The Preparation Of Core-shell Catalyst With Enhanced Catalytic Performance For Electrocatalytic Ammonia Synthesis

Ammonia（NH₃）is a vital raw material in the synthesis of fertilizer and an ideal carrier of carbon-free energy.Industrially,the large-scale commercial production of NH₃ is mainly carried out through the famous Haber-Bosch process that requires high temperature（350-550℃）and high pressure（150-350 atm）.In recent years,electrochemical reduction of N₂ at room temperature and atmosphere pressure has attracted widespread concern by virtue of its simplicity,environmentally-benign and ambient operating conditions.However,the yield of NH₃ and Faraday efficiency（FE）are usually very low owing to the loss of active electrocatalysts and competitive hydrogen evolution reactions.Therefore,there is a serious demand to explore NRR catalysts with low-cost,high efficiency and stability.Herein,this paper takes the application of ammonia as the research background,and takes the design and development of the electrochemical ammonia synthesis catalyst as the starting point.We successfully prepared efficient nickel based composite electrocatalysts by constructing the core-shell composite structure,regulating the micro morphology and the composition of electron acceptor,adjusting the electronic structure and surface local charge density of the catalyst.The effects of phase,composition and space charge distribution on the catalytic activity were studied.The main contents are as follows:In the third chapter,the Ni S@Mo S2 core-shell microspheres were synthesized via a Prussian-blue-analogue-sacrificed strategy.A series of characterizations indicate that the as-prepared Ni S@Mo S₂ core-shell microspheres possess sufficient active sites,high structural porosity,and convenient transport channels,all of which are favorable for electron transportation,mass transfer and structure stability.Additionally,the synergistic effect of Ni S and Mo S₂ promotes the interfacial charge transfer and accelerates the NRR reaction.As a result,the Ni S@Mo S₂ catalyst produces significant electrochemical activity,capable of exhibiting a high NH₃ yield of 9.66μg h^-1·mg^-1cat`at-0.3 V vs.RHE and an excellent FE of 14.8%at-0.1 V vs.RHE in 0.1M Na₂SO₄.The work offers a new approach for the exploration and synthesis of catalysts for synthetic ammonia technology.In the fourth chapter,Ag nanowires were used as templates to precisely control the uniform nucleation and growth of ZIF-67 on Ag nanowires,and a high quality one-dimensional composite structure（Ag@ZIF-67 core-shell nanowires）was synthesized.The Ag@Ag P₂@Ni-doped Co P core shell porous nanocomposites were formed by Ni ion exchange and a phosphating process.Ag@Ag P₂@Ni-doped Co P exhibited a conspicuously improved NRR performance with an NH₃ yield of 17.26μg h^-1·mg^-1cat at-0.4 V vs.RHE and a high Faradaic efficiency（FE）of 21.2%at-0.2 V vs.RHE.The unique core-shell porous structure provides more surface area for the material,thus providing sufficient active sites and promoting electrolyte diffusion.Importantly,the porous nanosheet outside the Ag@Ag P₂nanowires can adsorb and store gas,increasing the possibility of the electrochemical nitrogen reduction reaction.Ag nanowires can effectively improve the conductivity of the material,enhance the electrochemical stability of the electrode,and serve as a core structure to support the metal nanosheets,effectively preventing the agglomeration effect between the Co P nanosheets.Notably,the synergistic effect between different metal components also plays a vital role in charge transfer and improves the electrochemical ammonia synthesis performance.