Abstract
Capacitive deionization (CDI) is a water desalination technology in which ions are removed from water by creating a potential difference between two capacitive electrodes. Porous carbon has been extensively used as an electrode material in CDI. However, recent developments in the field of intercalation materials have led to their application in CDI due to their large ion storage capacity. One such intercalation material, nickel hexacyanoferrate (NiHCF), was used in this study as the electrode material. A symmetrical cell was assembled with two identical NiHCF electrodes separated by an anion-exchange membrane. The effect of operational parameters such as current density, feed concentration and flow rate on the desalination characteristics of the cell was investigated. The highest salt adsorption capacity of ≈ 35 mg/g was measured at a current density of 2.5 A/m2 in a 20 mM NaCl feed solution. Furthermore, a Nernst-Planck transport model was successfully used to predict the change in the outlet concentration and cell voltage of the symmetric CDI cell. Finally, performance of the symmetric NiHCF CDI cell was compared with an MCDI cell with porous carbon electrodes. The NiHCF cell, on average, consumed 2.5 times less energy than the carbon-based MCDI cell to achieve similar levels of salt removal from saline water in CDI.
Highlights
Capacitive deionization (CDI) is an electrochemical water desali nation technique in which the anions and cations are removed from water and temporarily stored in capacitive electrodes by creating a potential difference between them [1,2,3]
Regardless of the similarities between the design of different desalination setups, the mechanism of ion storage in the capacitive electrodes marks the difference between CDI and other electrochemical technologies such as electrodialysis, ED, where desalination is driven by faradaic reactions occurring at the electrodes
In this study we demonstrated the intrinsically high energy effi ciency of symmetric CDI cells with nickel hexacyanoferrate (NiHCF) electrodes compared to carbon-based MCDI cells by typically obtaining a 2.5-fold reduced energy consumption
Summary
Capacitive deionization (CDI) is an electrochemical water desali nation technique in which the anions and cations are removed from water and temporarily stored in capacitive electrodes by creating a potential difference between them [1,2,3]. Regardless of the similarities between the design of different desalination setups, the mechanism of ion storage in the capacitive electrodes marks the difference between CDI and other electrochemical technologies such as electrodialysis, ED, where desalination is driven by faradaic reactions occurring at the electrodes. Conventional CDI cells produce desalinated water in an intermittent manner and the electrode voltage changes with charge accumulation. The capacitive electrodes are regenerated by either short-circuiting or reversing the polarities. This regeneration step in terrupts the desalinated water supply. A different cell archi tecture such as flow electrodes and rocking chair can enable CDI to continuously produce desalinated water [4,5,6], similar to ED and reverse osmosis. The recovery of the energy released during the electrode regeneration can enhance the thermodynamic energy effi ciency of CDI [9], making capacitive electrodes attractive for water desalination applications
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