Delocalized Electronic States: The High-Shell Nitrogen Effects on Metal–Nitrogen–Carbon Catalysts

Higher-shell nitrogen is commonly present in metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction (ORR), yet its mechanistic role remains elusive owing to the intrinsic difficulty in resolving the atomic structures of most catalysts and existing models primarily describe the local electronic states of M1-Nx moieties. Here, beyond the conventional d-band center theory, we reveal that higher-shell nitrogen modulates delocalized electronic states, thus influencing the catalytic behavior of M-N-C catalysts. Inspired by these insights, we developed a molecular catalyst platform of Fe1-N4-C-Nx (x = 0, 2, 6) in which high-shell nitrogen species were deliberately engineered through precisely controlled synthesis routes. The results demonstrate that the ionization potential serves as a quantitative descriptor that captures the correlation between the delocalized electronic states and the activity and durability of the molecular catalysts. These findings expand the d-band center paradigm and establish a molecular platform to guide the rational design of advanced M-N-C catalysts.