Hybrid-flow batteries are a suitable storage technology for "green" electricity generated by renewable sources such as wind power and solar energy. Redox-active organic compounds have recently been investigated to improve the traditional metal-and halogen-based technologies. Here we report the utilization of a 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) derivative that is in particular designed for application in semiorganic zinc hybrid-flow batteries. The TEMPO derivative is synthesized and electrochemically characterized via cyclic voltammetry and rotating disc electrode measurements. This derivative features a high solubility in aqueous electrolytes; thus, volumetric capacities above 20 Ah L −1 are achieved. The fabricated hybrid-flow batteries feature over 1100 consecutive charge−discharge cycles at constant capacity retention, and current densities up to 80 mA cm −2 are applied.
The combination of
2,2,6,6-tetramethylpiperidinyl-N-oxyl and phenazine
yields an organic redox-active material for redox-flow
battery applications. This combined molecule (combi-molecule) features
a redox voltage of 1.2 V and facilitates the utilization of aqueous
electrolytes. It was synthesized from cost-efficient starting materials,
electrochemically characterized and applied as charge-storage material
in a symmetric aqueous redox-flow battery.
Herein, we present a novel copolymer (1), which incorporates (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) as a redox-active compound and the zwitterionic [(2-(methacryloxy)ethyl)dimethyl-(3-sulfopropyl)]ammonium hydroxide as a solubilizing comonomer, for the application as catholyte species within aqueous redox flow batteries (RFBs). The presented polymer-based redox-active material exhibits a high degree of oxidation and, compared to other commonly utilized active polymeric materials, a high solubility exceeding 20 Ah L −1 , while still featuring a low viscosity in 1.5 M NaCl aq solution. The electrochemical behavior was investigated by cyclic voltammetry, and a reversible redox reaction at E 0 = 0.7 V versus the Ag/AgCl reference electrode of the TEMPO/TEMPO + redox pair was observed. Symmetric design battery studies with two different types of membranes, a size-exclusion versus an anion-exchange membrane, were used to evaluate the applicability of this polymer in the RFB setup. Long-term stability tests over 1000 cycles indicate good stability with a capacity loss of ca. 0.08% per cycle utilizing a size-exclusion and an anion-exchange membrane, respectively. Finally, an allorganic aqueous RFB was operated utilizing 1 as the catholyte species and N,N′-dimethyl-4,4′-bipyridinium dichloride (MV) as the anolyte species. Such RFB exhibits Coulombic efficiencies of 99.01 ± 1.40% over 125 consecutive cycles, an energy efficiency of ca. 93%, and an initial energy density of 5.33 Wh L −1 during the studied discharge process.
The volumetric capacities and the lifetime of organic redox flow batteries (RFBs) are strongly dependent on the concentrations of the redox-active molecules in the electrolyte. Single-molecule redox targeting represents an efficient approach toward realizing viable organic RFBs with low to moderate electrolyte concentrations. For the first time, an all-organic Nernstian potential-driven redox targeting system is investigated that directly combines a single-electrode material from organic radical batteries (ORBs) with a single redox couple of an aqueous, organic RFB, which are based on the same redox moiety. Namely, poly(TEMPO-methacrylate) (PTMA) is utilized as the redox target ("solid booster") and N,N,N-2,2,6,6-heptamethylpiperidinyloxy-4ammonium chloride (TMATEMPO) is applied as the sole redox mediator to demonstrate the redox targeting mechanisms between the storage materials of both battery types. The formal potentials of both molecules are investigated, and the targeting mechanism is verified by cyclic voltammetry and state-of-charge measurements. Finally, battery cycling experiments demonstrate that 78−90% of the theoretical capacity of the ORB electrode material can be addressed when this material is present as the redox target in the electrolyte tank of an operating, aqueous organic RFB.
Flow Batteries (FBs) currently are one of the most promising large-scale energy storage technologies for energy grids with a large share of renewable electricity generation. Among the main technological challenges...
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