Improvements in desorption rate and electrode stability of membrane capacitive deionization systems by optimizing operation parameters

The operating parameters necessary to improve the desorption rate of a membrane capacitive deionization (MCDI) system while controlling the Faradaic reactions were studied. The total charge (QT) accumulated in the carbon electrode was set as the main operating parameter determining the desorption rate of the MCDI system. After adsorption was performed until the preset QT value was reached using the MCDI unit cell, desorption was performed at a cell potential of -0.2 V. As a result of this MCDI operation, the average desorption rate increased in proportion to the QT value. Additionally, the ratio of desorption charge according to the desorption time was consistent regardless of QT. Through this, it could be seen that the desorption process of the MCDI system is similar to the discharge characteristic of a series circuit comprising a resistor (R) and a capacitor (C). If the desorption time is too short during the MCDI operation, some charges will remain in the carbon electrode. When the adsorption charge (Qad) is supplied again, QT increases. When QT exceeds the maximum allowable charge (MAC), which is the total charge at the onset of Faradaic reactions, electrode reactions can occur. Through RC circuit analysis, a model equation for calculating the minimum desorption time required to operate a MCDI system without the occurrence of Faradaic reactions was derived. As a result of MCDI operation while changing the desorption time, the desalination performance almost matched the result predicted through the model equation. Additionally, it was found that the smaller Qad is, the shorter the desorption time, resulting in a higher desalination rate of the MCDI system.