Three-dimensional protonic conductivity in porous organic cage solids

Area of Science:

• Materials Science
• Chemistry
• Energy Storage

Background:

• Proton conduction is crucial for biological processes and energy devices like proton exchange membrane fuel cells.
• Existing crystalline porous solids often exhibit limited one-dimensional proton conduction pathways.
• Three-dimensional conduction pathways are desirable for maximizing proton conductivity.

Purpose of the Study:

• To engineer porous molecular cages that facilitate efficient three-dimensional proton conduction.
• To investigate the proton conductivity of these novel cage structures.
• To understand the molecular-level mechanisms governing proton transfer within the cages.

Main Methods:

• Synthesis and characterization of porous molecular cages.
• Proton conductivity measurements under varying humidity conditions.
• Crystallography, molecular simulations, and quasi-elastic neutron scattering (QENS) for mechanistic studies.

Main Results:

• Porous molecular cages demonstrated proton conductivities up to 10⁻³ S cm⁻¹ at high relative humidity, rivaling extended metal-organic frameworks.
• The cage structure intrinsically enforces three-dimensional proton conduction pathways.
• Cage molecules facilitate proton transfer by confining water and enabling hydrogen bond reorganization.

Conclusions:

• Porous molecular cages offer a viable platform for achieving high proton conductivity via 3D pathways.
• The findings provide fundamental insights into molecular-level proton transport mechanisms.
• These cages represent a promising starting point for developing high-temperature, anhydrous proton conductors by incorporating alternative guest molecules.