Unravelling the micro-mechanism of oxygen reduction reaction on Fe–N4 embedded in graphene

We conducted a first-principles density functional study on the FeN4 embedded in graphene (Fe–N4-Gra) for the Oxygen Reduction Reaction (ORR). Our study unraveled certain microscopic characteristics of the ORR process on the catalyst that are not readily observable in experiments. Firstly, we investigated the reduction preferences of the two oxygen atoms in the side-on form of O2. Our findings indicate that the 2p orbital of the Ob atom, located further from the central Fe atom, exhibits a significantly stronger overlap with the Fe 3d orbital compared to the Oa atom adjacent to Fe. This suggests a preferential reduction process occurring on the Ob atom. The Löwdin charge distribution between the two atoms further supports this observation. Secondly, we demonstrated that minimizing the orbital energy gap and enhancing the hybridization or resonance between the Fe-3d and O-2p orbitals is the origin of the activity of the Fe–N–C catalyst. We propose that future catalyst designs should consider expanding the Fe 3d or 4s orbital to overlap with the (σ2p)* orbital of O2 in order to achieve higher efficiency of 4 electron reduction process. These findings contribute to a deeper understanding of the ORR process on the Fe–N4-Gra catalyst and provide valuable insights for the development of more efficient catalysts in the future.