Electric Double-layer Capacitor Electrode Research Articles

Preparation of activated carbon from bamboo-cellulose fiber and its use for EDLC electrode material

Carbonization and post activation of bamboo-cellulose fiber was carried out. The carbonization was performed at 600°C, 800°C and 1000°C in argon atmosphere. Then, they were activated by heating solid mixture of carbonized bamboo and sodium hydroxide (NaOH) at 720°C in argon atmosphere. The largest specific surface area of the resulting activated carbon (carbonized at 600°C) was 2366m2/g with the micropore volume of 0.71cm3/g and mesopore volume of 0.06cm3/g. It was found that the carbonization temperature is very important to obtain nanoporous carbon with large specific surface area. The distributions of interlayer spacing were estimated from the power spectra of the TEM images of the carbonized samples. It showed the interlayer spacing of basic structural unit (BSU) decreased from 0.49nm (BC-0600) through 0.47nm (BC-0800) to 0.45nm (BC-1000). The activated carbon was used as the host material of the electrodes of coin type electric double-layer capacitor (EDLC) with organic electrolyte. The observed specific capacitance was 43F/g (23F/cm3) for the activated carbon (carbonized at 600°C), comparable to 44F/g (22F/cm3) of commercial activated carbon (MSP). The corresponding values for the activated carbons (carbonized at 800°C and 1000°C) were 40F/g (23F/cm3) and 17F/g (12F/cm3), respectively.

CVD grown graphene/CNT composite as additive material to improve the performance of electric double layer capacitors (EDLCs)

In this work, large area graphene-carbon nanotube (G-CNT) composite as additive material for EDLC performance was grown on copper foil at low temperature (500 °C) and at atmospheric pressure by using chemical vapor deposition (CVD). The growth state was characterized by field emission scanning electron microscope (FESEM), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy. EDLC electrodes based on high surface area activated carbon (YP50F) and G-CNT were then fabricated by a facile step. Coin-type EDLC cells with two symmetrical carbon electrodes were assembled using the prepared carbon materials. The electrochemical performance of G-CNT based eletrodes were examined in an electrolyte solution of tetraethylammonium tetrafluoroborate ((C2H5)4NBF4, TEABF4) in propylene carbonate (C4H6O3, PC) and measured by galvanostatic charge/discharge and cyclic voltammetry methods.

Macroporous carbon monoliths derived from phloroglucinol–sucrose resins as binder-free thick electrodes for supercapacitors

Herein, we report the preparation of phosphate-functionalized monolithic carbons containing interconnected pores of different sizes (macro- and micropores) by the pyrolysis of phloroglucinol–formaldehyde and phloroglucinol–sucrose–formaldehyde resins. Carbons were characterized by X-ray diffractometry, Raman spectroscopy, nitrogen adsorption–desorption and scanning electron microscopy. The addition of sucrose led to a significant decrease in the specific surface area of the carbon monoliths but improved their mechanical properties. This allowed their processability into disk-shaped monoliths of 1.1 mm thickness, which were directly tested as binder-free electrodes for electrical double-layer capacitors without the addition of any conductive additive. The electrochemical properties of the monoliths were studied by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy using a two-electrode configuration and 2 M H2SO4 aqueous solution as the electrolyte. The electrodes were cycled within the 1.4 V voltage window showing specific capacitances of ca. 250 and 110 F g−1 at current densities of 0.2 and 10 A g−1 (7 and 350 mA cm−2), respectively, and exhibited an excellent cycling stability with a capacity retention of 97% after 7500 charge–discharge cycles.

The ideal porous structure of EDLC carbon electrodes with extremely high capacitance.

We propose an ideal porous structure of carbon electrodes for electric double-layer capacitors (EDLCs). The porous carbon successfully improved the gravimetric capacitance above ∼200 F g-1 even in an organic electrolyte by utilizing the carbon nanopore surface more effectively. High-resolution transmission electron microscopy images and X-ray diffraction patterns classified 15 different porous carbon electrodes into slit-shape and worm-like-shape, and the pore size distributions of the carbons were carefully determined applying the grand canonical Monte Carlo method to N2 adsorption isotherms at 77 K. The ratio of pores where solvated ions and/or desolvated ions can penetrate also has a significant effect on the EDL capacitance as well as the pore shape. The detailed study on the effect of porous morphologies on the EDLC performance indicates that a hierarchical porous structure with a worm-like shaped surface and a pore size ranging from a solvated ion to a solvent molecule is an ideal electrode structure.

Cellulose and activated carbon based flexible electrical double-layer capacitor electrode: Preparation and characterization

Supercapacitors are efficient electric energy storage devices with a wide range of possible applications. In this study, a natural cellulose-activated carbon composite material that can be used as an electrode in a flexible supercapacitor is prepared using a phase inversion technique. The composite material preparation and testing methodology are described and the electrochemical characteristics of the composite material are analysed as a function of the activated carbon (AC) to natural cellulose (NC) mass ratio. Analysis of the influence of the AC/NC mass ratio on the electrochemical characteristics of the composite material demonstrated the importance of optimal mass ratio determination prior to manufacture. To prove an applicability of the NC binder, the electrochemical characteristics of the composite material prepared with a NC binder are compared with those of a composite material with a standard poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) binder. The experimental results confirm the applicability of using NC as a binder in the preparation of a composite material that can be used as a flexible self-standing supercapacitor electrode.

Solid-state NMR Study of Ion Adsorption and Charge Storage in Graphene Film Supercapacitor Electrodes.

Graphene film has been demonstrated as promising active materials for electric double layer capacitors (EDLCs), mainly due to its excellent mechanical flexibility and freestanding morphology. In this work, the distribution and variation pattern of electrolyte ions in graphene-film based EDLC electrodes are investigated with a 11B magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. For neutral graphene films soaked with different amounts of electrolytes (1 M TEABF4/ACN), weakly and strongly adsorbed anions are identified based on the resonances at different 11B chemical shifts. Unlike other porous carbonaceous materials, the strongly adsorbed anions are found as the major electrolyte anions components in graphene films. Further measurements on the ion population upon charging are carried out with applying different charging voltages on the graphene films. Results indicate that the charging process of graphene-film based EDLCs can be divided into two distinct charge storage stages (i.e., ejection of co-ions and adsorption of counter-ions) for different voltages. The as-obtained results will be useful for the design and fabrication of high performance graphene-film based EDLCs.

Relationships between pore size and charge transfer resistance of carbon aerogels for organic electric double-layer capacitor electrodes

Carbon aerogels are prepared from polycondensation of resorcinol and formaldehyde using sodium carbonate as a base catalyst while controlling the ratio of resorcinol to catalyst (R/C). Pore size of the carbon aerogel shows a tendency to increase with the R/C ratio because of the low addition reaction rate between two starting materials with a small amount of catalyst. In addition, the pore size of the carbon aerogels strongly affects their charge transfer resistance and their potential use as organic electric double-layer capacitor electrodes. Ionic resistance to facile mobility of electrolyte ions decreases with increasing pore size of the carbon aerogels, while their electronic resistance exhibited the opposite trend with respect to pore size. These compensation effects between ionic resistance and electronic resistance lead to a different optimum pore size of carbon aerogels for EDLC electrodes depending on the applied charge-discharge rate. A carbon aerogel with large pore size is favorable for use in a low-rate charge-discharge process, while one with small pore size have good performance in a high-rate charge-discharge process. Therefore, we suggest that designing an efficient mesoporous carbon material for an EDLC electrode should include significant consideration of its end use, especially the rate employed for the charge-discharge process.

Enhancing the Electrochemical Performance of Electric Double-Layer Capacitors by Applying Mesoporous Carbon Gel RFCX to the Electrode

Electric double-layer capacitor (EDLC) has been required high capacitance and high-rate performance. The capacitance of EDLC comes from the electrostatic charge accumulation at the interface between electrode and electrolyte. So the capacitance is highly dependent on the surface area of elecrode material. For this reason, microporous activated carbon had been used for many years as EDLC electrode materials. However, recent research use organic electrolyte to enhance the energy density of EDLC. In this case, microporous electrode materials are not good at ion transport and reduce rate perfermance because of the large ion diameter of organic electrolyte. Therefore, an optimal electrode material must provide an appropriate pore size for ion transport. Recently, mesoporous materials have been reported to be more effective in increasing capacitance at high charge/discharge rates. Based on the recent reports, we focused on the study of novel mesoporous materials. We prepared several kinds of mesoporous materials carbon gel RFCX (resorcinol formaldehyde carbon xerogel) with different pore size distribution. RFCX is a kind of mesoporous carbon material with a 3D interconnected network structure and under the control of the pore structure by varying certain conditions of the sol-gel process. We controled the concentration of catalyst and developed three types of mesoporous materials carbon gel RFCX with a large number of mesopores around 10 nm, 25 nm and 40 nm. For comparison, we used two kinds of commercial activated carbon products M (surface area 900 m2g-1) and Y (surface area 1500 m2g-1). Both of them are high-surface-area microporous activated carbon. And we examined each sample’s electrochemical behavior as the electrode for electric double-layer capacitors in organic electrolyte of tetra ethyl ammonium tetra fluoroborate in propylene carbonate (TEABF4/PC). Ion diameter of electrolyte ion TEA+ in solvation is 1.96 nm. And ion diameter of electrolyte ion BF4 - in solvation is 1.71 nm. Diameter was caculated by Car-Parrinello method. Carbon gel RFCX enhanced the electrolyte ion transport by the large pore diameter (2 ~ 50 nm). RFCXs showed higher surface areal capacitance of 4.0 ~ 6.2 μF cm−2 than the commercial product M (0.5 ~1.5 μF cm−2) in organic electrolyte. Though they are the similar surface area (about 800 m2g-1 and 900 m2g-1). Moreover, the RFCXs also showed higher surface areal capacitance than the commercial product Y (2.2 ~ 3.5 μF cm−2). Though surface area of RFCX is only a half of Y (1500 m2g-1). And RFCXs showed high-rate performance of 70% owing to the large mesopore volume. These results indicated that capacitance of EDLC in organic electrolyte indepandent of surface area of electrode material. Thus, we considered that the volume rate of mesopore of electrode material is one of the most important determinant of rate performance. In summary, EDLC based on the mesoporous carbon RFCX exhibited better electrochemical performance than microporous activated carbon in organic electrolyte. It is considered that RFCX has potential for high performance energy-related applications.

Structural Characterization and EDLC-Electrode Performance of Coal-Tar-Pitch Activated Carbon Using K2CO3 Treatment

Activated carbons (ACs) have been used as EDLC (electric double-layer capacitor) electrode materials due to their high specific area, stability, and ecological advantages. In order to prepare ACs with high density and crystallinity, coal tar pitch (CTP) was activated by K2CO3 and the textural and electrochemical properties of the obtained ACs were investigated. Although the CTP ACs formed by K2CO3 activation had much smaller specific surface area and pore volume than did the CTP ACs formed by KOH activation, their volumetric specific capacitance (F/cc) levels as electrode materials for EDLC were comparable due to their higher density and micro-crystallinity. Structural characterization and EDLC-electrode performance were studied with different activation conditions of CTP/K2CO3 ratio, activation temperature, and activation period.

Carbon Nanotube Capacitor Using Redox Reaction of Iodide Ions in Electrolyte

Owing to their high specific surface area, single-walled carbon nanotubes (SWCNTs) have been expected as electrode materials of electric double layer capacitors (EDLCs). Many researchers have investigated and reported EDLC electrode properties of SWCNTs. However, unfortunately, the reported EDLC capacitance values were not very high comparing with the capacitance of conventional activated carbon electrodes. On the other hand, recently new types of electrochemical capacitors using redox reactions of electrolytes have been reported. In this paper, we use the abbreviation EREC (Electrolyte Redox Electrochemical Capacitor) to express such capacitor. EREC is very attractive because it can store high energy in spite of the simple construction. In EREC, redox reaction of electrolyte occurs on one electrode while the other electrode works as a capacitor electrode (i.e. physi-sorptions of ions occur). Therefore, the redox electrode of EREC shows almost constant potential independent of the amount of charge, while the potential of the capacitor electrode is proportional to the charge. If we plot the potential difference between two electrodes as a function of accumulated charge, the plots show a straight line having half the slope value of the conventional symmetrical EDLC. It means the apparent capacity of EREC is twice as high as that of EDLC. Furthermore, only a small amount of active materials would be required for redox electrode of EREC compared to the capacitor electrode, because the redox electrode can use the entire active materials while the capacitor electrode can use only the surface of the materials. Taking it into account, it is expected that the energy density of EREC would be 2-4 times greater than that of EDLC. However, incidental shuttle reaction or charge migration of active redox species in an electrolyte between electrodes would cause the deterioration of the capacitor property of EREC. Probably due to the above reason, cycle performance has not been well discussed for the previous EREC cell using redox reaction of iodine molecules. To avoid such undesired migrations, the electrodes should have a trapping system for redox species. In the present study, we attempted to use hollow cores of SWCNTs as redox reaction sites. As well discussed in the previous reports, owing to their unique nanostructure, SWCNTs have fascinating physical and chemical properties for various kinds of application field such as energy storage, energy conversion, electronics, sensor, filter, gas storage, and medicine. Interestingly, various kinds of organic and inorganic molecules can be introduced into the hollow cores of SWCNTs, and the obtained encapsulation systems show unique electrochemical properties. We reported that it is very easy and effectively to prepare iodine molecules encapsulated in SWCNTs by an electrochemical method [1]. Using this encapsulation reaction, new type of EREC can be developed. We will talk about EREC electrode properties of SWCNTs for redox reaction of iodide ions [2].

Investigation of Self Supporting Paper-like Structures Fabricated with Few-Layer Exfoliated Graphene Platelets and Composites with Birnessite-MnO2 As Electrode Materials for Electric Double-Layer Capacitor and Redox Capacitor

Few-layer exfoliated graphene platelets are ultrathin particles of graphite that can also be thought of as short stacks of graphene sheets prepared through proprietary manufacturing processes. Several grades and sizes with thicknesses ranging from 1-20 nanometers and width ranging from 1-50µm are manufactured by XG Sciences, Lansing, MI, USA. Unique manufacturing processes for these platelet materials are non-oxidizing; thus, yielding a pristine graphitic surface of sp2 carbon molecules. This makes these platelet particles especially suitable for applications requiring high electrical and thermal conductivities. The surface areas of the resulting platelets predominantly consist of the open surface areas of the graphene basal planes. Open surface areas of the graphene basal planes as well the interlayer spaces between the graphene sheets coupled with high electrical conductivities are an important criteria for electrodes in many electrochemical energy storage technologies. Unique morphology of the platelets offer the potential to structure self-standing electrodes by doing away with inactive components such as current collectors and thus, enhance the energy density. Surface modification of the platelets through the deposition of transition metal oxides on the graphene basal planes has the potential to enhance the energy storage capacity due to the participation of two different components in the energy storage process. Few-layer exfoliated graphene platelets can be dispersed in various aqueous and non-aqueous solvents depending upon the nature of the interaction between their surfaces and the solvent molecules. Sometimes this nature of interaction can be altered by modifying the surfaces of the few-layer exfoliated graphene platelets with polymer surfactants that can control their hydrophobicity or hydrophilicity. Transition metal oxides can be deposited on few-layer exfoliated graphene platelet surface through the interaction with precursor materials in wet-chemical synthesis procedure. Presence of polymer surfactants in the synthesis procedure can bring in even further surface interaction phenomena and alter the structure of the deposited transition metal oxides. Dispersions of few-layer exfoliated graphene platelets and composites with transition metal oxides can be used to manufacture electrode structures either by coating directly onto metallic substrates with the help of a binder material or through the fabrication of self-standing structures without a binder material. Two types of few-layer exfoliated graphene platelets have been investigated in this research: Grade-M particles (15µm average diameter) have an average thickness of approximately 6-8 nanometers and a typical surface area of 120-150 m2/g; and Grade-C, generally consisting of aggregates of particles with diameter less than 2µm and a particle thickness of few nanometers with a surface area of 750 m2/g. First part of this research investigated platelets with different surface areas, particle sizes and structures combined with an aqueous electrolyte as electric double-layer capacitor electrode materials. A binder free self-standing bilayer composite electrode fabricated with Grade-M particles and Grade-C particles was found to retain near-ideal double-layer capacitive characteristics at high scan rate. Reasonably high specific capacitance values normalized with respect to both the weight of the Grade-C particles and the weight of the self-standing electrode were obtained at high current rate. Small equivalent series resistance (ESR) and charge transfer resistance (Rct) of the self-standing electrode was found to be the cause of this desirable behavior. Second part of this research investigated a composite of Birnessite-MnO2 with Grade-M platelet as a redox capacitor electrode material in a self-standing structure. It was found that a self-standing electrode with Birnessite-MnO2 coated platelet exhibits improved impedance characteristics compared to pure Grade-M platelet based electrode. This may be explained by the fact that the platelets remain separated in the electrode structure due to the metallic deposition and thus, improving the impedance characteristics.

Nickel Nanofoam/Different Phases of Ordered Mesoporous Carbon Composite Electrodes for Superior Capacitive Energy Storage.

Electrochemical energy storage devices based on electric double layer capacitors (EDLCs) have received considerable attention due to their high power density and potential for obtaining improved energy density in comparison to the lithium ion battery. Ordered mesoporous carbon (OMC) is a promising candidate for use as an EDLC electrode because it has a high specific surface area (SSA), providing a wider charge storage space and size-controllable mesopore structure with a long-range order, suppling high accessibility to the electrolyte ions. However, OMCs fabricated using conventional methods have several drawbacks including low electronic conductivity and long ionic diffusion paths in mesopores. We used nickel nanofoam, which has a relatively small pore (sub-100 nm to subμm) network structure, as a current collector. This provides a significantly shortened electronic/ionic current paths and plentiful surface area, enabling stable and close attachment of OMCs without the use of binders. Thus, we present hierarchical binder-free electrode structures based on OMC/Ni nanofoams. These structures give rise to enhanced specific capacitance and a superior rate capability. We also investigated the mesopore structural effect of OMCs on electrolyte transport by comparing the capacitive performances of collapsed lamellar, cylindrical, and spherical mesopore electrodes. The highly ordered and straightly aligned cylindrical OMCs exhibited the highest specific capacitance and the best rate capability.

Reduced graphene oxide incorporated NiWO4 for high-performance energy storage

In this work, reduced graphene oxide (RGO) was incorporated into NiWO4 to obtain the RGO/NiWO4 composites by a facile hydrothermal method. The supercapacitive properties of these composites were studied by cyclic voltammetry and galvanostatic charge/discharge. It is surprised that the RGO/NiWO4 composite exhibits twice specific capacitance and better stability than NiWO4. RGO acts as the conductive substrate for NiWO4 nanoparticles, which can not only inhibit the aggregations of the NiWO4 nanoparticles and RGO flakes to obtain the high specific surface area, but also improve the electrical conductivity to accelerate the charge transmission. Moreover, RGO is also an electrical double layer capacitor electrode material. Therefore, the RGO/NiWO4 composite is a good supercapacitive material.

Electrochemical behavior of pitch-based activated carbon fibers for electrochemical capacitors

In the present study, electrode materials for electrochemical capacitors were developed using pitch-based activated carbon fibers with steam activation. The surface and structural characteristics of activated carbon fibers were observed using scanning electron microscopy and X-ray diffraction, respectively. Pore characteristics were investigated using N2/77K adsorption isotherms. The activated carbon fibers were applied as electrodes for electrical double-layer capacitors and analyzed in relation to the activation time. The specific surface area and total pore volume of the activated carbon fibers were determined to be 1520–3230m2/g and 0.61–1.87cm3/g, respectively. In addition, when the electrochemical characteristics were analyzed, the specific capacitance was confirmed to have increased from 1.1F/g to 22.5F/g. From these results, it is clear that the pore characteristics of pitch-based activated carbon fibers changed considerably in relation to steam activation and charge/discharge cycle; therefore, it was possible to improve the electrochemical characteristics of the activated carbon fibers.

The synthesis of large area graphene/carbon nanotubes as additive material and their enhanced specific capacitance

Two-dimensional large area graphene/carbon nanotubes (LAG–CNTs) nanocomposites as additional material for carbon-based electric double layer capacitor (EDLC) performance have been fabricated by using chemical vapor deposition method. The as-prepared LAG–CNTs are characterized with Raman, field emission scanning electron microscope, transmission electron microscope, high-resolution transmission electron microscopy and atomic force microscopy images. Finally, EDLC electrodes based with high surface area activated carbon (YP50F) and LAG–CNTs were fabricated by a facile step. The prepared electrodes (8LAG–CNTs/AC) exhibit a specific capacitance of 15.5 F cc−1 at 0.5 mA cm−2, excellent cycling stability (96.1 % retention after 100 cycles) and high rate capability. These excellent capacitive performances may make the as-prepared nanocomposites as conductive material for preparation of promising electrode materials of EDLC.

Investigation of Hierarchically Structured Paper-like Electrodes Fabricated with Few-Layer Exfoliated Graphene Platelets and Composite with Birnessite-MnO2 for Electrochemical Energy Storage: Electric Double-Layer Capacitor and Redox Capacitor

Few-layer exfoliated graphene platelets are ultrathin particles of graphite that can also be thought of as short stacks of graphene sheets prepared through proprietary manufacturing processes. Several grades and sizes with thicknesses ranging from 1-20 nanometers and width ranging from 1-50µm are manufactured by XG Sciences, Lansing, MI, USA. Unique manufacturing processes for these platelet materials are non-oxidizing; thus, yielding a pristine graphitic surface of sp2 carbon molecules. This makes these platelet particles especially suitable for applications requiring high electrical and thermal conductivities. The surface areas of the resulting platelets predominantly consist of the open surface areas of the graphene basal planes. Open surface areas of the graphene basal planes as well the interlayer spaces between the graphene sheets coupled with high electrical conductivities are an important criteria for electrodes in many electrochemical energy storage technologies. Unique morphology of the platelets offer the potential to structure self-standing electrodes by doing away with inactive components such as current collectors and thus, enhance the energy density. Surface modification of the platelets through the deposition of transition metal oxides on the graphene basal planes has the potential to enhance the energy storage capacity due to the participation of two different components in the energy storage process. Few-layer exfoliated graphene platelets can be dispersed in various aqueous and non-aqueous solvents depending upon the nature of the interaction between their surfaces and the solvent molecules. Sometimes this nature of interaction can be altered by modifying the surfaces of the few-layer exfoliated graphene platelets with polymer surfactants that can control their hydrophobicity or hydrophilicity. Transition metal oxides can be deposited on few-layer exfoliated graphene platelet surface through the interaction with precursor materials in wet-chemical synthesis procedure. Presence of polymer surfactants in the synthesis procedure can bring in even further surface interaction phenomena and alter the structure of the deposited transition metal oxides. Dispersions of few-layer exfoliated graphene platelets and composites with transition metal oxides can be used to manufacture electrode structures either by coating directly onto metallic substrates with the help of a binder material or through the fabrication of self-standing structures without a binder material. Two types of few-layer exfoliated graphene platelets have been investigated in this research: Grade-M particles (15µm average diameter) have an average thickness of approximately 6-8 nanometers and a typical surface area of 120-150 m2/g; and Grade-C, generally consisting of aggregates of particles with diameter less than 2µm and a particle thickness of few nanometers with a surface area of 750 m2/g. First part of this research investigated platelets with different surface areas, particle sizes and structures combined with an aqueous electrolyte as electric double-layer capacitor electrode materials. A binder free self-standing bilayer composite electrode fabricated with Grade-M particles and Grade-C particles was found to retain near-ideal double-layer capacitive characteristics at high scan rate. Reasonably high specific capacitance values normalized with respect to both the weight of the Grade-C particles and the weight of the self-standing electrode were obtained at high current rate. Small equivalent series resistance (ESR) and charge transfer resistance (Rct) of the self-standing electrode was found to be the cause of this desirable behavior. Second part of this research investigated a composite of Birnessite-MnO2 with Grade-M platelet as a redox capacitor electrode material in a self-standing structure. It was found that a self-standing electrode with Birnessite-MnO2 coated platelet exhibits improved impedance characteristics compared to pure Grade-M platelet based electrode. This may be explained by the fact that the platelets remain separated in the electrode structure due to the metallic deposition and thus, improving the impedance characteristics.

Durian shell-based activated carbon electrode for EDLCs

Durian shell-based chemically treated and activated mesoporous-activated carbon had been successfully prepared by chemical activation using phosphoric acid (H3PO4) as an activating agent. The as-prepared activated carbon was systematically characterised by various techniques which include thermogravimetric analysis (TGA), scanning electron microscopy (SEM), nitrogen N2 adsorption-desorption isotherm, and X-ray diffraction. Subsequently, the activated carbon was used as electrode active material in electric double-layer capacitors (EDLCs) together with an aqueous neutral electrolyte. A specific capacitance of 120 mF cm−2 or equivalent to single electrode specific capacitance of 93.1 F g−1 was achieved. The energy density and power density were recorded at values of 11.3 Wh kg−1 and 170 kW kg−1, respectively, for the performance of the single electrode capacitance.

Preparation and characterization of porous carbon from expanded graphite for high energy density supercapacitor in aqueous electrolyte

In this work, we present the synthesis of low cost carbon nanosheets derived from expanded graphite dispersed in Polyvinylpyrrolidone, subsequently activated in KOH and finally carbonized in Ar/H2 atmosphere. Interconnected sheet-like structure with low concentration of oxygen (9.0 at.%) and a specific surface area of 457 m2 g−1 was obtained. The electrochemical characterization of the carbon material as supercapacitor electrode in a 2-electrode configuration shows high specific capacitance of 337 F g−1 at a current density of 0.5 A g−1 as well as high energy density of 37.9 Wh kg−1 at a power density of 450 W kg−1. This electrical double layer capacitor electrode also exhibits excellent stability after floating test for 120 h in 6 M KOH aqueous electrolyte. These results suggest that this activated expanded graphite (AEG) material has great potential for high performance electrode in energy storage applications.

Activated Carbon from Bio-wastes of Durian Fruits as Active Material for Electrodes of Electric Double-layer Capacitors

In this study, bio-wastes from durian fruits such as seeds and shells have been used as precursor materials to prepare activated carbon (AC). While the applicability of a one-step method of impregnation-activation has been investigated for the activation of durian shells, a two-step method of carbonization-impregnation-activation has been tried for the durian seeds. Durian shells based AC (DSh-AC) was found to have a BET surface area () of 2004 m2 g-1 and the durian seeds based AC (DS-AC) had of 1176 m2 g-1. A new approach to electrical double-layer capacitor (EDLC) fabrication has been attempted to avoid the use of polymer binders and organic solvents in the electrodes by coating the electrode material directly on the separator. Instead of coating onto metal current collector, the AC was coated on a glass microfiber filter which was used as the separator to form the electrode. As an electrode material in the EDLC, DSh-AC performed well with a specific capacitance () between 72 and 82 F g-1 whereas the DS-AC showed lower values of between 64 and 70 F g-1. The reasonable good results indicate that the simple approach of device fabrication can also produce EDLCs with satisfactory performance parameters.

Phenolic carbon cloth-based electric double-layer capacitors with conductive interlayers and graphene coating

Phenolic resin-derived activated carbon (AC) cloths are used as electrodes for large-scale electric double-layer capacitors or supercapacitors. To increase the energy and power density of the supercapacitor, the contact resistance between the carbon cloth and the aluminium foil current collector is reduced by modifying the Al current collectors. Different modified Al current collectors, including Toyal-Carbo®(surface-modified Al), DAG® (deflocculated Acheson™ graphite) coating and poly(3,4-ethylenedioxythiophene) (PEDOT) coating, have been tested and compared. The use of modified Al current collectors are shown to greatly reduce the contact resistance between the AC cloth and the Al foil. Another solution investigated in this study is to coat AC cloth with graphene through electrophoretic deposition (EPD). The graphene coated AC cloth is shown increased the capacitance and greatly reduced internal resistance.