Sodium-Decorated OctaNaphthylene as a Promising Material for Reversible Hydrogen Storage

Solid-state hydrogen storage is considered more advantageous than conventional physical storage methods for light-duty fuel cell electric vehicles (FCEVs). In this context, Na-decorated OctaNaphthylene (ONP) was investigated for reversible hydrogen storage using state-of-the-art density functional theory calculations. The pristine ONP monolayer adsorbs hydrogen molecules weakly, making it unsuitable for practical storage applications. In view of this, Na-decoration was employed in ONP to overcome this problem. Na dopants bind strongly to the ONP surface with a binding energy of −2.24 eV/atom. The diffusion energy barrier of the Na dopants on the ONP surface was calculated to be 0.495 eV, indicating suppressed metal clustering. The Na-decorated ONP system can adsorb up to 10 H₂ molecules with an average hydrogen adsorption energy of −0.208 eV/H₂, achieving a gravimetric hydrogen storage capacity of 7.14 wt%, which surpasses the Department of Energy (DOE) target. The hydrogen desorption temperature was estimated to be 265.93 K at 1 atm pressure. The hydrogen adsorption mechanism was analyzed using Bader charge analysis, charge density difference (CDD) plots, and electronic structure calculations, including band-structure and projected density of states (PDOS) analyses. The thermal stability and reversibility of hydrogen adsorption were examined using ab initio molecular dynamics (AIMD) simulations, while the dynamic stability was confirmed by the phonon dispersions. Based on these investigations, the Na-decorated ONP monolayer was identified as an efficient hydrogen storage material in accordance with DOE guidelines.