Energy Consumption and Recovery in Capacitive Deionization Using Nanoporous Activated Carbon Electrodes

Capacitive deionization (CDI) is an emerging desalination technology which utilizes porous electrodes to remove ions in water by electrosorption. Similar to electric capacitors, energy is stored and released during charging and discharging cycles, respectively. In this study, a nanoporous activated carbon coupled flow-through CDI device was used to evaluate energy consumption and recovery under various operational conditions by charging and discharging the cell at a constant current, respectively. Results indicated that the charging/discharging current, salt concentration and water flow rate were major factors impacting electrosorption and energy consumption, by changing the structure of the electrical double layer (EDL) and how ion transport occurs between the interface and bulk solution. A porosity-based EDL theory was applied to explain the experimental observations. Between 30 and 45% of the energy consumed during charging could be recovered depending on operational conditions, although thermodynamically more than 98% of the total energy should be recoverable. Results indicated that overpotential and faradaic reactions induced irreversible energy are the major reasons for gaps in observed energy losses. Energy consumption for reducing the salinity of brackish water from 32.7 to 5.5 mM by our device could be as low as 0.85 kWh/m3 under most optimized conditions (dependent on materials used and cell configuration). The energy consumption can be dramatically reduced by employing more electron-conductive and Faradaic-resistant electrode materials.