Flow Batteries

In a flow battery, the charged solutions are stored in external tanks and are pumped past the electrodes. This has several advantages:

  • it can offer almost unlimited capacity simply by using larger and larger storage tanks,
  • it can be left completely discharged for long periods with no ill effects,
  • it can be recharged simply by replacing the electrolyte if no power source is available to charge it,  
  • if the electrolytes are accidentally mixed the battery suffers no permanent damage.

Flow batteries can be divided into different types:

Redox has all the reactants in solution and is regarded at a rechargeable fuel cell. Examples are: vanadium redox flow battery, polysulfide bromide battery, and uranium redox flow battery.

Hybrid has one side in solution, and the other as the element. It is really the process of electro-plating, made reversible. E.g. zinc-bromine, zinc-cerium, and lead-acid flow batteries.

Membraneless avoids the need for a membrane separates the two liquid by using laminar flow.  usefule with strong acids.

Organic A quinone such as 9,10-anthraquinone-2,7-disulphonic acid (AQDS) can be used.

Metal Hydride -This battery stores hydrogen as a metal hydride.

Nano-Network The reactants are stored in nano particles that allow electricity to flow throughout the liquid.  Source


Small particles can be suspended in a viscous liquid.

Details of each type below.

A flow battery works on the same principle as other batteries: different atoms and ions atract electrons differently. And this difference give us the voltage we can expect, or hope for.

E.G. Copper ions in solution will attract electrons off the zinc metal. When the electrons leave the zinc, it will become an ion and go into solution.

This is reversed during charging.

In an electro chemical cell the solution are stored in the cell, usually as a paste. A flow battery stores th eliquid outside the cell.

Electrochemical cell

Flow battery


Vanadium redox flow battery

 Vanadium redox batteries are based on the ability of vanadium to exist in four different oxidation states (V2+, V3+, V4+, and V5+),

The version with Vanadium in a solution of sulfuric acid was developed by  Maria Skyllas-Kazacos - University of New South Wales in the 1980s.

 There are various developments since then. One is to modify the H2SO4 electrolyte by adding HCl (2.5:6). This allows 70% more Vanadium to be dissolved in solution, giving 70% more capacity. PNNL

There are a number of suppliers and Vanadium flow batteries: 


Iron Chromium flow battery

Developed by NASA and Mitsui in the 1970-80s. Use Fe2+ Fe3+ and Cr 2+ and Cr3+

.1.2 V and 70-80% efficient. Has the potential to be very cheap.


Polysulphide Bromide (PSB)

Sodium bromide and sodium polysulphide solutions are separated by a polymer membrane that only allows sodium ions to go through, producing about 1.5V. The net efficiency of this battery is about 75%. The positive electrolyte is sodium bromide, and the negative electrolyte is sodium polysulfide.   Source

The reactions are:

Positive Electrode: Br2 + 2e– → 2Br2- (discharging)

Negative Electrode: 2S2 → S42- + 2e– (discharging)

At the positive electrode, three bromide ions combine to form a tribromide ion. At the negative electrode, the sulfur in solution is shuttled between polysulfide and sulphide.


Zinc Bromine flow battery

The zinc-bromine battery is a hybrid redox flow battery, because the energy is stored by plating zinc metal as a solid onto the anode plates in the electrochemical stack during charge. Thus, the total energy storage capacity of the system is dependent on both the stack size (electrode area) and the size of the electrolyte storage reservoirs. The zinc-bromine flow battery was developed by Exxon as a hybrid flow battery system in the early 1970s.

In each cell of a zinc-bromine battery, two different electrolytes flow past carbon-plastic composite electrodes in two compartments, separated by a micro-porous polyolefin membrane. The electrolyte on the anode (negative) side is purely water-based, while the electrolyte on the positive side also contains an organic amine compound to hold bromine in solution.

During charge, metallic zinc is plated (reduced) as a thick film on the anode side of the carbon-plastic composite electrode . Meanwhile, bromide ions are oxidized to bromine and evolved on the other side of the membrane.  Bromine has limited solubility in water, but the organic amine in the catholyte reacts with the bromine to form a dense, viscous bromine-adduct oil that sinks to the bottom of the catholyte tank. The bromine oil must later be re-mixed with the rest of the catholyte solution to enable discharge. 

During discharge, the zinc metal, plated on the anode during charge, is oxidized to Zn2+ ion and dissolved into the aqueous anolyte. Two electrons are released at the anode to do work in the external circuit.
 The electrons return to the cathode and reduce bromine molecules (Br2) to bromide ions (2Br-), which are soluble in the aqueous catholyte solution.
The bromine in the catholyte is decomplexed from the amine and converted into two bromide (Br-) ions at the cathode, balancing the Zn2+ cation and forming a zinc bromide solution. The chemical process used to generate the electric current increases
the zinc-ion and bromide-ion concentration in both electrolyte tanks.
The net DC-DC efficiency of this battery is reported to be in the range of 65-75%.

The zinc-bromine redox battery offers one of the highest cell voltages and releases two electrons per atom of zinc.  These attributes combine to offer one of the highest energy density among flow batteries. However, the high cell voltage and highly oxidative element, bromine, require cell electrodes, membranes, and fluid handling components that can withstand the chemical conditions.  These materials are expensive.  Bromine is a highly toxic material through inhalation and absorption.  Maintaining a stable amine complex with the bromine is key to system safety.  Active cooling systems are provided by system manufacturers to maintain stability of the bromine-amine complex.  In addition, repeated plating of metals in general is difficult due to the formation of “rough” surfaces (dendrite formation) that can puncture the separator.
Special cell design and operating modes (pulsed discharge during charge) are required to achieve uniform plating and reliable operation.

Zinc bromine batteries from different manufacturers have energy densities ranging from 34.4–54 Wh/kg.Energy Storage Assn.

Voltage conversion by flow battery- quite remarkable!


Because cells can be used in series or in parallel, all in the same solution, then several remarkable conversions are possible: If say 120 cells at 2 volts each were set up, in the same solution, then:

In parallel they can be charged at 2 volts, then in series, discharged at 240 volts. Or vice versa. 240 could be converted to 120, 60, 30, 20,12, 8, 6, 4, 2.

With a zinc bromine battery the cell voltage is 1.67 V. so 144 cells would be needed to produce 240V. It could discharge at any voltage that is a multiple of the cell voltage and divides into 144V. There are complications and opportunities here, you'll need to think it through yourself.

With switching gear, an AC current can be fed into the cell as DC, then retrieved as DC, or through switching gear, as AC. Again at various voltages.




The primary features of the zinc bromine battery are:

  • Energy density of 15-39 Wh/L or 34-54 Wh/KG
  • Charge/discharge efficiency 76%
  • 100% depth of discharge capability on a daily basis
  • Life of > 2,000 cycles at 100% depth of discharge. Service adds 1,500 cycles
  • No shelf life limitations.
  • Three examples of zinc–bromine flow batteries are ZBB Energy Corporation's Zinc Energy Storage System (www.ensync.com), RedFlow Limited's Zinc Bromine Module (ZBM), and Premium Power's Zinc-Flow Technology.  Wikipedia

Zinc-polyiodide flow battery

"Lab tests revealed the demonstration battery discharged 167 watt-hours per liter of electrolyte. In comparison, zinc-bromide flow batteries generate about 70 watt-hours per liter, vanadium flow batteries can create between 15 and 25 watt-hours per liter, and standard lithium iron phosphate batteries could put out about 233 watt-hours per liter. Theoretically, the team calculated their new battery could discharge even more — up to 322 watt-hours per liter — if more chemicals were dissolved in the electrolyte.

.... PNNL's zinc-polyiodide battery is also safer because its electrolyte isn't acidic like most other flow batteries." PNNL

February 25, 2015

"Safe and versatile, but not perfect yet"


Polyiodide ranges from I4 - I29.

Flow assisted zinc manganese dioxide battery

Less expensive than most other batteries

The CUNY Energy Institute, in partnership with Rechargeable Battery Corporation (RBC) and Ultralife Corporation, is developing  a water-based flow-assisted battery using zinc and manganese dioxide. They claim 5,000 cycles, for under $100/kWh.    Source

Zinc-nickel oxide flow battery

Also being developed by CUNY.

The problem of zinc dendrite formation has been eliminated by cleaning cycles built into the flow process.


Hyrogen bromine flow battery - HBr, hydrobromic acid is corrosive and will destroy most membranes and batteries.

This design separates the two liquid by using laminar flow. This type of flow is in non mixing layers rather than tumbling as in turbulent flow.

Basically, the new battery was engineered with a slim channel between two electrodes. 

“Through the channel, the group pumped liquid bromine over a graphite cathode and hydrobromic acid under a porous anode. At the same time, the researchers flowed hydrogen gas across the anode. The resulting reactions between hydrogen and bromine produced energy in the form of free electrons that can be discharged or released.”

The membrane-less design enables power densities of 0.795 W cm−2 at room temperature and atmospheric pressure, with a round-trip voltage efficiency of 92% at 25% of peak power.


Source: Nature




Flow battery using quinones

Jan 2014. A team of Harvard scientists and engineers has demonstrated a new type of battery base on quinones instead of metal. They calculated the properties of more than 10,000 quinone molecules in search of the best candidates for the battery.

Quinones are abundant in crude oil as well as in green plants. The molecule that the Harvard team used in its first quinone-based flow battery is almost identical to one found in rhubarb. The quinones are dissolved in water, which prevents them from catching fire.

The new flow battery developed by the Harvard team already performs as well as vanadium flow batteries, with chemicals that are significantly less expensive, and with no precious metal electrocatalyst.

The battery’s anode uses a solution of sulphuric acid containing a quinone. The quinone is cheap and does not need a catalyst to form a higher-energy hydroquinone, thereby charging the battery. It has a cathode that alternates between bromine and hydrobromic acid.

The quinone–hydroquinone reaction is about 1,000 times faster than the vanadium reaction, allowing the battery to charge and discharge rapidly.

The problem is the hydrobromic acid as it is so corrosive. The team is working on replacing the bromine with a different quinone.



A quinone is a class of organic compounds that are formally "derived from aromatic compounds [such as benzene or naphthalene] by conversion of an even number of –CH= groups into –C(=O)– groups with any necessary rearrangement of double bonds," resulting in "a fully conjugated cyclic dione structure. The class includes some heterocyclic compounds.

Metal Hydride

Proton Flow Battery. A battery could be made by electrolysis of water to produce Hydrogen and oxygen, then these produce electrolysis in a fuel cell. But the problem is Hydrogen is difficult to store. This battery stores it as a metal hydride.

During charging, water is split into oxygen and hydrogen ions (protons). These attract electrons from a metal electrode, forming a metal hydride, which is solid and easily stores the hydrogen. When electricity is produced, the process is reversed and the protons are combined with oxygen from the air. Ref    Paper


Nano-Network flow battery 

Lithium–sulfur is extremely attractive as it has a theoretical energy density the same as petrol (Allowing for different efficiencies of electric motor vs ICE = 15/70). If the two elements are arranged in a network of nanoparticles then it avoids all sorts of difficulties. The network eliminates the requirement that charge moves in and out of particles that are in direct contact with a conducting plate. Instead, the nanoparticle network allows electricity to flow throughout the liquid.  Source

Nano Flowcell AG super sports car

NanoFlowcell AG are building a new car to be powered by their Nano-flow battery. They won't reveal what type of nano cell it is. Claim "It runs on salt water".  (How stupid do they think we are?)

Recharging is a matter of exchanging the used ioninc liquid for a recharged one. At the refuelling station, the liquid would then be recharged  from the mains. Interesting challenge to price the fuel.

They claim their car has a range of 600 - 1,000 KM. (Their claims vary).   Four motors, 2 x 200 litre tanks, 920-1,000 HP, and unbelievable acceleration figures. 2.8-second 0-62 mph (100 km/h) time and a potential 236 mph (380 km/h) top speed.

Nanoflowcell explains that its technology boasts five to six times the storage capacity of other flow cell designs or lithium-ion batteries.

Their Quant car design uses supercapacitors to release energy quickly, allowing for the sportiest performance. Gizmag

It all sound too good to be true, so until it has been independantly tested, skeptisim is permitted.

Nano flowcell powered car - 600-1,000 kM range.

If you want to sell a new technology, design a sexy looking car.

Refuelling is by plugging into an electricity supply or replacing the liquid. (Would you need to completely empty the tank first?)

Semi-solid flow battery

In a semi-solid flow cell, the positive and negative electrodes are composed of particles suspended in a carrier liquid. The positive and negative suspensions are stored in separate tanks and pumped through separate pipes into a stack of adjacent reaction chambers, where they are separated by a barrier such as a thin, porous membrane. The approach combines the basic structure of aqueous-flow batteries, which use electrode material dissolved in a liquid electrolyte, with the chemistry of lithium-ion batteries. Dissolving a material changes its chemical behavior significantly. However, suspending bits of solid material preserves the solid's characteristics. The result is a viscous suspension that flows like molasses.


Academic Papers

A new kind of flow battery is fueled by semi-solid suspensions of high-energy-density lithium storage compounds that are electrically ‘wired’ by dilute percolating networks of nanoscale conductor particles. Energy densities are an order of magnitude greater than previous flow batteries; new applications in transportation and grid-scale storage may result.  Abstract

A membrane-free lithium/polysulfide semi-liquid battery for large-scale energy storage

In this article, we develop a new lithium/polysulfide (Li/PS) semi-liquid battery for large-scale energy storage, with lithium polysulfide (Li2S8) in ether solvent as a catholyte and metallic lithium as an anode. Unlike previous work on Li/S batteries with discharge products such as solid state Li2S2 and Li2S, the catholyte is designed to cycle only in the range between sulfur and Li2S4.  Abstract

GE flow battery - (A vague PR claim)

"Scientists at GE Global Research and Lawrence Berkeley National Laboratory are developing a new kind of water-based “flow” battery for electric vehicles. the batteries could be 75 percent cheaper than car batteries available on the market today. GE engineers  say that unlike lithium-ion and other battery systems, the new technology will use water-based solutions of inorganic chemicals capable supplying high energy density by ferrying more than one electron at a time." Source: GEreports

Chemistries tried with flow batteries

Frontiers in Chemistry