Ultrahigh proton conductivity of four ionic hydrogen-bonded organic frameworks based on functionalized terephthalates

Area of Science:

• Materials Science
• Chemistry
• Nanotechnology

Background:

• Hydrogen-bonded organic frameworks (HOFs) are promising for proton conduction due to their crystallinity and H-bonding networks.
• Achieving HOFs with both high water stability and significant proton conductivity remains a challenge.
• Ionic HOFs (iHOFs) offer potential for enhanced proton transport properties.

Purpose of the Study:

• To synthesize highly water-stable ionic HOFs (iHOFs) with excellent proton conductivity.
• To investigate the influence of functional groups on proton conductivity in iHOFs.
• To elucidate the proton transport mechanism within the iHOF framework.

Main Methods:

• Synthesis of four iHOFs using functionalized terephthalic acids and dimethylamine.
• Alternating-current (AC) impedance testing under various temperature and humidity conditions.
• Analysis of crystal structure, water contact angles, and activation energy.

Main Results:

• Four water-stable iHOFs were successfully synthesized via a straightforward approach.
• All synthesized iHOFs demonstrated temperature- and humidity-dependent proton conductivity, with the highest reaching 1.72 × 10⁻² S·cm⁻¹.
• Proton conductivity decreased in the order: iHOF 1 > iHOF 2 > iHOF 3 > iHOF 4 under specific conditions (100 °C, 98% RH).

Conclusions:

• The presence of free carboxylic acid groups, crystallization water, and protonated units facilitates proton transport.
• The functional groups on terephthalic acid significantly influence the proton conductivity of the iHOFs.
• This study provides a foundation for developing advanced proton-conductive materials based on iHOFs.