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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[1].

## 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.

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

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

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

The overall cell reaction is therefore.

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

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

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.