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

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 degrees 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 - 196 degrees C 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 degrees 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 degrees 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 angstrom) micropores results in a dramatic increase in CO2 storage capacity; at 25 degrees 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). (C) 2015 Elsevier Ltd. All rights reserved.