DOE OSTI2011
The objective of the project was to exploit the unique morphology, tunable porosity and excellent metal supportability of single-walled carbon nanohorns (SWNHs) to optimize hydrogen uptake and binding energy through an understanding of metal-carbon interactions and nanoscale confinement. SWNHs provided a unique material to understand these effects because they are carbon nanomaterials which are synthesized from the 'bottom-up' with well-defined, sub-nm pores and consist of single-layer graphene, rolled up into closed, conical, horn-shaped units which form ball-shaped aggregates of {approx}100-nm diameter. SWNHs were synthesized without metal catalysts by the high-temperature vaporization of solid carbon, so they can be used to explore metal-free hydrogen storage. However, SWNHs can also be decorated with metal nanoparticles or coatings in post-processing treatments to understand how metals augment hydrogen storage. The project first explored how the synthesis and processing of SWNHs could be modified to tailor pore sizes to optimal size ranges. Nanohorns were rapidly synthesized at 20g/hr rates by high-power laser vaporization enabling studies such as neutron scattering with gram quantities. Diagnostics of the synthesis process including high-speed videography, fast pyrometry of the graphite target, and differential mobility analysis monitoring of particle size distributions were applied in this project to provide in situ process control of SWNH morphology, and to understand the conditions for different pore sizes. We conclude that the high-temperature carbon-vaporization process to synthesize SWNHs is scalable, and can be performed by electric arc or other similar techniques as economically as carbon can be vaporized. However, the laser vaporization approach was utilized in this project to permit the precise tuning of the synthesis process through adjustment of the laser pulse width and repetition rate. A result of this processing control in the project was to eliminate the large (2-3 nm) internal pores of typical SWNHs which were found not to store hydrogen effectively. Post processing treatments of the as-synthesized SWNHs focused on pore size, surface area, and metal decoration in order to understand the effects of each on measured hydrogen uptake. Wet chemistry and gas-phase oxidation treatments were developed throughout the life of the project to adjust the interstitial and slit pore sizes of the as-produced SWNHs, and increase the surface area to a maximum value of 2200 m2/g. In addition, wet chemistry approaches were used to develop methods to decorate the nanohorns with small Pt and Pd nanoparticles for metal-assisted hydrogen storage. Finally, oxygen-free decoration of SWNHs with alkaline earth metals (Ca) was developed using pulsed laser deposition and vacuum evaporation in order to produce surface coatings with high static electric fields sufficient to polarize and bind dihydrogen. Decoration of SWNHs with Pt and Pd nanoparticles resulted in enhanced binding energy (NREL, 36 kJ/mol), as well as enhancement in the room temperature uptake of 0.6 wt.% (for undecorated, oxidized, pure-C SWNHs at 20 bar), to 1.6 wt% for Pt- and Pd-decorated SWNHs at 100 bar, comparable to MOF-177 materials. NIST neutron scattering on gram quantity Pt- and Pd-decorated SWNHs showed clear evidence for 'spillover' type losses of molecular hydrogen and determined the onset temperature for this effect to be between 150K < T < 298K.High (2142 m2/g) surface area SWNH materials with variable pore sizes and metal-decorated SWNHs were demonstrated with metals (Pt, Pd) resulting in increased excess storage (3.5 wt. % at 77K). Compression results in bulk SWNH samples with density 1.03 g/cm3, and 30 g/L volumetric capacity. In summary, SWNHs were found to be unique carbon nanomaterials which can be produced continuously at high rates from vaporization of pure carbon. Their inherent pore structure exhibits significant room temperature hydrogen storage in sub-nm pores, and their morphology serves as an excellent metal catalyst support for small (2-3 nm nanoparticles). Pt- and Pd-nanoparticle-decorated SWNHs exhibit clear evidence for metal-assisted hydrogen storage which is activated at T>150 K, permitting additional room-temperature storage up to 1.8 wt.% at 100 bar. One of the key results of the project were theoretical predictions for doped, decorated, and filled nanostructures with distributed charge to maintain high static electric fields sufficient to polarize and bind hydrogen. This concept indicates a promising new direction for hydrogen storage materials.