A fully reduced {V^IV18 O₄₂} host and VO₄^3-, Cl^- as guest anions: Synthesis, characterization and proton conductivity

Fully reduced polyoxometalates (POMs) generally have a high negative charge that can be compensated by diverse cations including protons. The POMs with a large number of acidic protons often exhibit solid state proton conduction. We synthesized two polyoxovanadate (POV)-based compounds, [Na₇(H₂O)₁₄][H₈VIV18O₄₂(V^VO₄)]·N₂H₄·7H₂O (1) and [Na₅(H₂O)₁₆][H₈VIV18O₄₂(Cl)]·4N₂H₄·6H₂O (2), each of which accommodates eight acidic protons (per formula unit) acting as the cations. Compounds 1 and 2 have been characterized by routine analytical techniques, such as, IR, Raman, XPS, PXRD, TGA, etc., elemental analysis, and unambiguously by single crystal X-ray diffraction. As expected, compounds 1 and 2 having eight protons per formula unit exhibit proton conductivity in the solid state. Proton conductivity measurements of compounds 1 and 2 were performed (a) at variable temperature (20-50 °C) at constant humidity (70% RH) and (b) variable humidity (relative humidity from 40% to 70%) at constant temperature (30 °C). The proton conductivity was observed to increase with an increase in temperature and humidity. The best conductivity values were obtained at 50 °C at a RH of 70% and found to be 3.62 × 10^-3 S cm^-1 for compound 1 and 2.16 × 10^-4 S cm^-1 for compound 2. Reproducibility of proton conductivity was verified by repetitive data recording over a long time period and determining the mean value of conductivity of compounds 1 and 2 at 50 °C. The activation energies for 1 (23.34 kJ mol^-1) and 2 (18.40 kJ mol^-1) calculated from Arrhenius plots of temperature dependence of proton conductivity suggest a Grotthuss proton hopping mechanism for solid state proton conduction. Controlled experiments have shown that the synthesized compounds 1 and 2 are stable and robust under operational conditions.