Melamine resin containing boron was synthesized from boric acid, melamine, and formaldehyde. B/C/N materials were prepared by carbonization in an inert gas atmosphere at 800-1200 °C. The boron/carbon atomic ratio (B/C) increased from 0.02 to 0.42 as a result of the formation of more B-N bonds with increasing heat treatment temperature, and it is the formation of these bonds that fixes the nitrogen in the structure. XPS measurements indicate that nitrogen atoms are involved in the following bonding states: B-N, pyridine-type N, quaternary-N and B-N-C. It is clear that B-N-C and B-N bonds are formed during carbonization at temperatures >800 °C, where the elimination of nitrogen is usually observed, and these are responsible for the N retention. As a result, a high N/C atomic ratio of 0.09-0.42 was observed for a carbonization temperature greater than 800 °C. The EDLC capacitance of a material heat treated at 1000 °C with N/C of 0.24 had a high value of 500 F·g^-1 in spite of having a smaller specific surface area compared to that of activated carbon widely used in EDLC.

View full abstract

To make use of the strong advantage of electric double layer capacitors (EDLCs), the authors developed two EDLCs; a rapid type EDLC of 0.1 ΩF class and a higher temperature durable type EDLC at 105 °C. The time constant (ΩF) of the rapid type EDLC was far lower than that of a conventional one because it has thin carbon layers on the electrodes and low resistance especially between the current collector and the electrode. An elliptical spiral type EDLC (160 F class) and a module (40 cells) was developed. Most of the electrical energy that is produced when a motor starts and stops (regenerative energy) goes to waste as heat. To save on the electrical power requirement of a motor, a device with rapid (<1 second) charge and discharge energy storage (0.1 ΩF class) is necessary. Durability at high temperatures is required for an electric power storage device placed near the motor and the power electronics devices. An elliptical spiral type EDLC (1000 F class) that is durable at 105 °C and a module (10 cells) was developed. The EDLC modules were connected in parallel with lithium ion battery modules, and were charged and discharged at 10-mode (one of the running patterns of a car). It could extend the life of the lithium ion battery by sharing the current with the EDLC.

View full abstract

We propose a novel electrochemical capacitor (EC) that used electrolyte-based redox reactions of halogen species, which are induced by a novel pretreatment of an activated carbon positive electrode. This treatment, consisting of an impregnation of pores at a positive electrode of activated carbon fiber cloth (ACFC) with a halogenated solution such as bromine water before cell assembly, is simple and convenient. An aqueous sodium bromide solution is applied to an EC cell containing the treated positive electrode and a non-treated negative electrode of ACFC. Few studies have so far obtained ECs using an “electrolyte” charge storage system with practical performance due to diffusive loss of charged species from an electrode into an electrolyte or its shuttle migration between electrodes. Nonetheless, we have attained excellent capacitor performance with such a concept by suppressing an undesirable diffusion of electrolyte reactive species. This concept, electrolyte-based charge storage, is applicable to not only the above aqueous EC but also non-aqueous EC and lithium-ion capacitor application with dramatically improved performance.

View full abstract

Electrochemical capacitors use activated carbons for both positive and negative electrodes that show a non-faradaic, double-layer charge-discharge mechanism in a symmetric configuration. Thus electrochemical capacitors are efficient energy storage devices that exhibit long lifespans and extremely rapid charge-discharge characteristics compared with batteries. Today, capacitor technology is regarded as promising and has an additional advantage with increasing effectiveness when combined with solar and wind regenerative energy sources. In recent years, composite battery materials have been vigorously researched in the hope of improving their energy density. Hybridizing battery and capacitor materials overcomes the energy density limitation of existing generation-I capacitors without much sacrifice of the cycling performance. Normal battery-capacitor hybrids use a high-energy and sluggish redox electrode and low-energy and fast double-layer electrodes, possibly producing a larger working voltage and higher overall capacitance. In order to smoothly operate such asymmetric systems, however, the rates of the two different electrodes must be highly balanced. Especially, the redox rates of the battery electrodes must be substantially increased to the levels of double-layer process. In this perspective we summarize various hybrid systems and show representative aqueous and non-aqueous asymmetric configurations for their energy-power improvement. We attempt to identify the essential issues for the realizable hybrids and suggest ways to overcome the rate increase by exemplifying ultrafast performance of the Li₄Ti₅O₁₂ nanocrystal prepared by a unique in-situ material processing technology under ultra-centrifugation.

View full abstract

The lithium ion capacitor (LIC) is a new hybrid capacitor, in which the materials and the charge and discharge processes are different in the positive and negative electrodes. Generally, positive electrode materials are a kind of activated carbon as used for an EDLC electrode, and negative electrode materials are a kind of carbon materials as used for an LIB negative electrode. Also the negative electrode of the LIC is doped in advance with lithium ions using a pre-doping process. Because of the difficulty of the pre-doping process, early LICs had a small capacitance, but the development of a new pre-doping method is a breakthrough, and the commercialization and mass production of a large capacitance LIC has been accelerated. With the negative electrode doped with lithium ions, the LIC achieves a four times higher energy density than that of an EDLC, and also maintains the superior performance of EDLC, i.e. high power density and very long cycle life. The LIC has already started to be used in various industries such as the energy, electrical power and automobile industries. The LIC will replace not only EDLCs but also other secondary batteries such as the lead acid battery or the LIB in many industries in the near future.

View full abstract

We show, through some examples, that chemical activation by alkaline hydroxides permits the preparation of activated carbons with tailored pore volume, pore size distribution, pore structure and surface chemistry, which are useful for their application as electrodes in supercapacitors. Examples are presented discussing the importance of each of these properties on the double layer capacitance, on the kinetics of the electric double-layer charge-discharge process and on the pseudo-capacitative contribution from the surface functional groups or the addition of a conducting polymer.

View full abstract

Various porous carbons were produced with a small mass-production scale hydroxide-activation furnace using different carbon materials. Some products derived from soft-carbons showed high electric double layer capacitances, large liquid phase iodine adsorption and large methanol vapor adsorption in spite of poor BET surface area.

View full abstract

Carbon nanofibers were prepared by the exfoliation of natural graphite flake in a propylene carbonate solution containing a lithium salt and sonication. The electrochemical behavior of the carbon nanofibers used as the negative electrode in lithium-ion batteries was investigated. Carbon nanofibers heat-treated at 600 °C had a diameter of several tens of nanometers. From X-ray diffraction measurements and Raman spectroscopy, it was found that a graphitic structure derived from natural graphite flake remained in the carbon nanofibers. Carbon nanofibers heat-treated at 400 °C showed a higher reversible capacity than NG-7 (natural graphite powder) and a rapid response in the cyclic voltammogram. This is on account of the nanosize effect. The best electrochemical performance among all the carbon nanofibers was obtained by heat treatment at 400 °C. The rate performance of the carbon nanofiber heat-treated at 400 °C was also examined.

View full abstract

MgO-templated mesoporous carbons with different heat treatments are examined as both negative and positive electrodes in lithium-ion capacitors. MgO-templated carbon heated at 2200 °C has a specific surface area of 800 m²/g and a total pore volume of 1.2 cm³/g, and shows superior performance as a negative electrode of a Li-ion capacitor when compared to artificial graphite. MgO-templated carbon heated at 1000 °C shows a larger capacitance as a positive electrode than commercial activated carbons for an electric double layer capacitor. This unique performance of MgO-template carbons is attributed to a combination of both high surface area and a highly uniform mesoporosity with a narrow pore size distribution.

View full abstract