Zinc–bromine battery

The zinc–bromine flow battery is a type of hybrid flow battery. A solution of zinc bromide is stored in two tanks. When the battery is charged or discharged, the solutions (electrolytes) are pumped through a reactor stack and back into the tanks. One tank is used to store the electrolyte for the positive electrode reactions, and the other for the negative. Zinc–bromine batteries from different manufacturers have energy densities ranging from 60 to 85 W·h/kg.^[1]

The predominantly aqueous electrolyte is composed of zinc bromide salt dissolved in water. During charge, metallic zinc is plated from the electrolyte solution onto the negative (carbon felt in older designs, titanium mesh in modern) electrode surfaces in the cell stacks. Bromide is converted to bromine at the positive electrode surface and is stored in a safe, chemically complexed organic phase in the electrolyte tank. Older ZBRFB cells used polymer membranes (microporous polymers, Nafion etc.) More recent designs do not use membrane at all.^[3] The battery stack is typically made of carbon-filled plastic bipolar plates (e.g. 60 cells), and it is enclosed into a high-density polyethylene (HDPE) container. The zinc–bromine battery can be regarded as an electroplating machine. During charging, zinc is electroplated onto conductive electrodes, while at the same time bromine is formed. On discharge, the reverse process occurs: the metallic zinc plated on the negative electrodes dissolves in the electrolyte and is available to be plated again at the next charge cycle. It can be left fully discharged indefinitely without damage.

A new type of zinc–bromine battery, called a zinc–bromine gel battery, is currently being developed in Australia. It is lighter, safer, quicker to charge, and flexible.^[4]

Features[]

RedFlow ZBM2 10kWh flow batteries in a performance testing lab

The primary features of the zinc–bromine battery are:

• High-energy density relative to lead–acid batteries.
• 100% depth of discharge capability on a daily basis.^[5]
• No shelf-life limitations, as zinc–bromine batteries are non-perishable, unlike lead–acid and lithium-ion batteries, for example.^[5]
• Scalable capacities.

Drawbacks include:

• The need to be fully discharged every few days to prevent zinc dendrites that can puncture the separator.^[5]
• The need every 1–4 cycles to short the terminals across a low-impedance shunt while running the electrolyte pump, to fully remove zinc from battery plates.^[5]
• Low areal power (<0.2 W/cm²) during both charge and discharge, which translates into a high cost of power.^[6][7][8]

Zinc–bromine flow battery providers include:

• Primus Power – Hayward, California, USA.
• RedFlow Limited – Brisbane, Australia.
• Smart Energy – Shanghai, China.
• EnSync (Formerly ZBB)^[9] – Menomonee Falls, Wisconsin, USA.
• ZBEST Power – Beijing, China.

These battery systems compete to provide energy storage solutions at a lower overall cost than other energy storage systems such as lead–acid, vanadium redox, sodium–sulfur, lithium-ion and others^{[citation needed]}.

Electrochemistry[]

At the negative electrode zinc is the electroactive species. Zinc has long been used as the negative electrode of primary cells. It is a widely available, relatively inexpensive metal, which is electropositive, with a standard reduction potential E° = −0.76 V vs SHE. However, it is rather stable in contact with neutral and alkaline aqueous solutions. For this reason, it is used today in zinc–carbon and alkaline primaries.

In the zinc–bromine flow battery the negative electrode reaction is the reversible dissolution/plating of zinc:

${\displaystyle {\ce {Zn_{(s)}<=>{Zn^{2}+}_{(aq)}+2e^{-}}}}$

At the positive electrode bromine is reversibly reduced to bromide (with a standard reduction potential of +1.087 V vs SHE):

${\displaystyle {\ce {{Br2_{(aq)}}+2e^{-}<=>{2Br^{-}}_{(aq)}}}}$

The overall cell reaction is therefore

${\displaystyle {\ce {{Zn_{(s)}}+Br2_{(aq)}<=>{2Br^{-}}_{(aq)}+{Zn^{2}+}_{(aq)}}}}$

The measured potential difference is around 1.67 V per cell (slightly less than that predicted from the standard reduction potentials).^{[citation needed]}

The two electrode chambers of each cell are divided by a membrane (typically a microporous or ion-exchange variety). This helps to prevent bromine from reaching the positive electrode, where it would react with the zinc, causing the battery to self-discharge. To further reduce self-discharge and to reduce the vapor pressure of bromine, complexing agents are added to the positive electrolyte. These react reversibly with the bromine to form an oily red liquid and reduce the Br
₂ concentration in the electrolyte.^{[citation needed]}

Applications[]

Remote telecom sites[]

Significant fuel savings are possible at remote telecom sites operating under conditions of low electrical load and large installed generation using multiple systems in parallel to maximize the benefits and minimize the drawbacks of the technology.^[10]

Zinc–bromine gel batteries[]

Zinc–bromine batteries use a liquid to transport the changed particles, which makes them unsuitable for mobile use. A new development, by Thomas Maschmeyer from the University of Sydney, replaces the liquid with a gel. Gel is neither a liquid nor a solid, but has the advantages of both. Ions can move quicker, decreasing charging time. It is also more efficient, longer-lasting, and cheaper than lithium, and the gel is fire-retardant.^[11] As of April 2016, Gelion, which is the spin-off company of Sydney University, is developing the battery for commercial use. The company was boosted by an $11 million investment from UK renewables group .^[12] As the batteries are also flexible, they can be incorporated into the fabric of buildings. This creates the possibilities for new housing developments to be completely powered by solar systems that are off the grid.

See also[]

• Electrochemical engineering
• List of battery types

References[]

Further reading[]

• Bromine Complexation in Zinc–Bromine Circulating Batteries D. J. Eustace, J. Electrochem. Soc. 127(3), 528–32 (1980)
• Handbook of batteries, 3rd edition. D. Linden, T. B. Reddy. 39.1–39.8 (2002)
• Rose, David M.; Ferreira, Summer R. "Performance Testing of Zinc-Bromine Flow Batteries for Remote Telecom Sites" (PDF). Sandia National Laboratory. Update
• RedFlow.

External links[]

• ZnBr Batteries at the Electricity Storage Association