Construction Of Heteroatom-doped Porous Carbon Materials For Electrochemical Energy Storages

Owing to the advantages such as high specific surface area,excellent electrical conductivity and tunable pore structure,porous carbon materials have attracted more and more attentions.To further improve their applications,the porous carbon materials can be well-designed in structure and modified with heteroatoms doping.However,those two strategies always involve specific templates and surfactants,which inevitably increase the complexity of the whole production process as well as the cost.In this paper,focusing on the preparing of heteroatoms doped carbon materials in a convenient way and regulating their pore/surface structure,we used low-cost biomass materials such as bacterial cellulose?BC?and glucose as the carbonaceous materials to address the above issues.We proposed new strategies for preparing heteroatoms doped porous carbon materials,investigated their formation mechanisms,simplified the synthetic procedures,reduced the production cost and aimed at achieving industrial production.Based on the structural features and morphologies,the synthesized heteroatoms doped porous carbon materials were then applied in supercapacitors,lithium/sodium ion batteries and lithium sulfur batteries.The main contents of the dissertation are as follows:?1?The natural nanomaterial BC was selected as both hard template and carbonaceous material,which was then uniformly coated with polypyrrole?PPy?by using an in-situ polymerization method.Afterwards,the BC/PPy composite film was obtained by extraction filtration and then freeze dried.Finally,N doped carbon nanofibers?NDCN?material was prepared by simple calcination treatment in Ar.Owing to the unique three dimensional fibrous networks and high N content,the synthesized NDCN material was then used in both supercapacitors and lithium ion batteries.Finally,the structural and N doping effects of prepared NDCN were investigated on its capacitance/Li⁺storage performances.?2?Owing to the excellent water holding capacity,abundant surface functional groups and unique three dimensional fibrous network of BC,an adsorption-swelling strategy was employed by using never-dried BC pellicle to absorb urea solutions with different content.BC cuboids were obtained and then calcined directly in Ar to prepare a series of N doped carbon nanofibers?NDCF,in order to distinguish with NDCN?.A nitrogen doping content as high as 21.2 at.%could be achieved by varying the content of urea solution.Characterization methods such as XPS and Raman were used to investigate the different N doping contents on the structure effects of NDCFs.CV and galvanostatic charge/discharge tests were further used to investigate the structure effects of NDCFs on the Na⁺storage performances.Meanwhile,according to the DFT calculation,the structural defects created by N doping were proved to show stronger adsorption energy towards Na⁺,which was supposed to contribute the enhanced storage capacity on the N-doped model.?3?Based on the successful synthesis of NDCF by using urea as the N source,we used the same adsorption-swelling strategy to synthesize N/S co-doped carbon nanofibers?NSDF?.The experimental procedures were exactly the same except using thiourea instead of urea.The synthesized NSDF material was then used as electrode materials for supercapacitors,and the N/S co-doping effects on the electrochemical performances were investigated.?4?To cope with the complicated procedures during the preparation of heteroatoms doped porous carbon materials,in this section,we took advantages of the multi-functional peculiarities of P₂O₅ when reacting with glucose,and synthesized large-size 2D phosphorus-doped porous carbon nanosheets?2D-PPCN?through the solid state hydrothermal process and subsequent calcination.By investigating the reaction mechanisms during different stages,we verified the importance of P₂O₅ to the morphology and porous structure of obtained 2D-PPCN.Considering the success of preparing 2D-PPCN from glucose,an extended selection of saccharous carbon sources including fructose,sucrose,starch and cellulose were also investigated,and similar morphology and porous structure could be realized.Moreover,the surface area and pore structure could be adjusted by simply controlling the ratio of P₂O₅/glucose.This strategy provides a convenient and effective route for preparing heteroatom doped porous carbon materials as well as the new approach for regulating the pore structure.Finally,the effects of different pore structure on the capacitance behavior of 2D-PPCN material were discussed.?5?Based on the unique morphological and structural features of 2D-PPCN,in this section,we synthesized the P-doped porous carbon materials?PCS?by using the same strategy,except that sucrose was used as the carbonaceous materials.Meanwhile,the ratio of sucrose/P₂O₅ was adjusted to obtain high specific surface area as well as the robust pore structure.Subsequently,the PCS was used as the sulfur carrier for lithium sulfur batteries,aiming at investigating its restriction in the shuttle effect of polysulfides and exploring its capability of higher sulfur mass loading.Comparing with the traditional 2D material graphene,we proposed a conception of vertical ion pathway for fast Li⁺transportation,providing a new strategy for structural designing of heteroatom doped porous carbon materials applied in high performance lithium sulfur batteries.