Heat Treatment-Induced Structural Changes in SiC-Derived Carbons and their Impact on Gas Storage Potential

We investigate the effect of heat treatment on the structure of carbide-derived carbons (CDC) prepared by chlorination from nanosized beta SiC particles and on their methane as well as hydrogen storage and delivery performance. Pore size and pore wall thickness distributions of the CDCs are obtained from interpretation of argon adsorption data using the finite wall thickness (FWT) model. The adequacy of the FWT model for adsorption modeling in the SiC-CDC samples is demonstrated by satisfactory prediction of subatmospheric and high pressure adsorption isotherms of CO2 and CH4 at 313 and 333 K. From the characterization results, it is observed that the SiC-CDC particles are predominantly amorphous with slight graphitization of the external surface. The degree of graphitization is more pronounced in the sample prepared at 1000 degrees C and increases slowly with heat treatment time. During this time the accessibility of methane molecules is found to increase, as a result of short-range ordering and opening up of pore entrances. Nevertheless, methane storage capacity is unsatisfactory, despite the high surface area and porosity, clue to accessibility problems. On the other hand improvement in high pressure H-2 uptake (4.61 wt % at 77 K) is obtained for SiC-CDC chlorinated at 800 degrees C and heat treated for one day. The recently predicted optimal delivery temperature of 115 K for hydrogen storage is found to be appropriate for this material. It is demonstrated that accessibility is an important issue to be addressed for methane storage in carbons, but which has hitherto not received attention for this application.