Accurate modeling of platinum group metal free (PGM-free) electrocatalysts for the oxygen reduction reaction (ORR) is challenging due to the presence of solvent and applied potential, and the difficulty of observing reaction site mechanisms and surface structure under reaction conditions. These challenges are magnified when multiple active sites are present or other solvated molecules can compete with O2 adsorption. In this work, one promising PGM-free electrocatalyst composed of planar FeN4 moieties substituted into a graphene monolayer (FeN4@G) is modeled using self-consistent grand canonical density functional theory (GCDFT) to obtain information about this catalyst at experimentally relevant conditions. H, OH, H2O, O2, HSO4, SO4, and NO ligands were screened over multiple Fe, C, and N adsorption sites in order to determine the thermodynamically favored species at each applied bias. We find that both the working state of the catalyst and adsorption energies of ligands can be highly potential dependent, that O2 can be outcompeted for adsorption to Fe, and that spectator ligands significantly modify adsorption to Fe. The origins of these shifts in adsorption energy with potential are explained within the context of ligand dependent changes to the electronic structures and importance of using the self-consistent GCDFT formalism. Finally, these results are used to show why canonical ensemble calculations approximate GCDFT results better for Pt than FeN4@G.

Grand canonical density functional theory insight into adsorption on Pt and PGM-free electrocatalysts

ACS Fall 2021

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