Computational Evaluation of Al-Decorated g-CN Nanostructures as High-Performance Hydrogen-Storage Media

Density functional theory (DFT) calculations were employed to solve the electronic structure of aluminum (Al)-doped g-CN and further to evaluate its performance in hydrogen storage. Within our configurations, each 2 × 2 supercell of this two-dimensional material can accommodate four Al atoms, and there exist chemical bonding and partial charge transfer between pyridinic nitrogen (N) and Al atoms. The doped Al atom loses electrons and tends to be electronically positive; moreover, a local electronic field can be formed around itself, inducing the adsorbed H₂ molecules to be polarized. The polarized H₂ molecules were found to be adsorbed by both the N and Al atoms, giving rise to the electrostatic attractions between the H₂ molecules and the Al-doped g-CN surface. We found that each 2 × 2 supercell can adsorb at most, 24 H₂ molecules, and the corresponding adsorption energies ranged from -0.11 to -0.31 eV. The highest hydrogen-storage capacity of the Al-doped g-CN can reach up to 6.15 wt%, surpassing the goal of 5.50 wt% proposed by the U.S. Department of Energy. Additionally, effective adsorption sites can be easily differentiated by the electronic potential distribution map of the optimized configurations. Such a composite material has been proven to possess a high potential for hydrogen storage, and we have good reasons to expect that in the future, more advanced materials can be developed based on this unit.