Sustainable porous carbons have been prepared by chemical activation of hydrothermally carbonized polysaccharides (starch and cellulose) and biomass (sawdust). These materials were investigated as sorbents for CO 2 capture. The activation process was carried out under severe (KOH/precursor=4) or mild (KOH/precursor=2) activation conditions at different temperatures in the 600-800ºC range. Textural characterization of the porous carbons showed that the samples obtained under mild activating conditions exhibit smaller surface areas and pore sizes than those prepared by employing a greater amount of KOH. However, the mildly activated carbons exhibit a good capacity to store CO 2 , which is mainly due to the presence of a large number of narrow micropores (< 1 nm). A very high CO 2 uptake of 4.8 mmol⋅g-1 (212 mg CO 2 ⋅g-1) was registered at room temperature (25ºC) for a carbon activated at 600ºC using KOH/precursor=2. To the best of our knowledge, this result constitutes the largest ever recorded CO 2 uptake at room temperature for any activated carbon. Furthermore, we observed that these porous carbons have fast CO 2 adsorption rates, a good selectivity for CO 2-N 2 separation and they can be easily regenerated. Broader context The mitigation of carbon dioxide emissions is attracting widespread attention due to the fact that this gas is the main anthropogenic contributor to climate change. Among the possible strategies for CO 2 abatement, that of capture and storage has attracted keen interest. In this regard, the use of solid sorbents to capture CO 2 by means of pressure, temperature or vacuum swing adsorption systems constitutes a promising alternative. To accomplish this objective the sorbents need to satisfy important conditions: i) low-cost and high availability, ii) a large CO 2 uptake, iii) a high sorption rate, iv) a good selectivity between CO 2 and other competing gases (i. e. N 2) and v) an easy regenerability. However, the development of a solid sorbent that satisfies all these conditions has so far proved to be complex. Here we present a novel route for the preparation of carbon-based porous sorbents of CO 2 from a low-cost sustainable biomass product (sawdust). The results obtained show that these carbon sorbents exhibit very large CO 2 adsorption uptakes of up to 4.8 mmol⋅g-1 (212 mg CO 2 ⋅g-1) at room temperature (25ºC), a value that far exceeds those reported in the literature for activated carbons. Furthermore, these carbon sorbents exhibit high sorption rates, a good CO 2-N 2 selectivity and they can be easily regenerated.

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A novel activating agent for the production of highly microporous carbons with textural properties that match those of superactivated carbons prepared by KOH activation, i.e. BET surface areas of 2600-3000 m 2 g-1 , pore volumes of ~ 1.3-1.6 cm 3 g-1 and pore size distributions in the supermicropore-small mesopore (< 3 nm) region, is studied. It consists of a mixture of melamine and potassium oxalate, a substance which is less corrosive than KOH, imposing less technical restrictions. Additional advantages of this activating agent are that the morphology of the particles is not altered and, importantly, the product yield is almost double that of KOH activation. The advantageous textural characteristics of the produced materials are combined with a relatively good electronic conductivity of ~2-3 S cm-1. When tested as supercapacitor electrodes using conventional electrolytes such as H 2 SO 4 and TEABF 4 /AN, and less conventional ones such as EMImTFSI/AN, these carbons match the performance of benchmark KOH activated carbons and surpass that of commercial activated carbons specifically designed for supercapacitor applications.

Sustainable synthesis schemes for the production of porous carbons with appropriate textural properties for use as supercapacitor electrodes are in high demand. In this work a greener option to the widely used but corrosive KOH is proposed for the production of highly porous carbons. Hydrochar products are used as carbon precursors. It is demonstrated that a mild alkaline potassium salt such as potassium bicarbonate is very effective to generate porosity in hydrochar to lead to materials with large surface areas (> 2000 m(2)  g(-1) ) and a tunable pore size distribution. Furthermore, the use of KHCO3 instead of KOH gives rise to a significant 10 % increase in the yield of activated carbon, and the spherical morphology of hydrochar is retained, which translates into better packing properties and reduced ion diffusion distances. These features lead to a supercapacitor performance that can compete with, and even surpass, that of KOH-activated hydrochar in a variety of electrolytes.

A procedure for the fabrication of N-doped hollow carbon spheres with a high rate capability for supercapacitors has been developed. The approach is based on a nanocasting method and the use of a nitrogen-rich compound (pyrrole) as a carbon precursor. The carbon particles thus produced combine a large BET surface area ($1500 m 2 g À1 ) with a porosity made up of mesopores of $4 nm, a high nitrogen content ($6 wt%) and a capsule morphology which entails short ion diffusion paths derived from the shell morphology (thickness $60 nm). The porous properties of these hollow particles can be enhanced by means of an additional activation step with KOH. The activation process does not alter the hollow structure or spherical morphology, but strongly modifies the pore structure from a mesoporous network to a microporous one. The N-doped carbon capsules were tested in aqueous and organic electrolytes. In an aqueous medium (1 M H 2 SO 4 ), the mesoporous carbon capsules offer the best performance due to the pseudocapacitive contribution of the N-groups, exhibiting a specific capacitance of $240 F g À1 at 0.1 A g À1 and a capacitance retention as high as 72% at 80 A g À1 . In contrast, in an organic electrolyte(1 M TEABF 4 /AN), where the charge storage mechanism is based on the formation of the electric double-layer, the microporous capsules perform better due to the larger specific surface area. Thus, the microporous carbon capsules display a specific capacitance of up to 141 F g À1 at 0.1 A g À1 and an outstanding capacitance retention of 93% for an ultra-high discharge current density of 100 A g À1 .