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.