Highly functionalized carbonaceous materials were produced by means of the hydrothermal carbonization of cellulose at temperatures in the 220-250ºC range. The formation of this material follows essentially the path of a dehydration process, similar to that previously observed for the hydrothermal transformation of saccharides such as glucose, sucrose or starch. The materials so formed are composed of agglomerates of carbonaceous microspheres (size ~ 2-5 μm), as evidenced by SEM. The combination of the results of the elemental analysis with that obtained by different spectroscopic techniques (infrared and Raman spectroscopy, and XPS) has allowed us to inferred that, from a chemical point of view, the solid product consists of small clusters of condensed benzene rings that form stable groups with oxygen in the core (i. e. ether, quinone, pyrone), whereas the shell possesses more reactive/hydrophilic oxygen functionalities (i. e. hydroxyl, carbonyl, carboxylic, ester).

Carbon-rich-quick scheme: A carbon-rich solid product made up of uniform micrometer-sized spheres of tunable diameter has been synthesized by the hydrothermal carbonization of saccharides. These microspheres possess a core-shell chemical structure based on the different nature of the oxygen functionalities between the core and the outer layer (see figure).A carbon-rich solid product, here denoted as hydrochar, has been synthesized by the hydrothermal carbonization of three different saccharides (glucose, sucrose, and starch) at temperatures ranging from 170 to 240 degrees C. This material is made up of uniform spherical micrometer-sized particles that have a diameter in the 0.4-6 mum range, which can be modulated by modifying the synthesis conditions (i.e., the concentration of the aqueous saccharide solution, the temperature of the hydrothermal treatment, the reaction time, and type of saccharide). The formation of the carbon-rich solid through the hydrothermal carbonization of saccharides is the consequence of dehydration, condensation, or polymerization and aromatization reactions. The microspheres thus obtained possess, from a chemical point of view, a core-shell structure consisting of a highly aromatic nucleus (hydrophobic) and a hydrophilic shell containing a high concentration of reactive oxygen functional groups (i.e., hydroxyl/phenolic, carbonyl, or carboxylic).

Highly porous N-doped carbons have been successfully prepared by using KOH as activating agent and polypyrrole (PPy) as carbon precursor. These materials were investigated as sorbents for CO 2 capture. The activation process was carried out under severe (KOH/PPy = 4) or mild (KOH/PPy = 2) activation conditions at different temperatures in the 600-800 ° C range. Mildly activated carbons have two important characteristics: i) they contain a large number of nitrogen functional groups (up to 10.1 wt% N) identifi ed as pyridonic-N with a small proportion of pyridinic-N groups, and ii) they exhibit, in relation to the carbons prepared with KOH/PPy = 4, narrower micropore sizes. The combination of both of these properties explains the large CO 2 adsorption capacities of mildly activated carbon. In particular, a very high CO 2 adsorption uptake of 6.2 mmol · g − 1 (0 ° C) was achieved for porous carbons prepared with KOH/PPy = 2 and 600 ° C (1700 m 2 · g − 1 , pore size ≈ 1 nm and 10.1 wt% N). Furthermore, we observed that these porous carbons exhibit high CO 2 adsorption rates, a good selectivity for CO 2 -N 2 separation and it can be easily regenerated.

This review presents the state-of-the-art with respect to synthesis of activated carbons, and their use as electrode materials in supercapacitors and as hydrogen storage materials.