Hydrogen storage in alkali-decorated N-doped biphenylene: a DFT study

The hydrogen storage performance of alkali-decorated nitrogen-doped biphenylene monolayers was investigated using density functional theory calculations. Two possible nitrogen substitution configurations were examined, and the 2-N structure was identified as the energetically preferred configuration. Among the investigated adsorption sites, the octagonal ring provides the most stable anchoring for Li, Na, and K atoms. The hydrogen storage properties were evaluated in terms of adsorption energy and gravimetric density. By adopting 0.2 eV per H $_2^{}$ as the lower limit for practical reversible adsorption, the maximum hydrogen storage capacities were determined to be 9, 12, and 11 H $_2^{}$ molecules for Li-, Na-, and K-decorated structures, corresponding to gravimetric densities of 5.72, 7.12, and 6.27 wt%, respectively. Furthermore, double-sided hydrogen adsorption on the Na-decorated monolayer increases the gravimetric density to 12.50 wt%. These results suggest that Na-decorated nitrogen-doped biphenylene is a promising candidate for reversible hydrogen storage applications.

Density functional theory calculations were performed using the SIESTA package. Exchange-correlation effects were described within the generalized gradient approximation using the Perdew-Burke-Ernzerhof functional together with Grimme's dispersion correction. A Monkhorst-Pack $21\times 21\times 1$ k-point mesh, a double- $\zeta$ plus polarization basis set, and an energy cutoff of 200 Ry were employed. Geometry optimizations were carried out until the total energy converged to better than ${10}^{-5}$  eV and the residual force on each atom was below 0.01 eV/Å.