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.     

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.     

Preparation of ordered mesoporous nickel oxide film electrodes via lyotropic liquid crystal templated electrodeposition route

A novel electrochemical route to fabricate ordered mesoporous metal oxide film electrodes has been investigated with particular reference to nickel oxide. Ordered mesoporous nickel oxide films are successfully synthesized by templated electrodeposition of H_I-e nickel hydroxide and followed by heat-treatment in air at various temperatures. The films are characterized physically by thermogravimetry (TG), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The applicability of this film as inexpensive and high-performance supercapacitor electrode material is demonstrated by the electrochemical characterization using cyclic voltammetry (CV) and chronopotentiometry technique. The specific capacitance of the nickel oxide film depends on the annealing temperature, showing a maximum value of 590 F g⁻¹ when the as-deposited film is heat-treated at 250 °C for 1.5 h.     

PtCo supported on ordered mesoporous carbon as an electrode catalyst for methanol oxidation

A catalyst for methanol oxidation, PtCo supported on graphitized mesoporous carbon, has been synthesized and its electrochemical activity for methanol oxidation has been investigated. The graphitized mesoporous carbon support with ordered pore structure and high surface area of 585 m² g⁻¹ was prepared by one-step melt casting method using Al doped hexagonal mesoporous silica as hard templates and mineral pitches as carbon precursors followed by carbonization at 800 °C. The materials were characterized by X-ray diffraction, Raman spectra, field emission scanning electron microscopy, transmission electron microscopy and nitrogen sorption techniques. Cyclic voltammetry and amperometric i-t tests were adopted to characterize the electro-catalytic activities of the materials for methanol oxidation. The results show that the graphitized mesoporous carbon exhibits large electrochemical capacitance and good electric property. After supported with 20 wt%Pt or 20 wt%PtCo nanoparticles, the resultant mesostructured composites show 26-97% higher electrochemical catalytic activity for methanol oxidation than commercial catalyst 20 wt%Pt/C in mass activity (mA mg Pt⁻¹).     

Simple synthesis of magnetic mesoporous FeNi/carbon composites with a large capacity for the immobilization of biomolecules

An easy co-impregnation method has been developed to synthesize magnetic mesoporous FeNi alloy/carbon composites with an ordered hexagonal structure. Furfuryl alcohol and metal nitrates were used as the carbon and magnetic particle precursors and were impregnated into the silica template in one step using a simple ethanol solution of furfuryl alcohol and metal nitrates. Compared to the co-casting route with two-step impregnation, the co-impregnation makes the synthesis simpler and eliminates the difficult second impregnation step. The mesoporous FeNi/carbon composites obtained have large surface areas, large pore volumes and a bimodal pore size distribution. The FeNi alloy nanoparticles were well dispersed in the mesoporous carbon walls. The composites exhibit excellent superparamagnetic behavior and the saturation magnetization strength can be adjusted from 3.4 to 5.7 emu/g by increasing the content of alloy. Such bimodal mesoporous composites show a large immobilization capacity for the biomolecules of cytochrome c (up to 732 mg/g) and lysozyme (up to 790 mg/g).     

Nitrogen-containing microporous carbons prepared from anionic surfactant-melamine/formaldehyde composites

Nitrogen-containing microporous carbons have been synthesized by the carbonization of anionic surfactant-melamine/formaldehyde (MF) composites, which were themselves formed by an electrostatic organic-organic interaction. The carbons prepared from sodium dioctyl sulfosuccinate-MF and sodium dodecyl sulfate-MF mixtures have high surface area of 464 and 539 m² g⁻¹, respectively. The N/C molar ratios of the carbons are 0.11. The resultant carbons showed capacitances higher than 200 F g⁻¹ in an acidic solution of 1 M H₂SO₄ at a scan rate of 1 mV s⁻¹.     

Synthesis of monolithic 3D ordered macroporous carbon/nano-silicon composites by diiodosilane decomposition

Monolithic carbon/nano-silicon composites with a three-dimensionally ordered macroporous (3DOM) structure were synthesized via a nanocasting route using a macro- and mesoporous silica monolith as a hard template for carbon with similar hierarchical porosity, followed by chemical vapor deposition to infiltrate mesopores with silicon nanoparticles. Diiodosilane was used as the silicon precursor to produce nano-silicon upon thermal decomposition. X-ray photoelectron spectroscopy revealed the presence of both elemental silicon and oxidized silicon in the porous carbon. Silicon was dispersed uniformly inside 3DOM carbon without forming large agglomerates. The as-synthesized material was X-ray amorphous. A lithiation experiment showed an initial charge capacity of 920 mAh g⁻¹ and a reversible Li⁺ capacity of 332 mAh g⁻¹ for the carbon/nano-silicon composite. The lower-than-expected capacity is attributed to partial oxidation of nanosized silicon in the composite structure. A mechanism for decomposition of diiodosilane in this system is proposed.     

Exploring the use of near infrared reflectance spectroscopy to predict minerals in straw

The use of near infrared reflectance spectroscopy (NIRS) to predict minerals concentration (K, Na, Ca, Mg, Fe) in straw samples was investigated in this study. A total of 222 straw samples were collected in rural area of most provinces in China. Two types of straw samples were prepared, directly cut specimens and oven-dried, milled specimens. The spectra of two kinds of samples were employed to correlate with minerals concentration. Different spectral pre-treatments and regression methods were trialled to optimize the calibration. Coefficient of determination in prediction and standard error of prediction (SEP) were 0.69, 0.54, 0.73, 0.79, 0.41 and 3.77 mg g⁻¹, 0.69 mg g⁻¹, 0.58 mg g⁻¹, 0.31 mg g⁻¹, 0.11 mg g⁻¹ for directly cut straw; 0.85, 0.70, 0.82, 0.85, 0.63 and 2.35 mg g⁻¹, 1.46 mg g⁻¹, 0.47 mg g⁻¹, 0.27 mg g⁻¹, 0.13 mg g⁻¹ for dried milled samples, respectively.     

Adsorption of Hg(II) ions from aqueous chloride solutions using powdered activated carbons

Activated carbons were developed from Casurina equisetifolia leaves, by chemically treating with sulfuric acid (1:1) or zinc chloride (25%), at low (425 °C) and high (825 °C) temperatures. The resulting powdered activated carbons were applied for removing mercuric ions from aqueous solution at different agitation times and mercuric ion concentrations. The equilibrium data fitted well the Langmuir adsorption isotherm. The Langmuir adsorption capacities were 12.3 and 20.3 mg g⁻¹ for low temperature carbons and 43.9 and 38.5 mg g⁻¹ for high temperature carbons impregnated with H₂SO₄ and ZnCl₂, respectively. Studies of the effects of carbon dosage, NaCl concentrations and solution pH values were carried out for the more effective, high temperature carbons. Increasing NaCl concentration resulted in a significant decrease in the adsorption efficiency. Adsorption was high from solutions with low and neutral pH values and lower for solutions with alkaline pH values for the high temperature carbons.     

Electrochemical study of NiO nanoparticles electrode for application in rechargeable lithium-ion batteries

Nickel oxide nanoparticles were synthesized via a simple and inexpensive microwave-assisted synthesis method within a fast reaction time of less than 20 min. The calcination of as-prepared precursor at 600 °C produces single phase nickel oxide. The lattice structure and morphology of the sample were investigated by X-ray diffraction, field-emission scanning electron microscopy and field-emission transmission electron microscopy. The particle size range of the nickel oxide nanoparticles varied from 50 to 60 nm. Nickel oxide nanoparticles exhibited good electrochemical performances as an anode material for lithium-ion batteries. The prepared nickel oxide anode revealed a large initial discharge capacity of 1111.08 mAh g⁻¹ at 0.03 C rate and retained 80% of initial capacity (884.30 mAh g⁻¹) after 20 cycles. Furthermore, at elevated rate of 3.7 C, the charge capacity of the nickel oxide electrode was as high as 253.1 mAh g⁻¹, which was 35% greater than that of commercial bulk nickel oxide (188 mAh g⁻¹). The enhancement of the electrochemical performance was attributed to the high specific surface area, good electric contact among the particles and easier lithium ion diffusion.     

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.     

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.     

Mesoporous polyaniline or polypyrrole/anatase TiO2 nanocomposite as anode materials for lithium-ion batteries

A functional composite as anode materials for lithium-ion batteries, which contains highly dispersed TiO₂ nanocrystals in polyaniline matrix and well-defined mesopores, is fabricated by employing a novel one-step approach. The as-prepared mesoporous polyaniline/anatase TiO₂ nanocomposite has a high specific surface area of 224 m² g⁻¹ and a predominant pore size of 3.6 nm. The electrochemical performance of the as-prepared composite as anode material is investigated by cyclic voltammograms and galvanostatic method. The results demonstrate that the polyaniline/anatase nanocomposite provides larger initial discharge capacity of 233 mAh g⁻¹ and good cycle stability at the high current density of 2000 mA g⁻¹. After 70th cycles, the discharge capacity is maintained at 140 mAh g⁻¹. The excellent electrochemical performance of the polyaniline/TiO₂ nanocomposite is mainly attributed to its special structure. Furthermore, it is accessible to extend the novel strategy to other polymer/TiO₂ composites, and the mesoporous polypyrrole/anatase TiO₂ is also successfully fabricated.     

Determination of selenium by ICP-MS and HG-ICP-MS in coal, fly ashes and sorbents used for flue gas cleaning

The determination of selenium in solid materials related with the use of coal for energy production was evaluated by two methods; ICP-MS and HG-ICP-MS. Coals, fly ashes and various solids used as sorbents in gas cleaning processes (kaolin, limestone, alumina, metal oxide mixtures and activated carbons), were the materials analysed. In several of these materials, selenium could be determined by either method with similar results. However in coal, fly ashes and some activated carbons, the use of a HG-ICP-MS is necessary in order to avoid interferences. The results obtained by both methods in samples whose selenium content ranges from μg g⁻¹ to mg g⁻¹, are discussed. In general, good (±2-4%) to excellent (±1-2%) agreement between the reference and the obtained concentrations was achieved in the analysis of the certified and reference samples, the relative standard deviation being lower than 10% in all cases.     

Preparation and characterization of charcoals that contain dispersed aluminum oxide as adsorbents for removal of fluoride from drinking water

Charcoals adsorbents that contain dispersed aluminum and iron oxides have been synthesized by impregnating wood with salt solutions followed by carbonization at 500 °C, 650 °C or 900 °C. The adsorbents were characterized and their performance for fluoride removal from aqueous solution was evaluated. Aluminum and iron oxides were well dispersed into the porous charcoals. The carbons were amorphous and highly porous. XRD of the adsorbents showed crystalline iron oxide but did not show any form of crystalline aluminum oxides. All the adsorbents showed acidic surface properties. The efficiency of defluoridation was found to depend on the carbonization temperature, the pH of point of zero charge (pHPZC), and the co-existing ions. Substrates prepared at 650 °C with aluminum and iron oxides exhibited the best efficiency with a fluoride sorption capacity of 13.64 mg g⁻¹. More than 92% removal of fluoride was achieved within 24 h from a 10 mg L⁻¹ solution at neutral pH. Fluoride adsorption kinetic was well fitted by a pseudo-second order model. The amounts of residual Al and Fe in treated solution were pH dependant. At neutral pH, the amounts of dissolved Al and Fe were found to be 0.67 and 1.8 mg L⁻¹, respectively.     

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.     

Ru oxide/carbon nanotube composites for supercapacitors prepared by spontaneous reduction of Ru(VI) and Ru(VII)

A novel method based on spontaneous reduction of Ru(VI) and Ru(VII) is reported for the deposition of Ru oxide on multi-walled carbon nanotubes (MWCNT). Both purified and acid functionalized nanotubes (p-MWCNT and a-MWCNT) have been used to produce composite materials for use in high power aqueous supercapacitors. Specific capacitances of 213 ± 16 F g⁻¹ and 184 ± 11 F g⁻¹ were obtained for Ru oxide/p-MWCNT and Ru oxide/a-MWCNT composites, respectively. Specific capacitances for the Ru oxide component were 704 ± 62 F g⁻¹ and 803 ± 72 F g⁻¹, respectively. Current vs. potential curves exhibited capacitance peaks at ca. +0.5 V vs. Ag/AgCl. The Ru oxide/p-MWCNT composite was shown to be stable over 20,000 charge/discharge cycles. An advantage of the method is that no pre-treatment of the MWCNT is required for optimum performance.     

Tubular structured ordered mesoporous carbon as an efficient sorbent for the removal of dyes from aqueous solutions

Tubular structured ordered mesoporous carbon CMK-5 was investigated for the adsorption of the industrial dyes reactive blue 19, acid red 57 and fuchsin basic in aqueous solutions at room temperature. It was found that CMK-5 exhibits an ultrahigh adsorption rate and superior adsorption capacities for these dyes. Its maximum adsorption capacities for reactive blue 19, acid red 57 and fuchsin basic were 733, 1131 and 1403 mg g⁻¹, respectively, and significantly greater than other literature reported results on porous carbons. Following adsorption of reactive blue 19, CMK-5 carbon could be regenerated by either ethanol extraction or thermal annealing at 600 °C, reaching ∼51% and ∼77%, respectively of the adsorption capacity of the original carbon. For comparison, ordered mesoporous carbon CMK-3 (rod structure), polymer based disordered mesoporous carbon, and steam and CO₂ activated commercial coconut carbons were investigated for the adsorption of reactive blue 19. The fast adsorption rate of CMK-5 carbon is due to its unique properties of tubular mesostructure, bimodal mesopore system and high surface area. In the case of requiring emergency removal of large amount of dyes in aqueous solution, CMK-5 would be an ideal choice.     

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.     

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.