Preparation of a graphene nanosheet/polyaniline composite with high specific capacitance

A graphene nanosheet (GNS)/polyaniline (PANI) composite was synthesized using in situ polymerization. The morphology and microstructure of samples were examined by scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. Electrochemical properties were characterized by cyclic voltammetry (CV) and galvanostatic charge/discharge. GNS as a support material could provide more active sites for nucleation of PANI as well as excellent electron transfer path. The GNS was homogeneously coated on both surfaces with PANI nanoparticles (∼2 nm), and a high specific capacitance of 1046 F g⁻¹ (based on GNS/PANI composite) was obtained at a scan rate of 1 mV s⁻¹ compared to 115 F g⁻¹ for pure PANI. In addition, the energy density of GNS/PANI composite could reach 39 W h kg⁻¹ at a power density of 70 kW kg⁻¹.     

LiPF6 based ethylene carbonate-dimethyl carbonate electrolyte for high power density electrical double layer capacitor

Electrochemical characteristics of the electrical double layer capacitor based on the two identical microporous carbide derived carbon C(TiC 950) electrodes in 1 M LiPF₆ ethylene carbonate-dimethyl carbonate (1:1 by volume) mixture have been studied using cyclic voltammetry and electrochemical impedance spectroscopy. Specific capacitance, phase angle, series and parallel resistances, characteristic time constant, energy and power densities etc. have been calculated and found to be dependent on the cell potential applied. Wide region of ideal polarisability ΔE ≤ 3.2 V, short characteristic time constant and high limiting capacitance 129 F g⁻¹, complex power and maximal energy and power density values have been obtained, indicating that this electrolyte can be used for high energy and power density supercapacitors. Additionally, the supercapacitors based on the partially graphitized C(VC) (applied as negatively charged electrode) and amorphous C(TiC 950) (applied as positively charged electrode) were completed and tested. The calculated energy and power densities were for asymmetrical C(VC 1100)|C(TiC 950)|1 M LiPF₆ + EC + DMC cell 26.2 Wh kg⁻¹ and 57.2 kW kg⁻¹, respectively, but for symmetrical C(TiC 950)|C(TiC 950)|1 M LiPF₆ + EC + DMC cell somewhat higher energy density 36.7 Wh kg⁻¹ and power density 83.6 kW kg⁻¹ values were established.     

Polyaniline nanofibers obtained by interfacial polymerization for high-rate supercapacitors

Polyaniline (PANI) nanofibers were fabricated by interfacial polymerization in the presence of para-phenylenediamine (PPD). The additives cannot only have a profound impact on the polymers morphology, but can also improve their specific energy and specific capacitances. It was found that PANI nanofibers prepared in the presence of PPD were longer and less entangled than those in the absence of PPD due to a much faster polymerization rate in initial stage. A specific capacitance value of 548 F g⁻¹, a specific power value of 127 W kg⁻¹ and a specific energy value of 36 Wh kg⁻¹ were obtained in polyaniline nanofibers prepared in the present of PPD at a constant discharge current density of 0.18 A g⁻¹.     

Superior electric double layer capacitors using ordered mesoporous carbons

This paper reports for the first time superior electric double layer capacitive properties of ordered mesoporous carbon (OMCs) with varying ordered pore symmetries and mesopore structure. Compared to commercially used activated carbon electrode, Maxsorb, these OMC carbons have superior capacitive behavior, power output and high-frequency performance in EDLCs due to the unique structure of their mesopore network, which is more favorable for fast ionic transport than the pore networks in disordered microporous carbons. As evidenced by N₂ sorption, cyclic voltammetry and frequency response measurements, OMC carbons with large mesopores, and especially with 2-D pore symmetry, show superior capacitive behaviors (exhibiting a high capacitance of over 180 F/g even at very high sweep rate of 50 mV/s, as compared to much reduced capacitance of 73 F/g for Maxsorb at the same sweep rate). OMC carbons can provide much higher power density while still maintaining good energy density. OMC carbons demonstrate excellent high-frequency performances due to its higher surface area in pores larger than 3 nm. Such ordered mesoporous carbons (OMCs) offer a great potential in EDLC capacitors, particularly for applications where high power output and good high-frequency capacitive performances are required.     

Ordered mesoporous carbons synthesized by a modified sol-gel process for electrosorptive removal of sodium chloride

Capacitive deionization (CDI) represents an alternative process to remove the ions from the brackish water. In this study two series of ordered mesoporous carbons (OMCs) that demonstrated the potential use for capacitive desalination have been synthesized by a modified sol-gel process involving nickel salts. It was shown that the preferred formation of crown-ether type complexes between nickel ions and triblock copolymers resulted in higher BET surface area and smaller mesopores. As the electrode materials for CDI, OMC obtained by the addition of NiSO₄ · 6H₂O exhibited best electrochemical performance compared with other OMCs and a commercial activated carbon either in 0.1 M NaCl solution or in 0.0008 M NaCl solution, plus the amount of adsorbed ions measured by a flow through apparatus reached 15.9 μmol g⁻¹ and the ions could be fully released into the solution. The excellent electrosorption desalination performance of OMC obtained by the addition of NiSO₄ · 6H₂O was ascribed to its high BET surface area of 1491 m² g⁻¹ and ordered mesopores of 3.7 nm. Based on these results, it is deduced that the modified sol-gel process might be a potential method of obtaining the excellent electrode materials for CDI.     

Polyfurfuryl alcohol derived activated carbons for high power electrical double layer capacitors

Polyfurfuryl alcohol (PFA) derived activated carbons were prepared by the acid catalysed polymerization of furfuryl alcohol, followed by potassium hydroxide activation. Activated carbons with apparent BET surface areas ranging from 1070 to 2600 m² g⁻¹, and corresponding average micropore sizes between 0.6 and 1.6 nm were obtained. The porosity of these carbons can be carefully controlled during activation and their performance as electrode materials in electric double layer capacitors (EDLCs) in a non-aqueous electrolyte (1 M Et₄NBF₄/ACN) is investigated.Carbon materials with a low average pore size (<∼0.6 nm) exhibited electrolyte accessibility issues and an associated decrease in capacitance at high charging rates. PFA carbons with larger average pore sizes exhibited greatly improved performance, with specific electrode capacitances of 150 F g⁻¹ at an operating voltage window of 0-2.5 V; which corresponds to 32 Wh kg⁻¹ and 38 kW kg⁻¹ on an active material basis. These carbons also displayed an outstanding performance at high current densities delivering up to 100 F g⁻¹ at current densities as high as 250 A g⁻¹. The exceptionally high capacitance and power of this electrode material is attributed to its good electronic conductivity and a highly effective combination of micro- and fine mesoporosity.     

An activated carbon monolith as an electrode material for supercapacitors

Activated carbon binderless monoliths with high consistency and large porosity, synthesised from a mesophase pitch, are studied as electrodes for supercapacitors. The electrochemical cells prepared provided high capacitance values in sulphuric acid media (334 F g⁻¹) and very low electrical resistivity, which results in a very efficient energy storage device (12 Wh Kg⁻¹ maximum energy density and 12,000 W Kg⁻¹ maximum power density). Long-term cycling experiments showed excellent stability with a reduction of the initial capacitance values of 19% after performing 23,000 galvanostatic cycles at ∼300 mA g⁻¹.     

Mesoporous RuO2 for the next generation supercapacitors with an ultrahigh power density

The 3D mesoporous, well crystalline RuO₂ film prepared via the evaporation-induced self-assembled method (EISA) successfully demonstrates the extremely high power performances (e.g., excellent capacitive behavior at 10,000 mV s⁻¹, ultrahigh-frequency capacitive responses (the absence of a knee point in the Nyquist plot), and 2.6 MW kg⁻¹ with an acceptable energy density of 4.6 Wh kg⁻¹). These excellent capacitive performances were identified by means of voltammetric and electrochemical impedance spectroscopic (EIS) analyses. The mesoporous (with mean pore spacing of 18.1 nm) and crystalline nature of this film was characterized by means of the field emission scanning electron microscopy (FE-SEM), Brunaur-Emmett-Teller (BET) method, small-angle X-ray scattering (SAXRS), high-resolution transmission electron microscopy (HR-TEM), electron diffraction (ED), and X-ray diffraction (XRD) analyses. This mesoporous, well crystalline RuO₂ film constrains the redox transition to the superficial region meanwhile the tailored mesoporous structure increases the electrochemically active centers, promotes the penetration of electrolytes, provides the “proton reservoirs”, and enhances the rate of electron transport simultaneously for the ultrahigh power application. The specific capacitance of this mesoporous RuO₂ can be enhanced from 84 to 185 F g⁻¹ after the microwave-assisted hydrothermal treatment.     

Porous polythiophene as a cathode material for lithium batteries with high capacity and good cycling stability

Polythiophene (PTh) has been synthesized by chemical oxidative polymerization and used as an active cathode material in lithium batteries. The lithium batteries are characterized by cyclic voltammetry (CV), galvanostatic charge/discharge cycling and electrochemical impedance spectroscopic studies (EIS). The lithium battery with the PTh cathode exhibits a discharge voltage of 3.7 V compared to Li⁺/Li and excellent electrochemical performance. PTh can provide large discharge capacities above 50 mA h g⁻¹ and good cycle stability at a high current density 900 mA g⁻¹. After 500 cycles, the discharge capacity is maintained at 50.6 mA h g⁻¹. PTh is a promising candidate for high-voltage power sources with excellent electrochemical performance.     

Activated carbon fiber cloths as electrodes for high performance electric double layer capacitors

Activated carbon fiber cloth (ACFC) electrodes with high double layer capacitance and good rate capability were prepared from polyacrylonitrile (PAN) fabrics by optimizing the carbonization temperature prior to CO₂ activation. The carbonization temperature has a marked effect on both the pore structure and the electrochemical performances of the ACFCs. Moderate carbonization at 600 °C results in higher specific surface area and larger pore size, and hence higher capacitance and better rate capability. The specific capacitance of the ACFCs in 6 mol L⁻¹ KOH aqueous solution can be as high as 208 F g⁻¹. It remains 129 F g⁻¹ as the current density increases to 10 000 mA g⁻¹.     

Porous carbons prepared by using metal-organic framework as the precursor for supercapacitors

Several kinds of porous carbons were easily prepared by using metal-organic framework as both template and carbon precursor. Nanocasting is chosen to adjust the textures and structures with phenolic resin or carbon tetrachloride and ethylenediamine as the additional carbon sources. The carbon materials were further activated by potassium hydroxide (KOH). The electrochemical capacitance behaviors of these carbon materials were investigated in both aqueous and organic electrolytes. Energy densities of 9.4 W h kg⁻¹ in 6 M KOH and 31.2 W h kg⁻¹ in 1.5 M tetraethylammonium tetrafluoroborate acetonitrile solution can be obtained for one of the prepared porous carbon materials (MAC-A) with the surface area of 2222 m² g⁻¹ and the total pore volume of 1.14 cm³ g⁻¹. Due to its high packing density of 0.93 g cm⁻³, the related volumetric specific energy densities of 8.8 and 29.0 W h L⁻¹ can be got.     

Highly mesoporous carbonaceous material of activated carbon beads for electric double layer capacitor

The activated carbon beads (ACB) are prepared by a new preparation method, which is proposed by mixing the coal tar pitch and fumed silica powder at a certain weight ratio and activation by KOH at different weight ratios and different temperatures. The BET surface area, pore volume and average pore size are obtained based on the nitrogen adsorption isotherms at 77 K by using ASAP 2010 apparatus. The results show that our samples have much high specific surface area (SSA) of 3537 m² g⁻¹and high pore volume value of 3.05 cm³ g⁻¹. The percentage of mesopore volume increases with the weight ratio of KOH/ACB ranging from 4% to 72%. The electrochemical double layer capacitors (EDLCs) are assembled with resultant carbon electrode and electrolyte of 1 mol L⁻¹ Et₄NBF₄/PC. The specific capacitance of the ACB sample could be as high as 191.7 F g⁻¹ by constant current charge/discharge technique, indicating that the ACB presents good characteristics prepared by the method proposed in this work. The investigation of influence of carbon porosity structure on capacitance indicates that the SSA plays an important role on the capacitance and all the pore sizes of less than 1 nm, from 1 to 2 nm and larger than 2 nm contribute to the capacitance. Mesopore structure is beneficial for the performance at high current density.     

Environmental assessment of natural radionuclides and heavy metals in waters discharged from a lignite-fired power plant

Gross alpha, gross beta and ²²⁶Ra activities as well as the concentration of trace metals (V, Cr, Mn, Ni, Cu, Zn, Mo and Pb) in the discharge waters of the major lignite-fired power plant in Greece were measured during the period October 2004 to May 2006. Gross alpha activity of particulate matter in the discharge waters was 0.75 ± 0.40 Bq g⁻¹ (0.3-1.2 Bq g⁻¹) while the beta activity was 1.54 ± 0.50 Bq g⁻¹ (1.2-1.7 Bq g⁻¹). The ranges of water gross alpha, beta and ²²⁶Ra activities were 0.062-0.268 Bq L⁻¹, 0.064-0.268 Bq L⁻¹ and 0.021-0.062 Bq L⁻¹, respectively. The mean concentration of ²²⁶Ra in the discharge waters was at least one order of magnitude higher than in natural water bodies. Soil samples were collected from fields irrigated with discharge waters and 29.2 ± 2.2 Bq kg⁻¹ of ²³⁸U, 1.2 ± 0.2 Bq kg⁻¹ of ²³⁵U, 26.8 ± 0.8 Bq kg⁻¹ of ²²⁶Ra, 36.8 ± 1.5 Bq kg⁻¹ of ²³²Th and 492.6 ± 25.8 Bq kg⁻¹ of ⁴⁰K were determined. The concentration values of dissolved metals in the discharge waters were higher than those usually observed in water streams near coal-fired power plants or rivers due to metal leaching from lignite or/and by-products. However, the leaching at high pH’s as those observed in the discharged waters does not raise the concentration of the studied metals to values higher than the criteria maximum concentrations and criterion continuous concentration (CCC) values of the US EPA water quality criteria. Statistical analysis was further applied to reveal the correlations between the different water components. Hierarchical cluster analysis revealed four different clusters: the first cluster was primarily composed of radioactivity and physicochemical parameters; the second cluster consisted of Cu, Ni and Zn, the third of Mn, Fe, Mo and Pb and the fourth of V and Cr. This clustering agrees with the associations suggested for elements in most coals or with the Goldschmidt classification.     

Improvement of the structural and chemical properties of a commercial activated carbon for its application in electrochemical capacitors

The present paper shows that the performance of an inexpensive activated carbon used in electrochemical capacitors can be significantly enhanced by a simple treatment with KOH at 850 °C. The changes in the specific surface area, as well as in the surface chemistry, lead to high capacitance values, which provide a noticeable energy density.The KOH-treatment of a commercial activated carbon leads to highly pure carbons with effective surface areas in the range of 1300-1500 m² g⁻¹ and gravimetric capacitances as high as three times that of the raw carbon.For re-activated carbons, one obtains at low current density (50 mA g⁻¹) values of 200 F g⁻¹ in aqueous electrolytes (1M H₂SO₄ and 6M KOH) and around 150 F g⁻¹ in 1M (C₂H₅)₄NBF₄ in acetonitrile. Furthermore, the resulting carbons present an enhanced and stable performance for high charge/discharge load in organic and aqueous media.This work confirms the possibilities offered by immersion calorimetry on its own for the prediction of the specific capacitance of carbons in (C₂H₅)₄NBF₄/acetonitrile. On the other hand, it also shows the limitations of this technique to assess, with a good accuracy, the suitability of a carbon to be used as capacitor electrodes operating in aqueous electrolytes (H₂SO₄ and KOH).     

Bimodal, templated mesoporous carbons for capacitor applications

Several high capacitance ordered mesoporous carbon (OMC) materials, containing a bimodal pore distribution, were synthesized directly using hexagonal mesoporous silicas (HMS) as the template material. The HMS templates were formed using amine surfactants (C_nH_2n+1NH₂) with hydrophobic chain lengths containing 8-16 carbons (n = 8-16). These HMS structures were found to have an interconnected wormhole structure, high textural mesoporosity, a surface area ranging from 910 to 1370 m²/g, and a total pore volume of 1.09-1.83 cm³/g. Also, evidence for a change in structure from hexagonally ordered to layered (for surfactants of chain length with n > 12) was found. The resulting OMCs, formed using sucrose as the carbon precursor, contain bimodal pores 1.6-1.8 and 3.3-3.9 nm in diameter and have a very high surface area (980-1650 m²/g). The OMCs were evaluated as electrode materials for electrochemical capacitors using cyclic voltammetry in 0.5 M H₂SO₄ solution, giving a tunable gravimetric capacitance that increased linearly with BET area (and surfactant chain length), up to 260 F/g, among the highest yet reported for ordered carbon formed from an HMS templated precursor. All OMCs studied in this work displayed a specific capacitance of ∼0.15 F/m².     

High-capacitance carbon electrode prepared by PVDC carbonization for aqueous EDLCs

Porous carbons with high-volumetric capacitance in aqueous electric double layer capacitors (EDLCs) were simply prepared by poly(vinylidene chloride) (PVDC) carbonization at high temperature without activation or any other additional processes. The PVDC-derived carbon is microporous with Brunauer-Emmett-Teller (BET) surface area about 1200 m² g⁻¹. As it possesses not only high-gravimetric capacitance (262 F g⁻¹) but also high-electrode density (0.815 g cm⁻³), the PVDC-derived carbon present an outstanding high-volumetric capacitance of 214 F cm⁻³, twice over of the commercial carbon Maxsorb-3 with a high-surface area of 3200 m² g⁻¹. The PVDC-derived carbon also exhibit good rate performance, indicating that it is a promising electrode material for EDLCs.     

Electrochemical characteristics of a new electric double layer capacitor with acidic polymer hydrogel electrolyte

The article presents the results of a study on activated carbon fiber cloth (AC) and a hydrophobic microporous polypropylene (PP) membrane, both chemically modified with acidic acetone aldol condensation products, and on testing their use as an electrode material and separator in electric double layer capacitors (EDLCs). Polymer hydrogel used was based on poly(acrylamide) (PAAM), H₂SO₄ and water. Electrochemical characteristics of EDLCs were investigated by cyclic voltammetry and galvanostatic charge–discharge cycle tests and also by impedance spectroscopy. As a result, the capacitor with polymer hydrogel was found to exhibit similar capacitance as that with the H₂SO₄ aqueous solution and an excellent high-rate dischargeability. At the 4000th cycle of potential cycling (0.5 A g⁻¹) the specific capacitance of 132 F g⁻¹ was obtained with a energy density of 3.45 Wh kg⁻¹ at a power density of 56 W kg⁻¹. The above results provide valuable information to explore the novel composition of EDLCs.     

The effect of the interlayer anions on the electrochemical performance of layered double hydroxide electrode materials

Both high energy density and high power density are vitally required for new applications such as electric vehicles. Here we present a comparison of two well-crystallised layered double hydroxides, [Ni₄Al(OH)₁₀]OH and [Ni₄Al(OH)₁₀]NO₃, which shows that the former can maintain a better discharge capacity, 294-299 mAh g⁻¹, than the latter, 233-287 mAh g⁻¹ at a current density of 2000 mA g⁻¹ within about 300 cycles, although both electrodes deliver a similar capacity of 326 mAh g⁻¹ at 200 mA g⁻¹ initially. It is believed that both the more watery interlayer space in [Ni₄Al(OH)₁₀]OH than in [Ni₄Al(OH)₁₀]NO₃ and the morphologic changes induced by anion exchange of NO₃⁻ by OH⁻ during electrochemical cycles play key roles in their behaviour.     

Effect of pore size distribution of coal-based activated carbons on double layer capacitance

A series of coal-based activated carbons representing a wide range of mesopore content, from 16.7 to 86.9%, were investigated as an electrode in electric double layer capacitors (EDLCs) in 1 mol l⁻¹ H₂SO₄ and 6 mol l⁻¹ KOH electrolytic solutions. The activated carbons (ACs) used in this study were produced from chemically modified lignite, subbituminous and bituminous coals by carbonization and subsequent activation with steam. The BET surface area of ACs studied ranged from 340 to 1270 m² g⁻¹. The performance of ACs as EDLC electrodes was characterized using voltammetry, galvanostatic charge/discharge and impedance spectroscopy measurements. For the carbons with surface area up to 1000 m² g⁻¹, the higher BET surface area the higher specific capacitance (F g⁻¹) for both electrolytes. The surface capacitance (μF cm⁻²) increases also with the mesopore content. The optimum range of mesopore content in terms of the use of ACs studied for EDLCs was found to be between 20 and 50%. A maximum capacitance exceeding 160 F g⁻¹ and a relatively high surface capacitance about 16 μF cm⁻² measured in H₂SO₄ solution were achieved for the AC prepared from a sulfonated subbituminous coal. This study shows that the ACs produced from coals exhibit a better performance as an electrode material of EDLC in H₂SO₄ than in KOH electrolytic solutions. For KOH, the capacitance per unit mesopore surface is slightly lower than that referred to unit micropore surface (9.1 versus 10.1 μF cm⁻²). However, in the case of H₂SO₄ the former capacitance is double and even higher compared with the latter (23.1 versus 9.8 μF cm⁻²). On the other hand, the capacitance per micropore surface area is the same in both electrolytes used, about 10.0 μF cm⁻².     

Characterisation of activated nanoporous carbon for supercapacitor electrode materials

Commercial nanoporous carbon RP-20 was activated with water vapor in the temperature range from 950 °C to 1150 °C. The XRD analysis was carried out on nanoporous carbon powder samples to investigate the structural changes (graphitisation) in modified carbon that occurred at activation temperatures T ? 1150 °C. The first-order Raman spectra showed the absorption peak at 1582 cm⁻¹ and the disorder (D) peak at 1350 cm⁻¹. The low-temperature N₂ adsorption experiments were performed at −196 °C and a specific surface area up to 2240 m²g⁻¹ for carbon activated at T = 1050 °C was measured. The cell capacitance for two electrode activated nanoporous carbon system advanced up to 60 F g⁻¹ giving the specific capacitance ∼240 F g⁻¹ to one electrode nanoporous carbon ∣1.2 M (C₂H₅)₃CH₃NBF₄ + acetonitrile solution interface. A very wide region of ideal polarisability for two electrode system (∼3.2 V) was achieved. The low frequency limiting specific capacitance very weakly increases with the rise of specific area explained by the mass transfer limitations in the nanoporous carbon electrodes. The electrochemical characteristics obtained show that some of these materials under discussion can be used for compilation of high energy density and power density non-aqueous electrolyte supercapacitors with higher power densities than aqueous supercapacitors.