Multilevel Computational Studies Reveal the Importance of Axial Ligand for Oxygen Reduction Reaction on Fe–N–C Materials

Abstract

The systematic improvement of Fe–N–C materials for fuel cell applications has proven challenging, due in part to an incomplete atomistic understanding of the oxygen reduction reaction (ORR) under electrochemical conditions. Herein, a multilevel computational approach, which combines ab initio molecular dynamics simulations and constant potential density functional theory calculations, is used to assess proton-coupled electron transfer (PCET) processes and adsorption thermodynamics of key ORR intermediates. These calculations indicate that the potential-limiting step for ORR on Fe–N–C materials is the formation of the FeIII–OOH intermediate. They also show that an active site model with a water molecule axially ligated to the iron center throughout the catalytic cycle produces results that are consistent with the experimental measurements. In particular, reliable prediction of the ORR onset potential and the Fe­(III/II) redox potential associated with the conversion of FeIII–OH to FeII and desorbed H2O requires an axial H2O co-adsorbed to the iron center. The observation of a five-coordinate rather than four-coordinate active site has significant implications for the thermodynamics and mechanism of ORR. These findings highlight the importance of solvent–substrate interactions and surface charge effects for understanding the PCET reaction mechanisms and transition-metal redox couples under realistic electrochemical conditions.

Graphite-Supported Ptn Cluster Electrocatalysts: Major Change of Active Sites as a Function of the Applied Potential

“…Nonetheless, our main focus is on the geometry of Pt n clusters and its dependence on the applied potential, rather than on the specific electrolyte–electrode interaction, and we expect that the conclusions extracted from this work will hold when applying more elaborate electrochemical models. In this regard, we highlight that modeling the electrochemical reconstruction of interfaces is very challenging, and new theoretical methods continue to be developed. − …”

What is the Real Origin of the Activity of Fe–N–C Electrocatalysts in the O2 Reduction Reaction? Critical Roles of Coordinating Pyrrolic N and Axially Adsorbing Species

“…13 However, the coadsorbing species would reduce the activity of pyrrolic FeN 4 C. Upon the acceptance of this work, we noticed the following papers published very lately. Hutchison et al 46 demonstrated the effect of axial ligation on ORR by multilevel computations. Specifically, Ni et al 47 found that the active site of Fe−N−C catalysts for ORR is the iron site coordinated with pyrrolic N by Mossbauer spectroscopy, which is consistent with our computations.…”

Correlation between Electronic Descriptor and Proton-Coupled Electron Transfer Thermodynamics in Doped Graphite-Conjugated Catalysts

“…We determined the proton-coupled redox potentials for GCC-phenazine using the constant potential strategy discussed in previous work of our group and others. ,,− In this approach, the grand potentials are calculated as a function of applied potential E for the reactant and product states of a PCET reaction. The change in free energy for a reaction within the grand canonical approach is given as the difference between the product grand potential, Ω P ( E ), and the reactant grand potential, Ω R ( E ): normalΔ normalΩ ( E ) = normalΩ normalP ( E ) − normalΩ normalR ( E ) …”

“…In this work, we investigate PCET at GCC-phenazine, shown in Figure , in the presence of heteroatom dopants and defects on the graphitic carbon surface. The changes in chemical reactivity are tracked by computing the proton-coupled redox potential, E PCET , using a constant potential approach rooted in the grand canonical ensemble. ,, The variations in E PCET are explored in connection with changes in the density of states (DOS) of GCC-phenazine upon both electron and proton transfer. The electronic states directly involved in PCET are shown to be related to the corresponding electronic states of molecular phenazine, corroborating the quasi-molecular character of the GCC site.…”

“…Pristine GCC-phenazine was modeled by attaching the phenazine active site to a H-terminated armchair graphene nanoribbon (Figure ), corresponding to a stoichiometry of C 80 H 18 N 2 . , We introduced dopants to the substrate in a manner that ensures they are relatively well isolated from the GCC active site (Figure ). Dopants were chosen to both donate and withdraw electrons from the graphitic surface.…”