Theoretical Study On Two-dimensional Single-atom/Bi-atom Electrocatalysts

With the development of human society,the continuous consumption of traditional fossil fuels such as coal,oil and gas inevitably intensifies the energy crisis and environmental pollution.Hydrogen（H₂）has been considered as the most potential sustainable energy carrier with the advantages of high calorific values,clean and renewable.In addition,as a stgreen fliel’’,ammonia（NH₃）has been known as an important clean energy carrier to replace non-renewable fossil fuels,and has a widespread use in chemical industry production.Therefore,the technologies of highly efficient energy production and storage,such as water-splitting,metal-air batteries and electrochemical nitrogen fixation have attracted extensive attention worldwide.These energy conversion processes involve hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）,oxygen reduction reaction（ORR）and nitrogen reduction reaction（NRR）.In practice,an overpotential is usually needed to drive these reactions.Meanwhile,the effective electrocatalysts play pivotal rules to maximize energy conversion efficiency and accelerate reaction kinetics.In summary,it is of great importance to systematically explore new catalysts and study the related catalytic mechanisms.In this paper,based on first-principles calculations,a systematic theoretical study was carried out on the single-atom catalysts（SACs）and bi-atom catalysts（BACs）,which are composed of transition metal（TM）atoms and two-dimensional（2D）substrate materials.We demonstrate the geometric and electronic structures of catalytic center have great influence on the binding strength of reaction intermediates and have a further impact on catalytic perfonnance.We hope these studies would provide valuable theoretical guidance for the development of related experiments.The main research contents and conclusions of this paper are as follows:（1）Highly efficient single-atom catalysts（SACs）with maximal utility rate of metal atoms hold great promise in the energy storage devices.Support materials which can not only fimily anchor the active metal atoms but also modulate the catalytic activity are crucial for SACs.Here,we demonstrate from first-principles that the unique pores of pyrazine-modified graphyne（pyGY）which has been synthesized in recent experiments are quite promising for anchoring transition-metal（TM）atoms to achieve SACs due to the TM-N₂ units.Among the ten TM@pyGYs,we filtered out two biflinctional SACs,Co@pyGY and Pt@pyGY,as efficient water splitting（HER/OER）and metal-air battery（0ER70RR）catalysts,respectively.More interestingly,Ni@pyGY and Pd@pyGY can act as efficient HERVOER/ORR trifiinctional electrocatalysts.These multifunctional catalysts have the overpotentialscomparable or superior to those of the conventional uni-functional catalysts.This work provides a new group of promising multifimctional SACs,as well as a usefhl guidance for electrocatalyst design.（2）An economical and environment-friendly approach for N₂ reduction reaction（NRR）under mild conditions is currently urgent The development of single-atom catalysts（SACs）provides a new strategy for reaching this goal.However,the single active site of SAC makes limitation for achieving high NRR performance.The synergetic reaction centers in bi-atom catalysts（BACs）are proposed to further boost reaction activity.Based on a newly-synthesized two-dimensional（2D）binuclear Cu-salphen covalent organic framework（COF）,we predict four promising BACs for NRR with transition metal（TM）atoms（TM=Fe,Co,Mo,W and Ru）embedded in this COF.The low limiting potentials（-0.29 ^-0.57 V）and high theoretical Faradaic efificiency（FE）（76^100%）of these BACs can be correlated to the asymmetrical charge depletion of metal dimers and the synergistic effect between the metal atoms and the 2D COR The thermal stability and realistic feasibility of M1M2-COFS were also assessed.Our work offers a group of promising candidates of efiBcient COF-based NRR BACs,as well as a rational guidance for catalyst design.