Atomic-Scale Understanding of Electrified Interfacial Structures and Dynamics during the Oxygen Reduction Reaction on the Fe–N4/C Electrocatalyst

Understanding electric double layers (EDLs) and electrochemical processes represents significant challenges in electrocatalysis. In this study, we employed classic molecular dynamics (MD) and ab initio molecular dynamics (AIMD) simulations with an explicit water solvent to investigate interfacial structures on the Fe–N4–C catalyst and acquire dynamic observation of the oxygen reduction reaction (ORR). The orientation and population of interfacial water are potential-dependent. When potential shifts positively, water molecules evolve from structurally “two O–H down” to ‘one O–H parallel, one O–H down,’ “two O–H parallel,” and eventually to “two O–H up.” Our finding also suggests that hydrogen bonds (denoted as H-bonds) vary depending on the potential and follow an asymmetric M-shape pattern. It confirms that interfacial water with “two O–H parallel” structures maximizes the number of hydrogen bonds (H-bonds), while more water with ‘one O–H down, one O–H up’ suppresses H-bond formation. We provided detailed information on how the electrode potential influences H-bonds by impacting the orientations of interfacial water molecules. The above analysis of interfacial water is completely general and could be applicable to any water-based energy conversion and storage systems. Then, we focused on the reaction process and local environments around the ORR reaction center. Our simulations show that the proton transfer to oxygenous intermediates directly occurs at the applied potential. We also demonstrate that the applied potential induces charge redistribution and water reorientation around the reaction center. We further identified a quadratic function relationship between the reaction free energy/activation barrier and the potential for the key elementary ORR steps, in which the hydrogenation of the oxygenous intermediate is less favorable at higher potentials as the local water is stabilized and pulled away from the reaction center through the H-bond interaction. Our analysis provides a profound understanding of electric double layers and electrochemical processes, which are critical to experimental exploration and electrocatalyst application.