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Aqueous Symmetrical Redox Flow Batteries Based Upon a pH-Responsive Cobalt Complex

submitted 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.


Climate Change Innovation Technology Framework

Alberta Innovates Technology Futures (grant number CTDP-G2018000919)

Graduate scholarship from Alberta Innovates Technology Futures to HW

NSERC (grant numbers RGPIN-2014-05195 and RGPIN-2019-04346)

Future Energy Systems of the University of Alberta (; grant number T12-P04)

Alberta Innovates Technology Futures Industry Chair, (grant number AITF iCORE IC50- T1 G2013000198)

Western Economic Diversification (WD, grant number 000014328)

Canada Research Chairs program (CRC 207142)


Email Address of Submitting Author


University of Alberta



ORCID For Submitting Author


Declaration of Conflict of Interest


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First version