Flow-Electrode Capacitive Deionization Using an Aqueous

For preparing the flow-electrodes at various NaCl concentrations, the weight ... and height of the graphite current collectors are 110 mm, 66 mm, and ...
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Flow-Electrode Capacitive Deionization Using an Aqueous Electrolyte with a High Salt Concentration SeungCheol Yang,∥,† Jiyeon Choi,∥,† Jeong-gu Yeo,‡ Sung-il Jeon,‡ Hong-ran Park,† and Dong Kook Kim*,§ †

Marine Energy Convergence and Integration Laboratory, Jeju Global Research Center, Korea Institute of Energy Research, 200, Haemajihaean-ro, Gujwa-eup, Jeju-si, Jeju-do 63357, Republic of Korea ‡ Advanced Materials and Devices Laboratory, Energy Materials and Process Research Division, Korea Institute of Energy Research, 152, Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea § Energy Materials and Process Research Division, Korea Institute of Energy Research, 152, Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea S Supporting Information *

ABSTRACT: Flow-electrode capacitive deionization (FCDI) is novel capacitive deionization (CDI) technology that exhibits continuous deionization and a high desalting efficiency. A flowelectrode with high capacitance and low resistance is required for achieving an efficient FCDI system with low energy consumption. For developing high-performance flow-electrode, studies should be conducted considering porous materials, conductive additives, and electrolytes constituting the flow-electrode. Here, we evaluated the desalting performances of flow-electrodes with spherical activated carbon and aqueous electrolytes containing various concentrations of NaCl in the FCDI unit cell for confirming the effect of salt concentration on the electrolyte of a flow-electrode on desalting efficiency. We verified the necessity of a moderate amount of salt in the flow-electrode for compensating for the reduction in the performance of the flow-electrode, attributed to the resistance of water used as the electrolyte. Simultaneously, we confirmed the potential use of salt water with a high salt concentration, such as seawater, as an aqueous electrolyte for the flow-electrode.

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deionization (MCDI).14 However, instead of the fixed electrode used in conventional CDI, the carbon suspension flows between the ion-exchange membranes and current collectors. Hence, meandering flow channels for the flow electrodes are carved on the current collectors. Recently, Hatzell et al.15 have proposed a membrane-free FCDI, which has a cell structure similar to that employed in an electrochemical flow capacitor (EFC).16 The two FCDI cells only differ with respect to whether salt water (influent or effluent) and flow electrodes are separated by ion-exchange membranes. Regardless of their cell geometries, FCDI exhibits continuous desalting behavior and a high desalting efficiency, attributed to the infinite ion-adsorption capacity of the flow-electrode as compared to the limited capacity of the fixed electrode used in conventional CDI. In contrast to that of conventional CDI, the operation of FCDI is simple as the deionization and regeneration does not occur in the same unit cell; conventional

INTRODUCTION Capacitive deionization (CDI) is an electrochemical water treatment technology based on the formation of an electrical double layer (EDL) on the surface of porous electrode materials under an electrical field. As compared to other water treatment technologies, such as reverse osmosis (RO), CDI utilizes less energy for the desalination of brackish water because the direct electrosorption of ions in salt water on porous electrodes leads to low operating voltage of CDI.1,2 Several researchers have investigated CDI from various perspectives with the aim of realizing a more energy-efficient CDI system. Hence, studies are mainly focused on the maximization of desalting efficiency. Several studies have focused on the performance improvement of the CDI desalting performance by the development of novel materials,3−10 such as carbon materials, ion-exchange membranes, operation modes,11 such as a constant voltage vs current operation mode, and new types of cell geometry,12−15 such as flowthrough CDI, wire-based CDI, and flow-electrode CDI. Flow-electrode capacitive deionization (FCDI) is a novel CDI technology using a flow-electrode made of a carbon suspension (slurry). The first proposed FCDI exhibited a cell geometry similar to that utilized in membrane capacitive © XXXX American Chemical Society

Received: September 22, 2015 Revised: March 31, 2016 Accepted: May 10, 2016

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DOI: 10.1021/acs.est.5b04640 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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EXPERIMENTAL SECTION Preparation of Flow-Electrodes with Varying Salt Concentrations. For preparing flow-electrodes, spherical AC (BEAPS-AC0830, Asahi Organic Chemicals, Japan) was added into the aqueous electrolyte at various NaCl concentrations (Table 1). Campos et al. prepared flow-electrodes with

CDI involves a complicated operation (repeated deionization and regeneration) in the same unit cell. Recently, novel studies have been reported with respect to the efficient operation of FCDI systems in various aspects. Gendel et al. have demonstrated the continuous operation of the desalination module and regeneration or concentration module by the recirculation of flow-electrodes.17 Jeon et al. have suggested the closed-loop operation of a flow-electrode for regenerating the flow-electrode by the simple mixing of the used anode and cathode flow-electrodes.18 Porada et al. have shown that the salt removal efficiency can be improved by the optimization of operation conditions, such as water residence time (flow rate of salt water), in an FCDI system.19 In addition, Rommerskirchen et al. have suggested an FCDI system using a single flow-electrode and a single module for decreasing both energy and investment cost.20 The flow-electrode, used in recent FCDI and EFC studies, mainly consisted of porous activated carbon, a conductive additive, and an aqueous or organic electrolyte. Recent studies on the flow-electrode have investigated the modification and development of carbon materials for the high performance of FCDI and EFC,21−28 and expansion of the applications of flowelectrodes to salinity gradient power generation.19,29 In particular, flow-electrodes for the performance improvement of EFCs, such as the synthesis and modification of spherical activated carbon (AC),21−23 pseudocapacitive electrodes,24,25 graphene addition,26 and metal oxide electrodes,27 have been widely investigated relative to those of FCDI.28 Studies focusing on the development of high-performance flow-electrodes for achieving an efficient FCDI system with low energy consumption have focused on the reduction of ionic resistance for the easy access of ions to the porous particles and efficient electron transfer from the current collector to the particles in the flow-electrode. For satisfying two requirements, the high loading of porous particles in the flow-electrode has been studied by various methods. Porada et al. have fabricated flow-electrodes with a high loading of AC particles (up to 20 wt %) by simple mixing without any additional treatment for AC particles.19 Hatzell et al. have fabricated a flow-electrode with a high loading of AC particles up to 28 wt % by chemical oxidation method.28 In contrast to studies reported on porous materials utilized for the flow-electrodes in FCDI, those reported on electrolytes for the improvement of ionic resistance and conductive additive for efficient electron transfer are scarce. In the cases of electrolytes, although several research groups have fabricated flow-electrodes with an aqueous electrolyte with NaCl concentrations ranging from 0.0 to 0.2 M,14,17−20,28 specific studies on electrolytes for the improvement of the FCDI desalting efficiency have not been conducted in detail. In this study, flow-electrodes were fabricated using spherical AC, deionized water, and NaCl. For preparing the flowelectrodes at various NaCl concentrations, the weight ratio of deionized water to spherical AC was maintained constant at 10:1, and varying amounts of NaCl from 0.00 wt % to 6.98 wt % were added in deionized water. Desalting experiments were conducted using these flow-electrodes with the aim of obtaining a relationship between the salt concentration of the electrolyte in the flow-electrodes and desalting efficiency. We confirmed the importance of the appropriate salt concentration of the electrolyte in the flow-electrode for stable desalting performance and the possibility of fabricating a flow-electrode using highly concentrated water, such as seawater.

Table 1. Compositions of Flow-Electrodes Used in This Study NaCl concentration (wt %)

deionized water (g)

spherical activated carbon (g)

NaCl (g)

0.00 0.50 1.48 2.44 3.38 4.31 5.21 6.10 6.98

200

20

0 1 3 5 7 9 11 13 15

spherical AC and observed improvement in rheological properties.21 The weight ratio of deionized water to spherical activated carbon was 10:1. The mixtures were stirred using a magnetic bar for 24 h for achieving a homogeneous carbon suspension. The shape of spherical AC was verified by fieldemission scanning electron microscopy (FESEM, JSM-6700F, JEOL Ltd., Japan) (Supporting Information Figure S1). The average size of the spherical AC particles, measured by a particle size analyzer (Mastersizer 3000, Malvern instruments Ltd., UK), was 12.83 μm at D50. The specific surface area, average pore diameter, and total pore volume of the spherical AC particles calculated by Brunauer−Emmett−Teller (BET) method (ASAP2010, Micromeritics) were 3011 m2g−1, 2.49 nm, and 1.63 cm3g−1, respectively. Assembly of the FCDI unit cell. Figure 1 shows the assembly of an FCDI unit cell; its structure is similar to that used in MCDI; it is composed of a pair of graphite current collectors, cation- or anion-exchange membranes, a gasket, a

Figure 1. Schematic of the flow-electrode capacitive deionization (FCDI) set up used in this study. B

DOI: 10.1021/acs.est.5b04640 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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flow-electrode and salt water were uniformly loaded into the channel of a static cell, an EIS test was performed in a frequency range of 10 kHz to 10 mHz with a sine wave signal amplitude of 10 mV using a potentiostat (ZIVE SP5, Wonatech, Republic of Korea). The omhic resistance (R) of the cell was calculated by the interception of the x-axis of the Nyquist plot at a high frequency. The capacitance was calculated by the following equation:

spacer, and one pair of end plates (Figure 1). The width, length, and height of the graphite current collectors are 110 mm, 66 mm, and 12 mm, respectively. The dimensions of the flow channel carved on the collectors are a width of 2 mm and a depth of 2 mm. The column length of the flow channel is 30 mm, and the number of columns is 23. The thickness of the ion-exchange membranes was around 160 μm (Neosepta CMX and AMX, Tokuyama, Japan). The contact area between the ion-exchange membranes and flow-electrode was 12.7 cm2. A silicone gasket and a 0.3 mm thick polyester spacer were used between cation- and anion-exchange membranes for assembling the FCDI unit cell. All parts were held together using polyvinyl chloride (PVC) end plates. Operation of the FCDI Unit Cell. Salt water with a concentration of 35 g L−1 was passed through the spacer at a flow rate of 3 mL min−1 between the ion-exchange membranes. The salt water was operated under open-cycle conditions with two reservoirs (salt water and desalinated water reservoirs). The flow rate of the flow-electrode, operated by a closed cycle, was maintained constant at 25 mL min−1. The fresh cathode and anode flow-electrode came out from one flow-electrode reservoir to the FCDI unit cell. As shown in Figure 1, the used cathode and anode flow-electrodes were assembled into the one flow-electrode reservoir. As shown in our previous study,18 this operation method originated from the automatic release of ions electrostatically adsorbed on the surface of AC after desalting by the mixing and neutralization of the charged cathode and anode flow-electrodes. Consequently, the desalting performance of flow-electrodes was restored by the simple mixing of the charged cathode and anode flow-electrodes. Using one flow electrode in Table 1, desalting experiments were continuously conducted three times with a time interval of 15 min between each experiment for verifying the accumulation of salt in the flow-electrode. Each desalting experiment was performed for 30 min. The time interval between each experiment was introduced for attaining the initial electrical conductivity of the effluent after the release of voltage. After conducting the desalting experiments three times, the same experiment was repeated using other flow-electrodes. A constant voltage of 1.2 V was applied to the FCDI unit cell using a potentiostat (ZIVE SP5, Wonatech, Republic of Korea) for desalting experiments. The current from the FCDI unit cell was measured using a potentiostat during desalting experiments. Deionization and Electrochemical Characterization. The desalting efficiency of salt water was calculated by the following equation and normalized variations of the NaCl concentration from electrical conductivities of the effluents, which were measured every 10 s using a conductivity meter (S47, Mettler-Toledo, Switzerland). E=

Ci − Cs × 100 Ci

C=

1 ωZ″

(2)

Here, ω is the angular frequency of the applied ac signal, and Z″ is the imaginary resistance of the impedance.30

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RESULTS AND DISCUSSION Relationship between Desalting Efficiency and Electrolyte Concentration. Figure 2 shows the results obtained

Figure 2. Correlation data between desalting efficiencies and electrical currents with varying NaCl concentrations in the flow-electrode after 1st/2nd/3rd desalting.

from the correlation between desalting efficiency and current by varying the NaCl concentrations in the flow-electrode after 1st/2nd/3rd desalting. Figure 2 summarizes the results from Supporting Information Figure S2 and Figure S3, which show the desalting efficiencies and electrical currents obtained during desalting experiments. As can be observed in Figure 2, the desalting efficiencies of the flow-electrodes with NaCl concentrations ranging between 0.00 and 1.48 wt % sharply increased with experimental time, with no saturation observed at a certain point. In contrast, those of the flow-electrodes with NaCl concentrations of greater than or equal to 2.44 wt % were constant despite the increasing number of desalting experimental times. Moreover, for the FCDI unit cell using the flowelectrode with an NaCl concentration of less than 2.44 wt %, the electrical current tended to increase with experimental time, while the electrical currents of the FCDI unit cell using the flow-electrode with an NaCl concentrations greater than 2.44 wt % were nearly constant without significant variation regardless of experimental time. Furthermore, the desalting efficiency of the flow-electrode was proportional to the electrical current of the FCDI unit cell. In particular, the increase of electrical current at a low NaCl concentration in the flow-electrode indicated that the resistance drop of the aqueous electrolyte is attributed to an increased NaCl concentration in

(1)

Here, E is the desalting efficiency of the FCDI unit cell (%), Ci is the normalized initial NaCl concentration in the effluents, and Cs is the normalized NaCl concentration in the effluents at 30 min. Electrochemical impedance spectrocopy (EIS) was employed for the characterization of each flow-electrode and for the examination of the effects of salt concentration on ohmic resistance and capacitance. Electrochemical characterization was performed using a symmetric two-electrode cell under the same structure as those employed in the desalting test. After the C

DOI: 10.1021/acs.est.5b04640 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Table 2. Variations of NaCl Concentration (Content) in the Flow-Electrodes Calculated from the Accumulated Quantity of Electric Charge after 1st/2nd/3rd Desalting calculated NaCl concentration (content) initial NaCl concentration (content) 0.00 0.50 1.48 2.44 3.38 4.31 5.21 6.10 6.98

wt wt wt wt wt wt wt wt wt

% % % % % % % % %

(0 g) (1 g) (3 g) (5 g) (7 g) (9 g) (11 g) (13 g) (15 g)

after 1st desalting 0.10 0.75 1.79 2.78 3.71 4.63 5.54 6.43 7.30

wt wt wt wt wt wt wt wt wt

% % % % % % % % %

after 2nd desalting

(0.20 g) (1.51 g) (3.65 g) (5.71 g) (7.71 g) (9.71 g) (11.72 g) (13.74 g) (15.74 g)

0.26 1.03 2.12 3.12 4.04 4.96 5.87 6.75 7.62

wt wt wt wt wt wt wt wt wt

% % % % % % % % %

(0.52 g) (2.08 g) (4.32 g) (6.43 g) (8.42 g) (10.43 g) (12.46 g) (14.48 g) (16.50 g)

after 3rd desalting 0.48 1.33 2.45 3.46 4.37 5.29 6.20 7.08 7.95

wt wt wt wt wt wt wt wt wt

% % % % % % % % %

(0.96 g) (2.70 g) (5.02 g) (7.16 g) (9.14 g) (11.16 g) (13.21 g) (15.23 g) (17.27 g)

the flow-electrode as Na+ and Cl− are transferred from salt water to the flow-electrode via the ion-exchange membrane. For analyzing the detailed relationship between desalting efficiency and NaCl concentration in the flow-electrode, we calculated the NaCl concentration in the flow-electrodes with each composition after 1st/2nd/3rd desalting under the assumption of an electrical capacitor.31 First, electrical current−time graphs in Supporting Information Figure S3 were changed to the accumulated charge−time graphs by the following equation. Q=

∫0

τ

I(t )dt

(3)

Here, Q is the accumulated charge (C) of ions transferred through ion-exchange membranes, I is the electrical current (A) of the FCDI unit cell during desalting, and τ is the operation time (s). Supporting Information Figure S4 shows the transferred accumulated charge−time result. At low NaCl concentrations (0.00−1.48 wt %) in the flow-electrode, slopes of the accumulated charge as a function of the operation time increased with increments of desalting experimental times. In contrast, those results maintained constant gradients at high NaCl concentrations of greater than 2.44 wt %. Then, we calculated the NaCl concentration (content) in the flow-electrode after 1st/2nd/3rd desalting with the accumulated charge at 30 min (1800 s) by the following equations.

WNaCl =

Q × MNaCl F

(4)

C NaCl =

W0 + WNaCl × 100 WDI water + W0 + WNaCl

(5)

Figure 3. Results from the correlation between desalting efficiency and calculated NaCl concentration after 1st/2nd/3rd desalting.

0.50 wt %. At 1.48 wt % NaCl concentration, the desalting efficiency reached saturation at the third desalting experiment, similar to that observed for the flow-electrode having an NaCl concentration of 2.44 wt %. This result indicated that a NaCl concentration of 2.44 wt % in the aqueous electrolyte is required to maximize the original performance of the flowelectrode when it is prepared at a weight ratio of 10:1 of deionized water and spherical AC having a BET area of 3011 m2g −1. In contrast, desalting performances of flow-electrodes with NaCl concentrations ranging between 2.44 and 6.98 wt % gradually increased with increments of NaCl concentration in the flow-electrodes. The gradient distinction on increasing of the desalting efficiencies at a reference point of an NaCl concentration of 2.44 wt % in the flow-electrode can be explained by the path of ion diffusion from the membrane to AC through the aqueous electrolyte and within porous AC. At a low NaCl concentration region of 0.00−1.48 wt %, diffusion of ions from the membrane to AC through the aqueous electrolyte was inefficient under an electrical field, attributed to the long diffusion path between the membrane and AC originating from the shortage of NaCl salt in the flow-electrode. Hence, it is highly effective to add NaCl in the flow-electrode for improving desalting efficiency as existing ions near the AC particles preferentially shift toward AC particles for adsorption on its surface. On the other hand, at a high NaCl concentration region of greater than 2.44 wt %, the long diffusion path between the membrane and AC marginally affected desalting efficiency, attributed to the existence of sufficient NaCl salt in the flow-electrode. Ions in the flow-electrode with a high NaCl concentration can be diffused relatively easily within porous AC compared to those in the flow-electrode of a low NaCl

Here, WNaCl is the increased weight (g) of NaCl transferred via ion-exchange membranes from salt water to the flow-electrode, F is the Faraday constant (C mol−1), MNaCl is the molecular weight (g mol−1) of NaCl, W0 is the initial weight (g) of NaCl in the flow-electrode before the desalting experiment, WDI water is the weight (g) of DI water, and CNaCl is the final NaCl concentration in the flow-electrode after the desalting experiment. Table 2 lists the calculated NaCl concentrations (contents) in the flow-electrode after 1st/2nd/3rd desalting. Figure 3 shows the change in the desalting efficiency with varying NaCl concentrations after 1st/2nd/3rd desalting. As can be observed, the desalting efficiency of the flow-electrode sharply increased with increments of NaCl concentrations in the flow-electrode when the flow-electrode was fabricated with aqueous electrolytes having NaCl concentrations of 0.00 and D

DOI: 10.1021/acs.est.5b04640 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology concentration under an electrical field. Thus, the desalting performances gradually increase with increments of NaCl concentration in the flow-electrode at high NaCl concentrations between 2.44 and 6.98 wt %. Furthermore, in Figure 3 and Supporting Information Figure S2, the flow-electrode at high NaCl concentrations between 4.31 and 6.98 wt % did not exhibit any decrease of desalting efficiencies by spontaneous ion reversal, attributed to the ion concentration gradient between salt water (influent or effluent) and the flow-electrode regardless of the increase of NaCl concentration in the flowelectrode by repeated 1st/2nd/3rd desalting experiments. This result is attributed to the charge neutralization of the flowelectrode, membrane co-ion blocking, and counterion attraction by the electric field. On the basis of the uniform desalting performance of the flow-electrode with a high NaCl concentration of greater than 2.44 wt %, the potential use of seawater, which comprises most of the water on the surface of the earth, was verified as an aqueous electrolyte. Electrochemical Analysis As a Function of Electrolyte Concentration. For the detailed analysis of these results, electrochemical impedance spectroscopy analysis was conducted using the flow-electrodes with NaCl concentrations of 0.00/1.48/2.44/3.38/6.98 wt % before desalting. Supporting Information Figure S5 also shows the results obtained from measurement with aqueous electrolytes without AC. Figure 4 (a) shows the Nyquist plots according to NaCl concentrations in the flow-electrode. The x-axis intercept of the Nyquist plot in the high-frequency region is the equivalent series resistance (ESR), which is combined with the ionic resistance of the electrolyte and interfacial resistance at the active material/ current collector.22 The ESR values (Ω) obtained from the Nyquist plot were 1.731 (0.00 wt % NaCl), 1.275 (1.48 wt % NaCl), 0.500 (2.44 wt % NaCl), 0.518 (3.38 wt % NaCl), and 0.451 (6.98 wt % NaCl). As the same amount of the active material spherical AC was loaded in channels across all tested samples, these differences are attributed to the decrease of the ionic resistance of the electrolyte. Thus, the higher the NaCl concentration in the flow-electrode, the lower the ESR values. In the high-frequency region, semicircles with different diameters corresponding to the electrolyte concentration were also observed. With increasing NaCl concentration from 1.48 to 6.98 wt % in the flow-electrode, the diameter of the semicircle slightly decreased; on the other hand, it dramatically decreased between 0.00 and 1.48 wt %. The size of the semicircle is related to either the resistance at the interface between the electrolyte and electrode or the contact between internal particles.32 The extent of the diameter of the semicircle is indicative of the ionic conductivity of the electrolyte, which affects the electrolyte−electrode interfacial resistance, caused by the same content of spherical AC in the flow-electrode.33 In the low-frequency region, the Nyquist plots indicate semi-infinite diffusion, known as the Warburg resistance, the characteristic of which is a slope at 45°.34 In addition, the Warburg resistance accounts for ion diffusion depending on frequency within porous AC. Similar trends were observed in the range from 158 mHz to 10 mHz in response to increasing NaCl concentrations from 1.48 wt % to 6.98 wt %. As compared to 0.00 wt % NaCl concentration in the flow-electrode, all samples exhibited a decrease in the Warburg resistance, which is because the electrolyte concentration is beneficial to the decrease in the ion diffusion path and the increase in charge transport. Moreover, with decreasing frequencies, the electrolyte ions can easily penetrate the intrapores as the ac signal progressively

Figure 4. (a) Nyquist plots of flow-electrodes. Inset shows the highfrequency region. (b) Bode plots and (c) capacitance of flowelectrodes as a function of frequency. Results were obtained using flow-electrodes having NaCl concentration of 0.00, 1.48, 2.44, 3.38, and 6.98 wt % before desalting.

propagates into the intrapores of the flow-electrode.34,35 This means that efficient signal propagation leads to long pathways of ions into intrapores.35 Hence, it is thought that the results of resistance and capacitance in the low-frequency region can account for the effect of the electrolyte over capacitive behavior in the intrapores. As shown in Figure 4(b), at 10 mHz, the total resistances (Ω) were 24.259 (0.00 wt % NaCl), 5.036 (1.48 wt E

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were used. The ions near AC preferentially attached on the surface of charged AC. In the flow-electrode with appropriate or high NaCl concentration, the long diffusion path between the membrane and AC was not the dominant factor affecting desalting efficiency. Ions in the flow-electrode with a high NaCl concentration more easily diffused in the intrapores of AC in the flow-electrode under an electrical field as compared to those in the flow-electrode with an appropriate amount of NaCl. Thus, NaCl in the flow-electrode primarily served to decrease the resistance of the aqueous electrolyte at low NaCl concentrations and diffuse ions near the AC surface well within the intrapores at a high NaCl concentration region under an electrical field. In conclusion, FCDI desalting experiments were conducted using flow-electrodes having various NaCl concentrations for verifying the effect of salt concentration in the flow-electrode on the desalting performance of FCDI. On the basis of our experimental results, the following conclusions were made. As water in the flow-electrode acts as a resistor blocking the easy migration of the ions, the salt in the flow-electrode is necessary. In addition, an appropriate amount of salt should be added in the flow-electrode for maximizing desalting performance of the flow-electrode. The flow-electrode can be fabricated using an aqueous solution with a high salt concentration because desalting performances of the flow-electrode using a highly concentrated aqueous electrolyte of greater than 2.44 wt % NaCl remain constant regardless of experimental times. This result indicated that seawater, occupying the majority of the earth, can be utilized as an aqueous electrolyte for the flowelectrode. Practical Implication. Several seawater desalination plants have been installed and operated near the waterfront of scarce water areas, such as deserts and islands, for producing fresh water. For commercializing the FCDI system for the desalination of seawater, basic studies should be preferentially conducted in terms of various perspectives, such as the development of high-performance flow-electrodes, efficient regeneration of flow-electrodes, novel cell structure for minimizing clogging while extracting the maximum performance of flow-electrodes, and the improvement of system performance. The factors to be considered should be related to the actual installation and operation of FCDI systems as seawater desalination systems. Flow-electrodes should be prepared using easily available, cost-effective porous materials, conductive additives, and electrolytes at installation sites. In terms of electrolytes, easily available seawater near the waterfront of scarce water areas is suitable as the actual electrolyte for the flow-electrode. By our current study, we successfully overcame the limitations of flow-electrodes using low concentrated salt water as an electrolyte for the use of FCDI as a seawater desalination system.

% NaCl), 4.230 (2.44 wt % NaCl), 4.335 (3.38 wt % NaCl), and 4.068 (6.98 wt % NaCl). Figure 4(c) shows the result of capacitance derived from the imaginary part (Z″). The capacitance increased with increasing electrolyte concentration irrespective of frequency. Particularly, the capacitances (F) at 10 mHz were 1.46 (0.00 wt % NaCl), 2.24 (1.48 wt % NaCl), 6.22 (2.44 wt % NaCl), 6.28 (3.38 wt % NaCl), and 6.89 (6.98 wt % NaCl). Moreover, capacitance was observed to significantly increase at the NaCl concentrations of greater than 2.44 wt % as compared those observed in the range of 0.00 wt % to 1.48 wt %. Furthermore, in the case of aqueous electrolytes having an NaCl concentration of 0.00 or 6.98 wt % without AC (Supporting Information Figure S5(c)), capacitance was hardly observed over the entire range of frequencies. Thus, it can be explained that at high frequency, ions came close to the pores near the AC surface, whereas at low frequency, ions migrated deep inside the intrapores.33 Moreover, at an NaCl concentration of 1.48 wt % in the flowelectrode, although the resistance over the low-frequency region did not appear to be significantly different as compared to that at greater than 2.44 wt % NaCl, the NaCl concentration in the flow-electrodes was affected by the amounts of ions that can be adsorbed on the intrapores, as demonstrated by the results obtained for capacitance. Based on these results, according to the increase in the electrolyte concentration at values greater than 2.44 wt %, the migration of ions in the intrapores seemed to be significantly favorable. Based on Figures 4, the desalting behaviors of the flowelectrodes with or without NaCl salt in the FCDI unit cell are described in Figure 5. When desalting experiments were

Figure 5. Desalting behavior of flow-electrodes with or without NaCl salt in the FCDI unit cell.

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conducted under an electrical field using flow-electrodes without NaCl salt, ions of salt water, which were passed through ion-exchange membranes, moved long distance toward AC through the electrolyte (deionized water) of the flowelectrode. Then, ions were adsorbed on the surface of charged AC. Thus, electrolyte (deionized water), which does not have salt in the flow-electrode, acts as a resistor in the FCDI unit cell. On the other hand, ions that transferred through the ionexchange membrane were not primarily adsorbed on the surface of charged AC when flow-electrodes with NaCl salt

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b04640. SEM image of spherical AC (Figure S1); variation of NaCl concentration in the effluents (Figure S2); electrical current variations of the FCDI unit cell (Figure S3); accumulated charge−time graph calculated from the current−time graph (Figure S4); Nyquist plot, Bode F

DOI: 10.1021/acs.est.5b04640 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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plot, and capacitance of aqueous electrolytes without spherical activated carbon (Figure S5) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +82 42 860 3152; fax: +82 42 860 3133; e-mail: [email protected]. Author Contributions ∥

S.Y. and J.C. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was conducted by the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER) (B6-2457).

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DOI: 10.1021/acs.est.5b04640 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.est.5b04640 Environ. Sci. Technol. XXXX, XXX, XXX−XXX