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Enhanced Electrochemical Stability of a Zwitterionicpolymer-functionalized Electrode for Capacitive Deionization Youngsuk Jung, Yooseong Yang, Taeyoon Kim, Hyun Suk Shin, Sunghoon Hong, Sungmin Cha, and Soonchul Kwon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14609 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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ACS Applied Materials & Interfaces

Enhanced Electrochemical Stability of a Zwitterionic-polymerfunctionalized Electrode for Capacitive Deionization

Youngsuk Jung,a Yooseong Yang,b,∗ Taeyoon Kim,c Hyun Suk Shin, c Sunghoon Hong,c Sungmin Cha,d Soonchul Kwonc,∗

a

Analytical Science Group, Samsung Advanced Institute of Technology, Suwon, Gyeonggi 16678, Korea b Energy Lab, Samsung Advanced Institute of Technology, Suwon, Gyeonggi 16678, Korea c Department of Civil and Environmental Engineering, Pusan National University, Busan 46241, Korea d Jeolla Namdo Environmental Industries Promotion Institute, Jeollanam-do, 527-881, Korea

Keywords: capacitive deionization, zwitterionic polymer, surface modification, minimum conductivity, density functional theory

∗

Corresponding author. Tel.: +82 3180618448; E-mail address: [email protected] (Y. Yang) Tel.: +82 515107640 ; E-mail address: [email protected] (S. Kwon)

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Abstract In capacitive deionization, the salt-adsorption capacity of the electrode is critical for efficient softening of brackish water. To improve the water-deionization capacity, the carbon electrode surface is modified with ion-exchange resins. Herein, we introduce the encapsulation of zwitterionic polymers over activated carbon to provide a resistant barrier that stabilizes the structure of electrode during electrochemical performance and enhances the capacitive deionization efficiency. Compared to conventional activated carbon, the surface-modified activated carbon exhibits significantly enhanced capacitive deionization, with a salt adsorption capacity of ~2.0×10-4 mg/mL and a minimum conductivity of ~43 µS/cm in the alkali-metal ions solution. Encapsulating the activated-carbon surface increased the number of ions adsorption sites and surface area of the electrode, which improved the charge separation and deionization efficiency. In addition, the coating layer suppresses side reactions between the electrode and electrolyte, thus provides stable cyclability. Our experimental findings suggest that the well-distributed coating layer leads to a synergistic effect on the enhanced electrochemical performance. In addition, density functional theory calculation reveals that a favorable binding affinity exists between the alkali-metal ion and zwitterionic polymer, which supports the preferable salt ions adsorption on the coating layer. The results provide useful information for designing more efficient capacitive-deionization electrodes that require high electrochemical stability.


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1. Introduction Water softening requires the removal of calcium, magnesium, and certain other metal cations in hard water. The resulting softened water is more compatible with soap and its sufficient supply is critical to extract better performance of consumer electronic products. Capacitive deionization (CDI) is an attractive ion-removal technology for the softened water supplies, which uses electrostatic forces to transfer ions to a charged electrode, with the ions then adsorbed onto the porous structure of the electrode.1-5 Water deionization processes, such as adsorption, ion exchange, and reverse osmosis, utilize differences in the physical or chemical potentials of the different ions.6-7 Among the various ion-removal methods, CDI using adsorption is an emerging technology for ion removal in water desalination and hard water softening without using membrane-based filters. In principle, an electric field is applied to induce the adsorption of ions on the surface of a porous electrode and subsequent elimination of the electric field is intended to the desorption of the adsorbed ions, leading to a regeneration of the electrode.8-14 CDI possesses various advantages compared with conventional desalination techniques,6-7 such as low water wastage, cost effectiveness, environmental friendliness, and retention of adequate minerals required for the human body. In addition, CDI does not require the use of any chemical products during operation.8, 14-18 In particular, the simple operating process and low energy consumption can lead to CDI systems that can be used on a commercial scale. The charge-separation efficiency of CDI strongly depends on the surface properties of the electrodes used, with beneficial properties including a large surface area, a large number of adsorption sites, and effective adsorption properties of the relevant ions, which is found in carbon based materials such as graphene,19-23 carbon nanotubes,5,21 activated carbon,14 and so on. Among the various materials, activated carbon is suitable for use in CDI electrodes owing


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to its large surface area, electrical conductivity, chemical stability, and favorable shape on adsorption, as well as the cost benefits.24-29 Porous activated carbons have been widely used as adsorbents for odors, colors, and organic impurities for a long time because their random porous structure with an enormous internal surface area.24-,25,27 However, their enhanced capacity and stability for water deionization over many cycles still need to be improved. Recently, significant efforts aimed at improving the electrical properties of CDI electrodes have drawn attention to develop electrodes that provide minimal chemical degradation within the voltage window applied for CDI. Activated carbon electrodes exhibits limited deionization efficiency over many cycles due to inhibition of their activity leading to low efficiency and cyclability fading. To ameliorate this issue, modifying the chemical properties of carbon surface of these electrodes, for example by applying surface coatings or doping with other foreign elements, is broadly considered to be a viable remedy to alleviate such problems. Activated carbon electrodes modified with a deposited ion-selective membrane,1,30 water soluble binder,31 and other materials have provided the enhanced deionization performance, which is attributed to an increased electrode conductivity. Highly compacted structures such as carbon aerogel sheets and carbon honeycombs are advantageous for achieving a larger contact area of ion binding.18,32 Activated carbon has been combined with other materials, including an ion-exchange resin,33 MnO2,34 titania,35 and graphene,36 to improve its capacitance and deionization performance. In particular, the addition of an ion-exchange resin to the activated carbon electrode increases the hydrophilicity of the electrode, exposing the ion-exchange sites and increasing the ionremoval efficiency. The application of an ion-exchanging charge barrier attracts the desired ions to the active surface by pushing the counter ions to the opposite pole.37-41 Previously, ion-exchange resins, such as polyvinyl alcohol (PVA), were applied to the electrode as a binder, which improved its deionized-water adsorption performance.38-39 Since activated


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carbon contains metal compounds such as Al2O3, standing the weak charge of the surface,38 the charge is not strong enough for the hard water softening. Therefore, the surface of the activated carbon can be modified with more polar organic functional groups, in particular polymers with both positive and negative ionic groups.37 The spontaneous coagulation of amphiphilic zwitterionic polymers on the surface of carbon particles, which contain pyridinelike nitrogen, carboxylate, and potassium oxides, can prevent the movement of ions and, meanwhile, produce strong dipoles.42-45 Consequently, the zwitterionic character should facilitate the separation and release of charges, thus providing pronounced CDI efficiency. Herein, we introduce the simultaneous encapsulation of activated carbon composites with zwitterionic polymers to stabilize these electrodes and promote electrochemical performance. The surface of activated carbon was coated with polymeric materials including zwitterionic groups in the form of phosphoric acid salts, carboxylic acid, amide, and amine groups (Figure 1). It should be noted that a PVA binder with succinic acids and anhydrides also exhibits charge separation. Therefore, the well-distributed coating layers on the activated carbon play an important role in inhibiting reactions between the electrode and electrolyte. The coating layer and its effect on the structural stability and surface chemistry of the electrode were verified using several analytical techniques. In addition, cycling performance tests were conducted to determine the improved deionization efficiency. In studies involving interaction energetics at the atomic level, quantum chemical density functional theory (DFT) calculations have been widely performed with high accuracy to achieve a fundamental understanding of intrinsic energetic properties such as the adsorption energy with an optimized geometry. In this study, we additionally investigated the mechanism of alkali-metal ions (e.g., Mg2+, Ca2+) adsorption on amorphous carbon served as activated carbon and zwitterionic polymer/amorphous carbon by analyzing the energyminimized geometry of the structure, electronic properties, and the adsorption energy of the


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atomistic units using DFT calculations. These calculations, in turn, allowed a comprehensive analysis of the favorable molecular interactions between alkali-metal ions and the coating layer, compared with the pure amorphous carbon surface, at the atomistic scale; this can help us understand the accompanying characteristics and the role of coating layer. Both experimental and theoretical studies suggested that the nature of well-distributed zwitterionic coating layer influences the attractive intermolecular interactions with alkali-metal ions to promote structurally stable electrodes during CDI.

2. Materials and Methods 2.1 Electrode fabrication The CDI electrodes were prepared using PGW activated carbon (named PGW in this study), supplied by Kuraray Chemical Co. Ltd. Binders for the electrode were prepared with succinic anhydride, succinic acid, and poly vinylalcohol (PVA), purchased form SigmaAldrich. The surface-modifying solution for binder wetting into the activated carbon, AntiTerra 250® (AT250), was purchased from BYK-Chemie. Conductive carbon black, Super-P®, was provided by Timcal Ltd.


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Figure 1. Schematic illustration of a capacitive deionization cell using activated carbon electrodes modified with a zwitterionic polymer. Saline water flows through the spacer and adsorption of the anions and cations on the electrode is induced by application of an electric field. Molecular structure denotes zwitterionic polymer (i.e., zwitterionic functional groups in AT250 polymers).

The electrode material was synthesized according to the following procedure. First, PGW and AT250 were vigorously mixed in water with adding a homogenizer to ensure the two components in full contact with one other. AT250 was added at ratios of 10 and 20 wt.% of the PGW. After drying the PGW and AT250 composite to form a fine powder, 2.49 g of the composite was added to a binder solution composed of 0.74 g of succinic anhydride, 2.89 g of succinic acid, and 0.3 PVA in 2.7 g of water. Super-P (0.45 g) was added as a conducting agent. The mixed slurry was spread on a graphite foil and heated up to 120 °C for 6 h, which produced a sheet type of electrode with a thickness around 250 µm (See supplementary Figure S1).

2.2 Fabrication of CDI cell Figure 1 shows a schematic illustration of the CDI single cell containing activated carbon electrode coating with the well-distributed zwitterionic polymer. The prepared


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electrodes were cut into square pieces of 100 × 100 mm2 with a hole in the middle of each electrode. There was also a 15 × 15 mm2 hole in the middle of the spacer, which was a 200 mesh customized nylon sheet of 100 µm thickness. The cathode, spacer, and anode were alternately stacked so as to allow the passage of hard water. 150 ×150 mm2 size of an acrylic plate was attached to the outside of both cathode and anode. The entire assembly was tightened with screws to configure the CDI single cell.46 A hard water solution, composed of 1 M CaCl2 and 1 M MgCl2, was passed through the CDI cell at a rate of 1 mL/s while constantly applying an electrode potential of 1.5 V.

2.3 Characterization Surface modification using the zwitterionic polymer (AT-250) was confirmed using various component analyses. The components in the zwitterionic polymer/activated carbon composite were analyzed using an Agilent GC-MS 6890/5973 system equipped with a pyrolyzer and UA-1 column. In addition, the atomic composition of the material surface was verified by X-ray photoelectron spectroscopy (XPS) measurements using a Quantum 2000 instrument. Bright-field images of the surface-modified electrode were obtained using a Hitachi 4700 SEM. The quantity of AT250 in the electrode was measured with a TA Instruments TGA Q5000IR instrument by heating up to 600 °C at a heating rate of 10 °C/min under a N2 atmosphere. The surface charge distribution was determined by zeta-potential measurements using a HORIBA SZ-100 instrument. The BET surface area was determined from N2 adsorption/desorption isotherm measurements at -196 °C using volumetric apparatus supplied by Bell SorpMax (Bell, Japan). The prepared samples were degassed at 200 °C for 24 h before the measurements. Pore size distributions were determined from the adsorption isotherms using the Barrett-Joyner-Halenda (BJH) model with cylindrical pore geometry, and


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the pore volumes were measured at P/P0 > 0.97. The concentration of dissolved calcium ions in the effluent softened water were measured using SCM-6500-Ca, supplied by Sechang Instruments, South Korea and the concentration of magnesium ions was assumed as equal as that of calcium ions.

2.4 Electrochemical analysis We performed CV measurement in acidic medium (0.5 M H2SO4) using a standard threeelectrode cell (RDE0018, EG&G) and a potentiostat/galvanostat (PGSTAT 100, Autolab, The Netherlands). Pt wire was served as a counter-electrode, and saturated Ag/AgCl, KCl with a Luggin capillary used as a reference electrode. We used the frequency response analyzer of the potentiostat/galvanostat to determine the resistances between a working electrode and the Luggin capillary. The ohmic drops in the solutions have been compensated. We carried out the resistance measurement of the CDI cell by electrochemical impedance spectroscopy (EIS) with BioLogic VMP3 potentiostat (Bio Logic Science Claix, France), used in a two terminal configuration without a reference electrode due to the symmetric electrodes in the cell. The frequency of the signal was modulated from 2 MHz to 10 mHz with a 10 mV sinusoidal potential perturbation amplitude. For electrochemical analysis, the cell was filled with 2M NaCl and was aged for 30 min before EIS measurements to allow the cell to equilibrate with the NaCl solution. The Warburg element in the transmission line model (TLM) was used to model the long tail in the low-frequency region, which is widely used to fit Nyquist plots in order to describe porous electrodes.

2.5 Density functional theory calculations


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In this study, we employed quantum mechanical density function theory (DFT) calculations which is a method for evaluating electronic structures and properties of the materials using Dmol3 in Materials Studio.47 DFT has been widely applied for studying condensed matter systems including surfaces. Among the various functionals available for DFT calculations, we used the generalized gradient approximation (GGA) method with the non-empirical local functional, Perdew-Burke-Ernzerhof correlation (PBE), which has been widely used in the fields of materials science as well as physics.48 Here we used all-electron Kohn–Sham wave functions and the double numerical basis set with polarization (DNP). For the calculations, three-dimensional models were designed under the periodic boundary condition. The vacuum separation was set to 20 Å along the z direction to avoid interactions beyond the periodic boundary condition. A self-consistent field (SCF) convergence was set to 1×10-5 Ha. After the initial atomic positions were assigned, the geometry was optimized to refine the model structure. Once the optimized unit structures were obtained, the binding energy and the distances of the ionic bonding were determined from the molecular interactions. The adsorption energy, Ead, between the zwitterionic polymer and the alkali-metal ions was determined by three single-point energy calculations: (i) geometry optimization of the alkali-metal ions (Mg2+/Ca2+), (ii) geometry optimization of the zwitterionic polymer without the alkali-metal ions, and (iii) geometry optimization of the zwitterionic polymer with the alkali-metal ions. The adsorption energy was determined using Equation 1:

∆ =

+ + 1 2 1 2

,

where ΔEad denotes the adsorption energy of Mg2+ or Ca2+ on the zwitterionic polymer surface; n1 is the number of Mg2+ atoms; n2 is the number of Ca2+ atoms; Ep+Mg+Ca is the energy of the integrated system; and Ep, EMg, and ECa are the energy of the individual


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molecules, bare zwitterionic polymer surface, Mg2+, and Ca2+ atoms, respectively. The adsorption energy provides an important energy profile for evaluating the adsorption properties of both alkali-metal ions.

3. Results and Discussion 3.1. Structure and morphology of surface-coated activated carbon Zwitterionic polymer-encapsulated activated carbon (AT250/PGW) structures were fabricated as described in the experimental section and characterized using several analytical techniques to elucidate their structure and morphology prior to preparing the CDI electrodes. Figure 2b presents a scanning electron micrograph of the AT250-20/PGW (20 wt.% of PGW to AT250 over PGW) electrode.

Figure 2. Scanning electron microscopy (SEM) images of (a) bare PGW activated carbon (PGW) and (b) zwitterionic polymer-coated activated carbon (AT250-20/PGW). The scale bar denotes 1 µm.

It reveals that the sharp and uneven surface and pores of pristine PGW (Figure 2a) were covered with the well-distributed soft material (Figure 2b), which would be part of the


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electrode. Surface modification of the activated carbon using the zwitterionic polymer was also verified by X-ray absorption spectroscopy (XPS) measurements, which determine the binding energy of electron by core-electron excitation to the bonding orbitals (See Figure S2 in the Supporting Information). XPS spectra show the core levels of the elements present and allow direct determination of the chemical states on the surface of the coating layer. After coating AT250 on the PGW surface (AT250-20/PGW), the C 1s XPS spectrum exhibited an intense peak at ~284.1 eV, attributed to C–C and C=C bonds, and a shoulder peak at ~285.7 eV, attributed to C–O and C–OH groups. In addition, the peak at 532 eV in the O 1s spectrum, attributed to C=O and COOH groups, was significantly more intense for AT250/PGW compared with pristine PGW. These results confirm the bond generation of oleic acid and various lactones to the PGW surface via both single and double bonds, which indicates the stable encapsulation of AT250 over the PGW. For quantitative analysis of the surface coating layer, we evaluated the loading weight of AT250 coated on the PGW surface using thermogravimetric analysis (TGA), as shown in Figure S3. The weight percentage of AT250 was 10 (AT250-10) and 20 wt.% (AT250-20) of activated carbon, which corresponds to ~7 and 9 wt.% weight loss, respectively, up to 600 °C. This indicates that some of zwitterionic polymer was aggregated on the activated carbon surface. Hereafter, we designated the two composite electrodes as AT250-10/PGW and AT250-20/PGW, which corresponds to adding zwitterionic polymer at 7 and 9 wt.%, respectively, of the activated carbon. Modifying the electrode surface, through either mechanical or chemical surface treatment, with ionic resins that contain a lot of ionic functional groups is a suitable way to enhance the CDI efficiency. The method used to functionalize the PGW surface with ionic groups was designed with ultrasonification to generate surface charging on the PGW and provide improved pigment wetting on the electrode surface (Figure S1).49


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Figure 3. Mass spectrum of AT250 obtained by pyrolysis gas chromatography: (a) powder and (b) dispersed solution. AT250 is a zwitterionic polymer containing phosphoric acid salts in long chains of carboxylic acid and polyamine amides. Fragmented organic functional groups are oleic acids, caprolactones, amides, etc.

We elucidated the surface functionality of AT250 on the PGW surface using pyrolysis gas chromatography (PGC) analysis to obtain an AT250 mass spectrum (Figure 3), which represents the chemical components of AT250 for the surface modification. Figure 3 shows the unsaturated carboxylic acid and polyamine amides for the amphiphilic groups in coating layer. The fragmented functional groups correspond to either the negative or positive charges of oleic acid, lactone, various amides, and other groups, which confirms that the amphiphilic ion resins were encapsulated on the PGW surface.

3.2. Surface stability of composite electrode in dispersion During the manufacturing process, a mixed solution of the composites with PVA binder (PGW-PVA and AT250/PGW-PVA) was prepared to obtain electrodes after solvent drying. To ensure the stability of the electrodes in dispersion, we measured the electro-kinetic potential of the prepared solution. When dispersed in solution, the amphiphilic charged


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surface of the composite electrode particles possesses an electrical double layer to neutralize the surface charge. This simultaneously forms one charged ion strongly bound to the core of the particle that develops a stern layer and another oppositely charged ion weakly bound to the outer diffusion layer of the particle. To elucidate the degree of electrostatic repulsion between the adjacent charged particles in dispersion, we measured the potential at the diffusion layer, called the zeta potential (ζ)50 that indicates the magnitude of stability and resistance to coagulation in dispersion. Table 1 and Figure S4 show that the zeta potentials of pristine PGW-PVA and AT250-20/PGW-PVA are –36.8 mV and –60.3 mV, respectively. The increased zeta potential for AT250-20/PGW-PVA clearly indicates that the PGW surface is coated with AT250 and more electrochemically stable than the pristine PGW-PVA because the zwitterionic polymer chains of AT250 are supposed to aggregate readily on the PGW surface and act as a charge barrier for PGW to keep the charge on the surface. Unfortunately, using higher than 20 wt.% of AT250 did not provide highly increased zeta potential further; thus, we focused on AT250/PGW-PVA composites using less than 20 wt.% of AT250 (AT25020/PGW-PVA). Table 1. Zeta potential of pristine PGW and AT250/PGW with PVA binder. Zeta-potential (mV) Pristine PGW-PVA

-36.8

AT250-10/PGW-PVA

-54.2

AT250-20/PGW-PVA

-60.3

The various analyses of the composite electrode confirmed that the PGW is functionalized with zwitterionic polymeric materials. We also expect that the surface modification of PGW will improve the efficiency of the CDI electrodes through an increase in the number of adsorption sites.51 This is in line with the changes in the BET surface area (Table 2), which is


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correlated to the number of available adsorption sites. The BET surface area monotonically increased with increasing amounts of AT250, which indicates that the zwitterionic polymers of AT250 effectively created adsorption sites on the PGW surface to bring ions adsorption with the role of a charge barrier. From analyses of the physical properties, we suggest that the AT250-20/PGW-PVA sample is the optimized electrode for CDI performance. Table 2. Physical properties of pristine PGW and AT250/PGW and their minimum conductivities. BET surface area (m2/g)

Pore vol (cc/g) @ P/Po 0.990

T-plot u-pore vol (cc/g)

29.9

0.025

0.016

AT250-10/PGW-PVA

37

0.034

0.022

AT250-20/PGW-PVA

46.8

0.113

0.038

AT250-30/PGW-PVA

50.2

0.067

0.042

PGW-PVA

3.3 Electrochemical characterization To unambiguously evaluate the electrochemical performance of PGW-PVA and AT250-20/PGW-PVA

for

CDI

applications,

we

initially

investigate

the

cyclic

voltammograms of the CVs using both electrodes as the working electrodes in acidic medium (0.5 M H2SO4) in the potential range of 0.7–1.5 V vs. NHE at a potential sweep rate of 100mV/s (Figure 4a).


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Figure 4. The electrochemical configuration of (a) cyclic voltammograms in acidic medium (0.5 M H2SO4) and (b) Comparison of Nyquist plots of impedance with 2M NaCl

During a couple of cycles, AT250-20/PGW-PVA electrodes showed a dramatically changed voltammogram due to the hydration of the coating surface. The shape of voltammogram became consistently stable afterward. Especially after 100 cycles, the voltammogram remained consistency, supporting the high electrochemical stability of AT250-20/PGW-PVA. The improvement of electrochemical performance for AT25020/PGW-PVA can be ascribed to readily releasing ions from the zwitterionic functional groups by minimizing travel time of ions, which plays a significant role in electric stability against aggregation to enhance the electrochemical activity for the adsorption of ions. Low interfacial contact resistance is also an important attribute for any electrode in a CDI cell, since sufficient conductivity is required to allow percolation pathways for minimizing the interfacial contact resistance. To evaluate the electrochemical characteristics of the AT25020/PGW-PVA electrode for electrical contact configuration, we performed electrochemical impedance spectroscopy (EIS) analysis for measuring the charge transfer resistance (Rct), as shown in Figure 4b. The plot shows two comparative EIS experiments of assembled cell performed with 100 mM NaCl solution to simulate CDI performance. In each Nyquist plots of the CDI cells using both PGW-PVA and AT250-20/PGW-PVA electrode, a semi-circle is observed along with a long frequency tail, corresponding to the predominant resistance of the interface between the current collector and the porous electrode. The interfacial impedance of AT250-20/PGW-PVA electrode (62.4 Ω), obtained from the semicircles in the highfrequency region, was significantly reduced compared to PGW-PVA electrode (74.5 Ω) because ion-exchange resins expose the percolation pathways, reducing the interfacial contact resistance. The affinitive adsorption of the ions on the surface causes a higher ion concentration, but the zwitterionic polymer matrix does not diminish the contact area while


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maintains the active sites for specific resistance. It is noteworthy that the zwitterionic polymers which functionalized activated carbon facilitates superior electric stability with minimal resistance contacts compared to the conventional activated carbon, which provides a continuous conduction pathway in the CDI system. The electrical configuration clearly suggests that the functionalized electrode with zwitterionic polymers affords improved electric stability to maintain the deionization efficiency.

3.4 DFT calculations Concomitant with the experimental results discussed in sections 3.1-3.3, we also attempted to investigate the adsorption characteristics of alkali-metal ions on the PGW electrodes to determine the effects of the zwitterionic polymer on improving adsorptive deionization, with a particular focus on the nature of the alkali-metal ion binding at the molecular level. To provide a comprehensive correlation analysis of the molecular interactions between the alkali-metal ions (Ca2+ and Mg2+) and the electrodes, we performed DFT calculations to investigate the structural arrangement and binding affinity between the alkali-metal ions and the two electrodes: bare amorphous carbon (Figure 5a), which represents PGW activated carbon, and zwitterionic polymer-functionalized activated carbon (Figure 5b), which represents AT250/PGW. It is worth noting that the favorable electronic attraction between PGW and AT250 is responsible for the facile and solid coating of AT250 on the PGW surface to develop AT250/PGW.


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Figure 5. Optimized geometry of (a) amorphous carbon as PGW activated carbon and (b) zwitterionic polymerfunctionalized amorphous carbon as AT250/PGW using DFT calculations.

Using these structure models, we carried out geometry optimization of the alkalimetal ions on the electrodes to investigate the adsorptive characteristics of Ca2+ and Mg2+ ions on bare amorphous carbon (PGW, Figure 6) and zwitterionic polymer-functionalized amorphous carbon (AT250/PGW, Figure 7). Overall, the optimized geometry of the two interfaces indicated a high adsorptive interaction, which shows that favorable binding takes place with a strong binding energy (Eb < –2.74 eV) in accordance with the strong and short interactive bond lengths between the alkali-metal ions and both electrodes. The result represents that both electrodes supply a high energetic affinity to alkali-metal ions. The Ca2+ ion is absorbed more favorably on both adsorbents surfaces (–5.42 eV for PGW and –6.33 eV for AT250/PGW) than the Mg2+ ion (–3.08 eV for PGW and –4.45 eV for AT250/PGW), owing to its high potential energy. In particular, AT250/PGW exhibited considerably stronger binding properties toward both alkali-metal ions, compared with PGW, which suggests that


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the alkali-metal ions interact more favorably with AT250/PGW. Simultaneous adsorption of the alkali-metal ions was also more energetically favorable on AT250/PGW (–2.74 eV for PGW and –4.13 eV for AT250/PGW). The bond lengths also showed the same pattern: 2.03~2.25 Å for the stronger bond between the alkali-metal ions and AT250/PGW and 2.15~2.80 Å for the bond between the alkali-metal ions and PGW. This confirms that the zwitterionic polymer layer leads to a reduction in the ionic bond length. The zwitterionicpolymer layer on AT250/PGW clings effectively to the alkali-metal ions, owing to the strong intermolecular interactions between the ions and the oxygen sites of the zwitterionic polymer.

Figure 6. Binding energies and bond lengths between the pristine PGW and alkali-metal ions. (a) Ca2+, (b) Mg2+, and (c) simultaneous Ca2+ and Mg2+ adsorption.

These strong chemical bonds are attributed to electron sharing between the ion atoms and the adsorbent surface. To determine the most favorable adsorption mechanism, we interpreted energy-level on the basis of molecular orbitals. Figures 6 and 7 show the energetics of the bands and a localization of HOMO (highest occupied molecular orbital)/LUMO (lowest unoccupied molecular orbital) in the ions-adsorbent systems, which represents the orbital energies and the size of HOMO-LUMO gap in the adsorbent with alkali-metal ions adsorbed. After alkali-metal-ion adsorption, both adsorbates exhibit


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differences in the molecular orbital energetics due to the binding interactions. For simultaneous Ca2+ and Mg2+ adsorption, the band-gap is smaller for the zwitterionic-polymercoated AT250/PGW (0.07 eV) compared with PGW (0.11 eV), which is a result of greater π− orbital overlap and increased π−electron delocalization. The small band gap does not require of much energy to excite the molecule, allowing alkali-metal ions to bond on AT250/PGW more favorably. The DFT calculation trends agree with the experimental results. In conclusion, the theoretical findings suggest that the alkali-metal ions are adsorbed strongly on AT250/PGW, with a small HOMO-LUMO bandgap and a short binding distance. This confirms that functionalization with a zwitterionic polymer enhances the CDI performance of an activated carbon electrode.

Figure 7. Binding energies and bond lengths between a zwitterionic polymer-coated activated carbon (AT250/PGW) and alkali-metal ions. (a) Ca2+, (b) Mg2+, and (c) simultaneous Ca2+ and Mg2+ adsorption.

3.5 Deionization performance In order to evaluate the electrochemical characteristics of the prepared electrodes, we designed a CDI cell architecture in which the inlet water flows between the positively and negatively charged electrodes, as illustrated in Figure 1. Note that a positive cell voltage is


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applied between the stack of current collectors and the electrodes to induce ion adsorption. Once the CDI cell is charged, it should then be discharged for subsequent deionization cycles. During charge-discharge cycling, the ions electrostatically held in electrical double layers (EDLs) are released and the ion conductivity repeatedly follows a rising and falling pattern. As such, the minimum conductivity, which represents the repeatable purification ability of the electrode, is one of the most important criteria in determining the cell performance.52-53 Table 3 lists the measured minimum conductivity in each electrode system over three cycles under conditions of 28 mL/min water flow rate, charging at 1.5 V for 1 min, and discharging at –0.8 V for 5 min. We found that the increased weight (%) of AT250 caused a marked reduction in the minimum conductivity. For AT250-30/PGW-PVA, the enhancement in the electrical property was lowered to almost 60% of that of pristine PGW-PVA, attributed to the reduction in minimum conductivity on increasing the amount of coating to an increased thickness of zwitterionic polymer layers that insulate the electrode. Therefore, we focus on the optimized electrode, AT250-20/PGW-PVA, for studying the electrochemical performance. As the surface of carbon is covered with the zwitterionic polymers, the deionization rate increased markedly as shown in Figure 8. We can induce how much the water is purified from the important parameter, deionization rate. The zeta-potential value does not make any difference between the carbon covered with zwitterionic polymers 20% and 30% as indicated in Table 1. The deionization rate assumed to show a little difference between the zwitterionic polymer of 20 wt.% and 30 wt.%. To clarify the CDI performance with the amount of the zwitterionic polymers on the carbon in this study, the deionization rate could be evaluated. Current efficiency is also a suitable tool for the formation of EDLs, which represents the ratio of current that is used for ion removal compared to the total current supplied. In Table 3, the current efficiency showed maximum at the binder content of 20 wt.%. Higher


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binder content indicates larger amount of ion exchange functional groups are contained. This potential charge barrier might be the origin of the increased current efficiency. On the other hand, charge efficiency was measured to be inverse proportional to the binder content. Here, binder can act as an electrical impedance and the current getting smaller with the increase of the binder content at constant voltage. Ultimately, 20 wt.% of binder content showed largest amount of charge current with higher current efficiency and appears to have higher deionization rate (Figure 8) and larger capacity in salt adsorption (Table 3).

Figure 8. Deionization rate as a function of the binder content.

Table 3. Minimum conductivity, salt adsorption capacity, and current efficiency of pristine PGW and AT250/PGWs in MgCl2 and CaCl2 solution. Salt adsorption capacity Salt adsorption capacity (mg mL−1) (mg g−1)

Minimum conductivity (mS/cm)

Current efficiency (wt.%)

PGW-PVA

110.1

62

1.62 × 10-4

26.1

AT250-10/PGWPVA

94.8

82

1.90 × 10-4

30.6

AT250-20/PGWPVA

85.4

92

2.14 × 10-4

34.5

AT250-30/PGW-

43.2

84

1.95 × 10-4

31.4


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PVA Activated carbon2-4

1.3 × 10-4

Graphene5

1.5 × 10-4

Figure 9. Electrochemical CDI performance of AT250-20/PGW-PVA. (a) Minimum conductivity of PGW and AT250-20/PGW-PVA, (b) charge-discharge cycling of AT250-20/PGW-PVA, and (c) duration of minimum conductivity for AT250-20/PGW-PVA electrode during charge-discharge cycling.

Figure 9 represents electrochemical performances including minimum conductivity, lifetime performance, and duration time of minimum conductivity for the AT250-20/PGW-


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PVA electrodes in a CDI cell. The duration of minimum conductivity in the CDI cell with pristine PGW-PVA electrodes was extremely short and exhibited sharp peaks, while the duration time was almost 50 s when using AT250-20/PGW-PVA electrodes (Figure 9a). This confirms the stability of the AT250-20/PGW-PVA electrode and its potential for repeatable purification. To evaluate the long-term stability of this electrode, we performed a cycling performance test on the CDI cell. Interestingly, the cell was stable for over 270 cycles (Figure 9b), with a minimum conductivity of

Enhanced Electrochemical Stability of a ... - ACS Publications

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