applied sciences

Article Enhancing the Performance of Capacitive Deionization Using a Layered Double Hydroxide Coated Electrode

Jaehan Lee 1 , Seoni Kim 2, Nayeong Kim 2, Choonsoo Kim 3,* and Jeyong Yoon 2,4,*

1 Department of Biological and Chemical Engineering, College of Science and Technology, Hongik University, 2639 Sejong-ro, Sejong-si 30016, Korea; [email protected] 2 School of Chemical and Biological Engineering, College of Engineering, Institute of Chemical Process, Seoul National University (SNU), 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea; [email protected] (S.K.); [email protected] (N.K.) 3 Department of Environmental Engineering and Institute of Energy/Environment Convergence Technologies, Kongju National University, 1223-24, Cheonan-daero, Cheonan-si 31080, Korea 4 Korea Environment Institute, 370 Sicheong-daero, Sejong-si 30147, Korea * Correspondence: [email protected] (C.K.); [email protected] (J.Y.)

 Received: 25 November 2019; Accepted: 2 January 2020; Published: 5 January 2020 

Abstract: Capacitive deionization (CDI) is a promising desalination technology because of its simple, high energy efficient, and eco-friendly process. Among several factors that can affect the desalination capacitance of CDI, wettability of the electrode is considered one of the important parameters. However, various carbon materials commonly have a hydrophobic behavior that disturbs the ion transfer between the bulk solution and the surface of the electrode. In this study, we fabricated a layered double hydroxide (LDH) coated activated carbon electrode using an in-situ growth method to enhance the wettability of the surface of the carbon electrode. The well-oriented and porous LDH layer resulted in a better wettability of the activated carbon electrode, attributing to an enhanced capacitance compared with that of the uncoated activated carbon electrode. Furthermore, from the desalination tests of the CDI system, the LDH coated carbon electrode showed a higher salt adsorption capacity (13.9 mg/g) than the uncoated carbon electrode (11.7 mg/g). Thus, this enhanced desalination performance suggests that the improvement in the wettability of the carbon electrode by the LDH coating provides facile ion transfer between the electrode and electrolyte.

Keywords: capacitive deionization; layered double hydroxide; desalination; activated carbon; wettability

1. Introduction Capacitive deionization (CDI) is considered as a promising desalination technology because of its environmental benign and energy-efficient characteristics. In a typical CDI system, while the feed water passes through the porous carbon electrodes, ions in the feed water are captured on the electrodes when a potential is applied between the electrodes shown in Figure1. The water desalination process of CDI is based on the principle of the electrical double layer capacitor (EDLC), and ions in feed water are removed by the electrical adsorption onto the surface of the electrodes [1–11]. The conventional CDI system consists of a pair of porous carbon electrodes and open-meshed spacers for a flowing influent, and the desalination process in the CDI system is an interfacial process between the electrolyte and the electrode surfaces. Thus, the characteristics of a porous carbon electrode such as the surface area, pore size, electrical conductivity, chemical stability, and wettability are the major parameters to determine the desalination performance of the CDI system [5,6,12–15]. To develop

Appl. Sci. 2020, 10, 403; doi:10.3390/app10010403 www.mdpi.com/journal/applsci Appl.Appl. Sci. Sci.2020 2020, 10, 10, 403, x FOR PEER REVIEW 2 of2 10 of 9

are the major parameters to determine the desalination performance of the CDI system [5,6,12–15]. a CDI electrode with high performance, a porous carbon electrode should have a large specific surface To develop a CDI electrode with high performance, a porous carbon electrode should have a large area, optimal pore size distribution, high electrical conductivity, and good wettability to the electrolyte. specific surface area, optimal pore size distribution, high electrical conductivity, and good wettability Variousto the carbonelectrolyte. materials Various with carbon different materials surface with areas differen and poret surface size distributions areas and pore have size been distributions investigated, andhave among been them,investigated, activated and carbons among arethem, one activated of the most carbons suitable are one materials of the most for CDI suitable electrodes materials because for ofCDI their electrodes large surface because area of and their economic large surface merits. area However, and economic CDI electrodes merits. However, based on activatedCDI electrodes carbon materialsbased on exhibit activated a hydrophobic carbon materials behavior, exhibit disturbing a hydrophobic the accessibility behavior, disturbing of ions to the the accessibility pore structure. of Accordingly,ions to the various pore structure. surface modifications Accordingly, of various the activated surface carbon modifications electrodes of the have activated been reported carbon to overcomeelectrodes this have limitation been reported by treating to overcome with acidic this and limitation alkaline solutions, by treating attaching with acidic functional and alkaline groups, andsolutions, coating attaching the organic functional or inorganic groups, materials and coating [16–18 the]. organic or inorganic materials [16–18].

FigureFigure 1. 1.Principle Principle of of a a capacitive capacitive deionization deionization system. system. The The c capacitiveapacitive deionization deionization (CDI (CDI)) system system consistsconsists of of a apair pair ofof porous carbon carbon electrodes, electrodes, and and the the influent influent flows flows between between the two the electrodes. two electrodes. The Theions ions are are attracted attracted onto onto the thesurface surface of the of theelectrodes electrodes by applying by applying a potential. a potential.

LayeredLayered double double hydroxides hydroxides (LDHs),(LDHs), knownknown as anion exchangeable clay clayss containing containing transition transition metalmetal hydroxides, hydroxides, are are a a lamellar lamellar typetype ofof compound built built by by two two-dimensional‐dimensional positive positive charged charged layers. layers. II III x+ n TheThe typical typical LDH LDH structure structure is expressedis expressed with with the the chemical chemical formula formula [M [M1 xII1M−xMxIIIx(OH) (OH)22]]x+(A(An−−)x/n)x·yH/n yH2O,2O, II III − n · wherewhere M MandII and M MIIIrepresent represent thethe divalentdivalent andand trivalenttrivalent metal cations, and and A An− −meansmeans the the interlayer interlayer anionanion [19 [19–21–21].] LDH. LDH materials materials have have many many potential potential applications including including adso adsorbents,rbents, catalysts, catalysts, anticorrosive,anticorrosive, metal metal oxide oxide precursors, precursors, and and supercapacitors in aqueousin aqueous solutions solutions not not only only because because of theirof aniontheir exchange anion exchange capacity capacity and high and stability high stability but also but because also because of their of exceptional their exceptional hydrophilicity hydrophilicity [22,23 ]. Recently,[22,23]. usingRecently, the using LDH the precursor, LDH precursor, various substratesvarious substrates were successfully were successfully coated withcoated a well-orientedwith a well‐ porousoriented LDH porous structure, LDH attributingstructure, at totributing the enhanced to the durabilityenhanced asdurability well as theiras well wettability. as their wettability. [24,25]. [24,25Hence,]. the purpose of this paper is to improve the wettability of the porous carbon electrodes throughHence, LDH the coating purpose and of to this enhance paper the is desalinationto improve the performance. wettability of The the LDH porous crystals carbon on electrodes the surface ofthrough the activated LDH coating carbon wereand to aligned enhance vertically the desalinat usingion an performance. in-situ growth The method LDH crystals under aon hydrothermal the surface condition.of the activated The characteristics carbon were aligned of the LDH vertically coated usi activatedng an in‐situ carbon growth electrode method were under then a hydrothermal analyzed with thecondition. scanning The electron characteristics microscopy of the (SEM), LDH the coated cyclic activated voltammetry, carbon and electrode the contact were anglethen analyzed measurements. with Furthermore,the scanning ion adsorptionelectron microscopy/desorption (SEM),tests were the conducted cyclic voltammetry, to evaluate the and desalination the contact performance. angle measurements. Furthermore, ion adsorption/desorption tests were conducted to evaluate the 2.desalination Experimental performance. Section

2.1.2. FabricationExperimental of the Section Activated Carbon Electrodes The activated carbon electrode was prepared by mixing activated carbon with a polymer binder. 2.1. Fabrication of the Activated Carbon Electrodes The activated carbon powder (CEP-21, Power Carbon Technology Co., Gumi-Si, Korea) was mixed with carbonThe activated black (Super carbon P, Timcalelectrode Graphite was prepared and Carbon, by mixing Bironico, activated Switzerland) carbon with and asolution polymer dispersed binder. polytetrafluoroethyleneThe activated carbon powder (PTFE, (CEP Sigma‐21, Aldrich, Power Carbon USA) at Technology a mass ratio Co., of 86:7:7.Gumi‐Si, A sheetKorea) type was electrode mixed withwith a 300 carbonµm thickness black (Super was P,obtained Timcal by Graphite pressing and the Carbon, resulting Bironico, mixture usingSwitzerla a roll-pressnd) and machine. solution Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 10

dispersed polytetrafluoroethylene (PTFE, Sigma Aldrich, USA) at a mass ratio of 86:7:7. A sheet type electrode with a 300 μm thickness was obtained by pressing the resulting mixture using a roll‐press Appl. Sci. 2020, 10, 403 3 of 9 machine.

2.2.2.2. LDH LDH Coating Coating on on the the Activated Activated Carbon Carbon Electrode Electrode Surface Surface TheThe LDH LDH coating coating process process on on the the activated activated carbon carbon electrode electrode is is shown shown in in Figure Figure2. 2.The The LDH LDH precursorsprecursors were were prepared prepared by by a a method method similarsimilar toto thatthat showed in in the the previous previous study study [24] [24.]. Briefly, Briefly, 60 60mL mL of of aqueous aqueous solution solution containing containing Zn Zn (COOH) (COOH)2·2H2H2O (0.2O (0.2 M) M)and and Al(NO Al(NO3)3·9H) 2O9H (0.1O M) (0.1 were M) weremixed 2· 2 3 3· 2 mixedwith 240 with mL 240 of mL NaOH of NaOH solution solution (0.15 (0.15M) under M) under vigorous vigorous stirring stirring at room at roomtemperature temperature for 1 h. for The 1 h.resulting The resulting solution solution was then was rinsed then with rinsed deionized with deionized water using water a centrifuge using a centrifuge to remove to any remove remaining any remainingions from ions the fromLDHthe precursor LDH precursor and re‐dispersed and re-dispersed in 300 mL in 300of deionized mL of deionized water. water.For the For LDH the coating LDH coatingon the onsurface the surface of the ofactivated the activated carbon carbon electrode, electrode, the prepared the prepared activated activated carbon carbon electrode electrode was sealed was sealedon one on side one by side a Teflon by a Teflon plate plateand immersed and immersed vertically vertically in the in prepared the prepared LDH LDHprecursor precursor solution. solution. Then, it was hydrothermally treated at 70 °C for 18 h in an oven. Finally, the LDH coated electrode was Then, it was hydrothermally treated at 70 ◦C for 18 h in an oven. Finally, the LDH coated electrode was washed with deionized water several times and dried in vacuum oven at 60 °C for 12 h. washed with deionized water several times and dried in vacuum oven at 60 ◦C for 12 h.

Figure 2. Schematic representation of the fabrication process of the layered double hydroxides (LDH) coatedFigure activated 2. Schematic carbon representation electrode. of the fabrication process of the layered double hydroxides (LDH) coated activated carbon electrode. 2.3. Characterization of the Activated Carbon Electrodes 2.3.The Characterization morphology of and the wettability Activated Carbon of uncoated Electrodes (pristine) and LDH coated activated carbon electrodes were measuredThe morphology by a field and emission wettability scanning of uncoated electron microscope (pristine) and (FESEM, LDH JEOL coated JSM activated 6700 F, Japan) carbon andelectrodes a drop shapewere measured analyzer (DSA by a field 100, KRemissionUSS˝ GmbH, scanning Germany). electron microscope The cyclic voltammetry (FESEM, JEOL (CV) JSM tests 6700 wereF, Japan) conducted and a on drop a potentiostat shape analyzer/galvanostat (DSA 100, (PARSTAT KRŰSS 2273, GmbH, Princeton Germany). Applied The Research,cyclic voltammetry USA) to investigate(CV) tests the were electrochemical conducted on performance a potentiostat/galvanostat in a typical three-electrode (PARSTAT system. 2273, The Princeton uncoated Applied and LDHResearch, coated USA) activated to investigate carbon electrodes the electrochemical (d = 18 mm) performance were used asin thea typical working three/counter‐electrode electrodes, system. andThe a KCl uncoated saturated and Ag LDH/AgCl coated electrode activated was used carbon as a reference electrodes electrode. (d = 18 The mm) specific were capacitance used as of the theworking/counter uncoated and LDH electrodes, coated and electrodes a KCl saturated were calculated Ag/AgCl using electrode Equation was (1)used as as follows: a reference electrode. The specific capacitance of the uncoated and LDHR coated electrodes were calculated using Equation I dV (1) as follows: C = | | (1) 2v∆Vm ∫|| e = (1) where C is the specific capacitance (F/gelectrode); I is the2∆ current (A); v is the potential scan rate (V/s); ∆V is the potential window (V), and me is the mass of the electrode (g). where C is the specific capacitance (F/gelectrode); I is the current (A); v is the potential scan rate (V/s); ΔV is the potential window (V), and me is the mass of the electrode (g). Appl. Sci. 2020, 10, 403 4 of 9

Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 10 3. Desalination Performance Tests 3. Desalination Performance Tests The CDI system experiments were evaluated using a CDI unit cell that has been described in The CDI system experiments were evaluated using a CDI unit cell that has been described in Figure3 and previous papers [ 6,18,26], and the electrodes were soaked in 10 mM NaCl for 10 min. Figure 3 and previous papers [6,18,26], and the electrodes were soaked in 10 mM NaCl for 10 min. under a vacuum condition before the desalination tests. The uncoated and LDH coated activated under a vacuum condition before the desalination tests. The uncoated and LDH coated activated carbon electrodes (d = 5 mm) was used as the electrodes, and a nylon spacer (thickness = 0.2 mm) was carbon electrodes (d = 5 mm) was used as the electrodes, and a nylon spacer (thickness = 0.2 mm) was placed between the electrodes. A hole with a diameter of 4 mm was provided in the middle of the placed between the electrodes. A hole with a diameter of 4 mm was provided in the middle of the lower compartment electrode and the current collector so that the influent would flow between the lower compartment electrode and the current collector so that the influent would flow between the two electrodes. two electrodes.

Figure 3. Schematic diagram of the CDI unit cell. Figure 3. Schematic diagram of the CDI unit cell. The 10 mM NaCl aqueous solution was pumped at a flow rate of 10 mL/min. using a peristaltic pumpThe (MINIPLUS10 mM NaCl 3, aqueou Gilsons Inc.,solution Middleton, was pumped USA). at The a flow ion adsorptionrate of 10 mL/min./desorption using tests a peristaltic were carried pumpout by(MINIPLUS applying 3, a Gilson potential Inc., (charging: Middleton, 1.2 USA). V; discharging: The ion adsorption/desorption 0 V) for 10 min attests each were step carried using an out by applying a potential (charging: 1.2 V; discharging: 0 V) for 10 min at each step using an automatic battery cycler (WBC S3000, WonA Tech Corp., Seoul, Korea). The conductivity of the effluent automatic battery cycler (WBC S3000, WonA Tech Corp., Seoul, Korea). The conductivity of the was measured by a flow-type conductivity meter (F-54 BW, HORRIBA, Kyoto, Japan). The tests were effluent was measured by a flow‐type conductivity meter (F‐54 BW, HORRIBA, Kyoto, Japan). The held in a thermostatic chamber (KCL-2000, EYELA, Tokyo, Japan) to maintain the room temperature tests were held in a thermostatic chamber (KCL‐2000, EYELA, Tokyo, Japan) to maintain the room (298 K) during the operation. The accumulated salt adsorption capacity was calculated from the temperature (298 K) during the operation. The accumulated salt adsorption capacity was calculated conductivity changes during the charging step. The representative desalination performance results from the conductivity changes during the charging step. The representative desalination from triplicate experiments are presented in this study. performance results from triplicate experiments are presented in this study. 4. Results and Discussion 4. Results and Discussion Figure4 shows the surface images of the uncoated activated carbon ((a) and (b)) and LDH coated Figure 4 shows the surface images of the uncoated activated carbon ((a) and (b)) and LDH coated carbon electrode ((c) and (d)) using FESEM. As shown in Figure4c,d, the LDH structures were aligned carbon electrode ((c) and (d)) using FESEM. As shown in Figure 4c,d, the LDH structures were vertically on the surface of the activated carbon electrode, resulting in a porous LDH layer. The LDH aligned vertically on the surface of the activated carbon electrode, resulting in a porous LDH layer. coating layer was obtained under a hydrothermal reaction with an in situ growth method for better The LDH coating layer was obtained under a hydrothermal reaction with an in situ growth method LDH crystal porous structure without blocking the pores on the surface of the activated carbon powder. for better LDH crystal porous structure without blocking the pores on the surface of the activated carbonDuring powder. this coating During process, this thecoating LDH process, nanosheets the in LDH the LDH nanosheets precursor in werethe LDH attached precursor onto the were surface attachedof the substrate onto the surface and quickly of the formedsubstrate a thinand quickly LDH film. formed After a thin that, LDH larger film. LDH After particles that, larger began LDH to grow particlesand crystalize began onto grow top of and the primary crystalize LDH on top film of under the primary the hydrothermal LDH film condition under the gradually hydrothermal forming a conditionporous and gradually well-oriented forming LDH a porous layer. and It was well observed‐oriented that LDH the layer. one sideIt was of observed the activated that carbonthe one electrodeside ofsurface the activated was well carbon coated electrode with the surface LDH layer was withwell coateda depth with of approximately the LDH layer 50 withµm (verified a depth ofby the approximately 50 μm (verified by the cross‐sectional SEM images in Figure 4e,f. The detailed Appl. Sci. 2020, 10, 403 5 of 9

Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 10 cross-sectional SEM images in Figure4e,f. The detailed mechanism of the porous LDH layer formation mechanism of the porous LDH layer formation from the LDH precursor has been reported in from the LDH precursor has been reported in previous [24]. previous [24].

FigureFigure 4. 4. FieldField emission emission scanning scanning electron electron microscope microscope (FESEM (FESEM)) images images of that of activated that activated carbon (CEP carbon‐ a b c d 21)(CEP-21) sheet (a sheet,b), LDH ( , ),coated LDH activated coated activated carbon sheet carbon (c, sheetd), and ( , cross), and‐sectional cross-sectional view of viewthe LDH of the coated LDH coated activated carbon sheet (e,f). activated carbon sheet (e,f). Figure5a,b shows the contact angle images of the activated carbon and LDH coated activated Figure 5a,b shows the contact angle images of the activated carbon and LDH coated activated carbon electrodes using distilled water. As seen in the images of the water droplets, the contact carbon electrodes using distilled water. As seen in the images of the water droplets, the contact angles angles of the uncoated and LDH coated activated carbon electrodes were approximately 130 and 35 , of the uncoated and LDH coated activated carbon electrodes were approximately 130° and◦ 35°,◦ respectively. These results show that the LDH coated activated carbon electrode has a better wettability respectively. These results show that the LDH coated activated carbon electrode has a better than the uncoated one. Since the LDH materials have a hydrophilic characteristic because of their wettability than the uncoated one. Since the LDH materials have a hydrophilic characteristic because hydroxyl groups, the LDH coating layer results in an enhanced wettability of the activated carbon of their hydroxyl groups, the LDH coating layer results in an enhanced wettability of the activated electrode [19,27]. carbon electrode [19,27]. Appl. Sci. 2020, 10, 403 6 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 10

Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 10

Figure 5. 5. TThehe contact contact angle angle of of the the activated activated ca carbonrbon (a (a) )and and LDH LDH coated coated activated activated carbon carbon electrode electrode (b (b).).

Figure 66 showsshows thethe cyclic voltammetry (CV) curves for the unc uncoatedoated and and LDH LDH coated coated activated activated Figure 5. The contact angle of the activated carbon (a) and LDH coated activated carbon electrode (b). carbon electrodes electrodes with with a a potential potential range range of of 0 0–0.6–0.6 V V in in 1 1 M M NaCl NaCl solution. solution. Both Both CV CV curves curves are are nearly nearly rectangularFigure in in shape, shape, 6 shows showing showing the cyclic that that voltammetry the the ions ions are (CV) areadsorbed adsorbedcurves ontofor the onto the unc surface theoated surface and of both LDH of uncoated bothcoated uncoated activated and LDH and coatedLDHcarbon coated electrodes electrodes electrodes by with forming by a forming potential an electrical an range electrical of 0 double–0.6 double V in layer 1 M layer NaCl as as ideal solution. ideal capacitors capacitors Both CV [6,28curves [6,28]. ]. are The The nearly specific specific rectangular in shape, showing that the ions are adsorbed onto the surface of both uncoated and LDH capacitance of the LDH coated activated carboncarbon electrode calculatedcalculated byby EquationEquation (1)(1) waswas 124124 FF/g/g electrode, , coated electrodes by forming an electrical double layer as ideal capacitors [6,28]. The specificelectrode which shows a a slightly increased increased (6%) (6%) value value compared compared with with that that of of the the uncoated uncoated activated activated carbon carbon capacitance of the LDH coated activated carbon electrode calculated by Equation (1) was 124 F/gelectrode, electrode (116 F/gelectrode). These results indicate that the LDH coating layer improves the wettability electrodewhich (116 shows F/g electrodea slightly). Theseincreased results (6%) indicate value compared that the with LDH that coating of the layer uncoated improves activated the wettabilitycarbon of the electrode without disturbing the pore structure in the activated carbon, and this enhancement of theelectrode electrode (116 without F/gelectrode disturbing). These results the pore indicate structure that the in the LDH activated coating carbon,layer improves and this the enhancement wettability of of the wettability is attributed to the increase in the capacitance. the wettabilityof the electrode is attributed without disturbing to the increase the pore in thestructure capacitance. in the activated carbon, and this enhancement of the wettability is attributed to the increase in the capacitance.

Figure 6. The cyclic voltammetry (2 mV/s) of the activated carbon electrode and LDH coated activated FigureFigure 6. The 6. cyclic The cyclic voltammetry voltammetry (2 mV/s) (2 mV/s) of ofthe the activated activated carbon carbon electrode and and LDH LDH coated coated activated activated carbon electrode in 1M NaCl solution. carboncarbon electrode electrode in 1M in NaCl1M NaCl solution. solution. Appl. Sci. 2020, 10, 403 7 of 9

Furthermore, Figure7a,b shows the conductivity changes during the ion adsorption /desorption tests and the salt adsorption capacity curves during the third charging step. As seen in Figure7a, the conductivity rapidly decreased as an electrical potential (1.2 V) was applied during the charging step, while, it increased as the adsorbed ions were released during the discharging step. From the conductivity data, it was observed that the LDH coated activated carbon electrode had a better performance than that of the uncoated activated carbon electrode for overall cycles. Figure7b shows the accumulated salt adsorption capacity during the third cycle of the charging step. Note that the salt adsorption capacity was expressed as the accumulated mass of adsorbed ions per total mass of the electrodes. The calculated salt adsorption capacity of the LDH coated activated carbon electrode was 13.9 mg/gelectrode, which showed a better desalination capacity than the uncoated activated carbon electrode (11.7 mg/gelectrode). Also, the value was a comparable with that of the commercially available activated carbon materials as shown in Table1[ 29]. This enhanced salt adsorption capacity indicates the improvement of the wettability of the porous carbon electrodes because of a rapid ion transfer at the interface between the electrode and electrolyte [16,18]. In conclusion, the hydrophilic LDH layer on the activated carbon electrode increases the accessibility of ions into the pores resulting in an improved desalinationAppl. Sci. performance 2020, 10, x FOR PEER of the REVIEW CDI system. 8 of 10

Figure 7.Figure(a) The 7. (a) conductivity The conductivity changes changes of of the the effl effluentuent in in anan activated carbon carbon electrode electrode and and LDH LDH coated coated activatedactivated carbon carbon electrode electrode system system during during operations operations (1.2 (1.2 V V for for charging; charging; 0 V for 0 V discharging) for discharging) with 10 with 10 mL/minmL/min flow flow rate rate (source (source water: water: 10 10 mM mM NaCl). NaCl). (b)) Accumulated Accumulated salt salt adsorption adsorption capaci capacityty during during the the third cyclethird of cycle the of discharging the discharging step. step.

5. Conclusions In summary, the LDH coated activated carbon electrode was simply manufactured by an in‐situ growth method under a hydrothermal condition, and the LDH layer was delicately formed on the surface of the activated carbon powder without blocking the porous sites of the carbon electrodes. This LDH layer significantly enhances the wettability of the surface of the electrode and increases the specific capacitance of the activated carbon electrode. Moreover, the desalination test in the CDI system showed that the LDH coated electrode has a higher salt adsorption capacity (13.9 mg/gelectrode) than the uncoated activated carbon electrode (11.7 mg/gelectrode). The facile ion transport between the bulk solution and the surface of the electrodes may contribute to the improvement of the desalination performance of the CDI system. Appl. Sci. 2020, 10, 403 8 of 9

Table 1. Electrochemical and CDI performance reported for commercially available activated carbon materials for CDI.

Electrochemical Performance CDI Performance Activated Carbon Applied Cell NaCl Concentration Salt Adsorption Ref. Specific Capacitance (F/g) Voltage (V) (mg/L) Capacity (mg/g) S-51HF 44 1.2 ~58.4 4.1 6 SX PLUS 63 1.2 ~58.4 6.2 6 YS-2 81 1.2 ~58.4 7.0 6 MSP-20 124 1.2 ~58.4 13.6 6 CWZ-22 — 1.2 ~290 5.3 29 S-TE11 — 1.2 ~290 9.1 29 YP-50F — 1.25 ~2900 9.6 29 CEP-21 116 1.2 ~58.4 11.7 This work CEP-21 (LDH coated) 124 1.2 ~58.4 13.9 This work

5. Conclusions In summary, the LDH coated activated carbon electrode was simply manufactured by an in-situ growth method under a hydrothermal condition, and the LDH layer was delicately formed on the surface of the activated carbon powder without blocking the porous sites of the carbon electrodes. This LDH layer significantly enhances the wettability of the surface of the electrode and increases the specific capacitance of the activated carbon electrode. Moreover, the desalination test in the CDI system showed that the LDH coated electrode has a higher salt adsorption capacity (13.9 mg/gelectrode) than the uncoated activated carbon electrode (11.7 mg/gelectrode). The facile ion transport between the bulk solution and the surface of the electrodes may contribute to the improvement of the desalination performance of the CDI system.

Author Contributions: Conceptualization, J.L. and C.K.; Investigation, J.L. and S.K.; Supervision, C.K. and J.Y.; Visualization, N.K.; Writing—original draft, J.L.; Writing—review & editing, S.K., N.K. and C.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: This work was supported by the Hongik University new faculty research support fund. Conflicts of Interest: This research has no conflict of interest.

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