Dung Tran Phuong, Chihaia Viorel, Son Do Ngoc
Department of Chemistry, University of Science, Vietnam National University Ho Chi Minh City Vietnam.
Department of Chemistry, Ho Chi Minh City University of Education Ho Chi Minh City Vietnam.
RSC Adv. 2023 Mar 14;13(13):8523-8534. doi: 10.1039/d2ra08007a.
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 (-CH), and amino (-NH) functional groups at the 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 O 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-(CH) < porphyrin-(NH) < original porphyrin < porphyrin-(COOH) for the associative mechanism and porphyrin-(NH) < porphyrin-(CH) < porphyrin-(COOH) < original porphyrin for the dissociative pathway. The findings suggested that porphyrin-(CH) and porphyrin-(NH) 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 , d , and d 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.
阴极上氧还原反应（ORR）的活性是质子交换膜燃料电池性能的主要因素之一。铁卟啉成本低、环境友好且金属利用率最高。因此，这种材料有望成为燃料电池阴极的单原子金属分散催化剂。已证明官能团的变化能有效改变铁卟啉上ORR的活性。然而，官能团对ORR机理的影响仍不明确。这项工作关注卟啉环β位上羧基（-COOH）、甲基（-CH₃）和氨基（-NH₂）官能团的取代情况。通过使用范德华密度泛函理论（vdW-DF）计算，我们发现ORR机理可分别遵循缔合和离解途径。吉布斯自由能图表明，对于所有考虑的官能团，限速步骤在第一条途径的第二次氢化步骤以及第二条途径的O解离步骤发生。发现限速步骤的热力学能垒按以下顺序排列：对于缔合机理，卟啉-（CH₃）＜卟啉-（NH₂）＜原始卟啉＜卟啉-（COOH）；对于离解途径，卟啉-（NH₂）＜卟啉-（CH₃）＜卟啉-（COOH）＜原始卟啉。研究结果表明，在考虑的底物中，卟啉-（CH₃）和卟啉-（NH₂）应该是氧还原反应的最佳选择。此外，ORR中间体与底物之间的相互作用归因于Fe d轨道的dₓ、dᵧ和d_z分量以及底物的C和N p轨道与中间体中氧原子的p轨道的共振。最后，吸附剂与被吸附物之间相互作用的本质是电荷转移。
