Computational Design for Enhanced Hydrogen Storage on the Newly Synthesized 2D Polyaramid via Titanium and Zirconium Decoration.

2D polyaramid (2DPA) is a porous and polymeric material that has been synthesized recently. Titanium and zirconium decoration over 2DPA increases their affinity for hydrogen substantially, making them suitable for onboard and reversible hydrogen storage, particularly in light-duty vehicles. By decorating a single unit cell of 2DPA with two transition metal (TM) atoms, hydrogen storage of up to 6.422 and 6.792 wt % of H2 with average binding energies of -0.399 and -0.480 eV is predicted for 2DPA + Ti and 2DPA + Zr, respectively. The binding of Ti and Zr with 2DPA is accompanied by a flow of charge (-1.474e for Ti and -1.696e for Zr) from the TM toward the 2DPA sheet. Further, the interaction between H2 and the TM may proceed via Kubas interaction between the d orbital of the TM in 2DPA + TM and H 1s orbitals of H2, with a net flow of charge from the TM toward H2 (-0.218e for Ti and -0.391e for Zr). The desorption of H2 bound to 2DPA + Zr is endothermic (∼0.57 eV) and close in magnitude to the binding energy of the first H2 (∼-0.544 eV). The 2DPA + TM systems show structural and dynamic stability at high temperatures, as evident from ab initio molecular dynamics simulations and phonon spectra. The movement of TM atoms across the 2DPA sheet to form clusters may be hindered by the considerable barrier energy (∼4.9 eV for Ti). Through these systematic density functional theory simulations, we predict that Ti- and Zr-decorated 2DPA are high-performance hydrogen storage materials and can be explored by experimentalists.