Katsuhiko Naoi

Second generation ‘nanohybrid supercapacitor’: Evolution of capacitive energy storage devices

Nanoscience and nanotechnology can provide tremendous benefits to electrochemical energy storage devices, such as batteries and supercapacitors, by combining new nanoscale properties to realize enhanced energy and power capabilities. A number of published reports on hybrid systems are systematically reviewed in this perspective. Several potential strategies to enhance the energy density above that of generation-I electric double layer capacitors (EDLC: activated carbon/activated carbon) are discussed and some fundamental issues and future directions are identified. We suggest a new hybrid supercapacitor system that is able to meet the energy and power demands for a variety of applications, ranging from microelectronic devices to electrical vehicles, which presents itself as a breakthrough improvement. Two practical hybrid supercapacitor systems, namely, a lithium-ion capacitor (LIC: graphite/activated carbon) and a nanohybrid capacitor (NHC: (nc-Li4Ti5O12/CNF composite)/activated carbon), are featured and compared. The proposed NHC can pave the way toward generation-II supercapacitor systems by taking advantage of a novel, high quality, high efficiency and inexpensive nanomaterial preparation procedure. With such a breakthrough in nanofabrication–nanohybridization technology, the NHC, which utilizes an ultrafast nano-crystalline Li4Ti5O12, is considered to be an alternative for conventional generation-I EDLCs.

Electrochemical Polymerization of Electroactive Polyaniline in Nonaqueous Solution and Its Application in Rechargeable Lithium Batteries

An electroactive polyaniline (PAn) was successfully prepared from nonaqueous propylene carbonate (PC) solution containing acid and electrolyte . PAn deposition and the doping‐undoping process of deposited PAn film were investigated by consecutive cyclic voltammetry in various electropolymerization solutions. It is confirmed by infrared spectral measurement that there is no essential difference in the manner of bonding between the electroactive PAn films deposited from aqueous and PC solutions. Inspection by scanning electron microscope revealed that PAn films deposited from aqueous solution had a fibrous structure, whereas the PAn film deposited from PC solution had a porous surface which resembled polypyrrole films. The shape of the electro‐oxidation curve is found to be essentially different, depending on the solvent or the value of acid in an electropolymerization solution. An electroactive PAn film deposited from PC solution was applied to an effective cathode material of a rechargeable lithium battery . The (deposited from PC solution) battery showed a high coulombic efficiency and high‐energy density in charging‐discharging behavior. The electroactivity of PAn film deposited from PC solution was found to be as high as, or higher than, that of PAn film deposited from aqueous solution.

Supercapacitor Performance of Hydrous Ruthenium Oxide Electrodes Prepared by Electrophoretic Deposition

Electrodes of hydrous ruthenium oxide were prepared by electrophoretic deposition (EPD) without binder and carbon additives. Through EPD and heat-treatment (250°C), a network of nanoparticles (ca. 10 nm) was developed with porous structure (packing density ca. 1.9 g/cm 3 ). As a result, high specific capacitance (734 F/g at I mV/s and 608 F/g at 50 mV/s) was obtained, where specific energy and power were as high as 25 Wh/kg (at 92 W/kg) and 21 kW/kg (at 12 Wh/kg), with loading weight of 0.19 mg/cm 2 . At higher loading weight (>0.4 mg/cm 2 ), specific capacitance decreased gradually. Also, volumetric energy was high because low-density additives, such as porous carbon and binder, were not used. From electrochemical impedance spectroscopy, specific energy and power were calculated, showing good agreement with values measured by CV. The decrease of rate capability with thicker electrode could be correlated by distribution of the time constant (τ), which was calculated from impedance spectra by using an equivalent circuit with multiple Warburg elements.

Hydrous RuO2/Carbon Black Nanocomposites with 3D Porous Structure by Novel Incipient Wetness Method for Supercapacitors

For supercapacitor electrode material, hydrous RuO 2 /carbon black nanocomposites were prepared by the novel incipient wetness method using a fumed silica nanoparticle. First, hydrous RuO 2 /fumed silica/Ketjen black (KB) was synthesized by the sol-gel-based method. After dissolving the fumed silica, the hydrous RuO 2 /KB nanocomposite, which is composed of RuO 2 nanoparticles (20-60 nm) dispersed on the high-surface-area KB (1180 m 2 g - 1 ), was formed with 3D porous structure at high loading of 60 wt % RuO 2 (Ru content is 46 wt %). The hydrous RuO 2 /KB nanocomposite electrode exhibited a specific capacitance of 647 F g - 1 with high charge utilization of RuO 2 (72%). which is significantly higher than reported values by other workers at similar loading of RuO 2 . The high capacitance and the high charge utilization were probably due to enhanced proton paths within the 3D porous structure of the nanocomposite materials.

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.

Electrochemistry of Poly(1,5-diaminoanthraquinone) and its application in electrochemical capacitor materials

A poly(1,5-diaminoanthraquinone) [poly(DAAQ)], which is a conducting polymer condensed with 1,4-benzoquinone, was studied electrochemically and was proposed as a new category of electrochemical capacitor material. In common nonaqueous media, the poly(DAAQ) showed two sets of reproducible redox couples for repeated cycles. The potential window of the material was in the range from ca. {minus}1.5 to +1.0 V (vs. Ag/AgCl). In situ UV-visible spectroscopy suggested that the redox couple observed around {minus}1.5 V is responsible for the quinone group and that around +1.0 V is due to the {pi}-conjugated system. The poly(DAAQ) showed high conductivity (0.3--2.0 {Omega}{sup {minus}1} cm{sup {minus}1}) over a wide range of potential ({minus}2.0 to 0.8 V), which suggests extension of the {pi}-conjugated system. It is proposed here that one can construct an electrochemical capacitor device by utilizing poly(DAAQ) as both positive and negative electrodes. The authors constructed the poly(DAAQ)/poly(DAAQ) electrochemical capacitor for test purposes. The poly(DAAQ)/poly(DAAQ) electrochemical capacitor exhibited high specific energy (25--46 Wh/kg) and high specific power (10,200--30,500 W/kg) at discharge rates from 30 to 90 C.

The surface film formed on a lithium metal electrode in a new imide electrolyte, lithium bis(perfluoroethylsulfonylimide) [LiN(C2F5SO2)2]

A newly developed imide electrolyte salt, LiN(C 2 F 5 SO 2 ) 2 (LiBETI) was found to give very uniform, thin, and stable surface films on a lithium metal electrode in the propylene carbonate (PC) solution. LiBETI/PC was studied and compared to determine its ability to form such a stable surface film, with conventional electrolyte systems such as LiCF 3 SO 3 /PC, LiPF 6 /PC, and LiN(CF 3 SO 2 ) 2 /PC (LiTFSI/PC). The surface film formed in LiBETI/PC system was a hemispherical, and the composition of the film consisted mainly of LiF, which is similar to that in a LiPF 6 /PC system. Quartz crystal microbalance (QCM) and cyclic voltammetry (after the tenth cycle) indicated that the surface film formed in LiBETI/PC (ca. 50 nm) was thinner than those in LiPF 6 /PC (ca. 90 nm), LiTFSI/PC (ca. 140 nm), or LiCF 3 SO 3 /PC (ca. 255 nm). The variation of the resonance resistance (AR) obtained from in situ CV/QCM measurement, which has been demonstrated to be a good measure of the surface roughness, also suggested that LiBETI/PC system gave a compact and smooth surface topology during lithium deposition-dissolution cycles. Impedance spectroscopy together with preliminary cycling tests showed that the LiBETI/PC system provides the highest cycling efficiency and improved cycleability among existing electrolyte salt systems in rechargeable battery systems employing lithium metal anodes.

Simultaneous electrochemical formation of Al2O3/polypyrrole layers (I): effect of electrolyte anion in formation process

We studied the capability of various kinds of sulfonate-based electrolytes to form a bilayered Al2O3/polypyrrole (PPy) film on an aluminum substrate in aqueous solution. As electrolytes, we selected sodium p-toluenesulfonate (monovalent sulfonate), sodium naphthalene-trisulfonate (trivalent sulfonate), poly(sodium 4-styrenesulfonate), poly(sodium vinylsulfonate) (sulfonate polymer), sodium n-dodecylbenzenesulfonate (SDBS), sodium butylnaphthalenesulfonate(BNS), and sodium n-dodecylsulfate (SDS, surfactant sulfonate/sulfate). Among the sulfonates we investigated, SDBS, BNS and SDS appeared to form a barrier-type Al2O3 layer with high coulombic efficiency in the absence of pyrrole. In the presence of pyrrole, anodization of these electrolytes resulted in the formation of Al2O3 and PPy layers, simultaneously.

Conducting polymer films of cross-linked structure and their QCM analysis

The authors investigated polypyrrole (PPy) films for the purpose of obtaining microporous structures for high power-density supercapacitor applications. The PPy films were formed with dopants based on a naphthalene ring substituted with multiple sulfonate groups, viz. mono, di, and trisulfonates. Using the quartz crystal microbalance (QCM) method, we obtained in situ and semi-quantitative data for these PPy films during their electropolymerization and charge–discharge (doping–undoping) processes. The electropolymerization process of the PPy films in the presence of the multisulfonates (disulfonates and/or trisulfonates) involves an electrostatic cross-link, which is considered to produce microporous structures. AC impedance analysis indicated that the PPy/trisulfonate film had a higher diffusivity and a higher capacitance than the PPy/disulfonate and PPy/monosulfonate films.

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.