We report on novel aqueous redox flow batteries (RFBs) based on 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPOL) as an active material.  TEMPOL is a stable and highly water-soluble nitroxyl radical compound.  Electrochemical and UV-VIS measurements highlight that the pH value of electrolyte has a significant impact on the redox reversibility of TEMPOL.  The half-wave potential of TEMPOL was determined to be 0.61 V vs. Ag/AgCl (0.83 V SHE.). Furthermore, its peak separation of 73 mV at a sweep rate of 10 mV s^-1 is close to the theoretical value (59 mV) expected for a reversible one-electron reaction.  We demonstrate that the diffusion coefficient of TEMPOL in neutral aqueous solution is comparable to the vanadium ions used in practical RFBs.  TEMPOL also exhibits a quasi-reversible redox reaction, defined by a distinct kinetic rate constant, in near-neutral aqueous solutions.

  We show that RFBs incorporating TEMPOL catholyte and Zn-based anolyte have an average discharge voltage of 1.46 V and an energy efficiency of 80.4% during the initial cycle, when subject to a constant current of 10 mA cm^-2.  We also introduce a green aqueous RFB using TEMPOL catholyte and anthraquinone-2,7-disulfonic acid disodium salt (AQ27Na₂) as anolyte.  This green RFB contains no toxic or flammable compounds, and exhibits an open-circuit voltage of 1.22 V and an average discharge voltage of 0.74 V.  The TEMPOL/Zn and TEMPOL/AQ27Na₂ RFBs both show stable and long-term cycling at 10% SOC. Our approach offers a promising direction to develop inexpensive, environmentally-friendly, and safe RFBs.