Pore structure carbon membranes

Koresh, J.E. and A. Sofer, Study of molecular sieve carbon membranes, Part 1. Pore structure, gradual pore opening, and mechanism of molecular sieving, /. Chem. Soc., Faraday Trans. I, 76,2457,1980. 

Commonly used GDL materials are made of porous carbon fibers, including carbon cloth and carbon paper. Carbon cloth is more porous and less tortuous than carbon paper and has a rougher surface. Experimental results showed that carbon cloth GDL has better performance under high-humidity conditions because its low tortuosity (of the pore structure) and rough textural surface facilitate droplet detachment. " However, under dry conditions, carbon paper GDL has shown better performance than carbon cloth GDL because it is capable of retaining the membrane hydration level with reduced ohmic loss. 

The MSC membranes are produced by carbonization of PAN, polymide, and phenolic resins. They contain nanopores, which allow some of the molecules of a feed gas mixture to enter the pore structure at the high pressure side, adsorb, and then diffiise to the low pressure side of the membrane, while excluding the other molecules of the feed gas. Thus, separation is based on the difference in the molecular sizes of the feed gas components. The smaller molecules preferentially diffuse through the MSC membrane as shown by Table 4 [16,17]. 

The SSF membranes, which are produced by carbonization of PVDC, contain nanopores that allow all of the molecules of a feed gas mixture to enter the pore structure. However, the larger and more polar molecules are selectively adsorbed on the carbon pore walls at the high pressure side, and then th dif se selectively to the low pressure side. The smaller molecules are enriched at the high pressure side. These membranes can be used to enrich H2 from mixtures with C1-C4 hydrocarbons or from mixtures with CO2 and CH4. They can also be used to separate CH4-H2S and H2S-H2 mixtures. Table 5 compares performances of SSF carbon and polymeric PTMSP membranes for H2 enrichment from FCC off gas [15]. Clearly, the SSF membrane is much superior for this application. 

Carbon molecular sieve membranes. Molecular sieve carbons can be produced by controlled pyrolysis of selected polymers as mentioned in 3.2.7 Pyrolysis. Carbon molecular sieves with a mean pore diameter from 025 to 1 nm are known to have high separation selectivities for molecules differing by as little as 0.02 nm in critical dimensions. Besides the separation properties, these amorphous materials with more or less regular pore structures may also provide catalytic properties. Carbon molecular sieve membranes in sheet and hollow fiber (with a fiber outer diameter of 5 pm to 1 mm) forms can be derived from cellulose and its derivatives, certain acrylics, peach-tar mesophase or certain thermosetting polymers such as phenolic resins and oxidized polyacrylonitrile by pyrolysis in an inert atmosphere [Koresh and Soffer, 1983 Soffer et al., 1987 Murphy, 1988]. 

Materials commonly used for the gas diffusion layers are carbon paper or woven carbon mats (examples of which are shown in Fig. 3.41). They combine the cormectivity allowing electron transport with a pore structure suitable for hydrogen or oxygen gas access to the catalyst layer. In cell manufacture, the catalysts may be deposited either on the gas diffusion layer or on the membrane. 

The nanoporous carbon membrane consists of a thin layer (<10pm) of a nanoporous (3-7 A) carbon film supported on a meso-macroporous solid such as alumina or a carbonized polymeric structure. They are produced by judicious pyrolysis of polymeric films. Two types of membranes can be produced. A molecular sieve carbon (MSC) membrane contains pores (3-5 A diameters), which permits the smaller molecules of a gas mixture to enter the pores at the high-pressure side. These molecules adsorb on the pore walls and then they diffuse to the low-pressure side of the membrane where they desorb to the gas phase. Thus, separation is primarily based on differences in the size of the feed gas molecules. Table 7 gives a few examples of separation performance of MSC membranes. ° Component 1 is the smaller component of the feed gas mixture. 

Several attempts to characterize quantitatively pore structures in ultrafiltration membranes have been described in the literature. Preusser(lJ analyzed surface porosities of Amicon membranes, using a carbon replica technique and a high-resolution transmission electron microscopy (TEM). A similar approach was... 

Highly porous carbons can be produced from a variety of natural and synthetic precursors [11, 12]. Relatively inexpensive activated carbons are useful adsorbents, but generally their surface and pore structures are exceedingly complex [11, 13]. However, it is now possible to prepare a number of more uniform carbonaceous adsorbents. For example, molecular sieve carbons (MSCs) are available with narrow distributions of ultramicropores, which exhibit well-defined molecular selectivity [11], and carbon nanotubes, aerogels, and membranes are also amongst the most interesting advanced materials for research and development [12, 14]. 

More promising for reactive separations involving gas phase reactions appears to be the development and use in such applications of microporous zeolite and carbon molecular sieve (Itoh and Haraya [2.25] Strano and Foley [2.26]) membranes. Zeolites are crystalline microporous aluminosilicate materials, with a regular three-dimensional pore structure, which are relatively stable to high temperatures, and are currently used as catalysts or catalyst supports for a number of high temperature reactions. One of the earliest mentions of the preparation of zeolite membranes is by Mobil workers (Haag and Tsikoyiannis... 

A suitable polymer material for preparation of carbon membranes should not cause pore holes or any defects after the carbonization. Up to now, various precursor materials such as polyimide, polyacrylonitrile (PAN), poly(phthalazinone ether sulfone ketone) and poly(phenylene oxide) have been used for the fabrication of carbon molecular sieve membranes. Likewise, aromatic polyimide and its derivatives have been extensively used as precursor for carbon membranes due to their rigid structure and high carbon yields. The membrane morphology of polyimide could be well maintained during the high temperature carbonization process. A commercially available and cheap polymeric material is cellulose acetate (CA, MW 100 000, DS = 2.45) this was also used as the precursor material for preparation of carbon membranes by He et al They reported that cellulose acetate can be easily dissolved in many solvents to form the dope solution for spinning the hollow fibers, and the hollow fiber carbon membranes prepared showed good separation performances. 

The CMS membranes are prepared by carbonizing (under pyrolysing conditions) the precursor membranes in a high temperature tube furnace, as shown in Figure 15.4. The step-by-step method (several dwells) most commonly used as the protocol for the carbonization process is described elsewhere. Many researchers report different carbonization conditions in their research works illustrating very well that each precursor will need different protocols in order to be pore tailored for specific applications. " The carbonization process is the most important step for fabrication of CMS membranes and is used to tailor the pore size and structure of the carbon membranes. Therefore, how to control the carbonization conditions is crucial for the resulting CMS membrane performance. Su and Lua reported that the statistical 2" factorial... 

For Knudsen diffusion to take place, the lower limit for pore diameter has usually been set to 4>ore > 20 A. Gilron and Soffer have, however, discussed thoroughly how Knudsen diffusion may contribute to transport in even smaller pores, and from a model considering pore structure, shown that contributions to transport may both come from activated transport and Knudsen through one specific fiber. It may thus be difficult to know exactly when transport due to Knudsen diffusion is taking place. One way to approach this problem is to calculate the Knudsen number, Knudsen. for the system, which is 2/fi pore> where X is the mean free path. If Knudsen > 10, then the separation can be assumed to take place according to Knudsen diffusion. Therefore, if the preparation of the carbon membranes has been unsuccessful, one may get Knudsen diffusion. 

Like most of the materials (GDL, membranes) used in DMFC, the applicatiOTi of carbon black as catalyst support is a direct extrapolation of their use in hydrogen fuel cell. However, the processes occurring in the anode side are more complex. The carbonous support besides promoting a high dispersion of the catalyst (catalytic area), should allows also the free entrance of the liquid alcohol to the catalyst and avoid the occluding of the gas close to the catalytic zone. Nevertheless the pore structure should not hinder the triple phase boundary formation. 

Sample Preparation for SEM. Samples for the scanning electron microscopy (SEM Model Joel JXA-8600) were prepared by first immersing the membrane into a liquid nitrogen. This made the membrane brittle. The membrane was broken into small pieces and some of the pieces were mounted on the metal cylinder that forms part of the stage inside the microscope. The membrane was mounted such that its cross-section (or the broken face) was perpendicular to the electron beam. This way we were able to study the pore structure of the membrane directly. Our focus was to study the edge that was in contact with the bottom wall of the bottle. All membranes were coated with carbon to make them conductive prior to any microscopy work. 

Fuel cell electrodes are a particularly interesting catalyst application. The active layers of the proton conducting membrane fuel cells must have the ability to transport both gases and hydrogen ions to the catalyst as well as conducting water away. Mixtures of carbon blacks with special polymers create the desired hydrophilic and hydrophobic pore structures. 

Pilatos, G., Vermisoglou, E. C., Romanos, G. E., Karanikolos, G. N., Boukos, N., Eikodimos, V., and Kanellopoulos, N. K. (2010). A closer look inside nanotubes Pore structure evaluation of anodized alumina templated carbon nanotube membranes through adsorption and permeability studies. Adv. Fimt. Mater. 20(15), 2500-2510. 

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