Morphology Regulation And Electrochemical Performance Of Novel Covalent Organic Framework Materials

During the combustion of coal and other energy sources and the transportation process,the excessive release of NOX,volatile organic compounds and other air pollutants and greenhouse gas CO2 into the atmosphere have caused serious harm to the human living environment,such as acid rain and the greenhouse effect.If these gas pollutants can be effectively detected in real time,and even realize further transformation and utilization,then energy and environmental problems will be alleviated or solved in time.Therefore,researchers propose to solve these problems through gas sensing and electrocatalytic transformation,as follows: 1)Design highly efficient and sensitive gas sensors to effectively monitor and control the composition and content of ambient gas in real time;2)Propose the effective use of carbon resources,electrocatalytic reduction of CO2 into fuels or high value-added chemicals(CH4,C2H4,etc.),which can effectively alleviate a series of environmental problems caused by the greenhouse gas CO2 while realizing the carbon cycle;3)Use CO2 to further develop a new type of energy storage system Li-CO2 battery to achieve energy storage and conversion,thereby alleviating energy and environmental problems at the same time.In summary,it is of great significance to develop a catalyst with a clear structure,high catalytic activity and low cost to achieve the above goals.As a new class of porous crystalline materials,covalent organic framework materials(COFs)have potential applications in many fields such as sensing,catalysis and energy storage due to their low density,high stability and tunable pore structure.However,COFs are often closely packed or interspersed,resulting in low utilization of active sites or slow mass transfer,which may lead to insufficient development of their properties.Therefore,it is necessary to design and synthesize COFs with different special morphologies to fully expose the active sites and improve their performance in the application field.However,research on the morphological control mechanism and application of COFs is still in preliminary development so far,so the controlled synthesis of COFs with different morphologies is very attractive in different fields.Based on the above ideas,this thesis has conducted the following research based on the study of the morphology control and electrochemical performance of the new COFs:1)By synthesizing a series of ultra-thin COFs nanosheets modified with different metals,the gas-sensitive sensing performance of various gas pollutants is studied.Among them,H2-TPCOF(NS)showed high selectivity to NO2 gas at room temperature,rapid response and recovery(4.25 min/3.36 min)with a response of 89.00.In addition,M-TPCOF(M = Co,Cu or Zn)modified with different transition metals showed different sensing properties for NO2 under the same experimental parameters,indicating that different metal centers in the materials have different affinity with NO2 molecules.Notably,the sensitivity of Zn-TPCOF(NS)to NO2 sensing reached the highest(1594.22)at 100 ppm concentration,which was nearly 18 times higher than that of metal-free H2-TPCOF(NS),but it was difficult to recover.Moreover,the sensitivity of Cu-TPCOF(NS)reached 28.84 ppm-1,but it almost had no response to low concentration of NO2.And CoTPCOF(NS)also showed high response(302.61),fast response/recovery(4.22 min/4.12 min)and low detection limit(130 ppb)at room temperature,achieving high selectivity and sensitivity for NO2 molecular sensing.This work provides a new path for the design and manufacture of high-performance NO2 sensors in the future.2)Two kinds of AAN-COF(NF)and OH-AAN-COF(HT)materials with tunable 1D superstructures were designed and synthesized.This type of porous nanostructure has the characteristics of excellent stability,high porosity and strong CO2 adsorption/activation ability.After this type of COFs nanostructure is modified with different transition metals(such as Cu,Co and Zn),it can be applied to CO2 electricity.Reduction performance research.Among them,AAN-COF-Cu(NF)and OH-AAN-COF-Cu(HT)showed excellent CO2 RR properties and could be selectively reduced to high value-added CH4 with the Faraday efficiency of 77%(-128.1 m A cm-2,-0.9 V)and 61%(-99.5 m A cm-2,-1.0 V),respectively.As far as we know,the main reduction product of most crystalline COFs is CO,while CH4 has rarely been reported.It is worth noting that AAN-COF-Cu(NF)maintains a FECH4 of more than 53% in a wide voltage range(-0.8 ~-1.0 V),and the highest FECH4 can reach up to 77%(-0.9 V),which is the highest in crystalline COFs at present.In addition,in order to study the influence of morphology on CO2 RR,it was found that the morphology of nanofiber was more conducive to the generation of CH4,while the morphology of the hollow tube had a certain promotion effect on the generation of ethylene(C2H4).The preparation of this anthraquinone-based COF with 1D tunable nanostructure and its application of CO2 RR may provide a reference for the design of high-performance CO2 RR catalysts.3)A series of OH-AAN-COF-x(X = 1,2,3)were prepared by modifying OH-AANCOF-X(X = 1,2,3)with different amounts of Mn2+.The effects of OH-AAN-COFX(X = 1,2,3)on the performance of Li-CO2 cells were further studied by using them as positive electrode catalysts.Among them,OH-AAn-COF-2 has the best performance,showing smaller overpotential and good cycle stability.When the cut-off capacity is 1000 m Ah g-1 and 100 m A g-1,the first lap discharge voltage and charge voltage are 2.79 V and 4.12 V,respectively,achieving a low overpotential of 1.33 V;after 30 cycles,the discharge platform still maintains 2.71 V.The first lap discharge and charge voltages of OH-AAn-COF without metal at 100 m A g-1 and cut-off capacity of 1000 m Ah g-1 are 2.47 V and 4.67 V,respectively,but the reversible cycle cannot be realized.This may be attributed to the effective synergy between the hollow structure of OH-AAn-COF and the active center of Mn2+.The active center of Mn2+ can efficiently promote CO2 activation and the decomposition of discharge products;OH-AAn-COF has regular and orderly open channels and hollows.The morphology is conducive to fully exposing the active sites and improving the catalytic activity.The effective synergy between the two promotes a significant improvement in the performance of the Li-CO2 battery.