Hydrogen Storage in High Surface Area Carbons with Identical Surface Areas but Different Pore Sizes: Direct Demonstration of the Effects of Pore Size

We present experimental data that directly shows the effect of pore size on hydrogen uptake in high surface area porous carbons. A direct study of the influence of pore size has been made possible by comparing the uptake capacity of porous carbons with identical surface areas but with different pore sizes and pore size distributions. A variety of synthesis methods have been used to prepare carbon materials with similar surface areas with pore sizes ranging from the micropore range (12 angstrom) to supermicropore/lower mesopore (23 angstrom) and lower mesopore (31 angstrom) range. This allowed a simple and straightforward analysis of the influence of pore size without any changes in total surface area. The pore size essentially defines the hydrogen uptake with no apparent regard to the similar surface areas. The excess and total hydrogen uptake (at -196 degrees C and 20 bar) of carbons with identical surface areas of 3340 m(2)/g, increased from 3.7 and 5.4 wt % (31 angstrom sample), to 4.8 and 6.3 wt % (23 angstrom sample) and to 6.3 and 7.3 wt % for a 12 angstrom sample. The excess hydrogen storage density (mu mol H2 center dot m(-2)) decreases linearly with pore size from 9.5 at 12 angstrom to 7.3 at 23 angstrom and 5.5 at 31 A. Thus at a surface area of 3340 m2/g, a change of pore size from 31 to 12 angstrom improves the excess hydrogen storage by a staggering 70%. The pore size effect has general applicability; for carbons with similar surface areas of 2770 m(2)/g, the excess and total hydrogen storage was 1.7 and 3.0 wt % for a 28 A sample and increased to 5.6 and 6.4 wt % for and 15 angstrom sample. In this case, a change of pore size from 28 to ca. 15 angstrom results in a more than 3-fold increase in excess hydrogen storage. Therefore, to improve hydrogen storage capacity of carbons, we need to increase the surface area, but with pores of the right size. A high surface area and pore volume associated with large pores cannot compensate for unfavorably sized pores.