A fully reduced {V18IVO42} host and VO43-, 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, [Na-7(H2O)(14)][(H8V18O42)-O-IV((VO4)-O-V)]center dot N2H4 center dot 7H(2)O (1) and [Na-5(H2O)(16)][(H8V18O42)-O-IV(Cl)]center dot 4N(2)H(4)center dot 6H(2)O (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 degrees C) at constant humidity (70% RH) and (b) variable humidity (relative humidity from 40% to 70%) at constant temperature (30 degrees C). The proton conductivity was observed to increase with an increase in temperature and humidity. The best conductivity values were obtained at 50 degrees C at a RH of 70% and found to be 3.62 x 10(-3) S cm(-1) for compound 1 and 2.16 x 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 degrees 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.