Removing salt from Water
Removing charged species from aqueous media is of interest in diverse applications including desalting of saline water. Capacitive deionization (CDI) devices governed by the principles of a supercapacitor use two conductive electrodes which are preferably nanostructured to provide large surface area for adsorption of ions. Practical capacitive deionization systems operate at very low voltages, lower than the dissociation potential of water and unlike Reverse Osmosis, which is popularly used for desalination, capacitive deionization has been shown to be energetically favorable for desalting brackish water.
While it is generally accepted that the specific surface area of the electrode is the primary factor governing the salt removal capacity of the electrode, researchers at Sultan Qaboos University have broken the myth and demonstrated that in practical applications, it is the specific capacitance of the electrode and not the surface area which regulates electrode performance in a capacitive device. Additionally the work highlights the effect of asymmetry in terms of specific capacitance between the two electrodes and proposes a simple electrical model and its dependencies to qualitatively assess the desalting performance. The results show that the electrode with smaller capacitance is the limiting factor, indicating that anode-cathode capacitance should be matched for practical capacitive deionization units to achieve maximum desalting capacities.
Karthik Laxman1,2, Laila Al Gharibi,1, Joydeep Dutta1,3Show Affiliations
- Chair in Nanotechnology, Water Research Center, Sultan Qaboos University, PO Box 17, Al- Khoudh, Muscat 123, Oman
- Department of Electrical and Computer Engineering, College of Engineering, Sultan Qaboos University, PO Box 33, Al-Khoudh, Muscat 123, Oman
- Functional Materials Division, School of Information and Communication Technology, KTH Royal Institute of Technology, Isafjordsgatan 22, SE-16440 Kista, Stockholm, Sweden
Asymmetry of electrodes on the equilibrium salt adsorption capacity in a capacitive configuration was studied. Experiments were carried out by using activated carbon cloth (ACC) with a specific surface area and specific capacitance of ∼1000 m2/g and 44 F/g as the anode and ACC coated with zinc oxide nanorods (ZnO NR) with a specific surface area and specific capacitance of 637 m2/g and 57 F/g as the cathode. The electrodes were characterized electrically and their salt adsorption capacities measured for various anode-cathode configurations to conclude that for multimodal electrodes, specific capacitance and not specific surface area regulates the salt adsorption capacity. The adsorption trends were analyzed and equated to an electrical model to qualitatively predict the equilibrium salt adsorption capacity, where the smaller capacitance was observed to be the limiting factor. The results in this work are especially useful for practical capacitive deionization units, where anode-cathode capacitance should be matched to achieve maximum salt removal efficiency.Go To Electrochimica Acta