Synthesis Of Carbon Nitride-Based Catalyst And Research Of Electrocatalytic Reduction Performance

The problem of environmental pollution and energy exhaustion caused by the use of large amounts of fossil energy has become increasingly serious.The use of environmental technology to develop renewable no-carbon clean energy storage and conversion has become an urgent need for current development.Electrochemistry,as a clean and efficient industrial technology,has important application value in the field of green energy storage and conversion.In the research on the conversion and storage of electrocatalytic green energy,such as the development of preparation of hydrogen energy and reduction of nitrogen to ammonia technology,the electrocatalytic reduction reaction of cathode is involved.Therefore,the development of electrocatalysts with high activity,good stability and low cost is the key to improve the efficiency of cathodic reduction reaction.The intrinsic catalytic activity of electrocatalysts is determined by the adsorption capacity and electrical conductivity of the active centers.The catalytic performance can be enhanced effectively by adjusting the electronic structure and geometric structure of the materials.Recent studies have shown that carbon nitride can be used as an ideal platform for regulating the electronic structure and geometric structure of catalysts due to its good stability,rich nitrogen content with easy coordination metals,and rich nano structure with easy regulation.In this paper,through a reasonable design and treatment process,high-activity non-noble metal and metal-free electrocatalyst based on carbon nitride are synthesized to replace the noble metal catalyst.The origin of activity and structure-activity relationship between the catalyst and the reaction are revealed by physical characterization and theoretical calculation.The main research content includes the following four parts.1.The metal is easily corroded and deactivated in the highly acidic electrolyte,and the stability and catalytic activity of the metal can be improved by coating the metal with carbon nitride shell.Therefore,through metal-nonmetal coordination strategy,Cu SCN encapsulated in C₃N₄(Cu SCN/C₃N₄)is synthesized in situ on copper foam by electropolymerization and calcination,which is used as catalytic/working electrode for hydrogen evolution reaction(HER).The experimental results show that Cu SCN/C₃N₄ has almost identical HER activity as commercial Pt/C(the overpotential is only 85 m V at the current density of 10 m A cm^-2)and good stability(above 20 h).EXAFS analysis indicates that interfacial N-Cu-S coordination is formed between Cu CN and C₃N₄,which changes the electronic structure and becomes the main active site for catalytic HER.By controlling the time of electropolymerization,the coordination structure of Cu¹⁺can be changed significantly,thus illustrating the influence of different coordination structures on the HER performance.Density functional theory(DFT)calculations indicate that electrons can penetrate and enrich from Cu SCN to the surface of C₃N₄ shell to provide electrons for HER.Adsorption capacity of hydrogen on Cu SCN is stronger,while the adsorption capacity on C₃N₄ is weak.After Cu SCN with C₃N₄ coupling can make?G_H*values closer to 0 e V.In addition,the electrocatalyst is supported by the three-dimensional porous copper foam structure,which facilitates the penetration of the electrolyte and further enhances its catalytic activity.2.On the basis of previous research,using metal and non-metal coordination,the MoS₂/C₃N₄ with Mo-N coordination is synthesized by one-step pyrolysis method to form MoS₂ in situ on C₃N₄,and then applied to the electrocatalytic nitrogen reduction reaction(NRR).The electrocatalyst exhibits high NRR catalytic activity(NH₃ yield of19.86?g h^-1 mg^-1_cat.,Faraday efficiency of 6.87%)and good stability(above 18 h).Further structural analysis reveals that during the formation of MoS₂ layer and C₃N₄layer,they interact strongly to form interface Mo-N coordination,which promotes electron transfer at the MoS₂/C₃N₄ interface.Mo-N coordination acts as the active sites for catalyzing NRR,which can promote the adsorption and activation of N₂.In addition,because the MoS₂ layer is perpendicular to the C₃N₄ layer and presents a three-dimensional honeycomb structure,which increases the specific surface area of the catalyst and promotes the full access between the reactants and the active site.The C₃N₄ substrate provides the electrocatalyst with good stability and provides a pathway for electron transport.3.Since the performance of most NRR electrocatalysts is restricted to the competition of cathode HER.On the basis of the above research,it is further explored that the metal-free C₃N₄ is applied for electrocatalytic NRR by structural adjustment.The porous 2D C₃N₄-NV with abundant nitrogen vacancies and large specific surface area is prepared by regulating the structure of C₃N₄,and it is used as an efficient metal-free NRR electrocatalyst.The electrocatalyst shows an excellent catalytic activity and stability(over 20 h)for NRR in 0.1M HCl electrolyte.The NH₃ yield is17.85?g h^-1mg^-1_cat.,and the Faraday efficiency is 10.96%.Structural characterization and Density functional theory(DFT)calculation reveal that nitrogen vacancies in 2D C₃N₄-NV can enrich electrons,actively capture and activate nitrogen molecules,and become the active center of efficient catalyzing NRR.Meanwhile,the electrocatalyst can reduce energy barrier of potential-determining step(?G_PDS)in NRR and stabilize intermediates;through the study on the mechanism of HER,it indicate that strong?G_H*hinder the formation of H^*?H₂(Heyrovsky step)and effectively inhibit cathodic HER competition.In addition,porous structure and two-dimensional ultrathin nanosheet structure further increase the specific surface area of the electrocatalyst,which make more active sites expose,thus improving its NRR performance.4.The metal-free NRR electrocatalyst with high activity and good stability under neutral conditions is further studied.Therefore,a highly efficient and stable metal-free NRR electrocatalyst P-NV-C₃N₄ in neutral electrolyte is synthesized successfully through the P doping in C₃N₄ combined with the design of nitrogen vacancy structure.The experimental result shows that the electrocatalyst has excellent catalytic activity and stability(over 20 h)for NRR in 0.1M Na₂SO₄ electrolyte.The yield of NH₃ is28.67?g h^-1 mg^-1_cat.,and the corresponding Faraday efficiency is 22.15%.Structural characterization and Density functional theory(DFT)calculation reveal that P doping can effectively promote the water decomposition and provide hydrogen source for nitrogen reduction.Meanwhile,the N-vacancy structure can significantly inhibit the formation of HER pathway,hinder the Heyrovsky step,and promote the adsorption and activation of N₂.In addition,P-doped and N-vacancy structure of P-NV-C₃N₄ can reduce the Gibbs free energy(?G)barrier in NRR process and PDS energy barrier,and stabilize the reaction intermediates,thus improving its NRR performance.