Microwave-assisted hydrothermal synthesis of crystalline WO3–WO3·0.5H2O mixtures for pseudocapacitors of the asymmetric type

Crystalline tungsten oxide mixtures, WO3–WO3·0.5H2O, prepared by microwave-assisted hydrothermal (MAH) synthesis at 180 °C for various periods, show capacitive-like behavior at 200 mV s−1 and CS ≈ 290 F g−1 at 25 mV s−1 in 0.5 M H2SO4 between −0.6 and 0.2 V. Oxide rods can be obtained via the MAH process even when the synthesis time is only 0.75 h while WO3·0.5H2O sheets with poor capacitive performances are obtained by a normal hydrothermal synthesis process at the same temperature for 24 h. The aspect ratio of tungsten oxide rods is found to increase with prolonging the MAH time while all oxides consist of WO3 and WO3·0.5H2O. The oxide mixtures prepared by the MAH method with annealing in air at temperatures ≤400 °C show promising performances for electrochemical capacitors (ECs). Due to the narrow working potential window of the oxide mixtures, an aqueous EC of the asymmetric type, consisting of a WO3–WO3·0.5H2O anode and a RuO2·xH2O cathode, with a potential window of 1.6 V is demonstrated in this work, which shows the device energy and power densities of 23.4 W kg−1 and 5.2 kW kg−1, respectively.