Metal-Organic Frameworks Containing Missing-Linker Defects Leading to High Hydroxide-Ion Conductivity

Area of Science:

• Materials Science
• Solid-State Chemistry
• Nanotechnology

Background:

• Metal-organic frameworks (MOFs) offer tunable structures for various applications.
• Ionic conductivity in MOFs is crucial for energy storage and conversion devices.
• Understanding structure-property relationships in MOFs is key to optimizing performance.

Purpose of the Study:

• To investigate the ionic conductivity of face-centered cubic [Ni8 (OH)4 (H2O)2 (BDP_X)6] MOF systems.
• To evaluate the impact of post-synthetic modification, specifically introducing missing-linker defects via KOH treatment, on ionic conductivity.
• To correlate changes in material properties (porosity, basicity, hydrophilicity) with enhanced conductivity.

Main Methods:

• Synthesis of pristine MOF systems [Ni8 (OH)4 (H2O)2 (BDP_X)6] (X=H, OH, NH2).
• Post-synthetic modification using KOH to create K[Ni8 (OH)5 (EtO)(BDP_X)5.5] and K3 [Ni8 (OH)3 (EtO)(BDP_O)5] materials with missing-linker defects.
• Variable temperature AC impedance spectroscopy to measure ionic conductivity and activation energy (Ea) under varying relative humidity (RH).

Main Results:

• Modified MOFs (1@KOH, 2@KOH, 3@KOH) exhibited up to a four-order-of-magnitude increase in ionic conductivity compared to pristine MOFs (1-3).
• For MOF 2, conductivity increased from 5.86 × 10⁻⁹ S cm⁻¹ (Ea = 0.60 eV) at 22% RH to 2.75 × 10⁻⁵ S cm⁻¹ (Ea = 0.40 eV) for 2@KOH.
• At 100% RH, 2@KOH showed a further conductivity increase to 1.16 × 10⁻² S cm⁻¹ with a reduced Ea of 0.20 eV.

Conclusions:

• Post-synthetic modification introducing missing-linker defects significantly enhances ionic conductivity in Ni-based MOFs.
• Increased porosity, basicity, and hydrophilicity in modified MOFs contribute to their improved ionic transport properties.
• These findings highlight the potential of defect engineering in MOFs for developing advanced ion-conducting materials.