Capillary Densification of Hydrogen in Nanoporous, Amorphous Carbons

Abstract

We have studied the effect of synthesis conditions on the nanostructure of advanced carbons that show moderate to high hydrogen storage. We have performed in situ studies of the dynamic pyrolysis of palladium catalyzed multifunctional polymers (MFP) in nitrogen environments, to understand the effect of process parameters and to utilize the results in the development of optimum nanostructures to meet the storage requirements. Carbon sorbents have been synthesized from polysaccharides including chitosan, chitin, and cellulose. Pyrolysis in a hydrogen atmosphere at 1373 K and 24 h has lead to microporous carbons with an optimum surface area and pore volume exceeding 2000 m² g⁻¹ and 2.0 cc g⁻¹, respectively. Gravimetric microbalance studies indicate that these materials physically absorb hydrogen and store between 7 to 10 mass percent at 10 bar and 100 K. TEM micrographs indicate that the amorphous carbon shows additional ordering with prolonged hydrogen treatment from 2 to 24 h which has proven beneficial in increasing hydrogen storage. X-ray diffraction, TEM analysis, and porosimetry measurements also indicate that extending hydrogen exposure beyond 24 h leads to decreasing hydrogen storage due to the continuation of a loss in carbon relative to metal contaminants (P, Ca, Na) in the starting materials and increased crystallinity. Comparison of the results have shown that for advanced carbons derived from polymers or natural products treated with hydrogen at high temperatures, the optimum nanostructure is substantially disordered (turbostratic) graphite, whereas for materials derived from MFP, it is primarily amorphous carbon. The experiments have also revealed the optimum temperature region for storage where sorption/desorption curves show evidence of capillary condensation/densification.