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Evaluating the desalination performance and efficiency of capacitive deionization with activated carbon electrodes Chung, Ting-Chih

Abstract

Capacitive deionization (CDI) is an incipient desalination technology based on the principle of electrical double layer capacitors. When a constant voltage is applied to high surface area and electrically conductive electrodes, electrodes become oppositely charged and ions are adsorbed onto the electrode surfaces under the presence of the electric field, thereby producing a purified stream of water. When the electrodes are saturated with ions, the applied voltage is removed or the polarity is reversed to desorb the ions and generate a stream of waste concentrate. For brackish water with intermediate salinities, CDI technology has advantages over conventional desalination technologies because of operation at ambient temperatures and pressures, high water recovery, and no chemical usage. However, there are issues with translating CDI technology from laboratory to practice because of the lack of experience with its operation and uncertainties about its robustness and durability. To address these challenges, this thesis investigated a laboratory-scale CDI cell with the aim to holistically evaluate its desalination performance and efficiency using desalination metrics, kinetic models, and circuit models. The activated carbon electrodes used in the CDI cell were purchased commercially as well as fabricated in-house, and were analyzed with cyclic voltammetry, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Operating parameters including applied voltage, NaCl concentration, and flow rate were varied to study their effects. Lastly, the effect of including ion-exchange membranes was examined and preliminary tests were performed to explore the long-term desalination performance and efficiency of CDI technology. Commercial electrodes were found to be superior to the in-house fabricated electrodes. For operating parameters, higher applied voltages were found to increase the salt adsorption and capacitance but decrease the energy efficiency. Increasing NaCl concentration also increased salt adsorption but did not affect capacitance or energy efficiency. No trends were observed for flow rate and kinetic parameters. Ion-exchange membranes boosted the electrode performance considerably, with salt adsorption improving by 1.70 – 1.94 times and energy efficiency by 1.11 – 1.35 times. Long-term tests showed that electrode performance degraded steadily and reached half its original performance at 40 cycles but could be regenerated with NaOH washing.

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