Porous carbons have been extensively investigated for hydrogen storage but, to date, appear to have an upper limit to their storage capacity. Here, in an effort to circumvent this upper limit, we explore the potential of oxygen-rich activated carbons. We describe cellulose acetate-derived carbons that combine high surface area (3800 m2 g−1) and pore volume (1.8 cm3 g−1) that arise almost entirely (>90%) from micropores, with an oxygen-rich nature. The carbons exhibit enhanced gravimetric hydrogen uptake (8.1 wt% total and 7.0 wt% excess) at −196 °C and 20 bar, rising to a total uptake of 8.9 wt% at 30 bar, and exceptional volumetric uptake of 44 g l−1 at 20 bar, and 48 g l−1 at 30 bar. At room temperature they store up to 0.8 wt% (excess) and 1.2 wt% (total) hydrogen at only 30 bar, and their isosteric heat of hydrogen adsorption is above 10 kJ mol−1.

absorbents; [ 2 ] the better effi ciency of solid state absorbents is related to the ease (with respect to energy penalty) of their regeneration via energy effi cient pressure or temperature swing changes. Porous solid state materials, namely, zeolites, [ 3 ] metal organic frameworks (MOFs), [ 4 ] covalent organic frameworks (COFs), [ 4 ] and carbons [ 5,6 ] are currently the most studied CO 2 absorbers for postcombustion capture and storage. Of the three classes of porous materials, carbons are attractive due to their abundance, low cost, and rich availability in activated [5][6][7] or templated [ 8,9 ] form. Activated carbons are particularly attractive as they may be prepared from biomass via valorisation processes that convert low value waste into valuable carbonaceous materials. [ 5f ] Recent studies have shown that KOH activated carbons, including biomass-derived examples, can achieve some of the best CO 2 uptake under conditions (i.e., 0.1-1 bar and 25-50 °C) relevant to industrial postcombustion capture. [ 2,[5][6][7] It is now well established that pore size is the main factor that determines CO 2 uptake of activated carbons under postcombustion capture conditions. [5][6][7]9 ] The critical importance of pore size arises from the fact that the CO 2 -carbon interaction emanates from short-range attractive forces which are maximized when CO 2 adsorption takes place in very narrow pores where overlapping potential fi elds from neighboring walls exert a positive infl uence. For slit-shaped pores, such as those present in activated carbons, enhancement of the adsorption potential may be maximized for micropore widths that are two times the diameter of the CO 2 molecule, [ 10 ] which translates to an optimal pore size of 6-7 Å. Such pores can be obtained in lowly or mildly activated carbons prepared at KOH/carbon ratio of 2 and between 600 and 700 °C. [ 2,[5][6][7]9,11 ] Indeed, to date, such activated carbons are among materials that show the highest CO 2 uptake at 25 °C and low pressure (up to 1 bar). [ 2,[5][6][7]9 ] Conversely, though, such lowly activated carbons tend to have low surface area, which limits the maximum amount of CO 2 that they can store. For this reason, there appears to be a limit on the amount of CO 2 that can be stored by biomass-derived or other activated carbons at 25 °C of ≈1.5 mmol g −1 at 0.15 bar, and 4.5 mmol g −1 at 1 bar. [ 2,[5][6][7]9,12 ] Increase in surface area can be achieved at higher activation levels (higher KOH amounts Novel mechanochemical activation generates biomass-derived carbons with unprecedented CO 2 storage capacity due to higher porosity than analogous conventionally activated carbons but similar pore size. The mechanochemical activation, or so-called compactivation, process involves compression, at 740 MPa, of mixtures of activating agent (KOH) and biomass hydrochar into pellets/disks prior to thermal activation. Despite the increase in surface area and pore volume of between 25% and 75% compared to conventionally activated carbons, virtually all o...

The direct conversion of biomass to activated carbons in a simple and lower cost one step process, which negates the need for hydrothermal carbonisation or pyrolysis, generates activated carbons with properties and CO2 uptake comparable or superior to those of conventionally prepared activated carbons.

A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. AbstractThe first attempt at activation of air-carbonized carbon reveals unusual resistance to activation and unprecedentedly high yields (32 -80 wt%) of high packing density (0.7 -1.14 g cm -3 ) microporous carbon dominated by 5.5 -7 Å pores, which are just right for CO2 uptake (up to 5.0 mmol g -1 ) at 1 bar and 25 o C. The high gravimetric uptake and packing density offer exceptional volumetric storage, and unprecedented performance for low pressure swing adsorption (PSA)with working capacity of 6 -9 mmol g -1 for a pure CO2 stream (6 to 1 bar) and 3 -4 mmol g g l -1 (PSA) and 179 -233 g l -1 (VSA). For flue gas conditions, the working capacity is 120 to 160 g l -1 (PSA). The performance of the activated air-carbonized carbons is higher than the best carbons and benchmark zeolites or MOFs.2

Carbons derived from pre-mixed precursors (polypyrrole and sawdust) have surface area and hydrogen uptake not achievable for equivalent single precursor samples; pre-mixing allows hitherto impossible modulation of porosity in a predictable manner.