Redox couples are the working chemistry of a flow battery — the dissolved species whose oxidation state cycles between charge and discharge, setting cell voltage, energy density, and the bounds of every other component choice. This collection groups the active electrolytes and precursor salts used to build aqueous and non-aqueous flow cells at lab and pilot scale, organized by chemistry family so you can match a couple to your stack, membrane, and target duty cycle.
All-vanadium
The reference chemistry for stationary storage. VOSO4 and vanadium pentoxide precursors dissolved in concentrated sulfuric acid give you a single-element system on both sides — the V2+/V3+ negolyte and VO2+/VO2+ posolyte — which removes cross-contamination as a failure mode and is why all-vanadium remains the baseline for benchmarking new membranes and electrodes. Use this family when you are testing PFSA or hydrocarbon ion-exchange membranes, characterizing felt or paper electrodes, or running long-cycle stability work where capacity fade has to be unambiguously attributable.
Iron-chromium and all-iron
Lower-cost aqueous couples aimed at price-per-kWh rather than peak energy density. Iron-chromium uses FeCl2/FeCl3 against CrCl3 in HCl; all-iron pairs Fe(II)/Fe(III) with iron plating on the negative side. Suited to long-duration storage research where abundance of the active material matters more than volumetric energy.
Zinc-bromine and zinc-based hybrids
Hybrid flow chemistries where zinc plates onto the negative electrode while a soluble bromine complex cycles on the positive. ZnBr2 with bromine-sequestering quaternary ammonium agents is the classic formulation; useful when you need higher cell voltage and energy density than all-vanadium and can accept a plating-limited negative.
Organic and quinone-based couples
Aqueous and non-aqueous organic actives — anthraquinone sulfonates, viologens, TEMPO derivatives, ferrocene derivatives — for groups developing metal-free or tunable-potential systems. Solubility, stability against nucleophilic attack, and crossover through the separator are the dominant questions here, and the couple choice drives membrane selection more than in vanadium work.
If you are building a vanadium baseline cell, start with the vanadium precursors and pair them with carbon felts and PFSA membranes from Electrodes and Membranes. For organic-active research, also see Supporting Electrolytes and the broader Redox Flow Battery section.
