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.     

Influence of oxygen treatment on electric double-layer capacitance of activated carbon fabrics

Oxygen treatment at 250°C on polyacrylonitrile-based activated carbon fabric was conducted to explore the influence of carbon-oxygen complexes on the performance of capacitors fabricated with the carbon fabric. Surface analysis showed that most of the oxygen functional groups created from the oxygen treatment were the carbonyl or quinone type. The performance of the capacitors was tested in 1 M H₂SO₄, using potential sweep cyclic voltammetry and constant current charge-discharge cycling. It was found that the Faradaic current, the contributor of pseudocapacitance, increased significantly with the extent of oxygen treatment, while the increase in the double-layer capacitance was minor. Due to the treatment the overall specific capacitance showed an increase up to 25% (e.g., from 120 to 150 F g⁻¹ at a current density of 0.5 mA cm⁻²). However, the distributed capacitance effect, the inner resistance and the leakage current were found to increase with the extent of oxidation. It is suggested that due to the local changes of charge density and the increase in redox activity the presence of the carbonyl- or quinone-type functional groups may induce double-layer formation, Faradaic current, surface polarity, and electrolyte decomposition.     

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.     

Electrochemical responses from surface oxides present on HNO3-treated carbons

Carbons of different surface oxide compositions, which were prepared from HNO₃ oxidation of polyacrylonitrile-based carbon fabric followed by heat treatment at different temperatures, were subjected to electrochemical response analysis using a voltammetric method. Temperature programmed desorption was employed to analyze the surface oxide composition. A significant amount of unstable oxides that can be irreversibly removed with reduction using a cathodic potential sweep were found to be present on the oxidized carbons. About 2.6 electrons are required to remove one oxygen atom from the carbon surface, and this may be the first quantitative study ever reported. After the initial cathodic sweep, no irreversible reduction peak response was observed for the subsequent electrochemical measurements. The capacitive performance of the carbons is related to the population of stable CO-desorbing complexes that can be determined with thermal desorption after the cathodic sweep. The present work has shown that carbon capacitance increases from 170 to 190 F/g with an increase of the CO-desorbing oxides from 1.31 to 1.56 mmol/g. This means that each stable CO-desorbing oxide is able to store 0.8 electron charge per volt within the potential window employed, suggesting the effective role of CO-desorbing oxides in improving the capacitance.     

Adsorption of dyes on activated carbons: influence of surface chemical groups

The surface chemistry of a commercial activated carbon has been selectively modified, without changing significantly its textural properties, by means of chemical treatments, using HNO₃, H₂O₂, NH₃, and thermal treatments under a flow of H₂ or N₂. The resultant samples were characterized in terms of their surface chemistry and textural properties, and subsequently tested in the removal of different classes of dyes. It was shown that the surface chemistry of the activated carbon plays a key role in dye adsorption performance. The basic sample obtained by thermal treatment under H₂ flow at 700 °C is the best material for the adsorption of most of the dyes tested. For anionic dyes (reactive, direct and acid) a close relationship between the surface basicity of the adsorbents and dye adsorption was shown, the interaction between the oxygen-free Lewis basic sites and the free electrons of the dye molecule being the main adsorption mechanism. For cationic dyes (basic) the acid oxygen-containing surface groups show a positive effect but thermally treated samples still present good performances, showing the existence of two parallel adsorption mechanisms involving electrostatic and dispersive interactions. The conclusions obtained for each dye individually were confirmed in the colour removal from a real textile process effluent.     

Effects of activated carbon surface chemistry and pore structure on the adsorption of organic contaminants from aqueous solution

The objective of this research was to develop activated carbon selection criteria that assure the effective removal of trace organic contaminants from aqueous solution and to base the selection criteria on physical and chemical adsorbent characteristics. To systematically evaluate pore structure and surface chemistry effects, a matrix of activated carbon fibers (ACFs) with three activation levels and four surface chemistry levels was prepared and characterized. In addition, three granular activated carbons (GACs) were studied. Two common drinking water contaminants, relatively polar methyl tertiary-butyl ether (MTBE) and relatively nonpolar trichloroethene (TCE), served as adsorbate probes. TCE adsorbed primarily in micropores in the 7-10 Å width range while MTBE adsorbed primarily in micropores in the 8-11 Å width range. These results suggest that effective adsorbents should exhibit a large volume of micropores with widths that are about 1.3 to 1.8 times larger than the kinetic diameter of the target adsorbate. Hydrophobic adsorbents more effectively removed both TCE and MTBE from aqueous solution than hydrophilic adsorbents, a result that was explained by enhanced water adsorption on hydrophilic surfaces. To assure sufficient adsorbent hydrophobicity, the oxygen and nitrogen contents of an activated carbon should therefore sum to no more than about 2 to 3 mmol/g.     

Effects of carbon surface chemistry and solution pH on the adsorption of binary aromatic solutes

Adsorption of p-cresol, nitrobenzene and p-nitrophenol on treated and untreated carbons is investigated systematically. The effects of carbon surface chemistry and solution pH are studied and discussed. All adsorption experiments were carried out in pH-controlled solutions to examine the adsorption properties of the adsorption systems where the solutes are in molecular as well as ionic forms. Using the homogeneous Langmuir equation, the single solute parameters are determined. These parameters are then used to predict the binary solute adsorption isotherms and gain further insights into the adsorption process.     

Oxidation of pinane over phthalocyanine complexes supported on activated carbon: Effect of the support surface treatment

The oxidation of cis-pinane with tert-butyl hydroperoxide, at room temperature and atmospheric pressure, was carried out in the presence of iron-phthalocyanines supported on activated carbons. The carbon supports were prepared from a NORIT activated carbon, which was modified by different chemical and thermal treatments (including oxidation in the gas and liquid phases). The carbon samples were characterized by nitrogen adsorption, mass titration and temperature programmed desorption (TPD). The TPD profiles were analysed by a simple deconvolution method, allowing for the determination of the amount of oxygen containing functional groups on the carbon surface. The main reaction product is 2-pinane hydroperoxide (77% selectivity at 91% conversion). Formation of 2-pinanol, pinocampheol and verbanol and the respective ketones was also observed. The influence of the surface chemistry of the carbon supports on catalytic activity and product selectivity is studied. The catalysts prepared from supports with very high or very low oxygen content exhibit low activity, whereas for supports with intermediate oxygen contents a good correlation between the amount of phenols and lactones and catalytic activity is obtained.     

Influence of pore size distribution on methane storage at relatively low pressure: preparation of activated carbon with optimum pore size

Activated carbons prepared by KOH activation of an anthracite were studied for methane storage applications. The effect of the different variables of the activation process (KOH/anthracite ratio, pyrolysis temperature and time) on methane storage and methane delivery was analyzed. Methane delivery was obtained in two different ways: calculated from the isotherm and measuring the volume of methane delivered from a carbon-filled vessel (5 cm³). Both methods give similar values. In addition to the well-known effect of the micropore volume and packing density, special attention was paid to the effect that the micropore size distribution has in methane storage performance. It was shown that this parameter is also a key parameter in the application of activated carbons for methane storage applications. Activated carbons prepared from a cheap raw material and using a single stage activation process have reached very high values of methane storage (155 V/V) and delivery (145 V/V).     

On the nature of surface acid sites of chlorinated activated carbons

Two activated carbons containing different amounts of chlorine were obtained by chlorination of an activated carbon prepared from olive stones. Variations in surface physics and chemistry of the samples were studied by N₂ and CO₂ adsorption, mercury porosimetry, TPD, XPS, pH_PZC measurements, and by testing their behaviour as catalysts in the decomposition reaction of isopropanol. Our results indicate that chlorination of activated carbon increases its Lewis acidity but decreases its Brönsted acidity, which can be explained by the resonance effect introduced into the aromatic rings of graphene layers by the chlorine atoms covalently bound to their edges. This resonance effect could also explain the changes observed in the thermal stability of C-Cl and C-O bonds.     

Influence of the surface chemistry of modified activated carbon on its electrochemical behaviour in the presence of lead(II) ions

Cyclic voltammetric studies of the influence of surface chemistry on the electrochemical behaviour of granulated and powdered activated carbon samples in the presence of lead(II) ions both in bulk solution and pre-adsorbed on carbon were carried out. Variety in surface chemical character was achieved through modification of carbon samples by heat treatment in vacuum, ammonia and ammonia-oxygen atmospheres, as well as by oxidation in moist air and with concentrated nitric acid. For the samples obtained, the surface area (BET), acid–base neutralization capacities and sorption capacity towards Pb²⁺ ions were estimated. The states of the deposited Pb species were assessed by means of FTIR and XPS spectra as well as cyclic voltammetry. The importance of the surface chemistry of the carbon electrode materials are discussed in terms of their electrochemical properties and the mechanism of adsorption processes. The Cπ-metal and heteroatom-metal interaction are dominant in amphoteric and basic carbons, but in oxidized samples adsorption take place mainly by ion-exchange. Other forms of adsorption, such as the formation metal hydroxide species, are also covered buy this paper. Various forms of adsorbed lead species exhibit different electrochemical activities.     

Influence of activated carbon pore structure on oxygen reduction at catalyst layers supported on rotating disk electrodes

Carbon materials are often used as catalyst supports, and for catalysts in electrodes of a polymer electrolyte fuel cell, carbon black has been used. Recently, it was found, however, that activated carbon could replace carbon black and besides, significantly improve the activity of the electrode catalyst layer for oxygen reduction. In the present study, to optimize the pore structure of activated carbon for further activity improvement, the influence of the pore structure on the activity was investigated using activated carbon of various specific surface areas and mean pore diameters. A catalyst layer was formed from activated carbon loaded with platinum and a polymer electrolyte. The activity of the layer was measured in an oxygen-saturated perchloric acid solution, supporting the layer on a rotating glassy carbon disk electrode. We found that increases in the specific surface area and mean pore diameter increased the activity and that the latter was more effective than the former mainly due to the enhanced mass-transfer in the pores; the catalyst layer formed from activated carbon with the largest mean pore diameter was the most active. Unless pores excessively develop and lose connections between particles, a large pore diameter is therefore desired for the fuel cell electrodes.     

Exponential growth of electrochemical double layer capacitance in glassy carbon during thermal oxidation

The evolution of the electrochemical double layer capacitance of glassy carbon during thermochemical gas phase oxidation was studied with electrochemical impedance spectroscopy. Particular attention was paid to the initial oxidation stage, during which the capacitance grows exponentially. This stage could be experimentally assessed by lowering the reaction temperature and oxidant partial pressure. After a specific oxidation time the capacitance growth experiences a cross-over to a logistic growth.     

Preparation and properties of phenolic resin-based activated carbon spheres with controlled pore size distribution

Three kinds of phenolic resin-based activated carbon spheres (P-ACS) with different pore size distribution were prepared successfully by adding pore-forming agents to novolac-type phenolic resin. Polyethylene glycol and polyvinyl butyral, serving as pore-forming agents, evaporated during pyrolysis and left a small amount of carbon residue in the matrix of the phenolic resin-based carbon, thus changing the carbonization and activation behavior of the resin. Mesopores between 3 and 5 nm were created in the P-ACS, which possessed excellent adsorption properties for creatinine. Ferrocene has little effect on the carbonization process of the phenolic resin, but has a great impact on the activation process. Mesopores and macropores with a range from 3-5 to 10-90 nm were produced in the P-ACS, which exhibited large adsorption properties for VB₁₂, a larger molecule than creatinine. P-ACS without pore-forming agents exhibited a small specific surface area and mainly micropores, which resulted in a very small amount of creatinine and VB₁₂ adsorbed.     

Effect of pore size distribution of coal-based activated carbons on double layer capacitance

A series of coal-based activated carbons representing a wide range of mesopore content, from 16.7 to 86.9%, were investigated as an electrode in electric double layer capacitors (EDLCs) in 1 mol l⁻¹ H₂SO₄ and 6 mol l⁻¹ KOH electrolytic solutions. The activated carbons (ACs) used in this study were produced from chemically modified lignite, subbituminous and bituminous coals by carbonization and subsequent activation with steam. The BET surface area of ACs studied ranged from 340 to 1270 m² g⁻¹. The performance of ACs as EDLC electrodes was characterized using voltammetry, galvanostatic charge/discharge and impedance spectroscopy measurements. For the carbons with surface area up to 1000 m² g⁻¹, the higher BET surface area the higher specific capacitance (F g⁻¹) for both electrolytes. The surface capacitance (μF cm⁻²) increases also with the mesopore content. The optimum range of mesopore content in terms of the use of ACs studied for EDLCs was found to be between 20 and 50%. A maximum capacitance exceeding 160 F g⁻¹ and a relatively high surface capacitance about 16 μF cm⁻² measured in H₂SO₄ solution were achieved for the AC prepared from a sulfonated subbituminous coal. This study shows that the ACs produced from coals exhibit a better performance as an electrode material of EDLC in H₂SO₄ than in KOH electrolytic solutions. For KOH, the capacitance per unit mesopore surface is slightly lower than that referred to unit micropore surface (9.1 versus 10.1 μF cm⁻²). However, in the case of H₂SO₄ the former capacitance is double and even higher compared with the latter (23.1 versus 9.8 μF cm⁻²). On the other hand, the capacitance per micropore surface area is the same in both electrolytes used, about 10.0 μF cm⁻².     

The role of adsorbent pore size distribution in multicomponent adsorption on activated carbon

The impact of adsorbent pore size distribution (PSD) on adsorption mechanism for the multi solute system was evaluated in this study. Anoxic and oxic adsorption equilibrium for the single solute (phenol), binary solute (phenol/2-methylphenol) and ternary solute (phenol/2-methylphenol/2-ethylphenol) systems on one granular activated carbon (GAC) F400 and two types of activated carbon fibers (ACFs), namely, ACC-10 and ACC-15, were determined. F400 has a wide PSD, while ACC-10 and ACC-15 have narrow PSD and their critical pore diameters are 8.0 Å and 12.8 Å, respectively. In single solute adsorption, the increase of adsorptive capacity under oxic conditions as compared to anoxic ones was related to the PSD of the adsorbent. Binary solute adsorption on ACC-10 and ternary solute adsorption on ACC-15 indicated no impact of the presence of molecular oxygen on the adsorptive capacity and the adsorption isotherms were well predicted by the ideal adsorbed solution theory (IAST). Significant differences between oxic and anoxic isotherms were noticed for other multicomponent adsorption systems. The narrow PSD of ACFs was effective in hampering the oligomerization of phenolic compounds under oxic conditions. Such a phenomenon will provide accurate predictions of fixed bed adsorbers in water treatment systems.     

Influence of progressive surface oxidation of nitrogen-containing carbon on its electrochemical behaviour in phosphate buffer solutions

The effects of the nitric acid surface oxidation of nitrogen-containing carbons (SCN-type) on the chemical structure as well as the electrochemical properties of powdered electrodes prepared from them were studied. The oxidation efficiency was dependent on the duration of the oxidative modification. The surface chemistry was characterized using standard neutralization techniques and spectroscopic methods (FTIR and XPS). Electrochemically active surface groups obtained on carbon materials during oxidation were investigated by cyclic voltammetry. Some noteworthy relations between the electrochemical activity of carbon electrode material and the degree of surface oxidation are reported.     

Influence of HNO3 oxidation on the structure and adsorptive properties of corncob-based activated carbon

An investigation of the impact of strong oxidation with HNO₃ on the porosity and adsorption characteristics of char and activated carbons, derived from corncobs, is presented. Texture parameters, as obtained from N₂ adsorption at 77 K, showed a considerable decrease in surface area of the activated carbons with enhanced pore widening. The extent of porosity modification was found to depend on the scheme of activation of the precursor, simple carbonization, steam pyrolysis, steam gasification of the char, or chemical activation with H₃PO₄. Surface-chemical changes were detected by FTIR spectroscopy, where absorption bands assigned to carboxyl, carboxylate, carbonyl, and phenolic groups were observed. A SEM study demonstrated the erosive effect of HNO₃, detected by the presence of disintegration of the carbon grains, with the porous structure probably containing very large macropores. As a consequence of the oxidation process, elemental analysis showed high contents of O, H and N, and TG confirmed that the weight loss distribution in the thermogram becomes slower at higher temperatures. The removal of phenol decreased as a result of the formation of oxygen functionalities. Mono-nitrophenols were adsorbed in smaller amounts than phenol, and p-nitrophenol showed a relatively higher uptake than the other two mono-nitrophenols, whereas the uptake of Methylene Blue was improved. Removal of Pb²⁺ from aqueous non-buffered solution was considerably enhanced by chemical oxidation, which may be related to pore widening, increased cation-exchange capacity by oxygen groups, and the promoted hydrophilicity of the carbon surface.     

Influence of surface properties and iron addition on the SO2 adsorption capacity of activated carbons

When iron derivatives are added to low ash activated carbons having basic surface characteristics (obtained by suitable oxidation at 800°C with 2% of O₂ in N₂), certain materials are obtained showing high SO₂ sorbent properties from gaseous mixtures having a composition close to that of the flue gases. This behaviour seems to be related to the simultaneous presence of both basic surface sites promoting the initial adsorption of SO₂ and iron promoting the transformation of the adsorbed SO₂ into other, more stable forms. The sorbent properties of these activated carbons are more stable, following consecutive cycles, in the processes of adsorption and desorption of SO₂, than those shown by similar carbons with different surface characteristics.