Modification Of Two-dimensional Ti₃C₂T_x And Study On Electrocatalytic Ammonia Synthesis At Room Temperature

Ammonia（NH₃）is an important source of active nitrogen in the manufacture of fertilizers,drugs,dyes,explosives,resins and other materials.Although nitrogen is an inexhaustible molecule in the atmosphere,its high N≡N(941 k J.mol^-1)bond energy make it difficult to convert to ammonia.Haber-Bosch industrial ammonia production process consumes a lot of energy and causes serious carbon dioxide emissions.Therefore,it is very necessary to explore environmentally friendly alternatives to ammonia synthesis with low energy consumption.Electrochemical nitrogen reduction is an attractive alternative to ammonia synthesis.However,the nitrogen reduction reaction（NRR）at room temperature requires an effective electrocatalyst to activate nitrogen for the purpose of nitrogen fixation.Despite some progress in catalyst exploitation,there is still a great need to develop earth-rich NRR electrocatalysts that simultaneously achieve high ammonia yields and high Faraday efficiency.First,we used a co-doping strategy to design the electronic configuration and structural mechanics of Ti₃C₂T_x catalyst to achieve the purpose of efficient catalytic ammonia synthesis in NRR.NS-Ti₃C₂T_x nanosheets were successfully prepared by sintering the N and S doped into the Ti₃C₂T_x.At the same time,Ti₃C₂T_x,S-Ti₃C₂T_x and N-Ti₃C₂T_x catalysts were designed as control group to study the mechanism of electrochemical reduction of ammonia synthesis.As expected,the synergistic effect of N and S dopants in NS-Ti₃C₂T_x significantly increased electron transport capacity and increased catalytic active sites.Specifically,in 0.05 M H₂SO₄electrolyte,the prepared NS-Ti₃C₂T_x nanosheets exhibited a catalytic NH₃ yield of 34.23μg·h^-1·mg^-1,Faraday efficiency of 6.6%and excelent electrocatalytic stability at-0.55 V with standard hydrogen electrode（RHE）.The synergistic effect of nitrogen and sulfur co-doping can adjust the electron configuration of the catalyst and improves the electronic conductivity,thus accelerating the reaction kinetics.Secondly,co-doping induces a large number of mesopores and a large specific surface area in the NS-Ti₃C₂T_x of layered structure,which increases the active sites and is beneficial to charge transfer during NRR process.The MoO₃-Ti₃C₂T_x nanosheets of supported MoO₃ prepared by precipitation sintering,for high performance electrocatalytic nitrogen reduction.MoO₃ is a low-cost,simple to prepare and environmentally friendly catalyst.Since Ti₃C₂T_x（T=F,OH）MXene is a two-dimensional（2 D）material with high conductivity and large surface area,it can enhance the electron transfer of molybdenum trioxide and act as a strong support to prevent structural changes during electrochemical processes.MoO₃-Ti₃C₂T_x showed high catalytic effect on NRR in neutral electrolyte of 0.1 M Na₂SO₄,with an extremely high Faraday efficiency of 29.2%and NH₃ yield of 11.3μg·h^-1·mg^-1 at-0.35 V,as well as long-term stability and excellent recoverability in neutral electrolyte.Hence,this study not only provides non-precious metal-based electrocatalysts for ammonia synthesis,but also proposes a simple method to design Ti₃C₂T_x based nanocatalysts for nitrogen fixation.