Preparation Of Hierarchical Porous Carbon Materials And Their Application In Capacitive Deionization

Water scarcity is one of the most serious problems worldwide,and it will become worse with the population growth,the industrial development,and the contamination of available freshwater resources.Currently,the desalination of seawater and brackish water is the most effective way to solve this issue.Traditional desalination methods,such as reverse osmosis,thermal evaporation,and electrodialysis,limited by the high cost,excessive energy consumption,and being less environmentally friendly.Capacitive deionization（CDI）has attracted more attention for desalination and water treatment for its high energy efficiency,low cost,and easy regeneration.The CDI relies on the principle of electric double layer capacitor:When the external electric field is applied,ions move to the oppositely charged electrode surface,after desalination,the CDI electrode can be easily regenerated by short-circuiting the electrodes or reversing the potential and then return into the bulk solution for further use without secondary waste.To achieve high desalination capacity,therefore,ideal electrode materials should possess some properties,such as a high specific surface area,electric conductivity,tuned pore structure and so on.Compared with conventional carbon materials,for example,activated carbon,carbon aerogels,carbon nanotubes and carbon fibers,etc.,graphene hydrogels,porous carbon spheres and their complexes are considered as potential electrode materials due to their high specific surfac e area,good conductivity and abundant pore structure.This thesis focus on the preparation of N doped hollow mesoporous carbon spheres and holey graphene hydrogel with in-plane pores and further etched hollow mesoporous carbon spheres,and systematically studied its structure,electrochemical properties and desalination properties.The main contents and results in this thesis are listed as follows:1.This chapter designs and prepares hierarchical composite of N-doped hollow mesoporous carbon spheres（N-HMCS）and holey graphene hydrogel with in-plane pores for high-performance capacitive deionization.The preparation of N-HMCS/HGH composite is mainly composed of three steps,including synthesis of the N-HMCS,preparation of the GO sheets and etching in-plane pores therein,and hydrothermal reduction of HGO sheets in the presence of N-HMCS.The composite has the following characteristics:firstly,the incorporation of N-HMCS into HGH inherits abundant macropores,mesopores,and micropores into the resultant3-dimensional（3D）hierarchical structure of the composite,thereby providing a large surface area for the adsorption of ions.Meanwhile,the mesopores in N-HMCS and the in-plane pores in graphene sheet together provide abundant ion transport channels to access the entire internal surface of the composite.Secondly,the HGH has a monolithic,3D network structure with abundant graphene sheet-graphene sheet contacts,providing inherent electron pathways.This not only endows high conductivity to the composite but also avoids the use of electrode binder.Lastly,owing to N-doping,the resultant N-HMCS/HGH composite exhibits improved conductivity and hydrophilicity,thereby improving the EDL capacitance.With these unique characteristics,the N-HMCS/HGH composite shows great potential for CDI application.Upon 35 CDI-regeneration cycles,the electrosorption capacity of the N-HMCS/HGH electrode shows no obvious attenuation,exhibiting excellent cycling stability.These results indicate that the hierarchical N-HMCS/HGH composite shows great potential for practical application.2.On the basis of the successful preparation of hollow mesoporous carbon spheres,a series of the surface rough hollow mesoporous carbon spheres（ECS）were prepared by further etching with H₂O₂.In this chapter,the etching degree was controlled by changing the temperature and the content of the H ₂O₂,and finally H₂O₂ with a ratio of 10:1 to porous carbon spheres,a high performance ECS was prepared at 120℃for 8 h and named it as ECS-100.We used SEM,TEM,EDS,Mapping,N₂ adsorption-desorption isotherm,Raman,XRD and contact angle test to characterize the ECS-100.The electrochemical properties of ECS-100 were analyzed by CV and EIS.Finally,the properties of electrosorption capacity,average adsorption rate,charge efficiency and cycle stability were studied in details by assembling two ECS-100 electrodes into a CDI device.The results show that after H₂O₂ etching,the specific surface area of ECS-100 can reach 1995 m²·g^-1,the pore volume is as high as 4.26 cm³·g^-1,which provides more adsorption sites for ions,what’s more,the abundant pores reduces the transport resistance of salt ions.Besides,the oxygen-containing functional groups on the surface of ECS-100 increase,improving its hydrophilicity and greatly increasing the effective contact area of electrode materials and salt solution.Based on these characteristics,the specific capacitance of ECS-100 in 0.5 M NaCl reached 186 F·g^-1,and the salt adsorption capacity,adsorption rate and charge efficiency,in the initial concentration of 50mg·L^-1 NaCl solution,were 6 mg·g^-1,0.5 mg·g^-1·min^-1,29.3%,respectively.after10 cycles of CDI adsorption-desorption,the conductivity showed almost no change,indicating its good stability.