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ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-28880-x AB 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8020406080100 C 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8020406080100 E The power densities of the CFB at different current densities. The model was then used to visualize the species distribution in the NaCl/H2O and in Cl2-CCl4. During charge, the Cl− in NaCl/ H2O was consumed inside the porous carbon electrode (Fig. 3C) and limits the reaction kinetics; during discharge, Cl2 in CCl4 is consumed and limits the reaction kinetics. The Cl− concentration gradients are more significant than the Cl2 concentration gradient in the porous electrode for both charge and discharge (Fig. 3C, D), which is the result of a smaller diffusivity of Cl− (1.5 × 10−5 cm2/s for Cl−, 2.0 × 10−5 cm2/s for Cl2 in NaCl/H2O and 3 × 10−5 cm2/s for Cl2 in CCl453–55 and lower volume percentage of NaCl/H2O than CCl4 in the porous carbon electrode. The distinct species that control charge and discharge kinetics thus generate the asym- metric charge and discharge overpotentials (Fig. 2B, C). Since Cl− and Cl2 are in different phases, increasing the flow rate of NaCl/ H2O during charge and that of the Cl2-CCl4 during discharge enhance the mass transport of the limiting species accordingly, in which not only the overpotentials reduce, but the current density range allowing steady cell voltage extends (inset of Fig. 3A, B). At the highest flow rate examined, the voltage efficiency could be postulated to >93% at 20 mA/cm2. The high voltage efficiency of the cell is attributed not only to the fast reaction kinetics but also the membrane-free configura- tion. The potential gradient in the NaCl/H2O was determined by the model (Fig. 4A, B), and the potential difference across the cell at half-cell height was plotted. The potential drop of ~20 mV at 10 mA/cm2 and ~250 mV at 100 mA/cm2 (Fig. 4C) are equivalent to proton transport but over 5 times smaller than Na+ and K+ transport in Nafion ion-permeable membranes in aqueous flow batteries with similar cell dimensions56. Thus, removing the ion- selective membrane opens a range of chemistries to be investigated, as the charge carriers can be chosen arbitrarily. The CFB demonstrates the round-trip energy efficiency of 91% (calculated by voltage efficiency × Coulombic efficiency) at 10 mA/cm2 and provides an energy density of 125.7 Wh/L (see Methods), which is among the highest of the flow battery systems reported in past 10 years (Table S5). It is worth noting that the Cl2-CCl4 is different from bromine used in flow batteries that faces the serious self-discharge due to the diffusion of Br2 to the negative electrodes in the form of polybromide. When ion- permeable membranes were used to decrease Br2 cross-over, voltage efficiency was significantly limited by the transport of ions in the membrane, resulting in <80% energy efficiency in overall performance57–59. Figure 5A, B show the measured cell voltage profile and stable round-trip cycling for this battery at 20 mA/ cm2 with a charge storage capacity of 600 mAh and the stable capacity retention for 500 cycles. Discussion In this study, CCl4 was used as a proof of concept, it can be replaced by other liquids with high Cl2 solubility and are immiscible with NaCl/H2O. The candidates include heptane (chlorine solubility = 0.173 mole fraction at ambient tempera- ture), octane (chlorine solubility = 0.168 mole fraction at ambient temperature), tetradecane (chlorine solubility = 0.254 mole frac- tion at ambient temperature)29 and mineral spirit. Mineral spirit demonstrates good wettability (CA = 9.1°) with carbon current collector (Fig. S12A), low viscosity (1.24 mPa.s), low toxicity, and is cheaper than CCl460. When CCl4 was replaced by mineral spirit in the CFB, a volumetric capacity of 91.6 Ah/L was delivered at 20 °C (Fig. S12B). The removal of the ion-permeable membrane also allows multivalent ions as charge carriers. When ZnCl2 is added to the electrolyte, NaTi2(PO4)3 can be replaced by zinc metal electrode, increasing the cell operating voltage to 1.9 V (Fig. S13). Cost is one of the significant concerns to implementing flow batteries on a large scale for stationary energy storage. Con- sidering that the ion-permeable membrane (mainly per- fluorinated polymers) takes up more than 30% of the cost of flow batteries, significant cost reduction is expected with the membrane-free design20. The total material cost for energy sto- rage with the proposed CFB is estimated to be ~$5/kWh, which is the cheapest among all the current flow battery systems (Fig. 5C and Table S5). In addition, the RuO2 catalyst for chlorine evo- lution reaction (CER) can also be replaced by tin, zinc, cobalt, and other cheap metal oxides partially30. Therefore, the proposed CFB design leaves significant space to meet the stringent target of ~$100/kWh for RFB applications61. 4 NATURE COMMUNICATIONS | (2022)13:1281 | https://doi.org/10.1038/s41467-022-28880-x | www.nature.com/naturecommunications D 100 95 90 85 80 75 70 65 SOC (100%) 0 50 100 150 200 Current density (mA/cm2) E 350 300 250 200 150 100 50 0 SOC (%) 0 100 200 300 400 500 Current density (mA/cm2) Fig. 2 Schematic and electrochemical performance of chlorine flow battery (CFB). A Schematic of the CFB, the inner diameter of the tube containing CCl4 and RuO2-TiO2@C electrode is 2.0 mm, the thickness of the RuO2-TiO2@C electrode is 1.0 mm, the distance between the working and counter electrode is 3.0 mm. The thickness of the counter electrode is 3.0 mm. The height of the cell is 2.0 cm, and the volume capacity of the cell is around 2.0 mL. The total volumes of the CCl4 reservoir and the NaCl/H2O reservoir are 6.0 mL and 2.0 mL, respectively. Qaq = 0.02 mL/s and Qorg = 0.002 mL/s. Galvanostatic charge B and discharge C profiles of the CFB at different current densities. The state of charge (SOC) of the battery is normalized to the maximum reversible capacity at 10 mA/cm2, in which 100% SOC represents charge to 600 mAh. D The voltage efficiencies of the CFB at different current densities. 2 10 mA/cm 2 20 mA/cm 2 30 mA/cm 2 50 mA/cm 2 80 mA/cm 2 100 mA/cm 2 200 mA/cm 2 10 mA/cm 2 20 mA/cm 2 30 mA/cm 2 50 mA/cm 2 80 mA/cm 2 100 mA/cm 2 200 mA/cm Voltage efficiency (%) Cell voltage (V) Power density (mW/cm2) Cell voltage (V)PDF Image | High-energy and low-cost membrane-free CFB
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