Effect of Surface Oxygen Groups in Carbons on Hydrogen Storage by Spillover

Hydrogen adsorption properties of two different carbon materials (superactivated carbon, AX-21, and graphite oxide) doped with Pd nanoparticles have been studied. The effect of surface oxygen groups in AX-21 on hydrogen storage was investigated, and the results showed Pd supported on oxygen-modified AX-21 (Pd/AX-21-O) had a higher hydrogen storage capacity than Pd supported on unmodified AX-21 (Pd/AX-21). Overall heats of adsorption were obtained from the temperature dependence of the isotherms, and higher values were shown on the oxygen-modified AX-21 sample than the unmodified one, showing that the surface oxygen groups were favorable for hydrogen adsorption. Furthermore, a sample comprising Pd supported on graphite oxide with more surface oxygen (Pd/graphite oxide) was studied. The hydrogen adsorption on this sample showed a hydrogen storage capacity of 0.95 wt % at 100 atm and 298 K. Normalized by the BET surface area, the Pd/graphite oxide exhibited 2.8 x the storage capacity of Pd/AX-21-O and 3.4 x that of the Pd/AX-21 sample. Molecular orbital calculations showed that the presence of oxygen groups increased the binding energies of the spiltover H on graphite, which is consistent with the experimental results.