Selectivity Promotion Strategy Of Electrochemical Ammonia Synthesis Under Ambient Conditions

Ammonia is the basic material for industry,agriculture and defense industry.At present,most ammonia synthesis still relies on the energy-intensive Haber-Bosch process（300-500 ^oC,200-300 atm）,which consumes 1-2%of the world’s energy annually.And because hydrogen is converted from fossil fuels,the process inevitably releases to CO₂ emissions（about 1.5%of the world’s annual greenhouse-gas emissions）.In recent years,using nitrogen and water from a wide range of sources as raw materials and using renewable electricity to electrocatalytic reduction of nitrogen to ammonia（NRR）under mild conditions has been considered as a potential Haber-Bosch ammonia synthesis replacement technology.However,the major challenge that electrochemical nitrogen-fixing faced is ultra low efficiency（including ammonia production rate and current efficiency）,mainly because the triple bond of N₂ is very strong,the nitrogen under atmospheric pressure and the reaction of H on the dynamics is very difficult,but due to the formation of hydrogen and nitrogen reduction potential is very close,hydrogen evolution as a competitive reaction will seriously restrain reduction efficiency of ammonia nitrogen.In view of this,starting from the design of highly selective nitrogen reduction catalysts and of efficient activation path of nitrogen,this paper studied the general strategies to improve the current efficiency of electrochemical ammonia synthesis,and the main research results are as follows:（1）In this paper,a design strategy of highly selective nitrogen reduction catalyst is presented,which is to protect the nitrogen activation center and improve the performance of ammonia synthesis by purposefully introducing strong hydrogen evolution active sites into the nitrogen reduction catalyst to share the hydrogen evolution competitive reaction.In this paper,two-component Se@Ni was used as the model catalyst to prove the feasibility of the catalyst design strategy.The experimental results show that compared with the single component Se catalyst,the ammonia yield rate of ammonia synthesis is increased by 3 times after the introduction of nickel with strong hydrogen evolution.The separation of nitrogen activation and hydrogen evolution activity sites,while balancing or inhibiting HER competitive reaction,can effectively avoid the synchronous weakening of the catalyst’s nitrogen activation capacity,especially when compared with NRR catalyst with single active center.This strategy provides a new idea for the design of highly selective nitrogen reduction catalysts.（2）An organic electrochemical ammonia synthesis system based on metal Li was proposed,and the in-situ stable preparation of Li⁺→Li was realized by using the good compatibility between the organic electrolyte and metal Li and Li⁺,and the efficient synthesis of ammonia was realized by taking metal Li as the active center（Li⁺→Li+N₂→Li₃N→NH₃）.Based on the Li Cl O₄（0.2 M）+THF/Et OH（199:1 v/v,100 m L）electrolyte,and Fe₃O₄ with excellent lithium storage capacity was used as Li⁺host material to construct Li active sites of high-density metal.The experimental results show that,under ambient temperature and pressure,the ammonia yield of 40.73μg h^-1 cm^-2 can be achieved by using Li pathway（-7.5 V vs.Ag/Ag⁺）.In addition,based on the organic electrolyte system,the accurate quantitative and performance evaluation method of synthetic ammonia was established in this paper,which is expected to provide an important reference for the study of electrochemical ammonia synthesis in organic systems.