Microporous carbon fibers

Figure 1. FIRTEM image of a microporous carbon fiber obtainedfrom the pyrolysis of viscose at 1000°C.

Increases In qm and C values with the Increase In temperature of column conditioning Indicates the presence of micropores. Carbon fiber surface areas, calculated with Groszek s values for... 

Fig. XVII-31. (a) Nitrogen adsorption isotherms expressed as /-plots for various samples of a-FeOOH dispersed on carbon fibers, (h) Micropore size distributions as obtained by the MP method. [Reprinted with permission from K. Kaneko, Langmuir, 3, 357 (1987) (Ref. 231.) Copyright 1987, American Chemical Society.]...

Figure 3 c. HRTEM image of a cross-section of a carbon fiber after propylene pyrolysis the black line represents the boarder between the lamellar pyrocarbon (at the top) and the microporous fiber (at the bottom). 

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. 

Table 1. Micropore structure development during steam activation for CFCMS monoliths manufactured from P-200 carbon fibers.

Isotropic fibrous materials, 11 176-177 Isotropic microporous membranes, 15 798 Isotropic moldings, 23 397 Isotropic pitch-based carbon fibers, 26 734-735... 

Details about the porous texture properties of the studied materials can by found in our previous papers 4 18. In general, all activated carbons, activated carbon fibers and activated carbon monoliths are essentially microporous materials with a negligible contribution of meso- and macroporosity. 

Another important point regarding the fabrication process of MPLs is that, typically, when carbon fiber paper is used as the DL, the MPL is coated on only one surface of the CFP. However, when a carbon cloth DL is used, it is normally coated on both sides with MPLs. Section 4.3.S.4 will discuss these DLs with multiple microporous layers in more detail. 

We have an excellent activated carbon of fiber morphology, so called activated carbon fiber ACF[3]. This ACF has considerably uniform slit-shaped micropores without mesopores, showing characteristic adsorption properties. The pore size distribution of ACF is very narrow compared with that of traditional granular activated carbon. Then, ACF has an aspect similar to the regular mesoporous silica in particular in carbon science. Consequently, we can understand more an unresolved problem such as adsorption of supercritical gas using ACF as an microporous adsorbent. 

The coarse texture of the fibrous gas diffusion media can further amplify the contact stress exerted on the MEA. Figure 3 shows the relative size of a carbon fiber with respect to the typical thickness of the electrode and the electrolyte membrane. It can be seen that the diameter of the carbon fiber in the gas diffusion media is comparable to the thickness of the electrode. The rigid carbon fiber pressed onto the porous electrode layer can produce in-prints which can later become a stress-concentration and defect-initiation sites at the electrode-electrolyte interface. A microporous layer, if used, tends to smooth out the surface of the GDM and reduces fiber inprint. Thicker electrode layer also offers protection against fiber in-prints. 

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