4. Unique exohedral porosity

3. Tailoring the microporosity

74 Porosity - Process, Technologies and Applications

As stated above, mesopores and macropores, which are the voids between the nodules of polymer that make up the organic xerogel polymeric structure, are predetermined during the synthesis of the organic xerogel. However, in order to obtain the carbon xerogel, it is necessary to subject the organic gel to a carbonization process or, in other words, to a thermal treatment in an inert atmosphere. During the carbonization, as the temperature increases, diverse volatile compounds are released from the organic xerogel, while reorganization and condensation reactions occur, leading to a thermally stable material that is mainly composed of carbon with a dominant sp2 structure. The release of the volatile molecules (mainly H2, CO, CO2 and light hydrocarbons) leaves behind small holes or micropores. This microporosity is allocated to the carbonized nodules that constitute the carbon xerogel. This process is schematized in Figure 6. Although the amount (pore volume) and size of these micropores may differ depending on the carbonization conditions and the type of organic xerogel being carbonized, in practice, for carbonization temperatures between 800 and 1000C, the microporosity is similar for any carbon xerogel no matter the starting organic xerogel used [9, 27, 30, 43]. However, the microporosity can be modified by performing an activation process, which can be performed either during or after the carbonization. Either chemical activation (with KOH, phosphoric

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

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 conform to predetermined specifications.

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 (Figure 7).

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 the power density of supercapacitors.

Figure 7. Different kinds of pores according to the curvature of their surface.
