Preparation Of Fe₃O₄/Graphene Nanocomposites Using Solvothermal Method And Its Capacitors

Fe₃O₄ presents a wide application prospect for application on supercapacitor and electrode materials thanks to its advantages of relatively high theoretical specific capacitance,low price,easy access,environmentalfriendness.The preparation conditions play a vital role on the structure of Fe₃O₄,and further in its capacitive property.In order to obtain good electrode materials with excellent capacitance performance,it is necessary to optimize the preparation conditions and discuss the structure-activity relationship.In this thesis,the experiment researches in three aspects were carried out as following.?1?The solvothermal method was used to prepare Fe₃O₄ under concentration of reactants: 0.0500 mol/L anhydrous ferric chloride?AF system for short?;another formula contains: 0.0580 mol/L anhydrous ferric chloride?BF system for short?.The cyclic voltammetry,constant-current charge-discharge,and AC impedance propersities of samples were measured by an electrochemistry workstation equipment to calculate their specific capacitance.X-ray diffraction,nitrogen adsorption and SEM were carried out to characterize the phase composition,pore structure and surface topography of the sample.Based on the results obtained above,the nucleation,growth,and morphology evolution of Fe₃O₄,and the impact of the preparation time on the capacitive property of the sample were investigated,and the Fe₃O₄ sample with relatively good capacitive property was optimized.?2?Four graphene oxide samples were prepared by Hummers and Eigers Methods using 325-mesh and 5,000-mesh flake graphite as the raw material.And four graphene samples were obtained by the reduction of graphene oxide samples using ethylene glycol as the reductant.The effect of the particle size of raw material and preparation methods on the capacitive property of the samples were stuided.Fe₃O₄/graphene oxide samples were synthesised from compounding the above 8 samples with the selected Fe₃O₄ samples.The effect of graphere content and the synthesis methods on capacitive property and cyclic stability of Fe₃O₄/graphene composite was discussed.The Fe₃O₄/graphene sample with high specific capacity and good cyclic stability was optimized.?3?Different electrolytes of sodium sulfate,sodium sulfite,and potassium hydroxide acqeous solutions with concentration of 0.7-1.2 mol/L were used to evaluate the capacitive property of Fe₃O₄/graphene sample optimized above.The main conclusions are summarized as follows.?1?In the solvothermal system,Fe₃O₄ will undergo an "aggregate-flake sphere-hollow sphere" morphology evolution process.In the initial reaction period,Fe₃O₄ would experience a nucleation growth and develop into a flake aggregate in the solution;in the mid-term,the surface energy of the Fe₃O₄ flake aggregate would keep decreasing with the preparation time increases,and further assemble into a sphere;in the later-term,Ostwald Ripening effect starts to emerge,the crystalline grains in the sphere dissolves,making it finally become a hollow sphere.When the preparation time is more than 16 h,the pore volume of Fe₃O₄ increases as the preparation time extends,the specific surface area gradually spreads,the crystallinity continuously rises up.Moreover,relatively large pore volume and specific surface area will facilitate the migration of the electrolyte in the materials.The regularly-shaped crystal form would enhance the stability of the material,and make the specific capacitance of Fe₃O₄ keep increasing as the time extends.The specific capacity of Fe₃O₄ samples prepared in AF and BF systems are all higher than 150 F/g under the current density of 0.4 A/g.AF24 and BF24 samples obtained in AF and BF systems with the preparation of 24 h show relatively high specific capacitances,234 F/g and 222 F/g respectively.?2?Compared with that of the mechanical method and two-step method,the flake Fe₃O₄,which is fixed on the surface of the graphene by use of onestep method,grows a loose laminated structure that benefits the contact between the composite and electrolyte,and provides more pseudocapacitance.Meanwhile,the Fe₃O₄ on the surface of the graphene may prevent the graphene from stacking,thus enhancing the stability of the Fe₃O₄/graphene in the charge-discharge processes.The optimized concentration of following reactants has been obtained: anhydrous ferric chloride,0.0580 mol/L;anhydrous sodium acetate,0.2333 mol/L;anhydrous sodium citrate,0.0200 mol/L;ethylene glycol,60 mL;graphene oxide,0.4052 g.Under such concentration environment,sample BF24G60 S prepared under 205 oC shows a highest specific capacitance of 300 F/g under a current density of 0.4 A/g,and its capacitance could remain above 93% of the initial value after 500 recycles.?3?BF24G60S has the advantage of high environmental adaptability independent of electrolyte and temperature,and it would matain its excellent pseudocapacitance characteristics whether in Na2SO4,Na2SO3 or KOH electrolytes with concentrations of 0.7-1.2 mol/L.The KOH solution with concentration of 0.9 mol/L is more suitable as electrolyte,in which the specific capacitance of the sample reaches 330 F/g.