ASYMMETRIC HYDROUS RUO_2 ULTRACAPACITORS WITH SELF-CHARGING CAPABILITIES

摘要

Hydrous ruthenium oxide (RuO_2·xH_2O or RuO_xH_y) has a high specific capacitance, making it an attractive material for ultracapacitor electrodes. Hydrous RuO_2 ultracapacitors are usually fabricated with symmetric positive and negative electrodes, which can hold a potential difference up to 1.4 V when fully charged. A disadvantage of the electrodes is that the high protonic and metallic conductivity plus the catalytic nature of hydrous RuO_2 makes self-discharge and potential recovery a significant problem in practical applications. In this manuscript, we demonstrate that the potential recovery and self-discharge phenomena in hydrous RuO_2 and Pt, respectively, can be used as an asset for self-recharging in asymmetric systems. Asymmetric ultracapacitors are fabricated with positive electrodes of hydrous RuO_2 on Pt foil and negative electrodes of Pt foil. After assembly, the cells have open circuit voltages (OCVs) of 0.2 to 0.7 V, which vary with the concentrations of sulfuric acid in the electrolyte, the composition of the electrodes and the number of cycles performed. The cells are discharged from their OCV to 0 V in pulses of 4 to 400 μA/cm~ to yield mean powers ranging from 0.28 μW/cm~2 to 0.7 mW/cm~2. While resting, the cells recharge to their original OCV in 30 to 240 min due to self-discharge and potential recovery reactions. To date, these asymmetric ceils have been operated for over 800 hours and more than 200 cycles. In the future, these self-recharging hydrous RuO_2/Pt systems may be useful as power sources for autonomous microsystems.