Carbon electrodes with ionic functional groups for enhanced capacitive deionization performance

Journal Pre-proof Carbon electrodes with ionic functional groups for enhanced capacitive deionization performance Oneeb ul Haq, Da-Seul Choi, Jae-Hwan Choi, Youn-Sik Lee

PII:

S1226-086X(19)30611-2

DOI:

https://doi.org/10.1016/j.jiec.2019.11.021

Reference:

JIEC 4863

To appear in:

Journal of Industrial and Engineering Chemistry

Received Date:

25 April 2019

Revised Date:

2 October 2019

Accepted Date:

11 November 2019

Please cite this article as: Haq Ou, Choi D-Seul, Choi J-Hwan, Lee Y-Sik, Carbon electrodes with ionic functional groups for enhanced capacitive deionization performance, Journal of Industrial and Engineering Chemistry (2019), doi: https://doi.org/10.1016/j.jiec.2019.11.021

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Carbon electrodes with ionic functional groups for enhanced capacitive deionization performance

Oneeb ul Haq a, Da-Seul Choi a, Jae-Hwan Choi b, *, Youn-Sik Lee a, *

Division of Chemical Engineering, Chonbuk National University, 567 Baekje-Daero, Deokjin-

gu, Jeonju, Jeonbuk 561-756, Republic of Korea b

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a

Department of Chemical Engineering, Kongju National University, 1223-24, Cheonan-daero,

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Seobuk-gu, Cheonan, Chungnam, 31080, Republic of Korea

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Graphical abstract

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*Corresponding authors: [email protected] (J.-H. Choi), [email protected] (Y.-S. Lee).

Operation of the AC-CDI cell assembled using the A-AC and S-AC electrodes where salts are adsorbed when potential is applied (charged step) and are desorbed at short circuit (discharged step).

Abstract

Capacitive deionization (CDI) has proved to be a clean and green technology; however, the charge leakage in the electrode pores during the adsorption-desorption cycles lowers the desalination efficiency. In this work, commercially available activated carbon was chemically treated to give the activated carbon surface either immobilized amino groups or sulfonic acid groups. The amino group-containing and sulfonic group-containing carbon materials were used as anode and cathode electrodes, respectively, in assembling a CDI (AC-CDI) cell. The salt

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adsorption capacity, salt removal efficiency, and charge efficiency of the AC-CDI cell were over two times higher than those of the untreated carbon-based CDI cell and comparable with those of the membrane CDI (MCDI) cell. The greatly improved desalination performance of the AC-CDI cell was attributed to enhanced wettability, faster diffusion of counter ions to each

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electrode via electrostatic attraction, and the prevention of re-adsorption of ions via repulsion

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between the charged layers and co-ions during adsorption-desorption cycle. Thus, it can be concluded that chemically modified carbon-based CDI cell is highly promising for practical

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application.

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Keywords: capacitive deionization, ion exchange, functionalized activated carbon, salt

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removal.

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1. Introduction

Although earth contains 332.5 million cubic miles of water from which 96 % is saline

and only about 4% is freshwater which directly used for drinking, household, agriculture, and industry. With a world rising population and industrialization, water scarcity and quality issues become unavoidable universal challenge [1,2]. According to UN water scarcity is present on

every continent, approximately 1.2 billion people lack access to clean water [3]. That’s where the dream of water desalination turned to reality. Capacitive deionization (CDI) has emerged as an economical electro-deionization technology that can remove dissolved salts from ground water which is the 30% of the fresh water available on the planet and CDI consume less energy than its competitor like brackish reverse osmosis and electrodialysis [4,5]. CDI can be operated at little as 0.2 V which able to

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create enough of an electrical potential difference to effectively remove dissolved salts [6]. However, it has edged other desalination technologies due to the low-pressure operation, lower operational and maintenance costs, and no use of thermal energy, all of which can contribute to reducing secondary pollutants [7,8].

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In the CDI technology, counter-ions that are opposite to the electrode charge are

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adsorbed onto the electrode surface and co-ions are repelled. This indicate that ion adsorption and desorption occur at the same time in the pores of the carbon electrodes, greatly reducing

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electrosorption efficiency. However, the efficiency can be improved significantly by placing ion-exchange membranes (IEMs) between either anode or cathode and a spacer, which is called

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membrane CDI (MCDI). The IEMs increase counter-ion flux by allowing only counter-ions to permeate through them during adsorption step [9]. IEMs prevent oxidation and reduction

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reactions at cathode and anode and mitigate the faradic reactions that lead to progressive performance decay [10]. To reduce the electrical resistance, ultra-thin membranes that can be

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prepared via pore-filled polymerization [11] and IEM-coated electrodes have been introduced [12].

For CDI cells, various types of carbon materials have been employed: activated carbon

(AC) [13-15], carbon fiber (CF) [16-18], carbon aerogel [19,20], carbon cloth [21,22], carbon nanotube (CNT) [23-25], and graphene [26,27]. However, CNT or modified CNT [28, 29] and graphene have some limitations in application as electrodes of CDI cells due to insufficient

porosity and thus low surface area [30,31]. Carbon fibers may not be appropriate either as electrode material because they are highly expensive due to their high manufacturing cost. In contrast, AC is one of the best electrode materials for CDI cells due to its good electrical conductivity and high chemical stability as well as high specific surface area. AC is generally used for commercial CDI electrodes because it is cheap and easy to handle. Gao et al. prepared chemically modified carbon cloth materials and used them as

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electrodes in the assembling CDI cell [32]. In brief, amino group and carboxylic groups were introduced on woven fibers of carbon cloth electrode that enhanced the salt adsorption capacity (SAC) up to 1.6 times when used in pairs, from 9.6 mg/g to 15.4 mg/g. However, the application was limited to the small pH range because carboxyl groups are dissociated to

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carboxylate anions above pH 4.5. Furthermore, carbon cloth is expensive and can limit

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practical application.

Min et al. coated AC with TiO2 particles and functionalized the intermediate material

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using disodium 4,5-dihydroxy-1,3-benzenedisulfonate to give the AC sulfonic acid groups on its surface [33]. The chemically modified and pristine AC were used as cathode and anode,

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respectively, in assembling the CDI cell, and their cell exhibited SAC of 10 mg/g, which is 1.4 times higher than that of the pristine AC-based cell. The authors attributed the improved cell

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performance to the enhanced wettability and ion selectivity of the treated AC electrode. However, if amino group and sulfonic acid group-immobilized AC materials are used as anode

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and cathode, respectively, the resulting cell is expected to exhibit more improved performance. In this study, we chemically treated AC to give its surface either amino groups (−NH2)

or sulfonic acid groups (−SO3H), and the resulting modified ACs are denoted as A-AC and SAC, respectively. The chemical modifications were confirmed by Fourier-transform infrared (FT-IR), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and elemental analysis. We further characterized the A-AC and S-AC with respect to ion-exchange

capacity (IEC) and electrochemical properties. Finally, we used the A-AC and S-AC as anode and cathode materials, respectively, in assembling AC-CDI cells as shown in Fig. 1. We compared the performance of the modified AC-CDI cell with that of commercial ion-exchange membranes-based MCDI cell as well as the pristine AC-based CDI cell.

2. Experimental

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2.1 Materials Activated carbon (AC, CEP-21K) was purchased from Power Carbon Technology Co, Gumi, Korea. Ethylenediamine (anhydrous 99%) was purchased from Junsei Chemical Co., Tokyo, Japan. Nitric acid (60%), hydrochloric acid (35-37%), sulfanilic acid (99%), and sodium nitrite

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(98.5%) were obtained from Samchun Pure Chemical Co., Gangnam-gu, Korea. Graphite

Co., Hwasung, Korea.

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2.2 Surface functionalization of AC

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sheets (F02511, thickness: 300 μm; current collector) were acquired from Dongbang Carbon

Briefly, AC (10 g) was treated with concentrated nitric acid at room temperature for 24 h,

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separated, and washed using a large amount of deionized water, followed by drying at 180 ℃ in air for 48 h. The resulting material (N-AC) was dispersed in 300 mL of nitrogen-purged

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ethylenediamine heated to ~ 120 ℃ until the dispersion dried. The treated AC was washed with DI water and followed by drying at 100 ℃ in a nitrogen atmosphere; the prepared sample was

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designated as A-AC. A p-sulfonylphenyldiazonium salt solution was prepared by adding sulfanilic acid (0.5 g) with sodium nitrite (0.2 g) in water (100 mL) and 1 M HCl (5 mL) solution in a cold bath. The N-AC was dispersed in the salt solution (50 mL) and kept in an cold bath for 3 h with stirring. The resulting AC was separated by centrifugation, washed and followed by air drying, and designated as S-AC. 2.3 Electrode fabrication

We mixed the pristine or surface-functionalized AC powder with a polyurethane dispersion binder prepared in our laboratory (12%) and N-methyl-2-pyrrolidone (NMP) in a paste mixer. We used the obtained slurry to fabricate the sheet-like electrodes on the graphite sheets with thickness of 420 um by screen printing and dried them for 4 h in an oven at 100 oC. Finally, we cut the electrodes into circular geometry with diameter of 9.5cm for CDI cells. 2.4 Characterizations of surface-treated ACs and carbon electrodes

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To confirm the attachment of functional groups at the surface of treated carbon samples FT-IR spectroscopy (FTIR spectrometer 4100E, JASCO Deutschland GmbH, Pfungstadt, Germany) was ran at similar conditions over the wavenumber range 400-4000 cm−1, similarly TGA (SDT-

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Q600, TA Instruments, Linden UT, USA) was also ran at a 10 ℃ min-1 of heating rate in the range 25–800 ℃ in the presences of nitrogen. We also confirmed the incorporation of functional

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group elements with elemental analysis (IT/Flash 2000, Thermo Fisher Scientific, Waltham,

2.4.1 Ion-exchange capacity

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MA, USA).

We determined the IEC values of the A-AC and S-AC using the standard titration method [11].

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The A-AC and S-AC were immersed in 0.05 M NaOH and HCl solution, respectively, sonicated for half an hour, and left for 24 h at room temperature for complete substitution of

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ions. Later, carbon materials were filtered out from the solutions, we titrated the residual

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solutions against a 0.05 M aqueous standard solution of HCl or NaOH with phenolphthalein as an indicator. We calculated IEC with the following equation: IEC (𝑚𝑒𝑞/𝑔) =

(𝐶𝑇 × 𝑉𝑇 ) 𝑊𝑑𝑟𝑦

where, 𝐶𝑇 is the concentration of the titrant solution, 𝑉𝑡 is the volume of the titrant solution, and 𝑊𝑑𝑟𝑦 is the dry mass of the carbon sample. The IECs we report are the averages of three samples. 2.4.2 Electrochemical measurements To estimate the electrochemical properties of the electrode materials, we used a three-electrode cell with a 1 M NaCl solution. We performed cyclic voltammetry (CV) with scan rates (v) from

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5 to 100 mV/s within the defined potential window of -0.2 V to 0.5 V to avoid redox reactions. We calculated the specific capacitance (Ccv) of the electrode (F/g) using the following equation where I is the current (A), 𝑣 is the scan rate, m is the mass of the working electrode (g), and

𝐸2

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E1 and E2 are the low and high ends of the potential window [34]

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∫𝐸1 𝐼𝑑𝑣 𝐶cv = 2𝑣𝑚(𝐸 2−𝐸1)

We evaluated the point of zero charge (pHPZC) by dispersing 0.01 g of the samples (AC, A-AC,

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and S-AC) in an NaCl solution at various pH for 24 h; to adjust the pH, we mixed X g of 1 M NaCl solution with Y g of either an HCl or an NaOH solution. We measured zeta potential with

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a Zetasizer ZS90 (Malvern Panalytical Ltd, Malvern, UK). We measured each sample three times and plotted the averages against the equilibrium pH; we then estimated the pHPZC values

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with those plots.

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2.5 Desalination test of CDI unit cells We assembled the AC-CDI cell using the A-AC and S-AC electrodes as an anode and cathode, respectively, and we assembled two other CDI cells for comparison: one assembled with the pristine AC materials (CDI) and another one with a commercial anion- and cation-exchange membrane along with the pristine AC materials (MCDI).

We performed desalination experiments using the AC-CDI cell as shown in Fig. 1. The cell was composed of electrodes, i.e. a cathode and an anode backed with current collector and a spacer in between; the spacer used as water flow passage and prevents electrical short circuit too. The effective area of the carbon electrodes was 70 cm2. A flow channel was created by punching a hole with a diameter of 11 mm in the center of top electrode to allow the feed solution to run through the spacer and immerse the carbon electrodes surface; Plexiglas plates

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were used to assemble the unit cell which based on lower and upper part. The desalination experiments were performed in a flow-through system consisting of a feed solution tank, a peristaltic pump, a CDI cell, and a conductivity meter. An NaCl salt solution having a feed concentration of 8.55 mM passed through the CDI cell at a controlled flow rate of 20 mL/min

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by using a peristaltic pump. Each desalination cycle consists of two steps: adsorption and

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desorption. During the adsorption step, the constant cell potential of 1.0 V was applied for 300 s. During the desorption step, a constant potential of 0.0 V was applied for 300 s through a

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potentiostat (WPG100, WonATech Co., Seoul, Korea). The change in conductivity at the outflow of solution was measured at interval of 2 s via conductivity meter (Labquest, Vernier

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Software & Technology, Beaverton, OR, USA) installed at the exit of the CDI cell. Each desalination experiment was performed for three adsorption-desorption cycles which is done

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to stabilize the electrodes so that CDI unit cell could reached to a dynamic steady state and to

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calculate the mean value of salt removal performance. 3. Results and discussion 3.1 Synthesis: Surface and chemical properties

The pristine AC was oxidized by nitric acid to generate carboxyl and other oxygen-containing groups on the AC surface. The resulting material (N-AC) was treated with either ethylenediamine or p-sulfonylphenyldiazonium salt to yield material that was rich in either –

NH2 or –SO3H groups, respectively. The characteristic absorption peaks in their FT-IR spectra were similar to those reported for carbon nanotubes and graphene modified under similar conditions [31,32]. For example, the FT-IR spectrum of N-AC shows a strong peak near 3500 cm-1 that was absent in that of AC due to the existence of amino and hydroxyl groups (Fig. 3). Furthermore, it shows new peaks at 1730 and 1150 cm-1, owing to C=O and C-O stretching vibrations, respectively, indicating that carboxylic groups (-COOH) are attached to its surface.

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The A-AC spectrum shows peaks in the range of 1660-1580 cm-1, presenting overlapped traces for C=O stretching, NH2 scissoring, and C=C stretching. In contrast, the S-AC spectrum shows new peaks at 1300-900 cm-1 and 650-450 cm-1, corresponding to S=O and S-O moieties of – SO3H groups. Based on the FT-IR spectra, the chemical structures of A-AC and S-AC are

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schematically proposed in Fig. 4, where either amino or sulfonic acid groups are rich, as

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reported in the literature [35,36].

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We conducted an XPS experiment to characterize the surface functional groups of the samples, and the obtained N1s and S2p spectra are presented in Fig. 5. The N1s core-level spectrum of

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A-AC can be deconvoluted into two peaks at 397.6 and 399.3 eV, which are attributed to C=N and C-N bonds, respectively [37]. The presence of sulfur atoms in the S-AC is verified by the

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S2p peak appearing at 168.9 eV in the deconvolution spectrum [38,39]. It was reported that by introducing oxygen in to the carbon matrix surface is beneficial for the electrosorption which

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is mainly due to the wettability enhancement [40] and for the electrode’s potential of zerocharge (Epzc) dislocation [41]. The fitted O1s spectra of A-AC and S-AC show surface oxygen peaks owing to carboxyl, phenolic and carbonyl groups at 533±0.3, 532±0.3, and 531±0.3 eV, respectively. By analyzing the shift at Epzc it has been proposed that carbonyl groups are inert electrochemically [42]. Whereas, carboxyl and phenolic groups are polar in nature, thus they are responsible for the electrode’s hydrophilic behavior [43].

The performed TGA results are shown in Fig. 6. The weight losses below 150 ℃ observed in N-AC, A-AC, and S-AC (but not in AC) are due to the evaporation of water molecules, even though the samples were dried under identical conditions, indicating that the chemically modified AC materials are much more hydrophilic than the pristine AC. The thermal degradations above 150 ℃ corresponded to the decomposition of the carboxyl, amino, and

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sulfonyl groups depending on the specific sample. The residual weights of all the chemically treated AC samples are similar in the range of 72-74 wt%, indicating that organic contents of the modified samples are similar to each other (28-26%) even though any further quantitative

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analysis of the functional groups may be difficult.

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Fig. 7 shows FE-SEM images and element mappings for N-AC, A-AC, and S-AC; carboxyl groups are found on the AC surface. In the mapping of A-AC, amino groups are observed well

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spread at the molecular level on the surface of A-AC; similarly, sulfonyl groups are highly concentrated and well dispersed on the S-AC surface. These observations support that the N-

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AC was successfully modified such that the A-AC and S-AC sample surfaces had immobilized

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amino and sulfonyl groups, respectively.

We performed elemental analysis to analyze the immobilized functional groups at the surface

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of the chemically modified samples, and the results are shown in Table 1. The content of nitrogen in N-AC is 0.57% but increased to 10.7% for A-AC, indicating that the amination procedure greatly introduced nitrogen atom on the AC surface; similarly, the sulfur content increased greatly (from 0% to 6.5%) with the sulfonation procedure. Even though it was not possible to identify exactly which functional groups were present on the material surfaces, this

result suggests that the chemically modified samples may have much higher –NH2 or –SO3H group content than the pristine AC material.

3.2 Ion-exchange capacity and zero point charge We measured the IEC of the modified carbon materials using the titration method described in the experimental section, and the results are shown in Table 1. The IECs of AC and N-AC were

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below the measurement limit, but those of A-AC and S-AC were 11.3 and 13.4 meq/g, respectively. Considering that reported IECs of most commercial ion-exchange polymer membranes are 1.4-2.8 meq/g, these values seem to be very high even though they cannot be compared with any known materials because IECs of chemically treated AC materials have not

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been reported in the literature. The high IEC values may result from efficient chemical

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conversion reactions and highly porous nature of the AC itself.

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Figure 8. Zeta potential of AC, N-AC, A-AC, and S-AC.

We measured the zeta potentials of the samples in aqueous solutions with different pH values,

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and the results are shown in Fig. 8. The pHpzc of AC is approximately pH 4, indicating that the AC was positively charged below pH 4 but negatively charged above 4. The pHpzc of A-SC

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was ~ 9.5, indicating it remained positively charged until pH 9.5 due to amino groups on its surface. As expectedly, the S-AC remained negatively charged in the whole pH range from 1

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to 12, due to the sulfonic acid groups that were completely ionized in the pH range. In contrast, however, it was reported that the pHpzc values of oxidized and aminated carbon cloth were 5.5 and 9.8, respectively [44]. For the oxidized carbon cloth, the pHpzc of 5.5 was attributed to the immobilized carboxyl groups. As expected, the pHpzc of A-AC was close to that of the aminated carbon cloth because the immobilized amino groups were responsible for the basicity of both A-AC and the aminated carbon cloth. The pH values of most seawater and drinking water is

between 6 and 8. Thus, the measured zeta potential values indicate that the A-AC and S-AC are positively and negatively charged in this pH range, and they can adsorb both negatively (e.g., Cl- ions) and positively (e.g., Na+ ions) charged species via electrostatic attraction. 3.3 Electrochemical properties We performed the CV experiment at 5 mV/s scan rate of for the AC, A-AC, and S-AC electrodes, and these findings are shown in Fig. 9 All the data indicate that the capacitive

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processes of adsorption and desorption of ions were highly reversible. In addition, the rectangular shapes imply excellent electrochemical double-layer capacitance behaviors [33]. The current densities of A-AC and S-AC were larger than that for the pristine AC, indicating that they were highly positively or negatively charged due to the presence of ionized functional

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groups (-NH3+, -SO3-). We also carried out the CV experiments at different scan rates so that

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the effect of mass transfer can be detect for every electrode. As it was predicted, by increasing the scan rate, the capacitance could be decreased it occurred due to the limited diffusion in the

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ion transfer to the electric double layer [45]. The drop in S-AC capacitance was less noticeable as compared with the drop observed in the capacitance of A-AC. Besides the resistance in mass

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transfer there could be an additional reason for this output, that could be the higher conductivity in S-AC electrodes, which has a through effect on the electric field generated in the polarized

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electrode. Hence, as the electrode resistivity get higher the electric field get lesser and therefore, the capacitance get decreases on the higher scan rate. Therefore, A-AC was the most resistive

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electrode, showing the maximum capacitance drop. 3.4 Surface wettability To find the degree of hydrophilicity of material surfaces, contact angle examination is commonly used. In this work, however, water diffused into the surfaces of AC, A-AC, and SAC slowly and ultimately completely adsorbed by the material. Therefore, we measured in the dynamic state to observe the rate of change of the water drop angle [31]; we monitored the

changes in the contact angle over time, and the results are shown in Fig. 10; we detected appreciable differences and change trends for different samples. For instance, the pristine AC showed an initial contact angle of 75, which is higher than those of A-AC (70) and S-AC (58), suggesting improved wettability by the amination and sulfonation. After that, the contact angle declined slowly to zero, which resulted from the interactions between the water molecules and the positively or negatively charged groups (–NH3+, –SO3−). The contact angle

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reduction rates of A-AC and S-AC were significantly higher than that of AC, indicating their increased wettability; the times required to reach zero contact angle in the A-AC (4 min) and S-AC (3 min) electrodes were much shorter than that for the pristine AC (18 min). The images in Fig. 10 suggest that the water droplets fully diffused into A-AC and S-AC within 4 min but

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only scarcely diffused into the pristine AC.

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3.5 Desalination performance of surface treated carbon electrode

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We assembled a CDI cell using the A-AC and S-AC electrodes, which we called an AC-CDI cell; for comparison, we also assembled CDI and MCDI cells using the pristine AC and

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commercial anion- and cation-exchange membranes, respectively, as described in section 2.4. We conducted the desalination experiments by supplying 8.55 mM NaCl solution at the flow

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rate of 20 mL min-1. Fig. 11 displays the changes of the NaCl concentrations at outflow and current that passed through the three different cells at 1.0 V adsorption and 0.0 V desorption

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potential. In the CDI cell, the initial concentration of the effluent (8.55 mM) decreased to 3.8 mM within 24 s, but in the AC-CDI cell, it decreased to 1.8 mM within 21 s. The MCDI cell, in contrast, showed a slightly more rapid decrease of the initial effluent NaCl concentration than the AC-CDI cell. For the CDI cell, when an adsorption potential was applied, the initial currents of about 0.6 A decreased rapidly to about 0.2 A within 30 s and continued to decrease gradually during

the adsorption process. Because the CDI cell did not use ion-exchange membranes, the resistance of the CDI cell was lower than that of the MCDI cell; as a result, more current was supplied to the CDI cell than the MCDI cell at the beginning of the adsorption process. However, the concentration of effluent in the CDI cell was higher than that in the MCDI cell, indicating the decrease in the adsorption amount. When an electric potential was applied to the CDI cell, co-ions inside the carbon electrodes migrated to the feed stream, resulting in lower

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charge efficiency [5]. In contrast, the effluent concentration and the current change for the ACCDI cell were similar to those for the MCDI cell; the electric current results show that the electrical resistances of the S-AC and A-AC electrodes increased with the introduction of the functional groups on the surfaces of the carbon particles. In addition, we observed that the

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carbon electrode that had ionic functional groups functioned well as ion-exchange membranes

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in the MCDI cell.

At the end of the adsorption process (300 s), the effluent concentrations for all the cells

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converged to the influent concentration, which indicated that the adsorption of ions at the carbon electrodes had reached saturation. The currents in the MCDI and AC-CDI cell reduced

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to almost zero. In the CDI cell, however, the current of approximately 0.08 A was continuously supplied until the adsorption was completed. The current in the CDI cell shows that faradaic

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reactions occurred at the carbon electrodes. Coulombic efficiency is the ratio of output charge

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during discharging a cell over the input charge, and a value below unity indicates faradaic reactions occurring in a cell [4]. For the MCDI and AC-CDI cells, the total charges supplied to the cell during adsorption and desorption were similar, indicating that no electrode reactions occurred in those cells. In the case of the CDI cell, however, the total charge with adsorption was much larger than that with desorption due to the faradic current during the adsorption. Tang et al. [47] reported that electrode reactions can easily occur in a CDI cell but that ionexchange membranes can suppress the electrode reactions in a MCDI cell. Comparing the

currents in the MCDI cell and in the AC-CDI cell, it is noteworthy that the electrode reactions could be suppressed without using an ion-exchange membrane. We observed that the carbon electrode doped with ionic functional groups on the carbon particles effectively suppressed the electrode reactions. To evaluate and compare the performance of desalination for CDI cells assembled at different combination of electrodes SAC is used as the main signifying factor, the equation is given

SAC (mg/g) =

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below, (𝐶𝑜 − 𝐶𝑎𝑣𝑔 ) × 𝑉 𝑚

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Where, 𝐶𝑜 is the feed concentration (mg/L), 𝐶𝑎𝑣𝑔 is the average effluent concentration (mg/L), V is the total volume of solution (L) that passed through the cell during the adsorption

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step, and m is the mass (g) of the carbon electrode pair.

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By using the stated equation, the salt removal efficiency (SRE) was calculated [11]: SRE(%) = (1 − 𝐶𝑎𝑣𝑔 /𝐶𝑜 ) × 100.

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The charge efficiency (CE) can be calculated as the ratio of ions adsorbed versus to the current

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passed through the electrodes during the adsorption step by using the following equation [46], (𝐶𝑜 − 𝐶𝑎𝑣𝑔 ) ∙ 𝑉 ∙ 𝐹 × 100 ∫ 𝐼𝑑𝑡

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CE (%) =

where I is the supplied current and F is Faraday's constant. The calculated SAC, SRE, and CE values are listed in Table 2. The CDI cell exhibited a SAC of only 7.0 mg/g, which is good agreement with the value reported (6.9 mg/g) by Min et al. [33]. The AC-CDI cell showed much higher adsorption-desorption capability than the CDI cell. As shown in the table, the SAC, SRE, and CE of the AC-CDI cell are over two times higher

than those of the conventional CDI cell and very close to those of the MCDI cell. The fact that AC-CDI cell performance compared favorably with that of MCDI cells is the first report in the literature as far as we know. This result indicates that the chemically immobilized amino and sulfonic acid groups on the AC surface had a significantly favorable effect on the performance of the desalination cell. For example, in the A-AC electrode, the surface became covered by –NH3+ groups in the aqueous solution and more hydrophilic than the AC electrode. Based on the very high IEC (11.3 meq/g),

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the electrode is densely covered with –NH3+ (anion-exchange) groups. The greatly increased hydrophilicity (wettability) and high density of charged groups of the A-AC electrode should be more accessible to Cl- ions than the pristine AC electrode. Furthermore, Cl- ions are

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electrostatically more strongly attracted to the A-AC electrode than to the pristine AC electrode,

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which is nearly electrostatically neutral; this means that Cl- ions may transport faster towards the A-AC electrode in the AC-CDI cell than in the conventional CDI cell. It should also be

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noted that the A-AC electrode electrostatically repelled nearby Na+ ions during the desorption process; that is, the surface-immobilized organic functional groups exhibited a similar role to

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that of an anion-exchange membrane in MCDI cells where Na+ ions cannot pass through the cation-exchange membrane from the water path (a spacer). The same phenomena also occur in

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the S-AC electrode, where Na+ ions are electrostatically more strongly attracted and more rapidly transported to the S-AC electrode than they are to the pristine AC electrode. In contrast,

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Cl- ions are electrostatically repelled from the S-AC electrode. Conversely, the greatly improved performance of the AC-CDI cell can be attributed to the enhanced wettability of the electrodes and the increased electrostatic interaction between the electrodes and salt ions in the solution, leading to a faster diffusion of ions to electrodes. As discussed in the introduction, Min et al. found that when only one of the two AC electrodes was replaced with the sulfonic acid-containing AC, the resulting CDI cell SAC was

1.4 times higher than the capacity in the untreated AC-based cell. However, in the AC-CDI cell, where both electrodes were functionalized such as the anode and cathode are positively and negatively charged exhibited even higher SAC (14.7 mg/g) than Min et al.’s cell (10 mg/g), as expected. Moreover, considering that MCDI cells are costly because commercial ionexchange membranes are expensive (approximately $100-200/m2), our chemically modified AC material-based CDI cell has very high potential for practical application because its

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performance is very close to that of MCDI cells.

4. Conclusion

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We successfully modified a commercial AC to incorporate amino (A-AC) or sulfonic acid (SAC) groups on its surface. We measured the IECs of A-AC and S-AC to be 11.3 and 13.4

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meq/g, respectively, indicating that the electrodes were densely covered with the ion-exchange functional groups and highly hydrophilic. Based on the zeta potential measurement, the A-AC

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electrode was positively charged up to pH 9, while the S-AC one is negatively charged over pH above 1; both specific capacitances of the modified electrodes were higher than that of the

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pristine AC. The SACs, SREs, and CEs of the AC-CDI cell were over two times higher than those of the CDI cell and very close to or comparable with those of the MCDI cell. This finding

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may be the first report that the performance of a CDI cell can compare with that of MCDI cells. The greatly improved performance of the chemically modified AC-based CDI cell was

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attributed to the immobilized organic functional groups that make the electrodes’ surfaces significantly more hydrophilic and act as ion-exchange layers. Because our CDI cell does not require expensive ion-exchange polymeric membranes and electrode materials, it is highly cost-effective and has very promising applicability for desalination.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement This study was by Basic Science Research Program through the National Research

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Foundation of Korea (NRF) funded by the Ministry of Education (Grant No. 17011000686).

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Figure 1. Operation of the AC-CDI cell assembled using the A-AC and S-AC electrodes where

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salts are adsorbed when potential is applied (charged step) and are desorbed at short circuit

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(discharged step).

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Figure 2. Schematic procedures for preparing A-AC and S-AC from a commercial AC.

Figure 3. FT-IR spectra of AC, N-AC, A-AC and S-AC: (left) 4000-400 cm-1 region and (right) 2000-400 cm-1 region.

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Figure. 4 Schematic chemical structures of the chemically treated AC materials: (left) A-AC

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and (right) S-AC.

Figure 5. Deconvoluted high-resolution XPS spectra: (top) A-AC and (bottom) S-AC.

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Figure 6. TGA curves of AC, N-AC, A-AC, and S-AC under nitrogen atmosphere.

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Figure 7. FE-SEM images and element mappings of AC, N-AC, A-AC, and S-AC.

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Figure 8. Zeta potential of AC, N-AC, A-AC, and S-AC.

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Figure 9. CV curves and specific capacitance of AC, A-AC, and S-AC.

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Figure 10. Dynamic contact angle changes over time.

Figure 11. Changes in (left) NaCl concentration in the effluents and (right) current passed in

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electrodes during the final cycles of the three different cells at potential of 1.0/0.0 V at the

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adsorption/desorption cycle.

Table 1. Elemental analysis and IEC data of AC, N-AC, A-AC, and S-AC. C%

H%

N%

S%

O%

meq/g

AC

95.2

0.24

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-

-

-

N-AC

77.2

0.57

0.57

-

15.0

-

A-AC

70.6

2.50

10.7

-

12.0

11.3

S-AC

63.0

2.30

0.70

6.50

24.0

13.8

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Sample

Table 2. SAC, SRE, and CE values for the CDI, MCDI, and AC-CDI cells at the cell potential of 1.0 V. SAC (mg/g)

SRE (%)

CE (%)

CDI

7.0

19.2

37.5

MCDI

16.4

42.4

89.2

AC-CDI

14.7

40.6

85.8

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Cells