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# Zinc-bromine flow 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 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 have energy densities of 75 to 85 Wh/kg.

The primary features of the zinc bromine battery are superior energy density; depth of discharge; cycle life; large capacity scale (50 kWh) stackable to 500 kWh systems; and the ability to store energy from any prime mover generating source.

One example of zinc bromine technology is ZBB Energy Corporation’s Zinc Energy Storage System (ZESS), a regenerative fuel cell-type storage system that provides technical performance at a lower overall cost than other energy storage systems such as lead-acid, vanadium redox, sodium sulphur, lithium ion and others. ZESS’ core zinc-bromide energy storage system, based on fuel cell technology, is an aqueous flow battery using a circulation loop to continuously feed reactants to the battery stacks.

The ZESS relies on a flowing electrolyte system. 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 electrode surfaces in the cell stacks. Bromide is converted to bromine at the positive electrode surface of the cell stack and is immediately stored as safe, chemically complexed organic phase in the electrolyte tank. Each fully recyclable high density polyethylene (HDPE) cell stack has 60 bipolar, plastic electrodes between a pair of anode and cathode end blocks.

When the ZESS discharges, the metallic zinc plated on the negative electrodes dissolves in the electrolyte and is available to be plated again at the next charge cycle. In the fully discharged state, it can be left indefinitely for later charge.

## 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 electronegative, 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, according to the following equation.

$Zn _\left\{\left(s\right)\right\} leftrightarrow Zn^\left\{2+\right\} _\left\{\left(aq\right)\right\} + 2e^-$

At the positive electrode bromine is reversibly reduced to bromide, (with a standard reduction potential of +1.087 V vs. SHE) according to the following equation.

$Br _\left\{2\left(aq\right)\right\} + 2e^- leftrightarrow 2Br^- _\left\{\left(aq\right)\right\}$

The overall cell reaction is therefore.

$Zn _\left\{\left(s\right)\right\} + Br _\left\{2\left(aq\right)\right\} leftrightarrow 2Br^- _\left\{\left(aq\right)\right\} + Zn^\left\{2+\right\} _\left\{\left(aq\right)\right\}$

The measured potential difference is around 1.67 V per cell (slightly less than that predicted from the standard reduction potentials).

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 the self-discharge, and also 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 Br2 concentration in the electrolyte.

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