Sub-Nanometer Fe Nanoclusters: Unlocking Ultrafast Kinetics, Exceptional Stability, and Unambiguous Mechanism in...

Area of Science:

• Materials Science
• Electrochemistry
• Nanotechnology

Background:

• Ultrasmall metal nanoclusters (MNCs, <2 nm) possess unique properties but synthesizing non-coinage MNCs, especially refractory metals, is difficult.
• Existing methods often require harsh conditions, limiting their applicability in areas like electrochemical desalination.
• Global water scarcity necessitates innovative and efficient water purification technologies.

Purpose of the Study:

• To develop a mild and general synthesis strategy for non-coinage MNCs, specifically sub-nanometer iron nanoclusters (Fe NCs).
• To investigate the application of these Fe NCs as an anode material for faradaic capacitive deionization (FDI).
• To elucidate the atomic-level mechanism of chloride storage in Fe NCs for desalination.

Main Methods:

• A universal "pore-mediated vapor diffusion" (PVD) method was employed to synthesize 0.8 nm Fe NCs within mesoporous carbon spheres.
• The synthesized Fe NCs were fabricated into an electrode for testing in electrochemical desalination (FDI).
• Operando X-ray spectroscopy and Density Functional Theory (DFT) calculations were used to study the chloride storage mechanism.

Main Results:

• The PVD method successfully produced sub-nanometer Fe NCs, bypassing high-temperature requirements and applicable to other refractory metals.
• The Fe NC electrode demonstrated a record salt adsorption capacity (116.83 mg_Cl g^-1) and an ultrahigh rate (0.57 mg_Cl g^-1 s^-1).
• Exceptional stability was observed, with 86.47% retention after 200 cycles, attributed to ultrafast ion diffusion and stress mitigation enabled by the sub-nanometer structure.

Conclusions:

• The PVD method offers a versatile platform for synthesizing non-coinage MNCs, particularly for refractory metals.
• The Fe NC anode significantly advances faradaic capacitive deionization performance, offering a promising solution for water desalination.
• The study identified the chloride storage mechanism as a conversion reaction (Fe NCs + Cl^- ⇌ FeOCl), providing crucial atomic-level insights for future material design.