The Design And Synthesis Of Coal-based Carbon Materials For Capacitive Deionization

With the continuous development of economy and the growth of population,it is urgent to solve the problem of fresh water shortage.It is considered an effective way to convert the abundant seawater into available fresh water.Traditional seawater desalination technologies such as distillation,electrodialysis,reverse osmosis,and multi-stage flash evaporation still disadvantages such as high energy consumption,high maintenance costs,and complex processes.As a new type of desalination technology,capacitive deionization(CDI)is favored by researchers because of its low cost,high efficiency,simple operation,and no secondary pollution.The performance of CDI is largely determined by the electrode materials.Hence,exploring electrode materials with simple preparation process,low cost and high adsorption capacity has important theoretical research significance and practical application value.In this paper,three coal-based carbon materials,three-dimensional interconnected porous carbon,nitrogen-phosphorus-doped porous carbon,and boron-doped carbon fiber,are successfully prepared by sol-gel method and electrospinning method using Xinjiang coal as raw material.The adjustment of morphology,composition and structure can further improve its CDI performance and provide theoretical basis and technical support for the design and preparation of high-efficiency CDI electrode materials.The specific research work and results are as follows:(1)Using K₂CO₃ as the activator,liquid-phase oxidation modified coal(OC)as the carbon source,and polyvinylpyrrolidone(PVP)as the cross-linking agent,the hydrogel is prepared by the sol-gel method,and the hydrogel is freeze-dried and carbonized obtain three-dimensional interconnected coal-based porous carbon.The morphology and structure of the sample can be effectively controlled by adjusting the amount of K₂CO₃added.When the mass ratio of K₂CO₃ to OC is 1.5:1,the obtained sample HPC_1.5:1 has a high specific surface area of 2142 m² g^-1 and 1.478 cm³ g^-1 total pore volume.Thanks to the three-dimensional interconnected hierarchical pore structure,high specific surface area and excellent ion mobility,the mass adsorption capacity of HPC_1.5:1 as a CDI electrode in a 500 mg L^-1 Na Cl solution under 1.2 V voltage can reach 35.3 mg g^-1,the maximum adsorption rate reached 3.25 mg g^-1 min^-1,showing good adsorption performance and adsorption rate.(2)Using(NH₄)₂HPO₄ as the nitrogen source,phosphorus source and activator,OC as the carbon source,and PVP as the crosslinking agent,the hydrogel prepared by the sol-gel method is carbonized to obtain the nitrogen-phosphorus double-doped coal-based porous carbon.When the mass ratio of(NH₄)₂HPO₄ to OC is 1:1,the specific surface area and total pore volume of NPHPC_1:1 are 400 m² g^-1 and 0.257 cm³ g^-1,respectively,which significantly improved the specific surface area and pore volume of the sample.X-ray photoelectron spectroscopy(XPS)results show that NPHPC_1:1 contains 6.72 at.%nitrogen and 1.63 at.%phosphorus.Nitrogen and phosphorus double doping can improve the conductivity and electrochemical capacity of NPHPC_1:1.The specific capacitance of NPHPC_1:1 increases from 78 F g^-1 to 171 F g^-1 at 1 m V s^-1.When NPHPC_1:1 is used as a CDI electrode,its desalination capacity for 500 mg L^-1 Na Cl solution reaches 28.0 mg g^-1under 1.2 V voltage.The desalination capacity for 500 mg L^-1 KCl,Ca Cl₂,and Mg Cl₂solutions reaches 28.0 mg g^-1,30.4 mg g^-1,19.7 mg g^-1,15.5 mg g^-1,respectively,indicating that NPHPC_1:1 has excellent desalination performance when used as a CDI electrode.(3)Using H₃BO₃ as the boron source,OC as the carbon source,and polyacrylonitrile(PAN)as the forming aid,the electrospun fiber membrane prepared by the electrostatic spinning method is pre-oxidized and carbonized to prepare the boron-doped carbon fiber membrane.The hydrophilicity,graphitization degree and specific surface area of boron-doped carbon nanofiber are improved obviously.When boron-doped carbon fiber membrane is used as CDI electrode material,its desalination capacity can reach 33.5 mg g^-1 in 1 000 mg L^-1 Na Cl solution and 1.2 V applied voltage.At the same time,it has good desalination performance for KCl,Mg Cl₂ and Ca Cl₂ solutions,which provides a feasible solution for the separation and recovery of salt ions in seawater.Boron-doped carbon fiber film prepared by this method has the advantages of simple preparation process and easy reuse.The obtained carbon fiber film can be used as a promising CDI electrode material.