Enhancement of Hydrogen Storage Capacity of Zeolite-Templated Carbons by Chemical Activation

Chemical activation of zeolite-templated carbons (ZTCs) with KOH increases their textural properties and enhances hydrogen storage capacity. The KOH activation, at a carbon/KOH weight ratio of 1:4, was performed at 800 degrees C for 1 h. The surface area of the zeolite Y templated carbons, which possessed moderate zeolite-like structural ordering, increased by up to 84%, while the pore volume more than doubled. The surface area increased from 1400-1650 to between 1850 and 3100 m(2)/g, whereas the pore volume rose from 0.8-1.1 to 1.5-1.75 cm(3)/g. In general, small pore channels with sizes <1.5 nm already present in the ZTCs were retained after activation, along with an increase in the proportion of pores sized between 1.5 and 2.5 nm and, in some cases, up to 4 nm. The KOH activation mainly acts by generating large micropores and small mesopores with the actual size of pores generated being dependent on the level of graphitization of the ZTC; graphitization favored the formation of larger pores. The greatest increases in surface area and microporosity were, therefore, observed for the least graphitic ZTCs. The KOH activation caused an enhancement in hydrogen uptake capacity (at -196 degrees C and 20 bar) of between 60 and 96% from 2.4-3.5 wt % for ZTCs to between 4.3 and 6.1 wt % for activated ZTCs. Although the increase in hydrogen storage depends on rises in surface area and pore volume after activation, the changes in storage capacity also show that, although a high pore volume is desirable for hydrogen storage, it is more advantageous if a significant proportion of the volume is from micropores.