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.     

Production of fibrous activated carbons from natural cellulose (jute, coconut) fibers for water treatment applications

Different fibrous activated carbons were prepared from natural precursors (jute and coconut fibers) by physical and chemical activation. Physical activation consisted of the thermal treatment of raw fibers at 950 °C in an inert atmosphere followed by an activation step with CO₂ at the same temperature. In chemical activation, the raw fibers were impregnated in a solution of phosphoric acid and heated at 900 °C in an inert atmosphere. The characteristics of the fibrous activated carbons were determined in the following terms: elemental analysis, pore characteristics, SEM observation of the porous surface, and surface chemistry. As the objective of this study was the reuse of waste for industrial wastewater treatment, the adsorption properties of the activated carbons were tested towards pollutants representative of industrial effluents: phenol, the dye Acid Red 27 and Cu²⁺ ions. Chemical activation by phosphoric acid seems the most suitable process to produce fibrous activated carbon from cellulose fiber. This method leads to an interesting porosity (S_BET up to 1500 m² g⁻¹), which enables a high adsorption capacity for micropollutants like phenol (reaching 181 mg g⁻¹). Moreover, it produces numerous acidic surface groups, which are involved in the adsorption mechanisms of dyes and metal ions.     

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.     

Surface chemistry of phosphorus-containing carbons of lignocellulosic origin

Activated carbon adsorbents were prepared by phosphoric acid activation of fruit stones in an argon atmosphere at various temperatures in the 400-1000 °C range and at different acid/precursor impregnation ratios (0.63-1.02). The surface chemistry of the carbons was investigated by elemental analysis, cation exchange capacity (CEC, measured by neutralization of NaOH with acidic surface groups), infrared spectroscopy and potentiometric titration. Porous structure was derived from adsorption isotherms (N₂ at −196 °C and CO₂ at 0 °C). It was demonstrated that all carbons show considerable cation exchange capacity, the maximum (CEC = 2.2 mmol g⁻¹) being attained at 800 °C, which coincides with the maximum contents of phosphorus and oxygen. The cation exchange properties of phosphoric acid activated carbons from fruit stones are chemically stable in very acidic and basic solutions. Proton affinity distributions of all carbons show the presence of three types of surface groups with pK at 2.0-3.3, 4.6-5.9 and 7.6-9.1. These pK ranges were ascribed primarily to: (a) phosphorus-containing and carboxylic groups; (b) lactonic groups, and (c) phenolic groups, respectively. Phosphoric acid activated carbons are microporous with a relatively small contribution of mesopores. A maximum BET surface area of 1740 m² g⁻¹ was attained at 400 °C.     

Preparation and characteristics of rice-straw-based porous carbons with high adsorption capacity

To prepare porous carbons with high adsorption capacity from rice straws, two different kinds of precursors, i.e. one as the raw rice straws (one-stage process) and the other as pre-carbonized rice straws (two-stage process), were activated with KOH of various impregnation ratios. The two-stage process was found very effective for manufacturing porous carbons with high surface area and adsorption capacities for MB and I₂. For example, the porous carbon that was carbonized at 700°C and subsequently activated at 900°C exhibited the surface area of 2410 m²/g, the adsorption capacities of 800 and 1720 mg/g for MB and I₂, respectively, and the total pore volume of 1.4 ml/g. In the two-stage method, there was a preferential optimum impregnation ratio of KOH to a precursor carbon, i.e. 4:1, with which high surface area of porous carbons could be achieved. The formation of uni- and bidentate carboxylic salt structure, induced by reaction between KOH and oxygen containing carbon, that facilitates the formation of azo group (-NN-) on a subsequent heat treatment was considered as one of the key factors for the presence of optimum impregnation ratio of KOH. In contrast, the porous carbons of only moderate adsorption capacity could be obtained from the one-stage method. The original morphology of rice straw was sustained during the two-stage process, yet not during the one-stage process.     

Synthetic carbons activated with phosphoric acid: II. Porous structure

Synthetic activated carbons were prepared by H₃PO₄ activation of a chloromethylated and sulfonated copolymer of styrene and divinylbenzene, using an impregnation weight ratio of 0.75 and carbonization temperatures in the 400-1000 °C range. Other impregnation ratios (0.93 and 1.11) were also used at a carbonization temperature of 800 °C. The porous texture of the resulting carbons was characterized by N₂ adsorption at −196 °C and CO₂ adsorption at 0 °C. All carbons exhibited a multimodal pore size distribution with maxima in the micropore and meso/macropore regions. Maxima in pore volume were attained at 900 °C for micropores and at 500 and 900 °C for mesopores. The mesopore volume was less sensitive than the micropore volume to changes in the impregnation ratio. It is concluded that the porous texture is not a prime factor in determining the outstanding cation exchange capacities of these carbons.     

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.     

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).     

Fabrication of activated carbons with well-defined macropores derived from sulfonated poly(divinylbenzene) networks

Poly(divinylbenzene) (PDVB) monoliths with well-defined macropores that have been sulfonated and carbonized to obtain macroporous carbon monoliths. The original macroporous PDVB networks have been synthesized by living radical polymerization accompanied by spinodal decomposition. Sulfonation prevents polymer networks from large shrinkage and weight loss during carbonization by heat-treatment in an inert atmosphere. In the case of PDVB gels sulfonated at 120 °C using conc. H₂SO₄, mesopores in the original skeletons as well as macropores are retained after carbonization. The obtained carbon monoliths are subsequently activated by CO₂, which resulted in activated carbons. The specific surface area of the obtained activated carbons reaches up to 2360 m² g⁻¹.     

Carbon adsorbents from waste ion-exchange resins

A series of activated carbons was prepared from different waste commercial ion-exchange resins and studied by means of adsorption, SEM and IR methods. Samples were additionally washed or washed/frozen. This resulted in increases in micro- and mesoporosity in comparison with initial activated carbons. For some samples, the latter treatment gives enhancement of mesoporosity but reduction of microporosity and vice versa comparing with only washed carbons due to different localization of water droplets in mesopores or micropores. Changes in the morphology of chars and activated samples depended on resin composition and history. Relatively high values of porosity (V_p ≈ 0.4 cm³/g) and specific surface area (S_BET ≈ 600 m²/g) show that activated carbons prepared from waste ion-exchange resins can be utilized for different purposes, especially after additional treatment (such as washing, impregnation by certain compounds and subsequent thermal activation).     

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.     

Oxygen and phosphorus enriched carbons from lignocellulosic material

Activated carbons were prepared by phosphoric acid activation of fruit stones in air at temperatures 400-1000 °C. The surface chemistry was investigated by elemental analysis, cation exchange capacity, infrared spectroscopy and potentiometric titration. The porous structure was analyzed from adsorption isotherms (N₂ at 77 K and CO₂ at 273 K). It was demonstrated that all carbons show considerable cation exchange capacity, the maximum (2.2 mmol g⁻¹) being attained at 700 °C, which coincides with the maximum contents of phosphorus and oxygen. The use of air instead of argon during thermal treatment increased the amount of cation exchangeable surface groups for carbons obtained at 400-700 °C. Proton affinity distributions of all carbons show the presence of three types of surface groups with pK 1.8-3.1 (carboxylic and polyphosphates), 4.8-6.3 (second dissociation of carboxylic, weak acid in polyphosphates and enol structures) and 8.1-9.7 (phenols and enol structures). Carbons obtained in air at 400-600 °C show enhanced copper adsorption from 0.001 mol L⁻¹ Cu(NO₃)₂ in acidic solutions as compared to carbons obtained in argon. Carbons obtained in air show well-developed porous structure that is equivalent or higher as compared with carbons obtained in argon; the difference being progressively increased with increasing treatment temperature.     

HEMP-derived activated carbon fibers by chemical activation with phosphoric acid

Activated carbon fibers were prepared by chemical activation of hemp fibers with phosphoric acid at different carbonization temperatures and impregnation ratios. Surface properties of the activated carbons fibers were significantly influenced by the activation temperature and the impregnation ratio. An increase of either of these parameters produced a high development of the porous structure of the fibers. Activated carbon fibers with apparent surface area of 1350 m²/g and mesopore volume of 1.25 cm³/g were obtained at 550 °C with an impregnation ratio of 3. The activated carbon fibers presented a high oxidation resistance, due to the presence of phosphorus compounds on the carbon surface. The oxidation resistance results suggest that C-O-PO₃ and mainly C-PO₃ and C-P groups act as a physical barrier, blocking the active carbon sites for the oxidation reaction.     

Production of activated carbons from almond shell

The production of activated carbons from almond shell, using physical activation by CO₂ is reported in this work. The used method has produced activated carbons with apparent BET surface areas and micropore volume as high as 1138 m² g⁻¹ and 0.49 cm³ g⁻¹, respectively. The activated carbons produced have essentially primary micropores and only a small volume of wider micropores. By FTIR analysis it was possible to identify, in the surface of the activated carbons, several functional groups, namely hydroxyls (free and phenol), ethers, esters, lactones, pyrones and Si-H bonds. By the analysis of the XRD patterns it was possible to calculate the microcrystallites dimensions with height between 1.178 and 1.881 nm and width between 3.106 and 5.917 nm. From the XRD it was also possible to identify the presence of traces of inorganic heteroatoms such as Si, Pb, K, Fe and P. All activated carbons showed basic characteristics with point of zero charge between 9.42 and 10.43.     

Experimental evidence of an upper limit for hydrogen storage at 77 K on activated carbons

An upper limit for hydrogen storage at 77 K on activated carbons was clearly observed in the present experimental work. Such a limit is around 6.4 wt.%, i.e., close to the theoretical limit of 6.8 wt.%. Results of hydrogen storage were obtained in three independent laboratories using volumetric and gravimetric devices. Lab-made activated carbons (ACs) were found to have higher capacities than those of the commercial material AX-21. A maximum excess hydrogen storage capacity of 6.0 wt.% at 77 K and 4 MPa was obtained. This maximum was reduced to 0.6 wt.% at 298 K and 5 MPa. ACs with surface areas (S_BET) as high as 3220 m² g⁻¹ were prepared from chemical activation of anthracites with alkali (Na and K) hydroxides. At 77 K and 4 MPa, excess hydrogen storage capacity was directly correlated with S_BET for ACs having S_BET values lower than 2630 m²/g. Hydrogen uptake at 77 K also correlated with micropore volume and strongly depended on average pore diameter.     

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.     

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.     

Methane reforming reaction with carbon dioxide over SBA-15 supported Ni-Mo bimetallic catalysts

A series of mesoporous molecular sieves SBA-15 supported Ni-Mo bimetallic catalysts (xMo1Ni, Ni = 12 wt.%, Mo/Ni atomic ratio = x, x = 0, 0.3, 0.5, 0.7) were prepared using co-impregnation method for carbon dioxide reforming of methane. The catalytic performance of these catalysts was investigated at 800 °C, atmospheric pressure, GHSV of 4000 ml·g_cat^− 1·h^− 1 and a V(CH₄)/(CO₂) ratio of 1 without dilute gas. The result indicated that the Ni-Mo bimetallic catalysts had a little lower initial activity compared with Ni monometallic catalyst, but it kept very stable performance under the reaction conditions. In addition, the Ni-Mo bimetallic catalyst with Mo/Ni atomic ratio of 0.5 showed high activity, superior stability and the lowest carbon deposition rate (0.00073g_c·g_cat^− 1·h^− 1) in 600-h time on stream. The catalysts were characterized by power X-ray diffraction, N₂-physisorption, H₂-TPR, CO₂-TPD, TG and TEM. The results indicate that the Ni-Mo bimetallic catalysts have smaller metal particle, higher metal dispersion, stronger basicity, metal-support interaction and Mo₂C species. It is concluded that Mo species in the Ni-Mo bimetallic catalysts play important roles in reducing effectively the amount of carbon deposition, especially the amount of shell-like carbon deposition.     

Gas diffusion electrodes containing sulfonated poly (arylene ether) ionomer for polymer electrolyte fuel cells: Part 2. Improvement of the cathode performance

The performances of gas diffusion electrodes (GDEs) containing Pt/C catalyst (48 wt.% and 68 wt.%-Pt) and sulfonated poly (arylene ether) (SPAE) ionomer (ion exchange capacity, IEC = 1.8 and 2.5 meq g⁻¹) as a proton-conducting binder (SPAE-GDE) were examined in a PEFC at 80 °C and relative humidities (RH) from 60% to 100%. Based on our analyses in Part 1, we have succeeded in improving the cathode performance over the whole range of current densities examined by using a high Pt-loading for the catalyst (68 wt.%-Pt/C), in place of the previously used 48 wt.% one, for the reduction of thickness of the catalyst layer, which enabled us to increase the O₂ gas diffusion rate and to suppress the adsorption of the SPAE binder on the Pt surface via an effective utilization of generated water. The performance, especially at low RH, was improved further by employing an SPAE binder with a lower IEC, 1.8 meq g⁻¹ [SPAE(1.8)]. It was demonstrated by cyclic voltammetry that the specific adsorption of the sulfonate or organic moiety on the Pt surface was indeed suppressed for the case of SPAE(1.8). Hence, for the SPAE-GDEs, the use of a high Pt-loading catalyst, together with a binder with an appropriate IEC, is very important.     

Synthetic carbons activated with phosphoric acid III. Carbons prepared in air

Synthetic activated carbons were prepared by phosphoric acid activation of a styrene-divinylbenzene copolymer in an air atmosphere at various temperatures in the 400-900 °C interval. The carbons were characterized by elemental analysis, cation-exchange capacity measurement, infrared spectroscopy, potentiometric titration, copper adsorption from solution and physical adsorption of N₂ at −196 °C and CO₂ at 0 °C. It was shown that, similarly to synthetic phosphoric acid activated carbons obtained in argon, the synthetic carbons activated with phosphoric acid in air possess an acidic character and show considerable cation-exchange properties. The contribution of oxygen-containing surface groups along with phosphorus-containing groups to CEC is higher for carbons obtained in air. Three types of surface groups were identified on carbons prepared at temperatures up to 600 °C, and four types on carbons prepared at higher temperatures. These groups were assigned to ‘super-acidic’ (pK<0), phosphorus-containing (pK=1.1-1.2), carboxylic (pK=4.7-6.0) and phenolic (pK=8.1-9.4) groups. The cation-exchange capacity was at a maximum for the carbon prepared at 800 °C. Copper adsorption by synthetic phosphoric acid activated carbons obtained in air at temperatures lower than 800 °C is higher than for similar carbons obtained in argon. The increase is due to additional formation of oxygen-containing surface groups. Calculated copper binding constants revealed the importance of phosphorus-containing and carboxylic groups for adsorption of copper from aqueous solution. All carbons show a multimodal pore size distribution including simultaneously micropores and mesopores, but the porous texture is not a prime factor in determining the cation-exchange capacities of these carbons. Synthetic phosphoric acid activated carbons show a greater development of porosity when obtained in air as compared to carbons carbonized in argon.