Hydrogen Evolution Reaction in Borophene: Puckering Mechanism and the Role of Coordination Number

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