Tuning Hydrogen Adsorption in B₄CN₃ Monolayers: The Role of Metal Decoration and Vacancy Defects

This study investigates the hydrogen storage performance of various metal-decorated pristine and defective B₄CN₃ monolayers using first-principles calculations. The selected metals span alkali, alkaline earth, and 3d transition-metal (TM) series. All metal-decorated B₄CN₃ systems exhibit thermodynamic stability, as illustrated through their negative binding energies. The adsorption behavior and interaction strength of hydrogen are influenced by the type of metal, with alkali and alkaline earth metals showing weak physisorption and TMs demonstrating moderate to strong interactions via Kubas adsorption modes. The adsorption strength between metal atoms and hydrogen is crucial in determining the efficiency of hydrogen storage materials. In particular, Li-decorated pristine B₄CN₃ achieves a maximum gravimetric hydrogen storage capacity of 12.59 wt %, but its desorption temperature is too low due to the weak physisorption. To improve the hydrogen storage properties, vacancy defects were introduced. Among the investigated vacancy defects, the carbon vacancy (V_C) is the most energetically favorable. V_C leads to a stronger hydrogen adsorption energy and higher desorption temperature. This improvement is attributed toa shift in the Fermi level toward the vacuum level, which increases the polarizability of the substrates and enhances the H₂ adsorption. In addition, practical hydrogen storage assessed using ab initio molecular dynamic simulations at various desorption temperatures and pressures reveals that Mg, Ca, and Sc are promising candidates for pristine B₄CN₃, while Li, Na, K, and Ca were identified for defective B₄CN₃. This work provides valuable insights for the development of advanced hydrogen storage systems that leverage defective B₄CN₃ monolayers.