New charge storage mechanism for electrochemical capacitors and charge storage density vs. crystalline structure of metal oxides

The hydrous ruthenium oxide (RuO₂·xH₂O) powders with different contents of H₂O have been formed by a sol-gel process and have been annealed at different temperatures. It was found that the specific capacitance of RuO₂·xH₂O is strongly dependent on the crystalline structure. The specific capacitance of over 720 F/g was obtained in an amorphous RuO₂·xH₂O. This value is at least two times greater than ever reported of this type of material. The specific capacitance dropped rapidly as the amorphous phase transforms into the crystalline phase at high temperatures. In contrast to the crystalline phase the specific capacitance of an amorphous RuO₂·xH₂O was not dependent on the specific surface area. The charge storage mechanism in amorphous RuO₂·xH₂O is different from that in the crystalline RuO₂. The current in the cyclic voltammetric curve of RuO₂·xH₂O is nearly constant and is symmetrical with the potential in a range of 1 volts. The resistivity of pellets made from these powders is in the order of 10^-3 Ω-cm.