Microporous Carbon Adsorbents for Hydrogen Storage

The desirable characteristics of activated carbons for gas-storage applica tions are: (1) high microporosity (pores smaller than 2 nm) ; and (2) high sorbent packing density, i.e., low voidage in the storage container (e.g., the use of mono lithic sorbent elements). A cyclic chemisorption-desorption char activation tech nique was used to maximize micropore formation and minimize mesoporosity. Several carbons were prepared at different degrees of burn-off, and the BET surface areas were found to be up to 2000 m2/g. The carbons were prepared in the form of powders and pellets to demonstrate that the future use of shaped mono lithic elements can lead to the reduction of voidage in the storage container by up to 40%. The adsorption isotherms of the produced carbons showed high microporosity and no appreciable mesoporosity, even at high burn-offs. The advantage of using the cyclic chemisorption-desorption char activation technique over the traditional steady-state gasification was experimentally demonstrated. The paper also discusses the evolution of sorbent microporosity as a function of carbon burn-off and the duration of the chemisorption step. Preliminary data on hydrogen-storage capacity are also reported.