Precise tuning of heteroatom positions in polycyclic aromatic hydrocarbons for electrocatalytic nitrogen fixation

The electrochemical dinitrogen reduction represents an attractive approach of converting N₂ and water into ammonia, while the rational design of catalytic active centers remains challenging. Investigating model molecular catalysts with well-tuned catalytic sites should help to develop a clear structure-activity relationship for electrochemical N₂ reduction. Herein, we designed several polycyclic aromatic hydrocarbon (PAH) molecules with well-defined positions of boron and nitrogen atoms. Theoretical calculations revealed that the boron atoms possess high local positive charge densities as Lewis acid sites, which are beneficial for N₂ adsorption and activation, thus serving as major catalytic active sites for N₂ electrochemical reduction. Furthermore, the close vicinity of two boron atoms can further enhance the local positive density and subsequent catalytic activity. Using the PAH molecule with two boron atoms separated by two carbon atoms (B-2C-B), a high NH₃ production rate of 34.58 μg·h^-1·cm^-2 and a corresponding Faradaic efficiency (5.86%) were achieved at -0.7 V versus reversible hydrogen electrode, substantially exceeding the other PAHs with single boron or nitrogen-containing molecular structures.