Polyaniline/porous carbon electrodes by chemical polymerisation: Effect of carbon surface chemistry

Polyaniline/porous carbon composite electrodes were prepared by chemical polymerisation and characterized in terms of porosity and performance as electrochemical capacitors.To obtain the composite electrodes two methods were used. The first method consisted of mixing, directly, the activated carbon with chemically polymerised polyaniline. The second one consisted of mixing the activated carbon with aniline and subsequent chemical polymerisation. Additionally, the second process was carried out with the porous carbon previously thermally treated in N₂ up to 900 °C in order to remove surface oxygen groups.Changes in porosity with the polyaniline addition were analysed. It has been proved that the method used strongly affects the porous structure. Dealing with the electrochemical performance, polyaniline and carbon mechanically mixed seem to work independently, being the composite behaviour a combination of the corresponding performance of both materials separately. The composites prepared by the second method (polymerisation over carbon) reveal the key role of surface chemistry in polyaniline coating. Aniline reacts with the oxygen complexes and their positive effect in capacitance is not observed.The second method (polymerisation over carbon) using a thermally treated carbon seems to be the best one since a more porous (or thinner) polyaniline film is produced.     

Surface functional groups of carbons and the effects of their chemical character, density and accessibility to ions on electrochemical performance

Wood origin activated carbon was oxidized and then treated with melamine and urea followed by carbonization at 950 °C in an inert atmosphere. The samples were characterized using elemental analysis, adsorption of nitrogen, Boehm titration, FTIR and XPS. Testing the carbons as the electrode materials in supercapacitors indicated that the electrochemical behavior of modified samples is governed mainly by the specific types of functional groups. Both surface chemistry and texture of carbons are affected by the nitrogen source and the type of oxygen functionalities preexisting on the surface. The modified carbons revealed significantly enhanced capacitances in 1 M H₂SO₄ reaching 300 F/g and the capacitance retention ratio is 86% at the current load of 1 A/g. Perfect correlations were found between the number of basic groups and the gravimetric capacitance and between the normalized capacitance in micropores and the distribution of quaternary and pyridinic-N-oxide nitrogen species on the surface of the micropores. The pseudocapacitance on N and O atoms is particularly dominant at higher current loads and the charge on quaternary nitrogen and pyridinic-N-oxide enhances the electron transport through the electrode improving the rate performance of treated samples. The micropores were found to be most effective in a double-layer formation.     

Effects of thermal treatment of activated carbon on the electrochemical behaviour in supercapacitors

This paper studies the electrochemical behaviour of activated carbons with different oxygen content and investigates the contribution of pseudocapacitance to the global behaviour of the samples. A mesophase-derived activated carbon was further heat treated to 600 or 1000 °C in nitrogen. The changes in texture and surface chemistry induced by the thermal treatment were deeply studied. The electrochemical behaviour of the samples was studied in two- and three-electrode cells. The contribution of pseudocapacitance was evaluated by cyclic voltammetry and by the differences of specific capacitance obtained from galvanostatic tests performed in acidic (H₂SO₄) and basic (KOH) media. The presence of an extra capacitance due to redox reactions has been proved both in acidic and basic media for the samples with high oxygen content, although its contribution in basic media is significantly lower. The results obtained clearly indicate that the oxygen responsible for CO-evolution participates in redox reactions, whereas the oxygen responsible for the CO₂-evolution is of minor importance.     

Effect of electrochemical treatments on the surface chemistry of activated carbon

The effect of the electrochemical treatment (galvanostatic electrolysis in a filter-press electrochemical cell) on the surface chemistry and porous structure of a granular activated carbon (GAC) has been analyzed by means of temperature-programmed desorption and N₂ (at 77 K) and CO₂ (at 273 K) adsorption isotherms. The anodic and cathodic treatments, the applied current (between 0.2 and 2.0 A) and the type of electrolyte (NaOH, H₂SO₄ and NaCl) have been studied as electrochemical variables. Both anodic and cathodic treatments lead to an increase in the surface oxygen groups. A suitable choice of the electrochemical variables allows a selective modification of the amount and the nature of the surface oxygen groups of the GAC. In general, the electrochemical treatment does not modify significantly the textural properties of the GAC. However, an increase in the porosity of the activated carbon occurs during the cathodic treatment in oxygen-saturated solutions. This result is interpreted as a consequence of carbon gasification driven by reaction with peroxide species generated by electroreduction of oxygen. The anodic treatment in NaCl produces oxidation degrees comparable to those achieved by classical chemical oxidations.     

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).     

Role of surface chemistry on electric double layer capacitance of carbon materials

A large number of porous carbon materials with different properties in terms of porosity, surface chemistry and electrical conductivity, were prepared and systematically studied as electric double layer capacitors in aqueous medium with H₂SO₄ as electrolyte. The precursors used are an anthracite, general purpose carbon fibres and high performance carbon fibres, which were activated by KOH, NaOH, CO₂ and steam at different conditions. Among all of them, an activated anthracite with a BET surface area close to 1500 m²/g, presents the best performance, reaching a value of 320 F/g, using a three-electrode system. The results obtained for all the samples, agree with the well-known relationship between capacitance and porosity, and show that the CO-type oxygen groups have a positive contribution to the capacitance. A very good correlation between the specific capacitance and this type of oxygen groups has been found.     

Modified activated carbons for catalytic wet air oxidation of phenol

This study aims at testing several activated carbons for the catalytic wet air oxidation (CWAO) of phenol solutions. Two commercial activated carbons were used both as received and modified by treatment with either HNO₃, (NH₄)₂S₂O₈, or H₂O₂ and by demineralisation with HCl. The activated carbons were characterised by measuring their surface area, distribution of surface functional groups and phenol adsorption capacity. The parent and treated activated carbons were then checked for CWAO using a trickle bed at 140 °C and 2 bar of oxygen partial pressure. The treatments increase the acidic sites, mostly creating lactones and carboxyls though some phenolic and carbonyl groups were also generated. Only (NH₄)₂S₂O₈ treatment yields a significant decrease in surface area. CWAO tests show that catalytic activity mainly depends on the origin of the activated carbon. The modifications generally had a low impact on phenol conversion, which correlates somewhat with the increase in the acidity of the carbons. Characterisation of the used activated carbon evidences that chemisorbed phenolic polymers formed through oxidative coupling and oxygen radicals play a major role in the CWAO over activated carbon.     

Preparation, characterization and Methylene Blue adsorption of phosphoric acid activated carbons from globe artichoke leaves

Activated carbons were prepared by the pyrolysis of artichoke leaves impregnated with phosphoric acid at 500 °C for different impregnation ratios: 100, 200, 300 wt.%. Materials were characterized for their surface chemistry by elemental analysis, “Boehm titrations”, point of zero charge measurements, infrared spectroscopy, as well as for their porous and morphological structure by Scanning Electron Microscopy and nitrogen adsorption at 77 K. The impregnation ratio was found to govern the porous structure of the prepared activated carbons. Low impregnation ratios (~ 100 wt.%) led to essentially microporous and acidic activated carbons whereas high impregnation ratios (> 100 wt.%) gave essentially microporous-mesoporous carbons with specific surface areas as high as 2038 m²·g^− 1, pore volume as large as 2.47 cm³·g^− 1, and a slightly acidic surface. The prepared activated carbons were studied for their adsorption isotherms of Methylene Blue at pH = 3 and pH = 9. The supermicroporous structure of the material produced at 200 wt.% H₃PO₄ ratio was found to be appropriate for an efficient adsorption of this dye controlled by dispersive and electrostatic interactions depending on the amount of oxygen at the surface.     

Microstructure effects on the electrochemical corrosion of carbon materials and carbon-supported Pt catalysts

The electrochemical corrosion behavior of a set of porous carbonaceous materials of interest as catalyst supports for polymer electrolyte membrane fuel cells was examined in 2 M H₂SO₄ at 80 °C at constant electrode potential of 1.2 V vs. RHE. Correlations have been observed between the specific rates of corrosion of carbon materials and carbon-supported Pt catalysts on the one hand and their substructural characteristics derived from X-ray diffraction analysis on the other hand. Carbon supports of the Sibunit family and catalytic filamentous carbons possess lower specific (i.e., surface area normalized) corrosion currents compared to conventional furnace black Vulcan XC-72 and better stabilize Pt nanoparticles.     

On the preparation and characterization of chars and activated carbons from orange skin

Activated carbons were obtained by carbonization of orange skin waste and partial gasification with CO₂. The orange skin contains a significant amount of inorganic matter mainly potassium, calcium and phosphorus. CO₂ gasification is catalyzed by potassium and calcium, resulting in carbons with a microporous structure. Thermal treatment up to 900 °C applied to orange skin-derived activated carbons yields carbons with a highly developed porous structure, and a significant contribution of mesopores, due to the activation effect of potassium compounds. This porous structure is initially blocked by the inorganic matter that is removed by a subsequent acid wash, opening the porous structure of the final carbon; an activated carbon with a very wide porous structure and a specific surface area of around 1200 m²/g was obtained. The activated carbon with high potassium content shows relatively high NO adsorption capacities in the presence of oxygen at 120 °C, probably due to the catalytic effect of potassium on the oxidation of NO. The breakthrough times of the NO adsorption in the presence of oxygen at 120 °C were predicted by the Bohart and Adams model with a relevant agreement between the calculated and the experimental times.     

Measuring cycle efficiency and capacitance of chemically activated carbons in propylene carbonate

The cycle efficiency and capacitance of two KOH-activated carbons have been tested in 0.5 M tetraethylammonium tetrafluoroborate in propylene carbonate (TEABF₄/PC) under different conditions of temperature (room temperature and 70 °C) and voltage (0-3 V). The materials tested include a KOH-activated carbon before and after treatment with different acids and hydrogen at high temperature to remove most of the oxygen groups and inorganic impurities. Porous texture and surface chemistry characterization have been carried out before and after cycling to understand the changes in the properties of activated carbons during the cycles. It has been observed that the treatment with different acids and hydrogen at high temperature produces materials with higher cycling stability. The results point out that the observed electrode degradation is mainly due to some pore blockage caused by reaction of the solvent with the functional groups on the carbon. The blockage of porosity is higher for the sample with higher oxygen content, indicating that the treatment carried out to remove the oxygen groups has an important positive effect, giving electrodes with a much lower degradation under very aggressive conditions in a TEABF₄/PC medium.     

Ordered mesoporous carbons synthesized by a modified sol-gel process for electrosorptive removal of sodium chloride

Capacitive deionization (CDI) represents an alternative process to remove the ions from the brackish water. In this study two series of ordered mesoporous carbons (OMCs) that demonstrated the potential use for capacitive desalination have been synthesized by a modified sol-gel process involving nickel salts. It was shown that the preferred formation of crown-ether type complexes between nickel ions and triblock copolymers resulted in higher BET surface area and smaller mesopores. As the electrode materials for CDI, OMC obtained by the addition of NiSO₄ · 6H₂O exhibited best electrochemical performance compared with other OMCs and a commercial activated carbon either in 0.1 M NaCl solution or in 0.0008 M NaCl solution, plus the amount of adsorbed ions measured by a flow through apparatus reached 15.9 μmol g⁻¹ and the ions could be fully released into the solution. The excellent electrosorption desalination performance of OMC obtained by the addition of NiSO₄ · 6H₂O was ascribed to its high BET surface area of 1491 m² g⁻¹ and ordered mesopores of 3.7 nm. Based on these results, it is deduced that the modified sol-gel process might be a potential method of obtaining the excellent electrode materials for CDI.     

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.     

Kinetic study of the oxidation resistance of phosphorus-containing activated carbons

Chemical activation of different lignocellulosic materials with phosphoric acid produces activated carbons with higher oxidation resistance than that observed for catalyst free chars obtained at similar conditions from the same biomass, even though the activated carbons have a more developed porous structure. The main reason for such behavior is probably related to the presence of thermally stable phosphorus complexes that remain on the carbon surface after the activation process. XPS analyses point out the oxidation of the phosphorus reduced groups to C–O–PO₃/(CO)₂PO₂ prior to carbon gasification. The latter complexes seem to stabilize the active carbon sites. Moreover, the presence of these phosphorus inhibitors produces a change in the gasification mechanism of the activated carbons with respect to that for char. Char oxidation seems to proceed in the entire available particle surface, while activated carbon gasification is better explained by the shrinking unreacted core model. Such a difference in mechanism is attributed to the inhibition effect of the phosphorus complexes, which reduces the reactivity of carbon active sites, and could act as a physical barrier for oxygen diffusion in the micropores.     

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.     

Electrochemical capacitors based on highly porous carbons prepared by KOH activation

Various coal and pitch-derived carbonaceous materials were activated for 5 h at 800 °C using potassium hydroxide and 1:4 component ratio. Porosity development of the resultant activated carbons (ACs) was assessed by N₂ sorption at 77 K and their capability of the charge accumulation in electric double layer was determined using galvanostatic, voltammetric and impedance spectroscopy techniques. ACs produced from different precursors are all microporous in character but differ in terms of the total pore volume (from 1.05 to 1.61 cm³ g⁻¹), BET surface area (from 1900 to 3200 m² g⁻¹) and pore size distribution. Very promising capacitance values, ranging from 200 to 320 F g⁻¹, have been found for these materials operating in acidic 1 mol l⁻¹ H₂SO₄ electrolytic solution. The variations in the electrochemical behavior (charge propagation, self-discharge, frequency response) are considered in relation to the porous texture characteristics, elemental composition but also possible effect of structural ordering due to various precursor materials used. Cycling investigation of all the capacitors has been also performed to compare ability of the charge accumulation for different carbon materials during subsequent cycles.     

Surface-Treated Activated Carbon for Removal of Phenol from Water

Abstract

Adsorption of phenol from dilute solutions has been studied on porous and nonporous carbons, as well as on ion-exchange resins. At a given equilibrium concentration, uptake of phenol on nonporous carbons per unit area is determined by the nature of the carbon surface. Phenol uptake on porous activated carbons decreases sharply upon surface oxidation. However, progressive elimination of chemisorbed oxygen from the oxidized carbon upon heat treatment at increasing temperatures in N₂ increases the phenol adsorption capacity. The capacity is further enhanced if following heat treatment in N₂ at 950 [ddot]C the samples are reacted with H₂ at 300 [ddot]C. The mechanism of phenol adsorption on carbons has been discussed. Activated carbons are more effective adsorbents for phenol than commercial ion-exchange resins.     

The effect of activated carbon surface moisture on low temperature mercury adsorption

Experiments with elemental mercury (Hg⁰) adsorption by activated carbons were performed using a bench-scale fixed-bed reactor at room temperature (27°C) to determine the role of surface moisture in capturing Hg⁰. A bituminous-coal-based activated carbon (BPL) and an activated carbon fiber (ACN) were tested for Hg⁰ adsorption capacity. About 75-85% reduction in Hg⁰ adsorption was observed when both carbon samples’ moisture (∼2 wt.% as received) was removed by heating at 110°C prior to the Hg⁰ adsorption experiments. These observations strongly suggest that the moisture contained in activated carbons plays a critical role in retaining Hg⁰ under these conditions. The common effect of moisture on Hg⁰ adsorption was observed for both carbons, despite extreme differences in their ash contents. Temperature programmed desorption (TPD) experiments performed on the two carbons after adsorption indicated that chemisorption of Hg⁰ is a dominant process over physisorption for the moisture-containing samples. The nature of the mercury bonding on carbon surface was examined by X-ray absorption fine structure (XAFS) spectroscopy. XAFS results provide evidence that mercury bonding on the carbon surface was associated with oxygen. The results of this study suggest that surface oxygen complexes provide the active sites for mercury bonding. The adsorbed H₂O is closely associated with surface oxygen complexes and the removal of the H₂O from the carbon surface by low-temperature heat treatment reduces the number of active sites that can chemically bond Hg⁰ or eliminates the reactive surface conditions that favor Hg⁰ adsorption.     

Specific anion and cation capacitance in porous carbon blacks

Porous carbon black was modified to introduce oxygen and nitrogen surface functionality and characterized using wet titration methods, elemental analysis, XPS, TEM, thermal analysis, adsorption of nitrogen and carbon dioxide. Then the electrochemical capacitance was measured for cations and anions in 1 M H₂SO₄. The results were compared to those obtained on activated carbons. The modified carbon black samples have four times higher adsorption of anions than cations. It is hypothesized that the graphitic microstructure of carbon blacks with structural defects is responsible for intercalation of anions in-between the graphene layers which take place at potentials higher than 0.75 V vs. Ag/AgCl and at moderately low current loads. This process is reversible and the deintercalation occurs at approximately 0.4 V vs. Ag/AgCl during the cathodic reduction. Introduction of nitrogen have generally a detrimental effect on the anion adsorption capacitance due to a structural defects blockage. At high current loads this phenomenon of enhanced anion adsorption capacitance becomes less pronounced due to the kinetic limitations. For amorphous activated carbons enhanced anion electrosorption at low current loads is governed by proton assistance while at high current loads larger cation capacitance is due to the pseudocapacitive interactions of protons with nitrogen and oxygen functional groups.     

Lead(II) adsorption onto the graphene layer of carbonaceous materials in aqueous solution

Lead(II) adsorption from an aqueous solution onto a graphene layer (Cπ electrons) was investigated using activated carbon and charcoal. The carbonaceous materials were treated by several steps to prepare ash free and acidic oxygen free graphite surface by washing with HCl and H₂F₂ solution followed by out gassing at 1273 K. Changes in Pb(II) adsorption capacity were checked at each step to maximize the area of the graphene layer. As received activated carbon and charcoal and their HNO₃ oxidized counterparts were also used for the adsorption experiments for comparison with the ash free and the acidic oxygen free carbons. Boehm titration and Langmuir isotherms were used to evaluate the Pb(II) adsorption onto the adsorbents. The experimental results indicate that an acidic oxygen free graphene layer exhibits a basic character caused by Cπ electrons. When only a small amount of acidic oxygen groups was present, the Pb(II) adsorption strength onto the graphene layer (Cπ electrons) significantly diminished, and the Pb(II) adsorption sites were switched from the graphene layer to carboxylic and lactonic groups on the carbons in the results.