Structure Engineering Of Metal Carbide-based Electrocatalysts Towards Efficient Ammonia Synthesis

As one of the important intermediate products in the biological nitrogen cycle,ammonia（NH₃）is expected to become a new generation of fuel with the advantages of high hydrogen content,high energy density,no carbon,and easy liquefaction.It plays an irreplaceable role in industrial,agricultural,pharmaceutical and other industries.The current industrial ammonia production mainly relies on the Haber-Bosch industry with high energy consumption and high carbon dioxide emissions.In contrast,the process of electrocatalytic ammonia production is more green and clean.Electrocatalytic reduction of nitrogen and nitrate to produce ammonia has attracted extensive attention due to its advantages of low cost,mild reaction conditions and friendly environment Although great progress has been made in this field,there are still many unsolved problems:（1）low reactivity and low yield due to high reaction energy barrier.（2）Serious hydrogen evolution reaction and complex byproducts lead to low Faraday efficiency and selectivity.（3）Multi-proton and multi-electron transfer in the reaction process makes the kinetic process slow and the reaction path unclear.Aiming at the problems above,this article choose high conductivity high stability and highly reactive metal carbide as the research object.Through valence state regulation and heterogeneous elements doping,the electronic structure of catalyst and intermediate adsorption were optimized,which improves improve the performance of electrocatalytic ammonia production and provides a new way of thinking for the development of green sustainable strategy and the realization of carbon neutrality.The main research contents include:（1）Improving the selectivity of electrocatalytic reduction of N₂ to ammonia on Mo₂C quantum dots:S was introduced into the Mo₂C particles loaded on carbon substrate by solvothermal method,which reduced the size of Mo₂C and exposed more active sites;At-0.5 V,it was found that the ammonia yield of 29.5μg h^-1 mg_cat^-1with S doping was similar to that without S doping(31.8μg h^-1 mg^-1_cat),but FE increased from 4.9%to59.8%.The current density can remain unchanged for 10 h.（2）Optimization of the electronic structure of Fe₃C nanosheets and study on the electrocatalytic performance of NH₃ produced by nitrate reduction:First,Fe₃C/NC nanosheets supported on graphene-like substrates（Fe₃C/NC）were synthesized by solvothermal-calcination method.At-0.5 V,the yield,FE and selectivity of ammonia were 1.19 mmol h^-1 mg^-1,96.7%and 79.0%of nitrate reduction to ammonia.Then a series of control samples with different oxidation states were obtained by oxidizing or reducing atmosphere treatment.Compared with the control sample,Fe₃C/NC has the maximum electrochemically active area,the equilibrium constant of nitrate adsorption and the appropriate d-band center value optimizing the adsorption of intermediate products.The lower desorption temperature is beneficial to the generation of ammonia.The minimum impedance and activation energy,as well as the maximum rate constant,accelerate the reaction kinetics process.The transfer number of 2 and 6 electrons indicates that the reaction path is NO₃^-→NO₂^-→NH₃.