Thermogravimetric Measurement of Hydrogen Absorption in Alkali-Modified Carbon Materials

Abstract

We have prepared Li-doped multiwall carbon nanotubes and Li- and K-intercalated graphite and measured their hydrogen storage properties using a thermogravimetric analyzer (TGA). In a flowing H2 atmosphere Li-doped nanotubes and Li-intercalated graphite both exhibit a cyclable weight gain between 200 and 400 °C and weight loss between 400 and 500 °C characterized by a distinct and unusual temperature profile. We find, however, that neither H2 nor carbon is required to generate this TGA feature; we observe it even in Li-containing samples measured in flowing Ar without H2 and in LiOH samples measured in either H2 or Ar. Potassium-intercalated graphite shows mass cycling with a different thermal character between 40 and 250 °C, but as with Li, observation of a large cyclable feature does not rely on the presence of H2. In both cases we identify the cycling mass to be absorption/desorption of H2O present as an impurity in the TGA atmosphere. The temperature signatures we observe are strikingly similar to those reported in a recent study of Li- and K-doped carbon nanofibers in which mass uptakes as large as 20 wt % were attributed to hydrogen absorption. When the impurities in the TGA atmosphere are reduced as much as possible we do detect modest weight changes in K-intercalated graphite which we interpret as true hydrogen absorption at 1.3 wt %, of which 0.2 wt % is cyclable. This level of hydrogen absorption is consistent with pressure−composition isotherm measurements on the same material using a gas reaction controller (1.0 wt % total absorption with 0.3 wt % cyclable). We do not detect any evidence of hydrogen absorption by Li-containing carbon materials under our experimental conditions.