First-Principles Study of the Ligand Substituent Effect on ORR Catalysis by Metallocorroles

Metallocorroles (M-N-C) and especially Co-corroles are some of the best molecular catalyst alternatives to the expensive platinum-group metals (PGM) for oxygen reduction reaction (ORR) catalysis in polymer electrolyte membrane (PEM) fuel cells. In this work, we study the M-N-C corroles (M = Mn, Fe, Co) and the ligand (L) substitution (L-M-N-C, L = H, CH3, CF3, and imidazole) on the metal site as ORR catalysts based on the free energies of the*OOH,*O, and*OH ORR pathway intermediates. We also examine the influence of the basis set size, density functional theory exchange-correlation functional, and solvent environment on the calculated energies for the*OOH,*O, and*OH intermediates. We find that improved catalytic performance is expected in the ligand-substituted Mn-N-C that can be further fine-tuned by changing the nature of the ligand and the substituent group on the corrole. By contrast, the catalytic activity of the Co-corrole is decreased by the ligand substitution. The obtained ORR energetics are sensitive to basis set size, exchange-correlation functional, and solvent environment with the best agreement with the experiment obtained for large (6-311++G**) basis set, PBE functional, and gas-phase ORR intermediate calculations.