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ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-28880-x Integrating renewable energy, such as solar and wind power, is essential to reducing carbon emissions for sustainable devel- opment. However, large-scale utilization is hindered by the intermittence and uneven distribution of these power sources1–3. Implementation of grid-scale energy storage is essential to mitigate the mismatch between electricity production and consumption4. Different technologies are developed for this purpose, including supercapacitors, sodium–sulfur batteries, pump hydro, flywheels, and superconducting magnetic energy5. Redox flow battery (RFB) is considered one of the most attractive energy storage systems for large-scale applications due to the lower capital cost, higher energy conversion efficiency, and facile modularity6,7. The cores of flow cells are the circulating electro- lytes that carry the redox-active materials for energy storage and release. Currently, the all-vanadium RFB is the most researched and developed RFB chemistry; however, the market adoption of this system has been hampered by high-cost chemicals (material cost close to 60% of the overall system cost8 and low energy density. Although aqueous soluble organic redox species offer a potential option for low-cost materials9–15, the synthetic processes required to customize the molecular structure for high solubility and optimal potential will again limit the material cost and availability6,16–18. Also, they rely on the costly ion-permeable membranes to reduce cross-over, further increasing capital and maintenance costs19. Recently, polymer redox couples were developed to circumvent ion-permeable membranes20, and the semi-solid Li-ion (suspen- sions of Li-ion battery active materials in nonaqueous electro- lytes) systems have been explored for higher energy density and efficiency. However, high viscosity, lower peak power operation time, and high material cost emerged with these systems4,21,22. To meet the needs of RFB chemistries with the naturally abundant and low-cost redox-active materials, we report a new RFB system that capitalizes the electrolysis of saltwater or aqu- eous NaCl electrolyte using the Cl2/Cl− redox couple as the active material for the positive electrode. The Cl2/Cl− has a theoretical capacity of 755 mAh/g, more than two times that of vanadium oxides (VO2+/VO2+, 226 mAh/g) used in current RFBs. Cl2/Cl− redox chemistry is a fast single-electron transferred reaction with an activation energy of 35.5 kJ/mol23,24, which is comparable to or even smaller than that of VO2+/VO2+25, thus is suitable for high power applications. In addition, sodium chloride is one of the cheapest commodities available due to the abundant source in seawater and large-scale production (~$40 per metric ton)26,27. These features enable Cl2/Cl− redox reaction to be a promising candidate for RFB. − Rarely heard in the battery history is that Cl2/Cl redox couple was used in the RFB to power the first fully controlled airship La France in 188428. The Cl2/Cl− based batteries are often typified by low Coulombic efficiency (CE) of 40–70%29–33 due to Cl2 dissolution in the electrolytes and large voltage hysteresis (0.7 V at 32 mA/cm2) due to non-wettability between electrolytes and electrodes34,35, which limits the energy efficiency to around 60%. Graphite was reported as chlorine storage host via intercalation36. However, the instability of Cl2 intercalated graphite at room temperature results in low storage capacity (35–40 mAh/g) and limited cycle life. After that, no other materials with appropriate stability, storage capacity, and reaction kinetics have been reported to enable reversible Cl2 electrochemical reaction. Our objective is to develop a new RFB with the highly reversible Cl2/Cl− redox species through electrolyzing the saturated NaCl aqueous electrolyte (NaCl/H2O) and storing the as-produced Cl2 in water-immiscible organic phases such as carbon tetrachloride (CCl4) or mineral spirits. These organic phases provide several desirable properties: (1) Cl2 in CCl4 (Cl2-CCl4) delivers a volumetric capacity of 97 Ah/L due to high solubility of Cl2 in CCl4 (0.184 mole/mole CCl437, which is a 2 to 4 times improve- ment over the current vanadium-based catholyte (22.6–43.1 Ah/ L38; (2) The Cl2-CCl4 is immiscible to NaCl/H2O, thus requires no membrane to prevent cross-over, further reducing costs; (3) The Cl2-CCl4 has low and constant viscosity of 0.819 mPa.s, in con- trast to high and varying viscosity of aqueous vanadium-based catholyte (1.4–3.2mPa.s39, thus is easy to flow; (4) Cl2-CCl4 can wet carbon porous electrodes easily, which significantly enhances the surface area for Cl2 storage and reaction; (5) Cl2 has high diffusivity in CCl4, minimizing energy dissipation for mass transport. Results Storage and electrochemical performance of Cl2-CCl4. The Cl2/ Cl− redox reaction in NaCl/H2O was evaluated in a concentric cell with RuO2-TiO2 coated porous carbon (RuO2-TiO2@C) as a working electrode, activated carbon as a counter electrode (Fig. S1), and Ag/AgCl as the reference elec- trode (Fig. 1A). The RuO2-TiO2 catalysts on porous carbon (Figs. S2, S3) are used to promote the oxidation kinetics of chloride40–43 (Fig. S4). CCl4 was pumped through the working electrode, and the NaCl/H2O through the interstitial space between the working and counter electrodes to ensure adequate Cl- supply. While CCl4 and NaCl/H2O entered the RuO2- TiO2@C electrode as separate flows, they both wet the car- bon electrode, demonstrated by <90° contact angles (CAs) on a graphite plate electrode (Fig. 1B, C). And the two liquids take up 66.2% and 33.8% of the void volume in the RuO2-TiO2@C electrode, respectively (see the determination of percentage volume in Supplementary Note 1). The ion-permeable membrane used in traditional RFBs to prevent cross-contamination15,16,44–46 is not required here since the Cl2-CCl4 and NaCl/H2O are phase separated. During charge, the Cl2 was generated from oxidizing the Cl− in the RuO2-TiO2@C electrode. The reaction shows a constant potential at 1.2 V versus Ag/AgCl reference electrode [1.36 V versus normal hydrogen electrode (NHE)]. During discharge, the Cl2 in CCl4 was reduced to Cl− in the working electrode and entered the NaCl/H2O (see the formulation for positive electrode reaction). The presence of CCl4 flow significantly enhances the coulombic efficiency (CE) from 8 to 97% (Fig. 1D). Because the solubility of Cl2 in CCl4 is three orders of magnitude higher than that in NaCl/H2O (0.184 mole/mole CCl4 versus 0.0005 mole/mole NaCl/H2O38) (Fig. 1E), the Cl2 generated during the charging process can be stored in CCl4, which prevents Cl2 diffusion into NaCl/H2O as supported by Raman spectroscopy (Fig. S5) and the positive Gibbs free energy to transfer Cl2 from CCl4 to NaCl/H2O (Fig. S6). When 6.0mL CCl4 was used, a maximum reversible capacity for Cl2/Cl− conversion is 600 mAh (Fig. S7), rendering the capacity of 97 Ah/L for the Cl2-CCl4. 2 NATURE COMMUNICATIONS | (2022)13:1281 | https://doi.org/10.1038/s41467-022-28880-x | www.nature.com/naturecommunications Positive electrode reaction. 2Cl 2e $ Cl2 E0 1⁄4 1:36 Vðversus NHEÞ The Cl2-CCl4 positive electrode has a low and almost consistent viscosity. When the concentration of Cl2 increases from 0 to 0.184 mole/mole CCl4 (saturation), the viscosity even slightly decreases from 0.894 to 0.819 mPa.s (Fig. 1F) in accord to Eyring’s absolute reaction rate theory for gas–liquid mixtures47,48. On the other hand, the viscosity of common catholyte could increase by several or even dozen times as the concentration of solute increases49. The low viscosity of Cl2-CCl4 reduces the pumping loss40, and the steady viscosity minimizes thePDF Image | High-energy and low-cost membrane-free CFB
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