In a Flow battery we essentially have two chemical components that pass through a reaction chamber where they are separated by a membrane. A significant benefit is that the charged fluids can be stored in containers, significantly extending the energy storage capacity.
Cell Design
Tubular Redox Flow Cell Design
In a remarkable study led by Dr. Thorsten Struckmann at the Hamburg University of Applied Sciences and colleagues, published in the Journal of Power Sources (2025), the team demonstrated that extruded tubular redox flow cells integrated into stacks can deliver volumetric power densities above 750 W/L, with almost no performance loss from cell to stack.
Chemistry / Type
All-Iron Flow Battery
A redox flow battery using iron in different oxidation states on both sides of the cell, giving very low material cost since only one element is needed.
Aqueous Organic Redox Flow Battery (AORFB)
Replaces metal-based redox species with water-soluble organic molecules, avoiding metal costs and supply constraints entirely.
Hybrid Flow Battery
Flow battery in which one or both electrode reactions involve plating and stripping a solid phase rather than a purely dissolved redox couple on both sides.
Quinone Flow Battery
A specific aqueous organic redox flow battery chemistry using quinone molecules as the redox-active species, one of the most studied metal-free flow chemistries.
Redox-Targeting Flow Battery
A flow battery that uses a soluble redox mediator to access the capacity of solid storage material, extending energy density beyond what dissolved species alone could provide.
TEMPO Flow Battery
A flow battery chemistry using nitroxide-radical TEMPO molecules as the redox-active species, valued for fast reaction kinetics.
Vanadium Flow Battery
- Round trip efficiency ~60 to 80%
- Footprint ~ 20 to 50kWh/m2
Zinc Bromide Flow Battery
- Round trip efficiency ~60 to 70%
- Footprint ~ 2 to 43kWh/m2
Zinc-Iodine Flow Battery
A flow battery chemistry pairing a zinc electrode with an iodine/iodide redox couple.
Zinc-Iron Flow Battery
A hybrid flow battery combining a zinc-plating anode with an iron-based redox cathode.
Zinc-Nickel Flow Battery
Hybrid flow battery combining a zinc-plating anode with a nickel-based redox cathode, offering a higher cell voltage than zinc-bromine flow batteries.
Zinc Slurry Flow Battery
Flow battery that circulates zinc nanoparticles dispersed in a stabilizing ligand-based slurry, aiming for durable long-duration zinc-based energy storage.
Failure Mechanisms
Addressing Precipitation and Clogging Vanadium Redox Flow Battery Systems
One of the lesser-discussed failure mechanisms in Vanadium Redox Flow Battery (VRFB) systems is precipitation and clogging within the positive electrolyte side.
In many cases, stack leakage or system faults do not originate from the electrochemical cell itself, but from electrolyte flow restrictions caused by precipitation that gradually blocks the felt electrode or manifolds.
Lifetime
Its not easy to compare degradation 1:1 with Li-ion. It is hard to even define SOH unambiguously. Electrolyte in RFB either vanadium or iron for example can be rebalanced and capacity restored to 100% for tens of tousends of cycles, that is an impossible task for Li-ion. The cycle life is not chemistry limited but rather dependant on materials stability and serviceability of stacks, hydraulics, etc. Therefore, direct comparison with Li is misleading.
Flow batteries don’t really show a “knee point” in the same way lithium-ion cells do because their degradation mechanisms are mostly chemical and reversible rather than structural or interfacial. In vanadium systems, the capacity fade tends to be linear over time, mainly linked to electrolyte imbalance, ion crossover and gradual carbon surface oxidation.
Some operators indeed extend operation well below 80% SOH, since performance remains predictable as long as electrolyte rebalancing and maintenance are done properly. That flexibility is one of the reasons why flow batteries are gaining space in long-duration storage and heavy cycling applications.
See also: Iron-Chromium Flow Battery, Vanadium Redox Flow Battery (VRFB)