Giant Polyoxometalate {W₇₂Fe₃₀} into Pure Inorganic Gel and Xerogel: Rheology and Proton Conduction

Area of Science:

• Materials Science
• Inorganic Chemistry
• Electrochemistry

Background:

• Polyoxometalate (POM) gels are typically organic-inorganic hybrids.
• Pure inorganic POM gels remain an underexplored area in materials research.

Purpose of the Study:

• To synthesize and characterize a novel pure inorganic POM gel.
• To investigate the proton conductivity of the resulting xerogel material.

Main Methods:

• Hydrothermal synthesis of a straw-color hydrogel by mixing sodium tungstate and ferric chloride solutions.
• Rheological studies of the hydrogel.
• Dehydration of the hydrogel to form a xerogel.
• Characterization of the xerogel using techniques to confirm its structure and properties.
• Proton conductivity measurements under varying temperature and humidity conditions.

Main Results:

• Formation of a {W72Fe30} type giant Keplerate-based POM compound, [Fe(H2O)6]14[W72Fe30O252(H2O)72(OH)60]·166H2O ({W72Fe30}XG).
• The xerogel is a macroporous material with 60 acidic hydroxyl groups per formula unit.
• The {W72Fe30}XG xerogel exhibits solid-state proton conduction.
• Super proton conductivity of 1.71 × 10-2 S cm-1 was achieved at 80°C and 98% relative humidity, with an activation energy of 0.18 eV.

Conclusions:

• A novel pure inorganic POM xerogel ({W72Fe30}XG) was successfully synthesized.
• This inexpensive metal-oxide-based material functions as an inorganic acid with significant proton conductivity.
• The material shows potential for applications in solid-state proton-conducting devices.