Effects of functional groups in iron porphyrin on the mechanism and activity of oxygen reduction reaction

Abstract

The activity of the oxygen reduction reaction (ORR) on the cathode is one of the dominant factors in the performance of proton exchange membrane fuel cells. Iron porphyrin has low cost, environmental benignity, and maximum efficiency of metal usage. Therefore, this material can be a promising single-atomic metal dispersion catalyst for fuel cell cathodes. The variation of functional groups was proven to effectively modify the activity of the ORR on the iron porphyrin. However, the influences of functional groups on the mechanisms of the ORR remain ambiguous. This work paid attention to the substitution of carboxyl (–COOH), methyl (–CH₃), and amino (–NH₂) functional groups at the meso positions of the porphyrin ring. By using van der Waals density functional theory (vdW-DF) calculations, we found that the ORR mechanisms can follow the associative and dissociative pathways, respectively. The Gibbs free energy diagrams revealed that the rate-limiting step occurs at the second hydrogenation step for the first pathway and the O₂ dissociation step for the second pathway for all considered functional groups. The thermodynamic energy barrier at the rate-limiting step was found to be in the following order: porphyrin–(CH₃)₄ < porphyrin–(NH₂)₄ < original porphyrin < porphyrin–(COOH)₄ for the associative mechanism and porphyrin–(NH₂)₄ < porphyrin–(CH₃)₄ < porphyrin–(COOH)₄ < original porphyrin for the dissociative pathway. The findings suggested that porphyrin–(CH₃)₄ and porphyrin–(NH₂)₄ should be the best choices among the considered substrates for the oxygen reduction reaction. Furthermore, the interaction between the ORR intermediates and the substrates was attributed to the resonance of the d_z², d_xz, and d_yz components of the Fe d orbital and the C and N p orbitals of the substrates with the p orbitals of the oxygen atoms in the intermediates. Finally, the nature of the interaction between the adsorbent and adsorbate was charge transfer.