Phase Behavior and Proton Conductivity in Crown Ether-Based Supramolecular Sodium Hydrogen Sulfate Complexes

Area of Science:

• Solid-state chemistry
• Supramolecular chemistry
• Materials science

Background:

• Sodium hydrogen sulfate (NaHSO4) is a known proton conductor.
• Crown ethers are macrocyclic ligands capable of coordinating metal cations.
• Understanding phase transitions is key to developing advanced materials.

Purpose of the Study:

• To synthesize and characterize supramolecular complexes of NaHSO4 with 15-crown-5 and benzo-15-crown-5.
• To investigate the structural, thermal, and phase transition properties of these complexes.
• To explore the proton conductivity of the resulting superprotonic phases.

Main Methods:

• Single-crystal X-ray diffraction (XRD) for structural elucidation.
• Microcalorimetry, hot-stage microscopy, and variable-temperature powder XRD for thermal analysis.
• Solid-state NMR and electrochemical impedance spectroscopy for proton dynamics and conductivity measurements.

Main Results:

• Two supramolecular complexes, [15-crown-5·Na]-HSO4 (1) and [benzo-15-crown-5·Na]-HSO4 (2), were successfully synthesized and structurally characterized.
• The formation of these complexes triggers solid-solid phase transitions, leading to the development of superprotonic phases.
• Variable-temperature NMR indicated proton dynamics associated with phase transitions, and electrochemical impedance spectroscopy confirmed enhanced proton conductivity.

Conclusions:

• Supramolecular complex formation is an effective strategy to induce phase transitions in NaHSO4-based materials.
• The resulting superprotonic phases exhibit significantly improved proton conductivity.
• These findings open avenues for designing novel solid-state proton conductors for various applications.