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D.-H. Nam, M.A. Lumley and K.-S. Choi Energy Storage Materials 37 (2021) 556–566 tion is minimized. All electrodes used in our dual-purpose ESS exhibited excellent cyclabilities because of the new electrode fabrication methods developed in this study, which can be improved even further with con- tinued optimization efforts. The integration of a high voltage seawater ARNB and desalination/salination cells demonstrated in this study of- fers a new opportunity to simultaneously address challenges related to renewable electricity storage and seawater desalination. Compared with typical ARNBs where the electrolyte is held within the battery, our dual-purpose ESS that achieves desalination in addition to energy storage/release will require additional engineering efforts for practical and efficient operation; the desalinated and salinated water need to be replaced after each charging and discharging step. However, the operation of a three-step desalination cell is not much more com- plex than the operation of a two-step desalination cell. This is because the three-step system does not necessarily require the construction of three physically separate cells, and various engineering approaches can be used to design an efficient and practical system. For example, all three electrodes can be placed in the same container and a different pair of electrodes will be electrically connected depending on the pro- cess that will be performed. The container will be filled with a desired electrolyte before each process, and the desalinated or salinated water that is produced will be drained and collected through a designated out- let. Thus, the added complication does not come from the three-step sys- tem itself but from the added desalination capability of the cell, which justifies the extra complexity necessary to achieve the additional bene- fit of simultaneous desalination. The unique advantages provided by the dual-purpose ESS presented in this study, which allows for simultaneous membrane-free desalination and energy storage, considerably increase the benefit of ARNBs and encourage the continued development of this technology. 4. Experimental 4.1. Materials Bi2O3 (99.999%, PURATREM), graphite (99.995%, Sigma-Aldrich), C16 H36 O4 Ti (97%, Sigma-Aldrich), H2 O2 (30% solution, EMD Milli- pore), NH4OH (28−30% NH3 basis), C6H8O7 (99.0-102.0%, Alfa Aesar), NH4 H2 PO4 (≥98%, Sigma-Aldrich), Na2 CO3 (Macron), HNO3 (70%, Sigma-Aldrich), NiCl2 ∙6H2 O (98%, Alfa Aesar), Na3 C6 H5 O7 ∙2H2 O (≥98%, Mallinckrodt), Na4 Fe(CN)6 ∙10H2 O (≥99%, Sigma-Aldrich), polytetrafluoroethylene (PTFE) (60 wt % dispersion in H2O, Sigma- Aldrich), colloidal graphite (isopropanol, Ted Pella, Inc.), NaCl (99%, Macron), KNO3 (99.0%, Alfa Aesar) and EtOH (200 proof, Decon Labs Inc.) were used without further purification. Deionized water (Barnstead E-pure water purification system, resistivity >18 MΩ∙cm) was used to prepare all solutions. 4.2. Synthesis of NaTi2(PO4)3 and NiHCF NaTi2 (PO4 )3 was synthesized by a sol-gel method reported in a pre- vious study [17]. First, 0.02 M C16 H36 O4 Ti and 0.04 M C6 H8 O7 were dissolved in a solution containing 280 mL H2O2 and 120 mL NH4OH. Next, 0.13 M NH4 H2 PO4 and 0.01 M Na2 CO3 were dissolved in 80 mL containing 0.10 M NiCl2 and 0.70 M Na3 C6 H5 O7 . The resulting solution was stirred for 5 h and was then aged at room temperature for 20 h. A pale blue precipitate formed and was centrifuged at 5000 rpm, rinsing alternately with water and ethanol. Finally, the resulting precipitate was dried at 70°C for 24 h in a vacuum furnace held at a pressure of ~13.6 psi. The as-prepared NiHCF powder has a rhombohedral structure (space group: R-3, a = b = 7.389 Å and c = 17.310 Å) (Fig. S6a-b). The XRD pat- tern of NiHCF is also shown in Fig. S6c. The details of the structure are summarized in Table S1. EDS analysis reveals that the chemical formula of the as-prepared NiHCF powder is Na1.20 Ni[Fe(CN)6 ]0.85 ∙nH2 O. 4.3. Preparation of sheet-type electrodes The Bi, NaTi2(PO4)3, and NiHCF electrodes used in this study were prepared through a ball milling process followed by a rolling-pressing procedure. The active material (Bi2O3, NaTi2(PO4)3, or NiHCF) and graphite powder were first mixed with a mortar and pestle (the ratio of the active material to graphite powder was ~4:1 by mass), and then the mixture was ball milled for 1 h at a rate of 1060 cpm using a High- Energy Ball Mill (8000 M Mixer/Mill from SPEX SamplePrep). The re- sulting composite (Bi2O3/C, NaTi2(PO4)3/C, or NiHCF/C) was mixed with a PTFE binder (ratio of ~1.6:1 by mass) using water as the solvent to form a thick slurry. The slurry was repeatedly kneaded, folded, and pressed using a mortar and pestle, followed by the use of a roll-press to form a thin sheet with desired dimensions. Finally, the electrode sheet was dried on a hot plate at 80°C for at least 6 h to remove water and residual organic compounds. The dried electrode sheet was cut into a 1 cm2 electrode and was then attached onto a graphite current collector with carbon paint to perform electrochemical tests. 4.4. Characterization The morphology and crystal structure of the active materials were ex- amined using a LEO Supra55 VP Scanning Electron Microscope (SEM) at an accelerating voltage of 2 kV and powder X-ray diffractometer (XRD) (Bruker D8 Advanced PXRD, Ni-filtered Cu K𝛼 radiation, 𝜆 = 1.5418 Å), respectively. Energy dispersive X-ray spectroscopy (EDS) was per- formed using the same SEM equipped with an EDS (Noran System Seven, Thermo Fisher) at an accelerating voltage of 12 kV. To investigate the phase change of the Bi electrode after oxidation (chlorination) and re- duction (dechlorination), ex-situ XRD analysis was performed on the cycled samples. 4.5. Cyclability Tests The cycle performances of all electrodes were examined in 0.6 M NaCl (salinity = 3.5%), which mimics the salinity of seawater (~3.5%). An undivided three-electrode cell was used, and the electrolyte was not circulated or stirred during the measurements. The cycle performance of the Bi electrode was examined galvanostatically at a current density of ± 4 mA cm−2 with cutoff potentials of −1.45 V and 0.7 V vs Ag/AgCl. The Bi electrode used as the working electrode (geometric area of 1 cm2) contained ~6.9 mg of Bi. Thus, 4 mA cm−2 is equivalent to ~1.5 C-rate based on the theoretical capacity of 384.75 mAh g−1. A NiHCF electrode (geometric area of 4 cm2) was used as the counter electrode. The counter electrode had a sufficient amount of active material rela- tive to the working electrode to ensure that the current flow was not limited by the counter electrode during the electrochemical tests. The cycle performance of the NaTi2 (PO4 )3 electrode was examined galvano- statically at ± 2 mA cm−2 with cutoff potentials of −0.93 V and −0.2 V vs Ag/AgCl. The NaTi2 (PO4 )3 electrode used as the working electrode (geometric area of 1 cm2 ) contained ~10 mg of NaTi2 (PO4 )3 . Thus, ± 2 mA cm−2 is equivalent to ~1.5 C-rate based on the theoretical capacity of 133 mAh g−1. A NiHCF electrode (geometric area of 4 cm2) was used as the counter electrode. The cycle performance of the NiHCF electrode DI water and 100 mL HNO3 , Na2 CO3 solutions were added and C6 H8 O7 . The solution was aged at 140°C for 2 h to obtain a yellow precipitate and the resulting powder was annealed at 800°C for 12 h (ramp rate = 2°C min−1 ). The NaTi2 (PO4 )3 powder was ground with a mortar and pestle and was annealed again at 800°C for 12 h to improve the uniformity and crystallinity of the sample. The crystal structure and XRD pattern of NaTi2(PO4)3 (space group: R-3c, a = b = 8.502 Å and c = 21.833 Å) are shown in Fig. S4. NiHCF was synthesized by a co-precipitation method following the procedure reported in a previous study [41]. A 150 mL solution con- taining 0.10 M Na4Fe(CN)6 was slowly added into a 150 mL solution respectively. Then, the NH4 H2 PO4 and into the solution containing C16 H36 O4 Ti 564PDF Image | seawater battery with desalination capabilities
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