Porous Carbon Electrodes For Supercapacitor And Capacitive Deionization:Synthesis And Application

Clean water resources and abundant energy are fundamental for human activities and economic development.Based on the capacitance of porous carbon materials,the electrochemical energy storage（Supercapacitor）and the electrosorption desalination（Capacitive deionization,CDI）technologies are more energy-efficient and environmentally friendly than other technologies.Due to the wide variety of porous carbon materials and their uneven performance,carbon materials in capacitive deionization and supercapacitors have been greatly developed in terms of pore structure modification,surface chemical treatment,carbon nanomaterials,composite materials,and metal oxide doping methods,etc.Several methods for improving the capacitive properties of porous carbon materials are proposed for higher specific capacitance or high salt adsorption capacity from the perspective of new material development and ions migration enhancement.The main research contents and conclusions are as follows:（1）The effects of titanium and vanadium oxide substrates on the morphology,structure and capacitive performance of electrospun carbon nanofibers were investigated.Through the simultaneous loading of organic and inorganic oxide substrates,Hyb Ti@CNFs and VO/VOx/CNFs electrodes with superior properties were prepared.The two kinds of free-standing electrodes contain multi-phase titanium（Ti O₂）or vandium oxides（VOx）,which are beneficial for simultaneous enhancing the pore structure and electrical conductivity.In the two-electrode cell using 6 M KOH,high specific capacitance for Hyb Ti@CNFs(280.3 F g^–1)and VO/VOx/CNFs(325.7 F g^–1)were achieved.（2）Highly porous carbon material（HPAC）rich in macropores and micropores was prepared by template-assisted and in-situ KOH activated thermal cross-linking,freeze-drying,and ultrasonic techniques.Compared with the performances of activated carbon with wide pore size distribution and ordered mesoporous carbon,HPAC posesses both more excellent electrochemical energy storage(0.5 A g^–1,309 F g^–1)and desalination performance(1.2 V,16.3 mg g^–1).The results indicate that the effect of the pore structure on the supercapacitor and CDI performance is in consistent,and high concentration is more favorable for higher electrode capacitance.（3）Based on the above conclusion,the desalination performance of CDI and MCDI devices at high concentrations and overpotentials were investigated to enhance ions migration in CDI devices.This study has found that CDI is not suitable for high-concentration and overpotential desalination due to the exacerbation of coions rejection,while OP-MCDI assisted with reverse voltage（RP）,i.e.OP-MCDI-RP,can increase the salt adsorption capacity of mesoporous carbon（MC）and the carbon aerogel by 8folds(64.7 vs 8.2 mg g^–1)and 4 folds(43.4 vs 10.6 mg g^–1),respectively.By using OP-MCDI-RP technology with MC as electrodes,practical seawater was successfully desalted into freshwater.（4）The effects of overpotentials and carbon powder types（e.g.activated carbon,mesoporous carbon and single-walled carbon nanotubes）on the desalination performance of flow-electrode CDI（FCDI）were evaluated.It was found that the high concentration and overpotential operation are not suitable for FCDI due to the direct contact of the flow-electrode with the electrolyte.The optimized electrolyte concentration and operating voltage is 0.2 M and 1.6 V,respectively.The FCDI performance of SWCNTs is superior to other carbon materials mainly due to its superior electric conductivity,the unique ions adsorption both on the interior and exterior nanotube walls and the bridging effects between tubular particles.