High capacity supercapacitors based on modified activated carbon aerogel

Carbon aerogel derived from a modified resorcinol–formaldehyde (RF) method without supercritical drying step was activated under CO₂ flow and further modified with a surfactant sodium oleate solution. Carbon aerogel, activated carbon aerogel (ACA) and modified activated carbon aerogel (MACA) were characterized by Brunaver, Emmett and Teller(BET) surface measurement, constant-current charge–discharge and cyclic voltammetry(CV). It was found that the specific surface area of the ACA was twice that without activation. At relatively low discharge rates the specific capacitance and energy delivered from the capacitor were improved greatly by the activation of carbon aerogel. To apply the ACA at high discharge rate, a surface modification was introduced. After the surface modification the wettability of the organic electrolyte based on non-polar organic solvent (i.e. propylene carbonate) to the ACA was improved greatly and, as a result, the internal resistance of the capacitor decreased and the specific capacitance and energy delivered increased at all the test discharge rates. The effects from the modification become more marked at higher discharge rates, i.e. at 48mAcm^–2, the energy delivered increased by ca. 70%, which indicates the MACA more suitable as electrode material in electric double-layer capacitors (EDLCs) for high current applications.     

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.     

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.     

Structure and electrochemical properties of resorcinol-formaldehyde polymer-based carbon for electric double-layer capacitors

A nano-porous carbon was prepared by carbonization of a novel synthetic resorcinol-formaldehyde (RF) polymer without any additional activation process, and used as electrode materials for aqueous electric double-layer capacitors (EDLCs). This novel RF polymer-based carbon shows high specific surface area with large carbonization yield (∼50%), and excellent specific dc capacitance over 200 F/g. The effect of R/CA ratio (i.e. molar ratio of resorcinol to curing agent) on the specific surface area, pore size distribution, nanostructure and electrochemical capacitance was studied, respectively. The results showed that a higher R/CA ratio yielded carbon with higher specific surface area, larger specific capacitance, and broader pore size distribution. The highest specific surface area of 825 m²/g and specific capacitance exceeding 200 F/g were found to occur at R/CA ratio of 50. The electrochemical behaviors were characterized by means of galvanostatic charging/discharging, cycle voltammetry and impedance spectroscopy. The correlation between electrochemical properties and pore structure was investigated. Due to the excellent capacitance properties, low cost and simple process, this RF polymer-derived carbon would be a promising material for EDLCs applications.     

Redox poly[Ni(saldMp)] modified activated carbon electrode in electrochemical supercapacitors

The complex (2,2-dimethyl-1,3-propanediaminebis(salicylideneaminato))-nickel(II), [Ni(saldMp)], was oxidatively electropolymerized on activated carbon (AC) electrode in acetonitrile solution. The poly[Ni(saldMp)] presented an incomplete coated film on the surface of carbon particles of AC electrode by field emission scanning electron microscopy. The electrochemical behaviors of poly[Ni(saldMp)] modified activated carbon (PAC) electrode were evaluated in different potential ranges by cyclic voltammetry. Counterions and solvent swelling mainly occurred up to 0.6 V for PAC electrode by the comparison of D^1/2C values calculated from chronoamperometry experiments. Both the Ohmic resistance and Faraday resistance of PAC electrode gradually approached to those of AC electrode when its potential was ranging from 1.2 V to 0.0 V. Galvanostatic charge/discharge experiments indicated that both the specific capacitance and energy density were effectively improved by the reversible redox reaction of poly[Ni(saldMp)] film under the high current density up to 10 mA cm⁻² for AC electrode. The specific capacitance of PAC electrode decreased during the first 50 cycles but thereafter it remained constant for the next 200 cycles. This study showed the redox polymer may be an attractive material in supercapacitors.     

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.     

Electrochemical study of activated carbon-semiconducting oxide composites as electrode materials of double-layer capacitors

The energy storage of activated carbon modified with a semiconducting oxide TiO₂ is studied. The composite was prepared by mixing nanosize TiO₂ and activated carbon through a means of ultrasonic vibration in ethanol solution for 30 min. It was found that with modification of TiO₂, the specific capacitance of activated carbon measured at 0.65 mA/cm² was increased from 47.2 to 63.1 F g⁻¹. This method is unique in comparison the conventional method because it uses semiconducting TiO₂ other than electrochemically active materials such as RuO₂. The later has been adopted to make electrochemical-double-layer hybrid supercapacitors, however, the former is attributed to a pure double-layer supercapacitor.     

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.     

Carbon nanofibres and activated carbon nanofibres as electrodes in supercapacitors

Carbon nanofibres have been prepared by a floating catalyst procedure at industrial scale in a metallic furnace. The nanofibres (50-500 nm diameter and 5-200 μm length) are grown from the Fe particles used as catalyst. Soot appears together with the carbon nanofibres. The sample has been chemically activated using KOH as activating agent. Scanning electron microscopy has shown a smooth surface for the as-prepared carbon nanofibres but a rough surface for the activated ones. The specific surface area increases from 13 to 212 m²/g due to the activation. The volume of the micropores (in the 1-2 nm range) and the mesopores (2-5 nm range), as deduced by density functional theory methods, also increases after the activation. Electrochemical behaviour of the as-prepared and activated carbon nanofibres has been tested in a supercapacitor at laboratory scale using 6 M KOH aqueous solution as electrolyte. The specific capacitance, which is less than 1 F/g for the as-prepared sample, increase up to ≈60 F/g for the activated sample. Only a slight decrease in capacitance has been observed as the current density increases. Specific power of ≈100 W/kg at specific energy of 1 Wh/kg has been found in some particular cases. We have compared the electrochemical parameters of our activated carbon nanofibres with those of activated carbon nanofibres coming from a commercial sample; the latter was activated by the same way as our sample.     

Electrochemical performance of carbon onions, nanodiamonds, carbon black and multiwalled nanotubes in electrical double layer capacitors

This paper describes the electrochemical performance of carbon onions, nanodiamonds, carbon black and multiwalled nanotubes as electrodes in electrical double layer capacitors with organic electrolyte. Onions were formed by vacuum annealing of 5 nm nanodiamond (ND) powder at 1200-2000 °C with the goal to investigate the effect of carbon microstructure on specific capacitance and ion transport. In contrast to micro- or mesoporous activated carbons, the outer surface of carbon onions is fully accessible to electrolyte ions and the size of pores between carbon onions or nanotubes does not depend on the annealing temperature. Charge-discharge measurements revealed a two times decrease in the specific capacitance of onions and nanotubes upon graphitization and formation of polyhedral particles after annealing at 1800 °C and above. However, the capacitance became less current dependant. The carbon onion cells are able to deliver the stored energy under a high current density with a capacitance twice than the one obtained with MWCNT. Electrical measurements and impedance spectroscopy showed about two orders of magnitude increase in conductivity of electrodes and twofold decrease in the equivalent series resistance of the assembled cells after heat treatments of ND. The time constant extracted from the impedance data is around 10 times smaller for ND annealed at above 1800 °C than for activated carbons and is closely approaching the one for MWCNT. This shows that the open structure of carbon onions leads to an increased ability to quickly deliver the stored energy.     

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.     

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.     

High performance of nanoporous carbon in cryogenic hydrogen storage and electrochemical capacitance

Nanoporous carbon materials were synthesized by a two-step casting process using zeolite 13X as template. The nanoporous structures were characterized by X-ray diffraction, high resolution transmission electron microcopy and nitrogen sorption at 77 K, and the results show that pore filling in the zeolite channels could play an important role in the replication of zeolite-like structural order. Better pore filling led to a more ordered structure as well as higher surface area and pore volume. Further potassium hydroxide (KOH) activation improved the microporous texture to the carbon framework and resulted in higher surface area and pore volume. A large hydrogen uptake capacity of 6.30 wt.% has been achieved at 77 K and 20 bar. Besides, a high gravimetric capacitance of up to 160 F g⁻¹ and an energy density of 30 W h kg⁻¹ have been obtained when tested as an electrode for supercapacitors. The high performance in cryogenic hydrogen storage and electrochemical capacitance were closely correlated with the pore structures of the carbon materials.     

A comparison of the aging of electrochemical double layer capacitors with acetonitrile and propylene carbonate-based electrolytes at elevated voltages

The aging behavior of electrochemical double layer capacitors (EDLCs) based on activated carbon electrodes bound with poly(tetrafluoroethylene) (PTFE) was tested in electrolyte solutions based on acetonitrile (AN) and propylene carbonate (PC) at a constant elevated cell voltage of 3.5 V. The aging was quantified in terms of capacitance loss and resistance increase for the full cell and the individual electrodes. It is shown that the enhanced aging rate of symmetric EDLCs in either solvent at elevated voltages is dominated by the aging of a single electrode, and that the polarity of this limiting electrode depends directly on the solvent. In AN, the positive electrode ages much more rapidly than the negative, while in PC the negative electrode exhibits faster aging than the positive. After aging, the electrodes were investigated by nitrogen adsorption and X-ray photoelectron spectroscopy, revealing significant modifications of the electrode surface and providing clear evidence for the deposition of electrolyte degradation products on the electrodes.     

Effect of activation time on the properties of activated carbons prepared by microwave-assisted activation for electric double layer capacitors

Activated carbons (ACs) were prepared by microwave-assisted heat treatment of petroleum coke with KOH as activation agent, and characterized by infrared spectroscopy and nitrogen adsorption technique with the aim of studying the effect of activation time on the properties of ACs for electrodes in electric double layer capacitors (EDLCs). The electrochemical properties of AC electrodes in EDLCs were studied by cyclic voltammetry, constant current charge-discharge and electrochemical impedance spectroscopy. The results show that the specific surface area (S_BET) and total pore volume of ACs goes through a maximum as the activation time increases. At 35 min of the activation time, the as-made AC (denoted as AC-35) has a S_BET of 2312 m²/g. With AC-35 as the electrode, its specific capacitance in EDLC at a current density of 50 mA/g can reach 342.8 F/g, and remains at 245.6 F/g even after 800 cycles while the energy density of the capacitor remains at 8.0 Wh/kg. The results have demonstrated that the microwave-assisted heat treatment is an efficient approach to the preparation of ACs with high performance for EDLCs.     

活化温度对酚醛基活性炭纤维的孔结构和电化学性能的影响

从酚醛纤维出发,经过炭化和KOH活化制备了酚醛基活性炭纤维（PACF）,并对不同温度下活化样品的比表面积、孔结构以及所制备的双电层电容器（EDLC）的电化学性能之间的关系进行了探讨。氮气（77K）吸附法测定PACF活性炭纤维的孔结构和比表面积;采用循环伏安、交流阻抗、恒流充放电等测试对超级电容器的电化学性能进行了测试。结果表明：900℃是KOH活化酚醛纤维制备用于EDLC电极材料的最佳活化温度,该温度下活化样品具有最佳的循环性,稳定性和较小的内阻,比表面积为2311m^2·g^-1和比电容264．IF·g^-1（充放电电流为1000mA·g^-1）。PACF系列样品均呈现出典型的微孔炭的特征,不同活化温度下制备的PACF,虽然表现出不同的比表面积和比电容,但是其整体孔径分布范围基本相同,都在0.5nm～3．0nm之间。随活化温度的升高,样品的电容性能和功率特性越来越好,内阻也随活化温度的升高而降低。     

酚醛树脂为原料制备双电层电容器用电极材料的工艺研究

以酚醛树脂为原料，NaOH为活化剂制取双电层电容器用高比表面积活性炭电极材料，考察了炭化温度、活化温度、活化剂用量、活化时间等工艺参数对活性炭比电容的影响。实验结果表明，在炭化温度为600℃，活化温度为900℃，碱炭比为4，活化时间为1h的工艺条件下，制得的高比表面积活性炭比电容可达58．8F／g，用它组装成的电容器具有良好的充放电性能和循环性能，既能在大电流下快速充放电也能在小电流下缓慢充放电，但存在微孔所占比例较高引起的分散电容效应，这是大电流下放电容量有所下降的主要原因。     

Spatio-temporal charge behavior in electric double layer capacitors observed by pulsed electro acoustic method

The use of the pulsed electro acoustic (PEA) method allowed us to perform the direct observations of spatio-temporal charge distributions in EDLCs based on polarizable nanoporous carbonaceous electrode. The negative charge density became the maximum, about −173 C/m³ at the region where was near to anode layer in EDLCs for case of dc voltage 2.5 V, while the positively charged density became the maximum, about 70.7 C/m³ at the region where it was located around the cathode layer. The behavior of negative and positive charges measured by the PEA method reflected the hetero-charges distribution in EDLCs. The performance of the sample was found to be better in terms of the capacitance (C_s) with a maximum value of 55.4 F/g than general EDLCs which use organic electrolyte.     

High power density electrodes for Carbon supercapacitor applications

This paper presents results obtained with 4 cm² Carbon/Carbon supercapacitors cells in organic electrolyte. In the first approach, a surface treatment for Al current collector foil via the sol-gel route has been used in order to decrease the Al/active material interface resistance. Performances obtained with this original process are: a low equivalent series resistance (ESR) of 0.5 Ω cm² and a specific capacitance of 95 F g⁻¹ of activated carbon.Then, supercapacitors assembled with treated Al foil and active material containing activated carbon/carbon nanotubes (CNTs) with different compositions have been studied. Galvanostatic cycling measurements show that when CNTs content increases, both ESR and specific capacitance are decreased. Fifteen percent appears to be a good compromise between stored energy and delivered power with an ESR of 0.4 Ω cm² and a specific capacitance of 93 F g⁻¹ of carbonaceous active material.Finally, cells frequency behaviour has been characterized by Electrochemical Impedance Spectroscopy. The relaxation time constant of cells decreases when the CNTs content increases. For 15% of CNTs, the time constant is about 30% lower as compared to a cell using pure activated carbon-based electrodes leading to a higher delivered power.     

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).