Single-Crystal Covalent Organic Frameworks for Anhydrous Proton Conduction Above 200°C

Area of Science:

• Materials Science
• Electrochemistry
• Chemical Engineering

Background:

• Proton-exchange membrane fuel cells (PEMFCs) require advanced proton-conducting materials for efficient operation above 150°C.
• Developing materials with high chemical stability and proton conductivity at elevated temperatures remains a significant challenge.

Purpose of the Study:

• To construct novel three-dimensional covalent organic frameworks (COFs) with covalent phosphonate modification.
• To investigate the proton conduction properties of these functionalized COFs at high temperatures.

Main Methods:

• Solvent-free, melt-phase post-synthetic modification (PSM) strategy was employed to introduce phosphonate groups.
• Single-crystal X-ray diffraction (SCXRD) was used to analyze the structural changes and hydrogen-bonding networks.
• Proton conductivity measurements were performed at elevated temperatures (up to 230°C).

Main Results:

• The functionalized single-crystal COFs demonstrated excellent anhydrous proton conduction along the crystallographic c-axis.
• Proton conductivities of 8.91 × 10⁻³ S cm⁻¹ at 210°C (COF-300-DMP) and 5.65 × 10⁻³ S cm⁻¹ at 230°C (COF-300-DEP) were achieved.
• Low activation energies (0.196–0.229 eV) indicated a Grotthuss-type hopping mechanism.

Conclusions:

• The study presents a generalizable route for COF functionalization via covalent phosphonate modification.
• The developed COFs serve as a blueprint for designing advanced proton conductors for high-temperature fuel cells and extreme environments.