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D.-H. Nam, M.A. Lumley and K.-S. Choi Energy Storage Materials 37 (2021) 556–566 the use of a high-energy ball mill allowed for more uniform and intimate carbon coating of the NaTi2 (PO4 )3 particles. The rolling-pressing proce- dure also facilitated better contact between carbon-coated NaTi2 (PO4 )3 particles within the electrode, resulting in the formation of high-quality electrodes that demonstrated good cyclability. With further optimiza- tion, we believe that the lifetime of NaTi2(PO4)3 can be further im- proved. NiHCF was also fabricated as a sheet-type electrode using the same procedure. Detailed synthesis conditions and structural characterization of NiHCF powder, as well as the electrode fabrication procedure can be found in the experimental section and SI (Fig. S6-S7, Table S1). The cy- cle performance of the sheet-type NiHCF electrode was investigated in 0.6 M NaCl at a current density of ± 4 mA cm−2 (~6 C-rate) with cutoff potentials of 0.0 V and 0.9 V vs Ag/AgCl for sodiation and desodiation, respectively. The potential-capacity plots obtained during sodiation and desodiation are shown in Fig. 3g. The average potentials observed dur- ing sodiation and desodiation were 0.36 V and 0.56 V vs Ag/AgCl, re- spectively. The long-term cycle performance of the NiHCF electrode is shown in Fig. 3f. The initial sodiation capacity of the NiHCF electrode was 50.25 mAh g−1 and the sodiation capacity after 5000 cycles was 49.6 mAh g−1, corresponding to a capacity retention of 98.7% (Fig. 3f). Furthermore, the NiHCF electrode demonstrated an impressive Coulombic efficiency of 99.8% throughout the cycling test. A Coulombic efficiency near 100% illustrates the highly reversible redox properties of NiHCF and indicates that no parasitic side reactions or irreversible phase transitions occur during the cycling test. The cycle performance of NiHCF shown here is a significant improvement compared with the cycle performance of NiHCF reported in our previous study [41]. In this previous study, the NiHCF electrode was prepared by mixing NiHCF powder with a conduc- tive carbon additive and PTFE binder by hand in a mortar and pestle, and the capacity retention of this electrode after 200 cycles was 95%. The improved performance demonstrated by sheet-type NiHCF electrodes again highlights the advantages of our new processing method. The results presented in this section show that the electrodes chosen for our new electrochemical system can serve as practical component electrodes for our dual-purpose ARNB. We believe that the cyclability of the electrodes shown here can be further improved by continued op- timization of the electrode fabrication process. In the next section, we will describe how these electrodes can be used to construct a combined desalination/ARNB system, and we will examine the energy storage and desalination capabilities of the system. 2.3. Operation of Charging and Discharging Cells The energy storage and release processes and the desalination perfor- mance of the aqueous rechargeable system combining Bi, NaTi2 (PO4 )3 , and NiHCF electrodes were investigated galvanostatically. During the cell operation, the individual potentials of the anode and cathode were monitored against an Ag/AgCl reference electrode. The performance of Charging Cell 1 composed of Bi and NaTi2 (PO4 )3 electrodes was investigated in 0.6 M NaCl at a current density of 1.33 mA cm−2 (~0.5 C-rate based on the Bi electrode) until the capacity of the Bi electrode reached 100 mAh g−1 (Fig. 4a). The individual poten- tial profiles show that the reduction of NaTi2 (PO4 )3 (sodiation/cathode) occurs at −0.807 V vs Ag/AgCl while the oxidation of Bi to BiOCl (chlo- rination/anode) occurs at −0.067 V vs Ag/AgCl. Thus, the average input voltage required to operate Charging Cell 1 is 0.74 V. Charging Cell 1 also removed Na+ and Cl− from the electrolyte to achieve desalination. The performance of Charging Cell 2 composed of Bi and NiHCF elec- trodes was examined in an acidic solution containing 70 mM HCl (pH 1.3). The use of an acidic solution was necessary to improve the dechlori- nation kinetics of BiOCl [17,24,33,34], and therefore decrease the input voltage required to operate Charging Cell 2. In our previous study we thoroughly investigated the electrochemical properties of Bi and found that while the chlorination kinetics of Bi are fast in neutral solutions, the dechlorination kinetics of BiOCl are slow and require a large over- potential in neutral solutions. This is because dechlorination of BiOCl involves the release of both Cl− and O2− from the BiOCl lattice (reverse of Fig. 2c). We discovered that the use of an acidic electrolyte where H+ can serve as an O2− acceptor can drastically reduce the overpotential required for the reduction of BiOCl (Fig. S8). (Detailed overpotential analyses for the conversion between Bi and BiOCl using linear sweep voltammograms can be found in our previous study [17].) While Charg- ing Cell 1 is designed to achieve seawater desalination and must use seawater as the feedwater, Charging Cell 2 is designed to regenerate the Bi electrode and does not need to use seawater as the feedwater; the choice of electrolyte for Charging Cell 2 is flexible. Therefore, an acidic electrolyte was used to minimize the energy input required for Charg- ing Cell 2. (Acidic wastewater or acidified seawater can be used for the practical implementation of Charging Cell 2.) Other than the electrolyte type, the performance of Charging Cell 2 was investigated with the same operating conditions as Charging Cell 1 (Fig. 4b). The individual potential profiles show that the reduction of BiOCl to Bi (dechlorination/cathode) occurs at −0.147 V vs Ag/AgCl on average while the oxidation of NiHCF (desodiation/anode) occurs at 0.367 V on average, indicating that the Bi/NiHCF cell requires an average input voltage of 0.514 V (Fig. 4b). The performance of Discharging Cell composed of NiHCF and NaTi2(PO4)3 electrodes was investigated in 0.6 M NaCl at a current den- sity of 1.33 mA cm−2. The individual potential profiles show that the re- duction of NiHCF occurs at 0.41 V (sodiation/cathode) on average while the oxidation of NaTi2 (PO4 )3 occurs at −0.78 V (desodiation/anode) on average, resulting in an average output voltage of 1.19 V. Considering that the thermodynamic electrochemical stability window of aqueous electrolytes is 1.23 V due to the occurrence of water oxidation and wa- ter reduction, the output voltage reported here approaches the thermo- dynamic limit. Therefore, the combination of NiHCF and NaTi2 (PO4 )3 electrodes is suitable to construct an ANRB with a maximum output voltage. The fact that this cell is truly a discharging cell and can gen- erate electricity was visually demonstrated by connecting the cell to a small LED bulb. The LED bulb was illuminated when the NiHCF and NaTi2 (PO4 )3 electrodes were simply immersed in seawater (no external electrical energy was supplied) (Fig. S9). The energy efficiency of the system (energy output divided by en- ergy input, multiplied by 100%) was examined by dividing the output energy generated by Discharging Cell by the sum of the input energies required to operate Charging Cells 1 and 2. The energy output and in- puts required for the electrochemical reactions were calculated by in- tegrating the areas between the cell voltage (the difference between the cathode and anode potentials) and capacity plots (the shaded re- gions in Fig. 4a-c). We note that the capacities of the three cells were slightly different (100 mAh gBi−1 for Charging Cell 1, 105.29 mAh gBi−1 for Charging Cell 2, and 105.36 mAh gBi−1 for Discharging Cell) (Fig. S10a-c). This is because the Coulombic efficiency of the Bi electrode is slightly higher than 100% (Fig. 3d) (i.e. the dechlorination capacity is slightly greater than the chlorination capacity). A detailed explanation of the effect of this phenomenon on the capacities of the three cells is provided in the experimental section. When the as-obtained potential- capacity plots of Charging Cells 1 and 2 and Discharging Cell were used to calculate the energy efficiency of the system, the energy efficiency was overestimated (~99%) (Fig. S10d). In order to obtain a more ac- curate energy efficiency, we normalized the capacities of all three cells to be identical (100 mAh). With the potential-normalized capacity plots shown in Fig. 4a, the energy inputs required for Charging Cell 1 and Charging Cell 2 were calculated to be 74.10 mW and 51.44 mW, respec- tively, giving a total energy input of 125.54 mW. The energy output generated by Discharging Cell was calculated to be 118.73 mW. There- fore, the energy efficiency of the system was found to be 94.6% for the first cycle. The performance of the complete system was monitored for 15 consecutive cycles, and the energy input and output showed neg- ligible changes throughout the duration of the test to give an average 561PDF Image | seawater battery with desalination capabilities
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