Ligand-Functionalized MIL-68-type Indium(III) Metal-Organic Frameworks with Prominent Intrinsic Proton Conductivity

Area of Science:

• Materials Science
• Electrochemistry
• Porous Materials
• Proton Conduction

Background:

• Indium-based metal-organic frameworks (In-MOFs) are attractive porous solids with diverse structures and applications.
• Developing crystalline MOFs with enhanced proton-conductive properties is crucial for advanced electrochemical devices.
• Functionalized organic linkers offer a pathway to tune MOF properties.

Purpose of the Study:

• To efficiently fabricate five isostructural, three-dimensional In-MOFs using terephthalic acid and its functionalized derivatives.
• To investigate the structural stability, including thermal and water stability, of the synthesized In-MOFs.
• To comprehensively assess and compare the water-mediated proton conductivities of these novel In-MOFs.

Main Methods:

• Fabrication of five isostructural In-MOFs (MIL-68-In and MIL-68-In-X, where X = NH2, OH, Br, or NO2) using terephthalic acid-based linkers.
• Verification of structural stability through thermal analysis and powder X-ray diffraction.
• Assessment of proton conductivity (σ) under varying temperature and relative humidity (RH) conditions.

Main Results:

• All five synthesized In-MOFs exhibited excellent thermal and water stability.
• Proton conductivity showed a significant positive correlation with temperature and RH.
• The highest proton conductivity reached 1.72 × 10⁻³ S/cm (MIL-68-In-OH) at 100 °C/98% RH, with conductivity influenced by organic ligand functional groups.

Conclusions:

• The synthesized In-MOFs possess remarkable structural rigidity and promising proton conductivity.
• Functional groups on organic linkers significantly impact proton conduction properties.
• These In-MOFs represent a meritorious material foundation for future electrochemical applications, particularly in proton conduction.