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ACS Omega Article washed with distilled water and ethanol several times, and the collected particles were dried in an oven at 70 °C under a vacuum condition. NaFeHCF particles were synthesized by mixing 100 mL of a 0.05 M FeCl2 + 0.20 M Na-citrate solution and 100 mL of a 0.05 M Na4Fe(CN)6 solution. The mixing solution was stirred for 3 h and aged for 20 h at room temperature. The resulting particles were filtered and rinsed with distilled water and ethanol several times and then dried in an oven at 70 °C to eliminate the remaining solvent. The prepared particles were characterized using inductively coupled plasma atomic emission spectrometer (ICP-AES; VARIAN 730ES, Australia), field emission scanning electron microscopy (FESEM; JEOL JSM 6700 F, Japan), and X-ray powder diffraction (D8 Discover). All of the reagents were purchased from Sigma-Aldrich Corporation. 2.2. Electrode Fabrication and Cell Assembly. The NaNiHCF and NaFeHCF electrodes were made by mixing 70 wt % active material, 20 wt % carbon black (Super P, Timcal, Switzerland), and 10 wt % poly(tetrafluoroethylene) (PTFE; Sigma-Aldrich) in an ethanol solvent. The resulting slurry was pressed using a roll press machine to obtain sheet-type electrodes with a thickness of approximately 300 μm. The fabricated Prussian blue electrodes were dried using a vacuum oven at 60 °C for 12 h to eliminate the remaining solvent. Rectangular-shaped NaNiHCF (2.0 cm × 2.0 cm, weight: 90 ± 5 mg) was used as the positive electrode, and the same-sized NaFeHCF (2.0 cm × 2.0 cm, weight: 86 ± 4 mg) rectangle was used as a negative electrode. The Prussian blue electrodes were attached onto titanium plates (thickness: 0.20 mm; Sigma- Aldrich) using carbon paint (DAG-T-502; Ted Pella). The rocking chair desalination battery consisted of two Prussian blue electrodes, an anion-exchange membrane (AMX; ASTOM Co., Japan), and polyamide woven spacers (2.0 cm × 2.0 cm, thickness: 0.6 mm). The electrolyte of the cell consisted of a positive-electrode compartment and a negative-electrode compartment divided by the anion-exchange membrane, as shown in Figure S3, and the volume of each compartment is approximately 0.4 cm3. Before assembling the cell, the NaFeHCF electrode was charged to 0.3 V for 30 min in a 0.5 M NaCl solution using a three-electrode cell with a Ag/ AgCl (KCl sat′) reference electrode and a stainless steel counter electrode to extract Na ions from NaFeHCF to obtain a ready-to-use negative electrode. The cell was covered by a PTFE plate and silicon rubber. To avoid the negative cell voltage during the discharging step, the battery cell was precharged in a 0.5 M NaCl solution at a constant current (2 mA) for 10 min before starting the test. The seawater used in this study was obtained from the East Sea (Sockcho, Gangwon Province, Korea). 2.3. Desalination Performance Test. To probe the possibility of application for seawater desalination processes, 0.6 mL of seawater was put into the positive and negative compartment of the cell (0.3 mL in each compartment). The desalination process was conducted at a constant current (±0.5 mA/cm2) for 1 h (removal of 40% Na ions) and 40 min (removal of 25% Na ions). Each solution in the positive and negative compartments was extracted and exchanged with virgin seawater (0.3 mL in each compartment) by reassembling the cell after the charging and discharging processes. To investigate the maximum ion removal capacity, an additional experiment was carried out at constant current operation (±0.5 mA/cm2) in a 0.5 M NaCl aqueous solution with a voltage range of 0.05−0.85 V, and the results of the desalination 1655 performance are provided in the Supporting Information (see Figure S2 and Table S2). The concentration of various ions was measured by ion chromatography (ICS-1100 and DX-120; Dionex) with 50 μL of samples after each process. The desalination results from triplicate experiments with standard deviations are presented in this work. Energy consumption during the operation can be calculated by the amount of energy consumed during the charging step minus the energy generated during the discharging step and is demonstrated by the path integral of a voltage versus charge plot, as given by previous research24,33−36 W = ∮ ΔV dq c (1) where ΔV is the cell voltage (V) and q is the charge (C) during the operation. The ion removal efficiency reported in the table is based on the equation ion removal (%) = ci − ct c × 100 (2) i where ci is the ion concentration of the initial source water and ct is the ion concentration of treated water. The Na ion removal efficiency of the solution was represented by the average removal efficiency percent of Na ions by repeating the first cycle of the system using virgin Prussian blue electrodes. The efficiency of salt removal per total charge is expressed by the Coulombic efficiency according to the following equation ηc (%) = ziF(nI − nF) × 100 ∑ (3) where zi is the ion valance, F is the Faraday constant, nI − nF is the molar change in ions, and ∑ is the total charge transferred at the charging and discharging steps. 2.4. Electrochemical Characterization. The electro- chemical performance of the Prussian blue electrodes was examined in an electrochemical cell that was manufactured with a pair of graphite current collectors (d = 18 mm) and a glass fiber separator (GF/A; Whatman), as shown in the Supporting Information (see Figure S4). Cyclic voltammetry (CV) was performed in a 1 M NaCl solution or seawater with a three- electrode system. Round Prussian blue electrodes (d = 9 mm) were used as the working electrode, and the reference electrode was a Ag/AgCl (KCl sat′) electrode. A sheet-type Ag/AgCl electrode (d = 18 mm) with a large specific capacity was used as the counter electrode. Galvanostatic charging/discharging tests were carried out using a two-electrode or three-electrode cell in seawater electrolytes, and, before the test, 0.3 V (vs Ag/AgCl) was applied to the NaFeHCF electrode for 30 min in a 0.5 M NaCl solution. NaNiHCF and NaFeHCF electrodes were used as positive and negative electrodes. In a three-electrode cell, a Ag/AgCl (KCl sat′) electrode was used. To examine the stability of the NaNiHCF and NaFeHCF cell in seawater, a galvanostatic cycling test was conducted in an electrochemical cell at a current of ±1.05 mA (0.1 A/g of NaFeHCF) in a voltage range of 0.10−0.80 V. The electrochemical analyses were conducted using a battery cycler (WBCS3000; WonA Tech, Korea). 3. RESULTS AND DISCUSSION Figure 2a,b shows the voltage profiles during the charging and discharging steps at a current density of ±0.5 mA/cm2 for 1 h DOI: 10.1021/acsomega.6b00526 ACS Omega 2017, 2, 1653−1659PDF Image | Rocking Chair Desalination Battery Prussian Blue Electrodes
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