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 electrochemicalprocesses represents significant challenges in electrocatalysis. Inthis study, we employed classic molecular dynamics (MD) and ab initio molecular dynamics (AIMD) simulationswith an explicit water solvent to investigate interfacial structureson the Fe-N-4-C catalyst and acquire dynamicobservation of the oxygen reduction reaction (ORR). The orientationand population of interfacial water are potential-dependent. Whenpotential 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 alsosuggests that hydrogen bonds (denoted as H-bonds) vary depending onthe 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), whilemore water with 'one O-H down, one O-H up'suppresses H-bond formation. We provided detailed information on howthe electrode potential influences H-bonds by impacting the orientationsof interfacial water molecules. The above analysis of interfacialwater is completely general and could be applicable to any water-basedenergy conversion and storage systems. Then, we focused on the reactionprocess and local environments around the ORR reaction center. Oursimulations show that the proton transfer to oxygenous intermediatesdirectly occurs at the applied potential. We also demonstrate thatthe applied potential induces charge redistribution and water reorientationaround the reaction center. We further identified a quadratic functionrelationship between the reaction free energy/activation barrier andthe potential for the key elementary ORR steps, in which the hydrogenationof the oxygenous intermediate is less favorable at higher potentialsas the local water is stabilized and pulled away from the reactioncenter through the H-bond interaction. Our analysis provides a profoundunderstanding of electric double layers and electrochemical processes,which are critical to experimental exploration and electrocatalystapplication.