Subnanometer iron clusters confined in a porous carbon matrix for highly efficient zinc-air batteries

At the molecular level, metal coordinates are crucial for stabilizing an appropriate electronic configuration for high-efficiency oxygen reduction reaction (ORR) electrocatalysts. In this work, an excellent platform to realize the decoration of Fe coordinates at the subnanometer scale into nitrogen-doped carbon networks (designated as Fe-Fe@NC) is provided. X-ray absorption spectroscopy confirmed the precise configuration of Fe coordinates with Fe-Fe and Fe-N coordinations at the molecular level. As a cathode catalyst, the newly developed Fe-Fe@NC exhibited superior ORR performance and a higher peak power density of 175 mW cm-2 in Zn-air batteries. Unlike most reported pristine Fe-based catalysts, Fe-Fe@NC also showed good oxygen evolution reaction (OER) activity, with a low operating potential (1.67 V vs. RHE) at a current density of 10 mA cm-2. Calculations based on density functional theory revealed that the Fe-Fe coordination in Fe subclusters favored the 4e- transfer pathway and, thus, achieved highly active catalytic performance. This work reveals that iron clusters at the subnanometer scale provide an optimized electronic structure for enhanced ORR activity.