Aminopolycarboxylate chelates are emerging as a promising class of electrolyte materials for aqueous redox flow batteries, offering tunable redox potentials, solubility, and pH stability through careful selection of ligands and transition metal ions. Despite their potential, the impact of molecular structure modifications on the electronic and electrochemical properties of these chelates remains underexplored. Here, we examine how introducing a hydroxyl group, often employed to enhance solubility, on the backbone of the reference chelate CrPDTA significantly changes the thermodynamics and kinetics of its redox process. We correlate changes in molecular and electronic structures with different electrochemical responses resulting from the hydroxyl addition and show that the introduction of this functional group leads to a distortion in the octahedral coordination of chromium. This distortion leads to increased anisotropy in spin density, weaker metal–ligand orbital overlap, and a non-integral change in chromium oxidation state, collectively resulting in significantly slower electron transfer kinetics. Our findings underscore the importance of maintaining a stable hexacoordinate geometry across operational pH ranges and caution against ligand modifications that compromise the octahedral structure. These insights inform future design strategies for high-performance APC-based negative electrolytes in redox flow batteries.

Jonathan R. Thurston, Keith P. White, Max Kudisch, Luis Kitsu Iglesias, Julia Lorenzetti, Francesco Bernasconi, Michael F. Toney, Michael P. Marshak, David Reber, Batteries & Supercaps, 2025, e202500250