Hydrogen transfer and quinone/hydroquinone transitions in graphene-based materials

A computational quantum chemistry study, guided by critical experimental facts and issues, is carried out in an effort to provide a long-missing link between the very much intertwined processes of hydrogen, oxygen and electron transfer on the surface of graphene-based materials. Density functional theory is used to identify representative ground and transition states of prototypical graphene clusters. In particular, we analyze the effect of heteroatoms (oxygen, nitrogen and boron) on the electron density distribution surrounding the carbon active sites. Hydrogen transfer, especially the much invoked quinone/hydroquinone transition, is found to be very sensitive to, and significantly facilitated by, their presence. Thus, for example, the energy barrier for H-transfer at an aromatic armchair edge is reduced from 84 kcal/mol to 61 kcal/mol in the presence of a phenolic group, and to 20 kcal/mol if a contiguous carbene-type site is available; in the presence of substitutional N and B the analogous barriers are 17 and 44 kcal/mol. The implications of these findings for important oxygen/electron transfer processes such as the oxygen reduction reaction (ORR), including the controversial identification of the active sites, are presented and discussed.