The use of tannin to prepare carbon gels. Part II. Carbon cryogels

Organic and derived carbon cryogels were prepared by sol-gel polycondensation of resorcinol (R) and formaldehyde (F) in the presence of mimosa tannin (T). We show that 2/3 in weight of the resorcinol can advantageously be replaced by tannin, and that resultant TRF organic gels can be freeze-dried and pyrolysed. The resultant low-cost carbon gels present similar pore textures than those of their expensive aerogel counterparts prepared from pure RF resin. However, the presence of tannin allows additional features that are encountered in a much lower extent in RF carbon gels. These materials can indeed be prepared from a much broader range of pHs (2-8), leading to pore-size distributions that are progressively shifted towards narrower pores when the pH increases. Such drastic change of porosity at fixed composition has never been reported so far. At pH 8, the carbons are poorly porous, whereas surface areas close to 650 m(2) g(-1) are found at low and moderate pHs. (C) 2011 Elsevier Ltd. All rights reserved.