Fabrication And Electrochemical Properties Of Graphene Aerogels And Their Composites

Graphene has broad application prospects in supercapacitor electrode materials due to its superior physical and chemical properties such as ultra-high electrical conductivity and specific surface area.However,the two-dimensional graphene is susceptible to irreversible agglomeration and stacking due toπ-πinteraction and van der Waals force,which affects its electrochemical performance as an electrode material.The two-dimensional graphene is spatially assembled to form a three-dimensional graphene aerogel,which can prevent agglomeration.In addition,due to the advantages of high yield,controllable number of layers and stable dispersion of aqueous solution of graphene oxide,it is a hot spot to prepare high-performance three-dimensional graphene aerogel by assembly and reduction of graphene oxide.Yet it is difficult to obtain redox three-dimensional graphene with high specific surface area,good degree of reduction,less oxidative defects,and low contact resistance in the current preparation process.In this paper,the effects of preparation parameters on the pore structure and electrochemical properties of three-dimensional graphene were studied systematically by spark plasma sintering process.The mechanism of reduction and"welding"of graphene oxide was different from thermal reduction was found.On this basis,using graphene oxide as a template,nano-nickel and nickel oxide particles are formed in situ on the surface,which increases the effective surface area of the electrode material capable of adsorbing charge and realizes the energy storage mechanism of electric double layer capacitor and Faraday pseudocapacitor in combination.A nickel-loaded graphene-based composite material with good electrochemical properties was prepared.At the same time,the pore structure of the nickel oxide/graphene composites was controlled by the freeze casting process and the electrochemical performance was greatly improved by the optimization of the pore structure.The results showed that the X-ray photoelectron spectroscopy analysis of pure graphene aerogels prepared by our study showed that the atomic ratio of C/O was4.23 and the functional groups containing oxygen were almost completely removed forming a three-dimensional structure with overlapping lamellae.The specific surface area was up to 1965.37m²/g.Graphene aerogels with good electrochemical properties were obtained at 600-9min-40MPa in SPS process.The mass specific capacitance of 1mol/L Na₂SO₄ was the highest in 162.75F/g,and the highest energy density in 1mol/L EMIMBF₄/AN was 68.56Wh/kg.On this basis,the electrochemical properties of graphene-based composites synthesized in situ with nano-nickel and nickel oxide particles have been greatly improved.For nano-nickel/graphene composites,when the grain size of nickel is about 15.58 nm,the electrochemical test results showed that the specific capacitance in the 3 mol/L KOH electrolyte is 487.50 F/g,which is 199%higher than that of pure graphene aerogel,the energy density is 159.89Wh/kg,which is 133%higher than that of pure graphene aerogel.The nickel oxide/graphene composite improves the problem o f non-wetting of graphene and aqueous electrolyte.When the crystal size of nickel oxide is about 18.07 nm,the electrochemical test results showed that the specific capacitance is 448.81F/g in 3 mol/L KOH electrolyte,which is 173%higher than that of pure graphene aerogel,the energy density was 147.45Wh/kg,which is 115%higher than that of pure graphene aerogel.The nickel oxide/graphene composites with oriented layer arrangement were successfully prepared by the freeze casting method.The electrochemic al properties of the composites were improved by the control of the pore structure.The results showed that the specific capacitance in the 3mol/L KOH electrolyte can reach504.17F/g,which is 209%higher than that of pure graphene aerogel,and the energy density is 193.83Wh/kg,which is 182%higher than that of pure graphene aerogel.