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

Polyoxometalate (POM) gel is well-known but mostly with organic molecules. Pure inorganic POM gel, that is, a combination of a "POM anion and a metal cation," is hardly known and unexplored area of materials research. When an aqueous solution of sodium tungstate is mixed with an aqueous solution of ferric chloride, the resulting suspension forms a straw-color hydrogel. The behavior of this hydrogel {W₇₂Fe₃₀}_HG has been studied by performing rheology studies. Dehydration of hydrogel at room temperature brings about the corresponding xerogel, characterization of which confirms that the xerogel is a {W₇₂Fe₃₀} type giant Keplerate-based POM compound [Fe(H₂O)₆]₁₄[W₇₂Fe₃₀O₂₅₂(H₂O)₇₂(OH)₆₀]·166H₂O ({W₇₂Fe₃₀}_XG) and the basic building unit of the gel is {W₇₂Fe₃₀} cluster. The xerogel is macroporous material characterized with 60 hydroxyl groups per formula unit and these hydroxyl groups are acidic in nature. Interestingly, the title xerogel {W₇₂Fe₃₀}_XG, which is nothing but an inorganic acid and an inexpensive metal-oxide-based material, exhibits proton conduction in its solid state. The xerogel material shows super proton conductivity of 1.71 × 10^-2 S cm^-1 at 80 °C and 98% relative humidity with an activation energy (E_a) of 0.18 eV.