Characterization of RuO₂·xH₂O with various water contents

Hydrous ruthenium oxides (RuO₂·xH₂O) with different contents of water (x) were prepared by annealing commercial RuO₂·2.6H₂O powders at different temperatures. The morphologies and crystalline structures of RuO₂·xH₂O were investigated using transmission electron microscope (TEM) and selected area electron diffraction (SAED) techniques. From the TEM images, it was observed that the particle size of RuO₂·xH₂O increased with increasing annealing temperature. From the SAED patterns, it was observed that RuO₂·xH₂O powders became an amorphous phase at annealing temperatures <116 °C and became a crystalline phase at annealing temperatures above 116 °C. Amorphous RuO₂·xH₂O prepared at 116 °C reached its maximum specific capacitance as a result of proton insertion into the bulk of RuO₂ but with smaller Ru-Ru distance in the local structure. The more disordered structure induced by proton insertion was obtained by SAED pattern from a sample annealed at 116 °C. The possible connection between the microstructure and specific capacitance of RuO₂·xH₂O is discussed.