2. Tailoring the meso- and macroporosity

Perhaps the most relevant property of carbon xerogels is that their mesoporosity (i.e., pores with an average size between 2 and 50 nm) and macroporosity (i.e., pores with an average size greater than 50 nm) can be tailored during the synthesis of the organic xerogels by selecting the appropriate synthesis conditions and, above all, by predetermining the characteristics of the precursor solution.

During the gelation stage, the polycondensation of resorcinol and formaldehyde leads to the formation of quasi-spherical nodules (nucleation) that become attached to each other during the curing step (cross-linking). Then, when the solvent is removed, during the drying step, the empty interspaces between the nodules form pores with a size that depends on the size of the nodules formed in the previous stages of the synthesis. A sketch representing the formation of the nodules and the way they cross-link with the pores formed in the resorcinol-formaldehyde (RF) gel is presented in Figure 4.

As reported in many studies, the size of the nodules and hence of the mesopores (or macropores) can be varied by changing the synthesis conditions, such as the pH [5, 19, 33–36], type and amount of solvent [6, 37], the concentration of the reactants [6, 20, 37], the type of catalyst [7, 25], the temperature and time of synthesis [25, 28], etc. As there are so many variables involved, the meso- or macroporosity of the xerogels can be tailored with a relatively high degree of precision by adjusting these variables. This is, of course, a great advantage, as it means that it is possible to design specific porous textures for specific applications. However, these variables

Figure 4. Schematic representation of pore formation in RF xerogel synthesis.

Under cryogenic drying, the solvent used must be frozen and then eliminated by sublimation. As in the first case, when water is the dissolvent used for the synthesis, before the freezing process, it must be replaced by an organic solvent in order to avoid the formation of ice crystals inside the polymer structure, which would lead to the uncontrolled formation of megalopores or voids [18, 26]. The gels obtained in this drying process have high pore volumes [18] and are known as cryogels. Despite the fact that this procedure is more affordable than supercritical drying, it is still expensive due to the need for using exchanging solvents, very low operating

The performance of the third drying method is based on the direct evaporation of the solvent and the generation of gels known as xerogels. Unlike the two cases mentioned above, under subcritical drying conditions, a liquid-vapor interphase takes place. This interphase generates high superficial tensions that may cause the collapse of the porous structure of the gel. When water is the solvent used during the synthesis of the gel, one possible solution to this problem can be to replace the water by another solvent with less surface tension, such as acetone or cyclohexane. However, some authors [18, 27–32] have demonstrated that a suitable choice of operating conditions during the drying stage minimizes the shrinkage of the gel structure. In other words, the porosity of the gel is preserved without the need for solvent exchanges. For this reason, subcritical drying is the cheapest, easiest and fastest method, and consequently the

Perhaps the most relevant property of carbon xerogels is that their mesoporosity (i.e., pores with an average size between 2 and 50 nm) and macroporosity (i.e., pores with an average size greater than 50 nm) can be tailored during the synthesis of the organic xerogels by selecting the appropriate synthesis conditions and, above all, by predetermining the characteristics of the

During the gelation stage, the polycondensation of resorcinol and formaldehyde leads to the formation of quasi-spherical nodules (nucleation) that become attached to each other during the curing step (cross-linking). Then, when the solvent is removed, during the drying step, the empty interspaces between the nodules form pores with a size that depends on the size of the nodules formed in the previous stages of the synthesis. A sketch representing the formation of the nodules and the way they cross-link with the pores formed in the resorcinol-formaldehyde

As reported in many studies, the size of the nodules and hence of the mesopores (or macropores) can be varied by changing the synthesis conditions, such as the pH [5, 19, 33–36], type and amount of solvent [6, 37], the concentration of the reactants [6, 20, 37], the type of catalyst [7, 25], the temperature and time of synthesis [25, 28], etc. As there are so many variables involved, the meso- or macroporosity of the xerogels can be tailored with a relatively high degree of precision by adjusting these variables. This is, of course, a great advantage, as it means that it is possible to design specific porous textures for specific applications. However, these variables

temperatures and numerous long steps.

72 Porosity - Process, Technologies and Applications

most upscalable alternative for producing gels on a large scale.

2. Tailoring the meso- and macroporosity

precursor solution.

(RF) gel is presented in Figure 4.

are not independent of each other. Therefore, in order to be able to predict how changes in more than one of them will affect the porosity of the resultant xerogel, it is necessary to know the way in which they are interrelated, which is not a straightforward task [34, 37, 38].

The complexity of the problem can be explained briefly as follows. Resorcinol is responsible for the formation of the nodules or clusters and so the greater the amount of resorcinol that is used, the more the clusters that will be generated, while formaldehyde strengthens the gel by generating a structure that is more branched and/or interconnected. The volume of solvent added affects the distance between the nodules and so the greater the volume of solvent that is used, the more segregated the nodules will be, while the pH influences the speed of the reaction, i.e., the higher the pH is, the faster the resorcinol anions will be formed and consequently more nodules of small size will be created. All of these differences in the polymer structure affect the final porosity of the RF gel. Finally, the type of catalyst and the composition of the formaldehyde solution also influence the porosity formed by the RF gel [34]. However, as mentioned above, it is obvious that the modifications in porosity brought about by changes in the pH of the precursor solution, for example, will not be the same if the amount of solvent also changes. This is exemplified in Figure 5, where scanning electron microscope photographs, and the sketches in the insets, show for instance how increasing the pH of the precursor solutions from 5 to 5.8 leads to a polymeric structure with larger nodules (and hence wider pores) if the dilution is increased at the same time from 5.7 to 11.7.

Interestingly, the mesoporosity (or macroporosity) of the organic xerogels is preserved with only slight variations when thermal treatments such as carbonization and/or activation are applied [39]. The meso- or macroporosity of the carbon xerogels that is designed before and formed during the synthesis of the organic xerogel persists in the carbon xerogel, occasionally with a slight shrinkage of the pores when the carbonization or activation temperature is higher than 900C [40–42]. However, as this shrinkage is a function of the temperature and heating rate, it can easily be predicted. Therefore, by taking this into account, it is possible to predesign the mesoporosity (or macroporosity) of the carbon xerogels before they are produced.

acid, etc.) or physical activation (with CO2, steam or both) can be employed [40, 42]. An appropriate selection of the activation process is essential for tailoring the microporosity of the carbon xerogel. Thus, different micropore volume and micropore size distributions can be obtained by varying the xerogel/activating agent ratio, the type of activating agent or the

Carbon Xerogels: The Bespoke Nanoporous Carbons http://dx.doi.org/10.5772/intechopen.71255 75

Figure 6. Micropore formation in a carbon xerogel by either carbonization or activation of the organic xerogel.

A particularity of carbon xerogels is their unique porous structure, which completely differs from that of most porous carbons. Thus, carbon xerogels have a hierarchical porous structure that is composed of both micro- and mesopores (or macropores). Moreover, as stated in the previous sections, the size, and to some extent the pore volume, of the larger pores (meso- or macropores) can be tailored during the synthesis of the organic xerogel, whereas the pore volume and the size of the micropores can be predetermined during the carbonization or activation processes that give rise to the carbon xerogels. Consequently, the entire porosity (micro- and meso-/macroporosity) of carbon xerogels can be independently tailored to con-

In addition, most porous carbons have slit-shaped pores, with more or less flat walls, or cylindrical pores, with negative surface curvatures (endohedral). However, the mesopores (or macropores) of carbon xerogels, due to their polymeric structure of interconnected nodules, have a positive surface curvature (exohedral), which is quite unusual for porous carbons

The exohedral geometry of mesopores may have important implications for some applications where carbon xerogels are used [44], because the heterogeneous interaction of this kind of positive surface with gases or liquids differs from the interactions that occur with other types of carbon surfaces. For example, the positive curvature facilitates double layer formation in electrochemical capacitors, which is the subject of this chapter. The double layer, which is the basis of charge storage in this kind of device, is favored due to the reduced electrical field near to the positive surface, as a result of which the driving force behind counter-ion adsorption and co-ion desorption is decreased [45]. This has a positive influence on energy, but especially on

reaction time and temperature [40, 42].

4. Unique exohedral porosity

form to predetermined specifications.

the power density of supercapacitors.

(Figure 7).

Figure 5. SEM microphotographs of carbon xerogels obtained under different synthesis conditions (adapted from [39]).
