Exploring Proton Conductivity Studies of a Copper-based Hydroxylated Polyoxometalate

A sole inorganic framework-material formulated as [Li(H₂O)₄][{Cu^I(H₂O)_1.5} {Cu^II(H₂O)₃}₂{W^VI₁₂O₃₆(OH)₆}]·N₂·H₂S·3H₂O (1), consisting of a hydroxylated polyoxometalate (POM) anion, [{W^VI₁₂O₃₆(OH)₆}]⁶⁻ and a mixed-valent Cu(II)- and Cu(I)-aqua cationic complex species, [{Cu^I(H₂O)_1.5}{Cu^II(H₂O)₃}₂]⁵⁺, has been utilized to explore its proton conductivity studies. The POM cluster anion is functionalized with six hydroxyl groups, meaning there are six W^VI-OH groups per cluster unit. Compound 1, insoluble in water, can be described as an inorganic acid because its water suspension shows an acidic pH. The presence of these six hydroxyl groups per cluster unit having six labile protons has encouraged us to perform proton conductivity studies of this system. The highest proton conductivity of compound 1 is 1.72 ×10⁻² S cm⁻¹ at a temperature of 80 °C at 98% relative humidity (RH). Arrhenius plot has been constructed from the temperature-dependent proton conductivity values. The concerned Arrhenius plot has shown good linearity throughout the temperature range of 40 to 80 °C, revealing an activation energy of 0.41 eV. The compound has also shown remarkable stability in the conductivity value for 40 h.

Graphical Abstract

The proton conductivity studies of a framework-material [Li(H₂O)₄][{Cu^I(H₂O)_1.5} {Cu^II(H₂O)₃}₂{W^VI₁₂O₃₆(OH)₆}]·N₂·H₂S·3H₂O (1) have been performed. Compound 1 exhibits proton conductivity value of 1.72 ×10⁻² S cm⁻¹ at a temperature of 80 °C at 98% relative humidity. The concerned Arrhenius plot gives the activation energy value of 0.41 eV.