Fabrication Of Nano Modified Capacitive Deionization Composite Electrodes And The Mechanism Research On Brine Water Desalination

Capacitive deionization(CDI)is a new potential desalination technology with rapid development.CDI drives ions by electric field and other forces,and uses the electric double layer formed between electrode and electrolyte to capture ions in electrolyte,and ions can be separated from electrolyte.Unlike the conventional desalination methods,only the small amount of solute ions in brine is extracted and separated in CDI process,rather than to extract large amount of solvent water molecules in the brine.However,some disadvantages in previous CDI technology research should be overcomed to meet the requirements of further commercial application.Firstly,it is ignored that the flow characteristics of salt solution in the process of CDI desalination,the CDI device is fabricated by the conventional method of making double layer capacitors(EDLCs).The characteristics of CDI device as a water treatment device are not considered.Secondly,the main research direction is to enlarge the specific surface area of the electrode active materials,and the influence of current collector of CDI electrode is less considered.Thirdly,the desalination performance and mechanism under constant voltage and constant current density operation are less considered.To conquer above problems,the design of nano-modification on CDI electrode current collector,CDI electrode and CDI cell,configuration of CDI electrode active material precursor,optimization of control parameters in CDI desalination,adsorption and desorption mechanism under constant voltage and current density CDI desalination process,and forces condition have been analyzed and compared.The main results are described in the following:(1)To enhance the corrosion resistance quality and guarantee the conductivity of pure aluminum sheet,the alkali etching and Zinc and Nickel plating nano-modified method have been conducted.As a current collector,nano-modified aluminum sheet has better adhesion to active materials and lower internal resistance was obtained.By using superconducting carbon black and modified carbon nanotubes as conductive agent,the latter CDI electrode capacitance value increased by 42%.The optimum mass proportion of active coke powder,modified carbon nanotubes,binder PVDF and hydrophilic agent PEG-400 as electrode active material was 10:0.5:1.3:0.5.Theprecursor of active material was mixed according to the optimum proportion and then coated on the nano-modified aluminum sheet to fabricate a CDI cell with good expansibility,optimized water distribution channel layout and long service life.(2)The effects of single and multiple factors on the desalination performance of CDI were compared and analyzed.It was found that increasing of operating voltage from 0.8 V to 1.6V,influent concentration from 100mg/L to 300mg/L,and the lower surface velocity(1.78cm/h)of salt solution on the electrode surface have positive effects on desalting capacity.The higher solution temperature have a negative impact on the desalination performance.The variation of pH value of salt solution with operating voltage during the desalination of CDI was measured,and the mechanism of the variation of solution pH was verified.By comparing the operating conditions of constant voltage and constant current density,it was found that under the condition of constant current density,the CDI device can produce water with the lowest conductivity for longer time.Through the multi-factor analysis in the process of CDI desalination,the main influencing factors on the adsorption amount of CDI electrode include operating voltage,operating current density and solution concentration,while the factors such as electrode surface velocity have little influence.After three cycles,the desorption and regeneration efficiency of CDI composite electrode was over 99%after saturated adsorption.(3)Adsorption isotherm model,pseudo-first-order kinetics and pseudo-second-order kinetics adsorption model,intra-particle diffusion model,the electric migration mechanism and force analyzing was studied.It was found that the Langmuir adsorption isotherm model is more accord with the actual situation of CDI electrode adsorption ion in NaCl solution.The fitting applicability of intra-particle diffusion kinetics was poor.The electric migration between CDI electrodes can be well fitted by exponential equations.While other operating parameters remain unchanged,the higher the concentration of salt solution,the higher the average ion valence of salt solution,and the higher the inflow flow rate,the higher the concentration of electrolyte will be.However,the higher the charging and discharging efficiency and the higher the current density will cause the lower concentration.It is proved that electrostatic force dominants in the CDI desalination process.(4)The use of multiple pairs of electrodes and a multistage CDI device wasbeneficial to improve the overall desalination capability of the CDI system.Through the analysis of the selective adsorption ability of CDI to different valence ions,it was found that when the radius of hydrated ions increases,the resistance force of hydrated ions in the CDI process increases,so it has a negative effect on the ability of CDI.CDI had more removal ability to the same type of monovalent ions,and more removal ability to the cations with the same valence than to the anions.