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D.-H. Nam, M.A. Lumley and K.-S. Choi Energy Storage Materials 37 (2021) 556β566 charging. However, the output voltage generated by desalination batter- ies during discharging is typically lower than that of ARNBs [16,17,28]. This is because the chlorination potential of the Cl-storage electrode typ- ically lies between the sodiation potentials of the two Na-storage elec- trodes chosen to maximize the output voltage of ARNBs [16,17,28]. The low output voltage is not a problem for desalination batteries because the primary purpose of desalination batteries is not to maximize the out- put voltage but to achieve desalination with a minimum energy input [28]. However, the low output voltage will prevent the use of desalina- tion batteries as efficient ESSs. In the new ESS demonstrated in this study, a seawater ARNB was constructed to achieve a maximum output voltage during discharging. The charging process was performed in two separate cells that use the desalination and salination reactions to store energy with a low input voltage. During the discharging process, the ARNB discharging cell gen- erates an output voltage that is the sum of the input voltages of the two charging cells, which is much greater than the output voltage that can be generated by traditional desalination batteries. We note that the charging process that requires an energy input is a necessary step for any battery to enable energy storage. As the energy consumed for the desalination and salination processes is not truly consumed but rather stored in the system through the charging process, and the majority of the energy stored during charging is recovered during discharging, the extra energy consumed for desalination is minimized. The new ARNB system presented in this study provides a dual-purpose ESS that can op- erate using seawater as the electrolyte and generates both electricity and desalinated water as useful products. 2. Results and Discussion 2.1. Operating Principles Our system uses two Na-storage electrodes and one Cl-storage elec- trode to construct two charging cells and one discharging cell. Charg- ing Cell 1 is composed of NASICON-type NaTi2(PO4)3 [29β32] as the Na-storage electrode and Bi [17,24,33,34] as the Cl-storage electrode (Fig. 1a). In this cell, Na-storage by NaTi2(PO4)3 and Cl-storage by Bi occur through the following reactions to achieve desalination: Cathode reaction: BiOCl+2H+ +3πβ βBi+Clβ +H2O (3) Anode reaction: 3Na2 NiFeII (CN)6 β 3NaNiFeIII (CN)6 + 3Na+ + 3πβ (4) In this cell, the cathode potential (dechlorination potential of BiOCl) is more negative than the anode potential (desodiation potential of Ni- HCF), which means that Ecell is negative (Fig. 1e). Thus, the overall reaction is non-spontaneous and requires an energy input, equivalent to charging. Because Na+ and Clβ are released into the feedwater during the cell reaction, Charging Cell 2 is also a salination cell. Discharging Cell is composed of a sodiated NaTi2(PO4)3 electrode and a desodiated NiHCF electrode. In this cell, Na+ will be released from NaTi2 (PO4 )3 (reverse reaction of Eq. 1) and inserted into NiHCF (reverse reaction of Eq. 4). In this cell, the cathode potential (sodi- ation potential of NiHCF) is more positive than the anode potential (desodiation potential of NaTi2 (PO4 )3 ), which means that Ecell is pos- itive (Fig. 1f). Thus, the overall reaction is spontaneous and generates an energy output, equivalent to discharging. As the desodiation poten- tial of NaTi2(PO4)3 is very close to the water reduction potential, and the sodiation potential of NiHCF is very close to the water oxidation potential, the combination of these two electrodes enables the gener- ation of the maximum output voltage allowed for ARNBs. When the discharging process is complete, the desodiated NaTi2 (PO4 )3 electrode can be used in Charging Cell 1, and the sodiated NiHCF electrode can be used in Charging Cell 2 to allow for repeated charging and discharging cycles. In a typical ARNB, the same two electrodes are used for both dis- charging and charging, and the input voltage required for charging is comparable to the output voltage generated during discharging. In our new system, rather than combining NaTi2(PO4)3 and NiHCF to perform the charging process, the charging process is divided and performed in two separate cells. In the charging cells, NaTi2(PO4)3 and NiHCF are each combined with Bi/BiOCl that has a chlorination/dechlorination potential that lies between the sodiation/desodiation potentials of NaTi2(PO4)3 and NiHCF. This new cell design provides two major ad- vantages. First, while the typical charging process is used only to store energy, in our new design the two charging cells perform desalination (Charging Cell 1) and salination (Charging Cell 2) concurrently with en- ergy storage. As a result, our new device achieves desalination during charging and generates electricity during discharging. We note that all conventional desalination methods (distillation, RO, electrodialysis) al- ways consume energy to convert the feedwater to desalinated water and have no ability to store energy. Second, the input voltages required for the charging process of Charging Cell 1 and Charging Cell 2 are approx- imately half of the input voltage that would be required to charge the cell composed of NaTi2 (PO4 )3 and NiHCF electrodes. While the total en- ergy required to charge the system does not change, the use of charging cells that require a lower input voltage may allow these cells to utilize a greater fraction of renewable electricity with fluctuating power. These two advantages increase the efficacy of the proposed ARNB system and make it a highly attractive novel candidate for ESSs. 2.2. Enhanced Cyclability of Electrodes Enabled by New Fabrication Method While our new ARNB system is conceptually plausible, it cannot be considered as a practically viable ESS unless all the component elec- trodes exhibit long-term cyclability. The long-term cyclability of the Bi electrode is of a particular concern. This is because, unlike NaTi2 (PO4 )3 and NiHCF that achieve sodiation through intercalation with a mini- mal structural change of the host material, chlorination of Bi results in a 158% volume expansion, forming a different phase, BiOCl (Fig. S1) [17]. Therefore, repeated conversion between Bi and BiOCl can result Cathode reaction: ()+β () 3β2NaTi2 PO4 3 + 3Na + 3e β 3β2Na3 Ti2 PO4 3 Anode reaction: Bi+Clβ +H2OβBiOCl+2H+ +3πβ (1) (2) In Charging Cell 1, the cathode potential (sodiation potential of NaTi2 (PO4 )3 ) is more negative than the anode potential (chlorination potential of Bi), which means that the cell voltage (Ecell, Ecell = Ecathode β Eanode) is negative (Fig. 1d). Thus, the overall reaction is non- spontaneous and requires an energy input, equivalent to charging. Dur- ing operation of the cell, Na+ and Clβ are removed from seawater that is used as the feedwater and so Charging Cell 1 is also a desalination cell. Because the removal of Na+ and Clβ in the desalination charg- ing cell occurs through ion-specific electrode reactions, desalination is achieved without the use of a membrane. We note that reverse osmosis (RO) and other electrochemical desalination methods (e.g. electrodial- ysis) are based on the use of a membrane, and extra procedures and treatment steps are required to alleviate membrane fouling [35,36]. Charging Cell 2 is composed of a sodiated nickel hexacyanoferrate (NiHCF) electrode and a chlorinated Bi electrode, BiOCl (Fig. 1b). NiHCF is a type of Prussian Blue Analogue (PBA) with the nominal formula Ax MFe(CN)6 βnH2 O (A: alkali metal ion, M: divalent transition metal ion, 1 β€ x β€ 2) [38β41]. In this cell, Clβ is released from BiOCl through the reverse reaction of Eq. 2 (Eq. 3), and Na+ is released from NiHCF (Eq. 4). 557PDF Image | seawater battery with desalination capabilities
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