Aqueous 2,2,6,6-Tetramethylpiperidine-N-oxyl Catholytes for a High-Capacity and High Current Density Oxygen-Insensitive Hybrid-Flow Battery

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.