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 (Q_T) 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 Q_T 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 Q_T value. Additionally, the ratio of desorption charge according to the desorption time was consistent regardless of Q_T. 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 (Q_ad) is supplied again, Q_T increases. When Q_T 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 Q_ad is, the shorter the desorption time, resulting in a higher desalination rate of the MCDI system.