Hydrogen Storage in Carbon Nanotubes at High Pressures LDRD Final Report 03-ERD-047

Abstract

This goal of this project was to perform feasibility experiments and measurements of the fundamental interactions between hydrogen and single wall carbon nanotubes (SWNT) at high pressures. High-pressure is an adjustable experimental parameter for tuning interaction strengths, thereby elucidating and providing insights into the fundamental nature of the H{sub 2}/SWNT system. We have developed and utilized systems and methodologies to make x-ray scattering, optical spectroscopic and electrical transport measurements. These activities have been productive in demonstrating capabilities and measuring properties of SWNTs under high-pressure conditions. We have also developed strong cooperative and complementary relationships with academic research colleagues at Stanford University. Building on these results and relationships, we hope to continue and expand our research as co-investigators in a joint Harvard-LLNL-Stanford proposal to the DOE ''Grand Challenge'' for Basic and Applied Research in Hydrogen Storage (Solicitation No. DE-PS36-03GO93013). Hydrogen storage is an active research topic with important basic science implications and a crucial enabling technology for advanced energy systems. Measurements of the H{sub 2} storage capacity indicate that it may achieve or exceed the storage capacity level (6.5 wt-%) mandated by the DOE hydrogen plan for fielding a hydrogen-fueled vehicle. The H{sub 2}/SWNT system has been the subject of intensive study and controversy regarding this storage capacity, with various conflicting measurements ranging from 1 wt-% up to values exceeding 7 wt-%[1-5]. The mechanism and details of the hydrogen storage in SWNT systems is poorly understood and several key questions have not been definitively determined including: (1) importance of SWNT structural parameters (tube length, diameter, end termination, bundling); (2) adsorption sites, mechanism, and binding energy; and (3) optimal storage capacity and conditions. This project sought to demonstrate techniques to address these key issues using high-pressure methodologies. Applying high-pressure conditions leads to an enhancement of the interaction between the hydrogen and the SWNTs. We use this capability in combination with optical, x-ray and electrical transport diagnostics to study the lattice parameters, vibrational energies, and electrical properties of hydrogen, SWNTs and the H{sub 2}/SWNT system. Additionally high-pressure/low-temperature conditions allow us to immerse SWNT samples in liquid hydrogen, thereby maximizing the hydrogen uptake. Specifically, we directed efforts at developing a high-pressure cell for electric transport measurements, a cryostat for loading liquid hydrogen and optical studies SWNTs in high-pressure diamond anvil cells (DACs), and x-ray scattering experiments to measure SWNT structural and packing parameters.