A Study Of The Preparation And The Electrochemical Properties Of Graphene/Transition Metal Oxide Composites

With the rapid development of human society,science and technology,it has always been an important issue to meet the ever-increasing energy demand.In order to solve the energy crisis,the new energy storage technologies such as supercapacitors and lithium-ion batteries have been developed in recent years.Among them,supercapacitors have become popular in the research field of energy storage materials due to their high charge-discharge efficiency,high power density,and long cycle life,et al.The emerging technological products have put forward new requirements for supercapacitors:such as flexibility and transparency.Therefore,the preparation of supercapacitors with high energy density,high cycle life,and high flexibility has become an important problem to be solved.In the past few years,graphene has attracted much attention in the field of energy storage materials.Since GO is feasible to be modified and to produce composite materials with other phases,and it is easier to be prepared and can be produced on a large scale,if the energy storage performance of GO based composite materials can be greatly improved,it can provide an effective way for the practical application of GO based high-performance supercapacitors.In this paper,GO-based composites supercapacitor electrodes were prepared by compounding graphene with a variety of transition metal oxides（TMOs）.The synergetic effect between the graphene matrix and the TMOs on the electrochemical properties was thoroughly studied.The high-performance flexible supercapacitors were fabricated by the use of the GO-based composite films.A series of GO/LTO composites were prepared with the lanthanum titanate（LTO）fibers prepared by electrospinning and GO by a hydrothermal method.In the"sandwich-like"structure of the composite films,the LTO fibers were uniformly distributed on and between the GO sheets,reducing the agglomeration of the GO sheets and providing a large number of electrochemically active sites.The high electrochemical performance of LTO itself also provides large pseudocapacitance and energy density for the composites.Among them,GO/LTO-8 exhibited the highest electrochemical performance.The electrochemical performance of GO/LTO-8 electrodes was further improved by the"water-in-salt"electrolyte.The stable potential window of the GO/LTO-8 electrode was further increased to 1.8 V,and the maximum specific capacitance at a current density of 1 A g^-1 reached 900.6 F g^-1 by using a 1:1 volume ratio of sucrose and 1 M H₂SO₄ electrolyte.The symmetric supercapacitor devices by the use of the GO/LTO-8 composite electrodes and sucrose-H₂SO₄ electrolyte provided a large stable potential window and specific capacitance,which obtained a large energy density,and the device exhibited good cycling stability.Co₃O₄ nanoparticles,La Co O₃ nanoparticles,Sr Ti O₃ nanoparticles,Ni Co₂O₄ nanoflakes and Ni Mo O₄ nanorods were prepared and used to modify GO matrix by an electrostatic self-assembly method to prepare various GO/TMO composite electrodes.The energy storage performance and enhancement mechanism of different GO/TMO was studied in detail.Due to the difference in the oxygen stabilization of various TMOs,there were different synergistic effects on the enhancement of electrochemical performance and cycling stability.For Co₃O₄,a TMO with mainlyπ-stabilized oxygen,when combined with GO,the enhancement of electrochemical performance was limited,but showed excellent cycling stability.For binary TMOs such as LCO,the oxygen functional groups on the surface of the GO sheets promoted theσ-stabilization of the TMO in the composites,thereby enhancing the electrochemical performance of the composites.However,the change of the oxygen functional groups on the GO sheets during the charge-discharge cycles would destabilize theσ-stabilization,resulting in the decrease of cycling stability.On the other hand,a series of GR/LCO composites were prepared by electrostatic self-assembly method by using different contents of GO and r GO as the matrix to composite with La Co O₃ nanoparticles.The synergistic effect between oxygen functional groups on graphene sheets and LCO on the electrochemical performance was explored.With the increase of GO content in the GR/LCO matrix,the number of oxygen functional groups in the composites gradually increased.The increase of oxygen functional groups had two main effects on the electrochemical performance of GR/LCO composites:one was the increase of charge transfer resistance,which would reduce the electrochemical performance.The other was the increase of the synergistic effect byσ-stabilizing.The oxygen functional groups enhanced the redox reaction on the surface of the GO sheet and would improve the electrochemical performance of the composites.There was a competitive mechanism between the two effects on the electrochemical properties of the composites.When the GO in the matrix was relatively low,the effect of the charge transfer resistance took the advantage,so the introduction of a small amount of GO increased the charge transfer resistance,resulting in a decrease in the electrochemical performance of the composites.When the GO content was sufficient,the synergistic effect dominated the compete,which enhanced the electrochemical properties.Therefore,with the increase of GO content,the electrochemical performance of GR/LCO first decreased and then increased.In conclusion,it can be demonstrated that both graphene and transition metal oxides played an important role in the electrochemical performance of composite electrode materials through the study of the synergistic effect between graphene materials and transition metal oxides.Through the appropriate selection and use of transition metal oxides,the energy storage performance of GO-based composites can be greatly improved.It provides a new idea for the development of GO-based high-performance supercapacitor materials in a simple and low-cost way for practical application.