Preparation Of MXene-based Nanocomposites And Their Electrochemical Performance For Ammonia Synthesis

Ammonia（NH₃）is an important chemical substance and carbon-free energy carrier that plays a crucial role in modern economic development.Currently,the main method for large-scale synthesis of NH₃ in industry is the energy-intensive Haber-Bosch process,which has become a serious burden on global energy consumption and CO₂ emissions.Electrochemical nitrogen reduction reaction（NRR）driven by renewable energy electricity and using water and N₂ as raw materials can sustainably synthesize NH₃ at normal temperature and pressure.However,due to the high energy barrier for the cleavage of the N≡N bond and the presence of intense competing hydrogen evolution reaction（HER）during the NRR process,the NH₃ production rate and Faradaic efficiency（FE）of NRR are far from satisfactory,which seriously hinders the development of electrochemical NH₃ synthesis technology.Therefore,in order to improve the catalytic ability of NRR,it is necessary to rationally design efficient and inexpensive electrocatalysts,promote the adsorption and activation of N₂,suppress the occurrence of HER,and enhance the efficiency and FE of NH₃production.MXene is a novel two-dimensional layered nanomaterial with good hydrophilicity,conductivity and stability,and is widely used in the field of electrocatalysis.The design strategies of NRR catalysts mainly include morphology size control,vacancy engineering and heterojunction construction,all of which are conducive to improving the catalytic activity of NRR.In this paper,two efficient MXene-based nanocomposites are prepared as catalysts and their catalytic performance and mechanism were systematically and deeply studied,with the aim of exploring the preliminary application of MXene-based catalysts in the field of electrocatalytic NRR from basic research to practical application.The main research contents of this paper are as follows:（1）MoO₃/MXene nanocomposites are successfully prepared by microwave-assisted hydrothermal method using sodium molybdate and Ti₃C₂T_x/MXene as precursors,followed by annealing to obtain oxygen-deficient MoO_3-x/MXene nanocomposites.The experimental results showed that MoO_3-xnanoparticles were uniformly and densely loaded on the surface of MXene.The electrochemical tests showed that the NH₃ yield of MoO_3-x/MXene was highest at-0.4 V potential,reaching 95.8μg h^-1 mg^-1,and the FE was highest at-0.3 V,reaching22.3%.DFT calculations revealed the mechanism of the excellent NRR performance of MoO_3-x/MXene:OV is the main active center for N₂ adsorption and activation.MXene further regulates the OV sites to balance the adsorption energies of*N₂/*N₂H and*NH₂/*NH₃.OV and MXene synergistically reduce the reaction barrier,thereby promoting NRR performance.（2）Bi₂S_3-x/MXene nanocomposites were prepared by microwave-assisted hydrothermal method using bismuth nitrate,thiourea,and Ti₃C₂T_x/MXene as precursors.The electrochemical tests show that the NH₃ yield of Bi₂S_3-x/MXene is 68.3μg h^-1 mg^-1 at-0.6 V potential,and the faradaic efficiency is 22.5%at-0.4 V potential.DFT calculations found that sulfur vacancy（SV）and the heterojunction interface with Bi atoms acted as dual-active sites to synergistically activate N₂ molecules and enhance the adsorption of key intermediate*N₂H,thereby reducing the reaction barrier in NRR and improving the NRR performance of Bi₂S_3-x/MXene.The electronic structure of the catalyst can be effectively regulated by heterojunction and vacancy,thereby improving the catalytic activity of NRR.The MoO_3-x/MXene,Bi₂S_3-x/MXene nanocomposites prepared in this paper exhibit excellent NRR performance.