Inherent Ultrahigh Proton Conductivity of Two Highly Stable COOH-Functionalized Hafnium-Based Metal-Organic...

Area of Science:

• Materials Science
• Chemistry
• Nanotechnology

Background:

• Recent interest in proton conductivity (σ) of stable carboxyl metal-organic frameworks (MOFs) for fuel cells and sensing.
• Limited research on hafnium(IV)-based MOFs for these applications.
• Need for expanded research into hafnium(IV) MOFs with tunable properties.

Purpose of the Study:

• To synthesize and characterize novel hafnium(IV)-based MOFs with varying numbers of free carboxylic groups.
• To investigate the structural stability and proton conductivity of these new MOFs.
• To explore the relationship between MOF structure, functional groups, and proton conduction mechanisms.

Main Methods:

• Synthesis of two hafnium(IV)-based MOFs, UiO-66-COOH-Hf (1) and UiO-66-(COOH)₂-Hf (2), using phenylpoly(carboxylic acid) ligands and hafnium tetrachloride.
• Structural rigidity and stability assays (thermal and water stability).
• Alternating current impedance experiments to measure proton conductivity under varying humidity and temperature.

Main Results:

• Both synthesized MOFs exhibit excellent structural rigidity, including thermal and water stability.
• Proton conductivity (σ) increases with humidity and temperature, reaching up to 4.35 × 10⁻³ S·cm⁻¹ for MOF 2.
• MOF 2, with more free carboxyl groups, demonstrates approximately double the proton conductivity of MOF 1.

Conclusions:

• The synthesized hafnium(IV)-based MOFs possess high proton conductivity and excellent stability.
• The number of free carboxylic groups significantly influences proton conductivity, offering a tuning parameter.
• These materials provide a strong foundation for developing advanced proton-conductive materials for energy applications.