Application Of Porous Carbon-Based Materials In Electrochemical Energy Storage

Recently,environmental issues and energy crises have become increasingly prominent.As the storage device for electrical energy,electrochemical power supplies are particularly important for efficient storage and output.The main chemical power sources include fuel cells,secondary batteries and capacitors.With excellent conductivity,high specific surface area and good stability,porous graphene can be used as a carrier of electrode active materials,conductive additives and electrode materials for supercapacitorsand,which has promising application foreground.In this study,porous graphene was applied to lithium ion batteries and supercapacitors to investigate the electrochemical properties of porous carbon materials.In addition,as a supplement to the study of supercapacitor electrodes,the capacitive energy storage applications of doped activated carbon materials were further investigated.?1?The periodical Li storage behavior delivered by porous graphene was studied.Generally,porous graphene is not directly used as the cathode material of lithium ion batteries,but it is widely used as the carrier of active materials in batteries.Therefore,it is very important to study the lithium storage behavior of porous graphene.The 3D porous graphene material was prepared by chemical vapor deposition with magnesium sulfate whisker as template agent,and its electrochemical properties were tested as the negative electrode of lithium ion battery to study the periodical Li storage behavior delivered by porous graphene.Through careful observation of various performance indexes during the cycle,we found that the specific capacity of lithium storage and CEs of porous graphene would change periodically,and the changes of specific capacity and CEs were not synchronous.The change period was related to the mechanical structure characteristics of materials and charging and discharging rate.The periodic variations of lithiation/delithiation capacities and CEs actually correspond to the periodic accumulation and release of lithium ions in semi-closed pore structures.This project reveal a clear relationship between the elasticity of a porous carbon material and its Li storage performance.?2?The composite of porous graphene and lithium iron phosphate can significantly increase the specific capacity of lithium iron phosphate cathode.The porous graphene material was prepared by chemical vapor deposition with boiled magnesium oxide as template agent,and then mixed with lithium iron phosphate to obtain lithium iron phosphate/graphene composite.The composite was used as cathode of lithium ion battery to studyits electrochemical performance.The capacity of commercially available cathode carbon-coated lithium iron phosphate by surface-modified with 2 wt%of nanomesh graphene,is able to reach 217 m Ah g^-1.The nanomesh graphene wrapped around carbon-coated lithium iron phosphate can improve conductivity which assist the electron migration during the charge/discharge processes.Also the nanomesh graphene provides channels for Li⁺motion due to the porous structure.?3?The capacitive energy storage properties of sulfur-doped porous graphene were investigated.A post-treatment methodology is developed to producing S-doped porous grapheme?SPG?via the S doping reaction between graphene and Mg SO₄.In order to evaluate the capacitive performance of SPG,symmetric supercapacitors were assembled with KOH,Na₂SO₄ and TEATFB.The maximum capacitances of 94.2,103.6 and 98.3 F g^-1 are achieved at 0.5 A g^-1,respectively.The surface defects of doped porous graphene increase the active potential of the electrode.The surface electron density of doped porous graphene increased,which is favorable for charge transfer rate and electron transfer reaction.Besides,the hydrophilic group enhanced the wettability of graphene electrode.?4?Preparation and activation mechanism analysis of doped activated carbon was studied.S,N dual-doped carbon material was activatedby acidic?CO₂?and alkaline?KOH and NH₃?activators respectively,which can be regarded as a typical acid-base reaction process.When activated by alkaline activator,the sulfur-containing groups in the doped carbon materials show higher reactivity compared with the nitrogen-containing groups,while when activated by acid activator,the trend is opposite.Compared to the activation by KOH,activation by CO₂ and NH₃ can lead to a high retention of S and N content,thus resulting in higher area-normalized capacitance in a three-electrode supercapacitor.In conclusion,porous graphene and doped porous graphene were prepared by CVD method and applied to lithium ion batteries and supercapacitors respectively to investigate the electrochemical properties.The excellent physical and chemical properties of porous graphene can significantly improve the performance of lithium ion batteries and supercapacitors.Therefore,porous graphene has a promising application prospect in lithium ion batteries and supercapacitors.Due to the difficulties in low-cost mass production of porous graphene,activated carbon is widely used in supercapacitors at present.Therefore,the preparation and activation mechanism of doped activated carbon are also studied in this study,which provides further theoretical basis for its application in capacitors.