Charge/discharge efficiency 75.9% Nominal cell voltage 1.8 V | Cycle durability >2,000 cycles | |
Specific energy 34.4–54 W·h/kg (124–190 J/g) Energy density 15.7–39 W·h/L (56.5–140 kJ/L) Energy/consumer-price US$400/kW·h (US$0.11/kJ) | ||
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 34.4–54 W·h/kg.
Contents
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 and is stored in a safe, chemically complexed organic phase in the electrolyte tank. Each high-density polyethylene (HDPE) cell stack has up to 60 bipolar, plastic electrodes between a pair of anode and cathode end blocks.
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.
Features
The primary features of the zinc bromine battery are:
Drawbacks include:
Zinc–bromine flow battery providers include:
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.
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.
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.
The overall cell reaction is therefore.
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 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.
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.
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. Gelion, which is the spin-off company of Sydney University, is currently developing the battery for commercial use. The company was boosted recently by an $11 million investment from UK renewables group Armstrong Energy. 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 which are off the grid.