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| Fig. 1: The operation of a CDI cell involves a two-step process: electrosorption of ions via the charging of the double layer under an applied potential difference across both electrodes, and the regeneration of the electrodes and release of contaminants once the potential is reversed. (Source: Y. Cornejo Carrillo) |
Capacitive deionization (CDI) is a rising desalination technology that removes minerals and salts from saline water by capturing charged species during the charging of the electrodes and recovering them during discharge. [1] This unique approach makes CDI a more energy-efficient and cost-effective option for desalination compared to conventional methods. The concept behind CDI relies in the formation of electrical double layers (EDLs) inside of the porous electrodes under an external voltage to immobilize ions and selectively extract them from saline water. [2] Once the maximum ion storage capacity of the porous electrodes is reached, the cell voltage is reversed to release the electrosorbed ions and regenerate the electrodes for continuous operation (see Fig. 1).
The performance of CDI technologies is primarily based on the properties of the electrode supports. Some vital electrode requirements include large ion-accessible specific surface area (to maximize salt electrosorption capacity), high (electro)chemical stability over the operating voltage range and solution pH (to ensure longevity of the process), fast ion mobility within the network of pores (to avoid diffusion and kinetic limitations) and high electronic conductivity (to minimize energy losses). [3]
All these components will contribute to performance indicators commonly reported in CDI literature such as salt adsorption capacity (SAC) and specific energy consumption (SEC). SAC measures the amount of ions (in milligrams) that are electro-adsorbed per gram of active electrode material. Mathematically,
where M is the molar mass of the salt (g/mol), m is the total mass of electrodes in the CDI cell (g), and ΔNd is the salt removed over one cycle related to the feed stream (moles). [4]
The SEC (in Wh/g) quantifies how much electricity is needed to remove a contaminant from water and directly determines the operation costs of the CDI system. One can estimate it minimum value via the following equation,
where U (V) refers to the applied voltage and F (96,485 C/mol) is the Faraday constant. [5] This equation does not account for inefficiencies in the system such as ohmic losses and charge efficiencies.
Activated carbon (AC): Activated carbon is one of the most utilized electrodes materials due to its high specific surface area, which significantly enhances its ability to adsorb salts, and its affordability, making it well-suited for large-scale applications. The SAC value for AC is 2.88 mg/g when using a 200 mg/L sodium chloride (NaCl) solution supplied at 20 mL/min at a constant voltage of 1.5 V. [6] This resulted in a theoretical SEC value of 0.69 Wh/gNaCl.
Carbon nanotubes (CNT): Carbon nanotubes have been widely utilized taking the form of a composite electrode for supercapacitor applications; thus, they are also regarded suitable electrode materials for CDI. A study found that the SAC for CNTs was close to 3.32 mg/g under 1.2 V and feed flow rate of 14 mL/min. [7] Using CNTs, the theoretical CDI cell SEC was calculated to be close to 0.55 Wh/gNaCl.
Ordered mesoporous carbon (OMC): Unlike activated carbon which has a more random, disordered pore structure, ordered mesoporous carbon has well-defined, organized array of larger mesopores which can result in better adsorption capabilities for larger molecules. One report found a maximum electrosorptive capacity for OMC of 10.1 mg/g at an applied voltage of 1.2 V and feed flow rate of 40 mL/min, which yielded a minimum energy requirement of 0.55 Wh/gNaCl. [8]
Capacitive deionization presents a promising outlook for efficient removal of charged contaminants in brackish water feeds. Two important aspects that dictate desalination efficiency and energy consumption are the salt adsorption capacity of the porous electrodes and the specific energy consumption in the CDI system. Among the carbon-based electrodes reported in CDI literature are activated carbon, carbon nanotubes, and ordered mesoporous, all of which show enhanced desalination capabilities due to their high surface and tunable properties. However, comparing to state-of-the-art technologies such as reverse osmosis, which has a reported specific energy consumption of 0.071 - 0.11 Wh/gNaCl, the current CDI framework is not yet competitive. [9] Thus, innovation in material synthesis, surface modification and scaling up production will pay a significant role in advancing CDI technologies for large-scale desalination and water purification in the future.
© Yamile Cornejo Carrillo. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. Ntakirutimana et al., "Activated Carbon Electrode Design: Engineering Tradeoff with Respect to Capacitive Deionization Performance," J. Electrochem. Soc. 167, 143501 (2020).
[2] C. Zhang et al., "Faradaic Reactions in Capacitive Deionization (CDI) - Problem and Possibilities: A Review," Water Res. 128, 314 (2018).
[3] S. Porada et al., "Review on the Science and Technology of Water Desalination by Capacitive Deionization," Prog. Mater. Sci. 58, 1388d (2013).
[4] S. A. Hawks et al., "Performance Metrics For the Objective Assessment of Capacitive Deionization Systems," Water Res. 152, 126 (2019).
[5] Y. Jiang et al., "Energy Consumption in Capacitive Deionization for Desalination: A Review," Int. J. Environ. Res. Public Health. 19, 10599 (2022).
[6] Y.-J. Kim and J.-H. Choi, "Enhanced Desalination Efficiency in Capacitive Deionization with an Ion-Selective Membrane," Sep. Purif. Technol. 71, 70 (2010).
[7] X. Z. Wang et al., "Electrosorption of NaCl Solutions with Carbon Nanotubes and Nanofibers Composite Film Electrodes," Electrochem. Solid-State Lett. 9, E23 (2006).
[8] F. Duan et al., "Capacitive Deionization by Ordered Mesoporous Carbon: Electrosorption Isotherm, Kinetics, and the Effect of Modification," Desalin. Water Treat. 52, 1388 (2014).
[9] J. Kim et al., "A Comprehensive Review of Energy Consumption of Seawater Reverse Osmosis Desalination Plants," Appl. Energy 254, 113652 (2019).