Well-Defined Boron/Nitrogen-Doped Polycyclic Aromatic Hydrocarbons Are Active Electrocatalysts for the Oxygen Reduction Reaction

Top-down synthesized B- and B,N-doped carbons (e.g., graphenes) have been previously reported as catalysts for the oxygen reduction reaction (ORR), with activity superior to Pt electrocatalysts also previously reported in some cases. Such doped carbon materials are, however, chemically complex and contain multiple sites, which complicate the development of structure–activity relationships and subsequent catalyst optimization. Herein, a number of well-defined B- and B,N-doped polycyclic aromatic hydrocarbons (PAHs), prepared by a "bottom-up" approach, are shown to be active catalysts for the ORR in alkaline solution when deposited on carbon electrodes in contrast to the all carbon-based PAH perylene. Six dissimilar B-doped PAHs have been tested on three working electrodes, and the merits of each electrode for assessing ORR catalytic activity have been determined. A boron-doped diamond electrode was found to have the lowest background activity (relative to glassy carbon and highly ordered pyrolytic graphite ) and thus proved optimal for determining the ORR catalytic activity of the PAHs. Of the six B-doped PAHs studied, two PAHs with the highest lowest unoccupied molecular orbital (LUMO) energy were found to be inactive, whereas the other PAHs with lower LUMO energies were found to be active catalysts for the ORR. Doping of two heteroatoms, doubly B-doped and a B,N-codoped PAH containing separate (nonbonded) B and N atoms, was found to lead to the most active ORR catalysts from this set. This suggests that two proximal (separated only by one or two carbons) electrophilic sites improve the ORR activity of doped PAHs. This is the first study, to the best of our knowledge, which uses well-defined doped PAHs as models to identify potential ORR electrocatalytic moieties present in doped carbons; this approach thus enables definitive structure–activity relationships to be developed in this important area.