MOF-derived Transition Metal Oxide/carbon Electrocatalysts And Their Nitrogen Fixation Performance

As a common chemical,ammonia is widely used in industrial,agricultural production and the field of energy storage and conversion.Nowadays,industrial ammonia synthesis mainly relies on Haber-Bosch process,which needs to be carried out under high temperature and pressure.This process not only consumes a lot of energy,but also emits a large amount of greenhouse gas CO2.Thus,finding a green,low-energy and high-efficiency method for ammonia synthesis is very necessary to the sustainable development.With the development of electrochemical energy conversion technology,the method of electrocatalysis is used to make thermodynamic non-spontaneous nitrogen reduction reaction(NRR)get rid of thermodynamic equilibrium limit under the excitation of electric energy and achieve clean ammonia synthesis.Electrocatalytic NRR is an important direction for both basic research and industrial development.In this study,the defect engineering and interface structure optimization were investigated to improving the NRR performance of catalysts.Considering the characteristics of electrocatalytic reaction,metal organic frameworks(MOFs)were selected as the precursor to prepare transition metal oxides(TMOs)electrocatalysts.Moreover,TMOs were modified by defect engineering to greatly improve the NRR performance.The mechanism of NRR was also studied.This study provided a new idea and experimental basis for improving the NRR performance of electrocatalysts.The main research contents are as follows:(1)Co3O4@NC catalyst was successfully prepared through a two-step calcination using cheap ZIF-67 as the precursor.In 0.05 M H2SO4 electrolyte,it achieves a high NH3 yield of 42.58 μg h-1 mg-1 and a Faradaic eficiency(FE)of 8.49%,which are higher than that of most reported catalysts.The experimental results show that Co3O4@NC has a core-shell structure,the "core" consists of nitrogen-doped carbon and the "shell" mainly consists of Co3O4.This structure can provide a large number of active sites for NRR.For example,N2 is restricted in the "shell",which is more conducive to high-frequency collisions and increases the steady-state concentration of the reactants in the rate-determining step of NRR.The XPS,EPR and N2-TPD results indicate Co3O4@NC has a lot of O-vacancies,which can strongly adsorb and activate N2 to increase the NRR performance.Moreover,the NRR performance of Co3O4@NC catalyst exhibits negligible change in the 24 h NRR tests,showing high stability and long cycle life.(2)C@NiO@Ni catalyst was successfully prepared through a two-step calcination using cheap Ni-BTC as the precursor.In 0.1 M KOH electrolyte,it achieves a high NH3 yield of 43.15 μg h-1 mg-1 and a Faradaic efficiency(FE)of 10.9%,which are higher than that of most reported catalysts.Mechanism studies show that the introduction of O-vacancies can not only increase the electron trapping capacity for NRR,but also inject electrons into the anti-bonding orbitals of N2 molecules,which significantly improved the NRR performance.In addition,the experimental results indicate that the interfacial interaction of Ni/NiO heterojunction could provide abundant active sites for NRR and effectively suppress the hydrogen evolution reaction.In summary,the carbon in catalyst plays the role of conducting electrons.The abundant O-vacancies in NiO provides a lot of catalytic sites for the activation of N2,while Ni plays the role of activating and transporting protons.The activated N atoms migrated to the NiO/Ni interface and encountered H+to form NH3.This study provides a new idea for the structure design of NRR catalyst.(3)C@ZrO2 catalyst was successfully prepared through a one-step calcination using cheap UiO-66 as the precursor.Moreover,C@YSZ was prepared by calcining Y3+doped UiO-66 precursor.In 0.1 M Na2SO4 electrolyte,C@ZrO2 and C@YSZ achieve the NH3 yield of 22.4 and 24.6 μg h-1 mg-1,respectively.The experimental results indicate the O-vacancies in catalysts are the main active sites for NRR Introducing Y3+ into the ZrO2 lattice has a significant effect to increase and stabilize the O-vacancies.Meanwhile,C@YSZ catalyst displays remarkable stability and durability for NRR,showing negligible change after 7 days reaction,which is better than most reported NRR electrocatalysts.The in-situ electrochemical quartz-crystal microbalance(EQCM)was applied in the NRR field for the first time and was successfully combined with density functional theory(DFT)calculations to reveal the deactivation mechanism.In summary,the surface of C@ZrO2 is unstable,the surface O-vacancies is easily filled by the O species in the electrolyte,resulting in the deactivation of catalyst.However,the C@YSZ with Y doping has more stable surface to keep catalytic activity in the long-term tests.This study firstly discussed the deactivation mechanism of oxide-based catalysts,which is one of the important advances in the field of NRR.