Adsorption Site Screening on a PGM-Free Electrocatalyst: Insights from Grand Canonical Density Functional Theory

Platinum-group-free
Fe-N-C catalysts have been shown to be a promising
class of electrocatalysts for the oxygen reduction reaction (ORR)
in acidic media, with the Fe atom being regarded as the site most
responsible for ORR activity. Despite previous density functional
theory (DFT) modeling of this class of catalysts, the effect of the
self-consistent applied potential and relevance of competing adsorbates
to the ORR are still not well understood. In this work, we used grand
canonical DFT to calculate the adsorption thermodynamics of H, O,
OH, O2, H2O, NO, SO4, HSO4, and ClO4 ligands to symmetrically unique Fe, C, and
N sites for an FeN4 moiety hosted within a graphene monolayer
(FeN4@G). We find that the use of the applied potential
within the grand canonical ensemble (GCE) can significantly alter
the adsorption energies of these ligands in comparison to their canonical
adsorption energies for all adsorption sites. Under a GCE applied
potential, O, OH, H2O, NO, SO4, and HSO4 can all have a lower adsorption energy to Fe than O2, suggesting that other ligands may be persistently bound to Fe atoms
represented by this active site model during the ORR. Lateral spectator
OH ligands can adsorb near the Fe site, and these ligands can modulate
the grand canonical adsorption energies by approximately 0.1–0.4
eV. In addition, the use of a more oxidative applied potential affects
the adsorption energies of ligands qualitatively differently, with
the adsorption energies of H, O, O2, and NO being destabilized,
while the adsorption energies of the other ligands are stabilized.
A term-by-term comparison of the grand canonical and canonical adsorption
energies shows that despite both the clean catalyst and adsorbed ligand
systems oxidizing to similar (significant) degrees under an applied
potential, this effect only partially cancels when calculating grand
canonical adsorption energies. Additionally, we show that the choice
of the molecular referencing scheme is important, and multiple schemes
using both charged and neutral species with different implicit solvation
models are compared.