Dynamic study of the electrochemical hydrogenation of carbon atoms at the edge of a vacancy in a graphene bilayer

A single vacancy in a graphene bilayer has three carbon atoms with dangling bonds at the edge. We have studied the successive electrochemical hydrogenation of these sites in acidic media by quantum molecular dynamics at room temperature in the presence of water. In order to mimic the electrochemical environment, charged and uncharged systems have been studied. All three processes occur spontaneously and exothermally even though the total system is uncharged. In order to represent an acidic environment, we have analysed the electrosorption processes from a hydrated proton approaching the surface. The presence of water favours hydrogenation. Two carbon atoms at the edge may form a Jahn-Teller distortion, whose formation and stabilization play an important role in the hydrogenation. As a consequence the hydrogenations of the first and third carbon at the edge of the vacancy are more favourable than that of the second carbon, which involves the breaking of a distortion. Different configurations of the adsorbed hydrogen located above or below the top graphene layer have been observed. Contrary to the case of hydrogenated pristine graphene, where hydrogen can desorb easily at negative charges by the Heyrovsky mechanism or interaction with anions, in the case of hydrogenated carbons at the edge of the vacancy we have never observed nor the formation of molecular hydrogen neither its displacement by interaction with anions. Although we focus only on the electrochemical behaviour of graphene, our results also contribute to a better understanding of the electrocatalysis of the hydrogen evolution / oxidation reactions in general. The interplay between the applied charge at the interface and the polarization of the hydrogen - substrate bond determines the reaction mechanism.