Aromatic Polyimides Derived Porous Carbons For Capacitive Deionization

Capacitive deionization(CDI)is a technique that could produce fresh water from the brackish water by adsorbing the aqueous ions onto the parallel porous electrodes under a small voltage.The great advantage of CDI is its ion absorption under an ambient pressure at room temperature,avoiding significant energy consumption.The adsorption electrodes are the critical component of a CDI cell.Both physical and chemical properties of electrode materials,such as composition,morphology and conductivity,play a key role in desalination capacity and energy efficiency of CDI.Therefore,a series of nitrogen-doped porous carbons derived from the flower-like or spherical polyimide precursors were prepared for improving the deionization performance.All of them have good adsorption capacities(SAC)and high charging efficiency(?)under the voltage of 1.0 V.However,accumulated ammonia could be found in the flow at exit,when the operating voltage is set as 1.2 V.The reason has a preliminary discussion.Details of the experiment and characterization are introduced as follows:1)In this chapter,the flower-like polyimide microspheres are firstly prepared in one-step process by polymerizing ethylenediamine(EDA)with pyromellitic dianhydride(PMDA).Then,the carbonization temperature and KOH activation conditions of the porous flower-like carbon spheres(A-FCMs)are optimized to enhance their deionization performance.After optimization,A-FCMs has the BET specific surface area of 1870.18 m²/g,and specific capacitance of 228.18 F/g in 1.0 M Na Cl solution.When A-FCMs are casting into working electrodes,their flower-like structure could form continuous channels for ion-diffusion and their micropores provide numerous adsorption sites.Finally,the salt absorption capacity(SAC)of Na Cl and Li Cl of A-FCMs are 14.64 mg/g and 7.73 mg/g.Their charge efficiency(?)for Na Cl and Li Cl removing are 69%and 56%,respectively.The adsorption performance is stable after 10 cycles.2)In this chapter,an spherical hyperbranched polyamide acid precursor could directly prepared by polymerization of tris(4-aminophenyl)amine(TAPA)with PMDA in dichloromethane(DCM).Porous carbon spheres(A-SCMs)are finally achieved after carbonization and activation.The BET specific surface area of A-SCMs reaches 1949.67 m²/g,containing both micropores and mesopores.Their specific capacity is 209.86 F/g in 1.0 M Na Cl solution.The SACNa Cl and SACLi Cl are 5.94 mg/g and 6.14 mg/g,and their?Na Cl and?Li Cl were 37%and 56%,respectively.No significant reduction in the adsorption capacity after 6 cycles is observed.3)In this chapter,ammonia in the outlet flow has been characterized when the working voltage is set as 1.2 V,and the reason are primarily discussed.The ammonia production rate(?)in Na Cl and Li Cl electrolyte is 0.079?g mg^-1 h^-1 and 0.011?g mg^-1 h^-1,respectively.The faraday efficiency(FE)is 67.7%and 6.6%.XPS characterization proves that the nitrogen comes from the pyrrole-N of FCMs.Thus,our experiments indicates that the working voltage of CDI using nitrogen-doped carbon materials must be lower than that of traditional carbon materials.Because the generation of ammonia could effectively reduce the SAC and even invalids the deionization.In conclusion,polyimide has the advantages of controllable morphology and high nitrogen content as the precursor of nitrogen-doped porous carbons.After carbonization and activation,all the N-doped porous samples have good SAC and?.However,the N-dope samples are restricted by ammonia generation under higher working voltage.