The Study Of Electrocatalytic Nitrogen Reduction Performance And In-situ Electrochemical TEM Observation Of FeMo Oxides And Sulfides

Ammonia synthesis through Haber-Bosch process takes place under high temperature and high pressure,consuming a great amount of energy and producing greenhouse gases such as CO2.Electrochemical reduction of nitrogen to ammonia,as a novel strategy for ammonia synthesis,is an environmental-friendly process with low energy consumption.Unfortunately,due to the high binding energy of N≡N,electrochemical ammonia synthesis process suffers from low ammonia yield.Meanwhile,the high overpotential leads to severe hydrogen reduction reaction,resulting in a low Faradic efficiency.Therefore,the design of nitrogen reduction electrocatalyst with high activity and high selectivity is the key to achieve large-scale application of electrochemical synthesis of ammonia.Nitrogenase,which consists of Fe and Mo elements,exhibits excellent ammonia synthesis activity.Besides,research shows that Fe and Mo favor the nitrogen reduction reaction with a low overpotential for the potential determine step.Till now,the ammonia yield and faradic efficiency of electrocatalysts for nitrogen reduction reaction remain very low.Nevertheless,the active site of electrocatalysts is still controversial,and the mechanism of the ammonia synthesis process remains unclear.Thus,the relationship between the structural transformation and activity of catalysts during the electrocatalytic nitrogen reduction reaction(NRR)is vital to reveal the active sites of catalysts and NRR mechanism.In this work,Fe-Mo oxides and sulfides are designed as electrocatalysts for ammonia synthesis.By adjusting the structure and component of the catalysts,the NRR activity has been improved.In the meantime,the NRR process of the catalysts have been studied by in-situ electrochemical liquid cell TEM,and the structure-activity relationship have been revealed,providing a guidance for the development of high NRR activity electrocatalyst.Main research content includes two aspects as follows:(1)FeMoO4/Fe2(MoO4)3 catalyst was synthesized by hydrothermal method,which showed far better NRR performance than Fe2O3 and MoO3,owing to the synergistic effect of Fe and Mo.Under ambient conditions,FeMoO4/Fe2(MoO4)3 achieved an ammonia yield at 25.95 μg h-1mgcat.-1 and Faradic efficiency at 3.57%in 0.1 M Na2SO4 electrolyte at-0.6 V vs.RHE.Besides,FeMoO4/Fe2(MoO4)3 can maintain an almost constant current density for 20 h of continuous electrolysis,while MoO3 suffers from survere decline.When the pressure increased to 0.5 MPa,ammonia yield and Faradic efficiency increased to 35.33 μg h-1mgcat.-1 and 9.19%,respectively.In addition,in-situ electrochemical liquid cell TEM was used to explore the NRR process of FeMoO4/Fe2(MoO4)3,MoO3 and Fe2O3.It was found that the initial structure of Fe2O3 and FeMoO4/Fe2(MoO4)3 remained,while MoO3 suffered from obvious corrosion,which also confirmed that Fe2O3 and FeMoO4/Fe2(MoO4)3 had excellent stability.(2)Compared with metal oxides,chalcogenides have higher NRR activity.A series of FeMoS catalysts have been prepared.The component and crystal structure of FeMoS was compared.The m-FeS crystallinity in FeMoS-4/1 was the highest among all four samples,followed by FeMoS-5/1 and FeMoS-3/2,while FeMoS-2/3 did not show mFeS phase.The NRR activity decreased as the order of FeMoS-4/1>FeMoS-5/1>FeMoS-3/2>FeMoS-2/3.In 0.1 M Na2SO4 solution,FeMoS-4/1 reached the highest ammonia yield at 32.6 μg h-1mgcat.-1 and Faradic efficiency at 5.61%at-0.5 V vs.RHE,indicating that m-FeS phase is contributed to the NRR activity of FeMoS.In-situ electrochemical liquid cell transmission electron microscopy technique was used to explore the NRR process of FeMoS-4/1.No obvious change of the initial flower-like structure of FeMoS-4/1 was observed during the NRR process,which revealed the high stability of FeMoS-4/1.