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Cobalt dicarboxylate bipyridine complexes bearing low redox potential and low permeability were designed and synthesized for high-voltage and cycling-stable neutral AORFBs. [Display omitted] [ABSTRACT FROM AUTHOR]

Designing flow fields that can lead to uniform distributions of reactants at a minimum pump work is critical to enhancing the performance of redox flow batteries. This paper reports on an improved design of conventional serpentine flow fields, in which the channel depth is linearly reduced from the inlet to the outlet, speeding up the flow speed along the flow path and enhancing the under-rid convection downstream toward the outlet. Three-dimensional numerical simulations reveal that the optimized gradient at 25% (the channel-depth ratio between the outlet and inlet) can lead to the highest pump-based voltage efficiency. Experimental validations demonstrate that the application of the optimized flow field to a vanadium redox flow battery leads to significant improvements in both energy efficiency and electrolyte utilization, which is 5.0% and 27.7%, respectively, higher than that with the conventional serpentine flow field at a relatively high current density and low flow rate (400 mA cm−2, 12 mL min−1 cm−2). The effectiveness of the flow field design in boosting the uniform reactant distribution provides a feasible approach for scaling up high-performance redox flow batteries. [Display omitted] • An improved design strategy of flow field is proposed for VRFB. • Uniform distributions of electrochemical reactants are achieved. • Energy efficiency is improved by 5.0% for optimized design. • Electrolyte utilization is improved by 27.7% compared with SFF. • Operating with optimized current density and flow rate is evaluated. [ABSTRACT FROM AUTHOR]

Polymer-based aqueous redox flow batteries (RFBs) have emerged as a promising technology for large-scale energy storage. The development of novel polymeric electrode materials with high aqueous solubility to enhance the capacity and stability of batteries is the foremost concern of this area, yet remains challenging. Here we describe a stable polymer-based aqueous RFB assembled in air atmosphere, which uses a low-cost anionic terpolymer (P4) as the active cathode material, N , N ′-dimethyl-4,4′-bipyridinium dichloride (MV) as the anode material, and environmentally benign aqueous NaCl solution (NaCl aq) as the electrolyte. P4 exhibits high solubility (28.8 Ah L−1) and low viscosity in 1.5 M NaCl aq. The theoretical storage capacity of such stable RFB is as high as 12 Ah L−1. At the current density of 10 mA cm−2 and over 170 consecutive cycles in air atmosphere at room temperature, the P4/MV RFB exhibits the actual energy density of 6.43 Wh L−1, the material utilization of 71.3%, Coulombic efficiency over 95%, and the average attenuation per cycle of 0.12%. [Display omitted] • An anionic terpolymer P4 is synthesized and used as cathode material for aqueous redox flow battery (RFB). • P4 exhibits the excellent solubility and low viscosity in 1.5 M aqueous NaCl solution. • P4 -based aqueous RFB assembled in air exhibits stable manner with excellent capacity and low attenuation per cycle. • The theoretical storage capacity of stable P4-based aqueous RFB is as high as 12 Ah L−1. [ABSTRACT FROM AUTHOR]

Neutral aqueous organic redox flow battery (NAORFB) stands out as one of the most prospective mass energy storage options. The main limitations of large-scale storage regarding the NAORFB lie in the low Na+/Cl− conductivity of membrane in pH7 systems, which leads to a lower voltage efficiency. Herein, quaternary ammonium functionalized fluorinated poly (arylene ether) (QAFPAE-x) anion exchange membrane with hydrophilic and hydrophobic blocks was designed successfully. The results demonstrated that the grafting of quaternary ammonium and fluorine-containing block enables QAFPAE-x membrane low swelling, high Cl− conductivity, and promising chemical stability characteristics. The voltage efficiency of QAFPAE-100 membrane (93.8%–60.7%) is much higher than the AMVN membrane (88.4%–37.8%) at 20–160 mA cm−2. Furthermore, the NAORFB with the QAFPAE-100 membrane steadily runs at 60 mA cm−2 over 1500 cycles. Therefore, the ionic highway induced excellent performance of QAFPAE-100 membrane to render this material a promising candidate for practical NAORFB applications. [Display omitted] • A membrane with hydrophilic groups and hydrophobic block was designed. • The membrane has low swelling, high conductivity and chemical stability. • The membrane support over 1500 cycles of cells. [ABSTRACT FROM AUTHOR]

Salt cavern with a volume of hundreds of thousands of cubic meters for storing electrolyte has been considered as a promising large-scale storage technology. However, the solubility and electrochemical stability of active organic species are significantly limited in saturated brine solution. Here, an unsymmetrical two-electron viologens with high solubility in aqueous system is synthesized via a simple two-step reaction route. As a novel two-electron storage anolyte, its electrochemical properties are investigated in detail by cyclic voltammetry and rotating disk electrode voltammetry. Paired with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) derivatives, the battery delivers an exceptionally high cell voltage of 1.63 V, an energy efficiency of ~80%, and an average 99.95% capacity retention per cycle at 30 mA/cm2. To extend in saturated brine solution, the battery also exhibits a high cell efficiency and stable cycle performance, providing a promising strategy for developing the large scale salt-cavern redox flow batteries. • An unsymmetrical two-electron storage anolyte viologens was fabricated via two successive steps. • The molecule presented a high solubility and delivered a reversible two electron reduction reaction. • An average capacity retention rate of 99.95% per cycle was retained for 200 cycles. • The battery exhibited a high coulombic efficiency (~99%) and energy efficiency (~78%) in saturated brine solution. [ABSTRACT FROM AUTHOR]