Flow-Electrode Capacitive Deionization (FCDI) Using Suspensions of 2D Materials As Electrodes
Authors: Carmona Orbezo Aranzazu , Dryfe Robert AW 
Published: February 2020

The increasing shortage of potable water in many part of the world is leading the development of various approaches to water purification, with desalination of sea water the prime target. The most common approach to desalination is reverse osmosis (RO), being used in 65% of currently installed plants[1], but its high costs due to the use of membranes, leading to high-energy requirements, is pushing the desalination industry to find other solutions. An alternative to the high-energy requirements of RO is a method called capacitive deionization (CDI). In this process, two porous electrodes are arranged parallel to one to the other, letting brackish water flow between them. When a voltage is applied, the ions from the feed water migrate to the surface of the electrodes and are adsorbed capacitively, reducing the overall concentration of salts in the water stream. When the voltage is removed, or reversed, the ions desorb from the surface of the electrodes, resulting in a by-product of this process, brine. One of the greatest advantages of this system is that the applied energy can be recovered during the discharge step, just like an electrochemical capacitor, that is used to store energy by means of a pair of electrodes[2].

There are a number of recent reports in the literature describing an innovative CDI technique that focuses on improving the operation of these desalination systems. A suspended electrode material flowing in a path carved on the current collector plays the same role as the porous carbon electrodes fixed on the current collector in a typical CDI process, and this has been termed “flow-electrode capacitive deionization (FCDI)”[3].The flow operates continuously by providing fresh flow electrodes with increased ion capacitance, showing a continuous desalination behaviour and high desalting efficiency that originates from this.

In this study, we have tested FCDI systems using graphene and other 2D materials in suspension, as well as optimizing the FCDI system parameters, to try to improve the desalination performance of the system. The goal is to treat feed water with high salinity, close to that of seawater (35 g/L approximately). Studies of FCDI to date have used activated carbon and carbon black suspensions [4,5,6,7,8,9,10]. Graphene and 2D materials are expected to improve the overall desalination capacity of FCDI systems because of their high theoretical surface area and superior electrical conductivity. In the case of traditional CDI setups, graphene was able to increase the salt adsorption capacity 2.3 times[11], compared to previous studies made on activated carbon and carbon black electrodes.

References

[1] Virgili, Francisco and Pankratz, Tom. IDA Desalination Yearbook 2016-2017 for Global Water Intelligence. Media Analytics, Ltd. Publishers. (2017)

[2] Porada et al., Progress in Materials Science, 58(8), 1388-1442 (2013)

[3] Jeon et al., Energy & Environmental Science, 6, 1471-1475 (2013)

[4] Porada et al., Journal of Materials Chemistry, 2 (2), 9313-9321 (2014)

[5] Hatzell et al., Environmental Science and Technology, 49, 3040-3047 (2015)

[6] Jeon et al., Journal of Materials Chemistry, 2, 6378-6383 (2014)

[7] Gendel et al., Electrochemistry Communications, 46, 152-156 (2014)

[8] Rommerskirchen et al., Electrochemistry Communications, 60, 34-37 (2015)

[9] Yang et al., Sustainable Chemistry & Engineering, 4, 4174-4180 (2016)

[10] Liang et al., Desalination, 420, 63-69 (2017)

[11] Jia et al., Chemical Physics Letters, 548, 23-28 (2012)