applied sciences Article Enhancing the Desalination Performance of Capacitive Deionization Using a Layered Double Hydroxide Coated Activated Carbon 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 variouspore structure. surface modificationsAccordingly, ofvarious the activated surface carbonmodifications 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 deionizationdeionization 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 havehave manymany potentialpotential applications including including adso adsorbents,rbents, catalysts, catalysts, anticorrosive,anticorrosive, metal metal oxide oxide precursors, precursors, and and supercapacitors 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