Partially graphitic micro- and mesoporous carbon microspheres for supercapacitors

Partially graphitic micro-and mesoporous carbon microspheres(GMMCMs)were synthesized using hydrothermal emulsion polymerization followed by KOH activation and catalytic graphitization.The resulting GMMCMs show micro-and mesopores with a specifc surface area of 1113 m2/g,regular spherical shape with diameters of 0.5–1.0 mm and a partially graphitic structure with a low internal resistance of 0.34 V.The graphitic carbons as electrode for supercapacitor exhibit a fast ion-transport and rapid charge–discharge feature,and a high-rate electrochemical performance.The typical GMMCM electrode shows a specifc capacitance of 220 F/g at 1.0 A/g,and 185 F/g under a high current density of20.0 A/g in a 6 mol/L KOH electrolyte.     

Effects of pyrolysis conditions on the physical characteristics of oil-palm-shell activated carbons used in aqueous phase phenol adsorption

Oil-palm shells, a biomass by-product from palm-oil mills, were converted into activated carbons by vacuum or nitrogen pyrolysis, followed by steam activation. The effects of pyrolysis environment, temperature and hold time on the physical characteristics of the activated carbons were studied. The optimum pyrolysis conditions for preparing activated carbons for obtaining high pore surface area are vacuum pyrolysis at a pyrolysis temperature of 675 °C and 2 h hold time. The activation conditions were fixed at a temperature of 900 °C and 1 h hold time. The activated carbons thus obtained possessed well-developed porosities, predominantly microporosities. For the pyrolysis atmosphere, it was found that significant improvement in the surface characteristics of the activated carbons was obtained for those pyrolysed under vacuum. Adsorption capacities of activated carbons were determined using phenol solution. For the activated carbons pyrolysed under optimum vacuum conditions, a maximum phenol adsorption capacity of 166 mg/g of carbon was obtained. A linear relationship between the BET surface area and the adsorptive capacity was shown.     

Nitrogen enriched mesoporous carbon spheres obtained by a facile method and its application for electrochemical capacitor

A kind of mesoporous carbon spheres (MCS) containing in-frame incorporated nitrogen has been prepared by a facile polymerization-induced colloid aggregation method. As the electrode material for electric double layer capacitor (EDLC) in 5 mol/L H₂SO₄, the MCS products present excellent specific capacitance as 211 F/g much larger than that of the most popularly applied activated carbon at a high discharge current density of 1 A/g. Its specific capacitance can still remain 200 F/g at 20 A/g. The superior electrochemical performance of MCS is associated with the following characteristics: high specific surface area (∼1330 m²/g) contributed mainly by the mesopores, uniform pore size as large as 29 nm and moderate content of nitrogen (10 wt%), which are the requirements for ideal supercapacitors.     

Outstanding room-temperature capacitance of biomass-derived microporous carbons in ionic liquid electrolyte

A remarkable capacitance of 180 F·g^− 1 (at 5 mV·s^− 1) in solvent-free room-temperature ionic liquid electrolyte, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, was achieved in symmetric supercapacitors using microporous carbons with a specific surface area of ca. 2000 m²·g^− 1 calculated from gas sorption by the 2D-NLDFT method. The efficient capacitive charge storage was ascribed to textural properties: unlike most activated carbons, high specific surface area was made accessible to the bulky ions of the ionic liquid electrolyte thanks to micropores (1–2 nm) enabled by fine-tuning chemical activation. From the industrial perspective, a high volumetric capacitance of ca. 80 F·cm^− 3 was reached in neat ionic liquid due to the absence of mesopores. The use of microporous carbons from biomass waste represents an important advantage for large-scale production of high energy density supercapacitors.     

An activated carbon with high capacitance from carbonization of a resorcinol–formaldehyde resin

A polymeric activated carbon (PAC) was synthesized from the carbonization of a resorcinol–formaldehyde resin with KOH served as an activation agent. The nitrogen adsorption–desorption at 77 K, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the prepared PAC. Compared with the commercial activated carbon (Maxsorb: Kansai, Japan), PAC shows superior capacitive performance in terms of specific capacitance, power output and high energy density as electrode materials for supercapacitors. PAC presents a high specific capacitance of 500 F g^?1 in 6 mol l^?1 KOH electrolyte at a current density of 233 mA g^?1 which remained 302 F g^?1 even at a high current density of 4.6 A g^?1. The good electrochemical performance of the PAC was ascribed to well-developed micropores smaller than 1.5 nm, the presence of electrochemically oxygen functional groups and low equivalent series resistance.     

Electrochemical characterization on cobalt sulfide for electrochemical supercapacitors

High capacitance at a high charge–discharge current density of 50 mA/cm² for a new type of electrochemical supercapacitor cobalt sulfide (CoS_x) have been studied for the first time. The CoS_x was prepared by a very simply chemical precipitation method. The electrochemical capacitance performance of this compound was investigated by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge tests with a three-electrode system. The results show that CoS_x has excellent electrochemical capacitive characteristic with potential range −0.3 ∼ 0.35 V (versus SCE) in 6 M KOH solution. Charge–discharge behaviors have been observed with the highest specific capacitance values of 475 F/g at the current density of 5 mA/cm², even at the high current density of 50 mA/cm², CoS_x also shows the high specific capacitance values of 369 F/g.     

A study of ion charge transfer on electrochemical behaviors of poly(vinylidene fluoride)-derived carbon electrodes

In this work, porous carbon with a high specific surface area as electrode materials for supercapacitors are obtained by a carbonization process at various temperatures from 700 °C to 1000 °C without activation process using poly(vinylidene fluoride) (PVDF) as a carbon precursor. The electrochemical performance is characterized by cyclic voltammetry and galvanostatic charge–discharge cycling performance using two-electrode system in 6.0 M KOH as an aqueous electrolyte. The results indicate that carbonization temperature significantly affected the specific surface area and pore volume of the PVDF-derived carbons and their capacitive behavior. In particular, the electrochemical performance of the prepared PVDF-derived carbon is determined by both the electric double-layer capacitance and the pseudo-capacitance resulting from the residual surface functional groups on PVDF-derived carbons.     

The effects of hydrogen on KOH activation of petroleum coke

KOH activation of petroleum coke (PC) was conducted with 30 vol%H₂ + 70 vol% N₂ as carrier gas. TG-DTG, FTIR, elemental analysis, N₂ adsorption, GC and XRD techniques were used to investigate the effects of hydrogen on the activation. During the initial stage of the activation, i.e. the carbonization of the PC, additional CH and CH₂ species were formed due to the chemisorption of hydrogen on the nascent sites of the PC created by the removal of the surface heteroatom groups. The formation of the CH and CH₂ species increased the quantity of ‘active sites’ which is favorable to the further activation reaction, and developed the porous structure of the activated carbons. The micropore volume and BET surface areas of the activated carbon prepared under 30 vol% H₂ + 70 vol% N₂ and with a relatively low KOH/PC weight ratio of 2:1 have been increased from 0.78 cm³/g and 1936 m²/g to 0.97 cm³/g and 2477 m²/g, respectively, compared to that prepared in pure N₂ atmosphere with the same KOH/PC ratio.     

Effects of potassium distributions in carbonizations of bituminous coal

The carbonization of coal/KOH mixtures were investigated to identify the influence of potassium distributions on characteristics of the final products. The products were characterized using TGA, BET, TEM and adsorption of lead from its aqueous solutions with initial concentrations of 10–100 ppm. For the activated carbon obtained at 600 °C, the potassium distribution affected both the BET surface areas (661–1994 m²/g) and the meso- and micro-pore volumes ratios (0.48–0.91). There were also evolutions of nanostructures of both straight and curved tubular morphologies as evidenced by TEM micrograph. The samples exhibited different adsorptive capacities when tested in adsorption of lead from aqueous solutions. The adsorption followed second order kinetics and the equilibrium data were better described by empirical Freudlich isotherm model. The amount of lead adsorbed ranges from 4.3 to 47.3 mg/g. Thus, different degrees of potassium effects led to activated carbons with different surface and adsorptive properties.     

A high-performance carbon derived from polyaniline for supercapacitors

Activated carbon derived from rod-shaped polyaniline (the diameter of 170 nm) was synthesized by carbonization and subsequent activation with KOH. The obtained activated carbon exhibits a high specific capacitance (455 F g^?1) and remarkable rate capability due to its high specific surface area (1976 m² g^?1), narrow pore size distribution (< 3 nm) as well as short diffusion length. It is indicated that the promising synthetic method used in this work can pave the way for designing new carbon based materials from different polymers for high-performance energy applications.     

Highly porous electrodes from novel corn grains-based activated carbons for electrical double layer capacitors

In this study, novel corn grains-based activated carbons (CG-ACs) were prepared and their use as electrodes in the electrical double layer capacitor (EDLC) performed successfully. The structural properties, energetic heterogeneities and surface functional groups of CG-ACs were characterized using different techniques like nitrogen sorption data, adsorption energy distribution (AED) and X-ray photoelectric spectroscopy (XPS). The electrochemical properties of various CG-ACs were evaluated by using cyclic voltammetry. The maximum specific capacitance value as 257 F g⁻¹ was obtained in 6 M KOH electrolyte solution. The effects of various properties of the porous carbon materials on the EDLC performance were discussed.     

Effects of feedstock pre-drying on carbonization of KOH-mixed bituminous coal in preparation of activated carbon

Activated carbon samples of different porosities and other characteristic features have been prepared using bituminous coal–KOH feedstocks of different degrees of pre-drying. The feedstocks were subjected to thermogravimetric analysis from 30 to 1000 °C. Different weight loss profiles and carbon burnt-off were obtained. This was associated with different level of potassium intercalations in the coal matrix and concomitant different extent of reactions with carbon. Another set of feedstocks was carbonized at 600 or 800 °C to obtain samples of activated carbon. The activated carbons were characterized using BET surface area, SEM, FTIR and XRD. These reflected the effects of pre-drying on the products. The surface area and V_meso/V_micro ratio increase with increase in duration of pre-drying. XRD profiles indicated lower proportion of aromatic stacking in the sample with highest surface area (1994 m²/g) and V_meso/V_micro ratio (0.91) than in the other samples. The pre-drying could be used as a key variable for obtaining activated carbons of different porosities.     

Ordered mesoporous silicon carbide-derived carbon for high-power supercapacitors

Micro/mesoporous carbon was prepared by chlorination of ordered mesoporous silicon carbide derived from magnesio-thermal reduction of templated carbon-silica precursors. These materials were then used as active materials for electrochemical capacitors and characterized in 1.5 M NEt₄BF₄/AN. The electrodes showed outstanding rate capability (90% of capacity retention at 1 V/s and time constant of 1 s) with high specific areal capacitance (0.5 F/cm² of electrode), that makes such hierarchical porous carbons promising for high power and energy density supercapacitors.     

Mesoporous germanium as anode material of high capacity and good cycling prepared by a mechanochemical reaction

Mesoporous Ge was prepared by mechanochemical reaction of GeO₂ and Mg powders followed by an etching process with HCl solution. It was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and charge–discharge measurement. With a pore-distribution concentrated around 10 nm, the product presents a BET surface area of 49.98 m²/g. When using as an anode material for lithium ion battery, the mesoporous Ge exhibits a reversible capacity of 950 mA h/g and retains a capacity of 789 mA h/g after 20 cycles at a current density of 150 mA/g. The cycleability is significantly improved compared with non-porous Ge.     

Synthesis of nickel selenide thin films for high performance all-solid-state asymmetric supercapacitors

As a significant semiconductor, nickel selenide shows enormous potential and extensive application prospects in the field of sensor, photocatalysis and supercapacitor. In this paper, nickel selenide (Ni₃Se₂, NiSe) thin films were successfully fabricated on stainless-steel sheet using a facile, effective electrodeposition technique. The morphologies, microstructures and chemical compositions of the thin films are characterized systematically. Electrochemical tests exhibit that the Ni₃Se₂ and NiSe possess high specific capacitance of 581.1 F/g and 1644.7 F/g, respectively. A flexible, all-solid-state asymmetric supercapacitor is assembled by utilizing NiSe film as positive electrode and activated carbon as negative electrode. The solid device delivers a high areal capacitance of 27.0 mF/cm² at the current density of 0.7 mA/cm². The maximum volumetric energy density and power density of the NiSe//AC asymmetric SCs can achieve 0.26 mWh/cm³ and 33.35 mW/cm³, respectively. The device shows robust cycling stability with 84.6% capacitance retention after 10,000 cycles, outstanding flexibility and satisfactory mechanical stability. Moreover, two devices in series can light up a red light-emitting diode, which displayed great potential applications for energy storage.     

KOH activated carbon derived from biomass-banana fibers as an efficient negative electrode in high performance asymmetric supercapacitor

Here we demonstrate the fabrication, electrochemical performance and application of an asymmetric supercapacitor(AS) device constructed with β–Ni(OH)_2/MWCNTs as positive electrode and KOH activated honeycomb-like porous carbon(K-PC) derived from banana fibers as negative electrode. Initially,the electrochemical performance of hydrothermally synthesized β–Ni(OH)_2/MWCNTs nanocomposite and K-PC was studied in a three-electrode system using 1 M KOH. These materials exhibited a specific capacitance(Cs) of 1327 F/g and 324 F/g respectively at a scan rate of 10 m V/s. Further, the AS device i.e.,β–Ni(OH)_2/MWCNTs//K-PC in 1 M KOH solution, demonstrated a Cs of 156 F/g at scan rate of 10 m V/s in a broad cell voltage of 0–2.2 V. The device demonstrated a good rate capability by maintaining a Cs of 59 F/g even at high current density(25 A/g). The device also offered high energy density of 63 Wh/kg with maximum power density of 5.2 kW/kg. The AS device exhibited excellent cycle life with 100% capacitance retention at 5000 th cycle at a high current density of 25 A/g. Two AS devices connected in series were employed for powering a pair of LEDs of different colors and also a mini fan.     

Double-layer capacitance of waste coffee ground activated carbons in an organic electrolyte

Using ZnCl₂ activation we prepared a series of carbon electrodes from waste coffee grounds to study the effect of mesopores on double-layer capacitance in a tetraethyl ammonium tetrafluoroborate/acetonitrile electrolyte. The activated carbon with the largest mesopore volume achieved an energy density of 34 Wh kg^?1 at low current loads, and significantly retained an energy density of 16.5 Wh kg^?1 and specific capacitance of more than 100 F g^?1 at fast charge–discharge rates (20 A g^?1). The effect of mesopores on capacitance at fast charge–discharge rates is discussed.     

Constructing a hierarchical graphene–carbon nanotube architecture for enhancing exposure of graphene and electrochemical activity of Pt nanoclusters

Stacking of individual graphene sheets (GS) is effectively inhibited by introducing one-dimensional carbon nanotubes to form a 3-D hierarchical structure which enhances the utilization of GS-based composites. From SEM images, CNTs are useful nanospacers for diminishing the face-to-face aggregation of GS. The specific electrochemically active surface area (SECSA) and specific double-layer capacitance (C_S,DL) of Pt/GS–CNTs (127.9 m²/g, 171.3 F/g) is much higher than that of Pt/GS (105.4 m²/g, 104.7 F/g) and Pt/CNTs (51.5 m²/g, 37.1 F/g), revealing the synergistic effects between GS and CNTs on enhancing the electrochemical activity of Pt nanoparticles and electrolyte-accessible surface area.     

Macro- and microporous carbon monoliths with high surface areas pyrolyzed from poly(divinylbenzene) networks

Carbon monoliths with well-defined macropores and high surface areas were prepared by carbonization of macroporous poly(divinylbenzene) (PDVB) monoliths. The carbonization reactions of PDVB networks are studied by thermal analysis and FT-IR measurements. According to the measurement results, the PDVB networks are mostly pyrolyzed at 430 °C and their structures dynamically change to graphite-like structure between 600 and 700 °C. The macropore structure retained while the mesopores disappeared after carbonization. In addition, the surface area of the obtained carbons dramatically increased over 900 °C. The typical carbon monolith carbonized at 1000 °C for 2 h had a surface area of 1500 m² g^?1 and uniform macropores with a diameter of 1 μm.     

Nitric acid activation of graphite granules to increase the performance of the non-catalyzed oxygen reduction reaction (ORR) for MFC applications

Nitric acid and thermal activation of graphite granules were explored to increase the electrocatalytic performance of dissolved oxygen reduction at neutral pH for microbial fuel cell (MFC) applications. Electrochemical experiments showed an improvement of +400 mV in open circuit potential for graphite granules when they were activated. The improvement of ORR performance observed with activated granules was correlated to the increase of Brunauer–Emmett–Teller (BET) surface of the activated material and the emergence of nitrogen superficial groups revealed by X-ray photoelectron spectroscopy (XPS) analysis on its surface.The use of activated graphite granules in the cathodic compartment of a dual-chamber MFC led to a high open circuit voltage of 1050 mV, which is among one of the highest reported so far. The stable performance of this cathode material (current density of 96 A m^?3 at +200 mV/Ag–AgCl) over a period of 10 days demonstrated its applicability as a cathode material without any costly noble metal.