EDLC performance of carbide-derived carbons in aprotic and acidic electrolytes

This study shows that carbide-derived carbons (CDCs) with average pore size distributions around 0.9-1 nm and effective surface areas of 1300-1400 m² g⁻¹ provide electrochemical double-layer capacitors with high performances in both aqueous (2M H₂SO₄) and aprotic (1M (C₂H₅)₄NBF₄ in acetonitrile) electrolytes.In the acidic electrolytic solution, the gravimetric capacitance at low current density (1 mA cm⁻²) can exceed 200 F g⁻¹, whereas the volumetric capacitance reaches 90 F cm⁻³. In the aprotic electrolyte they reach 150 F g⁻¹ and 60 F cm⁻³.A detailed comparison of the capacitive behaviour of CDCs at high current density (up to 100 mA cm⁻²) with other microporous and mesoporous carbons indicates better rate capabilities for the present materials in both electrolytes. This is due to the high surface area, the accessible porosity and the relatively low oxygen content.It also appears that the surface-related capacitances of the present CDCs in the aprotic electrolyte are in line with other carbons and show no anomalous behaviour.     

Capacitive performance of mesoporous carbons derived from the citrates in ionic liquid

Mesoporous carbons with different pore structures are prepared via a simple pyrolysis process using citrate salts as precursors. BaC-T carbons derived from barium citrate possess large pore volume and typical multimodal pore size distribution (PSD), while MgC-T carbons derived from magnesium citrate possess high specific surface areas due to their dominant small mesopores and micropores. The capacitive performance of the prepared carbons is investigated in ionic liquid and high specific capacitances (maximum of 180.3 F g⁻¹ for MgC-700) are achieved. Experimental data demonstrate that porosity and surface chemistry corporately determine the capacitive performance. BaC-700 and BaC-800 present good rate performance, and exhibit energy densities of about 50 Wh kg⁻¹. The good capacitive performance of these carbons is attributed to their bimodal PSD with large pore size and appropriate surface properties. The MgC-800 and BaC-800 carbons present good durability of capacitance during three thousand of charge/discharge cycles.     

Investigation of polymer electrolyte hybrid supercapacitor based on manganese oxide-carbon electrodes

A hybrid supercapacitor based on manganese oxide, activated carbon and polymer electrolyte was developed and electrochemically investigated. The capacitive performance obtained from the polymer electrolyte based supercapacitor was similar to that of an aqueous electrolyte based supercapacitor, tested for comparison in the same operative conditions. A durability test carried out for 2500 cycles showed stable and slowly increasing performance. The specific capacitance of hybrid supercapacitor was 48 F g⁻¹ (192 F g⁻¹ as a mean one electrode capacitance), in which that of the positive electrode was 384 F g⁻¹ of MnO₂ and that of negative electrode 117 F g⁻¹ of carbon. The impedance analysis evidenced that although the polymer electrolyte based hybrid supercapacitor showed higher resistance compared to that of the liquid electrolyte based supercapacitor, this drawback was counterbalanced by better ion transport features, which were evident at lower frequencies, where similar values of capacitances were obtained from the different 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.     

The effect of Mo2C derived carbon pore size on the electrical double-layer characteristics in propylene carbonate-based electrolyte

Carbide-derived carbon (CDC) was synthesised from molybdenum carbide by extracting Mo atoms in a high-temperature chlorine atmosphere. A systematic study of the influence of pore size on the electrical double layer (EDL) performance was carried out with carbons synthesised in the temperature interval of 500-900 °C. Strong effect of chlorination conditions on the pore-size distribution was noticed that gives wide possibilities to vary the pore structure of Mo₂C derived carbons. An average pore size of carbons varied between 1 nm and 2 nm depending on chlorination temperature. The relationships were established between the pore-size distribution and the electrochemical performance of micro/mesoporous carbons. The EDL characteristics of carbon materials in a propylene carbonate solution of triethylmethylammonium tetrafluoroborate were obtained using the cyclic voltammetry at ΔE of 3.8 V and the constant current methods in a 3-electrode test cell. A novel test method was developed to demonstrate the power characteristics of the electrode materials. The results of this study affirmed the great potential of Mo₂C derived carbons, whose EDL capacitance reaches ∼65 F cm⁻³ and 132 F g⁻¹ and the 20-s discharge power density is 2.1 W cm⁻³.     

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.     

Performance of templated mesoporous carbons in supercapacitors

By analogy with other types of carbons, templated mesoporous carbons (TMCs) can be used as supercapacitors. Their contribution arises essentially from the double layer capacity formed on their surface, which corresponds to 0.14 F m⁻² in aqueous electrolytes such as H₂SO₄ and KOH and 0.06 F m⁻² for the aprotic medium (C₂H₅)₄NBF₄ in CH₃CN. In the case of a series of 27 TMCs, it appears that the effective surface area determined by independent techniques can be as high as 1500-1600 m² g⁻¹, and therefore exceeds the value obtained for many activated carbons (typically 900-1300 m² g⁻¹). On the other hand, the relatively low amount of surface oxygen in the present TMCs, as opposed to activated carbons, reduces the contribution of pseudo-capacitance effects and limits the gravimetric capacitance to 200-220 F g⁻¹ for aqueous electrolytes. In the case of non-aqueous electrolyte, it rarely exceeds 100 F g⁻¹.It is also shown that the average mesopore diameter of these TMCs does not improve significantly the ionic mobility compared with typical activated carbons of pore-widths above 1.0-1.3 nm.This study suggests that activated carbons remain the more promising candidates for supercapacitors with high performances.     

Optimisation of supercapacitors using carbons with controlled nanotexture and nitrogen content

In order to optimize the performance of supercapacitors, the capacitance of the carbon materials used as electrodes was strictly related to their pores size and also to their redox properties. Well-sized carbons have been elaborated through a template technique using mesoporous silica. For a series of template carbons, a perfect linear dependence has been found for the capacitance values versus the micropore volume determined by CO₂ adsorption. The redox properties of carbons were enhanced by substituting nitrogen for carbon up to ca. 7 wt.%. For carbons with similar nanotextural characteristics, the electrochemical measurements showed a proportional increase of the specific capacitance with the nitrogen content in acidic electrolyte. For an activated carbon from polyacrylonitrile with a specific surface area of only 800 m² g⁻¹, but with a nitrogen content of 7 wt.%, the capacitance reaches 160 F g⁻¹, with very little fading during cycling.     

Physical and electrochemical characterization of CuO-doped activated carbon in ionic liquid

A series of CuO-doped activated carbons (CDACs) were prepared by chemical deposition. The electrochemical behavior of CDACs was investigated in electrochemical capacitors based on ionic liquid 1-ethyl-3-methylimidazolium thiocyanate ([EMIm]SCN) as electrolyte. The results indicated that a diffusion-controlling, reversible redox reaction of CuO particles happened in ionic liquid and porous carbon. When the amount of CuO-doped activated carbon with a specific surface area of 2460 m² g⁻¹ reached 2%, the single electrode average specific capacitance can reach the maximal value of 210 F g⁻¹, about 20% higher than the one used pure activated carbon as electrode material.     

Improved capacitance characteristics during electrochemical charging of carbon nanotubes modified with polyoxometallate monolayers

By modification of surfaces of multi-walled carbon nanotubes with ultra-thin monolayer-type films of phosphododecamolybdic acid, H₃PMo₁₂O₄₀, an electrode material with improved capacitance properties is produced. It is apparent from three distinct test experiments (based on cyclic voltammetry, galavanostatic charging-discharging and AC impedance) that capacitors utilizing H₃PMo₁₂O₄₀-modified carbon nanotubes are characterized by specific capacitances and energy densities on the levels of 40 F g⁻¹ and 1.3 Wh kg⁻¹, whereas the respective values for the systems built from bare carbon nanotubes are lower, 22 F g⁻¹ and 0.7 Wh kg⁻¹. It is reasonable to expect that fast and reversible multi-electron transfers of the Keggin-type H₃PMo₁₂O₄₀ account for the pseudocapacitance effect and significantly contribute to the observed overall capacitance.     

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.     

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.     

Fast and reversible surface redox reaction of graphene-MnO2 composites as supercapacitor electrodes

We present a quick and easy method to synthesize graphene-MnO₂ composites through the self-limiting deposition of nanoscale MnO₂ on the surface of graphene under microwave irradiation. These nanostructured graphene-MnO₂ hybrid materials are used for investigation of electrochemical behaviors. Graphene-MnO₂ composite (78 wt.% MnO₂) displays the specific capacitance as high as 310 F g⁻¹ at 2 mV s⁻¹ (even 228 F g⁻¹ at 500 mV s⁻¹), which is almost three times higher than that of pure graphene (104 F g⁻¹) and birnessite-type MnO₂ (103 F g⁻¹). Interestingly, the capacitance retention ratio is highly kept over a wide range of scan rates (88% at 100 mV s⁻¹ and 74% at 500 mV s⁻¹). The improved high-rate electrochemical performance may be attributed to the increased electrode conductivity in the presence of graphene network, the increased effective interfacial area between MnO₂ and the electrolyte, as well as the contact area between MnO₂ and graphene.     

Electric double layer capacitors with gelled polymer electrolytes based on poly(ethylene oxide) cured with poly(propylene oxide) diamines

Using a gel electrolyte for electric double layer capacitors usually encountered a drawback of poor contact between the electrolyte and the electrode surface. A gel electrolyte consisting of poly(ethylene oxide) crosslinked with poly(propylene oxide) as a host, propylene carbonate (PC) as a plasticizer, and LiClO₄ as a electrolytic salt was synthesized for double layer capacitors. Diglycidyl ether of bisphenol-A was blended with the polymer precursors to enhance the mechanical properties and increase the internal free volume. This gel electrolyte showed an ionic conductivity as high as 2 × 10⁻³ S cm⁻¹ at 25 °C and was electrochemically stable over a wide potential range (ca. 5 V). By sandwiching this gel-electrolyte film with two activated carbon cloth electrodes (1100 m² g⁻¹ in surface area), we obtained a capacitor with a specific capacitance of 86 F g⁻¹ discharged at 0.5 mA cm⁻², while the capacitance was 82 F g⁻¹ for a capacitor equipped with a liquid electrolyte of 1 M LiClO₄/PC. The capacitance decrease with the current density was less significant for the gel-electrolyte capacitor. We found that the less restricted ion diffusion near the electrolyte/electrode interface led to the smaller overall resistance of the gel-electrolyte capacitor. The high performance of the gel-electrolyte capacitor has demonstrated that the developed polymer network not only facilitated ion motion in the electrolyte bulk phase but also gave an intimate contact with the carbon surface. The side chains of the polymer in the amorphous phase could stretch across the boundary layer at the electrolyte/electrode interface to come into contact with the carbon surface, thus improving transport of Li⁺ ions by the segmental mobility in polymer.     

Varying carbon structures templated from KIT-6 for optimum electrochemical capacitance

Bicontinuous ordered mesoporous carbons (OMCs), fabricated from a KIT-6 template using aluminosilicate as catalyst and furfuryl alcohol as carbon source, were successfully prepared and studied as electrodes in supercapacitors. Their structures were characterized by transmission electron microscopy (TEM), small-angle X-ray diffraction (SAXD) and N₂ cryosorption methods. Using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), the capacitive performance of the OMCs was found to be strongly dependent on the mesostructure. Specific capacitance value greater than 130 F g⁻¹ at 20 mV s⁻¹ were obtained from an OMC that featured high surface area with the existence of additional large pores to enhance the specific capacitance at high discharge rate. For the OMC with the best performance, we found that a power density as high as 4.5 kW kg⁻¹ at an energy density of 6.1 Wh kg⁻¹ can be delivered when the discharge current density is 20 A g⁻¹ and can also be continuously charged and discharged with little variation in capacitance after 2500 cycles. These results indicate that this OMC with optimized structure has potential to be used as a power component in electric vehicles.     

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).     

The role of textural characteristics and oxygen-containing surface groups in the supercapacitor performances of activated carbons

It is suggested that the specific capacitance C₀ of activated carbons at low current densities (d ∼ 1 mA cm⁻²) consists, to a good first approximation, of two contributions. For the H₂SO₄ electrolyte they correspond to approximately 0.080 F m⁻² from the total accessible surface area and an additional pseudo-capacitance of 63 F mmol⁻¹ from the surface species generating CO in thermally programmed desorption (TPD). The new correlation proposed here is an alternative to Shi's earlier approach which considered contributions from the microporous and the external surface areas. Furthermore, it appears that the variation of the specific capacitance C at high current densities d (up to 100-150 mA cm⁻²) depends essentially on the CO₂-generating surface groups and on 1/L₀, the inverse of the average micropore width.     

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.     

Multiwall carbon nanotube supported poly(3,4-ethylenedioxythiophene)/manganese oxide nano-composite electrode for super-capacitors

MWCNT-PSS/PEDOT/MnO₂ nano-composite electrodes were fabricated by generating pseudo-capacitive poly(3,4-ethylenedioxythiophene) (PEDOT)/MnO₂ nano-structures on poly(styrene sulfonate) (PSS) dispersed multiwalled carbon nanotubes (MWCNTs). PSS dispersed MWCNTs (MWCNT-PSS) facilitated the growth of PEDOT and MnO₂ into nano-rods with large active surface area and good electrical conductivity. The ternary MWCNT-PSS/PEDOT/MnO₂ nano-composite electrode was studied for the application in super-capacitors, and exhibited excellent capacitive behavior between −0.2 V and 0.8 V (vs. saturated Ag/AgCl electrode) with high reversibility. Specific capacitance of the nano-composite electrode was found as high as 375 F g⁻¹. In contrast, specific capacitance of MWCNT-PSS/MnO₂ and MWCNT-PSS nano-composite electrodes is 175 F g⁻¹ and 15 F g⁻¹, respectively. Based on cyclic voltammetric studies and cycle-life tests, the MWCNT-PSS/PEDOT/MnO₂ nano-composite electrode gave a highly stable and reversible performance up to 2000 cycles. Our studies demonstrate that the synergistic combination of MWCNT-PSS, PEDOT and MnO₂ has advantages over the sum of the individual components.     

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.