Research on redox‐flow batteries (RFBs) is currently experiencing a significant upturn, stimulated by the growing need to store increasing quantities of sustainably generated electrical energy. RFBs are promising candidates for the creation of smart grids, particularly when combined with photovoltaics and wind farms. To achieve the goal of “green”, safe, and cost‐efficient energy storage, research has shifted from metal‐based materials to organic active materials in recent years. This Review presents an overview of various flow‐battery systems. Relevant studies concerning their history are discussed as well as their development over the last few years from the classical inorganic, to organic/inorganic, to RFBs with organic redox‐active cathode and anode materials. Available technologies are analyzed in terms of their technical, economic, and environmental aspects; the advantages and limitations of these systems are also discussed. Further technological challenges and prospective research possibilities are highlighted.

The combination of a polymer-based 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) catholyte and a zinc anode, together with a cost-efficient size-exclusion membrane, builds a new type of semi-organic, "green," hybrid-flow battery, which features a high potential range of up to 2 V, high efficiencies, and a long life time.

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.

A novel redox-active polymer based on a 9,10-di(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (exTTF) system in combination with a conjugated backbone was synthesized via rhodium (Rh)-catalyzed polymerization of 2-ethynyl(exTTF), leading to polymers with low polydispersities. Composite electrodes containing this polymer exhibited chemically reversible two-electron oxidation in aqueous media. The application of these electrodes as active cathode materials in hybrid zinc-organic batteries using an aqueous electrolyte enabled the production of air-stable charge storage systems with a theoretical capacity of 133 mAh g − 1. These batteries featured high performance, charge/discharge rates of up to 120 C (30 s) and an ultra-long lifetime, of over 10 000 charge/discharge cycles (accompanied by a minor capacity loss of 14%). Finally, the polymer was compared with its nonconjugated derivative, revealing the positive influence of the conjugated backbone on the material activity owing to improved electron transfer within the polymer chain.

The utilization of boron-dipyrromethene (BODIPY) as active group for the charge storage process in a battery application is reported. Two BODIPY-containing copolymers were synthesized and electrochemically characterized. The polymers feature redox processes at 0.7 V and −1.5 V vs AgNO3/Ag, which enable the application in a redox-flow battery setup.

A poly(TEMPO methacrylate)-poly(styrene) block copolymer was utilised as catholyte in polymer/zinc hybrid flow batteries.

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