Synthesis And Properties Of Carbon-based Electrode Materials For Supercapacitors

With the rapid development of our human society,there has been an increasing and urgent demand for exploring highly efficient,green and sustainable energy storage and management systems.Supercapacitor is emerging as an advanced energy storage device for widespread and far-reaching applications,due to its unique characteristics like high power density,rapid charge-discharge capability,high energy conversion efficiency,long life span and safety.However,there are still some limitations of supercapacitor,especially in the case when high energy density is required.As the key component of a supercapacitor,the adopted electrode materials mainly determine its classification and overall electrochemical performance.Carbon materials are the most commonly used electrode materials for supercapacitors,among which porous carbon and graphene show the best application value and potential.Developing the preparation techniques for carbon materials has always been a hot topic in the related research fields,and it is also the precondition and key to their practical application and breakthrough.On the one hand,to obtain high-performance carbon electrode materials,we need to accomplish the reasonable design and optimization of material structure,as well as effectively regulating its surface chemical properties.On the other hand,it is necessary to balance the quality of products with the production cost and scale,and also ensure the non-pollution,safety and high efficiency during the preparation process.Based on the fully knowledge and understanding of supercapacitors,combining with the international research hotspots of their electrode materials,we conducted systematic and in-depth researches on the synthesis,structure,surface chemistry regulation and energy application of porous carbon and graphene.In this dissertation,a one-step strategy to synthesize three-dimensional porous graphitic biomass carbon was established;chitosan-derived hierarchically porous graphitic carbon with N-self doping was prepared;a simple and fast approach for producing high-quality graphene by electrochemical exfoliation and spark plasma sintering was proposed;the influence of different chemically modified graphenes on the microstructure and electrochemical performance for supercapacitors was studied comparatively and systematically.The above works enrich the research content for canbon-based supercapacitors and promote their development.This dissertation includes seven chapters.In chapter one,the research background,motivation,necessity of the subject are elucidated,followed by introductions including the classification and application of supercapacitors and their electrode materials.Subsequently,the research status and progress about preparation techniques of porous carbon and graphene are also summarized.The experimental details are described in the second chapter,including chemicals and preparation methods,characterizations and related equipments,fabrication of electrodes as well as assembly and testing of supercapacitors.In the third chapter,we established a one-step strategy to synthesize three-dimensional porous graphitic biomass carbon(PGBC)from bamboo char,and studied its electrochemical performance as electrode materials for supercapacitors.Using potassium ferrate(K₂Fe O₄)to fulfil the synchronous carbonization and graphitization of bamboo carbon,this method is less time-demanding,highly efficient and pollution-free,when compared with a conventional two-step strategy.Remarkably,the PGBC electrode exhibited outstanding capacitive performance,showing great potential as a candidate for advanced electrode materials of supercapacitors.Moreover,the assembled solid-state symmetric supercapacitor in aqueous electrolyte and the coin-type symmetric supercapacitor in ionic liquid electrolyte both exhibited considerable synergetic energy–power output properties for practical applications.In the forth chapter,the chitosan-derived hierarchically porous graphitic carbon(CS-HPGC)with N-self doping is synthesized via a simple and green approach with the assist of K₂Fe O₄.Different from conventional templating or two-step strategies,this method is less time-demanding and more efficient by accomplishing the carbonization,graphitization and heteroatom modification in one step.The above structural features gave the CS-HPGC electrode a remarkable capacitive performance,and the assembled CS-HPGC symmetric supercapacitors in both aqueous(KOH)and ionic liquid(EMIM TFSI)electrolytes exhibited outstanding synergetic energy-power output properties.In the fifth chapter,we report a simple and fast approach for producing high-quality graphene(HQG)by electrochemical exfoliation and spark plasma sintering.The process is simple,fast,effective and environment friendly,which is expected to meet the industrial level requirement of graphene applications.The HQG exhibitedvery low defects density,extremely high C/O ratios and superior conductive properties.Moreover,the free-standing graphene electrodes with excellent electrochemical performance provide a possibility for the exploitation of graphene-based ultrathin energy management devices.This process holds great promise for further large-scale applications of graphene with low cost and high quality.In the sixth chapter,the influence of different chemically modified graphenes on the microstructure and electrochemical performance for supercapacitors was studied comparatively and systematically.The present work reveals that,though graphene possesses superior and stable electrochemical properties,its different chemical properties still exhibit great influences on the microstructures and electrochemical performances.Therefore,it is very important to select the right and effective graphene-based electrode materials for practical application.The seventh chapter is a conclusion of this dissertation.At last,brief introductions of published papers,patents,participated projects,honors and awards during my Ph.D.education are listed,as well as the curriculum vitae and acknowledgements.