Capacitive deionization with asymmetric electrodes: Electrode capacitance vs electrode surface area

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.

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