Double layers of H₂ adsorption on an AlN sheet induced by electric field

The hydrogen storage properties on a single nano-scale AlN sheet with and without an applied electric field are investigated by using first-principles method based on density functional theory. The H₂ molecule on top of a N atom is the most stable adsorption state, while the H₂ molecule on top of an Al atom is a metastable one in the absence of an applied electric field. The diffusion barrier of the H₂ molecule from the metastable state to the most stable state is about 0.002 eV, which can be easily overcome in experiments. It is found that two layers of H₂ molecules can be steadily adsorbed on and below (upper H₂ molecules on the N atoms and lower H₂ molecules on the Al atoms) the sheet, respectively, by applying a suitable electric field. The reliability of H₂ adsorption on the AlN sheet is evaluated by investigating the changes of H₂ adsorption energy, bond lengths, and density of states. The stored H₂ molecules can be controllably released by reducing the strength of the applied electric field, thus making reversible hydrogen storage on the AlN sheet.