First-principles Study On Several Two-dimensional Carbon-based Atomistic-level Catalysts For Electrocatalytic Nitrogen To Ammonia Synthesis

Ammonia（NH₃）is not only a key chemical in the fertilizer industry,but also a clean energy carrier for chemical energy storage applications.Currently,industrial ammonia synthesis mainly depends on the Haber-Bosch process under high temperature and pressure,which consumes totally 1%～2%of global energy and releases plenty of CO₂.Therefore,it is highly desirable and quite urgent to explore ecofriendly and sustainable alternatives for green and energy-saving ammonia production.The recent emerging electrocatalytic nitrogen for ammonia synthesis has drawn great attention and demonstrates promising prospect.However,the high overpotential,low Faraday efficiency and low yields are the big challenges of electrocatalytic nitrogen for ammonia synthesis towards practical applications.The fundamental way out for the rapid development of electrocatalytic nitrogen for ammonia synthesis is the creation and innovation of highly efficient catalysts.This paper focuses on the catalyst regulation strategy and aims to obtain efficient catalysts for electrocatalytic nitrogen reduction reaction（e NRR）.Based several two-dimensional carbon-based materials as the substrate,different regulation strategies were used to construct atomistic-level catalysts.The reaction mechanism,catalytic activity,stability,selectivity,and structure-property relationship of catalysts were investigated by the combination of first-principles calculation,high-throughput screening,and ab initio molecular dynamics（AIMD）simulation.The details were as follows:（1）The possibility of transition metal atoms（TM=3d～5d transition metal）embedded g-C₄N₃ as a new class of electrocatalysts towards e NRR was systematically investigated.A volcano curve between onset potential(U_onset)and the adsorption energy of N₂(ΔE_ads（*N₂）)was established,and thusΔE_ads（*N₂）can be used as a descriptor to characterize the activity of catalysts.TM@g-C₄N₃（TM=V,Tc,Os,Pt）exhibited the highest activity(|U_onset|≤0.52V),which can be attributed to their moderate adsorption energies for N₂.The increasing order of catalytic activity was consistent with the increasing order of d-band center（ε_d）of the four catalysts.Among them,V@g-C₄N₃ had the highest catalytic activity(U_onset=-0.37V),and it possessed good electronic conductivity,thermal stability and selectivity,which indicates that V@g-C₄N₃ is a promising and efficient catalyst for e NRR.（2）A comprehensive study on the coordination engineering of 3d～5d transition metal atom embedded N/O-codoped graphene as electrocatalysts（TM-O_xN_y@Gra,x+y=4）for e NRR was performed.Among 210 possible materials（30 TM atoms and seven coordination environments）,ten types of highly efficient catalysts with|U_onset|<0.5 V were selected out.V-O₂N₂^γ@Gra had the lowest U_onsetof-0.38 V.Interestingly,a descriptorΔE_ads[（*N₂-*N）+（*NNH-*N）+（*NH₂-*N）]was proposed to unveil the structure-property relations,and a volcano curve between this descriptor and U_onset was established.The more charges adsorbed N₂ obtains,the longer the N-N bond length is elongated,and the higher the degree of N₂ activation will be.The ten catalysts possessed good electronic conductivities and high thermal stability.Overall,through coordination engineering,the coordination environment and electronic structure of the active centers can be modulated properly,thereby,regulating the performance of the catalyst.（3）240 combinations of bimetallic pairs obtained by composition engineering anchored on graphdiyne（GDY）as electrocatalysts（TM₂@GDY and TM-TM’@GDY）for e NRR were systematically investigated.43 diatomic catalysts were selected out as promising candidates with high stability,activity(|U_onset|≤0.40 V)and selectivity.Surprisingly,Rh-Hf@GDY and Rh-Ta@GDY displayed ultralow onset potentials of 0 and-0.04 V,respectively.The underlying mechanism of excellent performance can be attributed to the synergistic effects of bimetallic pairs.The combinations of a relatively weak（d-band center in low energy region）and a relatively strong（d-band center in high energy region）components can lead to the optimal adsorption strengths of reaction intermediates,thereby having high probabilities to achieve the optimal catalytic activity.