Reversible Hydrogen Storage via Na-Decorated Group III Nitride Naphthylene Analog: Toward Practical Desorption and Elemental Sustainability

Studies indicate so far that materials with low gravimetric capacity yet high volumetric hydrogen (H2) density, such as certain metal hydrides and borohydrides, remain viable in practical applications due to their capacity to store and release substantial quantities of H2 per unit volume while minimizing system complexity. Herein, the H2 storage potential of the proposed pristine and Na-decorated XN (X = Al, Ga, and In) was examined using ab initio density functional theory calculations. The stability of these direct band gap materials was confirmed by ab initio molecular dynamics (AIMD) and projected Crystal Orbital Hamilton Population (pCOHP) calculations. Although Na could be firmly attached to all proposed monolayers with adsorption energies ranging from −1.08 to −2.08 eV, only the absolute value for InN greatly surpassed the cohesive energy of bulk Na. Improved interaction between infused H2 and Na adatom results in an average adsorption energy between −0.11 and −0.37 eV, aligned with the US Department of Energy (DOE) 2025 standards (−0.1 to −0.6 eV). Additionally, minimal elongation of the average H–H bond length is observed, indicating a nondissociative adsorption of the H2 molecules. The findings demonstrate that the five and six adsorbed H2 molecules on 2Na@InN, with a desorption temperature of 230–303 K under delivery pressure, exhibit a favorable equilibrium of trade-off performances. Subsequent thermodynamic analysis revealed that the number of adsorbed H2 molecules could be elevated to 19H2 under practical temperature and pressure. Despite the gravimetric capacity being significantly below 5.0 wt %, the equilibrium among adsorption energy, desorption temperature, volumetric density exceeding 40 kg H2/m3, and material sustainability renders this system a possible candidate for scalable H2 storage under low-energy, ambient-pressure circumstances where volumetric capacity dominates system design, such as in modular tanks and stationary systems that enable on-site and power plants applications.