Sieverts apparatus and methodology for accurate determination of hydrogen uptake by light-atom hosts

Accurate determination of the quantity of hydrogen absorbed by a potential hydrogen storage material is crucial to progress in the field. The most common techniques for measuring hydrogen uptake from the gas phase by a solid host, the Sieverts technique and gravintetry, both become susceptible to systematic errors as the density of the host material decreases. We focus here on the Sieverts technique, which in a poorly designed apparatus may produce errors similar to 100% in the quantity of absorbed hydrogen owing to a realistic 25 % error in the density of a light-atom sample. Using hydrogen absorption isotherms measured for low-density materials, including carbon nanotubes, potassium-intercalated graphite and lithium nitride, we show that designing the Sieverts apparatus with carefully chosen volumes greatly lessens the impact of uncertainty in the sample density. Rules-of-thumb for the volumes in the apparatus and the volume occupied by the sample itself, and a figure of merit for the sensitivity of the system to changes in the hydrogen content of the sample, are introduced. (C) 2007 Elsevier B.V. All rights reserved.