Research On The Design, Preparation And Performance Of Graphene/MOFs Supercapacitor Electrode Materials

With the rapid development of emerging technologies and the pressing need for clean and sustainable energy,efficient and more powerful energy storage systems have become an urgent challenge for us today.Supercapacitors have shown great promise as an electrochemical energy storage device due to their high power density,good cycle stability,low cost,fast charge-discharge,and non-polluting operation.The electrode material for the supercapacitor is the most important parameter.As a star material,graphene has high electron conductivity,low mass density,excellent thermal/mechanical/chemical stability,good optical properties and large specific surface area.It has been widely used in the field of supercapacitor,but its capacitance is low.As an emerging material,metal-organic frameworks?MOFs?have the advantages of adjustable structure,large size,large surface area,high porosity,and good thermal stability.MOFs have attracted interest for constructing the supercapacitor as electrode materials,but their electrical conductivity is not good.Therefore,it should be an effective way to composite graphene and MOFs as the electrode material of the supercapacitor by a facile method.The main work is as follows:1.Graphene oxide?GO?was synthesized by the Hummers method,and then reduced graphene oxide?RGO?was obtained by the hydrazine hydrate reduction method.The characteristic of GO,RGO and carboxylated graphene oxide?GO-COOH?purchased from NanoInnova Technologics was studied by infrared spectroscopy?IR?,X-ray diffraction?XRD?and Raman spectroscopy.The results showed that GO and RGO were successfully synthesized in our work and the characteristic of GO-COOH was also consistent with the results given by NanoInnova Technologics.In our following study,all these graphene materials were be used as a precursor of composite materials.2.A nickel-based metal-organic framework?Ni-MOF?compound was successfully synthesized by a one-step hydrothermal method with nickel nitrate hexahydrate as the metal source and trimellitic acid as the organic ligand.The three graphene materials were further composited with Ni-MOF respectively,and Ni-MOF/GO,Ni-MOF/RGO,and Ni-MOF/GO-COOH materials were obtained by changing the amounts of GO,RGO,and GO-COOH added.The three series of materials were characterized by IR,XRD,scanning electron microscopy?SEM?and electrochemical techniques.?1?In the Ni-MOF/GO series,IR and XRD characterization showed that GO and Ni-MOF have been successfully composited.It was seen from the images of SEM that the morphologies of Ni-MOF/GO-1 and Ni-MOF/GO-3 and Ni-MOF power are spherical,and the surface of composite material are much rougher than the single Ni-MOFs.The spherical morphology of Ni-MOF/GO-1 is composed of many hexagonal prisms which are more favorable to ion diffusion and electron conduction.The electrochemical results also indicated that the Ni-MOF/GO-1 material showed the best electrochemical performance when the amount of GO added was 1%.At a current density of 1 A g^-1,the specific capacitance of Ni-MOF is 1003.57 F g^-1,while the specific capacitance of Ni-MOF/GO-1is 1232.14 F g^-1,which is increased 20%in contrasted with the specific capacitance of Ni-MOF.The specific capacitance retention rate of Ni-MOF/GO-1 was 75%,which is higher than that of single Ni-MOF 61%after 1000 cycles at a current density of 10 A g^-1.At the same time,the specific surface area of Ni-MOF/GO-1 was measured to be 8.02 m² g^-1.All these results shows that the specific surface area of the material,the stability of the material and the specific capacitance are improved due to the synergistic effect of GO and Ni-MOF.The material of Ni-MOF/GO-1 is hopeful to be used as a potential material for energy storage devices.?2?In the Ni-MOF/RGO series,the IR and XRD characterization of the composites show that RGO and Ni-MOF have been successfully combined.When the amount of RGO added was 1%,the best electrochemical performance was exhibited.At a current density of 1 A g^-1,the specific capacitance of Ni-MOF/RGO-1 is 1096.43 F g^-1,and the specific capacitance retention rate is 67%after 1000 cycles at a current density of 10 A g^-1.Although the specific capacitance and the cycle stability have some improvement,compared with the single Ni-MOFs,the results are not ideal compared to the Ni-MOF/GO series.The main reason is that the double-layer capacitance behavior of RGO does not play a leading role,and the preparation method of RGO needs to be improved.?3?In the Ni-MOF/GO-COOH series,the IR and XRD characterization of the composites show that GO-COOH and Ni-MOF have also been successfully combined.When the addition amount of GO-COOH was 1%,the best electrochemical performance was exhibited.At a current density of 1 A g^-1,the specific capacitance of Ni-MOF/GO-COOH-1is 1332.14 F g^-1,and the specific capacitance retention ratio is 37%after 1000 cycles at a current density of 10 A g^-1.Although the specific capacitance is larger than that of the Ni-MOF,the cycle performance decreases instead,indicating that the material needs to be combined with other materials to improve the performance.3.In this study,a nanocube of Co-based zeolitic imidazolate framework-67?ZIF-67?was easily synthesized by mixing two aqueous solutions of 2-methylimidazole and cobalt nitrate with a certain amount of cetyltrimethylammonium bromide?CTAB?.Then,GO wrapped ZIF-67 nanocomposites?ZIF-67/GO?were synthesized by a facile stir method at an ambient temperature.The morphology and micro-structure of ZIF-67 and its GO nanocomposites were studied using SEM,XRD technique and Brunauer-Emmett-Teller?BET?specific surface area measurements.Electrochemical capacitance properties of ZIF-67 and GO/ZIF-67 were characterized by cyclic voltammetry and galvanostatic charge/discharge technique.The experimental results indicated that the content of GO of nanocomposites affected the specific capacity while they were constructed as electrode materials.Compared with single ZIF-67 and single GO,nanocomposites had better specific capacity performance and ZIF-67/GO-2 had a specific capacitance of 100.41 F g^-1 at a sweep rate of 5 mV s^-1.The specific capacitance of the ZIF-67/GO-2 nanocomposites increases from 46.15 F g^-1 to 69.30 F g^-1 after 1000 cycles of 10 A g^-1,indicating that the material has good cycle stability.Nitrogen adsorption-desorption tests showed that the specific surface area of ZIF-67/GO-2 was 1222.15 m² g^-1.The experimental results showed that the ZIF-67/GO-2 has excellent electrochemical performance,large specific surface area,and is expected to become a potential material for electrochemical energy storage.