Applications of Active Carbon Fibers

Nawa, M., Nogami, T. and Mikawa, H., Application of activated carbon fiber fabrics to electrodes of rechargeable battery and organic electrolyte capacitor, J. Electrochem. Soc., 1984, 131(6), 1457 1459. 

Detailed accounts of fibers and carbon-carbon composites can be found in several recently published books [1-5]. Here, details of novel carbon fibers and their composites are reported. The manufacture and applications of adsorbent carbon fibers are discussed in Chapter 3. Active carbon fibers are an attractive adsorbent because their small diameters (typically 6-20 pm) offer a kinetic advantage over granular activated carbons whose dimensions are typically 1-5 mm. Moreover, active carbon fibers contain a large volume of mesopores and micropores. Current and emerging applications of active carbon fibers are discussed. The manufacture, structure and properties of high performance fibers are reviewed in Chapter 4, whereas the manufacture and properties of vapor grown fibers and their composites are reported in Chapter 5. Low density (porous) carbon fiber composites have novel properties that make them uniquely suited for certain applications. The properties and applications of novel low density composites developed at Oak Ridge National Laboratory are reported in Chapter 6. 

The major potential application of active carbon fibers is as an adsorbent in environmental control, as outlined in the previous section. However, there is a number of smaller scale, niche applications that seem to be particularly suited to ACF. These emerging applications include the use of ACF in medicine [111 (see also 59,60),112], as capacitors [113-119] and vapor sensors [120], and in refrigeration [121]. The first two of these applications are summarized below. However, there are not many detailed, publicly-available sources describing any of these applications, partly for commercial reasons and partly because the technology is emerging, so any summary is necessarily limited in scope. 

There is growing interest in the development and application of activated carbon fibers (ACF), whose unusual properties can be advantageous in certain applica-... 

Most research and development has focused on the use of granular or extruded activated carbons, and all commercial processes to date use this technology. However, within the last several years there has been appreciable research to examine the application of activated carbon fibers. Activated carbon fibers (ACEs) have... 

Lee, S. (2010). Application of Activated Carbon Fiber (ACF) for Arsenic Removal in Aqueous Solution. Korean, JowwaZ of Chemical Engineering 27, 110-115. 

S02 and NOx in flue gas from coal combustion contribute to smog and acid rain. Methods to remove these pollutants include alkaline wet scrubber systems that fix S02 to solid CaS04, and selective catalytic reduction by metal/metal oxide systems of NO/NOz to N2 and steam in the presence of ammonia. Particulate active carbons have also been used in flue gas decontamination, especially as they avoid costly scrubber processes and can operate at lower temperatures. The potential of active carbon fibers in this application has been explored by a... 

Brasquet, C., Roussy, J., Subrenat, E. and Le Cloirec, P., Adsorption and selectivity of activated carbon fibers application to organics, Environ. Technol., 1996, 17(11), 1245 1252. 

In this chapter, we present in some detail gas adsorption techniques, by reviewing the adsorption theory and the analysis methods, and present examples of assessment of PSDs with different methods. Some examples will show the limitations of this technique. Moreover, we also focus on the use of SAXS technique for the characterization of porous solids, including examples of SAXS and microbeam small-angle x-ray scattering (pSAXS) applications to the characterization of activated carbon fibers (ACFs). We remark the importance of combining different techniques to get a complete characterization, especially when not accessible porosity exists. 

To date, activated carbon is the most universal adsorbent for VOCs control. However, some disadvantages for the application of activated carbon include its flammability, difficulty in regenerating high-boiling point solvents and required humidity control. On the positive side, activated carbon fiber has uniform size and dimension, higher adsorption capacity, faster adsorption and desorption rates than activated carbon, and ease of handling [1,2]. These features obtain adsorptive system size reduction and added adsorbed vapor selectivity. In these respects, activated carbon fiber, as alternative to activated carbon inefficiencies, is an excellent micro-pore adsorbent. 

In the present work, positron annihilation lifetime spectroscopy has been applied to characterize the porosity of activated carbons fibers. These materials are essentially microporous [16], with slit shaped pores and with a homogeneous pore size distribution. Because of that, they seem to be the most appropriate materials to analyze the application of PALS technique to the characterization of porous carbon materials. 

Much less ordered than PAN-based high-strength CFs are the isotropic CFs. They are produced by the carbonization of isotropic pitch fibers (or other fibrous precursors such as phenolic resins or cellulose, including rayon), without any attempt to obtain a preferred orientation of the polyaromatic molecules in the fiber direction. Consequently, they have a random nanotexture and belong to the low modulus class of CFs [16]. Rather than being used for high-performance reinforcement purposes, they find their application as thermal insulators for furnaces or as reinforcements for cement [1]. Another important use of isotropic CFs is as a feedstock for the production of activated carbon fibers, a material dealt with in Section 2.4.4. 

The next two chapters deal mainly with the use of adsorption to characterize porous solids. In the case of activated carbon fibers (Chapter 17), methods to characterize microporosity, and particularly ultramicroporosity, by physical adsorption are of particular relevance for understanding the behavior of these adsorbents and extending the range of their applications. Moreover, in Chapter 18 the pore structure of ordered mesoporous carbons is shown to differ greatly from that of conventional activated carbons for which most of the available data treatment methods have been developed. Therefore, suitable procedures for correctiy analyzing the pore structure of these novel carbons are proposed in this chapter. 

Among the mechanical properties of greatest practical impact on catalysis applications is the attrition and crushing resistance of powdered or granular activated carbons, the most commonly used catalytic carbon materials, versus that of activated carbon fibers (ACFs) or of other, less-surface-active carbons (e.g.. 

Other advantage of ACFs is the possibility to prepare woven clothes and nonwoven mats, which developed new applications in small purification systems for water treatment and also as a deodorant in refrigerators in houses, recently reported. In order to give the fibers an antibacterial function and to increase their deodorant fimetion, some trials on supporting minute particles of different metals, such as Ag, Cu and Mn, were performed. Table 5. 2 presented comparison between properties of activated carbon fibers and granular activated carbons [13-46]. 

Our previous papers [15,16] and the current work show that die imprinting of mesophase pitch particles with colloidal silica is an efficient technique to prepare mesoporous carbons with uniform spherical pores as well as carbons with bimodal pore size distributions. These carbons exhibit negligible amount of micropores, which can be further eliminated during graphitization process. If micropores are need, they can be created by controlled oxidation analogous to that used in the preparation of activated carbon fibers. The possibility of tailoring the size of uniform spherical mesopores is of great importance for catalysis, adsorption and other advanced applications such as die manufacture of hi -quaiity electrochemical double-layer capacitors, fuel cells and lidiium batteries. 

This linear form of Ihe BET equation has been used by Parra et to calculate the BET parameters, viz. the surface area and the c constant and also to determine the relative pressure range for the applicability of the BET equation in the case of two series of activated carbons and two samples of activated carbon fibers. The results obtained by the alternative equation have been compared with those obtained from the classical linear form of Equation 2.74. 

For applications that do not require exceptional mechanical properties, carbon fibers made from high performance aramid polymers show considerable potential. These aramid fibers do not require stabilization prior to carbonization, which substantially simplifies the production process. Rayon-based carbon fibers continue to appear in some composite applications, but have become key substrates for the development of activated carbon fibers. These ACFs develop a microp-orous surface structure that is ideal for adsorption of low levels of volatile organic compounds. 

Monolithic composites of activated carbon fibers with phenolic resin as the binder have been prepared for a variety of possible applications, including gas separation (Burchell, 1999 Kimber et al., 1996). The low-density composites with densities <0.25 g/cm are particularly promising for gas separation as well as energy storage (CH4 and H2) applications. A detailed discussion on these types of materials has been given by Burchell (1999). Methane storage will be discussed in Chapter 10. 

Activated carbon fibers (ACFs) are a fibrous form of activated carbon with carbon content more than 90%. ACFs are relatively new adsorbents for filtration or purification techniques. The unique characteristics of ACFs compared with GAC and RAC could increase the application of activated carbons in various areas. The fiber shape of ACFs can significantly improve the intraparticle adsorption kinetics as compared with RAC and GAC, which are commonly employed in gas-phase and aqueous-phase adsorption. Therefore, ACFs adsorption is a promising technique used for designing adsorption units where intraparticle diffusion resistance is the dominant adsorption factor. As a consequence, the size of adsorption units can be decreased by using ACFs (Yue et al. 2001). 

In addition to the particulate adsorbents listed in Table 16-5, some adsorbents are available in structured form for specific applications. Monoliths, papers, and paint formulations have been developed for zeolites, with these driven by the development of wheels (Fig. 16-60), adsorptive refrigeration, etc. Carbon monoliths are also available as are activated carbon fibers, created from polymeric materials, and sold in the forms of fabrics, mats, felts, and papers for use in various applications including in pleated form in filters. Zeolitic and carbon membranes are also available, with the latter developed for separation by selective surface flow [Rao and Sircar, J. Membrane Sci., 85, 253 (1993)]. 

---