Preparation And Electrochemical Catalytic Performance Of Heteroatom-doped Carbon Material For ENRR

As an important chemical product,ammonia is widely used in the fields of agriculture and new energy.The traditional Haber-Bosch synthetic ammonia process needs to consume large amounts of fossil fuels and emits a huge amount of CO₂,which results in the serious energy and environment issues.Therefore,it is of great significance to explore new technology for the green synthesis of ammonia.The electrochemical N₂ reduction reaction（eNRR）to ammonia is a promising technology for green ammonia synthesis with the advantages of mild reaction conditions and zero carbon emissions,which has attracted much attention.However,eNRR technology usually suffers from the low ammonia yield rate and unsatisfactory faradaic efficiency.The development of high performance eNRR electrocatalysts is an effective solution to this dilemma.Heteroatom-doped carbon nanomaterials have become a hot topic in the field of eNRR catalysis owing to their hierarchical pore structure,adjustable heteroatom-doping configurations,and capable of inhibiting the competitive hydrogen evolution reaction.In this thesis,3,6-bis（3,5-dimethylpyrazole-1-yl）-1,2,4,5-tetrazine and 3,5-dimethylpyrazole-based metal complex were used as the precursors to prepare two types of heteroatom-doped carbon electrocatalysts with high eNRR activity,in which the relationships between the compositions and structures of such catalysts and the eNRR catalytic performance were explored.The main research results include the following two aspects:（1）Preparation and eNRR performance of the B/N-codoped carbon（BNC）.In this chapter,a series of B/N-codoped carbon（BNC-x,x denotes pyrolysis temperature）were fabricated by the pyrolysis of the mixture of 3,6-bis（3,5-dimethylpyrazole-1-yl）-1,2,4,5-tetrazine C/N source and ammonium tetraborate tetrahydrate B/N dopant.The BNC-750 electrocatalyst fabricated under the optimal conditions exhibited a relatively high specific surface area(282 m² g^-1),a broad pore-size distribution,and a relatively high level of B/N-codoping.As such,the BNC-750 electrocatalyst showed relatively high eNRR ability in acidic electrolyte（0.1 M HCl）,in which the ammonia yield and faradaic efficiency were found to be 18.8μg h^-1 mg^-1_cat and 10.1%,respectively,at the ambient temperature（25°C）and at the applied potential of-0.1 V（vs.RHE）.In addition,the BNC-750 electrocatalyst also showed the outstanding stability with only11%of catalytic activity reduction after 8 consecutive cycles of eNRR process.Such a high eNRR performance of the BNC-750 electrocatalyst can be attributed to the synergistic effect of B/N-codoping.（2）Preparation and eNRR performance of Zn（II）complex derived NDC catalyst.The 3,5-dimethylpyrazole/Zn（II）complex（Zn-dmpz H）was first prepared by reacting3,5-dimethylpyrazole with Zn（NO₃）₂ under the solvothermal conditions.The N-doped carbon（NDC-x,x denotes pyrolysis temperature）electrocatalysts were then fabricated by the pyrolysis of the mixture of Zn-dmpz H and melamine.The NDC-850 catalyst prepared under the optimal conditions displayed an ultrathin nanosheet structure,an ultrahigh N content（16.1 at%）,and a high electrochemically-active-surface area(15.1m F cm^-2).Owing to these reasons,the NDC-850 electrocatalyst displayed excellent eNRR activity in acidic electrolyte（0.1 M HCl）,in which the ammonia yield and faradaic efficiency were estimated to be 24.70μg h^-1 mg^-1_cat and 26.3%,respectively,at the ambient temperature（25°C）and at the applied potential of-0.1 V（vs.RHE）.Moreover,NDC-850 electrocatalyst also exhibited good stability with only 13%of activity reduction after 8 consecutive cycles of eNRR process.