High-energy Beam Induced Preparation Of Porous Graphene And Its Supercapacitor Application

As a new type of eco-friendly electrical energy storage device,supercapacitor(SC)possesses the characteristics of high power density and long cycle life,but the energy density still needs to be improved.While fully maintaining the advantages,improving its energy density is the spot of current research,and the key lies in the design and construction of high-performance electrode materials.Graphene has proven to be a potential ideal SC electrode material due to its large specific surface area,high electrical conductivity and many other special chemical and physical properties.However,the two-dimensional(2D)sheet structure of graphene is prone to stacking,hindering charge transport and causing performance degradation.Constructing graphene electrodes with three-dimensional(3D)porous structures is expected to become a new solution to above problems.Nevertheless,the traditional synthesis strategy of 3D graphene often requires expensive metal foam templates or complex powdered graphene assembly processes,which is not conducive to extensive commercial production.Using high-energy beams(such as laser,electron beam,etc.)to bombard carbon-containing substrates(like polymers and biomass)and induce in-situ carbonization of their surfaces to form porous graphene films has become a new and effective scheme for rapid and green preparation of 3D graphene.Laser is the most used high-energy beam in the current research,and laser-induced graphene(LIG)electrode still faces many shortcomings,such as relatively few active sites,difficulty in pore size distribution adjusting,limited graphene film thickness,which greatly restrict the SC performance of the LIG electrode.In view of the above problems,firstly,this thesis starts with the doping modification of LIG materials,using precursor in-situ composite method and laser induction technology to prepare nitrogen-doped LIG.By introducing heteroatoms,the active sites of LIG are increased,thereby enhancing the SC performance of LIG electrode.On the other hand,the pore size distribution of the SC electrode is also an key factor affecting its SC performance.Therefore,proceeding from the pore size distribution of LIG material itself,by introducing the ultimate nanopores(pore size<1 nm),relying on the desolvation enhancement effect,the SC performance of the LIG electrode is improved.Finally,considering the shortcomings of preparation efficiency and penetration depth of laser,this thesis successfully uses high-energy e-beam instead of laser as the energy source to induce the preparation of graphene films with good quality and millimeter-sized macro thickness(higher than LIG)on polymers for the first time.The SC electrode constructed by it exhibits excellent electric double layer capacitance characteristics.The main research contents and progress are as follows:(1)Preparation of nitrogen-doped LIG electrode material and its supercapacitor application.The melamine/polyimide(M/PI)composite film was synthesized by the means of precursor compounding,and during the laser pyrolysis of the M/PI film,the nitrogen atoms formed by the pyrolysis of melamine were doped into the graphene structure,and then the one-step,in-situ nitrogen-doped laser-induced graphene(N-LIG)electrode material was prepared.Compared with undoped LIG,the electrochemical performance of N-LIG was significantly improved.When the addition amount of melamine in the polyamide acid(PAA)precursor was 6 wt%,the area specific capacitance of the obtained N-LIG electrode reached 56.3 mF cm-2 under the current density of 0.4 mA cm-2 in a three electrode system with 1 M H2SO4 as electrolyte,which was 7.2 times that of the undoped LIG electrode under the same condition.In addition,the N-LIG electrode obtained on the M/PI substrate kept good flexibility,which laid a foundation for the industrial roll to roll production of N-LIG.This method possessed the advantages of environmental protection,economy and convenience.It showed a good application prospect in the field of flexible electrochemical energy storage devices.(2)Preparation of desolvation effect enhanced LIG electrode material and its supercapacitor application.Polyamide acid(PAA)precursor was doped with expandable graphite,and expandable graphite/polyimide(EG/PI)composite film was formed after heating and curing.Expanded graphite composite laser-induced graphene(EG-LIG)electrode was prepared by patterned laser scribing on EG/PI film.The expandable graphite rapidly released gas when heated in the process of laser induction,which affected the pore size distribution of LIG,realizing the introduction of the ultimate nanopore(pore size less than 1 nm)of LIG,and greatly enhancing the electric double-layer capacitance of LIG through desolvation effect.In addition,the properties of the samples obtained with different EG doping amounts were compared.The EG-LIG composite electrode obtained with 4 wt%doping amount showed an area specific capacitance of 52.7 mF cm-2(current density,0.2 mA cm-2)in the three electrode system with 1 M Na2SO4 as electrolyte,while the area specific capacitance of the undoped LIG electrode under the same conditions was only 6.1 mF cm-2.This strategy provided a new idea for the enhancement of the specific capacitance of the LIG electrode and expanded the application range of LIG in practical devices.(3)E-beam induced synthesis of macroscopic 3D porous graphene and its supercapacitor application.E-beam induced graphene(EIG)films with macro thickness(～0.66 mm)were directly synthesized by bombarding polyimide(PI)substrate with high-energy e-beam instead of laser as energy source.EIG possessed abundant 3D pore structure and good electrical conductivity(1100 S m-1).At the same time,the few-layer structure,low defects,high quality and high synthesis rate(84 cm2 min-1)made the prepared EIG film show the potential for large-scale applications.The performances of EIG in SC energy storage and photothermal were studied.The EIG SC showed an area specific capacitance of up to 67.1 mF cm-2 at 0.1 mA cm-2.Moreover,EIG showed ideal photothermal characteristics at-40℃ and could be used in anti-icing/deicing and other fields.EIG materials possessed excellent SC and photothermal properties at the same time,and exhibited great potential in the field of solar self-heating SC in the future.