Proton conductive amorphous thin films of tungsten oxide clusters with acidic ligands

Three kinds of peroxo-polytungstic acid (PPTA, C-PPTA and N-PPTA) were obtained by reacting hydrogen peroxide with metallic tungsten, tungsten carbide or tungsten nitride, respectively. Polytungstates, C-PPTA and N-PPTA, were found to contain oxalate and nitrate ligands. Their proton conductivities were compared using thin film specimens spin-coated from their water solution. Conductivity of each as-coated film was in the range from 10⁻³ to 10⁻⁴ S cm⁻¹ under the relative humidity of 40% (25 °C). A sharp decrease in conductivity (to less than 10⁻⁷ S cm⁻¹ at 25 °C) was observed for PPTA without acidic ligands after thermal treatment at 80 °C. However, the effect of thermal treatment on C-PPTA or N-PPTA was much milder. A 80 °C-treated C-PPTA film showed the conductivity of 1.0 × 10⁻⁵ S cm⁻¹ (25 °C) with a very weak dependency on ambient humidity.