Electrochemical surface oxidation of carbon nanofibers

Carbon nanofiber (CNF) surfaces were functionalized with oxygen-bearing groups through electrochemical oxidation. The electrode was prepared without a binder, allowing easy separation of the functionalized CNFs for subsequent applications. The relationships between the applied potential and the CNF structure with the resulting O/C atomic ratio and the distribution of oxygen functional groups were investigated. Surface groups were identified and characterized by elemental analyses, X-ray photoelectron spectroscopy, micro-attenuated total reflectance FTIR, and cyclic voltammetry. The oxidation of herringbone CNFs was initiated at a relatively low potential at both the anodic and cathodic electrodes, while the O/C atomic ratio remained relatively constant within the range of potentials investigated. The relative concentration of carbonyl and hydroxyl groups increased with increasing potential while the amount of carboxylic groups decreased. The structure of the CNF was important in determining the O/C atomic ratio, which was especially dependent on the spatial arrangement of graphene layers. Tubular CNFs exhibited low O/C atomic ratios while herringbone CNFs, which have a higher surface area, exhibited the largest ratios. The dispersion of the CNFs in water was much more homogeneous following electrochemical oxidation.     

A comparison of surface morphology and chemistry of pyrolytic carbon blacks with commercial carbon blacks

The surface morphology and chemistry of CBp obtained by pyrolysis of waste tyres at 500 and 700 °C, respectively was studied compared with a commercial tyre carbon blacks by laser particle size analyzer, X-ray diffractogram (XRD) and electron spectroscopy for chemical analysis (ESCA). The distribution of CBp aggregates was the mixed particle distribution of commercial carbon blacks added to tyres in fabrication. The concentration of inorganic compounds and carbonaceous deposits (the organic compounds deposited on the surface of the CBp) depends on the pyrolysis temperature. The chemical nature of the CBp from pyrolysis at 700 °C was found to be closer to the commercial tyre carbon blacks than the CBp from pyrolysis at 500 °C.     

The chemistry and physics of coking

The causes of coke formation during petroleum refining are only now beginning to be understood. They are closely related to the mechanism of the thermal decomposition of the petroleum Constituents and to changes in the character of the liquid medium. It was formerly believed that coke formation was, a polymerization reaction whereupon the chemical precursors to coke immediately formed macromolecules when subject to the processing temperatures. This is not so. And it is the initial stages of the thermal decomposition which determine the ultimate path of the reaction. Coke formation is a complex process involving both chemical reactions and thermodynamic behavior. Reactions that contribute to this process are cracking of side chains from aromatic groups, dehydrogenation of naphthenes to form aromatics, condensation of aliphatic structures to form aromatics, condensation of aromatics to form higher fused-ring aromatics, and dimerization or oligomerization reactions. Loss of side chains always accompanies thermal cracking, and dehydrogenation and condensation reactions are favored by hydrogen deficient conditions.     

吸附电活性染料的活性炭电极电化学性能

采用商用活性炭（AC）吸附二元混合染料亚甲基蓝（MB）和胭脂红（AR18）,制备得到AC/(MB+AR18)电极材料。比较单一活性炭（AC）和吸附不同浓度的二元混合染料后的活性炭[AC/(MB+AR18)]的电化学性能。三电极体系的测试结果表明：在1mol/L H₂SO₄电解液中,当电流密度为1A/g时,吸附了浓度为400mg/L污染物的AC/(MB+AR18)比电容为182F/g,高于单一AC的比电容（109F/g）。随后选用性能最优的AC/(MB+AR18)-400作为电极材料,组装对称超级电容器器件,发现工作电压窗口从只用AC组装的对称超级电容器的1.1V提高到1.5V,电流密度为0.75A/g时,功率密度为843.84W/kg,能量密度可达32.23W·h/kg,远远高于AC组装的超级电容器(4.74W·h/kg),说明MB和AR18不仅为AC提供额外的法拉第电容,同时有助于提高其工作电压窗口。     

Steel surface chemistry affecting the performance of organic coatings

The surface chemistry of steel sheets plays an important role in industrial operations such as cold forming, phosphatizing and painting. Although the surface of the steel sheet is uniformly covered with a thin (about 10 nm or less) iron oxide film, carbonaceous contaminants and various enriched elements such as manganese and silicon are present on it. The surface layers are ultimately formed when the steel sheets are annealed for softening in the sheet production process. Since the carbonaceous contaminant causes deterioration of the pre-treatment (phosphatizing), it facilitates paint failure in a corrosive environment (salt spray test). One of the possible reasons why the phosphatability is sensitive to the surface condition is that the formation of finely grained phosphate crystals depends greatly on the ability to adsorb titanium colloids, which act as nucleating agents.     

Screen-printed carbon electrode modified on its surface with amorphous carbon nitride thin film: Electrochemical and morphological study

The surface of a screen-printed carbon electrode (SPCE) was modified by using amorphous carbon nitride (a-CN_x) thin film deposited by reactive magnetron sputtering. Scanning electron microscopy and photoelectron spectroscopy measurements were used to characterise respectively the morphology and the chemical structure of the a-CN_x modified electrodes. The incorporation of nitrogen in the amorphous carbon network was demonstrated by X ray photoelectron spectroscopy. The a-CN_x layers were deposited on both carbon screen-printed electrode (SPCE) and silicon (Si) substrates. A comparative study showed that the nature of substrate, i.e. SPCE and Si, has a significant effect on both the surface morphology of deposited a-CN_x film and their electrochemical properties. The improvement of the electrochemical reactivity of SPCE after a-CN_x film deposition was highlighted both by comparing the shapes of voltammograms and calculating the apparent heterogeneous electron transfer rate constant.     

Electrochemical studies of octadecanethiol and octanethiol films on variable surface area mercury sessile drops

Octadecanethiol (ODT) and octanethiol (OT) films at the mercury-electrolyte interface are examined using cyclic voltammetry and differential capacitance measurements at a single frequency. A mercury flow-system is used to alter the volume, and therefore, the surface area and surface pressure of the mercury electrode. Manipulation of the mercury electrode's volume enables the introduction and removal of defects in the insulating thiol films. OT and ODT film behavior are contrasted under conditions of expansion and contraction. ODT forms extremely impermeable layers that allow 1000 time less redox probe current than seen on uncoated drops. Expansion of the mercury electrode to increase the electrode surface area produces defects and pinholes in the thiol film. These defects are almost completely removed when the drop is compressed back to its initial surface area. OT also forms insulating films on mercury sessile drops, however these films contain more defects than ODT films. While expansion of an OT-coated mercury drop increases redox probe current, recompression of the drop does not return the film to its initial condition. Pinholes and defects in the OT and ODT films can also be produced by cycling to negative potentials, which produce abrupt stripping peaks.     

Electrochemical performance of nano-Pt-supported carbon anode for lithium ion batteries

A nano-Pt-supported carbon anode was prepared by supporting Pt nanoparticles onto carbon powder. Ultrafine Pt nanoparticles could be well distributed on the surface of carbon particles. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), cyclic voltammetry (CV), FTIR, and impedance spectroscopy were used to study on the electrode structure and electrochemical performance. Nano-Pt-supported carbon anode enhanced the Li discharge reaction and suppressed the solvent decomposition reaction, which are favorable for lithium batteries.     

New insights into dye chemistry and physics

A review of the academic dye research performed by the Port Sunlight group and its coworkers over the past 15 years is presented. The work is focused on three areas: (1) substrate structure; (2) dye interactions in aqueous solution and on substrates; (3) dye degradation and products. For substrates, a detailed model of the nanoenvironment experienced by chemicals within cellulose fibre is given, showing the different environments and remarkable mobility of absorbed chemicals. Advanced nuclear magnetic resonance diffusion measurements provide the complex pathways by which compounds find their way in and out of the fibre. For dye interaction, detailed theoretical and experimental studies are reported on three model dye systems, the anionic monoazo dye Orange II, the bisazo anionic dye CI Direct Blue 1, and cationic monoazo thiazolium dyes, providing a comprehensive picture of their structure. A quantitative mechanism of dye binding to cellulose is shown. Resonance Raman provides an effective forensic tool for dye identification, even from single fibres. The products and kinetics of Orange II dye degradation by one‐electron reduction in aqueous solution is given, with the identification of an indophenol dye end‐product. In cellulosic materials the reduction mechanism is similar to solution, when the higher microviscosity is accounted for. Hydrolysis of thiazolium dyes occurs at both aromatic rings of the dye but on different timescales. Measurement and calculations of the electronic structures of one‐electron‐reduced and ‐oxidised dyes are presented. The mechanism of photooxidation by sunlight of azo dyes in cotton is delineated.     

Enhanced electrochemical and structural properties of carbon cryogels by surface chemistry alteration with boron and nitrogen

Resorcinol-formaldehyde (RF) derived carbon cryogels (CCs) with narrow pore size distribution were obtained via chemical modification using ammonia borane (AB). This chemical modification was achieved by adding AB to the hydrogels during the solvent exchange stage. Nitrogen sorption analysis, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy are used to investigate the pore structure, morphology, and electrochemical properties of the modified CCs. After pyrolysis, the AB modified CCs show an increased surface area and larger mesopore volume in comparison to the untreated precursor. In addition, a uniform porous structure with a narrow pore size distribution is produced with a mesopore diameter that does not change significantly during pyrolysis. Moreover, electric double-layer supercapacitors (EDLS) used to characterize the AB modified samples shows pseudocapacitive behavior, higher current density and capacitance.     

Role of activated carbon structural properties and surface chemistry in mercury adsorption

In this work the performance of activated carbons prepared from raw and demineralised lignite for gas-phase Hg° removal was evaluated. A two-stage activation procedure was used for the production of the activated samples. In order to study the effect of mineral matter on pore structure development and surface functionality of the activated carbons, a demineralisation procedure involving a three-stage acid treatment of coals, was used, prior to activation. Hg° adsorption tests were realized in laboratory-scale unit consisted of a fixed-bed reactor charged with the tested activated samples. The examined adsorbent properties that may affect removal capacity were the pore structure, the surface chemistry and the presence of sulphur on the surface of activated carbons. The obtained results revealed that activated carbons produced from demineralised lignite posses a high-developed micropore structure with increased total pore volume and BET surface area. These samples exhibit enhanced Hg° adsorptive capacity. In all cases, mercury removal efficiency increased by sulphur addition. Finally, the starting material properties and activation conditions affect the concentration and the type of the oxygen groups on activated carbon surface, that have been determined with TPD-MS experiments.     

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.     

碳纤维在不同电解质体系下的电化学表面改性

使用电化学氧化法对PAN基碳纤维进行表面改性,通过扫描电子显微镜（SEM）、Raman光谱和X射线光电子能谱仪（XPS）表征了碳纤维表面物理化学结构,同时结合力学性能分析,评价了碳酸氢铵、硫酸铵和复合铵盐溶液3种电解质体系的改性效果。实验结果表明,碳酸氢铵溶液下的电化学改性有利于界面粘结强度的提高,而硫酸铵溶液下的电化学改性有利于降低抗拉伸强度的损失,当采用复合溶液改性时,则可以同时提高碳纤维的抗拉伸强度和其复合材料的抗层间剪切强度。     

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.     

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.     

Studies in surface chemistry of carbon blacks-vi. adsorption isotherms of benzene on carbons associated with different surface oxygen complexes

Adsorption isotherms of benzene (35°C) on carbon blacks are of Type II and show complete reversibility. The presence of CO₂-complex, which imparts polar and hydrophilic character to the surface, suppresses the sorption of benzene but its elimination and the emergence of CO-complex as the only predominant complex enhances the sorption at all r.v.p's. The additional sorption amounts roughly to one molecule of benzene per quinonic oxygen as obtained by sodium borohydride treatment. This indicates probability of interaction of π electrons of benzene ring with partial positive charge on carbonyl carbon atom. The isosteric heat of adsorption is also higher in the presence of quinonic oxygen. The use of benzene isotherms for estimating surface area or pore volume of carbons may, therefore, be looked upon with caution. The isotherm on sugar charcoal also shifts bodily upward with the emergence of CO-complex but the adsorbed benzene cannot be recovered even on prolonged evacuation, showing that the benzene complexed at the quinonic sites within the microporous system requires activation energy to diffuse out of the pores. The interaction of benzene, due to the presence of π electrons, seems to be specific, since the isotherms of cyclohexane on a given carbon black are identical irrespective of the nature or amount of any of the surface oxygen complexes present in it.     

Characterization of the surface chemistry of carbon materials by potentiometric titrations and temperature-programmed desorption

Two methods currently used to characterize oxygen-containing functional groups on the surface of carbon materials are compared, namely potentiometric titration (PT) and temperature-programmed desorption (TPD). Two materials were used, activated carbon and multi-walled carbon nanotubes, which were subsequently modified by oxidative treatments in order to produce samples with different amounts of surface groups. The concentrations of carboxylic acid groups determined by both techniques are in good agreement, but the quantitative result obtained by TPD is closer to the analytical concentration of the species obtained by elemental analysis. Discrepancies between the quantitative results are more pronounced at higher pK_a values (weak acids), where the concentrations determined by PT are lower than those obtained by TPD. This directly reflects the effects of neglecting the electrostatic interaction parameter. The TPD method was particularly suited for the characterization of samples modified with ethylenediamine, which is anchored to specific oxygenated groups. PT results are useful to describe the material behaviour in aqueous solutions, where the activity of the surface groups depends not only on their concentrations, but also on their environment.