| At present,the global energy mainly depends on fossil resources,as a non-renewable resource,the human society will eventually face the problem of its depletion.At the same time,the use of fossil fuels will release a large amount of harmful substances,threatening the living environment of human beings.Owing to the continuous development of the renewable energy field,clean and reneable electricity can be generated through the conversion of wind or solar energy,and it has become quite competitive in the market.The rational utilization of renewable electricity can optimize and adjust the current energy structure,which is of great significance to the sustainable development of human society.Ammonia is a very important chemical,which is mainly used as the raw material for the industrial synthesis of nitrogen-containing fertilizers.In addition,ammonia molecule contains a high hydrogen content of 17.6 wt%and is easy to be liquefied,which makes it a fuel with high energy density and has great potential to be widely used in the new energy system.The large-scale industrial synthesis of ammonia is usually dominated by Haber-Bosch method through the reaction of nitrogen and hydrogen under high temperature and high pressure.Although the preparation of ammonia by this method has made a great contribution to the development of human society in the past century,it still suffers from the drawbacks of high energy consumption and environmental pollution.Electrocatalytic nitrogen reduction reaction is driven by clean and renewable electric energy,which can realize ammonia synthesis under ambient conditions,and is one of the effective strategies to alleviate energy and environmental crisis.The electrocatalyst is the key component of the catalytic system and the efficient electrocatalytic reaction cannot be separated form the careful design of catalyst,especially for the activation and reaction of chemically inert nitrogen molecules,meanwhile,the suppression of competitive hydrogen evoluation reaction should be taken into considerations.Carbon-based materials are widely used in the field of electrocatalysis due to the low cost and high adjustability.Among the modification strategies,the heteroatom doping could induce the reasonably redistribution of charge on the surface of material,thus providing abundant active sites for electrocatalytic nitrogen reduction reaction.Carbon-based catalysts with various bonding structures can be synthesized by optimizing the preparation process,and the relation ships between catalyst composition,bonding structure,active sites and electrochemical performance are determined by theoretical calculations and experimental results,serving as guidance for the synthesis of efficient electrocatalysts.As the main utilization of ammonia,the synthesis of urea is also operating under harsh conditions,and the ammonia obtained by electrocatalytic process faces great challenges in the subsequent separation and purification.Based on the reasonable design of electrocatalysts,the one-step green synthesis of urea was realized by the eletrocatalytic coupling of nitrogen and carbon dioxide molecules,which provided an original way for the fixation,conversion and utilization of inert nitrogen and carbon dioxide molecules and the green synthesis of special chemicals.The specific research contents are as follows:（ⅰ）Carbon materials are widely used in the field of electrocatalysis due to their low cost and high adjustability.Fluorine atom can achieve stronger electron pulling action on adjacent carbon atoms due to its strong electronegativity,so that the electronic structure of the material can be further optimized,exposing abundant active sites for electrocatalytic nitrogen reduction reaction.Fluorine-doped carbon catalyst was synthesized by thermal treatment of polytetrafluoroethylene and commercial carbon black.At the potential of -0.2 V versus reversible hydrogen electode（RHE）,the ammonia formation rate can reach a high value of7.03μg h-1 mg-1cat..Owing to the suppresion of hydrogen evolution reaction by fluorine doping,the material also presents a high selectivity of ammonia products（Faradic efficiency is 9.35%）.The active sites of electrocatalytic nitrogen reduction in the system were analyzed by theoretical calculation.It was indicated that the optimal configuration consisted of two fluorine atoms combining with two carbon atoms respectively and the co-regulation of two fluorine atoms on the active carbon atoms in this configuration is the source of catalytic activity.This configuration can promote the adsorption of *N2H,reduce the free energy change in the rate-determing step,and improve the catalytic nitrogen reduction performance of the material.（ⅱ）The charge distribution on the surface of the carbon-based materials can be adjusted by the introduction of heteroatoms,thus providing active sites for the catalytic reaction.However,the active sites of carbon-based catalysts to electrocatalytic nitrogen reduction have not been fully recognized currently,and barely any reasonable and efficient modification strategies have been designed to improve the electrocatalytic nitrogen reduction performance of materials.The co-doping of heteroatoms is an effective strategy for the further improvement of electrocatalytic performance,the theoretical calculation shows that the substitution of boron and nitrogen pairs into carbon material can provide efficient active sites towards nitrogen reduction reaction,and can greatly suppress the activity of competitive hydrogen evolution reaction.The electrocatalysts with various bonding structure have been fabricated and the above results have been confirmed by the experimental characterizations.The active sites were further analyzed and the results indicated that the active sites for nitrogen reduction reaction were the edge carbon atoms adjacent to the boron-nitrogen bonds,and the role of the boron-nitrogen bonds were the active triggers.On this basis,the boron-nitrogen enriched defective carbon materials were synthesized by the pyrolysis of organic precursors.More edge active sites were exposed and the electrocatalytic performance of the materials was further improved.On the basis of deepening the understanding of the active sites towards nitrogen reduction reaction,the relationship between the composition and bonding structure of catalyst,the active sites and the electrochemical properties was clarified,providing a new idea for the design of carbon-based electrocatalysts in this field.（ⅲ）About 80%of the world’s annual produced ammonia is used in urea synthesis.Currently,large-scale industrial synthesis of urea is realized through the reaction of ammonia and carbon dioxide under harsh conditions.The synthesis of ammonia via Haber-Bosch method suffers from the disadvantages of high energy consumption and high pollution,meanwhile,the electrocatalytic ammonia synthesis is still in the initial stage of research with low yielded rate and it is faced with the difficulties of subsequent separation and purification.The green synthesis of urea can be realized by the electrocatalytic coupling of nitrogen and carbon dioxide molecules under ambient conditions.The two-step process of industrial synthesis with high energy consumption and high pollution can be optimized into a one-step electrocatalytic process.Through the reasonable design of the electrocatalyst,nitrogen and carbon dioxide can be adsorbed and activated together on the catalyst（Pd Cu alloy nanoparticles loaded on the defective TiO2 nanosheets）,which greatly promotes the electrocatalytic coupling reaction.In situ synchrotron radiation infrared spectroscopy was used to trace the formation of the key intermediates and the evolution of the bonding structure in the electrocatalytic process,and the products were characterized quantitatively and qualitatively by isotopic labeling,confirming the products were generated from the electrocatalytic process of gas molecules.Based on the theoretical calculations and experimental results,the pathways of nitrogen reduction reaction and carbon dioxide reduction reaction are both optimized,and the reaction of absorbed nitrogen molecules（*N=N*）with the migrated carbon monoxide（CO）induces the generation of key intermediates（*NCON*）,and this process is the key to the generation of carbon-nitrogen bond and urea,which is thermodynamical and kinetical feasible.The urea can be obtained by the subsequent hydrogenation reaction of*NCON*.In this paper,the synthesis of carbon-nitrogen bond and urea under ambient conditions has been successfully achieved,which provides an original strategy for the electrocatalytic coupling of gaseous molecules and the green synthesis of chemicals. |
