Investigation And Improvement Of Desalination Stability Of Capacitive Deionization

Capacitive deionization?CDI?is a novel desalination technology based on the theory of supercapacitor,whose structure consists of a water channel and a pair?several pairs?of electrodes coated with active materials.Currently,besides metal oxide and conducting polymers,carbons are used as the main electrode active materials owing to their simple fabrication process,low cost and good performance.However,carbons are easy to be corroded during CDI operational process and the desalination stability of CDI is reduced.Consequently,CDI technology is limited in practical application.Therefore,to realize the commercialized application of CDI,it is important to develop strategies to improve the long-term desalination stability of CDI,such as changing the structure of CDI system,modifying operational mode and introducing novel electrode materials.In this dissertation,a series of works have been conducted to study the influence factors during the long-term desalination of CDI.Then,based on above works,electrode materials and CDI structure are chosen as the breakthrough to enhance the long-term desalination stability of CDI.The detailed work is described in the dissertation as followed:1.The long-term desalination experiment of CDI was conducted using activated carbon?AC?as electrode material?50 charging-discharging cycles?.Consequently,the changes of electrosorption capacitance and the variations of pH value were monitored during the long-term desalination.The results indicates that the electrosorption capacitance of CDI was reduced over time,and pH values in the first cycle were influenced by the side reactions occurred during the charging process.To investigate the relationship between the side reactions and the electrosorption capacitance of CDI,several characteristic tools were used to analyze the changes of electrode materials in structure,element composition and electrochemical performance before and after long-term operation.Besides,anion-exchange membrane was introduced into CDI module to study the effect of dissolved oxygen on anode material.It shows that the electrosorption capacitance reduction of CDI was caused by the oxidation of anode electrode material which was mainly induced by the side reactions during desalination process..2.The long-term desalination experiments were conducted in anion-exchange membrane CDI?AEM-CDI?using AC as anode material,and AC or carbon sphere?CS?as cathode material.The results indicate that two AEM-CDIs exhibited different electrosorption capacitances.The electrosorption capacitance of AEM-CDI with AC cathode was reduced over time,and that of AEM-CDI with CS cathode showed an opposite trend.By comparing the structures,specific surface areas and potentials of zero charge?PZC?of cathode materials before and after long-term operation,it is found that the performance variations of two AEM-CDIs were caused by the oxidation of cathode materials.Owing to the cathode oxidation,the specific surface area of AC cathode was reduced and the PZC of CS cathode was enhanced from 0.09V to 0.32 V.In conclusion,a strategy was developed in improving the electrosorption capacitance of AEM-CDI by using cathode materials with low PZC values,such as CS.3.N-doped carbon spheres,MRF-x?melamine-resorcinol-formaldehyde resin sphere,x means the heating temperature?,were fabricated by the heating treatment of resin spheres,which were synthesized from melamine,resorcinol and formaldehyde solution.After a series of electrochemical experiments of MRF-x composites were conducted,it is found that MRF-1000 exhibited the best oxygen reduction capacity,which means that the oxygen reduction capacity of MRF-x composites could be enhanced by improving the heating temperature.However,the results also indicate that the heating temperature was important to the specific capacitance of MRF-x composites.Especially,when the temperature was higher than 800?,the specific capacitance was reduced with the decrease of the specific surface area of MRF-x.Therefore,considering the oxygen reduction capacity and the specific capacitance of MRF-x composites,MRF-800 was chosen as cathode material of CDI with AC as anode material.It is found that even after 50 charging-discharging cycles,the CDI still exhibited a good electrosorption capacitance of 3.6 mg g^-1.Besides,owing to the oxygen reduction effect of MRF-800,the long-term operational stability of CDI was improved with a high electrosorption capacitance retention of 79.6%.4.A novel battery type materials were synthesized by reducing the mixture of nickel hexacyanoferrate?NiHCF?and graphene oxide?GO?,namely NiHCF/rGO-x,and NiHCF/rGO-10 was found to display an excellent electrochemical performance of271 F g^-1 by modifying the ratio of NiHCF and GO.Then,a hybrid CDI?HCDI?testing system consisted of NiHCF/rGO-10 as cathode material and AC as anode material.The results indicate that,compared to the electrosorption capacitance of conventional CDI(6.01 mg g^-1)in the first charging-discharging cycle,HCDI exhibited higher electrosorption capacitance of 21.4 mg g^-1.Besides,the side reactions were inhibited due to the low operational voltage?0.6 V?.Cconsequently,the long-term operational stability of HCDI was improved.The electrosorption capacitance retention of HCDI could reach 60.3%even after 50 charging-discharging cycles,which is higher than that of conventional CDI?18.0%?.