Decisive Roles of Interfacial Water Orientation and Hydrogen Bonds in the Dramatic Activity Gap of Metal-Nitrogen-Carbon Catalysts for ORR in Alkaline and Acid

Abstract There is a standing question in electrochemistry and energy conversion that why the metal-nitrogen-carbon (M-N-C) catalysts generally exhibit dramatic activity drop for the oxygen reduction reaction (ORR) when changing the electrolyte pH from alkaline to acid regime, which has severely inhibited the application of these nonprecious metal materials as viable electrocatalysts for the proton exchange membrane fuel cells (PEMFCs). Herein, taking FeCo-N6-C double-atom catalyst as a model system and by combining the ab initio molecular dynamics simulation and the slow-growth enhanced free-energy sampling approach, the interfacial double-layer microenvironments, ORR pathways, energetics of various elementary steps, and barriers of proton-coupled electron transfer (PCET) steps in alkaline and acid medias have been meticulously investigated and compared to understand the pH-dependent ORR activity on the molecular scale. We conclude that the greatly different orientations of interfacial water under alkaline and acid ORR conditions play a decisive role in the formation of hydrogen bonds between the surface oxygenated intermediates and the interfacial water molecules, thereby vitally control the kinetics of the PCET steps. The present finding that it is the interfacial double-layer structure, rather than the energetics of the multistep reaction pathway, that brings about the dramatic electrocatalytic activity gap of M-N-C catalysts for ORR in alkaline and acid electrolytes, not only emphasizes the significance of interfacial microstructures in electrocatalysis, but also opens new and feasible avenues for the design of advanced M-N-C catalysts for PEMFCs.