Using hydrogen activated by microwave plasma vs. molecular hydrogen for hydrogen storage in tungsten disulfide inorganic nanotubes

In this work we investigate the inorganic nanotubes of layered tungsten disulfide, as material for hydrogen storage. These nanotubes may allow hydrogen to be either chemi- or physisorbed inside their crystalline structure (in between the layers), inside hollow core of nanotubes, on the surface or in the open interstitial pore spaces of nanotubes' powder mesh. While exposure to molecular hydrogen was found to have measurable but limited absorption rate – up to 0.13 wt.%, the exposure to hydrogen activated by microwave (MW) plasma resulted in much higher value of adsorbed hydrogen of ∼1 wt.%. These observations could be attributed to more effective interaction of activated vs. molecular hydrogen with nanotubes surface due to the strong chemisorption of activated hydrogen compared to weaker physisorption of molecular hydrogen. We report here the results of such exposures and analyze the absorption and diffusion of hydrogen by different methods: adsorption–desorption curves obtained by pressure–composition–temperature isotherm measurements, and hydrogen depth profiles measured by Secondary Ion Mass Spectroscopy. We found that 5 min exposure to MW plasma at 400 W and 60 Torr (causing local heating up to ∼100 °C) results in substantial hydrogen retention, though some etching of the substrate material may occur during such treatment.