Page 63 - Energize March 2022

TECHNICAL

During charge, metallic zinc gets deposited on the anode in a The redox chemistry of the FeB is based on the iron (II)/
reduction reaction. Bromide ions are oxidized to bromine gas, chloride/iron (III) chloride couple at the positive electrode and the
shedding electrons to the cathode. The electrolyte in the cathode iron (II) chloride/metallic iron couple at the negative electrode.
half is added with an amine compound to help hold the bromine. During discharge of the battery, iron (III) chloride is reduced to iron
Reverse actions take place on discharge with zinc layer on the (II) chloride at the positive electrode. At the negative electrode,
anode getting depleted. Thus, the amount of zinc on the anode metallic iron is dissolved into the electrolyte as iron (II) chloride;
limits the capacity of the battery. these processes are reversed during battery charging.

Reactions are as follows:
Positive electrode: 2FeCl+ 2e ↔ 2FeCl+ 2Cl
Negative electrode: Fe + 2Cl ↔ FeCl + 2e -
Overall cell reaction: Fe + 2 FeCl ↔ 3FeCl E = 1,21V

Storage capacity is limited because iron is plated out during the
charge process, although claims of 15 times maximum power
output have been made. There is currently one company in the
6
USA producing iron redox flow batteries, and there are several
working installations. 6

Costs
Electrolyte constitutes a large portion of the flow battery, and
the cost of the electrolyte can typically be 35% of the total cost.
Electrolyte costs increase as storage capacity is increased and the
Figure 5: A typical zinc bromine battery use of cheaper electrolytes may provide a cost advantage for high
storage capacity batteries.
When the ZB is charged, the overall chemical reaction involves the
reduction of zinc and evolution of bromine, as simplified form in Modular systems
eqn. (1). RFBs on the market are typically supplied as standard sized units in
modular form, either as individual units, or integrated into containers
ZnBr2 → Zn + Br2 (1) as power units, with standard power output and duration.

Similarly, zinc and bromine recombine to form ZnBr2 , when the Future developments
ZBM is discharged, Future developments are taking place in the direction of organic
RFBs, which look promising although no product has yet reached
Zn + Br 2 → Zn Br 2 (2) commercialisation stage.

There are several manufacturers world-wide. In South Africa a References
bank Of ZB redox flow batteries with a capacity of 140 kWh is in 1. M Barnard: “Flow batteries are an area of strong innovation and
operation at PCB manufacturer Bosco. opportunity”, Cleantechnica, August 2021.
2. E Sanchez Diez, et al: “Redox flow batteries: Status and
Iron redox battery ( FeB) perspective towards sustainable stationary energy storage”, JPS
The iron redox battery is a recent development that uses commonly 481 2021.
available, cheap and non-toxic iron compounds for the electrolyte. 3. ESS: “ESS iron flow chemistry”, https://essinc.com/iron-flow-
Figure 6 shows a typical all iron flow battery. chemistry/
4. Bushveld Energy Selects Abengoa For A Solar PowerPlant
With Vanadium Flow Battery Storage, Energy and mines.com,
December 2020
5. A Colthorpe: “China’s largest solar-plus-flow battery project will
be accompanied by VRFB ‘gigafactory’”, Energy storage news,
March 2021
6. R Narayan: “All iron redox flow battery”, USC Dornsife.
7. A Dinesh: “Iron-based flow batteries to store renewable
energies”, Environmental Chemistry Letters,·February 2018

Figure 6: The all iron FeB 7 Send your comments to rogerl@nowmedia.co.za

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