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Synthesis, Structure, Characterization, and Redox Properties of the Porous MIL-68(Fe) Solid
Summary:
The authors from Institut Lavoisier (UMR CNRS 8180), Université de Versailles Saint Quentin en Yvelines, and other institutions developed a porous iron(III)-based metal-organic framework (MIL-68(Fe)) with thermal stability and redox properties, achieving results in the fields of gas adsorption and lithium-ion battery electrodes.
Background:
1. Metal-organic frameworks (MOFs) have diverse applications, but for the Fe/H₂BDC system, only MIL-53(Fe), MIL-88B(Fe), and MIL-101(Fe)-NH₂ have been reported. There is a need to explore new Fe-based MOFs with different structures and properties.
2. The authors synthesized MIL-68(Fe) through subsolvothermal conditions, characterized its structure and properties, and studied its application as a positive electrode material for lithium-ion batteries, obtaining valuable results.
Research Content:
1. Synthesis:
The authors synthesized MIL-68(Fe) using a subsolvothermal method. A mixture of iron(III) perchlorate hexahydrate, hydrofluoric acid, hydrochloric acid, and terephthalic acid (H₂BDC) in N,N'-dimethylformamide (dmf) was heated at 100°C for 120 hours.
2. Characterizations:
1) BET results showed that the maximum BET surface area of MIL-68(Fe) was 665(10) m²·g⁻¹ after activation at 300°C under secondary vacuum for one day. The theoretical surface area was 1450 m²·g⁻¹ (including triangular pores) and 1200 m²·g⁻¹ (excluding triangular pores).
2) SEM was not explicitly mentioned, but single-crystal analysis showed the crystal size was 0.34×0.02×0.02 mm.
3) X-ray diffraction confirmed it crystallized in the orthorhombic Cmcm space group with unit cell parameters a=21.301(1) Å, b=36.873(2) Å, c=6.8883(3) Å; ⁵⁷Fe Mössbauer spectrometry indicated octahedral high-spin Fe³⁺ sites; IR spectroscopy showed the presence of dmf and free terephthalic acid in pores; thermal analysis revealed stability up to 300-320°C.
3. Application:
The material was tested as a positive electrode for lithium-ion batteries. At a C/10 rate, about 0.35 Li per Fe atom could be inserted, corresponding to a capacity of 30 mAh·g⁻¹; at C/50, the capacity increased to 40 mAh·g⁻¹.
4. Mechanism:
The structure of MIL-68(Fe) consists of chains of corner-sharing FeO₆ octahedra connected by BDC linkers, forming triangular and hexagonal 1D pores. The lower lithium insertion capacity compared to MIL-53(Fe) may be due to the small triangular pores limiting electrolyte entry and lithium diffusion.
Outlook:
This research reports the synthesis and characterization of MIL-68(Fe), revealing the influence of pore structure and framework rigidity on its properties, providing a reference for the design and application of Fe-based MOFs in energy storage and adsorption.
Synthesis, Structure, Characterization, and Redox Properties of the Porous MIL-68(Fe) Solid
Authors: Alexandra Fateeva, Patricia Horcajada, Thomas Devic, Christian Serre, Jérôme Marrot, Jean-Marc Grenèche, Mathieu Morcrette, Jean-Marie Tarascon, Guillaume Maurin, Gérard Férey
DOI: 10.1002/ejic.201000486
Link: https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/ejic.201000486
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