A capacitor is an electrical device that can store energy in an electric field between a pair of closely spaced conductors. Its application as an energy storage device —an alternative to rechargeable batteries— has been receiving considerable attention. There are chemical capacitors using liquid electrolytes such as aluminum electrolytic capacitors and double-layer capacitors, and they utilize quaternary ammonium salts in their nonaqueous electrolytes. After a brief explanation of the working principles of these chemical capacitors and their requirements for electrolytes, the practical applications of quaternary ammonium salts for aluminum electrolytic capacitors and double-layer capacitors are reviewed from the historical and technological viewpoints based on research conducted in our laboratory.

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Formation of Nb₂O₅ films on aluminum was attempted by sol-gel coating, using NbCl₅ as a precursor. The structure and the dielectric properties of anodic oxide films formed on aluminum by sol-gel coating and the subsequent anodizing were examined. Anodic oxide film formed by this process was composed of an inner Al₂O₃ layer and an outer Nb₂O₅ layer. The capacitance of the anodic oxide films formed on Nb₂O₅-coated specimens was at most 34% higher than that of anodic oxide films on aluminum without coating.

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A new concept was proposed as the energy storage rubber to develop the batteries involving electrode materials in rubber matrix. The cathode active material (LiMn₂O₄) and conductive carbon were mixed with rubber material to give flexible electrode. Some rubber materials were tested for this purpose, and the acrylic rubber matrix cathode gave the good charge/discharge cycle. The initial discharge capacity was 123 mAh/g, which value corresponded to that obtained in electrolyte solution.

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An acidic polymer hydrogel electrolyte was successfully prepared from 4 M H₂SO₄ aqueous solution, poly(vinyl alcohol) and glutaraldehyde. The polymer hydrogel electrolyte showed higher ionic conductivity than that prepared from 1 M H₂SO₄ aqueous solution in our previous study. An electric double layer capacitor (EDLC) cell assembled using the polymer hydrogel electrolyte exhibited higher discharge capacitance in the wide range of current density than that using the hydrogel electrolyte with 1 M H₂SO₄ aqueous solution.

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The amorphous cobalt-nickel oxide nano-rods (diameter: 20∼30 nm, length: 100∼150 nm) were potentiodynamically deposited onto a stainless-steel electrode. Their structure and surface morphology were investigated by means of X-ray diffraction analysis and scanning electron microscopy. Their capacitive characteristics were evaluated in 1 M KOH electrolyte by using cyclic voltammetry and constant current charge-discharge methods. It was found that the capacitive characteristics of deposited cobalt-nickel oxide were strongly influenced by the deposition condition. A high specific capacitance value of ∼370 F/g and a specific power of 29 kW/kg were obtained when cobalt-nickel oxide was deposited in the aqueous solution of 0.15 M NiCl₂·6H₂O+0.05 M CoCl₂·6H₂O in potentiodynamic conditions. In comparison, the specific capacitance values for the deposited cobalt oxide and nickel oxide were 184 F/g and 118 F/g, respectively. The cycle-life test showed that the specific capacitance of cobalt-nickel oxide was highly stable even after 100,000 cycles, indicating its high potential for supercapacitor applications.

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A vertically aligned multi-walled carbon nanotube (MWCNT) electrode and a disordered MWCNT electrode are prepared to compare their structural differences and such capacitive performances as frequency response. The frequency response of a capacitor assembled with aligned MWCNT electrodes and a conventional PC-based electrolyte is much superior to a corresponding capacitor with disordered MWCNT electrodes. Such a merit of aligned MWCNT electrodes is observed even for an ionic liquid electrolyte system with high viscosity, diethylmethyl (2-methoxyethyl) ammonium tetrafluoroborate (DEMEBF₄). The obtained results demonstrate that the aligned MWCNT electrode has ideal ion-accessible structure with excellent electron pathways that provide a high-rate capability regardless of electrolytes.

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Origins of cathodic stability of an ionic liquid electrolyte, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄) containing lithium tetrafluoroborate (LiBF₄), are investigated mainly by applying electrochemical impedance spectroscopy for a glassy-carbon electrode interface. Its Helmholtz layer capacitance in the EMIBF₄ electrolyte containing Li⁺ becomes much higher than that in neat EMIBF₄ with increasing cathodic polarization. On the basis of such analyses as well as our hypotheses, a reasonable mechanism for the stabilizing effect induced by the Li salt addition is discussed.

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A novel electrode composed of activated carbon and DNA together with fluorinated binder has been prepared for an electric double layer capacitor (EDLC) with aqueous electrolytes. The DNA-loaded electrodes improve the rate capability and discharge capacitance of EDLC containing aqueous, neutral salt electrolytes. In contrast, the DNA-composite electrodes have a poor effect on the capacitance enhancement in acidic or basic electrolytes. The enhancement of discharge capacitance is significant especially at high-rate cycling in neutral salt electrolytes, because the presence of DNA reduces the internal resistance of an electrode and hence improves its rate capability.

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The pseudo-capacitance of ruthenium oxide electrode has been compared in organic solvent electrolytes having different solutes, tetrafluoroborate salts with tetraethylammonium and ethylmethylimidazolium cations, and various solvents using cyclic voltammetry. Some kinds of solvents such as acetonitrile promote substantial capacitance of ruthenium oxide in imidazolium-based electrolyte salt over tetraethylammonium-based one, while propylene carbonate exhibited no such difference between these two electrolyte systems. The pseudo-capacitance of ruthenium oxide in imidazolium-based electrolyte is stable over cycles under limited anodic potential range of 0.5 V vs. Ag. The contamination of ca. 1000 ppm of water induces further substantial capacitance only for imidazolium-based electrolyte.

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The effect of the addition of surfactants to an aqueous electrolyte on discharge property of the capacitor was investigated. Dodecylbenzene sulphonate surfactant drastically improved the specific capacitance of electric double layer capacitor (EDLC) electrode. The addition of surfactants promoted the electrolyte to access the interior portion of micropores. EDLC effective area was increased and subsequently specific capacitance was also increased. However, the specific capacitance depended on chemical structure and concentration of the surfactants added. The adsorbed surfactants interfered with mobillity of ions. This is another factor for decreasing the specific capacitance.

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A nanocomposite material based on polyfluorene (PF) loaded with a carbon black, namely Ketjen Black (KB), was investigated electrochemically as a cathode material for high-energy electrochemical capacitors. The nanocomposite was prepared by a chemical oxidation of fluorene monomer dissolved in the KB suspension. From TEM observation, thin PF films with 5-15 nm in thickness were loaded onto the surface of the aggregated KB particles. The nanocomposite based capacitor electrode exhibited high specific capacitance of 160 F g⁻¹ (260 F g⁻¹ per PF mass) in an electrolyte of 1 M tetraethylammonium tetrafluoroborate/propylene carbonate. More importantly, the charge was found to be stored at high potential ranged from 0.4 to 1.0 V vs. Ag/Ag⁺, which is higher than the redox potentials for conventional conducting polymers.

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A co-deposited film consisting of activated carbon (AC) and ketjenblack powder (KB) particles was prepared using the electrophoretic deposition (EPD) method and utilized as the electrode for a capacitor cell. As a result of evaluating various organic solvents as the dispersion solvent for the EPD bath, it was found that the best deposition condition was obtained using acetonitrile (AN). However, there was the problem that the co-deposited film consisting of AC and KB particles had a poor adhesion. The co-deposited film fabricated by adding polyvinylidine difluoride (PVdF) as a binder to the bath was able to be used as the electrode for a capacitor cell. The specific capacitance per unit weight of the electrode was almost equal to that of the commercial sheet electrode.

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The use of partially fluorinated compounds as alternative solvents can improve the performance of electric double layer capacitors (EDLCs). Electrolytic conductivity of fluoroethylene carbonate (FEC) is slightly lower than that of propylene carbonate (PC), which is commonly used as a solvent for EDLCs. However, the capacitance of a coin cell is higher than that obtained for PC.

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Propylene carbonate (PC), which shows high viscosity, is commonly used as a solvent for electric double-layer capacitors (EDLCs). The use of acetonitrile (AN) as an alternative solvent improves the performance of the EDLCs. Partial fluorination of organic solvents often increases their polarity. We focus our attention on the use of fluoroacetonitorile (FAN) as an alternative solvent for the EDLCs. Although electrolytic conductivity of FAN is slightly lower than that of AN, the capacitance of a model cell is higher than those obtained for AN and PC.

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The use of an asymmetric borate anion, difluoro(oxalato)borate (DFOB⁻), improves the solubility of the tetramethylammonium salt in propylene carbonate (PC) and the capacitances of electric double-layer capacitors (EDLCs). Electric conductivity of the PC solution containing tetramethylammonium difluoro(oxalato)borate (TMADFOB) was slightly lower than that obtained for tetraethylammonium tetrafluoroborate (TEABF₄), but higher than those obtained for tetramethylammonium bis(oxalato)borate (TMABOB) and tetraethylammonium bis(oxalato)borate (TEABOB). The gravimetric capacitances of measurement cells increased with decreasing the ion sizes of the electrolytes. The PC solution containing TMADFOB afforded the highest gravimetric capacitance.

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The double layer capacitance of an activated carbon nanofiber (ACNF) with a 100∼200 nm fiber-diameter, prepared by the polymer blend spinning technique, was investigated using 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF₄) as the electrolyte. The conventional activated carbon fiber (ACF) with the fiber-diameter of ∼10 µm and narrow micropores (∼0.7 nm pore-width) showed a significant irreversible adsorption of EMIm⁺ cation in EMImBF₄, however, the ACNF effectively suppress the irreversibility even with the pore structure comparable to the conventional ACF. This suggests the effect of the short pass length on the ion adsorbing/desorbing process. The decreased capacitance by the cycling in EMImBF₄ was recovered by the addition of propylene carbonate as an organic solvent to the electrolyte. This means that the irreversible adsorbed ions can be desorbed using propylene carbonate.

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The capacitance recovery of Ta capacitor using poly(3,4-ethylenedioxythiophene) (PEDOT) was evaluated. Capacitance recovery is defined as the ratio of the capacitance of Ta/oxide/PEDOT/Ag to that of Ta/oxide measured in aqueous electrolyte solution. The capacitance recovery was strongly dependent on the oxide formation voltage. From the analysis of this dependence we found an additional capacitance in series with the oxide capacitance which was independent of oxide formation voltage, and was present at the oxide/PEDOT interface. This additional capacitance was postulated by the presence of a depletion layer and poor contact because of the presence of polymerization byproduct at the oxide/PEDOT interface. However, the phenomenon of the additional series capacitance did not match well with the depletion layer capacitance calculated from semiconductor theory assumption.

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5-Azonia-spiro[4.4]nonane tetrafluoroborate [(CH₂)₄N(CH₂)₄]⁺BF₄⁻ is dissolved in dimethyl carbonate (DMC) to yield two liquid phases, DMC and salt-rich DMC solutions. The molecular structure of the [(CH₂)₄N(CH₂)₄]⁺ ion in the crystalline state and in the salt-rich DMC solution was studied by means of Raman spectroscopy at 298 K and theoretical DFT calculations. The [(CH₂)₄N(CH₂)₄]⁺ ion involves two pyrrolidinium (CH₂)₄N rings bridged through the N atom, and each pyrrolidinium ring involves various types of conformation. Theoretical DFT calculations for the [(CH₂)₄N(CH₂)₄]⁺ ion show that the distorted envelope-envelope conformers of the type E1-E1 and E1-E3 give relatively small energies. Observed Raman spectra of [(CH₂)₄N(CH₂)₄]⁺BF₄⁻ crystals and the salt-rich DMC solution were satisfactorily explained in terms of the favorable presence of the sole envelope-envelope conformer of the type E1-E1.

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Bimodal porous carbons consisting of interconnected macropores and spherical mesopores, were prepared by colloidal crystal templating method. The template was prepared by evaporation process of suspensions containing monodisperse poly[styrene-(co-2-hydroxyethyl methacrylate)] (PSHEMA) latex and colloidal silica in water. The carbonization of PSHEMA at 1000°C under inert atmosphere provided very thin carbon layer on the colloidal silica in the template, and the macropore corresponding to the PSHEMA particle size were formed simultaneously. After this procedure, bimodal porous carbons were obtained by removing the silica particles with hydrofluoric acid. Three kinds of bimodal porous carbons were prepared using PSHEMA latex of 450 nm and colloidal silica with three different average diameters of 10∼20 nm, 40∼50 nm, and 70∼100 nm, respectively. Bimodal porous structure was observed with a field emission-scanning electron microscope. Nitrogen adsorption/desorption measurements revealed that the prepared samples involved macropore and mesopore with small amount of micropore. The bimodal porous carbons were electrochemically evaluated as a negative electrode of lithium-ion capacitor in ethylene carbonate and diethyl carbonate containing 1 mol dm⁻³ LiClO₄. The bimodal porous carbon prepared using silica of 10∼20 nm showed a large capacitance of 360 F g⁻¹ at a high current density of 7.4 A g⁻¹.

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Novel non-aqueous proton (H⁺)-conducting polymeric gel systems have bee proposed for utilization as quasi-solid electrolytes in electrochemical capacitors. The gel systems consist of poly(vinylidenedifluoride) (PVdF) or poly(vinylidenedifluoride-co-hexafluoropropylene) (PVdF-HFP) matrix polymer swollen with dimethylformamide (DMF) solutions containing phosphoric acid (H₃PO₄) or trifluoromethanesulfonic acid (CF₃SO₃H). High ionic conductivity of 4.5 mS cm⁻¹ was obtained at 60°C for a PVdF-HFP-based polymeric gel containing 85 mass% of 0.5 M CF₃SO₃H/DMF with sufficient mechanical stability over the temperature range of 20–60°C. Redox process of hydrous ruthenium oxide, RuO₂·xH₂O, gave high pseudo-capacitance in the PVdF-HFP-based polymeric gel. The pseudo capacitance behavior was somewhat influenced by the composition of electrode/electrolyte interface. The discharge capacitance of ca. 350 F g⁻¹ (with respect to the mass of the oxide) was obtained for PVdF-HFP-based gel using RuO₂·xH₂O electrode prepared by a “wet process”.

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The effect of the H₂SO₄ concentration and cell temperature towards the pseudocapacitive behavior of layered ruthenic acid hydrate, H_0.2RuO_2.1·nH₂O, was examined. The increase in H₂SO₄ concentration resulted in a drastic decrease in the redox pair observed at 0.65 V vs. RHE, while the increase in cell temperature had little effect on this redox pair. The redox pair centered at ∼0.85 V vs. RHE was less dependent on the H₂SO₄ concentration and the charge associated with this peak increased with increasing cell temperature. It is suggested that the redox peak observed at ∼0.65 V vs. RHE is related to the adsorption of SO₄²⁻ ions on the outer surface of the microsized particles. The redox pair observed at ∼0.85 V vs. RHE is attributed to the penetration of protons or hydrated protons. The effect of cell temperature was negligible for nanoparticulate RuO₂·xH₂O, which indicates the difference in the charge storage mechanism of layered H_0.2RuO_2.1·nH₂O and nanoparticulate RuO₂·xH₂O.

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The electrochemical impedance Z and complex capacitance C for typical equivalent circuits were summarized systematically in order to support the frequency domain analysis of electric double layer capacitance (EDLC). In the present paper, the impedance and the complex capacitance were calculated in the case that the electrochemical response of porous electrode was presented by a transmission line model (TLM). The loci of Z and C on the complex were divided into the lumped constant and distributed constant ranges in low and high frequency ranges, respectively. The complex capacitance plot was superior to obtain the capacitance of porous electrode than the impedance plot.

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Tetraethylammonium bis(oxalato)borate (TEABOB), which is free from halogens, has the potential for use in electric double-layer capacitors (EDLCs). We have used a transmission-line network of finite length as the equivalent circuit of the average pore of an activated carbon electrode and derived the theoretical equation for a constant-current (CC) charge. We evaluated the performance of EDLCs by comparing the theoretical and the experimental charge curves. A 2025-type coin cell using TEABOB showed double-layer capacitance comparable to that obtained for tetraethylammonium tetrafluoroborate (TEABF₄). A very small distance between activated carbon electrodes in the EDLC may compensate for low ionic mobility of TEABOB in the bulk of the solution. The degree of ionic dissociation of TEABOB in the pores of the activated carbon electrode may not be appreciably lower than predicted from the bulk properties of the solution.

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An electrolyte composed of a tetra-alkyl ammonium cation having a spiro structure (spiro-(1,1′)-bipyrrolidinium: SBP) showed excellent performance in terms of electric conductivity, viscosity, and solubility in various solvents, especially propylenecarbonate (PC). The electric double-layer capacitor (EDLC) using spiro-(1,1′)-bipyrrolidinium tetrafluoroborate in PC (SBP-BF₄/PC) has a performance better than that using tetraethylammonium tetrafluoroborate in PC (TEA-BF₄/PC) and triethylmetylammonium tetrafluoroborate in PC (TEMA-BF₄/PC), especially at low temperatures and high discharging rates.

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Use of a novel electrolyte, a spiro-type quaternary ammonium salt (SBP-BF₄), in an electric double-layer capacitor (EDLC) has allowed the use of a linear-structure carbonate–which, although it has a low viscosity, had never been used as a solvent because it does not dissolve a conventional electrolyte–as a sub-solvent for an electrolytic solution in combination with a main solvent, propylenecarbonate (PC). Of such linear-structure carbonates, use of dimethylcarbonate (DMC) in an electrolytic solution attains not only low viscosity but also high conductivity of an electrolytic solution. Furthermore, an EDLC using the electrolytic solution exhibits very excellent capacitance, internal resistance, temperature properties, and discharging rate.

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