Sub‐Nanometer Fe Nanoclusters: Unlocking Ultrafast Kinetics, Exceptional Stability, and Unambiguous Mechanism in Faradaic Capacitive Deionization

Abstract Ultrasmall metal nanoclusters (MNCs, <2 nm) are emerging materials with unique properties, yet synthesizing non‐coinage MNCs, especially high‐melting‐point elements like Fe, Ti, and Mn, remains challenging due to the lack of mild, general strategies, hindering applications such as electrochemical desalination. Herein, a universal “pore‐mediated vapor diffusion” (PVD) method is reported to synthesize sub‐nanometer Fe NCs (0.8 nm) within mesoporous carbon spheres, tackling the critical bottleneck issue of faradaic capacitive deionization (FDI)–a promising approach for mitigating global water scarcity. This approach bypasses high‐temperature requirements and extends to other refractory metals. As a FDI anode, the Fe NC electrode achieves a record salt adsorption capacity of 116.83 mg Cl g −1 , an ultrahigh rate of 0.57 mg Cl g −1 s −1 , and exceptional stability (86.47% retention after 200 cycles). The sub‐nanometer structure enables ultrafast ion diffusion and stress mitigation, overcoming persistent kinetic and stability limitations in FDI. Through operando X‐ray spectroscopy and DFT, the chloride storage mechanism is identified as a conversion reaction (Fe NCs + Cl − ⇌ FeOCl), resolving key ambiguities at the atomic level in Fe electrochemistry. This work provides a versatile synthesis platform for non‐coinage MNCs and atomic‐level mechanistic insights, advancing next‐generation desalination technologies.