Surface Charge Modified Activated Carbon Electrode And Asymmetric Capacitance Deionization

Capacitive deionization(CDI),has been considered as an energy-efficient and eco-friendly desalination technology due to its merits such as low energy consumption,non-secondary pollution and easy regeneration in comparison to conventional technologies.A typical CDI is usually operated by controlling the charge-discharge process,and meanwhile the adsorption /desorption process of ions can be monitored.During charging process,ions are attracted towards the oppositely charged electrode and form an electric double layer(EDL)by electrostatic attraction.After that,ions are released from the electrodes to the bulk solution by short circuit or reversed chargingThe porous carbon is commonly used as CDI electrode material due to its high specific surface area and abundant pores.When the CDI cell is charged,the counter-ions are preferably adsorbed in the electrode,but the co-ions are repelled into the solution simultaneously,which is called co-ions repulsion effect.It results in reduced charge efficiency and desalination performance.Besides,the poor conductivity of AC electrode may decrease the ionic diffusion and transportation.Therefore,the theoretical study on the parameters of desalination process are necessaryand the development of new electrode materials with low cost and high performance is of great importance tothe popularization and application of capacitive deionization technology.In this paper,the effect of "co-ion repulsion effect" on the desalination performance of the capacitive deionizationhas been discussed.Firstly,the co-ion repulsion effects were analyzed by comparing the symmetrical CDIwith activated carbon as electrodes(AC||AC),the asymmetric CDIwith surfactant-modifiedactivated carbonas electrodes(CTAB-AC||LAS-AC)and membrane capacitive deionization(MCDI)at constant current mode.Then,the Nafion-AC composite electrode was prepared by the modification of AC with Nafion.The desalination performance of the modified composite activated carbon electrodes was improved with diminishing co-ion repulsion.The main results of this paper are as follows:1? Three types of CDI cell including symmetric AC||AC,asymmetric CTAB-AC||LAS-ACand MCDI were prepared in order to analyzethe co-ions repulsion effect during charging at constant current mode.The desalination tests of AC||AC CDI at various current denisities were conducted.The conductivity of Na Cl solution increased firstly then decreased and the corresponding charging quantity at adsorption equilibrium maintains at 4.4 m Ah,which indicates that the symmetrical AC||AC CDI cell has a specific adsorption balancedcharging quantity point and the charging quantity can be adjusted to increase the charge efficiency and to improve desalination performance.At lower current density,the solution conductivity of CTAB-AC||LAS-AC CDI cell was first decreased then remains flat during charging process.With the increase of current density,the solution conductivity of the CTAB-AC||LAS-AC CDI was first decreased then increased and the charging quantity at adsorption equilibrium point was larger than AC||AC CDI,which indicates the surfactant modification of AC electrodes can improve the co-ions repulsion effect.Due to charge selectivity of ion-exchange membrane,co-ions repulsion issues were avoided on MCDI cell.However,the voltage is much high,resulting in severe water electrolysis when extending the charging process,which may lead to unrecoverable damage of the electrode materials.Besides,high resistance from the ion-exchange membrane may lead to high energy consumption.2? The Nafion-AC composite was prepared by mixing Nafion solution with AC particles and Nafion can partially replace hydrophobic PTFE adhesive.The field emission scanning electron microscopy and energy dispersive spectrometer(SEM-EDS)results show that Nafion can be uniformly distributed on the surface of activated carbon electrode.N2 adsorption-desorption isotherm illustrates that there is a larger BET specific surface area and more micropore volume on Nafion-AC composites than on AC,indicating that Nafion molecules can well fill in the macroporous structure between the activated carbon electrodes and provide a bridge for ion transport.Contact angle characterization shows that the addition of Nafion can effectively improve the hydrophilicity of the electrode due to the rich sulfonic acid group from the Nafion surface,which promotes the ions migration and transmission.And the tensile strength of Nafion-AC composite activated carbon electrode is better than that of AC electrode.The asymmetric cells were assembled with Nafion-AC electrode as a negative electrode and AC electrode as positive electrode and the effects of Nafion contents on the electrochemical performance and desalination performance of the electrode were investigated.The results show that with 7.5% Nafion,the specific capacitance of the composite electrode can reach 148 F/g.When the Nafion-AC composite electrode was used as the negative electrode,it could effectively reduce the ion exclusion effect and improve the current efficiency.The desalination amount of Nafion-AC composite electrode could reach 10.8 mg/g,which was 1.6 times of activated carbon electrode,and the current efficiency is 2.0 times of activated carbon.