  Hydrogen is a promising element for applications in new energy sources like
fuel cells. One key issue for such applications is storing hydrogen. And, to
improve storage capacity, understanding the interaction mechanism between
hydrogen and possible storage materials is critical. This work uses DFT
simulations to comprehensively investigate the adsorption mechanism of H/H$_2$
on the 2D metallic porphyrins with one transition metal in its center. Our
results suggest that the mechanism for adsorption of H (H$_2$) is chemisorption
(physisorption). The maximum adsorption energy for atomic hydrogen was $-3.7$
eV for 2D porphyrins embedded with vanadium or chromium atoms. Our results also
revealed charge transfer of up $-0.43$ e to chemisorbed H atoms. In contrast,
the maximum adsorption energy calculated for molecular hydrogen was $-122.5$
meV for 2D porphyrins embedded with scandium atoms. Furthermore, charge
transfer was minimal for physisorption. Finally, we also determined that
uniaxial strain has a minimal effect on the adsorption properties of 2D
metallic porphyrins.
