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.     

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.     

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.     

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.     

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.     

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.     

Effect of mesoporosity on specific capacitance of carbons

The study compares the structural and electrochemical properties of 12 porous carbons based on phenolic resins, using both aqueous (H₂SO₄) and aprotic ((C₂H₅)₄NBF₄ in acetonitrile) electrolytes. It appears that they fit into the general pattern observed for other carbons. The present carbons have micropore volumes varying between 0.29 and 0.66 cm³ g⁻¹ and average pore widths L_o between 0.62 and 1.23 nm. Five samples are exclusively microporous, whereas seven also display a relatively important mesoporosity. This allows a direct comparison between pairs of carbons with similar micropore systems, with and without mesopores, in order to assess the role of mesoporosity in the electrochemical properties. It appears that mesopores have only a limited influence on the decrease in capacitance at high current density as opposed to earlier assumptions.     

High capacitance properties of polyaniline by electrochemical deposition on a porous carbon substrate

Electrochemical deposition of polyaniline (PANI) is carried out on a porous carbon substrate for supercapacitor studies. The effect of substrate is studied by comparing the results obtained using platinum, stainless steel and porous carbon substrates. PANI deposited at 100 mV s⁻¹ sweep rate by potentiodynamic technique on porous carbon substrate is found to possess superior capacitance properties. Experimental variables, namely, concentrations of aniline monomer and H₂SO₄ supporting electrolyte are varied and arrived at the optimum concentrations to obtain a maximum capacitance of PANI. Low concentrations of both aniline and H₂SO₄, which produce PANI at low rates, are desirable. The PANI deposits prepared under these conditions possess network morphology of nanofibrils. Capacitance values as high as 1600 F g⁻¹ are obtained and PANI coated carbon electrodes facilitate charge-discharge current densities as high as 45 mA cm⁻² (19.8 A g⁻¹). Electrodes are found to be fairly stable over a long cycle-life, although there is some capacitance loss during the initial stages of cycling.     

Morphology and organic EDLC applications of chemically activated AR-resin-based carbons

A morphological characterization of activated AR-resin was carried out. The time and temperature effect of the EDLC (electric double layer capacitor) properties of AR-resin were investigated. In order to clarify the relation between the electric double layer capacitance and the ion mobility in the organic solvent, a computer simulation was used to calculate the possible solvation size of Et₄NBF₄ in propylene carbonate. The sample (HTT 700) activated at a heat treatment temperature (HTT) of 700 °C for 2 h has a specific capacitance as high as 35.27 F g⁻¹ (equivalent to 141.08 F g⁻¹ for a single electrode), when it is charged to 2.5 V and discharged at 1 mA cm⁻². The capacitance increase by using a longer activation time at each HTT was as follows: 2.3 times at HTT 600, 1.6 times at HTT 700, 1.3 times at HTT 800 and 1.2 times at HTT 900. This variation could be forecast from image analysis using a TEM photograph. Furthermore, a possible ion size of electrolyte (Et₄NBF₄) in a solvent was calculated and it is related to the mobility of charge carriers in a micropore.     

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.     

Supercapacitive properties of a nanowire-structured MnO2 electrode in the gel electrolyte containing silica

Nanowire-structured MnO₂ active materials were prepared by a chemical precipitation method and their supercapacitive properties for use in the electrodes of supercapacitors were investigated by means of cyclic voltammetry in an aqueous gel electrolytes consisting of 1 M Na₂SO₄ and fumed silica (SiO₂). The MnO₂ electrode showed a maximum specific capacitance of 151 F g⁻¹ after 1000 cycles at 100 mV s⁻¹ when using the gel electrolyte containing 3 wt.% of SiO₂, which is higher than 121 F g⁻¹ obtained when using the 1 M Na₂SO₄ liquid electrolyte alone.     

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.     

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.     

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.     

Performance evaluation of CNT/polypyrrole/MnO2 composite electrodes for electrochemical capacitors

A ternary composite of CNT/polypyrrole/hydrous MnO₂ is prepared by in situ chemical method and its electrochemical performance is evaluated by using cyclic voltammetry (CV), impedance measurement and constant-current charge/discharge cycling techniques. For comparative purpose, binary composites such as CNT/hydrous MnO₂ and polypyrrole/hydrous MnO₂ are prepared and also investigated for their physical and electrochemical performances. The specific capacitance (SC) values of the ternary composite, CNT/hydrous MnO₂ and polypyrrole/hydrous MnO₂ binary composites estimated by CV technique in 1.0 M Na₂SO₄ electrolyte are 281, 150 and 35 F g⁻¹ at 20 mV s⁻¹ and 209, 75 and 7 F g⁻¹ at 200 mV s⁻¹, respectively. The electrochemical stability of ternary composite electrode is investigated by switching the electrode back and forth for 10,000 times between 0.1 and 0.9 V versus Ag/AgCl at 100 mV s⁻¹. The electrode exhibits good cycling stability, retaining up to 88% of its initial charge at 10,000th cycle. A full cell assembled with the ternary composite electrodes shows a SC value of 149 F g⁻¹ at a current loading of 1.0 mA cm⁻² during initial cycling, which decreased drastically to a value of 35 F g⁻¹ at 2000th cycle. Analytical techniques such as scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), Brunauer-Emmet-Teller (BET) surface area measurement and inductively coupled plasma-atomic emission spectrometry (ICP-AES) are also used to characterize the composite materials.     

Nanosized α-LiFeO2 as electrochemical supercapacitor electrode in neutral sulfate electrolytes

In this work we have explored the electrochemical properties of two lithiated iron oxide powders for supercapacitor purposes. These samples mainly consisted of α-LiFeO₂ in nanosized or micrometric form. Electrolyte was an aqueous 0.5 M Li₂SO₄ solution and voltage range studied was between 0 and −0.7 V vs. a Ag/AgCl reference electrode. As expected, electrochemical performance was dependent on the particle size. When electrolyte was deaerated a stable capacitance of ≈50 F g⁻¹ is provided by the nanosized sample for several hundred cycles. Other sulfate based salts (Na₂SO₄, K₂SO₄, Cs₂SO₄) were investigated as electrolytes but only Li₂SO₄ leads to a stable capacitance upon cycling, probably due to lithium intercalation. An hybrid cell consisting of this sample and MnO₂ as negative and positive electrodes, respectively, delivered 0.3 F cm⁻² (10 F g⁻¹). Although these values are lower than reported for other aqueous hybrid cell, α-LiFeO₂/MnO₂ asymmetric capacitor is interesting from both, an economic and an environmental point of view.     

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.     

Preparation of an electrochemically modified graphite electrode and its electrochemical performance for pseudo-capacitors in a sulfuric acid electrolyte

A graphite electrode was activated using recurrent electrochemical galvanic pulses. After activation, the original smooth surface was changed into an obviously porous, rough surface with microcrystalline flake structures with a larger number of O-containing functional groups. The material had a high capacitance and a satisfactory high-rate performance. The superior volumetric capacitance in active layer was ca. 428 F cm⁻³. Eighty-eight percentage of the capacitance remained after the discharge current increased from 2 mA (1.71 F cm⁻²) to 100 mA (1.50 F cm⁻²) in 2.3 M H₂SO₄.     

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.