3. Tailoring the microporosity

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

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 reaction time and temperature [40, 42].
