Theoretical Study Of Boron-doped Graphdiyne For Electrocatalytic Nitrogen Reduction Reaction (NRR)

Ammonia is not only a significant building-block in the manufacture of agriculture and industry,but also is recognized as a promising carbon-free energy carrier and energy storage intermediate.Currently,the primary method for ammonia synthesis is still the originally industrialized Haber-Bosch process,which has exhibited many disadvantages including large energy consumption,low ammonia yield,harsh reaction conditions,etc.Thus,as a promising alternative for N₂ fixing and reduction to ammonia under normal temperature and pressure conditions,electrochemical N₂ reduction reaction（NRR）has aroused researchers’wide concern in recent years.In this regard,the design and synthesis of electrocatalyst with excellent performance is a matter of the utmost importance.Recently,a new two-dimension allotrope of carbon,graphdiyne（GDY）was synthesized experimentally.By inserting a diacetylene bond between two hexagonal benzene rings,GDY monolayer contains both sp-and sp²-hybridized carbon atoms.The largeπ-conjugated structures impose GDY strong ability in promoting charge transfer and excellent conductivity,thus making GDY an ideal support material for electrocatalysts.Previously,A pathbreaking work found that when boron atom in the molecular catalyst with sp² or sp³ hybridization,its orbital properties are similar to those of transition metal atoms because it contains occupied and empty orbitals simultaneously.Therefore,boron atom can also act as an active center to catalyze the electrochemical N₂ reduction reaction.It also provides a new idea for the design and synthesis of metal-free catalysts for NRR with high catalytic performance.This work first systematically studied graphdiyne decorated by single B atom through different methods as NRR electrocatalysts and then evaluate their NRR electrocatalytic performance theoretically.The results of DFT calculation showed that the B（S₃）-GDY electrocatalyst formed by substituting sp-hybridized carbon atom with B atom exhibits highest catalytic performance.The overpotential value is calculated to be only 0.11V.In particular,it also has excellent selectivity for NRR.The electronic structure analysis further showed that the introduction of B atom not only reduces the band gap of GDY,but also improves the electron transport and greatly enhances the electrical conductivity,leading to its good catalytic activity.Further research has been done to evaluate the catalytic performance of graphdiyne decorated by double B atoms.By comparison,GDY-2B（S₂S₂’）,formed by substituting sp-hybrid carbon atoms on two adjacent carbon chains,exhibits good catalytic activity and high selectivity for NRR.The preferable reaction pathway is enzymatic pathway and its overpotential is only 0.12V.In addition,the DFT calculation results also show that the catalytic active site of GDY-2B（S₂S₂’）is composed of B atom and its adjacent sp-hybrid carbon atom.In the N₂ adsorption process,together with its adjacent sp-hybrid carbon atom,the B atom interact with N₂molecule strongly.Thus,the triple bond of N≡N is activated effectively,leading to the promotion of subsequent reduction process.