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NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-28880-x ARTICLE Fig. 1 Electrochemical performance and physical properties of Cl2-CCl4. A Schematic of the three-electrode cell. Inset shows the cylindrical structure of the cell from the top view, in which the inner diameter of the RuO2-TiO2@C working 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 and the thickness of the counter electrode is 3.0 mm. The height 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. B CA of CCl4 on graphite plate electrode. C CA of NaCl/H2O on graphite plate electrode. D Galvanostatic charge and discharge profiles of Cl2-CCl4 (red) and Cl2 without CCl4 (black) at the current density of 20 mA/cm2. Both cells ran with constant charge capacity of 600 mAh at Qaq (flow rate of NaCl/ H2O) = 0.02 mL/s and Qorg (flow rate of CCl4) = 0.002 mL/s. The differences between discharge and charge capacity are labeled as percentage capacity loss. E The solubility of Cl2 in CCl4 and NaCl/H2O. F The viscosities of Cl2-CCl4 with different concentrations of Cl2 (100% refers to saturation). volumetric transfer between catholyte and anolyte at different SOCs50,51. Full chlorine flow battery (CFB). To fabricate a full CFB, the activated carbon counter electrode was replaced by NaTi2(PO4)3 negative electrode (Fig. 2A). NaTi2(PO4)3 (Figs. S8, S9) was chosen as the negative electrode due to low potential (−0.5 V (versus NHE), rapid and reversible Na-ion insertion/extraction in NaCl/H2O demonstrated by the symmetric anodic and cathodic peaks with 60 mV separation in the cyclic voltammetry (negative electrode reaction and Fig. S10A)52. The NaTi2(PO4)3 shows a 65% capacity retention even at the C-rate of 315 C (1 C = fully discharge/charge within 1 hour, Fig. S10) and long cycle life of 1000 cycles (Fig. S11). to charge) of 93.6% at 10 mA/cm2 and ~77% at 100 mA/cm2. The multiplication of discharge capacity and voltage gives the cell power density that peaks at 325mW/cm2 when operated at 350 mA/cm2 (Fig. 2E). It is worth noting that polarizations for discharge are more significant than those for discharge (Fig. 2B, C). In the CFB, overpotentials are caused by redox reactions and concentration gradient. Since the symmetric factors for Cl−/Cl2 redox reactions are equal17,25, the overpotentials needed to drive the reduction and oxidation reaction are the same, the different overpotentials for charge and discharge observed here could only be attributed to the concentration gradient. A steady-state model was developed to understand the species distribution and controlling steps in the CFB. The Nernst-Plank equation was applied to the porous RuO2-TiO2@C electrode (cell width = 0–1.0 mm in Fig. 2A), and NaCl/H2O (cell width = 1.0–4.0 mm in Fig. 2A), Fick’s equation was applied to the Cl2-CCl4 phase (cell width = −2.0–0 mm in Fig. 2A). The negative electrode was involved implicitly at the boundary of the NaCl/H2O (cell width = 4.0 mm in Fig. 2A) (see model descrip- tion and Tables S1–S4 in Supplementary Note 1). The model was validated by the agreement between the simulated and experi- mental cell voltages (black lines and dots in Fig. 3A, B, experimental potential retrieved from Fig. 2B, C) at the same flow rates and current densities. Negative electrode reaction. Na3 Ti2 ðPO4 Þ3 2e 2Naþ $ NaTi2 ðPO4 Þ3 E0 1⁄4 0:5 Vðversus NHEÞ While the overpotentials enhanced (orange dash lines in Fig. 2B, C) as the current density increased, the discharge capacities did not vary (Fig. 2B, C), which could be attributed to the large reaction surface area endowed by the wetting between carbon electrode and Cl2-CCl4 (Fig. 1B, C). Fig. 2D demonstrates cell voltage efficiency (defined as the potential ratio of discharge NATURE COMMUNICATIONS | (2022)13:1281 | https://doi.org/10.1038/s41467-022-28880-x | www.nature.com/naturecommunications 3PDF Image | High-energy and low-cost membrane-free CFB
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