Porous texture microporous carbons

Nitrogen adsorption/desorption isotherms of all the activated carbons are of Type I, i.e. characteristic of basically microporous solids. There is a lack of adsorption/desorption hysteresis. More careful analysis permits to notice significant differences in the porous texture parameters depending on precursor origin. 

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. 

Immersion calorimetry is a very useful technique for the surface characterization of solids. It has been widely used with for the characterization of microporous solids, mainly microporous carbons [6]. The heat evolved when a given liquid wets a solid can be used to estimate the surface area available for the liquid molecules. Furthermore, specific interactions between the solid surface and the immersion liquid can also be analyzed. The appropriate selection of the immersion liquid can be used to characterize both the textural and the surface chemical properties of porous solids. Additionally, in the case zeolites, the enthalpy of immersion can also be related to the nature of the zeolite framework structure, the type, valence, chemistry and accessibility of the cation, and the extent of ion exchange. This information can be used, together with that provided by other techniques, to have a more complete knowledge of the textural and chemical properties of these materials. 

Similarly, there is a great potential in the use of water vapour for the analysis of the porous texture, because it has considerable potential due to both the easy experimental conditions (at room temperature the whole range of relative pressures can be covered) and the characteristics of the molecule itself (polar molecule and small kinetic diameter-0.28 nm). This vapour is widely used in the characterisation of inorganic porous solids, such as zeolites, silicas, and clays. However, its interaction with carbon materials (microporous carbons coals, activated carbon fibres, carbon molecular sieves and porous carbons activated carbons), is more complex than the interaction of non-polar molecules [8]. 

Although the activation with carbon dioxide or steam produces essentially microporous ACFs, strong differences have been found between these two activating agents regarding the porous texture and the mechanical properties of the ACFs [12, 13]. 

In the following, the usefulness of CO2 adsorption at 273 K to achieve a rather complete characterization of the porous texture of microporous carbons will be discussed. We will base our study on the results already published [33-35, 37] in which samples with different characteristics were used and CO2 adsorption experiments at high pressures (up to 4 MPa) were performed. In this study, the ACFs with different contents of microporosity have been very useful. The use... 

Denoyel et al. [45] derived the pore size distributions of two sets of activated carbons (one activated in water vapor and the other activated with phosphoric acid) using immersion calorimetric data. They concluded that immersion calorimetry is a convenient technique to assess the total surface area available for a given molecule and the micropore size distribution. More recently, Villar-Rodil et al. [46] have followed this approach to characterize the porous texture of a series of NomexO-derived carbon fibers activated to various bum-offs using liquids with different molecular dimensions as well as N2 and CO2 adsorption Isotherms. Table 3 includes the immersion enthalpies and corresponding surface areas. Relative changes in surface area accessible to the different adsorbates were ascribed to... 

While keeping in mind all these implications, the primary requirement in an attempt to store a huge charge based on the electrostatic forces seems to be high surface area of an activated carbon used. Among different ways of porosity development in carbons, the treatment with an excess of potassium hydroxide is most efficient in terms of microporous texture generation. Porous materials with BET surface areas in excess of 3000 m2/g could be prepared using various polymeric and carbonaceous type precursors [5,6]. 

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. 

Table 3 shows the textural characteristics of three carbon monoliths, two of which were produced by dipcoating a cordierite monolith with a solution of sucrose or PFA and one of which was produced by a CVD process resulting in a CNF coating. These are referred to as Cord-SUC, Cord-PFA, and Cord-CNF, respectively. From a texture analysis, it was concluded that the sucrose-derived carbon is highly porous, with pore diameters in a favorable range (t)q5ically, 11 nm). The PFA-derived carbon was microporous and, as a consequence, not suitable for adsorption of large species, such as enzymes. 

Since the texture of the final material obtained after drying and pyrolysis is strongly influenced by the reaction mechanism which is itself influenced by pH, the latter is a key variable for controlling texture. Indeed, depending on the pH interval used for synthesis, micro-macroporous, micro-mesoporous, only microporous or totally non-porous carbon materials can be obtained [54, 59, 66]. 

Conunonly, textural characterization of porous solids is carried out by physical adsorption of gases, which can be analyzed using several theories, to provide detailed information about the carbon micropore structure. A number of attempts have been made to establish standard procedures for the interpretation of the adsorption data in the characterization of porous solids [11-13]. However, there is still a lack of agreement on the assessment and interpretation of the adsorption data [14], and the results found in the literature depend upon the theory used to interpret the isotherms [15-18]. Usually, the PSD of porous solids is evaluated firom N2 adsorption at 77 K, and the structural heterogeneity of the microporosity is determined from the Dubinin-Radushkevich method (DR) and its modifications (Dubinin-Asthakov, DA,... 

Almazan-Almazan et al. attempted to change the textural properties of activated carbons from PET by controlling various variables and showed that these carbons can be tailored to range from molecular sieves to samples with variant pore sizes and high adsorption volume [70]. Chars were obtained after pyrolysis at 800 and 950°C with 19% yield. Subsequent activation under CO2 flow took place for 4 or 8 h. The activation resulted in further bum-offs of 81-87%. In the carbonization process at 800°C under carbon dioxide, the amorphous carbon is eliminated but the micropore system is not affected. These porous samples show molecular sieve behavior for cyclohexane/benzene as well as 2,2-DMB/benzene pairs. At 950°C, however, micropore textural characteristics are changed and the samples do not exhibit molecular sieve behavior due to constrictions at the entrance of the micropores [77]. 

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