Evidence for large hydrogen capacity in single-walled carbon nanotubes encapsulated by thin Pd layers onto a Pd substrate

We report a study of hydrogen storage and its mechanism in a novel material, representing single-walled carbon nanotubes (SWCNTs) encapsulated by thin Pd layers onto a Pd substrate. A synergetic effect resulting in combination of the Pd and the SWCNT properties with regard to hydrogen has been achieved. We showed that adding SWCNTs increases the H₂-capacity of the Pd–SWCNT composite under electrochemical loading only by up to 25% relative to the Pd metal alone. At the same time, with regard to the added SWCNTs, such synergetic approach (providing high H₂ pressure from highly H-loaded massive Pd substrate into a small fraction of deposited SWCNT) allowed us to achieve a net capacity of 8–12 wt.%. H₂, thus, bringing a unique chance to study hydrogen storage mechanism in highly H-loaded SWCNT. Using ESR technique it was established that the Pd–C_x π-complexes forming at the openings of SWCNTs could be considered as hydrogen adsorption sites, providing both high gravimetric capacity (H/C > 1) and low hydrogen binding energy in the Pd encapsulated SWCNT.