Proton conductivity of tungsten trioxide hydrates at intermediate temperature

Proton conductivity of layered hydrates WO3·2H2O and WO3·H2O was investigated in the temperature range of 273–423 K under various water vapor pressures (pH2O). Dihydrate exhibited conductivity as high as 7×10−3 S cm−1 at 423 K under pH2O/pS=1 (pS: equilibrium pressure, about 0.6 MPa at this temperature). Conductivity stayed at this level until the compound started decomposing with the release of inter-layer water at a low pH2O/pS. The activation energy of conductivity for the 323–423 K region was 0.36 eV. At lower temperature, however, conductivity depended very strongly on water pressure, especially in the region near pH2O/pS=1. The activation energy here was much smaller, 0.15 eV, which was typical of the surface water mechanism. For monohydrate WO3·H2O, the temperature dependence of conductivity was uniform with the activation energy of 0.13 eV through the range of 298–423 K. Its conductivity at 423 K and pH2O/pS=1 was about one order of magnitude lower than that of dihydrate.