How cations catalyse the hydrogenation of graphene vacancies

In alkaline solutions hydrogen adsorption involves the splitting of a water molecule, which can be catalysed by the presence of cations. We have analysed the adsorption processes from a water molecule approaching the surface of a graphene bilayer containing a vacancy, both in the presence and in the absence of cations (Na⁺, K⁺ and Mg²⁺) by quantum molecular dynamics using DFT based tight-binding. A cation near the interface induces a region with positive excess charge in its solvation shell, which facilitates the orientation of the reacting water molecule with the hydrogen towards the surface. This concerted mechanism catalyses the hydrogenation and stabilizes the resulting hydroxyl. Therefore in the presence of cations the hydrogen evolution reaction occurs spontaneously at less negative charges than in the absence. Thus our investigations suggest a detailed scenario for the catalytic effect of cations both on the spatial and temporal nano-scale.