Selective decoration of nitrogenated holey graphene (C₂N) with titanium clusters for enhanced hydrogen storage application

For an envisioned hydrogen (H₂) economy, the design of new multifunctional two-dimensional (2D) materials have been a subject of intense research for the last several decades. Here, we report the thriving H₂ storage capacity of 2D nitrogenated holey graphene (C₂N), functionalized with Ti_n (n = 1–5) clusters. By using spin polarized density functional theory (DFT) calculations implemented with the van der Waals corrections, the most favourable adsorption site for the Ti_n clusters on C₂N has been revealed. With the monomer Ti, the functionalization was evenly covered on C₂N having 5% doping concentration (C₂N–Ti). For C₂N–Ti sheet, Ti binds to C₂N with a strong binding energy of ~6 eV per Ti which is robust enough to hinder any Ti–Ti clustering. Bader charge analysis reveals that the Ti_n clusters donate significant charges to C₂N sheet and become cationic to polarize the H₂ molecules, thus act as efficient anchoring agents to adhere multiple H₂ molecules. Each Ti in C₂N–Ti could adsorb a maximum of 10H₂ molecules, with the adsorption energies in the range of −0.2 to −0.4 eV per H₂ molecule, resulting into a high H₂ storage capacity of 6.8 wt%, which is promising for practical H₂ storage applications at room temperature. Furthermore, Ti_m (m = 2, 3, 4, 5) clusters have been selectively decorated over C₂N. However, with Ti_m functionalization H₂ storage capacities fall short of the desirable range due to large molecular weights of the systems. In addition, the H₂ desorption mechanism at different conditions of pressure and temperature were also studied by means of thermodynamic analysis that further reinforce the potential of C₂N–Ti as an efficient H₂ storage material.