Vanadium Nitride-based Electrocatalysts For Nitrogen Reduction Reaction

Ammonia is an important chemical raw material and plays an important role in industrial and agricultural production as well as energy storage and conversion.The traditional Haber-Bosch ammonia production process in industry is very energy-intensive and emits a lot of greenhouse gases.Therefore,under the situation of increasingly prominent energy crisis and environmental problems,it is urgent to develop a new process for efficiently synthesizing ammonia under mild conditions.Electrochemical reactions provide an environmentally friendly and sustainable alternative process for ammonia production.However,due to the extremely high stability of N≡N triple bond and the slow nitrogen adsorption,the electrocatalytic nitrogen reduction to ammonia production has huge obstacles both thermodynamically and kinetically.Transition metal nitrides are evaluated as the most promising electrocatalysts for NRR.In recent years,researchers have screened out transition metal nitrides for electrochemical NRR through extensive theoretical calculations and experiments,and proposed a new Mars-van Krevelen（Mv K）mechanism for this system.Among them,vanadium nitride（VN）has a similar electronic structure to Pt and is considered to be a potential electrocatalyst to replace Pt.In addition,the application of VN to electrocatalytic nitrogen reduction is still limited by low material utilization,easy deactivation during the reaction processs.Due to the low cost,stable properties,diverse structures and easy modification and modification of VN,researchers have devoted themselves to designing vanadium nitride-based materials with special structures.In addition,strategies such as alloying,doping engineering,and surface reconstruction have also been demonstrated to enhance the catalytic activity of transition metal-based electrocatalysts.Accordingly,in this thesis,a VN-based nanocomposite with controllable electronic structure was prepared by hydrothermal self-assembly and chemical vapor deposition（CVD）technology,and its electrocatalytic nitrogen reduction performance was tested,and its performance improvement was studied by theoretical calculation.internal mechanism.The specific research contents are as follows:（1）Using bulk V₂O₅ as the precursor,combined with hydrothermal and CVD processes,the porous vanadium nitride with two-dimensional structure was prepared:firstly,the silk-like two-dimensional wrinkled V₂O₅ precursor was obtained by hydrothermal and freeze-drying,and then in CVD During the process,the V₂O₅ nanosheet structure was etched into a 2D porous structure with hierarchically hierarchical pores at high temperature in an NH₃/Ar atmosphere,and a 2D porous VN nanomaterial with abundant interfaces and active sites was obtained,which was used for mild Research on the performance of electrocatalytic ammonia synthesis under conditions.The electrocatalytic nitrogen reduction results show that the 2D porous VN achieves a Faradaic efficiency of 7.62%at-0.45 V and an NH₃ yield of 8.8μg h^-1 mg_cat.^-1at-0.55 V in an acidic solution.Its NRR activity is better than most transition metal nitride catalysts.In addition,the catalyst exhibits excellent cycling stability.Theoretical calculation studies show that the catalyst performs nitrogen reduction reaction according to the Mv K mechanism,and the active site of the reaction is N.（2）With ammonium heptamolybdate as the metal precursor and the two-dimensional porous vanadium nitride as the substrate,a Mo-O four-coordinate vanadium nitride composite catalyst was precisely constructed through a controllable hydrothermal self-assembly and CVD process.In Mo-VN,O-bridged Mo atoms are anchored to the VN surface through O atoms,thus ensuring the stability of the catalyst structure.Electrocatalytic nitrogen reduction results show that the optimized Mo-VN achieves Faradaic efficiency values of 18.8±0.57%（-0.45 V vs.RHE）and31.8±0.58μg h^-1 mg_cat.^-1（-0.5 V vs.RHE）for NH₃The yields,significantly higher than those of bare VN,suggest that Mo single atoms stabilized on VN nanosheets can significantly enhance the electrocatalytic performance.Its NRR activity is almost better than that of most catalysts for NRR reaction based on the Mv K mechanism.In addition,the catalyst exhibits excellent stability after multiple cycle tests.Theoretical calculation show that the catalyst utilizes the hydrogen bonds,which significantly accelerates the cleavage of the first bond of N₂,and the first step of hydrogenation of N₂changes from an endothermic reaction on the surface of VN to an exothermic reaction on the surface of Mo-VN,thereby significantly promoting N₂activation.（3）Introducing ammonium heptamolybdate and cobalt chloride precursors,and constructing O-coordinated Co and Mo diatomic supported two-dimensional porous VN catalysts（Co/Mo-VN）through hydrothermal reaction and controllable nitridation process.In Co/Mo-VN,the O-bridged Co/Mo diatomic system is anchored to the VN surface through O,which ensures the stability of the catalyst structure.The electrocatalytic nitrogen reduction results show that the optimized Mo-VN achieves a FE value of 37.56 at-0.3 V and a NH₃ yield of 45.34μg h^-1 mg_cat.^-1,which is higher than most of the catalysts reported so far,indicating that it is stable.Its NRR activity outperforms almost all Mo-and Co-based NRR catalysts.In addition,the catalyst also exhibits excellent cycle stability.