High Durability of a 14-Membered Hexaaza Macrocyclic Fe Complex for an Acidic Oxygen Reduction Reaction Revealed by In Situ XAS Analysis

Abstract

Nonplatinum metal (NPM) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) have been developed; however, NPM catalysts still need to be improved in terms of both their catalytic activity and durability. To overcome these problems, an Fe active site contained within a more compact ligand than conventional, porphyrinic, 16-membered ring ligands, or more specifically, a hexaaza macrocyclic ligand with a 14-membered ring (14MR), was developed. In this study, the durability of the Fe-14MR complex was compared to that of Fe phthalocyanine (FePc), which has a 16-membered ring ligand, using in situ X-ray absorption spectroscopy; demetalation of the Fe complexes was directly observed during electrochemical experiments performed under acidic ORR conditions. It was found that Fe-14MR is significantly more resistant to demetalation than FePc during the ORR.

Using Computational Chemistry To Reveal Nature’s Blueprints for Single-Site Catalysis of C–H Activation

“…The 57 Fe Mössbauer spectroscopic studies necessary to assign the precise oxidation/spin states of the FeN x C y moiety remain challenging due to the distribution of sites that must be obtained from spectral fits. , Typically, SACs are modeled with plane-wave, semilocal DFT, in which the metal oxidation and spin states are either erroneously predicted or not precisely defined (i.e., due to fractional occupation of frontier states with smearing using semilocal DFT). To reveal the electronic structure of the metal and its relationship to the catalytic reactivity, efforts to understand SAC reactivity via molecular mimics have probed the effects of nitrogen type (e.g., pyridinic or pyrrolic), typically embedded in 14- or 16-membered macrocycles. ,, DFT modeling of finite size nanoflakes that represent the infinite system also help address the challenges in periodic systems …”

“…To reveal the electronic structure of the metal and its relationship to the catalytic reactivity, efforts to understand SAC reactivity via molecular mimics have probed the effects of nitrogen type (e.g., pyridinic or pyrrolic), typically embedded in 14-or 16membered macrocycles. 208,228,229 DFT modeling of finite size nanoflakes that represent the infinite system also help address the challenges in periodic systems. 24 DFT has also been a useful tool for mechanistic understanding 230,231 and fast screening of new SACs.…”

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

“…This lower iron leaching in acid is in agreement with the greater stability experimentally established for the 14-membered hexa-aza macrocyclic complex in comparison with the electrocatalyst based on Fe-phthalocyanine (FePc). 1 To be able to obtain electrochemical results, both the 14-membered un-pyrolyzed hexa-aza and FePc complexes were impregnated on carbon black (/C). They were then cycled between 0 and 1 V in O 2 -saturated acid (0.5 M H 2 SO 4 ) solution.…”

“…Practically, all the studies were focused on the first four coordinating N‐atoms of the MetalN 4 sites. The family of coordination compounds of transition metals with hexa‐aza‐macrocyclic ligands is also of great interest due to its enhanced stability with regard to the electrolytic dissociation, their ability to catalyze various chemical reactions and, in electrochemistry, to catalyze the ORR, 1,2 the reduction of H 2 O to H 2 and of CO 2 to CO and/or formic acid 3,4 . In our previous work, 5 we showed that the initial fast decrease in the activities of porphyrin‐like (FeN 4 /C) and 1,10‐phenanthroline (Phen)‐like (FeN (2+2) /C) electrocatalytic centers in the molecular ORR in the acid medium of PEM fuel cells is most likely due to iron ions leaching from sites located in the micropores of the Fe–N–C catalyst (a demetallation reaction).…”

Lens-Shaped Carbon Particles with Perpendicularly-Oriented Channels for High-Performance Proton Exchange Membrane Fuel Cells

“…Proton exchange membrane fuel cells (PEMFCs) are considered promising power sources for achieving eco-friendly energy conversion. − However, the sluggish oxygen reduction reaction (ORR) requires a vast amount of scarce or expensive catalysts such as platinum (Pt) and has been a major hurdle for the commercialization of PEMFCs. − Thus, a great amount of effort has been devoted toward developing a highly active system with low Pt usage or Pt-free catalysts. − Nonetheless, the relatively thick catalyst layer is required to reduce Pt weight fraction, which severely impedes mass transport of reactants and products, limiting the PEMFC performance. − Although the introduction of pores inside carbon particles can increase the surface area, − it is essential to control the channel structures in terms of size and connectivity in the cathode to enhance the mass transport efficiency …”