Executive Summary

The zinc-bromine battery is a hybrid redox flow battery, because much of the energy is stored by plating zinc metal as a solid onto the anode plates in the electrochemical stack during charge. Thus, the total energy storage capacity of the system is dependent on both the stack size (electrode area) and the size of the electrolyte storage reservoirs. As such, the power and energy ratings of the zinc-bromine flow battery are not fully decoupled. The zinc-bromine flow battery was developed by Exxon as a hybrid flow battery system in the early 1970s.

Discussion

In each cell of a zinc-bromine battery, two different electrolytes flow past carbon-plastic composite electrodes in two compartments, separated by a micro-porous polyolefin membrane. The electrolyte on the anode (negative) side is purely water-based, while the electrolyte on the positive side also contains an organic amine compound to hold bromine in solution.

During charge, metallic zinc is plated (reduced) as a thick film on the anode side of the carbon-plastic composite electrode. Meanwhile, bromide ions are oxidized to bromine and evolved on the other side of the membrane.  Bromine has limited solubility in water, but the organic amine in the catholyte reacts with the bromine to form a dense, viscous bromine-adduct oil that sinks to the bottom of the catholyte tank. The bromine oil must later be re-mixed with the rest of the catholyte solution to enable discharge.

During discharge, the zinc metal, plated on the anode during charge, is oxidized to Zn2+ ion and dissolved into the aqueous anolyte. Two electrons are released at the anode to do work in the external circuit.  The electrons return to the cathode and reduce bromine molecules (Br2) to bromide ions, which are soluble in the aqueous catholyte solution. The bromine in the catholyte is decomplexed from the amine and converted into two bromide (Br-) ions at the cathode, balancing the Zn2+ cation and forming a zinc bromide solution. The chemical process used to generate the electric current increases the zinc-ion and bromide-ion concentration in both electrolyte tanks. The net DC-DC efficiency of this battery is reported to be in the range of 65-75%.

Practical Challenges

The zinc-bromine redox battery offers one of the highest cell voltages and releases two electrons per atom of zinc.  These attributes combine to offer the highest energy density among flow batteries.  However, the high cell voltage and highly oxidative element, bromine, demand cell electrodes, membranes, and fluid handling components that can withstand the chemical conditions.  These materials are expensive.  Bromine is a highly toxic material through inhalation and absorption.  Maintaining a stable amine complex with the bromine is key to system safety.  Active cooling systems are provided by system manufacturers to maintain stability of the bromine-amine complex when ambient temperatures may exceed 95 oF.  In addition, repeated plating of metals in general is difficult due to the formation of “rough” surfaces (dendrite formation) that can puncture the separator.  Special cell design and operating modes (pulsed discharge during charge) are required to achieve uniform plating and reliable operation.

Conclusion

Integrated Zn/Br energy storage systems have been tested on transportable trailers (up to 1 MW/3 MWh) for utility-scale applications. Multiple systems of this size could be connected in parallel for use in much larger applications. Zn/Br systems are also being supplied at the 5-kW/20-kWh Community Energy Storage (CES) scale, and now being tested by utilities, mostly in Australia.