Rational Design Of Efficient Catalysts Towards Electroreduction Of Nitrogen To Ammonia With Surface Strategies

Ammonia is not only the nutrient source for most of life on earth,but also an important intermediate for dye,medicine,and other industries.Generally,NH3 is produced through the Haber-Bosch process under high temperature and pressure in industry.This process relies on a large amount of fossil energy with huge CO2 emission,making the global environment and climate problems increasingly serious.Therefore,it is of great significance to explore a new method for the synthesis of ammonia under ambient conditions.At present,electroreduction of N2 into NH3 has been emerging as an attractive route,because this process operates under ambient pressure at room temperature and utilizes H2O as hydrogen source.However,owing to the high inertness of the N≡N covalent triple bond and the vigorous competing hydrogen evolution reaction,the ammonia yield rate and Faraday efficiency(FE)of the reported catalysts are very limited.Therefore,it remains a huge challenge to develop high-efficiency catalysts for the activation of N2 towards N2 electroreduction.The surface engineering of catalyst can optimize the electronic structure of the surface catalytic site,thus enhancing the adsorption and activation of nitrogen.Thus,it is an important solution for the design of electrocatalysts with effective surface strategies to improve the catalytic performance towards N2 electroreduction.In this paper,three kinds of electrocatalysts were designed and synthesized by different surface engineering strategies towards electroreduction of N2.The material system includes Ru single atom on nitrogen-doped porous carbon,LaCoO3 with oxygen vacancy,and F-doped porous carbon.These catalysts showed excellent catalytic performance towards N2 electroreduction,and the catalytic mechanism of the above catalysts was also studied.The main contents are concluded as follows:1.We reported a record-high activity for N2 electrochemical reduction over Ru single atoms distributed on nitrogen-doped carbon(Ru SAs/N-C).At-0.2 V vs reversible hydrogen electrode(vs RHE),Ru SAs/N-C achieved a FE of 29.6%for NH3 production.Notably,the yield rate of Ru SAs/N-C reached 120.9 μgNH3 mg-1 cat.h-1,which was one order of magnitude higher than the highest value ever reported.Mechanism studies showed that the Ru-N coordination structure in Ru SAs/N-C enhanced the adsorption of N2,reduced the energy barrier of N2 dissociation,and thus improved the catalytic performance towards N2 electroreduction.2.We designed an effective approach to promoting N2 activation by introducing oxygen vacancies into LaCoO3.In N2 electroreduction,LaCoO3 with oxygen vacancies(denoted as Vo-LaCoO3)exhibited a yield rate for NH3 of 182.2 μgNH3 mg-1 cat h-1 at-0.7 V vs RHE,which was 2.8 times than that(65.3 μgNH3 mg-1 h-1)of pristine LaCoO3.Density functional theory calculations revealed that enhanced activation of N2 over Vo-LaCoO3 originated from the increased charge density around the valence band edge via the introduction of oxygen vacancies.Furthermore,the analysis of the thermodynamic limiting potentials for N2 reduction and H2 evolution demonstrated the higher selectivity for N2 electroreduction over Vo-LaCoO3 relative to pristine LaCoO3.3.We developed a highly efficient metal-free catalyst by introducing F atoms into three-dimensional porous carbon framework(F-doped carbon)towards N2 electroreduction.At-0.2 V vs RHE,F-doped carbon achieved the highest FE of 54.8%for NH3,which was 3.0 times as high as that(18.3%)of pristine carbon frameworks.Notably,at-0.3 V vs RHE,the yield rate of F-doped carbon for NH3 reached 197.7μgNH3 mg-1 cat.h-1.Such value is more than one order of magnitude higher than those of other metal-free electrocatalysts under the near-ambient conditions for NH3 product up to date.Mechanistic studies revealed that the improved performance in N2 electroreduction for F-doped carbon originated from the enhanced binding strength of N2 and the facilitated dissociation of N2 into*N2H.F bonding to C atom created a Lewis acid site due to the different electronegativity between F and C atom.As such,the repulsive interaction between Lewis acid site and proton H suppressed the activity of H2 evolution reaction,thus enhancing selectivity of N2 electroreduction into NH3.