Experimental Study On Electrode Materials Of Zinc-Iron Flow Battery Based On MOFs

With the rapid development of the world economy,non-renewable energy sources are increasingly depleted.Low cost,high energy density,good cycle stability and environmentally friendly energy storage equipment have attracted extensive attention from researchers and society.Zinc-iron flow battery has become one of the hot technologies of electrochemical energy storage due to its advantages of safety,stability and low electrolyte cost.Electrode material is an important part of zinc-iron flow battery.The development of high-performance and lightweight electrode materials is the key to the development of flow battery technology.Graphene oxide(GO)has attracted extensive attention from researchers due to its excellent physical and chemical properties.Metal-organic frameworks(MOFs)have great application potential in energy storage,separation and analysis due to their high porosity,large specific surface area,simple and convenient preparation.However,both materials have limitations when used as electrodes alone.Therefore,the combination of graphene oxide and metal organic framework into MOF/GO material not only further improves the porosity,but also reduces the surface energy and enhances the stability.Based on the above research background,MOF/GO composites were synthesized by solvothermal method.The specific research contents and conclusions are as follows:1.Based on the research of the research group,GO was prepared by the improved Hummers method,and graphene oxide was characterized by scanning electron microscopy(SEM),transmission electron microscopy(TEM),Fourier transform infrared spectroscopy(FT-IR),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),Raman spectroscopy(Raman)and thermogravimetric analyzer(TG-DTG).The carbon-oxygen ratio of the prepared GO was calculated to be about 1.99;according to the XRD results,the interlayer spacing of GO is about 0.811 nm.Raman spectroscopy results show that the I_D/I_Gvalue of GO is about 0.94.The TG results clearly show that it can be divided into two weight loss stages and has the maximum degradation rate at 250°C.According to the characterization results,it was proved that GO with complete structure,high oxidation degree and rich oxygen-containing functional groups was prepared.2.Cobalt-based metal-organic framework materials(Co-Bpy)with good crystallinity were prepared by hydrothermal synthesis.The crystal structure and morphology of Co-Bpy particles were analyzed by SEM,FT-IR,XRD and XPS.The results show that Co-Bpy has an obvious hexagonal lamellar structure,and 2-picolinic acid as a bidentate ligand inhibits the longitudinal growth of Co-Bpy.FT-IR results showed that Co-Bpy contained the main chemical bond structure in 2-picolinic acid ligand,and cobalt ions coordinated with carboxyl and nitrogen atoms in 2-picolinic acid simultaneously.XRD results show that Co-Bpy has a good lattice structure.3.The MOF/GO-n composites were prepared by hydrothermal synthesis,and the crystal structure and morphology of MOF/GO-n were analyzed by various characterization methods.The results show that the Co-Bpy particles of hexagonal flakes are distributed in the intertwined GO sheets,and the GO nanosheets structure is obvious.With the increase of GO content,the Co-Bpy particles gradually decrease.The combination of GO and Co-Bpy retains the original structure and properties of the two materials.The proportion of GO will change the morphology and atomic composition of the composites.Co-Bpy and MOF/GO-n were loaded on nickel foam,respectively,and cyclic voltammetry(CV)and electrochemical impedance spectroscopy(EIS)were performed using a three-electrode test system.The results show that both Co-Bpy and MOF/GO-n exhibit pseudocapacitive characteristics.With the increase of GO content,the specific capacitance of MOF/GO-n increases in turn.EIS results show that both Co-Bpy and MOF/GO-n materials show linear curves in the low frequency region and quasi-semicircle curves in the high frequency region.The solution resistance of Co-Bpy material is 1.178Ω,and the solution resistance of MOF/GO-n composite material is 1.167Ω,1.158Ω,1.143Ω,1.129Ω,1.084Ω,respectively.The solution resistance of MOF/GO-n material decreases in turn,and it also shows extremely low charge transfer resistance characteristics.