Maximizing Volumetric Removal Capacity in Capacitive Deionization by Adjusting Electrode Thickness and Charging Mode

Abstract

Capacitive deionization (CDI) is an emerging desalination technology based on the same charge storage principles as in electrical double-layer supercapacitors (EDLC's). In this study, electrodes of differing thicknesses were tested using constant current (CC-CDI) and constant voltage (CV-CDI) operational modes in order to study the best way of sequestering the highest amount of ions under different salt concentration scenarios. CV-CDI was used to calculate electrode time constants (RC) and thereby determine a suitable current density for desalinating the solution. Results showed that the voltage pulse produced a fast but heterogeneous layer of ion adsorption, presumably, on the most accessible part of the electrode surface. Thus, volumetric specific capacitance under CV-CDI mode might vary from 17-24 F cm −3 and 14-20 F cm −3 for 50 μm and 180 μm electrodes, respectively. Nevertheless, results demonstrated that, under the constant current mode, it is possible to increase charge storage by 30% for a CDI cell consisting of thin electrodes and as much as 80% for thicker electrodes simply by controlling current density and, therefore, the rate capability. Moreover, the analysis of the charge efficiency indicated that a proper selection of the current density can result in efficiencies above 80% regardless the salt concentration scenario. The impact of water has on the production of energy and vice versa is unquestionable. Water is used to generate and transmit energy , and, at the same time, massive amounts of energy are needed to collect, clean, move and dispose of water. This link between water and energy production has been defined as the water-energy nexus. 1 During the last decades, the relevance of the water-energy nexus has been expanded with the introduction of new energy and water production technologies affecting the demands of water and energy. 2,3 In this context, brackish water desalination and treatment and reuse of effluents from different type of processes (municipal and industrial wastewater) for distinct applications (mainly agriculture, although in some areas like Singapore even human consumption) 4 using energy-efficient technologies is becoming of increasing concern. 5 With the aim of facing these issues, Capacitive Deionization (CDI) technology has been proposed as an opportunity to produce clean water while at the same time storing energy. 6 CDI is an electrochemically controlled desalination technology that removes ions from salt water by electrosorption via a two-step, non-faradaic process occurring in the electrical double layer region of porous electrodes. 6 In a first stage (charging), the electrodes are polarized (either by applying a constant voltage or current) forcing in this way the ions present in the feed water to be electro-adsorbed. Subsequently, as soon as the water reaches the demanded quality, the salt desorption step (discharg-ing) begins by switching the current/cell potential or short-circuiting the cell. 7-14 This means that CDI mechanism is based on the same principle as EDLC's which store energy in the double-layer formed at electrode/electrolyte interface. Thus, electric charges are accumulated on the electrode surface and ions of counter charge are arranged on the electrolyte side of that interface. 15-17 This idea of storing energy while delivering water has drawn the interest of the CDI scientific community leading to recent studies focused on the energy recovery approach. 8,18-21 Besides this advantage, CDI is also reported to have some other advantages over more established desalination methods. These include the operation at low voltages, in comparison with elec-trodialysis, or the lack of high pressures for water recovery, in contrast with membrane technologies. 22-26 Therefore, the study of CDI technologies based on the knowledge obtained from research conducted in * Electrochemical Society Member. z E-mail: julio.lado@imdea.org the field of EDLC could be considered a paradigm of the water-energy nexus. Following this idea, since the electrode materials in EDLC's are typically constructed using porous carbon materials with medium to high specific surface area (400-1100 m 2 · g −1) and a very low electrical resistivity (less than 40 m · cm 2), they could be considered also suitable for the electrosorption of anions and cations. 27-29 Thus, inexpensive activated carbons 30,31 having a large surface area and an adequate pore size distribution strongly contributes to an increase in the gravimetric capacitance of these electrodes. 32 In addition to the capacity of storing ions in the EDL, which is tied directly to salt ad-sorption capacity (SAC, mg · g −1), another relevant parameter is the rate capability of EDLC or the rate of sequestering these ions in the porous electrodes. In this case, the rate capability would be linked to the average salt adsorption capacity (ASAR, mg · g −1 · min −1), defined in the CDI literature as the amount of salt removed normalized by the mass of electrode/active material and duration of charging step or the entire CDI cycle. 14 The rate capability of electrodes employed as ELDC is usually assessed evaluating the time constant that is defined by two components, its capacitance and its equivalent series resistance (ESR). 33 Since these parameters constitute a highly relevant property of EDLC technology, the authors believe that this variable could be also used to tune electrode materials for optimizing the CDI process. 33-35 In this sense, previous studies have analyzed the effect of the electrode thickness in the rate capability of EDLC's. 33 Nevertheless, as far as the authors know, previous studies in CDI based on modifying the electrode thickness 36-40 have not employed the time constant approach as a way of optimizing the CDI process. In this work, the behavior of the electroadsorption-desorption processes using electrodes with different thickness were studied under differing values of water salinity and varying operational conditions employing constant voltage and constant current pulses. Thus, the main electrochemical parameters of CDI were evaluated in a flow cell assembled with electrodes prepared by a doctor-blading procedure that allowed us to control the film thickness of the carbon electrodes. In order to examine how the electrode thickness affects the rate of ion electrosorption into these porous carbon electrodes, the adsorption capacity of carbon film electrodes was theoretically derived from capac-itance as a function of the film thickness. The adequate ion transport velocity through the pores of these materials was separately analyzed, from which a current density-dependence has been predicted.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. 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