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Supercritical water (SCW) and steam were used to prepare activated carbon fibers (ACFs) to study the different behaviors of these two agents. The results showed that activation with SCW is much faster than with steam and that the lower activation energy observed when SCW is used suggests that the mechanism of gasification with SCW and with steam is different. All the ACFs prepared showed a highly developed micropore structure, with a large specific surface area. The ACFs prepared with SCW had a smaller micropore volume with a broader pore size distribution. In contrast, the ACFs prepared with steam had many small micropores. With SCW, external gasification was greater than with steam, fibers with smaller diameters being obtained, although these latter maintained their structure and their mechanical properties were scarcely altered. However, with steam, external gasification was less marked, but the fibers became fragile and friable.

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Two series of activated carbon have been prepared by reaction of a char (from olive stones) with supercritical water (SCW) with the objective of studying the effect of temperature and residence time on the development of porosity. The results have been compared with those obtained using the same char but with classical activation with steam. Both procedures develop porosity, but (i) the reaction rate is critical in the development of porosity for steam but not for SCW activation, and (ii) SCW activation produces a larger development of microporosity at low degrees of burnoff, whereas steam produces more meso- and macroporosity. The differences have been explained by assuming that the mechanism for the carbon-water reaction is common but the transport properties of water in the supercritical state are more favorable to facilitate the access of water to the interior of the char particles. In contrast, when steam is used for the activation of the char, the diffusion of the molecules cannot keep up with the chemical rate and, consequently, the reaction is preferentially taking place at the most accessible surface sites, thus facilitating the development of larger pores and the widening of microporosity.

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