Zr-Decorated Synthesized 2D Aza-Triphenylene for High-Capacity Hydrogen Storage: Insights from DFT Simulations

In our study, we explored the practical hydrogen storage properties of transition-metal (Zr)-doped two-dimensional conjugated covalent organic frameworks (COFs), i.e., Aza-triphenylene + Zr (AzaCOF + Zr). We computed the subsequent H2 adsorption energies and obtained an average adsorption energy of −0.38 eV/H2. The average desorption temperature at 5 bar pressure is 310.46 K. The AzaCOF + Zr system can store a maximum of seven H2 molecules per unit cell, resulting in a weight percentage of 7.26 wt %, which is higher than the gravimetric density requirement (6.5%) set by the US Department of Energy (US-DOE) for H2 storage. Using charge transfer and orbital density of states (DOS) analyses, we elucidated the mechanism of TM and H2 binding in Zr-decorated AzaCOF. A charge-transfer mechanism mediates the interaction between H2 and the AzaCOF + Zr system, wherein each Zr atom loses a net charge of 1.58e to the AzaCOF system, computed by the aid of Bader and charge density analysis. The structural stability of Zr-decorated AzaCOF is checked using ab initio molecular dynamics simulations at a temperature of 300 K. A high diffusion energy barrier (6.4 eV) encountered by the metal adatom nullifies the possibility of metal cluster formation. Thus, by the assistance of density functional theory simulations, we predict high-storage performance of hydrogen in Zr-doped AzaCOF. Such promising theoretical predictions may inspire the experimentalists to design Zr-doped AzaCOF as a high-capacity H2 storage material.