Fabrication Of 3D Graphene Materials For Supercapacitors Or Electrothermal Film

Due to the demands of modern society and the emerging ecological problems,we are increasingly demanding researches on novel,low-cost,environmentally friendly and high-performance energy storage systems.Graphene-based materials have great application potential in the field of supercapacitors due to their unique two-dimensional structure and excellent intrinsic physical properties,such as extremely high conductivity and large surface area.However,the two-dimensional graphene surface of the intact structure exhibits an inert state and has a very stable chemical property,which makes it weakly interact with other media.Moreover,it is affected by Van der Waals force and the interaction force between graphene sheets,which makes it easy to generate aggregation and heavy accumulation.So the specific surface area is reduced,and the transport of electrolyte ions is also hindered,which makes the electrochemical properties of the material have a sharp decline.In order to make full use of the excellent properties of graphene materials and make them more widely used,we need to overcome their inherent defects and to make an effectively functional design and preparation of them.Among many methods,the porous treatment of graphene is one of the effective methods for providing a large specific surface area,facilitating ion diffusion,and achieving high specific capacitance.In the second chapter,we use a green and mild synthesis method of hydrothermal synthesis reaction and select Na2MoO4 nanoparticles as a pore-forming agent to prepare a three-dimensional porous graphene hydrogel binderless integrated electrode.Etching on graphene nano-sheets to form nano-pores not only increases the specific surface area of the material,but also further accelerates the transport efficiency of electrolyte ions.The prepared porous graphene hydrogel electrode has a higher specific surface area and more pore volume than the unperforated gel,and can obtain a gravimetric capacitance of 473 F g-1 nearly three times of the hydrogel electrode without punching.In addition,MoO3 nanoparticles are ideal faradaic pseudocapacitance materials produced during hydrothermal reactions.At the same time,sodium molybdate is added to the acidic electrolyte,which also interacts with hydrogen ions to produce a faradaic pseudocapacitance.Therefore,the graphene-loaded MoO3 nanocomposite was used as the positive electrode,and the porous graphene hydrogel used as the negative electrode were assembled into an asymmetric supercapacitor.The energy density and power density of the device are increased.Doping and modification of graphene is also one of the methods to achieve effective functionalization of graphene.In the third chapter,we prepared a three-dimensional graphene hydrogel material and loaded cobalt porphyrin in its three-dimensional framework.The prepared composite gel can be used as a negative electrode of a supercapacitor.We firstly generate graphene gel by self-assembly reduction of graphene oxide,and soak it in cobalt porphyrin solution for adsorption.After adsorption reaches saturation,hydrothermal synthesis reaction is carried out.The composite gel electrode prepared achieved a specific capacitance of 335 F g-1,which is twice that of the graphene hydrogel which was not doped.At the same time,due to the introduction of cobalt porphyrin,the electrical resistance of the electrode material is reduced and the ion transfer is accelerated.After 10,000 charge-discharge cycles,the graphene-loaded cobalt porphyrin gel electrode still has a stable specific capacitance retention of 94.1 1%,indicating excellent cycle life.The ion diffusion directional assembly method can rapidly prepare micron-sized graphene oxide gel layers.The metal ions diffuse into the negatively charged GO dispersion,and a super-fast sol-gel conversion can produce a multilayer 3D gel film with a controlled thickness.The GO gel is then electrochemically reduced and used as an electrode for a supercapacitor.In the fourth chapter,when the selected metal ion in the adsorbed electrolyte is nickel ion and the electrolyte is potassium hydroxide,the electrode can be charged without distinguishing the polarity of the electrode.Under such a design,one side electrode acts through a Faraday redox reaction between nickel nanoparticles and OH-,while the other side electrode acts through electric double layer charge accumulation at the interface between graphene and the electrolyte,making the device owns asymmetrical characteristics,the voltage range is widened to 1.6 V.In the fifth chapter of this paper,we have created a new high performance graphene electrothermal film by a simple and versatile process.The electrothermal properties were investigated based on the applied voltage and heating rate.The graphene electrothermal film after electrochemical reduction shows good heating performance.For example,a graphene-based film that is electrically reduced at a reduction voltage of-1.3 V can achieve a saturation temperature up to 136 ? when a 12 V DC voltage is applied for 20 seconds.When a 15 V DC voltage is applied,the graphene-based film can exhibit a high steady-state temperature of 183 ?,and the maximum heating rate exceeds 7 ? s-1.It is heated for 1.5 hours at an electrothermal voltage of 8 V and has excellent heat retention.These excellent results combine with the high chemical stability and mechanical solubility of graphene,indicating that graphene-based electric heating elements have great prospects for many practical applications,such as heating clothes,heaters and other household appliances.Ionic liquids have good chemical stability,electrochemical stability potential window is greater than other water-soluble electrolytes,low flammability,negligible vapor pressure and high ionic conductivity.This makes it widely used in the area of energy storage and electrochemistry devices.In the sixth chapter,we explored and compared the effects of aqueous electrolytes and ionic liquid electrolytes on the reduction properties of graphene oxide.It is concluded that reduction in different types of electrolytes results in different packing densities of GO and different effects on capacitance.The ionic liquid as a template can enhance the capacitance performance and further expand the ion channel.