Optimizing the Energy Efficiency of Capacitive Deionization Reactors Working under Real-World Conditions

Abstract

Capacitive deionization (CDI) is a rapidly emerging desalination technology that promises to deliver clean water while storing energy in the electrical double layer (EDL) near a charged surface in a capacitive format. Whereas most research in this subject area has been devoted to using CDI for removing salts, little attention has been paid to the energy storage aspect of the technology. However, it is energy storage that would allow this technology to compete with other desalination processes if this energy could be stored and reused efficiently. This requires that the operational aspects of CDI be optimized with respect to energy used both during the removal of ions as well as during the regeneration cycle. This translates into the fact that currents applied during deionization (charging the EDL) will be different from those used in regeneration (discharge). This paper provides a mechanistic analysis of CDI in terms of energy consumption and energy efficiencies during the charging and discharging of the system under several scenarios. In a previous study, we proposed an operational buffer mode in which an effective separation of deionization and regeneration steps would allow one to better define the energy balance of this CDI process. This paper reports on using this concept, for optimizing energy efficiency, as well as to improve upon the electro-adsorption of ions and system lifetime. Results obtained indicate that real-world operational modes of running CDI systems promote the development of new and unexpected behavior not previously found, mainly associated with the inhomogeneous distribution of ions across the structure of the electrodes.

Evaluation of operational parameters for a capacitive deionization reactor employing asymmetric electrodes

“…Thus, ion electrosorption appeared to be limited to the most accessible sites of the surface. Desorption appeared to be also limited to the most accessible sites [49], which may be indicative of kinetic limitations or specific adsorption controlled by the potential used during the removal stage.…”

Electrospun carbon nanofiber networks from phenolic resin for capacitive deionization

“…The EIS analysis, representing as Nyquist plots, was performed to take a deep insight into the resistive and capacitive characteristics of the PhR CNF electrodes [48]. Fig.…”

Desalination and disinfection of inland brackish ground water in a capacitive deionization cell using nanoporous activated carbon cloth electrodes

“…Upon increasing the scan rate to 50 mV/s, capacitance reduced to 40 F/g (Fig. 1b), attributed to reduced time available for ion adsorption along with a denser mass transfer at the electrode-electrolyte interface leading to overlapping double layers at pore entrances [11,15]. The large capacitance values correspond well with the porous structure of ACC, giving it an active specific surface area (SSA) of~1000 m 2 /g (as measured from NMR experiments), which is comparable to SSA of activated carbon powders and other traditionally used electrode materials [18,43,44].…”

“…New stand-alone desalting techniques like capacitive deionization (CDI) have addressed the issue by specifically operating in the brackish water salt regimes [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], which are predicted to be present even offshore beneath the continental shelf [21]. CDI involves the potential mediated separation of salt from water, rather than water from salt, as practiced in traditional desalting techniques like RO and MSF [22,23].…”