Synthesis And Electrochemical Performance Of Based On Two-dimensional Metal-organic Framework Composite Materials

The rapid development of modern society has put forward new requirements for efficient energy supply.Therefore,it is essential to develop clean,safe,and sustainable energy storage and conversion technologies based on renewable energy.Among various novel techniques,electrochemical approaches through converting or storing energy,such as the electrocatalytic hydrogen evolution reaction,and supercapacitors,are regarded as one of the most efficient approaches.As a highly ordered coordination polymer,metal organic framework(MOF)acts as a self-assembly of metal ions and organic linkers.Due to its unique characteristics,such as nanoscale and adjustable thickness,high aspect ratio,large surface area,more accessible active sites,good mechanical flexibility,and optical transparency,it has been widely studied for electrochemical energy storage and electrocatalysis.In this thesis,a composite material with controllable morphology and structure was prepared using 2D Ni-ZIF as a template,and the electrochemical properties of the material were studied.The main research contents are as follows:(1)the optimized solvothermal reactions based on crystal growth dynamics are innovatively carried out to synthesize Ni-ZIF nanosheet with ultrathin morphology,more ordered crystal structure,high specific surface area,abundant diffusion channels,and adequate open space.Therefore,the active site can be significantly improved and the transfer rate of electrolyte ions and electrons and reaction kinetics can be accelerated.With these extraordinary characteristics,the prepared Ni-ZIF electrode has an excellent specific capacitance(the specific capacity is 1293 F g^-1at a current density of 1 A g^-1).More importantly,by directly using the optimized Ni-ZIF electrode as the positive electrode,an ASC device with excellent energy density(83 Wh kg^-1)and power density(3750 W kg^-1)was realized.Therefore,we believe that this research provides new ideas for the optimization of material structure modulation and crystal orientation.Due to its own shortcomings,the electrode material exhibits poor rate performance.Combining it with highly conductive materials will be the direction of our future work.(2)In order to further overcome the disadvantages of electrodes with additives/binders for the Ni-ZIF structure of supercapacitors,ultra-thin Ni-ZIF nanosheet arrays have been in situ grown on carbon cloth through solvothermal reaction.The structure of ultrathin Ni-ZIF nanosheet arrays can expose more active sites,provide abundant diffusion channels,and buffer the stress caused by phase transition during charge-discharge process of supercapacitors.The optimized solvothermal reactions can provide more ordered crystal orientations by keeping nanosheets on carbon cloth completely coming from in situ growth,which will decrease the inner resistance of ultrathin Ni-ZIF nanosheets and improve the efficiency and kinetics of electrons transfer.By the virtue of such remarkable features,the electrochemical results confirmed the rationality of the vertical array structure,and the specific capacitance of the composite material has been significantly improved,from 491 F g^-1to 1153 F g^-1.which is even more impressive,the assembled asymmetric supercapacitor(ASC)is based on the complementary potential window in the aqueous electrolyte.The device can work in the operating voltage range of 0–1.8 V and has an excellent energy density of 97.5 Wh kg^-1(900 W kg^-1),power density 4500 W kg^-1(11.9 Wh kg^-1)and high stability(capacitance retention rate after 5000 cycles is 85%).(3)An alkaline etching strategy was developed to fabricate defect-rich Ag NWs-Ni-ZIF core-shell nanostructures for integration of anode methanol oxidation and cathode HER.Due to the open unsaturated metal sites and unique structure,D-Ag NWs-Ni-ZIF exhibits high electrocatalytic activity and good durability.which can convert methanol to high value-added formic acid on the anode with high selectivity.It only needs a small potential of 1.313 V(vs RHE)to reach a current density of 10 m A cm^-2,which is nearly 200 m V lower than OER.It shows that the methanol upgrading process replaces the slow OER,greatly increases the output of H₂,and reduces the energy consumption required to produce clean energy.This work opens an economical and efficient way for the electrochemical production of hydrogen and formic acid,and provides a very promising example for the electrocatalytic conversion of small organic molecules using non-noble metal electrocatalysts.