Hydrogen Evolution\nReaction in Borophene: Puckering\nMechanism and the Role of Coordination Number

Using the free energy of hydrogen adsorption (Δ<i>G</i><sub><i>H</i></sub>) as the indicator, five borophene\nphases\nare previously shown to possess high catalytic activity for the hydrogen\nevolution reaction (HER). On these borophene phases, we investigate\nthe role of the coordination number (CN = 4, 5, 6) of the adsorption\nsites and the puckering of the adsorption site. CN is discovered to\nhave a profound effect on the Δ<i>G</i><sub><i>H</i></sub> distribution, charge, and puckering height (<i>h</i>) of adsorption sites. Employing bonding indicators, we\ninvestigated the puckering mechanism. An increase in <i>h</i> directly increases the bond order and bond strength of the adsorption\nbond, but the associated energy cost of bond stretching hinders further\npuckering. The occupation and localization of σ orbitals determine\nthe strength of local σ bondsthe key factors determining <i>h</i>. External biaxial strain is revealed to monotonically\nmodulate the 2<i>s</i> character in local σ bonds,\nthus affecting bond stretchability, altering <i>h</i> and\nΔ<i>G</i><sub><i>H</i></sub> subsequently.\nThese insights are valuable for borophene-based HER applications and\nprovide an analytical framework for the adsorption on 2D materials.