Blatter Radicals as Bipolar Materials for Symmetrical Redox-Flow Batteries br

Redox-active organic molecules are promisingcharge-storage materials for redox-flow batteries (RFBs), butmaterial crossover between the posolyte and negolyte and chemicaldegradation are limiting factors in the performance of all-organicRFBs. We demonstrate that the bipolar electrochemistry of 1,2,4-benzotriazin-4-yl (Blatter) radicals allows the construction ofbatteries with symmetrical electrolyte composition. Cyclicvoltammetry shows that these radicals also retain reversible bipolarelectrochemistry in the presence of water. The redox potentials ofderivatives with a C(3)-CF3substituent are the least affected bywater, and moreover, these compounds show >90% capacityretention after charge/discharge cycling in a static H-cell for 7 days(ca. 100 cycles). Testing these materials in aflow regime at a 0.1 Mconcentration of the active material confirmed the high cycling stability under conditions relevant for RFB operation anddemonstrated that polarity inversion in a symmetricalflow battery may be used to rebalance the cell. Chemical synthesis providesinsight in the nature of the charged species by spectroscopy and (for the oxidized state) X-ray crystallography. The stability of thesecompounds in all three states of charge highlights their potential for application in symmetrical organic redox-flow batteries.