Preparation And Electrochemical Performances Of Heteroatoms-doped Graphene Electrode Materials In Supercapacitors

It is urgently needed to research and develop new energy storage systems with low cost,environmental protection and high performance due to the development of modern society.Supercapacitors are supposed to be a promising candidate for alternative energy storage devices owning to their higher power density than batteries and higher energy density than conventional dielectric capacitors.Due to the unique 2D structure and outstanding intrinsic physical properties,such as extraordinarily high electrical conductivity and large surface area,graphene has been considered as a promising candidate electrode material for supercapacitors.However,the capacitive performance of graphene is far poorer than the expected.Therefore,it is undoubtedly of great theoretical value and practical significance to study and explore the technical ideas and methods to effectively improve the performance of graphene supercapacitors.It is shown that introducing heteroatoms into carbon materials is a very effective method to improve the supercapacitive performance of carbon materials.To prepare high-performance graphene supercapacitor electrode materials,this dissertation for the first time proposes the construction of heteroatoms-doped graphene electrode materials based on the preparation principle of GIC（graphite intercalation compound）,and the main study works and results are as follows:Single-atom doped graphene（S-RGO）and multi-atom doped graphene（SN-RGO）electrode materials are successfully prepared by the method of sulfuric acid intercalation,in which,two main procedures–the preparation of a sulfur doped graphite intercalation compounds（S-GIC）and the construction of S-RGO and SN-RGO–are included.And the results indicate that heteroatoms doping can lead to the formation of the wrinkles and floding in the graphene layers.These surface wrinkling and floding can not only generate lots of open edge sites,but also effectively prevent the aggregation of graphene sheets,which is facilitate to the ion transportation and the surface utilization.The specific surface area（SSA）of S-RGO is 279.7 m² g^-1,much higher than that(162.4 m² g^-1)of the non-doped RGO.And the calculated specific capacitance of S-RGO is up to 270.4 F g^-1,which is approximately 5.6 times higher than that(48.1 F g^-1)of RGO.Compared with the single sulfur atomic doping,the specific surface area,pore structure and electrochemical properties of SN-RGO are further improved by sulfur and nitrogen co-doping.The SSA of SN-RGO is reaching up to 566 m² g^-1.And the specific capacitance of SN-RGO is reaching up to 402.4 F g^-1,almost 4 times higher compared with that of RGO(101.7 F g^-1).In addition,both of the rate performance and the cycling stability of SN-RGO are significantly enhanced.In order to synthesize a more developed and reasonable electrolyte ion transmission channel and further enhance the supercapacitive performance of heteroatom-doped graphene electrode materials,a strategy to construct the sulfur and nitrogen co-doped porous graphene（SN-PRGO）is proposed by constructing the porous structure in natural graphite through KOH etching.The as-prepared SN-PRGO behaves relatively high SSA(734.7 m² g^-1)and large pore volume of micropores(0.21 m³ g^-1),significantly higher than that of unporous SN-RGO.In addition,the the doping content of sulfur and nitrogen in SN-PRGO is remarkably increased by the KOH etching.Owing to the high BET surface area and developed pore structure,which simultaneously providing through-plane and in-plane diffusion paths for electrolyte ions,SN-PRGO achieves an outstanding capacitive performance.The specific capacitance of SN-PRGO reaches up to 438.3 F g^-11 and also exhibits higher capacitance retention(almost 65.1%at the scan rate of 500 mV s^-1).Moreover,SN-PRGO also behaves superior cycling stability,almost 94%specific capacitance can be retained even after 10000 cycles.Up to now,the research on heteroatoms doping carbon material mainly focuses on the species and amounts of doped atoms,however,the influence of the chemical state of doped heteroatoms on the electrochemical performance has not been discussed.Therefore,this dissertation takes sulfur doped graphene（S-RGO）as the research object and systematically studies the effect of post-regulation of doped-S species on the supercapacitance performance of S-RGO electrode materials.It is found that the species of doped-S atom has more influence on the performance of supercapacitors than that of the doping amount.The thiophene sulfur species（-C-S-,-C=S-C-）are oxidized into a new doping species-oxidized sulfur species（-C-SOx-C-）by the Hummers oxidation.This dissertation has demonstrated that the unique oxidized-S species,which is transformed from the thiophene-S species,has superior contribution to the supercapacitive performances（e.g.,specific capacitance,rate performance and long-term cycling stability）.Interestingly,the S-doped graphene sample,which contains the highest content of the unique oxidized-S species,exhibits an outstanding long-term cycling stability,and the capacitance retention being up to 100.8%after 10000 consecutive cycling tests.