Zinc-Bromine (ZNBR) Flow Batteries

Executive Summary

The zinc-bromine battery1. Two or more electricAn adjective meaning “needing electricity to operate” such as electric motor or wire. IEEE: Containing, producing , arising from, actuated by or carrying electricity. cells connected together electrically. In common usage, the term “battery” is also applied to a single cell, such as a household battery. 2. A system comprised of identical electrochemical cells. is a hybrid redox flow battery, because much of the energy1. Energy is the potential of a physical system to perform work. (A common unit of work is foot-pound—the amount of energy needed to lift one pound up a distance of one foot.) Energy exists in several forms such as electromagnetic radiation ... is stored by plating zinc metal as a solid onto the anode plates in the electrochemical stack during chargeThe process of injecting energy to be stored into the storage system.. Thus, the total energy storage capacityThe rate at which equipment can either generate, convert or transfer energy. of the system is dependent on both the stack size (electrode area) and the size of the electrolyteFor electrochemical batteries; A chemical compound which, when fused or dissolved in certain solvents, usually water, will conduct an electric current. All electrolytes in the fused state or in solution give rise to ions which conduct the electric... storage reservoirs. As such, the powerThe rate at which energy is generated, converted, transmitted, distributed or delivered. 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 electrodeAn electrical conductor through which an electric current enters or leaves a conducting medium, whether it be an electrolytic solution, solid, molten mass, gas, or vacuum. For electrolytic solutions, many solids, and molten masses, an electrode is an.... 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 dischargeThe process of extracting stored energyThe energy available in the storage system to perform physical work through the conversion of its chemical or mechanical energy, stated in kWh or MWh. from the storage system.

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 densityThe amount of energy that a storage system can store per unit volume occupied by the system. among flow batteries.  However, the high cell voltage and highly oxidative element, bromine, demand1. The rate at which electric energy is delivered to or by a system or part of a system, generally expressed in kilowatts or megawatts, at a given instant or averaged over any designated interval of time. 2. The rate at which energy is being used by... 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 stabilityThe ability of an electric system to maintain a state of equilibrium during normal and abnormal conditions or disturbances. 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.