Metal-organic framework (MOF) as a template for syntheses of nanoporous carbons as electrode materials for supercapacitor

Five nanoporous carbons (NPCs) were prepared by polymerizing and then carbonizing carbon precursor of furfuryl alcohol accommodated in a porous metal-organic framework (MOF-5, [Zn₄O(bdc)₃], bdc = 1,4-benzenedicarboxylate) template. The Brunauer-Emmett-Teller (BET) surface areas for five NPC samples obtained by carbonizing at the temperatures from 530 to 1000 °C fall into the range from 1140 to 3040 m² g⁻¹ and the dependence of BET surface areas on carbonization temperatures shows a “V” shape. All the five NPC samples have a pore size distribution centered at about 3.9 nm. As electrode materials for supercapacitor, the NPC samples obtained at the temperatures higher than 600 °C display the ideal capacitor behaviors and give rise to almost constant specific capacitance (above 100 F g⁻¹ at 5 mV s⁻¹) at various sweep rates, which is associated with their mesoporous characteristics. However, the NPC sample with the highest BET surface area (3040 m² g⁻¹) obtained by carbonizing at 530 °C gives a unusually low capacitance (12 F g⁻¹ at 5 mV s⁻¹), which may be attributed to the poor conductivity of the carbon material due to the low carbonization temperature.     

Synthesis and characterisation of nanoporous carbide-derived carbon by chlorination of vanadium carbide

Nanoporous carbide-derived carbon (CDC) was synthesised from vanadium carbide (VC) powder via gas phase chlorination in the temperature range from 500 to 1100 °C. The XRD analysis of nanoporous carbon powder samples was carried out to investigate the structural changes (graphitisation) of nanoporous carbons synthesised. The first-order Raman spectra showed the absorption peak at ∼1582 cm⁻¹ and the disorder-induced (D) peak at ∼1345 cm⁻¹. The low-temperature N₂ adsorption experiments were performed and a specific surface area up to 1305 m² g⁻¹ and total pore volume up to 0.66 cm³ g⁻¹ were obtained.     

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.     

Electrical double-layer characteristics of novel carbide-derived carbon materials

A variety of nanoporous carbide-derived carbon materials possessing improved pore size distributions were synthesised from a mixture of titanium carbide and titanium dioxide. It was observed that TiO₂ caused partial oxidation of the carbon particles created during high-temperature chlorination of the TiC/TiO₂ mixture. The resulting carbon powder is characterised by narrow pore size distribution with a peak pore size of around 8 Å and a noticeably smaller amount of pores below 6-7 Å compared to the carbon derived from pure TiC. Electrochemical and electrical double-layer characteristics of novel carbon materials in the acetonitrile solution of triethylmethylammonium tetrafluoroborate were obtained by using cyclic voltammetry and constant current methods. Carbon electrode materials of this study were tested over the temperature range from −10 °C to +60 °C. Results of this study affirmed a great potential of the synthesised advanced carbide-derived carbon, whose specific double-layer capacitance reaches approximately 90 F cm⁻³ and 125 F g⁻¹.     

Cycling stability of a hybrid activated carbon//poly(3-methylthiophene) supercapacitor with N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid as electrolyte

A long cycle-life, high-voltage supercapacitor featuring an activated carbon//poly(3-methylthiophene) hybrid configuration with N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid, a solvent-free green electrolyte, was developed. The cyclability of a laboratory scale cell with electrode mass loading sized for practical uses was tested at 60 °C over 16,000 galvanostatic charge-discharge cycles at 10 mA cm⁻² in the 1.5 and 3.6 V voltage range. The reported average and maximum specific energy and power, specific capacitance and capacity, equivalent series resistance and coulombic efficiency over cycling demonstrate the long-term viability of this ionic liquid as green electrolyte for high-voltage hybrid supercapacitors.     

Overcoming the barrier to graphitization in a polymer-derived nanoporous carbon

A new pathway to synthesize a carbon with both nanoporosity and pre-graphitic structures has been discovered by annealing at 2000 °C a CO₂ activated, non-graphitizing, nanoporous carbon originally derived from polyfurfuryl alcohol. The activation process with CO₂ overcomes the barrier to graphitization normally present in this carbon even when treated at high temperature. Gas adsorption analysis, skeletal density measurements, X-ray diffraction, and transmission electron microscopy are utilized to probe the structure of both the non-activated and the activated carbons at 800, 1200, 1800, and 2000 °C. The influence of activation time is also examined. Prior to activation the nanopore walls are comprised of several layers of disordered graphenes. Activation eliminates the barrier to graphitization by reducing the number of layers below the limit of detection and by removing carbon material highly susceptible to oxidation. Annealing at 2000 °C of the carbon activated to 84% burnoff induces the formation of pre-graphitic domains amongst the nanoporous carbon. The (0 0 2) bands corresponding to 2θ = 24.3°, 26°, and 26.5° are identified and assigned to amorphous, turbostratic, and graphitic morphologies. A pore volume of 0.50 cm³ g⁻¹ localized in pores below 2 nm in size is preserved after annealing.     

Nanoscale fine-tuning of porosity of carbide-derived carbon prepared from molybdenum carbide

Micro- and mesoporous carbide-derived carbon (CDC) was synthesised from molybdenum carbide (Mo₂C) powder by gas phase chlorination in the temperature range from 400 to 1200 °C. Analysis of XRD results show that C(Mo₂C), chlorinated at 1200 °C, consist mainly on graphitic crystallites of mean size, L_a = 9 nm and L_c = 7.5 nm. The first-order Raman spectra showed the graphite-like absorption peak at ∼1587 cm⁻¹ and the disorder-induced (D) peak at ∼1348 cm⁻¹. The low-temperature N₂ adsorption experiments were performed and a specific surface area up to 1855 m² g⁻¹ and total pore volume up to 1.399 cm³ g⁻¹ were obtained. Sorption measurements showed the presence of both micro- and mesopores after chlorination at 400-900 °C and only mesopores after chlorination at 1000°-1200 °C. Stepwise formation of micro- and mesopores was achieved and the peak pore size can be shifted from 0.8 nm up to 4 nm by increasing the chlorination temperature.     

Relationship between the nanoporous texture of activated carbons and their capacitance properties in different electrolytes

A series of activated carbons (ACs) with progressively changing nanotextural characteristics was obtained by heat-treatment of a bituminous coal at temperatures ranging from 520 to 1000 °C, and subsequent activation by KOH at 700 °C or 800 °C. As the pre-treatment temperature increases, the total pore volume V_T decreases from 1.28 to 0.30 cm³ g⁻¹, and the BET specific surface area from 3000 to 800 m² g⁻¹. The specific capacitance determined for each series of ACs using symmetric two electrode cells in 6 mol L⁻¹ KOH varies almost linearly with the BET surface area, suggesting that the charge accumulation is controlled primarily by the surface area development. A further analysis of the electrochemical behaviour in different electrolytic media—aqueous and organic—shows that an adequate pore size is more important than a high surface area in order to obtain high values of capacitance. Theoretical values of volumetric capacitance could be evaluated without considering the size of ions, which is always uncertain in solution, and compared with the experimental data as a function of the pore width. The efficiency of pore filling, i.e., of double layer formation, is optimal when the pore size is around 0.7 nm in aqueous media and 0.8 nm in organic electrolyte. A study of the performance of the positive and negative electrodes during the charge/discharge of the capacitor, reveals an additional pseudo-faradaic contribution due to oxygenated functionalities within the working potential window of the negative electrode. This effect is more pronounced for the ACs series obtained at 700 °C, because of their higher oxygen content.     

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.     

Electrode properties of a double layer capacitor of nano-structured graphite produced by ball milling under a hydrogen atmosphere

Nano-structural graphite prepared by ball milling under H₂ or Ar atmosphere was studied as an electrode for electric double layer capacitors (EDLCs) by means of a conventional 2-electrode galvanostatic method. Especially, the product prepared under H₂ atmosphere using zirconia balls revealed 500 m² g⁻¹ surface area and showed 12 F g⁻¹ specific capacitance, which was comparable to that of an activated carbon with large specific surface area of 3000 m² g⁻¹ examined as a reference. A proper condition of the milling time is rather a shorter time than ∼8 h, where the graphitic feature is remained in the ball milled product. On the other hand, for the sample prepared by using steel balls, the specific capacitance per surface area was several hundreds times smaller than the others, indicating that the small amount of Fe contamination during milling played a negative role for the EDLC properties.     

Polypyrrole/carbon composite electrode for high-power electrochemical capacitors

Nano-thin polypyrrole (PPy) layers with thickness from ∼5 nm to several 10s nm were deposited on vapor grown carbon fibers (VGCF) by an in situ chemical polymerization. Using different concentrations of the pyrrole could control the thicknesses of deposited PPy layers. Surface morphology and thickness of the deposited PPy layers were confirmed by means of scanning electron microscopy and scanning transmission emission microscopy. Pseudo-capacitive behavior of the deposited PPy layers on VGCF investigated by means of cyclic voltammetry. Then, the PPy/VGCF composites were mixed with activated carbons (AC) at various mixing ratios. For the PPy/VGCF/AC composite electrodes, characteristics of specific capacitance and power capability were examined by half-cell tests. As results of this study, it was investigated that nano-thin PPy layer below ∼10 nm deposited on VGCF had high pseudo-capacitance and fast reversibility. Its specific capacitance per averaged weight of active material (PPy) was obtained as ∼588 F g⁻¹ at 30 mV s⁻¹ and maintained as ∼550 F g⁻¹ at 200 mV s⁻¹ of scan rate. Also, from the mixing 60 wt.% of the PPy/VGCF with 25 wt.% of AC, the PPy/VGCF/AC composite electrode exhibited higher power capability maintaining the specific capacitance per active materials of PPy and AC as ∼300 F g⁻¹ at 200 mV s⁻¹ in 6 M KOH.     

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.     

Titanium carbide derived nanoporous carbon for energy-related applications

High surface area nanoporous carbon has been prepared by thermo-chemical etching of titanium carbide TiC in chlorine in the temperature range 200-1200 °C. Structural analysis showed that this carbide-derived carbon (CDC) was highly disordered at all synthesis temperatures. Higher temperature resulted in increasing ordering and formation of bent graphene sheets or thin graphitic ribbons. Soft X-ray absorption near-edge structure spectroscopy demonstrated that CDC consisted mostly of sp² bonded carbon. Small-angle X-ray scattering and argon sorption measurements showed that the uniform carbon-carbon distance in cubic TiC resulted in the formation of small pores with a narrow size distribution at low synthesis temperatures; synthesis temperatures above 800 °C resulted in larger pores. CDC produced at 600-800 °C show great potential for energy-related applications. Hydrogen sorption experiments at −195.8 °C and atmospheric pressure showed a maximum gravimetric capacity of ∼330 cm³/g (3.0 wt.%). Methane sorption at 25 °C demonstrated a maximum capacity above 46 cm³/g (45 vol/vol or 3.1 wt.%) at atmospheric pressure. When tested as electrodes for supercapacitors with an organic electrolyte, the hydrogen-treated CDC showed specific capacitance up to 130 F/g with no degradation after 10 000 cycles.     

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.     

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.     

Preparation and electrochemical performance of polyaniline-based carbon nanotubes as electrode material for supercapacitor

Nitrogen-containing carbon nanotubes (CNTs) with open end and low specific surface area were prepared via the carbonization of polyaniline (PANI) nanotubes synthesized by a rapidly mixed reaction. On the basis of analyzing the morphologies and structures of the original and carbonized PANI nanotubes, the electrochemical properties of PANI-based CNTs obtained at different temperatures as electrode materials for supercapacitors using 30 wt.% aqueous solution of KOH as electrolyte were investigated by galvanostatic charge/discharge and cyclic voltammetry. It was found that the carbonized PANI nanotubes at 700 °C exhibit high specific capacitance of 163 F g⁻¹ at a current density of 0.1 A g⁻¹ and excellent rate capability in KOH solution. Using X-ray photoelectron spectroscopy measurement the nitrogen state and content in PANI-CNTs were analysed, which could play important roles for the enhancement of electrochemical performance. When the appropriate content of nitrogen is present, the presence of pyrrole or pyridone and quaternary nitrogen is beneficial for the improvement of electron mobility and the wettability of electrode.     

Facile solvothermal synthesis of a graphene nanosheet-bismuth oxide composite and its electrochemical characteristics

This work demonstrates a novel and facile route for preparing graphene-based composites comprising of metal oxide nanoparticles and graphene. A graphene nanosheet-bismuth oxide composite as electrode materials of supercapacitors was firstly synthesized by thermally treating the graphene-bismuth composite, which was obtained through simultaneous solvothermal reduction of the colloidal dispersions of negatively charged graphene oxide sheets in N,N-dimethyl formamide (DMF) solution of bismuth cations at 180 °C. The morphology, composition, and microstructure of the composites together with pure graphite oxide, and graphene were characterized using powder X-ray diffraction (XRD), FT-IR, field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), thermogravimetry and differential thermogravimetry (TG-DTG). The electrochemical behaviors were measured by cyclic voltammogram (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS). The specific capacitance of 255 F g⁻¹ (based on composite) is obtained at a specific current of 1 A g⁻¹ as compared with 71 F g⁻¹ for pure graphene. The loaded-bismuth oxide achieves a specific capacitance as high as 757 F g⁻¹ even at 10 A g⁻¹. In addition, the graphene nanosheet-bismuth oxide composite electrode exhibits the excellent rate capability and well reversibility.     

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).     

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.     

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.