Small angle neutron scattering is used to investigate materials which could be used for hydrogen storage.
Activated carbons are of interest for hydrogen storage because hydrogen molecules can be trapped on the carbon surface. Such trapping works best in nanoscale ‘slit pores’, where adsorbate molecules interact with both walls.
The standard method to measure porosity, helium pycnometry, overestimates pore volumes because condensation of the gas increases the apparent carbon density. The use of small angle neutron scattering (SANS) and contrast-matching liquids circumvents these difficulties. Using mixtures of H/D-toluene to match the scattering from carbon, we can obtain the carbon density at different length scales. On the scale of atomic distances, this density is close to that of graphite. At larger, nanometer length scales, the density drops owing to the presence of porous structure.
SANS data for carbon show fractal behaviour at the larger length scales, and a well-defined hump at atomic scales caused by the activation (oxidation) process. From these data, the minimum pore radius as a function of toluene partial pressure can be found. The SANS data also allow us to compare the accessible pore volumes at each partial pressure with those obtained gravimetrically.
Z Mileeva, DK Ross, D Wilkinson (University of Salford), S King (ISIS), T Ryan, H Sharrock (Chemviron Carbon Ltd)