Study of electrochemical capacitors utilizing carbon nanotube electrodes

Several types of block-form porous tablets of carbon nanotubes are fabricated to use as polarizable electrodes in electrochemical capacitors (ECs). These tablets are prepared by using moulded mixtures comprising carbon nanotubes and phenolic resin powders. Comparison of the effect of different processing on the performance of the capacitors is specifically investigated. Using these polarizable electrodes, ECs with a specific capacitance of about 15 to 25 F cm⁻³ are obtained with 38 wt.% H₂SO₄ as the electrolyte.     

Nickel hydroxide/activated carbon composite electrodes for electrochemical capacitors

In this paper, a nickel hydroxide/activated carbon (AC) composite electrode for use in an electrochemical capacitor was prepared by a simple chemical precipitation method. The structure and morphology of nickel hydroxide/AC were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that nano-sized nickel hydroxide was loading on the surface of activated carbon. Electrochemical performance of the composite electrodes with different loading amount was studied by cyclic voltammetry and galvanostatic charge/discharge measurements. It was demonstrated that the introduction of a small amount of nickel hydroxide to activated carbon could promote the specific capacitance of a composite electrode. The composite electrodes have good electrochemical performance and high charge–discharge properties. Moreover, when the loading amount of nickel hydroxide was 6 wt.%, the composite electrode showed a high specific capacitance of 314.5 F g⁻¹, which is 23.3% higher than pure activated carbon (255.1 F g⁻¹). Also, the composite electrochemical capacitor exhibits a stable cyclic life in the potential range of 0–1.0 V.     

Hybrid electrochemical capacitors based on polyaniline and activated carbon electrodes

The performance of a newly designed, polyaniline–activated carbon, hybrid electrochemical capacitor is evaluated. The capacitor is prepared by using polyaniline as a positive electrode and activated carbon as a negative electrode. From a constant charge–discharge test, a specific capacitance of 380 F g⁻¹ is obtained. The cycling behaviour of the hybrid electrochemical capacitor is examined in a two-electrode cell by means of cyclic voltammetry. The cycle-life is 4000 cycles. Values for the specific energy and specific power of 18 Wh kg⁻¹ and 1.25 kW kg⁻¹, respectively, are demonstrated for a cell voltage between 1 and 1.6 V.     

Effect of carbon types on the electrochemical properties of negative electrodes for Li-ion capacitors

The electrochemical properties of various carbon materials (graphite and hard carbon) have been investigated for use as a negative electrode for Li-ion capacitors. The rate capabilities of the carbon electrodes are tested up to 40C using both half and full cell configurations. It is found that the capacitance of the hard carbon material at 40C could be maintained up to 70% of that at 0.2C in full cells with an activated carbon positive electrode, which is the best among the carbon materials. The cycle performance of the hard carbon demonstrates that the initial capacitance is retained up to 83% even after 10,000 cycles. The outperforming results could be ascribed to the microstructure of hard carbon, which indicates that hard carbon is more suitable as negative electrode materials for high power energy storage applications.     

Anodic deposition of manganese oxide electrodes with rod-like structures for application as electrochemical capacitors

The electrochemical properties of nanocrystalline manganese oxide electrodes with rod-like structures were investigated to determine the effect of morphology, chemistry and crystal structure on the corresponding electrochemical behavior of manganese electrodes. Manganese oxide electrodes of high porosity composed of 1-1.5 μm diameter rods were electrochemically synthesized by anodic deposition from a dilute solution of Mn(CH₃COO)₂ (manganese acetate) onto Au coated Si substrates without any surfactants, catalysts or templates under galvanostatic control. The morphology of the electrodes depended on the deposition current density, which greatly influenced the electrochemical performance of the capacitor. Electrochemical property and microstructure analyses of the manganese oxide electrodes were conducted using cyclic voltammetry and microstructural techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The synthesized rod-like manganese oxide electrodes at low current densities exhibited a high specific capacitance due to their large surface areas. The largest value obtained was 185 F g⁻¹ for deposits produced at .5 mA cm⁻². Specific capacity retention for all deposits, after 250 charge-discharge cycles in an aqueous solution of 0.5 M Na₂SO₄, was about 75% of the initial capacity.     

Effect of Fe-contamination on rate of self-discharge in carbon-based aqueous electrochemical capacitors

The effect of Fe concentration on the Fe-induced self-discharge of electrochemical capacitor carbon electrodes in aqueous H₂SO₄ is presented. With an Fe-free system, the positive electrode self-discharges via an activation controlled self-discharge mechanism, while the negative electrode self-discharges with a diffusion control profile. This highlights that the self-discharge mechanism on each electrode of an electrochemical capacitor is likely different, and should be examined in a three-electrode (half cell) setup.     

High performance electrochemical capacitors from aligned carbon nanotube electrodes and ionic liquid electrolytes

We report a new class of electrochemical capacitors by utilizing vertically aligned carbon nanotubes as the electrodes and environmentally friendly ionic liquids (ILs) as the electrolytes. With their vertically aligned structures and well spacing, aligned carbon nanotubes showed a strong capacitive behavior in the ionic liquid electrolyte. Plasma etching played an important role in opening the end tips of nanotubes and in introducing defects and oxygenated functionalization to the nanotubes, further enhancing the capacitive behavior of carbon nanotubes. With the combined contribution from double-layer capacitance and redox pseudocapacitance, carbon nanotubes showed a remarkable capacitance in ionic liquid electrolyte. Combining the highly capacitive behavior of carbon nanotube electrodes with the large electrochemical window of ionic liquid electrolytes, the resultant capacitors showed a high cell voltage, high energy density, and high power density, potentially outperforming the current electrochemical capacitor technology. The device configuration incorporating vertically aligned nanostructured electrodes and inherently safe electrolytes would be useful for improving performances for new energy storage technologies.     

Activated-carbon electric-double-layer capacitors: electrochemical characterization and adaptive algorithm implementation

An activated-carbon electric-double-layer capacitor is characterized and an adaptive algorithm is derived and implemented that is commensurate with potential traction applications of these energy storage devices. The electrochemical characterization relies on a simplified equivalent circuit interpretation extracted from a more-complete mathematical representation of the capacitor system. In addition to the high power capability and potentially low costs of this class of capacitors, we clarify the substantial invariance of the device performance with respect to temperature, a distinct advantage over battery systems. The robustness of the equivalent circuit in terms of capturing the salient features of the experimental data over the temperature and voltage range of interest enables the formulation of a model-reference adaptive algorithm. The algorithm developed and implemented in this work is fully recursive in that the only variables required for on-line regression are those of the previous time step and the current time step. Successful comparisons of the algorithm's adapted state of charge and power predictions provide an initial validation of the algorithm.     

Carbon materials for electrochemical capacitors

The carbon materials used for electrochemical capacitors were reviewed and discussed the contribution of the surfaces owing to micropores and other larger pores to the capacitance and rate performance of the electric double-layer capacitors. The necessity to have an internationally accepted specification for the measurement of capacitor performance was emphasized.     

Textural and electrochemical characterization of porous carbon nanofibers as electrodes for supercapacitors

Porous carbon nanofibers (CNFs) enriched with the graphitic structure were synthesized by thermal decomposition from a mixture containing polyethylene glycol and nickel chloride (catalyst). The textural and electrochemical properties of porous CNFs were systematically compared with those of commercially available multi-walled carbon nanotubes (MWCNTs). The high ratio of mesopores and large amount of open edges of porous CNFs with a higher specific surface area, very different from that of MWCNTs, are favorable for the penetration of electrolytes meanwhile the graphene layers of porous CNFs serve as a good electronic conductive medium of electrons. The electrochemical properties of porous CNFs and MWCNTs were characterized for the application of supercapacitors using cyclic voltammetry, galvanostatic charge–discharge method, and electrochemical impedance spectroscopic analyses. The porous CNFs show better capacitive performances (C_S = 98.4 F g⁻¹ at 25 mV s⁻¹ and an onset frequency of behaving as a capacitor at 1.31 kHz) than that of MWCNTs (C_S = 17.8 F g⁻¹ and an onset frequency at 1.01 kHz). This work demonstrates the promising capacitive properties of porous CNFs for the application of electrochemical supercapacitors.     

Electrode and electrolyte materials for electrochemical capacitors

Among different electric energy storage technologies electrochemical capacitors are used for energy storage applications when high power delivery or uptake is needed. Their energy and power densities, durability and efficiency are influenced by electrode and electrolyte materials however due to a high cost/performance ratio; their widespread use in energy storage systems has not been attained yet.Thanks to their properties such as high surface area, controllable pore size, low electrical resistance, good polarizability and inertness; activated carbons derived from polymeric precursors are the most used electrode materials in electrochemical capacitors at present. Other electrode materials such as shaped nano-carbons or metal oxides are also investigated as electrode materials in electrochemical capacitors, but only as useful research tools.Most commercially used electrochemical capacitors employ organic electrolytes when offering concomitant high energy and high power densities. The use of aqueous based electrolytes in electrochemical capacitor applications is mainly limited to research purposes as a result of their narrow operating voltage. Recent studies on room temperature ionic liquids to be employed as electrolyte for electrochemical capacitor applications are focused on fine tuning their physical and transport properties in order to bring the energy density of the device closer to that of batteries without compromising the power densities.In this paper a performance analysis, recent progress and the direction of future developments of various types of materials used in the fabrication of electrodes for electrochemical capacitors are presented. The influence of different types of electrolytes on the performance of electrochemical capacitors such as their output voltage and energy/power densities is also discussed.     

A review on the use of carbon nanostructured materials in electrochemical capacitors

Sustainable and renewable energy resources, as well as energy storage systems (ESSs), are amongst the current and critical global requirements. A comparative discussion on batteries, fuel cells and electrochemical capacitors (ECs) is presented. The mechanisms involved in various classes of ECs are also elaborated. Additionally, a historical background highlighting some of the major steps associated with EC development over the years is discussed in this review. In particular, carbon nanostructured materials have high potential in the development of ESSs, and hence this review presents an insight on the current ESSs with a strong bias towards these materials as ECs. The current status of carbon nanomaterials, such as carbon nanotubes, nanofibers, nano‐onions, nanorods, fullerenes and graphene nanosheets, in ECs is reviewed. The associated effects of nanostructural parameters, such as pore sizes and specific electro‐active areas, amongst others, in terms of energy storage capabilities are also discussed. Typical physicochemical characterisation techniques, which enrich understanding of their characteristics, are also reviewed. The discussion views set platforms for a variety of unique carbon nanomaterial designs with high prospective specific capacitance. Key porosity tailoring protocols, such as chemical activation, introduction of heteroatoms in carbon nanostructures and template synthesis methods, are also reviewed. The effects of other device components, such as electrolyte ion size and solvent system, electrode design and use of binders, to the overall capability of EC, are also discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

Application of activated carbon/DNA composite electrodes to aqueous electric double layer capacitors

A novel capacitor electrode auxiliary, deoxyribonucleic acid (DNA), is applied to an electric double layer capacitor (EDLC) containing an aqueous 3.5 M NaBr electrolyte. The present electrode is composed of activated carbon (95 wt.%) and DNA (2.5 wt.%) with polytetrafluoroethylene (PTFE) as a binder (2.5 wt.%). An EDLC cell with the DNA-loading electrodes exhibits improved rate capability and discharge capacitance. An EDLC cell with DNA-free electrodes cannot discharge above a current density of 3000 mA g⁻¹ (of the electrode), while a cell with the DNA-loading electrodes can work at least up to 6000 mA g⁻¹. Moreover, an open-circuit potential (OCP) of the DNA-loading electrode sifts negatively with ca. 0.2 V from an OCP of the corresponding electrode without DNA. It is noteworthy that a small amount of DNA loading (2.5 wt.%) to the activated carbon electrode not only improves the rate capability but also adjusts the working potential of the electrode to a more stable region.     

Practical and theoretical limits for electrochemical double-layer capacitors

Two types of double-layer capacitors, based on carbon materials, were analysed: (1) an imaginary nano-capacitor assembled from single graphene sheets, separated by electrolyte layers (thickness of nanometers) and (2) a capacitor based on porous carbons. It has been shown that the maximum specific surface of a porous carbon material which may be used for the construction of a capacitor is ca. 2600 m² g⁻¹. The maximum energy density of an imaginary double-layer ‘nano-capacitor’, is close to 10 kJ kg⁻¹ at a voltage of U = 1 V (aqueous electrolyte) of ca. 40–45 kJ kg⁻¹ at U ≈ 2.3–2.5 V (organic electrolytes), and at the order of 100 kJ kg⁻¹ at voltages close to 4 V (ionic liquids as electrolytes). The real device consists of porous electrodes and a separator, both soaked with the electrolyte, as well as current collectors. Consequently, the maximum electric capacity expressed versus the mass of the device (ca. 20–30 F g⁻¹), is much smaller than the corresponding value expressed versus the mass of the carbon material (ca. 300 F g⁻¹). In order to obtain the energy density of the device at a level of 100 kJ kg⁻¹ (characteristic for the lead-acid battery), the capacitor with porous carbon electrodes should operate at voltages of ca. 4 V (ionic liquids as electrolytes). However, the specific power density of such a capacitor having an acceptable energy density (ca. 100 kJ kg⁻¹) is relatively low (ca. 1 kW kg⁻¹).     

Manganese oxide film electrodes prepared by electrostatic spray deposition for electrochemical capacitors from the KMnO4 solution

Manganese oxide film electrodes for electrochemical capacitors were deposited on the polished Pt foils by electrostatic spray deposition (ESD) from KMnO₄ precursor solution. The electrochemical properties of electrodes were systematically studied using cyclic voltammetry (CV), constant current charge–discharge tests, and electrochemical impedance spectroscopy (EIS). The specific capacitance (SC) of thick deposited film was 149 F g⁻¹ at the very high scan rate of 500 mV s⁻¹, in comparison with 209 F g⁻¹ at the low scan rate of 5 mV s⁻¹. The electrode shows good cyclic performance. The initial SC value was 163 F g⁻¹ and 103% of the initial SC can be retained after 10,000 cycles at the scan rate of 50 mV s⁻¹.     

Electrospun nickel oxide/polymer fibrous electrodes for electrochemical capacitors and effect of heat treatment process on their performance

Electrospinning is a versatile method for preparation of submicron-size fibers under ambient temperature. We demonstrate a new approach based on this method for preparing an electrode which consists of the fibers coated with nickel oxide (NiO) and acetylene black (AB) on their surfaces. The NiO/polymer fibrous electrodes show the electrochemical responses based on the electrochemical reaction of Ni(OH)₂ which is produced from NiO in alkaline aqueous solution. The capacitance of the test half cell with the as-prepared NiO/polymer fibrous electrode in 1 mol l⁻¹ KOH aqueous solution is 5.8 F g⁻¹ (per gram of NiO). Heat treatment (at 150 °C for 1 h in the air) of the NiO/polymer fibrous electrode increases the capacitance of the NiO/polymer fibrous electrode. The capacitance of the cell with the heat treated (HT) NiO/polymer fibrous electrode is 163 F g⁻¹ (per gram of NiO). SEM observation of the heat treated electrode suggests that partial melt of the fibers on the current collector forms the conducting passes and networks between the NiO particles and the collector and increases the specific capacity of the fibrous electrode.     

锂离子电池用无溶剂干法电极的制备及其性能研究

采用无溶剂电极制备技术成功制备了锂离子电池用LiNi0.8Co0.1Mn0.1O2干法电极片,采用扫描电子显微镜(SEM)和X射线能谱仪(EDS)对干法电极片的形貌和元素分布进行了分析测试,通过倍率充放电、交流阻抗、循环充放电等测试手段研究了干法电极片的电化学性能.结果表明:纤维状PTFE广泛地分布在LiNi0.8Co...     

Electric double-layer capacitors with sheet-type polarizable electrodes and application of the capacitors

Sheet-type polarizable electrodes with low sheet resistances for electric double-layer capacitors were outlined. The sheet-type electrodes consisted of activated carbon layers on aluminium foils. The sheet resistance of the sheet-type electrode mainly correlated with a filling density of activated carbons in the carbon layer. The species of activated carbons and the particle size of activated carbons affected the filling density of activated carbons in the layer. High filling ratio of activated carbons with small particle size resulted in the sheet-type electrodes with very low sheet resistance and high capacitance. Capacitors employing sheet-type electrodes showed very low internal resistance. Some application examples of the capacitor are described.     

Poly(3-methylthiophene)/MnO2 composite electrodes as electrochemical capacitors

Composite electrodes prepared by electrodeposition of manganese oxide on titanium substrates modified with poly(3-methylthiophene) (PMeT) were investigated and compared with Ti/MnO₂ electrodes. The polymer films were prepared by galvanostatic deposition at 2 mA cm⁻² with different deposition charges (250 and 1500 mC cm⁻²). The electrodes were characterized by cyclic voltammetry in 1 mol L⁻¹ Na₂SO₄ and by scanning electron microscopy. The results show a very significant improvement in the specific capacitance of the oxide due the presence of the polymer coating. For Ti/MnO₂ the specific capacitance was of 122 F g⁻¹, while Ti/PMeT₂₅₀/MnO₂ and Ti/PMeT₁₅₀₀/MnO₂ displayed values of 218 and 66 F g⁻¹, respectively. If only oxide mass is considered, the capacitances of the composite electrode increases to 381 and 153 F g⁻¹, respectively. The micrographs of samples show that the polymer coating leads to very significant changes in the morphology of the oxide deposit, which in consequence, generate the improvement observed in the charge storage property.     

Synthesis and characterization of tin oxide/carbon aerogel composite electrodes for electrochemical supercapacitors

Two types of carbon aerogel-based functional electrodes for supercapacitor applications are developed. To improve the electrochemical performance of the electrodes, carbon aerogels are doped with pseudocapacitive tin oxide either by impregnating tin oxide sol into resorcinol–formaldehyde (RF) wet gels (Method I), or by impregnating tin tetrachloride solution into carbon aerogel electrodes (Method II). The electrodes are heat-treated to 450 °C in air to activate the electrode surface and complete the oxidation of tin-precursors in the network structure of the aerogel. The effects of different impregnation methods on the physical/electrochemical properties of the composite electrodes are investigated. Microstructural and compositional variations of the electrodes with tin oxide doping are also examined by scanning electron microscopy and energy dispersive X-ray analysis. The tin oxide/carbon aerogel composite electrodes synthesized by both methods have similar specific capacitances (66–70 F g⁻¹). Composite electrodes synthesized via Method II showed better cyclic stability compared with electrodes synthesized via Method I.