Shuichi Ishimoto

Degradation Responses of Activated-Carbon-Based EDLCs for Higher Voltage Operation and Their Factors

To investigate the degradation mechanisms of electric double-layer capacitor (EDLC) components using 1.0 M triethylmethylammonium (TEMA) tetrafluoroborate (BF 4 ) in propylene carbonate (PC), the failure-mode processes of positive and negative electrodes were characterized as a function of the applied voltage (2.5-4.0 V). When the cell voltage ranges below 3.0 V, no impedance spectra or surface morphology changes were observed, indicating that no side reactions occur in this case. In the voltage range from 3.0 to 3.7 V, the exfoliation of graphene layers in activated carbon (AC) and the formation of cracks were observed in the positive electrode over 4.9 V vs Li/Li + possibly due to the gasification of surface functional groups with adsorbed water. On the negative electrode, the adsorbed water is electrochemically reduced to H 2 gas and OH ― . The generated OH ― induces the Hoffman elimination of TEMA + and activates the hydrolysis of PC. These water-induced side reactions could be the most critical factors for higher voltage operation. In the higher voltage range (over 3.7 V), the accumulation of solid electrolyte interface films by electrochemical oxidation and the reduction of PC were observed for both electrodes, indicating that the electrochemical oxidation and the reduction of PC on the AC surfaces occur above 5.2 V and below 1.5 V vs Li/Li + , respectively.

Encapsulation of Nanodot Ruthenium Oxide into KB for Electrochemical Capacitors

Nanosized hydrous RuO2/Ketjen Black KB composites were prepared using an in situ sol-gel process induced by ultracentrifugal mechanical force for supercapacitors. The hydrous RuO2 in the prepared samples were nanosized particles ca. 2 nm and were highly dispersed on conductive carbon KB even at high hydrous RuO2 content 50 wt % . The composite annealed at 150°C exhibited the high specific capacitance of 821 F g−1, which corresponds to a charge utilization as high as 96%. Such a high charge utilization could be because the nanoparticles have both outer and inner hydrous channels which facilitate ionic transport. A model capacitor assembled using the composite exhibited energy and power densities as high as 12 Wh kg−1 and 6 kW kg−1, respectively.

High-Voltage Asymmetric Electrochemical Capacitor Based on Polyfluorene Nanocomposite and Activated Carbon

An asymmetric electrochemical capacitor based on polyfluorene/carbon nanocomposite as positive electrode and activated carbon as negative electrode with nonaqueous electrolyte of 1 M tetraethylammonium tetrafluoroborate/propylene carbonate was investigated. From a galvanostatic charge-discharge test, the asymmetric capacitor test cell exhibited cell voltage as high as 3.2 V because of the high redox potential of the positive electrode (∼ 1.1 V vs Ag/Ag + ). Specific capacitance obtained for the test cell was 34 F g -1 (per electrode mass). The maximum energy density of the test cell was 47 Wh kg -1 (per electrode mass), which was two times higher than that of typical double-layer capacitors based on activated carbon/activated carbon electrodes.

Advanced hybrid capacitor with lithium titanium oxide for automobile

We have developed a next generation capacitor named Nano-hybrid capacitor (NBC), which uses a completely new lithium titanium oxide (LTO)-based negative electrode material comprising of nano-crystalline LTO grafted onto carbon nano-fibers (nano-LTO/CNF composite). The nano-LTO/CNF composite is simply prepared by using “Nano-hybrid technique”. The NBC has realized higher energy performance than electric double layer capacitor (EDLC) as a conventional capacitor, maintaining a good power performance as high as the EDLC.