4 C 12 moiety that bridges the zigzag edges in micropores ( Figure A) or integrates into a graphene layer (Figure 8B), and FeN 2+2 /C moiety that locates in micropores bridging the armchair sides (Figure 8C) or integrates into a graphene layer (Figure 8D) were modeled. [ 61 ]…”

“…C) FeN 2 +2/C sites bridging two armchair sides of graphene layers and D) FeN 2 +2/C sites in a graphene layer. [ 61 ] (A–D) Reproduced with permission under the terms of the CC‐BY‐NC‐ND license. [ 61 ] Copyright 2017, The Authors.…”

“…[ 61 ] (A–D) Reproduced with permission under the terms of the CC‐BY‐NC‐ND license. [ 61 ] Copyright 2017, The Authors. Published by the Electrochemical Society.…”

“…Through thermodynamic simulations, the equilibrium constant ( K c ) that represents the chemical stability can be calculated. Taking FeN 4 C 12 moiety as an example, the demetallation can be described by Equation (17) [ 61,63 ] where s and aq refer to the solid state and aqueous solution state, so that the K c is where a represents the activity coefficient. Similarly, the demetallation of FeN 2+2 /C moiety (modeled as [Fe II (N 2 R) 2 ] 2+ ) follows Equation (19) …”