Compactivation: A mechanochemical approach to carbons with superior porosity and exceptional performance for hydrogen and CO2 storage

Abstract

We describe a compactivation approach, which incorporates a mechanical compression step before thermochemical activation, to carbons that possess higher porosity than analogous conventionally activated carbons but without any significant changes in pore size. The method works for both highly activated and lowly activated carbons. For highly compactivated carbons (thermal treatment at 800 o C), enhanced porosity (surface area and pore volume up to 4000 m 2 g -1 and 3.0 cm 3 g -1 ) is achieved along with superior hydrogen uptake of 7.3 wt% (at -196ºC and 20 bar), rising to 9.6 wt% at 40 bar and 14.2 wt% at 150 bar, which corresponds to volumetric uptake of 38 g l -1 at 40 bar and 56 g l -1 at 150 bar, while at room temperature uptake reaches 3.6 wt% (14 g l -1 ). On densification, the highly compactivated carbons can retain a much greater proportion of their porosity (3200 -3500 m 2 g -1 and 2.0 -2.7 cm 3 g -1 ) whilst attaining high packing density, which translates to exceptional volumetric hydrogen storage; 49 g l -1 at 40 bar, 60 g l -1 at 80 bar and 72 g l -1 at 150 bar and -196 o C, while at room temperature and 150 bar the densified carbons can store 3.4 wt% (18 g l -1 ). For lowly activated carbons (thermal treatment at 600 o C), compactivation yields carbons with 35% higher surface area and pore volume but with no pore size expansion. The increase in surface area arising from small (5.9 Å) micropores results in a dramatic increase in CO2 storage capacity; at 25 o C the CO2 uptake rises from 1.3 to 2.1 mmol g -1 at 0.15 bar, and from 3.4 to 5.5 mmol g -1 at 1 bar. Due to their lowly activated nature, the highly microporous compactivated carbons have high packing density and thus exhibit very high volumetric CO2 uptake of 79 g l -1 and 206 g l -1 at 0.15 and 1 bar, respectively (cf to 52 g l -1 and 136 g l -1 for conventionally activated analogue).

High yield and high packing density porous carbon for unprecedented CO2 capture from the first attempt at activation of air-carbonized biomass

“…24,[55][56][57] It is therefore highly noteworthy, as shown in Indeed, at all levels of activation, the yields of activated CNL1 carbons are twice as high as those previously reported for precursors such as biochar or polypyrrole (Table S2). [55][56][57] Thus the CNL1 carbon may be described as being resistant towards activation. Compactivation has no clear trend on the yield as there is a mix of values with yields for compactivated samples being higher, lower, or identical to conventionally activated analogues.…”

“…The textural properties of conventionally activated and compactivated CNL1-xT carbons are summarized in The porosity data in Figure 3, Figure 4, and Figure S9) or ratio of 4 ( Figure S10), along with other carbons derived from grass, 74 carbon nanotube superstructures 61 or polypyrrole. 55,56 It is clear from the comparison ( Figure S9, Figure S10 and Table S3) that the retention of microporosity is far higher in the activated CNL1 carbons compared to the other four previously reported classes of carbons. Even when activated at KOH/carbon ratio of 4 at 800 o C, the resulting CNL1-4800 sample is still predominantly microporous compared to all the other analogous samples that are mesoporous ( Figure S10 and Table S3).…”

“…The typical yield of activated carbons with moderate to high porosity is 15 -40 wt%. 24,[55][56][57] As far as we can tell from available literature data, the highest yield reported so far for KOH activation is ca. 40 wt%, which occurs at the lowest levels of activation.…”

“…This is surprising because most previous studies have noted a widening of the adsorption knee at higher activation temperature. 29,56,57,[61][62][63][64][65][66][67][68][69][70][71][72][73][74] The level of porosity generated, as judged by the amount of nitrogen adsorbed, increases modestly for samples prepared at higher activation temperature despite the non-changing nature and shape of the isotherms. The isotherms of the compactivated carbons are similar to those of analogous conventionally activated carbons except that they exhibit higher amounts of nitrogen sorption.…”

“…The size of pores (or pore size distribution) is identical for conventionally activated and compactivated carbons ( Figure 3B and Table 1), which confirms that the higher porosity of the compactivated carbons is not accompanied by pore enlargement. 56,73 The nitrogen sorption isotherms obtained for activated CNL1-4T carbons prepared at KOH/CNL1 carbon ratio of 4 are shown in Figure 4A. Despite the increase in amount of KOH used, the resulting CNL1-4T samples exhibit isotherms that are still remarkable typical of microporous materials.…”