(Invited) Deionization and Energy Recovery in Capacitive Systems with Inverted Operation Characteristics

Abstract

Capacitive deionization (CDI) continues to emerge as a viable option for the treatment of saline streams with up to brackish levels of ionic contaminants. CDI functions similar to a capacitor, storing ions from a liquid stream onto highly porous electrodes with the aid of an applied electric field. When the pores become saturated, the electric field can be reduced or reversed to discharge the previously stored ions and form a concentrate stream [1]. At the same time, the electronic energy input to the cell during ion storage can be recovered to power auxiliary devices or coupled CDI units. The coupling of CDI cells offers not only the ability to lower the energy requirements for cell operation, but also the possibility of semi-continuous CDI operation. With much effort devoted to materials development and performance enhancements, there have been few reports on energy recovery in CDI literature. Of these, discharging a charged CDI cell over a resistive load, direct connection of CDI cells, and the use of buck-boost DC-DC converters have been demonstrated. In this work, parallel CDI cells are integrated with a DC-DC converter based on a Ćuk topology (Figure 1) to show active energy recovery from a discharging cell and subsequent deionization with a secondary cell using energy stored in the initial charged cell. Unlike buck-boost, the Ćuk topology incorporates a power supply that compensates for energy losses during the electron transfer process, thereby allowing the system to simultaneously transfer energy and meet its desalination goals. Inverted capacitive deionization (iCDI) was recently demonstrated where deionization was accomplished by a polarity difference between chemical surface charges on electrode pairs that facilitates adsorption of ions from solution [2]. Conversely to classical CDI, cell discharge is accomplished by applying an external electric field. We have improved upon the iCDI cell architecture by incorporating ion-selective membranes to form a new inverted membrane capacitive deionization cell (iMCDI). We will demonstrate energy recovery results for both the iCDI and iMCDI cells, and explore the influence of converter operation and cell architecture on energy recovery.