Coordination-state regulation of aqueous negolytes: electronic structure, degradation, and high-voltage flow-battery performance

Aqueous redox flow batteries are promising for long-duration energy storage, but their cell voltage is limited by the instability of highly reducing species in water. This review establishes a coordination-chemistry framework for low-potential aqueous negolytes, focusing on strong-field hexacyanometallates, multidentate metal chelates, and polyoxometalate-based clusters. The key premise is that negolyte performance depends not on formal potential alone, but on a reversible coordination trajectory that preserves solubility, kinetic accessibility, and chemical integrity throughout cycling and ageing. Ligand-field strength, spin state, Jahn–Teller distortion, denticity, coordinated water, free-ligand activity, protonation, counterion association, and membrane transport are therefore considered as coupled design variables. Chromium and manganese hexacyanides demonstrate how strong-field ligation stabilizes low-valent states and enables multielectron storage, whereas Fe–DTPA/Ce–DTPA and Fe(TPEN) systems highlight the roles of chelation, shared-ligand design, and ligand-exchange resistance. Polyoxometalates further show how proton-coupled electron transfer, reversible structural adaptation, and solvent-network regulation can expand the practical aqueous voltage window while maintaining low-temperature conductivity. Operando spectroscopy, electrochemical mass spectrometry, and molecular simulation are critically assessed as tools for resolving coordination-state evolution and degradation. Finally, evidence-based design principles and standardized reporting criteria are proposed to guide the development of durable, high-voltage aqueous negolytes.