There has been a major effort, all over the world, to increase an energy density of electrochemical capacitors to meet more demands for electric automotive and regenerative energy storage applications. Hybridizing battery-capacitor electrodes can certainly overcome the energy density limitation of the conventional electrochemical capacitors because they employ both the system of a battery-like (redox) and a capacitor-like (double-layer) electrode, producing a larger working voltage and capacitance. 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. There are increasing number of published reports on hybrid systems. Several potential strategies to enhance the energy density above that of Gen.1 supercapacitors is now being discussed and some fundamental issues and future directions for Gen.2 & 3 will be identified in near future.

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Highly n-doped silicon nanowires (SiNWs) have been grown by a chemical vapor deposition process and have been investigated as possible electrodes for electrochemical capacitors (ECs) micro-devices. Their performances have been compared to existing literature on the field, which shows the use of SiNWs fabricated via different techniques, SiC coated SiNWs and porous silicon layers. The double layer capacitance of n-doped silicon wafer is ≈6 µF cm⁻² in standard organic electrolyte, and this value can be increased by nanostructuration of SiNWs up to 440 µF cm⁻² by tuning deposition parameters. Similar values are found in the literature. Symmetrical microdevices based on two identical SiNWs electrodes can be operated in organic based electrolytes within a 1.2 V voltage window. The devices show excellent cycling efficiency over more than 2000 cycles, with capacitance value of 51 µF cm⁻² and an energy density of 10 nWh cm⁻² (37 µJ cm⁻²). The increase of specific surface area by different techniques may drastically boost these values in the near future.

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The compatibility of ionic liquid 1-ethyl-3-methyl imidazolium tetrafluroborate (EMIBF₄) with a redox capacitor system with redox species, nickel phthalocynine (NiPc) fixed at the interface between carbon nanotube (CNT) electrode and electrolyte has preliminarily been investigated. The redox of NiPc can be well reproduced by NiPc/CNT composite in EMIBF₄ electrolyte. The asymmetric cell with NiPc/CNT negative electrode with EMIBF₄ electrolyte exhibits significant contribution of redox charge on the cell capacitance even at high rate. The self-discharge behavior and electrolyte feature after long term charge-discharge reveal that the dissolution of NiPc is inhibited in EMIBF₄ electrolyte.

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The electrochemical capacitor properties of vanadium dioxide (VO₂) were studied at various temperatures near the metal-insulator transition (25–70°C). Cyclic voltammetry in 1.0 M Li₂SO₄ revealed an anomalous increase in capacitance with increasing temperature. The specific capacitance of conductive rutile VO₂ at 70°C was five times higher than that of the poorly conductive monoclinic VO₂ at 25°C. Electrochemical impedance spectroscopy showed a decrease in resistance with increasing temperature, consistent with the metal-insulator transition. The results are compared and discussed with other typical electrochemical capacitor materials.

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A poly(acrylonitrile) monolith having a three-dimensionally developed, continuous porous structure was uniformly coated with carbon nanotubes (CNTs) just by dipping the monolith in an aqueous dispersion of CNTs. This simple procedure successfully realized the modification of the monolith surface with the electron-conductive material. Cyclic voltammetry showed that electrochemical capacitance was indeed imparted to the CNT-coated monolith.

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Simple cobalt modifying on flower-like NiO positive electrode active materials for electrochemical capacitors was investigated for improving their rate capability. Soaking an NiO-deposited Ni foam positive electrode in a cobalt acetate aqueous solution and the following heat treatment realized uniform modification of Co oxides on the NiO positive electrode. A hybrid capacitor (HC) cell was constructed with the activated carbon negative electrode and cobalt modified or non-modified NiO positive electrode in 10 M KOH aqueous solution. The HC cell with the cobalt modified NiO positive electrode showed excellent high-rate dischargeability. The cobalt modifying was effective for reducing cell resistance due to the formation of cobalt oxyhydroxide with higher electrical conductivity than NiO during the first charging. Consequently, the HC cell with the cobalt modified NiO electrode showed high specific power even at high discharge currents.

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The electrochemical property of RuO₂ nanosheets in acetic acid-lithium acetate buffered electrolytes was studied in an attempt to understand the charge storage mechanism in neutral solutions and to improve the charge storage capability of aqueous hybrid capacitors. High specific capacitance and good rate capability were obtained for RuO₂ nanosheets in acetic acid buffered solution containing Li⁺ (pH ∼ 5.3). Maximum capacitance of 1038 F g⁻¹ was achieved, which is higher than the values in acidic electrolyte. Using RuO₂ nanosheet electrodes as positive electrode in acetic acid-lithium acetate buffered electrolyte with a multi-layered Li negative electrode, high specific energy of 724 Wh kg⁻¹ based on the positive electrode mass was attained.

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A novel ternary electrolyte system that consists of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (EMITFSA), LiTFSA, and tris(trifluoroethyl)phosphate (TFEP) has been developed as a nonflammable electrolyte for advanced electrochemical capacitors. The ionic conductivity of the system depended much on the electrolyte composition. Higher contents of EMITFSA gave higher conductivity in both binary EMITFSA-TFEP and ternary EMITFSA-LiTFSA-TFEP systems, whereas higher molar ratio of LiTFSA decreased the conductivity of the ternary system due to increased viscosity. In the systems keeping constant molar ratios of EMITFSA and LiTFSA, the conductivity showed maxima at ca. 70 mol% of TFEP, where the concentration of the charge carriers (ions) and the mobility of the species are balanced. Brief tests of a symmetric cell composed of activated carbon electrodes revealed that the present ternary component system work as a nonflammable electrolyte in carbon-based electrochemical capacitors.

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Poly(3,4-ethylenedioxythiophene) (PEDOT) films were produced by chemical oxidative polymerization (in-situ PEDOT) and coating aqueous dispersion (slurry PEDOT). Sheet resistance measured before and after heat treatment showed that slurry PEDOT film remained stable in air while in-situ film degraded resulting in a significant increase in resistance. Excellent stability of slurry film was credited to its PEDOT-rich core, Poly(styrenesulfonic acid) (PSS)-rich shell structure, and high molecular weight (M_w) PSS dopant while the degradation of in-situ PEDOT was attributed to oxidation by oxygen in air. Thermal analysis data supported this hypothesis. Fourier Transform Infrared (FT-IR) spectrometry and X-ray photoelectron spectroscopy (XPS) results indicated that C=C bond breaking and decreased doping level were responsible for poor stability of in-situ film in air.

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Capacitor performances and degradation behavior of porous carbon electrodes derived from α-cyclodextrin precursor have been investigated. Porous carbons with different pore size distribution have successfully been prepared from α-cyclodextrin by changing the heating rate of carbonization. The carbonaceous materials obtained by the heating rates of 10 and 50 K min⁻¹ exhibited similar crystalline structures and elemental compositions. However, the cells with different electrodes showed different capacitance degradation modes under high temperature and high voltage conditions. The α-cyclodextrin-derived carbon having a higher ratio of mesopore volume lost its capacitance significantly at initial cycles by the passivation of electrode.

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We performed MD calculations on ionic liquids (ILs) to investigate the effects of solvent and pore size. The slit graphite pore models with three pore sizes 1.6, 2.8, and 4.0 nm were constructed, which confine EMITFSI and EMITFSI/PC. We found that EMI⁺ and solvent PC form layering structures along the pore walls. For the effects of solvent, the layering of EMI⁺ is interfered by PC. It was also clarified, for the effects of pore size, that the diffusion coefficients of EMI⁺ and TFSI⁻ in 1.6 nm pore are significantly large compared with other pore sizes. In the 1.6 nm pore, EMI⁺ forms triple layering structures. Since this pore size nearly corresponds to the threefold space of molecular thickness, we concluded that the layering of ILs is important for the ionic mobility improvement in nanopores.

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The use of tetramethylammonium ions (TMA⁺) at moderately high concentrations of the electrolytes can increase the number of ions accumulated in pores of an activated carbon electrode and bring about high capacitance of an electric double-layer capacitor (EDLC). The solubility of tetramethylammonium difluoro(oxalato)borate (TMADFOB) in propylene carbonate (PC) was 2.0 M (M = mol dm⁻³) or more. The maximal ionic conductivity of a TMADFOB solution in PC was observed at about 1.6 M at 25°C. The increase in the concentration of the electrolyte resulted in the increase in the gravimetric capacitance. The gravimetric capacitance found for TMADFOB was slightly higher than that for triethylmethylammonium tetrafluoroborate (TEMABF₄).

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Propylene carbonate (PC) is commonly used as a single solvent for electric double-layer capacitors (EDLCs). Trimethylene carbonate (TMC) and PC are isomeric with each other: structural isomerism, which is especially called metamerism. TMC has a six-membered ring and is solid below 48°C. The relative permittivity of a PC-TMC equimolar binary mixture was highest in the four systems of solvents: PC-TMC > PC-ethylene carbonate (EC) > PC > buthylene carbonate (BC). Despite the lower ionic conductivity, the use of a tetraethylammonium tetrafluoroborate (TEABF₄) solution in PC-TMC increased gravimetric capacitance of a coin cell. TMC may weaken solvation of electrolyte ions, and the electrolyte ions can closely approach the electrode in a compact double layer.

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The use of partially fluorinated compounds as alternative solvents can increase the solubility of electrolytes and improve the performance of electric double-layer capacitors (EDLCs). The solubility of triethylmethylammonium tetrafluoroborate (TEMABF₄) in fluoroethylene carbonate (FEC) was 3.0 M (M = mol dm⁻³) or more at 25°C. The molar concentration of FEC in 2.5 M TEMABF₄ solution was higher than that of propylene carbonate (PC) in 2.0 M TEMABF₄ solution. The kinematic viscosity and ionic conductivity of 2.0 M TEMABF₄ solution in FEC were slightly lower than those of the counterpart in PC at room temperature. The use of TEMABF₄ at the high concentrations in FEC slightly increased the gravimetric capacitance of coin cells.

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A methoxymethyl group (CH₃OCH₂-) can exert the polar effect on the physical and electrochemical properties. We have investigated the effect of the substitution of a methyl group with a methoxymethyl group on the physical and electrolytic properties of propylene carbonate (PC or 4-methyl-1,3-dioxolan-2-one). The molar concentration of methoxypropylene carbonate (MetPC or 4-methoxymehyl-1,3-dioxolan-2-one) in an electrolytic solution was lower than that of PC. The viscosities of the electrolytic solutions in MetPC were considerably higher than those of the counterparts in PC. The ionic conductivity of the electrolytic solution was lower in MetPC. Nevertheless, the use of MetPC as an alternative single solvent led to the gravimetric capacitance comparable to that obtained for PC in the discharge of a measurement cell.

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Nano-crystalline hydrous RuO₂ particles that are hyper-dispersed within Ketjen Black (KB) matrix have been added as an alternative-conducting agent. In the present study, the authors succeeded in enhancing the EDLC’s withstanding voltage from 2.7 to 3.3 V by addition of small amount of MO_x/KB (MO_x = RuO₂) composite. Addition of 4 wt% RuO₂/KB only in the positive activated carbon electrode dramatically increased the voltage limitation up to 3.3 V. To date 3.3-V-rated EDLC (activated carbon based electrochemical capacitor) has never been attained. The test EDLC cell demonstrated a high energy density (18 Wh kg⁻¹) with prolonged charge-discharge cycling up to 9000 times in the voltage range of 0–3.3 V. In this study, the critical factors enabling such a high voltage operation have been investigated in relation to the mechanism that efficiently prevents consecutive water-induced chained failure mode reactions.

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Large amounts of spent coffee grounds (SCGs) are generated all over the world. Since SCGs contain a lot of carbon and inherently has a porous structure, SCGs are considered to be a valuable industrial resource by carbonization. According to our previous research, it was apparent that the specific surface area of SCGs-derived carbon was greatly improved by potassium hydroxide (KOH) activation. In this research, we prepared an electric double layer capacitor (EDLC) using SCGs-derived carbon activated with KOH, and compared them to phenol resin-derived activated carbon (MSP-20, Kansai Coke and Chemicals Co., Ltd.), which is commonly used as a reference material in EDLC research. Electrodes were prepared by mixing 80 wt% activated carbon, 10 wt% carbon black and 10 wt% polytetrafluoroethylene (PTFE) and the electrolyte used was 1 M triethylmethylammonium-tetrafluoroborate (TEMA-BF₄)/propylene carbonate (PC). The capacitive performance was evaluated by a constant current charge-discharge cycle measured with various electrical current loads from 5 to 150 mA/cm². Although the capacitance of SCGs-derived carbon activated with KOH was inferior to that of MSP-20 at low current load density, it had high capacitance in high rate charge-discharge cycles. This suggests that the EDLC consisting of SCGs-derived activated carbon electrodes is superior to MSP-20 in capacitance retention rate when used at a high electric current density.

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Surface oxygen groups play a key role on the performance of porous carbon electrodes for electrochemical capacitors in aqueous media. The electrooxidation method in NaCl electrolyte using a filter press cell and dimensionally stable anodes is proposed as a viable process for the generation of oxygen groups on porous carbon materials. The experimental set-up is so flexible that allows the easy modification of carbon materials with different configurations, i.e. cloths and granular, obtaining different degrees of oxidation for both conformations without the requirement of binders and conductivity promoters. After the electrooxidation method, the attained porosity is maintained between 90 and 75% of the initial values. The surface oxygen groups generated can increase the capacitance up to a 30% when compared to the pristine material. However, a severe oxidation is detrimental since it may decrease the conductivity and increase the resistance for ion mobility.

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Barrier-type anodic films are formed on magnetron sputtered Ta-W alloy films to various formation potentials in 0.1 mol dm⁻³ ammonium pentaborate electrolyte. The anodic films consist of two layers, comprising an outer thin Ta₂O₅ layer free from tungsten species and an inner layer containing both tantalum and tungsten species. Slower migration of W⁶⁺ ions with respect to Ta⁵⁺ ions results in the formation of the two-layered films. Because of the absence of more soluble tungsten species in the outer layer, the anodic films grow at high current efficiency. The reciprocal of capacitance of the anodic films changes linearly with formation voltage, as a consequence of linear thickening of the anodic films with the formation potential. The capacitance is enhanced by the addition of tungsten, particularly at low formation potential. The shift of the potential, at which the anodic film growth commences, to the noble direction, contributes to the enhanced capacitance at the low formation voltages.

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Mesoporous carbons (MPCs) with high specific surface area were synthesized by the heat-treatment and subsequent acid treatment of magnesium citrate. The MPCs obtained were examined as electrode materials for electric double layer capacitor and showed the huge gravimetric capacitance with superior rate performance in sulphuric acid electrolyte. The MPCs also realize the larger capacitance than conventional activated carbon in organic electrolyte and extraordinary high retention of capacitance at very low temperature, such as 80% of room temperature value at −60°C.

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Electroactive polymer actuators composed of ion-gel electrolyte (polymer gel containing ionic liquid) and composite carbon electrodes were prepared. The ion-gel actuators could be driven by charging and discharging the electric double-layer of the carbon electrodes. Four different carbon materials were used to prepare composite electrodes. The effects of physical properties of the carbon electrode materials on the actuator performance were explored. The actuators bent toward the anodic side by applying low voltages (<3 V) under atmospheric conditions. By utilizing a highly electron-conductive carbon electrode material having a high specific surface area, large deformation and fast response ability against electric stimulus of the actuator could be achieved.

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Impregnation is a cost-effective and scalable method for the non-covalent addition of different compounds on the surface of suitable substrates. For supercapacitors, it provides an alternative strategy for attaining higher energy densities by combining high surface area carbon electrodes with surface species adding a pseudocapacitive contribution. In this work, we study the performance of activated carbon electrodes impregnated with para-benzoquinone (p-BQ). It is observed that immobilized p-BQ can improve by more than 100% the specific capacitance of the bare carbon, reaching nearly 350 F g⁻¹ at 10 mV s⁻¹. The results here discussed are a step forward for the optimization of impregnation methods, which may lead to optimized carbon-based supercapacitors.

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The charge-discharge behavior of electric double-layer capacitors (EDLCs) composed of several ionic liquids and their lithium solutions were evaluated. The model EDLC cell with a Li⁺-containing ionic liquid electrolyte, LiTFSI/EMImFSI (LiTFSI = lithium bis(trifluoromethylsulfonyl)imide, EMImFSI = 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide), exhibited a higher discharge capacitance than the cell containing EMImFSI, whereas the discharge capacitance of the cell with LiTFSI/EMImTFSI was lower than that of the cell containing EMImTFSI. In addition, the cells composed of EMImBF₄-based electrolytes displayed similar charge-discharge characteristics irrespective of the presence of the corresponding lithium salts. The distinctive results for the EDLCs with lithium-containing ionic liquid electrolytes are also discussed based on the application of the electrochemical impedance technique to model carbon electrodes in ionic liquids. According to the obtained differential capacitance of the model glassy carbon electrode in each ionic liquid electrolyte, the electric double-layer (compact layer) structure at the electrode/electrolyte interface depends on the component anion species of the ionic liquid and the co-existence of the anion and Li⁺.

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Black Pearls (BP) carbon powder was chemically modified with chloroanthraquinone groups by spontaneous reduction of 1-amino, 5-chloroanthraquinone with the aim of determining the quinone content of the modified carbon. This information is essential to determine the increase of capacitance associated to the grafted quinone molecules. The capacitance of pristine carbon electrode was doubled due to the presence of the grafted electroactive molecules. The modified BP powder was characterized by thermogravimetric and elemental analyses. The modified powder was also used to fabricate composite electrodes that were characterized by electrochemistry to determine the loading of electroactive quinone molecules. The presence of the chlorine atom on the anthraquinone moiety allows an estimation of the loading from elemental analysis, which is in relatively good agreement with the value estimated by electrochemistry.

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Chitosan-based gel electrolytes with ionic liquids (Chi/IL) were prepared and investigated for use in electric double-layer capacitors (EDLCs). The voltage drop for the test cell with Chi/EMImBF₄ (EMImBF₄ = 1-ethyl-3-methylimidazolium tetrafluoroborate) or Chi/DEMEBF₄ (DEMEBF₄ = N,N-diethyl-N-methyl-N-2-methoxyethylammonium tetrafluoroborate) and activated carbon fiber cloth electrodes (ACFCs) was smaller than that with the corresponding ionic liquid electrolyte. To clarify the effect of the presence of Chi in contact with the carbon electrode, we also prepared the Chi-containing ACFC electrode (Chi+ACFC) and evaluated its charge-discharge characteristics in EMImBF₄. The results proved that the presence of Chi on the active materials reduces the internal resistances of the cell and plays an important role in the improvement of the EDLC performance. In addition, the high-voltage operation of the test cells with Chi-based gel electrolytes was investigated. These results suggest that the Chi is suitable for use in practical high-performance and safe EDLCs.

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The lateral size of individual graphene sheets is expected to impact the electrochemical properties of reduced graphite oxide nanosheets (rGOns). The size effect on the specific capacitance of rGOns was investigated by breaking down the size of rGOns using high-frequency ultrasonification. The specific capacitance of rGOns prepared by hydrogen reduction of GOns with an average equivalent diameter of 280 nm was 242 F g⁻¹ in 0.5 M H₂SO₄ and 205 F g⁻¹ in 0.5 M Na₂SO₄. The specific capacitance was 30% higher to that of larger-sized GOns with diameter of 920 nm. The area normalized capacitance ranged between 70 to 138 µF cm⁻², with larger-sized rGOns affording higher values. The cause of this size-effect is discussed based on the microstructure of the rGOns electrode as well as the edge/plane ratio.

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