Redox-active organic molecules are increasingly investigated as charge-storage materials for redox-flow batteries (RFBs), but material crossover between posolyte/negolyte and chemical degradation are limiting factors in the performance of all-organic RFBs. We demonstrate that the bipolar electrochemistry of 1,2,4-benzotriazin-4-yl (Blatter) radicals allows construction of batteries with symmetric electrolyte composition. Cyclic voltammetry shows that these radicals retain reversible bipolar electrochemistry also in the presence of water. The redox potentials of derivatives with a C(3)-CF<sub>3</sub> substituent are least affected by water and, moreover, these compounds show &gt;90% capacity retention after charge/discharge cycling for seven days (ca. 100 cycles) with the top-performers showing &lt;0.3% loss per day. Chemical synthesis provides insight in the nature of the charged species by spectroscopy and (for the oxidized state) X-ray crystallography, which confirms the high solution stability across all states-of-charge. Overall, this research demonstrates Blatter radicals to be promising candidates for symmetric RFBs based on organic active materials.

Blatter radicals as bipolar materials for symmetric redox-flow batteries

ACS Spring 2022

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