In terms of general social trends and technical trends in the electronic components industry, environmental problems and safety have caught attention and are progressing the acquisition of ISO-9000 and ISO-14000, lead-free soldering, dehalogenation, and the adoption of other environmental countermeasures. Technically, reduction in the voltages and costs of CMOS IC and LSI chips has rapidly increased new products related to mobile communications, such as mobile phones and electronic notebooks, and the tendencies towards more lightweight and more compact design of electronic components and towards their cost reduction are also progressing. Also, typical examples of new technical trends are cost reduction and functional enhancement of electronic components and the use of organic electrolytes, low-molecular-weight organic electrolytes, liquid-crystal polymers, and other new polymer materials, in electronic components as environmental protection measures. These tendencies and trends are significant in the capacitor field as well. The authors intend to give an overview of these tendencies and trends.
Aluminum coated with ZrO2, SiO2, and BaTiO3 films by a sol-gel dip coating was anodized to examine the structure and dielectric properties of anodic oxide films. Anodizing leads to the formation of anodic oxide films, which consist of an outer Al- (Zr, Si, BaTiO3) composite oxide layer and an inner Al2O3 layer, at the interface between the coated-oxide layer and Al substrate. The composite oxide converted into Al2O3 at the interface between the outer and inner layers during anodizing for the ZrO2- and BaTiO3-coated specimens, whereas Al2O3 converted into Al-Si composite oxide for the SiO2-coated specimens. The capacitance of the anodic oxide films formed on ZrO2- and SiO2-coated specimens was about 20% larger than that of anodic oxide films on aluminum without coating, and the capacitance for BaTiO3-coated specimens was almost identical to that without coating. Film formation mechanisms are discussed in terms of inward transport of Si-bearing anions and outward transport of Zr- and Ti-bearing cations across the composite oxide layer.
An idea of new electrical storage called ECS (Energy Capacitor System) composed from specially developed capacitors and electronic circuits, has opened up variety of applications and possibilities. A Copernican design of capacitor is provided by trading off internal resistance with energy density, and by charging serially connected capacitors from a current source to obtain better efficiency. Produced capacitors have achieved the energy density of 10 Wh/L at 2ΩF (ohms for each farad) and 16.6 Wh/L at 100ΩF, together with a record cell energy density of 27 Wh/L at 150ΩF. Problems inherent to the capacitor storage systems such as inefficient charging, imperfect utilization of stored energy, large change in terminal voltage and uneven voltage distribution to individual capacitor have been solved by combined electronic circuits. Experienced applications are ranging from solar systems to vehicles. A 15 t CNG hybrid bus powered by a 100 kW ECS has accomplished 2.36 times improvements in fuel mileage. As for power load levelers supported by Japanese national project, a 5.8 kWh system has been built and resulted 84% AC to AC, 94% DC to DC efficiency.
To use a polymer electrolyte successfully in the electric double-layer capacitor (EDLC), it is important to design the interface between the carbon electrode and the polymer electrolyte. In this work, interfacial properties of carbon/poly (ethylene oxide) (PEO) solid electrolyte and carbon (or activated carbon)/polymer gel electrolyte were investigated. The all-solid-state EDLC with a pair of isotropic high-density graphite (HDG) electrodes possesses a high capacitance in PEO /LiClO4 ([EO] / [Li+] =8) solid polymer electrolyte. The capacitance of HDG electrode was strongly influenced by temperature and the degree of crystallinity of PEO-LiClO4. Furthermore, various gel electrolytes were evaluated. The PMMA (poly(methyl methacrylate)), PVdF (poly(vinylidene fluoride)) and PVdF-HFP (poly(vinylidene fluoride-hexafluoropropylene) based gel electrolytes show a good electrochemical stability on the HDG electrode and a sufficient mechanical strength. In the case of using activated carbon (AC) powder (as well as ordinary carbon powder) as the electrode material for EDLC with PVdF-HFP gel electrolyte, the electrode composed of AC (or carbon powder) and the gel electrolyte exhibits a higher specific capacitance and a lower ion diffusion resistance than does the electrode prepared with a dry polymer binder. The highest specific capacitance of 123 F g-1 was achieved with a composite electrode containing AC powder with a specific surface area of 2500 m2 g-1.
Aiming to increase the capacitance with double-layer capacitance and pseudocapacitance, metal-loaded active carbon fiber (ACF) electrodes for use in electrochemical capacitors were prepared by the plasma enhanced chemical vapor deposition method. Platinum was used as the loaded material, and the preparation conditions such as substrate temperature, concentration of the aqueous Pt solution and plasma output were investigated. For the Pt-loaded ACF samples with a specific surface area of 1,500 m2/g, the capacitance increased by 24% compared to the untreated sample, and the highest electric capacity value of 306 F/g was achieved at a substrate temperature of the 250℃.
The rectangular-type electric double layer capacitor was obtained with sheet-type electrodes with a high capacitance density. The energy density and the power density were 11.6 Wh/L and 600 W/L, respectively. The capacitor showed high performance in a low temperature region of −20°C. In charge-discharge experiments, the capacitor also showed very stable cycle-life performance, from which the estimated life of the capacitor was over 30,000 cycles.
A Cu-porous carbon composite prepared from sawdust was applied to the electrode of the electric double-layer capacitor (EDLC). The Cu-carbon composites were characterized and the properties of the electrode of EDLC were investigated. The carbonaceous material prepared from the sawdust and impregnated with Cu(NO3)2 showed higher capacitance than the commercial activated carbon. The capacitance was dependent on the Cu-content in the composite. The highest capacitance was obtained when Cu-content was 0.086 Cu-g/g.
Polymer gel electrolytes, composed of poly(vinylidene fluoride) (PVdF) and propylene carbonate (PC) as a plasticizer with the asymmetric ammonium salt triethylmethylammonium tetrafluoroborate (TEMABF4), have been applied to electric double layer capacitors (EDLCs) with activated carbon cloth electrodes. The present capacitors with TEMABF4 have a higher capacitance, better coulombic efficiency, and lower leak current than those with a typical electrolytic salt, tetraethylammonium tetrafluoroborate (TEABF4). Ac impedance measurements show that the capacitors with TEMABF4 have not only lower electrolyte-bulk resistances but also lower electrolyte/electrode-interface resistances in comparison with the capacitors with TEABF4.
Mesoporous activated carbon fiber (ACF) was prepared from a phenolic resin containing a small amount (0.1 wt%) of an organic nickel complex through carbonization and steam activation. Microporous ACF as reference sample was also prepared from a phenolic resin without any agent. In both cases, for the mesoporous ACFs and the microporous ACFs, the electric double layer capacitance in propylene carbonate containing 0.5 mol dm-3 (C2H5)4NBF4 (0.5 M TEABF4/PC) was not proportional to the BET specific surface area. This is due either to the low permeability of nonaqueous electrolyte or the low mobility of the ions in the narrow micropores. However, the mesoporous ACF showed a higher double layer capacitance than the microporous ACF. This result suggests that the presence of many mesopores promotes the formation of an effective double layer or the transfer of ions in the micropore.
Output-power of a solar cell system and a windmill power generation system shows large fluctuations, which may cause a deterioration of a power quality of the grid connected. We have studied new systems with banks of electric double-layer capacitors (EDLCs) to decrease the fluctuations. Output fluctuations of a solar cell system and a windmill power generation system were suppressed sufficiently with banks of EDLCs. A peak shift effect of the output energy was also obtained in the solar cell system. An effective energy storage even during a low wind velocity region was obtained in the windmill power generation system.
Both soluble and electronically conductive polyanilines (PAns) have been successfully prepared by use of perfuloroimide super-acids for solid electrolytic capacitor. The perfuloroimide-doped PAns showed one order of magnitude higher solubilities (>2.0 wt%) than the conventional PAn doped with sulfuric acid (<0.1 wt%) in common organic solvents. Besides showing very high solubilities, the imide-doped PAns showed the same order of magnitude higher electronic conductivity values (up to 2.0×10−1 Ω−1 cm−1) as that of the sulfuric acid doped one (0.7×10−1 Ω−1 cm−1). From spectroscopic and elemental analyses, the causes of the high conductivity and high solubility of the imide-doped PAns were found to be due to the low degree of branching of the polymer chains, the low molecular weight (around 20,000), and the diminished association between the polymer chains.
Ta2O5/polyethylenedioxythiophene (poly (EDT)) bi-layered film as dielectric and electrolyte exhibited low impedance characteristics of a solid Ta capacitor. The bi-layered film was simultaneously formed by electrooxidizing a Ta electrode in EDT monomer aqueous solution containing sulfonic surfactant electrolytes, i.e., sodium salts of n-dodecylbenzenesulfonate (SDBS), butylnaphthalenesulfonate (BNS) and n-dodecylsulfate (SDS). The bi-layered film consisted of Ta2O5 sustaining 50 to 200 V of formation voltage and electroactive poly (EDT) film.
The anodic stability of a series of electrolytes consisting of propylene carbonate (PC) solvent and BF4− anion was shown to be partially influenced by cation species. A lithium cation lowered the anodic stability of electrolytes, particularly when the content of the cation was 1 mol dm−3 or higher. It was considered that lithium cation had the high ability to coordinate anion species that provided the lower anodic stability. When the interaction between cation and anion was small enough, the higher anodic stability of BF4− anion was observed.
A rotating disk electrode technique was applied to the detection of the known impurities in a nonaqueous electrolyte solution used for electrochemical capacitors. Propylene glycol and fluoride ion, which are both hydrolysis products of the nonaqueous electrolyte solution composed of propylene carbonate solvent and triethylmethylammonium tetrafluoroborate salt, were quantitatively detected by a glassy carbon based rotating disk electrode in the concentration range of 10 to 100 ppm, where normal linear sweep voltammetry cannot be applied.
Rate capability of activated carbon fiber with controlled pore structure has been investigated as the electrode of electric double layer capacitors using organic electrolyte solutions. The pore structure of carbon fiber much influenced the rate capability, depending on the composition of the organic electrolytes. The carbon fiber with developed mesopore structure that has been prepared from pitch containing acetylacetonatoyttrium, Y(acac)3, showed high rate capability, whereas the conventional carbon fiber with smaller average pore size gave poor rate capability. Effects of the cation size of the electrolyte on the rate capability was relatively small for the activated carbon fiber with mesopore structure.
Thin film of Li-doped nickel oxide was prepared by the electrostatic spray deposition (ESD) technique onto a Pt coated silicon wafer and Pt coated alumina substrate with different deposition temperatures. Crystal structure, surface morphology and electrochemical properties of the LixNi1-xO film were studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and cyclic voltammetry. It is found that the deposition temperature had a significant impact on the structure, surface morphology and the number of electrochemically active site of the Li-doped nickel oxide and their corresponding electrochemical properties for the Li-doped nickel oxide film prepared by the ESD technique. The Li0.1Ni0.9NiO film deposited at 200℃ exhibits the maximum specific capacitance of 251 F/g with a high rate capability in 1 M KOH solution. This relatively high specific capacitance of this film was originated from its unique highly porous surface morphology and amorphous structure.
Electric double layer capacitors (EDLCs) with aqueous diluted hydrochloric acid solution were prepared, and their capacitance and performance were estimated. The electrodes were activated carbon cloth having the apparent surface area of 0.385 cm2. The capacitance of the test EDLC with 1 mol dm−3 LaCl3/2 mol dm−3 HCl solution on discharge was about 65 F·g−l (carbon cloth) at 10 mA·cm−2 and about 40 F·g−l at 40 mA·cm−2. The results showed excellent capacitance and high power density at short discharge time.
The energy loss in aluminum oxide dielectric films on etched capacitor foil was measured during application of bipolar constant current pulses. Pulse conditions produced a ripple voltage with approximate triangular waveshape superposed on a dc bias voltage. The charge and discharge energies during a cycle were obtained by integration under the V-t trace, and the energy loss is expressed as the difference between these quantities divided by the charge energy. The energy loss increases with bias voltage and with ripple voltage. There are two factors contributing to the energy loss. One is a field-dependent oxide dissipation factor (DF), with either linear or quadratic dependence on bias voltage, depending on oxide thickness. The field dependence results from prior exposure of the oxide to elevated temperature. The other factor is a steady state absorption current that scales with the product of bias voltage and ripple voltage. Under conditions typical of capacitor applications the loss can be significantly greater than the usual small signal value of oxide DF measured with a LCR meter.
Aluminum foil of aluminum electrolytic capacitors are produced by continuous electrolytic surface finishing. Metallic conductor roll has been used to feed anodic current to aluminum foil, which is usually called direct contact process. This conventional process has some disadvantages. Since physical contact area between metallic roll and aluminum foil is so small that huge Joule's heat is generated locally during the current passes, which results in sometimes melting or cutting of the foil. This problem limits the production capacity of the existing manufacturing facilities. On the contrary, indirect contact through electrolyte called liquid contact current supply process (simply called liquid contact process) has the following advantages. Liquid contact process can completely eliminate the problem of melting or cutting of the product foil because there is no physical contact on the aluminum foil. The current load and the running speed of aluminum foil can be easily and intentionally increased depending on the production demand. Original idea itself goes back to around 1950, but it did not come true for lack of a suitable anode.
Apparently non-porous activated carbon with a specific surface area less than 100 m2/g(BET) prepared from calcined carbon of petroleum coke is studied as the polarized electrode for an Electric Double Layer Capacitor(EDLC). The non-porous carbon makes negligibly small electric double layer when it is dipped into an electrolyte solution in the beginning. During the initial charging process, however, "solvent co-intercalation of ions" builds double layers in the positive and the negative electrodes, and during the discharging process, the excess ions go out, while the opposite ions come in, to maintain the double layers electrically neutral. After that, the electrodes behave like conventional activated porous carbon electrodes with an extra high dense capacitance. The observed phenomenon is discussed from the viewpoints of the inter-layer distance in graphite-like structure, the molecular volume of solvent for electrolyte, the residual functional groups such as active oxidized hydrogen observed by NMR, and of the elimination method for the residual active oxidized hydrogen by heat treatment in H2 atmosphere, as well as the preparation method for source carbon.
The electrochemical properties of Ru-Ca-Ox/Ti electrodes prepared by a dip-coating method were evaluated. A Ru (40%)-Ca(60%)-Ox/Ti electrode exhibited high pseudocapacitance of 143 mC cm−2 in an alkaline electrolyte which was 20 times larger than that of RuO2/Ti. This value was equivalent to 638 F g−1 for the RuO2 loaded on the electrode. The present electrodes have a potential for use as electrochemical capacitors.