Study On Flow Behavior Of Graphene Nanostructure And Its Energy Storage Performance In Supercapacitor

The exploitation and utilization of traditional fossil fuels by human beings could cause the exhaustion of non-renewable power sources,and further exacerbate the global energy crisis and environment pollution.Developing renewable clean energy resources,high-efficiency energy conversion and storage technology faces enormous challenges,bringing about the development opportunity for the exploitation of renewable energy at the same time.Supercapacitors,based on the electric double-layer principle,achieve charge storage process by the electric double-layer electrostatic adsorption of electrolyte ions formed on the surfaces of active materials.In comparison with traditional capacitors and secondary batteries,supercapacitors exhibit outstanding advantages such as high power delivery,fast charge and discharge rates,wide operating temperature window and long cycle life.As a novel two-dimensional carbon nanomaterial,graphene possesses high specific surface area,excellent electrical conductivity and mechanical properties,which makes it as an ideal supercapacitor electrode material.The energy storage nature of supercapacitors is closely related to the interaction between the electrode material and electrolyte at the microscale.Increasing the effective surface area utilization of electrode material is of vital significance for fully exploiting the energy storage performance.From a microscopic point of view,the flow behavior of electrolyte in the graphene electrode material significantly affects the surface area utilization of the electrode material,and thus obviously affects the energy output of supercapacitors at the macroscale.The flow behavior of electrolyte in the graphene electrode material is rather complex.Owing to the limited visual analysis and accuracy,it is difficult to describe the electrolyte flow characteristics in the pore structure of the graphene in experimental studies.As an effective complement to experimental methods,numerical simulation technology can reveal the flow evolution based on the essential characteristics of flow process.In this work,study on flow behavior of graphene nanostructure and its energy storage performance in supercapacitor is investigated through numerical simulations and experimental methods.The basic flow behavior of electrolyte in graphene electrode materials is studied by Lattice Boltzmann method(LBM).Furthermore,the influences of various factors on the flow characteristics in horizontal graphenes and vertically-oriented graphenes are investigated.Based on the above simulation results,the influence of different factors on the energy storage performance of horizontal graphenes and vertically-oriented graphenes are further explored by means of experimental methods.The detailed research contents and main conclusions of this article are as follows:Firstly,the basic behaviors of electrolyte flow through the channels of continuous surface and discontinuous surface of graphenes are investigated by LBM.The results show that the flow behaviors for continuous surface and discontinuous surface in the graphene nanostructures are distinct to a wide extent.Capillary force in the continuous surface sheet channel plays a dominant role in electrolyte flow,allowing the channels to be sufficiently accessible to electrolyte.However,capillary force is invalid near the cracked structure formed by the discontinuous sheets and channels.Capillary force and additional pressure are both required to drive the electrolyte flow process.Secondly,based on quartet structure generation set(QSGS),the physical model for pore structure of horizontal graphenes is constructed,and the flow behavior of the electrolyte in horizontal graphenes is simulated and explored by LBM.The flow process of electrolyte in the discontinuous surface sheet channels of horizontal graphenes is investigated when the electrolyte is applied with varying flow-driving pressures.The above results indicate that when flow-driving pressure of electrolyte increases from 1 bar to 5 bar,the effective surface area utilization of the horizontal graphenes enhances from 14%to 78%.Furthermore,the flow behavior of electrolyte through the horizontal graphenes with different pore sizes is explored.As the pore size varies from 13.4 nm to 46.4 nm,the surface area utilization of horizontal graphenes increases from 14%to 87%.This phenomenon is ascribed to the decreased electrolyte flow resistance in the alleviated stacking structure of graphene sheets.Thirdly,the flow characteristics of electrolyte in vertically-oriented graphenes is investigated by LBM.Vertically-oriented graphenes exhibit open intersheet channels,which can avoid severe agglomeration and stacking of horizontal graphene sheets.Such features are beneficial for promoting the electrolyte flow in the channels of graphene sheets,increasing the surface area utilization of the graphenes.The results demonstrate that the electrolyte flow mechanism in the vertically-oriented graphenes is significantly different with that in the horizontal graphenes.Vertically-oriented graphenes possess continuous sheet channels,making it possible for electrolyte to flow in the graphene channels driven by capillary force.With the decrease of inter-sheet spacing from 322 to 12 nm,the effective utilization of the graphene surface area significantly increases by about 2 times.Fourthly,based on the simulation results of electrolyte flowing in horizontal graphene under different electrolyte flow-driving pressures,the energy storage performance of horizontal graphene electrode materials is investigated.Graphene paper is first prepared by chemical method using hydrazine hydrate solution as reducing agent.The obtained graphene paper is treated under the flow-driving pressures of 1 bar,3 bar,and 5 bar,respectively,and further employed as the electrode to investigate the electrochemical performances.The results show that with the electrolyte flow-driving pressure increasing from 1 bar to 5 bar,the specific capacitance increases from 103 F g-1 to 159 F g-1 at a scan rate of 20 mV s-1.The capacitance retention increases from 23%to 49%with the scan rate increasing from 20 to 1000 mV s-1.Moreover,horizontal graphenes with different pore sizes are fabricated by caffeic acid reduction and freeze-drying process.The as-prepared samples are assembled into supercapacitors for electrochemical performance test.The results show that with the average pore size increasing from 10.2 nm to 44.1 nm,the specific capacitance increases from 96 F g-1 to 167 F g-1 at a scan rate of 20 mV s-1.The capacitance retention increases from 31%to 45%as the scan rates range from 20 to 1000 mV s-1.Fifthly,based on the simulation results of electrolyte flow in vertically-oriented graphenes,the energy storage performance of vertically-oriented graphene electrode material is investigated.Vertically-oriented graphenes with different morphologies are prepared by plasma-enhanced chemical vapor deposition methods using DC discharge,inductive coupling,and microwave as the plasma sources,respectively,and electrochemical performance of vertically-oriented graphenes is tested.The results show that with the intersheet distance reducing from 306.2 to 14.5 nm,the specific capacitance increases from 82 to 147 F g-1 at a scan rate of 500 mV s-1.Due to the unique structure of the vertically-oriented graphenes,the capacitance retention reaches over 90%at the scan rates from 50 to 1000 mVs-1,which is far higher than that of the horizontal graphenes.To solve the problem of limited surface area utilization for graphene electrode materials,this work combines simulation with experimental methods to explore and analyze the electrolyte flow characteristics in graphene electrode materials.Furthermore,the energy storage properties of graphene-based supercapacitors are also investigated in detail.Finally,the flow and energy storage mechanism of graphene electrode materials are specifically described.The results of the current work could provide instructive information in the development of high performance graphene-based supercapacitors,and facilitate the revolution in energy storage research.