Characterization of activated carbon fiber/polyaniline materials by position-resolved microbeam small-angle X-ray scattering

Activated carbon fiber (ACF)/polyaniline (PANI) materials have been prepared using two different methods, viz. chemical and electrochemical polymerization. Electrochemical characterization of both materials shows that the electrodes with polyaniline have a higher capacitance than does a pristine porous carbon electrode. To analyze the distribution of PANI within the ACF, characterization by position-resolved microbeam small-angle X-ray scattering (μSAXS) has been carried out. μSAXS results obtained with a single ACF indicate that, for the experimental conditions used, a PANI coating is formed inside the micropores and that it is higher in the external regions of the ACF than in the core. Additionally, it seems that the penetration of PANI inside the fibers occurs in a larger extent for the chemical polymerization or, in other words, for the electrochemically polymerized sample there is a slightly larger accumulation of PANI in the external regions of the ACF.     

Characterization of pore distribution in activated carbon fibers by microbeam small angle X-ray scattering

In the present work, the results corresponding to the first experiments done with single activated carbon fibers (ACFs) at the microfocus beamline (ID13) in the ‘European Synchrotron Radiation Facility’ (Grenoble) are presented. The experiments done with CO₂ and steam ACFs have demonstrated the suitability of this technique to characterize a single ACF. The experiments show that scattering intensity increases with the burn-off degree, which agrees with SAXS experiments carried out using bigger amounts of fibers. Moreover, the two-dimensional scattering patterns show that, in this type of ACFs, the porosity development during the activation process is isotropic. In addition, it has been demonstrated that the use of an X-ray microbeam of 2 μm diameter allows the characterization of different regions of the same fiber with microscopic position resolution. The scans across the fiber diameter are the first direct proof for the previous results obtained by our research group. Thus, in the case of CO₂ ACFs, the scattering is high in different regions across the fiber diameter, confirming that CO₂ activation takes place within the fibers, generating a quite homogeneous development of porosity. On the other hand, in the case of steam ACFs, the scattering is much higher in the external zones of the fibers than in the bulk, which means that steam focuses the activation in the outer parts of the fibers.     

聚丙烯腈基活性碳纤维吸附CO2的性能

以国产聚丙烯腈(PAN)基碳纤维为原料,采用KOH为活化剂制备PAN基活性碳纤维。测定了不同ACF样品的CO2吸附量,并通过氮气吸附、碘吸附以及红外光谱对所得活性碳纤维的比表面积、孔结构及表面官能团进行表征。研究了活化温度、活化时间和表面改性对活性碳纤维CO2吸附量的影响。结果表明,活化温度是影响活性碳纤维CO2吸附量的主要因素。当活化温度为850℃时,所得活性碳纤维BET比表面积为1235m2/g,微孔比表面积为745 m2/g,在吸附温度为273 K、吸附相对压力P/P0为1时,CO2的吸附量达到87.29 mL/g。     

Activation of coal tar pitch carbon fibres: Physical activation vs. chemical activation

Activated carbon fibres (ACF) are obtained mainly by physical activation with steam or carbon dioxide. Additionally, there are many papers dealing with chemical activation of carbon fibres, or a polymeric raw material, with several chemical agents like for example, phosphoric acid, zinc chloride, aluminium chloride,… Nevertheless, although it is well known that hydroxides are good activating agents, there are few papers about the activation of carbon fibres with KOH or NaOH. In the present work, ACF with high surface area are obtained by chemical activation with KOH and NaOH. Both chemical agents present different behaviour; thus, NaOH developed the highest value of porosity and KOH developed samples with narrower micropore size distribution. In order to compare the results with those obtained by physical activation, some ACF have been prepared using CO₂ activation. The main conclusion of this work is that by using chemical activation it is possible to obtain similar, or even higher, porosity (∼1 ml/g, ∼3000 m²/g) than by physical activation. However, chemical activation presents two important advantages: (1) a much higher yield (27-47% for chemical activation and 6% physical activation for ∼2500 m²/g activated carbon fibres) and (2) the surface of the fibres prepared by chemical activation is less damaged than by physical activation.     

Effect of activation on the carbon fibers from phenol–formaldehyde resins for electrochemical supercapacitors

Activated carbon fibers (ACF) are prepared from phenol–formaldehyde resin fibers through chemical activation and physical activation methods. The chemical activation process consisted of KOH, whereas the physical activation was performed by activation in CO₂. The characteristics of the electrochemical supercapacitors with carbon fibers without activation (CF), carbon fibers activated by CO₂ (ACF-CO₂), and carbon fibers activated by KOH (ACF-KOH) have been compared. The activated carbon fibers from phenol–formaldehyde resins present a broader potential range in aqueous electrolytes than activated carbon and other carbon fibers. Activation does not produce any important change in the shape of starting fibers. However, activation leads to surface roughness and larger surface areas as well as an adapted pore size distribution. The higher surface areas of fibers treated by KOH exhibited higher specific capacitances (214 and 116 F g⁻¹ in aqueous and organic electrolytes, respectively) and good rate capability. Results of this study suggest that the activated carbon fiber prepared by chemical activation is a suitable electrode material for high performance electrochemical supercapacitors.     

KOH活化制备高比表面积窄孔径分布的活性炭纤维的研究

以ＫＯＨ溶液浸渍聚丙烯腈系预氧织物为原料,制备了高比表面积窄孔径分布的活性炭纤维,考察了ＫＯＨ浸渍量、活化温度、活化时间等因素的影响,并对活性炭纤维的孔结构及其吸附性能进行了分析。结果表明：ＫＯＨ的浸渍量,活化温度,活化时间都有一最佳值,苯吸附较碘吸附更能反应该活性的吸附性能。     

聚丙烯腈基碳纤维灰分的来源及其影响因素

研究了聚丙烯腈(PAN)原丝及PAN基碳纤维灰分的形态结构及化学成分,分析了碳纤维灰分产生的原因及其影响因素。结果表明:PAN原丝灰分及碳纤维灰分主要由C、O及Si三种元素组成,并且Si元素为灰分的主体成分;原丝灰分与碳纤维灰分之间存在线性关系;油剂种类及上油率是影响原丝及碳纤维灰分含量的主要因素;油剂中硅含量高,上油率高均会导致原丝及碳纤维灰分的大幅提高。     

New insights on the direct activation of isotropic petroleum pitch by alkaline hydroxides

The paper provides interesting evidences that a low softening point isotropic petroleum pitch can be used as a good carbon precursor for the preparation of activated carbons. The activation is carried out by KOH and/or NaOH and the resulting activated carbons present well developed porosity. Such hydroxide activations can be done directly on the pristine petroleum pitch (P) or on the pitch that has been submitted to an air stabilisation followed by a N₂ heat treatment (TAN). In general, KOH activation produces better results than NaOH, both in terms of porosity and yield, the results obtained for the activation of TAN being impressive because of the good porosity developments and high yields reached. The different treatments carried out over the petroleum pitch precursor clearly show that they significantly influence the extent of microporosity development. This is due to different changes occurring in the porous structure of the precursor as a function of the treatment carried out. The efficiency of the activation process increases as the mesophase content of the precursor decreases, as well as the mesophase formation during the activation process is avoided.     

Novel synthesis and characterization of activated carbon fiber and dye adsorption modeling

The main objective of this work is to prepare activated carbon fibers (ACF), analyze a synthesis mechanism of those fibers, and develop a new dye adsorption model. The surface chemical structures of the synthesized viscose rayon phosphates and ACF were analyzed using TOF-SIMS and ATR FT-IR. After steam-activation of carbon fiber at high temperature, the carboxyl group could not be observed due to the high temperature activation. Only the oxygen-contained carbon ring groups appeared. The adsorption mechanism of the developed model in this study, the bottle-neck model, was simple to understand and apply to the industrial adsorption plants. The model could predict theoretical concentration versus time or dye concentration in an ultra accurate manner in the medium and low concentration regions, which could not previously be attempted by other models.     

Distribution of potassium during chemical activation of petroleum coke: Electron microscopy evidence and links to phase behaviour

In this study, direct evidence for chemical penetration into petroleum coke particles during activation is presented. In addition, the porosity development was directly related to the sulphur loss and phase behaviour of the species present. Petroleum coke (petcoke, 6 wt.% sulphur) was activated with KOH and NaOH at temperatures between 400 and 800°C. The C S bonds were broken before 400°C in the presence of KOH and before 500°C in the presence of NaOH. Electron microscopy analysis of cross-sectioned and ultramicrotomed samples revealed that sulphur was still present within the particles and that the hydroxide activation agents had penetrated to the centre of the particles (90–150 μm). After heating to 800°C and washing with a weak acid aqueous solution, essentially all the sulphur was removed, as was any remaining chemical agent. The characterization results, phase diagrams, and complementary experiments with carbonate chemical agents or steam suggest that, during heating, a molten phase formed around the petcoke particles. The composition of this molten phase changed as activation proceeded and both sulphur and ash components were liberated from the petcoke. This better understanding of the activation process will improve the efficiency of preparing activated carbon.     

KOH and NaOH activation mechanisms of multiwalled carbon nanotubes with different structural organisation

New information was obtained on the mechanism of porosity development during chemical activation by KOH and NaOH using various multiwalled nanotubes (MWNTs) of different structural organization. The high purity MWNTs were prepared by acetylene decomposition on a cobalt-based catalyst at different temperatures. The obtained samples ranged from MWNTs with well organised graphitic walls to nanotubes with disorganised layers mixed with some pyrolytic carbon when decreasing synthesis temperature. The results of transmission electron microscopy (TEM) observations were linked with gas adsorption measurements and X-ray diffraction data. They show that NaOH is only effective with disordered materials whereas KOH is effective whatever the structural order. After reaction of the poorly ordered precursor with KOH, the nanotubular morphology is completely destroyed, whereas it is preserved when NaOH is used. However for the more ordered materials, the morphology remains unchanged with both reactants. Effects of activation are only seen with KOH, which generated a large concentration of defects in the nanotubes walls. The differences found between KOH and NaOH during activation are related with an additional intercalation step of metallic K or Na produced during the redox reactions. It is shown that metallic K has the ability to be intercalated in all materials in contrast with Na which can only intercalate in the very disorganised ones. The conclusions obtained from the study on ordered nanotubes were confirmed with an ordered carbon black, demonstrating that the structural organization of the carbon precursor is an important parameter which must be taken into account when alkali reactants are used for the activation.     

活性炭纤维及其吸附特性

活性炭纤维（ACF）作为活性炭（AC）的第三代产品,被用作高性能吸附剂。通过对ACF制备过程,孔结构与表面化学结构的介绍,充分说明ACF其优异的吸附性能,以及吸附的有关理论知识。     

Hydrogen adsorption by nanostructured carbons synthesized by chemical activation

In this work several samples of Quercus agrifolia activated carbon, the porous structure of which was nanostructured by chemical activation with NaOH and KOH, were evaluated for hydrogen adsorption at 77 K and atmospheric pressure. Hydrogen adsorption reached values in the order of 2.7 wt.% for KOH activated carbon. The mechanism of formation of the porous nanostructures was found to be the key factor in controlling the hydrogen adsorption capacity of chemically activated carbon.     

Analysis of the microporosity shrinkage upon thermal post-treatment of H3PO4 activated carbons

Starting from a commercial pelletized phosphoric acid based activated carbon, with a typical opened and developed micro and mesoporosity, a post-heat-treatment in KOH, at different KOH/activated carbon ratios, has been studied. In all the cases, a pore size shrinkage has been observed. To find an explanation for the reason of this micropore size distribution shrinkage different factors have been studied, among them: (a) effect of the presence of impurities coming from the activation process with phosphoric acid; (b) effect of the KOH post-treatment temperature; (c) heat-treatment temperature of the precursor (without chemical agent); (d) effect of the reagent nature (NaOH, NaCl and KCl vs. KOH). The variable that produces the most intense shrinkage effect, and the disappearance of the mesoporosity, is the heat-treatment in presence of hydroxide, which affects even using a low hydroxide/activated carbon ratio. Such a low hydroxide/activated carbon ratio does not produce activation, nor porosity development of the starting activated carbon during the treatment. This shrinkage phenomenon, which seems to be independent of the method of preparation used to prepare the activated carbon, can be understood considering our previous studies about the reactions involved during chemical activation by hydroxides.     

Preparation of carbon fiber from isotropic pitch containing mesophase spheres

Naphthalene based synthetic isotropic pitch was found to give spinnable precursors which carried 10–100 μm anisotropic spheres of 45 and 50 vol% in the isotropic matrix, when the pitch was heat-treated at 375 and 380 °C for 20 and 23 hours, respectively. The spun fiber was stabilized and carbonized into a carbon fiber, which showed anisotropic belts running along the fiber axis, suggesting that both anisotropic spheres and isotropic matrix are deformed to align within the nozzle. The structure and mechanical properties of the carbon fibers, thus prepared, were examined under microscopes and by mechanical testing.     

电子束辐射接枝对PAN基碳纤维的表面改性

采用电子束加速器辐射接枝方法对聚丙烯腈(PAN)基碳纤维进行表面改性,研究了接枝单体种类对接枝率及其环氧树脂基复合材料力学性能的影响,分析了辐射接枝前后PAN基碳纤维的表面形貌与化学结构以及其复合材料界面断口的形貌变化。结果表明:电子束辐射接枝改性的PAN基碳纤维表面粗糙度增加,表面活性官能团增多,与树脂的机械锲合作用增强,其树脂基复合材料断口表而较为平整;乙二胺/水溶液体系是辐射接枝改性的理想溶液,在200 kGy的电子束辐射下,PAN基碳纤维表面的接枝率为6.66%,复合材料的层间剪切强度提高了45.1%。     

碳化硅纤维制备技术研究进展

综述了工业化制备碳化硅 (Si C)纤维技术的研究进展 ,并对化学气相沉积 (CV D)法、先驱体转化法和活性炭纤维 (A CF )转化法这三种碳化硅纤维制备方法的原理、生产工艺路线和各自的优缺点作了系统的评述。认为 CV D法已被淘汰 ,先驱体转化法是目前 Si C纤维制备研究的方向 ,A CF法是实现Si C纤维的应用最佳方法     

国外PAN基碳纤维的研究现状及发展趋势

简介了目前世界PAN基碳纤维生产及消费情况,综述了近年来国外关于PAN基碳纤维的研究现状,指出我国在研发PAN基碳纤维时应注意降低成本及生产的环保性。     

Effects of improved porosity and electrical conductivity on pitch-based carbon nanofibers for high-performance gas sensors

Pitch-based carbon fibers with multi-walled carbon nanotubes (MWCNTs) were fabricated via an electrospinning method and used as gas sensor electrodes. The pitch-based carbon fibers were treated at various temperatures to investigate the effect of the reaction temperature. The electrospun fibers were thermally treated to produce carbon fibers, and the resulting material was chemically activated to increase the number of active sites for efficient gas adsorption. The activation process improved the porous structure by increasing the specific surface area by approximately 86-fold. Due to the improved porosity and electrical conductivity, gas adsorption sites were enlarged and electron transfer was improved, resulting in a high-performance NO gas sensor with improved sensitivity and rapid response time. The improved porosity was attributed to the chemical activation process, and the enhanced electrical conductivity was attributed to the heat treatment and the addition of MWCNTs.     

Conversion of polyacrylonitrile fibers to activated carbon fibers: Effect of activation

Polyacrylonitrile (PAN)-based activated carbon fibers were developed with the idea of increasing their potential and efficiency in industrial applications. The PAN-based fibers were first oxidized in air in a continuous multistage stabilization process, and then subjected to a continuous, low temperature carbonization and activation treatment in a mixture of steam and argon. The effect of the activation condition on the specific surface area, elemental composition, as well as the morphological structure of activated carbon fibers was studied. The surface area of the carbon fibers increased remarkably after the steam activation. It was found that steam activation promoted the elimination of nitrogen from the fiber. The bulk oxygen content of the fibers increased upon activation, probably due to formation of carbon-oxygen functionalities on the surface of the activated carbon fibers. The surface oxygen level of activated carbon fibers was greater than the bulk analysis of oxygen. It was observed that activation decayed the order of the carbon structure. © 1996 John Wiley & Sons, Inc.