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Aqueous Symmetrical Redox Flow Batteries Based Upon a pH-Responsive Cobalt Complex
preprintsubmitted on 07.12.2019, 22:47 and posted on 16.12.2019, 12:59 by Hao Wang, Sayed Youssef Sayed, Yuqiao Zhou, Brian Olsen, Erik J. Luber, Jillian Buriak
Aqueous symmetric redox flow batteries (RFB) are of great interest due to the non-flammability and high conductivity of the solvent, and avoidance of irreversible anolyte crossover seen in asymmetric cells. In this work, we introduce a simple octahedral Co(II) complex, termed BCPIP-Co(II), that has 4 appended carboxylic groups on the ligand periphery that render it both water-soluble and pH-sensitive in the range of pH 1.5 - 5.5. The complex has reversible BCPIP-Co(II-III) and BCPIP-Co(II-I) redox couples within the water splitting window, as well as fast kinetics. The overall charge of the complex varies from +3 to -3, resulting from the level of deprotonation of the carboxylic acid moieties and the oxidation state of the cobalt metal center, both of which affect the resulting redox properties. BCPIP-Co(II) was then incorporated, as both the posolyte and negolyte, into a symmetric aqueous RFB, demonstrating Coulombic efficiencies >99% for up to 100 cycles.