Impact Factors And Optimization Of Membrane Capacitive Deionization Desalination Performance

Membrane capacitive deionization(MCDI)is an energy-efficient desalination technology.When the voltage is applied across the anode and cathode,ions are driven to migrate to the electrodes and stored in the electric double layers(EDL)formed on the surface of the electrode.Flow-electrode capacitive deionization(FCDI)uses suspended carbon materials as electrodes in a MCDI.Driving force is one of the main factors that greatly impact MCDI's desalination performance.In this study,the desalination performance of MCDI and FCDI were investigated at different applied voltages.For FCDI,the conductive additives were added into flow-electrode electrolyte to enhance the desalination performance.The flow-by MCDI device employed carbon cloth as electrode in this study.The desalination rate of MCDI at 4.8 V applied voltage was 7.4 times than that at 1.2 V during adsorption stage.While the dynamic charge efficiency remained stable at first and then decreased regardless of applied voltages.During desorption phase,the MCDI's desalination rate at 4.8 V was 3.4 times than that at 1.2 V.The dynamic charge efficiency remained stable when the voltage was below 3.6 V,but it began to decrease after 180 s at 4.8 V.The high desalination rate and good stability were realized when the desalination time was 90 s at 4.8 V after 50 cycles at three applied voltage modes(1.2V/20 min,4.8 V/90 s,4.8 V/20 min).Factors that influence FCDI's desalination performance were investigated,including operation modes,applied voltages,concentration of flow-electrode and flow-electrode electrolyte.FCDI operated under short-circuited closed cycle mode,which ensured sufficient desorption of flow-electrode,delivered better performance than that under isolated closed cycle mode.Meanwhile,the desalination rate increased with the applied voltage,but the charge efficiency began to decrease when the voltage exceeded 1.2 V.Higher desalination rate was gained when the flow-electrode electrolyte increased from0.65 to 30 g/L and the applied voltage overtop 0.6 V,but the charge efficiency decreased greatly,which was attributed to the concentration diffusion between middle chamber and electrode compartments.The desalination performance of FCDI with additional carbon black(CB)was betterthan that with graphite because of the better conductivity of CB.With 1 wt% CB added,the desalination performance increased nearly 66% at 0.9 V,and the charge efficiency increased with CB concentration.The highest charge efficiency(96.5%)was obtained in FCDI with 1.5 wt% CB.The addition of CB reduced the ohmic resistance and interfacial charge transfer resistance of a flow-electrode,which facilitated the electronic charge transfer and strengthened the FCDI's desalination performance.However,the addition of CB couldn't restrain the concentration diffusion,thus it was invalid to promote the performance of FCDI when the flow-electrode electrolyte was 30 g/L.