Palladium-doped Nanoporous Carbon Fibers for Hydrogen Storage

Pd-free and Pd-containing activated carbon fibers (Pd-ACF) were synthesized from isotropic pitch. The source of Pd was a palladium salt that was premixed with pitch before fiber spinning and carbonization. Hydrogen adsorption was measured at near-ambient temperatures (5 to 80 C) and moderate pressures (up to 20 bar). It was found that adsorption on Pd-ACF is always higher than that on corresponding ACF without Pd, and in excess of what it would be expected based solely on formation of Pd hydride. Additional characterization provided arguments in support of the hydrogen spillover mechanism. It was also found that temperature and pressure have opposite effects on physisorption and spillover. It was hypothesized that a narrow temperature range exists, where the kinetic advantage of H2 spillover in PdACF overlaps synergistically with the thermodynamic advantage of physisorption, thus contributing to enhanced uptakes compared with the Pd-free carbons.