Synergistic light-metal activation in Aza-triphenylene covalent organic frameworks for reversible hydrogen uptake

Despite its immense potential as a clean and sustainable energy carrier, the widespread adoption of hydrogen (H₂) technologies is fundamentally constrained by the lack of safe, efficient, and high-capacity storage materials. In this study, we have performed spin-polarized density functional theory (DFT) calculations to explore the potential of a porous two-dimensional Aza-COF monolayer as an active material for storing H₂. Our results indicate that the H₂ adsorption energy on pristine Aza-COF falls below the threshold required for practical storage. Upon decoration with selected light metals (K, Ca), the H₂ adsorption energy is greatly enhanced. We have found that an Aza-COF unit cell can accommodate up to six K and Ca atoms. The metal-decorated Aza-COF structure can adsorb up to 30 H₂ molecules, with average adsorption energies of -0.251 eV for the 6K@Aza-COF and -0.173 eV for the 6Ca@Aza-COF, meeting the targets established by the U.S. Department of Energy. Furthermore, the gravimetric H₂ capacity reaches 6.54 and 6.49 wt% for the 6K@Aza-COF and 6Ca@Aza-COF, respectively. The thermodynamic analysis shows that the H₂ uptake in both systems increases with pressure and is strongly temperature-dependent, reaching maximum capacity at high temperature and pressure (∼30 H₂) at ∼400 K and 50 atm. 6K@Aza-COF shows a gradual increase across conditions, while 6Ca@Aza-COF exhibits a sharper rise above ∼340 K, indicating stronger adsorption sensitivity to thermal effects. Electronic and magnetic properties further indicate that Aza-COF holds promise as an excellent candidate for H₂ storage applications.