Exceptional Proton Conduction Performance under Low Humidity of Four Zirconium(IV)/Hafnium(IV) Metal-Organic...

Area of Science:

• Materials Science
• Chemistry
• Nanotechnology

Background:

• Metal-organic frameworks (MOFs) are advanced porous materials with tunable properties.
• Proton conductivity in MOFs is crucial for energy applications like fuel cells.
• Developing stable MOFs for low-humidity conditions remains a challenge.

Purpose of the Study:

• To synthesize and characterize novel isostructural 3D MOFs using isophthalic acid (IPA) and 5-hydroxyisophthalic acid (5-OH-IPA) as ligands.
• To evaluate the thermal, hydrolytic, and acid-base stability of the synthesized MOFs.
• To investigate the proton conductivity and transport mechanism of these MOFs, particularly under varying relative humidity (RH) conditions.

Main Methods:

• Synthesis of four isostructural MOFs: MIP-206-Zr (1), MIP-206-Hf (2), MIP-206-OH-Zr (3), and MIP-206-OH-Hf (4).
• Characterization of MOF stability (thermal, hydrolytic, acid-base) and proton conductivity (σ).
• Experimental and theoretical investigations including Density Functional Theory (DFT) for electrostatic potential (ESP) analysis and Hirshfeld surface analysis.

Main Results:

• All four MOFs exhibited excellent stability and high proton conductivity, with maximum σ values reaching up to 1.47 × 10⁻² S·cm⁻¹ at 100 °C and 68% RH.
• Stable conductivities (10⁻⁴ -10⁻³ S·cm⁻¹) were maintained even at 33% RH.
• DFT and Hirshfeld analyses revealed that hydroxyl groups enhance proton affinity and facilitate proton transfer via a denser H-bonding network, following the Vehicle mechanism.

Conclusions:

• The synthesized MOFs demonstrate superior proton conductivity and stability, making them suitable for low-humidity electrochemical applications.
• Hydroxyl functionalization significantly improves proton transport properties.
• These MOFs present a viable platform for developing next-generation proton-exchange membrane fuel cells and other electrochemical devices.