Hydrogen storage properties of catalyst metal-doped single-walled carbon nanotubes.

摘要

In the past decade, hydrogen storage in solid-state materials has been one of the biggest hurdles to meet the storage density, safety, reliability and cost reduction needed for a hydrogen fuel economy. Single-walled carbon nanotubes (SWNTs) are particularly intriguing for hydrogen storage because each carbon atom is a surface site, and calculations have indicated that hydrogen bond strength can be tuned by adjusting the nanotube diameter. However, exposure to molecular hydrogen has resulted in only modest hydrogen uptake, as opposed to exposure to atomic hydrogen where significant hydrogen bonding has been observed. This has motivated studies of catalyst metal-doped SWNTs in order to facilitate the disassociation process of hydrogen molecules to improve hydrogen storage in these nanotubes.; In this work, the hydrogen uptake and release properties were measured for undoped and catalyst metal-doped high pressure CO conversion (HiPco) SWNTs. Palladium and platinum catalytic nanoparticles were electrochemically (EC), UHV sputter (Sp)-deposited and e-beam evaporated (EE) on the surface of SWNT bundles. Rigorous and precise measurements were taken by employing a specially-designed Sieverts' volumetric apparatus up to 30 Bar of pressure that is capable of measuring hydrogen storage in milligram quantities. The undoped SWNTs exhibited a reversible hydrogen uptake capacity of 0.17 wt% in gravimetric basis at room temperature. The capacity of Pd-doped SWNTs was increased to 0.53 and 0.72 wt% for EC and Sp-doped samples, respectively. This corresponds to an increase of a factor of 3 to 4 over undoped material. For the case of Pt-doped SWNTs, the uptake capacities of EC and Sp-doped samples were also increased to 0.40 and 0.51 wt%, respectively. The increase of the stored hydrogen is explained by sequential processes of molecular hydrogen dissociation, spillover, and surface diffusion. Hydrogen uptake kinetics was also measured and compared between the samples. The formation of stable C-H bonds was confirmed by XPS and FTIR spectroscopy techniques. It was observed that the hydrogen stored in the metal catalyst-doped SWNTs could be completely removed after a prolonged evacuation process even at room temperature. A metal-catalyzed hydrogen desorption model was developed, and the curve fitting by the model replicated the experimental observation.