Axial heteroatom (P, S and Cl)-decorated Fe single-atom catalyst for the oxygen reduction reaction: a DFT study

An FeN₄ single-atom catalyst (SAC) embedded in a graphene matrix is considered an oxygen reduction reaction (ORR) catalyst for its good activity and durability, and decoration on the Fe active site can further modulate the performance of the FeN₄ SAC. In this work, the axial heteroatom (L = P, S and Cl)-decorated FeN₄ SAC (FeN₄L) and pure FeN₄ were comparatively studied using density functional theory (DFT) calculations. It was found that the rate-determining step (RDS) in the ORR on pure FeN₄ is the reduction of OH to H₂O in the last step with an overpotential of 0.58 V. However, the RDS of the ORR for the axial heteroatom-decorated FeN₄L is the reduction of O₂ to OOH in the first step. The axial P and S heteroatom-decorated FeN₄P and FeN₄S exhibit lower activity than pure FeN₄ since the overpotentials of the ORR on FeN₄P and FeN₄S are 1.02 V and 1.09 V, respectively. Meanwhile, FeN₄Cl exhibits the best activity towards the ORR since it possesses the lowest overpotential (0.51 V). The main reason is that the axial heteroatom decoration alleviates the adsorption of all the species in the whole ORR, thus modulating the free energy in every elementary reaction step. A volcano relationship between the d band center and the ORR activity can be determined among the axial heteroatom-decorated FeN₄L SACs. The d band center of the Fe atom in various FeN₄L SACs follows the order of FeN₄ > FeN₄Cl > FeN₄S > FeN₄P, whereas the overpotential of the ORR on various catalysts follows the order of FeN₄Cl > FeN₄ > FeN₄S ≈ FeN₄P. ΔG(*OH) is a simple descriptor for the prediction of the ORR activity of various axial heteroatom-decorated FeN₄L, although the RDS in the ORR is either the first step or the last step. This paper provides a guide to the design and selection of the ORR over SACs with different axial heteroatom decorations, contributing to the rational design of more powerful ORR electrocatalysts and achieving advances in electrochemical conversion and storage devices.