Anisotropic Single-Crystal Proton Conduction in a Sodium Chain-Based Coordination Polymer

Area of Science:

• Materials Science
• Solid-State Chemistry
• Proton Conduction

Background:

• Coordination polymers (CPs) offer tunable structures for proton conduction.
• Research on alkali metal-based CPs and their proton transport mechanisms is limited.
• Understanding anisotropic proton transport in single-crystal CPs is crucial for advanced applications.

Purpose of the Study:

• To synthesize and characterize a new alkali metal-based coordination polymer, Na-PyDat.
• To investigate the proton conduction properties of Na-PyDat, particularly its single-crystal behavior.
• To explore the potential of Na-PyDat in composite membranes for enhanced proton conductivity.

Main Methods:

• Hydrothermal synthesis of the coordination polymer [Na2(pytet)(Hdat)2(H2O)3]·2H2O (Na-PyDat).
• Structural characterization of the 3D network formed by [Na-O-Na] double strands and hydrogen bonds.
• Alternating current (AC) impedance analysis to measure single-crystal proton conductivity.

Main Results:

• Na-PyDat exhibits highly anisotropic single-crystal proton conduction along the [100] direction (1.04 × 10⁻⁴ S cm⁻¹ at 338 K, 90% RH).
• Proton conductivity is 1-2 orders of magnitude higher along [100] compared to other directions and the powder sample.
• Continuous hydrogen bond chains along the a-axis are identified as key to high proton conductivity.
• A composite membrane of Na-PyDat and Nafion shows enhanced proton conductivity (4.33 × 10⁻³ S cm⁻¹ at 338 K, 90% RH).

Conclusions:

• Na-PyDat is a promising alkali metal coordination polymer for single-crystal proton conduction.
• The study provides insights into anisotropic proton transport mechanisms in alkali metal CPs.
• Na-PyDat demonstrates potential for applications in proton exchange membranes for fuel cells.